@article{zeng_ma_pan_chen_ma_zhao_zhang_kim_shang_luo_et al._2021, title={A Chlorinated Donor Polymer Achieving High-Performance Organic Solar Cells with a Wide Range of Polymer Molecular Weight}, volume={6}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202102413}, abstractNote={Abstract In the field of non‐fullerene organic solar cells (OSCs), compared to the rapid development of non‐fullerene acceptors, the progress of high‐performance donor polymers is relatively slow. The property and performance of donor polymers in OSCs are often sensitive to the molecular weight of the polymers. In this study, a chlorinated donor polymer named D18‐Cl is reported, which can achieve high performance with a wide range of polymer molecular weight. The devices based on D18‐Cl show a higher open‐circuit voltage ( V OC ) due to the slightly deeper energy levels and an outstanding short‐circuit current density ( J SC ) owing to the appropriate long periods of blend films and less ([6,6]‐phenyl‐C71‐butyric acid methyl ester) (PC 71 BM) in mixed domains, leading to the higher efficiency of 17.97% than those of the D18‐based devices (17.21%). Meanwhile, D18‐Cl can achieve high efficiencies (17.30–17.97%) when its number‐averaged molecular weight ( M n ) is ranged from 45 to 72 kDa. In contrast, the D18‐based devices only exhibit relatively high efficiencies in a narrow M n range of ≈70 kDa. Such property and performance make D18‐Cl a promising donor polymer for scale‐up and low‐cost production.}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Zeng, Anping and Ma, Xiaoling and Pan, Mingao and Chen, Yuzhong and Ma, Ruijie and Zhao, Heng and Zhang, Jianquan and Kim, Ha Kyung and Shang, Ao and Luo, Siwei and et al.}, year={2021}, month={Jun} }
 @article{hu_li_zhang_peng_ma_xin_huang_ma_jiang_zhang_et al._2018, title={Effect of Ring-Fusion on Miscibility and Domain Purity: Key Factors Determining the Performance of PDI-Based Nonfullerene Organic Solar Cells}, volume={8}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.201800234}, abstractNote={Abstract Compared to the rapid development of nonfullerene organic solar cells (OSCs) based on the state‐of‐the‐art indacenodithiophene (IDT)‐based small molecule acceptors (SMAs), the progress for perylene diimide (PDI)‐based electron acceptors has lagged behind owing to the lack of understanding on the structure–morphology–performance relationship of PDI SMAs. Given the ease of synthesis for PDIs and their high intrinsic electron mobility, it is crucial to identify key material parameters that influence the polymer:PDI blend morphology and to develop rational approaches for molecular design toward high‐performance PDI‐based SMAs. In this study, three pairs of PDI‐based SMAs with and without ring‐fusion are investigated and it is found that ring‐fusion and domain purity are the key structural and morphological factors determining the fill factors (FFs) and efficiencies of PDI‐based nonfullerene OSCs. This data shows that nonfullerene OSCs based on the ring‐fused PDI‐based SMAs exhibit much higher average domain purity and thus increased charge mobilities, which lead to enhanced FFs compared to those solar cells based on nonfused PDIs. This is explained by higher Florry Huggins interaction parameters as observed by melting point depression measurements. This study suggests that increasing repulsive molecular interactions to lower the miscibility between the polymer donor and PDI acceptor is the key to improve the FF and performance of PDI‐based devices.}, number={26}, journal={ADVANCED ENERGY MATERIALS}, author={Hu, Huawei and Li, Yunke and Zhang, Jianquan and Peng, Zhengxing and Ma, Lik-kuen and Xin, Jingming and Huang, Jiachen and Ma, Tingxuan and Jiang, Kui and Zhang, Guangye and et al.}, year={2018}, month={Sep} }
 @article{zhang_guo_ma_ade_hou_2015, title={A Large-Bandgap Conjugated Polymer for Versatile Photovoltaic Applications with High Performance}, volume={27}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201502110}, abstractNote={A new copolymer PM6 based on fluorothienyl-substituted benzodithiophene is synthesized and characterized. The inverted polymer solar cells based on PM6 exhibit excellent performance with Voc of 0.98 V and power conversion efficiency (PCE) of 9.2% for a thin-film thickness of 75 nm. Furthermore, the single-junction semitransparent device shows a high PCE of 5.7%.}, number={31}, journal={ADVANCED MATERIALS}, author={Zhang, Maojie and Guo, Xia and Ma, Wei and Ade, Harald and Hou, Jianhui}, year={2015}, month={Aug}, pages={4655–4660} }
 @article{ma_reinspach_zhou_diao_mcafee_mannsfeld_bao_ade_2015, title={Tuning Local Molecular Orientation-Composition Correlations in Binary Organic Thin Films by Solution Shearing}, volume={25}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201500468}, abstractNote={A general impact of solution shearing on molecular orientation correlation is observed in polymer:fullerene organic solar cells in which one of the components forms fibrils or aggregates. Further investigation with polarized soft X‐ray scattering reveals that solution shearing induces more face‐to‐face orientation relative to the interface of two components compared to spin‐coating. This impact is shearing speed dependent, that is, slow shearing speed can induce more face‐to‐face orientation than a fast shearing speed. These results demonstrate that solution shearing is an effective method to control the relative molecular orientation. Solution shearing can also modify the domain size and average composition variations.}, number={21}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Ma, Wei and Reinspach, Julia and Zhou, Yan and Diao, Ying and McAfee, Terry and Mannsfeld, Stefan C. B. and Bao, Zhenan and Ade, Harald}, year={2015}, month={Jun}, pages={3131–3137} }
 @article{zhang_guo_ma_ade_hou_2014, title={A Polythiophene Derivative with Superior Properties for Practical Application in Polymer Solar Cells}, volume={26}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201401494}, abstractNote={A polythiophene derivative called PDCBT, which has a backbone of thiophene units and just carboxylate functional groups to modulate its properties, exhibits properties superior to those of poly(3-hexylthiophene), the classic polythiophene derivative, when used as an electron donor in polymer solar cells (PSCs). The best device, based on PDCBT/PC71BM (1:1), develops a good power conversion efficiency of 7.2%. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.}, number={33}, journal={ADVANCED MATERIALS}, author={Zhang, Maojie and Guo, Xia and Ma, Wei and Ade, Harald and Hou, Jianhui}, year={2014}, month={Sep}, pages={5880–5885} }
