Custom-tailored hole transport layer using oxalic acid for high-quality tin-lead perovskites and efficient all-perovskite tandems.

All-perovskite tandem solar cells (TSCs) have exhibited higher efficiencies than single-junction perovskite solar cells (PSCs) but still suffer from the unsatisfactory performance of low-bandgap (LBG) tin-lead (Sn-Pb) subcells. The inherent properties of PEDOT:PSS are crucial to high-performance Sn-Pb perovskite films and devices; however, the underlying mechanism has not been fully explored and revealed. Here, we report a facile oxalic acid treatment of PEDOT:PSS (OA-PEDOT:PSS) to precisely regulate its work function and surface morphology. OA-PEDOT:PSS shows a larger work function and an ordered reorientation and fiber-shaped film morphology with efficient hole transport pathways, leading to the formation of more ideal hole-selective contact with Sn-Pb perovskite for suppressing interfacial nonradiative recombination losses. Moreover, OA-PEDOT:PSS induces (100) preferred orientation growth of perovskite for higher-quality Sn-Pb films. Last, the OA-PEDOT:PSS-tailored LBG PSC yields an impressive efficiency of up to 22.56% (certified 21.88%), enabling 27.81% efficient all-perovskite TSC with enhanced operational stability.

Improving interface quality for 1-cm2 all-perovskite tandem solar cells.

All-perovskite tandem solar cells provide high power conversion efficiency at a low cost1-4. Rapid efficiency improvement in small-area (<0.1 cm2) tandem solar cells has been primarily driven by advances in low-bandgap (approximately 1.25 eV) perovskite bottom subcells5-7. However, unsolved issues remain for wide-bandgap (> 1.75 eV) perovskite top subcells8, which at present have large voltage and fill factor losses, particularly for large-area (>1 cm2) tandem solar cells. Here we develop a self-assembled monolayer of (4-(7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid as a hole-selective layer for wide-bandgap perovskite solar cells, which facilitates subsequent growth of high-quality wide-bandgap perovskite over a large area with suppressed interfacial non-radiative recombination, enabling efficient hole extraction. By integrating (4-(7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid in devices, we demonstrate a high open-circuit voltage (VOC) of 1.31 V in a 1.77-eV perovskite solar cell, corresponding to a very low VOC deficit of 0.46 V (with respect to the bandgap). With these wide-bandgap perovskite subcells, we report 27.0% (26.4% certified stabilized) monolithic all-perovskite tandem solar cells with an aperture area of 1.044 cm2. The certified tandem cell shows an outstanding combination of a high VOC of 2.12 V and a fill factor of 82.6%. Our demonstration of the large-area tandem solar cells with high certified efficiency is a key step towards scaling up all-perovskite tandem photovoltaic technology.

Improved Carrier Management via a Multifunctional Modifier for High-Quality Low-Bandgap Sn-Pb Perovskites and Efficient All-Perovskite Tandem Solar Cells.

All-perovskite tandem solar cells (TSCs) hold great promise in terms of ultrahigh efficiency, low manufacturing cost, and flexibility, stepping forward to the next-generation photovoltaics. However, their further development is hampered by the relatively low performance of low-bandgap (LBG) tin (Sn)-lead (Pb) perovskite solar cells (PSCs). Improving the carrier management, including suppressing trap-assisted non-radiative recombination and promoting carrier transfer, is of great significance to enhance the performance of Sn-Pb PSCs. Herein, a carrier management strategy is reported for using cysteine hydrochloride (CysHCl) simultaneously as a bulky passivator and a surface anchoring agent for Sn-Pb perovskite. CysHCl processing effectively reduces trap density and suppresses non-radiative recombination, enabling the growth of high-quality Sn-Pb perovskite with greatly improved carrier diffusion length of >8 µm. Furthermore, the electron transfer at the perovskite/C60 interface is accelerated due to the formation of surface dipoles and favorable energy band bending. As a result, these advances enable the demonstration of champion efficiency of 22.15% for CysHCl-processed LBG Sn-Pb PSCs with remarkable enhancement in both open-circuit voltage and fill factor. When paired with a wide-bandgap (WBG) perovskite subcell, a certified 25.7%-efficient all-perovskite monolithic tandem device is further demonstrated.

Rational design of Lewis base molecules for stable and efficient inverted perovskite solar cells.

Lewis base molecules that bind undercoordinated lead atoms at interfaces and grain boundaries (GBs) are known to enhance the durability of metal halide perovskite solar cells (PSCs). Using density functional theory calculations, we found that phosphine-containing molecules have the strongest binding energy among members of a library of Lewis base molecules studied herein. Experimentally, we found that the best inverted PSC treated with 1,3-bis(diphenylphosphino)propane (DPPP), a diphosphine Lewis base that passivates, binds, and bridges interfaces and GBs, retained a power conversion efficiency (PCE) slightly higher than its initial PCE of ~23% after continuous operation under simulated AM1.5 illumination at the maximum power point and at ~40°C for >3500 hours. DPPP-treated devices showed a similar increase in PCE after being kept under open-circuit conditions at 85°C for >1500 hours.

