- 學位論文
Non-toxic solution processed earth abundant kesterite based thin-film solar cells and their device engineering
利用無毒溶液製程鋅黃錫礦半導體材料應用於薄膜太陽能電池
摘要
本論文將介紹一製成。使用硫銅錫鋅結構作為吸收層，藉由使用無毒的溶膠凝膠法旋轉塗佈到表面達成，且做成一太陽能元件。 近年來，銅鋅錫硫(CZTS)或銅鋅錫硫硒(CZTSSe)系列的太陽能電池材料，因由地殼上豐度高的元素所組合而成，未來極有機會取代目前商業上已成功量產的銅銦鎵硫(CIGS)電池，而獲得龐大的關注。 在第二章，首先介紹旋轉塗佈的材料，此為一墨水，以1,3-二甲基-2-咪唑啉酮(1,3-dimethyl-2-Imadazolidinone)(DIM)作為溶劑，將銅鋅錫硫(以下簡稱CZTS)溶解於此溶劑中。透過旋轉塗佈得到CZTS薄膜。先將CZTS薄膜透過CVD沉積錫(Tin)，再者，CZTS薄膜快速熱退火的同時，通入經稀釋的硫化氫(H2S)氣體與6%氮氣(N2)。這樣外部提供的錫(Tin)與硫(S)，可以減少退火時錫(Tin)在CZTS薄膜中結構的損失，並以達到強化元件特性之目的。此晶粒狀的CZTS薄膜可達到0.7到1.5微米厚度，且在钼(Mo)與CZTS薄膜之間擁有低碳量與小晶粒層的特性。製造出的元件效率最高可達5.67%，擁有0.58伏的開路電壓(Voc)與每平方公分18.48毫安培的短路電流(Jsc)，且有53.14%的填充因子(Fill Factor)。 在第三章。我們介紹經硒化反應合成的銅鋅錫硫硒(CZTSSe)化合物(以下簡稱CZTSSe)作為吸收層材料，透過改變硫(S)與硒(Se)的比例，達成所要的能隙(Band Gap)。將CZTS薄膜快速熱退火的同時，通入經稀釋的硫化氫(H2S)氣體與10%氬氣(Ar)，再者由外部提供錫(Tin)與硒(Se)。我們成功地得到不同能隙的CZTSSe，藉由改變錫(Tin)與硒(Se)的總和(x= 274 mg, x/5, x/10)，我們更成功的將CZTSSe的能隙從1.14伏特提高到1.34伏特。此章最後，第一，優化的錫(Tin)與硒(Se)總量為54.8mg， 且CZTSSe最好的能隙坐落於1.22eV，元件效率可達到4.72%，擁有0.48伏的開路電壓(Voc)與22.2每平方公分毫安培的短路電流(Jsc)，且有44.3%的填充因子(Fill Factor)；第二，熱退火硒化時，在加入硒(Se)碇時僅通入氫氣與稀釋後的氬氣輔助，且觀察完整CZTSe的成長過程，在100sccm的氫氣，效率可達5.19%，擁有0.38伏的開路電壓(Voc)與27.6每平方公分毫安培的短路電流(Jsc)，且有49.5%的填充因子(Fill Factor)。 回顧我們先前研究的CZTS/CZTSSe元件。硫酸鎘(CdSO4)，曾被作為鎘(Cd)的提供源，透過CBD(根據穩定的沉積製成)沉積硫化鎘(CdS)薄膜，厚度為80正負20微米厚度。由穩定製成的製作出的元件，其顯示出元件在短波長波段的可見光光譜並沒有很好的電荷收集效率，原因是吸收層中做為緩衝層的CdS。在第四章，CdS層擁有37 微米的厚度沉積在CZTSSe，使用Cd(NO3)2作為前驅物，使用CBD在PH值11.8下製成。加上CdS層所做出完整的元件，效率最高可達到6.97%，相較於5.91%控制元件，因為是異質結界面的優化與對外部電子光譜電荷收集的強化，所以增加了電子密度。 在第五章，將展示出用溶液製成的CZTS做出p-n接面(CZTS/Zn) (S,O,OH)。一個n-type 鋅Zn(S,O,OH)厚度在40微米的緩衝層，利用13分鐘的CBD沉積在CZTS 上。其薄膜內的化學構成是由能譜儀並尋找硫與氧元素的梯度分布。在CBD之後p-n接面由NH4OH清洗，再由200C加熱10分鐘。由CBD做出的ZnS緩衝層製成的太陽能元件，效率可達4.1%，相較於5.67%標準CdS緩衝層的製成的元件。
並列摘要
This thesis demonstrates the deposition and growth of earth abundant kesterite (i.e., Cu2ZnSnS4, Cu2ZnSnSxSe4–x) absorber layers by using non-toxic sol-gel spin coating approach and their solar cells device engineering. In chapter 2, we have introduced 1,3-dimethyl-2-Imadazolidinone (DMI) as a solvent for the preparation of high viscosity homogeneous nontoxic Cu2ZnSnS4 (CZTS) ink. Annealing the spin coated CZTS thin film in diluted H2S (6% N2) gas with externally supplied tin and sulfur environment suppresses the tin loss from the thin-film surface and enhanced the device performance. Grain size of CZTS has been achieved to > 0.7 to 1.5 µm with no carbon rich or small grain layer at the Mo/CZTS interface. The fabricated champion device achieved 5.67% efficiency with open circuit voltage of 0.58 V, short circuit current density of 18.48 mA/cm2, and a fill factor of 53.14%. In chapter 3 – reactive gas selenization – we demonstrate the synthesis of CZTSSe absorber