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Coexistence of excitons and free charge carriers can complicate conventional spectroscopic studies of transport mechanisms in layered perovskite solar cells. Because of their large concentrations and absorbance cross sections, excitons tend to dominate spectroscopic signals and obscure observations of free charges in this class of systems. To investigate the effects of interstitial organic molecules on charge transport in photovoltaic devices, we apply a newly developed four-pulse transient grating method with photocurrent detection to layered perovskites possessing a range of quantum well thicknesses. In this method, a phase-stabilized “pump” pulse-pair photoexcites a carrier density grating in the active layer of a photovoltaic cell, whereas transport is time-resolved using the carrier density grating generated by a subsequent “probe” pulse-pair. Carrier diffusion mechanisms are revealed by measuring the recombination-induced nonlinear response of the device while varying the delay between pulse-pairs and phase difference between density gratings. Like drift velocity dispersion, our data suggest that encounters with inorganic–organic interfaces broaden the range of diffusivities in addition to skewing the distributions toward slower transit times. Rather than tunneling through the potential energy barriers associated with the organic material, the experimental measurements support a physical picture in which the photoexcited carriers traverse circuitous paths through the active layer while occupying the phases of the thickest quantum wells.
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Measuring carrier diffusion in MAPbI3 solar cells with photocurrent-detected transient grating spectroscopy
Conventional time-of-flight methods can be used to determine carrier mobilities for photovoltaic cells in which the transit time between electrodes is greater than the RC time constant of the device. To measure carrier drift on sub-ns timescales, we have recently developed a two-pulse time-of-flight technique capable of detecting drift velocities with 100-ps time resolution in perovskite materials. In this method, the rates of carrier transit across the active layer of a device are determined by varying the delay time between laser pulses and measuring the magnitude of the recombination-induced nonlinearity in the photocurrent. Here, we present a related experimental approach in which diffractive optic-based transient grating spectroscopy is combined with our two-pulse time-of-flight technique to simultaneously probe drift and diffusion in orthogonal directions within the active layer of a photovoltaic cell. Carrier density gratings are generated using two time-coincident pulse-pairs with passively stabilized phases. Relaxation of the grating amplitude associated with the first pulse-pair is detected by varying the delay and phase of the density grating corresponding to the second pulse-pair. The ability of the technique to reveal carrier diffusion is demonstrated with model calculations and experiments conducted using MAPbI3 photovoltaic cells.
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- PAR ID:
- 10511412
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 159
- Issue:
- 9
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0159301
