Thermal co-evaporation micro-chalcogenide solar panel technology

Singapore scientists have reviewed all the thermal evaporation techniques used to produce chalcogenide solar cells and modules and found that the new method achieves higher yields and product efficiencies, despite its limitations.

Scientists from the Energy Research Institute at Nanyang Technological University in Singapore have investigated how to use thermal evaporation (TE), a well-established technology commonly used in the microelectronics and photovoltaic industries to produce organic light-emitting diodes (OLEDs), metal contacts and coatings on a variety of materials, to make microcalcite solar modules.

"We analysed the use of several evaporation-based techniques to fabricate chalcogenide thin films," researcher Annalisa Bruno told Photovoltaics, "These ranged from relatively simple single-source deposition and multi-source co-evaporation, to more complex multi-step evaporation and a mix of employing thermal evaporation with gas reactions and solution treatment."

She says the combined approach can take advantage of the strengths of both methods, but also has some limitations, such as greater complexity and the necessity to use solvents.

Bruno said, "We believe thermal evaporation is the ideal method for rapid deposition of chalcogenide layers because it is easily scalable, does not contain harmful solvents and has been well integrated into existing photonics and microelectronics production lines."

One of the main problems with co-evaporation is the need to extensively optimise the parameters for deposition of chalcogenides with complex stoichiometry, especially when more than two or three precursors are evaporated simultaneously, the researchers said. They also noted that long deposition times can present a barrier to commercial production - a problem that most studies tend to ignore.

They then proposed a range of techniques for fabricating chalcogenide solar cells and mini-assemblies, which include one-step thermal evaporation, multi-step thermal evaporation and multi-step hybrid deposition.

The team said, "It is desirable to produce not only evaporated chalcogenide films, but also fully evaporated modules using the same manufacturing line."

Advantages of the technique include a high degree of process control, precise control of film thickness, ease of sequentially attaching multiple layers of film, and processability with low substrate temperatures. The researchers claim that the methods also allow for better purification of precursors during film formation, excellent spatial homogeneity in device batches, good repeatability and high yields during multiple rounds of manufacturing.

They say, "The whole process can be controlled automatically and is intrinsically attractive in terms of large-scale, high-volume manufacturing." They describe these production techniques in their recent publication 'Thermal evaporation and hybrid deposition of calcium titanite solar cells and micro-assemblies' in Joule.

Bruno says: "We believe that future research in this field should focus on new optimal device interlayers to improve the quality and transparency of the active material, passivate defects through vacuum technology and improve operational stability to maximise the full potential of thermally evaporated calcium titanite solar devices."