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Perovskite Solar Cells

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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 high-throughput, low-cost, open-air processes; advanced packaging and characterization techniques; and material and solar cell design strategies to address their mechanical and chemical instabilities are required.

Figure 1: 225 cm2 encapsulated open-air spray-deposited perovskite module.

Group Members ("Module Empire")

Open-Air Processing of Perovskite Solar Modules

Our program focuses on the development of high-throughput, low-cost processes for perovskite photovoltaics. We employ high-throughput deposition techniques such as spray coating alongside a suite of atmospheric pressure rapid curing techniques including low-temperature plasma and near-infrared rapid thermal processing (NIR-RTP) to create the fastest demonstrated perovskite deposition process with linear processing speeds of 12 m/min (Figure 2a). Cost-modeling is employed to validate the open-air processing techniques, demonstrating reductions in module cost even compared to other low-cost perovskite manufacturing techniques (Figure 2b). The entire perovskite device, from electrode and transport layer materials to the perovskite absorber itself, are explored with these open-air techniques to develop low-cost, manufacturable perovskite solar modules (Figure 2c).

Figure 2: a) Schematic of all open-air processed perovskite solar devices. b) Cost models of open-air deposited perovskite compared to conventional solution-processed techniques and alternative silicon-based technologies. c) Open-air deposited perovskite solar modules deposited on 100 cm2.

Rapid Spray Plasma Processing (RSPP) of Perovskites

The perovskite device absorber material is deposited with an open-air spray-plasma deposition technique, employing a range of perovskite precursors with an ultrasonic spray deposition system and blown-arc discharge plasma to rapidly deposit, cure, and anneal perovskite films. We designed a setup for in-situ wide-angle X-ray scattering (WAXS) measurements to characterize the crystal growth in RSPP perovskite thin films and measured complete perovskite conversion within a few tens of milliseconds, enabling linear manufacturing throughputs of 20 cm/s. 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, leading to lower defect densities and higher power conversion efficiencies (PCE) in air than traditional solution processing in an inert environment. The superior optoelectronic properties of these films allow for successful device integration with device performances of 18% PCE across 1 cm2 active areas.

Spray-Deposition of Charge Transport Layers

A successful perovskite device also requires efficient charge extraction away from the perovskite layer towards powering an external device. These charge transport layers sandwich the perovskite film and selectively extract either positive (holes) or negative (electrons) charges towards the device electrodes. Towards realizing the desired open-air manufacturing vision, we employ spray-deposited charge transport layers ranging from dense metal oxides to organic materials. Our spray-deposited films replace non-scalable and lower-throughput spin-coating and evaporation techniques, even enhancing charge extraction compared to conventional deposition methods. These materials are compatible with our demonstrated high-performing open-air deposited devices with performances >18% PCE and validated stability during maximum power point tracking across greater than one diurnal cycle of illumination.

Open-Air Synthesis of Transparent Conductors

Perovskite photovoltaic devices are traditionally fabricated on top of a glass substrate with a thin transparent conducting oxide material. Costly vacuum-based sputter deposition processes dominate the manufacturing of this transparent conducting layer, but we employ spray processing of uniquely low-temperature compatible chemistries with plasma treatment to enhance the final film conductivity. These open-air deposited films demonstrate sputter-quality transparency and conductivity at significantly higher throughputs for lower cost module production and have great potential in a host of optoelectronic applications employing transparent conductors.

Design of Scalable Perovskite Solar Cells with Improved Thermomechanical Reliability

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 perovskite solar cells with major improvements in reliability and service lifetimes that can compete with CIGS and c-Si cells.

Characterizing Mechanical Fragility of Perovskite Solar Cells

We study 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 3a). 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. 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 (Figure 3b). 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.

Figure 3: 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).

Plasma Curing for Reduced Film Stress

While successful in accelerating film conversion for high-throughput and scalable fabrication of perovskites, 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 similar morphologies 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 serially-interconnected 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.

Chemical and Structural Characterization of Perovskite Solar Cells 

Understanding structure-property relationships is critically important for engineering more robust perovskite modules. The characterization team works closely with the manufacturing team to understand the effects of processing parameters on the microstructure, nanostructure, and chemistry of each layer and interface. Scanning and Transmission Electron Microscopy are used to image the highly beam-sensitive perovskite in tandem with analytical techniques (e.g., electron diffraction and Energy Dispersive X-Ray Spectroscopy). Additionally, X-ray Photoelectron Spectroscopy and Nano SIMS are used to understand the chemistry of depth profiles, and X-ray diffraction is used to quantify crystallinity and degradation products after aging. 

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