Increasing bulk photovoltaic current by strain tuning

Photovoltaic phenomena are widely exploited not only for primary energy generation but also in photocatalytic, photoelectrochemistry, or optoelectronic applications. In contrast to the interface-based photovoltaic effect of semiconductors, the anomalous or bulk photovoltaic effect in ferroelectrics is not bound by the Shockley-Queisser limit and, thus, can potentially reach high efficiencies. Here, we observe in the example of an Fe-doped LiNbO3 bulk single crystal the existence of a purely intrinsic ``piezophotovoltaic'' effect that leads to a linear increase in photovoltaic current density. The increase reaches 75 under a low uniaxial compressive stress of 10 MPa, corresponding to a strain of only 0.005\%. The physical origin and symmetry properties of the effect are investigated, and its potential for strain-tuned efficiency increase in nonconventional photovoltaic materials is presented

Stress - modulated bulk photovoltaic effect in polar oxide crystals

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

Low-temperature surface phase transitions in multiferroic BiFeO3 nanocrystals probed via electron paramagnetic resonance

The low-temperature phase transitions observed in magnetoelectric bismuth ferrite (BiFeO3, BFO) have recently been a topic of interest to several researchers. This communication focuses on connecting recent structural revelations, such as the existence of a “skin” layer in BFO, the lattice contraction, and subsequent expansions in the skin layers with X-band electron paramagnetic resonance (EPR) spectra. A closer look at Lande’sg-factor and the EPR asymmetry parameters reveal vital information about the origin of the phase transitions at 140, 200, and 280 K. Correlating the EPR results with existing theoretical calculations indicates that oxygen vacancies (VO) accumulate at the skin layer, causing lattice contraction. This contraction causes local changes in the spin magnetic moment and translates to an anomaly in the resonant lines. The discussions imply that the phase transition at 140 K is due to spin reorientation caused by changes of interatomic distances and angles between the FeFe3+2+-VO••-FeFe3+2+sites. Transition at 200 K is observed to occur due to elastic distortion of the oxygen octahedra. The transition at 280 K is thought to be due to the freezing of spins in the lattice

About defect phenomena in ZnO nanocrystals

ZnO nanocrystals are receiving renewed attraction due to their multifunctional properties. Selective enhancement and tuning of their optical and electrical properties are essential for achieving novel devices with accurate sensing and conducting capabilities. The nature and type of intrinsic defects that occur in ZnO influence these properties. In this work, we investigate the intrinsic defect structure of ZnO via electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy and correlate the results with existing computational works. Mainly, the defects are analysed by taking the microscopic defect structure of the lattice into account. The results manifest the core-shell model of the defect structure in ZnO. By default, specifically for nanocrystals, oxygen vacancies localise on the surface, while zinc vacancies localise in the core. The investigations in this report demonstrate that the concentration of the intrinsic defects and their position can be tuned merely by changing the size of the nanocrystal. Additionally, the UV, green, orange and red emissions can be tuned by nanocrystal's size and post-annealing treatments

Spectroscopic probing of Mn-Doped ZnO nanowires synthesized via a microwave-assisted route

Dilute magnetic semiconductors such as transition-metal-doped ZnO are potential candidates for spintronic applications. Transition metals such as Mn, Fe, and Cu when doped in ZnO enable spin magnetic properties to conventional semiconductors. Although several techniques such as wet chemical and vapor deposition methods are employed to achieve homogeneous doping in ZnO, these methods have limits pertaining to solubility levels of dopant ion, morphology, competition between intrinsic and extrinsic defects and localization of the defect species. This manuscript is an addition to the vast knowledge of methods and protocols that present the synthesis of transition-metal-doped ZnO. In this report, manganese-doped ZnO is synthesized via a microwave-assisted hydrolysis technique. The defect structure of Mn-doped ZnO wires is investigated via electron paramagnetic resonance and photoluminescence techniques. The analysis indicates that Mn2+ substitutes the Zn ion and dominates the intrinsic defect species in ZnO

Competition between charge transfer and energy transfer: influence of intrinsic defects in ZnO nanocrystals on rhodamine-B dye color signals

The complexity of the competition between nonradiative energy transfer and charge transfer is presented in this work. This study uses ZnO nanocrystals as a donor component and rhodamine B (RhB) dye as an acceptor component. Investigations reveal that the concentration of intrinsic defects and their localization, particularly oxygen vacancies, zinc interstitials, and oxygen interstitials, play a vital role in nonradiative energy transfer from ZnO nanocrystals to RhB dye. Additionally, photoluminescent spectra indicate that ZnO nanocrystals degrade the emission signals of RhB dye via charge transfer. It is possible that a part of oxygen vacancies may also contribute to the photocatalytic oxidation of the RhB molecule. The correlation between electron paramagnetic resonance, photoluminescence, and the RhB emission decay rates indicates two processes that one may encounter when dealing with semiconductor-dye conjugates. First is a complex energy transfer from ZnO to RhB indicating a photoluminescent up-conversion and second is a charge transfer from photoexcited intrinsic defect species in ZnO suppressing the RhB emission signals, which occur together. The defect species involved in photoluminescent up-conversion in ZnO+RhB is due to a nonradiative energy transfer from photoexcited zinc interstitials or doubly charged oxygen vacancies in ZnO to RhB. Additionally, the suppression of RhB emission occurs due to charge transfer from oxygen vacancies in ZnO