All group PhD theses can be found in Spiral.
The stability of perovskite and organic solar cells remains a major challenge to realise their potential. To overcome the numerous degradation mechanisms, it is important to understand the underlying causes. This thesis will present device and material characterisation on two areas: firstly, elucidating perovskite environmental stability and, secondly, the effects of PCBM dimerisation in organic solar cells. The first results chapter investigates the environmental stability of methylammonium lead iodide. In this thesis, degradation under light and oxygen is found to be more detrimental than humidity. Injected charge (not photogenerated) also induce the degradation with oxygen; thus, all optoelectronic applications are vulnerable. Efficient electron extraction minimises superoxide generation and subsequent degradation. These results determine the dominant environmental instability and present pathways to curtail it. The second results chapter explores halide tuning of methylammonium lead iodide-bromide (MAPI-Br) to optimise environmental stability. The stability of thin films and solar cells for all halide ratios is investigated under light and oxygen stress and humid nitrogen stress. Partial bromide substitution does not improve stability. However, for MAPBr, the crystal structure and optical properties are unchanged after 120 h of light and oxygen stress. It is concluded that iodide limits mixed halide stability and MAPBr has excellent environmental stability for high voltage solar cell applications. The third results chapter investigates the morphological stability of polystyrene:PCBM blends under simultaneous light and thermal stress. Neutron reflectivity and atomic force microscopy are used to probe the vertical stratification and topography, respectively. It is found that PCBM dimerisation inhibits both surface relaxation and PCBM enrichment to the bottom PEDOT:PSS interface. Modelling the diffusion shows PCBM dimers to be effectively immobile over the timescales investigated. These results present a framework to understand polymer:fullerene morphological stability under operating conditions. The final results chapter develops a complete model to describe PCBM dimerisation. Predicting the photo-dimerisation and thermal decomposition within any polymer matrix, under any light and temperature profile. During operating conditions, the PCBM dimer will dominate the reaction equilibrium; therefore, clarifying the role of dimers is critical. Evidence of distinct dimer populations is observed with differing chemical and electronic properties. DFT simulations conclude this is due to PCBM dimer conformations. These results can rationalise the double-edged effects of PCBM dimerisation previously reported.
Camilo A. Mesa
In this thesis, the key factors governing the solar fuels catalytic process, i.e. photoelectrochemical water splitting, are studied by means of different spectroelectrochemical techniques. Photo-induced absorption spectroscopy together with transient photocurrent measurements are employed to study the catalytic function of different metal oxide photoanodes, focusing particularly on hematite, by the construction of rate law analyses. An introduction to the field is given in Chapter 1 followed by a description of the methods used herein in Chapter 2. Chapter 3, the first results chapter, focuses on the kinetics of water oxidation on hematite photoanodes at conditions of suppressed surface recombination. This chapter provides evidence of 2 different water oxidation mechanisms, with different activation energies, upon increasing the irradiation intensity. These mechanisms show faster kinetics upon surface deprotonation of the photoanode and a secondary kinetic isotope effect. In this chapter, a detailed water oxidation mechanism at a molecular level for hematite photoanodes is proposed and compared with biological systems such photosystem II. Further comparisons with other metal oxides such titania, bismuth vanadate and tungsten oxide indicate similar mechanisms whose kinetics are sensitive to the valence band edge of the materials. Chapter 4 explores the use of methanol oxidation on hematite, as a model organic substrate reaction, to improve the hydrogen production rate. This is proposed to be done concomitant with either, the degradation of organic pollutants or the synthesis of high value-added products when these reactions are selective. This chapter shows the selective oxidation of methanol to formaldehyde with kinetics faster than those for water oxidation and sensitive to the valence band edge when the reaction was performed on titania. The data collected allow the proposal of a detailed mechanism of reaction for the selective oxidation of methanol to formaldehyde. Finally, Chapter 5 focuses on the decay kinetics of the photogenerated holes in state-of-the-art thin, flat hematite films. This chapter discusses the recombination, accumulation and reaction dynamics of photogenerated holes upon laser excitation. Upon hole accumulation at the photoanode surface, this chapter provides evidence of a kinetic competition between surface recombination and trapping and hole extraction by the electrolyte.
