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Wide‐bandgap (WBG) perovskite solar cells (PSCs) have garnered significant attention for their potential applications in tandem solar cells. However, their large open‐circuit voltage (VOC) deficit and serious photo‐induced halide segregation remain the main challenges that impede their applications. Herein, a post‐treatment strategy without thermal annealing is presented to form a 2D top layer of 2‐thiopheneethylammonium lead halide (n = 1) on WBG perovskites. This thermal annealing‐free post‐treatment method can more effectively passivate the defects of WBG methylamine (MA)‐free formamidinium/cesium lead iodide/bromide perovskite films and suppress photo‐induced perovskite phase segregation, as compared with the thermal annealing method that yields multi‐2D phases. The resulting opaque and semi‐transparent 1.66 eV‐bandgap perovskite solar cells deliver maximum power conversion efficiencies of 21.47% (a small VOC deficit of 0.43 V) and 19.11%, respectively, both of which are among the highest reports for inverted MA‐free WBG PSCs. Consequently, four‐terminal all‐perovskite tandem cells realize a remarkable efficiency of 26.64%, showing great promise for their applications in efficient multi‐junction tandem solar cells. [ABSTRACT FROM AUTHOR]

Perovskite solar cells (PSCs) represent a promising next‐generation photovoltaic technology considering their high efficiency and low cost. At the current stage, resolving the stability bottleneck is extremely urgent to realize PSCs' commercialization since the efficiencies of these cells are improved to a level comparable to that of crystalline silicon solar cells. Similar to other functional layers, a proper choice of the rear electrode atop the perovskite layer is equally important for achieving the device's long‐term stability. This topic has not been comprehensively reviewed before. Here, recent progress in the development of rear electrodes based on metals, carbon‐based materials, transparent conductive oxides, and conductive polymers is summarized, especially focusing on their different impacts on the device's long‐term stability and associated degradation mechanisms. In the context of practical applications, the impacts of rear electrode materials on the device's overall efficiency and cost‐effectiveness are also discussed. [ABSTRACT FROM AUTHOR]

Optical loss is one of the main factors that restrict the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Anti-reflective coating and light-trapping structure is one of the effective methods to enhance light capture and thus improve PCE. Through the finite-difference time-domain (FDTD) method, polydimethylsiloxane (PDMS) anti-reflection coatings with different light-trapping structures were introduced in the air–glass interface of planar heterojunction-based PSCs. Calculating the optical absorption, the inverted-pyramid structure had the best anti-reflection performance. On this basis, in order to further improve photon absorption of perovskite layers, an optical coupling structure with double-decker inverted pyramids are designed by inserting an inverted pyramid structure between perovskite layer and TiO 2. According to the cross-sectional | E | 2 distribution of the EM wave inside the device, three different marshalling patterns of the optical coupling structure are proposed and studied to select the best one for PSCs. This kind of marshalling pattern can highly enhance light harvesting of perovskite solar cells, thus increasing the generation rate of charge carrier in perovskite layers and improving the PCE. [ABSTRACT FROM AUTHOR]

Abstract: This review provides an up‐to‐date review about the small molecule interlayers (SMIs) in organic solar cells (OSCs). Compared to polymer interlayers, SMIs exhibit intrinsic advantages such as easy synthesis and purification, monodispersity, well‐defined chemical structure, and high batch‐to‐batch reproducibility. Recently, various SMIs have been reported with landmark efficiencies of over 10% in both conventional and inverted OSCs, exhibiting promising potential in commercial application. In this review, the authors summarize the progress of SMIs from a device fabrication point of view, paying particular attention to the material categories, molecular design, preparation process, and applicable device structure. In addition, the working mechanisms of different SMIs are also discussed, including the structure–property relationships and the corresponding impact on device performance. Finally, a brief outlook is provided that includes opportunities and challenges in this emerging area. [ABSTRACT FROM AUTHOR]

