Earth-abundant Kesterite Based Chalcogenide Materials for Photovoltaics

Cheng-Ying Chen^1,2,3*, Wei-Chao Chen^1,3, Septia Kholimatussa'diah^1,2,3, Bandiyah Sri Aprillia^1,2,3, Yen-Ching Teng^1,2,3,5, Naili Saidatin^1,2,3,4, Chih-Yang Huang^1,2,3,5, Chih-Yuan Chiu^1,3, Ruei-San Chen⁴, Jih-Shang Hwang⁵, Kuei-Hsien Chen^1,3, Li-Chyong Chen^1,2

¹Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
²Center of Atomic Initiative for New Materials, National Taiwan University, Taipei, Taiwan
³Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
⁴Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan
⁵Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung, Taiwan

* Presenter:Cheng-Ying Chen, email:chen.chengying.cyc@gmail.com

Chalcogenide based thin-film PVs with a direct bandgap such as CdTe and Cu(In,Ga)Se2 (CIGSe) have achieved remarkable over 20% power conversion efficiency (PCE) but the toxicity of cadmium (Cd), and the scarcity of indium (In) and tellurium (Te) may restrict the production capacity for a growing worldwide power consumption (~terawatt). Therefore, kesterite based Cu2ZnSn(S,Se)4 (CZTSSe) have emerged as a potential alternative for CdTe and CIGSSe absorbers due to the use of non-toxic and earth-abundant elements and providing desirable optoelectronic properties, similar to CIGSSe.
For single junction solar cells, it has theoretically predicted that kesterite materials have potential to achieve efficiency more than 30% due to their exceptional physical properties such as high absorption coefficient (~104 cm-1), tunable (with S/Se ratio) and direct band gap of 1-1.5 eV. However, the best PCE, among all reported values, of CZTSSe solar cells is around ~12.6% by IBM’s toxic hydrazine method, still far from the expectation. The main bottleneck of CZTSSe is its lower open circuit voltage (open circuit voltage deficit, Eg/q-VOC < 0.5V in CIGSSe, whereas typical Eg/q-VOC > 0.6V in CZTSSe.), likely resulting (A) from bandgap fluctuation and electrostatic potential fluctuation: these fluctuations origin from (1) spatial variation in composition and (2) material defects or defect complexes (such as [CuZn- + ZnCu+]), which usually happen near the grain boundaries; (B) a non-optimal conduction band-alignment between the p-type CZTSSe absorber and the n-type buffer layers; and (C) contact losses/band-alignment due to the mismatch of the work function/bandgap offset. Actually, for non-Ohmic contacts in PV devices (i.e., existing Schottky barriers), the VOC will be limited by the work function difference between the top and the bottom electrodes, instead of quasi-fermi level splitting in absorbers.
In this talk, I will share the following several strategies to address the critical issue, (1) enhancing the inter-diffusion of each metals in precursors (Cu, Zn, and Sn), (2) CZTSSe defect-controlled engineering by introducing a few Ge to suppress SnZn antisite defect, and (3) inserting alkali metal fluoride interfacial layers as electron-selective contacts. Recently, the PCE of our earth-abundant/non-toxic CZTSSe solar cells can reach to 10.4%. If only considering the effective cell area, our PCE can be above 11.5%, which is very close to the world-record of 12.6% (by IBM).

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, 2016, 30, 762-770
[4] C.Y. Chen, B. S. Aprillia, W.C. Chen, Y.C. Teng, C.Y. Chiu, R.S. Chen, J.S. Hwang, K.H. Chen, and L. C. Chen, Nano Energy, 2018, 51, 597-603

Keywords: Earth-abundant, Chalcogenide, Photovoltaics, Cu2ZnSn(S,Se)4 (CZTSSe), thin-film