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 performanc e of completed devices. These absorber films were prepared by controll ed selenization of Cu/In/Cu metallic alloys in an atmosphere containin g H2Se and Ar. Transmission electron microscopy (TEM) indicated that t hese In-rich films consisted of small, highly defected (mainly stackin g faults and microtwins) grains. The photoluminescence (PL) responses from these layers were dominated by three relatively broad emission li nes (at 1.10, 0.975 and 0.89 eV) which were attributed to donor-accept or 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. T hese deep levels were observed in all our highly In-rich films and are believed to be detrimental to device operation, Quantum-efficiency me asurements revealed two distinct long-wavelength cut-off points (at ap proximately 1000 and 1200 nm) indicative of there being two different and parallel existing phases in these specific films. Completed CuInSe 2/CdS/ZnO devices displayed non-ideal I-V characteristics such as a cr oss over of dark and illuminated curves, strongly bias-dependent curre nt collections and, in extreme cases, even ohmic-like behaviour when e valuated under air mass (AM) 1.5 conditions. However, this behaviour w as a strongly intensity-related one and measurements under low illumin ation levels (2-5 mA cm(-2)) revealed a dramatic improvement in the de vice characteristics. This anomalous behaviour was confirmed by quantu m-efficiency measurements to be a function of the illumination level a nd temperature. Under normal operating conditions in the dark (corresp onding to a low intensity level) good carrier collection was observed. However, when the cells were light biased during measurements (or eva luated at low temperature) a dramatic drop in J(sc) was observed. This phenomenon is ascribed to a breakdown in the electrical field require d to ensure effective separation of photogenerated electron-hole pairs .