Search for: All records

Earth-abundant Cu2BaSnS4-xSex (CBTSSe) represents a recent alter- native for Cu2ZnSn(S,Se)4 for solar energy conversion with a lower level of disorder and band tailing. We report the heterogeneous excited-state and trap-state pattern in different solution-processed CBTSSe films using ultrafast two-color pump-probe diffuse reflec- tance microscopic imaging. The spectroscopy/microscopy method can visualize and correlate the microscopic compositional and elec- tronic variations (i.e., trap states) in real space with time-resolved photophysics. Heterogeneity patterns in TAM images show that some grains exhibit a positive excited-state absorption (ESA) signal, while others show negative ground-state bleaching (GSB). Our re- sults visualize that film processing, such as air annealing and Na addition, has a clear influence on the heterogeneity of the excited- state pattern. Importantly, we report stable charge carrier over 100 ps. We applied the image principal component and histogram for quantitative analysis of TAM images to deconvolute and visu- alize the contribution and fingerprints of minority free carriers and sub-band-gap trapped carriers. 

Melanin is a stable, widely light-absorbing, photoactive, and biocompatible material viable for energy con- version, photocatalysis, and bioelectronic applications. To achieve multifunctional nanostructures, we synthesized melanin nanoparticles of uniform size and controlled chemical composition (dopamelanin and eumelanin) and used them with titanium dioxide to fabricate donor–acceptor bilayers. Their size enhances the surface-to-volume ratio important for any surface-mediated functionality, such as photo- catalysis, sensing, and drug loading and release, while controlling their chemical composition enables to control the film’s functionality and reproducibility. Inkjet printing uniquely allowed us to control the de- posited amount of materials with minimum ink waste suitable for reproducible materials deposition. We studied the photochemical characteristics of the donor–acceptor melanin–TiO2 nanostructured films via photocatalytic degradation of methylene blue dye under selective UV-NIR and Vis-NIR irradiation con- ditions. Under both irradiation conditions, they exhibited photocatalytic characteristics superior to pure melanin and, under UV-NIR irradiation, superior to TiO2 alone; TiO2 is photoactive only under UV irradiation. The enhanced photocatalytic characteristics of the melanin–TiO2 nanostructured bilayer films, particularly when excited by visible light, point to charge separation at the melanin–TiO2 interface as a possible mechanism. We performed ultrafast laser spectroscopy to investigate the photochemical charac- teristics of pure melanin and the melanin–TiO2 constructs and found that their time-resolved photo- excited spectral patterns differ. We performed singular value decomposition analysis to quantitatively deconvolute and compare the dynamics of photochemical processes for melanin and melanin–TiO2 heterostructures. This observation supports electronic interactions, namely, interfacial charge separation at the melanin and TiO2 interface. The excited-state relaxation in melanin–TiO2 increases markedly from 5 ps to 400 ps. The results are remarkable for the future intriguing application of melanin-based con- structs for bioelectronics and energy conversion.