Narrow band-gap conjugated polyelectrolytes (NBGCPs) combine the advantages of narrow band-gap conjugated polymers and polyelectrolytes. However, they are limited reported and seldom used in polymer solar cells (PSCs). Herein, we design and synthesized two PTB7-based NBGCPs, cationic PTB7-NBr and zwitterionic PTB7-NSO 3 , as cathode interlayers (CILs) in conventional PSCs. Compared to poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2- b :4,5- b′ ]dithiophene-2,6-diyl- alt -ethylhexyl-3-fuorothithieno[3,4- b ]thiophene-2-carboxylate-4,6-diyl] (PTB7), both PTB7-NBr and PTB7-NSO 3 have similar absorption while PTB7-NBr has a higher HOMO level than PTB7 and PTB7-NSO 3 . With PTB7-NBr and PTB7-NSO 3 as CIL and Al as cathode, the devices both exhibited high efficiencies. The optimal power conversion efficiencies (PCEs) of PTB7-NBr device and PTB7-NSO 3 device are about 9%. However, PTB7-NBr devices are less sensitive to the CIL film thicknesses and the PCEs of PTB7-NSO 3 device decrease sharply when the thickness of PTB7-NSO 3 over 3 nm. What's more, when the devices with an Ag cathode PTB7-NBr exhibits a much better interfacial modification than PTB7-NSO 3 . Ultraviolet photoelectron spectroscopy (UPS) indicate that the work functions of PTB7-NBr and PTB7-NSO 3 film on Al are identical while on Ag they are different. We suggest the insensitivity to CIL thickness and good performance for Ag device of PTB7-NBr devices are attributed to the bromide anions in PTB7-NBr. The report on PTB7-based NBGCPs broads the types of organic cathode interfacial materials and is beneficial to the deep insight of ionic effect in electrolyte materials. [ABSTRACT FROM AUTHOR]

Two star-shaped oligofluorenes with hexakis(fluoren-2-yl)benzene as core are designed and synthesized, namely Tn0 and Tn1. Diethylamino groups are attached to the side chain of fluorene units of Tn0 and Tn1 and enable them alcohol solubility, additional hydrophobic nhexyl chains are grafted on the π-extended fluorene arms of Tn1. Power conversion efficiency (PCE) as high as 8.62% and 8.80% are achieved when utilizing Tn0 and Tn1 as cathode interlayers in inverted polymer solar cells, respectively. The work function of ITO effectively decreased by introducing interlayer, resulting in high Voc of the device, besides, the wetting properties of the interlayers can be tuned by modifying the oligofluorenes with π-extended structure, and the more hydrophobic interlayer will benefit the device performance with enhanced Jsc and FF. [ABSTRACT FROM AUTHOR]

Two alcohol‐soluble organic molecules are synthesized and introduced into inverted organic solar cells as the cathode interlayer. A power conversion efficiency as high as 9.22% is obtained by using the more hydrophobic molecule FTBTF‐N as the cathode interlayer. Morphological studies suggest that design of the backbone can help to enhance short‐circuit current density and fill factor. [ABSTRACT FROM AUTHOR]

Abstract: Lost-cost counter electrodes (CEs) based on Cu2ZnSnS(Se)4 nanoparticles have been successfully introduced to quantum dot-sensitized solar cells (QDSCs). The investigation has demonstrated that the catalytic activity of Cu2ZnSnS(Se)4 based CEs is sensitive to the composition and thermal annealing condition. The Cu2ZnSnSe4 CE with porous structure, fabricated by spray deposition method and sintered at 450 °C in Argon atmosphere, exhibits an excel power conversion efficiency of 4.35% under AM 1.5 solar (100 mW cm−2) irradiation in a QDSC, which is about 70% higher than that of device using the conventional thermally platinized conducting glass CEs. [Copyright &y& Elsevier]

