Organic photovoltaics has emerged as a promising technology for electricity generation. The essential component in an organic solar cell is the bulk heterojunction absorber layer, typically a blend of an electron donor and an electron acceptor. Efforts have been made to design new materials such as donor polymers and novel acceptors to improve the power conversion efficiencies. New fullerene free acceptors providing low cost synthesis routes and tenability of their optoelectronic and electrochemical properties have been designed. Despite the efforts, still not much is known about the photopysical processes in these blends that limit the performance. In this respect, time-resolved spectroscopy such as transient absorption and time-resolved photoluminescence, can provide in-depth insight into the various (photo) physical processes in bulk heterojunction solar cell. In this thesis, PCE10 was used as donor and paired with different non fullerene acceptors. In the first part of this thesis the impact of the core structure (cyclopenta-[2, 1-b:3, 4-b’]dithiophene (CDT) versus indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors, abbreviated as CDTBM and IDTTBM, on the photophysical characteristics of BHJ solar cells is reported. The IDTT-based acceptor achieves power conversion efficiencies of 8.4%, higher than the CDT-based acceptor (5.6%), due to concurrent increase in short-circuit current and open-circuit voltage. Using (ultra)fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10: IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that the higher energy of interfacial charge-transfer states observed for the IDTT-based acceptor blends is the origin of the higher open-circuit voltage. In the second part of this thesis, I report the impact of acceptor side chains on the photo-physical processes of BHJ solar cells using three different IDT-based acceptors, namely O-IDTBR, EH-IDTBR and O-IDTBCN blended with PCE10. Power conversion efficiencies as high as 10 % were achieved. The transient absorption spectroscopy experiments provide insight into sub-picosecond exciton dissociation and charge generation which is followed by nanosecond triplet state formation in PCE10:O-DTBR and PCE10:EH-IDTBR blends, while in O-IDTBCN triplets are not observed. Time delayed collection field experiments (TDCF) were performed to address the charge carrier generation and examine its dependence on the electric field.

Metal-halide perovskites show great promise as solution-processable semiconductors for efficient solar cells and LEDs. In particular, the diffusion range of photogenerated carriers is unexpectedly long and the luminescence yield is remarkably high. While much effort has been made to improve device performance, the barriers to improving charge transport and recombination properties remain unidentified. I first explore charge transport by investigating a back-contact architecture for measurement. In collaboration with the Snaith group at Oxford, we develop a new architecture to isolate charge carriers. We prepare thin films of perovskite semiconductors over laterally-separated electron- and hole-selective materials of SnOₓ and NiOₓ, respectively. Upon illumination, electrons (holes) generated over SnOₓ (NiOₓ) rapidly transfer to the buried collection electrode, leaving holes (electrons) to diffuse laterally as majority carriers in the perovskite layer. We characterise charge transport parameters of electrons and holes, separately, and demonstrate that grain boundaries do not prevent charge transport. Our results show that the low mobilities found in applied-field techniques do not reflect charge diffusivity in perovskite solar cells at operating conditions. We then use the back-contact architecture to investigate recombination under large excess of one charge carrier type. Recombination velocities under these conditions are found to be below 2 cm s⁻¹, approaching values of high quality silicon and an order of magnitude lower than under common bipolar conditions. Similarly, diffusion lengths of electrons and holes exceed 12 μm, an order of magnitude higher than reported in perovskite devices to date. We report back-contact solar cells with short-circuit currents as high as 18.4 mA cm⁻², giving 70% external charge-collection efficiency. We then explore the behaviour of charge carriers in continuously illuminated metal-halide perovskite devices. We show that continuous illumination of perovskite devices gives rise to a segregated charge carrier population, and we find that the distance photo-induced charges travel increases significantly under these conditions. Finally, we examine intermittancy in the photoluminescence intensity of metal-halide perovskite films.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23223号 / 工博第4867号 / 新制||工||1760(附属図書館) / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 大北 英生, 教授 辻井 敬亘, 教授 田中 一生 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

