This thesis presents a study of photo-active, semiconducting block copolymers for use in molecular solar cells. Current state-of-the-art organic devices utilise blends of two (or more) materials that are co-deposited from a common solution; the resulting structures formed are determined by material properties and deposition conditions, but often result in configurations that are detrimental to device performance. An answer to this problem comes in the form of the block copolymer; using these materials, devices can be formed from a single material active layer. In addition, the counterbalance of forces within films of block copolymer can lead to nano-scale self-assembly that allows for a strong degree of control over layer equilibrium morphology. Such control will be an important step forward in the evolution of molecular solar cells. The main body of this work is concerned with the study of the photo-physics of photo-conductive block copolymers, especially the generation of free charge. First, an investigation is made into the inherent structure-function relationship in block copolymers. A varying chain length is seen to drastically affect the photoluminescence quenching and yield of long-lived charges. Photovoltaic devices made using these materials show a peak efficiency of 0.11% and correlate with the spectroscopic results, subject to a trade off between charge generation and transport/collection. In a second investigation, the effects of post-fabrication annealing on block copolymer films are considered; studies on annealed samples lead to the conclusion that domain crystallinity is a significant factor in determining the yields of long-lived charge carriers. It is found that these yields are comparable with those of a standard blend (that achieve 75% photon to electron conversion efficiency). Annealing leads to increases in photovoltaic device performance over unannealed samples, although additional control over active layer morphology is necessary for these materials to attain their potential. Following this, a comparative study is made between a block copolymer and a similarly composed blend formed from well studied polyfluorene copolymers. Further advantages of block copolymers are highlighted, including the stability of morphologies generated under different deposition conditions. Finally, a novel tool set is introduced using a block copolymer sample to emphasise the experiments potential with regard to studying interfacial photophysical effects.

Organic devices based on polymer:fullerene blend films are attracting extensiveinterest as low cost solar cells, with power conversion efficiencies over 5%.Improvements in performance are dependent on developing a better understanding ofthe pertinent loss processes. This in turn requires the ability to reliably determinecharge densities (n) and carrier lifetimes (?n) in real devices under standard operatingconditions. In this thesis, we address the recombination dynamics in organic solarcells based on blends of poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM), P3HT:PCBM devices, one of the bestdevices to date, using both experimental and modelling studies. Initially, a drift-diffusion model was used to study the basic principles of solar celloperation, with particular focus on investigating the ?corrected photocurrent?, wherethe effects of dark injection are removed. We then have employed a series ofexperimental techniques ? including transient photovoltage and photocurrent,transient absorption spectroscopy and charge extraction ? to determine the carrierlifetimes and charge densities in standard annealed P3HT:PCBM devices underoperation. The results of our studies for a device under open-circuit conditions showthat the open-circuit voltage (Voc) is primarily governed by a trap dependentbimolecular recombination process. By applying charge extraction studies on devicesunder forward bias in the dark, we show that the dark current is also governed by thesame trap dependent bimolecular recombination mechanism which determines Voc. Based on the understanding of charge carrier dynamics at Voc and the forward biasdark current, a simple model has been developed to simulate ?light? current-voltage(J-V) curves. Despite the simplicity of this model, remarkably good agreement wasobserved with experimental J-V data.

Semiconductor quantum dots (also referred to as 'nanocrystals‘) are well suited as light-harvesting agents in solar cells because they are robust, have tuneable effective band gaps, and are easy to process. The research presented in this thesis is targeted towards the study of excitonic solar cells employing semiconductor nanocrystals as a light harvesting component. Gaining control of the interfacial charge transfer processes in operation in these devices forms a crucial part of any attempt to optimise their performance. In particular, the use of transient spectroscopic techniques reveals how efficient and long-lived charge separation can be achieved in these solar cell architectures. The primary focus of this research is to investigate the parameters influencing charge transfer in dye-sensitised solar cells (DSSCs) using colloidal quantum dots as light-absorbers. One aim is to study the impact of varying the thermodynamic driving forces provided for interfacial electron transfer on the yield of both the electron injection and hole regeneration reactions occurring within the DSSC; this can be achieved by varying the energetics of each component of the system (metal oxide, quantum dot and hole conductor) in turn. In addition, the interfacial morphology can be modulated by changing the passivating ligands present at the QD surface, and by modifying the structure of the redox mediator (or hole conductor). In doing so, we also attempt to improve our understanding of how charge carrier trapping in quantum dots impacts upon solar cell performance. Furthermore, new strategies towards solar cell design are presented, which show great potential as a result of their favourable photophysical properties. One of these approaches (presented in the final chapter) is to effect the in situ growth of CdS nanocrystals in a conducting polymer, a method which circumvents many of the processing issues associated with the use of nanocrystals in polymer blend solar cell architectures. It is hoped that the work presented in this thesis is used to develop design rules for the construction of semiconductor nanocrystal-based excitonic solar cells. By identifying which key parameters control the rates and yields of electron transfer at the nanocrystal interface, improvements in device efficiency can be realised. It is believed that these studies fill an important gap in our current understanding, and highlight some of the potential benefits and shortcomings of using semiconductor nanocrystals in cheap, solution-processed solar cells.

