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

Abstract

We explore the application of excitation correlation spectroscopy to detect nonlinear photophysical dynamics in two distinct semiconductor classes through time-integrated photoluminescence and photocurrent measurements. In this experiment, two variably delayed femtosecond pulses excite the semiconductor, and the time-integrated photoluminescence or photocurrent component arising from the nonlinear dynamics of the populations induced by each pulse is measured as a function of inter-pulse delay by phase-sensitive detection with a lock-in amplifier. We focus on two limiting materials systems with contrasting optical properties: a prototypical lead-halide perovskite (LHP) solar cell, in which primary photoexcitations are charge photocarriers, and a single-component organic-semiconductor diode, which features Frenkel excitons as primary photoexcitations. The photoexcitation dynamics perceived by the two detection schemes in these contrasting systems are distinct. Nonlinear-dynamic contributions in the photoluminescence detection scheme arise from contributions to radiative recombination in both materials systems, while photocurrent arises directly in the LHP but indirectly following exciton dissociation in the organic system. Consequently, the basic photophysics of the two systems are reflected differently when comparing measurements with the two detection schemes. Our results indicate that photoluminescence detection in the LHP system provides valuable information about trap-assisted and Auger recombination processes, but that these processes are convoluted in a nontrivial way in the photocurrent response and are therefore difficult to differentiate. In contrast, the organic–semiconductor system exhibits more directly correlated responses in the nonlinear photoluminescence and photocurrent measurements, as charge carriers are secondary excitations only generated through exciton dissociation processes. We propose that bimolecular annihilation pathways mainly contribute to the generation of charge carriers in single-component organic semiconductor devices. Overall, our work highlights the utility of excitation correlation spectroscopy in modern semiconductor materials research, particularly in the analysis of nonlinear photophysical processes, which are deterministic for their electronic and optical properties.

Exciton Bimolecular Annihilation Dynamics in Push–Pull Semiconductor Polymers

“…Although excitation correlation photoluminescence (ECPL) and photocurrent (PC) techniques are not as widely used as TA or time-resolved PL, their application has provided new insight on the photophysics of inorganic semiconductors, − carbon nanotubes, , two-dimensional transition-metal dichalcogenides and hybrid organic–inorganic perovskites − due to their sensitivity to nonlinear photophysical responses. Of particular relevance to organic semiconductors, Rojas-Gatjens et al recently investigated the nonlinear PL and PC responses of an organic small-molecule photovoltaic material, where the dominant source of charge-carrier generation is ascribed to the EEA process . Compared to the conjugated homopolymers, conjugated push–pull copolymers inherit strong charge-transfer character due to the differences in the electronegativities of the electron-deficient and -sufficient domains, which could contribute to the driving force for EEA .…”

“…Of particular relevance to organic semiconductors, Rojas-Gatjens et al recently investigated the nonlinear PL and PC responses of an organic small-molecule photovoltaic material, where the dominant source of charge-carrier generation is ascribed to the EEA process. 34 Compared to the conjugated homopolymers, conjugated push–pull copolymers inherit strong charge-transfer character due to the differences in the electronegativities of the electron-deficient and -sufficient domains, which could contribute to the driving force for EEA. 35 Here, our work provides new insights into exciton diffusion in conjugated push–pull polymers by comparing the TA and ECPL measurements, experimentally and via modeling, which can be further developed in new optoelectronic systems.…”

Nonlinear Optical Spectroscopy of Molecular Assemblies: What Is Gained and Lost in Action Detection?

“…This can be evaluated from the probability of the complementary event, that is, every molecule is in the ground state. This is true if each molecule undergoes either no transition (1 – p ) 2 or two transitions p 2 : S PuPr N = 1 − false[ false( 1 − p false) 2 + p 2 false] N The nonlinear signal is obtained by subtracting the pump–probe signal from that of the independent pump and probe pulses (Figure c): S NL N = 2 S Pu N − S PuPr N = ⁣ 2 false[ 1 − false( 1 − p false) N false] − { 1 − [ ( 1 − p ) 2 + p 2 ] N } From the spectroscopic point of view, the nonlinear signal can be extracted by modulating the amplitude of the pulses using a chopper (Figure b), as done in two-pulse action-detected experiments. − …”

“…This is true if each molecule undergoes either no transition (1 – p ) 2 or two transitions p 2 : The nonlinear signal is obtained by subtracting the pump–probe signal from that of the independent pump and probe pulses ( Figure 2 c): From the spectroscopic point of view, the nonlinear signal can be extracted by modulating the amplitude of the pulses using a chopper ( Figure 2 b), as done in two-pulse action-detected experiments. 29 − 32 …”