Project description:Recently, lead-free double perovskites have emerged as a promising environmentally friendly photovoltaic material for their intrinsic thermodynamic stability, appropriate bandgaps, small carrier effective masses, and low exciton binding energies. However, currently no solar cell based on these double perovskites has been reported, due to the challenge in film processing. Herein, a first lead-free double perovskite planar heterojunction solar cell with a high quality Cs2AgBiBr6 film, fabricated by low-pressure assisted solution processing under ambient conditions, is reported. The device presents a best power conversion efficiency of 1.44%. The preliminary efficiency and the high stability under ambient condition without encapsulation, together with the high film quality with simple processing, demonstrate promise for lead-free perovskite solar cells.
Project description:Cs2AgBiBr6 is a promising metal halide double perovskite offering the possibility of efficient photovoltaic devices based on lead-free materials. Here, we report on the evolution of photoexcited charge carriers in Cs2AgBiBr6 using a combination of temperature-dependent photoluminescence, absorption and optical pump-terahertz probe spectroscopy. We observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of 1.0 ps to an intrinsic small polaronic state. While the initially photogenerated delocalized charge carriers show bandlike transport, the self-trapped, small polaronic state exhibits temperature-activated mobilities, allowing the mobilities of both to still exceed 1 cm2 V-1 s-1 at room temperature. Self-trapped charge carriers subsequently diffuse to color centers, causing broad emission that is strongly red-shifted from a direct band edge whose band gap and associated exciton binding energy shrink with increasing temperature in a correlated manner. Overall, our observations suggest that strong electron-phonon coupling in this material induces rapid charge-carrier localization.
Project description:Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs2 AgBiBr6 , shows attractive optical and electronic features, making it promising for high-efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal-engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs2 AgBiBr6 under ambient conditions. The band-gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first-principles calculations indicate that enhanced Ag-Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band-gap narrowing effect. This work provides new insights for achieving lead-free double perovskites with suitable band gaps for optoelectronic applications.
Project description:Development of lead-free inorganic perovskite material, such as Cs2AgBiBr6, is of great importance to solve the toxicity and stability issues of traditional lead halide perovskite solar cells. However, due to a wide bandgap of Cs2AgBiBr6 film, its light absorption ability is largely limited and the photoelectronic conversion efficiency is normally lower than 4.23%. In this text, by using a hydrogenation method, the bandgap of Cs2AgBiBr6 films could be tunable from 2.18 eV to 1.64 eV. At the same time, the highest photoelectric conversion efficiency of hydrogenated Cs2AgBiBr6 perovskite solar cell has been improved up to 6.37% with good environmental stability. Further investigations confirmed that the interstitial doping of atomic hydrogen in Cs2AgBiBr6 lattice could not only adjust its valence and conduction band energy levels, but also optimize the carrier mobility and carrier lifetime. All these works provide an insightful strategy to fabricate high performance lead-free inorganic perovskite solar cells.
Project description:Lead-free Cs2AgBiBr6 double perovskite has emerged as a promising new-generation photovoltaic, due to its non-toxicity, long carrier lifetime, and low exciton binding energies. However, the low power conversion efficiency, due to the high indirect bandgap (≈2 eV), is a challenge that must be overcome and acts as an obstacle to commercialization. Herein, to overcome the limitations through the light trapping strategy, we analyzed the performance evaluation via FDTD simulation when applying the moth-eye broadband antireflection (AR) layer on top of a Cs2AgBiBr6 double perovskite cell. A parabola cone structure was used as a moth-eye AR layer, and an Al2O3 (n: 1.77), MgF2 (n: 1.38), SiO2 (n: 1.46), and ZnO (n: 1.9) were selected as investigation targets. The simulation was performed assuming that the IQE was 100% and when the heights of Al2O3, MgF2, SiO2, and ZnO were 500, 350, 250, and 450 nm, which are the optimal conditions, respectively, the maximum short-circuit current density improved 41, 46, 11.7, and 15%, respectively, compared to the reference cell. This study is meaningful and innovative in analyzing how the refractive index of a moth-eye antireflection layer affects the light trapping within the cell under broadband illumination until the NIR region.
