Understanding the Role of Grain Boundaries on ChargeāCarrier and Ion Transport in Cs 2 AgBiBr 6 Thin Films
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Abstract: Halide double perovskites have gained significant attention, owing to their composition of lowātoxicity elements, stability in air, and recent demonstrations of long chargeācarrier lifetimes that can exceed 1 Āµs. In particular, Cs2AgBiBr6 is the subject of many investigations in photovoltaic devices. However, the efficiencies of solar cells based on this double perovskite are still far from the theoretical efficiency limit of the material. Here, the role of grain size on the optoelectronic properties of Cs2AgBiBr6 thin films is investigated. It is shown through cathodoluminescence measurements that grain boundaries are the dominant nonradiative recombination sites. It also demonstrates through fieldāeffect transistor and temperatureādependent transient current measurements that grain boundaries act as the main channels for ion transport. Interestingly, a positive correlation between carrier mobility and temperature is found, which resembles the hopping mechanism often seen in organic semiconductors. These findings explain the discrepancy between the long diffusion lengths >1 Āµm found in Cs2AgBiBr6 single crystals versus the limited performance achieved in their thin film counterparts. This work shows that mitigating the impact of grain boundaries will be critical for these double perovskite thin films to reach the performance achievable based on their intrinsic singleācrystal properties.
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Funder: Cambridge Trust and Chinese Scholarship
Funder: Royal Academy of Engineering; Id: http://dx.doi.org/10.13039/501100000287
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1616-3028
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Royal Society through the Newton Alumni Fellowship (SERBāSRG/2020/001641)
EPSRC (EP/R025193/1)
SUNRISE (EP/P032591/1)
Engineering and Physical Sciences Research Council (EP/P032591/1)