Abstract
Phase-pure BiI3 films obtained by versatile gas-phase iodination of Bi2S3 are investigated as an absorber in photovoltaic devices. This preparation method leads to highly crystalline BiI3 films featuring a rhombohedral phase and a high-degree of stacking order. The films are composed of micrometer-sized flat grains distributed homogeneously across the F-doped SnO2 (FTO) substrate, exhibiting an indirect band gap transition at 1.72 eV. High-level calculations based on G0W0 approximation are used to rationalize the electronic structure of BiI3, confirming the band gap value estimated experimentally. The films show p-type conductivity with an acceptor density on the order of 1015 cm–3. Solar cells with the architecture glass/FTO/TiO2/BiI3/F8/Au, where F8 is poly(9,9-di-n-octylfluorenyl-2,7-diyl), display a record open-circuit voltage above 600 mV and overall power conversion efficiency of 1.2% under AM 1.5G illumination. The large open-circuit potential is rationalized in terms of carrier lifetimes longer th...
Highlights
Sustainable photovoltaic (PV) technologies beyond Si solar cells represent huge opportunities in key areas such as large-scale building integration and competitive tandem technologies close to 30%.1,2. This strategy requires new solar absorbers based on earth-abundant materials that can offer different alternatives to existing CdTe and CuInGa(S,Se)[2] (CIGS) technologies.[3−6] Hybrid perovskite solar cells have experienced an unprecedented rise from basic science to a technology close to commercialization, in the context of Si-tandem solar cells, significant challenges remain in terms of stability.[7]
Cu2ZnSn(S,Se)[4] has been the key target in the context of earth-abundant CIGS replacement; efficiency remains hindered by a loss mechanism yet to be fully elucidated.[8−10] These developments have led to the establishment of guiding principles for the design of solar absorbers based on the concept of “defect-tolerant materials”, with Bi being one of the elements in these strategies.[5,6]
Bi compounds such as methylammonium bismuth iodide, bismuth chalcohalides, bismuth sulfide, silver bismuth sulfide, and bismuth iodide have been tested as solar absorbers with power conversion efficiencies up to 6.3%.12−19 Previously, we reported on BiFeO3 all-oxide solar cells with a record power conversion efficiency of 4%
Summary
We have investigated cells employing F8 as the HTL but following previous protocols in which commercially available BiI3 powders were spin-coated to the TiO2 surface (so-called “direct” method).[20] The corresponding J−V curves (Figure s3a) exhibited JSC and FF values that were 10−15% lower, while the VOC dropped by 56% in comparison to the devices prepared by our gas-phase iodination method. The EQE spectra show significantly lower values at wavelengths close to the band edge for the devices prepared via the direct method (Figure s3b) These results strongly suggest that the minority carrier lifetime is significantly different in these two films. All data presented in this Letter can be freely accessed from the Bristol’s Research Data Repository (https://doi.org/10.5523/ bris.2b4tiz6tawpgy29o33jdjfxzk[0])
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