 @article{liu_zhao_li_mu_ma_hu_jiang_lin_ade_yan_2014, title={Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells}, volume={5}, ISSN={["2041-1723"]}, DOI={10.1038/ncomms6293}, abstractNote={Abstract Although the field of polymer solar cell has seen much progress in device performance in the past few years, several limitations are holding back its further development. For instance, current high-efficiency (>9.0%) cells are restricted to material combinations that are based on limited donor polymers and only one specific fullerene acceptor. Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near-ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains. This morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers and is insensitive to the choice of fullerenes. The uncovered aggregation and design rules yield three high-efficiency (>10%) donor polymers and will allow further synthetic advances and matching of both the polymer and fullerene materials, potentially leading to significantly improved performance and increased design flexibility.}, journal={NATURE COMMUNICATIONS}, author={Liu, Yuhang and Zhao, Jingbo and Li, Zhengke and Mu, Cheng and Ma, Wei and Hu, Huawei and Jiang, Kui and Lin, Haoran and Ade, Harald and Yan, He}, year={2014}, month={Nov} }
 @article{zhang_guo_ma_zhang_huo_ade_hou_2014, title={An Easy and Effective Method to Modulate Molecular Energy Level of the Polymer Based on Benzodithiophene for the Application in Polymer Solar Cells}, volume={26}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201304631}, abstractNote={Attaching meta-alkoxy-phenyl groups as conjugated side chains is an easy and effective way to modulate the molecular energy level of D-A polymer for photovoltaic application, and the polymer solar cells based on the polymer consisting meta-alkoxy-phenyl groups as conjugated side chain, PBT-OP, shows an enhanced open circuit voltage and thus higher efficiency of 7.50%, under the illumination of AM 1.5G, 100 mW/cm2. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.}, number={13}, journal={ADVANCED MATERIALS}, author={Zhang, Maojie and Guo, Xia and Ma, Wei and Zhang, Shaoqing and Huo, Lijun and Ade, Harald and Hou, Jianhui}, year={2014}, month={Apr}, pages={2089–2095} }
 @article{guo_zhang_ma_ye_zhang_liu_ade_huang_hou_2014, title={Enhanced Photovoltaic Performance by Modulating Surface Composition in Bulk Heterojunction Polymer Solar Cells Based on PBDTTT-C-T/PC71BM}, volume={26}, ISSN={["1521-4095"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84903160839&partnerID=MN8TOARS}, DOI={10.1002/adma.201400411}, abstractNote={For the blend film of PBDTTT-C-T:PC71BM, the use of 1,8-diiodooctane as the solvent additive enriches the polymer at the top surface, so that a power conversion efficiency of 9.13% is recorded in the inverted polymer solar cell based on the blend, which is much higher than that of the device with conventional structure. Recently, the power conversion efficiencies (PCEs) of the bulk heterojunction (BHJ) polymer solar cells, consisting of conjugated polymers as donor and fullerene derivatives as acceptor, have reached up to ∼9%.1 As known, morphology control of active layers of BHJ solar cells plays a critical role affecting the device performance.2 The ideal morphology formed in the active layer provides not only sufficient interfaces for efficient charge separation but also good percolation pathways for charge carrier transport to the respective electrodes so as to minimize the recombination of free charges.3 However, the efficient charge collection in a BHJ PSCs can also be assisted by the surface composition of the active layer.4 For example, the acceptor enrichment in the surface closed to the cathode would block the hole transport to the cathode,5 and the donor enrichment in the interface closed to the anode would block the electron transport to the anode.6 Therefore, it is essential to find a method to control not only the bulk morphologies but also the surface composition of the donor-acceptor blend film. Since the pioneer work reported by Bazan et al.,7 the addition of solvent additives with high boiling points (BP) have attracted much attention and has been used as a simple and effective way to improve the morphology of the active layers of BHJ PSC devices. According to the reported works, the effect of the solvent additives on morphology control is attributed to two properties: their selective solvency to the ingredients in the BHJ layers and their low volatility.[7],8-11 Therefore, solvent additives like 1, 8-Octanedithiol (OT), 1, 8-Diiodooctane (DIO) and N-methyl pyrrolidone (NMP) have been selectively used to optimize the bulk morphologies of active layers for PSCs based on different polymers.7-11 However, the effects of additives on the surface compositions of the active layers have not been reported in detail. Considering that the surface composition in BHJ layers plays an important role for charge transport and charge collection in PSCs, we hypothesized that the application of high BP solvent additives may be an effective method to control not only the bulk morphology but also the surface composition of the BHJ layers. In this work, the blend of PBDTTT-C-T (see Scheme 1) and [6, 6]-phenyl-C71-butyric acid methyl ester (PC71BM) (1:1.5, w/w) was chosen as the model system to explore the effects of variation of surface composition and device structures on photo­voltaic performance and also to investigate the mechanism for why the inverted structures12 are helpful to realize higher PCE in some of PSCs. Since the addition of DIO is a very effective method to improve photovoltaic properties of the PBDTTT-C-T: PC71BM system, herein the blend solvent of o-DCB and DIO was used to modulate the bulk morphologies and the surface compositions. In the PSC devices with conventional structure (c-PSC), PEDOT:PSS and Ca were used as the materials for making the buffer layers of the anode and the cathode (HTL and ETL), respectively; in the PSC devices with inverted structure (i-PSC), cross-linked water/alcohol soluble polymer, named as poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-bis(3-ethyl-(oxetane-3-ethyloxy)-hexyl)-fluorene)] (PFN-OX),13 and MoO3 were used as the ETL and HTL materials, respectively. We found that when DIO was used as the high BP additive, the donor material (PBDTTT-C-T) will be relatively enriched on the top surface of the blend film and this surface composition is more favorable to i-PSCs than c-PSCs. As expected, the i-PSC device showed an enhanced PCE of 9.13%, which is much higher than that of the c-PSC device. Overall, this work suggests that when high BP solvent additive is used, the BHJ films may have asymmetric surface compositions, and consequently, enhanced photovoltaic performance can be realized by selecting an appropriate device structure. In our previous work, we found that for the photovoltaic system of PBDTTT-C-T: PC71BM, the optimal D/A ratio is 1:1.5 (w/w) and the optimal ratio of DIO/o-DCB is 3% (v/v),14 and the optimal thickness of the blend films is ∼100 nm. Since we are focusing on investigating the correlations among the surface compositions, the device structures and the photovoltaic properties, these conditions for fabrication of the blend films were strictly followed. Initially, atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to investigate the morphology of the blend films in the real space. As shown in Figure 1a and 2d, compared to the blend film processed without DIO, the blend film processed with DIO show higher surface roughness, i.e., the Rq of the blend changed from 0.37 nm to 2.15 nm. Meanwhile, it seems that similar phase separation can be observed Figure 1b and 2e, and no big size aggregation can be distinguished in these two phase images. The TEM images (as shown in Figure 1c and 2f) of these two blends show very low contrast and no distinct phase separation can be observed. Overall, the AFM and TEM results indicate that the PBDTTT-C-T phase has good compatibility with the PC71BM phase, and for the blend films processed without and with DIO, the variations in their morphologies cannot be clearly distinguished by AFM and TEM measurements. The top surface compositions of the blend films processed without or with using DIO were characterized by X-ray Photoelectron Spectroscopy (XPS).