Abstract
Alternatives to lead- and tin-based perovskites for photovoltaics and optoelectronics are sought that do not suffer from the disadvantages of toxicity and low device efficiency of present-day materials. Here we report a study of the double perovskite Cs2TeI6, which we have synthesized in the thin film form for the first time. Exhaustive trials concluded that spin coating CsI and TeI4 using an antisolvent method produced uniform films, confirmed as Cs2TeI6 by XRD with Rietveld analysis. They were stable up to 250 °C and had an optical band gap of ∼1.5 eV, absorption coefficients of ∼6 × 104 cm–1, carrier lifetimes of ∼2.6 ns (unpassivated 200 nm film), a work function of 4.95 eV, and a p-type surface conductivity. Vibrational modes probed by Raman and FTIR spectroscopy showed resonances qualitatively consistent with DFT Phonopy-calculated spectra, offering another route for phase confirmation. It was concluded that the material is a candidate for further study as a potential optoelectronic or photovoltaic material.
Highlights
Following the first publication reporting a hybrid organic− inorganic perovskite solar cell in 2009, when a CH3NH3PbI3 absorber layer resulted in a 3.5% efficiency,[1] and breakthroughs delivering >10% efficient cells in 2012,2,3 the field has accelerated rapidly
The variables investigated were the type of solvent (DMSO, DMF, and dimethyl sulfoxide (DMSO)/DMF mixture), number of layers (1−4), precursor weight fraction (14, 26, 30, 35, 47, and 53 wt %), precursor molar ratio CsI:TeI4 (2:1, 1:1, and 1:1.5), solution temperature (RT and 50 °C), spin coating conditions, annealing temperatures (50−300 °C), and annealing time (1 min−10 min)
The scanning electron microscope (SEM) images of the Cs2TeI6 films shown in Figure 1 reveal that the films comprise large crystals (∼5 μm) surrounded by smaller crystals (∼1.4 μm), both having the same triangular bipyramidal shape
Summary
Following the first publication reporting a hybrid organic− inorganic perovskite solar cell in 2009, when a CH3NH3PbI3 absorber layer resulted in a 3.5% efficiency,[1] and breakthroughs delivering >10% efficient cells in 2012,2,3 the field has accelerated rapidly. In the past few years the efficiency has been increased to over 25.2%,4 which is comparable to silicon photovoltaics. These outstanding efficiencies can only be achieved with the use of lead which has known toxicity issues, and the devices demonstrate relatively poor stability in contact with moisture, UV light, and elevated temperatures.[5−7] Niu et al.[8] summarized the principal stability issues for leadbased perovskites arising in device structures, during solution processing and fundamentally in terms of the thermal stability of the crystal structure and the selection of their chemical components. Fewer studies have investigated substituting the B-site metal ion alone to replace the lead
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