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We have studied charge injection and charge transport in thin disordered films of CdSe nanocrystals between metal electrodes. Current–voltage characteristics of these devices are investigated as a function of electrode material, nanocrystal size, and temperature. We measure the photocurrent response of these structures and find that the photocurrent action spectra follow the quantum-confined absorption spectra of the nanoparticles. For dissimilar top and bottom electrodes, we find that the devices are highly rectifying. High work function materials such as gold and indium-tin oxide are found to be poor electron injectors, consistent with the estimated conduction and valence band levels of the nanocrystals. We observe that the current–voltage characteristics exhibit a history and time dependence which is characteristic of persistent photoconductivity, with current at constant bias decaying with time according to a stretched exponential form. We propose a model based on space-charge limited current dominated by mobile electrons which slowly fill deep traps. Numerical simulations show that the model is able to describe the observed time dependence. We also find that the conductivity is strongly temperature dependent, and is qualitatively consistent with an activated hopping process at temperatures above 180 K. We use the data and simulations to estimate the electron mobilities to be in the range of ∼10−4–10−6 cm2 V−1 s−1 and the trap densities to be approximately 2×1016 cm−3.

Efficient charge separation has been observed in blends of conjugated polymers and cadmium selenide nanocrystals. These composites, which can be used for photovoltaic applications, provide a unique medium for examining charge separation because the energy levels of both components can be tuned independently: those of the polymer by its side groups, and those of the nanocrystals by size-dependent quantum confinement effects. Here we study charge separation in such blends via photoluminescence quenching and photo-induced absorption measurements. The effects of polymer derivatization and nanocrystal size are investigated.

We study photoinduced electron transfer from derivatives of poly(p-phenylenevinylene) (PPV) and nanocrystals of cadmium selenide via photoluminescence (PL) quenching and photoinduced absorption (PIA) spectroscopy. Using size-dependent quantum confinement to vary the energy levels of the nanocrystal acceptors, and chemical substitution to vary the energy levels of the polymer donors, we present a systematic investigation of charge transfer in these polymer/quantum-dot composites. We observe efficient PL quenching in blends of poly[2-methoxy-5-(${2}^{\ensuremath{'}}$-ethyl-hexyloxy-p-phenylenevinylene)] (MEH-PPV) with nearly monodisperse CdSe samples for nanocrystal diameters from 2.5 to 4.0 nm. The observed PIA peaks, as well as their frequency and temperature dependence, are consistent with the formation of long-lived positive polarons on MEH-PPV following electron transfer to the nanocrystals. Both the PL quenching and the PIA features are insensitive to nanocrystal size. We have also studied blends of CdSe nanocrystals of 2.5--4.0 nm diameter with two high electron affinity cyano-substituted PPV derivatives. One of these polymers behaves similarly to MEH-PPV; however in the other polymer, which has different alkoxy side chains, we find neither efficient PL quenching nor any PIA features indicative of charge transfer. We explain the insensitivity of the electron transfer process to nanocrystal size in the context of the relevant polymer and nanocrystal energy levels and discuss the influence of the polymer side chains on the charge-transfer process.

± [1] Y. Kaminorz, E. Smela, O. Inganas, L. Brehmer, Semicond. Sci. Technol., 1998, 13, 433. [2] D. Pede, E. Smela, T. Johansson, M. Johanssen, O. Inganas, Adv. Mater. 1998, 10, 233. [3] Q. Pei, G. Yu, C. Zhang, Y. Yang, A. J. Heeger, Science 1995, 269, 1086. [4] W. Mammo, M. Andersson, O. Inganas, in preparation. [5] W. Vaterlein, B. Ziegler, W. Gebauer, H. Neureiter, M. Stoldt, M. S. Weaver, P. Bauerle, M. Sokolowski, D. D. C. Bradley, E. Umbach, Synth. Met. 1996, 76, 133. [6] B. H. Cumpston, K. F. Jensen, Synth. Met. 1995, 73, 195. [7] M. S. A. Abdou, S. Holdcroft, Macromolecules 1993, 26, 2954. [8] M. Yan, L. J. Rothberg, F. Papadimitrakopoulos, M. E. Galvin, T. M. Miller, Phys. Rev. Lett. 1994, 73, 744. [9] D. M. Taylor, H. L. Gomes, A. E. Underhill, S. Edge, P. I. Clemenson, J. Phys. D: Appl. Phys. 1991, 24, 2032. [10] M. S. A. Abdou, F. P. Orfino, Y. Son, S. Holdcroft, J. Am. Chem. Soc. 1997, 119, 4518. [11] M. S. A. Abdou, F. P. Orfino, Z. W. Xie, M. J. Deen, S. Holdcroft, Adv. Mater. 1994, 6, 838. [12] R. A. J. Janssen, L. Smilowitz, N. S. Sariciftci, D. Moses, J. Chem. Phys. 1994, 101, 1787. [13] H.-J. Egelhaaf, L. Luer, D. Oelkrug, G. Winter, P. Haisch, M. Hanack, Synth. Met. 1997, 84, 897. [14] H. Ishii, K. Seki, IEEE Trans. Electron. Devices 1997, 44, 1295. [15] W. Bantikassegn, O. Inganas, Synth. Met. 1997, 87, 5. [16] J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, A. B. Holmes, Nature 1990, 347, 539. [17] Z. D. Popovic, J. Chem. Phys. 1982, 77, 498. [18] M. Pope, C. E. Swenberg, Electronic Processes in Organic Crystals, Clarendon, Oxford 1982