Photocatalytic and photoelectrochemical water splitting processes remain hindered by fast recombination of photogenerated electrons and holes. Ferroelectric materials are increasingly being considered to address this issue; their internal electric fields have been shown to spatially separate electrons and holes, and thus should greatly reduce recombination rates. A kinetic understanding of the extent to which electron–hole recombination can be slowed in ferroelectric materials is essential to ascertain if they can play a significant role in achieving higher solar-driven water splitting efficiencies. The focus of this thesis is an experimental investigation of charge carrier dynamics in barium titanate (BaTiO3) to observe the effect of internal electric fields on recombination rates. Time-resolved spectroscopic techniques were used in conjunction with photocatalysis studies to determine whether ferroelectricity can significantly reduce recombination rates and lead to enhanced performance. It is found that, although the transient absorption spectrum of ferroelectric BaTiO3 is similar to previously reported metal oxides, the carrier lifetimes are significantly longer, indicating the potential for ferroelectrics to be used in devices limited by fast electron–hole recombination. In the first results chapter, the transient absorption spectrum of single crystal BaTiO3 is characterised under inert atmosphere over two timescales: femtosecond–nanosecond and microsecond–second. Absorption signals due to photogenerated holes and electrons are identified using electron and hole scavengers, respectively. Comparisons are drawn between BaTiO3 and other single crystal, but non-ferroelectric, metal oxides. It is found that, on timescales relevant for water oxidation, lifetimes in BaTiO3 are at least an order of magnitude longer. In the second results chapter, the origin of long carrier lifetimes in ferroelectric BaTiO3 is explored. When the polarisation is switched off by both temperature and nanostructuring, carrier lifetimes decrease by four orders of magnitude. Recombination rates in BaTiO3 exhibit a much stronger temperature dependence than other metal oxides, which is rationalised by considering the temperature dependence of the spontaneous polarisation. The third results chapter investigates the photocatalytic performance of BaTiO3 nanopowders. It is found that, in the presence of an electron scavenger, BaTiO3 photogenerated holes are reactive and can oxidise water to produce oxygen. Transient and photoinduced absorption spectroscopies indicated that hole accumulation in a BaTiO3 sample with a higher tetragonal (ferroelectric) content, which translates to higher rates of oxygen evolution. The final results chapter probes the influence of a ferroelectric BaTiO3 substrate on α-Fe2O3 thin films. Preliminary data suggests the internal field can penetrate through the film and slow electron–hole recombination rates in α-Fe2O3.
This thesis focuses on the factors determining the photocurrent generation efficiency of organic solar cells utilizing non-fullerene acceptors as opposed to the generally employed PCBM. This work explicates the photophysical properties of organic blends and devices incorporating such acceptors with transient absorption spectroscopy to understand their charge dynamics impact upon device performance and strategies to inhibit recombination detrimental to the photocurrent generation of such devices. The first results chapter introduces transient kinetic studies of a couple of promising blends with rhodanine based non-fullerene acceptors. These results show that the photocurrent generation in these acceptorblends is limited primarily by geminate recombination losses following efficient exciton separation and the losses are assigned to the difference in the nanomorphology of the blends. The next chapter reports an approach to suppress these losses in the device with non-fullerene acceptors: 1) An energetic offset increase is shown to suppress geminate recombination in blends with barbiturate acceptors, leading to a maximum EQE of ~84%. The third results chapter demonstrates a trade-off using a polymeric acceptor, N2200 which exhibits superior device thermal stability to PCBM based devices due to aggregation of PCBM upon thermal stress, but its initial device performance is limited by a geminate recombination and exciton decay to ground. The forth results chapter reports a comparison of charge generation for a series of non- fullerene acceptors blended with P3HT and PTB7-Th donors. A formation of an intermediate state prior to free dissociated charges is demonstrated to be bound polaron pair state and a high yield of such state is originated from the amorphous blend system. Geminate recombination losses suffered in the blend systems are suppressed by increasing the energy offset which in turn leads to an efficient photocurrent generation. The dissociation efficiency of bound-polaron pairs is shown to correlate with photocurrent generation efficiency. The last chapter provides concluding remarks and recommendations for further work.
The focus of this thesis is an experimental study of how material energetics and non-geminate recombination dynamics combine to define device performance, in two solution processed solar cell technologies; organic and perovskite photovoltaics. The work utilises transient optoelectronic measurements to elucidate the impact of non-geminate recombination losses on open-circuit voltage. The importance of both bulk recombination at the internal active layer heterojunction and surface recombination at the electrode interfaces is considered, particularly in high voltage systems. The first experimental chapter concerns the impact of blend morphology on recombination in OPV. Transient optoelectronic techniques are used as an in situ probe of energetics and recombination kinetics for different morphologies. The impact of polymer:fullerene blend ratio is studied for a range of donor polymers pBTTT, DPPTT-T and PBDTTT-C-T; while the increase in efficiency, despite a decrease in voltage with the use of the solvent additive DIO is studied for the PBDTTT-C-T system. The second experimental chapter reports a study of recombination in OPV blends utilising the recently developed rhodanine flanked non-fullerene acceptor family, particularly FBR. When blended with high performing polymers Pff4TBT-2OD and Pff4TBT-2DT, these acceptors demonstrate unusually high open-circuit voltages (> 1V) compared to PCBM while maintaining high current. If field-dependent geminate recombination, present with FBR but not with PCBM, can be reduced, NFA efficiencies beyond 10 % are achievable. In the final two experimental chapters, the methodology is extended to the new field of perovskite photovoltaics. The importance of charge accumulation and recombination in determining the voltage is investigated. A simple kinetic model allows the voltage behaviour to be reconstructed over a range of operational light intensities. Finally, the origin of voltage is investigated further by varying the halide composition and interlayer choice which impacts both energetics and recombination.
