Future perspective of polymer solar cells based on recent in-depth understanding of photovoltaic conversion mechanism

In this review, I will discuss the improvement of photovoltaic parameters, such as short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF), in terms of photophysical elementary processes of photovoltaic conversion in polymer solar cells. These elementary processes can be directly observed using time-resolved spectroscopic measurements. Thus, I will introduce the latest research topics, focusing on these spectroscopic analyses. Finally, I will mention future prospects for further improvements in the power conversion efficiency of polymer solar cells

π-Conjugated polymers and molecules enabling small photon energy loss simultaneously with high efficiency in organic photovoltaics

Organic photovoltaics (OPVs) are a topic of significant research interest in the field of renewable energy as well as organic electronics. The crucial issue in OPVs is the improvement of the power conversion efficiency (PCE). In addressing this issue, one of the most important factors is the photon energy loss (Eloss), which is defined as the difference between the bandgap of the materials and the energy corresponding to the open-circuit voltage. Typically, the Eloss for OPVs is considerably larger than that for inorganic and perovskite photovoltaics, which has prevented OPVs from generating larger photovoltages. In parallel, reducing the Eloss for OPVs causes a loss of driving-force energy for charge generation, which is detrimental to the generation of photocurrent. Thus, OPVs have been facing a trade-off between photocurrent and photovoltage. However, a number of recently developed π-conjugated materials for use as p-type and n-type organic semiconductors have been shown to enable small Eloss values that are close to those for inorganic systems, simultaneously with efficient charge generation. Here, we summarize recent progress in π-conjugated polymers and molecules that enable small Eloss and high PCE at the same time. We hope that this review will be of help to chemists and materials scientists who are involved in the design of materials and blends with an eye toward highly efficient OPVs

Sensitizer–host–annihilator ternary-cascaded triplet energy landscape for efficient photon upconversion in the solid state

In this paper, we introduce a new strategy for improving the efficiency of upconversion emissions based on triplet–triplet exciton annihilation (TTA-UC) in the solid state. We designed a ternary blend system consisting of a triplet sensitizer (TS), an exciton-transporting host polymer, and a small amount of an annihilator in which the triplet-state energies of the TS, host, and annihilator decrease in this order. The key idea underpinning this concept involves first transferring the triplet excitons generated by the TS to the host and then to the annihilator, driven by the cascaded triplet energy landscape. Because of the small annihilator blend ratio, the local density of triplet excitons in the annihilator domain is higher than those in conventional binary TS/annihilator systems, which is advantageous for TTA-UC because TTA is a density-dependent bimolecular reaction. We tracked the triplet exciton dynamics in the ternary blend film by transient absorption spectroscopy. Host triplet excitons are generated through triplet energy transfer from the TS following intersystem crossing in the TS. These triplet excitons then diffuse in the host domain and accumulate in the annihilator domain. The accumulated triplet excitons undergo TTA to generate singlet excitons that are higher in energy than the excitation source, resulting in UC emission. Based on the excitation-intensity and blend-ratio dependences of TTA-UC, we found that our concept has a positive impact on accelerating TTA

Triplet Exciton Dynamics in Fluorene-Amine Copolymer Films

Effect of aromaticity on triplet exciton dynamics was studied by transient absorption spectroscopy for two fluorene-based random copolymers with different aromatic amine, poly(9, 9′-di-n-octylfluorene-ran-N, N′-bis(4-n-butylphenyl)-N, N′-diphenyl-1, 4-benzenediamine) (F8-PDA) and poly(9, 9′-di-n-octylfluorene-ran-N, N′-bis(4-t-butylphenyl)-N, N′-diphenyl-9, 10-anthracenediamine) (F8-ADA). On a time scale of nanoseconds, triplet exciton was efficiently formed in F8-PDA through the intersystem crossing (ISC) from singlet exciton with a rate constant of 2.0 × 10⁸ s⁻¹. On the other hand, the ISC was not efficient in F8-ADA, resulting in efficient fluorescence emission. On a time scale of micro- to milliseconds, F8-PDA exhibited bimolecular triplet exciton decay due to triplet–triplet annihilation (TTA), but the TTA was negligible in F8-ADA, indicating that triplet excitons can diffuse freely in F8-PDA while they are trapped at ADA units in F8-ADA. The difference in the ISC efficiency and triplet exciton diffusion is discussed in terms of the aromaticity of the amine units. On the basis of these analyses, we discuss a strategy for further improvement in the efficiency of fluorene–amine copolymer based light-emitting diodes

Dye sensitization of polymer/fullerene solar cells incorporating bulky phthalocyanines

