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    Transparent Hole-Transporting Frameworks: A Unique Strategy to Design High-Performance Semitransparent Organic Photovoltaics
    (1970-01-01)
    Cheng, Pei
    ;
    Wang, Hao-Cheng
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    Zhu, Yuan
    ;
    Zheng, Ran
    ;
    Li, Tengfei
    Thanks to the nature of molecular orbitals, the absorption spectra of organic semiconductors are not continuous like those in traditional inorganic semiconductors, which offers a unique application of organic photovoltaics (OPVs): semitransparent OPVs. Recently, the exciting progress of materials design has promoted the development of semitransparent OPVs. However, in the perspective of device engineering, almost all reported works reduce the thickness of back/reflected electrode to obtain high average visible transmittance (AVT), which is a trade-off between power conversion efficiency (PCE) and the transmittance of the whole solar spectrum (visible and infrared), and therefore limit the further development. Herein, a unique strategy of "transparent hole-transporting frameworks" is proposed. A hole-transporting large-bandgap polymer (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA)) is employed to partially replace polymer donors in the active layer of PBDB-T/Y1. PTAA is a p-type polymer with a large bandgap of 2.9 eV; the partial substitution of PBDB-T by PTAA reduces the absorption of the active layer only in the visible region, keeping the hole-transporting pathways as well as the optimized film morphology. As a result, semitransparent OPVs with PCEs of 12% and AVTs of 20% are achieved, both on rigid and flexible substrates. To demonstrate the generality, this strategy is also used in three different active layers.
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    Enabling High-Performance Tandem Organic Photovoltaic Cells by Balancing the Front and Rear Subcells
    (1970-01-01)
    Cheng, Pei
    ;
    Wang, Hao-Cheng
    ;
    Zheng, Ran
    ;
    Zhu, Yuan
    ;
    Dai, Shuixing
    In tandem organic photovoltaics, the front subcell is based on large-bandgap materials, whereas the case of the rear subcell is more complicated. The rear subcell is generally composed of a narrow-bandgap acceptor for infrared absorption but a large-bandgap donor to realize a high open-circuit voltage. Unfortunately, most of the ultraviolet-visible part of the photons are absorbed by the front subcell; as a result, in the rear subcell, the number of excitons generated on large-bandgap donors will be reduced significantly. This reduces the (photo) conductivity and finally limits the hole-transporting property of the rear subcell. In this work, a simple and effective way is proposed to resolve this critical issue. To ensure sufficient photogenerated holes in the rear subcell, a small amount of an infrared-absorbing polymer donor as a third component is introduced, which provides a second hole-generation and transporting mechanism to minimize the aforementioned detrimental effects. Finally, the short-circuit current density of the two-terminal tandem organic photovoltaic is significantly enhanced from 10.3 to 11.7 mA cm(-2)(while retaining the open-circuit voltage and fill factor) to result in an enhanced power conversion efficiency of 15.1%.
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    Engineering the Core Units of Small-Molecule Acceptors to Enhance the Performance of Organic Photovoltaics
    (1970-01-01)
    Wang, Hao-Cheng
    ;
    Chen, Chung-Hao
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    Li, Ren-Hao
    ;
    Lin, Yu-Che
    ;
    Tsao, Cheng-Si
    Understanding the chemical structures of next-generation small molecules is a critical step for increasing the performance of organic photovoltaics (OPVs); an OPV's small molecule determines not only the extent of light absorption but also the morphology. Herein, four small molecules featuring different cores-indaceno dithiophene, dithienoindeno indaceno dithiophene (IDTT), substituted IDTT, and dithienothiophene-pyrrolobenzothiadiazole-denoted as ID-4Cl, IT-4Cl, m-ITIC-OR-4Cl, and Y7, respectively, are selected to form active layers with poly(quinoxaline) (PTQ10) and poly(benzodithiophene-4,8-dione) (PM6). The Y7 devices exhibit the best performance in both systems, with the power conversion efficiency (PCE) reaching 14.5%; in comparison, ID-4Cl device gives a PCE of 10.0% for blending with PTQ10 and a relative efficiency enhancement of 45%. The same trend occurs for the cases of PM6 blend devices. This enhancement is attributed to i) the improved short-circuit current density that is provided by the greater degree of conjugation in S, N-heteroarenes ladder-type fused-ring cores of Y7, ii) an induced face-on Y7 orientation and smaller domain sizes that result from the sp(2)-hybridized nitrogen side chain, and iii) smaller energy loss. This study reveals the importance of the core structure on the device performance and provides guidelines for the design of new materials for OPV technologies.
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    Mg Doped CuCrO2 as Efficient Hole Transport Layers for Organic and Perovskite Solar Cells
    (2019-09-01)
    Zhang, Boya
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    Thampy, Sampreetha
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    Dunlap-Shohl, Wiley A.
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    Xu, Weijie
    ;
    Zheng, Yangzi
    The electrical and optical properties of the hole transport layer (HTL) are critical for organic and halide perovskite solar cell (OSC and PSC, respectively) performance. In this work, we studied the effect of Mg doping on CuCrO2 (CCO) nanoparticles and their performance as HTLs in OSCs and PSCs. CCO and Mg doped CCO (Mg:CCO) nanoparticles were hydrothermally synthesized. The nanoparticles were characterized by various experimental techniques to study the effect of Mg doping on structural, chemical, morphological, optical, and electronic properties of CCO. We found that Mg doping increases work function and decreases particle size. We demonstrate CCO and Mg:CCO as efficient HTLs in a variety of OSCs, including the first demonstration of a non-fullerene acceptor bulk heterojunction, and CH3NH3PbI3 PSCs. A small improvement of average short-circuit current density with Mg doping was found in all systems.
