Tuning the electrical properties of Zn(O,S) thin films for buffer layer

Abstract

The solar cell technology is one of the cores of renewable energy industry. The solar PV industry will remain constant in the future because market formation is indispensable under various international agreements. In addition, the importance of solar PV technology has been further emphasized as the current trend of technological development is regarded as important for the connection of energy generation, storage and real life. One of them, the thin film solar cell technology can be applied everywhere such as portable devices or BIPV (Building-integrated Photovoltaic) and can have breakthrough power by forming solar cells in flexible, transparent substrates and various forms.

Due to the structure of solar cell based on PN junction, buffer layer technology is important in thin film solar cells. Buffer layers can relax the band offset difference between p-type absorber layer and n-type window layer. Thus, carrier recombination at the absorber/window interface can be reduced. The cadmium sulfide (CdS) as a representative buffer layer material, have well-matched band offset with CdTe and CIGS(Cu(In,Ga)(S,Se)2) absorber materials. However, the use of toxic element is a problem to be solved. To alternate the CdS, zinc oxysulfide (Zn(O,S)) has been studied in recent years. Zn(O,S) is composed of non-toxic materials and have larger transmittance than CdS due to the large band gap. Further, the composition of Zn(O,S) films can be controlled by the use of atomic layer deposition (ALD) method. Thus, we developed the ALD super cycle process for Zn(O,S) films and investigate the characteristics of Zn(O,S) films. Especially, insufficient studied electronic band structure of Zn(O,S) films were directly identified.

In this study, we identified the deposition characteristics of Zn(O,S) films according to the ratio of ZnS and ZnO cycle in the ALD process and compared their characteristics. Di-ethyl zinc (DEZ), H2O and H2S were used for ALD process of ZnO and ZnS. 100-nm thick Zn(O,S) thin films were deposited at 110 °C with ZnO:ZnS cycle ratios of 5:5, 4:1, 6:1, 9:1, 14:1 and 19:1. ZnS showed higher crystallinity than ZnO, and the crystallinity tended to decrease as the cycles of the two materials were mixed. As a result of AES analysis, the tendency of high ZnS content in the Zn(O,S) film compared to the low ZnS cycle fraction and high reactivity of ZnS by surface ion exchange reaction were identified. In the analysis of the Hall effect measurements and UV-vis, the crystallinity of Zn(O, S) films mainly affected on the optoelectronic characteristics of Zn(O,S) films. Hall measurement results showed that the Zn(O,S) film with 9:1 cycle ratio had best carrier concentration, but the mobility of the Zn(O,S) film with the 9:1 cycle ratio was poor. The UV-vis analysis showed increase of the band gap with the cycle mixing of ZnS and ZnO. The film with the 6:1 cycle ratio showed the biggest bandgap among all the samples. Lastly, the electronic band structure of Zn(O,S) films was determined by UPS analysis. The Zn(O,S) film using 14:1 cycle ratio showed the best conduction band offset with absorber materials, SnS. The difference of conduction band offset was 0.6 eV.

| 태양 전지 기술은 재생 에너지 산업의 핵심 중 하나입니다. 태양 광 산업은 앞으로도 다양한 국제 협약에 따라 시장 형성이 필수적이기 때문에 일정하게 유지될 것입니다. 또한 태양 광 기술의 중요성은 현재 기술 개발 추세가 에너지 생성, 저장 및 실제 연결과 관련하여 중요한 것으로 간주됨에 따라 더욱 강조되었습니다. 그 중 하나인 박막 태양 전지 기술은 휴대용 장치 또는 BIPV (Building-integrated Photovoltaic)와 같은 모든 곳에 적용될 수 있으며, 유연하고 투명한 기판과 다양한 형태로 태양 전지를 형성함으로써 획기적인 힘을 가질 수 있습니다.

대표적인 완충층 물질로서 CdS는 CdTe 및 CIGS (Cu (In, Ga) (S, Se) 2) 흡수제 물질과 잘 일치하는 밴드 오프셋을 갖는다. 그러나 독성 성분의 사용은 해결해야 할 문제이다. 그러므로 CdS를 대체하기 위해, 최근에 Zn (O, S)가 연구되어왔다. Zn (O, S)는 무독성 물질로 구성되며 큰 밴드갭으로 인해 CdS보다 투과율이 더 크다. 또한, Zn (O, S) 막의 조성은 원자층 증착 (ALD) 방법을 사용하여 제어될 수 있다. 따라서, Zn (O, S) 필름에 대한 ALD 슈퍼 사이클 프로세스를 개발하고 Zn (O, S) 필름의 특성을 조사했습니다. 특히, Zn (O, S) 필름의 불충분한 연구 전자 밴드 구조가 직접 확인되었다.

