@article{yin_ho_ding_fu_zhu_gullett_dong_so_2022, title={Enhanced Surface Passivation of Lead Sulfide Quantum Dots for Short-Wavelength Photodetectors}, volume={34}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.2c00293}, abstractNote={Lead sulfide (PbS) quantum dots are promising solution-processed materials for short-wave infrared (SWIR) photodetectors due to their tunable band gap and solution processability. Phase-transfer ligand exchange is a common method to prepare quantum dot (QD) inks used in device fabrication. For large-sized PbS QDs used for SWIR detection, the conventional phase-transfer ligand exchange has been problematic due to the densely packed organic ligands and charge-neutral (100) facets. Here, we report a new strategy to carry out the efficient phase-transfer ligand exchange in large-sized QDs. Specifically, using lead acetate trihydrate (PbAc2·3H2O) as a precursor and methylammonium acetate (MAAc) as an additive in the ligand solution, we can facilitate the efficient phase-transfer ligand exchange and epitaxial growth of perovskite intermediate (MAPbI3–xAcx) on the (100) facets, resulting in a significant improvement in film quality suitable for device fabrication. The resulting photodiodes show a 2.5× enhancement in external quantum efficiency (EQE) compared to devices using QD inks obtained using the conventional method. Considering the low transmittance of the ITO electrode in the SWIR regime, our devices exhibit an internal quantum efficiency of over 90%.}, number={12}, journal={CHEMISTRY OF MATERIALS}, author={Yin, Shichen and Ho, Carr Hoi Yi and Ding, Shuo and Fu, Xiangyu and Zhu, Liping and Gullett, Julian and Dong, Chen and So, Franky}, year={2022}, month={Jun}, pages={5433–5442} }
 @article{ho_pei_qin_zhang_peng_angunawela_jones_yin_iqbal_reynolds_et al._2022, title={Importance of Electric-Field-Independent Mobilities in Thick-Film Organic Solar Cells}, volume={10}, ISSN={["1944-8252"]}, url={http://dx.doi.org/10.1021/acsami.2c11265}, DOI={10.1021/acsami.2c11265}, abstractNote={In organic solar cells (OSCs), a thick active layer usually yields a higher photocurrent with broader optical absorption than a thin active layer. In fact, a ∼300 nm thick active layer is more compatible with large-area processing methods and theoretically should be a better spot for efficiency optimization. However, the bottleneck of developing high-efficiency thick-film OSCs is the loss in fill factor (FF). The origin of the FF loss is not clearly understood, and there a direct method to identify photoactive materials for high-efficiency thick-film OSCs is lacking. Here, we demonstrate that the mobility field-dependent coefficient is an important parameter directly determining the FF loss in thick-film OSCs. Simulation results based on the drift-diffusion model reveal that a mobility field-dependent coefficient smaller than 10-3 (V/cm)-1/2 is required to maintain a good FF in thick-film devices. To confirm our simulation results, we studied the performance of two ternary bulk heterojunction (BHJ) blends, PTQ10:N3:PC71BM and PM6:N3:PC71BM. We found that the PTQ10 blend film has weaker field-dependent mobilities, giving rise to a more balanced electron-hole transport at low fields. While both the PM6 blend and PTQ10 blend yield good performance in thin-film devices (∼100 nm), only the PTQ10 blend can retain a FF = 74% with an active layer thickness of up to 300 nm. Combining the benefits of a higher JSC in thick-film devices, we achieved a PCE of 16.8% in a 300 nm thick PTQ10:N3:PC71BM OSC. Such a high FF in the thick-film PTQ10 blend is also consistent with the observation of lower charge recombination from light-intensity-dependent measurements and lower energetic disorder observed in photothermal deflection spectroscopy.}, journal={ACS APPLIED MATERIALS & INTERFACES}, publisher={American Chemical Society (ACS)}, author={Ho, Carr Hoi Yi and Pei, Yusen and Qin, Yunpeng and Zhang, Chujun and Peng, Zhengxing and Angunawela, Indunil and Jones, Austin L. and Yin, Hang and Iqbal, Hamna F. and Reynolds, John R. and et al.}, year={2022}, month={Oct} }
