Organic solar cells have the potential to be portable power sources that are light-weight, flexible, and inexpensive. However, the highest power conversion efficiency for organic solar cells to date is ~8%, and most high-efficiency solar cells have an area of less than 1 cm². This thesis advances the field of organic solar cells by studying the physics and engineering of the devices to understand the reverse saturation current, which is related to efficiency, and the effects of area scaling. The most commonly accepted models to describe the physics of organic photovoltaic devices are reviewed and applied to planar heterojunction solar cells based on pentacene / C60 as a model system. The equivalent circuit model developed for inorganic solar cells is shown to work well to describe the behavior of organic devices and parameterize their current-voltage characteristics with five parameters. Changes in the parameters with different material combinations or device structures are analyzed to better understand the operation of the presented organic solar cells. A one-dimensional diffusion model for the behavior of excitons and treatment of the organic layers as planes is demonstrated to adequately model the external quantum efficiency and photocurrent in pentacene / C60 solar cells. The origin of the open-circuit voltage is studied using cells with different electrodes and different donor materials. While changing the electrodes does not affect open-circuit voltage, it is greatly modified by changes in the donor. Tests with additional semiconductors show the change in open-circuit voltage is not consistent from donor to donor as the acceptor is varied, suggesting a more complex relation than just the difference in energy levels. Study of the temperature dependence of the equivalent circuit parameters shows that the reverse saturation current, which has a significant role in determining the open-circuit voltage, has a thermally activated behavior. From this behavior, the reverse saturation current is related back to charge transfer at the donor / acceptor heterojunction to suggest that both the effective energy barrier presented by the energy levels and the electronic coupling are important in determining the reverse saturation current and open-circuit voltage. This marks a shift from just considering a built-in voltage or the energy levels to also considering the electronic coupling of the donor and acceptor materials. Temperature-dependent performance characteristics are also used to show key differences between organic and inorganic devices. Finally, the effect of area scaling is explored with pentacene / C60 solar cells having areas of 0.11, 7, and 36.4 cm². Analysis with the equivalent circuit model shows that performance decreases as area increases because of an increasing series resistance presented by the transparent electrode. A metal grid, to provide low resistance pathways for current, fabricated on top of the transparent electrode is proposed to reduce the effective resistance. The grid is unique in that it is placed between the electrode and the semiconductor layer and must be passivated to prevent shorts through the thin semiconductor to the back metal electrode. Analysis of the grid predicts greatly reduced series resistance, and experimental results show reduced resistance and improved performance for the 7 cm² and 36.4 cm² devices when including the grid.

The objective of the proposed research is to develop low-cost, screen-printed 20% efficient silicon solar cells. In the first part of this thesis, a ~19% efficient, screen-printed cell was fabricated using the commercially-dominant aluminum back surface field (Al-BSF) cell structure. Device modeling was then used to determine that increasing the efficiency to 20% required improvements in both back surface passivation and rear reflectance. In the second part of this thesis, a passivated, transparent boron BSF (B-BSF) structure was proposed as a high-throughput method for realizing these improvements. The first step in fabricating the proposed B-BSF cell involved the successful development of a water-based, spin-on solution of boric acid as a low-cost, non-toxic and non-pyrophoric alternative to common boron diffusion sources such as boron tribromide. A review of the literature shows that a common problem with boron diffusion is severe bulk lifetime degradation, with Fe contamination being commonly speculated as the cause. An experimental study was therefore devised in which the impact of boron diffusion and subsequent cell process steps on the bulk lifetime and bulk iron contamination was tracked. From this study, a model for boron diffusion-induced Fe contamination was developed along with methods for gettering Fe from the substrate. A key achievement of this thesis was the discovery of a novel, negatively charged, aluminum-doped spin-on glass (SOG) which can, in a short thermal step, simultaneously getter Fe and provide stable, high-quality passivation of planar, boron-diffused Si surfaces. Since past attempts at achieving low-cost, high-efficiency, boron-diffused cells have suffered from bulk lifetime degradation and difficulties with passivating a boron-diffused Si surface, the Al-doped SOG provides a solution to both challenges. Since a high rear reflectance is important for achieving high-efficiencies, an experimental study of various reflectors was undertaken and a silver colloid material was found which exhibits both high electrical conductivity and Lambertian reflectance >95%. The work on boric acid diffusion, iron gettering, surface passivation and rear reflectors was successfully integrated into a 20.2% efficient, screen-printed, B-BSF cell fabricated on 300 µm thick, p-type float-zone (FZ) Si wafers. Both device theory and modeling was used to show that, due to its well-passivated surfaces, this cell would suffer a large loss in efficiency due to light-induced degradation (LID) if it were fabricated on commercial p-type Czochralski (Cz) Si substrates. Since n-type Si substrates do not suffer from LID, the p-type process was slightly tweaked and applied to n-type FZ wafers, resulting in 20.3% efficient cells on 190 µm thick wafers. Computer modeling shows that both the p-type and n-type cells can maintain efficiencies of 20% for wafers as thin as 100 µm.

