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28th European Photovoltaic Solar Energy Conference and Exhibition; 14-19, We show that, by considering endothermic fission in a detailed balance formalism, the ultimate efficiency of solar cells is increased from 41.9% to 45.9% under AM1.5G illumination. An equivalent circuit formalism is used to elucidate this result. Endothermic fission has only been achieved at room temperature in organic materials such as tetracene, and, using a semi-empirical approach, we calculate an efficiency limit of ~36% for a tetracene(100 nm)/ideal inorganic absorber. Time-resolved photoluminescence studies are used to analyze the singlet fission mechanism in tetracene thin films. We present evidence of the existence of a luminescent bound triplet pair state, whose dissociation is thermally activated.

28th European Photovoltaic Solar Energy Conference and Exhibition; 352-355, Impressive efficiencies of triple-junction photovoltaic cells have come with prohibitive device production costs, while single-junction cells are inefficient due to (“Shockley-Queisser”) limitations[1] concerning inflexibility in acceptance energy of single-junction cells. If “excess” energy charge carriers can be tapped before they thermalize with the matrix, circumventing the Shockley-Queisser limit, triple-junction cell efficiencies at first generation photovoltaic cost should be possible. In this pursuit, two principal tasks await: retardation of carrier thermalization in the absorber lattice by preventing optical phonon decay/diffusion and/or hot carrier diffusion, and isoentropic collection of the carriers via devices known as “Energy Selective Contacts” (ESCs). We investigate the advantages of InN, with its large phononic band gap, as a suitable absorber, versus InGaAs, which possesses no significant gap between optical and acoustic phonon modes. To isolate the effects of carrier diffusion, we also study the relationship in InGaAs structures between barrier width and thermalization rate.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 60-64, Theoretical efficiencies of a hot carrier solar cell considering Indium nitride as the absorber material have been calculated in this work. In a hot carrier solar cell highly energetic carriers are extracted from the device before thermalisation, allowing higher efficiencies in comparison to conventional solar cells. Previous reports on efficiency calculations approached the problem using two different theoretical frameworks, the particle conservation model or the impact ionization model, which are only valid in particular extreme conditions. In addition an ideal absorber material with the approximation of parabolic bands has always been considered in the past. Such assumptions give an overestimation of the efficiency limits and results can only be considered indicative. In this report the real properties of wurtzite bulk InN absorber have been taken into account, including the actual dispersion relation. A new hybrid model that considers particle balance and energy balance at the same time has been implemented. Effects of actual impact ionization and Auger recombination lifetimes have been included in the calculations for the first time, considering the real InN band structure and thermalisation rates. A maximum efficiency of 43.6% has been found for 1000 suns, assuming thermalisation constants of 100 ps and ideal blackbody absorption.