原子層沉積寬能隙氧化鋅錫緩衝層用於高效/環境友善之銅鋅錫硫硒太陽能電池

Abstract

銅鋅錫硫硒(CZTSSe)薄膜太陽能電池由地球豐富且無毒的元素組成，是低成本且高產能的可再生能源。因CZTSSe太陽能電池常使用含有毒的硫化鎘做為N型緩衝層，其能隙較窄（約2.4eV），會吸收短波長之可見光，減少太陽能電池的光電流。為避免上述問題，本論文選擇使用寬能隙且對環境友善的緩衝層-氧化鋅錫(ZnSnO)取代有毒的硫化鎘。首先，藉由原子層沉積法來成長氧化鋅錫，目的藉由它能精準控制薄膜厚度且使薄膜大面積均勻地鋪覆，填補CZTSSe異質接面的孔隙，大幅減少介面的缺陷。我們選用超循環法來合成三元化合物-氧化鋅錫。並透過X-射線光電子能譜證明超循環法的成長方式可調控氧化鋅錫之Sn/(Zn+Sn)比例。接著，各別分析錫含量15%、20%、30%及40%的氧化鋅錫的能隙、能帶位置、載子濃度。在能隙方面，氧化鋅錫皆大於3eV，增加元件短波長處的光子吸收。我們用紫外光光電子能譜量測不同比例的氧化鋅錫能帶位置，計算與CZTSSe的導帶位置差異，由文獻得知其差異介於0~0.4eV，是較理想的情況。發現錫含量30%的氧化鋅錫有相差最小的導帶差異且為小尖峰態，有助於抑制介面載子複合，提升開路電壓及填充因子。同時對異質接面的材料與CZTSSe的晶格常數匹配對效率之影響性做討論，推測在富含硫硒的CZTSSe表面上沉積氧化鋅，易產生晶格匹配的硫化鋅介面，有助於鈍化介面缺陷以減少非輻射複合，對整體元件之開路電壓、串聯電阻有所改善。另外，透過電化學量測氧化鋅錫薄膜之載子濃度有1017cm-3 以上，易擴大空乏區，促使電子電洞對分離。綜合上述結果，皆是促使太陽能電池轉換效率能夠達到提升的重要因素。通過原子層沉積形成30% 錫含量的寬能隙氧化鋅錫緩衝層在CZTSSe太陽能電池表現優秀，其亮、暗電流間，無交叉現象，表示P-N接面處缺陷較少。在經過鍍製抗反射層MgF2 以減少光源反射出去，其開路電壓可達0.523 V，填充因子有53.51％，短路電流密度提升至35.16 mA/cm2。最終達到電池轉換效率9.83% (有效面積計算為10.86%)的高效率且環境友善的銅鋅錫硫硒薄膜太陽能電池。Copper-zinc-tin-sulfur-selenium (CZTSSe) thin-film solar cells are composed of earth-abundant and non-toxic elements. It is a low-cost and high-efficiency renewable energy source as well. Due to CZTSSe solar cells used cadmium sulfide as an N-type buffer layer, containing toxic cadmium metal, and its bandgap is narrow(~2.4eV), which absorbs short-wavelength visible light. It will reduce the photocurrent of the solar cells. To avoid the above problems. This paper uses a wide bandgap and Eco-friendly N-type buffer layers - zinc tin oxide (ZnSnO) instead of toxic cadmium sulfide.First of all, we use atomic layer deposition (ALD) to grow zinc tin oxide. It is easy to fill the voids of the CZTSSe heterojunction by precisely controlling the film thickness and large-area uniformly coverage, and reduce the interface defects. Through the supercycle method to synthesize the ternary compound-zinc tin oxide. We use X-ray photoelectron spectroscopy (XPS) to prove that the supercycle method can adjust the Sn/(Zn+Sn) ratio of zinc tin oxide. The second part, analyze the bandgap, band position, and carrier concentration of zinc tin oxide with 15%, 20%, 30%, and 40% Sn content individually. In terms of bandgap, zinc tin oxide is greater than 3 eV, which can enhance the photon absorption of the device at short wavelengths. We used Ultraviolet photoelectron spectroscopy (UPS) to measure the band positions of zinc tin oxide with different Sn/(Zn+Sn) ratios and calculate the conduction band offset (CBO) with CZTSSe. It is an ideal condition when CBO is between 0~0.4eV from the literature. The smallest CBO in ZnSnO Sn=30% and shows a little spike-like, which helps to reduce the interface carrier recombination and improve the open-circuit voltage and fill factor. At the same time, we discuss the effect of matching the interface lattice constant of CZTSSe with the heterojunction material. It is speculated that zinc oxide is deposited on the sulfur-selenium-rich surface, which is easy to produce the zinc sulfide at the interface which is a lattice match with CZTSSe. It helps to passivate the interface to reduce the non-radiative recombination and greatly improve the overall device the open-circuit voltage and series resistance. In addition, the carrier concentration of the zinc tin oxide film measured by electrochemistry is above 1017cm-3, which is easy to expand the depletion region and promote the separation of electron-hole pairs. The above results are all important factors that promote the improvement of solar cell conversion efficiency.It is excellent that the CZTSSe solar cells have a wide-bandgap zinc tin oxide buffer layer with 30% tin content by atomic layer deposition. There was no crossover phenomenon between the light and dark currents, indicating that there were fewer defects at the P-N junction. After depositing the anti-reflective coating MgF2 can reduce the reflection of sunlight. The open-circuit voltage achieved 0.523 V, the fill factor is 53.51%, and the short-circuit current density is increased to 35.16 mA/cm2. Eventually, achieving a highly efficient and Eco-friendly copper-zinc-tin-sulfur-selenium thin-film solar cell with a power conversion efficiency of 9.83% (10.86% in the effective area).