WNLO Developed a New Material System for the Research on Thin-film Photovoltaics

December 21, 2015

On May 18th, the authoritative magazine in optoelectronics-Nature Photonics published the whole paper Thin-film Sb2Se3 photovoltaics with oriented one-dimensional ribbons and benign grain boundaries, composed by the research group led by Professor Tang Jiang in WNLO.

The research developed a technique of rapid thermal evaporation by using the tube furnace to achieve the rapid evaporation preparation of Sb2Se3 thin -film in low vacuum condition (~1Pa)(the evaporate rate reaches 1micron per minute and deposition can be formed within 30 seconds). More importantly, through the deep analysis of the structure of Sb2Se3 crystal, the research team find that it’s formed by the accumulation of one-dimensional ribbon materials and there exist the covalent bonds in only one direction while in another two orthogonal directions they are combined through van der Waals' force. The paper pointed out that if the Sb2Se3 crystal is properly oriented, its grain boundaries (GBs) will become intrinsically benign due to free of  dangling bonds. This feature makes it stand out among all the known inorganic semiconductor materials such as GaAs, CIGS, ZnO, whose GBs must contain the dangling bonds which typically act as  recombination centers so they need to be treated with passivation. In cooperation with Professor Chen Youshi in East China Normal University, the research group conducted a deep theoretical calculation of the crystal face energy of Sb2Se3 ,  the result of which indicates that there don’t exist the dangling bonds in (hk0) surfaces of Sb2Se3.
In the process of experiment, master students  like Zhou Yin further proved that if Sb2Se3 thin film is properly oriented, its GBs will be intrinsically benign with a very low density of defect states through Kelvin Probe Force Microscopy(KPFM) and Electron Beam Induced Current(EBIC).  Through systematic optimization of the technique, the research group successfully obtained superstrate thin-film Sb2Se3 photovoltaics with the photoelectric conversion efficiency of 5.6%, which was independently certified by the third-party authoritative authentication from Newport Cooperation. Through damp-heat tests, the researchers find the device shows such good stability that after aging in the temperature of 85 ℃ and 85% humidity for 100 hours, the efficiency of the non-encapsulated device decreased by only 0.50%.

Nowadays, Thin-film Photovoltaics represented by CdTe and CIGS have been deeply researched and widely applied for such advantages as high conversion efficiency, low price, lightness and flexibility. However, its power generation efficiency cannot catch up with that of the thermal power and it still needs the subsidy from the government. Recently, organic–inorganic metal halide perovskites in which we had made breakthrough progress is the star material for photovoltaic applications. It can able to achieve  low costs and satisfy different needs such as  color, transparency, softness, power density and other different requirements, but it contains the poisonous element Pb and is not very stable, restricting its future large-scale ues. Professor Tang Jiang group has been devoted to the research and experiment in the solar power for the long term,   with the purpose of discovering the new semiconductor material with earth abundance, low cost and environmental friendliness to achieve the Photovoltaic power generation with high efficiency and low cost.
The research group found a new approach to solving this problem by fixing their research target on Sb2Se3 ,a V-VI compound which has not been widely explored. As the light absorbing layer material for photovoltaics, Sb2Se3 has the following advantages: 1) its band gap is around 1.1eV very near to that of silicon (1.12ev) and the theoretical  photoelectric conversion efficiency of the single-junction cell is over 30%; 2) it has a large absorption coefficient  of over 105 cm-1 in invisible light region, enabling full light absorption within a micrometer thick layer; 3) It is a simple binary compound with fixed composition and phase, and can achieve the high-quality thin-film growth in low temperature(<300 ℃) to minimize energy consumption during device fabrication; 4) The raw materials of Sb2Se3 are comparatively low-cost ( 2 RMB for every m2 of  Sb2Se3 thin film) , earth-abundant, green and non-toxicity ( None of US, China, EU lists Sb2Se3 as a toxic chemical  nor a carcinogen). Therefore, Sb2Se3 holds great potential for achieving high-efficiency and low-cost solar cells due to its excellent photoelectric and material properties, and yet possesses an important scientific value and a promising application prospect, deserved to explore.

Zhou Yin et al. in the group dissolved Se and Sb into hydrazine to obtain the precursor solution. By spin coating and sintering, pure thin-film Sb2Se3 could be obtained. Through the research on the basic physical and photovoltainc performances such as band structure, fluorescence spectra,mobility etc, they built  the first Sb2Se3 Thin-film solar cell with a superstrate structure of FTO/TiO2/Sb2Se3/Au achieving 2.26%efficiency. This work was published in Advanced Energy Materials.

Taking advantage of the high vapor pressure of Sb2Se3, Liu Xingsheng, Chen Jie et al. in the group systematically researched the technique of thermal evaporation method to prepare Sb2Se3 thin film and developed the first substrate Sb2Se3 solar cell achieving 2.1% efficiency. . The work was published in ACS Applied Materials and Interface.

Luo Miao et al. in the group fabricated the superstrate CdS/Sb2Se3 solar cell by thermal evaporation method. Moreover, through physical analysis, the device showed acceptable p-n junction quality and diode parameters. Finally, performance limiting factors were revealed and discussed. This work was published in Applied Physics Letters.

Leng Meiying et al. in the group further improved device performance to 3.7% by an additional selenization step. Photoresponse study indicated selenization probably compensated selenium loss during thermal evaporation, reducing VSe associated recombination loss and improving device performance. The work was published in Applied Physics Letters.

Liu Xinsheng, et al. introduced oxygen passivation heterjunction interfacial defect, and increased the devices’ efficiency to 4.8%.  At the same time, they applied the Secondary Ion Mass Spectrometry(SIMS) and bias voltage external quantum efficiency spectrum to  analyze the mechanism from material and device physical perspective. This work was published in Progress in Photovoltaics: Research and Applications.

The series of research published by Prof. Tang Jiang’s group shows that Sb2Se3 is a  very promising solar cells absorption material, further expand the understanding on  intrinsically benign GBs of oriented one-dimensional ribbons materials, and explored a new material system for thin film solar cells. All these papers above are accomplished with HUST as the first unit,  Prof. Tang Jiang is the only corresponding author, except the paper of Prog. Photovoltaics (with postdoctor Xue Ding-Jiang )
The above work gained the fund support by National 1000  Yong Talents project,, National Natural Science Foundation of China (Grant No .61274055、61322401、91433105),National 973 Program of China (Grant No. 2011CBA00703)and WNLO Director Foundation & HUST Startup Foundation for Advanced Talents.
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