We investigate the loss mechanisms in hybrid photovoltaics based on blends of poly(3-hexylthiophene) with CdSe nanocrystals of various sizes. By combining the spectroscopic and electrical measurements on working devices as well as films, we identify that high trap-mediated recombination is responsible for the loss of photogenerated charge carriers in devices with small nanocrystals. In addition, we demonstrate that the reduced open-circuit voltage for devices with small nanocrystals is also caused by the traps.

We report that the transit-time of photogenerated holes within an OPV device is significantly controlled by the interface between the photoactive layer and the electrode. We used time-resolved optical absorption spectroscopy to track the evolution and transport of photogenerated holes in situ within efficient PTB7:PC70BM (thieno [3,4-b] thiophene/benzodithiophene:[6,6]-phenyl C71-butyric acid methyl ester) devices. Our results demonstrate that the time required for 50% hole extraction is reduced from 2.3 μs for the standard devices to 0.9 μs either by introducing a thin layer of conjugated polyelectrolyte (CPE) layer at the electron-extracting cathode or simply by exposing the device to a polar solvent such as methanol which is used for spin coating the CPEs. We consider that the CPE modifies the interface between the photoactive layer and the cathode whereas the exposure of the device to associated polar solvent modifies the buried PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) hole extracting layer.

We investigate the resistance switching mechanism in memristors based on colloidal ZnO nanoparticles using electroabsorption (EA) spectroscopy. In this EA experiment, we incorporate a small amount of low-bandgap polymer, poly(9,9-dioctylfluorene-co-benzothiadiazole), as a probe molecule in ZnO-nanoparticle memristors. By characterizing this polymer, we can study the change of built-in potential (VBI) in the device during the resistance switching process without disturbing the resistance state by the EA probe light. Our results show that VBI increases when the device is switched to the high resistance state, suggesting a shift of effective workfunction of the electrode. Thus, we attribute the resistance switching to the field-dependent migration of oxygen vacancies associated with the adsorption and desorption of oxygen molecules at the Al/ZnO interface. This process results in the modulation of the interfacial injection barrier, which governs the resistance state of the device.

Colloidal quantum dot solar cells (CQDSCs) are attracting growing attention owing to significant improvements in efficiency. However, even the best depleted-heterojunction CQDSCs currently display open-circuit voltages (VOCs) at least 0.5 V below the voltage corresponding to the bandgap. We find that the tail of states in the conduction band of the metal oxide layer can limit the achievable device efficiency. By continuously tuning the zinc oxide conduction band position via magnesium doping, we probe this critical loss pathway in ZnO–PbSe CQDSCs and optimize the energetic position of the tail of states, thereby increasing both the VOC (from 408 mV to 608 mV) and the device efficiency.

Monte Carlo simulations are a valuable tool to model the generation, separation, and collection of charges in organic photovoltaics where charges move by hopping in a complex nanostructure and Coulomb interactions between charge carriers are important. We review the Monte Carlo techniques that have been applied to this problem, and describe the results of simulations of the various recombination processes that limit device performance. We show how these processes are influenced by the local physical and energetic structure of the material, providing information that is useful for design of efficient photovoltaic systems.

Recent research has pushed the efficiency of colloidal quantum dot solar cells toward a level that spurs commercial interest. Quantum dot/metal oxide bilayers form the most efficient colloidal quantum dot solar cells, and most studies have advanced the understanding of the quantum dot component. We study the interfacial recombination process in depleted heterojunction colloidal quantum dot (QD) solar cells formed with ZnO as the oxide by varying (i) the carrier concentration of the ZnO layer and (ii) the density of intragap recombination sites in the QD layer. We find that the open-circuit voltage and efficiency of PbS QD/ZnO devices can be improved by 50% upon doping of the ZnO with nitrogen to reduce its carrier concentration. In contrast, doping the ZnO did not change the performance of PbSe QD/ZnO solar cells. We use X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, transient photovoltage decay measurements, transient absorption spectroscopy, and intensity-dependent photocurrent measurements to investigate the origin of this effect. We find a significant density of intragap states within the band gap of the PbS quantum dots. These states facilitate recombination at the PbS/ZnO interface, which can be suppressed by reducing the density of occupied states in the ZnO. For the PbSe QD/ZnO solar cells, where fewer intragap states are observed in the quantum dots, the interfacial recombination channel does not limit device performance. Our study sheds light on the mechanisms of interfacial recombination in colloidal quantum dot solar cells and emphasizes the influence of quantum dot intragap states and metal oxide properties on this loss pathway.

