@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={Abstract 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={Abstract 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={•NF-OSCs with optimal miscibility are intrinsically stable against demixing•Crystallization of NF-SMA needs to be suppressed through proper vitrification•Polymers and NF-SMA with high Tg are needed to achieve long-term OSCs stability In recent years, the performance of organic solar cells (OSCs) has greatly improved with the development of novel non-fullerene small molecular acceptors (NF-SMA). The rapid increase in power conversion efficiency, now surpassing 15%, highlights an immediate and increasing need to understand the longevity and lifetime of NF-OSCs. However, the field relies mainly on a laborious trial-and-error approach to select polymer:NF-SMA pairs with desirable device stability. Here, we provide a structure-property relation that explains the morphological stability and burn-in degradation due to excessive demixing or crystallization. The framework presented in our study shows that a specific balance of interactions between polymer and NF-SMA can offer a short-term solution against excessive demixing. Long-term morphological stability that also suppresses crystallization can only be achieved by freezing in the initial quenched morphology through the use of polymers and/or NF-SMAs with low flexibility. 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. 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. 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Burn-in driven by over-purification of mixed domains and/or crystallization have been extensively studied and linked to morphological instability in fullerene-based systems,22Li 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; 8: 14541Crossref PubMed Scopus (263) Google Scholar, 30Mendaza A.D. Melianas A. Rossbauer S. Bäcke O. Nordstierna L. Erhart P. Olsson E. Anthopoulos T.D. Inganäs O. Müller C. High-entropy mixtures of pristine fullerenes for solution-processed transistors and solar cells.Adv. Mater. 2015; 27: 7325-7331Crossref PubMed Scopus (45) Google Scholar, 31Lindqvist C. Bergqvist J. Bäcke O. Gustafsson S. Wang E. Olsson E. Inganäs O. Andersson M.R. Müller C. Fullerene mixtures enhance the thermal stability of a non-crystalline polymer solar cell blend.Appl. Phys. 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Horiz. 2014; 1: 270-279Crossref Google Scholar For an amorphous polymer donor and with suppression of the crystallization of the small molecule acceptor (SMA), the phase diagram is asymmetric and there are only two domains: the acceptor-rich small molecule domain that is almost pure in sufficiently immiscible systems, and the donor rich, mixed amorphous domain.37Ye L. Hu H. Ghasemi M. Wang T. Collins B.A. Kim J.H. Jiang K. Carpenter J.H. 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 In addition to these two domains, a semi-crystalline donor based device has an additional pure polymer crystalline domain. The crystallization of the SMA (fullerene or NF-SMA) is usually prevented in fresh devices by quenching the acceptor into an amorphous, vitrified state.15de Zerio A.D. Müller C. Glass forming acceptor alloys for highly efficient and thermally stable ternary organic solar cells.Adv. Energy Mater. 2018; 8: 1702741Crossref Scopus (69) Google Scholar This two- or three-phase morphology must be carefully optimized to maximize the photon absorption, exciton separation, and charge transportation and extraction simultaneously. Such an optimization often creates mixed domains with an unstable composition (Figure 1).22Li 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; 8: 14541Crossref PubMed Scopus (263) Google Scholar, 38Liu Y. Zhao J. Li Z. Mu C. Ma W. Hu H. Jiang K. Lin H. Ade H. Yan H. Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells.Nat. Commun. 2014; 5: 5293Crossref PubMed Scopus (2727) Google Scholar, 39Müller C. 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This state depletes the mixed domains of SMA relative to the binodal because of the extra chemical potential of the crystals. Please note that we will use the term binodal or miscibility gap and liquidus for simplicity even in cases where the liquid phases have vitrified to an amorphous glass below the glass transition temperature Tg, and the phase boundaries correspond to solid-solid transitions. The binodal or miscibility gap is governed by the miscibility limit of the donor or acceptor materials in the majority phase, which can be parameterized in favorable cases by the effective amorphous-amorphous Flory-Huggins (F-H) interaction parameter χ.42Kouijzer 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, 43Kozub D.R. Vakhshouri K. Orme L.M. Wang C. Hexemer A. Gomez E.D. 