Combining wide-bandgap and narrow-bandgap perovskites to construct monolithic all-perovskite tandem solar cells offers avenues for continued increases in photovoltaic (PV) power conversion efficiencies (PCEs). However, actual efficiencies today are diminished by the subpar performance of narrow-bandgap subcells. Here we report a strategy to reduce Sn vacancies in mixed Pb–Sn narrow-bandgap perovskites that use metallic tin to reduce the Sn4+ (an oxidation product of Sn2+) to Sn2+ via a comproportionation reaction. We increase, thereby, the charge-carrier diffusion length in narrow-bandgap perovskites to 3 μm for the best materials. We obtain a PCE of 21.1% for 1.22-eV narrow-bandgap solar cells. We fabricate monolithic all-perovskite tandem cells with certified PCEs of 24.8% for small-area devices (0.049 cm2) and of 22.1% for large-area devices (1.05 cm2). The tandem cells retain 90% of their performance following 463 h of operation at the maximum power point under full 1-sun illumination. Improvements in the efficiency and stability of low-bandgap perovskite solar cells are key to enabling all-perovskite solar cells. Here, Lin et al. use metallic tin to prevent oxidation in such low-gap perovskite and demonstrate 24.8%-efficient tandems that are stable for over 400 h under operating conditions.

Monolithic all-perovskite tandem solar cells with 24.8% efficiency exploiting comproportionation to suppress Sn( ii ) oxidation in precursor ink

The application of liquid-exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene-based organic solar cells is reported. It is shown that solution processing of few-layer WS2 or MoS2 suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS2 are found to exhibit higher uniformity on ITO than those of MoS2 and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short-circuit current (JSC ), and lower series resistance than devices based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoS2 . Cells based on the ternary bulk-heterojunction PBDB-T-2F:Y6:PC71 BM with WS2 as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open-circuit voltage of 0.84 V, and a JSC of 26 mA cm-2 . Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS2 -based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high-efficiency organic photovoltaics.

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17% Efficient Organic Solar Cells Based on Liquid Exfoliated WS2 as a Replacement for PEDOT:PSS

Recent Progresses on Defect Passivation toward Efficient Perovskite Solar Cells

Monolithic all-perovskite tandem solar cells offer an avenue to increase power conversion efficiency beyond the limits of single-junction cells. It is an important priority to unite efficiency, uniformity and stability, yet this has proven challenging because of high trap density and ready oxidation in narrow-bandgap mixed lead–tin perovskite subcells. Here we report simultaneous enhancements in the efficiency, uniformity and stability of narrow-bandgap subcells using strongly reductive surface-anchoring zwitterionic molecules. The zwitterionic antioxidant inhibits Sn2+ oxidation and passivates defects at the grain surfaces in mixed lead–tin perovskite films, enabling an efficiency of 21.7% (certified 20.7%) for single-junction solar cells. We further obtain a certified efficiency of 24.2% in 1-cm2-area all-perovskite tandem cells and in-lab power conversion efficiencies of 25.6% and 21.4% for 0.049 cm2 and 12 cm2 devices, respectively. The encapsulated tandem devices retain 88% of their initial performance following 500 hours of operation at a device temperature of 54–60 °C under one-sun illumination in ambient conditions. Ensuring both stability and efficiency in mixed lead–tin perovskite solar cells is crucial to the development of all-perovskite tandems. Xiao et al. use an antioxidant zwitterionic molecule to suppress tin oxidation thus enabling large-area tandem cells with 24.2% efficiency and operational stability over 500 hours.

All-perovskite tandem solar cells with 24.2% certified efficiency and area over 1 cm2 using surface-anchoring zwitterionic antioxidant

Tin perovskite is rising as a promising candidate to address the toxicity and theoretical efficiency limitation of lead perovskite. However, the voltage and efficiency of tin perovskite solar cells are much lower than lead counterparts. Herein, indene-C60 bisadduct with higher energy level is utilized as an electron transporting material for tin perovskite solar cells. It suppresses carrier concentration increase caused by remote doping, which significantly reduces interface carriers recombination. Moreover, indene-C60 bisadduct increases the maximum attainable photovoltage of the device. As a result, the use of indene-C60 bisadduct brings unprecedentedly high voltage of 0.94 V, which is over 50% higher than that of 0.6 V for device based on [6,6]-phenyl-C61-butyric acid methyl ester. The device shows a record power conversion efficiency of 12.4% reproduced in an accredited independent photovoltaic testing lab.

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Ultra-high open-circuit voltage of tin perovskite solar cells via an electron transporting layer design.

