CZTSSe薄膜太陽能電池之長晶及表面硫化過程研究

Investigation of growth mechanism and surface sulfurization of CZTSSe thin film solar cell

王凱正(Kai-Cheng Wang)

指導教授 ： 陳學仕

國立清華大學/工學院/材料科學工程學系/碩士(2015年)

摘要

本研究利用磁控濺鍍系統將銅-鋅-錫合金靶及鋅靶濺鍍於鉬玻璃上製備銅-鋅-錫前驅物，接著前驅物進行硒化及後硫化製程(sulfurization after selenization process, SAS)。在SAS製程中，首先將銅-鋅-錫前驅物放進爐管，在H2Se/Ar氣氛中於475 ℃下硒化15分鐘，再升溫至495 ℃，於H2S/Ar氣氛中硫化15分鐘，最後可得Cu2ZnSn(S, Se)4 (CZTSSe)薄膜。為了探討SAS製程中之硒化及硫化之效應，本研究著重於兩部分，第一部分為銅-鋅-錫前驅物薄膜於不同硒化溫度下之成長，此部分研究利用掃描式電子顯微鏡(SEM)及能量分散光譜(EDX)來觀察薄膜表面形貌、厚度及元素分佈情況，由低掠角X光繞射(GIXRD)圖及拉曼光譜鑒定未知相，最後提出Cu2ZnSnSe4 (CZTSe)薄膜之可能成長機制。製備成元件後，由電流-電壓(I-V)及外部量子效率(EQE)之量測數據，可得之最佳元件效率為475 ℃下硒化之樣品(樣品編號：CZTSe-475)，在AM 1.5G下，開路電壓及短路電流分別為0.27 V及22.6 mA/cm2，轉換效率為2.1%；另一部分研究為經SAS製程之樣品性質分析(樣品編號：CZTSSe-7.5)，在AM 1.5G下，開路電壓及短路電流分別為0.39 V及33.1 mA/cm2，元件轉換效率達7.5%。比較CZTSSe-7.5及CZTSe-475兩元件於不同偏壓下EQE量測結果後發現，CZTSe-475之EQE (-0.5 V)/EQE (0 V)比值於短波長區域(400－600 nm)增加，然而CZTSSe-7.5之EQE比值並沒有明顯改變，此結果顯示硫化過程應扮演薄膜之鈍化作用，減少了表面缺陷，因而提升元件效率。從導電式原子力顯微鏡(conductive atomic force microscopy)觀察後發現，局部電流多出現於CZTSe-475及CZTSSe-7.5薄膜表面晶界附近，另外，光致螢光光譜(photoluminescence spectroscopy)顯示CZTSe-475及CZTSSe-7.5薄膜內可能存在VCu及ZnCu缺陷，而CZTSSe-7.5薄膜內亦可能存有ZnSn缺陷，可能為降低元件光轉換效率之原因。

關鍵字

CZTSe ； CZTSSe ； 太陽能電池 ； 磁控濺鍍 ； 硒化後硫化

並列摘要

In this study, Cu-Zn-Sn (CZT) precursor film was deposited on a Mo-coated soda-lime glass substrate by a DC-magnetron sputter with alloy Cu-Zn-Sn and Zn targets. As-prepared CZT precursor film was moved to a tube furnace for selenization and post sulfurization (sulfurization after selenization, SAS) process. Experimentally, CZT precursor film was first selenized at 475 ℃ in H2Se/Ar atmosphere for 15 minutes, followed by sulfurization at 495 ℃ in H2S/Ar atmosphere for 15 minutes to form Cu2ZnSn(S, Se)4 (CZTSSe) thin film. In order to understand the role of selenization and sulfurization in the SAS process, the study is divided into two parts. The first part is focused on the grain growth of CZT precursor film at different selenization temperatures. Surface morphology, thickness and element distribution were examined by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDX). The crystallography and phase information of the samples were studied by glazing angle X-ray diffraction (GIXRD) and Raman spectroscopy. Based on the experimental results, a plausible growth model of Cu2ZnSnSe4 (CZTSe) thin film was proposed. The CZTSe device properties were analyzed by I-V and external quantum efficiency measument systems. Among all samples, precursor film selenized at 475 ℃, which was called CZTSe-475, had the highest conversion efficiency of 2.1%, open circuit voltage of 0.27 V and short circuit current density of 22.6 mA/cm¬2 under AM 1.5G illumination. In the second part, a selenized sample treated with an additional sulfurization (SAS) process (device named as CZTSSe-7.5) had the conversion efficiency of 7.5%, open circuit voltage of 0.39 V and short circuit current density of 33.1 mA/cm¬2 under AM 1.5G illumination. Compared the CZTSe-475 device with the CZTSSe-7.5 one under a negative applied voltage (e.g., -0.5 V), the ratio of EQE (-0.5 V)/EQE (0 V) of the CZTSe-475 device increased in the short wavelength region (400－600 nm), but the ratio had no change in the CZTSSe-7.5 device. The result suggests that the defect density in the CZTSSe-7.5 device would be smaller than that in the CZTSe-475 device, which implies that the effect of the SAS process was passivation for the thin film. Moreover, conductive atomic force microscopy (CAFM) shows that local current appeared to exist near the grain boundaries (GBs). Photoluminescence study exhibits that both VCu and ZnCu defects existed in CZTSe-475 and CZTSSe-7.5 thin films. On the other hand, ZnSn defect, which might decrease the conversion efficiency of the device, was thought to exist only in CZTSSe-7.5 thin film.

並列關鍵字

CZTSe ； CZTSSe ； solar cells ； magnetron sputter ； sulfurization after selenization

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