Device characteristics of In-rich CuInSe2-based solar cells

In this study the material properties of in-rich CuInSe2 (>28 at% In) thin films were evaluated and correlated against the device performance of completed devices. These absorber films were prepared by controlled selenization of Cu/In/Cu metallic alloys in an atmosphere containing H2Se and Ar. Transmission electron microscopy (TEM) indicated that these In-rich films consisted of small, highly defected (mainly stacking faults and microtwins) grains. The photoluminescence (PL) responses from these layers were dominated by three relatively broad emission lines (at 1.10, 0.975 and 0.89 eV) which were attributed to donor-acceptor pair transitions. Admittance spectroscopy measurements revealed the presence of deep hole traps close to the midgap position of these In-rich (N-A similar or equal to 10(14) cm(-3)) CuInSe2 absorber films. These deep levels were observed in all our highly In-rich films and are believed to be detrimental to device operation, Quantum-efficiency measurements revealed two distinct long-wavelength cut-off points (at approximately 1000 and 1200 nm) indicative of there being two different and parallel existing phases in these specific films. Completed CuInSe2/CdS/ZnO devices displayed non-ideal I-V characteristics such as a cross over of dark and illuminated curves, strongly bias-dependent current collections and, in extreme cases, even ohmic-like behaviour when evaluated under air mass (AM) 1.5 conditions. However, this behaviour was a strongly intensity-related one and measurements under low illumination levels (2-5 mA cm(-2)) revealed a dramatic improvement in the device characteristics. This anomalous behaviour was confirmed by quantum-efficiency measurements to be a function of the illumination level and temperature. Under normal operating conditions in the dark (corresponding to a low intensity level) good carrier collection was observed. However, when the cells were light biased during measurements (or evaluated at low temperature) a dramatic drop in J(sc) was observed. This phenomenon is ascribed to a breakdown in the electrical field required to ensure effective separation of photogenerated electron-hole pairs.