Blistering of magnetron sputtered thin film CdTe devices

Magnetron sputtering is an industrially scalable technique for thin film deposition. It provides excellent coating uniformity and the deposition can be conducted at relatively low substrate temperatures. It is widely used in the manufacture of solar modules. However, its use for the deposition of thin film CdTe devices results in unusual problems. Blisters appear on the surface of the device and voids occur in the CdTe absorber. These problems appear after the cadmium chloride activation treatment. The voids often occur at the CdS/CdTe interface causing catastrophic delamination. This problem has been known for more than 25 years, but the mechanisms leading to blistering have not been understood. Using High Resolution Transmission Electron Microscopy we have discovered that during the activation process, argon trapped during the sputtering process diffuses in the lattice to form gas bubbles. The gas bubbles grow by agglomeration particularly at grain boundaries and at interfaces. The growth of the bubbles eventually leads to void formation and blistering

Activation of thin film CdTe solar cells using a cadmium bromide treatment

The activation of CdTe with a cadmium chloride annealing treatment is a vital step in the fabrication of high efficiency solar cells. Thin film MZO/CdTe cells have been activated using CdBr2 instead of CdCl2 with a lower activation process temperature. Using this method, CdBr2 does activate the cell as revealed by J-V and EQE measurements. TEM and EDX elemental maps from device cross-sections confirm that bromine is present in the grain boundaries. TEM shows that the treatment removes stacking faults at 425 °C. CdBr2 treatment resulted in a relatively modest conversion efficiency of 5.49% when treated at 375 °C. Nevertheless, the experiments shed further light on the mechanisms involved in the activation

Defect passivation and activation of thin film CdSeTe/CdTe solar cells

Multiple sustainable energy sources are of paramount importance in today's ever power hungry-world. Solar photovoltaics have provided cleaner, sustainable energy for nearly 70 years. While the market was previously purely silicon solar devices, the low cost thin film processing of cadmium telluride devices has allowed it to become the cheapest power per watt material for solar modules. The processing of the CdTe absorber has several important factors; firstly a CdCl2 treatment to remove stacking faults, passivate grain boundaries and give CdTe it's photovoltaic ability, secondly, the addition of Se at the front interface, further passivating grain boundaries, increasing the lifetimes and grading the bandgap in the device, and also the production of large grain CdTe and the incorporation of other dopants such as arsenic. The work presented in this thesis focuses on the effect of replacing Cl with Br in CdTe processing and separately, the pulsed DC sputtering (PDCMS) deposition method of CdSe as a Se source for CdTe devices. CdCl2 treatment of CdTe is a staple for almost all CdTe based photovoltaic devices. Studies of CdBr2 treatment of CdTe have been performed to observe similarities and differences in the effects of the halogens. An initial study was performed with varied pressure, temperature and treatment times to observe if CdBr2 would have an effect on CdTe doped with Cl. This was successful and led to an efficiency after CdBr2 that was higher than an anneal without a halide salt.  After this initial experiment, optimisation was attempted to find a temperature and time of CdBr2 that would lead to the best results. Finally, a higher quality back contact method including Cu was applied to attempt to increase the efficiency further.  CdBr2 treatment of CdTe leads to similar effects as seen with CdCl2, bromine decorating the grain boundaries is seen in high resolution compositional maps, stacking faults are removed and there is an increase in photoactivity. However, a major point is that bromine activation requires chlorine to be present in the bulk for it to work. This indicates two separate effects of the halides, one in the bulk with Cl creating p-type CdTe with A-centres, and one passivating the grain boundaries. Further investigations on the effects of CdCl2 treatment of CdSe polycrystalline films would be beneficial in the production of a CdSe absorber and full device. CdBr2 of CdTe was not viable alone, however it raises the possibility of combining halogens in a device to get benefits from both halogens. Sputter deposition of CdSe was optimised for high quality as deposited films, characterised by transmission electron microscopy and X-ray diffraction to observe the microstructural effects of the sputter conditions. After annealing these films, bubbles and blisters appeared. The sputter conditions were then optimised for reducing the subsequent voids and blistering after annealing. The bubbles and blisters were caused by sputter gas incorporation into the film, this is complemented by a theoretical molecular modelling study of the behaviour of Ar gas in CdSe films. After preventing the blisters and reducing the gas bubbles that would appear after anneal, the CdSe films were CdCl2 treated to promote greater structural rearrangement and to observe any phase changes.  Cl was found to decorate the grain boundaries of the CdSe and promoting a reasonable photoactivity when compared to untreated CdSe. The CdCl2 treatment also led to further Ar gas agglomerating into bubbles and blistering, while promoting grain growth and stacking fault removal. Evaporated CdSe was also analysed and this had none of the blister problems associated with sputter deposition, larger grains were formed after CdCl2 treatment, and the photoactivity and lifetimes were much higher than those found in sputtered treated CdSe.</p

