Hiroaki Benten

Near-IR Femtosecond Transient Absorption Spectroscopy of Ultrafast Polaron and Triplet Exciton Formation in Polythiophene Films with Different Regioregularities

The formation dynamics of polaron pairs, polarons, and triplet excitons in regiorandom and regioregular poly(3-hexylthiophene) (RRa-P3HT and RR-P3HT) films was comprehensively studied by transient absorption spectroscopy over the wide wavelength region from 500 to 1650 nm under various excitation intensities. In both RRa-P3HT and RR-P3HT films, polaron pairs were generated not from relaxed singlet exciton states but from hot excitons on a time scale of <100 fs and decayed monomolecularly by geminate recombination. In RRa-P3HT films, triplet excitons were rapidly generated on a picosecond time scale from higher exciton states produced by the singlet exciton-exciton annihilation as well as from the lowest singlet exciton states by the normal intersystem crossing. In RR-P3HT films, no triplet excitons were observed; polarons were also generated not from relaxed singlet exciton states but from hot excitons in competition with the formation of polaron pairs. The polarons formed in RR-P3HT can freely migrate and mainly recombine with other polarons bimolecularly in the nanosecond time domain. The ultrafast formation of triplet excitons can be explained by the singlet exciton fission into two triplets, and the ultrafast formation of polaron pairs and polarons can be explained on the basis of the hot-exciton dissociation model where the excess thermal energy of the initially formed hot excitons is necessary to overcome their Coulombic binding energy. The remarkably different formation dynamics in P3HTs with different regioregularities is discussed in terms of the film morphology of conjugated polymers.

Highly efficient charge-carrier generation and collection in polymer/polymer blend solar cells with a power conversion efficiency of 5.7%

A polymer/polymer blend solar cell with an external quantum efficiency approaching 60% and the best power conversion efficiency of 5.73% is fabricated. The efficient charge-carrier generation and collection, comparable to those of polymer/fullerene solar cells, are found to be the main reasons for the superior device performance.

Improvement of the Light-Harvesting Efficiency in Polymer/Fullerene Bulk Heterojunction Solar Cells by Interfacial Dye Modification

Enhancement of the light-harvesting efficiency in poly(3-hexylthiophene)/fullerene derivative (P3HT/PCBM) bulk heterojunction solar cells has been demonstrated by the introduction of near-infrared phthalocyanine molecules as the third component at the P3HT/PCBM interface. The introduction of silicon phthalocyanine derivative (SiPc) increased the short-circuit current density and hence improved the overall power conversion efficiency by 20%, compared to the P3HT/PCBM control device. For P3HT/PCBM/SiPc devices, two distinct external quantum efficiency (EQE) peaks were observed at wavelengths for the absorption bands of SiPc as well as P3HT before and after thermal annealing, suggesting that SiPc molecules are located at the P3HT/PCBM interface because of crystallization of the P3HT and PCBM domains. Furthermore, the EQE for the device increased even at wavelengths for the absorption band of P3HT by the introduction of SiPc molecules. This indicates that P3HT excitons can be dissociated into charge carriers more efficiently in the presence of SiPc molecules at the P3HT/PCBM interface by energy transfer from P3HT to SiPc molecules. These findings suggest that there are two origins for the increase in the photocurrent by the introduction of SiPc; SiPc molecules serve not only as a light-harvesting photosensitizer but also as an energy funnel for P3HT excitons at the P3HT/PCBM interface.

Multi-colored dye sensitization of polymer/fullerene bulk heterojunction solar cells

Multi-colored dye-sensitized polymer/fullerene solar cells with two different near-IR dyes, silicon phthalocyanine bis(trihexylsilyl oxide) (SiPc) and silicon naphthalocyanine bis(trihexylsilyl oxide) (SiNc), enhanced power conversion efficiency up to 4.3%, compared to that of the individual ternary blend solar cells with a single dye under AM1.5G illumination.

Photovoltaic Performance of Perovskite Solar Cells with Different Grain Sizes

Perovskite solar cells exhibit improved photovoltaic parameters with increasing perovskite grain size. The larger photocurrent is due to the enhanced absorption efficiency for thicker perovskite layers. The larger open-circuit voltage (VOC ) is ascribed to the reduced trap-assisted recombination for the larger grains. As a result, the power conversion efficiency exceeds 19% at best. Further improvement in VOC would be possible if the trap density were reduced.

