Anthony Vasko

Magnetron sputtered CdTe solar cells on flexible substrates

We describe the application of sputter deposition processes to the fabrication of CdS/CdTe solar cells on light weight, flexible substrates in both the substrate and superstrate configurations. We have fabricated CdS/CdTe cells on molybdenum foil and Kapton polyimide. The Mo-based devices are among the best reported for substrate configuration CdTe solar cells, with efficiencies of /spl sim/7% at AM1.5.

All-sputtered CdS/CdTe solar cells on polyimide

We have made CdS/CdTe solar cells on polyimides from different sources. These cells were fabricated using sputtered zinc oxide doped with aluminum as the transparent conducting oxide. The CdS and CdTe were also deposited by sputtering. In this paper we report properties of the sputtered AZO and its changes with CdCl2 heat treatments. We have achieved a cell efficiency of 10.5% at air mass 1.5G illumination.

Real time analysis of magnetron-sputtered thin-film CdTe by multichannel spectroscopic ellipsometry

Real time spectroscopic ellipsometry (RTSE) based on rotating-compensator modulation and multichannel detection has been implemented to characterize polycrystalline thin film CdTe deposition for photovoltaic applications. RTSE is capable of providing routine deposition information on substrate temperature T and deposition rate. It is also capable of providing detailed information on the thickness evolution of microstructure and optical properties. In this study, we highlight the differences in nucleation that occur under different CdTe deposition conditions on smooth crystalline Si wafer substrates. Differing behavior in the initial stages of deposition has been observed, ranging from layer-by-layer growth to nucleation and coalescence of 45 A thick clusters. We also consider the thickness and substrate dependence of the microstructure, comparing depositions on smooth Si wafer and rough thin film Mo substrates.

Temperature-Dependent Electroluminescence from CdTe/CdS Solar Cells

Electroluminescence (EL) from polycrystalline CdTe/CdS solar cells was studied over the temperature range – 30 C to 25 C. We are able to observe above-background EL at forward current densities as low as 3 mA/cm 2 , allowing us to explore the EL behavior at current-voltage regimes within the normal operating parameters of the device. The EL spectrum is very similar to the photoluminescence (PL) spectrum, and is independent of applied voltage. We show that the EL most likely originates from injected electron-hole recombination at the CdTe/CdS junction. The total EL intensity is found to vary as a power-law function of current, EL ∼ I b , where I is the forward current density and b is a constant. The value of b varies from sample to sample and decreases with increasing temperature. EL intensity typically is much more sensitive to device deterioration with light soak stress than is cell efficiency.

Visible and x-ray spectroscopy studies of defects in CdTe

We have used electroluminescence (EL), photoluminesence (PL), and synchrotron x-ray fluorescence (XRF) to study some of the properties of defects related to Cu and Na in polycrystalline thin films of CdTe. Some samples were prepared by ion implantation of CdTe single crystals followed by thermal anneal to remove the damage. Others were prepared by magnetron sputtering on quartz or borosilicate glass followed by vapor CdCl/sub 2/ heat treatment. EL was studied from sputtered CdS/CdTe solar cell structures. We find that EL is very sensitive to defects induced by light soaking; ion implantation-induced damage can be annealed at 400 C to yield good PL; and synchrotron XRF shows the presence of substantial copper in films sputtered from nominally pure CdTe.

Point admittance spectroscopy: New PV diagnostic

Typical admittance (e.g., capacitance-voltage-frequency) measurements are essentially one-dimensional measurements, characterizing an electronic device in the transverse dimension between its electrodes [1],[2]. Here, we discuss situations in which one of the electrodes has very small dimensions so that the device surface is not at equipotential and lateral dimensions influence the measurement. This point admittance spectroscopy (PAS) approach opens new venue in characterizing PV devices before their final metallization. We analyze the equivalent circuit and conduct numerical modeling underlying PAS. We show that the admittance technique is sensitive to lateral nonuniformities and the existence and location of shunts, and may also be used to determine a system lump parameters, such as series resistance, shunt resistance, and open circuit voltage.

Nonuniform degradation and hot spots in thin film PV

Patterns of efficiency degradation in thin film PV reveal noticeable variations between nominally identical cells. This and some other observations point at laterally nonuniform degradation driven by weak spots on a cell. These spots can be either preexisting or caused by fluctuation of stresses and temperature; they deteriorate in a runaway mode evolving towards effective linear or nonlinear shunts. Here we present a simplified analytical treatment and numerical modeling of such a nonuniform degradation tracking local deterioration and effects of weak spots on the integral cell parameters.

Transparent Back Contacts in CdTe/CdS: Evaluation for Tandem Cells

CdS/CdTe solar cells with 2.3 μm CdTe were prepared with four different types of p+/n+ transparent back contacts (TBCs) - ZnTe:Cu/ZnO:Al, ZnTe:N/ZnO:Al, ZnTe:Cu/ITO, and ZnTe:N/ITO. ZnTe:N/ITO was found to give the best results. This back contact was then used to make cells of lesser (1.8 μm and 0.7 μm) CdTe thickness, in those cases giving good performance up to 9.1% efficiency. Bifacial J-V was performed on all cells with the optimum ZnTe:N/ITO back contact.