device configuration usually adopting mesoporous TiO 2 scaffold for the purpose of increasing the contact area between electron transporting material (ETM) and perovskite materials. [1a,d,h] However, high-temperature sintering process (usually over 500 °C) required for preparing mesoporous TiO 2 films complicates device fabrication and increases energy consumption and thus device cost, which is incompatible with the fabrication of flexible PSCs. In order to overcome the above issues, low-temperature normal [1b,4] and inverted [5] planar PSCs were developed considering the long carrier diffusion length of commonly utilized perovskite compositions. [6] For low-temperature normal planar PSCs, developing low-temperature highquality ETMs are crucial to realize high PCE. Based on this consideration, several effective ETMs have been attempted and optimized in planar PSCs, such as TiO 2 , [1b,7] ZnO, [8] SnO 2 , [9] PCBM, [10] and so on. Among them, SnO 2 ETM as a promising alternative to TiO 2 possesses several appealing advantages, including wide optical bandgap (3.6-4.0 eV) beneficial for protecting UV-degradation, high bulk electron mobility (up to 240 cm 2 V −1 s −1 ), good band alignment with perovskites, low-temperature processability, and excellent chemical stability. [9] Consequently, SnO 2 ETM shows huge potentials in simultaneously achieving efficient and stable planar PSCs. To date, the PCEs over 21% have been reported for SnO 2 -planar PSCs based on optimization of SnO 2 film quality, [11] improvement of perovskite film quality, [1f ] and interface engineering. [12] In the past few years, various deposition methods have been developed to prepare high-quality SnO 2 film, such as solution process deposition, [1f,13] atomic layer deposition (ALD), [14] chemical bath deposition (CBD), [14,15] electrochemical deposition, [16] pulsed laser deposition (PLD), [17] etc. Particularly, solution process deposition from commercial SnO 2 nanoparticle colloidal dispersion solution attracts extensive attention because of high PCE and simple fabrication procedure. [1f,11,18] Most recently, a certified PCE up to 23.3% was reported based on SnO 2 -planar PSCs adopting commercial SnO 2 nanoparticle as ETM. [19] Although great progress has been made on SnO 2 -planar PSCs, the presently achieved PCE (over 23%) is still far from Chemical interaction at a heterojunction interface induced by an appropriate chemical linker is of crucial importance for high efficiency, hysteresis-less, and stable perovskite solar cells (PSCs). Effective interface engineering in PSCs is reported via a multifunctional chemical linker of 4-imidazoleacetic acid hydrochloride (ImAcHCl) that can provide a chemical bridge between SnO 2 and perovskite through an ester bond with SnO 2 via esterification reaction and an electrostatic interaction with perovskite via imidazolium cation in ImAcHCl and iodide anion in perovskite. In addition, the chloride anion in ImAcHCl plays a role in the improvement of crystallinity of perovskite film crystallinity. The...

Thermal stability of CHNHPbI (MAPbI)-based perovskite solar cells was investigated for normal structure including the mesoporous TiO layer and spiro-MeOTAD and the inverted structure with PCBM and NiO. MAPbI was found to be intrinsically stable from 85 °C to 120 °C in the absence of moisture. However, fast degradation was observed for the encapsulated device including spiro-MeOTAD upon thermal stress at 85 °C. Photoluminescence (PL) intensity and the time constant for charge separation increased with thermal exposure time, which is indicative of inhibition of charge separation from MAPbI into spiro-MeOTAD. A full recovery of photovoltaic performance was observed for the 85 °C-aged device after renewal with fresh spiro-MeOTAD, which clearly indicates that thermal instability of the normal structured device is mainly due to spiro-MeOTAD, and MAPbI is proved to be thermally stable. Spiro-MeOTAD with additives was crystallized at 85 °C due to a low glass transition temperature, and hole mobility was significantly deteriorated, which was responsible for the thermal instability. Thermal stability was significantly improved for the inverted structure with the NiO hole transporting layer, where the power conversion efficiency (PCE) was maintained at 74% of its initial PCE of 14.71% after the 80th thermal cycle (one cycle: heating at 85 °C for 2 h and cooling at 25 °C for 2 h). This work implies that the thermal stability of perovskite solar cells depends on selective contacts.

Here we report an efficient and reproducible multifunctional additive engineering strategy via methoxysilane cross-linking agents functionalized by the different terminal group, moderate electron-donating −SH, weak electron-donating −CH3, or strong electron-withdrawing −CN, into a PbI2 precursor solution. The power conversion efficiency (PCE) is increased from 18.4 to 20.8% after introduction of (3-mercaptopropyl)­trimethoxysilane (MPTS) containing a −SH group as a consequence of improved voltage and current density, while 3-cyanopropyltri­ethoxysilane (CPTS) containing a −CN group deteriorates the overall photovoltaic performance. Moreover, −SH in MPTS is found to passivate defects effectively through a Lewis acid–base interaction with PbI2, resulting in a larger grain size and a longer carrier lifetime. Owing to the formation of a cross-linking siloxane network as a protective layer on the grain boundary, the thermal and moisture stability of the device are improved remarkably. The present work provides a guideline for multifunctional additive engineering for the purpose of simultaneous achievement of a high PCE and long-term stability.

Vacuum evaporation is promising for the high-throughput fabrication of perovskite solar cells (PSCs) because of its solvent-free characteristic, precise control of film thickness, and compatibility with large-scale production. Nevertheless, the power conversion efficiency (PCE) of PSCs fabricated by vacuum evaporation lags behind that of solution-processed PSCs. Here, we report a Cl-containing alloy–mediated sequential vacuum evaporation approach to fabricate perovskite films. The presence of Cl in the alloy facilitates organic ammonium halide diffusion and the subsequent perovskite conversion reaction, leading to homogeneous pinhole-free perovskite films with few defects. The resulting PSCs yield a PCE of 24.42%, 23.44% (certified 22.6%), and 19.87% for 0.1, 1.0, and 14.4 square centimeters (mini-module, aperture area), respectively. The unencapsulated PSCs show good stability with negligible decline in performance after storage in dry air for more than 4000 hours. Our method provides a reproducible approach for scalable fabrication of large-area, high-efficiency PSCs and other perovskite-based optoelectronics.

Organo lead halide perovskite materials like methylammonium lead iodide (CH3NH3PbI3) and formamidinium lead iodide (HC(NH2)2PbI3) show superb opto-electronic properties. Based on these perovskite light absorbers, power conversion efficiencies of the perovskite solar cells employing hole transporting layers have increased from 9.7% to 20.1% within just three years. Thus, it is apparent that perovskite solar cell is a promising next generation photovoltaic technology. However, the unstable nature of perovskite was observed when exposing it to continuous illumination, moisture and high temperature, impeding the commercial development in the long run and thus becoming the main issue that needs to be solved urgently. Here, we discuss the factors affecting instability of perovskite and give some perspectives about further enhancement of stability of perovskite solar cell.