Thin film solar cells based on $Cu(In,Ga)Se_2$ (CIGS) continue to be a leading candidate for thin film photovoltaic devices due to their appropriate bandgap, long-term stability, and low-cost production. Over the past few years, many groups in the world have reported a variety of processes for CIGS film growth. To date, the most successful technique for the deposition of a CIGS absorber layer has been based on the co-evaporation of Cu, In, and Ga in the presence of Se, achieving an efficiency of greater than 19%. However, the evaporation process is difficult to scale-up for large-area manufacturing.
The selenization process has been a promising method for low-cost and large-scale production of high quality CIGS film. Also, it has an advantage of precise control of composition and film thickness. Conventionally, alloyed or stacked Cu-In-Ga metal precursor is deposited by sputtering, and it is followed by a selenization process in toxic $H_2Se$ ambient gas. Using the selenization process, the module efficiency of above 14% has been reported. Even though high efficiency is reported from some companies, the details of the selenization process are not fully understood yet. And also the effects of Ga incorporation on device behavior are not fully understood. The addition of Ga does not readily give a film with uniformly increased band gap. Instead, most Ga in the reacted film segregates near the Mo, so the solar cell behaves like CuInSe2 and lacks the increased open-circuit voltage. There was some reports that Ga grading was caused by In and Ga different reaction speed. Therefore, in this study we investigated Ga grading and Ga distribution in CIGS film.
In the first experiment, we have used $Cu_{40}In_{60}$ and $Cu_{50}Ga_{50}$ alloy targets for precursor deposition because pure In and Ga are soft materials. The precursor deposited by sputtering of $Cu_{40}In_{60}$ and $Cu_{50}Ga_{50}$ targets was selenized using two-step selenization process in a Se vapor in a vacuum chamber instead of conventional $H_2Se$ ambient for safe selenization. This study will investigate and characterize the effects of two-step selenization condition on Ga distribution in CIGS film and CIGS solar cells.
The Cu-In-Ga metal precursor was deposited by simultaneous sputtering of $Cu_{40}In_{60}$ and $Cu_{50}Ga_{50}$. This precursor was composed of $Cu_9(Ga,In)_4$ and In phases, and it had the soft and conglomerate morphology. The In concentration was high at the surface and low in the bulk. The Cu concentration was low at the surface and was high in the bulk even though the precursor had been deposited by co-sputtering. Before two-step selenization was applied to metal precursor, we investigated the InSe and the $Ga_2Se_3$ co-exist condition in 1st two-step selenization. Because it gives the sufficient temperature and time to form $Ga_2Se_3$. At 250$\degC$ substrate temperature in 1st two-step selenization, the XRD analysis indicated the existence of the InSe and the $Ga_2Se_3$ phases. After 2nd two-step selenization, we achieved CIGS film of average [Cu]/[In+Ga]~0.84, [Ga]/[In+Ga]~0.33 ratio. And it has been found that 1st two-step selenization condition co-existing the InSe and the $Ga_2Se_3$ seemed to show different Ga distribution in comparison with other conditions. Near the CIGS/Mo interface, [Ga]/[In+Ga] ratio was about 0.5 and from CIGS/Mo interface to the middle of film, the Ga grading was lower. Nevertheless, at the surface, [Ga]/[In+Ga] ratio was about 0.13 and still low. The difference in the Ga distribution at the surface made low performance in CIGS solar cell properties. The CIGS cell showed the conversion efficiency of 9.86 % with $J_{sc}$ = 34.31 mA/$cm^2$, $V_{oc}$ = 0.483 V, and F.F. = 0.60 for an active layer of 0.44 $cm^2$. Compared with CIGS cells using common selenization process, $V_{oc}$ is similar. So cell efficiency was not improved. It resulted from the lower Ga concentration at the CIGS surface. So using selenization process, modified precursor to restrict Ga segregation and diffusion to the CIGS/Mo interface was necessary. Additional suppling of Ga layer on the precursor was suggested to increase open circuit voltage and conversion efficiency.
