Andrea Crovetto

Modeling and Optimization of an Electrostatic Energy Harvesting Device

Modeling of energy harvesting devices is complicated by the coupling between electrical and mechanical domains. In this paper, we present a coupled electromechanical model for electret-based resonant energy harvesters where the two output pads are placed on the same device side (single-sided). An analytical analysis is complemented by 2-D finite element method simulations, where the fringing field effect on a plane capacitor is studied and accounted for by an effective area that is well fitted by a sinusoidal function of the displacement of the proof mass. From analytical calculations, we prove that the electrostatic transducer force is related to the voltage output and cannot be approximated by viscous damping or a Coulomb force as reported previously. The coupled model with two simultaneous differential equations is numerically solved for the voltage output and transduction force with given parameters. The model was verified both by practical measurements from our own fabricated device and results from a reference. An optimization study is carried out using this model to achieve the maximum output power by tuning the allowable movement (XM) of the proof mass. Finally, the effect of a standard power-conditioning circuit is investigated for both continuous and burst power supply applications.

Lattice-matched Cu2ZnSnS4/CeO2 solar cell with open circuit voltage boost

We report a reproducible enhancement of the open circuit voltage in Cu2ZnSnS4 solar cells by introduction of a very thin CeO2 interlayer between the Cu2ZnSnS4 absorber and the conventional CdS buffer. CeO2, a non-toxic earth-abundant compound, has a nearly optimal band alignment with Cu2ZnSnS4 and the two materials are lattice-matched within 0.4%. This makes it possible to achieve an epitaxial interface when growing CeO2 by chemical bath deposition at temperatures as low as 50 °C. The open circuit voltage improvement is then attributed to a decrease in the interface recombination rate through formation of a high-quality heterointerface.

Temperature dependent photoreflectance study of Cu2SnS3 thin films produced by pulsed laser deposition

The energy band structure of Cu2SnS3 (CTS) thin films fabricated by pulsed laser deposition was studied by photoreflectance spectroscopy (PR). The temperature-dependent PR spectra were measured in the range of T = 10–150 K. According to the Raman scattering analysis, the monoclinic crystal structure (C1c1) prevails in the studied CTS thin film; however, a weak contribution from cubic CTS (F-43m) was also detected. The PR spectra revealed the valence band splitting of CTS. Optical transitions at EA = 0.92 eV, EB = 1.04 eV, and EC = 1.08 eV were found for monoclinic CTS at low-temperature (T = 10 K). Additional optical transition was detected at EAC = 0.94 eV, and it was attributed to the low-temperature band gap of cubic CTS. All the identified optical transition energies showed a blueshift with increasing temperature, and the temperature coefficient dE/dT was about 0.1 meV/K.

Interface band gap narrowing behind open circuit voltage losses in Cu2ZnSnS4 solar cells

We present evidence that bandgap narrowing at the heterointerface may be a major cause of the large open circuit voltage deficit of Cu2ZnSnS4/CdS solar cells. Bandgap narrowing is caused by surface states that extend the Cu2ZnSnS4 valence band into the forbidden gap. Those surface states are consistently found in Cu2ZnSnS4, but not in Cu2ZnSnSe4, by first-principles calculations. They do not simply arise from defects at surfaces but are an intrinsic feature of Cu2ZnSnS4 surfaces. By including those states in a device model, the outcome of previously published temperature-dependent open circuit voltage measurements on Cu2ZnSnS4 solar cells can be reproduced quantitatively without necessarily assuming a cliff-like conduction band offset with the CdS buffer layer. Our first-principles calculations indicate that Zn-based alternative buffer layers are advantageous due to the ability of Zn to passivate those surface states. Focusing future research on Zn-based buffers is expected to significantly improve the op...

Sulfide perovskites for solar energy conversion applications: computational screening and synthesis of the selected compound LaYS3

One of the key challenges in photoelectrochemical water splitting is to identify efficient semiconductors with band gaps of the order of ∼2 eV to operate as the large-band-gap component in water splitting tandem devices. Here, we address this challenge by extensive computational screening of ternary sulfides followed by synthesis and confirmation of the properties of one of the most promising materials. The screening focusses on materials with ABS3 composition taking both perovskite and non-perovskite structures into consideration, and the material selection is based on descriptors for thermodynamic stability, light absorption, charge mobility, and defect tolerance. One of the most promising candidates identified is LaYS3. This material was synthesized directly in thin-film form demonstrating its stability, crystal structure, light absorption, and strong photoluminescence. These data confirms its potential applicability in tandem photoelectrochemical devices for hydrogen production.

On performance limitations and property correlations of Al-doped ZnO deposited by radio-frequency sputtering

The electrical properties of RF-sputtered Al-doped ZnO are often spatially inhomogeneous and strongly dependent on deposition parameters. In this work, we study the mechanisms that limit the minimum resistivity achievable under different deposition regimes. In a low- and intermediate-pressure regime, we find a generalized dependence of the electrical properties, grain size, texture, and Al content on compressive stress, regardless of sputtering pressure or position on the substrate. In a high-pressure regime, a porous microstructure limits the achievable resistivity and causes it to increase over time as well. The primary cause of inhomogeneity in the electrical properties is identified as energetic particle bombardment. Inhomogeneity in oxygen content is also observed, but its effect on the electrical properties is small and limited to the carrier mobility.

