A Large-Bandgap Conjugated Polymer for Versatile Photovoltaic Applications with High Performance.

Benzodithiophene-Based Small-Molecule Donors for Next-Generation All-Small-Molecule Organic Photovoltaics

Naphthalenothiophene imide-based polymer exhibiting over 17% efficiency

18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

Carrier Dynamics Engineering for High-Performance Electron-Transport-Layer-free Perovskite Photovoltaics

Oligothiophene-based photovoltaic materials for organic solar cells: rise, plateau, and revival

Surfactant-Encapsulated Polyoxometalate Complex as a Cathode Interlayer for Nonfullerene Polymer Solar Cells

Donor-acceptor (D-A) copolymers have attracted significant scientific interest recently as their electronic and optoelectronic properties can be manipulated through intramolecular charge transfer (ICT). Such polymers may have potential applications in various organic electronic devices, especially for field effect transistors and polymer solar cells. In order to obtain high-performance of these devices, it is essential to design and synthesize conjugated polymers with desirable properties, such as high carrier mobility, ordered and tunable morphology, and appropriate molecular energy levels. Utilizing highly electron-donating and coplanar aromatic fused-rings as a building block into D-A based system could probably meet above-methioned requirements, contructing ordered molecular packing and high charge transporting of conjugated polymers. Herein we summarized the systematic studies on the syntheses, optoelectronic properties, morphology, and their device characterizations of the three D-A conjugated systems embedded with coplanar donors, including: (1) indolocarbazole-based, (2) thiophene-phenylene-thiophene (TPT)-based, (3) two-dimensional thiophene (2D)-based alternating or random copolymers. In addition, the final subject is to introduce P3HT-based coil-rod-coil copolymer into bulk-heterojunction solar cell as a surfactant, leading to the improved efficiency and device stability. The details of each topic are summarized as below: 1. Synthesis of New Indolocarbazole-Acceptor Alternating Conjugated Copolymers and Their Applications to Thin Film Transistors and Photovoltaic Cells (Chapter 2): we report the synthesis, properties, and optoelectronic device characteristics of six new indolocarbazole-acceptor conjugated copolymers prepared by Suzuki coupling reaction. Two different linkages of indolocarbazole (28IC and 39IC) and four acceptors of 2,3-didodecylthieno[3,4-b]pyrazine (TP12), 2,3-bis(4-(2-ethylhexyloxy)phenyl)thieno[3,4-b]pyrazine (TPO), 2,1,3-benzothiadiazole (BT), and 2,3-bis(4-(2-ethylhexyloxy)phenyl)quinoxaline (QO) were used to explore the effects of acceptor structure, linkage, and side group on the electronic and optoelectronic properties. The hole mobility and on-off ratios of the studied copolymers were in the range of 1.66×10-5 ~ 4×10-4 cm2 V–1 s–1 and 40~46900, respectively. It basically depended on the degree of intromolecular charge transfer (ICT) between indolocarbazole and acceptor as well as the HOMO level. The power conversion efficiency (PCE) of the indolocarbazole-acceptor polymer/PC61BM or PC71BM based photovoltaic cells were in the range of 0.14-1.40% under the illumination of AM 1.5G (100 mW/cm2). P28IC-QO showed the best PCE among the studied copolymers because of its suitable HOMO/LUMO energy level, high molecular weight, good hole mobility, efficient PL quenching, and large Voc. 2. New Thiophene-Phenylene-Thiophene Acceptor Random Conjugated Copolymers for Optoelectronic Applications (Chapter 3): New low band-gap thiophene-phenylene-thiophene (TPT)-based donor-acceptor-donor random copolymers were synthesized for optoelectronic device applications by a palladium-catalyzed Stille coupling reaction under microwave heating. The acceptors included 2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-bis[5’-bromo-dithien-2-yl-quinoxa- lines] (DTQ) and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethyl-hexyl)pyrrolo[3,4-c] -pyrrole-1,4-dione (DPP). The prepared the random copolymers