The light-harvesting efficiency of P3HT:PCBM solar cells can be improved by incorporating near-IR dye molecules such as silicon phthalocyanine derivatives with bulky axial groups (SiPc). In order to study the size effect of the axial groups on the dye sensitization in P3HT:PCBM solar cells, we synthesized five SiPc derivatives with different axial groups: SiPc[OSi(C_{n}H_{2n+1})_{3}]_{2} (SiPcn, n = 2, 3, 4, 6) and SiPc[OSi(iBu)_{2}C_{18}H_{37}]_{2} (SiPcB18). The power conversion efficiency (PCE) increased in the order of n = 2–4, reached the maximum at around n = 4 and 6, and then decreased for SiPcB18 with the longest axial groups. As a result, the PCE was improved to 4.2%, which is larger by 10% than that of P3HT:PCBM control cells without dye molecules. We therefore conclude that the butyl or hexyl chain in the axial ligand is the most appropriate for the dye sensitization in P3HT:PCBM solar cells

Triplet sensitization via charge recombination at organic heterojunction for efficient near-infrared to visible solid-state photon upconversion

Realizing efficient near-infrared to visible photon upconversion in the solid state is pivotal for commercial applications in various fields. We previously reported a solid-state upconversion device which imitated the photovoltaic conversion mechanisms of organic solar cells. This leads to a significant improvement of up to 2.3% in the external quantum efficiency, which is two orders of magnitude higher than that of conventional devices. Here, we investigate the upconversion mechanism of this device. We examine exciton and charge dynamics using transient absorption spectroscopy and find that approximately 67% of incident photons are utilized owing to fast singlet exciton diffusion in the nonfullerene acceptor layer. Strikingly, triplet excitons are accumulated near the donor/acceptor interface, enabling accelerated triplet–triplet annihilation by a factor of more than 10

Enhanced Hole Transport in Ternary Blend Polymer Solar Cells

Recently, ternary blend polymer solar cells have attracted great attention to improve a short‐circuit current density (JSC) effectively, because complementary absorption bands can harvest the solar light over a wide wavelength range from visible to near‐IR region. Interestingly, some ternary blend solar cells have shown improvements not only in JSC but also in fill factor (FF). Previously, we also reported that a ternary blend solar cell based on a low‐bandgap polymer (PTB7‐Th), a wide‐bandgap polymer (PDCBT), and a fullerene derivative (PCBM) exhibited a higher FF than their binary analogues. Herein, we study charge transport in PTB7‐Th/PDCBT/PCBM ternary blend films to address the origin of the improvement in FF. We found that hole polarons are located in PTB7‐Th domains and their mobility is enhanced in the ternary blend film

Role of Energy Offset in Nonradiative Voltage Loss in Organic Solar Cells

The voltage loss incurred by nonradiative charge recombination should be reduced to further improve the power conversion efficiency of organic solar cells (OSCs). This work discusses the nonradiative voltage loss in OSCs with systematically controlled energy offset between optical bandgap and charge transfer (CT) states. It is demonstrated that the nonradiative voltage loss is a function of the energy offset; it drops sharply with decreasing energy offset. By measuring the quantum yields of electroluminescence from OSCs and decay kinetics of CT states, it is found that the radiative decay rate of CT states becomes larger when the energy offset is negligible compared with those in conventional OSCs with sufficient energy offset. This behavior is rationalized by hybridization between CT and local excited states, resulting in a considerable enhancement of the oscillator strength of CT states. Based on a trend observed in this study, the precise mechanism by which the energy offset affects the nonradiative voltage loss is discussed

Enhanced Charge Transport in a Conjugated Polymer Blended with an Insulating Polymer

Herein, the hole transport in a quinoxaline–thiophene based conjugated polymer (PTQ1) mixed with an insulating polystyrene (PS) was studied by macroscopic and local current density−voltage characteristics measurements. As a result, we found that the hole conductivity in PTQ1 : PS blends increases as the weight ratio of PTQ1 is reduced down to 20 wt%. This is mainly ascribed to increases in mobility because the charge carrier density would be constant in the insulating PS matrix. With decreasing PTQ1 weight ratio in the blends, the absorption bandwidth of PTQ1 and additional emission due to excimer decreased, suggesting that interchain interactions are suppressed. By measuring the temperature‐dependent conductivity, we also found that the activation energy for the hole conductivity is smaller in PTQ1 : PS blends than in PTQ1 neat films. These findings suggest that trap sites decrease because of the suppressed interaction between PTQ1 chains in blend films. We also measured conductive atomic force microscope images of the blend films to clarify the local conductive property. For PTQ1 neat films, a low conductive image was observed over the entire film. For PTQ1 : PS blends, on the other hand, many highly conductive spots were locally found. We thus conclude that the dilution of PTQ1 chains in the PS matrix leads to a lower formation of trap sites, resulting in more conductive transport in PTQ1 : PS blends than in PTQ1 neat films

Ternary Blend Polymer Solar Cells Based on Wide-Bandgap Polymer PDCBT and Low-Bandgap Polymer PTB7-Th

Light-harvesting efficiency can be prominently increased by using ternary blend polymer solar cells, in which a wide-bandgap crystalline polymer is incorporated into a binary blend of a low-bandgap polymer and a fullerene derivative. This is partly due to the complementary absorption bands over a wide wavelength range, and partly ascribed to the thick photoactive layer. As a result, the best power conversion efficiency of 9.40% was obtained for the ternary blend device with a thickness of ≈300 nm