Elucidation of Quantum-Well-Specific Carrier Mobilities in Layered Perovskites

Abstract

Layered organohalide perovskite films consist of quantum wells with concentration distributions tailored to enhance long-range charge transport. Whereas cascaded energy and charge funneling behaviors have been detected with conventional optical spectroscopies, it is not clear that such dynamics contribute to the efficiencies of photovoltaic cells. In this Letter, we use nonlinear photocurrent spectroscopy to selectively target charge transport processes within devices based on layered perovskite quantum wells. The photocurrent induced by a pair of laser pulses is directly measured in this “action” spectroscopy to remove ambiguities in signal interpretation. By varying the external bias, we determine carrier mobilities for quantum-well-specific trajectories taken through the active layers of the devices. The results suggest that the largest quantum wells are primarily responsible for photocurrent production, whereas the smallest quantum wells trap charge carriers and are a major source of energy loss in photovoltaic cells.

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 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 .…”

Probing Carrier Transport in Layered Perovskites with Nonlinear Optical and Photocurrent Spectroscopies

“…Second, we have developed nonlinear action spectroscopies to decompose the optical responses of photovoltaic cells into productive (photocurrent) and lossy (fluorescence) processes. − Applications of these techniques are motivated by the inability of transient absorption experiments to distinguish energy and charge transfer mechanisms. Most recently, we have shown that trajectory-specific carrier mobilities can be determined for layered perovskite systems by cycling the laser pulse sequences and external biases applied to a photovoltaic cell . These experiments suggest promise in a new family of techniques that combine multidimensional spectroscopies with device measurements.…”