Tallinn University of Technology

The future lies in solar cells that are produced sustainably and last for decades. In order to achieve this – to maintain a high level of performance for many years to come – it is necessary to develop solar cells that are based on robust inorganic absorber layers, a recent Tallinn University of Technology PhD Jako Siim Eensalu states.

Jako Siim Eensalu
Jako Siim Eensalu (screenshot from a video from the 2021 lecture competition “Science in 3 minutes”)

He provides solutions on how to achieve this in his recently defended doctoral thesis "Deposition of Sb2S3 Thin Films by Ultrasonic Spray Pyrolysis for Photovoltaic Applications".

Namely, the Sb2S3 solar cell film must be continuous and of appropriate thickness in order to be significantly efficient. The aim of Eensalu's thesis was to fabricate continuous phase pure Sb2S3 thin films by ultrasonic spray pyrolysis to assess their applicability in solar cells.

Solar panels can be seen in all sorts of places

"Generating energy from solar power is one of the key technologies to meet the ever-growing global demand for electricity in a climate-neutral way. The production of energy from sunlight is known not to pollute the environment or cause global warming," Eensalu stresses, in explaining why solar technologies are attracting so much attention and winning the hearts of consumers. In addition, new technologies under development are based on cheap and obtainable raw materials, in order to increase the self-sufficiency of energy production and reduce the environmental impact of transport.

When travelling around Estonia, solar panels can be seen in every possible and seemingly impossible place. However, the environmentally friendly production of solar cells entails a frugal consumption of materials and energy. According to Eensalu, this is why new solar cell solutions are being designed for different applications.

"Innovative applications include, for example, the integration of solar cells in transparent window glass, in tandem and bifacial devices. In addition to rooftop installations, building-
and product-integrated devices are also under development, e.g. a greenhouse glass covered with solar cells. So far, neither continuous phase pure Sb2S3 thin films nor relevant solar cells have been prepared using the ultrasonic spray pyrolysis method. Ultrasonic spray pyrolysis is well suited for this task because it is a fast, high-performance, high production capacity method that has been developed over many years and can be applied in the air," Eensalu stresses.

Different tests with films

In order to achieve the aim of the work, Eensalu explored how the deposition temperature, precursor solution composition and concentration of the phase composition affect the phase composition, elemental composition, optical properties and morphology of Sb2S3 thin films. In addition, solar cells were prepared based on the Sb2S3 thin films grown by ultrasonic spray pyrolysis, and the respective physical output parameters were studied.

In this thesis, films grown by spraying a solution of SbCl3 and thiourea in a molar ratio of 1/3 or 1/6 were investigated. Spraying a solution of SbCl3 and thiourea in a molar ratio of 1/3 at a temperature of 220 °C or a solution of SbCl3 and thiourea in a molar ratio of 1/6 at 210 °C or 220 °C yielded crystalline and phase pure, yet discontinuous, Sb2S3 films. In contrast, spraying a solution of SbCl3 and thiourea in a molar ratio of 1/6 at a temperature of 200°C yielded a discontinuous amorphous phase pure Sb2S3 thin film, which crystallised, but did not change in morphology after heat treatment in vacuum at 170 °C.

However, spraying a solution of SbCl3 and thiourea in a molar ratio of 1/3 at a temperature of 200 °C or 210 °C yielded a continuous amorphous 70–90 nm thick phase pure Sb2S3 thin film. Heat treatment of these films in vacuum at 170 °C resulted in continuous crystalline phase pure Sb2S3 thin films.

Two-step process necessary

Thus, a two-step process proved suitable to deposit continuous uniform phase pure Sb2S3 thin films. Sb2S3 thin films that had been deposited from a solution of SbCl3 and thiourea in a molar ratio of 1/3 at a temperature of 200 °C, were tested in planar thin film solar cells.

According to Jako Siim Eensalu, the thesis showed that the efficiency of a solar cell depends on the thickness of the Sb2S3 absorber layer. "The highest efficiency of up to 5.5% was achieved in planar solar cells of ITO(In2O3:Sn)/TiO2/Sb2S3/Poly(3-hexylthiophene-2,5-diyl) (P3HT)/Au by employing a 100-nm-thick Sb2S3 thin film. Preliminary results indicate that, in principle, the Sb2S3 thin films grown by ultrasonic spray pyrolysis are applicable as the absorber film in planar solar cells.

However, the author states that the research and development still need to be continued, as the films were not sufficiently uniform. The results of the storage time test indicate that the power conversion efficiency of an unencapsulated Sb2S3 solar cell is more stable over time compared to organic or perovskite photovoltaics. Based on the knowledge available to date, it was unknown whether and how phase pure Sb2S3 thin films could be deposited from solutions of other coordination complexes.

Proof that Sb2S3 film leads to the goal

In conclusion, the preparation of compact phase pure Sb2S3 thin films by ultrasonic spray pyrolysis in air from a halogenide-containing and -free coordination complex solution was reported for the first time in the scope of this study. Eensalu proved that highly efficient planar solar cells can be prepared from continuous Sb2S3 thin films grown by ultrasonic spray pyrolysis in air.

In addition, he proved that the back-contact-less stack of glass/ITO/TiO2/Sb2S3 is in principle suitable for semitransparent
photovoltaic applications, e.g. solar windows, bifacial and tandem devices, as well as building- and product-integrated devices. Therefore, the results of this study are useful for the
preparation of continuous phase pure Sb2S3 thin films in an air environment, and also for the research and development of conventional and semitransparent solar cells and products based thereon.

Looking back, Eensalu says that the most challenging part of the work was presenting the section of results and discussion concisely, yet thoroughly and clearly. In conclusion, many positive emotions were felt. "Most surprisingly, Sb2S3 films that meet the quality requirements of solar cells, can be easily prepared in an air environment by spray deposition from different combinations of starting materials."