Three n-type semiconducting polymers, P-0, P-50, and P-75 are investigated, in which the lactone group density were maximized by increasing the benzene content from 0% benzene (P-0), to 50% (P-50), and 75% (P-75), to enable co-planar polymers with deep-lying LUMO energy level. As a result, the polymer of P-75 exhibits a good polaron stability in air, allowing a high performing organic thermoelectrics with electrical conductivities (σ) of up to 12 S cm−1 and high power factor (PF) up to 13.2 μWm−1 K−2. These results suggest the significance of chemical design and provide new guidelines towards high performing n-type OTE devices.
The increasing demand for efficient solar water splitting devices calls for a mechanistic understanding of trap carrier dynamics in actual working photoelectrodes. Here, we design a pump-push-photocurrent experiment to optically manipulate and detect the in-situ dynamics of trap carriers in a model photoelectrochemical cell comprising monoclinic bismuth vanadate (BiVO4) as the photoanode. We show that a near-IR (1064 nm) push pulse can be used to reactivate the pump-induced electrons that are trapped by oxygen vacancies. Meanwhile, the effect of oxygen vacancies on carrier transport is strongly affected by external bias condition. These studies enable us to better understand the role of defects in the performance of BiVO4 photoanodes, and could be used to guide the design of other promising photocatalysts.
My talk will address three related topics in organic solar cells. I will initially address the issue of whether interfacial electron / hole pairs (i.e.: charge transfer states) are bound and unbound in organic donor / acceptor blends. In particular I will present a range of kinetic and structural data indicating that the behavior of such states depend strongly upon interface structure. Interfaces within molecular mixed domains can yield relatively bound states which can undergo significant geminate recombination losses. In contrast, for the same donor / acceptor materials, interfaces between domains can yield efficient charge separation with minimal geminate losses. I will then go on to address the importance of geminate recombination losses in limiting photocurrent generation in polymer / non-fullerene acceptor solar cells, and the role of energy level tuning in minimizing such losses. Finally I will go to address the challenge of achieving efficient charge collection in organic solar cells with active layer thicknesses large enough for scalable device manufacture. In particular I will address the role of space charge layer formation caused by charge trapping in shallow tail states, and the requirement that efficient photocurrent generation in thick organic solar cells requires minimizing the density of these tail states.
Electron-hole recombination determines photocurrent generation yields in polymer:fullerene blends, but the nature of this process and its timescales are not completely understood. In this study, we use a combination of spectroscopy techniques to probe how film structure and interface energetics control the charge generation and recombination dynamics in several polymer:fullerene blends. By varying film composition and comparing between different LUMO-LUMO offset systems, we identify geminate electron-hole recombination on the nanosecond timescale only in the blends consisting of finely-intermixed polymer:fullerene phases and low LUMO-LUMO offsets, whereas the formation of pure fullerene phases leads to the suppression of the geminate recombination process. Charge transfer state photoluminescence and electroluminescence data show similar dependencies confirming that geminate electron-hole recombination is controlled by interfacial enthalpic energy offsets and the density ratio between pure-fullerene and intermixed polymer-fullerene phases. Our results also indicate that electron-hole association probed with electroluminescence and photoluminescence derives from different interfaces playing distinct roles in the photocurrent generation process.
Polymer : fullerene blend films are the best studied materials systems for organic photovoltaic energy conversion. Whilst the variety of donor materials studied is huge, the vast majority of systems use either phenyl-C61 butyric acid methyl ester (PCBM) or its C71 analogues (PC70BM) as the electron acceptor. The choice places a constraint on the donors that may be used and may exclude high oxidation potential donor materials that would otherwise be good candidates for stable photovoltaic devices. Recently, Solenne BV has developed a new ketolactam fullerene (KL) which is a stronger acceptor than PCBM. Here, we study the photocurrent generation and charge generation properties of blend devices of the new fullerene with polymers which have exhibited insufficient driving force for excitons generated in the polymer to separate into free charge carriers, when blended with PCBM or PC70BM. An example is the low LUMO benzotrithiophene-co-diketopyrrolopyrrole polymer, BTT-DPP[1,2]. We compare the external and internal quantum efficiency spectra, current-voltage characteristics and charge generation yield for polymer:PCBM and polymer:KL systems, and thereby identify the factors limiting charge generation in these systems.