 @article{jones_ho_schneider_zhang_pei_wang_zhan_marder_toney_so_et al._2022, title={Insights into the Local Bulk-Heterojunction Packing Interactions and Donor-Acceptor Energy Level Offsets in Scalable Photovoltaic Polymers}, volume={7}, ISSN={["1520-5002"]}, url={http://dx.doi.org/10.1021/acs.chemmater.2c01121}, DOI={10.1021/acs.chemmater.2c01121}, abstractNote={An energy level offset in organic solar cells (OSCs) is necessary for efficient charge generation and separation. To date, there are several polymer donor–non-fullerene acceptor (NFA) bulk-heterojunction (BHJ) systems with a negligible ionization energy (IE) level offset achieving high power conversion efficiencies (PCEs) over 15%. Although these donor–acceptor pairs perform well in solar cells, there is little understanding on why some systems can achieve this phenomenon, and therefore, many of these BHJs are discovered through a trial-and-error process. Here, we investigate how OSC efficiencies can be modulated by adjusting the IE level offset in a series of PTQ10 n:m random terpolymer donors by means of solar cell performance (open circuit voltage (VOC) and short-circuit current (JSC)) when paired with Y6 and IDIC acceptors. PTQ10’s IE level was adjusted through a copolymerization of thiophene (n), bithiophene (m), and quinoxaline monomer units in different ratios, whereby 10% bithiophene leads to a 0.05 eV decrease in the polymer’s IE. The incorporation of 10% bithiophene (PTQ10 90:10) led to a 1.3 ± 0.5 mA/cm2 increase in JSC when paired with Y6 (PCE = 13.8 ± 0.4%) in conjunction with an incremental decrease in VOC and fill factor (FF) when compared to PTQ10 (PCE = 14.7 ± 0.1%). Increasing the bithiophene content to 20% (PTQ10 80:20) exacerbated the decrease in VOC and FF further without the benefit of increased JSC. The drop in FF with increasing bithiophene incorporation correlated with increasing edge-on orientation in the neat polymer and polymer:Y6 BHJ blend films, shown by grazing-incidence wide-angle X-ray scattering measurements. High-field solid-state (ss)NMR spectroscopy analysis of single component PTQ10, Y6, and PTQ10:Y6 BHJ blends provides a complementary insight into how a low IE level offset system (PTQ10:Y6) imparts high performance. By resolving inter- and intramolecular packing interactions at sub-nanometer distances, ssNMR results offer key insights into the changes in local structures and conformations in the vicinity of the alkoxy PTQ10 side chains and in the Y6 end group in BHJ blends when compared to the neat compounds. Despite the changes in local structures, the BHJ morphology maintains pure D-A domains and preserves the microstructure, which correlates with the high-performing solar cells. A synergic combination of chemical design, multiscale morphology characterization, and device physics shown in this study provides an excellent strategy to investigate the BHJ and its role in organic solar cell performance.}, journal={CHEMISTRY OF MATERIALS}, author={Jones, Austin L. and Ho, Carr Hoi Yi and Schneider, Sebastian A. and Zhang, Junxiang and Pei, Yusen and Wang, Jiayu and Zhan, Xiaowei and Marder, Seth R. and Toney, Michael F. and So, Franky and et al.}, year={2022}, month={Jul} }
 @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_kothari_fu_so_2021, title={Interconnecting layers for tandem organic solar cells}, volume={21}, ISSN={["2468-6069"]}, DOI={10.1016/j.mtener.Y021.100707}, journal={MATERIALS TODAY ENERGY}, author={Ho, C. H. Y. and Kothari, J. and Fu, X. and So, F.}, year={2021}, month={Sep} }
 @article{ngai_chan_ho_ho_cheung_yin_so_2020, title={A facile and robust approach to prepare fluorinated polymer dielectrics for probing the intrinsic transport behavior of organic semiconductors}, url={https://doi.org/10.1039/D0MA00175A}, DOI={10.1039/D0MA00175A}, abstractNote={Insulating polymers are often used as gate dielectric materials in all-solution processable organic thin-film transistors (OTFTs). Nonetheless, most of the polymers have poor resistance to common halogenated solvents and thus are not feasible for bottom-gate OTFT structures. In this contribution, we show that high molecular weight poly(2,3,4,5,6-pentafluorostyrene) (PPFS) is a gate dielectric material that can be free from this limitation. We