이 연구에서 우리는 ALD 공정에서 ZnS와 ZnO 사이클의 비율에 따라 Zn (O, S) 필름의 증착 특성을 확인하고 그 특성을 비교했습니다. 디 에틸 아연 (DEZ), H2O 및 H2S를 ZnO 및 ZnS의 ALD 공정에 사용하였다. 100 nm 두께의 Zn (O, S) 박막을 5 : 5, 4 : 1, 6 : 1, 9 : 1, 14 : 1 및 19 : 1의 ZnO : ZnS 사이클 비율로 110 ℃에서 증착 하였다. ZnS는 ZnO보다 결정성이 높았으며, 두 물질의 사이클이 혼합됨에 따라 결정성이 감소하는 경향이 있었다. AES 분석 결과 ZnS 사이클 비율이 낮은 Zn (O, S) 필름에서 ZnS 함량이 높은 경향과 표면 이온 교환 반응에 의한 ZnS의 높은 반응성이 확인되었다. 홀 효과 측정 및 UV-vis 분석에서 Zn (O, S) 필름의 결정도는 Zn (O, S) 필름의 광전자 특성에 주로 영향을 미쳤다. 홀 측정 결과는 9 : 1 사이클 비를 갖는 Zn (O, S) 필름이 최고의 캐리어 농도를 가졌지만 9 : 1 사이클 비를 갖는 Zn (O, S) 필름의 이동성은 열악한 것으로 나타났다. UV-vis 분석은 ZnS 및 ZnO의 사이클 혼합으로 밴드갭의 증가를 보여주었다. 6 : 1 사이클 비율을 갖는 필름은 모든 샘플 중에서 가장 큰 밴드갭을 나타냈다. 마지막으로, Zn (O, S) 필름의 전자 밴드 구조는 UPS 분석에 의해 결정되었다. 14 : 1 사이클 비율을 사용하는 Zn (O, S) 필름은 흡수제 물질 SnS로 최고의 전도대 오프셋을 보여 주었다. 이 때의 전도대역 오프셋의 차이는 0.6 eV였다.; The solar cell technology is one of the cores of renewable energy industry. The solar PV industry will remain constant in the future because market formation is indispensable under various international agreements. In addition, the importance of solar PV technology has been further emphasized as the current trend of technological development is regarded as important for the connection of energy generation, storage and real life. One of them, the thin film solar cell technology can be applied everywhere such as portable devices or BIPV (Building-integrated Photovoltaic) and can have breakthrough power by forming solar cells in flexible, transparent substrates and various forms.

Due to the structure of solar cell based on PN junction, buffer layer technology is important in thin film solar cells. Buffer layers can relax the band offset difference between p-type absorber layer and n-type window layer. Thus, carrier recombination at the absorber/window interface can be reduced. The cadmium sulfide (CdS) as a representative buffer layer material, have well-matched band offset with CdTe and CIGS(Cu(In,Ga)(S,Se)2) absorber materials. However, the use of toxic element is a problem to be solved. To alternate the CdS, zinc oxysulfide (Zn(O,S)) has been studied in recent years. Zn(O,S) is composed of non-toxic materials and have larger transmittance than CdS due to the large band gap. Further, the composition of Zn(O,S) films can be controlled by the use of atomic layer deposition (ALD) method. Thus, we developed the ALD super cycle process for Zn(O,S) films and investigate the characteristics of Zn(O,S) films. Especially, insufficient studied electronic band structure of Zn(O,S) films were directly identified.

In this study, we identified the deposition characteristics of Zn(O,S) films according to the ratio of ZnS and ZnO cycle in the ALD process and compared their characteristics. Di-ethyl zinc (DEZ), H2O and H2S were used for ALD process of ZnO and ZnS. 100-nm thick Zn(O,S) thin films were deposited at 110 °C with ZnO:ZnS cycle ratios of 5:5, 4:1, 6:1, 9:1, 14:1 and 19:1. ZnS showed higher crystallinity than ZnO, and the crystallinity tended to decrease as the cycles of the two materials were mixed. As a result of AES analysis, the tendency of high ZnS content in the Zn(O,S) film compared to the low ZnS cycle fraction and high reactivity of ZnS by surface ion exchange reaction were identified. In the analysis of the Hall effect measurements and UV-vis, the crystallinity of Zn(O, S) films mainly affected on the optoelectronic characteristics of Zn(O,S) films. Hall measurement results showed that the Zn(O,S) film with 9:1 cycle ratio had best carrier concentration, but the mobility of the Zn(O,S) film with the 9:1 cycle ratio was poor. The UV-vis analysis showed increase of the band gap with the cycle mixing of ZnS and ZnO. The film with the 6:1 cycle ratio showed the biggest bandgap among all the samples. Lastly, the electronic band structure of Zn(O,S) films was determined by UPS analysis. The Zn(O,S) film using 14:1 cycle ratio showed the best conduction band offset with absorber materials, SnS. The difference of conduction band offset was 0.6 eV.