Aenert. Research Laboratory news
With green energy sources becoming ever more important to meet the world's energy demands, several promising technologies have experienced renewed attention from scientists around the world. One of them is the thin-film solar cells, an overall efficient technology which, however, has certain drawbacks related to low space efficiency making them unsuitable for e.g. domestic applications. Some of the most effective thin-film cells are made of cadmium telluride (CdTe). In these devices, cadmium telluride is embedded in a thin semiconductor layer designed to absorb and convert sunlight into electricity. If the interface of cadmium telluride solar cells is treated with cadmium chloride, the devices become even more effective. However, in spite of the fact that the beneficial effects of the addition of cadmium chloride to the semiconducting layer have long been known, what exactly facilitated the improved performance has remained unclear up until recently when scientists found a means to analyse the structure of the material at the atomic level.

Now (2023), scientists at National Renewable Energy Laboratory (NREL) and colleagues at Khalifa University, Bowling Green State University, and First Solar — an American CdTe solar manufacturer — have been able to gain greater insights into the mechanisms of the CdCl₂ interface treatment. By modeling the behavior of individual atoms and electrons, the team simulated possible arrangements for CdCl₂-treated interfaces.

In order to get an idea of the electronic structure of the CdTe solar cell and its charge collection the scientists first determined the atomic arrangement CdCl₂ interface. To this end, the team implemented a structure prediction algorithm for interfaces. It used a random arrangement of atoms and then allowed them to arrange, with the help of a method called density functional theory to calculate the atomic forces.

They found that the CdCl₂ formed at the interface took a different structure than it would as a bulk material. It arranged itself into a 2D structure which could interact with both sides of the interface, its boundary conditions being different from those when it forms on its own.

To achieve higher-performing CdTe solar cells, the scientists connected them to the crystal structure on either side of the interface. The CdCl₂ was found to reduce the defects in the crystal structure that trap charges and reduce solar cell output. These findings could aid further improvements to CdTe solar cells.

Image: Interface structure prediction from first principles. Evolution of atomic structures at the (a) direct SnO₂/CdTe and (b) SnO₂/CdCl₂/CdTe interfaces. The initial seed structures of the CdTe film are still amorphous after DFT relaxation (left), but crystalline zincblende structures develop over the course of KLM energy minimization. The final lowest-energy structure is shown for each case (right). The corresponding evolution of the total film sheet energies ????tfs are shown in (c) and (d) as function of the sampling step, for a thickness between 8 and 18 fu of CdTe and 4–13 fu of CdCl₂ (with 12 fu CdTe), respectively. The most favorable configuration of the CdCl₂ mediated interface (6 fu) shows a much deeper energy minimum than that of the direct interface (14/17 fu), when compared to the spectrum of energies obtained for different thicknesses



Source: Abhishek Sharan, Marco Nardone, Dmitry Krasikov, Nirpendra Singh, and Stephan Lany/ Atomically thin interlayer phase from first principles enables defect-free incommensurate SnO₂/CdTe interface/ Applied Physics Reviews Volume 9, Issue 4, 17 June 2022/ doi.org/10.1063/5.0104008/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

Scientists have long tried to find out why the addition of a single substance, cadmium chloride, can improve the efficiency of cadmium telluride solar cells. In 2017, scientists analysed electrodeposited CdTe thin films which were fabricated using cadmium chloride precursor for applications in solar cells. Deposition of cadmium telluride (CdTe) from cadmium chloride (CdCl₂) and tellurium oxide was done by means of electroplating using a two-electrode configuration. The layers grown were analysed using X-ray diffraction (XRD), UV–Visible spectro-photometry, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), photoelectrochemical (PEC) cell and DC conductivity measurements. The electrodeposited CdTe layer was shown to be polycrystal-line in nature. Through UV–Visible spectrophotometry a bandgap of both as-deposited and heat-treated CdTe films was found to be in the range of (1.44–1.46) eV. The SEM showed grain growth after CdCl2 treatment. The EDX made the effect of growth voltage on the atomic composition of CdTe layers visible. The PEC results proved that p- as well as n-type CdTe could be electrodeposited and the DC conductivity revealed that the high resistivity is at the inversion growth voltage (Vi) for the as-deposited and CdCl₂ treated layers.

Image: SEM micrographs for CdTe layers grown at 1330, 1360 and 1400 mV, a–c for as-deposited and d–f for CdCl₂ treated layers at 400°C for 20 min in air



Source: A. A. Ojo, I. M. Dharmadasa/ Analysis of electrodeposited CdTe thin films grown using cadmium chloride precursor for applications in solar cells/ Journal of Materials Science: Materials in Electronics 28(2):1-11, October 2017/ DOI:10.1007/s10854-017-7264-0/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2023, scientists grew a cadmium telluride (CdTe) thin films using a low-cost two-electrode electrodeposition method in an aqueous acidic solution. The solution was made up of 1 M of cadmium acetate dihydrate (Cd (CH₃OO)₂. 2H₂O) as cadmium precursor and 2 ml of tellurium dioxide (TeO₂) as tellurium precursor. The thin films were deposited on coated glass florin doped tin oxide (FTO) substrate with a sheet resistance of 8 ohm/square. The deposition voltage was varied from 1200 to 1450 mV with a 50 mV increment to investigate the range of deposition voltage. By means of X-ray diffraction (XRD) it was revealed that CdTe thin films were polycrystalline cubic zinc blend structures. The best crystallinity was observed at deposition voltage of 1250 mV with crystallite size of 26 nm. Ultraviolet-visible (UV-VIS) measurements showed a maximum absorbance of 1250 mV. The energy band gap of CdTe thin films varied from (1.46 to 2.02) eV. Through photoelectrochemical cell (PEC) measurement the scientists proved that the conductivity of CdTe varied with deposition voltage.

Image: Two- electrode electrodeposition set-up



Source: A.U Yimamu, M A Afrassa, F. B. Dejene, K.G. Tshabalala/ The effect of growth voltage on the structure, electrical and optical properties of CdTe thin films prepared by electrodeposition method/ Project: Thin film solar energy materials, February 2023/ Open Access This is an Open Access article is distributed under the terms of the Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

The advantages of the findings are at hand: The CdCl₂ structure modeled at the interface exists only in very thin layers of the material. This ultrathin interface layer carries unique properties. Materials behave differently when they exist as atomically-thin layers on or between other materials than when they are in the bulk. Thin functional layers on top of a substrate, for example, can have unique two-dimensional crystal structures which are endowed with properties different from the bulk material. This, in turn, might give them new functionalities, useful, for example, for catalysis.

The scientists plan to continue to study how materials behave at interfaces. They believe that the potential applications stretch beyond photovoltaics, to catalytic materials, microelectronics, electrochemistry (water splitting used for making hydrogen), and detector materials. They are convinced that since semiconductor devices increasingly rely on the integration of different materials across interfaces, the enhanced ability to model and tune their structures will allow us to more intentionally design them for better performance.

By the Editorial Board