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However, an excessive repulsive molecular interaction between donor and acceptor materials can lead to over-purification of the mixed domains (i.e., with an SMA concentration below the percolation threshold), which would negatively affect device performance predominantly because of charge trapping and mono-molecular recombination.37Ye L. Hu H. Ghasemi M. Wang T. Collins B.A. Kim J.H. Jiang K. Carpenter J.H. 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, 40Ye 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, 44Bartelt 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, 45Ye L. Li S. Liu X. Zhang X. Ghasemi M. Xiong Y. Hou J. Ade H. Quenching to the percolation threshold in organic solar cells.Joule. 2018; 3: 443-458Abstract Full Text Full Text PDF Scopus (142) Google Scholar Schematics of the possible scenarios of morphology evolution in an upper critical solution temperature (UCST) polymer:SMA blend with an amorphous donor and crystallizable SMA with a low and an optimal miscibility are illustrated in Figure 1. Severe burn-in degradation can be expected when the optimal morphology is quenched near the percolation threshold and is far from the miscibility gap, referred to as a “low-” or “hypo-miscibility” system (Figure 1A). On the other hand, a device with a miscibility gap close to the percolation threshold (Figure 1B) during the normal device operation conditions is referred to as “optimal miscibility” and is expected to exhibit a relatively stable morphology and thus lower or slower burn-in degradation. Hyper-miscibility systems typically yield low performance37Ye L. Hu H. Ghasemi M. Wang T. Collins B.A. Kim J.H. Jiang K. Carpenter J.H. 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 and are not further considered. One example of a hypo-miscibility, high-performing polymer:SMA blend system (as illustrated in Figure 1A) is PffBT4T-2OD:PC71BM, with a PC71BM meta-stable equilibrium concentration in the mixed domains well below the percolation threshold.38Liu Y. Zhao J. Li Z. Mu C. Ma W. Hu H. Jiang K. Lin H. Ade H. Yan H. Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells.Nat. Commun. 2014; 5: 5293Crossref PubMed Scopus (2727) Google Scholar, 40Ye 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 Consequently, abnormally strong burn-in degradation is observed in PffBT4T-2OD:PC71BM solar cells as [6,6]-phenyl C61 or C71 butyric acid methyl ester (PCBM) readily diffuses even at RT.22Li 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; 8: 14541Crossref PubMed Scopus (263) Google Scholar In contrast, PCDTBT:PC71BM is a near optimally miscible system, which should provide improved shelf stability with regard to demixing of the mixed domain when compared to PffBT4T-2OD:PC71BM.37Ye L. Hu H. Ghasemi M. Wang T. Collins B.A. Kim J.H. Jiang K. Carpenter J.H. 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 However, PCDTBT:PC61BM blends, similar to other fullerene-based OSCs, are prone to crystallization of the fullerene, and although these blends are thermodynamically stabilized against amorphous demixing, they are only kinetically stabilized against degradation by crystallization, the second main morphological degradation pathway.46Li Z. Ho Chiu K. Shahid Ashraf R. Fearn S. Dattani R. Cheng Wong H. Tan C.H. Wu J. Cabral J.T. Durrant J.R. Toward improved lifetimes of organic solar cells under thermal stress: substrate-dependent morphological stability of PCDTBT: PCBM films and devices.Sci. Rep. 2015; 5: 15149Crossref PubMed Scopus (46) Google Scholar, 47Wong H.C. Li Z. Tan C.H. Zhong H. Huang Z. Bronstein H. McCulloch I. Cabral J.T. Durrant J.R. Morphological stability and performance of polymer–fullerene solar cells under thermal stress: the impact of photoinduced PC60BM oligomerization.ACS Nano. 2014; 8: 1297-1308Crossref PubMed Scopus (115) Google Scholar It is known that thermal annealing can boost the efficiency of many OSCs; however, heating may accelerate the transition of the morphology from the meta-stable miscibility gap to the liquidus or directly and simultaneously lead to crystallization failure as a result of nucleation or growth of SMA crystals.15de Zerio A.D. Müller C. Glass forming acceptor alloys for highly efficient and thermally stable ternary organic solar cells.Adv. Energy Mater. 