Earth-Abundant Cu2ZnSn(S,Se)4 Solar Cells with Efficiency over 9% by Defect-controlled Engineering
Cheng-Ying Chen1*, Wei-Chao Chen1, Septia Kholimatussa'diah1, Naili Saidatin1,2, Bandiyah Sri Aprillia1,2, Ruei-San Chen2, Kuei-Hsien Chen1,3, Li-Chyong Chen1
1Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan, Taiwan
2Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan, Taiwan
3Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan, Taiwan
* presenting author:Cheng-Ying Chen, email:chen.chengying.cyc@gmail.com
The efficiency of Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells can reach up to 22.3%, which is in the lead of thin film solar cells. However, In and Ga are rare-metal materials; so there is a clear quest for developing alternative compounds, such as earth-abundant Cu2ZnSn(Se,S)4 (CZTSSe). [1-3] Nevertheless, one of the issues in the kesterite based cells is the deficit of open circuit voltage due to deep trap states caused by multivalent Sn (2+ and 4+)

In this work, we demonstrated the high efficiency CZTSSe solar cells by introducing an interfacial Ge alloying layer between the absorber and the buffer layer (i.e., CdS) before sulfo-selenization processes. In the statistically studies, the Ge layers increase short‐circuit current density (Jsc) from 24.6 mA/cm2 to 29.5 mA/cm2 and open‐circuit voltage (Voc) from 455 mV to 492 mV, possibly resulting from the suppression of Sn+2 state formation by small quantities of Ge (Ge/(Sn+Ge) < 0.1). The defect energy sates of the absorber measured by admittance spectroscopy decrease from 220 to 112 meV with increasing Na contents. Finally, a 9.35% efficient CZTSSe solar cell with Voc of 500 mV, Jsc of 30.3 mA/cm2 and fill factor of 61.4 % was obtained, which is about 50% enhancement compared to the reference sample without the Ge alloying layers.

The morphology, elemental composition, and distribution of the absorber layers are being examined by scanning electron microscopy (SEM), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS),

References
[1] V. Tunuguntla, W.C. Chen, P.H. Shih, I. Shown, Y.R. Lin, C.H. Lee, J.S. Hwang, L.C. Chen and K.H. Chen, J. Mater. Chem. A, 2015,3, 15324-15330
[2] Y.R. Lin, V. Tunuguntla, S.Y. Wei, W.C. Chen, D. Wong, C.H. Lai, L.K. Liu, L.C. Chen and K.H. Chen, Nano Energy, 2015, 16, 438
[3] W.C. Chen, C.Y. Chen, V. Tunuguntla, S.H. Lu, C. Su, C.H. Lee, K.H. Chen and L.C. Chen, Nano Energy (DOI: http://dx.doi.org/10.1016/j.nanoen.2016.09.022) (2016)


Keywords: Earth-abundant , solar cells, CZTSSe, CIGS, Green energy