[8],15 The detailed results of XPS measurements of the blend films are collected in Table 1 and the spectra are shown in Figure 2a. The XPS measurement of the pure polymer film showed an atomic ratio of 8.10 for C 1s and S 2p peaks, which is close to the calculated values of C/S stoichiometric ratio (7.94), indicating that the XPS test can give reliable information of film surface composition. In the PBDTTT-C-T: PC71BM blend, sulfur can be used as the characteristic element of the polymer, because PC71BM does not contain sulfur. Therefore, the higher C/S ratio (RC/S) is observed, the more PC71BM is enriched on the top surface. The RC/S value of the blend film processed by pure o-DCB is significantly higher that of the blend film processed by o-DCB/DIO (RC/S, o-DCB > RC/S, o-DCB/DIO), meaning that the use of DIO will cause the enrichment of the polymer at the top surface. Furthermore, surface potentials of the top surfaces of these blend films were investigated by using the Peak Force-Kelvin Probe Force Microscopy (PF-KPFM).16 The PF-KPFM surface potential images and the corresponding analysis curves of contact potential differences on the top surfaces are shown in Figure 2b. The average surface contact potentials were deduced with a scale of 500 nm. According to the PF-KPFM measurements, the average contact surface potentials (ϕ) of the blend film casted from pure o-DCB was found to be ∼0.35 V higher than that of the film with DIO additive. Because the surface energy (γ)17 of PCBM (∼37.8 mN/m2) higher than that of polymer (∼26.9 mN/m2), we can get that when using DIO as additive, more polymer can be enriched at the surface. Grazing Incidence Wide-angle X-ray Scattering (GIWAXS) was used to analyze the crystallinity of the PBDTTT-C-T: PC71BM blend films processed without and with the use of DIO. The blend thin films were spin-cast on PSS-treated Si substrates and have a thickness of ∼100 nm.[18] Figure 3a and 4b show the 2D patterns and the out-of plane and in plane profiles of the films processed with or without additive. The GIWAXS patterns show only broad polymer lamellar reflections (100) at q ≈ 0.3 Å−1 for both two blend films. The PC71BM aggregation peaks are locate at q ≈ 1.3 Å−1. No clear π–π stacking reflection peak can be observed for the blend films. Both blends films exhibit weak peaks at q ≈ 0.4 Å−1, which originates from scattering from the PSS when incident angle of x-ray beam is large enough to penetrate the film. As a reference, PSS only film reflection profiles are plotted at the bottom of Figure 3b. In conclusion, these two blend films processed by different solvents show similar and very low crystallinities, implying that the use of DIO has little influence on molecular packing structure in the PBDTTT-C-T: PC71BM blend. Resonant Soft X-ray Scattering (R-SoXS) is employed to provide detailed statistical information about the bulk morphology of the blend films, i.e., the median characteristic length scales of the morphology and the average composition variations <Δc>.19 With the enhanced contrast near carbon K edge, R-SoXS overcomes the weakness of the AFM and TEM discussed above. A photon energy 284.2 eV was utilized to provide high material contrast between PBDTTT-C-T and PC71BM.19, 20 Figure 3c shows the R-SoXS profile of blend films processed with or without additives. The distribution of scattering profile can be fitted by one or two log-normal functions (see Figure S1 in SI) and represents the distribution function of spatial frequency, s (s = q/2π). The median of the distribution smedian corresponds to the characteristic median length scale, ξ, of the corresponding log-normal distributions in real space with ξ = 1/smedian, a model independent statistical quantity. When the blend film was processed with pure o-DCB, the ξ in the blend is 30 nm; when DIO was used as additive, the scattering profile shows two log-normal distributions, and the corresponding ξ are 70 and 30 nm. It is noted that the film processed by o-DCB/DIO shows hierarchical structure and exhibits similar phase separation at small length scale comparing with the blends without processed by additive. It is reported the hierarchical structure is a favorable morphology to enhance device performance.23 The detail characterization of its impact in our system is outside the scope in this work. The average composition fluctuations <Δc> (referred as relative domain purity within a two phase amorphous: amorphous morphology framework as suggested by the WAXS data) can be extracted by integrating the scattering profile and calculating the Total Scattering Intensity (TSI).20,[21],22 The relative purity is 0.74 and 1 for the blend processed without and with DIO additive, respectively. We find that domains become significant purer when DIO is used as additive. It is noted that the relative purity 1 does not indicate the domains are 100% pure, but is set to equal to ready allow the ratio of the composition variations to be compared. High composition variations are considered beneficial to reduce geminate and non-geminate recombination and consequently aid high FF and Jsc in the device.24 The c-PSCs with a structure of ITO/PEDOT: PSS (35 nm)/PBDTTT-C-T: PC71BM (1:1.5, w/w) (∼100 nm)/Ca (20 nm)/Al (100 nm) and the i-PSCs with a structure of ITO/PFN-OX (∼5 nm)/PBDTTT-C-T: PC71BM (1:1.5, w/w) (∼100 nm)/MoO3 (10 nm)/Au (100 nm) as shown in Scheme 1 were fabricated. Figure 4 shows the current density-voltage (J–V) curves of the PSCs under the illumination of AM 1.5G, 100 mW/cm2 and the external quantum efficiency (EQE) curves of the devices. The corresponding photovoltaic data of the devices are summarized in Table 2. For c-PSCs, in comparison with the devices processed without DIO, the device processed with DIO shows obvious decrease in VOC, from 0.84 V to 0.76 V, and great increase in JSC and FF, from 13.1 mA/cm2 to 15.2 mA/cm2 and 49.8% to 62.3%, respectively. As discussed above, the surface potential of the blend treated by the use of DIO is obvious lower than that of the blend processed without DIO, which should be the main reason for the reduced VOC.