Diverse simulations of time-resolved photoluminescence in thin-film solar cells: A SnO2/CdSeyTe1−y case study

Abstract

Time-resolved photoluminescence (TRPL) is widely used to measure carrier lifetime in thin-film solar cell absorbers. However, the injection dependence of data and frequent non-exponential decay shapes complicate the interpretation. Here, we develop a numerical model to simulate injection-dependent TRPL measurements in a SnO2/CdSeyTe1−y solar cell structure, considering parameters of interest to researchers in industry and academia. Previous simulations have shown that in low injection, excess electrons and holes injected by the laser pulse are rapidly separated in the electric field formed by the pn junction. As a result, at early times, the PL signal can decay faster than the Shockley–Read–Hall lifetime in the absorber bulk (τbulk). Prior simulations have shown that the charge stored in the junction can slowly leak out to affect decays at late times. However, it has not been clear if and to what degree charge storage can affect the slopes extracted from TRPL decays—τ2—commonly cited as the TRPL-measured lifetime. Here, we show that charge storage can, in some cases, result in τ2 values that substantially overestimate τbulk. Previous simulations indicate that high-injection conditions can screen the junction field and minimize charge separation. Here, we show that continued injection increases can drive down τ2 below τbulk as radiative recombination becomes dominant. We catalog charge storage and radiative recombination impacts for a diverse set of material parameters and compare results to double-heterostructure models.

20%-efficient polycrystalline Cd(Se,Te) thin-film solar cells with compositional gradient near the front junction

“…The much longer fast component of the TRPL lifetimes (τ 1 and τ 2 ) for the target sample suggests significantly reduced recombination at the front interface. The improvement of the slow component (τ 3 ), which is commonly related to the bulk recombination, implies a longer survival time for the carriers in the bulk, enabling the carriers coming back to the absorber from the TCO in the later times and contributing to the TRPL decay 41 . PLQY was further used to predict the ideal V OC (iV OC ) and the iV OC difference (ΔiV OC ) for the control and target devices 42 , 43 .…”

Band Bending at CdTe Solar Cell Contacts: Correlating Electro‐Optical and X‐Ray Photoelectron Spectroscopy Analyses of Thin Film Solar Cells

“…Next, we examine recombination dynamics from the glassside TRPL data (Figure 1c,d). Modeling for p-n junction devices after pulsed laser excitation supports using exponential fit to the tail of the TRPL decay to estimate total recombination rate [24]…”

“…Parameter values used in simulations are provided in Table S2, Supporting Information, and were based on Refs. [24] and [36]. Fermi-Dirac statistics were employed to account for the high doping levels in SnO 2 .…”

Impact of dopant-induced band tails on optical spectra, charge carrier transport, and dynamics in single-crystal CdTe

“…For solar cells, polycrystalline absorbers have a graded (depth-dependent) bandgap, different passivation at the front and back interfaces, and space charge fields. It is difficult to determine properties such as minority carrier lifetime and mobility in such complex absorbers 12 . Therefore, although first principles analysis has identified defects in As-doped CdTe 13 , such predictions largely have not been verified experimentally 14 .…”