prepare PPFS with a facile and efficient approach by using methyl isobutyl ketone (MIK) solvent in a basic wet-lab without the need of complex chemical equipment. Furthermore, the non-polar and hydrophobic nature of the PPFS surface allows us to probe the intrinsic transport behaviors of these acceptors. The MIK solvent-assisted polymerization method provides an alternative for low-cost effective gate polymer dielectric preparation. Such a novel but simple synthesis paradigm not only opens up a broad employment of solution-processable polymeric dielectrics for bottom-gate OTFTs, but also enables the full device potential with high-mobility semiconductors.}, journal={Materials Advances}, publisher={Royal Society of Chemistry (RSC)}, author={Ngai, Jenner H. L. and Chan, Cyril Chak Ming and Ho, Carr Hoi Yi and Ho, Johnny Ka Wai and Cheung, Sin Hang and Yin, Hang and So, Shu Kong}, year={2020} }
 @article{yi_peng_xu_seyitliyev_ho_danilov_kim_reynolds_amassian_gundogdu_et al._2020, title={Critical Role of Polymer Aggregation and Miscibility in Nonfullerene-Based Organic Photovoltaics}, volume={10}, ISSN={["1614-6840"]}, url={http://dx.doi.org/10.1002/aenm.201902430}, DOI={10.1002/aenm.201902430}, abstractNote={Abstract}, number={8}, journal={ADVANCED ENERGY MATERIALS}, author={Yi, Xueping and Peng, Zhengxing and Xu, Bing and Seyitliyev, Dovletgeldi and Ho, Carr Hoi Yi and Danilov, Evgeny O. and Kim, Taesoo and Reynolds, John R. and Amassian, Aram and Gundogdu, Kenan and et al.}, year={2020}, month={Feb} }
 @article{yi_ho_gautam_lei_chowdhury_bahrami_qiao_so_2020, title={Effects of polymer crystallinity on non-fullerene acceptor based organic solar cell photostability}, volume={8}, ISSN={["2050-7534"]}, DOI={10.1039/d0tc03969a}, abstractNote={While there has been rapid progress made in the performance of organic photovoltaic (OPV) cells in recent years, the device stability remains a major bottleneck for commercialization. In this work, we blended a stable acceptor (O-IDTBR) with two photostable donors (PTB7-Th and PffBT4T-2OD) having different polymer crystallinity, and the resulting devices show a significant difference in the OPV degradation rate. The OPV devices employing a highly crystalline polymer PffBT4T-2OD as an active layer show a good resistance against light soaking, maintaining 80% of the initial power conversion efficiency (PCE) up to 100 hours, while the devices employing an amorphous polymer PTB7-Th as an active layer show a significant PCE loss in the initial 20 hours mainly due to a rapid loss of the fill factor. By carrying out a comprehensive analysis of the device degradation mechanisms, we conclude that the origin for the PTB7-Th:O-IDTBR device degradation is the formation of mid-gap states under continuous sunlight illumination, leading to a significant drop in electron mobility. Device simulation revealed that deep traps act as charge recombination centers and increase the trap-assisted recombination rate, lowering the FF and Jsc.}, number={45}, journal={JOURNAL OF MATERIALS CHEMISTRY C}, author={Yi, Xueping and Ho, Carr Hoi Yi and Gautam, Bhoj and Lei, Lei and Chowdhury, Ashraful Haider and Bahrami, Behzad and Qiao, Qiquan and So, Franky}, year={2020}, month={Dec}, pages={16092–16099} }
 @article{firdaus_ho_lin_yengel_le corre_nugraha_yarali_so_anthopoulos_2020, title={Efficient Double- and Triple-Junction Nonfullerene Organic Photovoltaics and Design Guidelines for Optimal Cell Performance}, volume={5}, ISSN={["2380-8195"]}, DOI={10.1021/acsenergylett.0c02077}, abstractNote={The performance of multijunction devices lags behind single-junction organic photovoltaics (OPVs) mainly because of the lack of suitable subcells. Here, we attempt to address this bottleneck and demonstrate efficient nonfullerene-based multijunction OPVs while at the same time highlighting the remaining challenges. We first demonstrate double-junction OPVs with power conversion efficiency (PCE) of 16.5%. Going a step further, we developed triple-junction OPVs with a PCE of 14.9%, the highest value reported to date for this triple-junction cells. Device simulations suggest that improving the front-cell’s carrier mobility to >5 × 10–4 cm2 V–1 s–1 is needed to boost the efficiency of double- and triple-junction OPVs. Analysis of the efficiency limit of triple-junction devices predicts that PCE values of close to 26% are possible. To achieve this, however, the optical absorption and charge transport within the subcells would need to be optimized. The work is an important step toward next-generation multijunction OPVs.