3차원 정렬된 다공성 구조의 광전기화학 소자 적용 및 광전특성 분석
3D Ordered Pores Structures for Photoelectrochemical Devices: Fabrication and Photoelectrochemical Studies
- 주제(키워드) Photoelectrochemical cells , DSSCs , Water spliting , Inverse Opal , Hierarchical
- 발행기관 서강대학교 일반대학원
- 지도교수 문준혁
- 발행년도 2015
- 학위수여년월 2015. 2
- 학위명 박사
- 학과 및 전공 일반대학원 화공생명공학과
- 실제URI http://www.dcollection.net/handler/sogang/000000055437
- 본문언어 영어
- 저작권 서강대학교 논문은 저작권보호를 받습니다.
초록/요약
PEC devices are a sustainable and eco-friendly applications for the electricity generation and energy stroage. However, PEC performance of conventional NP TiO2 photoelectrodes was limited due to their disordered structure. Thus, there have been many efforts to study the structure engineering of photoelectrodes. This thesis focuses on fabricating the 3D OP structures for efficient PEC devices. The major contribution of research presented in this thesis focuses on the structure engineering of 3D OP photoelectrodes for light harvesting efficiency and charge collection efficiency enhancement and PEC performance evaluation of 3D OP photoelectrodes.. In chapter 2, The surface-modified TiO2 OP structure was applied as a 3D OP electrode. The morphology, crystalline structure and surface states of the 3D OP structure were characterized by SEM, TEM and XPS and compared to those of the conventional NP TiO2 structure. The performance of the 3D OP electrode was also evaluated by comparing the transport time and recombination lifetime to those of the conventional electrodes. Remarkably, the recombination lifetime in OP was found to be greater than in nanocrystalline TiO2 by 4.3–6.2 times, thus improving the electron collection efficiency by 10%. Comparing the photovoltaic performance, although the dye adsorption of the 3D-ordered porous electrode is lower, the electrode achieves a photocurrent density comparable to that of a nanoparticulate TiO2 electrode due to the higher light scattering as well as the higher collection efficiency. In chapter 3, we describe the preparation of three-dimensional hierarchical twin-scale OP (ts-OP) electrodes for DSSCs. The ts-OP TiO2 structure was obtained from a template fabricated via the assembly of mesoscale colloidal particles (40–80 nm in diameter) in the confined geometry of a macroporous OP structure. The photovoltaic properties of ts-OP electrodes were optimized by varying the layer thickness or the size of mesopores in the mesoscale colloidal assembly. Electron transport was investigated using impedance spectroscopy. The result showed that due to the competing effects of recombination and dye adsorption, the maximum efficiency was observed at an electrode thickness of 12 μm. The electrodes of smaller mesopores diameters yielded the higher photocurrent density due to the decrease in the electron transport resistance at the TiO2/dye interface. A maximum efficiency of 6.90% was obtained using an electrode 12 μm thick and a mesopore diameter of 35 nm. In chapter 4, alternative metal oxides are recently being studied that may provide advantages through facile synthesis, higher electron mobility and band alignment. We report that the charge transport properties of TiO2 shelled SnO2 OP electrodes in dye-sensitized solar cells can be enhanced by introduction of SnO2 core in TiO2 OP structures. Charge transport time and recombination lifetime in TiO2 shelled SnO2 OP electrodes were investigated using IMPS and IMVS. The morphology and crystalline structure of TiO2 shelled SnO2 OP were characterized by SEM and XRD and compared to those of the TiO2 OP structure. While TiO2 shelled SnO2 OP electrodes showed faster transport times and recombination time was slower in the TiO2 OP electrodes, indicating that the TiO2 shell SnO2 OP electrodes have higher charge collection efficiency than TiO2 OP electrodes. In chapter 5, we present a CdS nanoparticle-decorated TiO2 OP for use as an electrode for visible-light photoelectrochemical cells. A facile, low-temperature deposition of CdS nanoparticles was achieved via a sonochemical method. The use of TiO2 OP structures enabled the uniform deposition of CdS nanoparticles throughout the film. The quantity of deposited CdS nanoparticles was controlled by the concentration of the CdS precursor solution. The deposited layer of CdS nanoparticles sensitized the TiO2 surface to visible light; CdS-sensitized TiO2 OP films demonstrated absorption of up to approximately 550 nm light, with enhanced absorption for increased CdS deposits. The CdS-sensitized TiO2 OP film was employed as a photoanode for the PEC cell. We obtained a maximum photocurrent of 7.29 mA/cm2 at the optimum CdS deposition of 39.33 ms%. The optimum amount of deposited CdS was defined by balancing the light harvesting rate against the charge recombination rate, both of which increase with increasing CdS deposition. The CdS-sensitized TiO2 OP electrodes were compared with mesoporous TiO2 nanoparticulate electrodes; higher photocurrent was obtained with the OP, due to the uniform pores of the OP structures. In chapter 6, QDs possess promising characteristics that are important to light harvesting, but their mesoscale size limits their application in the direct sensitization of TiO2 porous films for photo-electrochemical cells. Here, tp-CdSe QDs of macroscale size (22 nm effective diameter) and OP TiO2 films were used in visible-light PECs. Because of the interconnected macropores in the OP structure, tp-CdSe penetrated the entire film and deposited on its surface. In contrast, infiltration was limited to the surface of conventional mesoporous TiO2 film. The amount of tp-CdSe deposited was dependent on the immersion time of the film in the tp-CdSe dispersion. Optimum deposition of tp-CdSe was observed at the highest photocurrent density. Light harvesting, thus the photocurrent, increased with increasing amount of deposit, but there was a corresponding decrease in electron lifetime. The maximum photocurrent density per Cd mass was 0.474 mA/cm2, which is greater than previous results from experiments using QD-sensitized PECs. We thus believe that a combination of tetrapod-shaped QDs and TiO2 OP film may provide a new platform for PEC electrodes.
