No description available.

No description available.

No description available.

The work presented in this thesis is concerned with organic photovoltaic devices (OPVs), in particular with the use of multi-wall carbon nanotubes (MWCNT) to improve device performance. The initial chapters introduce the subject areas of OPVs, carbon nanotubes (CNT) and experimental techniques. Within the thesis, it is demonstrated that MWCNT can be utilised as an interpenetrating, work function tuneable, electrode in bi-layer OPVs. A remarkably low concentration of MWCNT (~1 wt.%) uniformly distributed within the donor layer is shown to greatly increase the cell fill factor (FF) without complicating the process of device fabrication. Furthermore, functionalising the MWCNT with polar surface groups is shown to be an effective means of modifying the open circuit voltage (Voc). This study clearly demonstrates the potential of this approach to significantly increase the power conversion efficiency of bi-layer OPVs. We also introduce, a water soluble acid oxidised multi-wall carbon nanotube (o-MWCNT)-polythiophene composite for bi-layer OPVs. OPVs utilising this nanocomposite material as the donor layer exhibit a ~ 20 % increase in FF and commensurate increase in power conversion efficiency as compared to cells without o-MWCNT. Crucially o-MWCNT are incorporated into the cell structure using an environmentally compatible solvent without complicating the process of device fabrication. In this context the MWCNT enhances the conductivity of the donor layer, reducing cell series resistance and increasing FF. The growth of MWCNT directly onto indium-tin oxide (ITO) coated aluminosilicate glass via chemical vapour deposition as a large area semi-transparent electrodes bulk heterojunction OPV is also described. The rate of nanotube growth on this ternary oxide is shown to be greatly reduced as compared to that on silicon dioxide and soda lime glass enabling a high degree of control over CNT height. The strong potential of this nano-structured semi-transparent substrate as an interpenetrating hole-extracting electrode in bulk heterojunction OPVs is also demonstrated. Combining the work contained in the thesis, a hybrid OPV prototype is designed and fabricated, wherein MWCNT grown directly onto ITO coated glass function as an air stable cathode which contributes to exciton dissociation.

Electrodeposition was used to deposit thin film semiconductor materials for use in solar cell devices. Copper indium diselenide (CuInSe₂) was deposited from ethylene glycol at 150°C with the aim of improved material properties due to the elevated temperature. The broad nature of the X-ray diffraction (XRD) peaks before and after annealing indicated the layers were comprised of multiple phases identified as CuInSe₂ and Cu-Se binaries. Insufficient indium inclusion for device quality materials was incorporated into the layers over the explored growth range of -0.800 to -1.000 V vs Se reference electrode. The layers deposited at more positive deposition voltages were metallic and contained mainly Cu-Se binary phases. At more negative deposition voltages the formation of CuInSe₂ was confirmed although above -1.000 V vs Se the layers were often powdery and disintegrated on removal from the electrolyte. There were no noticeable improvements in the CuInSe₂ layers deposited from ethylene glycol compared to reports from aqueous media, which is less toxic and lower cost, therefore electrodeposition from aqueous solution is preferable. Undoped zinc oxide (ZnO) and aluminium doped ZnO (ZnO: Al) were deposited from zinc nitrate solutions with the aim of using electrodeposition for the ZnO bilayer in CuInSe₂ devices to unify the production process. ZnO was deposited at a range of deposition voltages from -0.900 to -1.050 V vs silver/silver chloride reference electrode as identified using XRD. Various morphologies were observed using scanning electron microscopy (SEM) and the electrical resistivity was determined at 6.9x 10⁶ Ω cm and decreased to 3.4x10⁵ Ω cm after Al doping. To make this method suitable for commercialisation more work would need to be carried out to address consistency issues mainly regarding the electrolyte conditions, including pH and oxygen concentration as a function of growth time. A comparison was made between electrodeposited and sputtered ZnO and ZnO: AI. Some differences in the material properties were found; all layers were identified as hexagonal wurtzite ZnO. A considerable change in morphology was observed by SEM between the electrodeposited and sputtered materials. Little change in the electrical resistivity was observed between electrodeposited and sputtered undoped ZnO, having 6.9x 10⁶ and 6.2x 10⁷ Ω cm. The electrical resistivity of ZnO: AI was 3.4 x l 0⁵ and 2.3 x 10⁵ Ω cm for electrodeposited and sputtered materials respectively. Further work would need to be carried out to quantify the concentration of Al dopant in the electrodeposition solution as a function of growth time if this method were to be used for commercialisation. Cadmium telluride (CdTe) was electrodeposited from aqueous solution onto glass/fluorine doped tin oxide/cadmium sulphide substrates. Little improvement in XRD spectra was observed for annealed layers compared to the as-deposited material and the CdTe was identified exhibited cubic phase having (111) preferential orientation. Working solar cell devices were fabricated over a range of growth voltages with superior performance being observed for materials deposited between -0.620 to -0.650 V vs saturated calomel electrode (SCE). Furthermore high uniformity over a2 cm² area completed with an array of 2 mm diameter contacts was observed for devices deposited in this growth voltage range. All devices fabricated using CdTe grown at -0.610 to -0.690 V vs SCE indicated photovoltaic activity although layers deposited between -0.630 and -0.650 V vs SCE indicated the highest performance, with device parameters of open circuit voltage = 420-540 mV, short circuit current density = 3.2-19.1 mA cm^-2 and fill factor = 0.48.