}, number={12}, journal={ACS ENERGY LETTERS}, author={Firdaus, Yuliar and Ho, Carr Hoi Yi and Lin, Yuanbao and Yengel, Emre and Le Corre, Vincent M. and Nugraha, Mohamad I and Yarali, Emre and So, Franky and Anthopoulos, Thomas D.}, year={2020}, month={Dec}, pages={3692–3701} }
 @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={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{jones_ho_riley_angunawela_ade_so_reynolds_2020, title={Investigating the active layer thickness dependence of non-fullerene organic solar cells based on PM7 derivatives}, url={https://doi.org/10.1039/D0TC03096A}, DOI={10.1039/D0TC03096A}, abstractNote={Power conversion efficiencies (PCEs) in organic solar cells (OSCs) have rapidly improved in the last 5 years owing largely to the development of novel small-molecule acceptors and polymer donors with several systems’ performances exceeding 17%. A key factor for these materials implementation into industrial relevant devices is their active layer thickness tolerance as solar cell performances are typically reported with thicknesses on the order of 100–150 nm, but thicker films (ca. 300 nm) are needed for printing and roll-to-roll processing. In this report, two PM7 isomeric derivatives were synthesized featuring a chlorinated benzodithiophene and ester functionalized terthiophene moieties for the incorporation into non-fullerene OSCs. The fundamental difference between the two isomeric polymers is the location of the ester side chains where the PM7 D1 esters are located on the outer thiophene units, whereas the esters on PM7 D2 are located on the central thiophene unit. This simple modification produced polymers with similar absorption profiles, electrochemical onsets, charge carrier mobilities when blended with ITIC-4F, and grazing-incidence wide-angle X-ray scattering patterns. Thin-film (100 nm) OSCs were fabricated resulting in average PCEs of 11.6% for PM7, 12.1% for PM7 D1, and 9.9% for PM7 D2 when blended with ITIC-4F. In contrast, large differences are observed in PCE when the active layer thickness is increased to 180 nm resulting in a decrease in average PCE for PM7 D2 (5.3%), whereas PM7 D1 was able to retain a 11.9% average PCE. The difference in active layer thickness tolerance between PM7 D1 and PM7 D2 is rationalized by extracting the energetic disorder (σ) for hole transport using temperature dependent space-charge limited current studies. In the end, this study conveys how small changes to polymer structure, such as side chain placement, may have a small effect on thin-film polymer properties and device performance, but significant differences are realized when charges are transported over longer distances (thicker films, >150 nm).}, journal={Journal of Materials Chemistry C}, publisher={Royal Society of Chemistry (RSC)}, author={Jones, Austin L. and Ho, Carr Hoi Yi and Riley, Parand R. and Angunawela, Indunil and Ade, Harald and So, Franky and Reynolds, John R.}, year={2020} }
 @article{dong_ho_yu_salehi_so_2019, title={Defect Passivation by Fullerene Derivative in Perovskite Solar Cells with Aluminum-Doped Zinc Oxide as Electron Transporting Layer}, volume={31}, ISSN={["1520-5002"]}, DOI={10.1021/acs.chemmater.9b01292}, abstractNote={ZnO is a potential replacement for TiO2 as an electron transport layer (ETL) used in low-temperature processed hybrid perovskite solar cells. However, poor thermal stability of perovskites on ZnO and interfacial traps introduced during processing are obstacles to achieve a good device performance. Here, we demonstrate perovskite solar cells using aluminum doped zinc oxide (AZO) nanoparticles for the ETL having a better thermal stability compared with ZnO. However, the device shows a lower short circuit current density and a large photocurrent hysteresis, which are attributed to the poor interfacial properties between the ETL and the perovskite layer. To address this issue, a thin interfacial modification layer of phenyl-C61-butyric acid methyl ester (PCBM) was employed. The resulting device shows the efficiency is improved from 13 to 17% along with a significant reduction in hysteresis. Results from our thermal admittance spectroscopy show that the interface defect states are significantly reduced with th...}, number={17}, journal={CHEMISTRY OF MATERIALS}, author={Dong, Qi and Ho, Carr Hoi Yi and Yu, Hyeonggeun and Salehi, Amin and So, Franky}, year={2019}, month={Sep}, pages={6833–6840} }