Heterojunctions of n-ZnO/p-Si solar cells were fabricated by RF sputtering ZnO:Al onto boron-doped (100) silicon (Si) substrates. Zinc Oxide (ZnO) films were also deposited onto soda lime glass for electrical measurements. Sheet resistance measurements were performed with a four-point-probe on the glass samples. Values for samples evacuated for 14 hours prior to deposition increased from 7.9 to 10.17 and 11.5 O/□ for 40 W, 120 and 160 W in RF power respectively. In contrast, those evacuated for 2 hours started with a higher value of 22.5 O/□, and decreased down to 7.6 and 5.8 O/□. Vacuum annealing was performed for both the glass and the Si samples. Current-voltage measurements were performed on the ZnO/Si junctions in the dark and under illumination. Parameters such as open-circuit voltage, Voc; short-circuit current, Isc; fill factor, FF; and efficiency, eta were determined. A maximum efficiency of 0.25% among all samples was produced, with an I sc of 2.16 mA, Voc of 0.31V and a FF of 0.37. This was a sample fabricated at an RF power of 80 W. Efficiency was found to decline with vacuum annealing. Furthermore, interfacial state density calculated based on capacitance-voltage measurements showed an increase in the value with vacuum annealing. The results found suggest that the interface states may be due to an interdiffusion of atoms, possibly those of Zn into the Si surface. The Electron Beam Induced Current (EBIC) method was used to determine diffusion length to be at a value ∼40--80 mum and therefore a minority carrier lifetime calculated of 3 musec. It was also used to determine the surface recombination velocity (SRV) of the fractured surface of the Si bulk from the fabricated solar cells. An SRV of ∼500 cm/sec was determined from the fractured Si surface, at a point located at 30 and 20 mum away from the junction interface.

Main objective of the present work is to develop wide-band gap InGaN solar cells in the 2.4 - 2.9 eV range that can be an integral component of photovoltaic devices to achieve efficiencies greater than 50%. In the present work, various challenges in the novel III-nitride technology are identified and overcome individually to build basic design blocks, and later, optimized comprehensively to develop high-performance InGaN solar cells. Due to the unavailability of a suitable modeling program for InGaN solar cells, PC1D is modified up to a source-code level to incorporate spontaneous and piezoelectric polarization in order to accurately model III-nitride solar cells. On the technological front, InGaN with indium compositions up to 30% (2.5 eV band gap) are developed for photovoltaic applications by controlling defects and phase separation using metal-organic chemical vapor deposition. InGaN with band gap of 2.5 eV is also successfully doped to achieve acceptor carrier concentration of 1e18 cm-3. A robust fabrication scheme for III-nitride solar cells is established to increase reliability and yield; various schemes including interdigitated grid contact and current spreading contacts are developed to yield low-resistance Ohmic contacts for InGaN solar cells. Preliminary solar cells are developed using a standard design to optimize the InGaN material, where the band gap of InGaN is progressively lowered. Subsequent generations of solar cell designs involve an evolutionary approach to enhance the open-circuit voltage and internal quantum efficiency of the solar cell. The suitability of p-type InGaN with band gaps as low as 2.5 eV is established by incorporating in a solar cell and measuring an open-circuit voltage of 2.1 V. Second generation InGaN solar cell design involving a 2.9 eV InGaN p-n junction sandwiched between p- and n-GaN layers yields internal quantum efficiencies as high as 50%; while sixth generation devices utilizing the novel n-GaN strained window-layer enhance the open circuit voltage of a 2.9 eV InGaN solar cell to 2 V. Finally, key aspects to further InGaN solar cell research, including integration of various designs, are recommended to improve the efficiency of InGaN solar cells. These results establish the potential of III-nitrides in ultra-high efficiency photovoltaics.