Surface segregation at the aluminum interface of poly(3-hexylthiophene)/fullerene solar cells

The effects of thermal annealing before and after Al deposition on poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend solar cells were investigated by current density-voltage measurements and x-ray photoelectron spectroscopy (XPS). Compared to the preannealed device, the postannealed device exhibited enhanced open-circuit voltage (VOC), which is ascribed to the decrease in the reverse saturation current density J0. The XPS measurements demonstrated that P3HT is dominant at the Al interface in the preannealed device while PCBM is instead dominant in the postannealed device. This surface-segregated PCBM formed in the postannealed device can serve as a hole-blocking layer at the Al interface to reduce J0, and therefore improve VOC.

Recent research progress of polymer donor/polymer acceptor blend solar cells

Polymer/polymer blend solar cells based on a blend of two types of conjugated polymers acting as an electron donor (hole transport) and acceptor (electron transport) have recently attracted considerable attention, because they have numerous potential advantages over conventional polymer/fullerene blend solar cells. The highest power conversion efficiency (PCE) was slightly above 2% five years ago, whereas PCEs of beyond 8% are the state-of-the-art today, and the efficiency gap between polymer/polymer and polymer/fullerene systems has closed very rapidly. In this review, we provide an overview of recent progress towards the performance enhancement of polymer/polymer blend solar cells. In addition, we discuss the future outlook and challenges regarding PCEs beyond 10%.

Bimodal Polarons and Hole Transport in Poly(3-hexylthiophene):Fullerene Blend Films

The bimolecular recombination dynamics in blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) has been studied by transient absorption spectroscopy. On a microsecond time scale, two polaron bands were observed at 700 and 1000 nm and exhibited different bimolecular recombination dynamics. The 700-nm band decayed with a time-independent bimolecular recombination rate of 10(-12) cm(3) s(-1). The activation energy was as small as approximately 0.078 eV independently of the carrier density. On the other hand, the 1000-nm band decayed with a time-dependent bimolecular recombination rate, which varied from 10(-12) to 10(-13) cm(3) s(-1), depending on time or carrier density. The activation energy decreased exponentially from 0.178 to 0.097 eV with the increase in the carrier density. Therefore, we assigned the 700-nm band to freely mobile delocalized polarons in crystalline P3HT domains and the 1000-nm band to localized polarons trapped in relatively disordered P3HT domains. At a charge density of 10(17) cm(-3), which corresponds to 1 sun open-circuit condition, some localized polarons exhibited trap-free bimolecular recombination due to trap-filling. These findings suggest that not only delocalized polarons but also some localized polarons play a crucial role in the efficient hole transport in P3HT:PCBM solar cells.

Exciton Diffusion in Conjugated Polymers: From Fundamental Understanding to Improvement in Photovoltaic Conversion Efficiency

Singlet exciton diffusion plays a central role in the photovoltaic conversion in organic photovoltaics (OPVs). Upon light absorption, singlet excitons are promptly generated in organic materials instead of charge carriers because the dielectric constant (εr) is small (∼3-4), which is in sharp contrast to inorganic and perovskite solar cells. In order to convert to charge carriers, excitons need to diffuse into an interface between electron donor and acceptor materials before deactivating to the ground state. Therefore, fundamental understanding of exciton diffusion dynamics is one of the most important issues to further improve OPVs. We highlight recent leading studies in this field and describe several approaches for efficient exciton harvesting at the interface in OPVs.

High-performance ternary blend all-polymer solar cells with complementary absorption bands from visible to near-infrared wavelengths

We developed high-performance ternary blend all-polymer solar cells with complementary absorption bands from visible to near-infrared wavelengths. A power conversion efficiency of 6.7% was obtained with an external quantum efficiency over 60% both in the visible and near-infrared regions. Our results demonstrate that the ternary blend all-polymer systems open a new avenue for accelerating improvement in the efficiency of non-fullerene thin-film polymer solar cells.