Lambertian back reflector in Cu(InGa)Se 2 solar cell: optical modeling and characterization Nir Dahan a,b , Zacharie Jehl a , Jean-Fran¸ cois-Guillemoles a , Daniel Lincot a , Negar Naghavi a , and Jean-Jacques Greffet b a Institut de Recherche et D´ eveloppement sur l’Energie Photovolta¨ ıque, IRDEP (EDF/CNRS/Chimie-ParisTechUMR7174), 6 quai Watier, 78401 Chatou, France; b Laboratoire Charles Fabry, Institut d’Optique, CNRS - Universit´ e Paris-Sud, Campus Polytechnique, RD128, 91127 Palaiseau Cedex, France ABSTRACT In the past years, reducing the thickness of the absorber layer in CIGS-based solar cells has become a key issue to reduce the global Indium consumption and thus increased its competitiveness. As the absorber thickness is reduced, less photons are absorbed and consequently the efficiency decreases. It is well known that scattering light in the absorbing layer increases the effective optical length, which results in enhanced absorption. In this study, we have deposited a transparent conductive oxide as a back contact to the cell with a white paint on the rear surface to diffuse the light back to the cell. A proof of concept device is realized and optically characterized. Modeling scattering by rough surfaces can be done by brute force numerical simulations but does not provide a physical insight in the absorption mechanisms. In our approach, we regard the collimated solar light and its specular reflection/transmission as coherent. On an irregular surface, part of the collimated light is scattered in other directions. To model this diffuse light, we adopt the formalism of the radiative transfer equation, which is an energy transport equation. Thus, interference effects are accounted for only in the coherent part. A special attention is dedicated to preserving reciprocity and energy conservation on the interface. It is seen that most of the absorption near the energy bandgap of CIGS is due to the diffuse light and that this approach can yield very significant photocurrent gains below 500 nm absorber thickness. Keywords: Solar energy, Photovoltaic, Optical modeling, Radiative transfer 1. INTRODUCTION Copper indium gallium diselenide (CIGS) is one of the most promising thin film solar cell technologies. 1 While increasing the development of commercial modules the issue of materials resources arises. Therefore, reducing the amount of indium by thinning the absorber layer in solar cells is of main interest. The typical thickness of CIGS in commercial cells is about 2–2.5 μm. For this thickness, most of the light in the visible range is absorbed in a single pass due to the high absorption coefficient. In a thin film with a thickness on the order of 0.5 μm or less, the thickness of the absorber layer is smaller than the absorption depth of light; therefore, optical management is of crucial importance. With current technology, reducing the thickness of CIGS results in increased parasitic losses in the Mo back contact. Hence, it was proposed to replace the back contact by a more reflective material such as silver or gold. 2–4 While the previous discussion has been focussed on CIGS photovoltaic cells, the issue of enhancing absorption by thin cells is relevant for all materials. It is well known that scattering light in the absorber layer enhance the absorption in the cell by light trapping effect. 5 Since the refractive index of semi-conductors is high, the absorption near its bandgap energy can be enhanced significantly by trapping rays by total internal reflection. While this mechanism is well understood in the ray optics regime for a slab with one or two rough interfaces, 5–9 modeling the scattering due to rough surfaces within the entire solar cell is much more complicated. 10–17 Of Further author information: E-mail: nir dahan@yahoo.com Physics, Simulation, and Photonic Engineering of Photovoltaic Devices II, edited by Alexandre Freundlich, Jean-Francois Guillemoles, Proc. of SPIE Vol. 8620, 862019 · © 2013 SPIE CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2003968 Proc. of SPIE Vol. 8620 862019-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/05/2015 Terms of Use: http://spiedl.org/terms