Chen Weijie

A highly‐transparent and true‐colored indoor light harvester should possess an absorbance, which is mismatched with the emission spectra of the light sources. Porphyrin‐based donor materials are potential candidates, and such a P2:PC71BM cell achieves a power conversion efficiencies (PCE) exceeding 10%, and an illuminance transparency ≈65%, while preserving the color perception of the light sources.

Abstract

Organic photovoltaic (OPV) cells are promising indoor light energy harvesters because organic materials absorb strongly in the visible range. An indoor photovoltaic (IPV) device is an effective tool for the remote off‐grid wireless charging. However, as the indoor light fluxes are much weaker than the 1‐Sun condition, high‐performance indoor cells should have large areas in order to generate appreciable energies. They would then appear as flat, but expansive and dark objects if deployed indoors. Their presence would then alter the indoor lighting environment and affect visual perceptions. This work addresses the lighting and perception issues of IPV cells in three ways. i) A concept is proposed such that a high‐efficiency, semitransparent indoor OPV cell should possess an absorbance which is mismatched with the emission spectra of the light sources. ii) It is demonstrated that bulk heterojunction (BHJ) OPV solar cells with porphyrin donors can serve as high‐transparency and high‐efficiency indoor light harvesters. iii) Quantitative assessment criteria are presented for the transparency and chromaticity of an indoor semitransparent OPV cell, demonstrating that porphyrin‐based P2:PC₇₁BM semitransparent BHJ cells can achieve a power conversion efficiency (PCE) exceeding 10%, and an illuminance transparency ~65%, while preserving the color perception of the light sources.

Transient photovoltage (TPV) experiments are performed on perovskite solar cells with different defect concentrations. The voltage decay time does not represent recombination dynamics but an RC‐limited charging and discharging process. Devices with inhomogeneous generation and recombination profiles can show a pronounced double‐exponential behavior. Drift‐diffusion device simulations reproduce these results and show the relevance of transport limitations in TPV.

Abstract

In all kinds of solar cells, transient photovoltage (TPV) decay measurements have been used to determine charge carrier lifetimes and to quantify recombination processes and orders. However, in particular, for thin‐film devices with a high capacitance, the time constants observed in common TPV measurements do not describe recombination dynamics but RC (R : resistance, C : capacitance) times for charging the electrodes. This issue has been revisited for organic and perovskite solar cells in the recent literature. Here, these discussions are extended by analyzing a perovskite model system (Bi defects in Cs_0.1FA_0.9Pb(Br_0.1I_0.9)₃ in which defect recombination can be tuned. It is found that TPV, intensity‐modulated photovoltage spectroscopy, and impedance spectroscopy yield the same time constants that do not describe recombination dynamics but are limited by the differential resistance of the diode and the geometric capacitance in common light intensity ranges (<1 sun). By employing numerical device simulations, it is found that low charge carrier mobility can furthermore limit the TPV time constants. In samples with spatially nonuniform recombination dynamics, two time constants are measured, which depend on the charge carrier generation profile that can be tuned by the wavelength of the incident light. In that case, numerical simulation provides insights into recombination and charge transport processes in the device.

Achieving uniform all‐inorganic halide perovskites (AIHP) with large size and ultrathin thickness simultaneously is a challenge. A novel deposition strategy is proposed where AIHP grows in a confined space to significantly enhance the growth stability. AIHP of ≈30 µm lateral length and ≈10 nm thickness are fabricated, showing high quality comparable to the reported top‐level AIHP.

Abstract

2D all‐inorganic halide perovskites (2D AIHP) have attracted huge attention for their excellent optoelectronic properties and superior atmospheric stability for organic–inorganic halide perovskites. 2D AIHP has been fabricated by varied strategies, whereas a satisfied way to simultaneously reach good quality, dimensions, and uniformity is still far from achieved yet. Here, a direct thermal deposition method to synthesize 2D AIHP of lateral length up to ≈30 µm and uniform thickness down to ≈10 nm, which shows excellent optoelectronic performance comparable to the reported top‐level 2D AIHP, is proposed. A vertical mass transport instead of conventional horizontal mode is adopted, significantly enhancing the deposition stability. Furthermore, the morphologies of AIHP are switchable between nanowires and nanoflakes under the control of deposition temperature. This method may pave the way for uniform fabrication of 2D AIHP or AIHP‐based 2D heterostructures for various applications.