And in second experiment, The precursor was deposited by sputtering of $Cu_{40}In_{60}$, $Cu_{50}Ga_{50}$ and Cu targets and then Ga and Se layer was deposited on Cu-In-Ga metal precursor. Cu target was selected to control the Cu composition and the reason using Se with Ga was pure Ga are soft and liquid material. The modified precursor was selenized using common selenization process in a Se vapor in a vacuum chamber. This study will investigate and characterize the effects of Ga and Se deposition layer thickness on Ga distribution in CIGS film.
The Cu-In-Ga metal precursor was deposited by simultaneous sputtering of $Cu_{40}In_{60}$, $Cu_{50}Ga_{50}$ and Cu. The Cu-In-Ga metal precursor was made to be Cu-rich and Ga-poor composition and this metal precursor composed of $Cu_{16}(In_{1-y}Gay)_9$ and In phase, and it made to increase the In solubility because of Cu-rich concentration. The In-rich and Cu-poor region was reduced at the surface and the Cu, In and Ga concentration was uniform in the bulk. Ga and Se layer was applied on the Cu-In-Ga metal layer to restrict Ga segregation and diffusion in CIGS film with making Ga and Se binding, reducing the supply of Ga with $Cu_{50}Ga_{50}$ target. There are two ways to deposit the Ga and Se layer. The first one is Ga and Se co-evaporation and the other is $Ga_2Se_3$ evaporation method. The layer made by both methods, that was showd to amorphous $Ga_2Se_3$ layer in the result of XPS and XRD measurement. As the thickness of $Ga_2Se_3$ layer increased in both methods, a small-grained CIGS layer was developed and phase separation was showed. The XRD and AES depth profile indicated that a small-grained Ga-rich CIGS layer was developed near CIGS/Mo interface and a little large-grained Ga-poor CIGS layer was developed near the surface.
When the thickness of Ga and Se layer was 360nm using by Ga and Se co-evaporation, the average [Cu]/[In+Ga] and [Ga]/[In+Ga] ratio was 0.92 and 0.28 respectively. The XRD analysis indicated the existence of the $Ga_2Se_3$ or $(InGa)_2Se_3$ and CuSe in the CIGS films. From the AES depth profile, Near the CIGS/Mo interface, [Ga]/[In+Ga] ratio was about 0.5 and from CIGS/Mo interface to the middle of film, the Ga grading was lower. And at the surface, [Ga]/[In+Ga] ratio was about 0.2. Compared with one-step selenization process, [Ga]/[In+Ga] ratio was increased at the surface. But it was necessary to increase selenization reaction time to form CIGS because of remained the $Ga_2Se_3$ or $(InGa)_2Se_3$ and CuSe.
When the thickness of $Ga_2Se_3$ layer was 220nm using by $Ga_2Se_3$ evaporation, the average [Cu]/[In+Ga] and [Ga]/[In+Ga] ratio was 0.89 and 0.31 respectively. The XRD analysis indicated the existence of the small CuSe phase in the CIGS films. From the AES depth profile, Near the CIGS/Mo interface, [Ga]/[In+Ga] ratio was about 0.5 and from CIGS/Mo interface to the middle of film, the Ga grading was lower. And at the surface, [Ga]/[In+Ga] ratio was about 0.2. Compared with one-step selenization process, [Ga]/[In+Ga] ratio was increased at the surface.