 @article{sen_yang_rech_feng_ho_huang_so_kline_you_kudenov_et al._2019, title={Panchromatic All-Polymer Photodetector with Tunable Polarization Sensitivity}, volume={7}, ISSN={["2195-1071"]}, DOI={10.1002/adom.201801346}, abstractNote={Abstract}, number={4}, journal={ADVANCED OPTICAL MATERIALS}, author={Sen, Pratik and Yang, Ruonan and Rech, Jeromy J. and Feng, Yuanxiang and Ho, Carr Hoi Yi and Huang, Jinsong and So, Franky and Kline, R. Joseph and You, Wei and Kudenov, Michael W. and et al.}, year={2019}, month={Feb} }
 @article{xu_yi_huang_zheng_zhang_salehi_coropceanu_ho_marder_toney_et al._2018, title={Donor Conjugated Polymers with Polar Side Chain Groups: The Role of Dielectric Constant and Energetic Disorder on Photovoltaic Performance}, volume={28}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201803418}, abstractNote={Abstract}, number={46}, journal={Advanced Functional Materials}, author={Xu, B. and Yi, X. and Huang, T. and Zheng, Z. and Zhang, J. and Salehi, A. and Coropceanu, V. and Ho, C.H.Y. and Marder, S.R. and Toney, M.F. and et al.}, year={2018}, pages={1803418} }
 @article{luo_tran_kadlubowski_ho_riley_so_mei_2018, title={Side-Chain Sequence Enabled Regioisomeric Acceptors for Conjugated Polymers}, volume={51}, ISSN={["1520-5835"]}, url={https://doi.org/10.1021/acs.macromol.8b01946}, DOI={10.1021/acs.macromol.8b01946}, abstractNote={Side-chain sequence enabled regioisomeric acceptors, bearing different side-chain sequences on the same conjugated backbone, are herein reported. Two regioregular polymers PTBI-1 and PTBI-2 and one regiorandom polymer PTBI-3 were synthesized from these two regioisomeric acceptors for a comparative study. UV–vis–NIR absorption spectroscopy and electrochemical study confirmed similar frontier molecular orbital levels of the three polymers in their solid state. More intriguingly, absorption profiles suggest that the sequence of side chains greatly governs the aggregation behaviors. Furthermore, the PTBI-2 film shows larger ordered domains than PTBI-1 and PTBI-3 films, as supported by AFM and GIWAXS measurements. As a result, PTBI-2-based FET devices achieved an average hole mobility of 1.37 cm2 V–1 s–1, much higher than the two polymers with other side-chain sequences. The regiorandom PTBI-3 exhibited the lowest average hole mobility of 0.27 cm2 V–1 s–1. This study highlights the significant impact of side-c...}, number={21}, journal={MACROMOLECULES}, publisher={American Chemical Society (ACS)}, author={Luo, Xuyi and Tran, Dung T. and Kadlubowski, Natalie M. and Ho, Carr Hoi Yi and Riley, Parand and So, Franky and Mei, Jianguo}, year={2018}, month={Nov}, pages={8486–8492} }
 @article{deng_wu_lei_zhou_ho_zhu_ong_2017, title={A readily-accessible, random perylene diimide copolymer acceptor for all-polymer solar cells}, volume={146}, url={https://doi.org/10.1016/j.dyepig.2017.06.058}, DOI={10.1016/j.dyepig.2017.06.058}, abstractNote={A new, structurally random perylene diimide (PDI) copolymer acceptor, PPDI, has been readily synthesized from Stille cross-coupling polycondensation of two pairs of regioisomeric dibromo-PDIs with 2,5-bis(trimethylstannyl)thiophene. PPDI possesses a complex structural configuration with a greatly twisted conformation, and thus exhibits weak intermolecular interactions. A solution of PPDI and a polymer donor such as PBT7-Th affords a smooth homogenous film without large crystalline domain formation. This composite film absorbs strongly throughout the visible spectrum, and when used as an active layer in all-polymer solar cells, provides a power conversion efficiency of over 5%.}, journal={Dyes and Pigments}, publisher={Elsevier BV}, author={Deng, Ping and Wu, Bo and Lei, Yanlian and Zhou, Dagang and Ho, Carr Hoi Yi and Zhu, Furong and Ong, Beng S.}, year={2017}, month={Nov}, pages={20–26} }
