The electronic structure of Cu(In 1 − x Ga x)Se 2 alloyed with silver

Abstract

We have examined the electronic properties of (Ag 1 − x Cu x)(In 1 − y Ga y)Se 2 (ACIGS) alloys over a wide range of compositions to assess whether such alloys might allow one to achieve larger values of V OC at larger band gaps compared to the Cu(In 1 − y Ga y)Se 2 (CIGS) alloys. Our studies employed junction capacitance techniques such as drive level capacitance profiling (DLCP) and transient photocapacitance (TPC) spectroscopy, as well as temperature dependent J–V measurements. The TPC spectra revealed not only that the band gap did indeed increase as the Ag-fraction was increased, but also that the bandtailing (or Urbach energies) in all ACIGS samples were substantially smaller than for CIGS samples of corresponding band gaps. This indicates that the Ag alloying somehow reduces the degree of disorder present. The DLCP measurements indicated very low free carrier densities, on the order of 10 14 cm − 3 , as well as evidence of defects located at the CdS/ACIGS junction. Temperature-dependent I–V measurements revealed a distinct “kink” in the V OC vs T characteristics, suggesting a transition from an interface-trap limited regime to a bulk-limited regime. At temperatures below 250 K, the V OC increased by up to 0.1 V as the sample was light soaked. This suggests that the interface traps limiting the V OC can be passivated by exposure to light.