[16],25,26 As shown as in Figure S2, Table S1 and Figure 2, compared to the blend processed without DIO, the blend processed with DIO has higher and symmetric hole and electron mobilities and favorable bulk and surface morphologies to keep efficient charge separation and also reduce the non-geminate recombination,27 and therefore, the JSC and the FF can be simultaneously improved by using DIO as additive. As known, in a PSC with conventional structure, electrons are transported towards the top surface of the BHJ blend through the channel formed by the acceptor material and then collected by the cathode, so if the donor material is enriched near to the interface between the blend and the cathode, i.e., the top surface of the blend in c-PSC, the electron transport and the electron collection will be impeded. Therefore, from the point of view of the top surface composition, the PBDTTT-C-T: PC71BM blend processed with DIO may be more suitable for electron transport and collection in i-PSC than in c-PSC. Therefore, as listed in Table 2, from c-PSC to i-PSC, the FF increased from 62.3% to 67.0%. On the other hand, from c-PSC to i-PSC, the buffer layer material for modification of the ITO electrode was changed from PEDOT: PSS to PFN-OX. Since PFN-OX has weaker absorption ranging from 500 nm to 1000 nm as shown in Figure S3 than PEDOT: PSS, the active layers can harvest more sunlight in i-PSC than in c-PSC. As shown in Figure 4b, from 550 nm to 750 nm, quantum efficiencies of the two i-PSCs are higher than these of the two c-PSCs, which agrees well with the reported results.[12],[13] As shown in Figure S3, it can be seen that the PEDOT: PSS layer shows higher transmittance than the PFN-OX layer in the range from 400 nm to 500 nm, while the former has stronger absorption than the latter in the long wavelength direction beyond 500 nm. Interestingly, although these two buffer layers show different transmittance in different region, for the devices processed by o-DCB/DIO, the EQE values can be improved in the whole response region from c-PSC to i-PSC, indicating that for the active layer processed by o-DCB/DIO, the i-PSC structure is more favorable than the c-PSC structure. In detail, from c-PSC to i-PSC, more improvement in quantum efficiency is observed in long wavelength direction than in short wavelength direction, which should be ascribed to the higher transmittance of the PFN-OX layer at long wavelength region. Overall, both two kinds of blend films processed without and with DIO additive show higher JSC values in i-PSCs than in c-PSCs, i.e., the JSC of the devices processed without and with DIO improved from 13.1 mA/cm2 to 14.2 mA/cm2 and from 15.2 mA/cm2 to 17.7 mA/cm2, respectively. Overall, when the inverted device structure was used instead of the conventional structure, the PCE of the PSC processed without DIO reduced from 5.48% to 5.19%, while the PCE of the PSC processed with DIO increased from 7.20% to 9.13%. In summary, we fabricated the conventional structure and inverted structure PSCs based on PBDTTT-C-T/PC71BM as the model system without or with the use of solvent additive DIO. We found that the application of high BP solvent additives may be an effective method to control not only the bulk morphology but also the surface compositions in the BHJ layers. After using DIO, the polymer will be enriched near to the top surface of the BHJ layer, which is favorable to the PSCs with inverted structure. The PCE of the inverted PSC with DIO additive reached 9.13% under the illumination of AM1.5G, 100 mW/cm2, which is much higher that of the device with conventional structure. This work suggests that the bulk morphologies and the surface compositions of the BHJ layers should be considered in designing highly efficient PSCs and also demonstrates a new mechanism for why the PSCs with inverted structures have superiorities in realizing higher photovoltaic performance. This work was supported by 973 and 863 projects, the National Natural Science Foundation of China (NSFC) and the Science and Technology Commission of Beijing. (Nos. 2014CB643501, 91333204, 2011AA050523, Z131100006013002, 51173189 and 51203168). R-SoXS and GIWAXS measurements and analysis by WM and HA are supported by the US Department of Energy, Office of Science, Basic Energy Science, Division of Materials Science and Engineering under contract DE-FG02–98ER45737. X-ray data is acquired at beamlines 7.3.3 (WAXS)18 and 11.0.1.2. (R-SoXS)19 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02–05CH11231. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.}, number={24}, journal={ADVANCED MATERIALS}, author={Guo, Xia and Zhang, Maojie and Ma, Wei and Ye, Long and Zhang, Shaoqing and Liu, Shengjian and Ade, Harald and Huang, Fei and Hou, Jianhui}, year={2014}, month={Jun}, pages={4043–4049} }
 @article{zhou_kurosawa_ma_guo_fang_vandewal_diao_wang_yan_reinspach_et al._2014, title={High Performance All-Polymer Solar Cell via Polymer Side-Chain Engineering}, volume={26}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201306242}, abstractNote={An average PCE of 4.2% for all-polymer solar cells from 20 devices with an average J SC of 8.8 mA cm−2 are obtained with a donor-acceptor pair despite a low LUMO-LUMO energy offset of less than 0.1 eV. Incorporation of polystyrene side chains into the donor polymer is found to assist in reducing the phase separation domain length scale, and results in more than 20% enhancement of PCE. We observe a direct correlation between the short circuit current (J SC) and the length scale of BHJ phase separation, which is obtained by resonance soft X-ray scattering. The performance of organic solar cells has rapidly improved over the past few years.1 Major efforts have been focused on developing a variety of donor materials to collect a broader wavelength range of the solar spectrum, to tune their energy levels, and to improve hole transport.2-4 On the other hand, the most widely used acceptors are still those from the fullerene family, including [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), PC71BM and indene C60 bis-adduct (ICBA).3, 5 The high cost of fullerenes and their potential lack of morphological stability will potentially hinder the future commercialization of fullerene-based organic solar cells.6 All-polymer solar cells, consisting of polymers for both the donor and acceptor, gained significantly increased interests recently, because of their ease of solution processing, potentially low cost, versatility in molecular design, and their potential for good chemical and morphological stability due to entanglement of polymers.7, 8 Unlike small molecular fullerene acceptors, polymer acceptors can benefit from the high mobility of intra-chain charge transport9, 10 and exciton generation by both donor and acceptor.11 Moreover, all-polymer bulk heterojunction (BHJ) systems are considered less likely to have "dead island" formation and the associated charge recombination.7 Despite extensive efforts on all-polymer solar cells in the past decade, however, only a few systems had reported power conversion efficiencies (PCE) over 2%.12-15 In early 2013, there have been two new reports with PCEs up to 3.3%16 and 3.6%.17-19 More recently, a PCE of 4.1% was reported.19 Additionally, Poly­era released an announcement of achieving a PCE as high as 6.47%, demonstrating the potential for high efficiency with all-polymer solar cells.7 Controlling phase separation is one of the most critical