Keisuke Tajima

Control of Miscibility and Aggregation Via the Material Design and Coating Process for High‐Performance Polymer Blend Solar Cells

A power conversion efficiency of 3.6% for an all-polymer solar cell, which is the highest ever reported, is achieved by introducing a conjugated side chain into a p-type polymer to improve the miscibility of the polymer blend and by adding small amounts of 1,8-diiodooctane to increase the aggregation of n-type polymer.

Organic Planar Heterojunctions: From Models for Interfaces in Bulk Heterojunctions to High-Performance Solar Cells

Recent progress regarding planar heterojunctions (PHJs) is reviewed, with respect to the fundamental understanding of the photophysical processes at the donor/acceptor interfaces in organic photovoltaic devices (OPVs). The current state of OPV research is summarized and the advantages of PHJs as models for exploring the relationship between organic interfaces and device characteristics described. The preparation methods and the characterization of PHJ structures to provide key points for the appropriate handling of PHJs. Next, we describe the effects of the donor/acceptor interface on each photoelectric conversion process are reviewed by examining various PHJ systems to clarify what is currently known and not known. Finally, it is discussed how we the knowledge obtained by studies of PHJs can be used to overcome the current limits of OPV efficiency.

Dominant Effects of First Monolayer Energetics at Donor/Acceptor Interfaces on Organic Photovoltaics

Energy levels of the first monolayer are manipulated at donor/acceptor interfaces in planar heterojunction organic photovoltaics by using molecular self-organization. A cascade energy landscape allows thermal-activation-free charge generation by photoirradiation, destabilizes the energy of the interfacial charge-transfer state, and suppresses bimolecular charge recombination, resulting in a higher open-circuit voltage and fill factor.

Synthesis and properties of D–A copolymers based on dithienopyrrole and benzothiadiazole with various numbers of thienyl units as spacers

Three kinds of donor–acceptor (D–A) type semiconducting copolymers in which electron donating units of dithienopyrrole (DTP) and accepting units of benzothiadiazole (BT) were connected with different numbers of thienyl spacers (x = 0–2) were synthesized and used as electron donor materials in polymer solar cells (PSCs) combined with fullerene derivatives. The optical band gaps of the polymers could be tuned from 1.41 eV to 1.61 eV by changing the number of thiophene spacers. Electrochemical measurements showed that the increase of band gap is mainly due to the change of the lowest unoccupied molecular orbital (LUMO) energy level. The power conversion efficiency (PCE) of the polymer solar cells based on the three polymers and PC70BM reached 3.12% with x = 2 under the illumination of AM 1.5, 100 mW cm−2. This approach could provide a simple strategy for designing high-performance D–A type photovoltaic polymers based on the existing polymers and a large potential to improve their performance further.

Fullerene-free organic photovoltaics based on unconventional material combination: a molecular donor and polymeric acceptors

In conventional organic photovoltaic cells, the active layer consists of a polymeric donor and a molecular acceptor (PD/MA). An unconventional material combination based on molecular donor/polymeric acceptor (MD/PA) emerged in 2014 but attracted limited attention. To broaden photovoltaic material systems and understand the crucial factors related to the photovoltaic performance, in this report, we adopted a molecular donor (p-DTS(FBTTh2)2) and three polymeric acceptors based on perylenediimide (PDI). We find that the high contents (70–80%) of p-DTS(FBTTh2)2 and the better crystallinity and larger grains in the blend films induced by the addition of 1,8-diiodooctane (DIO) play an important role in constructing the continuous and effective donor phase for charge transfer and hole transport in the active layers. The highest PCE of photovoltaic cells reached 3.01% with a VOC of 0.68 V, JSC of 7.59 mA cm−2, and FF of 0.58 for the p-DTS(FBTTh2)2u2006:u2006PSe-PDI active layer, although the hole and the electron mobilities are still unbalanced. Further optimization of the film morphology and improvement of the electron mobility by material design and device engineering are expected to boost the efficiency of MD/PA type fullerene-free solar cells.

