Tuning the bandgap of multiferroic solar cells made from Bi2FeCrO6 is achieved by cationic ordering and is shown to dramatically improve their performance.

Bandgap tuning of multiferroic oxide solar cells

Recently, the anomalous photovoltaic (PV) effect in BiFeO3 (BFO) thin films, which resulted in open circuit voltages (Voc) considerably larger than the band gap of the material, has generated a revival of the entire field of photoferroelectrics. Here, via temperature-dependent PV studies, we prove that the bulk photovoltaic (BPV) effect, which has been studied in the past for many non-centrosymmetric materials, is at the origin of the anomalous PV effect in BFO films. Moreover, we show that irrespective of the measurement geometry, Voc as high as 50 V can be achieved by controlling the conductivity of domain walls (DW). We also show that photoconductivity of the DW is markedly higher than in the bulk of BFO.

/pdf/role-of-domain-walls-in-the-abnormal-photovoltaic-effect-in-1076aj8hsl.pdf

Role of domain walls in the abnormal photovoltaic effect in BiFeO 3

This review presents the fundamental theoretical concepts concerning the photogalvanic effect (PGE)—the phenomenon of appearance of a direct current in a homogeneous medium under uniform illumination. This effect can occur in all media lacking a center of symmetry, in particular, in ferroelectrics, piezoelectrics, gyrotropic crystals, and in gases and liquid possessing natural optical activity. The starting point of a systematic microscopic theory is the asymmetry of the elementary electronic processes—their noninvariance with respect to spatial reflection. Within the framework of the theory, we study the most important mechanisms of the PGE in the regions of impurity, interband, and intraband light absorption. Possible observable manifestations of the PGE are discussed. Theoretical results are compared with experimental data.

https://iopscience.iop.org/article/10.1070/PU1980v023n03ABEH004703/pdf

The photogalvanic effect in media lacking a center of symmetry

The Shockley–Queisser limit for solar cells is overcome in the ferroelectric insulator BaTiO3.

Power conversion efficiency exceeding the Shockley–Queisser limit in a ferroelectric insulator

The bulk photovoltaic effect (BPVE) refers to the generation of a steady photocurrent and above-bandgap photovoltage in a single-phase homogeneous material lacking inversion symmetry. The mechanism of BPVE is decidedly different from the typical p–n junction-based photovoltaic mechanism in heterogeneous materials. Recently, there has been renewed interest in ferroelectric materials for solar energy conversion, inspired by the discovery of above-bandgap photovoltages in ferroelectrics, the invention of low bandgap ferroelectric materials and the rapidly improving power conversion efficiency of metal halide perovskites. However, as long as the nature of the BPVE and its dependence on composition and structure remain poorly understood, materials engineering and the realisation of its true potential will be hampered. In this review article, we survey the history, development and recent progress in understanding the mechanisms of BPVE, with a focus on the shift current mechanism, an intrinsic BPVE that is universal to all materials lacking inversion symmetry. In addition to explaining the theory of shift current, materials design opportunities and challenges will be discussed for future applications of the BPVE.

/pdf/shift-current-bulk-photovoltaic-effect-in-polar-materials-n1znw635rg.pdf

Shift current bulk photovoltaic effect in polar materials—hybrid and oxide perovskites and beyond

In ferroelectric melt-grown BaTiO3 single crystals steady-state short-circuit photocurrents have been observed parallel to the c-axis. At room temperature light polarized parallel and perpendicular to the c-axis produces short-circuit photocurrents with opposite sign. Along with the temperature dependence a physical model of this effect is outlined.

Anomalous photovoltage in BaTiO3

The charge transport giving rise to the photovoltaic effect is compared with the photoconductivity in the photon energy range from 2.5 eV to 5 eV. The photoconductivity at 2.5 eV is activated with about the same activation energy as the Hall mobility, which was measured for reduced samples between room temperature, where it is 1.1×10−3 cm2/Vs, and 200°C. The dependence on temperature, intensity, and time demonstrates different charge transport mechanisms for the short-circuit photocurrent and for the photoconductivity.

The photovoltaic effect and the charge transport in LiNbO3

Ferroelectric domains on TGS cleavage faces can be decorated by well conducting Te layers of more than 100 A thickness. The Te crystallites are large (up to 200 μm) on positive domains and small (∼ 1 μm) on negative domains. On both domains the c-axis of the Te crystallites is parallel to the TGS surface. In addition a fibre texture of the Te films is observed on negative domains with the c-axis of all Te crystallites parallel to the c-axis of the TGS. The mechanism for the decoration of domains on TGS is discussed.

Oriented growth of Te on tgs cleavage faces

In a ferroelectric with semiconducting electrodes the polarization is neutralized incompletely. The associated depolarization field leads to a reduction of the spontaneous polarization. The dependence of the polarization on the thickness of a unidomain ferroelectric is calculated for TGS in contact with one metal electrode and one semiconductor electrode in which the neutralization of the polarization leads to a depletion of majority carriers. The depolarization field manifests itself severely.

P. Wurfel

Papers

Anomalous photovoltage in BaTiO3

The photovoltaic effect and the charge transport in LiNbO3

Oriented growth of Te on tgs cleavage faces

Polarization reduction in thin ferroelectrics with semiconducting electrodes