The thermal stability and long‐term illumination stability both in N₂ and in the ambient condition of the perovskite solar cells are simultaneously improved using a 2,3,4,5,6‐pentafluorobenzoyl chloride‐capped ZnO as a modifier of PCBM electron‐transporting layer (ETL).

Stability is a big issue for the commercialization of perovskite solar cells. The degradation of perovskite solar cells is a complex physical–chemical process related to the photoactive layer, the interface layer, and the metal electrode. Zinc oxide (ZnO) is a popular material used as the electron‐transporting layer (ETL) in perovskite solar cells. A major problem of the ZnO ETL for perovskite solar cells is the thermal instability caused by the chemical reaction between ZnO and perovskite layer. Aiming to solve the degradation issue of perovskite solar cells a kind of ZnO nanoparticle that is chemically tailored with 2,3,4,5,6‐pentafluorobenzoyl chloride (ZnO@PFBC) is provided. Herein, the migration of halogen and zinc that takes responsibility for the thermal degradation of the p–i–n‐type perovskite solar cells through time of flight secondary‐ion mass spectrometry (ToF–SIMS) and X‐ray photoelectron spectroscopy (XPS) results is proved. Using ZnO@PFBC as a modifier of PC₆₁BM, both the ion migration and the chemical reaction of ZnO and perovskite are suppressed. The thermal stability and long‐term illumination stability both in N₂ and in ambient conditions are simultaneously improved, 75% of the initial efficiency remaining after 200 h of annealing at 85 °C.

A novel bimodal silver nanowire (AgNW) electrode comprising AgNWs of two different aspect ratios is produced to achieve superior optical and electrical performance. Semitransparent organic solar cells based on the bimodal AgNW top electrodes obtain an enhanced power conversion efficiency with high average visible transmittance. The ST‐devices also demonstrate an enhanced reproducibility and excellent color‐rendering index.

Semitransparent organic photovoltaics (ST‐OPVs) provide a potentially facile route for some applications in building integrated photovoltaics. One of the challenges in developing large‐scale, printable ST‐OPVs is to address the need for high‐performance and fully solution‐processed top electrodes, allowing the replacement of the evaporated thin metallic films (Ag, Au, and Al). Silver nanowire (AgNW) is considered a promising candidate for the substitution due to its excellent transparency, conductivity, and solution processability. Herein, a novel bimodal AgNW (AgNW‐BM) electrode is reported, comprising AgNWs of two different aspect ratios. It is shown that the AgNW‐BM film achieves lower sheet resistance and higher visible transmittance than each monodisperse AgNW film, respectively. Furthermore, ST‐OPVs based on PTB7‐Th:IEICO‐4F with AgNW‐BM top electrodes are fabricated, which can obtain a maximum power conversion efficiency (PCE) of 7.49% with an average visible transmittance (AVT) of 33%. The ST‐devices also demonstrate an enhanced reproducibility and excellent color‐rendering index of 90. In addition, the bimodal top electrode is successfully implemented in the PM6:Y6 system with a higher PCE of 9.79% and with an AVT of 23%, demonstrating the universality for various semiconductor systems. Our work provides a simple strategy to realize fully solution‐processed, highly efficient ST‐OPVs.

Herein, quasi‐heteroface perovskite solar cells (QHF‐PSCs) are reported. Compared with normal PSCs, QHF‐PSCs have better carrier separation capabilities and effectively suppress the nonradiative recombination. Meanwhile, middle band gap perovskite layer is obtained by combining a wide band gap perovskite layer with a narrow band gap perovskite layer. This strategy points out a new avenue to further expand the application of perovskite materials.

