「銅銦鎵硒(CIGS)」是當今最有發展潛力的薄膜太陽能電池的吸收層材料，並已成功地商業化進入全球各地市場。然而，「銦」與「鎵」的稀有性，以及近年來銦在工業界的高需求量，使得「銅銦鎵硒(CIGS)」產品的製造成本大幅上昇。因此，以其它較豐沛礦藏的材質取代(CIGS)中的「銦」與「鎵」遂有其必要性。目前的研究顯示，取代(CIGS)的最佳候選者係由鋅(Zn)與硒(Se)所合成的「銅鋅錫硫硒(CZTS/Se)」太陽能電池。

目前「銅銦鎵硒(CIGS)」太陽能電池的最高效率為21.7%，係德國ZSW集團採用「共蒸發(co-evaporation)」製程所產出。此外之最高效率者為「銅鋅錫硫(CZTS)」太陽能電池的12.6%，係美國IBM公司採用1.13eV吸收層能隙(Eg)的溶液態製程(solution process)所產出。兩大嚴重限制「銅鋅錫硫硒(CZTS/Se)」太陽能電池效率的因素為開路電壓不足及低填充係數。這是因為 MoSe_2 層可增加 CZTS/Se薄膜與Mo底層間的附著，並協助歐姆接面的形成。但 MoSe_2如果太厚，會損壞接電的導電特性，因而增加系統的串聯電阻，降低FF。「銅鋅錫硫硒(CZTS/Se)」中的鍚(Sn)有多價電性的, 會造成成深層缺陷的形成。這些缺陷(defects)提供受光激發而產生的電子-電洞對形成非放射性復合的地方, 進而降低開路電壓V_oc,以及整體的系統表現。

本論文的第一部分會著重於底層接面工程—使用Passivate層克服MoSe_2層過厚的問題。(CZTS/Se)吸收層與Mo底層接面中，可鍍上一層非常薄的MoO_3「犧牲」層，增加接面的特性。實驗數據顯示MoO3犧牲層的厚度，由350奈米降至100奈米，也可以減少MoSe_2接面上的s……，串聯電阻也隨之降低(從1.83至1.54)，而FF也隨之上升(從42.67%至52.12%)。使用MoO3薄膜也導致grain growth的增加，以及缺陷能階的降低。其他的太陽能電池參數也因增加一層MoO3犧牲層而有所進步。整體的系統功能轉換效率為7.78%。

本論文的第二部分會著重於缺陷控制的工程。Admittance spectroscopy數據顯示，一層非常薄的鍺合金可降低缺陷能階, 從228meV至137meV。Admittance Spectroscopy顯示，這或許是因為Ge合金層抑制了Sn^(2+)的形成，進而抑制了Sn^(4+)轉換為Sn^(2+)時所形成的深層缺陷 (它會傷害到太陽能電池的特性)。Ge合金也幫助在(CZTS/Se)吸收層的鈉(Na)擴散。Time-resolved光照(PL)光譜顯示，在使用Ge合金的樣本中的長(59.4ns)。這代表使用Ge合金的樣本可製造出電荷分離較好的樣本。短路電流Jsc與開路電壓Voc分別由24.59mV/cm2與458 mV增加至29.49mA/cm2與497mV，而由此製造出的(CZTS/Se) 太陽能電池功能轉換效率提升至9.35%。

Copper-Indium-Gallium-(di)Selenide (CIGS) is one of the most promising absorber materials for thin-film solar cells application, and has recently been successfully commercialized worldwide. However, the rarity of the elements indium (In) and gallium (Ga), and the high demand for indium in industries, has rendered CIGS production considerably and increasingly costly. Therefore, it is necessary to replace the indium and gallium in CIGS with some earth-abundant material. The strongest candidate for CIGS so far is a Copper-Zinc-Tin-Sulfide/Selenide (Cu2ZnSn(S,Se)4 or CZTSSe) solar cell created using zinc (Zn) and tin (Sn).

