Carrier-Density-Wave Multiple Lifetime Imaging in a Multicrystalline Silicon Solar Cell Using Quantitative Heterodyne Lock-In Carrierography and Localized Current–Voltage Characteristics

Abstract

Trap-state kinetic parameters of a multicrystalline silicon solar cell were investigated using dynamic heterodyne lock-in carrierography (HeLIC) imaging under various conditions of illumination intensity and load resistance. Physical relaxation times associated with free-carrier-density-wave (CDW) band-to-band recombination, capture in and emission from two intraband-gap traps were obtained and imaged using camera-based HeLIC pixel frequency responses and a nonlinear rate equation model which provided physical insight on dynamic interactions between the CDW and the traps in the solar cell. An optoelectronic analog to the conventional diode equation was used to determine the solar cell parameters and, thus, enable the measurement of local I–V characteristics imaged over the device surface. The dc photoluminescence image at an open circuit was used as a reference and its pixel statistics and electrical parameters (maximum power, series resistance, saturation current, and generation current) were imaged using the optoelectronic equivalent of the electrical Shockley equation. The optoelectronic lifetime images were then compared with lifetime images derived from these electrical solar cell parameters and the effects of the trap states were explored. Regions with maximum power were found to be associated with low trap concentrations exhibiting low thermal emission and capture rates, with the surface/shallow trap density affecting the maximum power. The trap type (surface versus bulk) limiting the solar cell maximum power was found to vary under different values of photovoltage from open circuit to short circuit.