Substantial progress has been made in achieving increasingly high organic photovoltaics (OPV) power conversion efficiencies. Progress has largely been derived from the development of new donor materials, as until recently a notable scarcity of successful electron acceptor materials remained. [6,6]-phenyl C60 butyric acid methyl ester (PC60BM) or other fullerene derivatives have been the centrepiece of the accepting materials used throughout OPV research, and they have only recently been accompanied by rare examples of novel non-fullerene alternatives. However, the current understanding of the properties of effective electron acceptors for OPV is still relatively limited. In this thesis, attention is paid to the photophysical properties of accepting materials with respect to charge photogeneration and recombination. The work is comprised of three closely linked studies. The first is a detailed exploration of the photophysical properties expressed by PCxBM. This is followed by a consideration of a newly synthesised PDI derivative, tasked with specific design rules to address prior limitations identified by previous perylene diimide (PDI) acceptors. The third study entails a broad comparison of a selection of accepting materials utilising bilayer fabrication to remove morphological variation. Chapter 3 outlines detailed understandings of the photophysical properties of PCxBM excitons. In particular, exciton characteristics are reported as a function of film morphology. Transient absorption measurements show a significant reduction in intersystem crossing and strong shifts in excited state absorption spectra in neat PCxBM films when compared to dispersed molecules within an amorphous polystyrene system. Additionally, neat PCxBM films show an increased formation of free charges when compared to the dispersed systems. An alternative model of excitonic formation from PCxBM systems between aggregated and dispersed systems is proposed. Chapter 4 introduces a newly synthesised amorphous perylene, PDISO, that is designed to remove the typical problem of aggregation in blends. Using a range of spectroscopic and microscopy techniques, the progression of absorption to charge formation is characterised for PCDTBT:PDISO blend films, drawing a direct comparison to the same processes in PCDTBT:PC70BM blend films. In this way, PDISO is demonstrated to overcome key loss mechanisms previously reported to limit efficiencies in PDI blends and determines the optimum blend ratio for charge generation with PCDTBT. Chapters 5 and 6 jointly establish an understanding of solution-processed bilayers, which is followed by a comparison across several acceptors with the two polymers PCDTBT & PBDTTT-CT (chosen for their ability to generate charges). The comparison demonstrates that recombination dynamics fundamentally illustrate dramatic differences between acceptors. This provides evidence that morphologically independent charge dynamics can significantly affect overall device performance post separation. Correlations among the bilayer device performance, acceptor dielectric-mobility product and recombination dynamics are demonstrated. The outcomes in this thesis together draw a complex picture of multiple factors that affect the performance of electron-accepting materials in OPV. This provides a suitable platform for identifying important parameters when designing and testing new accepting materials. It also highlights potentially critical gaps in the current experimental understanding of fundamental charge interaction and recombination dynamics.
Ernest Pastor Hernandez
In this thesis, optical and electrochemical techniques are used to study the factors controlling catalytic function in solar-to-fuel conversion systems. Chapter 3, the first results chapter, considers a system for CO2 reduction based on a Re photocatalyst anchored to TiO2. This chapter reports evidence that the immobilisation of the photocatalyst via covalent bonds improves the stability of one of the key reaction intermediates resulting in higher catalytic yields. This chapter also provides insight into the nature and timescale of the steps in the mechanism of CO2 reduction. Chapter 4 considers a proton reduction system based on a Ru absorber, a Ni electrocatalyst and a sacrificial electron donor. This chapter discusses the mechanism behind the strong pH dependence in this system. The results show that whereas the electron transfer between the dye anions and the electrocatalyst is pH independent, the generation of dye anions and the catalytic function of the electrocatalyts have opposite pH-dependencies. Chapter 5 considers a photocathode for proton reduction based on a Cu2O/Al:ZnO buried p-n junction with protection and catalyst layers. The results presented show that the buried junction controls charge separation and the photocurrent onset. Furthermore, the catalyst layer is found to slow down charge recombination and help achieve high reduction yields. This chapter also discuses the mechanism of proton reduction and how the nature of the rate-limiting step has an impact in the recombination kinetics. Chapter 6 discusses the use of transient absorption spectroscopy to study high refractive index materials with high quality interfaces. This chapter investigates light interference effects in TiO2, Cu2O and a CH3NH3PbI3 perovskite device. The results show that interference effects in these materials can dominate their transient spectra, hindering its interpretation. However, it is found that this spectroscopy can also be used to extract information about the changes of the refractive index.
