Environmental and economic factors demand radical advances in solar cell technologies. Organic and polymer photovoltaics emerged in the 1990's that have led to low cost per unit area, enabled in significant part by the convenient manufacturing of roll-to-roll-processible solution-cast semiconductors. Colloidal quantum dot solar cells dramatically increase the potential for solar conversion efficiency relative to organics by enabling optimal matching of a photovoltaic device's bandgap to the sun's spectrum. Infrared-absorbing colloidal quantum dot solar cells were first reported in 2005. At the outset of this study in 2007, they had been advanced to the point of achieving 1.8% solar power conversion efficiency. These devices degraded completely within a few hours’ air exposure. The origin of the extremely poor device stability was unknown and unstudied. The efficiency of these devices was speculated to be limited by poor carrier transport and passivation within the quantum dot solid, and by the limitations of the Schottky device architecture. This study sought to tackle three principal challenges facing colloidal quantum dot photovoltaics: stability; understanding; and performance. In the first part of this work, we report the first air-stable infrared colloidal quantum dot photovoltaics. Our devices have a solar power conversion efficiency of 2.1%. These devices, unencapsulated and operating in an air atmosphere, retain 90% of their original performance following 3 days’ continuous solar harvesting. The remarkable improvement in device stability originated from two new insights. First, we showed that inserting a thin LiF layer between PbS film and Al electrode blocks detrimental interfacial reactions. Second, we proposed and validated a model that explains why quantum dots having cation-rich surfaces afford dramatically improved air stability within the quantum dot solid. The success of the cation-enrichment strategy led us to a new concept: what if - rather than rely on organic ligands, as all prior quantum dot photovoltaics work had done - one could instead terminate the surface of quantum dots exclusively using inorganic materials? We termed our new materials strategy ionic passivation. The goal of the approach was to bring our nanoparticles into the closest possible contact while still maintaining quantum confinement; and at the same time achieving a maximum of passivation of the nanoparticles' surfaces. We showcase our ionic passivation strategy by building a photovoltaic device that achieves 5.8% solar power conversion efficiency. This is the highest-ever solar power conversion efficiency reported in a colloidal quantum dot device. More generally, our ionic passivation strategy breaks the past tradeoff between transport and passivation in quantum dot solids. The advance is relevant to electroluminescent and photodetection devices as well as to the record-performing photovoltaic devices reported herein.

This thesis reports a detailed characterization of indium arsenide antimonide (InAs1- xSbx) photodetectors grown on gallium arsenide (GaAs) substrate by molecular beam epitaxy. A combination of polyimide and sulphur and a single layer of polyimide were used as passivation films in this study. Two different epilayer structures were used for assessing the detector performance and comparing the above passivation layers. For the first structure, the optical measurements revealed that Sb mole fraction was 0.13 and the cut-off wavelength was around 4.1 &micro / m at 80 K. The Sb mole fraction of the second structure was 0.2, and the 77 K cut-off wavelength was 4.8 &micro / m. Detailed electrical and optical characterizations were performed on 33x33 &micro / m2 test diodes. The photodiodes yielded peak detectivities of ~3.65x1010 and ~1.22x1010 cmHz1/2/W at 80 K for the first and second structures, respectively. Considerable 1/f noise current related with trap-assisted tunneling mechanism was observed in both structures at 80 K. Dark current modeling study showed that the dark current was dominated by the shunt and trap-assisted tunneling mechanisms throughout the entire reverse bias voltages for both detectors. The dark current analysis of variable area detectors fabricated with both epilayer structures revealed that the reverse bias current was mainly generated by the surface leakage in small sized (33x33 &micro / m2) detectors for both passivation layers. However, the surface current contribution was observed to be lower for the double layer (polyimide and sulphur) passivated detectors suggesting that it is a better passivation technique.

付記する学位プログラム名: 京都大学卓越大学院プログラム「先端光・電子デバイス創成学」 / 京都大学 / 新制・課程博士 / 博士(工学) / 甲第23905号 / 工博第4992号 / 新制||工||1779(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 白石 誠司, 准教授 小林 圭 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Physics / D.Phil (Physics)

abstract: Layers of intrinsic hydrogenated amorphous silicon and amorphous silicon carbide were prepared on a polished, intrinsic crystalline silicon substrate via plasma-enhanced chemical vapor deposition to simulate heterojunction device relevant stacks of various materials. The minority carrier lifetime, optical band gap and FTIR spectra were observed at incremental stages of thermal annealing. By observing the changes in the lifetimes the sample structure responsible for the most thermally robust surface passivation could be determined. These results were correlated to the optical band gap and the position and relative area of peaks in the FTIR spectra related to to silicon-hydrogen bonds in the layers. It was found that due to an increased presence of hydrogen bonded to silicon at voids within the passivating layer, hydrogenated amorphous silicon carbide at the interface of the substrate coupled with a hydrogenated amorphous silicon top layer provides better passivation after high temperature annealing than other device structures. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2016

