Multidimensional time-of-flight spectroscopy

Abstract

Experimental methods based on a wide range of physical principles are used to determine carrier mobilities for light-harvesting materials in photovoltaic cells. For example, in a time-of-flight experiment, a single laser pulse photoexcites the active layer of a device, and the transit time is determined by the arrival of carriers at an acceptor electrode. With inspiration from this conventional approach, we present a multidimensional time-of-flight technique in which carrier transport is tracked with a second intervening laser pulse. Transient populations of separate material components of an active layer may then be established by tuning the wavelengths of the laser pulses into their respective electronic resonances. This experimental technique is demonstrated using photovoltaic cells based on mixtures of organohalide perovskite quantum wells. In these “layered perovskite” systems, charge carriers are funneled between quantum wells with different thicknesses because of staggered band alignments. Multidimensional time-of-flight measurements show that these funneling processes do not support long-range transport because of carrier trapping. Rather, our data suggest that the photocurrent is dominated by processes in which the phases of the thickest quantum wells absorb light and transport carriers without transitions into domains occupied by quantum wells with smaller sizes. These same conclusions cannot be drawn using conventional one-dimensional techniques for measuring carrier mobilities. Advantages and disadvantages of multidimensional time-of-flight experiments are discussed in the context of a model for the signal generation mechanism.

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

“…Notably, the nonlinearity represented in Figure b is fundamentally different from a cascade of lower-order responses, such as those observed in off-resonant 2D Raman spectroscopies , because it provides dynamical information that is not available when a material absorbs a single laser pulse. For example, recent studies of layered perovskite systems demonstrate that recombination-induced nonlinearities can be exploited to resolve energy and charge funneling dynamics in layered perovskite systems on time scales of 100s of ps. ,− …”

“…In recent works, we have presented several approaches for obtaining time-of-flight information with NLPC experiments. − The external biases are cycled in these techniques to probe the dynamics of free charges; however, excitons are the dominant electronically excited species in the present layered perovskite systems. The n = 2 and 3 quantum wells (i.e., the most concentrated systems in the present material) have exciton binding energies of approximately 251 and 177 meV, which suggests the numbers of excitons must greatly outweigh those of the free charge carriers .…”

“…In our experience, NLPC decay profiles involving small quantum wells typically exhibit a sub-ns phase of nonradiative decay, which we refer to as T 1 , followed by a ∼10 ns phase of carrier drift, T 2 . − To extract carrier transit times, the signals in Figure d are fit to sums of exponential functions: S ( τ ) = A 1 .25em exp ( − τ / T 1 ) + A 2 .25em exp ( − τ / T 2 ) The drift velocities are then computed using v drift = α –1 T 2 –1 , where α is the 7.48 μm –1 absorption coefficient of the film with 570 nm incident light (see the Supporting Information). With inspiration from conventional time-of-flight methods, , the potential is varied to extract a carrier mobility of μ = 0.083 ± 0.010 cm 3 /V/s from the data sets presented in Figure e. − Here, the uncertainty of 0.010 cm 3 /V/s in the mobility is the standard error of the regression slope for the three points shown in Figure e. To account for the variability in fabrication conditions, these three velocities represent averages computed with four data sets acquired using three separate photovoltaic cells (see Supporting Information).…”

“…To reduce ambiguities in experimental interpretations, action spectroscopies have been developed in which spontaneous processes such as the fluorescence and photocurrent generated by a device are measured instead of coherent light emission. Nonlinear fluorescence action spectroscopies have been employed in studies of molecular aggregates exhibiting Frenkel exciton electronic structure. − In addition, nonlinear photocurrent spectroscopies have been used to uncover ultrafast dynamics (e.g., exciton dissociation, charge transfer) − and long-range drift in photovoltaic cells. − These earlier works demonstrate that spontaneous processes can be exploited in creative ways to characterize photoinduced dynamics.…”

“…Higher order perturbative responses have been targeted in most previous applications of action spectroscopies; − ,,,− , however, alternate signal generation mechanisms can be advantageous depending on the material and desired information. − For example, two- and four-pulse implementations of nonlinear action spectroscopies are compared in Figure . In Figure a, a sequence of four laser pulses with experimentally controlled phases (Ω j ) is used to isolate signal components resembling the ground state bleach, excited state emission, and excited state absorption responses present in conventional transient absorption and 2D electronic spectroscopies .…”