Abstract
Creation of a partially filled intermediate band in a photovoltaic absorber material is an appealing concept for increasing the quantum efficiency of solar cells. Recently, we showed that formation of a partially filled intermediate band through doping a host semiconductor with a transition metal dopant is hindered by the strongly correlated nature of d-electrons and the antecedent Jahn–Teller distortion, as we have previously reported. In present work, we take a step forward and study the delocalization of a filled (valence-like) intermediate band throughout the lattice: a case study of Ti- and Nb-doped In2S3. By means of hybrid density functional calculations, we present extensive analysis on structural properties and interactions leading to electronic characteristics of Ti- and Nb-doped In2S3. We find that Nb creates an occupied doublet, which can become delocalized onto the crystal at high but feasible concentrations (around 2.5 at% and above). As a consequence, doping In2S3 with adequately high concentrations of Nb allows the subgap intermediate band to conduction band absorption, which leads to higher photocurrent densities compared to pure In2S3. Ti on the other hand forms an occupied singlet intermediate band, which remains strongly localized even at high concentration of 5 at%.
Highlights
The idea of multi-transition solar cells was first introduced by Wolf in 19591, 5 years after manufacturing of the first silicon p–n junction photocell
The results clearly indicate that Ti t+ state splits into an occupied singlet at 0.14 eV above the valence band maximum (VBM) and an empty doublet inside the conduction band (CB)
Our results indicate that the heat of solution, Hs, of Ti and Nb are positive in both In-rich and S-rich atmospheres, suggesting that their dissolution into In2S3 is an endothermic reaction
Summary
The idea of multi-transition solar cells was first introduced by Wolf in 19591, 5 years after manufacturing of the first silicon p–n junction photocell. For a better utilization of the solar spectrum, he suggested to introduce deep impurity levels within the absorber band gap, e.g., by means of transition metal doping[2] These levels would facilitate sequential absorption of lower-energy photons from the valence band (VB) to empty defect levels and from the filled defect levels to the conduction band (CB). In 1997, Luque and Martí[5] demonstrated theoretically that outperforming the Shockley–Queisser (SQ) efficiency limit[6] of solar cells is achievable by introducing a partially filled intermediate band (IB) instead of an isolated defect level inside the band gap The core of their idea is that through formation of partially filled IBs, two photons with sub-band gap energies can excite electrons and generate electron-hole pairs.
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