Jake W. Bowers

Hydrazine-Free Solution-Deposited CuIn(S,Se)2 Solar Cells by Spray Deposition of Metal Chalcogenides

Solution processing of semiconductors, such as CuInSe2 and its alloys (CIGS), can significantly reduce the manufacturing costs of thin film solar cells. Despite the recent success of solution deposition approaches for CIGS, toxic reagents such as hydrazine are usually involved, which introduce health and safety concerns. Here, we present a simple and safer methodology for the preparation of high-quality CuIn(S, Se)2 absorbers from metal sulfide solutions in a diamine/dithiol mixture. The solutions are sprayed in air, using a chromatography atomizer, followed by a postdeposition selenization step. Two different selenization methods are explored resulting in power conversion efficiencies of up to 8%.

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.

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.

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.

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.

Aluminium-doped zinc oxide deposited by ultrasonic spray pyrolysis for thin film solar cell applications

Aluminum-doped zinc oxide (AZO) thin films were deposited on glass substrates by ultrasonic spray pyrolysis, from metal salt precursors. The electrical and optical properties were investigated as a function of the deposition parameters and the optimum conditions were defined. The thin films exhibit ~80% transparency and a resistivity in the order of 2×10-2Ωcm. The electrical properties can be improved further with post-deposition annealing in vacuum, or with increase in thickness which causes insignificant transmission losses. AZO nanoparticles can be used as a seed layer and affect the optical properties of the material. The optimized process results in good quality AZO films for their application as the transparent conductive oxide (TCO) layer in thin film solar cells.

Inkjet and laser hybrid processing for series interconnection of thin film photovoltaics

Abstract Inkjet deposition can be a complementary technology to laser ablation to enable new processes. One such process is the One step interconnect for thin film photovoltaics, which is an improved method for series interconnection. The standard series interconnection process consists of three laser scribes between the deposition of the three key cell layers; transparent front contact, absorber layer and the metallic back contact. The one step interconnect allows the series interconnection to occur after the deposition of all layers significantly simplifying the manufacturing process. This is achieved by inkjet printing of conductive and insulative materials concurrently with depth selective laser scribes. The one step interconnect process has been shown to make effective interconnects on cadmium telluride photovoltaics with fill factors >60%. The benefits are many and include the reduction of capital equipment costs, reduced panel wastage and potentially improved material performance. The process is fully scalable and production ready.

Deposition of cupric oxide thin films by spin coating

Abstract Cupric oxide thin films were deposited onto soda lime glass by spin coating and subsequent annealing of copper nitrate dissolved in a glycerol–water solvent. It was found that the solution consistently gave reproducible films with good adhesion on glass. A range of band gaps were estimated between 0·8 and 1·17 eV, showing that this material has potential as a photoabsorber. Resistivity was successfully reduced from 1·47×105 to 7·02 Ω cm by doping the films with sodium. Dopant concentrations of 1 at-% gave the lowest resistivity, showing that the ideal doping is 1% or less. Film structure was found to improve with an increase in annealing time from 10 min to 1 h, although this did not have any noticeable effect on either the electrical or optical properties of the films.

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.