 @article{ho_cao_lu_lau_cheung_li_yin_chiu_ma_cheng_et al._2017, title={Boosting the photovoltaic thermal stability of fullerene bulk heterojunction solar cells through charge transfer interactions}, volume={5}, url={https://doi.org/10.1039/C7TA06530B}, DOI={10.1039/C7TA06530B}, abstractNote={Fullerene-based bulk heterojunction organic solar cells (BHJ-OSCs) represent one of the current state-of-the-art organic solar cells. Nonetheless, most of these devices still suffer from adverse performance degradation due to thermally induced morphology changes of active layers. We herein demonstrate that the photovoltaic performance stability of BHJ-OSCs can be profoundly enhanced with an appositely functionalized 9-fluorenylidene malononitrile. The latter, through charge transfer (CT) interactions with a donor polymer, enables the formation of a “frozen” 3-dimensional mesh-like donor polymer matrix, which effectively restrains free movement of embedded fullerene molecules and suppresses their otherwise uncontrolled aggregation. 9-Fluorenylidene malononitrile derivatives with multiple CT interaction sites are particularly effective as preservation of a power conversion efficiency of over 90% under severe thermal stress has been accomplished. The generality of this novel strategy has been affirmed with several common donor polymers, manifesting it to be hitherto the most efficient approach to stabilized fullerene-based BHJ-OSCs.}, number={45}, journal={Journal of Materials Chemistry A}, publisher={Royal Society of Chemistry (RSC)}, author={Ho, Carr Hoi Yi and Cao, Huanyang and Lu, Yong and Lau, Tsz-Ki and Cheung, Sin Hang and Li, Ho-Wa and Yin, Hang and Chiu, Ka Lok and Ma, Lik-Kuen and Cheng, Yuanhang and et al.}, year={2017}, pages={23662–23670} }
 @article{yin_chiu_ho_lee_li_cheng_tsang_so_2017, title={Bulk-heterojunction solar cells with enriched polymer contents}, volume={40}, url={http://www.sciencedirect.com/science/article/pii/S1566119916304578}, DOI={10.1016/j.orgel.2016.10.030}, abstractNote={In a high performance PTB7:PC71BM bulk-heterojunction (BHJ) solar cell, the commonly optimized polymer:fullerene (D:A) weight ratio is about 1:1.5, when PC71BM is used as the acceptor. This report explores alternative D:A weight ratios. We describe how to enrich the polymer contents of these BHJ solar cells to achieve high power conversion efficiencies (PCEs). The concentration of 1,8-diiodooctane (DIO), a solvent additive for the BHJ precursor solutions, is increased in order to re-optimize the BHJ cells. The PCEs of the re-optimized cells are improved for the PTB7 cells. Detailed charge transport measurements were carried out to examine the polymer-rich BHJs. We observed enhanced hole mobilities for the PTB7 BHJs. Additionally, the electron mobilities are preserved due to the dispersion of fullerene domains by increased DIO concentrations. Two other well-known polymer donors PCDTBT and PDTSTPD have been also investigated, and the improvements of hole mobilities and PCEs can be obtained for both polymer-rich BHJ solar cells.}, journal={Organic Electronics: physics, materials, applications}, author={Yin, H. and Chiu, K.L. and Ho, C.H.Y. and Lee, H.K.H. and Li, H.W. and Cheng, Y. and Tsang, S.W. and So, S.K.}, year={2017}, pages={1–7} }
 @article{deng_ho_lu_li_tsang_so_ong_2017, title={Naphthalene diimide-difluorobenzene-based polymer acceptors for all-polymer solar cells}, volume={53}, url={https://doi.org/10.1039/C6CC09724C}, DOI={10.1039/C6CC09724C}, abstractNote={Regio-random (P1) and -regular (P2) difluorobenzene-naphthalene-containing polymer acceptors were developed for bulk-heterojunction all-polymer solar cells (all-PSCs). P2 exhibited significantly higher crystallinity in thin films, providing high spectral absorptivity and electron mobility than P1. When used in all-PSC devices, P2 afforded a respectably higher power conversion efficiency of over 5%.}, number={22}, journal={Chemical Communications}, publisher={Royal Society of Chemistry (RSC)}, author={Deng, Ping and Ho, Carr Hoi Yi and Lu, Yong and Li, Ho-Wa and Tsang, Sai-Wing and So, Shu Kong and Ong, Beng S.}, year={2017}, pages={3249–3252} }
