As the efficiency potential for the industrial P-type Al-BSF silicon solar cell reaches its limit, new solar cell technologies are required to continue the pursuit of higher efficiency solar power at lower cost. It has been demonstrated in literature that among possible alternative solar cell structures, cells featuring a local BSF (LBSF) have demonstrated some of the highest efficiencies seen to date. Implementation of this technology in industry, however, has been limited due to the cost involved in implementing the photolithography procedures required. Recent advances in solar cell doping techniques, however, have identified ion implantation as a possible means of performing the patterned doping required without the need for photolithography. In addition, past studies have examined the potential for building solar cells on N-type silicon substrates, as opposed to P-type. Among other advantages, it is possible to create N-type solar cells which do not suffer from the efficiency degradation under light exposure that boron-doped P-type solar cells are subject to. Industry has not been able to capitalize on this potential for improved solar cell efficiency, in part because the fabrication of an N-type solar cell requires additional masking and doping steps compared to the P-type solar cell process. Again, however, recent advances in ion implantation for solar cells have demonstrated the possibility for bypassing these process limitations, fabricating high efficiency N-type cells without any masking steps. It is clear that there is potential for ion implantation to revolutionize solar cell manufacturing, but it is uncertain what absolute efficiency gains may be achieved by moving to such a process. In addition to development of a solar specific ion implant process, a number of new thermal processes must be developed as well. With so many parameters to optimize, it is highly beneficial to have an advanced simulation model which can describe the ion implant, thermal processes, and cell performance accurately. Toward this goal, the current study develops a process and device simulation model in the Sentaurus TCAD framework, and calibrates this model to experimentally measured cells. The study focuses on three main tasks in this regard: Task I - Implant and Anneal Model Development and Validation This study examines the literature in solar and microelectronics research to identify features of ion implant and anneal processes which are pertinent to solar cell processing. It is found that the Monte Carlo ion implant models used in IC fabrication optimization are applicable to solar cell manufacture, with adjustments made to accommodate for the fact that solar cell wafers are often pyramidally textured instead of polished. For modeling the thermal anneal processes required after ion implant, it is found that the boron and phosphorus cases need to be treated separately, with their own diffusion models. In particular, boron anneal simulation requires accurate treatment of boron-interstitial clusters (BICs), transient enhanced diffusion, and dose loss. Phosphorus anneal simulation requires treatment of vacancy and interstitial mediated diffusion, as well as dose loss and segregation. The required models are implemented in the Sentaurus AdvancedModels package, which is used in this study. The simulation is compared to both results presented in literature and physical measurements obtained on wafers implanted at the UCEP. It is found that good experimental agreement may be obtained for sheet resistance simulations of implanted wafers, as well as simulations of boron doping profile shape. The doping profiles of phosphorus as measured by the ECV method, however, contain inconsistencies with measured sheet resistance values which are not explained by the model. Task II - Device Simulation Development and Calibration This study also develops a 3D model for simulation of an N-type LBSF solar cell structure. The 3D structure is parametrized in terms of LBSF dot width and pitch, and an algorithm is used to generate an LBSF structure mesh with this parametrization. Doping profiles generated by simulations in Task I are integrated into the solar cell structure. Boundary conditions and free electrical parameters are calibrated using data from similar solar cells fabricated at the UCEP, as well as data from lifetime test wafers. This simulation uses electrical models recommended in literature for solar cell simulation. It is demonstrated that the 3D solar cell model developed for this study accurately reproduces the performance of an implanted N-type full BSF solar cell, and all parameters fall within ranges expected from theoretical calculations. The model is then used to explore the parameter space for implanted N-type local BSF solar cells, and to determine conditions for optimal solar cell performance. It is found that adding an LBSF to the otherwise unchanged baseline N-type cell structure can produce almost 1% absolute efficiency gain. An optimum LBSF dot pitch of 450um at a dot size of 100um was identified through simulation. The model also reveals that an LBSF structure can reduce the fill factor of the solar cell, but this effect can be offset by a gain in Voc. Further efficiency improvements may be realized by implementing a doping-dependent SRV model and by optimizing the implant dose and thermal anneal. Task III - Development of a Procedure for Ion Implanted N-type LBSF Cell Fabrication Finally, this study explores a method for fabrication of ion-implanted N-type LBSF solar cells which makes use of photolithographically defined nitride masks to perform local phosphorus implantation. The process utilizes implant, anneal, and metallization steps previously developed at the UCEP, as well as new implant masking steps developed in the course of this study. Although an LBSF solar cell has not been completely fabricated, the remaining steps of the process are successfully tested on implanted N-type full BSF solar cells, with efficiencies reaching 20.0%.

