@article{ghasemi_guo_darabi_wang_wang_huang_lefler_taussig_chauhan_baucom_et al._2023, title={A multiscale ion diffusion framework sheds light on the diffusion-stability-hysteresis nexus in metal halide perovskites}, ISSN={["1476-4660"]}, DOI={10.1038/s41563-023-01488-2}, journal={NATURE MATERIALS}, author={Ghasemi, Masoud and Guo, Boyu and Darabi, Kasra and Wang, Tonghui and Wang, Kai and Huang, Chiung-Wei and Lefler, Benjamin M. and Taussig, Laine and Chauhan, Mihirsinh and Baucom, Garrett and et al.}, year={2023}, month={Feb} }
 @article{ghasemi_balar_peng_hu_qin_kim_rech_bidwell_mask_mcculloch_et al._2021, title={A molecular interaction-diffusion framework for predicting organic solar cell stability}, volume={20}, ISSN={["1476-4660"]}, DOI={10.1038/s41563-020-00872-6}, number={4}, journal={NATURE MATERIALS}, author={Ghasemi, Masoud and Balar, Nrup and Peng, Zhengxing and Hu, Huawei and Qin, Yunpeng and Kim, Taesoo and Rech, Jeromy J. and Bidwell, Matthew and Mask, Walker and McCulloch, Iain and et al.}, year={2021}, month={Apr}, pages={525-+} }
 @article{peng_balar_ghasemi_ade_2021, title={Upper and Apparent Lower Critical Solution Temperature Branches in the Phase Diagram of Polymer:Small Molecule Semiconducting Systems}, volume={12}, ISSN={["1948-7185"]}, DOI={10.1021/acs.jpclett.1c02848}, abstractNote={Solution-processable semiconducting materials are complex materials with a wide range of applications. Despite their extensive study and utility, their molecular interactions as manifested, for example, in phase behavior are poorly understood. Here, we aim to understand the phase behavior of conjugated systems by determining phase diagrams spanning extensive temperature ranges for various combinations of the highly disordered semiconducting polymer (PTB7-Th) with crystallizable (IT-M and PC61BM) and noncrystallizable (di-PDI) small molecule acceptors (SMAs), with polystyrene as an amorphous control, a nonsemiconducting commodity polymer. We discover that the apparent binodal of the studied blends frequently consists of an upper critical solution temperature (UCST) and lower critical solution temperature (LCST) branch, exhibiting a sharp kink where the branches join. Our work suggests that phase diagrams might be a probe in combination with sophisticated models to understand the complexity of semiconducting materials, including microstructure and molecular interactions.}, number={44}, journal={JOURNAL OF PHYSICAL CHEMISTRY LETTERS}, author={Peng, Zhengxing and Balar, Nrup and Ghasemi, Masoud and Ade, Harald}, year={2021}, month={Nov}, pages={10845–10853} }
 @article{hu_ghasemi_peng_zhang_rech_you_yan_ade_2020, title={The Role of Demixing and Crystallization Kinetics on the Stability of Non-Fullerene Organic Solar Cells}, volume={32}, ISSN={["1521-4095"]}, url={https://doi.org/10.1002/adma.202005348}, DOI={10.1002/adma.202005348}, abstractNote={With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non-fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room-temperature (RT) diffusion coefficients of 3.4 × 10−23 and 2.0 × 10−22 are measured for ITIC-2Cl and ITIC-2F, two state-of-the-art non-fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC-2F and ITIC-2Cl. The differences as observed by secondary-ion mass spectrometry, differential scanning calorimetry (DSC), grazing-incidence wide-angle X-ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC-4F and ITIC-4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability.}, number={49}, journal={ADVANCED MATERIALS}, publisher={Wiley}, author={Hu, Huawei and Ghasemi, Masoud and Peng, Zhengxing and Zhang, Jianquan and Rech, Jeromy James and You, Wei and Yan, He and Ade, Harald}, year={2020}, month={Dec} }
 @article{carpenter_ghasemi_gann_angunawela_stuard_rech_ritchie_brendan t. o'connor_atkin_you_et al._2019, title={Competition between Exceptionally Long-Range Alkyl Sidechain Ordering and Backbone Ordering in Semiconducting Polymers and Its Impact on Electronic and Optoelectronic Properties}, volume={29}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201806977}, abstractNote={Intra- and intermolecular ordering greatly impacts the electronic and optoelectronic properties of semiconducting polymers. The interrelationship between ordering of alkyl sidechains and conjugated backbones has yet to be fully detailed, despite much prior effort. Here, the discovery of a highly ordered alkyl sidechain phase in six representative semiconducting polymers, determined from distinct spectroscopic and diffraction signatures, is reported. The sidechain ordering exhibits unusually large coherence lengths (≥70 nm), induces torsional/twisting backbone disorder, and results in a vertically multilayered nanostructure with ordered sidechain layers alternating with disordered backbone layers. Calorimetry and in situ variable temperature scattering measurements in a model system poly{4-(5-(4,8-bis(3-butylnonyl)-6-methylbenzo[1,2-b:4,5-b′]dithiophen-2-yl)thiophen-2-yl)-2-(2-butyloctyl)-5,6-difluoro-7-(5-methylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole} (PBnDT-FTAZ) clearly delineate this competition of ordering that prevents simultaneous long-range order of both moieties. The long-range sidechain ordering can be exploited as a transient state to fabricate PBnDT-FTAZ films with an atypical edge-on texture and 2.5× improved field-effect transistor mobility. The observed influence of ordering between the moieties implies that improved molecular design can produce synergistic rather than destructive ordering effects. Given the large sidechain coherence lengths observed, such synergistic ordering should greatly improve the coherence length of backbone ordering and thereby improve electronic and optoelectronic properties such as charge transport and exciton diffusion lengths.}, number={5}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Carpenter, Joshua H. and Ghasemi, Masoud and Gann, Eliot and Angunawela, Indunil and Stuard, Samuel J. and Rech, Jeromy James and Ritchie, Earl and Brendan T. O'Connor and Atkin, Joanna and You, Wei and et al.}, year={2019}, month={Feb} }
 @article{ghasemi_hu_peng_rech_angunawela_carpenter_stuard_wadsworth_mcculloch_you_et al._2019, title={Delineation of Thermodynamic and Kinetic Factors that Control Stability in Non-fullerene Organic Solar Cells}, volume={3}, ISSN={["2542-4351"]}, DOI={10.1016/j.joule.2019.03.020}, abstractNote={Summary Although non-fullerene small molecular acceptors (NF-SMAs) are dominating current research in organic solar cells (OSCs), measurements of thermodynamics drivers and kinetic factors determining their morphological stability are lacking. Here, we delineate and measure such factors in crystallizable NF-SMA blends and discuss four model systems with respect to their meta-stability and degree of vitrification. We determine for the first time the amorphous-amorphous phase diagram in an NF-SMA system and show that its deep quench depth can result in severe burn-in degradation. We estimate the relative phase behavior of four other materials systems. Additionally, we derive room-temperature diffusion coefficients and conclude that the morphology needs to be stabilized by vitrification corresponding to diffusion constants below 10−22 cm2/s. Our results show that to achieve stability via rational molecular design, the thermodynamics, glass transition temperature, diffusion properties, and related structure-function relations need to be more extensively studied and understood.}, number={5}, journal={JOULE}, author={Ghasemi, Masoud and Hu, Huawei and Peng, Zhengxing and Rech, Jeromy James and Angunawela, Indunil and Carpenter, Joshua H. and Stuard, Samuel J. and Wadsworth, Andrew and McCulloch, Iain and You, Wei and et al.}, year={2019}, month={May}, pages={1328–1348} }
 @article{hu_ye_ghasemi_balar_rech_stuard_you_brendan t. o'connor_ade_2019, title={Highly Efficient, Stable, and Ductile Ternary Nonfullerene Organic Solar Cells from a Two-Donor Polymer Blend}, volume={31}, ISSN={["1521-4095"]}, url={https://publons.com/wos-op/publon/18518240/}, DOI={10.1002/adma.201808279}, abstractNote={Organic solar cells (OSCs) are one of the most promising cost-effective options for utilizing solar energy, and, while the field of OSCs has progressed rapidly in device performance in the past few years, the stability of nonfullerene OSCs has received less attention. Developing devices with both high performance and long-term stability remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable mechanical durability. Building upon the ink (toluene:FTAZ:IT-M) that broke the 10% benchmark when blade-coated in air, a second donor material (PBDB-T) is introduced to stabilize and enhance performance with power conversion efficiency over 13% while keeping toluene as the solvent. More importantly, the ternary OSCs exhibit excellent thermal stability and storage stability while retaining high ductility. The excellent performance and stability are mainly attributed to the inhibition of the crystallization of nonfullerene small-molecular acceptors (SMAs) by introducing a stiff donor that also shows low miscibility with the nonfullerene SMA and a slightly higher highest occupied molecular orbital (HOMO) than the host polymer. The study indicates that improved stability and performance can be achieved in a synergistic way without significant embrittlement, which will accelerate the future development and application of nonfullerene OSCs.}, number={17}, journal={ADVANCED MATERIALS}, publisher={Wiley}, author={Hu, Huawei and Ye, Long and Ghasemi, Masoud and Balar, Nrup and Rech, Jeromy James and Stuard, Samuel J. and You, Wei and Brendan T. O'Connor and Ade, Harald}, year={2019}, month={Apr} }
