Doping limits, band gaps, work functions and energy band alignments of undoped and donor-doped transparent conducting oxides Zn0, In2O3, and SnO2 as accessed by X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) are summarized and compared. The presented collection provides an extensive data set of technologically relevant electronic properties of photovoltaic transparent electrode materials and illustrates how these relate to the underlying defect chemistry, the dependence of surface dipoles on crystallographic orientation and/or surface termination, and Fermi level pinning.

Transparent Conducting Oxides for Photovoltaics: Manipulation of Fermi Level, Work Function and Energy Band Alignment

The application of ferroelectric materials (i.e. solids that exhibit spontaneous electric polarisation) in solar cells has a long and controversial history. This includes the first observations of the anomalous photovoltaic effect (APE) and the bulk photovoltaic effect (BPE). The recent successful application of inorganic and hybrid perovskite structured materials (e.g. BiFeO3, CsSnI3, CH3NH3PbI3) in solar cells emphasises that polar semiconductors can be used in conventional photovoltaic architectures. We review developments in this field, with a particular emphasis on the materials known to display the APE/BPE (e.g. ZnS, CdTe, SbSI), and the theoretical explanation. Critical analysis is complemented with first-principles calculation of the underlying electronic structure. In addition to discussing the implications of a ferroelectric absorber layer, and the solid state theory of polarisation (Berry phase analysis), design principles and opportunities for high-efficiency ferroelectric photovoltaics are presented.

/pdf/ferroelectric-materials-for-solar-energy-conversion-19ffns6ifj.pdf

Ferroelectric materials for solar energy conversion: photoferroics revisited

/pdf/ferroelectric-materials-for-solar-energy-conversion-2y5gwkw07q.pdf

Ferroelectric Materials for Solar Energy Conversion: Photoferroics Revisited

The nature of the band gap of indium oxide is still a matter of debate. Based on optical measurements the presence of an indirect band gap has been suggested, which is 0.9 to $1.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ smaller than the direct band gap at the $\ensuremath{\Gamma}$ point. This could be caused by strong mixing of $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ and $\mathrm{In}\phantom{\rule{0.3em}{0ex}}4d$ orbitals off $\ensuremath{\Gamma}$. We have performed extensive density functional theory calculations using the LDA+U and the GGA+U methods to elucidate the contribution of the $\mathrm{In}\phantom{\rule{0.3em}{0ex}}4d$ states and the effect of spin-orbit coupling on the valence band structure. Although an indirect band gap is obtained, the energy difference between the overall valence band maximum and the highest occupied level at the $\ensuremath{\Gamma}$ point is less than $50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. It is concluded that the experimental observation cannot be related to the electronic structure of the defect free bulk material.

Band structure of indium oxide: Indirect versus direct band gap

Energy band alignment at interfaces of semiconducting oxides: A review of experimental determination using photoelectron spectroscopy and comparison with theoretical predictions by the electron affinity rule, charge neutrality levels, and the common anion rule

Transparent conductive In2O3 films were deposited by reactive evaporation of In and analyzed in-situ with photoelectron spectroscopy. The interface formation of In2O3 with evaporated CdTe has been investigated using the same technique. A valence band offset ΔEVB=2.1±0.1 eV is determined, resulting in a negligible conduction band offset. However, In2O3 will not provide an Ohmic contact to n-CdTe, due to the Fermi level position at the interface.

F. Rüggeberg

Papers

The In2O3/CdTe interface: A possible contact for thin film solar cells?