나노임프린트 공정 기반 유/무기 태양전지

국문 초록 (Abstract)

태양전지는 태양광에 의해 자유전자가 발생 된다는 광전효과를 이용한 장치로서 태양광을 전력으로 변환시켜주는 장치이다. 태양전지의 효율을 높일 수 있는 방법 중 한 가지는 태양전지에 의해 수집 될 수 있는 태양광의 양을 늘려줘서 태양광에 의해 생성될 수 있는 전자의 양을 늘려주는 것 이다. 본 연구에서는 나노임프린트 리소그래피(nanoimprint lithography; 이하 NIL)을 통해 만들어진 나노패턴 층을 유/무기 태양전지 제작 시 도입하여서 태양전지의 광 수집률을 증가시키고 이로 인하여 태양전지의 효율을 높이는 것에 주안점을 두었다. 먼저 유기 태양전지의 일종인 P3HT/PCBM 태양전지에 NIL 방식을 통해 나노패턴을 도입하여 태양전지의 효율이 2.62 %에서 3.12 %로 증가 하였음을 확인 하였고, 페로브스카이트 태양전지(perovskite solar cell; 이하 PSC)의 다공성 TiO2 층에 NIL의 몰드(mold) 깊이에 따른 나노패턴을 도입 하여 그 결과를 관찰 하였다. NIL에 사용한 mold의 깊이는 각각 130 nm, 200 nm 이다. 130 nm의 깊이를 가진 mold를 사용하여 나노패턴을 도입한 PSC에서 나노패턴이 도입되지 않은 PSC에 비해 단락전류 밀도(Jsc) 가 6.6% 상승한 25.24 mA/cm2로 측정되었다. 결과적으로 PSC의 효율 역시 약 7.6% 상승한 15.77%로 측정되었다. UV-vis. 분석을 통해서 130 nm의 mold를 사용하여 NIL 방식을 도입한 다공성 TiO2층의 광 투과율이 NIL 방식을 도입하지 않은 다공성 TiO2층 보다 더 높아졌음을 확인 하였다. 이를 통해 130 nm의 mold를 사용하여 NIL 방식을 도입한 다공성 TiO2층을 포함한 PSC의 광 수집률이 증가하여 효율이 상승 되었음을 알 수 있었다.
NIL 방식이 도입된 PSC의 효율을 더욱 향상시키기 위해서 풀러렌(fullerene) 유도체인 PCBM을 역(inverted)구조의 PSC의 활성 층인 perovskite 층에 도입하여 PCBM-perovskite 복합 층을 만들었다. PCBM은 perovskite의 결정구조 사이에 존재 할 수 있으며 전자의 흐름을 원활하게 해주어 PSC의 성능을 개선시킬 수 있다. 임피던스(impedance)분석을 통하여 PCBM-perovskite 복합 층의 전자 수송능력이 일반 perovskite 층의 전자 수송능력에 비해 상승되었음을 확인 할 수 있었다. 일반 PSC의 측정 효율 16.72 %에 비해서 NIL 방식이 도입 된 PSC의 효율은 17.44 %, NIL 방식과 PCBM-perovskite 복합 층이 함께 도입 된 PSC의 효율은 18.31 % 로 각각 상승 되었다.

다국어 초록 (Multilingual Abstract)

Requirements for a renewable energy source are increasing, with a wide interest in new types of solar cell technology due to their categorical testament for clean energy resource replacing non-renewable resources.
Third generation organic/inorganic solar cells have received much interest because of their low-cost and high power conversion efficiency (PCE). We have developed a method to improve the PCE of the third generation solar cells. The best way to improve the power conversion efficiency in solar cells is by increasing the light absorption of the photo-absorber. However, it is difficult to modify the materials to increase light absorption in solar cell structure, as they consist of thin film layers. In a nanopattern on thin film layers, the control of size, shape, and depth of such materials, with highly uniform and well-designed structures, is easily achieved. The nanopatterning thin film is prepared using the nanoimprint lithography (NIL) method to improve the PCE of organic/inorganic solar cells. We first adopted the NIL method in P3HT/PCBM bulk heterojunction (BHJ) solar cells and observed the effect of the pattern depth on the nanoimprinted layer in perovskite solar cells. The pattern depth of the nanoimprinted mesoporous TiO2 (MP-TiO2) layer is controlled by the Si master and replica mold (mold diameter: 265 nm, mold depth: 130 nm or 200 nm). Perovskite solar cells were fabricated with different nanoimprinted MP-TiO2 layer depths. The nanopatterning device with a depth of 130 nm showed improved performance by exhibiting a short-circuit current density (Jsc) of 25.24 mA/cm2, a 6.6 % enhancement. Consequently, the PCE was increased from 14.65 % to 15.77 %, a 7.6 % enhancement, due to the increased light harvesting to the perovskite structure by the high-transmittance nanoimprinted MP-TiO2 layer.
The PCE of perovskite solar cell was more improved by using PCBM-Perovskite composite films with the NIL method. Fullerene derivative (PCBM) was dissolved into perovskite precursors and the corresponding inverted perovskite solar cells were fabricated. The added small amount of PCBM filled the vacancies at the grain boundaries of perovskite, and produced continuous electron pathways to enhance performance. The impedance measurements confirm the enhanced electron transporting property in PCBM-Perovskite film. The perovskite solar cells incorporated with the nanoimprinted layer and PCBM-Perovskite layers resulted in higher PCEs of 17.44 % and 18.31 %, respectively, than that of the device with the plain perovskite solar cell (16.72 %).
The PCE of the perovskite solar cell could be improved with enhanced light harvesting by following the NIL method and using PCBM-Perovskite composite film to enhance electron transport properties.

