To extract charges more efficiently through charge-transporting layers (CTLs), various dopants are necessary. Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) is the most widely used dopant in electron- and hole-transporting layers. However, Li+ ions easily migrate into the perovskite and deteriorate the device performance. To address this issue, several efforts such as introducing a buffer layer have been tried, but the issue is still not fully resolved. Thus it is required to find a simple way without additional treatments. In this work, we propose a simple strategy to use defect-tolerant dopant in CTLs, sodium bis(trifluoromethanesulfonyl)imide (Na-TFSI), to improve both the efficiency and the stability of perovskite solar cells (PSCs). The PSCs with Na-TFSI for both the electron-transport layer and the hole-transport layer show the highest power conversion efficiency up to 22.4%. In addition, the device with Na-TFSI exhibited better long-term operating stability at 45 °C, maintaining >80% of the initial performance even after 500 h of continuous 1 sun illumination.

　For commercializing perovskite solar cells (PSCs), moisture-tolerant materials are required because a moisture-free environment cannot be maintained on an actual production line (large scale). Recently, PSCs with efficiency exceeding 22% have been fabricated using Li-doped mesoporous TiO2 as an electron transport layer (ETL). However, the use of Li can negatively influence device stability during the fabrication process under humid air because of its hydroscopic property. Here, we report a strategy for improving processing stability without sacrificing the power conversion efficiency (PCE) under a humid atmospheric environment by employing a mesoporous BaSnO3 as an ETL. Using the mesoporous BSO ETL, we achieved a certified efficiency of 21.3% and stabilized efficiency of 21.7%. Furthermore, the BSO-based PSCs also exhibited better processing stability than Li-doped TiO2-based PSCs under humid air. We believe that this strategy of introducing BSO into PSCs will accelerate the commercialization of PSCs.

　Perovskite solar cells (PSCs) with mesoporous TiO2 (mp-TiO2) as the electron transport material attain power conversion efficiencies (PCEs) above 22%; however, their poor long-term stability is a critical issue that must be resolved for commercialization. Herein, it is demonstrated that the long-term operational stability of mp-TiO2 based PSCs with PCE over 20% is achieved by isolating devices from oxygen and humidity. This achievement attributes to systematic understanding of the critical role of oxygen in the degradation of PSCs. PSCs exhibit fast degradation under controlled oxygen atmosphere and illumination, which is accompanied by iodine migration into the hole transport material (HTM). A diffusion barrier at the HTM/perovskite interface or encapsulation on top of the devices improves the stability against oxygen under light soaking. Notably, a mp-TiO2 based PSC with a solid encapsulation retains 20% efficiency after 1000 h of 1 sun (AM1.5G including UV) illumination in ambient air.

　Recent advances in perovskite solar cells (PSCs) have resulted in greater than 23% efficiency with superior advantages such as flexibility and solution-processability, allowing PSCs to be fabricated by a high-throughput and low-cost roll-to-roll (R2R) process. The development of scalable deposition processes is crucial to realize R2R production of flexible PSCs. Gravure printing is a promising candidate with the benefit of direct printing of the desired layer with arbitrary shape and size by using the R2R process. Here, flexible PSCs are fabricated by gravure printing. Printing inks and processing parameters are optimized to obtain smooth and uniform films. SnO2 nanoparticles are uniformly printed by reducing surface tension. Perovskite layers are successfully formed by optimizing the printing parameters and subsequent antisolvent bathing. 2,2′,7,7′-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene is also successfully printed. The all-gravure-printed device exhibits 17.2% champion efficiency, with 15.5% maximum power point tracking efficiency for 1000 s. Gravure-printed flexible PSCs based on a two-step deposition of perovskite layer are also demonstrated. Furthermore, a R2R process based on the gravure printing is demonstrated. The champion efficiency of 9.7% is achieved for partly R2R-processed PSCs based on a two-step fabrication of the perovskite layer.

