The thin CdS layer is normally deposited by a chemical bath step. Phases, composition and thickness, as well as impurities can be determined by XRD, XPS, RBS, XRR, and SIMS. Surface texture can be measured with AFM. Defects can be analyzed by STEM/EDS.
The thick CdTe layer is deposited by a variety of methods, but in all cases is a multi-grain structure. GDMS can be used to quantitatively measure impurities in the CdTe feedstock powder or granules. GDMS can also measure the impurities in Cd and Te feedstock. These impurities can include unwanted counter dopants, and elements that cause electron-hole recombination in the depletion region, or elements that can contribute to structural defects that interfere with current collection. The Cl-containing thermal treatment changes the grain structure. XRD can determine the structural phases of the CdTe and CdS before or after thermal treatment. SEM and TEM can provide structural information about the CdTe/CdS interface before and after the thermal treatment. SIMS can provide quantitative measurement of dopants and impurities in the CdTe layer. Because the CdTe surface is rough (textured), the accuracy of the SIMS profiles can be improved by a polish of the CdTe surface before obtaining the SIMS profile. This also helps in measuring the S diffusion profile from the CdS into the CdTe. The surface texture of the CdTe layer before forming the metal contact can be measured by AFM. The surface chemistry (e.g., Te bonding) of the CdTe before metal contact can be measured by XPS, and TOF-SIMS can provide surface chemistry as well.
The metal contact normally contains Cu which diffuses into the device. The amount of Cu and its location in the device can either be beneficial or detrimental to device performance. The Cu profile can be measured by SIMS through the device. The interface between the metal contact and the CdTe can be analyzed by STEM/EDS and FE-AES (for a profile through a single CdTe grain). There are a variety of approaches to the contact, including for example, Cu-doped graphite paste, Cu/Au, Cu/Mo, Cu/ITO, and ZnTe:Cu. Composition, thickness, and phases of the metal contact can be analyzed by a variety of techniques, such as RBS, XRR, XRD, XPS, and AES.
For the module there is usually some kind of encapsulation material used and this can be analyzed by GCMS and FTIR.