Abstract

Perovskite solar cells (PSCs) have attracted unprecedented attention due to their rapidly rising photoelectric conversion efficiency (PCE). In order to further improve the PCE of PSCs, new possible optimization path needs to be found. Here, quasi‐heteroface PSCs (QHF‐PSCs) is designed by a double‐layer perovskite film. Such brand new PSCs have good carrier separation capabilities, effectively suppress the nonradiative recombination of the PSCs, and thus greatly improve the open‐circuit voltage and PCE. The root cause of the performance improvement is the benefit from the additional built‐in electric field, which is confirmed by measuring the external quantum efficiency under applied electric field and Kelvin probe force microscope. Meanwhile, an intermediate band gap perovskite layer can be obtained simply by combining a wide band gap perovskite layer with a narrow band gap perovskite layer. Tunability of the band gap is obtained by varying the film thicknesses of the narrow and wide band gap layers. This phenomenon is quite different from traditional inorganic solar cells, whose band gap is determined only by the narrowest band gap layer. It is believed that these QHF‐PSCs will be an effective strategy to further enhance PCE in PSCs and provide basis to further understand and develop the perovskite materials platform.

An in situ grazing‐incidence wide‐angle X‐ray scattering experiment is conducted to reveal the crystallization kinetics and formation mechanism of 2D perovskite films, during which additives play a key role in regulating the nucleation and growth process. For the dual additive processing case, a novel sandwich‐type structure is achieved, which can effectively passivate defects at dual interfaces, finally resulting in a high device efficiency of 16.48%.

Abstract

2D perovskite solar cells with high stability and high efficiency have attracted significant attention. A systematical static and dynamic structure investigation is carried out to show the details of 2D morphology evolution. A dual additive approach is used, where the synergy between an alkali metal cation and a polar solvent leads to high‐quality 2D perovskite films with sandwich‐type structures and vertical phase segregation. Such novel structure can induce high‐quality 2D slab growth and reduce internal and surface defects, resulting in a high device efficiency of 16.48% with enhanced continuous illumination stability and improved moisture (55–60%) and thermal (85 °C) tolerances. Transient absorption spectra reveal the carrier migration from low n to high n species with different kinetics. An [PbI₆]⁴⁻ octagon coalescence transformation mechanism coupled with metal and organic cations wrapped is proposed. By solvent vapor annealing, a recrystallization and reorientation of the 2D perovskite slabs occurs to form an ideal structure with improved device performance and stability.

Herein the morphology and exciton/charge carrier dynamics in bulk heterojunctions of the donor polymer PTQ10 and molecular acceptor IDIC are investigated. The results emphasize the potential for high material crystallinity to enhance charge separation and collection in organic solar cells, but also that long exciton diffusion lengths are likely to be essential for efficient exciton separation in such high crystallinity devices.

Abstract

Herein the morphology and exciton/charge carrier dynamics in bulk heterojunctions (BHJs) of the donor polymer PTQ10 and molecular acceptor IDIC are investigated. PTQ10:IDIC BHJs are shown to be particularly promising for low cost organic solar cells (OSCs). It is found that both PTQ10 and IDIC show remarkably high crystallinity in optimized BHJs, with GIWAXS data indicating pi‐pi stacking coherence lengths of up to 8 nm. Exciton‐exciton annihilation studies indicate long exciton diffusion lengths for both neat materials (19 nm for PTQ10 and 9.5 nm for IDIC), enabling efficient exciton separation with half lives of 1 and 3 ps, despite the high degree of phase segregation in this blend. Transient absorption data indicate exciton separation leads to the formation of two spectrally distinct species, assigned to interfacial charge transfer (CT) states and separated charges. CT state decay is correlated with the appearance of additional separate charges, indicating relatively efficient CT state dissociation, attributed to the high crystallinity of this blend. The results emphasize the potential for high material crystallinity to enhance charge separation and collection in OSCs, but also that long exciton diffusion lengths are likely to be essential for efficient exciton separation in such high crystallinity devices.