issues in all-polymer solar cells as it limits the generation of free charge carriers and subsequently the device performance.7 Tuning processing parameters during device fabrication has been used to optimize phase separation.20, 21 However, limited success has been achieved in this regard. Recently, we introduced the use of polystyrene (PS) side-chains for conjugated polymers.22 The solubility and processibility were improved for the resulting polymers with <10 mol% PS-containing repeating units, yet, the thin film transistor mobility and solar cell performance with fullerene acceptors were the same or better than without PS side chains.22 In this paper, we report high performance all-polymer solar cells employing isoindigo-containing donor polymers and perylene tetracarboxlic di-imide (PTCDI)-containing acceptor polymers. Incorporation of polystyrene side chains into the donor polymer22 is found to assist in reducing the phase separation domain length scale. A direct correlation between the short circuit current (JSC) and the length scale of BHJ phase separation is observed. An average PCE of 4.2% from 20 devices with an average JSC of 8.8 mA cm−2 are obtained. The highest PCE is 4.4%, with a JSC as high as 9.0 mA cm−2, and VOC of 1.04 V. This result represents one of the highest performance in published literature for all-polymer solar cells.7, 16, 17, 19 The chemical structures and energy diagrams of the donor and acceptor polymers employed herein are shown in Scheme 1. The isoindigo polymers are selected as the donor polymers because of their low bandgaps, strong absorption, good hole charge transport, deep HOMO for potentially high VOC and good BHJ solar cell performance when combined with PCBM acceptors.23-26 A swallow-tail substituted PTCDI polymer is chosen as the acceptor polymer due to its good electron transporting property and good solubility.15, 27, 28 All the polymer samples were synthesized from Stille coupling polymerization reactions and purified by Soxhlet followed by preparative Size Exclusion Chromatography (SEC) fractionation. The molecular weight and polydispersity index (PDI) are listed in Table S1. As shown in Figure 1 and Table S1, the optical band gaps of the isoindigo polymers are close to 1.6 eV while PiI-BDT gives an optical gap of 1.7 eV. The thin films of polymeric donors show two major absorption bands at around 700 nm and 650 nm, respectively. The maximum absorption peak of P(TP) is centered at 563 nm, hence complementing the absorption profile of the polymeric donors. The energy levels of these active materials were measured by Ultraviolet Photoemission Spectroscopy (UPS) and Inverse Photoemission Spectroscopy (IPES). The LUMO levels of the donor polymers are all similar to each other at around −3.70 eV, despite the different aromatic co-monomers. The HOMO level of P(TP) was measured to be −5.72 eV by UPS, while the LUMO level was −3.80 eV by IPES. Based on the IPES measured LUMO levels, the LUMO-LUMO offset [(−3.70 eV) − (−3.80 eV)] between the donors and the acceptor is less than 0.1 eV, lower than the often mentioned empirical value of 0.3 eV needed for efficient exciton dissociation.29 It can be seen later that despite of this very low offset, the relatively high PCE can still be achieved. Grazing incident X-ray diffraction (GIXRD) is used to obtain the crystalline order and polymer orientation information in the polymeric donors. As shown in Figure S1, crystalline structures are found in all the donors in the thin film state while no crystalline order is observed for the acceptor polymer (data not shown). The d-spacing does not change between the neat films and blend films, for both the lamellar (100) distance and the π−π (010) distance, but the intensities of those diffraction patterns were notably lower. This data indicates that the crystallinity of the donor polymers was reduced by blending with the acceptor polymer, but the orientation of the crystalline regions remained unchanged. Our all-polymer solar cell is constructed in an inverted BHJ structure, in which an electron transporting layer of ZnO is at the bottom and the hole transporting layer of MoO3 is at the top. The various donor polymers with small changes in their chemical structures exhibited drastically different photovoltaic characteristics (Table 1). From the external quantum efficiencies (EQE) shown in Figure 2b, we clearly observed that features at wavelength shorter than 550 nm are overlapping with the acceptor polymer absorption region, which indicates that excitons generated on the acceptor polymer also contributed to the overall performance of the device. Normalizing the EQE over the double-pass absorption of the devices, the internal quantum efficiencies (IQEs) from 500 nm to 720 nm (Figure S2) are almost the same within each cells with different donor, suggesting that the excitons generated from donor and acceptor split with equal probability in these blends. Interestingly, although the energy levels and absorption coefficients of the employed donor polymers are very similar to each other, the current generated from the various junctions differed drastically. In addition to the absorption spectrum, JSC depends on the fraction of photogenerated excitons reaching the D-A interface, the dissociation efficiency of these excitons into free carriers and the transport efficiency of the resulting free carriers. To understand and identify the causes for the observed trend in JSC we turn to analyzing the absorption, phase separation, driving force for exciton dissociation into free charge carriers and carrier mobilities. In order to determine the percentage of excitons that can reach and dissociate at the donor/acceptor interface, the photoluminescence quenching efficiencies (PLQE) are measured by directly comparing the PL intensity of the polymer donor, acceptor and BHJ films under the same excitation wavelength, geometry and equipment parameters. The trend in PL quenching efficiency correlates well with the observed trend in JSC (shown in Figure 2c). A poor PL quenching might be due to either large domains or energy level mismatch, which inhibits charge transfer. Resonant Soft X-ray Scattering (RSoXS) is used to obtain the phase separation characteristics of the polymer blends. This technique is essential in this case because the contrast of the scattering signal of hard X-ray is too weak from the all-polymer blends.30-33 A resonant photon energy of 283 eV is used to enhance the contrast of the polymer:polymer blends. To confirm the RSoXS scattering is originating from the polymer:polymer phase separation, a non-resonant energy 270 eV that is less sensitive to polymer:polymer contrast but more sensitive to mass-thickness variations is also utilized. The stronger and more pronounced scattering peak at 283 eV indicates that the RSoXS measures polymer:polymer phase separation at that energy (Figure S8). The scattering profiles at 283 eV are displayed in Figure 2d and represent the distribution function of spatial frequency, s = q/2π, of the samples. The median of the distribution smedian corresponds to the characteristic median length scale, ξ, of