High rate deposition of CdSe thin films by pulsed DC magnetron sputtering

The efficiency of cadmium telluride solar cells has recently been increased by adding selenium as a CdSeTe alloy at the front of the device. In this paper, we report on the use of pulsed dc magnetron sputtering to deposit thin films of Cadmium Selenide from a compound target. The deposition rates are surprisingly high and exceed 10nm/sec at a power of only 1. SkW on a 6 inch diameter target. The deposited thin films are dense and columnar. The thin films have been characterized using xRay diffraction, UV-vis Spectrophotometry, SEM, and Hall effect to analyse the structural, optical and electrical properties. Magnetron sputtering is widely used in thin film manufacturing and the high rates reported here make the use of pulsed dc sputtering an attractive industrial production technique for CdSe deposition in the CdSeTe device stack

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

Effect of microstructure on the photoactivity of thin film CdSe

CdSe is increasingly being used as a Se source in CdSeTe/CdTe photovoltaic device fabrication. In this paper, a CdSe thin film has been deposited by sputtering These films have then been subject to a CdCl2 activation treatment. The as deposited and treated films have then been characterized with XRD, EBSD, TEM, EDX and PL to analyse the relationship between photoactivity and the microstructural changes that occur during CdCl2 activation. The as deposited CdSe is photo-inactive, the structure is columnar and is a heavily faulted mix of cubic and hexagonal. Cross-sectional TEM reveals a high density of stacking faults. After the CdCl2 treatment, CdSe is photoactive and the microstructure is clearly hexagonal and indexes well in EBSD. The stacking faults are removed. It is important that thin film CdSe at the front of the device is photoactive. It must be fully exposed to the cadmium chloride process and recrystallized if its use as a source of Se in devices is to be successful. Untreated CdSe will have a detrimental effect on device performance

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

Transient metastable behaviour in highly efficient MZO/CdSeTe/CdTe thin film solar cells

Metastability effects have been investigated in high efficiency MZO/CdSeTe/CdTe solar cells. Different preconditioning procedures have been studied that are used to recover the performance of the devices. J-V characteristics before preconditioning have shown an 'S' shaped behaviour which is removed during preconditioning. However, this recovery remained only for 3 days while the devices were maintained under vacuum in the dark. Temperature dependent J-V and capacitance measurements before and after preconditioning revealed the presence of recombination centres and defect levels at the MZO/absorber interface. Previous studies have shown degradation of MZO occurring if the layer is exposed to the atmosphere. Hall effect measurements on the MZO films showed no significant changes after any preconditioning or CdCl2 treatment

Chlorine passivation of grain boundaries in cadmium telluride solar cells

Cadmium Telluride is the most commercially important second generation thin film photovoltaic, with a record solar cell conversion efficiency of 22.1%. However as-deposited cells are <5% efficient and require a cell activation treatment with CdCl2 at about 400 ◦C to reach commercially viable efficiencies. Such a treatment is a routine process during CdTe module manufacturing. However, the precise mechanisms at work for this remarkable efficiency enhancement are not well understood. In this paper, atomistic modelling techniques are used to improve the fundamental understanding of the structural and electronic properties of CdTe by modelling the effects of chlorine and other elements with their interaction with extended defects and grain boundaries. Studies at high spatial resolution with NanoSIMS, TEM and Energy Dispersive X-ray analysis shows that chlorine atoms are concentrated at grain boundaries in CdTe after the CdCl2 treatment. DFT calculations show that both ClTe and for the first time Cli are stabilised at the grain boundaries compared to bulk CdTe. Similar defect formation energies of these defects suggests both will be present at the grain boundaries. As expected, four single particle levels are present in the Σ3 (112) GB band gap which explains the low efficiencies prior to treatment. ClTe substitutions passivate one of these levels and partially passivate another two. Remarkably further addition of Cli fully passivates the remaining single particle levels. This passivation of single particle levels is most likely to be the primary cause of the efficiency enhancement on chlorine treatment. Further to this, alternative halogens were then trialled as activation treatments. All halogens show similar electronic effects and their defect formation energies follow ionic radii trends

The origins of void formation in sputtered CdSe

The introduction of selenium to the front of thin film CdTe photovoltaic devices has led to a sharp increase in conversion efficiency. One way to introduce the selenium is to first deposit a layer of CdSe on to the buffer layer before depositing a CdTe layer at the back. The two layers are then inter-diffused during the cadmium chloride activation. Magnetron sputtering is an attractive deposition method to deposit the CdSe, but previous studies have revealed the presence of deleterious voids in the finished sputtered CdSe/CdTe devices. In this paper, we show that the voids are caused by the accumulation of argon into bubbles formed during activation with experimental and theoretical evidence. The bubbles are similar to those previously observed in sputtered CdTe. These bubbles can also develop into blisters causing significant exfoliation of the film surface