Large area survey grain size and texture optimization for thin film CdTe solar sells using xenon-plasma focused ion beam (PFIB)

Microstructural analysis of high efficiency thin film CdTe solar cells has been obtained over large areas. Analysis regions are device cross-sections approximately 0.325 mm in length. The samples have been prepared using a xenon-plasma focused ion beam (Xe-PFIB). The detailed images of the microstructure were obtained using backscattered electron imaging and electron backscatter diffraction (EBSD). As deposited devices and those with a low level of cadmium chloride treatment both show strong (111) growth texture. A high density of twins is seen in the columnar grains. Three As doped FTO/CdZnS/CdTe with varying process conditions we devices with 13.1 %, 16.3% and 17% conversion efficiency were investigated. Lowest efficiency device was CdCl2 treated at 420°C for 10 minutes while the 16.3 and 17% devices were both treated at 440°C for 10 minutes. The large area analysis revealed a partial recrystallisation state in the 16.3% efficient device which was induced by an incomplete chloride activation process. The analysis confirms that the efficiency of the devices tends to correlate with grain size. It also showed that a strong correlation exists between device efficiency and the randomization of the texture away from the (111) grain orientation. EBSD can be used to survey large areas and to mark out features for more detailed analysis using transmission electron microscopy (TEM). As an example, we show how using an EBSD scanned cross-sectional area can identify a partially recrystallized region which is then extracted and analyzed in detail using TEM

Absorber texture and the efficiency of polycrystalline thin film CdTe solar cells

A range of microstructural changes occur during the deposition and activation of CdTe based thin film solar cells. In particular, the cadmium chloride (CdCl2) activation treatment results in wholesale recrystallisation which transforms the conversion efficiency of the solar cell. One of the noticeable effects is the change of preferred orientation of the CdTe absorber. Highly orientated [111] texture is observed in as deposited or under-treated CdTe based devices. Optimized activation results in a more randomized texture and the [111] preferred texture component is significantly weakened. In this paper we use Electron Backscatter Diffraction to characterise absorber cross-sections. The focus is on how randomization of the absorber texture reflects device performance. We have had access to a range of CdTe devices using a variety of deposition techniques. We have observed a clear pattern that shows that devices with a highly orientated [111] texture have poor efficiency. Devices with a randomized texture have much higher efficiency. Here we illustrate this empirical correlation using devices deposited by Metal Organic Chemical Vapour Deposition with a range of efficiencies from 13.1 % to 17 %. We have also included the analysis of an absorber from a 18.7 % high efficiency CdSeTe/CdTe device to show that texture is similarly important in these advanced devices. We have been able to quantify the effect of texture by using multiples of uniform density or (MUD) values from the inverse pole figures. MUD figures close to 1 correlate with highest efficiency. Although the random texture of the absorber microstructure is only one of several important process factors, it appears to be a necessary feature for highest efficiency CdTe-based polycrystalline solar cells.</p

Measurement of band alignment between ZnO based front emitters and CdCl₂ treated CdSeTe/CdTe absorbers

Thin film CdSeTe/CdTe solar cells have achieved > 22% record efficiency and generated solar electricity at a cost as low as 3 US cents per kW.hr in large scale utilities. In this work, the use of ZnO based n-type emitters is considered with the aim of improving the thin film CdSeTe/CdTe photovoltaic device efficiency still further. The measured conduction band offsets (CBOs) between ZnO and CdSeTe are determined to be in the “cliff” conformation. These CBOs will be optimized to achieve a “spike” conformation by incorporating suitable dopants to achieve high device efficiency. This work identifies new pathways to highly efficient ZnO based n-type emitters for arsenic doped CdTe solar cells. In particular, we have identified Sn, Ce and Cs as suitable dopants to generate the preferred ‘spike’ in the band alignment between the emitter layer and the CdSeTe absorber. Suitably doped emitter layers will be used in device research to reduce the deficit in open circuit voltage.</p