Nanophotonic emission control for improved photovoltaic efficiency

With the necessary transition to renewable energy at hand, there is a renewed research focus on increasing solar cell efficiency, in order to reduce the cost of electricity. Nanomaterials are promising candidates to contribute to a new generation of low cost and highly efficient solar cells. Due to their wavelength-scale dimensions, nanomaterials display exceptionally strong light-matter interactions that lead to large perturbations in absorption and emission compared to their bulk counterparts. Although most work on nanostructured solar cells has focused on increasing the absorption, emission control may have even greater potential for improving efficiency of state-of-the-art solar cells. In this perspective article, we describe how nanostructures can be applied to improve solar cell efficiency, focusing on emission control. First, we analyze the requirements for making the most efficient solar cell, by looking at the thermodynamics of energy conversion. We show that an ideal solar cell at open circuit displays emission that is identical to its absorption. Comparing this to the emission of a typical silicon solar cell shows that there are three differences: the intensity, the angles in which light is emitted, and the spectrum. These differences lead to a reduction in efficiency, mainly due to a drop in open circuit voltage. For each loss mechanism, we discuss how nanomaterials can manipulate the emission and thereby reduce the voltage loss. Finally, we analyze the performance of two conceptual designs for solar cells based on nanomaterials. These give a large improvement in efficiency compared to conventional designs, showing the great potential of nanomaterials in solar cells.