Tin-based perovskites with excellent optoelectronic properties and suitable band gaps are promising candidates for the preparation of efficient lead-free perovskite solar cells (PSCs). However, it is challenging to prepare highly stable and efficient tin-based PSCs because Sn2+ in perovskites can be easily oxidized to Sn4+ upon air exposure. Here we report the fabrication of air-stable FASnI3 solar cells by introducing hydroxybenzene sulfonic acid or its salt as an antioxidant additive into the perovskite precursor solution along with excess SnCl2 . The interaction between the sulfonate group and the Sn2+ ion enables the in situ encapsulation of the perovskite grains with a SnCl2 -additive complex layer, which results in greatly enhanced oxidation stability of the perovskite film. The corresponding PSCs are able to maintain 80 % of the efficiency over 500 h upon air exposure without encapsulation, which is over ten times longer than the best result reported previously. Our results suggest a possible strategy for the future design of efficient and stable tin-based PSCs.

Antioxidant Grain Passivation for Air-Stable Tin-Based Perovskite Solar Cells

As a rising star of third-generation photovoltaic technology, organic–inorganic halide perovskite solar cells (PSCs) have exhibited high power conversion efficiency. However, the most investigated high-performance perovskites contain toxic lead, which may hinder their widespread applications. Among various alternative metal ions to replace lead for environmentally benign perovskites, tin has been successfully used in PSCs with the highest efficiency over 13% at present, making tin-based perovskites the most promising active materials for lead-free PSCs. In this review, we first summarize the device design of PSCs and the physical properties of tin-based perovskites. Then we state the major challenges for this field and discuss the essential techniques in film preparation. Specifically, tin rich conditions (tin halide) and suitable reducing agents are needed in the film preparation to prohibit Sn2+ oxidation, and solvent modulation is a feasible approach for controlling the film morphology. Meanwhile, chemical substitutions of A site and B site cations and X site anions to improve the optoelectronic properties of tin-based perovskites are comprehensively addressed. Moreover, tin–lead mixed perovskites that contain less lead than conventional perovskites for high-performance PSCs and tandem solar cells are discussed. This comprehensive review can shed light on the development of environmentally friendly PSCs and provide a guideline for realizing lead-free PSCs with high efficiency and stability via synergistic approaches.

Recent progress in tin-based perovskite solar cells

The quality of perovskite films is critical to the performance of perovskite solar cells. However, it is challenging to control the crystallinity and orientation of solution-processed perovskite films. Here, solution-phase van der Waals epitaxy growth of MAPbI3 perovskite films on MoS2 flakes is reported. Under transmission electron microscopy, in-plane coupling between the perovskite and the MoS2 crystal lattices is observed, leading to perovskite films with larger grain size, lower trap density, and preferential growth orientation along (110) normal to the MoS2 surface. In perovskite solar cells, when perovskite active layers are grown on MoS2 flakes coated on hole-transport layers, the power conversion efficiency is substantially enhanced for 15%, relatively, due to the increased crystallinity of the perovskite layer and the improved hole extraction and transfer rate at the interface. This work paves a way for preparing high-performance perovskite solar cells and other optoelectronic devices by introducing 2D materials as interfacial layers.

Solution-Phase Epitaxial Growth of Perovskite Films on 2D Material Flakes for High-Performance Solar Cells.

Perovskite solar cells have attracted much attention recently for their high efficiency, ease of preparation and low cost. Here, we report a novel laser-annealing method for perovskite films at a low substrate temperature by scanning laser spots on the film surfaces. An ultrafast crystallization process within a few seconds is realized under a laser with a high intensity and a fast scanning speed. Because the crystalline perovskite phase has a stronger light absorption than the amorphous phase, the fast laser annealing can induce a higher temperature in the former and lead to the selective growth of large perovskite grains. Under optimum conditions, perovskite films with high crystallinity are successfully fabricated, resulting in perovskite solar cells with high power conversion efficiency and good stability. Moreover, a faster laser-annealing process of perovskite films is achieved by using a linear laser beam, which is expected to be a promising technique for the mass production of large-scale perovskite solar cells.

Ultrafast laser-annealing of perovskite films for efficient perovskite solar cells

Maintaining the stability of tin halide perovskites is a major challenge in developing lead-free perovskite solar cells (PSCs). Adding extra SnX2 (X = F, Cl, or Br) in the precursor solution to inh...

Jiupeng Cao

Papers

Antioxidant Grain Passivation for Air-Stable Tin-Based Perovskite Solar Cells

Recent progress in tin-based perovskite solar cells

Solution-Phase Epitaxial Growth of Perovskite Films on 2D Material Flakes for High-Performance Solar Cells.

Ultrafast laser-annealing of perovskite films for efficient perovskite solar cells

Highly Air-Stable Tin-Based Perovskite Solar Cells through Grain-Surface Protection by Gallic Acid