more초록/요약
3차원 정렬된 다공성 구조는 광전기화학 소자에 적용 시, 기존 나노입자형 및 박막형 필름의 구조적 한계를 극복할 수 있는 차세대 전극 구조이다. 특히 3차원 정렬된 다공성 구조는, 빠르게 전자를 전달할 수 있는 경로를 제공하여 전자 재결합률을 줄일 수 있으며, 모든 기공이 연결되어 있기 때문에, 구조 내 전해질 등을 침투시켰을 때 전해질의 이온 및 물질 전달을 용이하게 할 수 있는 장점이 있다. 본 논문에서는 3차원 정렬된 다공성 구조를 제작하고, 그에 대한 전극의 두께, 기공 크기, 표면 모폴로지 및 전극 물질 등을 제어하였다. 또한 상기 구조를 태양전지, 물분해 등의 광전기화학 소자에 적용하여, 광전 특성 등을 분석하였다. 3차원 정렬 전극의 전자 전달 특성을 측정하여 기존 나노입자형 전극과 비교하였다. 3차원 정렬 전극의 전자 전달 시간은 1.9-2.3배 빠르며, 전자 생존 시간의 경우 3.2-4.6배 성능이 우수함을 확인하였다. 기존 논문에 따르면 전자 전달 시간과 전자 생존 시간은 전극의 비표면적에 의존하는 특성이 있다. 따라서, 비표면적을 고려한 전자 전달 특성을 계산하였으며, 전자 전달 시간은 3-25% 성능이 향상되지만, 전자 생존 시간의 경우 4.3-6.2배까지 성능이 우수함을 관찰하였다. 이는 동일한 TiO2물질을 사용하였기 때문에 전자 전달 시간 성능은 비슷하지만 3차원 정렬 구조로 인하여 표면 결함이 기존 나노입자형 전극 보다 적어 전자 생존시간이 증가하였다. 3차원 정렬 전극은 전자 전달 특성이 우수하지만 비표면적이 낮아 광전효율이 낮은 한계가 있다. 이와 같은 한계를 극복하기 위해 메조크기의 기공을 갖는 계층형 3차원 정렬 전극을 제작하여, 태양전지의 광전효율을 최적화하였다. 35-70 nm 메조 크기 및 8-18 µm 두께를 제어하여, 35 nm, 12 µm의 두께를 갖는 계층형 전극이 최대 6.90%의 광전효율을 확보하였다. SnO2를 코어로 갖는 3차원 TiO2정렬 전극을 제작하여, 3차원 전극의 전자 전달 특성을 더욱 향상시켰다. SnO2는 우수한 electron mobility로 인하여, 전자 전달 속도가 빠른 특징이 있다. SnO2를 코어로 하는 3차원 정렬 전극은 기존 3차원 정렬 전극 대비, 동일 비표면적에서 2.81배 빠른 전자 전달 시간과 2.14배 우수한 전자 생존시간을 갖음을 확인하였다. 또한, 3차원 전극 위에 광산란층을 도입하여 전극 내 조사된 빛의 확산 길이를 증가시켰으며, 최대 광전효율 8.39%를 달성하였다. 3차원 정렬 전극은 규칙적 구조 및 매크로 크기의 기공으로 인하여 양자점과 나노입자 등을 전극 표면에 코팅 또는 흡착하기 용이한 장점이 있다. CdS 나노입자 및 CdSe 테트라팟 양자점을 3차원 정렬 전극에 증착 및 흡착하였으며 이를 물분해에 적용하였다. CdS, CdSe의 흡착량을 조절하여 최적의 물분해 효율을 갖는 전극을 제조하였다. 또한 CdSe 테트라팟 양자점의 양자 구속 효과로 인하여 560, 640 nm의 빛을 더 흡수함을 관찰하였다. 이는 IPCE결과를 통하여 확인할 수 있으며 낮은 CdSe 질량 대비 높은 물분해 광전류 밀도 결과를 갖음을 확인하였다.
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