Elisa Collado Fregoso
This thesis addresses charge separation and charge recombination in different, mainly low bandgap, polymer/fullerene blend films and their relation to device performance. Charge separation and recombination was studied as a function of variables including the difference in the LUMO levels of the polymer and fullerene, the polymer/fullerene blend ratio, the presence of a fluorine atom on the polymer backbone and the use of a bulky fullerene acceptor (Indene-C60-trisadduct, ICTA). A key focus of the thesis is on the impact of film microstructural differences upon charge generation and recombination kinetics. Charge generation and recombination was studied via transient absorption spectroscopy (TAS) with time resolutions from femtoseconds to microseconds. In Chapter 1, an introduction to the field is presented. Basic concepts of polymer solar cells and the steps of light-to-electrical energy conversion are included. The chapter focuses on the current discussions on charge generation, separation and recombination and their relationships with other parameters such as material energetics and morphology. In Chapter 2, the experimental methodologies are presented. A description of the materials used, the techniques used to prepare the samples, and the mainly optical techniques used to study them: steady state photoluminescence (PL), TAS (fs and microsecond), X-ray diffraction (XRD) and device characterization. Chapter 3 to 6 present the results of each project. In Chapter 3, the role of the driving energy for charge separation (DEcs) for low bandgap DPP-based polymers is addressed. A us-TAS characterization of DPP-based polymer/fullerene blends is presented, and the yield of charges correlated with the experimentally obtained DEcs. The correlation was then extended to other low-bandgap polymers and the trend compared with that obtained for larger bandgap polymers. Chapter 4 deals with the effect of DPPTT-T/PC70BM blend ratio upon the film photophysics. With PL quenching and fs-TAS studies, it is demonstrated that the limitations in the performance of DPPTT-T polymer mainly come from an incomplete exciton quenching for all the compositions studied. The study is in agreement with morphology probes including transmission electron and atomic force microscopies, as well as with the changes in crystallinity, as observed by XRD. Chapter 5 deals with the effect of polymer backbone fluorination on a low-bandgap polymer (PGeDTBT). PL quenching and fs to us TAS data is presented and correlated with structural analyses and theoretical calculations to compare the properties of non-fluorinated and fluorinated version of the polymers. It was found that charge generation seems to be equally efficient, despite the lower driving energy for charge separation (DEcs) in the fluorinated polymer. A four-fold slowing down in non-geminate recombination was also observed upon fluorination, correlated with a larger polymer tendency to aggregate, thus demonstrating its multiple effects on material properties and photovoltaic behaviour. Chapter 6 deals with the effect of mixed and “flatter” interfaces upon charge separation. XRD data are presented to show the contrast in intercalation between the polymer and the acceptors (PC70BM and ICTA). These results are correlated with fs-TAS data to show the change in the regime of recombination: while the highly intercalated blends present a high predominance of geminate recombination, the blends with ICTA predominantly present non-geminate recombination. Finally in Chapter 7, the conclusions of every chapter are summarized. A general discussion on the relationship of the conclusions is given and the areas where further research is needed are discussed.
This thesis described an investigation of charge carrier dynamics in dense, flat bismuth vanadate (BiVO4) photoanodes using transient absorption spectroscopy and photoelectrochemical measurements including transient photocurrents. Transient absorption spectroscopy was employed to probe directly the photogenerated charge carrier population change as a function of time from microsecond (μs) to second (s) timescales. Transient photocurrent measurements were used to monitor charge extraction under chopped light conditions. Photo-induced absorption spectroscopy was employed to investigate charge carriers under working photo-electrochemical (PEC) conditions. The transient absorption signals due to photogenerated holes in BiVO4 were determined through using electron/hole scavengers and applied electrical bias in a complete photoelectrochemical cell. In ‘un-doped’ BiVO4, photogenerated holes were found to absorb from 500 nm to 900 nm. The dynamics of photogenerated holes were studied as a function of applied potential and excitation intensity. The population of long-lived (milliseconds–seconds) holes increased with increasing the width of space charge layer as a function of applied potential. A recombination process in kinetic competition with water oxidation on these long timescales was found to limit the photocurrent amplitude and onset potential in un-doped BiVO4 photoanodes. Using transient photocurrent measurements, this recombination process was identified as recombination of surface-accumulated holes with electrons from the bulk of the semiconductor (back electron/hole recombination). Doping molybdenum (MoVI) in un-doped BiVO4 has been reported to be an effective method to increase photocurrent amplitude. Impedance measurements were carried out to determine the donor density increased by the presence of MoVI doping. The increased donor density limited efficient generation of the space charge layer to retard fast recombination on microseconds to milliseconds timescales, thus limiting the long-lived hole yield under modest applied potentials. MoVI dopants were shown to improve the electron transport determined by front/back side illumination in PEC and transient absorption spectroscopy (TAS) measurements. Cobalt phosphate (CoPi) surface-modified un-doped BiVO4 photoanodes were also studied using transient absorption spectroscopy and transient photocurrent measurements. Transient absorption spectra of CoPi-modified BiVO4 were similar to those of unmodified BiVO4, and the kinetics on milliseconds to seconds did not change in the presence of CoPi surface modification. Both results indicated that photogenerated holes in BiVO4 rather than CoPi species were monitored by transient absorption spectroscopy. However, the negative shift of photocurrent onset and increased photocurrent could be explained by efficient suppression of back electron/hole recombination in BiVO4 photoanodes. In terms of the function of CoPi water oxidation catalysts on BiVO4, photo-induced absorption (PIA) was employed to further study the CoPi/BiVO4 system. CoPi species oxidised by BiVO4 holes were observed in PIA measurements due to the high extinction coefficient of oxidised CoPi and significant hole accumulation generated by continuous illumination. However, these oxidised CoPi species did not appear to drive catalytic water oxidation, as evidenced by results from spectroelectrochemical measurements of CoPi/FTO electrodes; water oxidation still occurred via BiVO4, consistent with the transient absorption results. Therefore, I concluded that in the CoPi/BiVO4 system, CoPi did not act as a catalyst, although hole transfer to CoPi can take place.
