以濺鍍法製備銅鋅錫硫硒薄膜及其應用於薄膜太陽能電池製備之研究

Abstract

本論文研究使用CuS、ZnS及SnS2二元相粉體為起始原料，以固態燒結法製備2吋Cu2ZnSnS4（CZTS）單一四元相靶材。粉末混和於不同Cu莫耳比（2、1.8及1.6）後壓成6 mm直徑圓錠，經200至600C、8小時之燒結，探討微觀結構、組成及表面形貌。X光繞射（X-ray Diffraction，XRD）與拉曼光譜（Raman Spectroscopy）分析顯示，Cu莫耳比為1.6之試片經600°C、8小時燒結可獲得單一Kesterite （KS） CZTS相；掃描式電子顯微鏡（Scanning Electron Microscopy，SEM）與能量散佈光譜儀（Energy Dispersive Spectroscopy，EDS）分析顯示其化學劑量比為Cu26.53Zn14.75Sn12.31S46.41之貧銅/富鋅（Cu-poor/Zn-rich）組成。利用6 mm圓錠建立之燒結製程製備2吋CZTS濺鍍靶材，結構與成分分析顯示，可獲得單一KS-CZTS相結構、化學劑量比為Cu26.55Zn15.00Sn12.32S46.14之貧銅/富鋅組成之2吋CZTS靶材。接著以濺鍍法在1、5及10 mtorr之工作壓力沉積CZTS光吸收層薄膜，其經570C、1小時硫化退火處理後，XRD及Raman分析顯示薄膜均為單一KS-CZTS相結構，霍爾效應（Hall Effect）量測顯示其均為p型化合物半導體，EDS分析顯示，在工作壓力為1 mtorr鍍成之薄膜為Cu24.15Zn14.73Sn10.75S50.37之貧銅/富鋅組成，紫外光-近紅外光光譜（UV-NIR Spectroscopy）分析顯示其能隙（Energy Bandgap，Eg）為1.5 eV，且此一薄膜具有最佳的載子濃度（Carrier Concentration，N）為2.81017 cm3及載子遷移率（Mobility，）為8.61 cm2V1sec1。含此一CZTS薄膜之太陽能電池元件之光電轉換效率（Conversion Efficiency，）為5.2%、短路電流（Short-circuit Current，Jsc）為19.17 mAcm2、開路電壓（Open-circuit Voltage，Voc）為0.52 V及填充因子（Fill Factor，FF）為52.7%，外部量子轉換效率（External Quantum Efficiency，EQE）約為60%。 為了近一步提升CZTS薄膜太陽能電池轉換效率，本研究將不同含量的Se元素加入CZTS薄膜而形成Cu2ZnSn(S1-xSex)4（CZT(SSe)）薄膜，並探討其對顯微結構、組成及光電特性之影響。利用先前製備之2吋CZTS靶材，以濺鍍法沉積CZTS薄膜後，經570C、1小時、不同硫/硒莫耳比（0、0.5、1、4及10）氣氛的退火處理後，可獲得硫/(硫+硒)比例由0.21至1的CZT(SSe)薄膜，且均為貧銅/富鋅組成，XRD與Raman分析顯示薄膜為KS-CZTS相及KS-CZTSe相之混合。紫外光-近紅外光光譜分析顯示，硫/(硫+硒)比例由0.21至1的CZT(SSe)薄膜的Eg範圍為1.06至1.45 eV；霍爾效應分析顯示CZT(SSe)薄膜均為p型化合物半導體，硫/(硫+硒)比例為0.46之CZT(SSe)薄膜具有最佳的N為2.171015 cm3及為8.9 cm2V1sec1。含此一CZT(SSe)薄膜之太陽能電池元件之為6.9%、Jsc為27.41 mAcm2、Voc為0.506 V及FF為50%，EQE約為70%。

This study prepares the 2-inch Cu2ZnSnS4 (CZTS) targets by the solid-state reactions of CuS, ZnS and SnS2 raw powders mixture. The powders were mixed at various Cu molar ratios of 2, 1.8 and 1.6, pressed into 6-mm pellet forms, and then sintered at temperatures ranging from 200 to 600C for 8 hrs. Accordingly, the correlations of powder constitution and heat-treatment conditions to the microstructure, composition and morphology of CZTS pellets were investigated. X-ray diffraction (XRD) and Raman spectroscopy analyses indicated that the sample prepared with the Cu molar ratio of 1.6 and sintered at 600C for 8 hrs forms the single-phase kesterite (KS) CZTS structure. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses revealed the Cu-poor/Zn-rich feature with the stoichiometry of Cu26.53Zn14.75Sn12.31S46.41. Afterward, the 2-inch CZTS sputtering target was prepared by utilizing the optimum sintering condition established by the study of 6-mm pellets. Microstructure and composition analyses indicated the CZTS target possesses the single-phase KS-CZTS structure and the stoichiometry of Cu26.55Zn15.00Sn12.32S46.14 with Cu-poor/Zn-rich feature. The 2-inch CZTS sputtering target was then transferred to a sputtering system for depositing the CZTS photo-absorption layers at working pressures of 1, 5 and 10 mtorr. Follwed by the annealing at 570C for 1 hr in sulfur (S) vapor ambient, the single-phase, Cu-poor/Zn-rich CZTS layer with the stoichiometry of Cu24.15Zn14.73Sn10.75S50.37 was achieved in CZTS layer deposited at 1 mtorr. Moreover, it possessed the bandgap (Eg) of 1.5 eV, p-type carrier concentration (N) of 2.81017 cm3 and mobility () of 8.61 cm2V1sec1. Such a CZTS layer was implanted in the thin-film solar cells with the device structure of Mo/CZTS/CdS/i-ZnO/IZO/Al. Under the AM1.5 illumination condition, the CZTS thin-film solar cell sample with the open-circuit voltage (Voc) of 0.52 V, short-circuit current (Jsc) of 19.17 mAcm2, the fill factor (FF) of 52.7%, the conversion efficiency () of 5.2% and the external quantum efficiency (EQE) of 60% was achieved. In order to further enhance the efficiency of CZTS device, selenium (Se) was added into the CZTS to form the Cu2ZnSn(SxSe1-x)4 (CZT(SSe)) photo-absorption layer and their microstructure, composition and physical properties were investigated. The CZT(SSe) layers were prepared by sputtering deposition using single-phase CZTS target followed by annealing treatment at 570C for 1 hr in the ambient with various Se/S ratios (0, 0.5, 1, 4 and 10). Analytical results indicated the Cu-poor/Zn-rich CZT(SSe) layers with S/(S+Se) ratios in the range of 0.21 to 1 could be achieved and the CZT(SSe) layers were the mixture of KS-CZTS and KS-CZTSe phases. UV-NIR spectroscopy indicated the Eg’s of CZT(SSe) samples are in the range of 1.06 to 1.45 eV when the S/(S+Se) ratio varies from 0.21 to 1. Hall effect measurement observed the best transport property with p-type conduction, N of 2.171015 cm3 and  of 8.9 cm2V1sec1 in CZT(SSe) layer with S/(S+Se) ratio of 0.46. Under the AM1.5 illumination condition, the CZT(SSe) thin-film solar cell sample with S/(S+Se) ratio of 0.46 exhibited the best performance with Voc of 0.506 V, Jsc of 27.41 mAcm2, FF of 50%,  of 6.9% and EQE of 70%.