the corresponding phase distribution in real space with ξ = 1/smedian. The ξ is 260 nm for PiI-tT/P(TP), 100 nm for PiI-2F/P(TP), 50 nm for PiI-BDT/P(TP) and 54 nm for PiI-2T/P(TP), respectively. The trend in the characteristic median length scale correlates well with the PL quenching efficiency as well as the JSC, i.e. a smaller domain size was found to give a higher JSC and a higher PLQE. The dissociation of excitons into free charge carriers involves electron transfer that forms the charge transfer (CT) state, in which the electron resides on the acceptor molecule while the hole remains on the donor molecule. When such a CT state is lower in energy than the neat donor or acceptor exciton, it is characterized by weak absorption and emission at energies below the optical gap of the neat materials. Sensitive EQE measurements in the sub-gap region of most studied organic solar cell BHJs therefore reveal CT absorption.34 The driving force for charge transfer, defined by the difference between the lowest optical gaps of the neat material and the CT state optical gap, can be deduced from such measurements.34, 35 For our all-polymer BHJs, highly sensitive EQE measurements did not reveal any distinct sub-gap CT absorption band (Figure S3a-d). On the other hand, when using regioregular poly(3-hexylthiophene) (P3HT) with a shallower LUMO as the donor material blended with P(TP), a CT band in the P3HT gap region was clearly visible (See Figure S3e). This indicates that the absence of a sub-gap CT band for the isoindigo polymers under investigation here is due to their deeper LUMO level as compared to P3HT. The smaller LUMO-LUMO offset between the donor and acceptor resulted in a lower driving force for charge transfer and subsequently CT state formation. Similarly, the electroluminescence spectra of the blends showed only emission spectra resembling that of the neat donor polymer, with no CT band observed (Figure S3). This again confirms that the CT state in these BHJs is very high in energy, close to the energy of the excitons in the neat material. In some cases (Figure S3a) the peak positions of EL and EQE of the blends were even blue shifted as compared to the pure material. This is probably due to a decrease in aggregation of the isoindigo polymer upon blending. The employments of both EQE and EL spectra enable the accurate determination of the optical gaps of the neat donor polymers and blends. Based on the above values, the VOC of the devices correlates well with the deduced optical gap of the blend (VOC =Eg-0.6 V). The absence of a subgap CT absorption or emission band correlation indicate that for all the blends studied in this work, the energy loss due to electron transfer is minimized, hence maximizing the VOC. It is remarkable that despite the lack of a large driving force for electron transfer, a decent charge generation (6.5 mA cm−2) is still possible in these materials. Similar observations have been reported before for other isoindigo polymer:PCBM BHJs.36, 37 The mobilities measured from space-charge limited current (SCLC) are listed in Table S3. The hole mobilities in the blend films are all around 2 × 10−4 cm2 V−1 s−1, while the acceptor polymer P(TP) has a lower electron mobility of 2 × 10−5 cm2 V−1 s−1. The hole mobilities of the donors do not correlate well to the JSC, indicating that the charge transport efficiency of the devices may be limited by the low electron mobility of the acceptor polymer. The absorption (A(E)) of the blend films was in the similar range, indicating that the differences of the JSC were not originated from the absorption difference either (Figure S4). The above characterizations suggest that phase separation domain size is the most significant factor that gives rise to the dramatically different device performance. Therefore, we proceed to modify the best polymer donor polymer by attaching a small percentage of PS side chains to demonstrate a new route to control phase separation behavior in all-polymer blends (Scheme S1).22 With such polymer side-chain modification, we expected to reduce the domain size in the blend film by reducing the strong tendency for self-aggregation in the D-A donor polymer. In fact, the average PCE of the PiI-2T-PS5/P(TP), in which 5 mol% of the repeating units in PiI-2T are attached with PS side chains, reached 4.2%, with a JSC as high as 8.8 mA cm−2, and a VOC of 1.04 V. The highest measured PCE was up to 4.4%, and JSC­ was as high as 9.0 mA cm−2. From RSoXS data in Figure 3c, the median domain size (30 nm) of these devices was indeed 45% smaller than that formed by the PiI-2T/P(TP) blend (54 nm). To test the applicability of the PS-side-chain modification approach to other donor polymers, the same modification on the worst performing donor polymer PiI-tT is performed. After being modified by 5 mol% of PS side-chain, the PCE of PiI-tT-PS5/P(TP) increased to 2.75% from 1.67%, with a JSC as high as 5.92 mA cm−2, and a VOC of 0.99 V(Shown in Figure S5). The phase separation length scale of the blend film, measured by RSoXS, was reduced from 260 nm to 50 nm by attaching 5% PS side chains in the donor polymer. In this case (Figure S5), the PLQE increased from 66% to 76% after the attachment of PS side-chain, confirming again a reduction of the domain length scale and a more efficient exciton dissociation. Different device fabrication conditions are utilized to optimize the device performance. Annealing temperatures from 80 to 140 °C are applied to the active films prior to the thermal evaporation of the electrodes. Similar J–V curves are obtained indicating that phase separations of the polymers blends were stable under different thermal annealing conditions (Figure S7). The PCEs of devices are not very sensitive to the donor/acceptor blend ratios from 5/4 to 4/5. These robust fabrication features are highly desirable for future large scale production. In conclusion, a side-chain engineering approach using polystyrene enables manipulation of phase separation domain size and enhances all-polymer solar cell performance to reach PCE as high as 4.4%. A JSC as high as 9.0 mA cm−2 is obtained with a donor-acceptor pair despite of a low LUMO-LUMO energy offset of less than 0.1 eV. The phase separation domain length scale correlates well with the JSC and is found to be highly sensitive to the aromatic co-monomer molecular structures used in the crystalline donor polymers. With the PS polymer side-chain engineering, the phase separation domain length scale decreased by more than 45%. The PCE and JSC of the devices increased accordingly by more than 20%. This work demonstrates that a better understanding of tuning polymer phase separation domain size provides an important path towards high performance all-polymer solar cells. The use of polymer side-chain engineering provides an effective molecular engineering approach that may be combined with additional processing parameter control to further elevate the performance of all-polymer solar cells. General Method: All the polymers were synthesized according to previously reported procedures.15, 22, 24 All the polymers were purified via preparation SEC at