Revealing the Role of Tin Fluoride Additive in Narrow Bandgap Pb-Sn Perovskites for Highly Efficient Flexible All-Perovskite Tandem Cells.

Tin fluoride (SnF2) is an indispensable additive for high-efficiency Pb-Sn perovskite solar cells (PSCs). However, the spatial distribution of SnF2 in the perovskite absorber is seldom investigated while essential for a comprehensive understanding of the exact role of the SnF2 additive. Herein, we revealed the spatial distribution of the SnF2 additive and made structure-optoelectronic properties-flexible photovoltaic performance correlation. We observed the chemical transformation of SnF2 to a fluorinated oxy-phase on the Pb-Sn perovskite film surface due to its rapid oxidation. In addition, at the buried perovskite interface, we detected and visualized the accumulation of F- ions. We found that the photoluminescence quantum yield of Pb-Sn perovskite reached the highest value with 10 mol % SnF2 in the precursor solution. When integrating the optimized absorber in flexible devices, we obtained the flexible Pb-Sn perovskite narrow bandgap (1.24 eV) solar cells with an efficiency of 18.5% and demonstrated 23.1% efficient flexible four-terminal all-perovskite tandem cells.

Pure 2D Perovskite Formation by Interfacial Engineering Yields a High Open-Circuit Voltage beyond 1.28 V for 1.77-eV Wide-Bandgap Perovskite Solar Cells.

Surface post-treatment using ammonium halides effectively reduces large open-circuit voltage (VOC ) losses in bromine-rich wide-bandgap (WBG) perovskite solar cells (PSCs). However, the underlying mechanism still remains unclear and the device efficiency lags largely behind. Here, a facile strategy of precisely tailoring the phase purity of 2D perovskites on top of 3D WBG perovskite and realizing high device efficiency is reported. The transient absorption spectra, cross-sectional confocal photoluminescence mapping, and cross-sectional Kelvin probe force microscopy are combined to demonstrate optimal defect passivation effect and surface electric-field of pure n = 1 2D perovskites formed atop 3D WBG perovskites via low-temperature annealing. As a result, the inverted champion device with 1.77-eV perovskite absorber achieves a high VOC of 1.284 V and a power conversion efficiency (PCE) of 17.72%, delivering the smallest VOC deficit of 0.486 V among WBG PSCs with a bandgap higher than 1.75 eV. This enables one to achieve a four-terminal all-perovskite tandem solar cell with a PCE exceeding 25% by combining with a 1.25-eV low-bandgap PSC.

MXene-Modulated Electrode/SnO2 Interface Boosting Charge Transport in Perovskite Solar Cells.

Interface engineering is imperative to boost the extraction capability in perovskite solar cells (PSCs). We propose a promising approach to enhance the electron mobility and charge transfer ability of tin oxide (SnO2) electron transport layer (ETL) by introducing a two-dimensional carbide (MXene) with strong interface interaction. The MXene-modified SnO2 ETL also offers a preferable growth platform for perovskite films with reduced trap density. Through a spatially resolved imaging technique, profoundly reduced non-radiative recombination and charge transport losses in PSCs based on MXene-modified SnO2 are also observed. As a result, the PSC achieves an enhanced efficiency of 20.65% with ultralow saturated current density and negligible hysteresis. We provide an in-depth mechanistic understanding of MXene interface engineering, offering an alternative approach to obtain efficient PSCs.

Beneficial effects of potassium iodide incorporation on grain boundaries and interfaces of perovskite solar cells.

Grain boundaries and interfacial impurities are the main factors that limit the further development of polycrystalline perovskite solar cells because their existence severely deteriorates the device performance. In order to optimize the efficiency of perovskite solar cells, it is essential to eliminate these defects. In the present work, potassium iodide (KI) is incorporated into the perovskite absorber. KI incorporation improves the crystallinity of the perovskite, increases the grain size, and decreases the contact potential distribution at the grain boundary, which are verified by X-ray diffraction, scanning electronic microscopy and Kelvin probe force microscopy. Besides, the activation energy of the recombination, estimated from the temperature dependent current-voltage of perovskite solar cells, is larger than the bandgap calculated from the temperature coefficient. These suggest that KI incorporation effectively passivates the grain boundaries and interfacial defects. As a result, charge trapping in the absorber as well as the bimolecular and trap-assisted recombination of the device are significantly suppressed. Consequently, the open circuit voltage and fill factor of the incorporated devices are greatly improved, enabling an optimized power conversion efficiency of 19.5%, in comparison with that of 17.3% for the control one. Our work provides an effective strategy of defect passivation in perovskite solar cells by KI incorporation and clarifies the mechanism of the performance optimization of KI incorporated devices.