Dipolar cations confer defect tolerance in wide-bandgap metal halide perovskites

Abstract

Efficient wide-bandgap perovskite solar cells (PSCs) enable high-efficiency tandem photovoltaics when combined with crystalline silicon and other low-bandgap absorbers. However, wide-bandgap PSCs today exhibit performance far inferior to that of sub-1.6-eV bandgap PSCs due to their tendency to form a high density of deep traps. Here, we show that healing the deep traps in wide-bandgap perovskites—in effect, increasing the defect tolerance via cation engineering—enables further performance improvements in PSCs. We achieve a stabilized power conversion efficiency of 20.7% for 1.65-eV bandgap PSCs by incorporating dipolar cations, with a high open-circuit voltage of 1.22 V and a fill factor exceeding 80%. We also obtain a stabilized efficiency of 19.1% for 1.74-eV bandgap PSCs with a high open-circuit voltage of 1.25 V. From density functional theory calculations, we find that the presence and reorientation of the dipolar cation in mixed cation–halide perovskites heals the defects that introduce deep trap states.

FA‐Assistant Iodide Coordination in Organic–Inorganic Wide‐Bandgap Perovskite with Mixed Halides

“…It should be noted that compared with the film without FA + , the FA + incorporated films exhibit a blueshift of the absorption on‐set from 684 nm in MAPb(I 0.6 Br 0.4 ) 3 to 670 nm in MA 0.7 FA 0.3 Pb(I 0.6 Br 0.4 ) 3 . Similar phenomenon has also been reported by Sargent and co‐workers, and they have found that it is due to the crystal distortion caused by the size difference between the organic cations and halide anions 25,35,45…”

“…It is corresponding to around 30% of Br − to the total halide composition 34. However, such high Br − content would cause phase impurity and high defect density issues in the resulting films, which are detrimental to the device performances 14,35,36. Sutter‐Fella et al reported that incorporating Br − will induce defects close to the mid‐gap of the perovskites using sub‐bandgap external quantum efficiency (EQE) measurement, and the defect density increases with increasing the Br − content 31.…”

“…It is well known that the crystallization of solution‐derived perovskite is very sensitive to the composition and processing conditions 25,35,39. In particular, for the mixed‐halide perovskites, there are ions with different sizes and complexities which would alter the coordination process 40.…”

Two‐Terminal Perovskites Tandem Solar Cells: Recent Advances and Perspectives

“…By introducing a highly polar additive formamide (CH 3 NO) into the precursor solution, the solubility of the Cs salt increased, which led to the less formation of yellow δ‐phases and reduced defects density in films. As a result, the optimized FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 PSC ( E g = 1.75 eV) exhibited a high stability and reduced hysteresis, with PCE of 17.8% and a V OC of 1.23 V. Recently, Tan et al showed that the presence and reorientation of the MA dipolar cation in CsFA‐based perovskites could heal the deep trap defects and then reduce the trap‐assisted nonradiative recombination. Via cation engineering, the Cs 0.05 MA 0.15 FA 0.8 Pb(I 0.75 Br 0.25 ) 3 ( E g = 1.65 eV) with an illumination area of 1.1 cm 2 delivered a stabilized PCE of 20.7%, a high V OC of 1.22 V and a low V OC deficit of 0.41 V. The Cs 0.12 MA 0.05 FA 0.83 Pb(I 0.6 Br 0.4 ) 3 perovskite ( E g = 1.74 eV) yielded an efficiency of 19.1%, with a high V OC of 1.25 V and FF of 81.5%.…”

Cesium Lead Inorganic Solar Cell with Efficiency beyond 18% via Reduced Charge Recombination

“…[26] Li et al invented a novel gradient thermal annealing to control the growth of CsPbI 2 Br film (band gap = 1.92 eV), and a champion PCE of 16.07% with a V OC of 1.23 V was realized. [29] It can be found that the opencircuit voltage loss (V oc loss) is still the main reason of low performance of inorganic perovskite solar cells, which strongly related to energy band matching and defects at the interface or in the bulk of perovskite. [28] Although there are significant progresses in inorganic perovskite solar cells, the PCE is still far behind the hybrid PSCs, even compared with the I-Br mixed hybrid perovskite with a similar band gap (1.75 eV).…”