Semiconducting ferroelectric perovskites with intermediate bands via $B$-site ${\mathrm{Bi}}^{5+}$ doping

We propose $B$-site ${\mathrm{Bi}}^{5+}$-doped ferroelectric perovskite materials as suitable candidates for the bulk photovoltaic effect and related solar applications. The low-lying $6s$ empty states of the electronegative Bi atom produce empty bands in the energy gap of the parent materials, effectively lowering the band gap by 1–2 eV, depending on the composition of the ferroelectric end member and the concentration of ${\mathrm{Bi}}^{5+}$ in the solid solution. The polarization decreases but survives upon doping, which enables the “shift-current” mechanism for photocurrent generation, while the decreased band gap allows absorption of much of the visible spectrum. The magnitude of shift-current response is calculated for $0.75{\mathrm{Pb}}_2{\mathrm{InNbO}}_6\text{−}0.25{\mathrm{Ba}}_2{\mathrm{InBiO}}_6$ (PIN-BIB) and $0.75{\mathrm{Pb}}_2{\mathrm{ScNbO}}_6\text{−}0.25{\mathrm{Ba}}_2{\mathrm{ScNbO}}_6$ (PSN-BSB) and is predicted to exceed the visible-light bulk photovoltaic response of all previously reported materials, including ${\mathrm{BiFeO}}_3$. Furthermore, the existence of their intermediate bands and multiple band gaps, combined with Fermi-level tuning by $A$-site co-doping, also allows for their potential application in traditional $p-n$ junction-based solar cells as broad-spectrum photoabsorbers.

Figure 1

LDA + $U$ band structures and PDOS of (a) KNO and (b) KNB; shaded areas indicate total DOS.

Figure 2

(a) LDA band gap and (b) polarization vs composition for PIN-BIB and PSN-BSB. $x$ is the concentration of end member BIB or BSB.

Figure 3

Band structures and PDOS for (a) PIN-BIB and (b) PSN-BSB; shaded areas indicate total DOS.

Figure 4

E-point intermediate-band wave functions along the Bi-O-$B^{\prime }$-O-Bi chain in (a) PIN-BIB and (b) PSN-BSB.

Figure 5

Comparison between (a),(c) DFT and (b),(d) tight-binding electronic bands for (a),(b) PIN-BIB and (c),(d) PSN-BSB. The intermediate bands are composed of circles whose centers reside on the corresponding energy levels and whose radii are proportional to the norm of the coefficients of the atomic orbital basis for the eigenstates at each k point.

Figure 6

Total bulk photovoltaic susceptibility $\left|\sigma \right|$ (solid line) and Glass coefficient $\left|G\right|$ (dashed line) of (a) PIN-BIB and (b) PSN-BSB.