Ilvydas Matulionis

Cadmium Telluride Solar Cells on Molybdenum Substrates

We report on the development of Mo/CdTe/CdS/indium-tin-oxide, thin-film solar cells grown by radio-frequency magnetron sputtering. This is an inverted configuration compared to the conventional glass/tin-oxide/CdS/CdTe/metal cells. Molybdenum was chosen as a substrate because its thermal expansion coefficient and the work function are close to those of CdTe. We have achieved AM1.5 conversion efficiencies of 7.8 percent on 0.05 cm 2 area devices. Our best cells had a nitrogen-doped ZnTe layer between the molybdenum and the CdTe for a somewhat improved back contact. However, we observe a significant rollover in the IV curve in forward current that indicates a back-diode effect. This implies the need for improvement of the electronic properties of the molybdenum - CdTe and possibly CdS - ITO interfaces.

Optimization of laser scribing for thin-film photovoltaics

In this paper, the authors report on studies of four different lasers for the scribing of thin-film photovoltaic materials including CdTe, CIS, SnO/sub 2/, ZnO, and Mo. They have used cw-lamp-pumped and flashlamp-pumped Nd:YAG lasers (/spl lambda/=532/1064 nm), a copper-vapor laser (511/578 nm), and an XeCl-excimer laser (308 nm), with pulse durations ranging from 180 nsec down to 8 nsec. The purpose of this work is to identify the effects of laser wavelength and laser pulse duration on the most effective energy utilization and on the quality of the scribe line. Ablation spots and scribe lines were examined by optical microscopy and stylus profilometry. The ablation threshold has been identified for the combinations of four lasers and five materials. The energy density for optimum removal of material has been identified for two of the laser systems (three wavelengths).

Pulse duration and wavelength effects in laser scribing of thin-film polycrystalline PV materials

This project is focussed on a study of wavelength-dependent effects and pulse-duration effects on laser scribing of polycrystalline thin-film PV materials. The materials studied here are CdTe, CI(G)S, SnO2, ZnO, molybdenum and gold. This paper provides a summary of thresholds and optimum scribing energy densities for two types of Nd:YAG lasers, a 308 nm excimer laser, and a copper vapor laser. A comparison is presented of glass-side vs. film-side scribing. Discussion is also given of scribing of multilayer films such as ZnO/CIS/moly and gold/CdTe/SnO2.

Effect of CdTe thickness reduction in high efficiency CdS/CdTe solar cells

High efficiency CdTe solar cells are typically grown with CdTe thicknesses from 3 to 15 μm, although the thickness required for 90% absorption of the incident irradiation at 800 nm is only ∼1 μm. In this paper, we present the effect of CdTe thickness reduction on the performance of CdS/CdTe solar cells in which both the CdS and CdTe films were grown by sputtering. We produced a series of cells with different CdTe thickness (from 0.5 to 3.0 μm), and held the CdS thickness and back-contact-processing constant. The effect of CdTe thickness reduction on the diffusion of CdS into CdTe was studied using optical absorption and x-ray diffraction techniques. Only slight decreases occur in open-circuit voltage, short-circuit current, and fill factor with decrease in CdTe film thickness to 1.0 μm. Almost 10% efficient cells were obtained with 1 μm CdTe. Below 1 μm, all cell parameters decrease more rapidly, including the red quantum efficiency.

Lasers and beam delivery options for polycrystalline thin-film scribing

We have investigated the use of several different types of lasers for scribing of the thin film materials: CdTe, CuInGaSe2, ZnO, SnO2, Mo, Al, and Au. The lasers included several types of Nd: YAG (1064 and 532 nm wavelengths), Cu vapor (511/578 nm), XeCl excimer (308 nm), and KrF excimer (248 nm). Pulse durations ranged from ∼0.1 nsec to ∼250 ns. We found that the Nd: YAG systems work well for almost all of the above materials except for the transparent conductor ZnO, for which the two excimer lasers showed good performance. Pulse duration was found generally not to be critical except for the case of CIGS on Mo where longer pulse durations appear advantageous.

Development of a corrosion-resistant amorphous silicon carbide photoelectrode for solar-to-hydrogen photovoltaic/photoelectrochemical devices

Photoelectrochemical (PEC) water splitting at a semiconductor-electrolyte interface using sunlight is of considerable interest as it offers a clean approach to hydrogen production. PEC cells require semiconductor photoelectrode materials fulfilling a number of important requirements, such as band-edge alignment, corrosion resistance to electrolyte, and adequate current generation. We report the development of RF-PECVD-deposited hydrogenated amorphous silicon carbide (a-SiC:H) photoelectrodes with improved durability, which, when combined with a-Si:H tandem photovoltaic devices, should produce hydrogen directly from water under sunlight. The a-SiC:H is commonly grown with a bandgap in excess of 2.0 eV and completes the PEC device by providing contact with the electrolyte, proper band-edge alignment, and acts as a buffer for the a-Si:H tandem structure. Effects of the pH of electrolyte, type of substrates, and a platinum nanoparticle coating on the durability of a-SiC photoelectrodes will be presented. From these studies we surmise that corrosion or damage mechanism occurring on a-SiC:H layer could be divided into different aspects of physical and chemical. From the physical point of view, defects associated with spikes in textured TCO substrates, roughness of stainless steel, or other sources of pinholes may initiate delamination as confirmed by SEM (Scanning Electron Microscopy) and EDS (Energy-Dispersive X-ray Spectroscopy) studies. Chemically, the production of hydrogen involves reactions that may etch the electrode, especially when physical defects are involved. We observe that reducing the acidity of the electrolyte (increasing the pH from 0 to 2) significantly reduces corrosion while the useful photocurrent output of the a-SiC:H p/i structure is unaffected.

Semi-Transparent Amorphous Silicon Solar Cells on Inexpensive Plastic Substrates

We report a monolithic series connected semi-transparent (transmission > 40%) amorphous silicon ( a -Si:H) solar cell panel (substrate area 30cm × 40cm), which has an i-layer thickness 3% constructed on inexpensive plastic substrates.