B. Maniscalco

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.

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.

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.

Formation of MoOx barrier layer under atmospheric based condition to control MoSe2 formation in CIGS thin film solar cell

ABSTRACT As part of the device fabrication process, selenization step is required to crystallise the CIGS absorber layer. However, during high temperature selenization process, excessive formation of MoSe2 can lead to delamination of the film and adverse effect on electrical properties of the solar cells. In this paper, a new method is proposed to form a Molybdenum Oxide (MoOx) barrier layer in between of the Mo back contact using plasma jet under atmospheric based conditions. The effect of MoOx compound (MoO2 and MoO3) towards the efficiency of the device is investigated. It has been proven that a thin layer of MoOx barrier layer is able to control the formation of MoSe2 effectively and provide a significant improvement in electrical properties of the devices. A power conversion efficiency of 5.24% with least efficiency variation across the champion device was achieved which demonstrates the importance of this methodology on small area devices.

Review on Substrate and Molybdenum Back Contact in CIGS Thin Film Solar Cell

Copper Indium Gallium Selenide- (CIGS-) based solar cells have become one of the most promising candidates among the thin film technologies for solar power generation. The current record efficiency of CIGS has reached 22.6% which is comparable to the crystalline silicon- (c-Si-) based solar cells. However, material properties and efficiency on small area devices are crucial aspects to be considered before manufacturing into large scale. The process for each layer of the CIGS solar cells, including the type of substrate used and deposition condition for the molybdenum back contact, will give a direct impact to the efficiency of the fabricated device. In this paper, brief introduction on the production, efficiency, etc. of a-Si, CdTe, and CIGS thin film solar cells and c-Si solar cells are first reviewed, followed by the recent progress of substrates. Different deposition techniques’ influence on the properties of molybdenum back contact for CIGS are discussed. Then, the formation and thickness influence factors of the interfacial MoSe2 layer are reviewed; its role in forming ohmic contact, possible detrimental effects, and characterization of the barrier layers are specified. Scale-up challenges/issues of CIGS module production are also presented to give an insight into commercializing CIGS solar cells.

Combined thin-film thickness measurement and surface metrology of photovoltaic thin films using Coherence Correlation Interferometry

Accurate measurement of the thickness of transparent and semi-transparent thin films is important in all thin film photovoltaic technologies. Surface roughness measurements are also important to interpret the performance of photovoltaic devices. Here we report on a new capability for coherence correlation interferometry that enables it to provide thin film thickness measurements with comparable accuracy to spectroscopic ellipsometry as well as measuring surface roughness with sub-nanometre precision. Applications of the techniques new capability for thin film thickness measurements are presented for a range of photovoltaic materials. In particular, we report on the close correlation between results obtained by coherence correlation interferometry and spectroscopic ellipsometry. Coherence Correlation Interferometry is a fast, sensitive, non-contacting, non-destructive metrology technique with the potential for use as an in-line quality assurance tool in the large scale production of photovoltaic modules.