Today, the highest CIGS solar cell efficiency is 21.7%, obtained by the ZSW Group in Germany using the co-evaporation process. On the other hand, the highest efficiency recorded for the Copper-Zinc-Tin-Sulfide (CZTS) solar cell is 12.6% from IBM Group in USA using a solution process with an absorber bandgap (Eg) of 1.13 eV. The two main problems that limit CZTSSe solar cell performance are open-circuit voltage (VOC) deficit and low fill factor (FF). The formation of MoS(e)2 at the back interface between the CZTSSe absorber and molybdenum (Mo) back contact reduces the FF. A very thin (~10 nm) MoS(e)2 layer may improve the adhesion between CZTSSe film and Mo back contact and help form an ohmic contact. However, too thick a MoS(e)2 layer will deteriorate the electrical contact and increase the series resistance, leading to a lower FF. On the other hand, the multivalence character of tin (Sn) in CZTSSe can create deep defects. These defects act as non-radiative recombination centers for the photoexcited electrons and holes and cause VOC deficit, which limits the device performance.
The first part of this thesis will be focused on back contact engineering by using a passivate layer to overcome the overly thick MoS(e)2 formation. A very thin MoO3 has been applied as the sacrificial layer to optimize the CZTSSe absorber/Mo back contact interface. The results clearly show that the MoO3 sacrificial layer can reduce the thickness of MoS(e)2 layer significantly from 350 to 100 nm, and also reduce a lot of secondary phases formed at the back interface of Mo back contact and CZTSSe absorber. As a result, the series resistance was reduced from 1.83 to 1.54 Ω.cm2 and the fill factor was increased from 42.67 to 52.12%. Introducing MoO3 also improves the CZTSSe absorber quality by increasing the grain growth and lowering the defect energy levels from 228 to 148 meV. Consequently, all the solar cell parameters, including short-circuit current (JSC), VOC, FF, series resistance (RS), and shunt resistance (RSH) are improved by inserting a 5 nm MoO3 sacrificial layer. As a result, a CZTSSe device with 7.78% power conversion efficiency (PCE) was achieved.

The second part of this thesis will be focused on defect-controlled engineering. A very thin germanium (Ge) alloying has been applied. The admittance spectroscopy study shows that Ge alloying helps reduce the defect energy level from 228 meV to 137 meV. This is possibly caused by suppressing the formation of Sn2+ species, which is further proved by X-ray Photoelectron Spectroscopy (XPS) study. Inside CZTSSe absorber, the transition of Sn4+ to Sn2+ inside the gap is detrimental for solar cell application due to the deep traps it creates. In addition, Ge alloying helps sodium (Na) interdiffusion inside the CZTSSe absorber. Time-resolved photoluminescence (PL) spectra of the Ge alloying sample shows longer lifetime (73.9 ns) than the pristine sample (59.4 ns). This indicates the CZTSSe absorber with the Ge alloying sample has better charge separation. Consequently, the JSC and VOC are both increased, from 24.59 to 29.49 mA/cm2 and from 458 to 497 mV, respectively. In the end, a CZTSSe solar cell with 9.35% PCE was achieved from Ge-5 alloyed CZTSSe.