2018; 8: 1702741Crossref Scopus (69) Google Scholar The propensity for this transition to occur will depend on Tg, which is an indicator of the degree of vitrification at RT. Conceptually, there are three main classes of systems for crystallizable NF-SMA: class I systems that are unstable as a result of demixing and crystallization (low Tg case in Figure 1A), class II systems that have meta-stable mixed domains but can crystallize (low Tg case in Figure 1B), and class III systems that are kinetically stabilized irrespective of whether they are meta-stable or not (high Tg cases). Class III can be subdivided into class IIIa when a hypo-miscibility system is vitrified and class IIIb when an optimal miscibility, meta-stable system is vitrified. In Figure 1, we measure the phase diagrams of NF-SMA OSC systems for the first time at least partially and delineate thermodynamic drivers and kinetic factors for stability in the three main classes of NF-SMA OSC systems delineated above. We select and utilize the well-known, prototypical NF-SMAs (i.e., EH-IDTBR and ITIC) selectively blended with the prototypical semiconductor polymers P3HT (semi-crystalline, ductile) and FTAZ (amorphous, ductile) to yield three different donor-acceptor blend-based systems in this study (P3HT:EH-IDTBR, FTAZ:EH-IDTBR, and FTAZ:ITIC) that systematically exemplify the characteristics of the different scenarios. We determine their device morphology and operational shelf-stability for variable processing conditions. We also investigate the thermodynamic drivers and kinetic factors of a previously reported relatively stable NF-SMA system based on another prototypical amorphous donor, namely PTB7-Th:EH-IDTBR,19Baran D. Gasparini N. Wadsworth A. Tan C.H. Wehbe N. Song X. Hamid Z. Zhang W. Neophytou M. Kirchartz T. et al.Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination.Nat. Commun. 2018; 9: 2059Crossref PubMed Scopus (134) Google Scholar as a comparison (the 4th system). It is found that P3HT:EH-IDTBR-based OSCs suffer from severe burn-in degradation in both as-cast and annealed samples. The severe efficiency loss of P3HT:EH-IDTBR devices is more pronounced in annealed devices compared to as-cast devices, even when these films are only briefly annealed at moderately elevated temperatures (120°C for 10 min). Even short periods of annealing are sufficient to nucleate crystals of the NF-SMA, which then assert their presence and negative influence over time. On the other hand, as-cast blend films based on FTAZ:EH-IDTBR exhibit low burn-in loss, whereas strong burn-in degradation occurs readily in annealed FTAZ:EH-IDTBR devices. In contrast, by substituting EH-IDTBR with ITIC, the FTAZ:ITIC-based systems provide OSCs with reduced burn-in degradation in both as-cast and low temperature annealed (120°C) devices, but with severe burn-in in high T annealed (180°C) devices. We determine χ(T) for P3HT:EH-IDTBR, the liquidus for FTAZ:EH-IDTBR, and FTAZ:ITIC-based systems, and the binodal at 110°C and 100°C for PTB7-Th:EH-IDTBR. In addition, we estimated RT diffusion coefficients of 1.7 × 10−17, 2.0 × 10−18, and 4 × 10−20 cm2/s for EH-IDTBR in P3HT, FTAZ, and PTB7-Th, respectively, and analyzed device stability in light of the thermodynamic and kinetic knowledge gained. The diffusion coefficient of EH-IDTBR in P3HT- and FTAZ-based systems is sufficiently high to enable demixing and crystallization burn-in after low T annealing or even in as-cast devices. The more robust morphology of PTB7-Th:EH-IDTBR devices is attributed to the smaller diffusion coefficient of EH-IDTBR in PTB7-Th. These results indicate that NF-SMA OSCs can be stabilized by employing an NF-SMA with a high Tg or a polymer with lower ductility that suppresses the crystallization of the NF-SMA. In order for NF-SMA-based OSCs to be a viable technology, the quenched morphology that gives high performance has to be stabilized against demixing or crystallization by a high degree of vitrification with low diffusion (∼1 × 10−22 cm2/s) or systems have to be meta-stable with a mixed composition near the percolation threshold, and the crystallization of the NF-SMA has to be suppressed or intr}, 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={Abstract 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={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.}, 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. 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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. 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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. 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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={Abstract 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 (oxylene/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 the blade-coating of several cases of OPVs in air. Remarkably, this single nonhazardous solvent yields ∼8.4% and ∼5.2% efficiency in OPVs by respectively blade-coating PBDT-TSR:PC71BM and all-polymeric PBDT-TS1:PPDIODT in ambient air, which are among the highest values for the respective kind of device. We postulate this simple nonhazardous solvent approach will also be applicable in the large area roll-to-roll coating and industrial scale printing of high-efficiency OPVs in air.}, 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} }