3D-printed concentrator arrays for external light trapping on thin film solar cells

After our recent demonstration of a 3D-printed external light trap on a small solar cell, we now consider its potential for large solar panels. An external light trap consists of a parabolic concentrator and a spacer that redirects the photons that are reflected by the solar cell back towards the solar cell. These retro-reflections enable higher absorptance and improved power conversion efficiency. Scaling a single external light trap such that it covers a large solar panel has disadvantages in terms of height and cost of the external light trap. These disadvantages can be overcome by deploying an array of concentrators as the top part of the external light trap. We present an optimization study of concentrator arrays for external light trapping. We fabricated 3D-printed external light traps with a square, hexagonal and circular compound parabolic concentrator to test their suitability for concentrator arrays. The 3D-printed traps were placed on top of an organic solar cell which resulted in a significant enhancement of the external quantum efficiency. The required transmittance of these concentrator arrays is calculated as a function of the parameters of both the concentrator and the solar cell. We compare the theoretical and experimentally determined optical performance of the different concentrators. Finally, the prospects of external light trapping are analyzed and we give guidelines for improvements of the external light trap design. (C) 2015 The Authors. Published by Elsevier B.V.