Optical Anisotropy and Strong H‐Aggregation of Poly(3‐Alkylthiophene) in a Surface Monolayer

Slab optical waveguide absorption spectra reveal that surface segregated monolayers of a vertically oriented poly(3-buthylthiophene) derivative have large optical anisotropy, and that confinement of the polymer chains in the isolated monolayer causes strong H-aggregation.

Inside-fused perylenediimide dimers with planar structures for high-performance fullerene-free organic solar cells

For perylenediimide derivatives it seems that twisted structures are essential to avoid excessive aggregation tendencies and realize high-performance fullerene-free solar cells. However, in this communication, we designed and synthesized two planar inside-fused perylenediimide dimers, TDI2 and BDT-TDI2. Theoretical calculations reveal that both TDI2 and BDT-TDI2 have a highly planar molecular conformation with small dihedral angles of 0.02° and 11.4° between two TDI segments, respectively. By using BDDT as the donor polymer, power conversion efficiencies (PCEs) of the photovoltaic cells reached 5.80% for TDI2 and 4.52% for BDT-TDI2, with high open-circuit voltages (VOC) of ∼1.0 V. These results indicate planar PDI-dimer derivatives are also possible electron acceptors to realize high-performance fullerene-free solar cells.

Effects of a side chain sequence on surface segregation of regioregular poly(3-alkylthiophene) and interfacial modification of bilayer organic photovoltaic devices

A regioregular poly(3-alkylthiophene) with a statistical sequence of alkyl/semifluoroalkyl side chains (stat-P3DDFT) was synthesized through the copolymerization of mixed monomers. The surface segregation behavior was compared with the corresponding alternating copolymer (alt-P3DDFT) that forms highly ordered dipole layers by surface segregation. X-ray and ultraviolet photoelectron spectroscopies of the polymer films revealed that alt-P3DDFT formed a much more ordered surface segregated monolayer than did stat-P3DDFT, indicating the importance of the alternating sequence of the side chains. When stat-P3DDFT was inserted into the donor/acceptor interfaces in bilayer organic photovoltaic devices, the short circuit current and the open circuit voltages of the devices were related to the disordered interface structures of stat-P3DDFT.

Interface-induced crystallization and nanostructure formation of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in polymer blend films and its application in photovoltaics

Crystallization of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in thin films and in blend films with various polymers was investigated by X-ray diffraction. Thermal annealing induced the crystallization of PCBM in the blend films only through direct contact with a crystallized pure PCBM layer beneath, suggesting that an epitaxial crystallite growth occurred from the bottom interface. The morphology of the crystals depended strongly on the mixing ratio and the crystal structure of the bottom layer, and nanorod-like PCBM crystallites with widths in the range of 100–150 nm and lengths in the range of 150–500 nm were observed. A bulk-heterojunction (BHJ) organic solar cell utilizing the PCBM crystallite as the acceptor showed the highest VOC of 0.83 V for a PTB7:PCBM device to date. These findings offer the ways to use the crystallized PCBM with the controlled nanostructures as the electron conducting materials in organic and hybrid perovskite photovoltaics.

Cumulative gain in organic solar cells by using multiple optical nanopatterns

Light wave manipulation by using nanostructures is a promising strategy for enhancing the light absorption of thin photoactive layers in organic photovoltaics (OPVs). Here, we propose a method for nanopatterning the multiple interfaces in bulk heterojunction (BHJ) OPVs by using soft imprint lithography at room temperature. The interfaces in the OPVs were separately modified in the front ZnO layers and the back metal electrodes with a grating pattern. Each nanopattern increased the light absorption and the power conversion efficiency of the OPVs by up to 32.5% depending on the materials. Moreover, the nanopatterning at both the front and the back cumulatively increased the light absorption, resulting in the highest efficiency increase of 38.5%. The increases were observed in various BHJ systems with different properties containing the polymers PTB7, PCE10, P3HT, or PNTz4T. A certified performance of 10.31% was achieved for the PNTz4T:PC71BM system in the presence of the nanopatterns. Detailed analysis by using the absorption spectra and optical simulations indicated that the origins of the optical gains from the nanopatterns on the front and the back are different. The front pattern increases the transmittance and the back pattern increases the scattering and excites the surface plasmon polaritons.