The major limitation of organic–inorganic perovskite solar cell performance is the existence of numerous charged defects at the absorption layer surface, which caused the charge carrier to recombine depravation. These defects have a remarkable influence on charge extraction, which further caused the instability of the device and induced severe hysteresis. Here, three low-cost anion-doped conductive fullerene derivatives, fullerene bis­(phenethyl alcohol) malonate (FMPE-I), fullerene bis­(ethylenediamine) malonamide (FEDA-I), and fullerene bis­(propanediamine) malonamide (FPDA-I), are developed for the first time as interfacial layers between perovskite and phenyl-C61-butyric acid methyl ester (PCBM) in planar invert perovskite solar cells by mild solution fabrication. The constituent Lewis basic halides and the specific amide groups of conductive fullerene derivatives efficaciously heighten the chemical interaction between perovskite and conductive fullerene derivatives since the iodide can combine with undercoordinated Pb2+ by electrostatic interaction and the amide group can facilely be combined with I by hydrogen bonding, improving the dual passivation of perovskite defects. Moreover, due to the well-matched energy level of conductive fullerene derivatives and the high conductivity of the perovskite/interlayer film, the electron extraction capacity can be effectively enhanced. Consequently, superior optoelectronic properties are achieved with an improved power conversion efficiency of 17.63%, which is considerably higher than that of the bare PCBM-based devices (14.96%), for the perovskite device with conductive interlayer treatment along with negligible hysteresis. Moreover, hydrophobic conductive fullerene derivatives improve the resistance of the device to moisture. The conductive fullerene derivative-based devices without encapsulation are maintained at 85% of the pristine power conversion efficiency value after storage under ambient conditions (25 °C temperature, 60% humidity) for 500 h.

Although organic small molecule spiro-OMeTAD is widely used as a hole-transport material in perovskite solar cells, its limited electric conductivity poses a bottleneck in the efficiency improvement of perovskite solar cells. Here, a low-cost and easy-fabrication technique is developed to enhance the conductivity and hole-extraction ability of spiro-OMeTAD by doping it with commercially available benzoyl peroxide (BPO). The experimental results show that the conductivity increases several orders of magnitude, from 6.2×10 S cm for the pristine spiro-OMeTAD to 1.1×10 S cm at 5 % BPO doping and to 2.4×10 S cm at 15 % BPO doping, which considerably outperform the conductivity of 4.62×10 S cm for the currently used oxygen-doped spiro-OMeTAD. The fluorescence spectra suggest that the BPO-doped spiro-OMeTAD-OMeTAD layer is able to efficiently extract holes from CH NH PbI and thus greatly enhances the charge transfer. The BPO-doped spiro-OMeTAD is used in the fabrication of perovskite solar cells, which exhibit enhancement in the power conversion efficiency.

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A high-quality precursor solution is essential for the fabrication of hybrid perovskite solar cells. This article reports a simple and efficient method for preparing a high-quality concentrated solution of methylammonium triiodoplumbate (MAPbI3) in N,N-dimethylformamide (DMF) by using MAPbI3 crystals instead of conventional lead iodine and methylammonium iodine blend. The MAPbI3 concentration of the precursor solution is easily and accurately adjusted from 0 up to 1.64 M. An investigation of the dissolution process of the MAPbI3 crystals reveals that the concentrated solution of MAPbI3 in DMF is metastable, and the transition from the concentrated solution to solvated intermediate MAPbI3·DMF determines the solubility of MAPbI3 in DMF. The high purity and precise stoichiometric ratio of the crystals eliminate the possible impurities that initialize the transition to MAPbI3·DMF and consequently suppress the transition and increase the stability of the concentrated solution. MAPbI3 films with different thicknesses up to 800 nm are prepared with the conventional film fabrication technique, and the highest power conversion efficiency of 20.7% is achieved on corresponding solar cells. This newly developed method for preparing a concentrated precursor solution can be easily combined with other fabrication techniques for further development of industrial-scale manufacture of solar cells.

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