 @article{dang_wang_ghasemi_tang_de bastiani_aydin_dauzon_barrit_peng_smilgies_et al._2019, title={Multi-cation Synergy Suppresses Phase Segregation in Mixed-Halide Perovskites}, volume={3}, ISSN={["2542-4351"]}, DOI={10.1016/j.joule.2019.05.016}, abstractNote={•Reveal phase segregation in mixed-halide perovskite films via in situ observation•Cs+ and Rb+ additions dictate the crystallization pathways and solar cell performance•Direct formation of perovskite phase is beneficial in minimizing halide segregation Having recognized the potential of hybrid organic-inorganic perovskites solar cells, in recent years the photovoltaic community has shifted its focus away from efficiency improvements to simplifying the processing and improving the stability of devices. In this work, we utilize in situ and time-resolved X-ray scattering to track various phase evolutions during the perovskite film solidification to link the microstructure to the composition. In particular, we unravel the crucial roles of Cs+ and Rb+ in promoting the in situ formation of the perovskite phase prior to thermal annealing, thus preventing segregation of halides and cations. Our study points to a significant new guideline for designing mixed-halide mixed-cation perovskites: the sol-gel formulation must possess the ability to convert directly into the targeted perovskite phase without transitioning through compositionally distinct intermediate phases in order to minimize halide segregation and yield-homogenized films. Mixed lead halide perovskite solar cells have been demonstrated to benefit tremendously from the addition of Cs+ and Rb+, but its root cause is yet to be understood. This hinders further improvement, and processing approaches remain largely empirical. We address the challenge by tracking the solidification of precursors in situ and linking the evolutions of different crystalline phases to the presence of Cs+ and Rb+. In their absence, the perovskite film is inherently unstable, segregating into MA-I- and FA-Br-rich phases. Adding either Cs+ or Rb+ is shown to alter the solidification process of the perovskite films. The optimal addition of both Cs+ and Rb+ drastically suppress phase segregation and promotes the spontaneous formation of the desired α phase. We propose that the synergistic effect is due to the collective benefits of Cs+ and Rb+ on the formation kinetics of the α phase and on the halide distribution throughout the film. Mixed lead halide perovskite solar cells have been demonstrated to benefit tremendously from the addition of Cs+ and Rb+, but its root cause is yet to be understood. This hinders further improvement, and processing approaches remain largely empirical. We address the challenge by tracking the solidification of precursors in situ and linking the evolutions of different crystalline phases to the presence of Cs+ and Rb+. In their absence, the perovskite film is inherently unstable, segregating into MA-I- and FA-Br-rich phases. Adding either Cs+ or Rb+ is shown to alter the solidification process of the perovskite films. The optimal addition of both Cs+ and Rb+ drastically suppress phase segregation and promotes the spontaneous formation of the desired α phase. We propose that the synergistic effect is due to the collective benefits of Cs+ and Rb+ on the formation kinetics of the α phase and on the halide distribution throughout the film. On the quest for high-efficiency solar cells at affordable cost, hybrid organic-inorganic metal-halide perovskites have emerged as promising thin-film photovoltaic materials, owing to their outstanding optoelectronic properties for photon-to-electron conversion and tunable bandgap.1Colella S. Mazzeo M. Rizzo A. Gigli G. Listorti A. The bright side of perovskites.J. Phys. Chem. Lett. 2016; 7: 4322-4334Crossref PubMed Scopus (104) Google Scholar, 2Correa-Baena J.-P. Abate A. Saliba M. Tress W. Jesper Jacobsson T.J. Grätzel M. Hagfeldt A. The rapid evolution of highly efficient perovskite solar cells.Energy Environ. Sci. 2017; 10: 710-727Crossref Google Scholar, 3Ono L.K. Juarez-Perez E.J. Qi Y. Progress on perovskite materials and solar cells with mixed cations and halide anions.ACS Appl. Mater. Interfaces. 2017; 9: 30197-30246Crossref PubMed Scopus (368) Google Scholar, 4Berry J. Buonassisi T. Egger D.A. Hodes G. Kronik L. Loo Y.-L. Lubomirsky I. Marder S.R. Mastai Y. Miller J.S. et al.Hybrid organic–inorganic perovskites (HOIPs): opportunities and challenges.Adv. Mater. 2015; 27: 5102-5112Crossref PubMed Scopus (339) Google Scholar Since their first use as sensitizers for solar cells in 2009,5Kojima A. Teshima K. Shirai Y. Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.J. Am. Chem. Soc. 2009; 131: 6050-6051Crossref PubMed Scopus (15167) Google Scholar perovskite solar cells have rapidly achieved impressive power conversion efficiencies (PCE), which now stands at 24.2% for single-junction solar cells.6https://www.nrel.gov/pv/assets/images/efficiency-chart.pngGoogle Scholar These results underline the remarkable progress both from the perspectives of chemical compositional engineering7Jeon N.J. Noh J.H. Yang W.S. Kim Y.C. Ryu S. Seo J. Seok S.I. Compositional engineering of perovskite materials for high-performance solar cells.Nature. 2015; 517: 476-480Crossref PubMed Scopus (4901) Google Scholar (or solid-state alloying8Li Z. Yang M. Park J.-S. Wei S.-H. Berry J.J. Zhu K. Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium and cesium lead iodide solid-state alloys.Chem. Mater. 2016; 28: 284-292Crossref Scopus (1279) Google Scholar, 9Zhou Y. Zhou Z. Chen M. Zong Y. Huang J. Pang S. Padture N.P. Doping and alloying for improved perovskite solar cells.J. Mater. Chem. A. 2016; 4: 17623-17635Crossref Scopus (131) Google Scholar) as well as process and device optimization.10Xiao M. Huang F. Huang W. Dkhissi Y. Zhu Y. Etheridge J. Gray-Weale A. Bach U. Cheng Y.B. Spiccia L. A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells.Angew. Chem. Int. Ed. 2014; 53: 9898-9903Crossref PubMed Scopus (1296) Google Scholar, 11Jeon N.J. Noh J.H. Kim Y.C. Yang W.S. Ryu S. Seok S.I. Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells.Nat. Mater. 2014; 13: 897-903Crossref PubMed Scopus (5280) Google Scholar The AMX3 crystallographic structure of these perovskites is characterized by a monovalent cation (A) such as methylammonium (MA+), formamidinium (FA+), or Cs+; a divalent metal cation (M) such as Pb2+ or Sn2+; and a halide anion (X) such as Cl−, Br−, or I−. Record efficiencies require the perovskite to crystallize in the α-phase, which has a trigonal symmetry (space group P3m1, often called black phase,9Zhou Y. Zhou Z. Chen M. Zong Y. Huang J. Pang S. Padture N.P. Doping and alloying for improved perovskite solar cells.J. Mater. Chem. A. 2016; 4: 17623-17635Crossref Scopus (131) Google Scholar, 12Stoumpos C.C. Malliakas C.D. Kanatzidis M.G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties.Inorg. Chem. 2013; 52: 9019-9038Crossref PubMed Scopus (3935) Google Scholar, 13Weller M.T. Weber O.J. Frost J.M. Walsh A. Cubic perovskite structure of black formamidinium lead iodide, α-[HC(NH2)2]PbI3, at 298 K.J. Phys. Chem. Lett. 2015; 6: 3209-3212Crossref Scopus (363) Google Scholar for the color of the perovskite film). Unwanted non-perovskite phases such as the hexagonal symmetry (P63mc) δ-phase (or yellow phase9Zhou Y. Zhou Z. Chen M. Zong Y. Huang J. Pang S. Padture N.P. Doping and alloying for improved perovskite solar cells.J. Mater. Chem. A. 2016; 4: 17623-17635Crossref Scopus (131) Google Scholar, 12Stoumpos C.C. Malliakas C.D. Kanatzidis M.G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties.Inorg. Chem. 2013; 52: 9019-9038Crossref PubMed Scopus (3935) Google Scholar) often lead to poor photovoltaic performance. Currently, optimal perovskite compositions are based on FA+ as the majority cation. The cubic perovskite α-FAPbI3 phase has a band gap of 1.48 eV, which is closer to the ideal single-junction device Shockley-Queisser bandgap (1.34 eV) than MAPbI3 (1.57 eV).13Weller M.T. Weber O.J. Frost J.M. Walsh A. Cubic perovskite structure of black formamidinium lead iodide, α-[HC(NH2)2]PbI3, at 298 K.J. Phys. Chem. Lett. 2015; 6: 3209-3212Crossref Scopus (363) Google Scholar However, the α-FAPbI3 phase is thermodynamically unstable in ambient conditions and undergoes a phase transition to a non-perovskite, photo-inactive, yellow δ-FAPbI3 phase. Notwithstanding recent progress in stabilizing the pure α-FAPbI3 phase using reduced dimensional and molecular passivants,14Niu T. Lu J. Tang M.-C. Barrit D. Smilgies D.M. Yang Z. Li J. Fan Y. Luo T. McCulloch I. et al.High performance ambient-air-stable FAPbI3 perovskite solar cells with molecule-passivated Ruddlesden-Popper/3D heterostructured film.Energy Environ. Sci. 2019; 11Google Scholar incorporation of MA+ has been shown to alleviate this phase instability issue, resulting in significantly more stable and efficient solar cells.15Pellet N. Gao P. Gregori G. Yang T.Y. Nazeeruddin M.K. Maier J. Grätzel M. Mixed-organic-cation perovskite photovoltaics for enhanced solar-light harvesting.Angew. Chem. Int. Ed. 2014; 53: 3151-3157Crossref PubMed Scopus (1037) Google Scholar In addition, mixing well-controlled Br− concentrations in the MAPbI3 lattice results in mixed-halide (I− and Br−) perovskites with a more stable crystal structure and better long-term durability. This can be attributed to the smaller ionic radius of Br− compared to I−, which transforms the tetragonal phase I4/mcm symmetry of MAPbI3 into the Pm3m cubic phase.16Zarick H.F. Soetan N. Erwin W.R. Bardhan R. Mixed halide hybrid perovskites: a paradigm shift in photovoltaics.J. Mater. Chem. A. 2018; 6: 5507-5537Crossref Google Scholar Therefore, despite the fact that the widened bandgap somewhat shifts away from the Shockley-Queisser optimum, the mixed-halide (I− and Br−) perovskites offer better performance when compared to triiodide perovskites as well as tunable bandgaps, which are critical in tandem application of wider bandgap perovskites. The latter depends on the halide ratio in the precursors. It is noted, however, that mixed-halide compositions are not a necessity for achieving high-performance devices, as recent reports on FAPbI3-based devices demonstrate.17Zhao Y. Tan H. Yuan H. Yang Z. Fan J.Z. Kim J. Voznyy O. Gong X. Quan L.N. Tan C.S. et al.Perovskite seeding growth of formamidinium-lead-iodide-based perovskites for efficient and stable solar cells.Nat. Commun. 2018; 9: 1607Crossref PubMed Scopus (250) Google Scholar Instead, mixed halides appear to significantly facilitate the processing and manufacture of high-quality perovskite thin films. Solar cells prepared from mixed-cation (MA+ and FA+) and mixed-halide (I− and Br−) precursors are more reproducible and thermally stable when adding ∼ 5% (molar) Cs+ to the perovskite precursor formulation.18Saliba M. Matsui T. Seo J.Y. Domanski K. Correa-Baena J.P. Nazeeruddin M.K. Zakeeruddin S.M. Tress W. Abate A. Hagfeldt A. et al.Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency.Energy Environ. Sci. 2016; 9: 1989-1997Crossref PubMed Google Scholar Addition of Cs+ was found to enhance phase stabilization by tuning the tolerance factor through solid-state alloying.8Li Z. Yang M. Park J.-S. Wei S.