A study on doping effect of semiconductor nanostructure’s charge transfer in solar energy conversion system

Abstract

Recently, many research has been actively conducted in the field of light-energy conversion systems. Selecting proper absorbing layer, synthesizing appropriate nanostructure, and modulating the semiconductor are studied for higher energy conversion efficiency. Various semiconductors have a unique bandgap and absorb light energy above the bandgap to generate electron-hole pairs. Generated charged can be used in many ways, including solar cells, photoelectrochemical (PEC) cells, optical sensors, and photocatalysts. In particular, the study for develop high-efficiency light-energy conversion system is actively conducted. Among them, hydrogen energy produced by water splitting has recently attracted attention as a renewable pollution-free energy candidate. In the energy conversion system, two factors are importantly determining the performance of the device. The band gap is related the width of the wavelength of absorbing light, and the charge transfer determined the movement of generated charge carrier. In order to widen the wavelength of light absorption, a study for heterojunction of two or more semiconductors was conducted, or studies have also been reported that the doping treatment forms a mid-gap state between band gaps. In addition, to prevent the recombination of generated electron-hole pairs in the process of charge transfer, a semiconductor heterojunction was formed to separated electrons and holes spatially. For increasing of charge carrier mobility, charge carrier density via semiconductor doping can help to increasing the efficiency of energy conversion system. Doping of the semiconductor has various effects depending on the selection of the dopant element. In addition to the aforementioned effects of increasing the charge density and charge mobility, and reducing the bandgap, various effects such as defect generation and adjustment of the Fermi level can be expected. Crystal defects are generated due to distorted crystal structure, and the Fermi level can be adjusted by the difference in electronegativity with the dopant. The effect of doping on the light energy conversion system was studied in the field of UV sensors, PEC cell, and photocatalyst. In addition, the efficiency improvement and its cause was analyzed in each field. In chapter 2, as a photoanode of a photoelectrochemical cell, ZnO and BiVO4 were doped respectively to increase the photocurrent of the heterojunction photoanode. ZnO nanorods synthesized by hydrothermal method were doped with N by reducing in an ammonia environment. BiVO4 synthesized by a metal organic decomposition method, doping was performed by adding a Mo precursor to the synthetic solution. After the synthesis of BiVO4, bunched nanorods array were observed by SEM, TEM, and XRD, and the doping state of N and Mo was analyzed through XPS. The optimized doped sample showed a photocurrent value of 3.62 mA/cm2 at 1.23 V vs RHE, which is 2.2 times higher than the undoped sample. As a result of Mott-Schottky analysis, it can be seen that N doping increased the charge carrier density of ZnO by 3.55 times. To analyze the effect of BiVO4’s Mo doping, charge transfer rate constant (kct) can be calculated through time resolved photoluminescence (TRPL). After doping, kct increased by more than 40%, and Mo doping prevents recombination of charge carriers to improve charge transfer efficiency. Through EIS analysis, it was confirmed that the charge transfer resistance was also reduced by more than 2 times after doping, thereby making it more suitable in many characteristic as photoanode. In chapter 3, a study was performed to change the charge transfer mechanism in heterojunction with C3N4/ZnO by controlling the Fermi level of the semiconductor through B doping of C3N4. B was selected as a dopant element because it has a low electronegativity compared to C, so the work function can increase when doping. The doping was proceeded by adding a precursor during synthesize C¬¬3N4, and XPS analysis confirmed that B was doped in place of C. As a measured result of hydrogen production reaction through GC, it was confirmed that the hydrogen production rate increased nearly 3 times after doping. When the hydrogen production efficiency was optimized while adjusting the doping concentration, the bandgap decreased while increasing the doping concentration, which resulted in a gradual increase in hydrogen production rate. In addition, an explosive increase in hydrogen production rate of more than 2 times was observed at a certain point, where it was predicted that the charge transfer mechanism was converted from Z-scheme to type-II heterojunction. For in-depth analysis, UPS and ESR analysis were employed, and after doping, the Fermi level of C3N4 reversed that of ZnO, confirming that the direction of band bending was changed and the charge transfer mechanism was switched as expected. Z-scheme's high redox potential can be used to proceed with various reactions, but it is possible to make high-efficiency hydrogen production by using the higher charge utilization of the type-II heterojunction mechanism. In summary, in this study, various effects of semiconductor doping at the several light energy conversion systems were studied. Doping was able to significantly improve the efficiency of PEC cells, and photocatalysts, and especially, it was found for the first time the charge transfer mechanism of heterojunction can be switched through doping. It is expected that in-depth research on doping can be active in various fields by strengthening and modulating the intrinsic properties of semiconductors.