실리콘 이종접합 태양전지에서 ITO와 구리 도금 전극 간의 Ohmic Contact 향상을 위한 Cu-Sn 합금 seed층에 대한 연구 = A study of Improvement of Ohmic Contact Between the Indium Tin Oxide and Copper-Plated Contact of Silicon Heterojunction Solar Cells by Using the Cu–Sn Alloy Film

국문 초록 (Abstract)

태양전지 효율 향상을 위해, 이종접합 (Hetero-junction) 구조에 기반 된 결정질 태양 전지는 많은 기관에서 꾸준히 연구되어왔다[1-4]. 현재까지 주로 사용되고 있는 실리콘 태양전지는 동종접합 (Homo-junction) 구조를 가지고 있다. 이러한 단결정 구조를 가지고 있는 태양전지는 에미터 층이 도핑되는 과정에서 800~1000℃의 고온에서 소성되는 공정이 요구된다. 이때, 고온에서의 공정은 실리콘 웨이퍼를 휘게 하는 현상의 원인이 되는데, 웨이퍼 두께가 얇을수록 휘는 현상이 더 심해지기 때문에 얇은 두께의 웨이퍼를 사용하는 것 에는 한계가 있다. 이와는 다르게 이종접합 태양전지는 200℃ 이하의 저온공정을 통해 제작되어 공정 과정에서 셀이 받게 되는 thermal stress가 상대적으로 적어 셀의 파손 가능성이 매우 낮기 때문에 웨이퍼 박형화에 유리하다. 이렇게 낮은 온도에 따른 공정과정에서 웨이퍼의 박형화를 통해 원가절감이 가능해지고, 또한 태양전지의 개방전압 상승으로 인한 셀의 효율 증가 효과도 볼 수 있다. 이종접합 태양전지의 제조에서 silver paste를 사용한 screen printing 방식은 단순하고 빠른 공정으로 주로 사용되어왔지만, 이종접합 태양전지 특성상 저온경화 폴리머 paste를 사용해야하므로 고온공정을 거친 일반적인 태양전지에 비해 전기 전도도가 낮은 단점이 있다. 또한, silver paste의 높은 가격은 비용적인 측면에서 효율적인 태양전지와의 비 호환성을 증가시키고 있다[5, 6]. 이에 대한 해결책으로, metallization 비용과 광학적 손실을 줄이기 위해 이종접합 태양전지에 대한 도금 된 구리 접촉이 활발히 연구되고 있고[7-18], 최근에 이종접합 태양전지에 대해 전기 도금된 구리 전면 접촉 metalliization이 입증되었다[19-21]. 이종접합 태양전지에 도금 된 구리 접촉을 적용하기 위해서는 태양전지의 높은 fill factor에 대한 접촉 비저항의 개선 및 샘플 표면에서의 좋은 접착력이 요구되는데, 이에 대한 해법으로 ITO를 사용할 수 있다. ITO는 구리 확산에 대해 효과적인 장벽 역할을 하는 것으로 알려져 있다[22, 23]. 본 논문에서는 Cu와 Sn을 ITO층 위에 co-evaporation 방법을 사용해서 합금 형태로 증착하여, 접촉저항과 접착력을 향상시키기 위한 seed층 물질로서 사용하였다. Cu와의 합금물질로서 Sn을 사용하는 이유는 열처리 이후에 접촉저항이 감소 될 가능성이 있기 때문이다. ITO층이 이미 Sn으로 도핑되어 있기 때문에 Cu-Sn 합금 seed층이 열처리 될 때, 주석이 확산 될 수 있고 ITO와 Cu-Sn 전극 사이에 더 나은 ohmic contact을 형성 할 것으로 예상된다. seed층 증착 이후에는 Light Induced Electro Plating (LIEP) 방식을 통해 Cu와 Ag를 도금하였고, 각 Cu-Sn 합금 seed층의 조성에 따른 접촉 비저항은 Transfer Length Method (TLM)을 사용하여 측정하였다. 이후, Cu-Sn 합금 seed층을 증착한 이종접합 태양전지를 제작하여 Light I-V 측정을 통한 태양전지 특성을 분석하였다.

다국어 초록 (Multilingual Abstract)