-H. Berry J.J. Zhu K. Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium and cesium lead iodide solid-state alloys.Chem. Mater. 2016; 28: 284-292Crossref Scopus (1279) Google Scholar In parallel, Cs+ suppresses the photo-inactive δ-phase of pure FAPbI3 and facilitates the formation of the desired photoactive α-phase, resulting in more homogenous, low-defect perovskite films.18Saliba M. Matsui T. Seo J.Y. Domanski K. Correa-Baena J.P. Nazeeruddin M.K. Zakeeruddin S.M. Tress W. Abate A. Hagfeldt A. et al.Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency.Energy Environ. Sci. 2016; 9: 1989-1997Crossref PubMed Google Scholar Gratia et al. showed that the crystallization sequence of mixed-halide perovskite films is 2H-4H-6H-3C and unveiled that the addition of 3% Cs+ provides a shortcut to the 3C phase by inhibiting the hexagonal intermediate phases.19Gratia P. Zimmermann I. Schouwink P. Yum J.H. Audinot J.N. Sivula K. Wirtz T. Nazeeruddin M.K. The many faces of mixed ion perovskites: unraveling and understanding the crystallization process.ACS Energy Lett. 2017; 2: 2686-2693Crossref Scopus (106) Google Scholar We adopt here the Ramsdell notation, widely used for oxide perovskites and also used by Gratia et al.,19Gratia P. Zimmermann I. Schouwink P. Yum J.H. Audinot J.N. Sivula K. Wirtz T. Nazeeruddin M.K. The many faces of mixed ion perovskites: unraveling and understanding the crystallization process.ACS Energy Lett. 2017; 2: 2686-2693Crossref Scopus (106) Google Scholar to accurately describe the intermediate phases during perovskite film formation. The classical AMX3 perovskite structure shows various polytypes depending on the stacking sequence of the close-packed AX3 layers. For example, the well-known yellow δ phase observed in lead halide perovskites is composed of pure hexagonal closed-packed AX3 layers, resulting in a 1D array of BX6 face-sharing octahedra, namely the 2H phase. Moreover, the photoactive α phase is composed of cubic closed-packed AX3 layers, resulting in a 3D framework of BX6 corner-sharing octahedral, namely the 3C phase. The perovskite can also crystallize in a 3R phase, resembling the 3C phase but with rhombohedral symmetry instead of cubic symmetry. The other two common intermediate phases, 4H and 6H, are composed of a combination of a hexagonal and cubic closed-packed AX3 stacking sequence, resulting in a 3D framework of both face-sharing and corner-sharing BX6 octahedra.19Gratia P. Zimmermann I. Schouwink P. Yum J.H. Audinot J.N. Sivula K. Wirtz T. Nazeeruddin M.K. The many faces of mixed ion perovskites: unraveling and understanding the crystallization process.ACS Energy Lett. 2017; 2: 2686-2693Crossref Scopus (106) Google Scholar Recently, Rb+ addition has been reported to further improve the performance of mixed-halide mixed-cation lead perovskite solar cells, exhibiting a PCE of 21.6% with remarkable stability under continuous illumination at elevated temperature.20Saliba M. Matsui T. Domanski K. Seo J.Y. Ummadisingu A. Zakeeruddin S.M. Correa-Baena J.P. Tress W.R. Abate A. Hagfeldt A. et al.Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance.Science. 2016; 354: 206-209Crossref PubMed Scopus (2765) Google Scholar Subsequently, several studies investigated the effect of Rb+ and Cs+ on the resulting perovskite films.21Kubicki D.J. Prochowicz D. Hofstetter A. Zakeeruddin S.M. Grätzel M. Emsley L. Phase segregation in Cs-, Rb- and K-doped mixed-cation (MA)x(FA)1–xPbI3 Hybrid Perovskites from Solid-State NMR.J. Am. Chem. Soc. 2017; 139: 14173-14180Crossref PubMed Scopus (262) Google Scholar, 22Jacobsson T.J. Svanström S. Andrei V. Rivett J.P.H. Kornienko N. Philippe B. Cappel U.B. Rensmo H. Deschler F. Boschloo G. Extending the compositional space of mixed lead halide perovskites by Cs, Rb, K, and Na doping.J. Phys. Chem. C. 2018; 122: 13548-13557Crossref Scopus (63) Google Scholar, 23Philippe B. Saliba M. Correa-Baena J.-P. Cappel U.B. Turren-Cruz S.-H. Grätzel M. Hagfeldt A. Rensmo H. Chemical distribution of multiple cation (Rb+, Cs+, MA+, and FA+) perovskite materials by photoelectron spectroscopy.Chem. Mater. 2017; 29: 3589-3596Crossref Scopus (156) Google Scholar, 24Yinghong H. M. Philipp H.E. Irene R. Jonas G. H. F. G. Matthias H.A. Thomas H. et al.Understanding the role of cesium and rubidium additives in perovskite solar cells: trap states, charge transport, and recombination.Adv. Energy. Mater. 2018; 8Google Scholar, 25Hu Y. Aygüler M.F. Petrus M.L. Bein T. Docampo P. Impact of rubidium and cesium cations on the moisture stability of multiple-cation mixed-halide perovskites.ACS Energy Lett. 2017; 2: 2212-2218Crossref Scopus (139) Google Scholar Kubicki et al. found that Rb+ is not incorporated into the perovskite structure as initially thought but may rather “passivate” the resulting thin film.21Kubicki D.J. Prochowicz D. Hofstetter A. Zakeeruddin S.M. Grätzel M. Emsley L. Phase segregation in Cs-, Rb- and K-doped mixed-cation (MA)x(FA)1–xPbI3 Hybrid Perovskites from Solid-State NMR.J. Am. Chem. Soc. 2017; 139: 14173-14180Crossref PubMed Scopus (262) Google Scholar Philippe et al.23Philippe B. Saliba M. Correa-Baena J.-P. Cappel U.B. Turren-Cruz S.-H. Grätzel M. Hagfeldt A. Rensmo H. Chemical distribution of multiple cation (Rb+, Cs+, MA+, and FA+) perovskite materials by photoelectron spectroscopy.Chem. Mater. 2017; 29: 3589-3596Crossref Scopus (156) Google Scholar employed hard X-ray photoelectron spectroscopy to investigate the chemical composition and chemical distribution at different probing depths of the perovskite films. They found that along with the unreacted formamidinium iodide (FAI), ∼3% Rb+ and ∼8% Cs+ are homogeneously distributed up to 18 nm below the surface, resulting in an overall improvement of the cell’s open-circuit voltage (VOC).23Philippe B. Saliba M. Correa-Baena J.-P. Cappel U.B. Turren-Cruz S.-H. Grätzel M. Hagfeldt A. Rensmo H. Chemical distribution of multiple cation (Rb+, Cs+, MA+, and FA+) perovskite materials by photoelectron spectroscopy.Chem. Mater. 2017; 29: 3589-3596Crossref Scopus (156) Google Scholar Using a combination of characterization techniques, Hu et al.24Yinghong H. M. Philipp H.E. Irene R. Jonas G. H. F. G. Matthias H.A. Thomas H. et al.Understanding the role of cesium and rubidium additives in perovskite solar cells: trap states, charge transport, and recombination.Adv. Energy. Mater. 2018; 8Google Scholar found that Rb+ addition enhances the film’s carrier mobility whereas Cs+ addition leads to a significant reduction of trap density in the perovskite crystals. In a recent study,26Yadav P. Dar M.I. Arora N. Alharbi E.A. Giordano F. Zakeeruddin S.M. Grätzel M. The role of rubidium in multiple-cation-based high-efficiency perovskite solar cells.Adv. Mater. 2017; 29Crossref Scopus (103) Google Scholar improvements in the VOC and fill factor (FF) were found for solar cells with Rb+ addition, which was argued to originate from lower recombination and improved extraction of holes at the interface of perovskite and spiro-OMeTAD. However, collectively, these studies have only probed the residual effect of added Cs+ and Rb+ on the properties of perovskite films and devices via ex situ characterization techniques, i.e., by characterizing the final state of the resulting films. In this work, we investigate in detail the impact of Cs+ and Rb+ addition upon perovskite film formation by tracking the evolution of the crystal phases in situ with the help of time-resolved grazing incidence wide-angle x-ray scattering (GIWAXS) measurements performed during spin coating.27Yang B. Keum J.K. Geohegan D.B. Xiao K. Kumar C.S.S.R. In-situ characterization techniques for nanomaterials. Springer, 2018: 33-60Crossref Scopus (1) Google Scholar, 28Munir R. Sheikh A.D. Abdelsamie M. Hu H. Yu L. Zhao K. Kim T. Tall O.E. Li R. Smilgies D.-M. et al.Hybrid perovskite thin-film photovoltaics: in situ diagnostics and importance of the precursor solvate phases.Adv. Mater. 2017; 29PubMed Google Scholar, 29Masi S. Rizzo A. Munir R. Listorti A. Giuri A. Esposito Corcione C. Treat N.D. Gigli G. Amassian A. Stingelin N. et al.Organic gelators as growth control agents for stable and reproducible hybrid perovskite-based solar cells.Adv. Energy Mater. 2017; 7Crossref PubMed Scopus (72) Google Scholar, 30Meng K. Wu L. Liu Z. Wang X. Xu Q. Hu Y. He S. Li X. Li T. Chen G. In situ real-time study of the dynamic formation and conversion processes of metal halide perovskite films.Adv. Mater. 2018; 30Crossref Scopus (45) Google Scholar, 31Schlipf J. Müller-Buschbaum P. Structure of organometal halide perovskite films as determined with grazing-incidence X-ray scattering methods.Adv. Energy Mater. 2017; 7PubMed Google Scholar, 32Richter L.J. DeLongchamp D.M. Amassian A. Morphology development in solution-processed functional organic blend films: an in situ viewpoint.Chem. Rev. 2017; 117: 6332-6366Crossref PubMed Scopus (133) Google Scholar Supported also by time-of-flight secondary ion mass spectrometry (ToF-SIMS) halide mapping, we reveal that the phase-segregation-induced chemical segregation—which inherently occurs in pristine formulations—can be suppressed when Cs+ and Rb+ are added together in the right amount. In doing so, Cs+ and Rb+ also promote the direct transformation of the precursor into the photoactive 3C (often known as α) phase without requiring thermal annealing to initiate the conversion process. This is reflected in the significantly improved photovoltaic performance of devices made from the optimal combinations of Cs+ and Rb+, reaching 20.1% PCE as opposed to 15.3% from the pristine precursor. We go on to show a direct link between the ease of direct transformation of the perovskite phase and the achievable PCE within mixed-halide mixed-cation solar cells. Such detailed knowledge of the crystallization processes highlights the key role of controlling phase segregation and benefits the development of new design rules for compositions containing atomic additives intended to mitigate halide segregation, facilitate phase transformation and overall ease of processing, and ultimately yield hybrid perovskite films with excellent optoelectronic properties and reproducibility. We synthesized a set of perovskite films containing different amounts of CsI and/or RbI added into the pristine precursor. The pristine perovskite precursor serving as the control sample has a stoichiometry of (FA0.83MA0.17)Pb(I0.83Br0.17)3. Thin films resulting from the addition of Cs+ and/or Rb+ are coded for convenience as (% Cs and % Rb), where % Cs and % Rb are, respectively, molar percentages of added CsI and RbI solution relative to the mixed cations. All test samples have a total addition of 10% of Cs+ and/or Rb+ [e.g., (1,9), (3,7), (5,5), (7,3), and (9,1)], with the exception of three cases: (5,0), (0,5), and the control (0,0). All perovskite films are prepared using a one-step spin-coating process with so-called antisolvent drip and used chlorobenzene as quenching solvent,10Xiao M. Huang F. Huang W. Dkhissi Y. Zhu Y. Etheridge J. Gray-Weale A. Bach U. Cheng Y.B. Spiccia L. A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells.Angew. Chem. Int. Ed. 2014; 53: 9898-9903Crossref PubMed Scopus (1296) Google Scholar followed by annealing at 100°C for 20 min. We refer to the Experimental Procedures for more details on solution preparation and device fabrication. To unveil the mechanism underlying the improved efficiency, as well as the roles of added Cs+ and Rb+, we took advantage of time-resolved GIWAXS to diagnose the solution-to-solid transformation of the various perovskite precursors, both with and without the addition of Cs+ and/or Rb+. To mimic the perovskite crystallization in as-cast films, we performed these measurements during spin coating using an earlier developed setup,28Munir R. Sheikh A.D. Abdelsamie M. Hu H. Yu L. Zhao K. Kim T. Tall O.E. Li R. Smilgies D.