layers with desired bandgap by tuning the composition of S to Se ratio. Annealing the spin coated CZTS thin film in diluted H2S (10% Ar) gas with externally supplied tin and selenium; we successfully have obtained CZTSSe absorber layers of different bandgaps. By changing Sn+Se amount (x= 274 mg, x/5, x/10), we successfully tuned the CZTSSe absorber layers bandgaps from 1.14 to 1.34 eV. Finally, by optimizing the Sn+Se (=54.8 mg) amount, the best CZTSSe (Eg = 1.22 eV) device efficiency was achieved to be 4.72% with Voc = 0.48 V, Jsc = 22.2 mA/cm2 and FF = 44.3 %. In second part, H2+Ar-assisted selenization, when annealed in H2 (diluted in Ar) and Se, we have observed formation of completely grown CZTSe absorber layer. In 100sccm H2, the device efficiency was achieved to 5.19%, Voc = 0.38 V, Jsc = 27.6 mA/cm2, FF=49.5 %. In order to obtain “stable CZTS” solution, we have changed copper source to copper formate to get preferred oxidation Cu2+1Zn+2Sn+4S4-2. H2-assisted selenization of the spin-coated film with such a sol-gel, gave an improved solar cell performance. The champion cell efficiency found to be 5.19%, Voc = 0.38 V, Jsc = 27.6 mA/cm2, FF=49.5 %. Eg = 1.06 eV. In our previous studies on CZTS/CZTSSe devices, cadmium sulfate (CdSO4) has been used as the cadmium source for depositing CdS layer (80±20 nm) via CBD (hereafter referred to as standard procedure). Devices fabricated with the standard procedure show poor charge collection at shorter wavelengths of the visible spectrum due to absorption by the CdS buffer layer. In chapter 4, a Cadmium sulfide (CdS) layer with a thickness of 37±5 nm is deposited onto a Cu2ZnSn(SSe)4 absorber layer using Cd(NO3)2 precursor at pH 11.8 via CBD process. Here the absorber is grown by sputtering process. Full devices fabricated with the thin CdS layer show improved champion efficiency of 6.97%, compared with 5.91% control device due to increased current density from optimized hetero-junction interface and enhanced charge collection in the external quantum efficiency spectrum. In chapter 5, preparation of solution processed earth abundant p-n junction, Cu2ZnSnS4/Zn(S,O,OH), is presented here. A thin, n-type Zn(S,O,OH) buffer layer of 40±5 nm thickness is chemical bath deposited (CBD) on Cu2ZnSnS4 absorber layer with 13 min deposition time. The chemical composition of the film is determined by energy dispersive spectroscopy and found gradient distribution of S and O across the film. After CBD, the p-n junction is rinsed in NH4OH and subsequently heated at 200 C for 10 min. The solar cell performance of CBD-ZnS buffer layer reached up to 4.1% efficiency compared with 5.67% of standard CdS buffer layer solar cell.