room temperature with chloroform as solvent. The molecular weight and PDI of all polymers were measured by high temperature GPC with 1,2,4-trichlorobenzene as the solvent and polystyrenes as the calibration standards at 160 °C. 2D-GIXD images were collected in reflection mode with a planar area detector in a He atmosphere at beamline 11–3 of the Stanford Synchrotron Radiation Lightsource. The sample−detector distance was nominally set to 400 mm, and the incidence angle was 0.12°; the X-ray wavelength was 0.9758 Å. Slits were set to 150 and 50 μm in the horizontal and vertical directions, respectively. R-SoXS measurements were performed at beamline 11.0.1.2 at the Advanced Light Source.38 The sample films used for R-SoXS were spin-cast on sodium polysulfonate covered glass as substrate. To carry out R-SoXS experiment in transmission, the film is floated off in deionized water and picked up with a 1.5 mm by 1.5 mm silicon nitride window. The film is then dried in air before being transferred into the vacuum chamber for R-SoXS. Photoemission spectroscopy of occupied and unoccupied states of the system was performed using a VG ESCA Lab system equipped with both UPS and IPES. The spectrometer chamber of the UHV system had a base pressure of 8 × 10−11 Torr. Occupied states (or valence band) spectra were obtained by UPS using the unfiltered He I line (21.2 eV) of a discharge lamp with the samples biased at -5.0 V to avoid the influence of the detector work function and to observe the true low-energy secondary cutoff. The typical instrumental resolution for UPS measurements ranges from 0.03 to 0.1 eV with photon energy dispersion of less than 20 meV. Unoccupied states (or conduction bands) were measured by IPES using a custom-made spectrometer composed of a commercial Kimball Physics ELG-2 electron gun and a bandpass photon detector prepared according to an existing design. IPES was done in the isochromat mode using a photon detector centered at a fixed energy of 9.8 eV. The combined resolution (electron + photon) of the IPES spectrometer was determined to be 0.6 eV from the width of the Fermi edge measured on a clean polycrystalline Au film. The UPS and IPES energy scales were aligned by measuring the position of the Fermi level on a freshly evaporated Au film. The position of the vacuum level, Evac, was measured for each surface using the onset of the secondary cutoff in the UPS spectra. All the measurements were done at room temperature. Absorption spectra were recorded on a Cary 6000i Spectrometer. The PL spectra were obtained from a Horiba Jobin-Yvon FluoroMax-4 Luminescence Spectrometer. Solar Cell Fabrication and Testing: Glass substrates coated with patterned indium-doped tin oxide (ITO) with a sheet resistance of 13 Ω/◻ was purchased from Xin Yan Technology Lt. Before device fabrication, the ITO/glass substrates were subjected to a series of wet-cleaning processes in an ultrasonic bath sequentially in acetone, detergent, deionized water and isopropanol. After dried in the vacuum oven at 80 °C for 10 mins and followed by a 20-min UV-Ozone treatment, ZnO sol-gel was spun-coated onto the ITO surface at a speed of 4000 rpm for 18 s.39 The ZnO was baked at 200 °C for 0.5 h in air to formed a 30 nm thick film. The polymers were dissolved into 1,2-dichlorobenzene with stirring over night at 100 °C. The concentration was 10 mg/mL for PiI-2T, PiI-2T-PS5 and PiI-2F, and 20 mg/mL for PiI-BDT, PiI-tT and P(TP), respectively. The blend solutions were freshly made prior spin-coating and kept at 50 °C. All the active layers were spun-coated at 700 rpm for 25 s in the glove box. The wet films were put into a covered Petri dish and allowed to dry at room temperature overnight. The dried films were subsequently annealed in the glove box at 100 °C for 5 mins before they were transferred to a vacuum evaporator for electrode deposition. The thickness of the active layer was 100 nm for PiI-2T/P(TP) and PiI-2F/P(TP), 93 nm for PiI-2T-PS5/P(TP), 120 nm for PiI-BDT/P(TP), 180 nm for PiI-tT/P(TP), measured by a Dektak surface profiler. A MoO3 layer (15 nm) followed by a Ag layer (70 nm) were thermally deposited at a pressure of 6 × 10−6 Torr. The device active area is 4.0 mm2. All the devices were tested inside a nitrogen glove box with less than 40 ppm O2 and 4 ppm H2O. The PCE was tested under AM 1.5G illumination with an intensity of 100 mW cm−2 (Newport Solar Simulator 94021A) calibrated by a Newport certified silicon photo diode covered with KG5 filter with active area as 0.0663 cm2, comparable to our device area of 0.04 cm2.40 The J–V curves were recorded with a Keithley 2400 semiconductor analyzer. The IPCE and double-pass absorption were measurements under monochromic illumination and the calibration of the incident light intensity was performed with a calibrated silicon photodiode. The double-pass absorptions measurements were carried out in an integrating sphere and the parasitic absorptions of the electrode materials calculated by transfer matrix were deducted. For the sensitive EQE measurements, the current from the devices was measured as a function of photon energy using a lock-in amplifier (Stanford Instruments SR 830). For IQE calculation, the parasitic absorptions were calculated by transfer matrix optical modelling.41 Electroluminescence spectra were measured using a spectrograph (Acton Research SpectraPro 500i) equipped with a silicon CCD (charge-coupled device) array detector (Hamamatsu). The spectra were corrected for the instrument response. Acknowledge support from the Office of Naval Research (N00014-14-1-0142), KAUST Center for Advanced Molecular Photovoltaics at Stanford and the Stanford Global Climate and Energy Program, NSF DMR-1303742 and the National Natural Science Foundation of China (Projects 21174004 and 21222403). Soft X-ray characterization and analysis by NCSU supported by the U.S. Department of Energy, Office of Science, Basic Energy Science, Division of Materials Science and Engineering under Contract DE-FG02–98ER45737. Soft X-ray data was acquired at beamlines 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02–05CH11231. We thank Professor Michael D. McGehee, Dr. George F. Burkhard and Dr. Eric T. Hoke for their help in discussion of the recombination mechanism. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.}, number={22}, journal={ADVANCED MATERIALS}, author={Zhou, Yan and Kurosawa, Tadanori and Ma, Wei and Guo, Yikun and Fang, Lei and Vandewal, Koen and Diao, Ying and Wang, Chenggong and Yan, Qifan and Reinspach, Julia and et al.}, year={2014}, month={Jun}, pages={3767–3772} }
 @article{mu_liu_ma_jiang_zhao_zhang_chen_wei_yi_wang_et al._2014, title={High-Efficiency All-Polymer Solar Cells Based on a Pair of Crystalline Low-Bandgap Polymers}, volume={26}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201402473}, abstractNote={All-polymer solar cells based on a pair of crystalline low-bandgap polymers (NT and N2200) are demonstrated to achieve a high short-circuit current density of 11.5 mA cm-2 and a power conversion efficiency of up to 5.0% under the standard AM1.5G spectrum with one sun intensity. The high performance of these NT:N2200-based cells can be attributed to the low optical bandgaps of the polymers and the reasonably high and balanced electron and hole mobilities of the NT:N2200 blends due to the crystalline nature of the two polymers.}, number={42}, journal={ADVANCED MATERIALS}, author={Mu, Cheng and Liu, Peng and Ma, Wei and Jiang, Kui and Zhao, Jingbo and Zhang, Kai and Chen, Zhihua and Wei, Zhanhua and Yi, Ya and Wang, Jiannong and et al.}, year={2014}, month={Nov}, pages={7224–7230} }