were named as PTPTDTQ0.55, PTPTDTQ0.34DPP0.14, and PTPTDTQ0.26DPP0.34 depending on the copolymer ratio and their corresponding Egopt (eV) were 1.74, 1.56, and 1.48 eV, respectively. The hole mobility obtained from the field effect transistor devices prepared from PTPTDTQ0.55, PTPTDTQ0.34DPP0.14, and PTPTDTQ0.26DPP0.34 were 2.2×10–3, 2.4×10–3, and 4.7×10–3 cm2 V–1s–1, respectively, with the on-off ratios of 4.0×104, 4.0×104, and 5.3×104. It suggested that the significant ICT effect between the TPT and acceptor led to the band gap reduction and hole mobility enhancement. Polymer solar cells of these TPT-based copolymers blended PC71BM exhibited power conversion efficiencies (PCEs) as high as 3.71 %. Besides, the near-infrared photodetector device prepared from PTPTDTQ0.26DPP0.34 showed a high external quantum efficiency exceeding 32% at 700 nm (under –3V bias) and fast-speed response. The present study suggests that the prepared TPT-based donor-acceptor random copolymers exhibited promising and versatile applications on optoelectronic devices. 3. New Two-Dimensional Thiophene-Acceptor Conjugated Copolymers for Field Effect Transistor and Photovoltaic Cell Applications (Chapter 4): we report the synthesis, properties and optoelectronic device applications of two-dimensional (2D) like conjugated copolymers, P4TBT, P4TDTBT, P4TDTQ, and P4TDPP, consisted of 2’,5’’-bis(trimethystannyl)-5,5’’’-di-(2-ethylhexyl) -[2,3’;5’,2’’;4’’,2’’’]quarterthiophene (4T) with the following four acceptors of BT, 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT), DTQ, and DPP. The 2D-like conjugated copolymers exhibited high hole mobilities in the range of 10-1 ~ 10-4 cm2 V–1s–1. Moreover, the FET electron mobilities were observed for P4TDTBT and P4TDPP, due to their relatively low-lying LUMO level suitable for electron injection. In particular, P4TDPP showed the ambipolar characteristics with the hole and electron mobilities of 0.115 cm2V–1s–1 (on/ff ratio : 2.49×104) and 3.08×10-3 cm2V-1s-1 (on/off ratio : 7.34×102), respectively, which was strongly related to its order intermolecular chain packing based on the DSC and XRD studies. The PCE could be reached to 2.43 % of P4TDPP/PC71BM (1:2) based device, due to the balanced hole/electron mobility. The above results indicate that these two-dimensional 4T-acceptor conjugated copolymers could enhance the charge-transport characteristics and are promising materials for organic optoelectronic devices. 4. Enhancement of P3HT/PCBM Photovoltaic Efficiency Using the Surfactant of Triblock Copolymer Containing Poly(3-hexylthiophene) and Poly(4-vinylphenylamine) Segments (Chapter 5): the well-defined coil-rod-coil triblock copolymer, PTPA-P3HT-PTPA, has been served as a surfactant for P3HT/PCBM (1:1) based solar cells. The device performance is enhanced in the presence of the 1.5% PTPA-P3HT-PTPA with optimized devices showing a power conversion efficiency of 4.4%. With the surfactant ratios in the range of 0% to 1% and 2% to 5%, the fiber-like structure could be observed. As the critical ratio of 1.5% is added, the sphere-like nanostructure were obtained resulting to smaller domain size and increasing the interfacial area for charge separation as compared to fibrous structure. On the other hand, the increasing hole mobility with the addition of surfactant maybe due to the donor characteristic of PTPA-P3HT-PTPA, leading to the balanced hole and electron mobility. Hence, the significant enhanced PCE of the 1.5% PTPA-P3HT-PTPA blended system as compared to the pristine P3HT/PCBM system (3.9%) could be attributed to the sphere-like structure formed and much more balanced mobility (μe/μh ~1.7). Additionally, the introduction of PTPA-P3HT-PTPA as a surfactant not only extents the life-time of solar cells but also reduces the PCBM aggregation upon annealing, resulting in better thermal stability. The surfactant effect also could be confirmed by DSC measurement revealing that the selective miscibility of coil segment with PCBM. These results indicate the superior compatibilizing effect for the triblock copolymer in solar cell application.

