熱效應對於銅銦鎵硒硫太陽能電池元件界面元素擴散之研究

Abstract

本研究致力於探討長時間的熱效應對於銅銦鎵硒硫[Cu(Inx,Ga1-x)(Sey,S1-y)2, CIGSSe]薄膜太陽能電池元件樣品之界面處的元素擴散行為。為了模擬真實環境，我們利用石英管通入空氣下加熱110度約40小時。為了可以觀測到熱處理後各種元素在薄膜界面的擴散分布，我們將樣品的薄膜層蝕刻一個梯度，並利用軟硬X光光電子能譜研究ZnO/CdS/CIGSSe的界面處的化學鍵結及電子能帶結構變化。 對於經過長時間受熱影響的樣品，我們觀察到Cd元素會往吸收層擴散，S元素會往下層吸收層方向擴散，而Se元素會往緩衝層與背電極方向擴散，Ga、In和Cu元素則會往緩衝層擴散。經過熱處理的樣品，在In 4d光電子能譜低束縛能位置產生未束縛價電子的化合物，此化合物為CdS與CIGSSe表面處因元素相互擴散形成的CdxInySzSe1-z，CdS/CIGSSe界面產生的In未束縛電子將會束縛正載子，使載子不易被收集到背電極。熱效應讓樣品元素濃度重組使得化學環境改變，導致能隙變小。這些結果清楚地說明熱效應的影響，會產生一些重要因素在太陽能電池效率退化方面。

This study is focused on understanding the elemental diffusion behaviors of Cu(In,Ga)(Se,S)2-based solar cells under the long-term thermal effect, To approach a real situation in application of solar cell, the sample was annealed to 110 oC and maintained this temperature for about 40 hr in the quartz tube. We chemically etched the ZnO/CdS/CIGSSe sample with a gradient thickness to observe the elemental distribution near/at the interfaces, the chemical environments and the electronic structure of the samples were also studied by means of soft and hard X-ray photoemission spectroscopies (XPS). Our results indicate that, the cadmium elements of the buffer layer diffuse into CIGSSe absorber layer, whereas the sulfur diffuses towards the absorber layer. On the other hand, the selenium elements of CIGSSe layer tend to diffuse towards both the buffer and the absorber layers, but Ga, In, and Cu diffuse towards the buffer layer. After thermal treatment, the In 4d photoelectron spectra reveal a lower binding energy component, which can be assigned to the interface phase CdxInySzSe1-z with the unbound valence electron of In. This compound is possibly formed at the interface of CdS and surface In-rich CIGSSe due to interdiffusion of elements. The creation of the unbound electron of In at the CdS/CIGSSe interface will trap the positive carriers, leading to lower probability of carrier collection by the back-side electrode. The redistributed concentration of all elements also leads to a smaller band gap. These results indicate some important factors of degradation in solar-cell conversion efficiency due to thermal effect.