To ensure that photovoltaics contributes significantly to future world energy production, the cost per watt of producing solar cells needs to be drastically reduced. The use of n-type silicon wafers in conjunction with industrial print technology has the potential to lower the cost per watt of solar cells. The use of n-type silicon is expected to allow the use of cheaper Cz substrates, without a corresponding loss in device efficiency. Printed metallisation is well utilised by the PV industry due to its low cost, yet there are few examples of its application to n-type solar cells. This thesis explores the use of n-type Cz silicon with printed metallisation and diffusion from printed sources in creating industrially applicable solar cell structures. The thesis begins with an overview of existing n-type solar cell structures, previous printed thick film metallisation research and previous research into printed dopant sources. A study of printed thick-film metallisation for n-type solar cells is then presented, which details the fabrication of boron doped p-type emitters followed by a survey of thick film Ag, Al, and Ag/Al inks for making contact to a p-emitter layer. Drawbacks of the various inks include high contact resistance, low metal conductivity or both. A cofire regime for front and rear contacts is established and an optimal emitter selected. A study of printed dopant pastes is presented, with an objective to achieve selective, heavily doped regions under metal contacts without significantly compromising minority carrier lifetime in solar cells. It is found that heavily doped regions are achievable with both boron and phosphorus, but that only phosphorus paste was capable of post-processing lifetime compatible with good efficiencies. The effect of belt furnace processing on n-type silicon wafers is explored, with large losses in implied voltage observed due to contamination of Si wafers from transition metals present in the belt furnace. Due to exposure to chromium in the belt furnace, no significant advantage in using n-type wafers instead of p-type is observed during the belt furnace processing step. Finally, working solar cells with efficiencies up to 16.1% are fabricated utilising knowledge acquired in the earlier chapters. The solar cells are characterised using several new photoluminescence techniques, including photoluminescence with current extraction to measure the quality of metal contacts. The work in this thesis indicates that n-type printed silicon solar cell technology shows potential for good performance at low cost.

Orientadores: Vitor Baranauskas, Helder José Ceragioli / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação / Made available in DSpace on 2018-08-27T01:34:46Z (GMT). No. of bitstreams: 1 Silva_ThiagoFranchiPereirada_D.pdf: 9373432 bytes, checksum: 13a36b35d789088396a134f65246b3a9 (MD5) Previous issue date: 2015 / Resumo: Células solares orgânicas e inorgânicas representam uma excelente alternativa como fonte de energia renovável. Este trabalho consiste em aplicar nanoestruturas de carbono obtidas pela técnica HFCVD (Hot Filament Chemical Vapour Deposition) como componentes utilizados na construção e melhoria de células solares orgânicas (organic photovoltaics - OPVs) e células sensibilizadas por corante (dye-sensitized cells - DSCs). Foi obtido óxido de grafeno reduzido (rGO), carbono tipo diamante (DLC - diamondlike carbon) e diamante condutor nanoestruturado. Estes materiais foram caracterizados por microscopia eletrônica de varredura (SEM-FEG), microscopia de transmissão de alta resolução (HRTEM), espectroscopia Raman e análise termogravimétrica (TGA). O rGO foi empregado na construção das células DSC misturado na pasta de TiO2 em diferentes concentrações, produzindo o aumento de fotocorrente gerada e, consequentemente, o rendimento. O mesmo material foi empregado nas OPVs, em diferentes concentrações, para a substituição do fulereno PCBM (1-(3-metoxicarbonil)-propil-1-1-fenil- (6,6)metanofulereno) e também em conjunto com o fulereno, sendo observada também a melhoria no desempenho dos dispositivos em função da concentração. Com finalidade de substituir os contraeletrodos das células DSC, carbono tipo diamante foi depositado em substrato de alumínio (Al) e diamante condutor nanoestruturado depositado em substratos de nióbio (Nb). As células com contraeletrodos de Al com filme de DLC apresentaram sensibilidade à luz, com possibilidade de aplicação em sensores ópticos, enquanto as células com contraeletrodos de Nb com filme de diamante condutor apresentaram excelente desempenho, tornando possível a substituição dos contraeletrodos de platina / Abstract: Organic and inorganic solar cells comprise a promising solution as a renewable energy source. This work consists of applying carbon nanostructures obtained by HFCVD technique (Hot Filament Chemical Vapour Deposition) as components used in the construction and improvement of organic solar cells (organic photovoltaics - OPVs) and dye sensitized cells (dye-sensitized cells - DSCs). Reduced graphene oxide (rGO), carbon diamond-like (DLC - diamond- like carbon) and nanostructured conductor diamond was obtained. These materials were characterized by scanning electron microscopy (SEM-FEG), high resolution transmission microscopy (HRTEM), Raman spectroscopy and thermal gravimetric analysis (TGA). Reduced graphene oxide was used for the construction of the DSC cell at the TiO2 layer mixed in different concentrations, producing an increase in photocurrent generated and thus conversion efficiency. The same material was used in the OPVs at different concentrations for the replacement of fullerene PCBM (1- (3-methoxycarbonyl) -propyl-1-1-phenyl- (6,6) metanofulereno) and with the fullerene was also observed improvement in performance of the devices as a function of concentration. With aim of replacing the counterelectrode of DSCs cells, diamond-like carbon was deposited on aluminum substrate (Al) and nanostructured conductive diamond deposited on niobium (Nb) substrates. Cells with Al/DLC counterelectrode showed sensitivity to light, with the possibility of application in optical devices while cells with Nb/nanostructured conductive diamond counterelectrode showed excellent performance, with possibility to replace platinum counterelectrodes / Doutorado / Eletrônica, Microeletrônica e Optoeletrônica / Doutor em Engenharia Elétrica

