developed silicon solar cells. It shows the great potential of PSCs as the dominator of next-generation photovoltaics. Whereas, during the evolution of PSC development, the metal oxide electron transporting layer (ETL), as well as the ETL/perovskite interface, [11][12][13][14] has always been an issue in regard to photovoltaic efficiency and device stability. [8] Derived from dye-sensitized solar cells (DSSCs), the combination of compact and mesoporous TiO 2 has been commonly utilized as ETLs during the early studies of PSCs. [15] But a high sintering temperature of ≈500 °C is normally required for the fabrication process, which is high energy consumption and incompatible with scalable depositions on flexible substrates. Worse still, TiO 2 is highly photocatalytic active under ultraviolet (UV) irradiation that severely hampers the long-term stability of PSCs under illumination. [16][17][18][19][20][21][22] Later on, a new ETL, SnO 2 , has been developed as a better candidate, due to its low-temperature processability [23] and high sustainability under UV illumination. [24][25][26][27] More importantly, SnO 2 film has superior crystallinity and carrier mobility in comparison to TiO 2 . [28,29] Thus, a single layer of compact SnO 2 could enable an efficient charge transport and suppressed recombination losses at the ETL/perovskite interface. Benefiting from these advantages, PSCs based on SnO 2 as ETL have reached PCE of 25.2% to date. [30] Whereas, considerable amount of oxygen vacancies on the SnO 2 surface would act as deep traps to capture the photogenerated carriers, which causes hysteresis and instability of the device. [31][32][33][34] And this intrinsic defect of SnO 2 needs to be resolved for a further PCE breakthrough of PSCs.In recent years, significant attempts of defect-passivation have been made to decrease the oxygen vacancies and trap states on SnO 2 surface. [35][36][37][38] Among them, n-type fullerene derivatives represent one of the most studied and efficacious passivator, [39][40][41][42] due to the ease of forming coordinate bonds between carboxylate group and SnO 2 surface. In addition, fullerene derivative is a common electron acceptor in organic solar cells (OSCs), [43] which grants an effective electron extraction from the perovskite active layer to ETL, thus contributing to higher PSC performances. Nevertheless, it should be noticed that π-cage structures of fullerene derivatives are prone to self-aggregate, [44] which strongly affects the validity and SnO 2 has been universally applied as electron transporting layer (ETL) towards the fabrication of highly efficient perovskite solar cells (PSCs), owing to its unique advantages including low-temperature solution-processability, high optical, transmittance and good electrical conductivity. Uncoordinated Sn-dangling bonds on SnO 2 surface exist as deep traps to capture the photogenerated carriers, causing hysteresis and device instability. Fullerene derivatives, though being widely utilized as the passivator for SnO 2 , are highly prone to...

Formamidinium lead triiodide (FAPbI 3 ) has been demonstrated as the most efficient perovskite system to date, due to its excellent thermal stability and an ideal bandgap approaching the Shockley-Queisser limit. Whereas, there are intrinsic quantum confinement effects in FAPbI 3 , which lead to unwanted non-radiative recombination. Additionally, the black α-phase of FAPbI 3 is unstable under room temperature due to the significant residual tensile stress in the film. To simultaneously address the above issues, a thermallyactivated delayed fluorescence polymer P1 is designed in the study to modify the FAPbI 3 film. Owing to the spectral overlap between the photoluminescence of P1 and absorption of the above-bandgap quantum wells of FAPbI 3 , the Förster energy transfer occurs at the P1/FAPbI 3 interface, which further triggers the Dexter energy transfer within FAPbI 3 . The exciton "recycling" can thus be realized, which reduces the non-radiative recombination losses in perovskite solar cells (PSCs). Moreover, P1 is found to introduce compressive stress into FAPbI 3 , which relieves the tensile stress in perovskite. Consequently, the PSCs with P1 treatment achieve an outstanding power conversion efficiency (PCE) of 23.51%. Moreover, with the alleviation of stress in the perovskite film, flexible PSCs (f-PSCs) also deliver a high PCE of 21.40%.