[1] T. Tinoco, C. Rincón, M. Quintero, G. Sánchez Pérez. Phase Diagram and Optical Energy Gaps for CuInyGa1−ySe2 Alloys. Physica Status Solidi (a) 124 (1991): 427 – 434.
[2] V. Fthenakis. Sustainability of photovoltaics: The case for thin-film solar cells. Renewable and Sustainable Energy Reviews 13 (2009): 2746 – 2750.
[3] K. Ito, T. Nakazawa. Electrical and Optical Properties of Stannite-Type Quaternary Semiconductor Thin Films. Japanese Journal Applied Physics 27 (1988): 2094 – 2097.
[4] S. Delbos. Kesterite thin films for photovoltaics: a review. EPJ Photovoltaics 3, 35004 (2012).
[5] K. Woo, Y. Kim, W. Yang, K. Kim, I. Kim, Y. Oh, J. Y. Kim, J. Moon. Band--gap-graded Cu2ZnSn(S1-x,Se(x))4 solar cells fabricated by an ethanol-based, particulate precursor ink route. Scientific Reports 3, 3069 (2013).
[6] J. B. Li, V. Chawla, B. M. Clemens. Investigating the role of grain boundaries in CZTS and CZTSSe thin film solar cells with scanning probe microscopy. Advanced Materials 24 (2012): 720 – 723.
[7] C. Wadia, A. P. Alivisatos, and D. M. Kammen. Materials Availability Expands the Opportunity for Large-Scale Photovoltaics Deployment. Environmental Science & Technology 43 (2009): 2072 – 2077.
[8] M. Patel and A. Ray. Enhancement of output performance of Cu2ZnSnS4 thin film solar cells—A numerical simulation approach and comparison to experiments. Physica B: Condensed Matter 407 (2012): 4391 – 4397.
[9] S. Adachi. Earth Abundant Materials for Solar Cells: Cu2-II-IV-VI4 Semiconductors. John Wiley & Sons (2015). E-book.
[10] B W. Veal and A. P. Paulikas. Optical properties of molybdenum experiment and Kramers-Kronig analysis. Physical Review B 10, 1280 (1974).
[11] P. Jackson, D. Hariskos, R. Wuerz, O. Kiowski, A. Bauer, T. M. Friedlmeier, and M. Powalla. Properties of Cu(In,Ga)Se2 solar cells with new record efficiencies up to 21.7%. Phys. Status Solidi RRL 9 (2015): 28 – 31.
[12] W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu, and D. B. Mitzi. Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency. Advanced Energy Material 4, 1301465 (2014).
[13] K. F. Tai, O. Gunawan, M. Kuwahara, S. Chen, S. G. Mhaisalkar, C. Hon, A. Huan, and D. B. Mitzi. Fill Factor Losses in Cu2ZnSn(SxSe1−x)4 Solar Cells: Insights from Physical and Electrical Characterization of Devices and Exfoliated Films. Advanced Energy Material 6, 1501609 (2016).
[14] S. Nishiwaki, N. Kohara, T. Negami, T. Wada. MoSe2 layer formation at Cu(In,Ga)Se2/Mo Interfaces in High Efficiency Cu(In1- xGa x)Se 2 Solar Cells. Japanese Journal of Applied Physics 37 (1998): L71 – L73.
[15] B. Shin, Y. Zhu, N. A. Bojarczuk, S. J. Chey, S. Guha. Control of an interfacial MoSe2 layer in Cu2ZnSnSe4 thin film solar cells: 8.9% power conversion efficiency with a TiN diffusion barrier. Applied Physics Letters 101, 053903 (2012).
[16] J. Li, Y. Zhang, W. Zhao, D. Nam, H. Cheong, L. Wu, Z. Zhou, and Y. Sun. A Temporary Barrier Effect of the Alloy Layer During Selenization: Tailoring the Thickness of MoSe2 for Efficient Cu2ZnSnSe4 Solar Cells. Advanced Energy Material 5, 1402178 (2015).
[17] H. Cui, X. Liu, F. Liu, X. Hao, N. Song, and C. Yan. Boosting Cu2ZnSnS4 solar cells efficiency by a thin Ag intermediate layer between absorber and back contact. Applied Physics Letters 104, 041115 (2014).
[18] W. Li, J. Chen, H. Cui, F. Liu, X. Hao. Inhibiting MoS2 formation by introducing a ZnO intermediate layer for Cu2ZnSnS4 solar cells. Materials Letters 130 (2014): 87 – 90.
