Fabiana Lisco

Multilayer Broadband Antireflective Coatings for More Efficient Thin Film CdTe Solar Cells

Reflection losses limit the efficiency of all types of photovoltaic devices. The first reflection loss occurs at the glass-air interface of the photovoltaic module. If no light trapping mechanism is used about 4% of the solar energy is lost at this surface. Currently, most commercial thin-film CdTe solar modules are manufactured using NSG TEC10 glass, with no light trapping mechanism addressing the reflection at the interface of the glass with the atmosphere. To minimize the losses, a broadband multilayer thin-film coating has been designed and deposited onto the glass surface of a thin-film CdTe solar cell. The coating consisted of four dielectric layers of alternating thin films of ZrO2 and SiO2 . The layers were deposited by using high-rate-pulsed dc magnetron sputtering. Spectrophotometer measurements confirm that the transmission increased by between 2% and 5% over the spectrum utilized by the thin-film CdTe solar cell. The weighted average reflection reduced from 4.22% to 1.24%. Standard test conditions (STC) solar simulator measurements confirmed a 0.38% increase in absolute efficiency and a 3.6% relative increase in efficiency.

Pinhole free thin film CdS deposited by chemical bath using a substrate reactive plasma treatment

Achieving a pinhole-free CdS layer is necessary to produce high performance thin film CdTe solar cells. Pinholes in the CdS layer can compromise the efficiency of a CdTe solar cell by causing shunts. We have investigated the use of a plasma treatment of a fluorine doped tin oxide coated glass substrate (NSG TEC 15) and its effect on pinhole reduction in thin film CdS layers grown by Chemical Bath Deposition. CdS films, <100 nm thickness, were deposited on both O2/Ar plasma cleaned and conventionally cleaned substrates. We show that the O2/Ar plasma treatment of the TEC 15 substrate reduced the water contact angle from ∼55° to less than 12° indicating a substantial increase in the surface energy. The CdS deposited on the plasma treated TEC 15 was pinhole free, very smooth and homogenous in morphology and composition. Scanning electron microscopy images show that the O2/Ar plasma treatment is effective in increasing film density and grain size. Corresponding spectroscopic ellipsometry measurements show an i...

InAs(100) Surfaces Cleaning by an As-Free Low-Temperature 100 ° C Treatment

Oxide removal from InAs(100) surfaces was achieved using a combination of HF:methanol wet etching followed by atomic hydrogen treatment at a temperature as low as 100°C without any stabilizing As flux. The process was monitored in real-time exploiting spectroscopic ellipsometry. Following this treatment, the surface morphology was found to be very smooth at the nanometer scale, with a reduced effective oxide thickness and no appreciable levels of elemental In and As. Furthermore, we demonstrate stability of such surfaces against oxide reformation.

Laser annealing of thin film CdTe solar cells using a 808 nm diode laser

We report on the effect of a new laser annealing treatment for thin film CdTe solar cells using a 808 nm diode laser. As-deposited, laser annealed and MgCl2 treated/laser annealed CdTe thin films have been analysed. One part of the work has been focused on understanding the efficacy of the activation treatment by laser annealing. The results show partial chlorine diffusion and associated partial re-crystallisation of the absorber. The second part of this work has been focused on the effect of the treatment on the chemical composition of the CdTe surface. It has been found that the process also contributes to the formation of a Te-rich layer on the surface of the CdTe absorber, which may provide a useful process to produce a back contact. This paper reveals the effect of the laser treatment on the microstructural properties of the CdTe absorber material. The microstructure has been analysed using STEM/EDX, HRTEM and XRD. Further work is required to optimise the process but it has the potential to provide much greater control than current activation methods and also to provide a Te back contact suitable for CdTe solar cells.

Surface activation of rigid and flexible substrates for thin film photovoltaics using atmospheric pressure plasma

Reducing fabrication costs is a major driving force in photovoltaic research. Atmospheric processes such as spin coating, spraying or printing are being developed to reduce the cost/Wp of CIGS, CZTS and perovskite solar technologies. For all technologies, surface cleaning and activation prior to thin film deposition is required and for this vacuum based low pressure plasma is a well-established technique. However, a vacuum based surface pre-treatment is not compatible with atmospheric deposition methods. We show that atmospheric-pressure plasmas are highly effective in activating the surface of substrates commonly used in photovoltaic device fabrication and demonstrate its effectiveness on both rigid and flexible substrates. The effectiveness of using atmospheric-pressure plasmas to increase surface energy is demonstrated using Water Contact Angle (WCA) measurements and chemical changes are analysed using X-ray Photoelectron Spectroscopy (XPS). Scanning Electron Microscopy (SEM) images show no alteration of the surface morphology of the substrates after the plasma treatment.

Oxygenated CdS window layers for thin film CdTe photovoltaics by pulsed DC magnetron sputtering

Absorption in the CdS window layer limits the photocurrent obtained from thin film CdTe solar cells. These absorption losses can be avoided by tailoring the band gap of the CdS material to improve the light transmission at shorter wavelengths. This can be achieved by adding oxygen to the CdS to create Oxygenated Cadmium Sulphide films (CdS:O). In this paper we report on a new process in which oxygen is incorporated into the thin film CdS during deposition by pulsed DC magnetron sputtering. Spectrophotometric measurements of transmission show that the absorption edge is shifted from 500nm to 350nm by incorporating the oxygen into the deposited films. In this way, light absorption in the window layer of the solar spectrum utilised by the CdTe solar cells was significantly reduced. Ellipsometric measurements showed that the refractive index of the CdS decreased from 2.45 to 2.1 at λ=632.8nm and the band gap shifted from 2.38eV to 3.1eV as a result of incorporating the oxygen into the film. The refractive index dispersion profiles suggest that the oxygenated films are amorphous/semi-amorphous and this was confirmed by X-ray Diffraction measurements and TEM cross-section images.