Maximizing Short Circuit Current Density and Open Circuit Voltage in Oxygen Vacancy-Controlled Bi_1–xCa_xFe_1–yTi_yO_3−δ Thin-Film Solar Cells

Abstract

Designing solid-state perovskite oxide solar cells with large short circuit current (J SC ) and open circuit voltage (V OC ) has been a challenging problem. Epitaxial BiFeO 3 (BFO) films are known to exhibit large V OC (>50 V). However, they exhibit low J SC (≪μA/cm 2 ) under 1 Sun illumination. In this work, taking polycrystalline BiFeO 3 thin films, we demonstrate that oxygen vacancies (V O ) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high J SC and V OC simultaneously. While aliovalent substitution (Ca 2+ at Bi 3+ site) is used to control the lattice V O , Ca and Ti cosubstitution is used to bring out only GB-V O . Fluorine-doped tin oxide (FTO)/ Bi 1−x Ca x Fe 1−y Ti y O 3−δ /Au devices are tested for photovoltaic characteristics. Introducing V O increases the photocurrent by four orders (J SC ∼ 3 mA/cm 2 ). On the contrary, V OC is found to be <0.5 V, as against 0.5−3 V observed for the pristine BiFeO 3 . Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-V O . This creates defect bands occupying the bulk band gap, as inferred from the diffused reflection spectra and band structure calculations, leading to a three-order increase in J SC . The cosubstitution, following a charge compensation mechanism, decreases the lattice V O concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve V OC (3−8 V) much larger than that of BiFeO 3 (0.5−3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice V O concentration dominates GB-V O concentration. Notwithstanding, both lattice and GB-V O contribute to the increase in J SC ; the former weakens ferroelectricity, and as a consequence, undesirably, V OC is lowered well below 0.5 V. Using optimum J SC and V OC , we demonstrate that the efficiency ∼0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.

Fe3+‐ and Nb5+‐Substituted Na0.5Bi0.5TiO3: A Route toward Enhanced Ferroelectric Photovoltaic Response in Al/NBFNT/Ag Solar Cell through Reduced Bandgap and Controlled Oxygen Vacancy

“…However, the mixing of the R3c phase along with slight tetragonal symmetry could affect small changes in V oc of the Al/NBFNT75/Ag system based on the reduction of symmetry breaking mechanism. [23] The results of this study demonstrated that the measured short-circuit current density increases linearly with reduced bandgap, as shown in Table 4. By increasing metal content, the absorption edge was shifted to higher wavelength regions (Figure S6, Supporting Information), indicating a wider range of solar absorption and thus enhancing PV efficiency.…”

“…When a bulk PV device is illuminated with light, charge carriers are introduced at the metal/FE/metal interface. [ 23,24 ] The work function difference between the two metal electrodes ( φ Al = 4.125 eV and φ Ag = 4.28 eV) creates an asymmetric charge accumulation at both interfaces, generating a built‐in electric field that separates the photogenerated charge carriers and produces a large open‐circuit voltage. [ 23 ] However, according to Nandy et al, maximum V and large V oc cannot exist in the same system.…”

“…Similar to the Schottky barrier effect that generates a built‐in electric field at the interface, vacancies in FE materials (such as oxygen vacancies) can also create a built‐in electric field that affects their photovoltaic performance. [ 23 ] However, oxygen vacancies have been reported to be advantageous for creating gap states for visible light absorption and detrimental to ferroelectricity. [ 24 ] The presence of oxygen vacancies (V) in a material not only adversely affects its FE properties, such as fatigue and domain pinning, but also leads to unstable photovoltaic behavior.…”

Controlling the Low-temperature Ionic Purification of a Silicon Surface by Electron Spectroscopy

“…One of the most popular materials in modern micro-and nanoelectronics is monocrystalline silicon and silicon dioxide [1,2]. Recently, thanks to the creation of quantum-dimensional structures on silicon [3][4][5], the di culty of using the above materials in photoelectronics, due to their low probability of radiative recombination, has been overcome [6][7]; thus, a new impetus has been obtained for its widespread use in the aforementioned elds of science and technology [8]. In the production of silicon integrated circuits, in addition to the quality of the surface structure, the choice of the crystallographic orientation of the substrate crystals is of great importance [9][10][11]; for example, the (111) planes are oxidized faster than the (100) planes due to the high surface packing density of atoms capable of entering the oxidation reaction [12].…”

Above-Band-Gap Voltage from Oriented Bismuth Ferrite Ceramic Photovoltaic Cells

“…With delicate dopings, open-circuit voltages were claimed to be tuned from 0.5–3 to 3–8 V. However, the reported data support an average voltage of 6 V with a widely dispersed voltage from 3 to 8 V, in which the highest voltage is claimed as the top voltage in the literature. Remarkable efficiencies of 0.11–0.22% were recorded for FTO/Bi 0.8 Ca 0.2 FeO 3−δ /Au cells with open-circuit voltages from 0.1 to 0.3 V and current densities of ∼2.6 to 3.2 mA·cm –2 . Although the enhanced ferroelectric properties account for the high voltage on doped BFO samples, the varied current and voltage on a definite component are vague, reflecting the uncertainty about the nature of the reported above-band-gap voltages.…”