 @article{ma_tumbleston_ye_wang_hou_ade_2014, title={Quantification of Nano- and Mesoscale Phase Separation and Relation to Donor and Acceptor Quantum Efficiency, J(SC), and FF in Polymer:Fullerene Solar Cells}, volume={26}, ISSN={["1521-4095"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84903724646&partnerID=MN8TOARS}, DOI={10.1002/adma.201400216}, abstractNote={Two characteristic length scales are revealed and quantified in a complex hierarchical polymer–fullerene blend by combining different X-ray scattering techniques. Anti-correlated composition variations between meso- and nanoscale separation are observed and impacted by the solvent mixture. Due to competition between the impact of the two length scales, the relation to device performance is complex and an ideal morphology is yet to be delineated.}, number={25}, journal={ADVANCED MATERIALS}, author={Ma, Wei and Tumbleston, John R. and Ye, Long and Wang, Cheng and Hou, Jianhui and Ade, Harald}, year={2014}, month={Jul}, pages={4234–4241} }
 @article{tumbleston_collins_yang_stuart_gann_ma_you_ade_2014, title={The influence of molecular orientation on organic bulk heterojunction solar cells}, volume={8}, ISSN={["1749-4893"]}, DOI={10.1038/nphoton.2014.55}, number={5}, journal={NATURE PHOTONICS}, author={Tumbleston, John R. and Collins, Brian A. and Yang, Liqiang and Stuart, Andrew C. and Gann, Eliot and Ma, Wei and You, Wei and Ade, Harald}, year={2014}, month={May}, pages={385–391} }
 @article{ma_ye_zhang_hou_ade_2013, title={Competition between morphological attributes in the thermal annealing and additive processing of polymer solar cells}, volume={1}, number={33}, journal={Journal of Materials Chemistry C}, author={Ma, W. and Ye, L. and Zhang, S. Q. and Hou, J. H. and Ade, H.}, year={2013}, pages={5023–5030} }
 @article{ma_tumbleston_wang_gann_huang_ade_2013, title={Domain Purity, Miscibility, and Molecular Orientation at Donor/Acceptor Interfaces in High Performance Organic Solar Cells: Paths to Further Improvement}, volume={3}, ISSN={["1614-6840"]}, DOI={10.1002/aenm.201200912}, abstractNote={Abstract Domain purity and interface structure are known to be critical for fullerene‐based bulk heterojunction (BHJ) solar cells, yet have been very difficult to study. Using novel soft X‐ray tools, we delineate the importance of these parameters by comparing high performance cells based on a novel naphtha[1,2‐c:5,6‐c]bis[1,2,5]thiadiazole (NT) material to cells based on a 2,1,3‐benzothiadiazole (BT) analogue. BT‐based devices exhibit ∼15 nm, mixed domains that differ in composition by at most 22%, causing substantial bimolecular recombination. In contrast, NT‐based devices have more pure domains that are >80 nm in size, yet the polymer‐rich phase still contains at least 22% fullerene. Power conversion efficiency >6% is achieved for NT devices despite a domain size much larger than the nominal exciton diffusion length due to a favourable trade‐off in the mixed domain between exciton harvesting, charge transport, and bimolecular recombination. The miscibility of the fullerene with the NT and BT polymer is measured and correlated to the purity in devices. Importantly, polarized x‐ray scattering reveals preferential face‐on orientation of the NT polymer relative to the PCBM‐rich domains. Such ordering has previously not been observed in fullerene‐based solar cells and is shown here to be possibly a controlling or contributing factor to high performance.}, number={7}, journal={ADVANCED ENERGY MATERIALS}, author={Ma, Wei and Tumbleston, John R. and Wang, Ming and Gann, Eliot and Huang, Fei and Ade, Harald}, year={2013}, month={Jul}, pages={864–872} }
 @article{qian_ma_li_guo_zhang_ye_ade_tan_hou_2013, title={Molecular Design toward Efficient Polymer Solar Cells with High Polymer Content}, volume={135}, ISSN={["1520-5126"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84878954003&partnerID=MN8TOARS}, DOI={10.1021/ja402971d}, abstractNote={A novel polythiophene derivative, PBT1, was designed, synthesized, and applied in polymer solar cells (PSCs). This work provides a successful example of using molecular structure as a tool to realize optimal photovoltaic performance with high polymer content, thus enabling the realization of efficient photoabsorption in very thin films. As a result, an efficiency of 6.88% was recorded in a PSC with a 75 nm active layer.}, number={23}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Qian, Deping and Ma, Wei and Li, Zhaojun and Guo, Xia and Zhang, Shaoqing and Ye, Long and Ade, Harald and Tan, Zhan'ao and Hou, Jianhui}, year={2013}, month={Jun}, pages={8464–8467} }
 @article{wu_li_ma_huang_huo_guo_zhang_ade_hou_2013, title={PDT-S-T: A New Polymer with Optimized Molecular Conformation for Controlled Aggregation and pi-pi Stacking and Its Application in Efficient Photovoltaic Devices}, volume={25}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201301174}, abstractNote={The correlation among molecular conformation, the crystallinity of the morphology, propensity for π–π stacking, J- versus H-aggregation, and photovoltaic performance have been studied based on two newly designed polymers, PBDTTT-S-T and PDT-S-T. The results show that more linear backbone structure is helpful to improve photovoltaic properties of the polymer, and therefore, molecular conformation should be considered for molecular design of photovoltaic polymers. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.}, number={25}, journal={ADVANCED MATERIALS}, author={Wu, Yue and Li, Zhaojun and Ma, Wei and Huang, Ye and Huo, Lijun and Guo, Xia and Zhang, Maojie and Ade, Harald and Hou, Jianhui}, year={2013}, month={Jul}, pages={3449–3455} }
 @article{ye_zhang_ma_fan_guo_huang_ade_hou_2012, title={From Binary to Ternary Solvent: Morphology Fine-tuning of D/A Blends in PDPP3T-based Polymer Solar Cells}, volume={24}, ISSN={["1521-4095"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84870872663&partnerID=MN8TOARS}, DOI={10.1002/adma.201202855}, abstractNote={For the PDPP3T/PCBM system investigated here, atomic force microscopy, resonant soft X-ray scattering, and grazing incidence wide angle X-ray scattering are used as an initial set of tools to determine the surface texture, the bulk compositional morphology, and the crystallization behavior, respectively. We find systematic variations and relate them to device performance. A solvent mixture of DCB/CF/DIO = 76:19:5 (v/v/v) yields a PCE of 6.71%.}, number={47}, journal={ADVANCED MATERIALS}, author={Ye, Long and Zhang, Shaoqing and Ma, Wei and Fan, Benhu and Guo, Xia and Huang, Ye and Ade, Harald and Hou, Jianhui}, year={2012}, month={Dec}, pages={6335–6341} }