Reducing Saturation‐Current Density to Realize High‐Efficiency Low‐Bandgap Mixed Tin–Lead Halide Perovskite Solar Cells

Abstract

The unsatisfactory performance of low‐bandgap mixed tin (Sn)–lead (Pb) halide perovskite subcells has been one of the major obstacles hindering the progress of the power conversion efficiencies (PCEs) of all‐perovskite tandem solar cells. By analyzing dark‐current density and distribution, it is identified that charge recombination at grain boundaries is a key factor limiting the performance of low‐bandgap mixed Sn–Pb halide perovskite subcells. It is further found that bromine (Br) incorporation can effectively passivate grain boundaries and lower the dark current density by two–three orders of magnitude. By optimizing the Br concentration, low‐bandgap (1.272 eV) mixed Sn–Pb halide perovskite solar cells are fabricated with open‐circuit voltage deficits as low as 0.384 V and fill factors as high as 75%. The best‐performing device demonstrates a PCE of >19%. The results suggest an important direction for improving the performance of low‐bandgap mixed Sn–Pb halide perovskite solar cells.

Realizing High Efficiency over 20% of Low‐Bandgap Pb–Sn‐Alloyed Perovskite Solar Cells by In Situ Reduction of Sn4+

“…Many efforts have been devoted to improving the PCE of low‐bandgap mixed Pb–Sn PSCs . However, it often exhibits a larger open‐circuit voltage ( V oc ) loss, and therefore, its efficiency is far behind that of pure Pb‐based PSCs with a medium bandgap.…”

Vacuum‐Assisted Growth of Low‐Bandgap Thin Films (FA0.8MA0.2Sn0.5Pb0.5I3) for All‐Perovskite Tandem Solar Cells

“…Making use of the large columnar LBG perovskite grains produced by VAGC in this work, PSCs with PCE as high as 18.2% are demonstrated, exceeding the 15.4% of reference devices prepared by the established antisolvent method (Figure 1d). It should be noted that, to date, this represents one of the highest PCEs of the LBG PSC based on pure iodine halogen without incorporation of Br − , [28] Cl − [31] nor incorporation of additives such as GuaSCN. The long-term MPP tracking of the prepared devices by VAGC and the antisolvent method shows similar photo-stability for PSC prepared by VAGC compared to the antisolvent method ( Figure S1, Supporting Information) for devices stored at 25 °C in N 2 atmosphere.…”

“…[24][25][26][27] While it was shown that these small grains (accompanied by a large number of grain boundaries in the film) do not cause severe problems in Pb-based perovskite thin films, they are reported to be detrimental in Sn-based perovskite thin films [24][25][26][27] due to lots of defects at the grain-boundaries. [28] In perovskite thin films, grain boundaries are known as being potential sites for nonradiative recombination of charge carriers. To further improve the morphology and crystal quality of LBG perovskite thin films, several studies investigated the ratio of halogens in the LBG perovskite thin films.…”

“…Therefore, controlling the nucleation and crystal growth in Sn-based LBG perovskites is essential for achieving thin films of high optoelectronic quality. [28] Several attempts were conducted to reduce the grain boundaries in LBG perovskites using two-step methods, [30] MACl vapor treatment, [19] and hot-casting methods. [19,28] It was reported that in the LBG PSCs, similar to perovskites with regular-bandgap (E G ≈ 1.5-1.6 eV), the addition of chloride anions increases the average grain size and reduces electronic disorder in the mixed perovskite absorber thin films.…”

“…In particular, employing the VAGC to fabricate LBG PSCs (E G ≈ 1.27 eV), a high PCE of 18.2% is demonstrated which is among one of the highest reported pure iodine-based LBG PSCs (with no additives such as Br − , [28] Cl − , [31] and GuaSCN [12] ). In particular, employing the VAGC to fabricate LBG PSCs (E G ≈ 1.27 eV), a high PCE of 18.2% is demonstrated which is among one of the highest reported pure iodine-based LBG PSCs (with no additives such as Br − , [28] Cl − , [31] and GuaSCN [12] ).…”