Diana Shvydka

Effects of nonuniformity in thin-film photovoltaics

We discuss the physical origin and effects of micrononuniformities on thin-film photovoltaics. The key factors are the large device area and the presence of potential barriers in the grain boundaries (for polycrystalline films) and in device junctions. We model the nonuniformity effects in the terms of random microdiodes connected in parallel through a resistive electrode. The microdiodes of low open circuit voltages are shown to affect macroscopically large regions. They strongly reduce the device performance and induce its nonuniform degradation in several different modes. We support our predictions by experiments, which show that the device degradation is driven by the light-induced forward bias and is spatially nonuniform.

Optical properties of CdTe1−xSx(0⩽x⩽1): Experiment and modeling

Spectral ellipsometry at 300 K, in the range 0.75–5.4 eV, has been used to determine the optical constants e(E)[=e1(E)+ie2(E)] of a series of CdTe1−xSx (0⩽x⩽1) films fabricated by a laser-deposition process. The measured e(E) data reveal distinct structures associated with critical points (CPs) at E0 (direct gap), spin-orbit split E1, E1+Δ1 doublet and E2. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Phys. Rev. B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs (also E0+Δ0 CP) plus excitonic and band-to-band Coulomb enhancement (BBCE) effects. In addition to evaluating the energies of these various band-to-band CPs, our analysis also makes it possible to obtain information about the binding energies of not only the three-dimensional exciton associated with E0 but also the two-dimensional exciton related to the E1, E1+Δ1 CPs. Our results will be compared to previous experiments and m...

Blocking thin-film nonuniformities: Photovoltaic self-healing

An approach is developed to block the effects of lateral nonuniformities in thin-film semiconductor structures. The nonuniformity modulates the surface photovoltage distribution. When exposed to light and immersed in a proper electrolyte, this distribution will generate laterally nonuniform electrochemical reactions. Such treatments result in a nonuniform interfacial layer that balances the original nonuniformity. This approach has been implemented for CdTe/CdS photovoltaic devices, in which it improved the device efficiency from 1%–3% to 11%–12%.

Back contact and reach-through diode effects in thin-film photovoltaics

The physics of back contact effects in photovoltaic devices is revisited. We show that the back contact Schottky barrier can act in either back-diode or reach-through diode regimes. This understanding predicts that rare local spots with low back barrier hole transparency and/or weak main junctions can shunt the photocurrent thus decreasing the measured open-circuit voltage and device efficiency. We derive several more specific predictions of our model and verify them experimentally for the case of thin-film CdTe photovoltaics. Our concept has practical implications: a simple recipe leading to an efficient (13%) copper-free CdTe solar cell.

Piezo-effect and physics of CdS-based thin-film photovoltaics

We report a strong reversible piezo-effect in CdTe∕CdS photovoltaics consistent with the piezo parameters of CdS. Our finding suggests a different understanding of CdS-based solar cells including CdTe- and CuIn(Ga)Se-based devices. Because the CdS film is put into compression in the device, the piezo coupling generates surface charges and the electric field opposing that of the absorber layer. The corresponding potential barrier makes CdS insulating and the device operate in a metal-insulator-semiconductor mode. Our understanding introduces the concept of piezo-photovoltaics and suggests specific practical implications.

Lock-in thermography and nonuniformity modeling of thin-film CdTe solar cells

We present the lock-in thermography study of thin-film CdTe/CdS solar cells. Several major features of thermal signal are identified, such as much higher intensity for cells under illumination, considerable inhomogeneity, and a bright contour line corresponding to the higher intensity at the cell edge. Light soak stress is shown to increase the device lateral nonuniformity. We model the solar cell as a two-dimensional system of random diodes connected in parallel through a resistive electrode. The simulated current distribution maps are consistent with the thermography data.

Surface and build-up region dose analysis for clinical radiotherapy photon beams.

The standard clinical approach of dose measurement using a Farmer type fixed plane parallel cylindrical ionization chamber produces erroneous results in the build-up region. We studied dose distribution in this region using a Monte Carlo simulation technique and compared our results with data measured using extrapolation, parallel plate, and cylindrical farmer type ionization chambers for 6 and 10 MV photon beams from two different accelerators. The extrapolation chamber data agreed favorably with the Monte Carlo results, suggesting that dose at the skin surface and a few mm beneath is significantly lower than conventionally accepted values.

Admittance spectroscopy revisited: Single defect admittance and displacement current

A general approach to semiconductor device admittance spectroscopy analysis is developed, which describes arbitrary defect distributions, and gives the geometrical capacitance limit and the relationship between the measured conductance and capacitance. A single defect capacitance concept is introduced that facilitates the analysis. Special attention is paid to accounting for the role of displacement current, which was overlooked in the preceding work. An experimental verification of the approach is given.

Bias-dependent photoluminescence in CdTe photovoltaics

We show that external bias significantly affects the photoluminescence (PL) in CdTe photovoltaics. The main observations are: (1) reverse bias suppresses PL, (2) PL increases with moderate forward bias and tends to saturate when it is above the open-circuit voltage, and (3) PL in the region of saturation is extremely sensitive to device degradation. We attribute the observed phenomena to the competition between the field-induced separation of electrons and holes and their nonradiative recombination. We have developed a model that describes bias-dependent PL more quantitatively and forms a basis for using it as an indicator of device degradation.

From photovoltaics to medical imaging: Applications of thin-film CdTe in x-ray detection

Recent developments in photovoltaics created a commercially viable technology of large area uniform thin-film structures, potentially extendable to the field of medical imaging, where the capability of producing large area x-ray detectors is essential for the successful diagnosis and treatment of cancer. We propose a large area thin-film CdTe structure suitable for radiation detection and discuss its main characteristics under x-ray beams used in radiation therapy. While based on the same principle, the details of its operations are very dissimilar from that of photovoltaics.