We study the processes of charge separation and transport in composite materials formed by mixing cadmium selenide or cadmium sulfide nanocrystals with the conjugated polymer poly(2-methoxy,5-(2\ensuremath{'}-ethyl)-hexyloxy-$p$-phenylenevinylene) (MEH-PPV). When the surface of the nanocrystals is treated so as to remove the surface ligand, we find that the polymer photoluminescence is quenched, consistent with rapid charge separation at the polymer/nanocrystal interface. Transmission electron microscopy of these quantum-dot/conjugated-polymer composites shows clear evidence for phase segregation with length scales in the range 10-200 nm, providing a large area of interface for charge separation to occur. Thin-film photovoltaic devices using the composite materials show quantum efficiencies that are significantly improved over those for pure polymer devices, consistent with improved charge separation. At high concentrations of nanocrystals, where both the nanocrystal and polymer components provide continuous pathways to the electrodes, we find quantum efficiencies of up to 12%. We describe a simple model to explain the recombination in these devices, and show how the absorption, charge separation, and transport properties of the composites can be controlled by changing the size, material, and surface ligands of the nanocrystals.

The photoluminescence properties of conjugated polymers are currently of great interest, since the radiative decay of singlet excitons is the process by which light emission occurs in polymer light-emitting diodes. We describe here accurate measurements of photoluminescence efficiencies in solid films of conjugated polymers, using an integrating sphere to collect the emitted light. Values in excess of 40% are measured for cyano-substituted derivatives of poly(p-phenylenevinylene) (PPV). In PPV itself, measured photoluminescence efficiencies of 27%, combined with time-resolved measurements of the luminescence decay, suggest that the species produced by photoexcitation is predominantly the singlet exciton responsible for luminescence. The photoluminescence spectrum and the rate of radiative decay can be strongly affected by the presence of dielectric or metal interfaces. We model here the effects of optical interference in a number of multi-layer structures. We compare these results with measured p...

Photoinduced charge transfer at a conjugated polymer donor-acceptor heterojunction controls the ultimate photogeneration efficiency in photovoltaic devices. We have studied this charge transfer by measurements of electric-field-induced photoluminescence (PL) modulation in polyfluorene-based bilayer devices at room and low temperatures. Depending on the field direction across the interface we observe field-induced quenching or enhancement of the PL, which we ascribe to modulation of the exciton dissociation rate through modification of the energy of the charge-transfer state formed after charge separation.

We report that we can enhance the performance of polymer photovoltaics by introducing materials with dipoles at the donor–acceptor interface of the polymer solar cells. We use a zwitterionic conjugated polyelectrolyte (ZCPE) as an interfacial layer in a model bi-layer system consisting of donor and acceptor polymers. We find an increase in short-circuit current and fill factor when a 2 nm thin layer of ZCPE is introduced between donor and acceptor polymers. Absorption of light in the ZCPE interlayer contributes to the photocurrent, and, as revealed by time-resolved pump–push optical spectroscopy, the interlayer reduces the probability of electron–hole recombination at the donor–acceptor interface.

Measurements of absolute photoluminescence (PL) efficiencies have been performed for solid films of several conjugated polymers commonly used for electroluminescence. In poly( p -phenylenevinylene) (PPV), a PL efficiency of 0.27 is measured in samples which show an initial PL decay time-constant of 320 ps. These values indicate that photoexcitation in PPV produces intra-chain singlet excitons with a high quantum yield. The PL efficiencies of derivatives of PPV have been investigated, and efficiencies in excess of 0.4 have been measured for cyano-substituted PPVs.

Spintronics based on organic semiconductor materials is attractive because of its rich fundamental physics and potential for device applications. Manipulating spins is obviously important for spintronics, and is usually achieved by using magnetic electrodes. Here we show a new approach where spin populations can be controlled primarily by energetics rather than kinetics. We find that exciton spin statistics can be substantially controlled by spin-polarizing carriers after injection using high magnetic fields and low temperatures, where the Zeeman energy is comparable with the thermal energy. By using this method, we demonstrate that singlet exciton formation can be suppressed by up to 53% in organic light-emitting diodes, and the dark conductance of organic photovoltaic devices can be increased by up to 45% due to enhanced formation of triplet charge-transfer states, leading to less recombination to the ground state., Control of spin statistics by spin injection from ferromagnetic electrodes has been shown to achieve only weak effects in organic optoelectronic devices. Wang et al. use instead polarization of spins after injection, at high magnetic fields and low temperatures, achieving a 50% change in device characteristics.