목차 (Table of Contents)

• Chapter 1 Introduction 1
• 1.1. Solar Cell 2
• 1.1.1. Structure of Si 3
• 1.1.2. Principle of p-n junction silicon solar cells 4
• 1.1.3. Classification of solar cells 9
• 1.1.4. Efficiency of solar cells 11
• 1.2. P3HT/PCBM bulk heterojunction (BHJ) solar cell 13
• 1.3. Perovskite solar cell 15
• 1.4. Nanoimprint lithography 17
• 1.4.1. UV- nanoimprint lithography 18
• 1.4.2. Gel-nanoimprint lithography 19
• 1.5. Motivations for the research 20
• Chapter 2 Experiment 22
• 2.1. Preparation of nanopatterned mold 23
• 2.2. Fabrication of P3HT/PCBM bulk heterojunction (BHJ) solar cells based on nanoimprint method 24
• 2.2.1. Introduction of nanoimprinted ZnO layer 24
• 2.2.2. P3HT-PCBM active layer 24
• 2.2.3. Preparation of PEDOT:PSS layer 25
• 2.2.4. Counter electrode 25
• 2.3. Fabrication of perovskite solar cells based on nanoimprint method 26
• 2.3.1. Preparation of compact TiO2 layer 26
• 2.3.2. Introduction of nanoimprinted mesoporous TiO2 layer 26
• 2.3.3. Perovskite layer 27
• 2.3.4. Preparation of hole transport layer 27
• 2.3.5. Counter electrode 28
• 2.4. Fabrication of PCBM-perovskite solar cells based on nanoimprint method 29
• 2.4.1. Preparation of compact NiO layer 29
• 2.4.2. Introduction of nanoimprinted mesoporous NiO layer 29
• 2.4.3. The PCBM-Perovskite composite layer 30
• 2.4.4. Preparation of electron transport layer 30
• 2.4.5. Counter electrode 30
• 2.5. Analysis 31
• 2.5.1. Field emission scanning electron microscope (FE-SEM) 31
• 2.5.2. Solar simulator 31
• 2.5.3. Ultravioletvisible spectroscopy (UV-Vis.) 31
• 2.5.4. Incident photon-to-current conversion efficiency (IPCE) 32
• 2.5.5. Fourier-transform infrared spectroscopy (FTIR) 32
• Chapter 3 Nanoimprinted ZnO Layers for P3HT/PCBM bulk heterojunction (BHJ) solar cells 33
• 3.1. Overall scheme of P3HT/PCBM BHJ solar cells based on nanoimprinted ZnO layer 34
• 3.2. Characteristics of P3HT/PCBM BHJ solar cells based on nanoimprinted ZnO layer 37
• 3.3. Measurements of P3HT/PCBM BHJ solar cells based on nanoimprinted ZnO layer 42
• 3.4. Summary 45
• Chapter 4 Nanoimprinted mesoporous TiO2 (MP-TiO2) layer as anti-reflective layers for perovskite solar cells 46
• 4.1. Overall scheme of perovskite solar cell based on nanoimprinted MP-TiO2 layer 47
• 4.2. Characteristics of perovskite solar cells based on nanoimprinted MP-TiO2 layer 50
• 4.3. Measurements of perovskite solar cells based on nanoimprinted MP-TiO2 layer 54
• 4.4. Summary 57
• Chapter 5 High-transmittance nanopatterning mesoporous TiO2 (MP-TiO2) layer to control mold depth for perovskite solar cells 58
• 5.1. Overall scheme of perovskite solar cells based on nanoimprinted MP-TiO2 layer with difference in mold depth 59
• 5.2. Characteristics and optimization of perovskite solar cells based on nanoimprinted MP-TiO2 layer with difference in mold depth 61
• 5.3. Measurements of perovskite solar cells based on nanoimprinted MP-TiO2 layer with difference in mold depth 73
• 5.4. Summary 76
• Chapter 6 PCBM-Perovskite solar cells with nanoimprinted mesoporous NiO (MP-NiO) layer 77
• 6.1. Overall scheme of PCBM-Perovskite solar cells based on nanoimprinted MP-NiO layer 78
• 6.2. Characteristics and optimization of PCBM-Perovskite solar cells based on nanoimprinted MP-NiO layer 80
• 6.3. Measurement of PCBM-Perovskite solar cells based on nanoimprinted MP-NiO layer 85
• 6.4. Electrochemical impedance spectroscopy (EIS) of PCBM-Perovskite solar cell based on NIL method 88
• 6.5. Summary 90
• Reference 91
• Abstract (Korean) 103