Solar cells based on 300 nm thick MAPbI₃ perovskite monocrystal are prepared, which show 3% enhancement in power conversion efficiency (PCE) compared to their polycrystalline counterparts. The suppressed charge recombination loss due to the reduction of grains and grain boundaries is believed to be the main reason for the PCE improvement.

Abstract

Grains and grain boundaries play key roles in determining halide perovskite‐based optoelectronic device performance. Halide perovskite monocrystalline solids with large grains, smaller grain boundaries, and uniform surface morphology improve charge transfer and collection, suppress recombination loss, and thus are highly favorable for developing efficient solar cells. To date, strategies of synthesizing high‐quality thin monocrystals (TMCs) for solar cell applications are still limited. Here, by combining the antisolvent vapor‐assisted crystallization and space‐confinement strategies, high‐quality millimeter sized TMCs of methylammonium lead iodide (MAPbI₃) perovskites with controlled thickness from tens of nanometers to several micrometers have been fabricated. The solar cells based on these MAPbI₃ TMCs show power conversion efficiency (PCE) of 20.1% which is significantly improved compared to their polycrystalline counterparts (PCE) of 17.3%. The MAPbI₃ TMCs show large grain size, uniform surface morphology, high hole mobility (up to 142 cm² V⁻¹ s⁻¹), as well as low trap (defect) densities. These properties suggest that TMCs can effectively suppress the radiative and nonradiative recombination loss, thus provide a promising way for maximizing the efficiency of perovskite solar cells.

CsPbI₃ perovskite quantum dot (PQD) hybrid nonfullerene organic solar cells are fabricated. The devices based on a PTB7‐Th:FOIC blend with PQDs yield higher efficiency of 13.2% even at near‐zero driving force than that without PQDs (11.6%). Incorporation of PQDs also leads to efficiency enhancement from 15.4% to 16.6% for a PM6:Y6 blend.

Abstract

To take advantages of the intense absorption and fluorescence, high charge mobility, and high dielectric constant of CsPbI₃ perovskite quantum dots (PQDs), PQD hybrid nonfullerene organic solar cells (OSCs) are fabricated. Addition of PQDs leads to simultaneous enhancement of open‐circuit voltage (V _OC), short‐circuit current density (J _SC), and fill factor (FF); power conversion efficiencies are boosted from 11.6% to 13.2% for PTB7‐Th:FOIC blend and from 15.4% to 16.6% for PM6:Y6 blend. Incorporation of PQDs dramatically increases the energy of the charge transfer state, resulting in near‐zero driving force and improved V _OC. Interestingly, at near‐zero driving force, the PQD hybrid OSCs show more efficient charge generation than the control device without PQDs, contributing to enhanced J _SC, due to the formation of cascade band structure and increased molecular ordering. The strong fluorescence of the PQDs enhances the external quantum efficiency of the electroluminescence of the active layer, which can reduce nonradiative recombination voltage loss. The high dielectric constant of the PQDs screens the Coulombic interactions and reduces charge recombination, which is beneficial for increased FF. This work may open up wide applicability of perovskite quantum dots and an avenue toward high‐performance nonfullerene solar cells.

Tandem solar cells hold promise for breaking the second law of thermodynamics and Shockley–Queisser limits. So far, such devices have to be made via costly methods. The advent of perovskite‐based absorbers enables the fabrication of various tandem devices through low‐cost techniques by combination with different subcells.