Singlet exciton fission-sensitized solar cells have the potential to exceed the Shockley-Queisser limit by generating additional photocurrent from high-energy photons. Pentacene is an organic semiconductor that undergoes efficient singlet fission--the conversion of singlet excitons into pairs of triplets. However, the pentacene triplet is non-emissive, and uncertainty regarding its energy has hindered device design. Here we present an in situ measurement of the pentacene triplet energy by fabricating a series of bilayer solar cells with infrared-absorbing nanocrystals of varying bandgaps. We show that the pentacene triplet energy is at least 0.85 eV and at most 1.00 eV in operating devices. Our devices generate photocurrent from triplets, and achieve external quantum efficiencies up to 80%, and power conversion efficiencies of 4.7%. This establishes the general use of nanocrystal size series to measure the energy of non-emissive excited states, and suggests that fission-sensitized solar cells are a favourable candidate for third-generation photovoltaics.

We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells.

We present the synthesis, purification, and characterization of all-conjugated block copolymers comprising poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (PF8TBT) and poly(3-hexylthiophene) (P3HT). Suzuki step-growth polycondensation is used for the synthesis of PF8TBT, which is subsequently terminated via the addition of narrow-distributed, monobrominated P3HT-Br. Purification via preparative GPC is carried out to reduce polydispersity and to remove excess P3HT. Wavelength-dependent GPC and careful NMR end group analysis, assisted by model compounds, reveal pure diblock copolymers of PF8TBT-b-P3HT. Insight into structure formation is given by temperature-dependent UV–vis absorption, DSC, and X-ray scattering. These indicate that PF8TBT-b-P3HT does not microphase-separate within the investigated range of composition and molecular weight. The critical role of introducing sufficient dissimilarity between the segments in all-conjugated block copolymers...

We consider the present understanding of the operation of light-emitting diodes which use conjugated polymers for both charge transport and emission. We highlight the improvement to the electroluminescence efficiency that can be produced by the use of two polymer layers selected so that the heterojunction between the two layers is able to confine charge and thus bring about efficient electron—hole capture to generate excitons. We also report recent results on the fabrication of diodes on top of non-transparent substrates such as silicon. This requires the deposition of a transparent top electrode, and we have used indiumtin oxide in this role.

We present a new fully conjugated diblock copolymer, P3HT-b-PFTBTT, containing donor and acceptor blocks with suitably positioned energy levels for use in a solar cell. This is the first block copolymer to be based on an existing high-performance polymer:polymer blend. We observe phase separation of the blocks and self-assembly behavior. In ternary blends with the respective homopolymers the diblock copolymer introduces lateral nanostructure without restricting P3HT crystallization in the charge transport direction, resulting in standing lamellae. By adding the diblock to the homopolymer blend as a compatibilizer, we prevent phase separation at elevated temperatures and benefit from a dramatic increase in P3HT ordering, allowing us to demonstrate polymer blend photovoltaics where the nanostructure is thermodynamically, rather than kinetically, controlled.

Photovoltaic devices made from organic thin fi lms of polymers and small molecules have received increasing attention due to their high fl exibility and the possibility of low-cost preparation of large-area devices by solution processing. The deposition of polymer mixtures from solution is accompanied by complex phase segregation phenomena caused by their low entropy of mixing. This results in non-equilibrium microstructures which depend on the deposition method, materials, solvents, substrate, temperature, and atmosphere. [ 1 , 2 ] For applications in bulk-heterojunction photovoltaic devices, a certain degree of phase separation is desired to allow effi cient charge separation and transport while retaining effi cient exciton dissociation. [ 3 ] The structural characterization of organic blends is rather challenging due to the high similarity of the components, e.g., in mass and electron density. Atomic force microscopy (AFM) and transmission/scanning electron microscopy (TEM/SEM) provide suffi cient resolution ( ∼ 3‐5 nm), but it is diffi cult for these techniques to differentiate between two organic components. [ 3 , 4 ] Where the blend components have different elemental composition, energyfi ltered TEM in combination with electron-energy-loss spectroscopy can be used to provide enhanced contrast between phases. [ 4 ] Other material-sensitive methods are limited by resolution (such as confocal photoluminescence microscopy, which is diffraction-limited) or are merely surface-sensitive (such as scanning probe microscopies). [ 2 , 5 ] Recent progress has been made using a soft X-ray microscopy technique that allows adequate resolution ( ∼ 35 nm), but requires synchrotron access. [ 6 ] Two common techniques in high-resolution imaging on otherwise indistinguishable materials are selective staining (as used for TEM) or selective etching (for SEM and AFM). [ 7 ] Both are commonly used on inorganic and organic structures and require an exclusive interaction of one of the components with a chemical compound, in the

We investigate the transient photocurrents of organic photovoltaic devices in response to a sharp turn-on of illumination, by numerical modeling of the drift-diffusion equations. We show that the photocurrent turn-on dynamics are determined not only by the transport dynamics of free charges, but also by the time required for the population of geminate charge pairs to reach its steady-state value. The dissociation probability of a geminate charge pair is found to be a key parameter in determining the device performance, not only by controlling the efficiency at low intensities, but also in determining the fate of charge pairs formed by bimolecular recombination at high intensities. Bimolecular recombination is shown to reduce the turn-on time at high intensities, since the typical distance traveled by a charge pair is reduced.