Project description:Double perovskites have shown great potentials in addressing the toxicity and instability issues of lead halide perovskites toward practical applications. However, fabrication of high-quality double perovskite thin films has remained challenging. Here, sequential vapor deposition is used to fabricate high-quality Cs2AgBiCl6 perovskite films with balanced stoichiometry, superior morphology, and highly oriented crystallinity, with an indirect bandgap of 2.41 eV. Using a diode structure, self-powered Cs2AgBiCl6 ultraviolet (UV) photodetectors are demonstrated with high selectivity centered at 370 nm, with low dark current density (≈10-7 mA cm-2), high photoresponsivity (≈10 mA W-1), and detectivity (≈1012 Jones). Its detectivity is among the highest in all double-perovskite-based photodetectors reported to date and surpassing the performance of other perovskite photodetectors as well as metal oxide in the UV range. The devices also show excellent environmental and irradiation stability comparable to state-of-the-art UV detectors. The findings help pave the way toward application of double perovskites in optoelectronic devices.
Project description:Cs2AgInBr6 is among the lead-free halide perovskites of interest, predicted by first-principles calculations to be stable with a direct band gap, but there has been only one report of its synthesis. Herein we report the formation of Cs2AgInBr6 thin films through thermal evaporation of CsBr, AgBr, and InBr3 and subsequent annealing between 130 °C and 250 °C. Cs2AgInBr6 appears stable in this temperature range. However, Cs2AgInBr6 thin films are thermodynamically unstable at room temperature, remaining cubic only long enough to be characterized but not long enough to be useful for practical devices. Cs2AgInBr6 decomposed into Cs2AgBr3, Cs3In2Br9, AgBr, and InBr3 upon cooling from 130 °C to 250 °C to room temperature. This conclusion did not depend on illumination, film thickness, annealing environment, or details of the film formation, pointing to an intrinsic thermodynamic instability of the material. Optical absorption measurements may be interpreted as Cs2AgInBr6 having a direct band gap of 1.57 ± 0.1 eV.
Project description:Cs2AgBiBr6 exhibits promising photovoltaic and light-emitting properties, making it a candidate for next-generation solar cells and LED technologies. Additionally, it serves as a model system within the family of halide double perovskites, offering insights into a broader class of materials. Here, we study various possible excited states of this material to understand its absorption and emission properties. We use time-dependent density functional theory (TD-DFT) coupled with nonempirical hybrid functionals, specifically PBE0(α) and dielectric-dependent hybrids (DDH) to explore direct, indirect, and self-trapped excitons in this material. Based on comparison with experiment, we show that these methods can give excellent predictions of the absorption spectrum and that the fundamental band gap has been underestimated in previous computational studies. We connect the experimental photoluminescence signals at 1.9-2.0 eV to the emission from self-trapped excitons and electron polarons. Finally, we reveal a complex landscape with energetically competing direct, indirect, and self-trapped excitons in the material.
Project description:Cs2SnCl6 double perovskite has attracted wide attention as a promising optoelectronic material because of its better stability and lower toxicity than its lead counterparts. However, pure Cs2SnCl6 demonstrates quite poor optical properties, which usually calls for active element doping to realize efficient luminescence. Herein, a facile co-precipitation method was used to synthesize Te4+ and Er3+-co-doped Cs2SnCl6 microcrystals. The prepared microcrystals were polyhedral, with a size distribution around 1-3 μm. Highly efficient NIR emissions at 1540 nm and 1562 nm due to Er3+ were achieved in doped Cs2SnCl6 compounds for the first time. Moreover, the visible luminescence lifetimes of Te4+/Er3+-co-doped Cs2SnCl6 decreased with the increase in the Er3+ concentration due to the increasing energy transfer efficiency. The strong and multi-wavelength NIR luminescence of Te4+/Er3+-co-doped Cs2SnCl6 originates from the 4f→4f transition of Er3+, which was sensitized by the spin-orbital allowed 1S0→3P1 transition of Te4+ through a self-trapped exciton (STE) state. The findings suggest that ns2-metal and lanthanide ion co-doping is a promising method to extend the emission range of Cs2SnCl6 materials to the NIR region.
Project description: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.