-M. et al.Hybrid perovskite thin-film photovoltaics: in situ diagnostics and importance of the precursor solvate phases.Adv. Mater. 2017; 29PubMed Google Scholar, 32Richter L.J. DeLongchamp D.M. Amassian A. Morphology development in solution-processed functional organic blend films: an in situ viewpoint.Chem. Rev. 2017; 117: 6332-6366Crossref PubMed Scopus (133) Google Scholar, 33Chou K.W. Yan B. Li R. Li E.Q. Zhao K. Anjum D.H. Alvarez S. Gassaway R. Biocca A. Thoroddsen S.T. et al.Spin-cast bulk heterojunction solar cells: a dynamical investigation.Adv. Mater. 2013; 25: 1923-1929Crossref PubMed Scopus (156) Google Scholar, 34Wei Chou K. Ullah Khan H. Niazi M.R. Yan B. Li R. Payne M.M. Anthony J.E. Smilgies D.-M. Amassian A. Late stage crystallization and healing during spin-coating enhance carrier transport in small-molecule organic semiconductors.J. Mater. Chem. C. 2014; 2: 5681-5689Crossref Google Scholar and also dripped chlorobenzene (CB) as solvent quencher during in situ measurements.29Masi S. Rizzo A. Munir R. Listorti A. Giuri A. Esposito Corcione C. Treat N.D. Gigli G. Amassian A. Stingelin N. et al.Organic gelators as growth control agents for stable and reproducible hybrid perovskite-based solar cells.Adv. Energy Mater. 2017; 7Crossref PubMed Scopus (72) Google Scholar, 35Niu T. Lu J. Munir R. Li J. Barrit D. Zhang X. Hu H. Yang Z. Amassian A. Zhao K. Stable high-performance perovskite solar cells via grain boundary passivation.Adv. Mater. 2018; 30Crossref Scopus (551) Google Scholar, 36Li J. Munir R. Fan Y. Niu T. Liu Y. Zhong Y. Yang Z. Tian Y. Liu B. Sun J. et al.Phase transition control for high-performance blade-coated perovskite solar cells.Joule. 2018; 2: 1313-1330Abstract Full Text Full Text PDF Scopus (138) Google Scholar, 37Zhang X. Munir R. Xu Z. Liu Y. Tsai H. Nie W. Li J. Niu T. Smilgies D.-M. Kanatzidis M.G. et al.Phase transition control for high performance Ruddlesden–Popper perovskite solar cells.Adv. Mater. 2018; 30Google Scholar, 38Quintero-Bermudez R. Gold-Parker A. Proppe A.H. Munir R. Yang Z. Kelley S.O. Amassian A. Toney M.F. Sargent E.H. Compositional and orientational control in metal halide perovskites of reduced dimensionality.Nat. Mater. 2018; 17: 900-907Crossref PubMed Scopus (271) Google Scholar See Experimental Procedures and Figure S1 for more details on GIWAXS measurements. The time evolution of the 2D intensity map with respect to the scattering vector q and process time t for the pristine (control) precursor is shown in Figure 1A. The 2D intensity map was constructed by cake integration (Figure S2). Prior to CB dripping, the initial dominant scattering feature is characterized by a halo at low q values (∼4–7 nm−1) associated with the colloidal state of the sol-gel precursor.28Munir R. Sheikh A.D. Abdelsamie M. Hu H. Yu L. Zhao K. Kim T. Tall O.E. Li R. Smilgies D.-M. et al.Hybrid perovskite thin-film photovoltaics: in situ diagnostics and importance of the precursor solvate phases.Adv. Mater. 2017; 29PubMed Google Scholar Upon CB dripping (indicated in Figure 1A with a vertical arrow), the sol-gel halo disappears and diffraction peaks appear instead. Scattering peaks at lower q, namely ∼5 and ∼7 nm−1 are associated with the presence of a solvated crystalline phase (S solvate phase).28Munir R. Sheikh A.D. Abdelsamie M. Hu H. Yu L. Zhao K. Kim T. Tall O.E. Li R. Smilgies D.-M. et al.Hybrid perovskite thin-film photovoltaics: in situ diagnostics and importance of the precursor solvate phases.Adv. Mater. 2017; 29PubMed Google Scholar, 39Barrit D. Sheikh A.D. Munir R. Barbé J.M. Li R. Smilgies D.-M. Amassian A. Hybrid perovskite solar cells: in situ investigation of solution-processed PbI 2 reveals metastable precursors and a pathway to producing porous thin films.J. Mater. Res. 2017; 32: 1899-1907Crossref Scopus (22) Google Scholar We detect a comparatively faint diffraction signal at ∼10 nm−1, where the 6H and 3C phases are known to be.28Munir R. Sheikh A.D. Abdelsamie M. Hu H. Yu L. Zhao K. Kim T. Tall O.E. Li R. Smilgies D.-M. et al.Hybrid perovskite thin-film photovoltaics: in situ diagnostics and importance of the precursor solvate phases.Adv. Mater. 2017; 29PubMed Google Scholar In the region between 8–9 nm−1, we see a broad scattering feature composed of two contributions at q = 8.4 and 8.8 nm−1. These two phases have distinct peak positions and compositions and will later be discussed to be the 2H and 4H hexagonal phases. Our GIWAXS results are supported by X-ray diffraction (XRD) measurements of the as-prepared thin films (see Figure S3) where solvated, 4H, 2H, and 6H crystalline phases are, respectively, located at 2θ values of ∼(6.8° and 9.2°), (11.6° and 13.0°), 11.8°, and (12.2° and 14.0°). We note that peaks of the 6H (102) and 3C (100) phases, both observed at q ∼ 10 nm−1 in the GIWAXS, are positioned at very close 2θ values of 14.0° and 14.1° respectively, in the XRD of thin films (Figure S3). We therefore distinguish the two phases by the presence of an additional 6H (101) peak at 12.2°. Briefly, we found that the 3C phase is observed in films prepared from Cs+-added precursors whereas the 6H phase is observed in other films (pristine, as well as 5% Rb+- and 10% Rb+-added films). Figure 1B shows 2D GIWAXS snapshots at select moments (10, 40, and 350 s) after initiating the spin coating of the precursor, revealing more details about the in-plane texture of the different phases in the as-cast film. The identity of the solvate (S) and intermediates, including so-called δ phases, are not always well-defined in the literature,7Jeon N.J. Noh J.H. Yang W.S. Kim Y.C. Ryu S. Seo J. Seok S.I. Compositional engineering of perovskite materials for high-performance solar cells.Nature. 2015; 517: 476-480Crossref PubMed Scopus (4901) Google Scholar, 40Kim J. Saidaminov M.I. Tan H. Zhao Y. Kim Y. Choi J. Jo J.W. Fan J. Quintero-Bermudez R. Yang Z. et al.Amide-catalyzed phase-selective crystallization reduces defect density in wide-bandgap perovskites.Adv. Mater. 2018; 30Google Scholar, 41Deng Y. Dong Q. Bi C. Yuan Y. Huang J. Air-stable, efficient mixed-cation perovskite solar cells with Cu electrode by scalable fabrication of active layer.Adv. Energy. Mater. 2016; 6Crossref Scopus (247) Google Scholar mainly because of the singular composition of the studied perovskite precursors and the presence of only one diffr}, number={7}, journal={JOULE}, author={Dang, Hoang X. and Wang, Kai and Ghasemi, Masoud and Tang, Ming-Chun and De Bastiani, Michele and Aydin, Erkan and Dauzon, Emilie and Barrit, Dounya and Peng, Jun and Smilgies, Detlef-M and et al.}, year={2019}, month={Jul}, pages={1746–1764} }
 @article{ye_li_liu_zhang_ghasemi_xiong_hou_ade_2019, title={Quenching to the Percolation Threshold in Organic Solar Cells}, volume={3}, ISSN={["2542-4351"]}, url={https://doi.org/10.1016/j.joule.2018.11.006}, DOI={10.1016/j.joule.2018.11.006}, abstractNote={•Quench depth of a high-efficiency nonfullerene system was determined•Quench depth, formation kinetics, and percolation threshold were correlated•Morphology needs to be kinetically quenched for deep quench depth systems Organic photovoltaic (OPV) cells are a potential clean-energy technology that provides an earth-abundant, light-weight, and low-energy-production photovoltaic solution. Particularly, OPVs based on emerging nonfullerene small-molecule acceptors have enjoyed significant attention in recent years. The fundamental relationships between molecular interaction, formation kinetics, and device performance remain unexplored for these nonfullerene solar cells and therefore become an imperative research goal in the community. A framework is highly desired for accelerating the development of more performant devices. Here, we discovered the need to kinetically quench the morphology of state-of-the-art nonfullerene systems if the thermodynamic interaction of constituent materials is too repulsive. Most fundamentally, these relations formulate basic rules for optimizing morphology in device performance by significantly guiding improvements in fabrication yield, reliability, and stability. The general lack of knowing the quench depth and the convolution with key kinetic factors has confounded deeper understanding of the respective importance of these factors in the morphology development of organic solar cells. Here, we determine the quench depth of a high-efficiency system and delineate the need to kinetically quench the mixed domains to a composition close to the percolation threshold. Importantly, the ability to achieve such a quench is very sensitive to structural parameters in polymer solar cells (PSCs) of the polymer PBDB-TF. Only the highest-molecular-weight polymer is able of earlier liquid-solid transition to “lock in” a high-performing PSC morphology with a composition above the miscibility limit and with an efficiency of over 13%. Systems with deep quench depths are therefore sensitive to molecular weight and the kinetic factors of the casting, likely impacting fabrication yield and reliability. They also need to be vitrified for stable performance. The general lack of knowing the quench depth and the convolution with key kinetic factors has confounded deeper understanding of the respective importance of these factors in the morphology development of organic solar cells. Here, we determine the quench depth of a high-efficiency system and delineate the need to kinetically quench the mixed domains to a composition close to the percolation threshold. Importantly, the ability to achieve such a quench is very sensitive to structural parameters in polymer solar cells (PSCs) of the polymer PBDB-TF. Only the highest-molecular-weight polymer is able of earlier liquid-solid transition to “lock in” a high-performing PSC morphology with a composition above the miscibility limit and with an efficiency of over 13%. Systems with deep quench depths are therefore sensitive to molecular weight and the kinetic factors of the casting, likely impacting fabrication yield and reliability. They also need to be vitrified for stable performance. A considerable amount of research has been devoted to creating new π-functional materials, for instance, organic small molecules and conjugated polymers for the use in bulk-heterojunction polymer solar cells (PSCs).1Zhang J.Q. Tan H.S. Guo X.G. Facchetti A. Yan H. Material insights and challenges for non-fullerene organic solar cells based on small molecular acceptors.Nat. Energy. 