Efficient and stable inverted polymer solar cells using TiO2 nanoparticles and analysized by Mott-Schottky capacitance

Toward high performance inverted polymer solar cells

Dimerized small-molecule acceptors enable efficient and stable organic solar cells

The performance of the inverted polymer solar cells (PSCs) were examined with solution-processed TiO2 nanoparticles (TiO2 NPs) as electron transport layer (ETL) and vapor deposited MoO3 as hole transport layer (HTL). The performance of the inverted PSCs based on P3HT:PCBM heterojunction was closely depend on the thickness of interface layer. The high power conversion efficiency (PCE) of 3.90% was achieved when the thickness of MoO3 is 5 nm. The interesting thing is that the PCE of these kinds of inverted PSCs were dramatically enhance after exposing them in air. The increase of the Ag work function, the reduction of MoO3 film work function, and the enlarged P3HT:PCBM grain size are deduced to the main reasons for the improvement of the devices’ performance. The very high PCE of 4.51% was achieved after the device was exposed in air for 144 h. In addition, we also measured the stability of this kind of devices in air for 1008 h. The devices show excellent stability and the PCE of the devices could maintain about 4.0% after 1008 h exposure. The results provide a new facile method for fabrication of highly efficient and stable inverted PSCs.

Bulk heterojunction (BHJ) polymer solar cells (PSCs) that can be fabricated by solution processing techniques are under intense investigation in both academic institutions and industrial companies because of their potential to enable mass production of flexible and cost-effective alternative to silicon-based solar cells. A combination of novel polymer development, nanoscale morphology control and processing optimization has led to over 8% of power conversion efficiencies (PCEs) for BHJ PSCs with a conventional device structure. Attempts to develop PSCs with an inverted device structure as required for achieving high PECs and good stability have, however, met with limited success. Here, we report that (1) solution-processed zinc oxide (ZnO) thin film as an electron extraction layer for inverted polymer solar cells. Operated at room temperature, no obviously degradation was observed from the PSCs with ZnO layer after continuously illuminating the devices for 4 hours. However, a significantly degradation was observed from the PSCs without ZnO buffer layer after illuminating the devices only for 1 hour. Furthermore, PSCs with ZnO buffer layer also show very good shelf stability; only 10 % degradation observed in PCEs after 6 months; (2) a high PCE of 8.4% under AM1.5G irradiation was achieved for BHJ PSCs with an inverted device structure. This high efficiency was obtained through interfacial engineering of solution-processed electron extraction layer, ZnO, leading to facilitate electron transport and suppress bimolecular recombination. All these results provided an important progress for solution-processed PSCs, and demonstrated that PSCs with an inverted device structure are comparable with PSCs with the conventional device structure.

ABX3 Perovskites for Tandem Solar Cells

ConspectusEmerging solar cells that convert clean and renewable solar energy to electricity, such as organic solar cells (OSCs) and perovskite solar cells (PSCs), have attracted increasing attention owing to some merits such as facile fabrication, low cost, flexibility, and short energy payback time. The power conversion efficiencies (PCEs) of OSCs and PSCs have exceeded 18% and 25%, respectively.Fullerene derivatives have high electron affinity and mobility with an isotropic transport feature. Fullerene-based OSCs yielded superior PCEs to other acceptors and have dominated electron acceptor materials from 1995 to 2015. However, some drawbacks of fullerenes, such as weak visible absorption, limited tunability of electronic properties, laborious purification, and morphological instability, restrict further development of OSCs toward higher PCEs and practical applications. The theoretical PCE of fullerene-based OSCs is limited to ∼13% due to the relatively large energy losses. Many efforts have been dedicated to developing new acceptor systems beyond fullerenes, and some successful systems such as rylene diimides have achieved PCEs up to ca. 11%.In 2015, our group pioneered a new class of electron acceptors, fused-ring electron acceptor (FREA), as represented by the star molecule ITIC. The chemical features of FREAs include: (1) a modular structure, consisting of an electron-donating core, electron-withdrawing end groups, π-bridges, and side chains, which benefits molecular tailoring; (2) facile synthesis, purification, and scalability. The physical features of FREAs include: (1) a broad modulation range of absorption and energy levels; (2) strong absorption, especially in the 700-1000 nm region; (3) high electron mobility. The device features of FREAs include: (1) low voltage loss; (2) high efficiency; (3) good stability. The FREAs boosted PCEs of the OSCs up to 18% and initiated the transformation from the fullerene to nonfullerene era of this field. FREAs can also be used in PSCs as interfacial layers, electron transport layers, or active layers, improving both efficiency and stability of the devices. Beyond photovoltaic applications, FREAs can also be used in photodetectors, field-effect transistors, two-photon absorption, photothermal therapy, solar water splitting, etc.In this Account, we review the development of the FREAs and their applications in OSCs, PSCs, and other related fields. Molecular design, device engineering, photophysics, and applications of FREAs are discussed in detail. Future research directions toward performance optimization and commercialization of FREAs are also proposed.