George F A Dibbs
The efficiency of an organic photovoltaic (OPV) device is determined by the shape of its current-voltage (J-V) curve. Previous studies showed that the J-V curve for the best-studied OPV system, P3HT:PCBM, is determined mainly by non-geminate recombination where free electrons and holes recombine before being collected, thereby reducing output current. In this thesis we study non-geminate recombination experimentally, as well as charge collection efficiency and rates of geminate recombination, in several polymer:fullerene material systems. The aim is to determine limits to the performance of OPV devices and to quantify the recombination losses. The first two experimental chapters investigate P3HT:PCBM devices. The first presents an analysis of non-geminate recombination, and the application of temperature dependent measurements and new transient techniques to probe the energetic distribution of trap states within the semiconductor. The second reconciles two apparently contradictory experimental results, namely, the highly non-linear dependence of non-geminate recombination rate upon charge density and the linear dependence of corrected photocurrent on light intensity. In the third experimental chapter of this thesis both geminate and non-geminate recombination processes are studied and quantified in several material systems. Specifically we study the extent to which the two recombination mechanisms can impact upon the generation, collection and recombination of charges in the devices and we relate this directly to the fill-factor of the devices. In the final experimental chapter, we study the effect of electronic doping space-charge accumulation upon the electrostatics of a device and hence on non-geminate recombination and charge collection. Through optical and electronic modelling we show that the doping of the photovoltaic active layer causes the accumulation of space-charge, which in turn alters the electric field within the solar cell, reducing the electric field driving collection of the minority carriers and consequently reducing charge collection.
Zheng Gang Huang
Polymer:fullerene blend microstructure has been recognised as a key, but poorly understood, factor in optimising performance of polymer:fullerene based solar cells. This thesis focuses on investigating the impact of material chemical structure upon the blend microstructure, and thereby on device efficiency and stability for polymer:fullerene solar cells. It will be shown in the first results chapter that polymer fluorination can promote phase segregation in polymer:fullerene blend, limiting its charge generation as demonstrated using transient absorption spectroscopy. The promotion in phase segregation is shown to vary from polymer to polymer. With a careful modification of the polymer backbone, the negative impact of polymer fluorination can be avoided, resulting in an improvement in device performance. The following chapter focuses on the impact of polymer side chain on device performance. Linear side chain for PTTV polymers is found to offer a better polymer:fullerene mixing in comparison to branched side chains, potentially due to its better ability to accommodate fullerene molecules. This subsequently addresses limitations of poor of fullerene exciton dissociation efficiency. The next chapter provides new insights into the effects of polymer molecular weight on device performance. A high molecular weight of two DPP-containing polymers is shown to be beneficial to device performance, by offering a higher degree of material mixing in the polymer:fullerene blend. The last results chapter compares two polymers with different crystallinity in terms of their blend microstructure and device performance upon thermal annealing. It is shown that devices employing a crystalline DPP based polymer exhibits a sharp collapse in efficiency for annealing at temperatures beyond 140 °C, which is assigned to the polymers poorer miscibility with fullerene, making it incapable to suppress fullerene clusters formation. In contrast, an amorphous IDT based polymer shows a smaller decrease in efficiency under the same condition, consistent with its greater miscibility with fullenere.
Competition between charge collection and non-geminate recombination in bulk heterojunction solar cells
Florent Gilles Henri Deledalle
This thesis is concerned with quantification of non-geminate recombination losses in organic bulk heterojunction solar cells. After description of the context of this work, the theoretical background and the methodology employed are presented. In this thesis, many different polymer:fullerene systems are investigated. In the next chapter, we show that the study of non-geminate losses using charge extraction/transient photovoltage analysis can be applied to many different systems away from P3HT/P3HS blends. We see to what extent ideality factors can give a more precise description of the exact recombination mechanism. Then, the change of optoelectronic properties of a high performance polymer:fullerene blend upon a blend ratio perturbation is investigated. The resulting shifts in energetics and dynamics of the blends are quantified. A quantitative agreement between two methods (charge extraction and electroluminescence) probing the shifts in the energetics at the heterojunction is presented. In the next two chapters, two limits of the common vision of polymer: fullerene systems are explored by combining experiment and 1D drift-diffusion modelling. First, the impact of the variations of the spatial distribution of carriers on the apparent reaction order is experimentally investigated. The study reconciles the apparent contradictions currently in the literature regarding the meaning of high reaction orders. In the following chapter, the often underestimated effects of unintentional doping in polymer blends are addressed experimentally. In particular, its effect on device optimisation, understanding of carrier collection and recombination dynamics are explored. This study suggests that many donor/acceptor blends are not, contrary to common belief, intrinsic semiconductors. Finally, an analysis of the Langevin and non-Langevin behaviour of some efficient systems is presented. The different interpretations of regular observations disproving non-geminate recombination following the Langevin type mechanism are reviewed. We suggest the ratio ’recombination over collision’ is often overlooked and question the underlying assumption that it should be unity.
