Influence of Zn excess on compositional, structural and vibrational properties of Cu2ZnSn0.5Ge0.5Se4 thin films and their effect on solar cell efficiency. (15th March 2020)

• Record Type: Journal Article
• Authors: Ruiz-Perona, A.; Sánchez, Y.; Guc, M.; Calvo-Barrio, L.; Jawhari, T.; Merino, J.M.; León, M.; Caballero, R.
• Is Part Of: Solar energy. Volume 199(2020)
• Page Start: 864
• Page End: 871
• Publication Date: 2020-03-15
• Subjects: Kesterite -- ZnSe -- Ge -- Solar cells; Solar energy -- Periodicals; Solar engines -- Periodicals; 621.47
• DOI: 10.1016/j.solener.2020.02.082
• Languages: English
• ISSNs: 0038-092X

Abstract

Graphical abstract: Highlights: CZTGSe thin films grown by co-evaporation and thermal treatment at T < 500 °C. CZTGSe thin films with Ge/(Ge + Sn) around 0.5 with Eg near 1.3 eV. The absence of secondary phases results in larger grain size and compact structure. ZnSe secondary phase at the surface and in the bulk of CZTGSe for >14 at % Zn. ZnSe secondary phase led to a decreased device performance, from 6.4% to 4.2%. Abstract: The effect of Zn content on compositional, structural and vibrational properties of Cu2 ZnSn1-x Gex Se4 (CZTGSe, x ~ 0.5) thin films is studied. Kesterite layer is deposited by co-evaporation onto 5 × 5 cm 2 Mo/SLG substrate followed by a thermal treatment at maximum temperature of 480 °C, obtaining areas with different composition and morphology which are due to the sample position in the co-evaporation system and to the non-uniform temperature distribution across the substrate. Kesterite layers with higher Zn amounts are characterized by lower Cu and Ge contents; however, a uniform Ge distribution through the absorber layer is detected in all cases. The excess Zn concentration leads to the formation of ZnSe secondary phase on the surface and in the bulk of the absorber as determined by Raman spectroscopy. When higher Ge content and no ZnSe are present in the absorber layer, a compact structure is formed with larger grain size of kesterite. This effect could explain the higher Voc of the solar cell. The Zn content does not affect the bandgap energy significantly (Eg near 1.3 eV), although the observed effect of Zn excess in CZTGSe results in a decreased device performance from 6.4 to 4.2%. This investigation reveals the importance of the control of the off-stoichiometric CZTGSe composition during the deposition process to enhance solar cells properties.