Integrating Ultrathin Bulk‐Heterojunction Organic Semiconductor Intermediary for High‐Performance Low‐Bandgap Perovskite Solar Cells with Low Energy Loss

Abstract

Mixed tin (Sn)-lead (Pb) perovskite is considered the most promising low-bandgap photovoltaic material for both pursuing the theoretical limiting efficiency of single-junction solar cells and breaking the Shockley-Queisser limitation by constructing tandem solar cells. However, their power conversion efficiencies (PCEs) are still lagging behind those of medium-bandgap perovskite solar cells (pero-SCs) due to their serious energy loss (E loss ). In this work, we used an ultra-thin bulkheterojunction (BHJ) organic semiconductor (PBDB-T:ITIC) layer as an intermediary between the hole transporting layer and Sn-Pb-based low-bandgap perovskite film to minimize E loss . It was found that this BHJ PBDB-T:ITIC intermediary simultaneously provided a cascading energy alignment in the device, facilitated high-quality Sn-Pb perovskite film growth, and passivated the antisite defects of the perovskite surface. In this simple way, the E loss of pero-SCs based on (FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 (bandgap ≈ 1.25 eV) was dramatically reduced below 0.4 eV, leading to a high open-circuit voltage (V oc ) of 0.86 V. As a result, the best pero-SC showed a significantly improved PCE of 18.03% with negligible J-V hysteresis and high stability. To the best of our knowledge, the PCE of 18.03% and V oc of 0.86 V are the highest values among the low-bandgap pero-SCs to date.

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

“…However, the proportion of crystal grains with sizes over 1 μm for perovskite film with Sn 4+ reduction seems slightly higher than that of the film without Sn 4+ reduction. Note that it has proven difficult to synthesize uniform, pinhole‐free, and Sn‐based perovskite thin films because Sn‐based perovskite films crystallized rapidly, the thickness of the (FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 perovskite films prepared in this work is about 800 nm which is thicker than most previous reports of mixed Pb–Sn PSCs . It seems that the average grain size of the film is comparable with the film thickness.…”

“…The crystallization of (FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 films on PEDOT:PSS without and with Sn 4+ reduction has been evaluated by X‐ray diffraction (XRD) measurements, as shown in Figure S1, Supporting Information. The two XRD patterns show strong diffraction peaks at 14.1° and 28.3° corresponding to (110) and (220) preferred orientation . There is no obvious change in the XRD patterns of the perovskite films without and with Sn 4+ reduction, which indicates that Sn powder reduction has little effect on the crystallization of (FASnI 3 ) 0.6 (MAPbI 3 ) 0.4 perovskite films.…”

“…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.…”

Highly Efficient and Stable Solar Cells Based on Crystalline Oriented 2D/3D Hybrid Perovskite

“…And V eff is defined as V eff = V 0 – V app , where V 0 is the voltage when J ph = 0 and V app is the applied bias. The ratio of J ph / J sat can be used to estimate the charge collection efficiency under operation conditions, where J sat is the saturation photocurrent density . Under short circuit conditions, the J ph / J sat ratios were calculated to be 0.958 and 0.991 for the control and 2D/3D perovskite devices, respectively.…”

Low‐Bandgap Mixed Tin‐Lead Perovskites and Their Applications in All‐Perovskite Tandem Solar Cells

“…Recently, Hayase and co‐workers reported low‐bandgap PSCs with a conduction band spike between the perovskite and electron transport layer (ETL), yielding a PCE of 17.6% . Xu et al reported mixed Sn‐Pb PSCs with a high PCE of 18.03%, achieved by incorporating an ultrathin organic semiconductor (PBDB‐T:ITIC) layer which sufficiently reduces the charge recombination at the front interface . More recently, Our group reported mixed Sn‐Pb PSCs with the highest PCE exceeding 19% through reducing the saturation current via Br incorporation .…”