Within the Dauskardt group at Stanford, Dr Watson’s research has focused on developing chemical strategies for enhancing the fracture resistance of perovskite solar cells and promoting adhesion of organic materials to inorganic oxide surfaces.
The remarkable optoelectronic properties of hybrid organo-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. Perovskites exhibit efficient photo-induced carrier generation, long carrier lifetimes, high carrier mobilities and internal quantum efficiencies while absorbing strongly over a broad region of the solar spectrum.
The fracture resistance of perovskite solar cells has unique implications on their reliability. The formation of a crack within the ionic perovskite material or within the ancillary carrier selection layers not only results in loss of ohmic contact, but provides an accelerated pathway for degradation through the ingress of water or through evolution of volatile organics like methylamine.
In our preliminary work, the fracture resistance (Gc) of a wide range of perovskite solar cells were measured and were found to be thermomechanically fragile (Gc < 2 J/m2). These values are lower than organic solar cells (~5-20 J/m2) generally recognized to be thermo-mechanically fragile and considerably lower than competing CIGS (~10 J/m2) and c-Si cells (~10-200 J/m2).
Analysis of the cells tested revealed that failure frequently occurred in the electron and hole selective contacts and charge-transporting layers. Of these layers, those comprised of fullerenes such as C60 or PC61BM exhibited the lowest resistance to fracture. Accordingly, we have developed a cross-linkable solvent resistant fullerene adduct, MPMIC60 that exhibits a 10-fold increase in fracture resistance in comparison to C60. MPMIC60 exhibits suitable electronic properties for use in perovskite solar cells, enabling the fabrication of efficient solar cells with increased fracture resistance.
The mechanical properties of organic polymers can be modified following deposition from the solution state via various cross-linking strategies. Cross-linking can imbue a material with enhanced fracture and solvent resistance. Designing highly symmetric nodes, which can be used as additives for transforming a broad array of organic polymers into tough, solvent resistant materials has immediate application for electronic devices utilizing highly refined semiconducting, organic polymers which exhibit poor cohesive properties, such as PTAA in perovskite solar cells.
This approach is generic and allows for the mechanical properties of already synthesized polymers to be tuned through addition of the cross-linking additive which produces points of hyper-connectivity within the organic material, without requiring the polymer itself to be redesigned.
Hybrid molecular materials containing both organic and inorganic components and synthesized via sol-gel chemistry exhibit unique and superior mechanical properties due to the intimate nanometer length-scale mixing of both components. As a result, these materials are best-suited for addressing the challenges related to improving the bonding between organic and inorganic materials.
We explore the adhesive and cohesive properties of a self-assembled and compositionally graded organic/inorganic hybrid layer produced via a low-cost sol-gel processing technique. This material serves as an interphase layer between an inorganic substrate and a polymeric material, and can dramatically improve the reliability of multilayer devices in microelectronic, photovoltaic and display technologies.
The evolution of the reactions occurring in solution during the sol-gel synthesis have significant influence on the structure and properties of the resulting hybrid layer. In particular, the influence of pH and sol-gel aging time must be tuned to achieve the deposition of a compositionally graded structure.