First principles approach to solar energy conversion efficiency of semiconductor heterojunctions

We present an approach to determine from first principles the expected efficiency of semiconductors heterojunctions in solar light absorption and electron-hole pairs generation for photocatalysis and solar cells applications. In a composite material, upon absorption of photons of appropriate wavelength, electrons and holes can migrate towards different components of the junction, thus reducing the electron-hole recombination probability. This occurs when the bands of the two semiconductors are properly aligned. However, a favorable band alignment is not a sufficient condition to have a device able to convert efficiently solar light into chemical or electrical energy. Using the Transfer Matrix Method and Density Functional Theory calculations we evaluate the transmittance and reflectivity of the charge carriers, and we determine two descriptors, the injection efficiency and the maximum conversion efficiency, that allow us to predict in a more realistic way the performance of the device. This method is based on the explicit inclusion of surface, interface, and quantum confinement effects that can be present when the heterojunction is formed. By using the YP/TiO2 (Y = Al, Ga, In) and the CsPbX3/TiO2 (X = Cl, Br, I) systems as examples we show that the approach provides an excellent agreement with the available experimental data.