The preparation of ultra-thin semi-transparent solar cells with potential applications in windows or transparent roofs entails several challenges due to the very small thickness of the layers involved. In particular, problems related to undesired inter-diffusion or inhomogeneities originated by incomplete coverage of the growing surfaces must be prevented. In this paper, undoped SnO₂, CdS, and CdTe thin films with thickness suitable for use in ultra-thin solar cells were deposited with a radiofrequency (RF) magnetron sputtering technique onto conductive glass. Preparation conditions were found for depositing the individual layers with good surface coverage, absence of pin holes and with a relatively small growth rate adapted for the control of very small thickness. After a careful growth calibration procedure, heterostructured solar cells devices were fabricated. The influence of an additional undoped SnO₂ buffer layer deposited between the conductive glass and the CdS window was studied. The incorporation of this layer led to an enhancement of both short circuit current and open circuit voltage (by 19 and 32%, respectively) without appreciable changes of other parameters. After the analysis of the cell parameters extracted from the current-voltage (I-V) curves, possible origins of these effects were found to be: Passivation effects of the SnO₂/CdS interface, blocking of impurities diffusion or improvement of the band alignment.

The development of semitransparent CdS/CdTe ultrathin solar cells has been delayed as a result of the activation annealing to which the device must be subjected, which may involve problems such as the sublimation of ultrathin films and the diffusion of Cd and S at the interface. In this work, CdS/CdTe ultrathin devices on soda-lime glass/SnO₂:F/ZnO substrates were obtained by RF magnetron sputtering. CdS/CdTe ultrathin heterostructures were obtained with the following thicknesses for the CdS thin film: 70, 110, and 135 nm. The CdTe thickness film was kept constant at 620 nm. Subsequently, activation annealing with CdCl₂ was carried out at 400 °C. Surface characterization was performed by scanning electron microscopy, which indicated that the CdCl₂ annealing tripled the CdTe thin films’ grain size. Raman characterization showed that CdS thin films deposited by RF sputtering present the first, the second, and the third longitudinal optical modes, indicating the good crystallinity of the CdS thin films. The study showed that the photovoltaic properties of the CdS/CdTe ultrathin devices improved as the CdS thicknesses decreased.

Pollycrystalline CdTe thin films deposited in large areas present some difficulties, such as non-uniformity over the entire area. It is well known that the ratio between the thickness and grain size of CdTe thin films is approximately one. In contrast, the pin holes in CdTe should be minimized. CdS thin films as window materials must guarantee a uniform transparent film free of pinholes to avoid short-circuiting in the photovoltaic device. In this work, CdTe and CdS thin films are reported in areas of 40 cm2 with optimal physical properties for potential application on solar cells. CdTe mini-modules were fabricated using CdTe and CdS layers obtained and characterized under outdoor conditions. The current-voltage measurements were performed under different charge and solar irradiance conditions.