Recent Advances in Modulating the Crystallization Dynamics of Sn-Pb Mixed Perovskites for High-Efficiency All-Perovskite Tandem Solar Cells.

Multi-junction all-perovskite tandem solar cells are a promising choice for next-generation solar cells with high efficiency and low fabrication cost. However, the lack of high-quality low-bandgap perovskite absorber layers seriously hampers the development of efficient and stable two-terminal monolithic all-perovskite tandem solar cells. Here, we report a bulk-passivation strategy via incorporation of chlorine, to enlarge grains and reduce electronic disorder in mixed tin–lead low-bandgap (~1.25 eV) perovskite absorber layers. This enables the fabrication of efficient low-bandgap perovskite solar cells using thick absorber layers (~750 nm), which is a requisite for efficient tandem solar cells. Such improvement enables the fabrication of two-terminal all-perovskite tandem solar cells with a champion power conversion efficiency of 21% and steady-state efficiency of 20.7%. The efficiency is retained to 85% of its initial performance after 80 h of operation under continuous illumination. Two-terminal monolithic all-perovskite tandem solar cells are attractive due to their flexible nature and low-cost fabrication. Here the authors develop a process to obtain high-quality Sn–Pb perovskite thin films by incorporating chlorine. Such layers are employed to fabricate 20.7%-efficient tandem cells with 80 h operational stability.

High-performance methylammonium-free ideal-band-gap perovskite solar cells

Role of conducting polymers in enhancing the stability and performance of perovskite solar cells: a brief review

Among many photovoltaic conversion technologies, perovskite solar cells have received significant research interests as effective photovoltaic materials owing to their high solar conversion efficiencies and low cost. However, the performance of perovskite solar cells is limited by the instability of CH3NH3PbI3 to water and ambient moisture. To address this issue, in this study, we introduced a new fundamental approach that utilizes 4-tert-butylpyridine (tBP) as the surface modification agent to enhance the performance and stability of CH3NH3PbI3-based perovskite solar cells fabricated in ambient air. The tertiary butyl group in tBP is highly hydrophobic, leading to the formation a hydrophobic layer on the surface of CH3NH3PbI3, thus increasing the moisture stability of perovskite solar cells. With this strategy, the performance of perovskite solar cells prepared at even >50% RH in ambient air was tremendously enhanced by as much as 200% compared to that without tBP additive. Besides, the stability of pero...

Small grains as recombination hot spots in perovskite solar cells

Multijunction tandem solar cells offer a promising route to surpass the efficiency limit of single-junction solar cells. All-perovskite tandem solar cells are particularly attractive due to their high power conversion efficiency, now reaching 28% despite being made with relatively easy fabrication methods. In this review, we summarize the progress in all-perovskite tandem solar cells. We then discuss the scientific and engineering challenges associated with both absorbers and functional layers and offer strategies for improving the efficiency and stability of all-perovskite tandem solar cells from the perspective of chemistry.

Organometal halide perovskite solar cells have demonstrated high conversion efficiency but poor long-term stability against ultraviolet irradiation and water. We show that rapid light-induced free-radical polymerization at ambient temperature produces multifunctional fluorinated photopolymer coatings that confer luminescent and easy-cleaning features on the front side of the devices, while concurrently forming a strongly hydrophobic barrier toward environmental moisture on the back contact side. The luminescent photopolymers re-emit ultraviolet light in the visible range, boosting perovskite solar cells efficiency to nearly 19% under standard illumination. Coated devices reproducibly retain their full functional performance during prolonged operation, even after a series of severe aging tests carried out for more than 6 months.

Decoupling the effects of defects on efficiency and stability through phosphonates in stable halide perovskite solar cells

Lead-free tin perovskite solar cells

Tandem solar cells using only metal-halide perovskite sub-cells are an attractive choice for next-generation solar cells. However, the progress in developing efficient all-perovskite tandem solar cells has been hindered by the lack of high-performance low-bandgap perovskite solar cells. Here, we report efficient mixed tin–lead iodide low-bandgap (∼1.25 eV) perovskite solar cells with open-circuit voltages up to 0.85 V and over 70% external quantum efficiencies in the infrared wavelength range of 700–900 nm, delivering a short-circuit current density of over 29 mA cm−2 and demonstrating suitability for bottom-cell applications in all-perovskite tandem solar cells. Our low-bandgap perovskite solar cells achieve a maximum power conversion efficiency of 17.6% and a certified efficiency of 17.01% with a negligible current–voltage hysteresis. When mechanically stacked with a ∼1.58 eV bandgap perovskite top cell, our best all-perovskite 4-terminal tandem solar cell shows a steady-state efficiency of 21.0%. All-perovskite tandem solar cells hold the promise of high efficiencies whilst safeguarding the ease of fabrication intrinsic to perovskites. Here, Zhao et al. present a certified 17% efficient tin and lead perovskite solar cell, which is integrated as the low-bandgap component of a tandem device with 21% efficiency.

