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To investigate the effects of polymer donors with isomeric alkyl chains on the interfacial charge-transfer (CT) processes, the rates of exciton dissociation (kED) and charge recombination (kCR) have been calculated for complexes PM6/Y6 and PM6i/Y6, where isomer PM6i was designed by shifting the side chains in the electron-withdrawing units to the thiophene-bridges based on PM6. For complex PM6/Y6, kED of PM6 and Y6 exciton are calculated to be 1010–1012 s−1 and 1012–1013 s−1, and kCR is 1011–1012 s−1 for recombination of the lowest CT (CT0) excitons into the lowest triplet states of Y6, which are consistent with the experimental results. When going to complex PM6i/Y6, kED of PM6i exciton is higher than that of PM6 exciton, and of Y6 exciton is decreased to 1011–1012 s−1, but kCR is decreased to 1010–1011 s−1. The calculated results reveal that isomeric alkyl chains of polymer donors can enhance the polymer exciton dissociation and suppress the charge-recombination processes, underlining the important role of isomeric alkyl chains in design of polymer donors for high performance non-fullerene organic photovoltaics. [ABSTRACT FROM AUTHOR]

Singlet fission describes the spin-conserving production of two triplet excitons from one singlet exciton. The existence of a spin-2 (quintet) triplet-pair state as a product of singlet fission is well established in the literature, and control of quintet formation is an important step towards applying singlet fission in photovoltaics and quantum information. However, a definitive mechanism for quintet formation is yet to be established, which makes it difficult to design materials for optimal quintet formation. Here we outline a mechanism in which inter-triplet exchange-coupling fluctuations drive fast and efficient quintet formation. We show that quintet formation is possible even in the strong-exchange regime, in accordance with recent experimental prediction. We evaluate the performance of this quintet formation mechanism in two regimes of conformational freedom, and relate quintet dynamics to material properties of singlet fission molecules. Quintet formation is a part of a photochemical energy conversion process, which could be exploited to help achieve higher efficiency values for photovoltaics. Here, the authors propose a stochastic and coherent resonance mechanism for formation of the quintet spin state in singlet fission materials, that is still viable in the strong-exchange regime. [ABSTRACT FROM AUTHOR]

This paper explores a novel way of charge separation (autoionization) of molecular excitons, by quantum tunneling through a p- n nanojunction. This mechanism can dominate the standard one (i.e., when Frenkel exciton is ionized at donor or acceptor impurity sites) for very short, nanosized, p- n junction, where the junction electric field can be strong for relatively small (on the order of 1 V) voltage drop. Within a simple one-dimensional model for the depletion region of the p- n junction (donor and acceptor reservoirs connected by a short molecular wire) we compute the quantum yield Y for the tunneling exciton autoionization in the 'bulk' of the depletion region. For modern organic photosensitive materials with p- n junction size on the order of 10-20 nm, Y could be close to 1. Such a high efficiency of the charge separation (one of the main factor entering figure of merit, indicating how good are photovoltaic conversion cells) makes this new mechanism potentially very perspective for the applications. [ABSTRACT FROM AUTHOR]

Photoconversion in planar-heterojunction organic photovoltaic cells (OPVs) is limited by a short exciton diffusion length (LD) that restricts migration to the dissociating electron donor/acceptor interface. Consequently, bulk heterojunctions are often used to realize high efficiency as these structures reduce the distance an exciton must travel to be dissociated. Here, we present an alternative approach that seeks to directly engineer LD by optimizing the intermolecular separation and consequently, the photophysical parameters responsible for excitonic energy transfer. By diluting the electron donor boron subphthalocyanine chloride into a wide-energy-gap host material, we optimize the degree of interaction between donor molecules and observe a ~50% increase in LD. Using this approach, we construct planar-heterojunction OPVs with a power conversion efficiency of (4.4 ± 0.3)%, > 30% larger than the case of optimized devices containing an undiluted donor layer. The underlying correlation between LD and the degree of molecular interaction has wide implications for the design of both OPV active materials and device architectures. [ABSTRACT FROM AUTHOR]

Quantum dot-sensitized solar cells based on fluorine-doped tin oxide (FTO)/Au/TiO/CdS photoanode and polysulfide electrolyte are fabricated. Au nanoparticles (NPs) as interfacial layer between FTO and TiO layer are dip-coated on FTO surface. The structure, morphology and impedance of the photoanodes and the photovoltaic performance of the cells are investigated. A power conversion efficiency of 1.62% has been obtained for FTO/Au/TiO/CdS cell, which is about 88% higher than that for FTO/TiO/CdS cell (0.86%). The easier transport of excited electron and the suppression of charge recombination in the photoanode due to the introduction of Au NP layer should be responsible for the performance enhancement of the cell. [ABSTRACT FROM AUTHOR]

Future polymeric or ‘plastic’ solar cells are very attractive for solar energy conversions where large area, low cost, lightweight, and flexible shape are desired. In this brief review, fundamental mechanisms of inorganic and organic/polymeric photovoltaic materials will be briefly compared at exciton levels. This study shows that the frontier orbital couplings at atomic and molecular levels are crucial for the mechanisms of photo carrier generation and transport, either exciton or band models. Optimization approaches of polymer based solar cells are also addressed. [ABSTRACT FROM AUTHOR]