Abstract
A combined electrical and time-resolved optical investigation of the perovskite formamidinium tin iodide (FASnI3) and its phenylethylammonium (PEA) derivative PEA0.08FA0.92SnI3, which recently achieved a power conversion efficiency of 9%, is presented to study the specifics of contact characteristics and charge carrier dynamics. Microstructured gold electrode arrays were used to investigate the charge transport across a metal-perovskite interface and through micrometers of the perovskite films. Symmetrical contact configuration enabled detailed polarization studies. Hysteresis in the current-voltage characteristics and a corresponding current-time behavior indicated limitations by charge transfer in the contacts. Hysteresis was less pronounced in PEA0.08FA0.92SnI3 compared with FASnI3. This is explained by a 2-dimensional interlayer at the contacts, which leads to decreased field-induced migration of ions at the contact. Carrier recombination in the bulk of FASnI3 films, however, was only slightly modified by the presence of PEA. Femtosecond broadband transient absorption experiments up to 1.5 ns provided rate constants for the Auger and bimolecular recombination processes in FASnI3 of k3 = 1 × 10−29 cm6 s−1 and k2 = 3.1 × 10−10 cm3 s−1, respectively. In PEA0.08FA0.92SnI3, no significant differences in k2 and an only slightly increased k3 = 2 × 10−29 cm6 s−1 were measured. In extension to previous photoluminescence studies, we found efficient cooling of hot carriers by coupling to optical phonons (τcop = 0.5 ps), which is even faster than in lead perovskites.
Highlights
Interdigitated microelectrodes with a gap width of 40 μm were prepared via photolithography onto highly n-doped silicon wafers with a thermally grown oxide layer of 300 nm, as described previously.22 50 nm of gold was deposited as a metal electrode by thermal evaporation
The samples for optical characterization were prepared on soda lime glass (Carl Roth) that was cleaned with RBS solution (Carl Roth), acetone, and n-propanol in an ultrasonic bath for 15 min, subsequently
For the FASnI3 films, 172 mg formamidinium iodide (FAI, Greatcell Solar), 372.5 mg SnI2, and 15.6 mg SnF2 (Sigma Aldrich, 99%) were mixed by stirring in 800 μl N,N-dimethylformamide (DMF, Alfa Aesar) and 200 μl dimethyl sulfoxide (DMSO, Alfa Aesar), resulting in a bright yellow solution
Summary
Interdigitated microelectrodes with a gap width of 40 μm were prepared via photolithography onto highly n-doped silicon wafers with a thermally grown oxide layer of 300 nm, as described previously.22 50 nm of gold was deposited as a metal electrode by thermal evaporation. We employ steady-state absorption and PL measurements as well as ultrafast transient broadband absorption experiments to understand the charge recombination and carrier cooling processes in FASnI3 and PEA0.08FA0.92SnI3 perovskite layers upon photoexcitation and to differentiate between an influence of PEAI on either bulk or interface properties.
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