A standard procedure for P3HT:PCBM bulk-heterojunction solar cells has been developed. Fabrication conditions, such as environment; solution concentration, thickness of active layer or post-treatment methods are systematically optimized. The best device performance is obtained by slow-drying spin-coated P3HT:PCBM (1:0.8) blend layer with DCB as solvent. C60:LiF composite films with up to 80% LiF concentration as hole blocking layer have been developed to significantly increase power conversion efficiencies of OPV devices. The short-circuit current density is greatly enhanced, without sacrificing open-circuit voltage and fill factor. Due to its superior oxygen diffusion blocking effect, the C60:LiF composite layer also can provide a more effective passivation film than a thin LiF layer, resulting in an impressive enhancement in air stability of devices.

The probability of recombination of photogenerated electron hole pairs in crystalline silicon is governed by the density of surface defect states and the density of charge carriers. Depositions of intrinsic hydrogenated amorphous silicon (a-Si:H) in dc saddle field (DCSF) PECVD system and hydrogenated amorphous silicon nitride (SiNx) in rf PECVD system forms a dual layer stack on c-Si, which results in an excellent passivation of the surface and an anti-reflection coating. Response Surface Methodology is used in this work to optimize the deposition conditions of SiNx. Optimization of the response surface function yielded deposition conditions that materialized in a surface recombination velocity of less than 4cm/s. The BACH (Back Amorphous Crystalline silicon Heterojunction) cell concept makes use of this dual layer a-Si:H/SiNx stack to form a high efficiency photovoltaic device. The high quality passivating structure can result in the BACH solar cell device with more than 20% conversion efficiency.

A standard procedure for P3HT:PCBM bulk-heterojunction solar cells has been developed. Fabrication conditions, such as environment; solution concentration, thickness of active layer or post-treatment methods are systematically optimized. The best device performance is obtained by slow-drying spin-coated P3HT:PCBM (1:0.8) blend layer with DCB as solvent. C60:LiF composite films with up to 80% LiF concentration as hole blocking layer have been developed to significantly increase power conversion efficiencies of OPV devices. The short-circuit current density is greatly enhanced, without sacrificing open-circuit voltage and fill factor. Due to its superior oxygen diffusion blocking effect, the C60:LiF composite layer also can provide a more effective passivation film than a thin LiF layer, resulting in an impressive enhancement in air stability of devices.

The probability of recombination of photogenerated electron hole pairs in crystalline silicon is governed by the density of surface defect states and the density of charge carriers. Depositions of intrinsic hydrogenated amorphous silicon (a-Si:H) in dc saddle field (DCSF) PECVD system and hydrogenated amorphous silicon nitride (SiNx) in rf PECVD system forms a dual layer stack on c-Si, which results in an excellent passivation of the surface and an anti-reflection coating. Response Surface Methodology is used in this work to optimize the deposition conditions of SiNx. Optimization of the response surface function yielded deposition conditions that materialized in a surface recombination velocity of less than 4cm/s. The BACH (Back Amorphous Crystalline silicon Heterojunction) cell concept makes use of this dual layer a-Si:H/SiNx stack to form a high efficiency photovoltaic device. The high quality passivating structure can result in the BACH solar cell device with more than 20% conversion efficiency.

Solution processed solar cells offer the promise of a low cost solution to global energy concerns. Colloidal quantum dots are one material that can be easily synthesized in and deposited from solution. These nanoparticles also offer the unique ability to select the desired optical and electrical characteristics, all within the same materials system, through small variations in their physical dimensions. These materials, unfortunately, are not without their limitations. To date, films made from colloidal quantum dots exhibit limited mobilities and short minority diffusion lengths. These limitations imply that simple device structures may not be sufficient to make an efficient solar cell. Here we show that through clever manipulation of the geometric and energetic structures, we can utilize the size-tunability of CQDs while masking their poor electrical characteristics. We further outline the physical mechanisms present within these architectures, namely the utilization of a distributed built-in electric field to extract current through drift rather than diffusion. These architectures have consequently exceeded the performance of legacy architectures such as the Schottky cell. Finally, we discuss some of the limiting modes within these architectures and within CQD films in general including the impact of surface traps and polydispersity in CQD populations. Through the development of these novel architectures, the power conversion efficiency of CQD solar cells has increased from ~3.5% to 7.4%; the highest efficiencies yet reported for colloidal quantum dot solar cells.

