Semiconductors Used in Photovoltaic and Photocatalytic Devices: Assessing Fundamental Properties from DFT Tangui Le Bahers,* ,† Michel Re ́ rat, ‡ and Philippe Sautet † † Universite ́ de Lyon, Universite ́ Claude Bernard Lyon1, ENS Lyon, Centre Nationale de Recherche Scientifique, 46 allé ed’Italie, 69007 Lyon Cedex 07, France ‡ Equipe de Chimie-Physique, IPREM UMR5254, Universite ́ de Pau et des Pays de l’Adour, Hé lioparc, 2 avenue du Pre ́ sident P. Angot, 64053 Cedex, France * S Supporting Information ABSTRACT: The photovoltaic and photocatalytic systems generally use at least one semiconductor in their architecture which role is to absorb the light or to transport the charge carriers. Despite the large variety of working principles encountered in these systems, they share some fundamental steps such as light absorption, exciton dissociation, and charge carrier diffusion. These phenomena are governed by fundamental properties of the semiconductor like the bandgap, the dielectric constant, the charge carrier effective masses, and the exciton binding energy. The ability of density functional theory to compute all of these properties is evaluated. From the particularly good results obtained with the HSE06 functional, it can be concluded that DFT is a reliable tool for the evaluation and prediction of these key properties which open nice perpectives for in silico design of improved semiconductors for solar energy application. In the light of these calculations, some experimental observations on the difference of efficiencies between semiconductors like TiO 2 anatase and rutile or ZnO are interpreted. 1. INTRODUCTION Harvesting energy from sunlight is a major challenge for sustainable development. Devices to convert the energy of light into other forms of energy such as electricity (photovoltaic systems) or chemical compound (photocatalysis, water splitting, CO 2 photoreduction, etc.) play a key role for that objective since they represent a real opportunity to produce electricity, chemicals, hydrogen fuels, etc. at low environmental and economical costs. 1−5 Generally these systems involve a semiconductor (SC) for the light absorption or for the conduction of the photo- generated charge carriers or both. Table 1 presents some examples of applications where a SC is used for the light conversion process. Although the working principle of this type of devices can be complicated and different from one system to another, some fundamental steps are common to all of them. These steps are summed up in Figure 1, each step inducing a specific requirement on the SC properties. Step 1: The light absorption. This is the first step for devices where a SC has to absorb the light. It promotes an electron (e − ) from the valence band to the conduction band of the SC, leaving an electronic vacancy in the valence band, called a hole (h + ). The electron and the hole interact through a Coulomb attraction, giving an entity called exciton. In the case of sunlight absorption and for photovoltaic application, an optimum gap between the valence and conduction bands of the SC exists. As the curve of the maximum efficiency as a function of the bandgap, E g , is pretty flat around the optimum, 26 one can defined a region of optimum, between 1.1 and 1.4 eV, 26 more than a unique optimum. This zone of E g optimum represents a compromise between a high photocurrent in the solar cells (obtained by diminishing the gap) and a high photovoltage (obtained by increasing the gap). This requirement on the gap can be different if the light does not belong to the sunlight spectrum (indoor application for instance) or if the objective is not electricity production but chemical reactions (water splitting for instance). In the case of chemical reaction, a compromise needs Received: September 30, 2013 Revised: February 25, 2014 Published: February 28, 2014 Table 1. Example of applications of the light conversion along with SCs used for this application. The stars indicate SCs that are used for the light absorption. applications example of semiconductors found for these application inorganic photovoltai ̈ c Si*, 6 Ge*, 7 GaAs*, 6,8 CdTe*, 6,9 CuInS 2 *, 10 CdS 11 dye-sensitized solar cells and quantum dots-sensitized solar cells TiO 2 , 3 ZnO, 12 SnO 2 , 13 NiO, 14 CdSe*, 15 CdTe* 15 CO 2 photoreduction TiO 2 , 16 TiO 2 *, 17,18 In(OH) 3 * 19 water splitting Fe 2 O 3 *, 20 CdS*, 21 NiO, 22 Bi 2 S 3 *, 2 Ta 3 N 5 * 23 waste photodegradation BiVO 4 *, 24 BiWO 6 *, 2 TiO 2 * 25 Article pubs.acs.org/JPCC © 2014 American Chemical Society 5997 dx.doi.org/10.1021/jp409724c | J. Phys. Chem. C 2014, 118, 5997−6008