Monolithically-integrated laterally-arrayed multiple bandgap solar cells for spectrum-splitting photovoltaic systems

Spectrum-splitting photovoltaics is an alternative to multi-junction tandem cells which has been the subject of renewed interest in recent years as researchers try to push the limits of efficiency and cost-reduction for solar energy production. A myriad of solutions have been proposed for the spectrum-splitting optics, yet the basic cell technologies for these systems have received comparatively little attention. This paper reports on and reviews the most recent progress on a fundamentally different approach to cell design and fabrication: that of Monolithically-Integrated Laterally-Arrayed Multi-Band gap (MILAMB) solar cells. The essence of this concept is to fabricate multiple cells simultaneously on a single substrate using composition-graded semiconductor alloy nanowires to simplify the process, cut costs, and eventually achieve high efficiencies. After a brief introduction and overview of the existing approaches to spectrum-splitting photovoltaics, we present results of theoretical design and numerical studies using two candidate materials, CdPbS and InGaN. These design studies show that the MILAMB cells are capable of similar efficiency levels to those of multi-junction tandem cells, with potentially much reduced cost. Proof-of-concept two-subcell devices fabricated simultaneously on a single substrate using CdSSe nanowire ensembles are reviewed. Their performance is compared to similar thin-film cells to illustrate the current limits and potential benefits of this new approach. Finally, future challenges and possible directions for developing a practical MILAMB system are outlined.