G. Claudio

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...

Effect of the tilt angle on antimony in silicon implanted wafers

A dose of 5.0×1014 antimony (Sb+) ions cm−2 was implanted into silicon wafers at an energy of 70 keV, at different tilt angles (0°, 15°, 30°, 45°, and 60°). One set of samples was preamorphized with 160 keV germanium (Ge+) ions with a dose of 1×1015 cm−2. The second set consisted of implants into single crystal silicon. After implantation the samples were annealed at 700 °C for 30 s in a pure nitrogen ambient. Secondary ion mass spectroscopy was performed to evaluate the atomic profile and the retained dose as a function of the tilt angle before and after annealing. Rutherford backscattering spectroscopy and ion channeling measurements were performed to quantify the retained dose and the subtitutional fraction of antimony in the preamorphized and crystalline samples. Hall measurements were performed in order to calculate the proportion of the dopant which was electrically active. It was found that the substitutional fraction was a function of the tilt angle for all of the implanted samples. A correlation ...

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.

Characterization of contacts produced using a laser ablation/inkjet one step interconnect process for thin film photovoltaics

A new laser ablation/inkjet process has been developed for the interconnect of thin film photovoltaic modules. This process involves laser ablation and inkjet printing of insulator and conductor materials carried out with high precision. Any error will compromise the device efficiency by increasing the series resistance or by causing shunting effects. Here we present a way of characterizing these interconnects using Scanning White Light Interferometry (SWLI). The SWLI technique allows the precise measurement of the laser scribe profile. We also present the use of a transmission line method (TLM) for the measurement of the contact resistance between the inkjet silver conductor and the Transparent Conducting Oxide.

Electrical behaviour of arsenic implanted silicon wafers at large tilt angle

Two sets of silicon wafers were implanted with 60 keV arsenic ions at a dose of 5×1015 cm-2, using an Applied Materials SWIFT implanter. The first batch consisted of five silicon pre-amorphised wafers, whilst the second batch was implanted under the same conditions into <100> single crystal wafers. The tilt angle was varied over the range 0° - 45°. After implantation the samples were cut into smaller pieces and annealed in the range 800 to 1000°C for times up to 60 seconds in flowing nitrogen. Van der Pauw measurements of the Hall effect and resistivity were made to obtain values of the sheet resistance (RS), Hall mobility (μH) and sheet carrier concentration (NS) as a function of the tilt angle. Electrical profiles were obtained from differential Hall effect measurements, whilst the retained dose was determined by Rutherford Back Scattering (RBS). The results show that the sheet resistance and the sheet carrier concentration are a function of the tilt angle and that preamorphisation results in a higher degree of electrical activation. A comparison of atomic and carrier concentration profiles as a function of tilt angle in conjunction with simulations are used to explain the observed results.

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.

Metrology of silicon photovoltaic cells using Coherence Correlation Interferometry

Surface metrology plays an important role in the development and manufacture of photovoltaic cells and modules. Coherence Correlation Interferometry (CCI) is a non-contacting surface metrology tool with potentially important applications in the characterization of photovoltaic devices. Its major advantages are that it is fast, non-destructive and it takes its data from a relatively large and hence representative area. A special mode, called “stitching x-y” can be used to provide information on a wider area, combining measurements taken in different zones of the sample. The technique is capable of providing surface roughness and step height measurements with sub-nanometre precision. It is also capable of measuring quantitatively surface texture and surface form in three dimensions and it now has a new capability to measure thin film and thick film thickness. CCI measurements are presented on a range of features on silicon photovoltaic cells including surface roughness, surface texture, the profile of laser grooves for buried contacts as well as the roughness and thickness of silicon nitride thin films. Complementary analysis using spectroscopic ellipsometry is also presented for verification. CCI is a sensitive, non-destructive metrology technique with potential use as an in-line quality assurance tool in the large scale production of photovoltaic modules.

Passivation of laser grooved buried contacts (LGBC) solar cells with silicon oxide or silicon nitride grown by a remote sputtering deposition system

Silicon nitride (SiNx) and Silicon Oxide (SiO2) thin films have been deposited by a new remote plasma deposition system HiTUS (High Target Utilisation Sputtering). The remote plasma geometry allows, pseudo separation of plasma/target-bias parameters, avoiding ion bombardment, and effectively eliminates poisoning, making it an attractive option for stable reactive sputtering of important electronic and photovoltaic films including SiNx or SiO2 for passivation of the front surface or as antireflective coating. The SiNx (SiO2) thin films are grown by reactive sputtering from a silicon target in an Ar/N2 (O2) atmosphere, negating the use of silane gas in difference to the more commonly used PECVD method. A deposition rates up to 7.8 Å/s have been obtained (see Fig.1).The deposition rate increase from 4 Å/s up to 7Å/s at 100% RF bias power with an error of ±5%. A good control of the refractive index (between 1.7 up to 2.3) has also been found. Photo conductive measurements by a WCT-100 were performed on silicon wafer after each thermal process to measure the effective lifetime of p-type crystalline silicon wafer Boron doped ≪100≫ to measure the effective lifetime of the minority carriers. Carrier lifetime up to 30 microseconds have been measured on c-Si wafer passivated by SiO2. We have shown that the SiO2 and SiNx thin film have high passivation quality making the HiTUS a viable new sputtering system for deposition of antireflective coatings for solar cells production.