[19] F. Liu, K. Sun, W. Li, C. Yan, H. Cui, L. Jiang, X. Hao, and M. A. Green. Enhancing the Cu2ZnSnS4 solar cell efficiency by back contact modification: Inserting a thin TiB2 intermediate layer at Cu2ZnSnS4/Mo interface. Applied Physics Letters 104, 051105 (2014).
[20] F. Zhou, F. Zeng, X. Liu, F. Liu, N. Song, C. Yan, A. Pu, J. Park, K. Sun, and X. Hao. Improvement of Jsc in Cu2ZnSnS4 solar cell by using a thin carbon intermediate layer at Cu2ZnSnS4/Mo interface. ACS Applied Material Interfaces 7 (2015): 22868 – 22873.
[21] D. Abou-Ras, D. Mukherji, G. Kostorz, D. Brémaud, M. Kälin, D. Rudmann, M. Döbeli, A. N. Tiwari. Dependence of the MoSe2 formation on the Mo orientation and the Na concentration for Cu(In,Ga)Se2 thin-film solar cells. Materials Research Society Symposia Proceedings 865, F8.1 (2005).
[22] Y. Guo and J. Robertson. Origin of the high work function and high conductivity of MoO3. Applied Physics Letters 105, 222110 (2014).
[23] P. Qin, G. Fang, W. Ke, F. Cheng, Q. Zheng, Jiawei Wan, Hongwei Lei, Xingzhong Zhao. In situ growth of double-layer MoO3/MoS2 film from MoS2 for hole-transport layers in organic solar cell. Journal of Material Chemistry A 2 (2014): 2742 – 2756.
[24] S. Chuang, C. Battaglia, A. Azcatl, S. McDonnell, J. S. Kang, X. Yin, M. Tosun, R. Kapadia, H. Fang, R. M. Wallace, A. Javey. MoS2 P-type transistors and diodes enabled by high work function MoOx contacts. Nano Letters 14 (2014): 1337 – 1342.
[25] J. M. Yun, Y. J. Noh, C. H. Lee, S. I. Na, S. H. Lee, S. M. Jo, H. I. Joh, D. Y. Kim. Exfoliated and Partially Oxidized MoS2 Nanosheets by One-Pot Reaction for Efficient and Stable Organic Solar Cells. Small 10 (2014): 2319 – 2324.
[26] S. McDonnell, A. Azcatl, R. Addou, C. Gong, C. Battaglia, S. Chuang, K. Cho, A. Javey, and R. M. Wallace. Hole Contacts on Transition Metal Dichalcogenides: Interface Chemistry and Band Alignments. ACS Nano 8 (2014): 6265 – 6272.
[27] P. Qin, G. Fang, W. Ke, F. Cheng, Q. Zheng, J. Wan, H. Lei and X. Zhao. In situ growth of double-layer MoO3/MoS2 film from MoS2 for hole-transport layers in organic solar cell. Journal of Material Chemistry A 2 (2014): 2742 – 2756.
[28] K. Biswas, S. Lany, and A. Zunger. The electronic consequences of multivalent elements in inorganic solar absorbers: Multivalency of Sn in Cu2ZnSnS4. Applied Physics Letters 96, 201902 (2010).
[29] Q. Guo, G. M. Ford, W. C. Yang, C. J. Hages, H. W. Hillhouse, R. Agrawal. Enhancing the performance of CZTSSe solar cells with Ge alloying. Solar Energy Materials & Solar Cells 105 (2012): 132 – 136.
[30] D. B. Khadka and J. H. Kim. Band Gap Engineering of Alloyed Cu2ZnGexSn1−xQ4 (Q = S,Se) Films for Solar Cell. Journal of Physical Chemistry C 119 (2015): 1706 – 1713.
[31] I. Kim, K. Kim, Y. Oh, K. Woo, G. Cao, S. Jeong, and J. Moon. Bandgap-Graded Cu2Zn(Sn1−xGex)S4 Thin-Film Solar Cells Derived from Metal Chalcogenide Complex Ligand Capped Nanocrystals. Chemistry of Materials 26 (2014): 3957 – 3965.
[32] D. B. Khadka, S. Y. Kim, and J. H. Kim. Effects of Ge Alloying on Device Characteristics of Kesterite-Based CZTSSe Thin Film Solar Cells. Journal of Physical Chemistry C 120 (2016): 4251 – 4258.
[33] C. J. Hages, S. Levcenco, C. K. Miskin, J. H. Alsmeier, D. Abou-Ras, R. G. Wilks, M. Bär, T. Unold and R. Agrawal. Improved performance of Ge-alloyed CZTGeSSe thin film solar cells through control of elemental losses. Progress in Photovoltaics: Research and Applications 23 (2015): 376 – 384.