2018; 3: 720-731Crossref Scopus (646) Google Scholar, 2Hou J. Inganäs O. Friend R.H. Gao F. Organic solar cells based on non-fullerene acceptors.Nat. Mater. 2018; 17: 119-128Crossref PubMed Google Scholar, 3Yan C. Barlow S. Wang Z. Yan H. Jen A.K.Y. Marder S.R. Zhan X. Non-fullerene acceptors for organic solar cells.Nat. Rev. Mater. 2018; 3https://doi.org/10.1038/natrevmats.2018.3Crossref PubMed Scopus (1785) Google Scholar, 4Huang Y. Kramer E.J. Heeger A.J. Bazan G.C. Bulk heterojunction solar cells: morphology and performance relationships.Chem. Rev. 2014; 114: 7006-7043Crossref PubMed Scopus (1040) Google Scholar, 5Cheng P. Li G. Zhan X. Yang Y. Next-generation organic photovoltaics based on non-fullerene acceptors.Nat. Photon. 2018; 12: 131-142Crossref Scopus (1278) Google Scholar In stark contrast, less attention has been placed on understanding the critical morphology drivers of PSCs and a general framework that guides the morphology optimization is especially lacking in the community.6Kouijzer S. Michels J.J. van den Berg M. Gevaerts V.S. Turbiez M. Wienk M.M. Janssen R.A.J. Predicting morphologies of solution processed polymer:fullerene blends.J. Am. Chem. Soc. 2013; 135: 12057-12067Crossref PubMed Scopus (241) Google Scholar, 7Vandewal K. Himmelberger S. Salleo A. Structural factors that affect the performance of organic bulk heterojunction solar cells.Macromolecules. 2013; 46: 6379-6387Crossref Scopus (135) Google Scholar, 8Müller C. Ferenczi T.A.M. Campoy-Quiles M. Frost J.M. Bradley D.D.C. Smith P. Stingelin-Stutzmann N. Nelson J. Binary organic photovoltaic blends: a simple rationale for optimum compositions.Adv. Mater. 2008; 20: 3510-3515Crossref Scopus (360) Google Scholar, 9Treat N.D. Chabinyc M.L. Phase separation in bulk heterojunctions of semiconducting polymers and fullerenes for photovoltaics.Annu. Rev. Phys. Chem. 2014; 65: 59-81Crossref PubMed Scopus (93) Google Scholar Sufficient electron percolating pathways for transport have been shown to be an important morphological requisite for achieving desirable performance in PSCs.10Ye L. Collins B.A. Jiao X. Zhao J. Yan H. Ade H. Miscibility-function relations in organic solar cells: significance of optimal miscibility in relation to percolation.Adv. Energy Mater. 2018; 8: 1703058Crossref Scopus (194) Google Scholar, 11Liu Y. Zhao J. Li Z. Mu C. Ma W. Hu H. Jiang K. Lin H. Ade H. Yan H. et al.Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells.Nat. Commun. 2014; 5: 5293Crossref PubMed Scopus (2727) Google Scholar, 12Li N. Perea J.D. Kassar T. Richter M. Heumueller T. Matt G.J. Hou Y. Güldal N.S. Chen H. Chen S. et al.Abnormal strong burn-in degradation of highly efficient polymer solar cells caused by spinodal donor-acceptor demixing.Nat. Commun. 2017; 8https://doi.org/10.1038/ncomms14541Crossref Scopus (263) Google Scholar The composition corresponding to the lower limit for electron transport is the percolation threshold.13Isichenko M.B. Percolation, statistical topography, and transport in random media.Rev. Mod. Phys. 1992; 64: 961-1043Crossref Scopus (1098) Google Scholar Recent progress in measuring amorphous-amorphous phase diagrams and thus quench depth, i.e., the depth that describes how deep a system is located inside the two-phase region of phase diagram at its initial composition (see Figure 1A),14Ye L. Hu H. Ghasemi M. Wang T. Collins B.A. Kim J.-H. Jiang K. Carpenter J. Li H. Li Z. et al.Quantitative relations between interaction parameter, miscibility and function in organic solar cells.Nat. Mater. 2018; 17: 253-260Crossref PubMed Scopus (432) Google Scholar has yielded quantitative understanding of structure-function relations in cases where the mixed domains have a purity above this percolation threshold. In such a case, device performance can be quantitatively correlated with phase purity and the Flory-Huggins interaction parameter χ as exemplified in the study of systems with shallow quench depths such as poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):phenyl-C71-butyric acid methyl ester (PC71BM) blend14Ye L. Hu H. Ghasemi M. Wang T. Collins B.A. Kim J.-H. Jiang K. Carpenter J. Li H. Li Z. et al.Quantitative relations between interaction parameter, miscibility and function in organic solar cells.Nat. Mater. 2018; 17: 253-260Crossref PubMed Scopus (432) Google Scholar (as illustrated in Figure 1A). Conversely, a substantial drop in average device efficiency is observed15Bartelt J.A. Beiley Z.M. Hoke E.T. Mateker W.R. Douglas J.D. Collins B.A. Tumbleston J.R. Graham K.R. Amassian A. Ade H. et al.The importance of fullerene percolation in the mixed regions of polymer-fullerene bulk heterojunction solar cells.Adv. Energy Mater. 2013; 3: 364-374Crossref Scopus (406) Google Scholar once the composition of the mixed amorphous domains is below the percolation threshold. For instance, thermal annealing of poly(di(2ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-co-octylthieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD):PCBM blends corresponds to a deep quench depth and leads to over-purification of the mixed domains, thereby leading to a poor performance. PffBT4T:PCBM even over-purifies at room temperature within 5 days,12Li N. Perea J.D. Kassar T. Richter M. Heumueller T. Matt G.J. Hou Y. Güldal N.S. Chen H. Chen S. et al.Abnormal strong burn-in degradation of highly efficient polymer solar cells caused by spinodal donor-acceptor demixing.Nat. Commun. 2017; 8https://doi.org/10.1038/ncomms14541Crossref Scopus (263) Google Scholar and is inferred to be a deep quench system. Collectively, fundamental and general guidelines are required to avoid undesired, laborious efforts and to rationally optimize material systems with various quench depths. The use of Hansen solubility parameters and surface energy measurements provide estimates, but both are unreliable.16Ghasemi M. Ye L. Zhang Q. Yan L. Kim J.H. Awartani O. You W. Gadisa A. Ade H. Panchromatic sequentially cast ternary polymer solar cells.Adv. Mater. 2017; 29: 1604603Crossref Scopus (78) Google Scholar Direct measurements of the amorphous phase diagram at operating and processing conditions are rare. The amorphous-amorphous phase diagram and quench depth have only been measured for one fullerene-based system14Ye L. Hu H. Ghasemi M. Wang T. Collins B.A. Kim J.-H. Jiang K. Carpenter J. Li H. Li Z. et al.Quantitative relations between interaction parameter, miscibility and function in organic solar cells.Nat. Mater. 2018; 17: 253-260Crossref PubMed Scopus (432) Google Scholar to date, and little is known about the required kinetic control (i.e., the timing of the liquid-liquid (LL) and liquid-solid (LS) phase transitions) to quench-in not only a small length scale, but to quench the mixed domains to the percolation threshold for PSC systems in general and for emerging nonfullerene small-molecule acceptors in particular. Among the π-conjugated polymers for high-performance nonfullerene PSCs, poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T) (Figure 1B) and its derivatives are the largest categories of donor polymers17Ye L. Jiao X. Zhou M. Zhang S. Yao H. Zhao W. Xia A. Ade H. Hou J. Manipulating aggregation and molecular orientation in all-polymer photovoltaic cells.Adv. Mater. 2015; 27: 6046-6054Crossref PubMed Scopus (254) Google Scholar, 18Zhao W. Li S. Yao H. Zhang S. Zhang Y. Yang B. Hou J. Molecular optimization enables over 13% efficiency in organic solar cells.J. Am. Chem. Soc. 2017; 139: 7148-7151Crossref PubMed Scopus (2294) Google Scholar, 19Li S. Ye L. Zhao W. Zhang S. Mukherjee S. Ade H. Hou J. Energy-level modulation of small-molecule electron acceptors to achieve over 12% efficiency in polymer solar cells.Adv. Mater. 2016; 28: 9423-9429Crossref PubMed Scopus (1244) Google Scholar, 20Li W. Ye L. Li S. Yao H. Ade H. Hou J. A high efficiency organic solar cell enabled by strong intramolecular electron push-pull effect of non-fullerene acceptor.Adv. Mater. 2018; 30: e1707170Crossref PubMed Scopus (341) Google Scholar, 21Fei Z. Eisner F.D. Jiao X. Azzouzi M. Röhr J.A. Han Y. Shahid M. Chesman A.S.R. Easton C.D. McNeill C.R. et al.An alkylated indacenodithieno[3,2-b]thiophene-based nonfullerene acceptor with high crystallinity exhibiting single junction solar cell efficiencies greater than 13% with low voltage losses.Adv. Mater. 2018; 30: e1800728Crossref PubMed Scopus (24) Google Scholar, 22Zhang S. Qin Y. Zhu J. Hou J. Over 14% efficiency in polymer solar cells enabled by a chlorinated polymer donor.Adv. Mater. 2018; 30: e1800868Crossref PubMed Scopus (924) Google Scholar, 23Fan Q. Zhu Q. Xu Z. Su W. Chen J. Wu J. Guo X. Ma W. Zhang M. Li Y. et al.Chlorine substituted 2d-conjugated polymer for high-performance polymer solar cells with 13.1% efficiency via toluene processing.Nano Energy. 2018; 48: 413-420Crossref Scopus (222) Google Scholar that were extensively used in the field and contributed to the latest record-efficiency tandem cell.24Meng L. Zhang Y. Wan X. Li C. Zhang X. Wang Y. Ke X. Xiao Z. Ding L. Xia R. et al.Organic and solution-processed tandem solar cells with 17.3% efficiency.Science. 