Silicon quantum dots (QDs) in SiO2 superlattices were fabricated by alternate deposition of silicon oxide (SiO2) and silicon-rich oxide (SRO), i.e. SiOx (x<2), and followed by high temperature annealing. A deposited SRO film is thermodynamically unstable below 1173oC and phase separation and diffusion of Si atoms in the amorphous SiO2 matrix creates nano-scaled Si quantum dots. The quantum-confined energy gap was measured by static photoluminescence (PL) using an Argon ion laser operating at 514.5 nm. The measured energy band gaps of crystalline Si QDs in SiO2 matrix at room temperature (300 K) show that the emission energies from 1.32 eV to 1.65 eV originating Si dot sizes from 6.0 nm to 3.4 nm, respectively. There is a strong blue-shift of the PL energy peak position with decreasing the quantum dot size and this shows the evidence of quantum confinement of our fabricated Si QDs in SiO2 matrix. The PL results indicate that the fabricated Si QDs in SiO2 matrix could be suitable for the device application such as top cell material for all-silicon tandem solar cells. Silicon QD superlattices in nitride matrix were fabricated by alternate deposition of silicon nitride (Si3N4) and silicon-rich nitride (SRN) by PECVD or co-sputtering of Si and Si3N4 targets. High temperature furnace annealing under a nitrogen atmosphere was required to form nano-scaled silicon quantum dots in the nitride matrix. The band gap of silicon QD superlattice in nitride matrix (3.6- 7.0 nm sized dots) is observed in the energy range of 1.35- 1.98 eV. It is about 0.3- 0.4 eV blue-shifted from the band gap of the same sized quantum dots in silicon oxide. It is believed that the increased band gap is caused by a silicon nitride passivation effect. Silicon-rich carbide (SRC, i.e. Si1-xCx) thin films with varying atomic ratio of the Si to C were fabricated by using magnetron co-sputtering from a combined Si and C or SiC targets. Off-stoichiometric Si1-xCx is of interest as a precursor to realize Si QDs in SiC matrix, because it is thermodynamically metastable when the composition fraction is in the range 0 < x < 0.5. Si nanocrystals are therefore able to precipitate during a post-annealing process. SiC quantum dot superlattices in SiC matrix were fabricated by alternate deposition of thin layers of carbon-rich silicon carbide (CRC) and SRC using a layer by layer deposition technique. CRC layers were deposited by reactive co-sputtering of Si and SiC targets with CH4. The PL energy band gap (2.0 eV at 620 nm) from 5.0 nm SRC layers could be from the nanocrystalline ??-SiC with Si-O bonds and the PL energy band gap (1.86 eV at 665 nm) from 6.0 nm SRC layers could be from the nanocrystalline ??-SiC with amorphous SiC clusters, respectively. The dielectric material for an all-silicon tandem cell is preferably silicon oxide, silicon nitride or silicon carbide. It is found that for carrier mobility, dot spacing for a given Bloch mobility is in the order: SiC > Si3N4 > SiO2. By ab-initio simulation and PL results, the band gap for a given dot size is in the order: SiC > Si3N4 > SiO2. However, the PL intensity for a given dot size is in the order: SiC < Si3N4 < SiO2.

This thesis focuses on improving the adhesion of electroless metal layers plated to pit contacts in interdigitated, backside buried contact (IBBC) solar cells. In an electrolessly plated, pit contact IBBC cell, the contact grooves are replaced with lines of pits which are interconnected by the plated metal. It is shown, however, that electroless metal layers, plated by the standard IBBC plating sequence, are not adherent on pit contact IBBC solar cells. The cause of this adhesion problem is investigated by examining the adhesive properties of each of the metal layers in the electroless metallization sequence on planar test structures. This investigation reveals that Pd activation of heavily P diffused Si impedes Ni silicide growth and that, in the absence of a silicide at the Ni/Si interface, an electrolessly plated Cu layer will cause the underlying Ni layer to peel away from the substrate. It is also found that the Ni silicidation process itself intermittently causes the unreacted Ni to spontaneously peel away from the substrate. An electroless metallization sequence that results in thick, adhesive Cu deposits on planar &lt 100&gt surfaces is developed in this thesis. It is shown that this process leads to the formation of a Ni silicide on both n- and p- type, heavily diffused surfaces. Fully plated, pit contact IBBC solar cells were not able to be fabricated during the course of this work but it is reasonable to expect that the modified plating sequence developed in this work will result in the metal layers being adhesive on these cells.