Charge generation and stability are key issues that have great impact upon the commercial viability of organic solar cells. In this thesis, a range of donor polymers, mainly of donor-acceptor class, were employed. Various materials characterisation, photophysical and photostability studies were performed on neat polymer films and polymer/fullerene blend films with the aim of establishing relationships between material structure and device function/stability. Transient absorption spectroscopy (TAS) was employed for the photophysical studies on neat films. The photophysics of triplet excitons are found to strongly correlate with relative polymer crystallinity as determined from wide-angled x-ray diffraction (WAXD), with the more amorphous polymers exhibiting longer triplet lifetimes. The rate constant and yield of oxygen quenching of these triplet states also showed clear correlations with material crystallinity. Charge generation in polymer/fullerene blend films was investigated using TAS and steady state optical spectroscopies. Compositional dependence studies with varying fullerene loadings were conducted on two polymers of different crystallinity, with a stronger dependence being observed in the more amorphous blend films. A comparison of charge generation pathways via electron or hole transfer suggests that the energetics between donor and acceptor can affect the efficiency of these pathways. This is consistent with the observation of a correlation between polaron yield and the energy offset driving charge separation for a series of blend films. Furthermore, the polaron yield estimated from TAS was correlated with device photocurrent. Photochemical stability is of significant concern in organic solar cells, as organic materials are susceptible towards photo-oxidation. Accelerated photodegradation in neat and blend films was monitored using steady state absorption spectroscopy under oxygen atmosphere. More crystalline polymers with shorter triplet lifetimes are found to be more stable. The mechanism of photodegradation involving triplet-mediated singlet oxygen generation was investigated with a molecular fluorescent probe, and found to be a significant photodegradation pathway.
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.
Stephanie R Pendlebury
Although the field of solar water splitting is now forty years old, in recent years there has been an upsurge of research in this area, with the aim of using sunlight to produce hydrogen cheaply and efficiently. Hematite (α-Fe2O3) is of particular interest as a photoanode material for solar water splitting, due to its optimum band gap (2.0-2.2 eV) and visible light absorption and stability. Various modifications – including nanostructuring and doping – have been investigated as routes to improved efficiencies, thought to be limited by long visible light absorption depths, low charge carrier mobilities and slow hole-transfer kinetics. Additionally, an anodic applied bias is required for water oxidation to occur on hematite. Improved understanding of the role of applied bias and the processes limiting the performance of hematite photoanodes will lead to more directed routes to photoanode architectures with increased efficiencies. This Thesis describes the results of transient absorption spectroscopy studies, in conjunction with photoelectrochemical measurements, of hematite photoanodes. Transient absorption spectroscopy on microsecond-second timescales allows direct monitoring of the recombination, trapping and reaction of photogenerated holes, both in isolated hematite films, and in photoanodes in a fully functional photoelectrochemical cell. Transient photocurrent measurements probe electron extraction from the photoanode on microsecond-millisecond timescales. The charge carrier dynamics are found to be strongly dependent on the electron density, which is controlled by applied electrical bias. The photocurrent generated is found to correlate with the population of long-lived holes, determined by the kinetics of electron-hole recombination. Generally, effects which lower electron density result in retarded electron-hole recombination kinetics, increasing the population of long-lived holes and hence increasing the photocurrent. Following an introduction and review of the literature, the first results chapter reports that the effect of a positive applied bias is to retard the otherwise dominant electron-hole recombination, increasing the lifetime of photogenerated holes such that water oxidation can occur. The relative timescales of recombination, electron extraction and water oxidation as a function of applied bias are discussed in the following chapter, in conjunction with the results of excitation density studies. The third results chapter compares the charge carrier dynamics in photoanodes with different nanomorphologies. The fourth results chapter discusses the effect of an energetic trap state on charge carrier dynamics, while the effects of surface treatment with cobalt, which is shown to retard recombination at low applied bias, is reported in the final results chapter. Overall conclusions are drawn and the implications of these for photoelectrode design are discussed.
