Effect of CdCl2 passivation treatment on microstructure and performance of CdSeTe/CdTe thin-film photovoltaic devices

Abstract

The effects of the CdCl 2 passivation treatment on thin-film CdTe photovoltaic films and devices have been extensively studied. Recently, with an addition of CdSeTe layer at the front of the absorber layer, device conversion efficiencies in excess of 19% have been demonstrated. The effects of the CdCl 2 passivation treatment for devices using CdSeTe has not been studied previously. This is the first reported study of the effect of the treatment on the microstructure of the CdSeTe /CdTe absorber. The device efficiency is <1% for the as-deposited device but this is dramatically increased by the CdCl 2 treatment. Using Scanning Transmission Electron Microscopy (STEM), we show that the CdCl 2 passivation of CdSeTe/CdTe films results in the removal of high densities of stacking faults and increase and reorientation of grains. The CdCl 2 treatment leads to grading of the absorber CdSeTe/CdTe films by diffusion of Se between the CdSeTe and CdTe regions. Chlorine decorates the CdSeTe and CdTe grain boundaries leading to their passivation. Direct evidence for these effects is presented using STEM and Energy Dispersive X-ray Analysis (EDX) on device cross-sections prepared using focused ion beam etching. The grading of the Se in the device is quantified using EDX line scans. The comparison of CdSeTe/CdTe device microstructure and composition before and after the CdCl 2 treatment provides insights into the important effects of the process and points the way to further improvements that can be made.

Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells

“…We find a strong correlation between selenium concentration, high material luminescence efficiency, sub-gap transitions at 1.36 eV, and longer diffusion lengths, all at the sub- around the front contact of the device. The setup accurately measures the J-V parameters and the agreement of these measurements with an externally certified photovoltaic device has been shown in previous work [18].…”

“…This is similar to the as-deposited device. However, in contrast to the as-deposited device, which has a sharp interface between the CdSeTe and CdTe layers (see [18], and Supplementary Figure 1), the CdCl 2 -treated cell shows a gradual decrease in the selenium concentration with distance away from the front interface. This means that selenium signal is detected towards the back of the cell, in the CdTe layer (for clarity now referred to as the 'interdiffused CdTe' region).…”

Analysis of a novel CuCl2 back contact process for improved stability in CdTe solar cells

“…Use of a chloride compound was chosen due to the well‐known chlorine‐related benefits of recrystallization, grain growth, and p‐type doping (bicomplexes with native defects) in CdTe. There is evidence that similar processes have been already applied in some laboratories but without a study of the process' effect …”

“…There is evidence that similar processes have been already applied in some laboratories but without a study of the process' effect. [24][25][26][27][28] Beach et al suggested that the defects induced by CdCl 2 , probably Cl-induced donor, increased the solubility of the Cu at Cd substitutional sites impurities Cu − Cd , which are considered acceptors. 29 The CuCl 2 is applied after the standard CdCl 2 activation treatment because combining activation treatment and copper doping in a single step is not possible: CdCl 2 activation treatment is applied at a temperature which would cause a very large Cu diffusion.…”

Effects of Successive CdCl2 Treatments on the Texture of Thin Film CdTe Solar Cells

“…The CdCl2 treatment is especially important with CST devices, as it causes the diffusion of the graded layer. This will also affect texture change [11].…”