The dye-sensitized solar cell (DSC) is a promising efficient low-cost molecular photovoltaic device. One of the key components in DSCs is the dye, as it is responsible for the capture of sunlight. State-of-the-art DSC devices, based on ruthenium dyes, show record efficiencies of 10-12 %. During the last decade, metal-free organic dyes have been extensively explored as sensitizers for DSC application. The use of organic dyes is particularly attractive as it enables easy structural modifications, due to fairly short synthetic routes and reduced material cost. Novel dye should in addition to the light-harvesting properties also be compatible with the DSC components. In this thesis, a series of new organic dyes are investigated, both when integrated in the DSC device and as individual components. The evaluation methods consisted of different electrochemical and photoelectrochemical techniques. Whereas the light-harvesting properties of the dyes were fairly easily improved, the behavior of the dye integrated in the DSC showed less predictable photovoltaic results. The dye series studied in Papers II and IV revealed that their dye energetics limited vital electron-transfer processes, the dye regeneration (Paper II) and injection quantum yield (Paper IV). Further, in Papers III-VI, it was observed that different dye structures seemed to alter the interfacial electron recombination with the electrolyte. In addition to the dye structure sterics, some organic dyes appear to enhance the interfacial recombination, possibly due to specific dye-redox acceptor interaction (Paper V). The impact of dye sterical modifications versus the use of coadsorbent was explored in Paper VI. The dye layer properties in the presence and absence of various coadsorbents were further investigated in Paper VII. The core of this thesis is the identification of the processes and properties limiting the performance of the DSC device, aiming at an overall understanding of the compatibility between the DSC components and novel organic dyes. / QC 20100730

The solid-state mesoscopic solar cells (sMSCs) have been developed as a promising alternative technology to the conventional photovoltaics. However, the device performance suffers from the low hole-mobilities and the incomplete pore filling of the hole transport materials (HTMs) into the mesoporous electrodes. A variety of HTMs and different preparation methods have been studied to overcome these limitations. There are two types of sMSCs included in this doctoral thesis, namely solid-state dye-sensitized solar cells (sDSCs) and organometallic halide perovskite based solar cells. Two different types of HTMs, namely the small molecule organic HTM spiro-OMeTAD and the conjugated polymer HTM P3HT, were compared in sDSCs. The photo-induced absorption spectroscopy (PIA) spectra and spectroelectrochemical data suggested that the dye-dye hole conduction occurs in the absence of HTM and appears to be of significant importance to the contribution of hole transport. The PIA measurements and transient absorption spectroscopy (TAS) indicated that the oxidized dye was efficiently regenerated by a small molecule organic HTM TPAA due to its excellent pore filling. The conducting polymer P3HT was employed as a co-HTM to transfer the holes away from TPAA to prohibit the charge carrier recombination and to improve the hole transport. An alternative small molecule organic HTM, MeO-TPD, was found to outperform spiro-OMeTAD in sDSCs due to its more efficient pore filling and higher hole-mobility. Moreover, an initial light soaking treatment was observed to significantly improve the device performance due to a mechanism of Li+ ion migration towards the TiO2 surface. In order to overcome the infiltration difficulty of conducting polymer HTMs, a state-of-the-art method to perform in-situ photoelectrochemical polymerization (PEP) in an aqueous micellar solution of bis-EDOT monomer was developed as an environmental-friendly alternative pathway with scale-up potential for constructing efficient sDSCs with polymer HTMs. Three different types of HTMs, namely DEH, spiro-OMeTAD and P3HT, were used to investigate the influence of HTMs on the charge recombination in CH3NH3PbI3 perovskite based sMSCs. The photovoltage decay measurements indicate that the electron lifetime (τn) of these devices decreases by one order of magnitude in the sequence τspiro-OMeTAD > τP3HT > τDEH.