Highlights: High-quality large-area perovskite films are prepared using a solid–liquid two-step film formation method combined with CsBr modification for the buried interface and Urea additive for perovskite crystallization. The inverted perovskite solar modules' performance is enhanced to 20.56% in 61.56 cm2 with improved stability. A considerable efficiency gap exists between large-area perovskite solar modules and small-area perovskite solar cells. The control of forming uniform and large-area film and perovskite crystallization is still the main obstacle restricting the efficiency of PSMs. In this work, we adopted a solid–liquid two-step film formation technique, which involved the evaporation of a lead iodide film and blade coating of an organic ammonium halide solution to prepare perovskite films. This method possesses the advantages of integrating vapor deposition and solution methods, which could apply to substrates with different roughness and avoid using toxic solvents to achieve a more uniform, large-area perovskite film. Furthermore, modification of the NiOx/perovskite buried interface and introduction of Urea additives were utilized to reduce interface recombination and regulate perovskite crystallization. As a result, a large-area perovskite film possessing larger grains, fewer pinholes, and reduced defects could be achieved. The inverted PSM with an active area of 61.56 cm2 (10 × 10 cm2 substrate) achieved a champion power conversion efficiency of 20.56% and significantly improved stability. This method suggests an innovative approach to resolving the uniformity issue associated with large-area film fabrication. [ABSTRACT FROM AUTHOR]

Non‐fullerene acceptors have shown great promise for organic solar cells (OSCs). However, challenges in achieving high efficiency molecular system with conformational unicity and effective molecular stacking remain. In this study, we present a new design of non‐fused tetrathiophene acceptor R4T‐1 via employing the encapsulation of tetrathiophene with macrocyclic ring. The single crystal structure analysis reveals that cyclic alkyl side chains can perfectly encapsulate the central part of molecule and generate a conformational stable and planar molecular backbone. Whereas, the control 4T‐5 without the encapsulation restriction displays cis‐ and twisted conformation. As a result, R4T‐1 based OSCs achieved an outstanding power conversion efficiency (PCE) exceeding 15.10 % with a high short‐circuit current density (Jsc) of 25.48 mA/cm2, which is significantly improved by ≈30 % in relative to that of the control. Our findings demonstrate that the macrocyclic encapsulation strategy could assist fully non‐fused electron acceptors (FNEAs) to achieve a high photovoltaic performance and pave a new way for FNEAs design. [ABSTRACT FROM AUTHOR]

Non‐fullerene acceptors have shown great promise for organic solar cells (OSCs). However, challenges in achieving high efficiency molecular system with conformational unicity and effective molecular stacking remain. In this study, we present a new design of non‐fused tetrathiophene acceptor R4T‐1 via employing the encapsulation of tetrathiophene with macrocyclic ring. The single crystal structure analysis reveals that cyclic alkyl side chains can perfectly encapsulate the central part of molecule and generate a conformational stable and planar molecular backbone. Whereas, the control 4T‐5 without the encapsulation restriction displays cis‐ and twisted conformation. As a result, R4T‐1 based OSCs achieved an outstanding power conversion efficiency (PCE) exceeding 15.10 % with a high short‐circuit current density (Jsc) of 25.48 mA/cm2, which is significantly improved by ≈30 % in relative to that of the control. Our findings demonstrate that the macrocyclic encapsulation strategy could assist fully non‐fused electron acceptors (FNEAs) to achieve a high photovoltaic performance and pave a new way for FNEAs design. [ABSTRACT FROM AUTHOR]

Perovskite solar cells (PSCs) have attracted much attention in the past decade and their power conversion efficiency has been rapidly increasing to 25.2%, which is comparable with commercialized solar cells. Currently, the long‐term stability of PSCs remains as a major bottleneck impeding their future commercial applications. Beyond strengthening the perovskite layer itself and developing robust external device encapsulation/packaging technology, integration of effective barriers into PSCs has been recognized to be of equal importance to improve the whole device's long‐term stability. These barriers can not only shield the critical perovskite layer and other functional layers from external detrimental factors such as heat, light, and H2O/O2, but also prevent the undesired ion/molecular diffusion/volatilization from perovskite. In addition, some delicate barrier designs can simultaneously improve the efficiency and stability. In this review article, the research progress on barrier designs in PSCs for improving their long‐term stability is reviewed in terms of the barrier functions, locations in PSCs, and material characteristics. Regarding specific barriers, their preparation methods, chemical/photoelectronic/mechanical properties, and their role in device stability, are further discussed. On the basis of these accumulative efforts, predictions for the further development of effective barriers in PSCs are provided at the end of this review. [ABSTRACT FROM AUTHOR]

