@article{ravishankar_charles_xiong_henry_swift_rech_calero_cho_booth_kim_et al._2021, title={Balancing crop production and energy harvesting in organic solar-powered greenhouses}, volume={2}, ISSN={["2666-3864"]}, DOI={10.1016/j.xcrp.2021.100381}, abstractNote={Adding semitransparent organic solar cells (ST-OSCs) to a greenhouse structure enables simultaneous plant cultivation and electricity generation, thereby reducing the greenhouse energy demand. However, there is a need to establish the impact of such systems on plant growth and indoor climate and to optimize system tradeoffs. In this work, we consider plant growth under OSCs and system-relevant design. We evaluate the growth of red leaf lettuce under ST-OSC filters and compare the impact of three different OSC active layers that have unique transmittance. We find no significant differences in the fresh weight and chlorophyll content of the lettuce grown under these OSC filters. In addition, OSCs provide an opportunity for further light and thermal management of the greenhouse through device design and optical coatings. The OSCs can thus affect plant growth, power generation, and thermal load of the greenhouse, and this design trade space is reviewed and exemplified.}, number={3}, journal={CELL REPORTS PHYSICAL SCIENCE}, publisher={Elsevier BV}, author={Ravishankar, Eshwar and Charles, Melodi and Xiong, Yuan and Henry, Reece and Swift, Jennifer and Rech, Jeromy and Calero, John and Cho, Sam and Booth, Ronald E. and Kim, Taesoo and et al.}, year={2021}, month={Mar} }
 @article{ho_kim_xiong_firdaus_yi_dong_rech_gadisa_booth_brendan t. o'connor_et al._2020, title={High-Performance Tandem Organic Solar Cells Using HSolar as the Interconnecting Layer}, volume={10}, ISSN={["1614-6840"]}, url={https://doi.org/10.1002/aenm.202000823}, DOI={10.1002/aenm.202000823}, abstractNote={Abstract Tandem structure provides a practical way to realize high efficiency organic photovoltaic cells, it can be used to extend the wavelength coverage for light harvesting. The interconnecting layer (ICL) between subcells plays a critical role in the reproducibility and performance of tandem solar cells, yet the processability of the ICL has been a challenge. In this work the fabrication of highly reproducible and efficient tandem solar cells by employing a commercially available material, PEDOT:PSS HTL Solar (HSolar), as the hole transporting material used for the ICL is reported. Comparing with the conventional PEDOT:PSS Al 4083 (c‐PEDOT), HSolar offers a better wettability on the underlying nonfullerene photoactive layers, resulting in better charge extraction properties of the ICL. When FTAZ:IT‐M and PTB7‐Th:IEICO‐4F are used as the subcells, a power conversion efficiency (PCE) of 14.7% is achieved in the tandem solar cell. To validate the processability of these tandem solar cells, three other research groups have successfully fabricated tandem devices using the same recipe and the highest PCE obtained is 16.1%. With further development of donor polymers and device optimization, the device simulation results show that a PCE > 22% can be realized in tandem cells in the near future.}, number={25}, journal={ADVANCED ENERGY MATERIALS}, publisher={Wiley}, author={Ho, Carr Hoi Yi and Kim, Taesoo and Xiong, Yuan and Firdaus, Yuliar and Yi, Xueping and Dong, Qi and Rech, Jeromy J. and Gadisa, Abay and Booth, Ronald and Brendan T. O'Connor and et al.}, year={2020}, month={Jul} }
 @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. 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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. 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{ye_xiong_chen_zhang_fei_henry_heeney_o’connor_you_ade_et al._2019, title={Sequential Deposition of Organic Films with Eco-Compatible Solvents Improves Performance and Enables Over 12%-Efficiency Nonfullerene Solar Cells}, volume={31}, ISSN={["1521-4095"]}, url={https://doi.org/10.1002/adma.201808153}, DOI={10.1002/adma.201808153}, abstractNote={Casting of a donor:acceptor bulk-heterojunction structure from a single ink has been the predominant fabrication method of organic photovoltaics (OPVs). Despite the success of such bulk heterojunctions, the task ofcontrolling the microstructure in a single casting process has been arduous and alternative approaches are desired. To achieve OPVs with a desirable microstructure, a facile and eco-compatible sequential deposition approach is demonstrated for polymer/small-molecule pairs. Using a nominally amorphous polymer as the model material, the profound influence of casting solvent is shown on the molecular ordering of the film, and thus the device performance and mesoscale morphology of sequentially deposited OPVs can be tuned. Static and in situ X-ray scattering indicate that applying (R)-(+)-limonene is able to greatly promote the molecular order of weakly crystalline polymers and form the largest domain spacing exclusively, which correlates well with the best efficiency of 12.5% in sequentially deposited devices. The sequentially cast device generally outperforms its control device based on traditional single-ink bulk-heterojunction structure. More crucially, a simple polymer:solvent interaction parameter χ is positively correlated with domain spacing in these sequentially deposited devices. These findings shed light on innovative approaches to rationally create environmentally friendly and highly efficient electronics.}, number={17}, journal={ADVANCED MATERIALS}, publisher={Wiley}, author={Ye, Long and Xiong, Yuan and Chen, Zheng and Zhang, Qianqian and Fei, Zhuping and Henry, Reece and Heeney, Martin and O’Connor, Brendan T. and You, Wei and Ade, Harald and et al.}, year={2019}, month={Apr} }
