A solvent-assisted methodology has been developed to synthesize CH3NH3PbI3 perovskite absorber layers. It involved the use of a mixed solvent of CH3NH3I, PbI2, γ-butyrolactone, and dimethyl sulfoxide (DMSO) followed by the addition of chlorobenzene (CB). The method produced ultra-flat and dense perovskite capping layers atop mesoporous TiO2 films, enabling a remarkable improvement in the performance of free hole transport material (HTM) carbon electrode-based perovskite solar cells (PSCs). Toluene (TO) was also studied as an additional solvent for comparison. At the annealing temperature of 100 °C, the fabricated HTM-free PSCs based on drop-casting CB demonstrated power conversion efficiency (PCE) of 9.73 %, which is 36 and 71 % higher than those fabricated from the perovskite films using TO or without adding an extra solvent, respectively. The interaction between the PbI2–DMSO–CH3NH3I intermediate phase and the additional solvent was discussed. Furthermore, the influence of the annealing temperature on the absorber film formation, morphology, and crystalline structure was investigated and correlated with the photovoltaic performance. Highly efficient, simple, and stable HTM-free solar cells with a PCE of 11.44 % were prepared utilizing the optimum perovskite absorbers annealed at 120 °C.Graphical Abstract

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We report efficient mixed halide perovskite solar cells using thermally evaporated Ag or AgAl alloy layers as back electrodes. The properties of AgAl alloy and Ag films deposited on a hole-transport material layer for use in CH 3 NH 3 PbI 3-x Cl x solar cells were investigated. The influence of the distance between the metal source and the sample on the performance of solar cells was determined. The cell with an Ag layer deposited at a distance of 20 cm displayed a power conversion efficiency (PCE) of 5.49 %. When the Ag layer was deposited at a distance of 30 cm, the resulted device achieved a 46.8 % enhancement in PCE compared to the cell with the Ag 2 prepared at 20 cm. Furthermore, the AgAl alloy based perovskite solar cell accomplished a 37.3 % enhancement in PCE compared to the optimized Ag electrode.The fabricated AgAl alloy perovskite cells show a fill factor of 59.6 %, open-circuit voltage of 0.88 V, short-circuit current density of 21.11 mA cm −2 , yielding an overall efficiency of 11.07%. The AgAl alloy layer exhibited high optical reflectivity and good adhesion on hole -transport material layer compared to a layer of Ag. The PCE enhancement mechanisms are discussed. Our work has demonstrated that AgAl is a promising back electrode material for high-efficiency perovskite solar cells.

We present for the first time the synthesis of Eu(3+)-doped β-phase sodium gadolinium fluoride (NaGdF4:Eu) nanocrystals (NCs) using a hydrothermal method and the application of down conversion (DC) NaGdF4:Eu NCs to efficient dye-sensitized solar cells (DSSCs). The as-prepared NaGdF4:Eu(3+) NCs were characterized by X-ray diffraction, photoluminescence spectrometry, and scanning and transmission electron microscopy. DC layers consisting of poly(methyl methacrylate) (PMMA) doped with luminescent NaGdF4:Eu(3+) were prepared and attached onto the back of a prefabricated TiO2 anode to form a more efficient DSSC, compared with a device based on a pure TiO2 electrode. The influences of both doped and undoped NaGdF4 NC layers on the photovoltaic devices were compared and evaluated by the measurement of the device's incident-photon-to-current efficiency (IPCE). An obvious increase in IPCE was observed when the DC layer was added in the device. As the down-converted photons can be reabsorbed within DSSCs to generate photocurrent, the DSSC with a 100 nm thick NaGdF4:Eu(3+) DC-PMMA layer improved photoelectric conversion efficiency by 4.5% relative to the uncoated solar cell. The experiments conclude that NaGdF4:Eu(3+) nanocrystals mainly act as luminescent DC centers and light scatterers in the ultraviolet and visible domains, respectively, for enhancing the spectral response of the device in the measured spectral regime.