Organo-lead trihalide perovskite compounds, represented by CH3NH3PbI3, exhibit many rare functions as narrow bandgap semiconductors (Fig. 1). In 2006, we have first employed CH3NH3PbBr3 and CH3NH3PbI3 as the sensitizer of electrochemical solar cell. 1a In 2008 this method was applied to make a first solid-state perovskite solar cell with carbon-polymer composite as a hole transport materialin.1c Recent rapid progress of perovskite-based photovoltaics (PV) has enabled power conversion efficiency (PCE) to reach 2２% (Fig. 2) . Our group has studied simple solution process for perovskite absorber preparation in ambient air conditions and achieved efficiency beyond 17%.2 Here, photocurrent-voltage (J-V) behavior of the perovskite PV cell was investigated to clarify the origin(s) of its hysteretic performance by focusing on the influence of interfacial structures.3 Perovskite cells can be fabricated by low temperature processes. Lightweight flexible photovoltaic cells will see enormous applications in power devices. We developed plastic film perovskite cells (PCE >13%), fabricated on ITO-PEN film by using brookite TiO2 as mesoporous layer, to demonstrate its high mechanical durability against repeated bending. As metal oxide electron collectors, ZnO/SnO2 composite films prepared at temperatures <120oC enabled PCE of perovskite cell exceeding 15%.4 Use of SnO2 collector is a new trend in perovskite PV in terms of high voltage and improvement of cell lifetime. The heat stability of perovskite device is an important issue for industrial applications. It is improved by using formamidinium (HC(NH2)2) that replaces CH3NH3. We made HC(NH2)2PbI3-based solar cells on low-temperature mesoporous ZnO. They exhibit relatively high stability for long time preservation with PCE as high as 16% (Fig. 3 left). HC(NH2)2PbI3 solar cell prepared on thin TiO2 layer exhibits high open circuit voltage (Voc)>1.1V with PEC >17% (Fig. 3 right). Enormous potential of perovskite-based device is not only expected for power devices but also for high performance optical sensing devices. Organic lead trihalide perovskite can work as a high function semiconductor photodiode. We found that the gain of CH3NH3PbI3-induced diode photocurrent can reach a high level of the order of 103, showing excellent light-switching and current amplifying performance (Fig. 4)5. Such rare functions of the perovskite devices provide a lot of rooms to explore their versatile applications in photovoltaics and optoelectronics. REFERENCES 1. a) A. Kojima, et al. #397, 210th ECS Meeting, 2006. b) ibid, #352, 212th ECS Meeting, 2007. c). ibid, PRiME 2008, #27, Honolulu,2008.2. T. Miyasaka, Chem. Lett., 44, 720 (2015).3. A. K. Jena, A. Kulkarni, M. Ikegami, T. Miyasaka, J . Power Sources, 309, 1-10(2016).4. J. Song, E. Zheng, X.-F. Wang, W. Tian, T. Miyasaka, Solar Ener . Mater . S olar Cells, 144, 623(2016).5. H.-W. Chen, N. Sakai, A. K. Jena, Y. Sanehira, M. Ikegami, K.-C. Ho, T. Miyasaka, J. Phys. Chem. Lett., 6, 1773(2015) Figure 1