 @article{xiong_ye_gadisa_zhang_rech_you_ade_2018, title={Revealing the Impact of F4-TCNQ as Additive on Morphology and Performance of High-Efficiency Nonfullerene Organic Solar Cells}, volume={29}, ISSN={1616-301X}, url={http://dx.doi.org/10.1002/ADFM.201806262}, DOI={10.1002/adfm.201806262}, abstractNote={Abstract Fluorinated molecule 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) and its derivatives have been used in polymer:fullerene solar cells primarily as a dopant to optimize the electrical properties and device performance. However, the underlying mechanism and generality of how F4‐TCNQ affects device operation and possibly the morphology is poorly understood, particularly for emerging nonfullerene organic solar cells. In this work, the influence of F4‐TCNQ on the blend film morphology and photovoltaic performance of nonfullerene solar cells processed by a single halogen‐free solvent is systematically investigated using a set of morphological and electrical characterizations. In solar cells with a high‐performance polymer:small molecule blend FTAZ:IT‐M, F4‐TCNQ has a negligibly small effect on the molecular packing and surface characteristics, while it clearly affects the electronic properties and mean‐square composition variation of the bulk. In comparison to the control devices with an average power conversion efficiency (PCE) of 11.8%, inclusion of a trace amount of F4‐TCNQ in the active layer has improved device fill factor and current density, which has resulted into a PCE of 12.4%. Further increase in F4‐TCNQ content degrades device performance. This investigation aims at delineating the precise role of F4‐TCNQ in nonfullerene bulk heterojunction films, and thereby establishing a facile approach to fabricate highly optimized nonfullerene solar cells.}, number={1}, journal={Advanced Functional Materials}, publisher={Wiley}, author={Xiong, Yuan and Ye, Long and Gadisa, Abay and Zhang, Qianqian and Rech, Jeromy James and You, Wei and Ade, Harald}, year={2018}, month={Nov}, pages={1806262} }
 @article{sen_xiong_zhang_park_you_ade_kudenov_brendan t. o'connor_2018, title={Shear-Enhanced Transfer Printing of Conducting Polymer Thin Films}, volume={10}, ISSN={["1944-8244"]}, DOI={10.1021/acsami.8b09968}, abstractNote={Polymer conductors that are solution-processable provide an opportunity to realize low-cost organic electronics. However, coating sequential layers can be hindered by poor surface wetting or dissolution of underlying layers. This has led to the use of transfer printing where solid film inks are transferred from a donor substrate to partially fabricated devices using a stamp. This approach typically requires favorable adhesion differences between the stamp, ink, and receiving substrate. Here, we present a shear-assisted organic printing (SHARP) technique that employs a shear load on a post-less polydimethylsiloxane (PDMS) elastomer stamp to print large-area polymer films that can overcome large unfavorable adhesion differences between the stamp and receiving substrate. We explore the limits of this process by transfer printing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films with varied formulation that tune the adhesive fracture energy. Using this platform, we show that the SHARP process is able to overcome a 10-fold unfavorable adhesion differential without the use of a patterned PDMS stamp, enabling large-area printing. The SHARP approach is then used to print PEDOT:PSS films in the fabrication of high-performance semitransparent organic solar cells.}, number={37}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Sen, Pratik and Xiong, Yuan and Zhang, Qanqian and Park, Sungjune and You, Wei and Ade, Harald and Kudenov, Michael W. and Brendan T. O'Connor}, year={2018}, month={Sep}, pages={31560–31567} }
 @article{ye_xiong_zhang_li_wang_jiang_hou_you_ade_2018, title={Surpassing 10% efficiency benchmark for nonfullerene organic solar cells by scalable coating in air from single nonhalogenated solvent}, volume={30}, DOI={10.1002/adma.201870054}, number={8}, journal={Advanced Materials}, author={Ye, Long and Xiong, Y. and Zhang, Q. Q. and Li, S. S. and Wang, C. and Jiang, Z. and Hou, J. H. and You, W. and Ade, H.}, year={2018} }
 @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{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} }