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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.

To that end and inspired by the compound eyes of an insect, we have developed a new concept in solar cell design, the compound solar cell (CSC), which addresses the intrinsic fragility of these materials with mechanically reinforcing internal scaffolds. The internal scaffold effectively partitions a conventional monolithic planar solar cell into an array of dimensionally scalable and reinforced individual perovskite cells.

Scaffold-reinforced compound solar cells inspired by the fly’s eye

We further demonstrate a scalable atmospheric plasma process to rapidly form mechanically robust photoactive perovskite films in open air at linear deposition rates exceeding 2 cm/s. In contrast to conventional solution-processing, our Rapid Spray Plasma Processing (RSPP) uses clean dry air to produce 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

Motivation and Aims: Perovskites are the leading class of materials for next-generation thin film solar cells based on their high power conversion efficiencies on lab-scale devices—comparable in performance to conventional solar technologies—but limitations in device stability and scalability have challenged the path to commercialization. Conventional solar modules exhibit 25 year service lifetimes while being resistant to the harsh environmental conditions that are often experienced in operation. We discovered that perovskites are the most mechanically fragile class of solar cells ever tested in their current planar design. Mechanical properties are excellent predictors for device resilience to environmental stressors such as thermal cycling, moisture ingress, and UV radiation that accelerate the evolution of internal defects and cause delamination and device failure in layered structures. As a result, perovskites in their current form are highly susceptible to degradation and limited in their path to market based on instabilities and low-throughput processing methods. Open-air fabrication methods offer low capital-expenditure routes to perovskite manufacturing, but achieving stable, high performing devices in ambient conditions with varying relative humidity remains a persistent challenge.

 

 

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).

Figure 2. Top and side view of the compound solar cell. The partitioning scaffold (gray) shields the fragile perovskite microcell array from mechanical stress. The individual microcells in the array are connected in parallel by the top and bottom electrode.

Approach: We have investigated the fundamental mechanical and material properties of perovskite solar cells to gain insight on relevant degradation modes and failure mechanisms in order to inform design criteria for robust, stable modules. After characterizing the mechanical fragility of perovskite materials and establishing a connection with device stability, we have developed strategies to improve the thermomechanical reliability and manufacturability of perovskite solar cells.

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.

Compound Solar Cell for Mechanical Stability

To improve perovskite fracture resistance, inspired by the compound eye of a fly—which comprises a close-packed array of independent photoreceptor units—we developed the compound solar cell using a reinforcing scaffold to partition a perovskite solar cell into a vast array of thousands of smaller, encapsulated, mechanically shielded, and chemically contained perovskite microcells (Figure 2). Instead of attempting to improve upon the intrinsically fragile nature of perovskites, which is related to their brittle and salt-like crystal structure, the scaffold provides extrinsic shielding from mechanical forces. A close-packed honeycomb structure was chosen for the scaffold, as the honeycomb geometry exhibits the best combination of mechanical properties, packing efficiency, and reduced internal cell perimeter in comparison to other geometries. The scaffold was patterned onto a substrate using a photocurable polymer resin and filled with perovskite ink. We fabricated measured a 30-fold increase in the fracture resistance of compound solar cells compared to planar perovskite films without a scaffold while maintaining efficiencies comparable to planar devices, repositioning perovskite-based solar cell technology into the same domain of mechanical resilience as conventional solar modules.

Scalable Barrier Films for Environmental Stability

The compound solar cell effectively shielded perovskite laterally from environmental stressors, but a top encapsulant or barrier film was still necessary to limit degradation pathways above the cell. Traditional methods for encapsulation use rigid, mm-scale packaging materials, while thinner barrier films are typically deposited with spin-coating processes that are not readily scalable. We developed a one-step process to deposit submicron barrier films for perovskite encapsulation using a rapid spray plasma process (RSPP), where an energy-rich plasma state is achieved from applying electricity to compressed air in ambient. A spray coater was placed in front of the plasma, creating an inline process for the deposition and formation of a thin film in seconds from the convective thermal energy and reactive species without any additional heating. The barrier film is composed of a mixture of common industrial organosilicate precursor with a fluorinated aromatic compound, and adjusting the precursor ratio tuned film adhesion, morphology, density, and hydrophobicity. After optimizing the precursor ratio, we performed wide angle X-ray scattering (WAXS) measurements of barrier-coated perovskite films while heating, observing no signs of degradation in the barrier-coated films when the controls had fully decomposed. The thin barrier films are also resistant to mechanical stress, exhibiting flexibility by withstanding >10,000 bending cycles without cracking or delamination.

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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