In this paper two extremely narrow band-gap polymers, based on naturally occurring indigo with high thin film crystallinity, have been examined using transient absorption spectroscopy. This was done in order to assess their charge photogeneration and recombination characteristics in blends with PC71BM. Two charge photogeneration mechanisms are found to be operating, depending on which component of the blend is photoexcited. Despite virtually isoenergetic LUMO levels, photoexcitation of the polymer causes standard electron transfer, albeit with a relatively low efficiency of 17 %. Photoexcitation of the fullerene, however, produces an exceptionally slow nanosecond timescale hole transfer.
Achieving efficient charge generation has been a challenge in the field of organic photovoltaics (OPVs). Therefore, it is
important to understand the photophysical behaviour of a system in relation to the material properties. In this study,
photophysical properties of a range of conjugated donor-acceptor polymers with varying crystallinity are investigated.
Preliminary study shows a correlation between photophysics and polymer crystallinity.
Herein we present an extension of our work on indacenodithiophenes (IDT) by replacing the central benzene ring
with a thieno(3,2-b) thiophene unit. This newly developed thieno[3,2-b]thieno bisthiophene (4T) donor moiety was
synthesized from commercially available reagents and incorporated into a series of donor-acceptor polymers. We
will discuss the pronounced donating character of 4T compared to IDT and the choice of bridging atom in those
new polymers with an emphasis on field effect transistor and photovoltaic device performance.
We focus upon the role of interfacial energetics and morphology in influencing the separation of CT states into dissociated charge carriers. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) fullerene electron acceptors. For the PDI film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron phenyl-C61-butyric acid methyl ester(PCBM). Furthermore, we compare the charge recombination dynamics observed in films comprising P3HT blended with three fullerene derivatives: PCBM and two alternative pyrazolinofullerenes. Transient absorption data indicate that replacement of PCBM with either of the pyrazolinofullerene derivatives results in a transition from nongeminate to monomolecular (geminate) recombination dynamics. We show that this transition cannot be explained by a difference in interfacial energetics. However, this transition does correlate with nanomorphology data that indicate that both pyrazolinofullerenes yield a much finer phase segregation with correspondingly smaller domain sizes than observed with PCBM. Our results therefore provide clear evidence of the role of nanomorphology in determining the nature of recombination dynamics in such donor/acceptor blends.
KEYWORDS: Polymers, Solar cells, Solid state electronics, Electron transport, Capacitance, Absorption, Solid state physics, Information operations, Solar energy, Heterojunctions
Time-of-flight (TOF) photocurrent measurements have been used to study charge transport in films of regioregular
poly(3-hexylthiophene) (P3HT). Devices in which the P3HT film had been deposited directly onto an indium tin oxide
(ITO) electrode produced high dark currents as a result of hole injection into P3HT from ITO. Photocurrent transients in
such devices were disperse. It was found however, that these dark currents could be significantly reduced by inserting a
dense TiO2 layer between the ITO and the polymer film. The resulting devices gave non-dispersive transients with hole
and electron mobilities in the range of 1 - 2 10-4 cm2 V-1 s-1 at room temperature. The mobility values were observed to
be almost independent of film thickness over the range of 350 nm to 4.3 μm. Temperature dependence studies showed a
weak dependence on temperature with a low energetic disorder parameter according to analysis using the Gaussian
Disorder Model (GDM) of 71 meV.
Solar cells fabricated from composites of conjugated polymers with nanostructured metal oxides are gaining interest on account of the stability, low cost and electron transport properties of metal oxides. Zinc oxide (ZnO)/polymer solar cells are promising compared to other metal oxide/polymer combinations, on account of the possibility of low temperature synthesis, as well as the potential for controlling interface morphology through simple processing from solution. Here, we focus on the effect of surface morphology of ZnO films on photovoltaic device performance. We have successfully grown ZnO nanorods standing almost perpendicular to the electrodes on a flat, dense ZnO "backing" layer. We studied structures consisting of a conjugated polymer in contact with three different types of ZnO layer: a flat ZnO backing layer alone; ZnO nanorods on a ZnO backing layer; and ZnO nanoparticles on a ZnO backing layer. We use scanning electron microscopy, steady state and transient absorption spectroscopies and photovoltaic device measurements to study the morphology, charge separation and recombination behaviour and device performance of the three types of structures. We find that charge recombination in the structures containing vertically aligned ZnO nanorods is remarkably slow, with a half life of over 1 ms, over two orders of magnitude slower than for randomly oriented ZnO nanoparticles. A photovoltaic device based on the nanorod structure which has been treated with an ambiphilic dye before deposition of poly(3-hexyl thiophene) (P3HT) polymer shows a power conversion efficiency over four times greater than for a similar device based on the nanoparticle structure. The best ZnO nanorods: P3HT device yields a short circuit current density of 2 mAcm-2 under AM1.5 illumination (100mWcm-2) and peak external quantum efficiency over 14%, resulting in a power conversion efficiency of 0.20%.
