Item Accepted version Open AccessSite-selective measurement of coupled spin pairs in an organic semiconductor.(Proceedings of the National Academy of Sciences, 2018-05-15) Bayliss, SL; Weiss, LR; Mitioglu, A; Galkowski, K; Yang, Z; Yunusova, K; Surrente, A; Thorley, KJ; Behrends, J; Bittl, R; Anthony, JE; Rao, A; Friend, RH; Plochocka, P; Christianen, PCM; Greenham, NC; Chepelianskii, AD; Surrente, A [0000-0003-4078-4965]From organic electronics to biological systems, understanding the role of intermolecular interactions between spin pairs is a key challenge. Here we show how such pairs can be selectively addressed with combined spin and optical sensitivity. We demonstrate this for bound pairs of spin-triplet excitations formed by singlet fission, with direct applicability across a wide range of synthetic and biological systems. We show that the site sensitivity of exchange coupling allows distinct triplet pairs to be resonantly addressed at different magnetic fields, tuning them between optically bright singlet ([Formula: see text]) and dark triplet quintet ([Formula: see text]) configurations: This induces narrow holes in a broad optical emission spectrum, uncovering exchange-specific luminescence. Using fields up to 60 T, we identify three distinct triplet-pair sites, with exchange couplings varying over an order of magnitude (0.3-5 meV), each with its own luminescence spectrum, coexisting in a single material. Our results reveal how site selectivity can be achieved for organic spin pairs in a broad range of systems. Item Published version Open AccessPerovskite/Colloidal Quantum Dot Tandem Solar Cells: Theoretical Modeling and Monolithic StructureGreenham, NC; Karani, Arfa; Di, Dawei; Yang, Le; Greenham, Neil [0000-0002-2155-2432]; Karani, Arfa [0000-0002-9038-1593]; Di, Dawei [0000-0003-0703-2809]Metal-halide perovskite-based tandem solar cells show great promise for overcoming the Shockley-Queisser single-junction efficiency limit via low-cost tandem structures, but so far they employ conventional bottom-cell materials that require stringent processing conditions. Meanwhile, difficulty in achieving low-bandgap (<1.1 eV) perovskites limits all-perovskite tandem cell development. Here we propose a tandem cell design based on a halide-perovskite top-cell and a chalcogenide colloidal quantum dot (CQD) bottom-cell, where both materials provide bandgap-tunability and solution-processability. Theoretical efficiency of 43% is calculated for a tandem-cell bandgap combination of 1.55 eV (perovskite) and 1.0 eV (CQDs) under 1-sun illumination. We highlight that inter-subcell radiative coupling contributes significantly (>11% absolute gain) to the ultimate efficiency via photon recycling. We report initial experimental demonstration of a solution-processed monolithic perovskite/CQD tandem solar cell, showing evidence for subcell voltage addition. We model that a power-conversion efficiency of 29.7% is possible by combining the state-of-the-art perovskite and CQD solar cells. Item Accepted version Open AccessOrganic solar cells based on non-fullerene acceptors.(Springer Science and Business Media LLC, 2018-01-23) Hou, Jianhui; Inganäs, Olle; Friend, Richard H; Gao, Feng; Friend, Richard [0000-0001-6565-6308]Organic solar cells (OSCs) have been dominated by donor:acceptor blends based on fullerene acceptors for over two decades. This situation has changed recently, with non-fullerene (NF) OSCs developing very quickly. The power conversion efficiencies of NF OSCs have now reached a value of over 13%, which is higher than the best fullerene-based OSCs. NF acceptors show great tunability in absorption spectra and electron energy levels, providing a wide range of new opportunities. The coexistence of low voltage losses and high current generation indicates that new regimes of device physics and photophysics are reached in these systems. This Review highlights these opportunities made possible by NF acceptors, and also discuss the challenges facing the development of NF OSCs for practical applications. Item Published version Open AccessOrder enables efficient electron-hole separation at an organic heterojunction with a small energy loss.(Springer Science and Business Media LLC, 2018-01-18) Menke, S Matthew; Cheminal, Alexandre; Conaghan, Patrick; Ran, Niva A; Greehnam, Neil C; Bazan, Guillermo C; Nguyen, Thuc-Quyen; Rao, Akshay; Friend, Richard H; Menke, S Matthew [0000-0003-4468-0223]; Rao, Akshay [0000-0003-0320-2962]; Friend, Richard H [0000-0001-6565-6308]Donor-acceptor organic solar cells often show low open-circuit voltages (V OC) relative to their optical energy gap (E g) that limit power conversion efficiencies to ~12%. This energy loss is partly attributed to the offset between E g and that of intermolecular charge transfer (CT) states at the donor-acceptor interface. Here we study charge generation occurring in PIPCP:PC61BM, a system with a very low driving energy for initial charge separation (E g-E CT ~ 50 meV) and a high internal quantum efficiency (η IQE ~ 80%). We track the strength of the electric field generated between the separating electron-hole pair by following the transient electroabsorption optical response, and find that while localised CT states are formed rapidly (<100 fs) after photoexcitation, free charges are not generated until 5 ps after photogeneration. In PIPCP:PC61BM, electronic disorder is low (Urbach energy <27 meV) and we consider that free charge separation is able to outcompete trap-assisted non-radiative recombination of the CT state. Item Open AccessSinglet exciton fission in solution.(Springer Science and Business Media LLC, 2013-12) Walker, Brian J; Musser, Andrew J; Beljonne, David; Friend, Richard H; Friend, Richard [0000-0001-6565-6308]Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight. Item Open AccessThe role of spin in the kinetic control of recombination in organic photovoltaics.(Springer Science and Business Media LLC, 2013-08-22) Rao, Akshay; Chow, Philip CY; Gélinas, Simon; Schlenker, Cody W; Li, Chang-Zhi; Yip, Hin-Lap; Jen, Alex K-Y; Ginger, David S; Friend, Richard H; Friend, Richard [0000-0001-6565-6308]In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.