Spatially resolved measurements of CdTe thin film photovoltaic solar cells were performed using both laser beam induced current (LBIC) and scanning probe microscopy (SPM) techniques. The triple wavelength LBIC system was used to assess the thickness uniformity of Cd1-xZnxS window layers incorporated into CdTe solar cells. A blue laser was used to reveal window ~r • layer absorption and transmission characteristics. This was observed to influence . photoresponse at longer wavelengths where lateral variations in minority carrier lifetime were leading to variable carrier collection. This was found to be caused by localised regions of ~ 50 run thin Cd1-xZnxS forming a defective depletion region. The moderate to high clustering of pin-holes in both thick and thin regions of Cd1-xZnxS and CdTe layers was found to contribute to shunt resistance losses independently of the Cd1-xZnxS thickness distribution. Quantum dot (QD) luminescent down shifting layers incorporated into Cd1_xZnxS/CdTe devices were studied using the LBIC technique where, using a 405 run excitation wavelength, QD isotropic emission was observed to increase the overall lateral carrier collection area of : the cell. Scanning Kelvin probe microscopy (SKPM) was used to study the Fermi level shift in Arsenic doped CdTe devices where the contact potential-difference (CPD) between probe tip and sample surface revealed that increasing As concentrations in CdTe led to a decrease in CPD. This highlighted a downward shift in the CdTe Fermi level and an increase in CdTe work function. Absolute CdTe work function values between 3.88 and 4.09 eV were calculated using a highly oriented pyrolytic graphite reference sample. A localised shift in CPD at grains boundaries with increased As doping was observed. This was proposed to reduce carrier recombination by channelling minority carriers away from the grain boundary. Conductive atomic force microscopy revealed differences in bulk grain and grain boundary conductivity. The localised CdTe Er and the barrier formed at the tip/surface interface was observed to determine the measured current.

No description available.

In an attempt to find the optimum number of wells for maximum conversion efficiency a pair of otherwise identical strain balanced samples, one containing 50 wells and the other 65 wells have been characterised. The 65 well sample is found to possess a lower predicted efficiency than the 50 well sample, suggesting that the optimum well number lies between these values. Devices grown using tertiary butyl arsine (TBAs) are found to possess comparable conversion efficiencies to the control cells grown using arsine and slightly superior dark IV characteristics, indicating that TBAs may be substituted for arsine without loss of device efficiency and may even be beneficial to cell performance. Several fundamental refinements to the existing quantum efficiency model of are explored. Firstly, expressions for the strained band gaps are derived. A value for the conduction band offset is determined using the difference in energy between the heavy and light hole exciton peaks in low temperature photo current scans and found to be 0.55±0.03. The magnitude of the el-hhl exciton binding energy is also estimated from these scans and found to be in excellent agreement with the value obtained from a simple, parameterized expression for the exciton binding energy. Finally, an absolute calculation for the absorption coefficient is incorporated into the quantum efficiency model and values for the heavy and light hole in-planes masses are obtained. The model is found to underestimate the level of absorption in the intrinsic region by an amount consistent with estimates of the magnitude of the reflection from the back surface. The conversion efficiency of a sample predicted using SOL is compared to an independently obtained value. Good agreement is observed between the two results (25.3% and 25.7% for 317 suns AM1.5D). Additionally, an optimum structure for illumination by the AM1.5D spectrum was found to be a 120A well ofIno.lGaAs.