 @article{thick-film high-performance bulk-heterojunction solar cells retaining 90% pces of the optimized thin film cells_2017, url={http://onlinelibrary.wiley.com/doi/10.1002/aelm.201700007/full}, DOI={10.1002/aelm.201700007/full}, journal={Advanced Electronic Materials}, year={2017}, month={Mar} }
 @article{ho_cheung_li_chiu_cheng_yin_chan_so_tsang_so_et al._2017, title={Using Ultralow Dosages of Electron Acceptor to Reveal the Early Stage Donor-Acceptor Electronic Interactions in Bulk Heterojunction Blends}, volume={7}, ISSN={["1614-6840"]}, url={http://onlinelibrary.wiley.com/doi/10.1002/aenm.201602360/full}, DOI={10.1002/aenm.201602360}, abstractNote={Tuning the donor–acceptor (D–A) weight ratio is an essential step to optimize the performance of a bulk heterojunction (BHJ) solar cell. The unoptimized regime with a low acceptor concentration is generally unexplored despite it may reveal the early stage electronic D–A interactions. In this study, PTB7:PC71BM is used to examine factors that limit the device performance in unoptimized regime. The key limiting factor is the creation of traps and localized states originated from fullerene molecules. Photothermal deflection spectroscopy is used to quantify the trap density. Starting with pristine PTB7, addition of small concentration of fullerene increases the electron trap density and lowers the electron mobility. When the D–A weight ratio reaches 1:0.1, fullerene percolation occurs. There is an abrupt drop in trap density and simultaneously a six orders of magnitude increase in the electron mobility. Furthermore, the fill factors of the corresponding photovoltaic devices are found to anticorrelate with the trap density. This study reveals that electron trapping is the key limiting factor for unoptimized BHJ solar cells in low fullerene regime.}, number={12}, journal={ADVANCED ENERGY MATERIALS}, publisher={Wiley-Blackwell}, author={Ho, C. H. Y. and Cheung, S. H. and Li, H. W. and Chiu, K. L. and Cheng, Y. H. and Yin, H. and Chan, M. H. and So, Franky and Tsang, S. W. and So, S. K. and et al.}, year={2017}, month={Jun} }
 @article{ho_dong_yin_leung_yang_lee_tsang_so_2015, title={Impact of Solvent Additive on Carrier Transport in Polymer:Fullerene Bulk Heterojunction Photovoltaic Cells}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84939457367&partnerID=MN8TOARS}, DOI={10.1002/admi.201500166}, abstractNote={The effects of a solvent additive, 1,8‐diiodooctane (DIO), on both hole and electron transport are investigated in a state‐of‐the‐art bulk‐heterojunction (BHJ) system, namely PTB7:PC71BM. For a polymer:fullerene weight ratio of 1:1.5, the electron mobility in the blend film increases by two orders of magnitude with the DIO concentration while almost no change is found in the hole mobility. For lower DIO concentrations, the electron mobility is suppressed because of large, but poorly connected PC71BM domains. For higher concentrations of DIO, the electron mobility is improved progressively and the hole mobility becomes the limiting factor. Between 1 and 5 vol%, the electron and hole mobilities are balanced. Using the Gaussian disorder model (GDM), we found that the DIO concentration modifies fundamentally the average hopping distances of the electrons. In addition, there exist alternative donor–acceptor ratios to achieve optimized PTB7:PC71BM based solar cells. It is demonstrated that the fullerene content of the BHJ film can be significantly reduced from 1:1.5 to 1:1 while the optimized performance can still be preserved.}, number={12}, journal={Adv. Mater. Interfaces}, author={Ho, Carr Hoi Yi and Dong, Qi and Yin, Hang and Leung, Winky Wing Ki and Yang, Qingdan and Lee, Harrison Ka Hin and Tsang, Sai Wing and So, Shu Kong}, year={2015}, month={Jul} }
 @inproceedings{using solvent additive to achieve charge carrier balance transport in polymer : fullerene bulk heterojunction photovoltaic cells_2015, booktitle={2015 MRS Fall Meeting and Exhibit, At Boston}, year={2015}, month={Nov} }