Electron transport materials (ETMs) play an important role in organic solar cells (OSCs) and the rapidly evolving non-fullerene acceptors have higher demands for ETMs. A novel naphthalene diimide-based conjugated polymer (PNDI-NN) with aliphatic amino side chains at the N-position of naphthalene diimide was synthesized and introduced as an ETM in non-fullerene OSCs. The only N-substituted aliphatic amino side chains endow PNDI-NN with good solubility in alcohol and an intense self-doping effect, which can not only reduce the work function of the metal electrode (Ag), but also have a good coverage on the active layer and promote electron transport. The device with PNDI-NN showed the best power conversion efficiency of 15.6% with an improved open-circuit voltage (VOC) and fill factor (FF). The device also exhibited good long-term stability in an inert atmosphere, in a humid environment and under continuous heating. [ABSTRACT FROM AUTHOR]

Planar heterojunction perovskite solar cells were fabricated through a low temperature approach. We find that the device performance significantly depends on the external bias before and during measurements. By appropriate optimization of the bias conditions, we could achieve an 8-fold increase in the power conversion efficiency. The significant improvement in device performance might be caused by the ion motion in the perovskite under the external electric field. [ABSTRACT FROM AUTHOR]

Organic light emitting diodes (OLEDs) employing organic thin‐film based emitters have attracted tremendous attention due to their widespread applications in lighting and as displays in mobile devices and televisions. The novel thin‐film photovoltaic techniques using organic or organic–inorganic hybrid materials such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs) have become emerging competitive candidates with regard to the traditional photovoltaic techniques on account of high‐efficiency, low‐cost, and simple manufacturing processing properties. However, OLEDs, OPVs, and PSCs are vulnerable to the undesired degradation induced by moisture and oxygen. To afford long‐term stability, a robust encapsulation technique by employing materials and structures that possess high barrier performance against oxygen and moisture must be explored and employed to protect these devices. Herein, the recent progress on specific encapsulation materials and techniques for three types of devices on the basis of fundamental understanding of device stability is reviewed. First, their degradation mechanisms, as well as, influencing factors are discussed. Then, the encapsulation technologies and materials are classified and discussed. Moreover, the advantages and disadvantages of various encapsulation technologies and materials coupled with their encapsulation applications in different devices are compared. Finally, the ongoing challenges and future perspectives of encapsulation frontier are provided. [ABSTRACT FROM AUTHOR]

Abstract The power conversion efficiency (PCE) of perovskite solar cells (PSCs) keeps decreasing after a period of time. This has remained a concern in commercializing PSCs. Researchers have attempted to slow down the degeneration of the cell. In this paper, a facile freeze treatment was applied to increase the PCE of degenerated PSCs. Through freeze treatment, the carrier recombination in the cell was significantly reduced, thus improving the open-circuit voltage by 3% and increasing the PCE from 8.34% to 9.23%. Compared with the nontreated one, the time spent was more than 200% for the PCE to decrease from 8% to 6% after three cycles of freeze treatment. [ABSTRACT FROM AUTHOR]

