Photocharging is a process in which electrons are accumulated in a semiconductor upon bandgap excitation followed by quenching of the photogenerated holes by reductants. In semiconductors with excess of electrons, negative charge is compensated by cations, among which the most ubiquitous is H+. Under anaerobic conditions photocharged semiconductors retain excess of electrons in the conduction band for days and even weeks. Molecular scientists could immediately recognize remarkable similarities between semiconductors photocharging with photoreduction of viologens, rhodamine dyes and acridinium salts to their persistent radicals.
Photocharging of semiconductors has been studied since 1980s both from a fundamental perspective and application—as a source of electrons and protons for reduction of organic compounds in dark, solar-to-electric energy conversion, and recently also in the design of autonomous microswimmers.
Obviously, the number of electrons that a single semiconductor particle can store depends on many parameters. These parameters are: type of a semiconductor – TiO2 or ZnO, particle diameter – sub nanometer clusters or micro-meter-sized particles, type of sacrificial reductant that is used to quench holes – water, alcohols or aliphatic amines, etc.
The vast amount of research data accumulated in this research field in the past 40 years allows performing quantitative analysis. In other words, to correlate, for example, number of electrons stored in a single semiconductor particle with its volume. Indeed, there is a rather strong correlation between these parameters.
Figure 1. Dependence of the number of electrons stored per semiconductor particle on its volume.
On the other hand, numbers of moles of electrons stored in one gram of a semiconductor material depends only weakly on particle volume.
Figure 2. Dependence of specific concentration of electrons in photocharged semiconductor particle on its volume.
As digitalization enters every aspect of our life and also science, I created an online Database of Photocharged Materials that provides an access to the processed data in the form of interactive vizes. Taking into account increasing interest in the area of materials photocharging, the database will be expanded in future. But already today data scientists, possibly in combination with machine learning, can use it to find other hidden dependencies between materials properties and their ability to store solar energy in the form of separated charges. Analysis of semiconductors structure and their ability to undergo photocharging will lead to the development of high-performing materials.
More examples of the database use are in the Advanced Energy Materials review article.
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