In broad terms, this thesis is concerned with the measurement and interpretation of carrier lifetimes in multicrystalline silicon. An understanding of these lifetimes in turn leads to a clearer picture of the limiting mechanisms in solar cells made with this promising material, and points to possible paths for improvement. The work falls into three broad categories: gettering, trapping and recombination. A further section discusses a powerful new technique for characterising impurities in semiconductors in general, and provides an example of its application. Gettering of recombination centres in multicrystalline silicon wafers improves the bulk lifetime, often considerably. Although not employed deliberately in most commercial cell processes, gettering nevertheless occurs to some extent during emitter formation, and so may have an important impact on cell performance. However, the response of different wafers to gettering is quite variable, and in some cases is non-existent. Work in this thesis shows that the response to gettering is best when the dislocation density is low and the density of mobile impurities is high. For Eurosolare material these conditions prevail at the bottom and to a lesser extent in the middle of an ingot. However, these conclusions can not always be applied to multicrystalline silicon produced by other manufacturers. Low resistivity multicrystalline silicon suffers from a concurrent thermally induced degradation of the lifetime. This had previously been attributed to the dissolution of precipitated metals, although we note that the crystallographic quality also appears to deteriorate. The thermal degradation effect results in an optimum gettering time for low resistivity material. Edge-defined Film-fed Growth (EFG) ribbon silicon was also found to respond to gettering, and even more so to bulk hydrogenation. Evidence for Cu contamination in the as-grown EFG wafers is presented. Multicrystalline silicon is often plagued by trapping effects, which can make lifetime measurement in the injection-level range of interest very difficult, and sometimes impossible. An old model based on centres that trap and release minority carriers, but do not interact with majority carriers, was found to provide a good explanation for these effects. These trapping states were linked with the presence of dislocations and also with boron-impurity complexes. Their annealing behaviour and lack of impact on device parameters can be explained in terms of the models developed. The trapping model allowed a recently proposed method for correcting trap-affected data to be tested using typical values of the trapping parameters. The correction method was found to extend the range of useable data to approximately an order of magnitude lower in terms of carrier density than would be available otherwise, an improvement that could prove useful in many practical cases. High efficiency PERL and PERC cells made on gettered multicrystalline silicon resulted in devices with open circuit voltages in the 640mV range that were almost entirely limited by bulk recombination. Furthermore, the injection-level dependence of the bulk lifetime resulted in decreased fill factors. Modelling showed that these effects are even more pronounced for cells dominated by interstitial iron recombination centres. Analysis of a commercial multicrystalline cell fabrication process revealed that recombination in the emitter created the most stringent limit on the open circuit voltage, followed by the bulk and the rear surface. The fill factors of these commercial cells were mostly affected by series resistance, although some reduction due to injection-level dependent lifetimes seems likely also. Secondary Ion Mass Spectroscopy on gettered layers of multicrystalline silicon revealed the presence of Cr and Fe in considerable quantities. Further evidence strongly implied that they resided almost exclusively as precipitates. More generally, injection-level dependent lifetime measurements offer the prospect of powerful contamination-monitoring tools, provided that the impurities are well characterised in terms of their energy levels and capture cross-sections. Conversely, lifetime measurements can assist with this process of characterising impurities, since they are extremely sensitive to their presence. This possibility is explored in this thesis, and a new technique, dubbed Injection-level Dependent Lifetime Spectroscopy (IDLS) is developed. To test its potential, the method was applied to the well-known case of FeB pairs in boron-doped silicon. The results indicate that the technique can offer much greater accuracy than more conventional DLTS methods, and may find applications in microelectronics as well as photovoltaics.

This thesis reports the spectroscopic and computational studies of two classes of compounds, which have applications in new optoelectronic materials technology. Substituted ligands of dipyrido-[3,2a:2�,3�c]phenazine (dppz), and their Cu(I), Re(I) and Ru(II) complexes have utility in organic electroluminescent devices. A series of Zn(II) tetraphenylporphyrins with conjugated functional groups at the β-position have been used with success in liquid heterojunction dye-sensitized solar cells. The vibrational spectra and optoelectronic properties of the two classes were investigated using Raman, resonance Raman and transient resonance Raman spectroscopy, in conjunction with density functional theory methods. Density functional theory frequency calculations were used to aid vibrational mode assignments for the dppz compounds, and show close agreement with the experimental non-resonance Raman spectra. The enhancement of modes which are localized on differing sections of the ligand was identified. The nature of the absorbing chromophores for the dppz ligands and complexes was established using resonance Raman spectroscopy in concert with vibrational assignments from calculations. Transient resonance Raman spectra of the ligands provided spectral signatures for the triplet ligand-centred state; these features were observed in the TR� spectra of the metal complexes, along with other features attributable to MLCT states. Electroluminescent devices were fabricated using the dppz ligands and complexes as emissive dopants, and their properties investigated. The optoelectronic behaviour of the devices was found to be influenced by the mechanism of exciton formation on the dopant. The device properties were also dependent on the dopant concentration, the concentrations of other components and the driving voltage. The electronic structure of the porphyrin compounds was investigated using time-dependent density functional theory methods. Comparison of calculated optical transitions with experimental data shows that the calculations predict trends in the optical absorption spectra with change of functional group and with increase in conjugation chain length. The calculations suggest that the electron-withdrawing substituent decreases the configuration interaction effect by breaking the degeneracy of the two lowest unoccupied MOs, and other configuration interaction effects come into play involving other frontier MOs. Interrupting the conjugation of the functional group is shown to mitigate the breakdown of the configuration interaction. The perturbation of the normal electronic structure of the porphyrin by the substituent was also investigated using resonance Raman spectroscopy. Vibrational analysis identified bands due to the substituent, implying coupling between the porphyrin and substituent chromophores. Changes in frequency of porphyrin core modes due to the differing substituents and different metal centres were reproduced by density functional theory calculations. This project has allowed the spectroscopic investigation of the active optical states in a number of polypyridyl and porphyrin compounds, and determined the efficacy of DFT and TDDFT calculations to predict the properties of these compounds.