Quantifying Regeneration in Dye Sensitized Solar Cells: A Step Toward Red Absorbing Dyes having Lower Energy Loss
Assaf Y Anderson
A limiting factor on DSSC efficiency is the lower fraction of the solar spectrum that is absorbed by the dye molecules developed to this point. Dye molecules that function well in DSSCs tend to have poor or no absorption to the red of 750 nm. Extending this absorption to the red by 100 nm, without losing efficiency in other ways, would result in a significant improvement in photocurrent. This challenge has proven difficult, in large part because of one slow reaction in the electron transfer cycle of DSSCs, the regeneration reaction. Better understanding of this reaction is thus critical. The kinetics of regeneration is understudied relative to the other processes in DSSCs, this is in part because the regeneration reaction produces no, as yet detected, measurable electrical signal. It must be studied by more difficult transient absorbance (TA) techniques. The first step of this thesis focuses on isolating a reliable transient signal that reflects the regeneration reaction. This is made by upgrading the conventional TA system to also acquire transient electrical (TE) signals simultaneously (TA-TE). The system is used to characterize dye-sensitized solar cells (DSSCs) under 1 sun illumination whilst the cells are fully operational and their stability is monitored. The second step of the work consists of the development of a methodology and a kinetic model which uses the isolated regeneration signal and a range of complimentary measurements on operating cells, to determine the quantum yield and the associated intrinsic rate constants and orders of the regeneration reaction. This enabled understanding of the regeneration mechanism and its optional rate limiting steps. Finally, the use of steady state photoinduced absorption (SSPA), as a complementary or alternative tool to assess regeneration, is also questioned. SSPA is compared with the regeneration TA –TE and charge extraction measurements.
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.
Christopher G Shuttle
Organic devices based on polymer:fullerene blend films are attracting extensive interest as low cost solar cells, with power conversion efficiencies over 5%. Improvements in performance are dependent on developing a better understanding of the pertinent loss processes. This in turn requires the ability to reliably determine charge densities (n) and carrier lifetimes (τn) in real devices under standard operating conditions. In this thesis, we address the recombination dynamics in organic solar cells 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 best devices to date, using both experimental and modelling studies. Initially, a drift-diffusion model was used to study the basic principles of solar cell operation, with particular focus on investigating the ‘corrected photocurrent’, where the effects of dark injection are removed. We then have employed a series of experimental techniques – including transient photovoltage and photocurrent, transient absorption spectroscopy and charge extraction – to determine the carrier lifetimes and charge densities in standard annealed P3HT:PCBM devices under operation. The results of our studies for a device under open-circuit conditions show that the open-circuit voltage (Voc) is primarily governed by a trap dependent bimolecular recombination process. By applying charge extraction studies on devices under forward bias in the dark, we show that the dark current is also governed by the same trap dependent bimolecular recombination mechanism which determines Voc. Based on the understanding of charge carrier dynamics at Voc and the forward bias dark current, a simple model has been developed to simulate ‘light’ current-voltage (J-V) curves. Despite the simplicity of this model, remarkably good agreement was observed with experimental J-V data.
Dye sensitised solar cells (DSSCs) have attracted extensive research over the last decade because of their potential for low cost production. However, fundamental studies aimed at direct practical application of the devices have been relatively limited to date. In the work presented in this thesis, fundamental studies based upon transient absorption spectroscopy (TAS) are employed to address the potential of a range of novel molecular materials for practical device development. These studies of charge transfer dynamics for novel molecular dyes and hole transport materials in DSSCs have revealed some key issues about controlling these kinetics for efficient energy conversion.
This thesis was divided into two parts; research on liquid electrolyte and solid state dye sensitised solar cells. In the first part, a range of dyes have been studied, including not only analogues of the established dye, Ru(dcbpY)2(NCS)2, but also alternative dyes such as Platinum complexes. Such studies have in particular focused on lengthening the distance between injected electrons and the dye cations in order to slow interfacial charge recombination. Of particular interest, it was shown that a supermolecular analogue of the Ru(dcbpY)2(NCS)2which included the addition of a secondary electron donor moiety, tri-methoxy phenyl amine, showed the longest lived charge-separated state reported to date, with decay half time of 0.7 s.
Research on solid cells has concentrated on both polymer electrolyte and molecular hole conductors, including a range of novel materials supplied by both industrial and academic partners. For the polymer electrolyte devices, it wasdetermined that kinetic competition between charge recombination and dye cation rereduction was critical in limiting device efficiency. In contrast, for molecular holeconductor, electron recombination with the holes in the HTMs was found to be ofprimary concern. A range of strategies were employed to address this issue. In particular a charge transfer cascade was developed based upon a bi-layer of two holeconductors differing in ionization potential resulting in a significant retardation ofinterfacial charge recombination.
Competition between charge separation and charge recombination processes is a key issue in molecular photovoltaic devices. In this thesis these issues are addressed in nanocrystalline titanium dioxide dye-sensitised (DSSC) and PPV-fullerene derivatives based solar cells.
In dye-sensitised devices photoinduced charge separation is initiated by electron injection from a molecular sensitiser dye to the semiconductor conduction band yielding the dye cation. This charge separation is subsequently stabilised by regeneration of the dye molecule by electron transfer from donor species, iodide ions of the iodide /triiodide redox couple in the electrolyte. Efficient device function requires the dye cation regeneration reaction by the iodide to be faster than dye regeneration by photoinjectedelectrons (charge recombination).
