Process sensitivities and interface optimisation of CdTe solar cells deposited by close-space sublimation

In a world where the need of clean and sustainable energy production has become a necessity, photovoltaic (PV) solar cells can provide a clean and cost-effective alternative to conventional fossil fuel energy sources. Recent technological advancements in PV technologies have improved their financial viability, making the PV industry the leading energy market at the moment in new installations. Thin film solar cells can potentially further reduce manufacturing costs through less material requirements and simpler deposition methods. CdTe solar cells are currently the most commercially successful thin film technology which have secured approximately half of the thin film market share. However, CdTe solar cells have only achieved 70% of their theoretical maximum efficiency, making this a promising area of research in the quest for improving the financial viability of this technology. This thesis aims to investigate possible ways of improving the performance of CdTe solar cells through interface optimisation. [Continues.

Fabrication of CdTe thin films by close space sublimation

The effects of substrate and source temperatures, deposition disruption and material preparation of CdTe thin films, fabricated in a home-made CSS system are investigated. It is demonstrated that packing CdTe powder prior to deposition has a positive effect on growth rate. Furthermore, grain size, thin film thickness and uniformity strongly depend on substrate and source temperatures. Growth disruption during the sublimation process showed island formation, island size increase and island coalition in consecutive steps

Chemical bath deposition of thin film CdSe layers for use in Se alloyed CdTe solar cells

Chemical bath deposition (CBD) was used as a method to deposit CdSe thin films for use in CdTe solar cells. Solution parameters such as precursor stoichiometry, concentration and deposition time, were varied to assess the impact on the morphology of the CdSe films deposited on FTO coated glass. The solution precursors were cadmium acetate and sodium selenosulphite with NH3 used as a complexant to control the release of ions into the solution. It was seen that particle size, surface coverage and thickness were successfully controlled. CdSe films were grown with a band gap of ~1.74 eV and were made into full devices with CdTe. A ternary compound of CdTe1-xSex formed with a band gap of ~1.40 eV, which was shown in an improved EQE in the IR, as well as an improved Jsc. The best device, with an efficiency of 12.3% was produced from a 280 nm thick film with a surface coverage of 59% and grain size of ~600 nm. An increased response at longer wavelengths due to the lowered band gap resulted in a high JSC value of 28.2 mA cm-2 sug

Combined electrical and optical characterisation of recombination mechanisms and minority carrier lifetime in solar cells

This work presents the development of a measurement system which combines optical and electrical characterisation of carrier lifetime and recombination mechanisms of thin film solar cells. It combines spectrally-resolved photoluminescence (PL), time-resolved photoluminescence (TRPL), and transient photocurrent (TPCD), which are measured using a single excitation laser source at the same spot of the solar cell. Initial PL, TRPL and TPCD measurements have been conducted on CdS/CdTe and MZO/CdTe samples. The resulting PL spectra show emission peaks corresponding to the material bandgap. Under the same excitation conditions and same measurement spot, initial TRPL and TPCD measurements were successfully conducted, yielding effective carrier lifetime values (τ1=1.39ns, τ2=3.91ns) and (τ=40.5ns), respectively for the CdS/CdTe sample and (τ1=0.98ns, τ2=3.3ns) and (τ=36.8ns), respectively for the MZO/CdTe sample. These values are affected by different recombination processes in solar cells. Therefore, as well as characterising solar cell material quality, the electronic properties of the finished PV device can also be characterised

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

Comparison of Cadmium Selenide Thin Films deposited by chemical bath and pulsed DC sputtering for use in Cadmium Telluride devices

Cadmium selenide (CdSe) thin films deposited using chemical bath deposition and pulsed DC magnetron sputtering are compared for use in cadmium telluride/selenide (CST) photovoltaic (PV) devices. Full devices were made from the bath and sputtered films using a cadmium chloride (CdCl2) treatment temperature of 425°C, this gave an overall efficiency of 9.3% and 3.2% respectively. Photoluminescence (PL) of the sputtered sample confirmed a bandgap was present of 1.58 eV which suggested poor diffusion at 425°C. A (CdCl2) treatment temperature of 465°C gave a large PL peak at 1.37 eV which corresponds to the bandgap of CST, indicating diffusion was more effective at this temperature.</div

Inert gas cluster formation in sputter-deposited thin film CdTe solar cells

Magnetron sputtering is widely used for thin film deposition because it is a relatively low temperature process which also produces films with excellent uniformity. Unfortunately, in its use for the deposition of thin film CdTe devices, the inert working gas from the magnetron can incorporate into the film during the growth process and aggregate into large subsurface clusters during post processing. The gas clusters often occur at the CdS/CdTe interface causing delamination and blisters up to about 30 µm in diameter are readily observable on the film’s surface. The surface blisters are observed after post processing with CdCl2 at an elevated temperature but smaller inert gas clusters of several nanometres in diameter can be observed using high resolution transmission electron microscopy before the CdCl2 treatment. In this paper, these effects are investigated both experimentally and using molecular dynamics modelling. Some suggestions are also made as to how the effect might be minimised and higher efficiency solar devices fabricated

Degradation of Mg-doped zinc oxide buffer layers in thin film CdTe solar cells

Cadmium Sulphide is the conventional n-type buffer layer used in thin film Cadmium Telluride solar cells. It is well known that Cadmium Sulphide causes optical losses and sulphur diffuses into the absorber during high temperature activation. Sputter-deposited Mg-doped ZnO (MZO) has been shown to be an attractive buffer layer for Cadmium Telluride solar cells due to its transparency and tuneable band gap. It is also stable to high temperature processing and avoids diffusion of elements into the cadmium telluride absorber during the cadmium chloride activation treatment. However, degradation is observed in solar cells incorporating MZO buffer layers. Analysis of the MZO film surface potential has revealed significant fluctuations in the thin film work function once the layer is exposed to the atmosphere following deposition. These fluctuations are attributed to the high reactivity to water vapour of the MgO contained in the MZO films. This has been analysed using X-ray Photoelectron Spectroscopy to determine corresponding changes in the surface chemistry. The Zinc Oxide component is relatively stable, but the analysis shows that MgO forms a Mg(OH)2 layer on the MZO surface which forms a secondary barrier at the MZO/CdTe interface and/or at the interface between MZO and the Fluorine-doped SnO2. This affects the Fill Factor and as a consequence it degrades the conversion efficiency

Photoluminescence imaging analysis of doping in thin film CdS and CdS/CdTe devices

The use of photoluminescence (PL) imaging analysis to assess the effectiveness of the passivation treatment due to the presence of chlorine in CdS thin films has been investigated. In this work, we show that the chlorine doping effect in the CdS window layer can be detected by PL imaging analysis, due to the formation of a defect complex of sulfur vacancy and ClS (VS-ClS) and complexes between halogen ions and cadmium vacancies (VCd-ClS). CdTe devices with differently doped CdS layers were investigated. PL imaging, TEM, IV performance indicators and EQE analysis were performed to understand the effect of the different dopants on the electrical performances of CdTe devices