2018; 361: 1094-1098Crossref PubMed Scopus (2020) Google Scholar As shown in Figure 1C, PBDB-TF is a doubly fluorinated version25Zhang M. Guo X. Ma W. Ade H. Hou J. A large-bandgap conjugated polymer for versatile photovoltaic applications with high performance.Adv. Mater. 2015; 27: 4655-4660Crossref PubMed Scopus (679) Google Scholar of PBDB-T and exhibits strong temperature-dependent aggregation in solution state. Very recently, power conversion efficiency (PCE) of ∼13.5% was reported20Li W. Ye L. Li S. Yao H. Ade H. Hou J. A high efficiency organic solar cell enabled by strong intramolecular electron push-pull effect of non-fullerene acceptor.Adv. Mater. 2018; 30: e1707170Crossref PubMed Scopus (341) Google Scholar, 26Fan Q. Su W. Wang Y. Guo B. Jiang Y. Guo X. Liu F. Thomas P.R. Zhang M. Li Y. et al.Synergistic effect of fluorination on both donor and acceptor materials for high performance non-fullerene polymer solar cells with 13.5% efficiency.Sci. China Chem. 2018; 61: 531-537Crossref Scopus (321) Google Scholar in single-junction, binary, nonfullerene PSCs by matching PBDB-TF with a fluorinated small-molecule acceptor named 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (IT-4F) (see Figure 1B). For these high-performance systems, the device performance metrics are often sensitive to their casting process,27Qian D. Ye L. Zhang M. Liang Y. Li L. Huang Y. Guo X. Zhang S. Tan Z.A. Hou J. et al.Design, application, and morphology study of a new photovoltaic polymer with strong aggregation in solution state.Macromolecules. 2012; 45: 9611-9617Crossref Scopus (587) Google Scholar, 28Ye L. Zhao W. Li S. Mukherjee S. Carpenter J.H. Awartani O. Jiao X. Hou J. Ade H. High-efficiency nonfullerene organic solar cells: critical factors that affect complex multi-length scale morphology and device performance.Adv. Energy Mater. 2017; 7: 1602000Crossref Scopus (215) Google Scholar which likely impacts repeatability for different materials batches and between laboratories and even co-workers using slightly different implementations of nominal procedures. So far, thermodynamic drivers and kinetic pathways of morphology formation of these critically important polymers and their dependence on structure parameters (i.e., polymer molecular weight) are not well understood and no single-phase diagram of nonfullerene small-molecule acceptor-based systems has been determined. Only different solvent mixtures are typically used in trial-and-error fashion to manipulate the casting kinetics by monitoring the final device performance. To this end, measurement and an in-depth understanding of quench depth, the key thermodynamic driver, and its relations with percolation threshold and kinetics is profoundly desired for accelerating the development of more high-performing PSCs. Such understanding is particularly important, as any system that needs to be kinetically quenched for best performance to a composition within the two-phase region of the phase diagram will also have to be vitrified for stable operation. Motived by these considerations, here we determine the quench depth at room temperature and use PBDB-TF with varied molecular weights to delineate the importance of kinetic factors and instructive morphology-performance relations in state-of-the-art nonfullerene PSC systems from the perspective of thermodynamics and thus are able to reveal general morphological benefits of high-molecular-weight polymer donors in PSCs. Utilizing a combination of characterization tools including time-of-flight secondary ion mass spectroscopy (TOF-SIMS) and hard/soft X-ray scattering, we systematically examine the molecular packing, lateral phase separation, vertical composition profiles, equilibrium composition, molecular aggregation, and quench depth of the model system PBDB-TF:IT-4F as a function of the polymer molecular weight. Crucially, we discover that the PSC morphology needs to be kinetically quenched if the thermodynamic interaction of constituent materials is unfavorable and too repulsive (with binodal composition well below the percolation threshold). It is found that the blend system with the highest-molecular-weight polymer exhibits the smallest long period and a composition of mixed domains near the percolation threshold and thus highest device PCE in PBDB-TF:IT-4F devices. This finding is also effective in explaining the morphology evolution previously observed in FTAZ:PCBM devices29Li W. Yang L. Tumbleston J.R. Yan L. Ade H. You W. Controlling molecular weight of a high efficiency donor-acceptor conjugated polymer and understanding its significant impact on photovoltaic properties.Adv. Mater. 2014; 26: 4456-4462Crossref PubMed Scopus (185) Google Scholar with varied molecular weights. Additional investigations of a nonfluorinated polymer PBDB-T with high- and low-molecular-weight batches also shows a similar trend, strengthening the validity of the molecular weight-morphology-device performance relations observed and the need to kinetically quench the composition in the mixed domains which can only be achieved effectively with the highest molecular weight. Most fundamentally, these new findings on formation kinetics in deep quench-depth systems together with prior quantitative interaction parameter χ-performance relations14Ye L. Hu H. Ghasemi M. Wang T. Collins B.A. Kim J.-H. Jiang K. Carpenter J. Li H. Li Z. et al.Quantitative relations between interaction parameter, miscibility and function in organic solar cells.Nat. Mater. 2018; 17: 253-260Crossref PubMed Scopus (432) Google Scholar formulate a more complete guideline for optimizing PSC morphology by relying much less on trial-and-error efforts. We first synthesize four batches of PBDB-TF (hereafter referred to as MWx, x = 1–4) as the model system to start our study. By tuning the polymerization parameters (solvent, temperature, and time), various molecular weights are obtained as confirmed by high-temperature gel permeation chromatography characterizations (see Figure S1). As shown in Scheme 1, the polymerization of PBDB-TF is carried out with two commercially available monomers via palladium-catalyzed Stille-coupling approach as reported.25Zhang M. Guo X. Ma W. Ade H. Hou J. A large-bandgap conjugated polymer for versatile photovoltaic applications with high performance.Adv. Mater. 2015; 27: 4655-4660Crossref PubMed Scopus (679) Google Scholar The number-average molecular weights of MW1–4 are determined to be 51.2, 39.5, 30.8, and 27.6 kDa, respectively. All of them show a very comparable polydispersity (PDI) of ∼2, which eliminates PDI as a significant variable and thus permits us to explore the molecular weight effect exclusively. The molecular weight of MW1 is about 1.9 times as high as that of MW4, and therefore the solubility of MW1 is lower. We note that MW1 is among the highest-molecular-weight batches that we are able to make and attaining higher-molecular-weight PBDB-TF is limited by the material solubility in common chlorinated solvents. This observation is generally consistent with a report by Li et al.29Li W. Yang L. Tumbleston J.R. Yan L. Ade H. You W. Controlling molecular weight of a high efficiency donor-acceptor conjugated polymer and understanding its significant impact on photovoltaic properties.Adv. Mater. 2014; 26: 4456-4462Crossref PubMed Scopus (185) Google Scholar on a benzodithiophene-based polymer PBnDT-FTAZ, where the highest MW guided by the Carothers equation is still less than 70 kDa even after careful purifications of monomers and catalysts. To understand the influence of structure parameter (polymer molecular weight) on the photovoltaic characteristics of nonfullerene PSC devices, we first fabricated nonfullerene PSCs with MWx polymers using an identical device configuration of glass/indium oxide/PEDOT:PSS/MWx:IT-4F/PFN-Br/Al at NCSU. Chlorobenzene is used as the host solvent and a trace amount (0.5% volume) of 1,8-diiodooctane is introduced as the solvent additive. Figure 2A shows the current density (J)-voltage (V) curves of optimized MWx:IT-4F devices under AM 1.5G simulated solar irradiation at 100 mW/cm2. The corresponding device parameters are averaged from the same batch of devices and listed in Table 1. MWx-based devices demonstrated very similar open-circuit voltage (Voc) of 0.84 ± 0.01 V, while clear changes in Jsc and fill factor (FF) values were observed. MW4:IT-4F devices show the lowest device efficiency with a relatively poor FF of ∼70% and short-circuit current density (Jsc) of ∼18.9 mA/cm2. The device PCE increases with the polymer molecular weight. Owing to the highest Jsc of ∼20.8 mA/cm2 and FF of ∼77%, the MW1 device shows the highest PCE of 13.4%. We note that the best device efficiency obtained here is very comparable with the certificated results reported earlier.20Li W. Ye L. Li S. Yao H. Ade H. Hou J. A high efficiency organic solar cell enabled by strong intramolecular electron push-pull effect of non-fullerene acceptor.Adv. Mater. 2018; 30: e1707170Crossref PubMed Scopus (341) Google Scholar The estimated Jsc values from external quantum efficiency curves as illustrated in Figure 2B show a consistent trend in the wavelength range (400–800 nm): MW1>MW2>MW3>MW4. In addition, independent device experiments on the same material batches were performed at the Institute of Chemistry, Chinese Academy of Sciences, and the obtained performance metrics are shown in Figure S2. Both datasets point to a consistent and monotonic trend that device PCEs vary inversely with the molecular weight of PBDB-TF. Even higher molecular weight might be beneficial, although synthesis of such materials is difficult. To understand the difference in photovoltaic performance and its morphological origins, we will show below the morphological details from the aspect of both thermodynamics and kinetics using a set of probing tools.Table 1Photovoltaic and Morphological Parameters of Nonfullerene PSCs Based on PBDB-TF (MWx):IT-4FBlendVoc (V)Jsc (mA/cm2)FF (%)PCE (%)aStatistical results of a full batch of devices are listed here and the highest values are shown in the parentheses.MW1:IT-4F0.840 ± 0.001 (0.839)20.61 ± 0.14 (20.79)77.09 ± 0.24 (76.86)13.35 ± 0.05 (13.41)MW2:IT-4F0.833 ± 0.002 (0.833)19.88 ± 0.25 (20.09)75.55 ± 1.15 (76.37)12.50 ± 0.13 (12.77)MW3:IT-4F0.845 ± 0.002 (0.850)19.56 ± 0.11 (19.51)74.53 ± 0.33 (74.96)12.32 ± 0.07 (12.43)MW4:IT-4F0.834 ± 0.002 (0.838)19.00 ± 0.14 (18.89)69.69 ± 0.33 (70.16)10.72 ± 0.04 (11.05)a Statistical results of a full batch of devices are listed here and the highest values are shown in the parentheses. Open table in a new tab First, TOF-SIMS was applied to measure the amorphous miscibility/quench depth of IT-4F in polymers with different molecular weights. The procedure is similar to that described in our previous study.14Ye L. Hu H. Ghasemi M. Wang T. Collins B.A. Kim J.