Orders of Recombination in Complete Perovskite Solar Cells – Linking Time‐Resolved and Steady‐State Measurements

Abstract

Ideally, the charge carrier lifetime in a solar cell is limited by the radiative free carrier recombination in the absorber which is a second-order process. Yet, real-life cells suffer from severe nonradiative recombination in the bulk of the absorber, at interfaces, or within other functional layers. Here, the dynamics of photogenerated charge carriers are probed directly in pin-type mixed halide perovskite solar cells with an efficiency >20%, using time-resolved optical absorption spectroscopy and optoelectronic techniques. The charge carrier dynamics in complete devices is fully consistent with a superposition of first-, second-, and third-order recombination processes, with no admixture of recombination pathways with non-integer order. Under solar illumination, recombination in the studied solar cells proceeds predominantly through nonradiative first-order recombination with a lifetime of 250 ns, which competes with second-order free charge recombination which is mostly if not entirely radiative. Results from the transient experiments are further employed to successfully explain the steady-state solar cell properties over a wide range of illumination intensities. It is concluded that improving carrier lifetimes to >3 µs will take perovskite devices into the radiative regime, where their performance will benefit from photon-recycling.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.202101823.

Nanoscale Photoexcited Carrier Dynamics in Perovskites

“…The use of pulsed optical excitation sources can be used to readily access fast and ultrafast dynamic processes, but their high peak field strength and fluence can complicate direct comparison to steady-state measurements . With our pulse duration on the order of 10s of picoseconds and modest pulse intensities <200 nJ/cm 2 , we do not expect to drive significant nonlinear optical processes, though our estimated per-pulse photogenerated carrier density of n p + = n e – ≈ 3 × 10 16 cm –3 is higher than typical steady-state values under 1-sun illumination on the order of 10 15 . These carrier densities are significantly below the threshold for third-order Auger recombination, and contributions from second-order processes are expected to be limited .…”

“…48 With our pulse duration on the order of 10s of picoseconds and modest pulse intensities <200 nJ/cm 2 , we do not expect to drive significant nonlinear optical processes, though our estimated per-pulse photogenerated carrier density of n p + = n e − ≈ 3 × 10 16 cm −3 is higher than typical steady-state values under 1-sun illumination on the order of 10 15 . 52 These carrier densities are significantly below the threshold for third-order Auger recombination, 48 and contributions from second-order processes are expected to be limited. 52 We emphasize that our careful control of pulse powers enables a direct comparison between time-domain methods to validate time-domain MIM and to provide complementary information to understand the electron and hole dynamics.…”

“…52 These carrier densities are significantly below the threshold for third-order Auger recombination, 48 and contributions from second-order processes are expected to be limited. 52 We emphasize that our careful control of pulse powers enables a direct comparison between time-domain methods to validate time-domain MIM and to provide complementary information to understand the electron and hole dynamics. Although we did not measure conventional absorption spectra, our illumination spectrum was calibrated using a spectrum analyzer, and the spectrally resolved lightinduced generation of photoexcited carriers provides a direct measure of the device-relevant photoconductive response.…”

Interpretation of the Recombination Lifetime in Halide Perovskite Devices by Correlated Techniques

“…It has been pointed out that some exponential dependencies and excess values are due to capacitive factors. [14][15][16]29 We now analyze the recombination model used in these references in order to obtain the excess factor of eq 1.…”

“…There is, therefore, a significant opportunity to reconcile different methods to obtain a robust set of system parameters. In the studies of previous hybrid photovoltaic technologies it was established that analyzing the mechanisms of the lifetime requires obtaining high-quality data over a wide variation of the splitting of the Fermi level. , Recombination in halide perovskites has been studied for many years, but recently some works have developed the correspondence of optical and electro-optical techniques over a wide voltage range. − In addition, progress has been obtained in the characterization of capacitances of halide perovskites. ,, Herein, we summarize the progress in this topic by formulating the conditions of observation of the lifetime in halide perovskites in terms of the capacitances in the system, and we show the correspondence of the models used in the time domain with the frequency domain methods. We also summarize the problems still existing for the measurement of the lifetime in order to point out further experimental determinations. There is a significant opportunity to reconcile different methods to obtain a robust set of system parameters.…”