Localized plasmon resonances of spherical nanovoid arrays strongly enhance solar cell performance by a factor of 3.5 in external quantum efficiency at plasmonic resonances, and a four-fold enhancement in overall power conversion efficiency. Large area substrates of silver nanovoids are electrochemically templated through self-assembled colloidal spheres and organic solar cells fabricated on top. Our design represents a new class of plasmonic photovoltaic enhancement: that of localized plasmon-enhanced absorption within nanovoid structures. Angularly-resolved spectra demonstrate strong localized Mie plasmon modes within the nanovoids. Theoretical modelling shows varied spatial dependence of light intensity within the void region suggesting a first possible route towards Third Generation plasmonic photovoltaics.

Surface states in ZnO nanoparticles play an important role in controlling their electrical transport properties, and these properties can be modified greatly by exposure to UV light. In order to investigate the origin of these effects, we investigate the change in the optical absorption of films of ZnO nanoparticles on exposure to UV light. The modulation spectrum changes from the first derivative to the second derivative of the absorption spectrum as the films are annealed. UV illumination changes the surface states of the nanocrystals, leading to a change in the electric field within the films, which we study using electrostatic force microscopy. The modulation of the optical absorption is found to be consistent with a Stark effect caused by the change in the electric field.

We present a spectroscopic and theoretical investigation of the effect of the presence and position of hexyl side-chains in the novel low-bandgap alternating donor-acceptor copolymer poly[bis-N,N-(4-octylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine-alt-5,5'-4',7',-di-2-thienyl-2',1',3'-benzothiadiazole] (T8TBT). We use electronic absorption and Raman spectroscopic measurements supported by calculations of chain conformation, electronic transitions, and Raman modes. Using these tools, we find that sterically demanding side-chain configurations induce twisting in the electronic acceptor unit and reduce the electronic interaction with the donor. This leads to a blue-shifted and weakened (partial) charge-transfer absorption band together with a higher photoluminescence efficiency. On the other hand, sterically relaxed side-chain configurations promote coupling between donor and acceptor units and exhibit enhanced absorption at the expense of luminescence efficiency. The possibility of tuning the donor-acceptor character of conjugated polymers by varying the placement of side-chains has very important ramifications for light emitting diode, Laser, display, and photovoltaic device optimization.

The nanoscale morphology in polymer:PCBM based photovoltaic devices is a major contributor to overall device performance. The disordered nature of the phase-separated structure, in combination with the small length scales involved and the inherent diffi culty of reproducing the exact morphologies when spin-coating and annealing thin blend fi lms, have greatly hampered the development of a detailed understanding of how morphology impacts photo voltaic device functioning. In this paper we demonstrate a double nanoimprinting process that allows the formation of nanostructured polymer:PCBM heterojunctions of composition and morphology that can be selected independently. We fabricated photovoltaic (PV) devices with extremely high densities (10 14 mm − 2 ) of interpenetrating nanoscale columnar features (as small as 25 nm; at or below the exciton diffusion length) in the active layer. By comparing device results of different feature sizes and two different polymer:PCBM combinations, we demonstrate how double imprinting can be a powerful tool to systematically study different parameters in polymer photovoltaic devices.

The effects of the side chains on the loss processes in three polymer:PC[61]BM organic blends are studied by comparing transient absorption spectroscopy and solar cell measurements. It is shown that, after efficient photoinduced charge transfer, charge pairs are the only significant species present in the blend and that their recombination can be followed by monitoring the PIA kinetics. Charges are found to decay following two regimes. The subnanosecond decay of the charge signal, responsible for the losses of 55–75% of the charges, is well described by an Onsager–Braun geminate recombination model. The rates are found to be strongly affected by the presence and position of side chains. We attribute these changes to a better aggregation of the side-chain-less polymer that provides a better delocalization of the holes and a better mobility. At longer times (after about 10 ns) the charges undergo bimolecular recombination. By modeling the time and intensity dependence of this process, we measure the rate co...

We report on measurements of electroluminescence (EL) in poly(3-hexylthienylene), and compare its EL properties with those of poly(3-dodecylthienylene) using calcium and aluminium as electron-injecting contacts. We find efficiencies significantly higher than those reported elsewhere, and achieve values of 0.2 % photons per electron in poly(3-dodecylthienylene). We present detailed EL spectra showing well-resolved phonon structure and highlight the role of defects in the performance of the devices.