Abstract Novel small-molecules, B2T and B2F, based on benzobis(thiadiazole) have been designed and synthesized as electron transporting materials for perovskite solar cells (PSCs). Benefiting from the high electron affinity of benzobis(thiadiazole) unit, both B2T and B2F exhibited an efficient electron transporting ability in CH 3 NH 3 PbI 3-x Cl x planar-heterojunction (PHJ) PSCs. The device based on B2T exhibited an optimal power-conversion efficiency (PCE) of 13.0%, while that of B2F was 15.0% with a relatively low hysteresis. Compared to B2T device, the higher efficiency of B2F device was mainly due to the impressive increase in short-circuit current density (J SC), from 19.36 mA cm−2 to 22.13 mA cm−2. By comprehensive analysis of the energy level and photoluminescence spectra of the devices, we inferred that the higher J SC was resulted from the lower highest occupied molecular orbital (HOMO) energy level of B2F than B2T, which induced a more efficient hole-blocking and less recombination of charge carriers. Through the molecular design (from thiophene to fluorene), we developed a highly efficient electron transporting material, this strategy may provide a new building block to develop novel efficient interface materials for PSCs. Graphical abstract Two benzobis(thiadiazole)-based small molecules (B2T and B2F) were designed and synthesized. Simply by tuning the donor groups from thiophenes to fluorenes, perovskite solar cells with B2F as electron transporting layer exhibited a high power conversion efficiency of 15%. fx1 Highlights • Simply by tuning the donor groups from thiophenes to fluorenes, two effective electron transport materials were designed and synthesized, the they were used to the inverted perovskite solar cells. this may provide a new building block to develop novel efficient interface materials. • The device based on B2T exhibited an optimal power-conversion efficiency (PCE) of 13.0%, while that of B2F was 15.0% with a relatively low hysteresis. the higher efficiency of B2F device was mainly due to the impressive increase in short-circuit current density (JSC), from 19.36 mA cm−2 to 22.13 mA cm−2. • The higher JSC of B2F device resulted from the lower highest occupied molecular orbital (HOMO) energy level of B2F than B2T, which induced a more efficient hole-blocking and less recombination of charge carriers. [ABSTRACT FROM AUTHOR]

Hole-transporting materials play an important role in perovskite solar cells. In this study, we introduced a novel hole-transporting material without p-type dopant, a tetrathiafulvalene derivative anchored with eight carboxyl groups (TTA) into inverted structure perovskite solar cells. The devices with TTA as hole-transporting layer showed high power conversion efficiency of 16.7%, which was 44.5% improvement compared with the reference devices without hole-transporting layer. The performance improvement was ascribed to three factors: 1) the better match of energy level between ITO and perovskite layer; 2) enhanced hole extraction at anode interface; 3) improved perovskite morphology. [ABSTRACT FROM AUTHOR]

Fullerene derivatives, which possess extraordinary geometric shapes and high electron affinity, have attracted significant attention for thin film technologies. This study demonstrates an important photovoltaic application using carboxyl-functionalized carbon buckyballs, C60 pyrrolidine tris-acid (CPTA), to fabricate electron transport layers (ETLs) that replace traditional metal oxide-based ETLs in efficient and stable n-i-p-structured planar perovskite solar cells (PSCs). The uniform CPTA film is covalently anchored onto the surface of indium tin oxide (ITO), significantly suppressing hysteresis and enhancing the flexural strength in the CPTA-modified PSCs. Moreover, solution-processable CPTA-based ETLs also enable the fabrication of lightweight flexible PSCs. The maximum-performing device structures composed of ITO/CPTA/CH3NH3PbI3/2,2′,7,7′-tetrakis-( N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD)/Au yield power conversion efficiencies of more than 18% on glass substrates and up to 17% on flexible substrates. These results indicate that the CPTA layers provide new opportunities for solution-processed organic ETLs by substantially simplifying the procedure for fabricating PSCs for portable applications. [ABSTRACT FROM AUTHOR]

A new carboxylate functionalized small molecule non-conjugated zwitterion, HDAC was proposed for its feasible one-step synthetic procedure. It can be used as an efficient interlayer to improve the fullerene/cathode interface of inverted planar heterojunction perovskite solar cells. A HDAC modified device could increase the power conversion efficiency from 15% to 17%. The main increase of the performance parameter lies on the increase of fill factor from around 72% to 80%, which could be attributed to the formation of interface dipole and elimination of potential barrier at the interface of electron transporting layer and cathode. This work brings up a simple design principle for efficient interfacial materials in perovskite solar cells. [ABSTRACT FROM AUTHOR]

