Quantum-kinetic perspective on photovoltaic device operation in nanostructure-based solar cells
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The implementation of a wide range of high-efficiency solar cell concepts is based on nanostructures with configuration-tunable optoelectronic properties. On the other hand, effective nano-optical light-trapping concepts enable the use of ultra-thin absorber architectures. In both cases, the local density of electronic and optical states deviates strongly from that of a homogeneous bulk material. At the same time, nonlocal and coherent phenomena like tunneling or ballistic transport become increasingly relevant. As a consequence, the semiclassical, diffusive bulk picture may no longer be appropriate to describe the physics of such devices. In this review, we provide a quantum-kinetic perspective on photovoltaic device operation that reaches beyond the limits of the standard simulation models for bulk solar cells. Deviations from bulk physics are assessed in ultra-thin film and nanostructure-based solar cell architectures by comparing predictions of semiclassical models with those of a more fundamental description based on nonequilibrium quantum statistical mechanics.
I. INTRODUCTION
Nanostructures have been considered for photovoltaic applications since the advent of epitaxial growth, with the quantum-well solar cell (QWSC) development as one of the pioneering activities.1 As early as 2001, the rapidly growing interest in nanostructures and the challenge of efficient as well as cost-effective photovoltaic energy conversion had led to a seminal workshop on nanostructures for photovoltaics in Dresden, where a large number of concepts and structures with different functionalities (optical, electronic, and vibrational) were discussed, for which the most part belonged to the then emerging field of third generation photovoltaics.2 This included quantum-well and quantum-dot heterostructures for tunable absorbers in multijunction solar cells,3 for the formation of intermediate bands for sequential multiphoton absorption,4 for the modification of carrier– phonon interaction to slow down cooling in hot-carrier solar cells,5 and for the engineering of efficient carrier multiplication processes in colloidal quantum dots.6 Additionally, a wide range of optical nanostructures have been proposed for nanophotonic or plasmonic lighttrapping, spectral splitting, and up-conversion or downshifting.7,8 In many cases, efficiencies far beyond the single junction Shockley–Queisser limit were predicted based on detailed balance (DB) calculations. Since then, research groups around the globe have been working on Contributing Editor: Sam Zhang a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.468
the implementation of those promising concepts. However, to date, efficiencies could be reached in very few cases only that are close to the DB limits. The poor performance of nanostructure-based solar cells is mainly attributed to a detrimental increase of recombination losses due to a larger number of internal interfaces that are prone to the accumulation of recombination-active defect states and to wave
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