We have been working to characterise the electronic excitations created by charge injection into conjugated polymers such as poly(p-phenylenevinylene) in electroluminescent devices. We report on the optical probing of electronic states injected into an indium-tin oxide/PPV/calcium electroluminescent device. We monitor changes in absorption in the polymer film due to charge carrier injection into the device by detecting the reflection of a probe beam off the back metal contact layer. These injected carriers are expected to reside in states with energy levels within the band gap and hence give rise to a change in the sub-band gap optical properties. We observe absorption features centred around 1.38 eV, 1.55 eV, 0.65 eV. These states closely resemble those seen in photoinduced absorption and have been assigned to optical transitions involving bipolarons (0.65 and 1.55 eV) and a triplet-exciton (1.38 eV).

In this letter, we examine the effect of charge trapping on geminate recombination and organic photovoltaic performance using a Monte Carlo model. We alter the degree of charge trapping by considering energetic disorder to be spatially uncorrelated or correlated. On correlating energetic disorder, and so reducing the degree of trapping, it is found that power conversion efficiency of blend and bilayer devices improves by factors of 3.1 and 2.6, respectively. These results are related to the experimental data and quantum chemical calculations for poly[9,9-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine] (PFB)/poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as well as poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl-C(61)-butyric acid methyl ester (PCBM) solar cell systems. The minimization of traps at the heterojunction between electron- and hole-accepting materials, perhaps by molecular design, appears to be a promising strategy to achieve large gains in PV performance. It is also shown that macroscopically measurable quantities such as mobility and energetic disorder are not necessarily good predictors of nanoscale geminate recombination process.

We have used space-charge limited current measurements to study the mobility of holes and electrons in two fluorene-based copolymers for temperatures from 100 to 300 K. Interpreting the results using the standard analytical model produced an Arrhenius-type temperature dependence for a limited temperature range only and mobility was found to be apparently dependent on the thickness of the polymer film. To improve on this, we have interpreted our data using a numerical model that takes into account the effects of the carrier concentration and energetic disorder on transport. This accounted for the thickness dependence and gave a more consistent temperature dependence across the full range of temperatures, giving support to the ex-tended Gaussian disorder model for transport in disordered polymers. Furthermore, we find that the same model adequately describes both electron and hole transport without the need to explicitly include a distribution of electron traps. Room-temperature mobilities were found to be in the region of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }{\text{cm}}^{2}\text{ }{\text{V}}^{\ensuremath{-}1}\text{ }{\text{s}}^{\ensuremath{-}1}$ in the limit of zero field and zero carrier density with disorders of $110\ifmmode\pm\else\textpm\fi{}10$ and $100\ifmmode\pm\else\textpm\fi{}10\text{ }\text{meV}$ for polymers $\text{poly}{9,9\text{-dioctylfluorene-co-bis}[N,{N}^{\ensuremath{'}}\text{\ensuremath{-}}(4\ensuremath{-}\text{butylphenyl})]\text{bis}(N,{N}^{\ensuremath{'}}\text{-phenyl-}1,4\text{-phenylene})\text{diamine}}$ and poly(9,9-dioctylfluorene-co-benzothiadiazole), respectively.

In this paper, we demonstrate a double nanoimprinting process that allows the formation of nanostructured polymer heterojunctions of composition and morphology that can be selected independently. We fabricated photovoltaic (PV) devices with extremely high densities (10(14)/mm(2)) of interpenetrating nanoscale columnar features in the active polymer blend layer. The smallest feature sizes are as small as 25 nm on a 50 nm pitch, which results in a spacing of heterojunctions at or below the exciton diffusion length. Photovoltaic devices based on double-imprinted poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2',2''-diyl) (F8TBT)/ poly(3-hexylthiophene) (P3HT) films are among the best polymer-polymer blend devices reported to date with a power conversion efficiency (PCE, eta(e)) of 1.9%.

We have constructed electroluminescent diodes using several layers of conjugated polymers with differing band gaps; these provide a range of different colour light-emitting layers and can be used to control charge injection and transport. Poly(1,4-phenylenevinylene, PPV, and derivatives have been used, with indium/tin oxide as hole-injecting layer and calcium as electron-injecting contact layer. For this selection of materials, we show that the ordering of the polymer layers allows control of the colour of device emission. Emission can be produced in more than one layer.

We present quasi-steady-state photoinduced absorption measurements on thin films of CdSe nanocrystals dispersed in a polystyrene matrix. For nanocrystals treated with pyridine we observe an intense, size-dependent absorption peaking in the mid-infrared when the samples are irradiated with visible light. This infrared absorption is associated with a size-dependent bleach in the visible, near the peak of the first excitonic absorption. We attribute the infrared absorption to an intraband electron transition in the quantum dots and measure the lifetime of the absorbing state to be 1.0 ms at 295 K.