　Optical films with antireflective and self-cleaning surfaces have much potential for applications in solar cells, architectural glasses, and outdoor displays. Here we demonstrate the fabrication of novel self-cleaning antireflection (AR) thin films by depositing plasma-polymerized fluorocarbon (PPFC) which is fluorinated polymer consisting carbon and fluorine formed under plasma environment on Nb2O5/SiO2/Nb2O5 (NSN) trilayers using mid-range frequency power source in a continuous roll-to-roll sputtering system. The reflectance of PPFC/NSN films with a PPFC thickness of 70 nm was 1.71% at a wavelength of 528 nm, and the PPFC/NSN films showed low reflectance in a wide range in the visible region. The PPFC/NSN AR films exhibited a water-repelling surface with water contact angle of 105° after the application of a top fluorocarbon layer with low surface energy. We tested and confirmed the AR and self-cleaning functions of the PPFC/NSN films through the incorporation to perovskite solar cells (PSC). The short-circuit current density and power conversion efficiency of PPFC/NSN/HC-PET/PSC were 20.6 mA cm−2 and 17%. By attaching a self-cleaning AR films to both side of PSC, photocurrent collection of the device was improved and applicability of protective films to PSC was demonstrated.

　Perovskite solar cells typically comprise electron- and hole-transport materials deposited on each side of a perovskite active layer. So far, only two organic hole-transport materials have led to state-of-the-art performance in these solar cells1: poly(triarylamine) (PTAA)2,3,4,5 and 2,2ʹ,7,7ʹ-tetrakis(N,N-di-p-methoxyphenylamine)-9,9ʹ-spirobifluorene (spiro-OMeTAD)6,7. However, these materials have several drawbacks in terms of commercialization, including high cost8, the need for hygroscopic dopants that trigger degradation of the perovskite layer9 and limitations in their deposition processes10. Poly(3-hexylthiophene) (P3HT) is an alternative hole-transport material with excellent optoelectronic properties11,12,13, low cost8,14 and ease of fabrication15,16,17,18, but so far the efficiencies of perovskite solar cells using P3HT have reached only around 16 per cent19. Here we propose a device architecture for highly efficient perovskite solar cells that use P3HT as a hole-transport material without any dopants. A thin layer of wide-bandgap halide perovskite is formed on top of the narrow-bandgap light-absorbing layer by an in situ reaction of n-hexyl trimethyl ammonium bromide on the perovskite surface. Our device has a certified power conversion efficiency of 22.7 per cent with hysteresis of ±0.51 per cent; exhibits good stability at 85 per cent relative humidity without encapsulation; and upon encapsulation demonstrates long-term operational stability for 1,370 hours under 1-Sun illumination at room temperature, maintaining 95 per cent of the initial efficiency. We extend our platform to large-area modules (24.97 square centimetres)—which are fabricated using a scalable bar-coating method for the deposition of P3HT—and achieve a power conversion efficiency of 16.0 per cent. Realizing the potential of P3HT as a hole-transport material by using a wide-bandgap halide could be a valuable direction for perovskite solar-cell research.

　Light-induced electron-spin-resonance (LESR) spin density in a perovskite solar cell (PSC) is ascribed to photo-induced charges accumulated at the interface of the perovskite and the hole transport layer (HTL). The LESR spin density was observed to be decreased as the temperature was lowered from room temperature to 170 K in a PSC; below ∼170 K, no LESR was found. Meanwhile, the photocurrent showed only an ∼25% decrease between room temperature and 170 K. The comparison of LESR in temperature-dependence with transient or steady photocurrents suggested that the decrease in photocurrent at the low temperatures stems mainly from trapping of delocalized holes at the interface of perovskite and HTL.

　The energy level alignment of the perovskite and hole transporting materials (HTMs) is essential for increasing the open-circuit voltage (Voc) and enhancing the performance of perovskite solar cells (PSCs). In this work, new sequentially fluorinated poly(triarylamine) polymers (PTAA, 1F-PTAA, and 2F-PTAA) with tuned highest occupied molecular orbital (HOMO) energy levels are developed and applied as HTMs into PSCs. The fluorination approach successfully leads to stepwise downshifting of the HOMO levels of PTAA derivatives, resulting in an obvious increase in the Voc and power conversion efficiency (PCE) of the PSCs. In particular, introduction of 1F-PTAA polymer in (FAPbI3)0.85(MAPbBr3)0.15-based mesoporous n-i-p structure PSCs achieves the high stabilized PCE of 21.2% at the maximum power point with improved Voc of 1.14 V. To elucidate the importance of the optimized degree of fluorination of PTAA polymers on the photovoltaic performances, the optical, electrical, photophysical properties, and doping behaviors of the fluorinated PTAA derivatives are investigated.