Silicon is widely used in optoelectronic devices, including solar cells. In recent years new forms of silicon have become available, including amorphous, microcrystalline and nano-crystalline material. These new forms have great promise for low cost, thin film solar cells and the purpose of this work is to investigate their preparation and properties with a view to their future use in solar cells. A Hot Wire-Deposition Chemical Vapour Deposition CVD (HW-CVD) system was constructed to create a multi-chamber high vacuum system in combination with an existing Plasma Enhanced Chemical Vapour Deposition (PECVD) system; to study the amorphous to crystalline transition in silicon thin films. As the two chambers were linked by a common airlock, it was essential to construct a transfer mechanism to allow the transfer of the sample holder between the two systems. This was accomplished by the incorporation of two gate valves between the two chambers and the common airlock as well as a rail system and a magnetic drive that were designed to support the weight of, and to guide the sample holder through the system. The effect of different deposition conditions on the properties and structure of the material deposited in the combined HW-CVD:PECVD system were investigated. The conditions needed to obtain a range of materials, including amorphous, nano- and microcrystalline silicon films were determined and then successfully replicated. The structure of each material was analysed using Transmission Electron Microscopy (TEM). The presence of crystallites in the material was confirmed and the structure of the material detected by TEM was compared to the results obtained by Raman spectroscopy. The Raman spectrum of each sample was decoupled into three components representing the amorphous, intermediate and crystalline phases. The Raman analysis revealed that the amorphous silicon thin film had a dominant amorphous phase with smaller contribution from the intermediate and crystalline phase. This result supported the findings of the TEM studies which showed some medium range order. Analysis of the Raman spectrum for samples deposited at increasing filament temperatures showed that the degree of order within the samples increased, with the evolution of the crystalline phase and decline of the amorphous phase. The Selected Area Diffraction (SAD) patterns obtained from the TEM were analysed to gain qualitative information regarding the change in crystallite size. These findings have been confirmed by the TEM micrograph measurements. The deposition regime where the transition from amorphous to microcrystalline silicon took place was examined by varying the deposition parameters of filament temperature, total pressure in the chamber, gas flow rate, deposition time and substrate temperature. The IR absorption spectrum for ƒÝc-Si showed the typical peaks at 2100cm-1 and 626cm-1, of the stretching and wagging modes, respectively. The increase in the crystallinity of the thin films was consistent with the evolution of the 2100cm-1 band in IR, and the decreasing hydrogen content, as well as the shift of the wagging mode to lower wavenumber. IR spectroscopy has proven to be a sensitive technique for detecting the crystalline phase in the deposited material. Several devices were also constructed by depositing the ƒÝc-Si thin films as the intrinsic layer in a solar cell, to obtain information on their characteristics. The p- layer (amorphous silicon) was deposited in the PECVD chamber, and the sample was then transferred under vacuum using the transport system to the HW-CVD chamber where the i-layer (microcrystalline silicon) was deposited. The sample holder was transferred back to the PECVD chamber where the n-layer (amorphous silicon) was deposited. The research presented in this thesis represents a preliminary investigation of the properties of ƒÝc-Si thin films. Once the properties and optimum deposition characteristics for thin films are established, this research can form the basis for the optimization of a solar cell consisting of the most efficient combination of amorphous, nano- and microcrystalline materials.