There is increasing evidence that the initially generated excited state species in bulk heterojunction solar cell photoactive layers are critical to device performance. At present however, an understanding of the nature and dynamics of such excited states still remains limited. This thesis presents a study of the ultrafast exciton dynamics in bulk heterojunction organic and hybrid organic-inorganic solar cells. Fluorescence upconversion is used to elucidate the dynamics of such transient species allowing internal properties of the blend systems to be probed including changes in film morphology and ultrafast energy loss mechanisms. An understanding of such processes is an important step forward in the evolution of molecular semiconductor based solar cells. The first chapter focuses on the main experimental technique, fluorescence upconversion, and how this can be employed to study excited states. In particular, this section addresses one of the main unanswered questions in the field and attempts to correlate the exciton dynamics with the structure of the common photoactive polymer poly(3-hexylthiophene) (P3HT). Three structural variations of P3HT are studied and their exciton dynamics associated with differing internal processes occurring within the polymers. These include self localisation, and different types of long-range energy transfer mechanisms. The following two chapters build upon the knowledge of exciton dynamics obtained from the first chapter. First, a study is made of amorphous polymers with different acceptors, all based on phenyl-C61-butyric acid methyl ester (PCBM). The distinct interactions of the PCBM-type molecules with the polymer results in different electron transfer dynamics, from which the exciton diffusion length of the polymer in real bulk heterojunction blends is extracted using a simple model. Second, the ultrafast excited state dynamics of a crystalline polymer with the same PCBM-type acceptors is studied. Correlation of these dynamics with thermal analysis of the blend films allows the morphology of the films to be extracted and allows two different mechanisms of microstructure development to be identified. In the final chapter, the effect of acceptor aggregation on exciton dynamics and charge generation yields in hybrid organic-inorganic blend films has been studied. Such aggregation has been shown to be essential for efficient charge generation in all-organic solar cells but has often been assumed to be less important in such inorganic hybrids. More aggregated acceptor nanoparticles are shown to not only result in greater than expected exciton quenching but are also shown to result in a greater yield of long-lived charges. This study is extended to show that in-situ grown inorganic nanoparticles exhibit superior performance to standard pre-synthesised inorganics.

Organic photovoltaic devices are receiving extensive interest, with device efficiencies now exceeding 8%. There is increasing evidence that the efficiency of dissociation of excitons (bound electron-hole pairs) into free charge carriers is a key factor determining device performance. Dissociation of these excitons occurs at the interface between donor and acceptor molecules in the photoactive layer of the device and is driven by a favourable difference in electronic energy levels between the two materials. Several factors can potentially determine the efficiency of this process, including interfacial energetic energies, molecular structure, film microstructure and device electric fields. This thesis employs optical spectroscopic techniques, including photoluminescence quenching and transient absorption spectroscopy to assay the efficiency of charge separation for a range of donor / acceptor blend films and devices, thereby providing new insights into the factors determining the efficiency of this process. The first results chapter focuses on the experimental technique, transient absorption spectroscopy, and how this can be employed to determine the yield of dissociated charges in donor / acceptor blend films. Three materials systems are studied, firstly looking at the effects of structure of the donor material, followed by a study into the effects of the temperature on charge generation and recombination. The following two chapters investigate the effects of film morphology on charge generation, specifically focusing upon the fullerene acceptor. Aggregation of phenyl-C61-butyric acid methyl ester (PCBM) is shown to be a key factor in the generation of free charges in the blend film, leading to proposal of a model regarding the role of this aggregation in charge generation. This study has then been extended by using additives to modify the concentration threshold for PCBM aggregation by preventing the intercalation of the PCBM between the polymer side chains, and hence inducing PCBM aggregation at a lower concentration. In the final chapter, the effects of an externally applied electric field on charge generation in devices have been studied. Such electric fields have been proposed to reduce geminate recombination losses, thereby increasing the dissociation of free charges, and thus leading to an increase in the device photocurrent. Polymer and small molecule blend systems have been studied and shown to exhibit different dependences of charge generation upon applied bias depending on the donor molecule and the morphology of the blend.