High-resolution laser beam induced current measurements on Cd0.9Zn0.1S/CdTe solar cells

An assessment of Cd0.9Zn0.1S window layer thickness and its impact on photoresponse uniformity in CdTe thin film photovoltaic (PV) devices is presented. A triple-wavelength laser beam induced current (LBIC) system provided a spatially resolved photocurrent mapping technique. Three diode lasers; lambda = 405, 658, 810nm gave photon absorption and carrier generation characteristics over the spectral range of the Cd0.9Zn0.1S/CdTe devices. Two contrasting device structures were grown by metal-organic chemical vapour deposition (MOCVD), where the uniformity of the Cd0.9Zn0.1S window layer was known to vary: 1. uniform 240 nm Cd0.9Zn0.1S/2 mu m CdTe and 2. a poorly nucleated 40 - 300 nm Cd0.9Zn0.1S/2 mu m CdTe. Calculated photon penetration depths, delta(p) allowed for the separation of identified defects within the device cross-section. Cd0.9Zn0.1S pin holes were identified in 240 nm Cd0.9Zn0.1S/2 mu m CdTe where 405 nm photon 'punch-through' into the absorber material was observed. These pin holes also led to a localised reduction in photoresponse at lambda = 658 and 810 nm. In the device structure where the Cd0.9Zn0.1S window layer thickness was known to vary from 40 to 300 nm, CdTe pin holes were identified where localised similar to 50 mu m regions of reduced photoresponse, at all wavelengths were observed. Local variations in both Cd0.9Zn0.1S and CdTe thickness were also identified where variable absorption led to a distribution of LBIC photoresponse. It was demonstrated that reduced photoresponse uniformity at all incident wavelengths was related to reduced device shunt resistance, R-sh and open-circuit voltage, V-oc. (C) 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Organizers of European Materials Research Society (EMRS) Conference: Symposium on Advanced Inorganic Materials and Concepts for Photovoltaics.