Since the first publication by Miyasaka in 2009 on the use of lead halide perovskite as a light-harvesting material (Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc.2009, 131, 6050), unprecedented successes have been achieved and great efforts have been made in the field of perovskite solar cells (PSCs) to push the power conversion efficiency (PCE) past 25%, which corresponds to ∼80% of this material’s theoretical bandgap limit determined on the basis of the Shockley–Queisser theory. Recent progress is mainly attributed to the development of key strategies that effectively reduce the defects on the surface of the perovskite layer and minimize non-radiative recombination at the interfaces, thereby enhancing device efficiency. For future development of PSCs with PCEs exceeding 26%, this Perspective highlights an investigation of the key factors that have allowed realization of efficient PSCs with state-of-the-art PCEs and includes a discussion of practical strategies, including full optimization of the electron-transport layer, minimization of defect loss related to non-radiative recombination, and enhancement of light-harvesting near the band-edge.

　The photovoltaic performance and scalability potential of a semitransparent perovskite solar cells (ST-PSCs) are primarily determined by the optoelectronic properties of the top transparent conducting electrode (TCE) used. Herein, we demonstrate the scalable fabrication of ST-PSC using a three-dimensional (3D) TCE consisting of (i) a sputtered amorphous indium-tin-oxide (a-ITO) film and (ii) silver (Ag) mesh subelectrodes prepared via a 3D direct-ink writing technique. At an optimized aspect ratio of 0.5, the a-ITO/Ag mesh 3D TCE exhibits a sheet resistance of < 1 Ω/□ and a transparency of ~85%. Utilizing the a-ITO/Ag mesh as a top contact, standard (0.07 cm2) and large-area (1.0 cm2) ST-PSCs achieved power conversion efficiencies (PCE) of 16.26% and 15.52%, respectively, with > 85% transmittance in the near-infrared region. Moreover, the ST-PSCs displayed superior ambient and thermal stability than the opaque PSCs due to the presence of a-ITO buffer that prevents moisture ingress and ions migration. Using ST-PSC as a top cell, the standard (0.07 cm2) and large-area (1.0 cm2) four-terminal ST-PSC/SiSC tandem cells achieved PCEs of 26.47% and 24.70%, respectively. To the best of our knowledge, our tandem cell showed the minimum efficiency roll-off among all the reported large-area tandem cells, manifesting the scalability potential of our ST-PSCs.

　To approach the theoretical efficiency of perovskite solar cells (PSCs), the defects in perovskites should be managed. Among different types of defects, halide vacancies easily form on the surface of perovskite grains (PGs), hindering perovskite stability and the charge-transport process by trapping charge carriers. In this work, oxidized black phosphorus quantum dots (O-BPQDs) are incorporated into a perovskite to resolve these issues. Oxygen atoms of the O-BPQDs interact with uncoordinated Pb (halide vacancies), forming grain interconnections. These interactions reduce halide vacancies and suppress the overall recombination kinetics. Along with defect reduction, the O-BPQDs offer an efficient charge-transport channel across individual PGs. We achieve a best power-conversion efficiency (PCE) of 22.34% for SnO2-based PSCs and of 23.1% for TiO2-based PSCs. These PSCs exhibit moisture stability in a relative humidity (RH) 40% environment comparable to 3D/2D perovskites. Our strategy provides practical applicability and versatility for PSCs to approach the theoretical PCE value.

　Perovskite solar cells (PSCs) are emerging next-generation photovoltaics, and some breakthroughs for the commercialization have been rapidly made. To develop the technologies for large-area modules, economically feasible fabrication using a roll-to-roll (R2R) solution process may be the ultimate target for the fabrication of PSCs. In order to achieve successful R2R production of PSCs, however, several issues still need to be addressed. Roll-based continuous and scalable deposition of perovskite and charge transporting layers on a flexible substrate needs to be developed to obtain high-quality R2R-processed PSCs. There are also critical factors involved in accomplishing R2R fabrication: heat treatment at low temperature and a short processing time over the whole process with industrial-compatible methods. We briefly discuss this perspective: scalable deposition of layers, considerations for the R2R process, and progress and challenges in the R2R fabrication of the PSCs.

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　This report addresses indium oxide doped with titanium and tantulum with high near-infrared transparency to potentially replace the conventional indium tin oxide transparent electrode used in semitransparent perovskite devices and top cells of tandem devices. The high near-infrared transparency of this electrode is possibly explained by the lower carrier concentration, suggesting less defect sites that may sacrifice its optical transparency. Incorporating this transparent electrode into semitransparent perovskite solar cells for both the top and bottom electrodes improved the device performance through possible reduction of interfacial defect sites and modification in energy alignment. With this indium oxide-based semitransparent perovskite top cell, we also demonstrated four-terminal perovskite–silicon tandem configurations with improved photocurrent response in the bottom silicon cell.

　Perovskite solar cells in an n-i-p structure record high power conversion efficiency, but issues of insufficient thermal stability and the high cost of p-type hole transporting materials have been raised as drawbacks. H2-phthalocyanine (Pc) is introduced as a hole transport material to ensure the thermal stability and simultaneously have served surface passivation effects on hybrid halide perovskites as a Lewis base. Pyrrolic nitrogen in the Pc reacts with uncoordinated Pb2+ ions on the perovskite surface. Upon enhancing the interfacial interaction between phthalocyanine and the perovskite, the open circuit voltage in devices increases as compared to that of devices using a metal-phthalocyanine complex. While the phthalocyanine-applied device maintains superior thermal long-term stability, the power conversion efficiency also exceeds 20%.