Colloidally grown CdSe nanocrystals with epitaxial ZnS shells show highly efficient, size-tunable luminescence. We report the incorporation of films of these core-shell nanocrystals into wavelength-scale, high-Q, planar microcavities. Under optical excitation, we find that emission from the nanocrystals couples to the discrete optical modes of the microcavity. The broad free-space emission spectrum of the nanocrystals is modified by the presence of the microcavity, giving a series of sharp emission lines with wavelengths determined by the cavity dimension. Our experiments demonstrate that microcavities with semiconductor emitters can be conveniently fabricated using spin-coating techniques. We find that, at room temperature, the microcavity emission spectrum is independent of excitation intensity for excitation densities up to approximately one electron–hole pair per nanocrystal.

We have investigated the effects of thickness variation and thermal treatment of the electrode polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) in photovoltaic and photodetector devices using conjugated polymer blends as the photoactive material. By variation of the PEDOT:PSS layer thickness between 25 and 150 nm, we found optimum device performance, in particular low dark current and high external quantum efficiency (EQE) and open-circuit voltage (Voc), at around 70 nm. This has been observed for two different active layers. Annealing studies on the PEDOT:PSS films, with temperatures varied between 120 and 400°C, showed an optimum device performance, in particular EQE and Voc at 250°C. This optimum performance was found to be associated with loss of water from the PSS shell of the PEDOT:PSS grains. For annealing temperatures above 260°C, device performance was dramatically reduced. This was associated with chemical decomposition leading to loss of sulfonic acid, although this did not significantly affect the in-plane conductivity. © 2009 American Chemical Society.

Although in principle very promising, photovoltaic technology has so far failed to deliver robust high efficiency modules at affordable prices. Despite considerable research, high efficiency silicon based cells remain expensive, while the more recent organic photovoltaics are still struggling with low efficiencies and short lifetimes. Meanwhile, over the last few years, the study of localized plasmons [1,2] has also received great attention due to the high field enhancements associated with confined fields , with a wide range of applications possible, from optical switches to substrates for surface enhanced Raman spectroscopy (SERS). Here we discuss how combining the structures normally used in photovoltaic devices with metallic cavities supporting localized plasmons can lead to considerable improvements in the performance of solar cells. In particular we show how by changing the shape and size of spherical voids on a metallic surface, one can tune the plasmon modes to obtain significant absorptions across the solar spectrum [3]. By coating one such nanocavity surface with a sub 100 nm-layer of semiconductor, we can create a nanostructured solar cell, where the localised Mie modes efficiently couple light into the semiconductor layer. As the plasmons electric field enhancement is largest very close to the surface, significant absorption can be maintained even when the semiconductor thickness is reduced to below the typical exciton diffusion length. In addition minority carrier transport is improved. That means we can beat the usual balance between light absorption and exciton recombination loses, and so significantly increase the overall efficiency of the photovoltaic devices.

We use a Monte Carlo model to predict the effect of composition, domain size, and energetic disorder upon the mobility of carriers in an organic donor-acceptor blend. These simulations show that, for the changes in local morphology expected within the thickness of a typical bulk heterojunction photovoltaic device, changes in mobility of more than an order of magnitude are expected. The impact of nonuniform mobility upon space-charge-limited diode and photovoltaic (PV) device performance is examined using a drift-diffusion model. The current passing through a space-charge-limited diode is shown to depend upon the position of the layers with differing mobility. Accurate modeling of the current in such devices can only be achieved using a drift-diffusion model incorporating nonuniform mobility. Inserting a 20 nm thick layer in which the mobility is less by one order of magnitude than in the rest of the 70 nm thick PV device reduced the device efficiency by more than 20%. Therefore it seems vital to exert a high degree of control over the morphology throughout the entire blend PV device, otherwise potential PV performance may be lost.

A Monte Carlo model is used to examine geminate pair dissociation in polymer-polymer photovoltaic devices. It is found that increasing one or both carrier mobilities aids geminate separation yield eta(GS) particularly at low fields. This, in turn, leads to improved maximum power output from polymer-polymer blend photovoltaics, even when carrier mobilities are unbalanced by a factor of 10. The dynamic behaviors of geminate charges that eventually separate and recombine are examined for the first time. It is shown that geminate pairs in a bilayer become effectively free when separated by approximately 4 nm, which is far smaller than the thermal capture radius of 16 nm here. This may lead one to expect that eta(GS) would not be limited by the separation allowed by the morphology once the domain size has increased above 4 nm. In fact it is found that eta(GS) in a blend improves continuously as the average domain size increases from 4 to 16 nm. We show that although a small degree of separation may be available in a blend, the limited number of possible routes to further separation makes charge pairs in blends more susceptible to recombination than charge pairs in a bilayer.

We investigate the rate constant for Langevin-type bimolecular recombination using a Monte Carlo model. It is found that filamentary transport, mobility anisotropy, restriction of transport in one dimension, and confinement of electrons and holes to one transporting component within a blend can cause significant deviations from classical Langevin behavior. We discuss in detail the reasons for the failure of the Langevin model for these instances, provide alternative formula, and comment upon the implications for polymer transistors, LEDs, and photovoltaics.