A conjugated polymer poly (9,9-dioctyfluorene-co-benzothiazole) (F8BT) was used as the electron transporting layer (ETL) to modify the cathode interface of inverted planar perovskite solar cell. Incorporation of the thin F8BT layer delivered greatly enhanced power conversion efficiency (PCE) of 13.9% compared to the PCE 7.9% of the device without F8BT, due to the improvements in various performance parameters like V oc , J sc and FF . Moreover, great elimination of the hysteresis was demonstrated in F8BT based devices. These results suggest promising potential of polymeric materials in perovskite solar cells and possible approach to eliminate the hysteresis of perovskite solar cells with this class of interfacial materials. [ABSTRACT FROM AUTHOR]

Hybrid organic/inorganic perovskite planar heterojunction (PHJ) solar cells are becoming one of the most competitive emerging technologies. Here, the devices (ITO/PEDOT:PSS/CH 3 NH 3 PbI 3− x Cl x /PC 60 BM/C 60 /Al) are fabricated based on solution process. A power conversion efficiency (PCE) up to 13.1% was obtained after isopropanol (IPA) solvent treatment, which was 9% improvement than that of the original devices. Photocurrent hysteresis could be reduced to a certain extent by introducing IPA treatment. Solvent treatment can remove as-grown impurities like CH 3 NH 3 I and defects formed during the device fabrication. We also demonstrated the electrical and optical properties of perovskite films were improved. The addition of IPA-treated facilitates the formation of a relatively smooth PC 60 BM films. This work provides a very simple but effective strategy to enhance the power conversion efficiency of perovskite solar cells. [ABSTRACT FROM AUTHOR]

In this work, a facile and low-temperature water evaporation approach to prepare columnar superstructures consisting of face centered cubic (fcc) Cu2− xSe nanoflakes stacked along 〈111〉 direction is reported. Formation of such unique stacked nanoflake assemblies is resulted from oriented attachment of isolated hexagonal CuSe nanoflakes along the 〈001〉 direction with a ripening effect driven by solvent evaporation, and then followed by a phase conversion into fcc Cu2− xSe. Evolution from hexagonal CuSe nanoflakes to fcc Cu2− xSe columnar superstructures results in obvious red-shift of band-gap absorption edge from 670 to 786 nm and dramatically decreased Raman resonance band intensity of the Se-Se stretching mode at 259 cm−1 due to the phase conversion and composition variation. Remarkably, the Cu2− xSe columnar superstructures are employed as low-cost and highly efficient counter electrodes (CEs) in quantum dot sensitized solar cells, exhibiting excellent electrocatalytic activity for polysulfide electrolyte regeneration. A ZnSe/CdSe cosensitized solar cell using the Cu2− xSe CE shows a significant increase in fill factor and short-current density ( JSC) and yields a 128% enhancement in power conversion efficiency as compared to the traditional noble metal Pt CE. [ABSTRACT FROM AUTHOR]

Three amino-functionalized fluorene oligomers with different solubility were developed as cathode interfacial materials for inverted polymer solar cells (I-PSCs). By side chain design, we solved the interface layer erosion problem for I-PSCs, and the devices exhibit a power conversion efficiency as high as 8.94%. [ABSTRACT FROM AUTHOR]

Replacement of the ZrO2 insulator layer in the state-of-the-art TiO2/ZrO2/carbon structure by mesoscopic p-type NiO particles led to 39% increase of energy conversion efficiency of hole-conductor-free organometallic perovskite heterojunction solar cells with carbon counter electrodes. In these cells, the light absorber, CH3NH3PbI3, formed instantly inside the pores of the entire TiO2/NiO/carbon layer upon sequential deposition of PbI2 and CH3NH3I. Photoluminescence, impedance spectroscopy and transient photovoltage decay measurements have revealed that introduction of NiO extended the electron lifetime and augmented the hole extraction of the counter electrode. As a result, the photocurrent and open-circuit voltage both increased, resulting in a cell with impressive energy conversion efficiency of 11.4% under AM1.5G conditions. [ABSTRACT FROM AUTHOR]