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    Rational Tuning of Molecular Interaction and Energy Level Alignment Enables High-Performance Organic Photovoltaics
    (1970-01-01)
    Wang, Rui
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    Yuan, Jun
    ;
    Han, Guangchao
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    Huang, Tianyi
    ;
    Huang, Wenchao
    The performance of organic photovoltaics (OPVs) has rapidly improved over the past years. Recent work in material design has primarily focused on developing near-infrared nonfullerene acceptors with broadening absorption that pair with commercialized donor polymers; in the meanwhile, the influence of the morphology of the blend film and the energy level alignment on the efficiency of charge separation needs to be synthetically considered. Herein, the selection rule of the donor/acceptor blend is demonstrated by rationally considering the molecular interaction and energy level alignment, and highly efficient OPV devices using both-fluorinated or both-nonfluorinated donor/acceptor blends are realized. With the enlarged absorption, ideal morphology, and efficient charge transfer, the devices based on the PBDB-T-F/Y1-4F blend and PBDB-T-F/Y6 exhibit champion power conversion efficiencies as high as 14.8% and 15.9%, respectively.
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    Emerging Organic and Organic/Inorganic Hybrid Photovoltaic Devices for Specialty Applications: Low-Level-Lighting Energy Conversion and Biomedical Treatment
    (2019-01-04)
    Chen, Fang-Chung
    ;
    交大名義發表
    ;
    光電工程學系
    ;
    National Chiao Tung University
    ;
    Department of Photonics
    This article reviews the progress made in emerging photovoltaic devices, including dye-sensitized solar cells, organic solar cells, and perovskite solar cells, for specialty applications. Because they all feature tunable band structures, which can be modified through molecular design and/or altered compositions, the properties of emerging photovoltaic devices can be tailored to fit the special requirements of a particular application. For instance, they can be designed according to the emission spectrum of an artificial light source (e.g., fluorescence lamp and white light-emitting diode), thereby efficiently converting the indoor lighting to electricity. Furthermore, because there exists a near-infrared windowranging from 650 to 1350 nmin which photons can penetrate deeply into tissues, the absorption bands of these photovoltaic devices can be tailored specially to this therapeutic window to harvest tissue-penetrating light, thereby wirelessly providing power to biomedical devices positioned beneath the skin. The rapid progress made in these two representative areas suggests further promising applications of such emerging photovoltaic devices.
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    Toward Long-Term Stable and Efficient Large-Area Organic Solar Cells
    (2017-07-10)
    Tsai, Pei-Ting
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    Lin, Kuan-Chu
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    Wu, Cheng-Yu
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    Liao, Chung-Hung
    ;
    Lin, Man-Chun
    Here, we report that long-term stable and efficient organic solar cells (OSCs) can be obtained through the following strategies: i) combination of rapid-drying blade-coating deposition with an appropriate thermal annealing treatment to obtain an optimized morphology of the active layer; ii) insertion of interfacial layers to optimize the interfacial properties. The resulting devices based on poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-2-carboxylate-2,6-diyl)] (PBDTTT-EFT):[6,6]-phenyl C-71 butyric acid methyl ester (PC71BM) blend as the active layer exhibits a power conversion efficiency (PCE) up to 9.57 %, which represents the highest efficiency ever reported for blade-coated OSCs. Importantly, the conventional structure devices based on poly(3-hexylthiophene) (P3HT):phenyl-C-61-butyric acid methyl ester (PCBM) blend can retain approximately 65% of their initial PCE for almost 2 years under operating conditions, which is the best result ever reported for long-term stable OSCs under operational conditions. More encouragingly, long-term stable large-area OSCs (active area = 216 cm(2)) based on P3HT:PCBM blend are also demonstrated. Our findings represent an important step toward the development of large-area OSCs with high performance and long-term stability.
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    Use of Ultrafast Time-Resolved Spectroscopy to Demonstrate the Effect of Annealing on the Performance of P3HT:PCBM Solar Cells
    (2015-03-04)
    Wang, Yu-Ting
    ;
    Chen, Mei-Hsin
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    Lin, Chao-Ting
    ;
    Fang, Jian-Jhih
    ;
    Chang, Che-Jui
    The organic solar cells of heterojunction system, ITO/PEDOT:PSS/P3HT:PCBM/Al, with a thermal annealing after deposition of Al exhibit better performance than those with an annealing process before deposition of Al. In this study, ultrafast time-resolved spectroscopy is employed to reveal the underlying mechanism of annealing effects on the performance of P3HT:PCBM solar cell devices. The analyses of all decomposed relaxation processes show that the postannealed devices exhibit an increase in charge transfer, in the number of separated polarons and a reduction in the amount of recombination between excited carriers. Moreover, the longer lifetime for the excited carriers in postannealed devices indicates it is more likely to be dissociated into photocarriers and result in a larger value for photocurrent, which demonstrates the physical mechanism for increased device performance.