A detailed charge recombination mechanism is presented for organic photovoltaic devices with a high open-circuit voltage. In a binary blend comprised of polyfluorene copolymers, the performance-limiting process is found to be the efficient recombination of tightly bound charge pairs into neutral triplet excitons. We arrive at this conclusion using optical transient absorption (TA) spectroscopy with visible and IR probes and over seven decades of time resolution. By resolving the polarization of the TA signal, we track the movement of polaronic states generated at the heterojunction not only in time but also in space. It is found that the photogenerated charge pairs are remarkably immobile at the heterojunction during their lifetime. The charge pairs are shown to be subject to efficient intersystem crossing and terminally recombine into F8BT triplet excitons within approximately 40 ns. Long-range charge separation competes rather unfavorably with intersystem crossing--75% of all charge pairs decay into triplet excitons. Triplet exciton states are thermodynamically accessible in polymer solar cells with high open circuit voltage, and we therefore suggest this loss mechanism to be general. We discuss guidelines for the design of the next generation of organic photovoltaic materials where separating the metastable interfacial charge pairs within approximately 40 ns is paramount.

The x-ray stability of photodiodes made of poly(9,9-di- n -octylfluorene-co-benzothiadiazole):perylene diimide, poly[2,7-(9,9-di- n -octylfluorene)-co-(1,4-phenylene-[(4-sec-butylphenyl)imino]-1,4-phenylene)]: perylene diimide and poly(3-hexylthiophene):([6,6]-phenylC61-butyric acid methyl ester) (P3HT:PCBM) blends has been examined up to lifetime doses equivalent to those used in medical x-ray digital imaging applications. Dark currents and external quantum efficiencies (EQEs) are not significantly affected after exposure to 500 Gy. Only in the case of P3HT:PCBM is a significant loss in EQE (17% of the initial value) observed. Possible reasons for the observed changes are proposed. When a scintillation layer is attached to the devices, a linear dependence of the photocurrent on the x-ray dose rate is observed for the three material systems. © 2008 American Institute of Physics.

The triplet excited-state $({T}_{1}\ensuremath{\rightarrow}{T}_{n})$ absorption cross section of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) was calculated from quasi-steady-state photoinduced absorption measurements of films of the polymer blended with iridium (III) [(bis 4,6-difluorophenyl)-pyridinato-$N,\text{ }\mathrm{C}{2}^{\ensuremath{'}}$] picolinate (FIrpic). Steady-state photoluminescence spectra and photoinduced absorption results showed that quenching of the FIrpic phosphorescence originated from triplet energy transfer from FIrpic to F8BT. By comparing the strength of the photoinduced absorption with the known rate of triplet generation in the F8BT, the triplet excited-state absorption cross section of F8BT was found to be $3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}\ifmmode\pm\else\textpm\fi{}20%$. This value is relatively small compared with that for poly($p$-phenylenevinylene)s, but is consistent with previous quantum-chemical calculations. Accurate knowledge of the triplet excited-state absorption cross section is valuable for quantifying the populations of nonemissive triplet excitons in polymer-based devices.

Solar cells based on conjugated-polymer and methanofullereneblended thin films are a promising route to low-cost solution-processedphotovoltaics.

Conformational disorder of conjugated polymers is an important issue to be understood and quantified. In this paper we present a new method to assess the chain conformation of conjugated polymers based on measurements of intrachain energy transfer. The chain conformation is modeled on the basis of monomer-monomer interactions, such as torsion, bending, and stretching of the connecting bond. The latter two potentials are assumed to be harmonic, while the torsional potential was calculated by density functional theory using B3-LYP functional with the SVP basis set. The energy transfer dynamics of excitons on these chains are quantitatively simulated using Forster-type line-dipole energy transfer. This allows us to compare the simulated ground state conformation of single polymer chains to ultrafast depolarization experiments of poly [3-(2,5-dioctylphenyl)thiophene] in solution. We identify torsional rotation as the main contributor to conformational disorder and find that this disorder is mainly controlled by the energy difference between syn and anti bonds.

In isolated conjugated polymers two explanations are in discussion for the redshift of the emission on a picosecond time scale-exciton energy transfer (EET) between conjugated segments along the chains and conformational changes of these segments themselves, i.e., torsional relaxation. In order to resolve this question we perform femtosecond time-resolved transient absorption measurements of the energy relaxation of poly[3-(2,5-dioctylphenyl)thiophene] in toluene solution. We show that torsional relaxation can be distinguished from EET by site-selectively exciting low-energy conjugated segments. We present a unified model that integrates EET and torsional dynamics. In particular, comparison to ultrafast depolarization measurements shows that torsional dynamics cannot be neglected when analyzing EET dynamics and furthermore reveals that the exciton extends itself by about 2 monomer units during torsional relaxation.