Thin Film Solar cells for Future Solar Power Electronics: A Review on CZTSSe

Figure 1:(a) Shows the disadvantages of solar cells and (b) advantages of CZTSSe thin film based solar cells.

Renewable energy sources like solar thermal energy, solar photovoltaic energy, and biomass conversion energy plays a vital role in everyday life, so these sources must be developed to meet the challenges of environment change. Among these energy sources, photovoltaic source is a very standard renewable source is foreseen to solve energy problem, which has the potential to convert a large amount of sunlight into electrical energy. A solar cell is an example of a photovoltaic device, i.e., a device that generates voltage when exposed to light. In such a device, absorption of photons results the excitation of charge carriers into conduction band. The basic mechanism regarding this light induced electron transition to a higher energy states is alike to that of photoelectric effect in which a photon carrying sufficient amount of energy frees an electron from metal surface. The devices which exploiting PV effect are called solar cells, also photovoltaic cells or photovoltaic devices. Solar cells based on thin films is a second-generation solar cell in which a thin layer or multilayer’s of semiconducting material deposited on different substrates like glass, plastic or metal. There are several factors like lower absorption, spectral mismatch, lower mobility etc. (Figure1a) which could affect a cell’s conversion efficiency. Amorphous silicon based solar cells ultimately degrade when exposed to light and their efficiency decreases by 10-20%. Modern-day technology calls for the development of eco-friendly compound semiconductor thin film having the tailor-made properties. However, chalcogenide based solar cells like CdTe, CIGS, CZTSSe etc. show good advantages over Silicon based solar cells. Selenium based quaternary kesterite compounds like mixed chalcogenide Cu²ZnSn (SxSe1−x)4 (CZTSSe) and Cu²ZnSnSe⁴ (CZTSe) have emerged as the potential alternative to the existing CIGS and CdTe absorbers in thin film solar cells. Among which CZTSSe is an alternative option for absorbing material in thin film solar cells due to its tunable direct band gap of 1.0–1.5eV (optimum band gap), earth abundant having p-type conductivity and large optical absorption coefficient (>104cm−1) [1-6] (Figure 1b). These advantages allow CZTSSe to complete head-to-head with silicon from a performance standpoint, but with the potential of lower cost due to the thin film nature of the solar cell device give extra superiority. CZTSSe conversion efficiency is also very stable over time, means its performance continuous unabated for many years. However, CZTSSe, has a PCE of 12.6%. Compared with CIGS, which has a PCE of 21.7% and CdTe, which has a power conversion efficiency (PCE) of 21.5% [7,8]. To improve the efficiency of a CZTSSe solar cell, it is important to minimize current and voltage losses. To do so, the grain crystallinity must be improved [9,10] the formation of secondary phases must be suppressed [10-13] and the presence of defects [13] the Na content, [13,14] and the band gap [13,15] in the absorber layer must be controlled. To overcome these disadvantages, various efforts had been made so far by different researchers for enhancing the PCE of CZTSSe solar cell up to 12.6% [7,8,13] Figure 2.

Figure 2:Shows schematic fabrication of CIGS solar cell.

1. Willoughby A (2014) Solar cell materials: developing technologies. Wiley Series in Materials for Electronic & Optoelectronic Applications, Wiley, Chichester, England.
2. Repins I, Beall C, Vora N, DeHart C, Kuciauskas D, et al. (2012) Coevaporated Cu²ZnSnSe⁴ films and devices. Sol Energy Mater Sol Cells 101: 154-159.
3. Guo Q, Ford GM, Yang WC, Walker BC, Stach EA, et al. (2010) Fabrication of 7.2% efficient CZTSSe solar cells using CZTS nanocrystals. J Am Chem Soc 132(49): 17384-17386.
4. Olekseyuk ID, Gulay LD, Dydchak IV, Piskach LV, Parasyuk OV, et al. (2002) Single crystal preparation and crystal structure of the Cu2Zn/Cd, Hg/SnSe4 compounds. J Alloy Compd 340(1-2): 141-145.
5. Wang K, Gunawan O, Todorov T, Shin B, Chey SJ, et al. (2010) Thermally evaporated Cu²ZnSnS4 solar cells. Appl Phys Lett 97(14): 143508.
6. Bag S, Gunawan O, Gokmen T, Zhu Y, Todorov TK, et al. (2012) Low band gap liquid-processed CZTSe solar cell with 10.1% efficiency. Energy Environ Sci 5: 7060-7065.
7. Green MA, Emery K, Hishikawa Y, Warta W, Dunlop ED (2015) Solar cell efficiency tables (Version 45). Prog Photovoltaics 23(1): 805-812.
8. Powalla M, Jackson P, Hariskos D, Paetel S, Witte W, et al. 3AO.4.2 - CIGS thin-film solar cells with an improved efficiency of 20.8%. 29th European Photovoltaic Solar Energy Conference, Amsterdam, Netherlands.
9. Son D, Kim D, Park S, Yang K, Nam D, et al. (2015) Growth and Device Characteristics of CZTSSe Thin-Film Solar Cells with 8.03% Efficiency. Chem Mater 27(15): 5180-5188.
10. Yang K, Sim J, Son D, Kim D, Kim GY, et al. (2015) Effects of the compositional ratio distribution with sulfurization temperatures in the absorber layer on the defect and surface electrical characteristics of Cu²ZnSnS4 solar cells. Prog Photovoltaics 23(12): 1771-1784.
11. Fairbrother A, Fontan´e X, Izquierdo-Roca V, Placidi M, Sylla D, et al. (2014) Secondary phase formation in Zn‐rich Cu²ZnSnSe⁴‐based solar cells annealed in low pressure and temperature conditions. Prog Photovoltaics 22(4): 479-487.
12. Vigil-Galan O, Espındola-Rodrıguez M, Courel X, Fontane M, Sylla D, et al. (2013) Secondary phases dependence on composition ratio in sprayed Cu²ZnSnS4 thin films and its impact on the high-power conversion efficiency. Sol Energy Mater Sol Cells 117: 246-250.
13. Yang KJ, Son DH, Sung SJ, Sim JH, Kim YI (2016) A band-gap-graded CZTSSe solar cell with 12.3% efficiency. J Mater Chem A 4(26): 10151- 10158.
14. Yang K, Sim J, Son D, Kim D, Kang J (2015) Two different effects of Na on Cu²ZnSnS4 thin-film solar cells. Curr Appl Phys 15(11): 1512-1515.
15. Woo K, Kim Y, Yang W, Kim K, Kim I, et al. (2013) Reproduction 3: 3069.