　Despite of enormous potential in niche applications, flexible perovskite solar cells (F-PSCs) still suffer from low power conversion efficiency (PCE) compared to rigid PSCs largely because of the reduced photocurrent driven by low transmittance of their flexible substrate and transparent electrode. This paper presents a plasma-polymerized-fluorocarbon (PPFC) thin film as an antireflection (AR) coating material for enhancing the efficiency of F-PSCs. The PPFC, which are deposited on flexible polyethylene terephthalate (PET) substrates at low temperature by a mid-range frequency sputtering process, are highly transparent because of their amorphous structure. Due to the very low refractive index (~ 1.38) of PPFC over a wide wavelength range, these thin films decrease the average reflectance by 2.22% and increase the average transmittance in the visible light region by 1.40%. As a result, the AR films increase the PCE of F-PSCs from 18.6% to 20.4%. The thin films have a self-cleaning effect due to their hydrophobic surface, excellent mechanical flexibility (bending radius: 4 mm), and high chemical stability. Accordingly, the thin films improve the long-term stability of F-PSCs in humid environments. Finally, these AR PPFC thin films can be manufactured by a roll-to-roll process, making them suitable for future use in highly efficient F-PSCs.

　Recent progress in highly efficient perovskite solar cells (PSCs) has been made by virtue of interfacial engineering on 3D perovskite surfaces for their defect control, however, the structural stability of the modified interface against external stimuli still remains unresolved. Herein, 4-dimethylaminopyridine (DMAP) is introduced to develop a facile technique for selectively passivating the grain boundary (GB) and controlling the topographical boundary of the perovskite surface near the GB. Through the surface treatment of DMAP, strongly bound DMAP crystals are selectively formed at the GB, which serves two functions: nonradiative recombination at GB is effectively reduced by healing the uncoordinated Pb2+ while adhesion strength between the perovskite and the poly(triaryl amine) (PTAA) polymer is significantly enhanced by a mechanical interlock effect. A planar PSC with DMAP treatment exhibits a champion power conversion efficiency of 22.4%, which is not only much higher than the 20.04% observed for a nontreated control device, but also the highest among the planar PSCs using PTAA polymers as a hole transport material. Furthermore, the use of DMAP leads to a substantial improvement in the device stability under damp-heat test and light irradiation.

　Metal halide perovskite solar cells (PSCs) are an emerging photovoltaic technology with the potential to disrupt the mature silicon solar cell market. Great improvements in device performance over the past few years, thanks to the development of fabrication protocols1,2,3, chemical compositions4,5 and phase stabilization methods6,7,8,9,10, have made PSCs one of the most efficient and low-cost solution-processable photovoltaic technologies. However, the light-harvesting performance of these devices is still limited by excessive charge carrier recombination. Despite much effort, the performance of the best-performing PSCs is capped by relatively low fill factors and high open-circuit voltage deficits (the radiative open-circuit voltage limit minus the high open-circuit voltage)11. Improvements in charge carrier management, which is closely tied to the fill factor and the open-circuit voltage, thus provide a path towards increasing the device performance of PSCs, and reaching their theoretical efficiency limit12. Here we report a holistic approach to improving the performance of PSCs through enhanced charge carrier management. First, we develop an electron transport layer with an ideal film coverage, thickness and composition by tuning the chemical bath deposition of tin dioxide (SnO2). Second, we decouple the passivation strategy between the bulk and the interface, leading to improved properties, while minimizing the bandgap penalty. In forward bias, our devices exhibit an electroluminescence external quantum efficiency of up to 17.2 per cent and an electroluminescence energy conversion efficiency of up to 21.6 per cent. As solar cells, they achieve a certified power conversion efficiency of 25.2 per cent, corresponding to 80.5 per cent of the thermodynamic limit of its bandgap.

　In semitransparent perovskite solar cells with n–i–p configuration, thermal evaporation is the common method to deposit the sputter buffer material, such as molybdenum oxide and tungsten oxide. Buffer layers are especially necessary when using organic hole transporting layers, as they are more susceptible to get damaged when sputtering the top transparent conducting oxide. However, there is a limited selection of possible materials and limited control of the materials properties by thermal evaporation, which leads to inefficient protection against sputtering and poor air stability. While there have been well-established buffer layers by atomic layer deposition, including tin oxide, for p–i–n structured semitransparent perovskite solar cells, this is not the case for n–i–p structured devices. Here, copper oxide is demonstrated by pulsed-chemical vapor deposition incorporated into perovskite solar cells for the sputter buffer layer, which result in stable encapsulated semitransparent devices maintaining over 95% of the maximum efficiency under AM 1.5 G at maximum power point tracking for 150 h without any temperature control.

　Perovskite solar cells (PSCs) have now achieved power conversion efficiencies (PCEs) over 25%, but their long-term stability under illumination and thermal stress is still a major barrier to commercialisation. Herein, we demonstrate the evaluation of light-induced degradation activation energy (Ea) of encapsulated semi-transparent PSCs by using the commonly employed method in crystalline silicon solar cells. Different parameters showed different activation energies where primary degradation is due to increase in series resistance, which also led to reduction in short-circuit current. Open-circuit voltage and shunt resistance also change with different Ea, suggesting the mechanism of the reduction is likely to be due to different reasons. Despite each parameter exhibiting slight variation over time for each temperature, the overall trend converges, indicating that each parameter is likely to be primarily reduced by a single dominant reaction. We also report the main cause of irreversible device degradation is not due to the decomposition of the perovskite layer as confirmed by X-ray diffraction characterisation. Instead, our pole figure map and absorption spectra analysis indicate that a loss of crystal symmetry occurs due to ion migration within the device that induce oxidation of 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD). Our work provides a better understanding through quantification of the degradation processes of encapsulated semi-transparent PSCs over time, which is essential for further progress and development of stable perovskite-Si tandem solar cells.