Earth-abundant Kesterite Based Chalcogenide Materials for Photovoltaics
Cheng-Ying Chen1,2,3*, Wei-Chao Chen1,3, Septia Kholimatussa'diah1,2,3, Bandiyah Sri Aprillia1,2,3, Yen-Ching Teng1,2,3,5, Naili Saidatin1,2,3,4, Chih-Yang Huang1,2,3,5, Chih-Yuan Chiu1,3, Ruei-San Chen4, Jih-Shang Hwang5, Kuei-Hsien Chen1,3, Li-Chyong Chen1,2
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,
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).

[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