<p>Nanoporous dye-sensitized solar cells (DSSC) are promising devices for solar to electric energy conversion. In this thesis photoelectron spectroscopy (PES), x-ray absorption spectroscopy (XAS) and photovoltaic measurements are used for studies of the key interfaces in the DSSC. </p><p>Photovoltaic properties of new combinations of TiO₂/dye/hole-conductor heterojunctions were demonstrated and their interfacial structures were studied. Three different types of hole-conductor materials were investigated: Triarylamine derivatives, a conducting polymer and CuI. The difference in photocurrent and photovoltage properties of the heterojunction due to small changes in the hole-conductor material was followed. Also a series of dye molecules were used to measure the influence of the dye on the photovoltaic properties. Differences in both the energy-level matching and the geometric structure of the interfaces in the different heterojunctions were studied by PES. This combination of photovoltaic and PES measurements shows the possibility to link the interfacial electronic and molecular structure to the functional properties of the device. </p><p>Three effective dyes used in the DSSC, Ru(dcbpy)₂(NCS)₂, Ru(tcterpy)(NCS)₃ and an organic dye were studied in detail using PES and XAS and resonant core hole decay spectroscopy. The results gave information of the frontier electronic structure of the dyes and how the dyes are bonded to the TiO₂ surface. </p><p>Finally, the hole-conductor mechanism in a conducting polymer was investigated theoretically using semi-empirical and ab-initio methods. </p>

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Robert G. Hunsperger, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

Nanoporous dye-sensitized solar cells (DSSC) are promising devices for solar to electric energy conversion. In this thesis photoelectron spectroscopy (PES), x-ray absorption spectroscopy (XAS) and photovoltaic measurements are used for studies of the key interfaces in the DSSC. Photovoltaic properties of new combinations of TiO2/dye/hole-conductor heterojunctions were demonstrated and their interfacial structures were studied. Three different types of hole-conductor materials were investigated: Triarylamine derivatives, a conducting polymer and CuI. The difference in photocurrent and photovoltage properties of the heterojunction due to small changes in the hole-conductor material was followed. Also a series of dye molecules were used to measure the influence of the dye on the photovoltaic properties. Differences in both the energy-level matching and the geometric structure of the interfaces in the different heterojunctions were studied by PES. This combination of photovoltaic and PES measurements shows the possibility to link the interfacial electronic and molecular structure to the functional properties of the device. Three effective dyes used in the DSSC, Ru(dcbpy)2(NCS)2, Ru(tcterpy)(NCS)3 and an organic dye were studied in detail using PES and XAS and resonant core hole decay spectroscopy. The results gave information of the frontier electronic structure of the dyes and how the dyes are bonded to the TiO2 surface. Finally, the hole-conductor mechanism in a conducting polymer was investigated theoretically using semi-empirical and ab-initio methods.

In the field of organic solar technology, there are two main problems, the stability of materials and the low power efficiency. By analyzing the power efficiency of organic solar cells, we can infer that efficiency of absorption and charge mobility are the key factors to these two problems. In this study, we focus on coupling carbazole with different low bandgap moieties. By using Suzuki Coupling, we synthesized new conjugated polymers with main chain structures of D-A sequence. It turns out that the copolymer can form a strong intramolecular charge transfer (ICT). We¡¦ve successfully synthesized two new low bandgap copolymers with D-A sequence, PCAMDT and PCAMDP. These two copolymers show us excellent thermal stabilities with decomposition temperature of 320¢Jand 355¢J,respectively.According to UV-Vis absorption spectrum, PCAMDT and PCAMDP own bandgaps at 1.85 eV and 2.22eV,respectively. Electrochemical analysis reveals that the HOMO and LUMO level of PCAMDT are found to be -5.69eV and -3.77eV,repectively, while the HOMO and LUMO level of PCAMDP are -5.87eV and -3.75eV. These properties make PCAMDT and PCAMDP advantageous materials while applied as high absorbing layers of organic solar cells.

This research is to synthesize polyaniline (PANI) thin film on the Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) by using potentiostatic deposition of electrochemical method. The hybrid film composed of PEDOT:PSS and PANI was fabricated to replace the ITO layer for polymer solar cells as an anode. In the future, the hybrid film can develop the flexible polymer solar cells. In this study, we fixed the total thickness of the hybrid film, and we investigated optical transmittance, conductivity, Highest Occupied Molecular Orbital (HOMO), surface roughness, and surface morphology of hybrid films by changing the ratio of PEDOT:PSS and PANI, and to discuss the factors on device efficiency. Then, we compared the device structures with anode made by PEDOT: PSS. We found the hybrid films fabricated with different ratio of PEDOT:PSS and PANI, and the HOMO results were similar. In addition, we found optical transmittance, conductivity, surface roughness, and surface morphology of hybrid films that varies with different ratio of PEDOT:PSS and PANI. The power conversion efficiencies of the device mainly were affected by the surface roughness and morphology of the hybrid film surface. Comparing to other parameters, the hybrid film fabricated by PEDOOT:PSS(280nm) and PANI(30nm) owns the most appropriate surface roughness and surface morphology. The power conversion efficiency(PCE) was up to 0.68%, and then via post-annealing of 90¢J 10 minutes the PCE was increase to 1.06% under AM 1.5G illumination based on PEDOT:PSS (280 nm) / PANI (30 nm) / P3HT: PCBM (100 nm) / Al (200 nm), and the device area of 0.16 cm2.