Transient Photovoltage Measurements on Perovskite Solar Cells with Varied Defect Concentrations and Inhomogeneous Recombination Rates

Abstract

In all kinds of solar cells, transient photovoltage (TPV) decay measurements have been used to determine charge carrier lifetimes and to quantify recombination processes and orders. However, in particular, for thin‐film devices with a high capacitance, the time constants observed in common TPV measurements do not describe recombination dynamics but RC (R: resistance, C: capacitance) times for charging the electrodes. This issue has been revisited for organic and perovskite solar cells in the recent literature. Here, these discussions are extended by analyzing a perovskite model system (Bi defects in Cs0.1FA0.9Pb(Br0.1I0.9)3 in which defect recombination can be tuned. It is found that TPV, intensity‐modulated photovoltage spectroscopy, and impedance spectroscopy yield the same time constants that do not describe recombination dynamics but are limited by the differential resistance of the diode and the geometric capacitance in common light intensity ranges (<1 sun). By employing numerical device simulations, it is found that low charge carrier mobility can furthermore limit the TPV time constants. In samples with spatially nonuniform recombination dynamics, two time constants are measured, which depend on the charge carrier generation profile that can be tuned by the wavelength of the incident light. In that case, numerical simulation provides insights into recombination and charge transport processes in the device.

Understanding Transient Photoluminescence in Halide Perovskite Layer Stacks and Solar Cells

“…Furthermore, the mistake of interpreting capacitive effects in devices as "lifetimes" should be avoided in a similar way as has been previously mentioned for the case of transient photovoltage measurements. [51,53,54] A key prerequisite for data analysis is the use of decay times as a function of carrier density or quasi-Fermi-level splitting. What should be avoided is the reduction of the data to a weighted sum of exponentials because those will be unable to capture the actual shape of the transients and the information contained in them.…”

“…These measurements also allow deriving differential decay times that are however frequently denoted as "lifetimes" in the literature. [49,51,53,54] Radiative and Auger recombination define the shape of the decay time at high Fermi-level splitting, i.e., directly after the laser pulse. The comparison of the analytical solution with and without Auger recombination illustrates that Auger recombination steepens the increase of τ TPL .…”

“…These measurements also allow deriving differential decay times that are however frequently denoted as “lifetimes” in the literature. [ 49,51,53,54 ]…”

“…Hence, τ TPL,HLI is also increasing exponentially toward smaller Δ E F as previously observed for large and small signal V oc decays. [ 50,51,53,54 ]…”

A Simple Cu(II) Polyelectrolyte as a Method to Increase the Work Function of Electrodes and Form Effective p‐Type Contacts in Perovskite Solar Cells

“…Another mechanism by which the Cu:PSS based HTLs can improve the J SC is by reducing charge carrier recombination. Techniques such as impedance spectroscopy, [ 60 ] transient photovoltage, [ 61 ] time‐resolved photoluminescence (TPRL) [ 62 ] have been used to measure charge carrier recombination in solar cell devices. This effect is also apparent in the improved FFs of devices.…”