Bulk Photovoltaic Effect at Visible Wavelength in Epitaxial Ferroelectric BiFeO3 Thin Films

Adv. Mater. 2010, 22, 1763–1766 2010 WILEY-VCH Verlag G T IO N While silicon-based diodes have been the dominant solar cell type, novel photovoltaic mechanisms are being explored in pursuit of lower cost or improved efficiency. In a semiconductor photodiode, such as a Si solar cell, photons with energy higher than the band gap are absorbed to produce electron-hole pairs, which are separated by the internal field in the p–n junction and collected with the electrodes. However, a p–n junction is not a prerequisite for the photovoltaic effect. For exitonic solar cells, photon absorption creates excitons, which dissociate at a heterojunction. In materials without a center of symmetry, such as ferroelectric materials, steady-state photocurrent can exist in a homogeneous medium under uniform illumination, a phenomenon called bulk photovoltaic effect (BPVE). BPVE is a fascinating mechanism with many unique features such as extremely large photovoltage, a photocurrent proportional to the polarization magnitude, and charge-carrier separation in homogeneous media. Observed in bulk ferroelectrics in as early as 1950s, BPVE has seen a resurgent interest recently, especially in ferroelectric thin films. It has been proposed that remarkably higher photovoltaic efficiency can be achieved in thin films. On the other hand, open-circuit voltage much larger than the band gap has also been achieved with ferroelectric thin films with in-plane interdigital electrodes, which has led to the development of UV sensors and dosimeters. The ferroelectric thin-film materials under the previous study, such as BaTiO3 and Pb(ZrTi)O3, have wide band gaps (typically larger than 3.3 eV) corresponding to the UV region. BPVE in visible wavelength could lead to the development of new photovoltaic cells or other novel optoelectronic devices. BiFeO3 (BFO), a multiferroic material at room temperature with a band gap near 2.74 eV and a very large remnant ferroelectric polarization, offers a unique opportunity for such an investigation. Appreciable photoconductivity in visible light has been reported in BFO. Optical studies by absorption spectroscopy and spectroscopic ellipsometry have shown that BFO has a direct band gap with high absorption coefficient. Recently, a switchable-diode effect and a visible-light photovoltaic effect has been observed in BFO bulk crystals. However, no value of photovoltage has been reported for BFO single crystals and significant bulk photovoltaic response has not been demonstrated in BFO thin film. It is also unclear if the photovoltaic response in BFO is due to the diode effect. Here, we studied the photovoltaic effect in epitaxial BFO thin films and obtained an open-circuit voltage Voc of 0.3 V. We further demonstrated that photocurrent direction can be switched by the polarization direction of the BFO film and that the ferroelectric polarization is the main driving force of the observed photovoltaic effect. Moreover, the as-deposited BFO films were self-polarized and they could readily function as a photovoltaic cell without any poling. Epitaxial BFO thin films of 170 nm were grown by radiofrequency (RF) magnetron sputter deposition on a (001)c SrTiO3 (STO) substrate, with a 60-nm layer of SrRuO3 (SRO) as the bottom electrode. The resulting films show good epitaxy as determined by high-resolution X-ray diffraction (HRXRD; Supporting Information, Fig. S1). The polarization–electric field (P–E) hysteresis measurement shows a remnant polarization (Pr) of more than 65mCcm 2 with a Au top electrode (Fig. S2). Devices with an indium tin oxide (ITO) top electrode have a slightly smaller Pr. Figure 1a shows the spectral response of the short-circuit current (Jsc) of the BFO film. Highest current density is detected at 460 nm, closely corresponding to the measured BFO band gap of 2.72 eV (Fig. S3). Incident light at 435 nm, slightly above band gap, was used for the current-density–voltage (J–V) measurement (Fig. 1b). The as-deposited samples were electrically poled before measurement. The poling direction is termed positive if a positive bias voltage is applied to the top electrode with the bottom electrode grounded. In the J–Vmeasurement, the applied voltage is positive if a positive bias voltage is applied to the bottom electrode. Fig. 1b shows that for the positively poled samples the photocurrent is positive (i.e., it flows out of the top electrode). In contrast, after the negative poling, the photocurrent direction is reversed. The magnitudes of both the photocurrent and photovoltage are smaller in positively poled samples than in negatively poled ones. Jsc is observed to increase almost linearly with the illumination intensity (Fig. 1c), whileVoc saturates at high illumination intensity (Fig. 1d). At the highest illumination intensitymeasured,Voc in the negatively poled film of 170-nm thickness is 0.286V. The substantialVoc obtainedhere is probably a result of the low conductivity of our samples, which is on the order of 10V 1 cm , six orders of magnitude smaller than that reported by Basu et al. and also much smaller than that reported by Choi. The photovoltaic response for the as-deposited films without any poling was also measured. The results are surprisingly