Polymer-small molecule blend films are of increasing interest in the field of organic solar cells. This thesis employs transient absorption spectroscopy as a mean to study charge photogeneration in these blend films. These studies allow identifying and addressing the charge photogeneration efficiency-limiting processes in polymer:perylene diimide organic solar cells. We approach the question by considering the influence of nanomorphology and phase segregation on charge photogeneration and recombination dynamics. We further report on yield of charge separated species in polythiophene / perylene diimide blend films as a function of electron acceptor’s energy levels. We find that, compared to polythiophene / PCBM blend films, charge photogeneration is significantly enhanced. Correlations between free energy for charge dissociation and charge photogeneration yield within different polymer:acceptor series are observed and indicate the generality of this relationship. Furthermore, the energetic model proposed to account for these results is consistent with the well-established Onsager and Marcus theories. It can therefore be concluded that the yield of photogenerated charges in polymer/acceptor systems is likely to be dependent upon the excess vibrational energy of the bound radical pair, such that the key kinetic competition in the photogeneration process is between vibrational relaxation and dissociation of this species Simultaneously, we investigate photoinduced charge separation in solid films of two perylene diimides and a donor-bridge-acceptor (D-B-A)molecule, exTTF-pcp-C60 relative to solution. First we find Intramolecular charge separation and recombination is correlated with a reduction in the yield of long-lived, intermolecular charge-separated species in the perylene diimide dyad. In the D-B-A system we observe the exTTF-pcp-C60 motif in this case leads to more charges than the reference compounds or a mixture of them but that the excited state of the electron acceptor, the fullerene, suffers from concentration self-quenching which severely affect the charge yield in solid films.

In this thesis, the dynamics and quantum yields of electron injection occurring in liquid and solid state dye sensitised solar cells (DSSCs) based on titanium dioxide (TiO2) anodes sensitised with Ru – polypyridyl or organic dyes have been measured. The electron injection process is investigated through both experimental and modelling studies. A transient emission technique based on time correlated single photon counting (TCSPC) has been developed to measure the kinetics and yields of injection occurring in both films and devices. Other processes occurring in the device are probed using a range of experimental techniques, including transient absorption spectroscopy and transient photovoltage. Initially the principles of the TCSPC measurement technique are introduced and the procedure for measuring the injection in samples is outlined. Comparison of appropriate control sample measurements, which show transient emission decay dynamics in the absence of electron injection, with the TiO2 sample traces enables the quantification of injection occurring in each experimental sample. TCSPC emission decays associated with each sample are then fitted using stretch exponential functions constrained by two degrees of freedom. This TCSPC technique for measuring electron injection dynamics is validated by showing agreement with previously published kinetics for an analogous system as measured by a well established ultrafast transient absorption technique. The fits to the TCSPC decay dynamics are also shown to be accurately replicated by Monte Carlo integrations based on a previously published model of the active dye / TiO2 interface in the DSSCs. The technique is extended to probing DSSCs employing a range of different sensitisers and measuring the kinetics under different operating conditions occurring within the DSSCs where injection is found to only depend strongly on the concentration of potential determining additives. The first results chapter describes the TCSPC technique and gives examples of the data analysis procedures associated with each transient emission decay measurement. The agreement between injection kinetics measured using TCSPC with those measured using ultrafast transient absorption technique is highlighted. The model of 5 the active dye / TiO2 DSSC interface is introduced and Monte Carlo integrations based on this physical model are shown to agree well with the experimental data. The second results chapter extends the measurement of injection kinetics to different Ru – polypyridyl based sensitisers. Injection kinetics are measured for a structure – function dye series and the observed variations in the kinetics and yields are explained with reference to the dye / TiO2 interface. The measurements are extended to completely solid state DSSCs and successful fitting of the TCSPC data with integrations based on the physical model show dispersive injection kinetics observed in solid state DSSCs are controlled by the same parameters as the liquid cells. The third chapter looks at a variety of factors which may affect injection in complete, operating DSSCs. The factors addressed include presence of the commonly used iodide / triiodide redox couple, residual effects of acid versus base film synthesis procedures, effect of increasing the Fermi level in the DSSC and changing the concentration of potential determining ions in the redox electrolyte. The major controlling factor is found to be the concentration of the potential determining, commonly used tert – butyl pyridine device additive and implications of this on DSSC performance are discussed. The last chapter compares device parameters for DSSCs based on successful organic sensitiser with DSSCs based on the commonly used Ru – polypyridyl N719. Features which control the performance of organic dyes in general are outlined and the reduced performance of DSSCs employing these dyes is explained.

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