This thesis focuses on the competition between these two electron transfer pathways.Transient optical absorption spectroscopy was employed to investigate this kinetic competition. Kinetics of dye-sensitised electrodes in a three-electrode electrochemical cell were measured as a function of an externally applied electrical bias and other parameters such as sensitiser employed and electrolyte composition.
The kinetic competition was successfully modelled by extending a continuous timerandom walk model for the recombination reaction to include a reaction pathway with the iodide anions. The implications of these findings are discussed in relation to the functioning of liquid electrolyte based cells. These studies were extended to completesolar cells where photoinjected electrons can also react with tri-iodide in the electrolyte.
Further studies assessed the function of solar cells fabricated with PPV and fullerene derivatives, blended on a nanometer scale. In these cells the transport of photogeneratedcarriers to the electrodes is in competition with charge recombination.Trapping of photogenerated positive polarons MDMO-PPV' is found to play an important role inthese film s and a "mobility edge" is found.
The dynamics of interfacial electron transfer in dye-sensitised titanium dioxide (TiO2) nanocrystalline films has been studied by means of the femtosecond to millisecond, transient absorption spectroscopy.
The project required the construction of a nanosecond transient absorption spectrometer, and the development of a femtosecond optical parametric amplifier to provide a tuneable excitation source and a pump-probe system for ultrafast transient absorption and operation of the laser are described in detail.
Interfacial electron transfer kinetics have been studied for the sensitiser dye cisruthenium"(2,2'-bipyridyl-4,4'-dicarboxylate)2(NCS)2(Ru(dcbpY)2(NCS)2) adsorbed onto the surface of nanocrystalline Ti02films. This system was chosen as it is attracting considerable attention for application in photo electrochemical solar energy conversion.
A method to assign the kinetics of electron transfer reactions at the dye / semiconductorinterface has been established. It has been found that the electron injection is multiexponential on femtosecond / picosecond timescales. In contrast, the charge recombination completes on micro second-millis econd timescales.The parameters influencing the electron injection kinetics in Ru(dcbPY)2(NCS)2 sensitised nanocrystalline Ti02films have been studied. Ru(dcbpY)2(NCS)2is compared with other sensitiser dyes in order to address the origin of the high efficiency achieved with this dye. Different experimental conditions were employed in order to compare the injection kinetics in Ru(dcbpY)2(NCS)2 sensitised Ti02 films, including excitation wavelength, solvent environment, electrolyte composition and application of external voltages.
The results of these experiments are discussed in terms of heterogeneous electron transfer theory. The relevance of these results to the function of dye sensitised semiconductor photoelectron chemical solar cells is also discussed, relating in particular to the energy conversion efficiency and the long-term stability of the cell.
James Robert Durrant
This thesis describes a study of the photoinduced electron and energy transfer pathways within the isolated photosystem two (PS II) reaction centre.
The experimental technique most used in this thesis is transient absorption spectroscopy. A femtosecond transient absorption spectrometer was constructed inorder to study the photoinduced primary charge separation events occurring in the PS II reaction centre. In addition, a nanosecond to millisecond transient absorption spectrometer was but in order to study the relaxation processes occurring in the PS II reaction centre after the primary charge separation.
Initial experiments with the isolated PS II reaction centre were hindered by the instability of the complex to illumination. This was found to be caused primarily by oxygen quenching of a photoinduced triplet state residing upon P680, the primary electron donor of PS II, resulting in the production of highly oxidising singlet oxygen. Removal of the oxygen from the preparation greatly stabilised the reaction centre.
The kinetics of charge recombination from the primary radical pair state were studied. This charge recombination results in a 30% yield of a chlorophyll triplet state, which was identified as the triplet state of P680. In the absence of oxygen, this triplet state decays primarily with a lifetime of 1ms. Analysis of the triplet-minus-singlet absorption difference spectrum of this P680 triplet state suggests that P680 is a pair of chlorophyll molecules whose singlet states are excitonically coupled. In a minority of reaction centres, energy transfer between the P680 triplet state and a carotenoid triplet state is observed. The resultant P680/carotenoid triplet equilibrium, which is formed in approximately 10% of reaction centres, decays with a lifetime of 12ps. An attempt to reconstitute a quinone as a secondary electron acceptor into the reaction centre is also described.
Transient absorption kinetics were observed with a (l.lps)-1 rate. This kinetics were tentatively assigned to the decay of the stimulated emission band of P680, associated with primary charge separation and leading to the formation of the primary radical pair. Slower kinetics, with a (18ps)-l rate, were assigned to energy transfer processes. Further experiments aimed at confirming these assignments are discussed. A recurring theme throughout this thesis is the similarities and differences between the observations made here on PS II reaction centres, and studies previously carried out upon reaction centres isolated from purple bacteria.