Light-induced halide segregation constrains the photovoltaic performance and stability of wide-bandgap perovskite solar cells and tandem cells. The implementation of an intermixed two-dimensional/three-dimensional heterostructure via solution post-treatment is a typical strategy to improve the efficiency and stability of perovskite solar cells. However, owing to the composition-dependent sensitivity of surface reconstruction, the conventional solution post-treatment is suboptimal for methylammonium-free and cesium/bromide-enriched wide-bandgap PSCs. To address this, we develop a generic three-dimensional to two-dimensional perovskite conversion approach to realize a preferential growth of wider dimensionality (n ≥ 2) atop wide-bandgap perovskite layers (1.78 eV). This technique involves depositing a well-defined MAPbI3 thin layer through a vapor-assisted two-step process, followed by its conversion into a two-dimensional structure. Such a two-dimensional/three-dimensional heterostructure enables suppressed light-induced halide segregation, reduced non-radiative interfacial recombination, and facilitated charge extraction. The wide-bandgap perovskite solar cells demonstrate a champion power conversion efficiency of 19.6% and an open-circuit voltage of 1.32 V. By integrating with the thermal-stable FAPb0.5Sn0.5I3 narrow-bandgap perovskites, our all-perovskite tandem solar cells exhibit a stabilized PCE of 28.1% and retain 90% of the initial performance after 855 hours of continuous 1-sun illumination.

Monolithic two-terminal (2-T) all-perovskite tandem solar cells are a promising candidate for the next generation solar cells with ultrahigh efficiency and low fabrication cost. Here, we report on the fabrication of 2-T all-perovskite tandem solar cells consisting of solution-processed wide-bandgap and low-bandgap perovskite subcells connected by a vacuum processed interconnection layer. We employed a bulk-passivation strategy to enlarge the grain size, increase carrier mobilities, reduce electronic disorder, and suppress trap-assisted recombination in the low bandgap mixed Sn-Pb perovskite films, which leads to improved performance of low bandgap perovskite solar cells with an absorber thickness over 750 nm. This advance in high-quality low-bandgap perovskite layers enables the fabrication of efficient 2-T all-perovskite solar cells with power conversion efficiencies of more than 21% and decent stability. Our approach provides a suitable pathway to realize ultra-high efficiency 2-T all-perovskite tandem solar cells.

Double junction tandem solar cells consisting of two absorbers with designed different band gaps show great advantage in breaking the Shockley-Queisser limit efficiency of single junction solar cell by differential absorption of sunlight in a wider range of wavelengths and reducing the thermal loss of photons. Owing to the advantages of adjustable band gap and low cost of perovskite cells, perovskite/crystalline silicon tandem solar cells have become a research hotspot in photovoltaics. We systematically review the latest research progress of perovskite/crystalline silicon tandem solar cells. Focusing on the structure of perovskite top cells, intermediate interconnection layers and crystalline silicon bottom cells, we summarize the design principles of high-efficiency tandem devices in optical and electrical aspects. We find that the optical and electrical engineering of each layer structure in perovskite/crystalline silicon tandem solar cells goes through the whole process of device preparation. We also summarize the challenges of limiting the further improvement of the efficiency of the perovskite/crystalline silicon tandem solar cells and the corresponding improvement measures, which covers the following respects: 1) Improving the balance between <i>V</i><sub>oc</sub> and <i>J</i><sub>sc</sub> of the broadband perovskite cell through additive engineering and interface engineering; 2) improving the bandgap matching between the electrical layers and reducing the carrier transport barrier through adjusting the work function or conductivity of layers; 3) improving the photocurrent coupling between sub-cells and the photocurrent of tandem solar cells by using light engineering and conformal deposition technology of perovskite cells. At present, there have been many technologies to improve the stability of perovskite solar cells, such as additive engineering and interface engineering, but the problem has hardly been solved. Therefore, improving the stability of broadband gap perovskite solar cells to the level of crystalline silicon solar cells will become an important challenge to limit its large-scale application. In terms of efficiency, the mass production efficiency of perovskite/crystalline silicon tandem solar cells is far lower than that of the laboratory level. One of the reasons is that it is difficult to achieve low-cost and deposition of uniform large area perovskite solar cells. Therefore improving the stability of broadband gap perovskite solar cells and developing low-cost large-area perovskite deposition technology will become extremely critical. Finally we look forward to the next generation of higher efficient low-cost tandem solar cells. We believe that with the increasing demand for higher efficiency photovoltaic devices, the triple junction solar cells based on the perovskite/crystalline silicon stack structure will become the future photovoltaics.

The interfacial contact is critical for the performance of perovskite solar cells (PSCs), leading to dense perovskite thin films and efficient charge transport. In this contribution, an effective interfacial treatment solution using polyethyleneimine (PEI) was developed to improve the performance and stability of PSCs. Inserting PEI between the s-VOx and perovskite layers can produce a high-energy hydrophilic surface to facilitate the formation of a high-quality perovskite layer by the solution method. Accordingly, the surface coverage of perovskite film on the s-VOx layer increased from 80% to 95%, and the PCE of the device improved from 12.06% (with an average of 10.16%) to 14.4% (with an average value of 12.8%) under an irradiance of 100 mW cm-2 AM 1.5G sunlight. More importantly, the stability of PSCs was further improved after adding another PEI layer between the electron transport layer and LiF/Al layer, less than 10% decay in efficiency during a 10-days observation. Since all layers of the PSCs were fabricated at low temperature (<150 °C), these PEI-treated PSCs based on the amorphous VOx layer have the potential to contribute significantly toward the development of efficient and stable solar cells on flexible substrates.

A simple off-the-shelf post-device ligand treatment is developed to simultaneously improve the performance and air stability of perovskite solar cells, as well as repair as-prepared ‘poor devices’ for the first time.