Abstract

Tandem solar cells (TSCs) comprising stacked narrow‐bandgap and wide‐bandgap subcells are regarded as the most promising approach to break the Shockley–Queisser limit of single‐junction solar cells. As the game‐changer in the photovoltaic community, organic–inorganic hybrid perovskites became the front‐runner candidate for mating with other efficient photovoltaic technologies in the tandem configuration for higher power conversion efficiency, by virtue of their tunable and complementary bandgaps, excellent photoelectric properties, and solution processability. In this review, a perspective that critically dilates the progress of perovskite material selection and device design for perovskite‐based TSCs, including perovskite/silicon, perovskite/copper indium gallium selenide, perovskite/perovskite, perovskite/CdTe, and perovskite/GaAs are presented. Besides, all‐inorganic perovskite CsPbI₃ with high thermal stability is proposed as the top subcell in TSCs due to its suitable bandgap of ≈1.73 eV and rapidly increasing efficiency. To minimize the optical and electrical losses for high‐efficiency TSCs, the optimization of transparent electrodes, recombination layers, and the current‐matching principles are highlighted. Through big data analysis, wide‐bandgap perovskite solar cells with high open‐circuit voltage (V _oc) are in dire need in further study. In the end, opportunities and challenges to realize the commercialization of TSCs, including long‐term stability, area upscaling, and mitigation of toxicity, are also envisioned.

In this paper, laboratory and rooftop performance of perovskite solar cells under changing temperature and irradiance is analyzed. By integrating laboratory data trends and measured weather data into optical energy yield model, the temperature‐dependent energy yield model is developed and validated, and can be used to predict generated energy of perovskite solar cells or track their degradation during field testing.

Abstract

Perovskite solar cells (PSC) have shown that under laboratory conditions they can compete with established photovoltaic technologies. However, controlled laboratory measurements usually performed do not fully resemble operational conditions and field testing outdoors, with day‐night cycles, changing irradiance and temperature. In this contribution, the performance of PSCs in the rooftop field test, exposed to real weather conditions is evaluated. The 1 cm² single‐junction devices, with an initial average power conversion efficiency of 18.5% are tracked outdoors in maximum power point over several weeks. In parallel, irradiance and air temperature are recorded, allowing us to correlate outside factors with generated power. To get more insight into outdoor device performance, a comprehensive set of laboratory measurements under different light intensities (10% to 120% of AM1.5) and temperatures is performed. From these results, a low power temperature coefficient of −0.17% K⁻¹ is extracted in the temperature range between 25 and 85 °C. By incorporating these temperature‐ and light‐dependent PV parameters into the energy yield model, it is possible to correctly predict the generated energy of the devices, thus validating the energy yield model. In addition, degradation of the tested devices can be tracked precisely from the difference between measured and modelled power.

Solid‐state ¹⁹F magic angle spinning nuclear magnetic microscopy and elemental mapping are introduced to probe the structures of ternary and quaternary blends. The presence of the individual guest paths minimizes the influence on charge generation and transport of the host system, allowing cooperation of the parallel‐like subcells, producing impressive 17.2% efficiency via a quaternary strategy.

Abstract

Ternary strategies show over 16% efficiencies with increased current/voltage owing to complementary absorption/aligned energy level contributions. However, poor understanding of how the guest components tune the active layer structures still makes rational selection of material systems challenging. In this study, two phthalimide based ultrawide bandgap polymer donor guests are synthesized. Parallel energies between the highest occupied molecular orbitals of host and guest polymers are achieved via incorporating selnophene on the guest polymer. Solid‐state ¹⁹F magic angle spinning nuclear magnetic spectroscopy, graze‐incidence wide‐angle X‐ray diffraction, elemental transmission electron microscopy mapping, and transient absorption spectroscopy are combined to characterize the active layer structures. Formation of the individual guest phases selectively improves the structural order of donor and acceptor phase. The increased electron mobility in combination with the presence of the additional paths made by the guest not only minimizes the influence on charge generation and transport of the host system but also contributes to increasing the overall current generation. Therefore, phthalimide based polymers can be potential candidates that enable the simultaneous increase of open‐circuit voltage and short‐circuit current‐density via fine‐tuning energy levels and the formation of additional paths for enhancing current generation in parallel‐like multicomponent organic solar cells.