To address the commonly poor short wavelength response of the conventional solar cell structure which consists of a highly doped thin emitter layer on top of a thicker and less doped base, the novel concept of the Buried-Windowed-Emitter is introduced. This new solar cell structure makes use of a high quality semiconductor layer on top of the traditionally made highly doped emitter and greatly enhances the spectral response of the solar cell by giving the superficially generated carriers a higher chance of collection at the junction. In the proposed BWE structure the emitter is windowed in order to electrically connect the top layer to the base for current collection. The efficacy of the proposed novel device is proven by computer aided device simulations using the available device simulation tools such as MEDICI. The results of simulation show that the proposed novel Buried-Windowed-Emitter solar cell will not only improve the short wavelength spectral response of the overall cell as expected, but also will boost the spectral efficiency for all the wavelengths. Another exciting conclusion from the results of the computer simulation of the BWE solar cell is that the minority carrier lifetime in the top layer does not need to be very high for a superb performance and values as low as 1µs can still boost the short circuit current of the cell to values close to the theoretical limit of the photo-current collectable by a silicon solar cell. This is indeed a good news for manufacturability of this device as it should be practically feasible to achieve epitaxial films with minority carrier lifetime in this range. In order to increase the understanding about the rather complex structure of the proposed Buried-Windowed-Emitter solar cell, an analytical circuit level model, similar to the case of the standard solar cell, is developed for the proposed device. The developed analytical model helps to understand the importance of the main design parameters such as the dimensions of the pattern of the windowed emitter. On the path to fabricate the proposed BWE solar cell, great deal of work is done on the development of a low temperature (<300°C) epitaxial silicon technology using the benefits of Plasma Enhanced Chemical Vapor Deposition (PECVD). Highly doped epitaxial silicon layers of up to around 1µm thickness are achieved with sheet resistivity as low as 7Ω/sq which is much lower than what is reposted in the literature in similar deposition conditions. Intrinsic, phosphorous doped n-type and boron doped p-type epitaxial films have been developed on silicon substrates. Measurement of reflection spectra of the deposited epitaxial films is proposed as a fast, non destructive and process-integrate-able method to assess the crystalline quality of the epitaxial films. Effects of higher temperature post deposition annealing have been studied on the develop epitaxial films A full technology is developed for the fabrication of the proposed novel solar cells. Photo-masks are designed to create 10 different architectures for the design of the windowed emitter in the BWE cell. All the steps taken in the successful fabrication of the novel BWE cells are presented in detail and the relevant findings are discussed and proposed as future research topics. Three kinds of cells are fabricated using the developed technology to separately study the effects of partial coverage of the windowed emitter, the optical performance of the developed epitaxial silicon films and the performance and manufacturability of the novel BWE solar cell The results show that the concept of windowed-emitter by itself (even without the top layer) is capable of enhancing the performance of the solar cell when compared to a standard design. It also promises high conversion efficiency for the BWE solar cell in case a high quality top layer can be deposited on top of the windowed emitter. The results further reveal the lower than expected quality of the low temperature epitaxial films despite the indication of their full crystallinity through other analyses. Use of the epitaxial films as the emitter of the solar cell is proposed as a direct and effective method of studying the photovoltaic performance of the low temperature epitaxial films. Further development of the epitaxial technology will lead to feasibility of a BWE solar cell with very high photovoltaic performance.

