Increasing Halide Perovskite Scalability and Stability with Food Additives

In this work, we report on a polymeric additive endemic to the food industry, gellan gum, as a means of enabling scalable, open-air manufacturing of halide perovskites with improved stability characteristics under high heat and humidity conditions.<br/>As the global demand for scalability escalates, traditional perovskite inks designed for small scale fabrication in inert environments like spin coating have been found to produce pinholed, incomplete, and shunted films when utilized in more scalable open-air processes like blade coating. Interestingly, gellan gum enables viable films through the critical transition of well-studied spin coating inks into solutions that are compatible with blade coating. By working as a non-toxic and efficient rheological modifier in combination with a pure dimethyl sulfoxide solvent system, gellan gum enables longer periods for crystallization which eliminates the formation of a patchy network of needle-shaped crystals attributed to the PbI₂ phase of perovskites in favor of larger spherulitic grains which results in smoother, more uniform films in a one-step coating method. In fact, the use of this additive reduces surface roughness of ambient blade coated films from 138nm using an unmodified ink to 35nm when gellan gum is incorporated.<br/>Similarly, hyperspectral photoluminescence data reveals that gellan gum decreases the defect density between grains. In films with low gum concentration the intensity of the photon flux decreases when moving from the grain centers to the boundaries which could be explained by thinner coverage in those areas; however, the opposite trend is true for samples with higher gum concentration. This indicates the additive decreases the defect density through passivation and induces a corresponding reduction in non-radiative recombination centers. This reduction in defects, and therefore improvement in stability, is supported by XRD data demonstrating increased crystallinity through ordered texturing.<br/>Additionally, open-air blade coated gellan gum samples feature an intrinsic compressive stress above 1wt% additive which contrasts with traditional tensile stress found in spin coated films. Not only does compressive stress indicate increased resistance to delamination and fracture under thermal and environmental conditions, but it is also associated with an increase in the activation energy for ion migration. For example, when aged under 85% relative humidity for 100 hours, a compressive film processed with gellan gum features very little change in compressive stress, from 50MPa to 75MPa, while a tensile sample can experience dramatic tensile relaxation from 450MPa to 200MPa. Under thermal aging at 85 degrees Celsius for 100 hours, a similar stress trend is observed, and the compressive film visually retains the black photoreactive phase while tensile samples turn completely yellow, indicative of degradation to PbI₂.<br/>XRD data confirms that aged tensile control samples feature an increase of PbI₂ and a weakening of the perovskite peaks while aged compressive gum samples generate spectrums with markedly less prominent PbI₂peaks and relatively unchanged perovskite peaks. Furthermore, when compared to a similar polysaccharide biopolymer like cornstarch, gellan gum enables a reduction in additive concentration from 5wt% with an associated compressive stress of 35 MPa, to only 1wt% inducing compressive stresses of 75 MPa.<br/>Overall, open-air blade coating of single-step gellan gum-halide perovskites reduces defect densities at grain boundaries and improves stability and manufacturability with compressive stresses that may limit ion migration leading to increased resiliency to heat, humidity, and illumination. In designing for reliability, this work mitigates some of the fundamental degradation mechanisms preventing halide perovskites from reaching service lifetimes comparable to incumbent PV technologies.