Rapid degradation of ion migration, measurement source‐induced damage, phase transition, and separation of perovskite materials hinder accurate evaluation by conventional characterization tools. Recent advanced characterization tools, such as cryogenic temperature assisted measurement, in situ observation, and multidimensional imaging/mapping are presented here that enable the correct diagnose perovskite properties.

Abstract

In the last 10 years, organic–inorganic hybrid perovskite solar cells have achieved unprecedented advances, to the point where they now exhibit extremely high efficiency. However, long‐term stability and areal scalability limitations impede the commercial application of perovskite materials, and appropriate diagnosistic tools have become necessary to evaluate perovskite materials. Characterization of perovskite materials is regularly misinterpretated, due to unique intrinsic and extrinsic factors: degradation from the measurement source, ion migration, phase transition, and separation. Herein, studies on perovskites are reviewed that have used advanced characterization tools to overcome characterization challenges. Cryogenic temperature assisted measurements mitigate degradation or phase transitions induced by the measurement source. In situ measurements can track the variation of perovskite materials depending on external stimuli. Spatial material properties are able to be evaluated by the use of multidimensional mapping techniques. An overview of these advanced characterization tools that can overcome the challenges associated with established tools provides the opportunity for further understanding perovskite materials and solving the remaining challenges on the road to commercialization.

Nature Energy, Published online: 20 July 2020; doi:10.1038/s41560-020-0652-3

Nature Energy, Published online: 20 July 2020; doi:10.1038/s41560-020-0653-2

The emergence of nonfullerene electron acceptors has rejuvenated the field of organic photovoltaics, with device efficiencies over 18% and 20% in sight. In this essay, the basic properties of these new nonfullerene acceptors are discussed. Perspectives and suggestions for further research endeavors toward successful commercialization are also provided.

Abstract

Efficient, low‐cost, and low‐embodied energy photovoltaics are key enablers of the global decarbonization agenda. In addition to the market‐leading crystalline silicon technology, several other promising candidates are under active investigation with the perovskites leading the way with single‐junction efficiencies exceeding 25% at the lab‐scale. So‐called organic photovoltaics (solar cells based upon organic semiconductors), particularly those that can be solution processed, have long promised the Nirvana of ultralow cost and very short energy payback times. However, relatively low efficiencies, poor long‐term stability, and issues with manufacturing at scale have so far prevented truly meaningful commercialization of the technology. The recent emergence of the so‐called nonfullerene electron acceptors is potentially about to shift this dynamic—they have delivered a step change in performance in a relatively short period of time. In this Essay, the basic properties of these new materials, their pros and cons, what we know and what we do not know are explored.

A highly efficient organic solar cell with a ternary architecture is successfully demonstrated by enhancing and balancing charge transport as well as matching integer charge transfer energy in a bulk heterojunction blend. As a result, a power conversion efficiency of 17.13% is obtained with the significantly improved fill factor of 0.813.

Abstract

Ternary architecture is one of the most effective strategies to boost the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, an OSC with a ternary architecture featuring a highly crystalline molecular donor DRTB‐T‐C4 as a third component to the host binary system consisting of a polymer donor PM6 and a nonfullerene acceptor Y6 is reported. The third component is used to achieve enhanced and balanced charge transport, contributing to an improved fill factor (FF) of 0.813 and yielding an impressive PCE of 17.13%. The heterojunctions are designed using so‐called pinning energies to promote exciton separation and reduce recombination loss. In addition, the preferential location of DRTB‐T‐C4 at the interface between PM6 and Y6 plays an important role in optimizing the morphology of the active layer.

