Device Performance of the Mott Insulator ${\mathrm{LaVO}}_3$ as a Photovoltaic Material

Searching for solar-absorbing materials containing earth-abundant elements with chemical stability is of critical importance for advancing photovoltaic technologies. Mott insulators have been theoretically proposed as potential photovoltaic materials. In this paper, we evaluate their performance in solar cells by exploring the photovoltaic properties of Mott insulator ${\mathrm{LaVO}}_3$ (LVO). LVO films show an indirect band gap of 1.08 eV as well as strong light absorption over a wide wavelength range in the solar spectrum. First-principles calculations on the band structure of LVO further reveal that the $d\text{−}d$ transitions within the upper and lower Mott-Hubbard bands and $p\text{−}d$ transitions between the O $2p$ and V $3d$ band contribute to the absorption in visible and ultraviolet ranges, respectively. Transport measurements indicate strong carrier trapping and the formation of polarons in LVO. To utilize the strong light absorption of LVO and to overcome its poor carrier transport, we incorporate it as a light absorber in solar cells in conjunction with carrier transporters and evaluate its device performance. Our complementary experimental and theoretical results on such prototypical solar cells made of Mott-Hubbard transition-metal oxides pave the road for developing light-absorbing materials and photovoltaic devices based on strongly correlated electrons.

Figure 1

(a) Schematic illustration of energy levels for the Mott insulator and charge-transfer insulator. (b) Shockley-Queisser limit as a function of band gap, along with the solar spectrum. The band gaps of various prototypical TMOs are marked.

Figure 2

(a) XRD $2\theta \text{−}\omega$ scan of the ${\mathrm{LaVO}}_x$ films grown on LSAT(001) substrates with different oxygen pressures. (b) RSM data collected around the LSAT and LVO $(\overline{1}03)$ reflections. (c) The $M\text{−}T$ curve measures at 100 Oe. The antiferromagnetic temperature $T_{\mathrm{AFM}}$ is marked by the solid arrow.

Figure 3

XPS spectra and fitting curves of the LVO films deposited at (a) ${10}^{-3}\mathrm{Torr}$ and below ${10}^{-6}\mathrm{Torr}$. The various valence states of vanadium cation are labeled.

Figure 4

(a) Experimental absorption spectrum measured from the $\mathrm{LVO}/\mathrm{LSAT}(001)$ film and fitting curves with five Gaussian peaks labeled $a$ to $e$. The inset shows the energy-dependent absorption coefficient data of LVO films grown on LSAT(001), LSAO(001), and quartz substrate, in comparison with those of $c\text{−}\mathrm{Si}$, $\alpha \text{−}\mathrm{Si}$, and CdTe. (b) ${(\alpha E)}^2$ and ${(\alpha E)}^{1/2}$ versus $E$ plots for analyzing the band gap of LVO. The solid (dashed) straight lines mark the indirect (direct) band edges. The inset shows the direct band gap analysis in the higher-energy range.

Figure 5

(a) Band structure of LVO calculated using the $\mathrm{GGA}+U$ method. Some important $k$ positions, ${\mathrm{\Gamma }}_1$ to ${\mathrm{\Gamma }}_4$ and $X_1$, are marked. (b) Calculated total DOS along with the partial contributions from O $2p$, V $3d$ (including the V $t_{2g}$ and V $e_g$) and La $5p$ bands. (c) Calculated curves of absorption coefficient ${\alpha }_{xx}$, ${\alpha }_{zz}$, and ${\alpha }_{\text{av}}$, in comparison with the experimental spectrum. The indirect band gap of 1.1 eV is marked by a dashed line, and the absorption tail below the band-gap energy is highlighted by an arrow. The inset shows the ${\alpha }_{\text{av}}\text{−}E$ curves calculated with various $U$ values. (d) Schematic band diagram of LVO. The proposed optical transitions (solid arrows) that may contribute to the light absorption are labeled $a$ to $e$, and the possible forbidden transition (dashed arrow) is also shown.

Figure 6

(a) $\rho \text{−}T$ curve (open circles) measured from the $\mathrm{LVO}/\mathrm{LSAT}(001)$ film along with the TAP and VRH fitting curves (solid lines). The top and lower inset show the $\ln (\rho /T)$ vs $1000/T$ and $\ln \rho$ vs $T^{-1/4}$ dependence, respectively, as well as the corresponding linear fitting curves. (b) Temperature-dependent carrier mobility and concentration data extracted from the Hall measurements.

Figure 7

Dark and illuminated $J\text{−}V$ curves of (a) the device with semitransparent Au electrodes deposited on the $\mathrm{LVO}/\mathrm{NSTO}(001)$ film and (b) the DSSC-like $c\text{−}{\mathrm{TiO}}_2/\mathrm{LVO}/\text{spiro}\text{−}\mathrm{OMeTAD}$ device.

Figure 8

(a) Cross-sectional SEM image of the $m\text{−}{\mathrm{TiO}}_2/\mathrm{LVO}/\text{spiro}\text{−}\mathrm{OMeTAD}$ heterojunction solar cell. (b) EDX elemental line scan profiles along the red line marked in (a). (c) Schematic of the device architecture. Note that the LVO layer (deep blue) covers only the top surfaces of $m\text{−}{\mathrm{TiO}}_2$. (d) Energy levels of the various components in the solar cell.

Figure 9

Dark and illuminated $J\text{−}V$ curves of (a) $m\text{−}{\mathrm{TiO}}_2/\mathrm{LVO}/\text{spiro}\text{−}\mathrm{OMeTAD}$ heterojunction solar cell and (b) $m\text{−}{\mathrm{TiO}}_2/\text{spiro}\text{−}\mathrm{OMeTAD}$ diode without LVO. $J_{\mathrm{SC}}$, $V_{\mathrm{OC}}$ FF, and PCE of these devices are also given.

Figure 10

$J\text{−}V$ curves of the $m\text{−}{\mathrm{TiO}}_2/\mathrm{LVO}/\text{spiro}\text{−}\mathrm{OMeTAD}$ heterojunction solar cells with different thicknesses of (a) LVO layer and (b) $m\text{−}{\mathrm{TiO}}_2$ layer. (c),(d) [(e),(f)] show the evolutions of $V_{\mathrm{OC}}$ [$J_{\mathrm{SC}}$] as functions of $t_L$ and $t_m$, respectively.