Thin film thickness measurements using Scanning White Light Interferometry

Scanning White Light Interferometry is a well-established technique for providing accurate surface roughness measurements and three dimensional topographical images. Here we report on the use of a variant of Scanning White Light Interferometry called coherence correlation interferometry which is now capable of providing accurate thickness measurements from transparent and semi-transparent thin films with thickness below 1 μm. This capability will have many important applications which include measurements on optical coatings, displays, semiconductor devices, transparent conducting oxides and thin film photovoltaics. In this paper we report measurements of thin film thickness made using coherence correlation interferometry on a variety of materials including metal-oxides (Nb2O5 and ZrO2), a metal-nitride (SiNx:H), a carbon-nitride (SiCxNy:H) and indium tin oxide, a transparent conducting oxide. The measurements are compared with those obtained using spectroscopic ellipsometry and in all cases excellent correlation is obtained between the techniques. A key advantage of this capability is the combination of thin film thickness and surface roughness and other three-dimensional metrology measurements from the same sample area

Test methods for hydrophobic coatings on solar cover glass

The world market for solar energy continues to expand. However, to be competitive with traditional energy sources, photovoltaic (PV) modules must be capable of continuous and reliable high performance. Performance losses occur due to the soiling of the cover glass on modules. Soiling can be reduced by using hydrophobic coatings. These decrease surface energy and thus minimize adhesion to soiling. These coatings can help reduce maintenance and retain consistent electrical output. It is not yet clear, how hydrophobic coatings can be assessed and compared. In this paper, test methods to simulate the stresses that coatings experience in their life-time are assessed. These test methods help to predict the durability and useful lifetime of the coatings when applied to solar cover glass. Various test methods from different standards have been applied to hydrophobic coated glass surfaces and optimized to simulate real-outdoor conditions. A sand impact test and a water drop simulation test have been devised to study the effect of sand and rain on hydrophobic performance and durability

Silicon Carbon-Nitride (SiCxNy:H) by High Target Utilisation System (HiTUS) for crystalline silicon solar cell anti-reflective coating and passivation

SiCxNy:H films were deposited as anti-reflective (ARC)– passivation layers on crystalline silicon photovoltaics. The thin films were deposited using a remote plasma sputtering system HiTUS (High Target Utilisation Sputtering). The HiTUS allows the deposition of SiCxNy:H avoiding the use of pyrophoric silane precursor. Minority carrier lifetime was monitored, the effective minority carrier lifetime increased up to 40.7 µs, which corresponds to an implied Voc of 640mV. Film roughness and thickness of the films were measured using coherence correlation interferometry (CCI) using a Taylor Hobson “Sunstar”. Deposited films were found to have rms roughness values below 2nm and the film roughness was found to be proportional to passivation quality but inversely proportional to deposition rates. Films were also characterised using Horiba UVISEL spectroscopic ellipsometer, which provided refractive index dispersion. The deposited films exhibited refractive indexes in the range of 2-2.5. Measured deposition rates were in range 1-25nm/min

Optimisation of cadmium chloride solution processing of close space sublimated thin film CdTe solar cells

In this paper, we describe the optimisation process for the Cadmium Chloride (CdCl2) wet chemical treatment of cadmium telluride (CdTe) photovoltaics (PV). Various experiments were performed on Close Spaced Sublimated (CSS) CdTe cells, including optimisation of the cadmium chloride deposition process and optimisation of the annealing process. The treatment consisted of dipping the sample for various time periods in a methanol solution containing CdCl2 using different concentrations and different temperatures. The samples were subsequently annealed in an oven at different temperatures and times. The optimum conditions were found to be: dipping for 15 minutes in a solution of 1%wt CdCl2 in methanol heated to 50°C and annealing for 15 min at 370°C in a pre-heated furnace

Process development of sublimated Cu-free CdTe solar cells

CdCl2 treatment by evaporation has been optimised for CdTe solar devices without a copper annealing process. In this work, we show that the performance of CdTe devices follow a curvilinear trend by increasing the amount of chlorine used for the passivation treatment. It has been observed that the maximum of the curve represented the best performing devices. However, excessive CdCl2 caused deleterious effect on electrical performances of the cells. The PL imaging analysis has been found to be a useful technique to assess the effectiveness of the activation treatment due to the presence of chlorine in CdS thin films. Electrical characterisation has been performed on each device and PL imaging analysis has been used to validate the performances of the CdTe solar cells. Results showed that the optimum performance corresponded to the highest PL image emission signal

The activation of thin film CdTe solar cells using alternative chlorine containing compounds

The re-crystallisation of thin film cadmium telluride (CdTe) using cadmium chloride (CdCl2) is a vital process for obtaining high efficiency photovoltaic devices. However, the precise micro-structural mechanisms involved are not well understood. In this study, we have used alternative chlorine-containing compounds to determine if these can also assist the re-crystallisation of the CdTe layer and to understand the separate roles of cadmium and chlorine during the activation. The compounds used were: tellurium tetrachloride (TeCl4), cadmium acetate (Cd(CH3CO2)2), hydrochloric acid (HCl) and zinc chloride (ZnCl2). TeCl4was used to assess the role of Cl and the formation of a Te-rich outer layer which may assist the formation of the back contact. (Cd(CH3CO2)2) and HCl were used to distinguish between the roles of cadmium and chlorine in the process. Finally, ZnCl2was employed as an alternative to CdCl2. We report on the efficacy of using these alternative Cl-containing compounds to remove the high density of planar defects present in untreated CdTe

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

Characterisation of laser scribes in thin film photovoltaics by coherence correlation interferometry

In this paper we present results measuring the precise shape of laser scribes in thin film photovoltaics using Coherence Correlation Interferometry (CCI). Laser ablation is used in interconnect processes in all types of thin film devices, including those based on CdTe, CIGS and amorphous silicon. The work presented in this paper was focused on the use of a laser ablation process in the interconnect of thin film CdTe modules. This process is known as monolithic integration. The laser scribe measurements presented here were obtained on a nanometre scale using Coherence Correlation Interferometry. The Coherence Scanning Interferometry (CSI) technique is able to provide three and two dimensional topographical images of the sample surface, with an ultimate vertical resolution of 0.01nm. It provides two dimensional profiles of the laser ablated trenches, from which it is possible to extract quantitative information of the detailed shape of the scribe including precise measurements of both depth and width. In addition, the CCI is able to provide detailed analysis of surface roughness within the bottom of the trench which is important for efficient electrical contact

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

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 MoSe2can 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 MoOxcompound (MoO2and MoO3) towards the efficiency of the device is investigated. It has been proven that a thin layer of MoOxbarrier layer is able to control the formation of MoSe2effectively 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