The potential for commercialization of organic solar cells (OSCs) has vastly increased in recent years as the device efficiency for small scale laboratory OSCs has been continuously increased. There are, however, still multiple challenges that needs to be addressed and overcome. Among them, upscaling of the device manufacturing techniques to be compatible with the potential attributes of low‐cost must be the pinnacle. Herein, we present a pathway for upscaling with an ink engineering approach toward in‐air optimization of large area OSCs. The optimized flexible indium tin oxide (ITO)‐free OSCs based on PTB7‐TH:IEICO‐4F:PC₇₁BM ternary blend show efficiencies up to 10.2 % (device active area 0.88 cm²), which is the highest value reported to date (for in‐air slot‐die coated devices). This was achieved through ink modifications and optimizations as well as electrode and active layer compositional optimizations, leading to an impressive efficiency retention of 0.86 compared to the in‐literature optimized small‐scale devices.

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The strategy of preparing large quantities of high‐photovoltaic‐performance electron transporting layers (ETLs) at low temperatures (<100 °C), including at room temperature, is an important step toward commercializing low‐cost, high‐efficiency perovskite solar cells (PSCs). Low‐temperature‐processed TiO₂ (Lt‐TiO₂) ETLs possess low conductivity and connectivity, resulting in poor phorovoltaic performance. Herein, an EtOH‐soluble, highly conducting fullerene derivative, C₆₀RT₆, which was designed and synthesized as an additive for Lt‐TiO₂ ETLs, is reported. A nanocomposite (Fu/Lt‐TiO₂) ETL was prepared simply by spin‐coating a C₆₀RT₆ and G‐TiO₂ NP (TiO₂ nanoparticles prepared by grounding the bulk TiO₂ powder at room temperature) mixture. Besides raising the valance band and conduction band of the Lt‐TiO₂ ETL to be better aligned with the frontier orbitals of the FA_xMA_1‐xPbI₃ (Psk), the C₆₀RT₆ additive improves the continuity, conductivity, flatness, and surface hydrophilicity of the Lt‐TiO₂ film. Psk films spin‐coated on more hydrophilic Fu/Lt‐TiO₂ ETLs also have slightly larger grains and thickness compared to those deposited on the less‐hydrophilic Lt‐TiO₂; therefore, electron extraction is enhanced and charge recombination may be suppressed at the Psk/ETL interface or in the bulk Psk. PSCs based on room‐temperature‐processed R‐Fu/Lt‐TiO₂ ETL possess higher power conversion efficiency (PCE, up to 20% on glass substrate), less (negligible) current hysteresis, and better long‐term stability compared to that (having a high PCE of approximately 16% with serious current hysteresis) used temperature‐processed R‐Lt‐TiO₂ as an ETL The merit of the C₆₀RT₆ additive for the Lt‐TiO₂ ETL is more significant when applied to a flexible Indium tin oxide/polyethylene terephthalate (ITO/PET) substrate. The flexible PSC with an R‐Fu/Lt‐TiO₂ ETL achieves a PCE of 18.06% and retains 90% of the initial PCE after 500 bending cycles with a bending radius of 6 mm. In contrast, the PCE of the flexible cell with a Lt‐TiO₂ ETL is only 8.2%, and loses 60% of the initial value after 500 bending cycles.

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As one of the most promising hole transporting materials for perovskite solar cell, NiO is widely used in the inverted p‐i‐n cell structure due to its high stability, decent hole‐conductivity, and easy processability for hysteresis‐free cells. However, the efficiency of NiO‐based perovskite solar cell is still low, due largely to the poor perovskite/NiO interface. Herein, we introduce a sulfur‐doping strategy to modify NiO surface via ion exchange reaction by a simple and scalable chemical bath deposition technique, which greatly improves the photovoltaic performance of the derived devices. A systematic investigation has shown that sulfur doping leads to favorable interfacial energetics with reduced V_oc loss. Sulfur doping at the interface also improved the contact between NiO and perovksite and facilitated the formation of high‐quality perovskite films. Carrier dynamics studies demonstrate reduced defect states and trap‐assisted recombination with sulfur doping, which promote photovoltaic performance of the devices. These merits contribute concurrently to low‐loss charge transfer across the perovskite/NiO interface and facilitate charge transport through the perovskite films, leading to a high champion efficiency at 20.43% of the p‐i‐n structure solar cell devices.

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