abstract: Silicon solar cells with heterojunction carrier collectors based on a-Si/c-Si heterojunction (SHJ) have a potential to overcome the limitations of the conventional diffused junction solar cells and become the next industry standard manufacturing technology of solar cells. A brand feature of SHJ technology is ultrapassivated surfaces with already demonstrated 750 mV open circuit voltages (Voc) and 24.7% efficiency on large area solar cell. Despite very good results achieved in research and development, large volume manufacturing of high efficiency SHJ cells remains a fundamental challenge. The main objectives of this work were to develop a SHJ solar cell fabrication flow using industry compatible tools and processes in a pilot production environment, study the interactions between the used fabrication steps, identify the minimum set of optimization parameters and characterization techniques needed to achieve 20% baseline efficiency, and analyze the losses of power in fabricated SHJ cells by numerical and analytical modeling. This manuscript presents a detailed description of a SHJ solar cell fabrication flow developed at ASU Solar Power Laboratory (SPL) which allows large area solar cells with >750 mV Voc. SHJ cells on 135 um thick 153 cm2 area wafers with 19.5% efficiency were fabricated. Passivation quality of (i)a-Si:H film, bulk conductivity of doped a-Si films, bulk conductivity of ITO, transmission of ITO and the thickness of all films were identified as the minimum set of optimization parameters necessary to set up a baseline high efficiency SHJ fabrication flow. The preparation of randomly textured wafers to minimize the concentration of surface impurities and to avoid epitaxial growth of a-Si films was found to be a key challenge in achieving a repeatable and uniform passivation. This work resolved this issue by using a multi-step cleaning process based on sequential oxidation in nitric/acetic acids, Piranha and RCA-b solutions. The developed process allowed state of the art surface passivation with perfect repeatability and negligible reflectance losses. Two additional studies demonstrated 750 mV local Voc on 50 micron thick SHJ solar cell and < 1 cm/s effective surface recombination velocity on n-type wafers passivated by a-Si/SiO2/SiNx stack. / Dissertation/Thesis / Ph.D. Electrical Engineering 2013

The ability to passivate metallic surfaces under non-UHV conditions is not only of fundamental interests, but also of growing practical importance in catalysis and microelectronics. In this work, the passivation effect of a surface iodine layer on air-exposed Ta for the copper electroless deposition was investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Although the passivation effect was seriously weakened by the prolonged air exposure, iodine passivates the Ta substrate under brief air exposure conditions so that enhanced copper wetting and adhesion are observed on I-passivated Ta relative to the untreated surface.

Defect passivation and surface modification of perovskite semiconductors play a key role in achieving highly efficient and stable perovskite solar cells (PSCs) and light-emitting diodes (LEDs). This dissertation describes three novel strategies for such defect passivation and surface modification. In the first strategy, we demonstrate a facile approach using inorganic perovskite quantum dots (QDs) to supply bulk- and surface-passivation agents to combine high power conversion efficiency (PCE) with high stability in CH3NH3PbI3 (MAPbI3) inverted PSCs. This strategy utilizes inorganic perovskite QDs to distribute elemental dopants uniformly across the MAPbI3 film and attach ligands to the film’s surface. Compared with pristine MAPbI3 films, MAPbI3 films processed with QDs show a reduction in tail states, smaller trap-state density, and an increase in carrier recombination lifetime. The strategy results in reduced voltage losses and an improvement in PCE from 18.3% to 21.5%, which is among the highest efficiencies for MAPbI3 devices. The devices maintain 80% of their initial PCE under 1-sun continuous illumination for 500 h and show improved thermal stability. In the second strategy, we reduce the efficiency gap between the inverted PSCs and regular PSCs using a trace amount of surface-anchoring, long-chain alkylamine ligands (AALs) as grain and interface modifiers. We show that long-chain AALs suppress nonradiative carrier recombination and improve the optoelectronic properties of mixed-cation mixed-halide perovskite films. These translate into a certified stabilized PCE of 22.3% (23.0% PCE for lab-measured champion devices). The devices operate for over 1000 hours at the maximum power point (MPP), under simulated AM1.5 illumination, without loss of efficiency. Finally, we report a strategy to passivate Cl vacancies in mixed halide perovskite (MHP) QDs using non-polar-solvent-soluble organic pseudohalide (n-dodecylammonium thiocyanate (DAT)), enabling blue MHP LEDs with enhanced efficiency. Density-function-theory calculations reveal that the thiocyanate (SCN-) groups fill in the Cl vacancies and remove deep electron traps within the bandgap. DAT-treated CsPb(BrxCl1-x)3 QDs exhibit near unity (~100%) photoluminescence quantum yields; and their blue (~470 nm) LEDs are spectrally stable with an external quantum efficiency (EQE) of 6.3% – a record for perovskite LEDs emitting at the 460-480 nm range relevant to Rec. 2020 display standards.

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