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The Shockley Queisser limit of single junction solar cells can be overcome by introducing an intermediate band (IB) in wide band gap materials. Thus thermalization losses can be reduced [1]. Furthermore sub-bandgap photons can be absorped by valence band to IB and IB to conduction band transitions. According to theoretical calculations $\text{In}_2\text{S}_3$ hyper-doped with vanadium is a suitable candidate to realize such an IB solar cell [2].
We grew $\beta$-$\text{In}_2\text{S}_3$ thin films by physical co-evaporation of the elements on glass and on a-, c-, m-, and r-plane sapphire substrates. The deposition parameters were varied in a wide range to optimize the structural properties of the films. At appropriate deposition parameters (103)-orientation of $\beta$-phase $\text{In}_2\text{S}_3$ was enhanced. Highest crystallinity and smoothest surfaces could be realized for samples epitaxially grown on a-sapphire substrates.
Electrical characterization reveals a strong persistant photoconductivity. To investigate the photovoltaic response we fabricate $pn$-heterojunctions using amorphous $p$-type zinc-cobalt-oxide.
Further we grew $\text{In}_2\text{S}_3\text{:V}$ on sapphire substrates using a combinatorial approach to cover within a single deposition process a wide range of doping concentrations reaching from $1.1 \, \text{at.}\%$ to $11.4 \, \text{at.}\%$ vanadium.
For samples with doping concentrations above a critical concentration of $3.2 \, \text{at.}\%$ vanadium we find an unusual temperature dependence in mobility and charge carrier concentration, which might give evidence to the formation of an IB.
[1] Luque and Martí, Phys. Rev. Lett., 1997, 78, 5014.
[2] Palacios et al., Phys. Rev. Lett., 2008, 101, 046403.
| Category | Solid State (Experiment) |
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