Optical confinement in chalcopyrite based solar cells

Potential gains in short-circuit current density related to improvements in optical confinement in chalcopyrite based solar cells are studied and quantified by means of optical simulations. In the first part idealised optical conditions - anti-reflection at front interfaces, high reflection at back contact and light scattering - are introduced by simulating realistic scenarios of Cu(In, Ga)Se2 (CIGS) solar cells with 2000 nm thick and 300 nm ultra-thin CIGS absorber, including the encapsulation at the front. For anti-reflection effect at front interfaces simulations revealed that in the photovoltaic module structure the most critical reflectance is the reflectance of the front surface of the protecting glass (possible 4.4% gain in short-circuit current density) and not the one at the front transparent conductive oxide contact, as in the case of non-encapsulated solar cell. Introduction of a highly reflective, highly diffusive back reflector is the most crucial point to improve the short-circuit current density of the ultra-thin devices. Potential for 15.8% gain in short-circuit current density related to ideal reflectance and additional 17.4% related to ideal scattering introduced at the back contact was revealed. A concrete example of light management structure was investigated in the second part by employing fully three-dimensional rigorous optical simulations. A semi-ellipsoidal texture was introduced to the substrate of the ultra-thin device. By using ZrN back reflector in simulations the gains in short-circuit current density related to the optimised size of the texture reach 10%, whereas in the case of an ideal highly reflective contact the gain is > 22% according to simulations.