Approaching Charge Separation Efficiency to Unity without Charge Recombination

Improving the efficiency of charge separation (CS) and charge transport (CT) is essential for almost all optoelectronic applications, yet its maximization remains a big challenge. Here we propose a conceptual strategy to achieve CS efficiency close to unity and simultaneously avoid charge recombination (CR) during CT in a ferroelectric polar-discontinuity (PD) superlattice structure, as demonstrated in ${({\mathrm{BaTiO}}_3)}_m/{({\mathrm{BiFeO}}_3)}_n$, which is fundamentally different from the existing mechanisms. The competition of interfacial dipole and ferroelectric PD induces opposite band bending in ${\mathrm{BiFeO}}_3$ and ${\mathrm{BaTiO}}_3$ sublattices. Consequently, the photoexcited electrons ($e$) and holes ($h$) in individual sublattices move forward to the opposite interfaces forming electrically isolated $e$ and $h$ channels, leading to a CS efficiency close to unity. Importantly, the spatial isolation of conduction channels in ${({\mathrm{BaTiO}}_3)}_m/{({\mathrm{BiFeO}}_3)}_n$ enable suppression of CR during CT, thus realizing a unique band diagram for spatially orthogonal CS and CT. Remarkably, ${({\mathrm{BaTiO}}_3)}_m/{({\mathrm{BiFeO}}_3)}_n$ can maintain a high photocurrent and large band gap simultaneously. Our results provide a fascinating illumination for designing artificial heterostructures toward ideal CS and CT in optoelectronic applications.

Figure 1

Schematic band diagrams for (a) conventional $p\text{−}n$ junction model, (b) BPV model, and (c) OCST model. Charge generation and recombination coexist in (I) and (III) zones (mixed red-green color), while zone (II) is free of recombination (green color). $M_1$ and $M_2$ refer to two constituting materials of the interface or superlattice.

Figure 2

(a) Band structure for ${({\mathrm{BaTiO}}_3)}_2/{({\mathrm{BiFeO}}_3)}_2$. (b) Layer-resolved DOS for ${({\mathrm{BaTiO}}_3)}_4/{({\mathrm{BiFeO}}_3)}_{10}$. (c) Schematic plot of ferroelectric polarizations and dipole-induced electric field (left) and electrostatic potential ($U$) along the superlattice direction (right). Hollow arrows are ferroelectric polarizations ($P_1$, $P_2$, and $P_1^{\prime }$), and pink arrow is electric field induced by dipolar polarization ($E_d$). Interface structural relaxation in ${\mathrm{BiFeO}}_3$ leads to $P_1^{\prime }<P_1$. CBM in ${\mathrm{BiFeO}}_3$ are marked by pink circles. (d) Charge density distributions at VBM (left) and CBM (right) for ${({\mathrm{BaTiO}}_3)}_4/{({\mathrm{BiFeO}}_3)}_{10}$.

Figure 3

Three-dimensional band structure combining both reciprocal energy space and real lattice space for ${({\mathrm{BaTiO}}_3)}_4/{({\mathrm{BiFeO}}_3)}_{10}$. Under photoexcitation, both ${\mathrm{BaTiO}}_3$ and ${\mathrm{BiFeO}}_3$ layers show intralayer excitation, then the built-in potential drives the generated $e$ or $h$ to drift to its own energy minima toward type-A or type-B interfaces. See text for more explanation.

Figure 4

(a) Calculated charge separation efficiency ($R$ value) as a function of energy integration area for ${({\mathrm{BaTiO}}_3)}_4/{({\mathrm{BiFeO}}_3)}_n$ ($n=4$, 6, 8, 10). $R$ values of ${\mathrm{MoSe}}_2/{\mathrm{MoS}}_2$ [136] and ${\mathrm{WS}}_2/{\mathrm{MoS}}_2$ [137] are also included for direct comparison. Inset: illustrates definition of $R$ value to estimate the charge separation by DOS [57], where $M$ and $M^{\prime }$ are integrated DOS near the VBM or CBM for materials 1 and 2, respectively, and the integration begins from the VBM (CBM) to a lower (higher)-energy position within an energy range of $\delta E$. The dashed line is Fermi level. (b) Comparison of calculated various photocurrents $J$ ($J_{\mathrm{SC}}$, $J_G$,$J_R$) for ${({\mathrm{BaTiO}}_3)}_2/{({\mathrm{BiFeO}}_3)}_2$ superlattice and reference single ${\mathrm{BiFeO}}_3$ based on macroscopic device simulations, along with experimental data obtained from Refs. [12, 72, 76, 77, 78, 79, 80, 81, 138, 139, 140].