-H. Jiang K. Carpenter J. Li H. Li Z. et al.Quantitative relations between interaction parameter, miscibility and function in organic solar cells.Nat. Mater. 2018; 17: 253-260Crossref PubMed Scopus (432) Google Scholar Figure S3 shows the depth profiles of unannealed and solvent-annealed PBDB-TF/IT-4F bilayers. By selectively measuring the C2N− molecular fragment that traces IT-4F, we are able to extract the local thermodynamic equilibrium stoichiometry (or binodal composition) as depicted in Figure 3A. Importantly, but not unexpectedly, a comparable value of ∼7% was obtained for all the MWx samples. It can be inferred that the local stoichiometry of PBDB-TF:IT-4F blend is not sensitive to polymer molecular weight. For all MWx polymers, the corresponding χ within a classic Flory-Huggins framework would be ∼2.3 (see Figure 1A). We conclude that the quench depth is the same for all PBDB-TF:IT-4F blends. Hence, the differences observed in the device performance are originating from kinetic factors during casting, the impact of which we will study by determining morphological parameters. In addition, the percolation threshold for electron transport can be estimated by monitoring the change of electron mobility of polymer-based films via gradually increasing the acceptor loading.30Vakhshouri K. Kozub D.R. Wang C. Salleo A. Gomez E.D. Effect of miscibility and percolation on electron transport in amorphous poly(3-hexylthiophene)/phenyl-c61-butyric acid methyl ester blends.Phys. Rev. Lett. 2012; 108: 026601Crossref PubMed Scopus (98) Google Scholar To experimentally determine the percolation threshold for electron transport, here we utilized electron mobilities derived from dark J-V curves of electron-only diodes (Figure S4). Electron mobility can be extracted properly by applying the Mott-Gurney law.31Jason A.R. Davide M. Saif A.H. Thomas K. Jenny N. Exploring the validity and limitations of the Mott-Gurney law for charge-carrier mobility determination of semiconducting thin-films.J. Phys. Condens. Matter. 2018; 30: 105901Crossref PubMed Scopus (89) Google Scholar Shown in Figure 3B is the electron mobility of PBDB-TF as a function of IT-4F loading, which is varied from 0% to 40% (weight percentage). The electron mobility remains at a low level of ∼10−5 cm2 V−1 s−1 when the IT-4F loading is below 20%. However, there is a sharp increase in electron mobility when the IT-4F loading is increased from 20% to 30%. This observation implies that the percolation threshold composition is in the 20%–30% range. As a result, the percolation threshold of IT-4F in PBDB-TF:IT-4F blends is thus approximately 25% by volume. As the thermodynamic equilibrium compositions of all batches are ∼7%, well below the percolation threshold, the thermodynamic interaction of PBDB-TF and IT-4F is unfavorable and exceedingly repulsive. The devices cannot have mixed domains that have reached local equilibrium and the morphology development of the mixed domains needs to be kinetically quenched to a composition inside the two-phase region of the phase diagram. To put morphological characterization and molecular packing on a solid footing to provide causative structure-function correlations, UV-visible absorption spectra of both neat and blend films were acquired. Figure S5 indicates that all the polymers exhibit a comparable absorption coefficient of 1 × 105 cm−1 in neat films. Also, the optimized MWx:IT-4F blend films do not show much difference in absorbance (Figure S6), which largely rules out the optical effect on device performance. Similarly, TOF-SIMS depth profiling was conducted on PBDB-TF:IT-4F blend films to measure their vertical composition profiles and its potential impact. No significant difference in vertical component distribution is observed for MW1-4, and vertical gradients can be also eliminated as an important parameter (for details, see Figure S7). Molecular-scale structure order of blend films was probed with grazing incidence wide-angle X-ray scattering (GIWAXS).32Hexemer A. Bras W. Glossinger J. Schaible E. Gann E. Kirian R. MacDowell A. Church M. Rude B. Padmore H. et al.A SAXS/WAXS/GISAXS beamline with multilayer monochromator.J. Phys. Conf. Ser. 2010; 247: 012007Crossref Scopus (502) Google Scholar The 2D patterns of MW1-4:IT-4F blends are summarized in Figure 4A. Figures 4B and 4C show the 1D profiles extracted along in-plane and out-of-plane directions from the 2D patterns, respectively. As neat IT-4F is highly disordered (see Figure S8), the scattering peaks of blend films at q values of 1.7–1.8 Å−1 in the out-of-plane direction are mainly due to the π-π stacking of polymer backbones, and (100), (200), and (300) peaks respectively at q = 0.32, 0.64, and 0.95 Å−1 correspond to lamellar stacking of alkyl side chains of PBDB-TF. The d spacings of these diffraction peaks remain constant for MWx:IT-4F blends. In addition, no pronounced difference of orientation ordering and peak intensities are detected. Consequently, molecular packing of PBDB-TF:IT-4F film is not sensitive to the polymer molecular weight. This can be also supported by the GIWAXS results of neat polymers (as depicted in Figure S9). Due to the quite comparable packing at the nanoscale and profiles in the vertical direction, lateral mesoscale morphology needs to be further investigated to understand the differences in device performance. Next, to quantify the in-plane phase separation of the MW1-4:IT-4F blends, we apply carbon K-edge resonant soft X-ray scattering (R-SoXS) on beamline 11.0.1.233Gann E. Young A.T. Collins B.A. Yan H. Nasiatka J. Padmore H.A. Ade H. Hexemer A. Wang C. Soft X-ray scattering facility at the advanced light source with real-time data processing and analysis.Rev. Sci. Instrum. 2012; 83: 045110Crossref PubMed Scopus (392) Google Scholar at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. A photon energy of 283.8 eV was selected to provide high material contrast (see Figure S10). Reduction of raw 2D R}, number={2}, journal={JOULE}, publisher={Elsevier BV}, author={Ye, Long and Li, Sunsun and Liu, Xiaoyu and Zhang, Shaoqing and Ghasemi, Masoud and Xiong, Yuan and Hou, Jianhui and Ade, Harald}, year={2019}, month={Feb}, pages={443–458} }
 @article{zhu_gadisa_peng_ghasemi_ye_xu_zhao_ade_2019, title={Rational Strategy to Stabilize an Unstable High-Efficiency Binary Nonfullerene Organic Solar Cells with a Third Component}, volume={9}, ISSN={["1614-6840"]}, url={https://doi.org/10.1002/aenm.201900376}, DOI={10.1002/aenm.201900376}, abstractNote={Long device lifetime is still a missing key requirement in the commercialization of nonfullerene acceptor (NFA) organic solar cell technology. Understanding thermodynamic factors driving morphology degradation or stabilization is correspondingly lacking. In this report, thermodynamics is combined with morphology to elucidate the instability of highly efficient PTB7-Th:IEICO-4F binary solar cells and to rationally use PC71BM in ternary solar cells to reduce the loss in the power conversion efficiency from ≈35% to <10% after storage for 90 days and at the same time improve performance. The hypomiscibility observed for IEICO-4F in PTB7-Th (below the percolation threshold) leads to overpurification of the mixed domains. By contrast, the hypermiscibility of PC71BM in PTB7-Th of 48 vol% is well above the percolation threshold. At the same time, PC71BM is partly miscible in IEICO-4F suppressing crystallization of IEICO-4F. This work systematically illustrates the origin of the intrinsic degradation of PTB7-Th:IEICO-4F binary solar cells, demonstrates the structure–function relations among thermodynamics, morphology, and photovoltaic performance, and finally carries out a rational strategy to suppress the degradation: the third component needs to have a miscibility in the donor polymer at or above the percolation threshold, yet also needs to be partly miscible with the crystallizable acceptor.}, number={20}, journal={ADVANCED ENERGY MATERIALS}, publisher={Wiley}, author={Zhu, Youqin and Gadisa, Abay and Peng, Zhengxing and Ghasemi, Masoud and Ye, Long and Xu, Zheng and Zhao, Suling and Ade, Harald}, year={2019}, month={May} }
 @article{kim_schaefer_ma_zhao_turner_ghasemi_constantinou_so_yan_gadisa_et al._2019, title={The Critical Impact of Material and Process Compatibility on the Active Layer Morphology and Performance of Organic Ternary Solar Cells}, volume={9}, ISSN={["1614-6840"]}, url={https://doi.org/10.1002/aenm.201802293}, DOI={10.1002/aenm.201802293}, abstractNote={Although ternary solar cells (TSCs) offer a cost-effective prospect to expand the absorption bandwidth of organic solar cells, only few TSCs have succeeded in surpassing the performance of binary solar cells (BSCs) primarily due to the complicated morphology of the ternary blends. Here, the key factors that create and limit the morphology and performance of the TSCs are elucidated. The origin of morphology formation is explored and the role of kinetic factors is investigated. The results reveal that the morphology of TSC blends considered in this study are characterized with either a single length-scale or two length-scale features depending on the composition of the photoactive polymers in the blend. This asymmetric morphology development reveals that TSC blend morphology critically depends on material compatibility and polymer solubility. Most interestingly, the fill factor (FF) of TSCs is found to linearly correlate with the relative standard deviation of the fullerene distribution at small lengths. This is the first time that such a correlation has been shown for ternary systems. The criteria that uniform sized and highly pure amorphous domains are accomplished through the correct kinetic path to obtain a high FF for TSCs are specifically elucidated. The findings provide a critical insight for the precise design and processing of TSCs.}, number={2}, journal={ADVANCED ENERGY MATERIALS}, author={Kim, Joo-Hyun and Schaefer, Charley and Ma, Tingxuan and Zhao, Jingbo and Turner, Johnathan and Ghasemi, Masoud and Constantinou, Iordania and So, Franky and Yan, He and Gadisa, Abay and et al.}, year={2019}, month={Jan} }