The solution-processable electron transport layer (ETL) is essential for the electron transport and extraction of inverted perovskite solar cells (PSCs). A new naphthalene diimide based copolymer with phosphite ester pendants (PFNDI) is developed as an ETL in inverted PSCs. The naphthalene diimide backbone and the phosphite ester pendants endow PFNDI a good electron transport property and a strong binding force on perovskite. By introducing PFNDI on the perovskite interface, we succeed in improving surface morphologies of perovskite film, reducing the trap density and facilitating electron transport properties. The PFNDI-based devices exhibit an optimal power conversion efficiency of 18.25% (with the V OC = 1.068 V, J SC = 22.53 mA cm−2 and FF = 75.85%) and fine long-term stabilities. The un-encapsulated PFNDI-based devices maintain 80% of their initial performance after storage for more than 500 h at 20 °C in air with a relative humidity of 25% and 75% of their initial efficiency after continuously heating for about 300 h. Our work demonstrated that NDI-based polymers with proper pendant groups can be the ideal n-type organic materials for high-performance PSCs. Image 1 • A new naphthalene diimide based copolymer was synthesized and introduced into PSCs. • The copolymer improves the charge transport properties to deliver a PCE of 18.25%. • Our findings may pave a new way to design promising non-fullerene-based ETL. [ABSTRACT FROM AUTHOR]

The hole-transport material (HTM) free carbon-based perovskite solar cells (C–PSCs) lacks a HTM layer inside, resulting in poor charge transport performance. The power conversion efficiencies (PCE) of the C–PSCs could be effectively improved by increasing the transmission of photo-generated electrons from the perovskite to the electron transport layer. Here, the nanoparticles with TiO 2 @ PbTiO 3 core-shell structure were prepared by in-situ reaction and used as mesoporous electron transport layer to improve electron transport performance. It is worth noting that the density of defect states on the surface of TiO 2 significantly reduced due to the coating of PbTiO 3 , which effectively inhibits the interfacial charge recombination caused by the defect as the carrier recombination center. The electron injection from perovskite to electron transport layer enhanced ascribing to the optimized energy band arrangement. The open circuit voltage (V oc) and short circuit density (J sc) were significantly improved, as well as the hysteresis retarded. Finally, the optimal PCE prepared in the air condition increased from 6.55% to 7.97%. This study provides a very feasible idea for obtaining high-performance C–PSCs. Image 1 • The TiO 2 @PbTiO 3 core-shell structure using for mesoporous layer was fabricated by the in-situ reaction. • The trap states density acted as the center of charge recombination reduced significantly ascribing to the PbTiO 3 shell. • The electron injection from perovskite to electron transport layer enhanced due to the optimized energy band arrangement. • The open circuit voltage and short circuit density of cells improved apparently, and retarded the hysteresis. [ABSTRACT FROM AUTHOR]

The hole transport layer (HTL) free carbon based perovskite solar cells (Carbon PSC) are known for low cost, good stability and commercial manufacturing technology. Compared to the structure of traditional cells, the carrier transport performance between the absorber layer and the carbon electrode (CE) is poor. As an excellent P-type material, NiO has been doped in CE for obtaining a better performance of PSCs. However, NiO will inevitably reduce the conductivity of the CE due to the poor conductivity, thus failing to fully exploit the advantages of NiO in hole extraction. In this paper, rGO/NiO composites were successfully prepared by doping rGO with good conductivity, which not only improved the conductivity of the CE, but also ensured good hole extraction of the doped CE. The carrier transport in the cell was significantly improved, resulting in 32% increase in the short circuit current density (J sc). The performance of perovskite solar cells was optimized by doping the CE, and the power conversion efficiency (PCE) of the cell was increased from 5.48% to 8.50%. [ABSTRACT FROM AUTHOR]