This thesis undertakes the development, characterization and optimization of boron diffusion for silicon solar cells. Heavy diffusions (sheet resistance < 40 Ohm/square) to form a back surface field, and light diffusions (sheet resistance > 100 Ohm/square) to form oxide-passivated emitters were developed. Test structures and solar cells were fabricated to assess uniformity, lifetime and recombination effects due to the light and heavy boron diffusions. It was found that the growth of a thin ~200 ??, thermal oxide, during stabilization ??? immediately prior to the boron diffusion - was required to maintain high lifetime and reduce surface recombination (reducing the emitter saturation current density) for all boron diffusions. The heavy boron diffusion process was incorporated into the single side buried contact solar cell processing sequence. The solar cells fabricated had both boron diffused and Al/Si alloyed P+ regions for comparison. This research conclusively showed that boron diffused solar cells had significantly higher open circuit voltage compared to Al/Si alloyed devices. Fabrication of n-type solar cells, and their subsequent characterization by overlayed secondary electron image and the electron beam induced current map showed that the Al/Si alloy varied in depth from 5 to 25 micrometers deep. Methodology and characterization for light, oxide-passivated boron diffusions are also presented. This study yielded boron diffused emitters (sheet resistance > 100 Ohm/square) with low emitter saturation current. It was observed that this was possible only when the thermal oxidation after the boron diffusion was minimal, less than 1,000 ??. This was due to the segregation effect of boron with oxide, decreasing the surface concentration that in turn decreased the electric field repulsion of electrons from the surface. Device modelling of n-type solar cells is presented where the parameters of the modelling include the results of the light, oxide-passivated boron diffusions. This modelling shows n-type-base material with light oxide-passivated boron diffusion has higher potential conversion efficiency than forming a solar cell from phosphorous diffused p-type material.

abstract: As the 3rd generation solar cell, quantum dot solar cells are expected to outperform the first 2 generations with higher efficiency and lower manufacture cost. Currently the main problems for QD cells are the low conversion efficiency and stability. This work is trying to improve the reliability as well as the device performance by inserting an interlayer between the metal cathode and the active layer. Titanium oxide and a novel nitrogen doped titanium oxide were compared and TiOxNy capped device shown a superior performance and stability to TiOx capped one. A unique light anneal effect on the interfacial layer was discovered first time and proved to be the trigger of the enhancement of both device reliability and efficiency. The efficiency was improved by 300% and the device can retain 73.1% of the efficiency with TiOxNy when normal device completely failed after kept for long time. Photoluminescence indicted an increased charge disassociation rate at TiOxNy interface. External quantum efficiency measurement also inferred a significant performance enhancement in TiOxNy capped device, which resulted in a higher photocurrent. X-ray photoelectron spectrometry was performed to explain the impact of light doping on optical band gap. Atomic force microscopy illustrated the effect of light anneal on quantum dot polymer surface. The particle size is increased and the surface composition is changed after irradiation. The mechanism for performance improvement via a TiOx based interlayer was discussed based on a trap filling model. Then Tunneling AFM was performed to further confirm the reliability of interlayer capped organic photovoltaic devices. As a powerful tool based on SPM technique, tunneling AFM was able to explain the reason for low efficiency in non-capped inverted organic photovoltaic devices. The local injection properties as well as the correspondent topography were compared in organic solar cells with or without TiOx interlayer. The current-voltage characteristics were also tested at a single interested point. A severe short-circuit was discovered in non capped devices and a slight reverse bias leakage current was also revealed in TiOx capped device though tunneling AFM results. The failure reason for low stability in normal devices was also discussed comparing to capped devices. / Dissertation/Thesis / M.S. Materials Science and Engineering 2011

