Thermally stable, efficient, vapor-deposited inorganic perovskite solar cells
Perovskite solar cells have garnered a great deal of attention in the solar cell research community. Their attractive optical and electronic properties—such as their high absorption coefficient, long diffusion lengths, low defect densities, and low exciton binding energies—make perovskites an excellent choice for photovoltaics. The fact that their optical and electronic properties can be tuned through changing the perovskite composition provides researchers with a vast playing field.
The meteoric rise of the power conversion efficiencies of hybrid organic-inorganic perovskites, from 3.8% to 23.6% in less than a decade of research, has attracted huge interest from the academic community. Despite the superior efficiencies of hybrid organic-inorganic perovskite solar cells, their thermal instability, primarily caused by the volatility of the organic
cation, remain a serious limitation in their deployment into real world conditions. This report eschews the use of solution processing for the growth of the perovskite layer due to its inherent problems with reproducibility and scalability. Instead, the research described
herein utilized a commercially viable and scalable vapor deposition technique by sequential layer-by-layer vacuum deposition of the perovskite layer using a custom-built vacuum deposition system.
The initial part of this report displays our work on replacing the thermally volatile
organic cation with the inorganic cation, Cesium, to form fully inorganic cesium lead iodide
(CsPbI3) perovskites. We optimize the device performance of the cesium lead iodide perovskites
solar cells to fabricate devices with a photoconversion efficiency of 11.2% using a p-i-n
architecture with the structure ITO/PTAA/ CsPbI3/PCBM/Al.
The second part of this report focuses on the partial substitution of the iodide anion with a
bromide anion to improve the thermodynamic instability of the cesium lead iodide perovskite.
We employ Indium doped Cadmium Sulphide as the electron transport layer and P3HT as the
hole transport layer to fabricate n-i-p devices with a photoconversion efficiency of 11.7%.
Most importantly, we have demonstrated the thermal stability of these cesium lead mixed
halide perovskites through X-Ray diffraction analysis, showing no compositional or phase
degradation at 200°C for extended periods of time. Further, we have shown that our device
performance exhibits no degradation during the thermal stability test at 200°C for over 72 hours.
Throughout this report, we study the effect of several key parameters of the perovskite
fabrication process that control the intermixing of the perovskite layers and their effect on device
efficiency and hysteresis. Further, we employed several characterization techniques to study
important material parameters of the deposited perovskite material, such as the density of deep
defects and the Urbach energy of the valence band tail states necessary for optimal device
performance. The characterizations indicate the high quality of the material grown with our
layer-by-layer vapor deposition technique with a density of deep defects in the 7x1015/cm3/eV range, an Urbach energy of 21meV, and a dielectric constant of 28.