In the recent years area organic photovoltaics is generating a lot of interests because whole process of synthesis and fabrication is less energy intensive process as well as it is cost effective compared to conventional inorganic Si based photovoltaic technology. This work mainly deals with the fabrication and optimization of device fabrication conditions for organic photovoltaic materials. In first part of the work, the solar cell fabrication conditions were optimized for the commonly used system P3HT – PCBM. The fabricated device was optimized for the solvents used for the active material, concentration of the active material solution, donor- acceptor ratio of the active material, annealing conditions of the active layer and the metal evaporation conditions for the cathode. All the optimization procedures were carried out in controlled atmosphere to minimize the environmental effect inference during fabrication of the solar cell devices. All the characterization was carried out at ambient conditions. The efficiency of the solar cell was improved from 0.009% to 6.2%. the environmental stability of the fabricated devices were carried out after encapsulating it with epoxy based resin in both ambient conditions as well as extreme conditions like 85% RH at 25°C inside the humidity chamber. It was observed that both the data matches well with each other indicating proper encapsulation required to safe guard the device for the better performance over the period of time. Second part of this work mainly deals with understanding the structure property relationship of thiophene based donor- acceptor- donor molecule 2,5-dithienyl-3,4-(1,8-naphthylene) cyclopentadienone (DTCPA), which is highly crystalline, low band gap organic molecule which absorbs over entire visible region of the solar spectra. DTCPA crystals of various morphologies were prepared by various recrystallization routes. It was observed that macro scale morphology of these crystals differs from each other. Also depending on the method of recrystallization sizes of the crystals also varies. All the recrystallized DTCPA shows strong orientation toward (001) direction. However, it was observed that lattice parameters of these crystals slightly differ from each other owing to the recrystallization methodology. These variations in crystal parameters are more than 0.02 which is significant. It was also observed that the crystallite sizes depend on the recrystallization routes. Slow evaporation of concentrated solution (SEC) grown crystals has the larger crystallite size of 170nm. It was observed that absorption range of these crystals slightly differ from each other owing to the change in the crystallite sizes and crystal parameters. Third part of this work deals with the fabrication and optimization of thermal evaporation process of DTCPA for photovoltaic applications. DTCPA is stable at higher temperatures as well as has sharp melting point which make it ideal candidate for thermal evaporation. In this work films of DTCPA were fabricated for various evaporation rates by thermal evaporation technique. Chemical integrity of the molecules upon evaporation is found to be intact as observed from FTIR spectroscopy. XRD shows that at lower (25 W/m2) as well as higher (40 W/m2) films are oriented to (001), (400) as well as (311) directions, at 30 W/m2 and 35 W/m2 there is a strong orientation towards (311) and (001) directions respectively. Photo luminescence studies indicate that there is strong 410 nm emission for films deposited at the power of 25 W/m2 and 40 W/m2. Microscopic studies confirm that morphology is dependent on the deposition rates as it changes with the change in deposition rate. This in turn reflects in the device characteristics of these films. It was observed that films deposited at high deposition rates show better device characteristics with high VOC and current density values. All these device fabrication and characterizations were carried out in ambient conditions. Fourth part of this work deals with P3HT - DTCPA composites which exhibit wide range of light absorption. It was observed that DTCPA act as nucleating centers for the P3HT molecules and increases crystallinity in the composite. Furthermore, DTCPA helps in exciton separation because of donor and acceptor moieties present in the molecule. It also helps in charge transportation because of its crystalline nature and further it induces molecular ordering in the P3HT matrix. The band diagram of P3HT- DTCPA suggests that the band edges of both materials are ideal for charge separation. In addition, crystalline nature of the DTCPA molecule helps in effective charge transportation. J-V characteristics shows that there is large built in potential in the devices from these blends leading to large Voc. Composites with lower DTCPA loadings show higher efficiency than with higher loadings. These devices were prepared in ambient conditions and needs to be optimized for obtaining better device properties. In the fifth part of the work two types of system were studied to understand the band edge matching on the photovoltaic properties, carbazole based copolymers and DTCPA based copolymers. In the case of carbazole based copolymers it was observed that by copolymerizing carbazole with thiophene based derivatives lowers the band gap and modifies the HOMO and LUMO levels for better suit for the photovoltaic device fabrication. It was observed that that is two orders of improvements in the efficiency by co polymerizing carbazole with benzothiodizole as improves the JSC and VOC. Also the copolymerization of carbazole with both benzothiodiazole and bithiophene results in better light harvesting as the optical band gap was lowered. In the case of DTCPA copolymers with DTBT and DHTBT as both are random copolymers the solubility was low as well as their HOMO band edge was mismatched with the PEDOT: PSS which is a hole transport layer. However, the alternate polymerization of DTCPA with DTBT improved the band edge matching and also the solubility. As a result there was tenfold improvement in the charge collection and hence the efficiency was improved from 0.02% to 2.4%. Many of the conducting polymers have good material property but poor filmability. In the sixth part of this work deals with fabrication of device quality films by alternate deposition technique like pulsed laser deposition. Two types of system were studied in this work (i) polypyrrole- MWCNT nanocomposites and (ii) Poly DTCPA polymer. In both the cases it was observed that chemical integrity of the polymer retained during ablation. PolyDTCPA films were fabricated by pulsed laser deposition by both IR (Nd-YAG) and UV (KrF) laser source. Morphological studies indicate that IR laser ablated films were particulate in nature whereas UV laser ablated films were grown as continuous layers as polyDTCPA absorbs better in UV region. As a result the IV characteristics indicate that IR laser ablated films are resistive in nature and UV laser ablated films are good rectifiers indicating the suitability of the process for fabrication of device quality films.

Photovoltaic cells are a promising solution to the current energy crisis. Among the different photovoltaic cell technologies developed, dye-sensitised solar cells (DSSC) are emerging as viable low-cost alternatives to Si PV technology. This thesis presents studies on fabrication and characterisation of TiO2 based dye-sensitised solar cells. Chapter 1 gives an overview of different photovoltaic cell technologies and a review of the state-of-the art DSSC technology. Chapter 2 describes the techniques used for characterisation of DSSCs. Chapter 3 describes the fabrication of TiO2 based dye-sensitised solar cells. Chapter 4 presents the analysis of measurements obtained by the characterisation techniques. Finally chapter 5 summarises the work done and suggests directions for future work.