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

摘要

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; λ = 405, 658, 810 nm 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 μm CdTe and 2. a poorly nucleated 40 - 300 nm Cd0.9Zn0.1S/ 2 μm CdTe. Calculated photon penetration depths, δp 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 μm 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 λ = 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 ∼ 50 μm 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, Rsh and open-circuit voltage, Voc.