Surface passivation and carrier selectivity of the thermal-atomic-layer-deposited TiO2 on crystalline silicon

Here, we demonstrate the use of an ultrathin TiO2 film as a passivating carrier-selective contact for silicon photovoltaics. The effective lifetime, surface recombination velocity, and diode quality dependence on TiO2 deposition temperature with and without a thin tunneling oxide interlayer (SiO2 or Al2O3) on p-type crystalline silicon (c-Si) are reported. 5-, 10-, and 20-nm-thick TiO2 films were deposited by thermal atomic layer deposition (ALD) in the temperature range of 80–300 °C using titanium tetrachloride (TiCl4) and water. TiO2 thin-film passivation layers alone result in a lower effective carrier lifetime compared with that with an interlayer. However, SiO2 and Al2O3 interlayers enhance the TiO2 passivation of c-Si surfaces. Further annealing at 200 °C in N2 gas enhances the surface passivation quality of TiO2 tremendously. From these findings, design principles for TiO2–Si heterojunction with optimized photovoltage, interface quality, and electron extraction to maximize the photovoltage of TiO2–Si heterojunction photovoltaic cells are formulated. Diode behaviour was analysed with the help of experimental, analytical, and simulation methods. It is predicted that TiO2 with a high carrier concentration is a preferable candidate for high-performance solar cells. The possible reasons for performance degradation in those devices with and without interlayers are also discussed.

ZnS top layer for enhancement of the crystallinity of CZTS absorber during the annealing

Pulsed Laser Deposition (PLD) of thin films of Cu2ZnSnS4 (CZTS) has not yet led to solar cells with high efficiency. The reason for the relative low efficiency is discussed and a way to overcome this issue is presented. The present thin film absorbers of CZTS suffer from loss of volatile Zn during the plasma-assisted transfer with PLD. This can be compensated by adding a thin layer of ZnS (~ 80 nm) on top of the CZTS layer before the annealing. In this work the stack ordering of the two layers CZTS and ZnS is investigated, indicating that the configuration with ZnS on top of a CZTS film gives a better crystalline quality of CZTS after the annealing, as demonstrated by X-ray diffraction and Raman spectroscopy.

Estimating complete band diagrams of non-ideal heterointerfaces by combining ellipsometry and photoemission spectroscopy

In this work, we show that spectroscopic ellipsometry can be combined with photoemission spectroscopy to obtain complete interface band diagrams of non-ideal semiconductor heterointerfaces, such as interfaces between thin-film polycrystalline materials. The non-destructive ellipsometry measurement probes the near-interface bandgap of the two semiconductors (including the buried semiconductor) after the interface has formed. This is important in the non-ideal case where chemical processes during interface growth modify the electronic properties of the two separated surfaces. Knowledge of near-interface bandgaps improves accuracy in conduction band offset measurements of non-ideal interfaces, and it sheds light on their device physics. Both of those positive outcomes are demonstrated in the Cu2ZnSnS4/CdS interface used here as a case study, where the bandgap of both materials decreases by up to 200 meV from the bulk to the near-interface region. This finding reveals a preferential electron-hole recombination channel near the interface, and it yields corrected values for the interfacial conduction band offset.In this work, we show that spectroscopic ellipsometry can be combined with photoemission spectroscopy to obtain complete interface band diagrams of non-ideal semiconductor heterointerfaces, such as interfaces between thin-film polycrystalline materials. The non-destructive ellipsometry measurement probes the near-interface bandgap of the two semiconductors (including the buried semiconductor) after the interface has formed. This is important in the non-ideal case where chemical processes during interface growth modify the electronic properties of the two separated surfaces. Knowledge of near-interface bandgaps improves accuracy in conduction band offset measurements of non-ideal interfaces, and it sheds light on their device physics. Both of those positive outcomes are demonstrated in the Cu2ZnSnS4/CdS interface used here as a case study, where the bandgap of both materials decreases by up to 200 meV from the bulk to the near-interface region. This finding reveals a preferential electron-hole recombinatio...

Nondestructive Thickness Mapping of Wafer-Scale Hexagonal Boron Nitride Down to a Monolayer

The availability of an accurate, nondestructive method for measuring thickness and continuity of two-dimensional (2D) materials with monolayer sensitivity over large areas is of pivotal importance for the development of new applications based on these materials. While simple optical contrast methods and electrical measurements are sufficient for the case of metallic and semiconducting 2D materials, the low optical contrast and high electrical resistivity of wide band gap dielectric 2D materials such as hexagonal boron nitride (hBN) hamper their characterization. In this work, we demonstrate a nondestructive method to quantitatively map the thickness and continuity of hBN monolayers and bilayers over large areas. The proposed method is based on acquisition and subsequent fitting of ellipsometry spectra of hBN on Si/SiO2 substrates. Once a proper optical model is developed, it becomes possible to identify and map the commonly observed polymer residuals from the transfer process and obtain submonolayer thickness sensitivity for the hBN film. With some assumptions on the optical functions of hBN, the thickness of an as-transferred hBN monolayer on SiO2 is measured as 4.1 Å ± 0.1 Å, whereas the thickness of an air-annealed hBN monolayer on SiO2 is measured as 2.5 Å ± 0.1 Å. We argue that the difference in the two measured values is due to the presence of a water layer trapped between the SiO2 surface and the hBN layer in the latter case. The procedure can be fully automated to wafer scale and extended to other 2D materials transferred onto any polished substrate, as long as their optical functions are approximately known.