To improve solar cell efficiency, crystalline solar cells based on heterojunction structures have been steadily studied in many institutions[1-4]. Silicon solar cells, which are mainly used to date, have a homo-junction structure. Solar cell having such a single crystal structure is required to be fired at a high temperature of 800~1000℃ in the process of doping the emitter layer. At this time, the process at high temperature causes the phenomenon of bending the silicon wafer, but the thinner the wafer thickness, the more severe the bending phenomenon is, so there is a limit to using a thin wafer. In contrast, heterojunction solar cells are fabricated through a low temperature process of 200°C or less, which is advantageous for thinning wafers because the thermal stress of the cell is relatively low. In such low temperature process, cost reduction is possible through thinning of the wafer, and the efficiency of the cell can be increased by increasing the open circuit voltage of the solar cell. The screen printing method using silver paste has been mainly used as a simple and fast process in the manufacture of heterojunction solar cells. However, because of the characteristics of heterojunction solar cells, low temperature curing polymer paste should be used, which results in lower electrical conductivity than general solar cells undergoing high temperature processes. There are disadvantages. In addition, the high price of silver paste increases the incompatibility with the solar cell, which is cost effective[5, 6]. As a solution, plated copper contacts for heterojunction solar cells have been actively studied to reduce metallization costs and optical losses [7–18], and recently electroplated copper front contact metalliization for heterojunction solar cells has been developed. Proven [19-21]. Application of plated copper contacts to heterojunction solar cells requires improved contact resistivity for the high fill factor of the solar cells and good adhesion on the sample surface. ITO can be used as a solution. ITO is known to act as an effective barrier against copper diffusion [22, 23]. In this paper, Cu and Sn were deposited on the ITO layer in the form of alloy by using co-evaporation method and used as seed layer material to improve contact resistance and adhesion. The reason for using Sn as an alloying material with Cu is that the contact resistance may decrease after heat treatment. Since the ITO layer is already doped with Sn, it is expected that when the Cu-Sn alloy seed layer is heat treated, tin can diffuse and form a better ohmic contact between the ITO and Cu-Sn electrodes. After seed layer deposition, Cu and Ag were plated by Light Induced Electro Plating (LIEP), and the contact resistivity of each Cu-Sn alloy seed layer was measured using a Transfer Length Method (TLM). Subsequently, a heterojunction solar cell having a Cu-Sn alloy seed layer deposited thereon was fabricated and analyzed for solar cell characteristics through light I-V measurement.

목차 (Table of Contents)

• 제1장 서 론···················································1
• 1. 소개·························································1
• 1.1. 태양에너지 및 태양전지·····················································1
• 1.2. 태양전지의 종류······························································3
• 1.2.1. 단결정 실리콘 태양전지·······················································3
• 1.2.2. 다결정 실리콘 태양전지························································3
• 1.2.3. 비정질 실리콘 태양전지·······················································4
• 1.2.4. 화합물형 태양전지·······························································5
• 1.2.5. 신소재 및 유기물 태양전지···················································5
• 1.3. 고효율 실리콘 태양전지··················································6
• 2. 태양전지 기초이론······················································8
• 2.1. 태양전지의 구조 및 발전원리·············································8
• 2.2. 태양전지의 전류-전압 곡선···············································10
• 2.3. 단락전류 (short circuit current)···········································11
• 2.4. 개방전압 (open circuit voltage)···········································12
• 2.5. 곡선인자 (fill factor)·······················································12
• 2.6. 변환효율 ···································································13
• 2.7. 직렬 저항 및 병렬 저항 ···············································14
• 제2장 이 론···················································17
• 1. 이종접합 태양전지·····················································17
• 1.1. 이종접합 태양전지 배경···········································17
• 1.2. 이종접합 태양전지 제조공정······································21
• 2. 도금···································································24
• 2.1. 전해 도금 및 무전해 도금·········································24
• 2.2. 광유도 도금 (LIP, Light-induced Plating)······················25
• 2.3. 광유도 전해 도금 (LIEP, Light-induced Electro Plating)·····25
• 3. 에미터 특성분석 방법·················································27
• 3.1. 4-point probe 분석·················································27
• 3.2. EDS (Energy Dispersive Spectrometer) 분석·················28
• 3.3. SIMS (Secondary Ion Mass Spectrometer) 분석···············28
• 3.4. XRD (X-Ray Diffraction) 분석·································29
• 3.5. TLM (Transmission Line Method) 분석·······················30
• 제3장 실험 방법···················································32
• 1. 기판 제작·····················································32
• 2. 실험 진행······················································33
• 2.1. 열처리 온도 및 시간 결정·········································33
• 2.2. 접촉 비저항 측정 및 접착력 측정································34
• 2.3. XRD, EDS 측정·················································36
• 2.4. SIMS 측정······················································37
• 2.5. 태양전지 특성 측정················································39
• 3. 도금 전극 형성을 위한 주요 공정·······························42
• 3.1. photolithography 공정·········································42
• 3.2. Deposition 공정·················································43
• 3.3. Plating 공정····················································47
• 제4장 실험 결과···················································48
• 1. EDS 측정 결과············································48
• 2. XRD 측정 결과··············································50
• 3. 접촉 비저항 측정 결과············································51
• 4. Work function 분석 결과······································52
• 5. 열처리 테스트 결과···········································53
• 6. 열처리 이후의 접촉 비저항 측정 결과·······················54
• 7. 열처리 전/후 SIMS 측정 결과·····························56
• 8. 접착력 측정 결과···········································57
• 9. 이종접합 태양전지 특성 분석 결과···························58
• 제5장 결 론·························································63
• 참고 문헌··································································65
• Abstract·····································································71