Charge Carrier Lifetime Determination in Graded Absorber Solar Cells Using Time-Resolved Photoluminescence Simulations and Measurements

Thin-film photovoltaic device efficiencies are limited by carrier recombination, thus understanding recombination mechanisms is critical for performance improvements. Bulk minority carrier lifetime (τ _bulk) is a critical parameter for solar cells but is difficult to determine in P–N junction devices, especially for high doping. As doping ≥10¹⁶ cm⁻³ is required for efficient drift-charge-carrier-collection devices, a method for τ _bulk determination in doped P–N junction devices is necessary. This work utilizes time-resolved photoluminescence (TRPL) simulations to quantify bulk and interface recombination properties in highly doped, graded absorber CdSeTe structures. The two methods developed here for τ _bulk determination include utilization of an instantaneous lifetime representation to guide TRPL fitting and direct comparison between measured and simulated decays. Simulations verified that both methods are valid for state-of-the-art device architectures which include graded bandgap absorbers, graded doping, and graded lifetimes. Shifts in the dominant recombination mechanism are identified for sufficiently long τ _bulk, where front and back interface quality plays a more prominent role. Evaluation of surface recombination velocities and conduction band offset illustrate electro-optical advantages of a positive conduction band offset and highlight the necessity of improved interfaces as bulk quality in photovoltaic devices improves.