Highly Efficient Earth-abundant Cu2ZnSn(S,Se)4Solar Cells by Defect-controlled and Interface/Contact Engineering
Cheng-Ying Chen1,2*, Bandiyah Sri Aprillia1,2,3, Septia Kholimatussa'diah1,2,3, Wei-Chao Chen1,2, Yen-Ching Teng1,2,4, Jih-Shang Hwang4, Ruei-San Chen3, Kuei-Hsien Chen2,1, Li-Chyong Chen1
1Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
2Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
3Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan
4Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung, Taiwan
* Presenter:Cheng-Ying Chen, email:chen.chengying.cyc@gmail.com
Cu2ZnSn(S,Se)4(CZTSSe) is the earth-abundant/non-toxic alternative compound for the commercialized metal chalcogenides (i.e., CdTe and Cu(In,Ga)(S,Se)2) thin-film solar cells. To boost the performance of CZTSSe based photovoltaics, much effort has been applied to improving the quality of absorbers, band alignments, front and back interfaces/contacts. [1,2,3]
In this talk, I will mainly talk about highly efficient CZTSSe solar cells by simply introducing an interfacial alkali metal fluoride (AMF) layers (~ several nm NaF, LiF) between the buffer layer (i.e., CdS) and the front transparent conductive electrode (i.e., ITO) without extra 50nm ZnO layers. In statistical studies (10 cells), the AMF layers increase power conversion efficiency from 5.77±0.54 % to 9.68±0.5 %, short circuit current density (Jsc) from 30.1±0.3 mA/cm2 to 32.2±0.2 mA/cm2 and open circuit voltage (Voc) from 400±20 mV to 480±10 mV, resulting from the front AMF modified ITO as electron-selective contacts. After the AMFs modification, the kelvin probe measurement confirmed that the work function of the front ITO decreases from 4.82 eV to 3.39 eV (NaF)/ 3.65 eV (LiF), which causes the beneficial band alignment for electron collection (/hole blocking) on top electrodes. According to the temperature-dependent current-density voltage measurement, the AMF based devices reduce the contact voltage loss, leading to the larger implied Voc of 900 meV compared with 740 meV of pristine devices. After the thickness dependent studies of AMFs, a 10.4 % efficient CZTSSe solar cell with Voc of 490 mV, Jsc of 32.8 mA/cm2 and fill factor (FF) of 63.2 % was obtained. [4]
Nevertheless, one of the issues in the kesterite based cells is the deficit of Voc due to deep trap states caused by multivalent Sn (2+ and 4+). We also demonstrated the enhanced Voc in CZTSSe solar cells by introducing an interfacial Ge alloying layer between the absorber and the buffer layer (i.e., CdS) before sulfo-selenization processes. The defect energy states 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. [5]

[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, 3, 15324-15330 (2015)
[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, 16, 438 (2015)
[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, 30, 762-770 (2016)
[4] C.Y. Chen, W.C. Chen, B. S. Aprillia, N. Saidatin, R.S. Chen, K.H. Chen and L.C. Chen, manuscript in preparation (2017)
[5] C.Y. Chen, W. C. Chen, S. Kholimatussa'diah, Y.R. Lin, K. H. Chen and L. C. Chen, manuscript in preparation (2017)

Keywords: CZTSSe, solar cell, earth-abundant, defect, interface