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Perovskite Solar Cells

The remarkable optoelectronic properties of hybrid organolead-halide perovskite materials hold tremendous promise for use as the active layer in low-cost solar cells and have attracted extraordinary attention for next-generation PV. For the promises of perovskite photovoltaics to be realized, however, dramatic advances in the understanding of their thermomechanical properties along with the development of material and solar cell design strategies to address their mechanical and chemical instabilities are required.

Our program focuses on solar cell design strategies along with improvements in the active and charge transport layers themselves to demonstrate mechanically and thermally robust working perovskite solar cells with major improvements in reliability and service lifetimes that can compete with CIGS and c-Si cells.

We demonstrate a scalable atmospheric plasma process to rapidly form mechanically robust photoactive perovskite films in open air at high linear deposition rates. In contrast to conventional solution-processing, our Rapid Spray Plasma Processing (RSPP) uses a combination of reactive species (photons, metastables, and radicals) and thermal energy to rapidly convert the perovskite film after spray-coating. We use RSPP to deposit pinhole-free, large-area, robust perovskite films with a ten-fold increase in fracture toughness and improved efficiency compared to control devices.

Large-area perovskite devices deposited by RSPP with an average efficiency of 13.4%

 

Design of Scalable Perovskite Solar Cells with Improved Thermomechanical Reliability

 

Figure 1. (a) The measured resistance to fracture (Gc) and degradation rate as a function of solar cell active material, showing a correlation between mechanical integrity and long-term reliability. (b) Measured film stress for perovskite formed at 25, 60, and 100 °C compared to predicted stress from thermal expansion mismatch between the perovskite and substrate (dashed lines).

Characterizing Mechanical Fragility of Perovskite Solar Cells

We first studied the resistance to fracture of perovskite solar cells processed from solution using a variety of perovskite device architectures, fabrication methods, and charge transport layers. Prior to our work, the mechanical properties of perovskites were not at all understood. Among all cells studied, the resistance to fracture observed was so low as to altogether rule out monolithic perovskite solar cells as a viable solar technology. These values are lower than organic solar cells—widely recognized to be thermomechanically fragile and susceptible to environmental stressors—and considerably lower than competing CIGS and c-Si cells (Figure 1a). In addition, we found that perovskite layers can accumulate high tensile film stresses during elevated temperature processing. We modeled the film stress and found that the large difference in thermal expansion coefficient between the perovskite and substrate accounts for the film stress that develops during processing, as film stress was linearly correlated with annealing temperature. Since typical processing temperatures are at least 100 °C, stresses can reach values comparable to the yield strength of copper and increase even further during operation. Additionally, since the driving force for cracking scales as the square of the tensile film stress, the mechanical fragility of perovskite films leads to increased device instability. Processing techniques are therefore needed that enhance fracture resistance and reduce film stresses.

Plasma Curing for Reduced Film Stress

In order to accelerate film conversion for high-throughput and scalable fabrication of perovskites, we found that RSPP is also effective at forming the perovskite layer in ambient, leading to lower defect densities and higher power conversion efficiencies in air than traditional solution processing in an inert environment. We designed a setup for in-situ WAXS measurements to characterize the crystal growth in RSPP perovskite thin films and measured complete perovskite conversion within a few tens of milliseconds. The rapid crystallization kinetics reduce the thermal load needed to form the perovskite, resulting in RSPP perovskite films with film stress values >5X lower than spin coated films. We also measured a 10-fold increase in perovskite fracture resistance compared to spin-coated films, attributed to the morphology induced from the rapid crystallization producing a polycrystalline grain structure that enables crystallites to interlock more effectively. Our current focus is scaling RSPP perovskite films while maintaining a similar morphology and optoelectronic properties on much larger areas. The influence of precursor composition, processing speed, and RSPP curing kinetics on perovskite film microstructure and crystallinity enables improved optoelectronic properties, performance, and stability for large-area series-connected perovskite modules with high tolerance to humidity compared to modules produced using traditional solution processing when increasing the substrate size and active area by >10X.

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