Resolving Nonlinear Recombination Dynamics in Semiconductors via Ultrafast Excitation Correlation Spectroscopy: Photoluminescence versus Photocurrent Detection

Abstract

We explore the application of excitation correlationspectroscopyto detect nonlinear photophysical dynamics in two distinct semiconductorclasses through time-integrated photoluminescence and photocurrentmeasurements. In this experiment, two variably delayed femtosecondpulses excite the semiconductor, and the time-integrated photoluminescenceor photocurrent component arising from the nonlinear dynamics of thepopulations induced by each pulse is measured as a function of inter-pulsedelay by phase-sensitive detection with a lock-in amplifier. We focuson two limiting materials systems with contrasting optical properties:a prototypical lead-halide perovskite (LHP) solar cell, in which primaryphotoexcitations are charge photocarriers, and a single-componentorganic-semiconductor diode, which features Frenkel excitons as primaryphotoexcitations. The photoexcitation dynamics perceived by the twodetection schemes in these contrasting systems are distinct. Nonlinear-dynamiccontributions in the photoluminescence detection scheme arise fromcontributions to radiative recombination in both materials systems,while photocurrent arises directly in the LHP but indirectly followingexciton dissociation in the organic system. Consequently, the basicphotophysics of the two systems are reflected differently when comparingmeasurements with the two detection schemes. Our results indicatethat photoluminescence detection in the LHP system provides valuableinformation about trap-assisted and Auger recombination processes,but that these processes are convoluted in a nontrivial way in thephotocurrent response and are therefore difficult to differentiate.In contrast, the organic-semiconductor system exhibits moredirectly correlated responses in the nonlinear photoluminescence andphotocurrent measurements, as charge carriers are secondary excitationsonly generated through exciton dissociation processes. We proposethat bimolecular annihilation pathways mainly contribute to the generationof charge carriers in single-component organic semiconductor devices.Overall, our work highlights the utility of excitation correlationspectroscopy in modern semiconductor materials research, particularlyin the analysis of nonlinear photophysical processes, which are deterministicfor their electronic and optical properties.