Este trabalho tem como principal objetivo comparar o comportamento fotovoltaico de heterojunções de P3HT/PCBM e heterojunções de P3HT/PCBM contendo nanopartículas de ouro (P3HT/PCBM/Au nanop). Inicialmente foi encontrada a melhor condição para deposição dos filmes de P3HT/PCBM, através do estudo do efeito do solvente, da temperatura e pressão durante a formação do filme de P3HT. Os resultados mostram que a melhor organização estrutural é obtida quando o polímero é disperso em clorobenzeno, e solidificado a temperatura 110°C em condição de baixa pressão. Os materiais foram caracterizados por voltametria cíclica, espectroscopia Uv-vis, espectroscopia no infravermelho, microscopia eletrônica de transmissão e difratometria de raios-x. Os dispositivos montados foram caracterizados por curvas de corrente versus potencial e eficiência de conversão de fóton incidente. Com o objetivo de avaliar o efeito das nanopartículas metálicas na estabilidade estrutural do dispositivo, foram realizadas obtidos espectros de FTIR dos filmes de P3HT na presença e na ausência de nanopartículas de ouro, após diferentes períodos de exposição à luz. Os filmes de P3HT/PCBM/Aunanop apresentaram maior absorção de luz na região do visível que resultou em maior eficiência de geração de fotocorrente quando comparada a P3HT/PCBM. Ainda através dos gráficos de IPCE foi observado um aumento da geração de fotocorrente na mesma região espectral do modo plasmônico. Através destes resultados nós observamos que a presença das nanoparticulas metálicas melhora a geração de fotocorrente, aumentando a XV intensidade de luz absorvida e o aprisionamento da luz dentro da camada fotoativa. / The main goal of this work was to compare the effect of gold nanoparticles on the photovoltaic properties of the P3HT/PCBM heterojunction. Initially we have studied the best condition to obtain the P3HT/PCBM films evaluating the effect of solvent, temperature and pressure during the formation of the film. The results show that the best structural organization is obtained from in chlorobenzene, at 110 oC at low pressure condition. The materials were characterized by cyclic voltammetry, Uv-vis spectroscopy, infrared spectroscopy, transmission electron microscopy and x-ray diffractometry. The assembled devices were characterized by current versus potential curves and incident photon to current efficiency. Aiming to evaluate the effect of gold nanoparticles on the structural stability of the device, we have performed FTIR experiments of the films with and without gold nanoparticles after different exposition periods. The P3HT/PCBM/Au nanop film presented improved optical behavior and enhanced photocurrent generation when compared to P3HT/PCBM, in addition by IPCE we have observed improved generation of photocurrent in the region near the absorption range of the gold nanoparticles. From the results we observe that the presence of gold nanoparticles improve the generation of photocurrent, by increasing light absorption and improving light entrapment in the photoactive film.

abstract: CdTe/MgxCd1-xTe double heterostructures (DHs) have been grown on lattice matched InSb (001) substrates using Molecular Beam Epitaxy. The MgxCd1-xTe layers, which have a wider bandgap and type-I band edge alignment with CdTe, provide sufficient carrier confinement to CdTe, so that the optical properties of CdTe can be studied. The DH samples show very strong Photoluminescence (PL) intensity, long carrier lifetimes (up to 3.6 μs) and low effective interface recombination velocity at the CdTe/MgxCd1 xTe heterointerface (~1 cm/s), indicating the high material quality. Indium has been attempted as an n-type dopant in CdTe and it is found that the carriers are 100% ionized in the doping range of 1×1016 cm-3 to 1×1018 cm-3. With decent doping levels, long minority carrier lifetime, and almost perfect surface passivation by the MgxCd1-xTe layer, the CdTe/MgxCd1-xTe DHs are applied to high efficiency CdTe solar cells. Monocrystalline CdTe solar cells with efficiency of 17.0% and a record breaking open circuit voltage of 1.096 V have been demonstrated in our group. Mg0.13Cd0.87Te (1.7 eV), also with high material quality, has been proposed as a current matching cell to Si (1.1 eV) solar cells, which could potentially enable a tandem solar cell with high efficiency and thus lower the electricity cost. The properties of Mg0.13Cd0.87Te/Mg0.5Cd0.5Te DHs and solar cells have been investigated. Carrier lifetime as long as 0.56 μs is observed and a solar cell with 11.2% efficiency and open circuit voltage of 1.176 V is demonstrated. The CdTe/MgxCd1-xTe DHs could also be potentially applied to luminescence refrigeration, which could be used in vibration-free space applications. Both external luminescence quantum efficiency and excitation-dependent PL measurement show that the best quality samples are almost 100% dominated by radiative recombination, and calculation shows that the internal quantum efficiency can be as high as 99.7% at the optimal injection level (1017 cm-3). External luminescence quantum efficiency of over 98% can be realized for luminescence refrigeration with the proper design of optical structures. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2016