 @article{ye_hu_ghasemi_wang_collins_kim_jiang_carpenter_li_li_et al._2018, title={Quantitative relations between interaction parameter, miscibility and function in organic solar cells}, volume={17}, ISSN={["1476-4660"]}, url={https://doi.org/10.1038/s41563-017-0005-1}, DOI={10.1038/s41563-017-0005-1}, number={3}, journal={NATURE MATERIALS}, publisher={Springer Nature}, author={Ye, Long and Hu, Huawei and Ghasemi, Masoud and Wang, Tonghui and Collins, Brian A. and Kim, Joo-Hyun and Jiang, Kui and Carpenter, Joshua H. and Li, Hong and Li, Zhengke and et al.}, year={2018}, month={Mar}, pages={253–260} }
 @article{bin_yang_zhang_ye_ghasem_chen_zhang_zhang_sun_xue_et al._2017, title={9.73% Efficiency Nonfullerene All Organic Small Molecule Solar Cells with Absorption-Complementary Donor and Acceptor}, volume={139}, ISSN={["0002-7863"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=MEDLINE&KeyUT=MEDLINE:28322045&KeyUID=MEDLINE:28322045}, DOI={10.1021/jacs.6b12826}, abstractNote={In the last two years, polymer solar cells (PSCs) developed quickly with n-type organic semiconductor (n-OSs) as acceptor. In contrast, the research progress of nonfullerene organic solar cells (OSCs) with organic small molecule as donor and the n-OS as acceptor lags behind. Here, we synthesized a D–A structured medium bandgap organic small molecule H11 with bithienyl-benzodithiophene (BDTT) as central donor unit and fluorobenzotriazole as acceptor unit, and achieved a power conversion efficiency (PCE) of 9.73% for the all organic small molecules OSCs with H11 as donor and a low bandgap n-OS IDIC as acceptor. A control molecule H12 without thiophene conjugated side chains on the BDT unit was also synthesized for investigating the effect of the thiophene conjugated side chains on the photovoltaic performance of the p-type organic semiconductors (p-OSs). Compared with H12, the 2D-conjugated H11 with thiophene conjugated side chains shows intense absorption, low-lying HOMO energy level, higher hole mobility and ordered bimodal crystallite packing in the blend films. Moreover, a larger interaction parameter (χ) was observed in the H11 blends calculated from Hansen solubility parameters and differential scanning calorimetry measurements. These special features combined with the complementary absorption of H11 donor and IDIC acceptor resulted in the best PCE of 9.73% for nonfullerene all small molecule OSCs up to date. Our results indicate that fluorobenzotriazole based 2D conjugated p-OSs are promising medium bandgap donors in the nonfullerene OSCs.}, number={14}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, publisher={American Chemical Society (ACS)}, author={Bin, Haijun and Yang, Yankang and Zhang, Zhi-Guo and Ye, Long and Ghasem, Masoud and Chen, Shanshan and Zhang, Yindong and Zhang, Chunfeng and Sun, Chenkai and Xue, Lingwei and et al.}, year={2017}, month={Apr}, pages={5085–5094} }
 @article{zhao_ye_li_liu_zhang_zhang_ghasemi_he_ade_hou_et al._2017, title={Environmentally-friendly solvent processed fullerenefree organic solar cells enabled by screening halogen-free solvent additives}, volume={60}, ISSN={["2199-4501"]}, url={https://publons.com/wos-op/publon/5290953/}, DOI={10.1007/s40843-017-9080-x}, abstractNote={Though the power conversion efficiencies (PCEs) of organic solar cells (OSCs) have been boosted to 12%, the use of highly pollutive halogenated solvents as the processing solvent significantly hinders the mass production of OSCs. It is thus necessary to achieve high-efficiency OSCs by utilizing the halogen-free and environmentally-friendly solvents. Herein, we applied a halogen-free solvent system ( o -xylene/1-phenylnaphthalene, XY/PN) for fabricating fullerene-free OSCs, and a high PCE of 11.6% with a notable fill factor (FF) of 72% was achieved based on the PBDB-T:IT-M blend, which is among the top efficiencies of halogen-free solvent processed OSCs. In addition, the influence of different halogen-free solvent additives on the blend morphology and device performance metrics was studied by synchrotron-based tools and other complementary methods. Morphological results indicate the highly ordered molecular packing and highest average domain purity obtained in the blend films prepared by using XY/PN co-solvent are favorable for achieving increased FFs and thus higher PCEs in the devices. Moreover, a lower interaction parameter ( χ ) of the IT-M:PN pair provides a good explanation for the more favorable morphology and performance in devices with PN as the solvent additive, relative to those with diphenyl ether and N -methylpyrrolidone. Our study demonstrates that carefully screening the non-halogenated solvent additive plays a vital role in realizing the efficient and environmentally-friendly solvent processed OSCs.}, number={8}, journal={SCIENCE CHINA-MATERIALS}, author={Zhao, W. C. and Ye, Long and Li, S. S. and Liu, X. Y. and Zhang, S. Q. and Zhang, Y. and Ghasemi, M. and He, C. and Ade, H. and Hou, J. H. and et al.}, year={2017}, month={Aug}, pages={697–706} }
 @article{ghasemi_ye_zhang_yan_kim_awartani_you_gadisa_ade_2017, title={Panchromatic Sequentially Cast Ternary Polymer Solar Cells}, volume={29}, ISSN={0935-9648}, url={http://dx.doi.org/10.1002/ADMA.201604603}, DOI={10.1002/adma.201604603}, abstractNote={A sequential-casting ternary method is developed to create stratified bulk heterojunction (BHJ) solar cells, in which the two BHJ layers are spin cast sequentially without the need of adopting a middle electrode and orthogonal solvents. This method is found to be particularly useful for polymers that form a mechanically alloyed morphology due to the high degree of miscibility in the blend.}, number={4}, journal={Advanced Materials}, publisher={Wiley}, author={Ghasemi, Masoud and Ye, Long and Zhang, Qianqian and Yan, Liang and Kim, Joo-Hyun and Awartani, Omar and You, Wei and Gadisa, Abay and Ade, Harald}, year={2017}, month={Jan}, pages={1604603} }
 @article{ye_xiong_li_ghasemi_balar_turner_gadisa_hou_o’connor_ade_et al._2017, title={Precise Manipulation of Multilength Scale Morphology and Its Influence on Eco-Friendly Printed All-Polymer Solar Cells}, volume={27}, ISSN={1616-301X}, url={http://dx.doi.org/10.1002/ADFM.201702016}, DOI={10.1002/adfm.201702016}, abstractNote={Significant efforts have lead to demonstrations of nonfullerene solar cells (NFSCs) with record power conversion efficiency up to ≈13% for polymer:small molecule blends and ≈9% for all-polymer blends. However, the control of morphology in NFSCs based on polymer blends is very challenging and a key obstacle to pushing this technology to eventual commercialization. The relations between phases at various length scales and photovoltaic parameters of all-polymer bulk-heterojunctions remain poorly understood and seldom explored. Here, precise control over a multilength scale morphology and photovoltaic performance are demonstrated by simply altering the concentration of a green solvent additive used in blade-coated films. Resonant soft X-ray scattering is used to elucidate the multiphasic morphology of these printed all-polymeric films and complements with the use of grazing incidence wide-angle X-ray scattering and in situ spectroscopic ellipsometry characterizations to correlate the morphology parameters at different length scales to the device performance metrics. Benefiting from the highest relative volume fraction of small domains, additive-free solar cells show the best device performance, strengthening the advantage of single benign solvent approach. This study also highlights the importance of high volume fraction of smallest domains in printed NFSCs and organic solar cells in general.}, number={33}, journal={Advanced Functional Materials}, publisher={Wiley}, author={Ye, Long and Xiong, Yuan and Li, Sunsun and Ghasemi, Masoud and Balar, Nrup and Turner, Johnathan and Gadisa, Abay and Hou, Jianhui and O’Connor, Brendan T. and Ade, Harald and et al.}, year={2017}, month={Jul}, pages={1702016} }
 @article{kim_gadisa_schaefer_yao_gautam_balar_ghasemi_constantinou_so_o'connor_et al._2017, title={Strong polymer molecular weight-dependent material interactions: impact on the formation of the polymer/fullerene bulk heterojunction morphology}, volume={5}, ISSN={2050-7488 2050-7496}, url={http://dx.doi.org/10.1039/C7TA03052E}, DOI={10.1039/c7ta03052e}, abstractNote={The morphological evolution is initiated by L–L or L–S phase separation (left) and further developed by molecular mobility, governed by polymer–solvent interactions which determine the final domain size of the BHJ layer (right).}, number={25}, journal={Journal of Materials Chemistry A}, publisher={Royal Society of Chemistry (RSC)}, author={Kim, Joo-Hyun and Gadisa, Abay and Schaefer, Charley and Yao, Huifeng and Gautam, Bhoj R. and Balar, Nrup and Ghasemi, Masoud and Constantinou, Iordania and So, Franky and O'Connor, Brendan T. and et al.}, year={2017}, pages={13176–13188} }
 @article{ye_xiong_yao_dinku_zhang_li_ghasemi_balar_hunt_o'connor_et al._2016, title={High Performance Organic Solar Cells Processed by Blade Coating in Air from a Benign Food Additive Solution}, volume={28}, ISSN={0897-4756 1520-5002}, url={http://dx.doi.org/10.1021/ACS.CHEMMATER.6B03083}, DOI={10.1021/acs.chemmater.6b03083}, abstractNote={Solution processable conjugated organic materials have gained tremendous interest motivated by their potential of low cost, lightweight and especially easy manufacturing of large-area and flexible electronics. Toxic halogen-containing solvents have been widely used in the processing of organic electronics, particularly organic photovoltaics (OPVs). To transition this technology to more commercially attractive manufacturing approaches, removing these halogenated solvents remains one of the key challenges. Our morphological (hard/soft X-ray scattering) and calorimetric characterizations reveal that using o-methylanisole, a certified food additive, as processing solvent can achieve similar crystalline properties and domain spacing/purity with that achieved by widely used binary halogenated solvents (chlorobenzene and 1,8-diiodooctane), thus yielding comparable photovoltaic performance in spin-casted films. To move a step forward, we further present the potential of o-methylanisole as processing solvent in th...}, number={20}, journal={Chemistry of Materials}, publisher={Link}, author={Ye, L. and Xiong, Y. and Yao, H. and Dinku, A.G. and Zhang, H. and Li, S. and Ghasemi, M. and Balar, N. and Hunt, A. and O'Connor, B.T. and et al.}, year={2016}, pages={7451–7458} }