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All‐inorganic perovskite solar cells (PSCs) have developed rapidly in the field of photovoltaics due to their excellent thermal and light stability. However, compared with organic–inorganic hybrid perovskites, the phase instability of inorganic perovskite under humidity still remains as a critical issue that hampers the commercialization of inorganic PSCs. We originally propose in this work that microstrains between the perovskite lattices/grains play a key role in affecting the phase stability of inorganic perovskite. To this end, we innovatively design the π‐conjugated p‐type molecule bis(2‐ethylhexyl) 3,3′((4,8‐bis(5‐(2‐ethylhexyl)‐3,4‐difluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(3,3″‐dioctyl[2,2′:5′,2″‐terthiophene]‐5″,5‐diyl))(2E,2′E)‐bis(2‐cyanoacrylate) (BTEC‐2F) to covalent with the Pb dangling bonds in CsPbI2Br perovskite film, which significantly suppress the trap states and release the defect‐induced local stress between perovskite grains. The interplay between the microstrains and phase stability of the inorganic perovskite are scrutinized by a series of characterizations including x‐ray photoelectron spectroscopy, photoluminescence, x‐ray diffraction, scanning electron microscopy, and so forth, based on which, we conclude that weaker local stresses in the perovskite film engender superior phase stability by preventing the perovskite lattice distortion under humidity. By this rational design, PSCs based on CsPbI2Br perovskite system deliver an outstanding power conversion efficiency (PCE) up to 16.25%. The unencapsulated device also exhibits an exceptional moisture stability by retaining over 80% of the initial PCE after 500 h aging in ambient with relative humidity of (RH) 25%.

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The minimum interface recombination and maximum carrier extraction of perovskite solar cells (PSCs) are important for achieving a better power conversion efficiency (PCE). According to the recent investigations, 3D/2D hybrid perovskite systems have been recognized as an effective approach to improve the efficiency and stability of PSCs. However, a large highest occupied molecular orbital energy level gap between a 2D perovskite and the hole transport layer (HTL) spiro-OMeTAD would cause energy losses at the interface, which limit open-circuit voltage (V oc) and thus PCE of the PSCs. In this work, we utilized a thermally activated delayed fluorescence molecule M1 stacked on the 3D/2D hybrid perovskite films to engineer the 3D/2D perovskite/HTL interface. The ultrathin interfacial layer of M1 forms a cascade energy alignment between 3D/2D perovskites and a HTL, as a means to circumvent energy losses, which consequently improves the efficiency of PSCs from 19.56 to 21.48% with an outstanding increase of V oc from 1.18 to 1.23 V. The charge separation and carrier recombination in PSCs were analyzed by photoluminescence and impedance characterization, from which, we deduce that a suitable energy level structure can reduce interface charge recombination and promote a minimal open-circuit voltage (V oc) loss, which facilitate the improvement of PSC performances.

As an important part of perovskite solar cells (PSCs), hole transporting layer (HTL) has a critical impact on the performance and stability of the devices. In an attempt to alleviate the moisture and thermal stability issues from the commonly used HTL Spiro-OMeTAD with dopant, it is urgent to develop novel HTLs with high stability. In this study, a new class of polymers D18 and D18-Cl are applied as undoped HTL for CsPbI 2 Br-based PSCs. In addition to the excellent hole transporting properties, we unveil that D18 and D18-Cl with larger thermal expansion coefficient than that of CsPbI 2 Br could impose a compressive stress onto the CsPbI 2 Br film upon thermal treatment, which could release the residual tensile stress in the film. As a result, the efficiency of CsPbI 2 Br-based PSCs with D18-Cl as HTL reaches 16.73%, and the fill factor (FF) exceeds 85%, which is one of the highest FF records for the conventionalstructured device to date. The devices also show impressive thermal stability with over 80% of the initial PCE retained after 85 °C heating for 1500 h.