Uncovering Transport Mechanisms in Perovskite Materials and Devices with Recombination-Induced Action Spectroscopies

Abstract

Although valuable insights are derived from conventional spectroscopic approaches, the understanding of a photovoltaic device’s operation mechanisms can be limited in ex situ measurements. For example, the signals measured in transient absorption experiments reflect the concentrations and extinction coefficients of all photoexcited species in a material regardless of functional relevance. Elimination of such ambiguities has motivated the development of various “action spectroscopy” techniques in which the response of a photovoltaic device to a sequence of laser pulses is directly detected. The class of action spectroscopies described in this Perspective leverages recombination-induced nonlinearities to distinguish lossy (fluorescence) and productive (photocurrent) processes within the active layers of photovoltaic cells. Although recombination processes are problematic in alternate approaches for conducting action spectroscopies, our experiments show that this type of nonlinearity can be exploited to reveal transport mechanisms on nanosecond time scales. Applications to mixtures of layered perovskite quantum wells are presented to demonstrate signatures of energy funneling and long-range carrier drift in photovoltaic devices.

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

“…Earlier, ECS-like experiments have been interpreted using Feynman diagrams. , We emphasize that this is not precise since Feynman diagrams indicate optical transitions among states and their coherent correlation but do not include the interactions among their populations. This imprecision is addressed in recent literature, recognizing recombination dynamics as the only origin of the measured nonlinearity . Due to their distinct origin, they provide distinct information.…”

Unifying Nonlinear Response and Incoherent Mixing in Action-2D Electronic Spectroscopy

“…Nevertheless, the presence of incoherent mixing can be informative of the interaction network at play in the system. Indeed, it has been shown that dynamics-induced nonlinearities can be exploited to study the long-range transport mechanism in photovoltaic devices using a pump–probe setting. − …”

“…Indeed, it has been shown that dynamics-induced nonlinearities can be exploited to study the long-range transport mechanism in photovoltaic devices using a pump−probe setting. 42 A further point deserving attention is the generality of the incoherent mixing mechanism. Indeed, just as the mixing of linear signals can enter in the fourth-order response, higherorder contributions may appear in the linear signal, e.g., × * ( ) ( ) R 43 is modulated at Ω R +Ω 43 = Ω 21 , as recently observed in ref 32, or the mixing between fourth-order and linear signals may contribute to sixth-order response.…”

“…Whereas in the one-particle representation, the focus is on the single chromophore and cross-peaks arise from the spurious interaction with another system, the two-particle representation supports the dimeric nature of the system, even when the interaction is weak, and cross-peaks are part of the nonlinear response of the system as a whole. In the literature about A-2DES, the former view has been traditionally adopted to interpret the response of extended solid-state systems, , while the latter has been privileged for analyzing the response of small molecular aggregates. − …”

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. − …”