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See detailStress - modulated bulk photovoltaic effect in polar oxide crystals
Nadupalli, Shankari UL

Doctoral thesis (2019)

Light-induced phenomena in ferroelectric materials have been exploited for decades for optoelectronic applications. Homogeneous illumination of a non-centrosymmetric ferroelectric material creates ... [more ▼]

Light-induced phenomena in ferroelectric materials have been exploited for decades for optoelectronic applications. Homogeneous illumination of a non-centrosymmetric ferroelectric material creates anomalously high voltages exceeding a value which is usually limited by its band gap. This phenomenon is called the bulk-photovoltaic effect (BPVE). Lithium niobate is a prototypical material for BPVE. The only limiting factor in lithium niobate is its low photo-current values, which can be improved by doping the crystal with donor metals. This study focuses primarily on light induced processes in mono-domain lithium niobate single crystals doped with transition metal ions, particularly the influence of stress on the BPVE. The effect of stress on BPVE is termed the piezo-photovoltaic effect (PPVE). This thesis report is framed to systemically introduce topics which cause, influence and aid in understanding the PPVE. Topics such as the symmetry in crystals, their physical properties, the intrinsic bulk photovoltaic effect (BPVE) are introduced and the structure, defects, light-induced charge transport in donor doped lithium niobate and the reason behind the appearance of BPVE are discussed in this report. The techniques and experimental arrangements used in this work are detailed in this thesis. A direct evidence of BPVE and the influence of stress is shown in the results. Transition metal doped lithium niobate crystals are oriented via x-ray diffraction (XRD) and a basic chemical characterization is undertaken using secondary ion mass spectrometry (SIMS) to identify dopant elements. Absorption spectroscopy in the UV/VIS/NIR range revealed windows in the spectra indicating photo-excitation of the donor doped ions. The absorption lines show that a shift in the fundamental band-edge occurs in lithium niobate for different dopant elements. Electron paramagnetic resonance (EPR) spectrometry is performed on the samples to confirm the location of the dopant ion in the crystal matrix by indicating its symmetry. The difference in the dopant concentration and the change in the oxidation state of the dopant ion under light illumination is obtained from EPR study. Direct measurements to obtain bulk photovoltaic current density in iron doped- lithium niobate single crystals are performed at increasing intensities at different wavelengths to determine the BPV coefficients. This study provides a quantitative analysis of different components of the BPV tensor values. The highest BPV component measured along the polar axis with extraordinary light polarisation is observed when iron doped lithium niobate is illuminated with light wavelength 450 nm. Obtained BPV tensor components are corroborated by the influence of the structural environment and the dipole interactions on charge transport mechanism of BPVE. The charge transport mechanism and the obtained values of the BPV tensor components are justified and discussed on the basis of the polaronic charge transport phenomena existing in the literature. The influence of stress on BPVE is measured using a custom-designed set-up. The PPV components in lithium niobate are experimentally investigated for stress levels in the 1MPa - 10MPa range. A detailed discussion on the experimental observations are given in this report. The prime discovery of this thesis is the intrinsic character of the piezo-photovoltaic effect (PPVE), where increase in the light induced current is observed when the crystal is subjected to uniaxial compressive stress. The Young's modulus of lithium niobate is 202 GPa. Applying 10 MPa compressive stress translates to strain levels of just 50 ppm. 10 MPa of compressive stress along the polar axis of the crystal increased the short-circuit photo-current by 73%. When stress is applied perpendicular to the polar axis, about 370% increase in short-circuit photocurrent was observed with just 50 ppm of strain, which is a drastic for such moderate amounts of stress levels. This study proves the vitality of strain tuning to increase the PV properties in crystalline solar cells. Extrapolating the observed effect, PPVE is envisioned as a phenomenon which could be exploited in other polar oxide ceramics and thin-films where large photovoltaic energy generation can be made possible beating the existing limits. [less ▲]

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