All-perovskite tandem solar cells with improved grain surface passivation

Contemporary thin-film photovoltaic (PV) materials contain elements that are scarce (CIGS) or regulated (CdTe and lead-based perovskites), a fact that may limit the widespread impact of these emerging PV technologies. Tin halide perovskites utilize materials less stringently regulated than the lead (Pb) employed in mainstream perovskite solar cells; however, even today's best tin-halide perovskite thin films suffer from limited carrier diffusion length and poor film morphology. We devised a synthetic route to enable in situ reaction between metallic Sn and I2 in dimethyl sulfoxide (DMSO), a reaction that generates a highly coordinated SnI2·(DMSO)x adduct that is well-dispersed in the precursor solution. The adduct directs out-of-plane crystal orientation and achieves a more homogeneous structure in polycrystalline perovskite thin films. This approach improves the electron diffusion length of tin-halide perovskite to 290 ± 20 nm compared to 210 ± 20 nm in reference films. We fabricate tin-halide perovskite solar cells with a power conversion efficiency of 14.6% as certified in an independent lab. This represents a ∼20% increase compared to the previous best-performing certified tin-halide perovskite solar cells. The cells outperform prior earth-abundant and heavy-metal-free inorganic-active-layer-based thin-film solar cells such as those based on amorphous silicon, Cu2ZnSn(S/Se)4 , and Sb2(S/Se)3.

One-Step Synthesis of SnI 2·(DMSO) x Adducts for High-Performance Tin 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

Perovskite solar cells (PSCs) emerging as a promising photovoltaic technology with high efficiency and low manufacturing cost have attracted the attention from all over the world. Both the efficiency and stability of PSCs have increased steadily in recent years, and the research on reducing lead leakage and developing eco-friendly lead-free perovskites pushes forward the commercialization of PSCs step by step. This review summarizes the main progress of PSCs in 2020 and 2021 from the aspects of efficiency, stability, perovskite-based tandem devices, and lead-free PSCs. Moreover, a brief discussion on the development of PSC modules and its challenges toward practical application is provided.

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The Main Progress of Perovskite Solar Cells in 2020-2021.

Expanding the near-infrared (NIR) response of perovskite materials to approach the ideal bandgap range (1.1–1.4 eV) for single-junction solar cells is an attractive step to unleash the full potential of perovskite solar cells (PSCs). However, polycrystalline formamidinium lead triiodide (FAPbI3)-based absorbers, used in record-efficiency PSCs, currently offer the smallest bandgap that can be achieved for lead-halide perovskite thin films (>100 meV larger than the optimal bandgap). Here, we uncover that utilizing a mixed-cation single-crystal absorber layer (FA0.6MA0.4PbI3) is capable of redshifting the external quantum efficiency (EQE) band edge past that of FAPbI3 polycrystalline solar cells by about 50 meV – only 60 meV larger than that of the top-performing photovoltaic material, GaAs – leading to EQE-verified short-circuit current densities exceeding 26 mA cm−2 without sacrificing the open-circuit voltage (VOC), and therefore, yielding power conversion efficiencies of up to 22.8%. These figures of merit not only set a new record for SC-PSCs and are among the highest reported for inverted-structured-PSCs, but also offer an avenue for lead halide PSCs to advance their performance toward their theoretical Shockley–Queisser Limit potential.

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22.8%-Efficient single-crystal mixed-cation inverted perovskite solar cells with a near-optimal bandgap

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

Despite the well-known implications in the field of III-V semiconductors, lattice strain in halide perovskite materials has been largely overlooked until recently. Here we review the effect of latt...

Strain Engineering in Halide Perovskites

Most efficient perovskite solar cells are based on polycrystalline thin films; however, substantial structural disorder and defective grain boundaries place a limit on their performance. Perovskite...

Perovskite Single-Crystal Solar Cells: Going Forward

Lead (Pb) in conventional perovskite solar cells (PSCs) is toxic and has to be replaced. Situated in one group of the periodic table of elements, tin (Sn) has the same valence electrons’ configurat...

Tin Halide Perovskites Going Forward: Frost Diagrams Offer Hints

All-organic infrared (IR)-to-visible upconversion organic light-emitting diodes (OLEDs) with an IR sensitivity up to 1100 nm were fabricated using a low-band-gap polymer as the organic IR sensitizi...

Vishal Yeddu

Papers

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

Strain Engineering in Halide Perovskites

Perovskite Single-Crystal Solar Cells: Going Forward

Tin Halide Perovskites Going Forward: Frost Diagrams Offer Hints

Low-Band-Gap Polymer-Based Infrared-to-Visible Upconversion Organic Light-Emitting Diodes with Infrared Sensitivity up to 1.1 μm