Control of charge interfacial charge transfer is central to the design of photovoltaic devices. A an elegant approach to control those dynamics, is the use of an insulating metal oxide blocking layer at a nanocrystalline inorganic / organic semiconductor interface. We show that the conformal growth of a ~1 nm thick overlayer of MgO on a preformed nanocrystalline SnO2 film results in a ~4-fold retardation in the rate of charge recombination at such an interface This observation shows a good correlation with the current/voltage characteristics of dye sensitised nanocrystalline solar cells fabricated from such films, with the MgO coating resulting in ~ 50% improvement in overall device efficiency.
This study focuses on systems consisting of high hole-mobility MEHPPV based polymers or a fluorene-bithiophene co-polymer in contact with different nanocrystalline TiO2 films. We use photoluminescence quenching, time of flight mobility measurements and optical spectroscopy to characterize the exciton transport, charge transport and light harvesting properties, respectively, of the polymers, and correlate these material properties with photovoltaic device performance. We find that the polymer properties with greatest influence on device efficiency are the polymer exciton diffusion length and absorption range, followed by the hole mobility. We have also studied the photovoltaic performance of these TiO2/polymer devices as a function of active layer thickness. Device performances are significantly improved by introducing a PEDOT layer between the polymer and the top Au electrode and by reducing the thickness of the active layers. The optimized devices have peak external quantum efficiencies ≈ 40 % at the polymer's maximum absorption wavelength and yield short circuit current densities ≥ 2 mA cm-2 for air mass (AM) 1.5 conditions (100 mW cm-2, 1 sun). The AM 1.5 open circuit voltage reaches 0.64 V and the fill factor 0.43, resulting in an overall power conversion efficiency of 0.58 %.
Interfacial electron transfer dynamics of dye sensitised metal oxide films have been widely studied by transient optical techniques. In this paper, we extend such studies to complete dye sensitised solar cells, and show how such transient optical studies can be correlated both with transient photovoltage studies and current / voltage analyses of device photovoltaic performance.
Control of charge interfacial charge transfer is central to the design of photovoltaic devices. We report herein the application of insulating metal oxide blocking layers to control the charge recombination kinetics at a solid-state dye sensitised nanocrystalline inorganic/organic semiconductor interface. We show that the conformal growth of a ~1 nm thick overlayer of Al2O3 on a preformed nanocrystalline TiO2 film results in a ~3-fold retardation in the rate of charge recombination at such an interface. This observation shows a good correlation with the current/voltage characteristics of dye sensitized nanocrystalline solar cells fabricated from such films, with the Al2O3 coating resulting in a 40% improvement in overall device efficiency
We present a study of charge recombination in polyfluorene:[6,6]-Phenyl C61-butyric acid methyl ester (PCBM) blend photovoltaic cells. The recombination kinetics of photogenerated charge carriers are investigated at room temperature by frequency and time domain photoinduced absorption. Monomolecular and bimolecular recombination processes are discussed in detail. Finally, a discussion on what kind of recombination can be dominant in our photovoltaic cells is addressed.
We study the charge recombination kinetics and photovoltaic performance of composites of poly (9,9-dioctylfluorene-co-bithiophene) polymer with nanocrystalline TiO2. Transient optical spectroscopy confirms that photoexcitation of the polymer leads to electron transfer to the TiO2 and indicates that charge recombination is slow with a half-time of 100 μs to 10ms. Polymer penetration into thick porous TiO2 layers is improved by melt-processing and treatment of the TiO2 surface. We study the photovoltaic characteristics of devices with different layer thickness and interface morphology. Quantum efficiency (QE) of all devices is increased by reducing the TiO2 and polymer layer thickness. Inserting a thin porous TiO2 layer in to a thin bi-layer device increases the QE by a factor of five. The improved device shows peak QE and monochromatic power conversion efficiencies of over 11% and 1% at 440nm respectively. The device produced a short-circuit current density of 300μAcm-2, a fill factor of 0.24 and an open-circuit voltage of 0.8V under AM1.5 illumination. The fill factor is increased from 0.24 to 0.40 by introducing an additional dip-coating layer and overall power conversion efficiency is increased by 50%. However, the device produced degraded current-voltage characteristics. We investigate this using an alternative polymers and different top contact metals.
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