　Perovskite solar cells (PSCs) require both high efficiency and good long-term stability if they are to be commercialized. It is crucial to finely optimize the energy level matching between the perovskites and hole-transporting materials to achieve better performance. Here, we synthesize a fluorene-terminated hole-transporting material with a fine-tuned energy level and a high glass transition temperature to ensure highly efficient and thermally stable PSCs. We use this material to fabricate photovoltaic devices with 23.2% efficiency (under reverse scanning) with a steady-state efficiency of 22.85% for small-area (~0.094 cm2) cells and 21.7% efficiency (under reverse scanning) for large-area (~1 cm2) cells. We also achieve certified efficiencies of 22.6% (small-area cells, ~0.094 cm2) and 20.9% (large-area, ~1 cm2). The resultant device shows better thermal stability than the device with spiro-OMeTAD, maintaining almost 95% of its initial performance for more than 500 h after thermal annealing at 60 °C.

　The presence of excess lead iodide in halide perovskites has been key for surpassing 20% photon-to-power conversion efficiency. To achieve even higher power conversion efficiencies, it is important to understand the role of remnant lead iodide in these perovskites. To that end, we explored the mechanism facilitating this effect by identifying the impact of excess lead iodide within the perovskite film on charge diffusion length, using electron-beam-induced current measurements, and on film formation properties, from grazing-incidence wide-angle X-ray scattering and high-resolution transmission electron microscopy. Based on our results, we propose that excess lead iodide in the perovskite precursors can reduce the halide vacancy concentration and lead to formation of azimuthal angle-oriented cubic α-perovskite crystals in-between 0° and 90°. We further identify a higher perovskite carrier concentration inside the nanostructured titanium dioxide layer than in the capping layer. These effects are consistent with enhanced lead iodide-rich perovskite solar cell performance and illustrate the role of lead iodide.

　Methylammonium lead iodide perovskites have limited practical applications due to a lack of stability under operating conditions. Environmental stability under heating has not yet been achieved, although recent studies modifying perovskites with long organic cations have reported the progress of stabilisation under humid conditions. In this work, we report the structural evolution of long alkylammonium-modified MAPbI3 and its functions for highly efficient and stable solar cells. As an encapsulating agent, octylammonium (OA) cation produced individual MAPbI3 grains in full armour without the formation of layered structures in contrast to butylammonium (BA) and phenethylammonium (PEA) cations. Our OA-armoured MAPbI3 achieved a stabilised power conversion efficiency of 20.1% without a deterioration of charge transport properties due to highly preferential orientation suppressing non-radiative recombination. The structural features also led to a much improved thermal stability at 85 °C in ambient atmosphere retaining 80% of the initial efficiency after 760 h without any encapsulation, as well as water tolerance. This work addresses widespread concerns associated with the photovoltaic efficiency and stability of MAPbI3 by exploring the inter-relationship between structural features and their functions in surfactant-modified perovskites.

　As the efficiency of perovskite solar cells (PSCs) reached more than 22%, the large-area fabrication of PSCs became another issue receiving growing attention. For large-area PSCs, more reproducibility is required to precisely control the crystallization behavior of perovskites. A two-step process has been preferred to apply large-area coatings of perovskite because of its better reproducibility, but the process has suffered from slow and incomplete conversion of PbI2 to perovskite. In this paper, we propose a fast, simple, two-step method—mediator extraction treatment (MET)—for the preparation of a high-quality perovskite film. In MET, a pre-deposited PbI2–DMSO complex film is converted into a peculiar PbI2 film with a porous morphology and unusual crystallographic orientation via the removal of DMSO. PbI2 could be completely converted into MAPbI3 by a fast reaction with MAI molecules. We demonstrate that this MET process in MAPbI3-based PSCs can achieve 18.8% of the maximum power conversion efficiency (PCE) using spin-coating, and 18.3% of the maximum PCE using slot-die coating with a uniform distribution in a 10 ×10 cm2 substrate at a laboratory scale. Moreover, over 18% of PCE could be achieved in only 100 s, and with room-temperature processing.