Development of zinc oxide based heterojunctions patterned with maskless laser lithography for photodetector applications

The presented thesis study is based on developing the vision of photodetector (PD) applications with high sensitivity and performance, environmentally friendly and multifunctional minimal device design approaches that can be adapted to different systems by focusing on nanostructured heterojunctions. In addition to, the materials selected for high PD performance, expanding the surface area of nanomaterials is also important. Providing regular alignment of patterned nanostructures, strong light absorption, and charge transfer acceleration compared to no patterned nanostructures is an interesting approach to PD performance. With this approach, the thesis study is divided into two sections according to the pattern design. In other words, ZnO NRAs that are perfectly aligned vertically were grown in the circle patterns and the mesh patterns using maskless laser lithography. Firstly, ZnO NRAs were grown in circular patterns having different diameters (D=20 µm, 2D=40 µm, 4D=80 µm) and different distances between circles (L=20 µm, L/2=10 µm) for liquid-state systems. The effects of pattern geometry and the performance of heterojunctions formed with indium sulfide (In2S3) to improve the light-absorbing properties in the visible region were investigated in photoelectrochemical (PEC) PDs. Although the no patterned ZnO NRAs were coated with In2S3, no patterned ZnO NRAs exhibited lower photocurrent density than D-L patterned structures. It was observed that the current density increased by 224% in the D-L design after the spray pyrolysis of 20 passes (20P) of In2S3. ZnO NRAs/In2S3-20P design showed the best performance by providing the highest charge transfer at the photoanode/electrolyte interface when the diameter of circles is D in pattern and the distance between the circles is decreased (L/2). After that, this design was also produced as a portable PEC PD for quasi-solid-state systems using ionic gel electrolyte and showed the highest performance at 400 nm in the between 367 nm to 535 nm wavelengths. The effects of the distance between the centers of the lines (L) in the mesh pattern on the PEC PD performance device performance were investigated in the liquid-state systems. The mesh patterned PEC PD device (L=20 µm) exhibited higher performance compared to the circle patterned (D-L/2) in the ZnO NRAs/In2S3-20P heterojunction. Finally, mesh-patterned ZnO NRAs and inorganic halide perovskite (IHP) heterojunctions were developed in the thesis study. Lead-based CsPbBr3-Cs2PbBr5 dual phase and lead-free Cs2AgBiBr6 double perovskite were used as IHP. According to the performance results for the quasi-solid-state system with gel electrolyte, it was concluded that lead-free halide perovskite is an alternative material for PDs. Especially, the open circuit potential (VOC) of mesh patterned lead-free PEC PD was obtained about 5 times higher than lead-containing perovskite. The light response of lead-free PEC PD extended through more visible regions compared to the lead-containing perovskite. In addition, it was confirmed that the response times were much faster than In2S3-coated PEC PD. In solid-state systems, by coating Cs2AgBiBr6 on mesh patterned ZnO NRAs, it was observed that the maximum photocurrent value increased about 13.65 times compared to pristine ZnO NRAs under +5 V and AM 1.5 illumination. The highest FD performances were obtained at 382 nm and +5 V in the between 367 nm and 522 nm wavelengths. Under these conditions, approximately 2.5 times higher photocurrent was achieved compared to lead-free. In this thesis study, the pattern shape, geometry, the effect of different electrolytes and active areas were examined together, the compact device architecture is promising for self-powered PEC PD applications, the determination of lead-free perovskite as an alternative material and the obtaining of stable and environmentally friendly PDs with millisecond response times have brought an innovative approach to the literature and a strong strategy for efficient light management mechanism for future new generation PDs. Keywords: ZnO, In2S3, IHP, Maskless Laser Lithography, Photodetector