태양전지의 광흡수층으로서 적합한 CIGS 박막을 제조하기 위하여 $Cu_{40}In_{60}$, $Cu_{50}Ga_{50}$과 Cu 타겟으로부터 증착된 금속 층 위에 Ga과 Se을 후속 공급한 뒤 selenization하였다. Ga과 Se을 공급한 이유는 CIGS 박막 표면의 부족한 Ga의 분포를 보완하여 CIGS 태양전지의 개방 전압을 향상시키기 위함이다. Ga과 Se을 공급하기 전의 전구체 내의 조성은 In이 Cu내 고용하여 기존의 4장에서의 금속 전구체에서와 달리 Cu-rich 조성으로 제조하였고 Ga의 후속 보충되는 것을 고려하여 Ga-poor 조성으로 제조하였다. 이렇게 함으로써 비교적 평탄한 금속 전구체 위에 Ga과 Se을 공급한 precursor를 만들 수 있었다. Ga과 Se을 공급하는 실험은 첫 번째, Ga과 Se source를 co-evaportaion 방법으로 하는 것과 $Ga_2Se_3$를 이용하여 증착하는 두 번째 실험을 진행하였다. 먼저 후속 공급되는 막의 특성을 알아보기 위해, XRD와 XPS 분석결과 amorphous 형태의 $Ga_2Se_3$ 막으로 증착되는 것을 확인할 수 있었다.
첫 번째 실험은 금속 전구체 위에 Ga과 Se을 co-evaporation 하여 금속 전구체 위에 공급한 막을 selenization하여 CIGS의 특성을 분석하였다. Ga과 Se을 공급하는 두께가 커질수록 Cu조성비는 줄어들고, Ga조성비는 증가하는 것을 보였고, 이에 따라 결정립의 크기도 줄어드는 경향을 보였다. Ga과 Se을 490nm 이상 공급한 조건에서는 CIGS 박막이 두층으로 존재하였고, X선 회절 분석과 AES depth profile에서도 CIGS 상이 분리되어 존재하고 두층으로 나뉘어 있는 것을 확인할 수 있었다. CIGS 박막내의 Ga의 분포는 360nm를 공급한 조건에서 가장 균일한 분포를 보였다. one-step selenization 공정과 비교하여 표면에서 조금 더 상승된 것을 확인할 수 있었고 박막 내부에서도 비교적 균일한 분포를 보였다. 하지만 그림 4-15의 NREL의 CIGS 박막 표면의 Ga조성비(~0.3)에 비해 여전히 낮았다.
두 번째 실험인 $Ga_2Se_3$를 evaporation한 뒤 selenization 처리한 CIGS 박막 역시 $Ga_2Se_3$를 공급하는 두께가 커질수록 Cu조성비는 줄어들고, Ga조성비는 증가하는 것을 보였고, 이에 따라 결정립의 크기도 줄어드는 경향을 보였다. $Ga_2Se_3$를 400nm 공급한 조건에서는 CIGS 박막이 두층으로 존재하였고, X선 회절 분석과 AES depth profile에서도 Ga의 grading이 심하여 상이 분리되어 존재하고 두층으로 나뉘어 있는 것을 확인할 수 있었다. CIGS 박막내의 Ga의 분포는 220nm를 공급한 조건에서 가장 균일한 분포를 보였다. 마찬가지로 one-step selenization 공정과 비교하여 표면에서 조금 더 상승된 것을 확인할 수 있었고 박막 내부에서도 비교적 균일한 분포를 보였다. 하지만 그림 4-15의 NREL의 CIGS 박막 표면의 Ga조성비(~0.3)에 비해 여전히 낮았다.
본 연구를 통하여 Cu-rich, Ga-poor한 금속 전구체 위에 Ga과 Se을 co-evaporation 또는 $Ga_2Se_3$를 evaporation한 Ga과 Se을 후속 공급한 층을 적용한 후 selenization처리한 CIGS 박막을 제조하였고, 두 조건에서 Ga의 grading 현상을 줄일 수 있었고, 어느 정도 표면에서의 Ga 분포를 상승([Ga]/[In+Ga]~0.2)시킬 수 있었다. 그림 5-24는 [Ga]/[In+Ga] 조성비를 실험한 조건에 대해 비교 요약한 AES depth profile 이다. 앞으로 후속 공급한 Ga과 Se에 의해 제조된 CIGS 태양전지를 제조하여 광전압 특성에 기여하는 정도를 실험하여야 할 것이다.