To cope with energy requirements the utilization of renewable energies, particularly the Sun supplies the biggest and abundant energy source in Earth. Photo-voltaic and solar cell are the well advance and burning technology and a field of hot research. Majority of research centers and universities are working in this field. 1G, 2G, 3G and next generation of photo-voltaic cells have been developed and still to improve its efficiency and to decrease it 0.2 $/W cost. Our work mainly based on the theoretical and physical analysis of thin-film Photovoltaic devices. We will explore different software used for the analysis of PV cells, and will analyse different simulation related to solar cells like open circuit voltage VOC, Short circuit current JSC, Fill Factor FF (%) and external Quantum efficiency (%) for thin film solar cell including CIGS, CIS, CGS, CdTe, SnS/CdS/ZnO etc. To have different analysis for different combination and different replacement for materials used in the solar cell fabrication. To cope with the PV cost and environmental hazards we have to find alternate solutions. / Ullah, H. (2015). Simulation studies of photovoltaic thin film devices [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/48800 / TESIS

Atomically thin 2D layered semiconductor materials such as Transition Metal Di-Chalcogenides (TMDCs) have great potential for use as flexible, ultra-thin photovoltaic materials in solar cells due to their favorable photon absorption and electronic transport properties. In this dissertation, the electronic properties, such as band structure and bandgap, and optical absorption properties of a TMDC known as Tungsten Disulfide (WS2) were obtained from Density Functional Theory (DFT) calculations to design conventional and unconventional solar cells. Using these properties, a 1 μm thick heterojunction solar cell based on monolayer and bulk WS2 together with amorphous silicon (a-Si) was modeled using numerical calculations and simulations. The maximum efficiency of this cell is 23.3% with Voc = 0.84 V and Jsc = 33.5 mA/cm2 under the AM1.5G terrestrial solar spectrum. Next, a similar but even thinner solar cell with a thickness of 200 nm, together with a light trapping structure and an anti-reflection coating layer, was modeled under the AM0 space solar spectrum; similar device performance efficiencies around 21-23% were obtained. The performance of these solar cell models is comparable to many commercial cells in both terrestrial and space photovoltaics. As conventional photovoltaics approach the Shockley-Queisser limit, the need for unconventional materials and approaches has become more apparent. Hybrid alloys of TMDCs exhibit tunable direct bandgaps and significant dipole moments. Dark state protection induced by dipole-dipole interactions forms new bright and dark states in the conduction band that reduce radiative recombination and enhance photon-to-electron conversion, leading to significantly higher photocurrents. In our work, current enhancement of up to 35% has been demonstrated by modeling dark state protection in a solar cell composed of Tungsten Diselenide (WSe2) and Tungsten Sulfo-Selenide (WSeS), with the potential to exceed the Shockley-Queisser limit under ideal conditions.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20405号 / 工博第4342号 / 新制||工||1673(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 赤木 和夫, 教授 木村 俊作 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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