P.M. Kaminski

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...

Cadmium chloride assisted re-crystallization of CdTe: The effect of annealing over-treatment

Although the cadmium chloride treatment is an essential process for high efficiency thin film cadmium telluride photovoltaic devices, the precise mechanisms involved that improve the cadmium telluride layer are not fully understood. The treatment parameters have a narrow window, deviating from these even slightly can be detrimental to cell performance. In this investigation we apply advanced microstructural characterization techniques to study the effects of varying two parameters: the temperature of the substrate during the cadmium chloride treatment and the length of time of the treatment. In both cases, the devices have been deliberately over-treated. The effect of the over-treatment on the microstructure of cadmium telluride solar cells, deposited by close spaced sublimation is investigated and related to cell performance. A range of techniques has been used to observe the changes to the microstructure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the microstructural properties of the cells have also been undertaken. Techniques used include Transmission Electron Microscopy (TEM) for sub-grain analysis, EDX for chemical analysis and XPS for composition-depth profiling.

Broadband anti-reflection coatings for thin film photovoltaics

Light reflection from the glass surface of thin film photovoltaic (PV) modules is a source of significant energy loss for all types of thin film devices. The reflection at the glass and air interface accounts for ~4% of the total energy. Currently, most of the commercial thin film solar modules are manufactured using a glass superstrate/substrate with no light trapping strategy to reduce the reflection loss. To minimise these losses, a series of broadband multilayer thin film coatings has been designed and tested. Anti-reflection coatings for CdTe, CIGS, perovskite and a-Si thin film PV are presented in this paper. The coatings consist of four to six dielectric layers of alternating thin films of ZrO2 and SiO2. The layers were deposited by using a high deposition rate pulsed DC magnetron sputtering tool. Spectrophotometer measurements confirm that the transmission increased by between 2% and 5% over the spectrum utilized by each thin film solar cells. The weighted average reflection reduced from around 4.22% to 0.62% (a-Si-4 layers), 0.80% (perovskite-4 layer), 1.22% (CdTe-4 layers), and 1.49% (CIGS-6 layers). This results in an increase in current density and cell efficiency. These dielectric coatings are highly durable and scratch resistant.

Cadmium chloride assisted re-crystallization of CdTe: The effect of the annealing temperature

The aim of this investigation is to apply advanced microstructural characterization techniques to study the effects of varying the cadmium chloride annealing temperature on the microstructure of cadmium telluride solar cells deposited by close spaced sublimation (CSS) and relate this to cell performance. A range of techniques have been used to observe the morphological changes to the microstructure as well as the chemical and crystallographic changes as a function of treatment parameters. Electrical tests that link the device performance with the microstructural properties of the cells have also been undertaken. Techniques used include Transmission Electron Microscopy (TEM) for sub-grain analysis, X-ray Photoelectron Spectroscopy (XPS) depth profiles to show the effect of temperature on the diffusion of chlorine into the CdTe. Grain orientation data as well as grain size change has been obtained using Electron Backscatter Diffraction (EBSD) on Focused Ion Beam (FIB) prepared planar sections.

One step thin-film PV interconnection process using laser and inkjet

Monolithic series interconnection is a key advantage of thin-film PV. The standard approach is to alternate layer deposition and laser scribing. Here the M-Solv patented One Step Interconnect (OSI) process is introduced as an alternative to the conventional all laser interconnect. OSI interconnects TF-PV modules in a single step after all deposition is complete with considerable advantages: reduced capital expenditure; better process control; less vacuum/air interfaces, reduced line footprint and faster panel transit. OSI employs a combination of laser scribing and inkjet printing of functional materials. OSI mini-modules have been fabricated on CdTe with good electrical performance. Although the focus here has been CdTe, OSI is applicable to all thin-film technologies. The laser and inkjet processes presented are fully scalable for industrial production.

Refractive index determination by coherence scanning interferometry

Coherence scanning interferometry is established as a powerful noncontact, three-dimensional, metrology technique used to determine accurate surface roughness and topography measurements with subnanometer precision. The helical complex field (HCF) function is a topographically defined helix modulated by the electrical field reflectance, originally developed for the measurement of thin films. An approach to extend the capability of the HCF function to determine the spectral refractive index of a substrate or absorbing film has recently been proposed. In this paper, we confirm this new capability, demonstrating it on surfaces of silicon, gold, and a gold/palladium alloy using silica and zirconia oxide thin films. These refractive index dispersion measurements show good agreement with those obtained by spectroscopic ellipsometry.

The effect of a post-activation annealing treatment on thin film cdte device performance

The cadmium chloride activation treatment of cadmium telluride solar cells is essential for producing high efficiency devices. The treatment has many effects but the most significant is the complete removal of stacking faults in the cadmium telluride grains and the diffusion of Chlorine along the grain boundaries of the device. Chlorine decorates all cadmium telluride and cadmium sulphide grain boundaries and also builds up along the CdTe/CdS junction.. This paper reveals that by annealing devices to temperatures of 400°C to 480 °C for times ranging from 30 to 600 seconds in moderate vacuum results in the re-appearance of stacking faults and the removal of Choline from the grain boundaries. STEM analysis confirms the re-appearance of the stacking faults and SIMS and EDX confirm the removal of chlorine from the grain boundaries. This directly corresponds to a lowering in cell efficiency. The study provides further evidence that CdCl2 diffusion and certain microstructural defects directly affect the performance of cadmium telluride photovoltaic devices.

Performance and durability of broadband antireflection coatings for thin film CdTe solar cells

Light reflection from the glass surface of a photovoltaic (PV) module is a significant source of energy loss for all types of PV devices. The reflection at the glass and air interface accounts for ∼4% of the total energy. Single layer antireflection coatings with sufficiently low refractive index have been used, such as those using magnesium fluoride or porous silica, but these are only effective over a narrow range of wavelengths. In this paper, the authors report on the design, deposition, and testing of multilayer broadband antireflection coatings. These coatings reduce the weighted average reflection over the wavelength range used by thin film CdTe devices to just ∼1.22%, resulting in a 3.6% relative increase in device efficiency. The authors have used multilayer stacks consisting of silica and zirconia layers deposited using reactive magnetron sputtering. Details of the stack design, sputter deposition process parameters, and the optical and microstructural properties of the layers are provided. Anti...

Initiation of the cadmium chloride assisted re-crystallization process of magnetron sputtered thin film CdTe

The re-crystallization of thin film cadmium telluride (CdTe) using the cadmium chloride (CdCl2)annealing procedure is a vital process for obtaining high efficiency photovoltaic devices. Although the process is crucial, the precise micro-structural mechanisms at work are poorly understood. Recently it has been observed that untreated CdTe contains a high density of stacking faults and that these are removed in the CdCl2 assisted re-crystallization. In this paper, we report on experiments using magnetron sputtered CdTe to determine the effects of the re-crystallization process. In particular, we have focused on how the re-crystallization process initiates by using ultra-low concentrations of CdCl2 (0.06% and 0.03% of saturated CdCl2 in methanol). With these low concentrations a partial re-crystallization occurs, with the initiation occurring at the CdS/CdTe interface and not at the surface of the CdTe layer.