Photoelectrolysis Using Type-II Semiconductor Heterojunctions

Abstract

The solar-powered production of hydrogen for use as a renewable fuel is highly desirable for the world’s future energy infrastructure. However, difficulties in achieving reasonable efficiencies, and thus cost-effectiveness, have hampered significant research progress. Here we propose the use of semiconductor nanostructures to create a type-II heterojunction at the semiconductor–water interface in a photoelectrochemical cell (PEC) and theoretically investigate it as a method of increasing the maximum photovoltage such a cell can generate under illumination, with the aim of increasing the overall cell efficiency. A model for the semiconductor electrode in a PEC is created, which solves the Schrödinger, Poisson and drift–diffusion equations self-consistently. From this, it is determined that ZnO quantum dots on bulk n-InGaN with low In content x is the most desirable system, having electron-accepting and -donating states straddling the oxygen- and hydrogen-production potentials for x < 0.26, though large variance in literature values for certain material parameters means large uncertainties in the model output. Accordingly, results presented here should form the basis for further experimental work, which will in turn provide input to refine and develop the model.

Utilizing Band Diagrams To Interpret the Photovoltage and Photocurrent in Photoanodes: A Semiclassical Device Modeling Study

“…We can also equate V ph, OC as the offset between E Fn light at the bulk of the photoanode and E L since E L retains the same distance from E Ref as does E Fn dark ; this can be seen by comparing Figure a and Figure b . Indeed, eq is often the approach by which the photovoltage is measured in the literature. ,,, However, as we shall see shortly, the photovoltage can be extracted at nearly any bias voltage. On the other hand, the open-circuit PEC voltage is given by the quasi-Fermi level splitting under illumination Δ V F, OC , as shown in Figure a.…”

“…First, it was established through capacitive analysis that the externally measured photovoltage is an electrostatic potential change that develops in response to illumination [which lifts up (down) the working photoanode (photocathode) electrochemical potential ( E WE ) relative to that of the counter electrode ( E CE )]. From this, it was emphasized, building upon the work of earlier researchers, that the open-circuit photovoltage can be measured using reference electrodes as V ph, OC = V RHE, OC dark – V RHE, OC light . ,− However, the illumination of photoanodes is also accompanied by an electrochemical potential change (Δ V F ), which can be termed the PEC voltage. The PEC voltage arises due to the splitting of quasi-Fermi levels ,− inside the semiconductor.…”

“…39 Indeed, eq 8 is often the approach by which the photovoltage is measured in the literature. 32,35,34,37 However, as we shall see shortly, the photovoltage can be extracted at nearly any bias voltage. On the other hand, the open-circuit PEC voltage is given by the quasi-Fermi level splitting under illumination ΔV F, OC , as shown in Figure 2a.…”

“…Nevertheless, there exists a wide degree of disagreement in the field regarding the physical origin and meaning of the photovoltage ,,− While many researchers in the field describe the photovoltage as an internal electrochemical perturbation directly within the photoanode arising from illumination, ,− many others describe the photovoltage as an offset between the electrochemical potential of the semiconducting photoanode and that of the aqueous media. ,− , From these contradicting viewpoints in the conceptualization of the photovoltage, there arise discrepancies in uncovering the fundamental origins of the photocurrent since the two concepts are deeply intertwined; at a given bias, one produces a given current and vice versa. − Crucially, to further improve the efficiency of photoanodes, a general design consensus needs to emerge in the field as to how one should formally conceptualize the photovoltage and photocurrent. , Indeed, an improper description of the photovoltage can lead to inaccurate interpretations regarding the performance of PEC cells. Only through the development of such a consensus can universal performance metrics be specified and overall design efficiency goals be met. , …”

Desenvolvimento de heterojunções >i<n>/i<-BiVO>sub<4>/sub</>i<p>/i<-FeMO>sub<x>/sub< (M = Ni, Co e Mn) por pulverização catódica: elucidação da estrutura eletrônica e alinhamento interfacial de bandas na fotoeletrólise da água

“…Na heterojunção do tipo II, ambas BC e BV do Semicondutor 2 ficam acima (se localizem em energias menores) das do Semicondutor 1. 49,[79][80][81][82] E na heterojunção do tipo III, ambas BC e BV do Semicondutor 2 ficam acima da BC do 1, e, consequentemente, acima de sua banda de valência, como pode ser visto na Figura 16:…”