“Nonfullerene” acceptors are proving effective in bulk heterojunction (BHJ) solar cells when paired with selected polymer donors. However, the principles that guide the selection of adequate polymer donors for high-efficiency BHJ solar cells with nonfullerene acceptors remain a matter of some debate and, while polymer main-chain substitutions may have a direct influence on the donor–acceptor interplay, those effects should be examined and correlated with BHJ device performance patterns. This report examines a set of wide-bandgap polymer donor analogues composed of benzo[1,2-b:4,5-b′]dithiophene (BDT), and thienyl ([2H]T) or 3,4-difluorothiophene ([2F]T) motifs, and their BHJ device performance pattern with the nonfullerene acceptor “ITIC”. Studies show that the fluorine- and ring-substituted derivative PBDT(T)[2F]T largely outperforms its other two polymer donor counterparts, reaching power conversion efficiencies as high as 9.8%. Combining several characterization techniques, the gradual device performance improvements observed on swapping PBDT[2H]T for PBDT[2F]T, and then for PBDT(T)[2F]T, are found to result from (i) notably improved charge generation and collection efficiencies (estimated as ≈60%, 80%, and 90%, respectively), and (ii) reduced geminate recombination (being suppressed from ≈30%, 25% to 10%) and bimolecular recombination (inferred from recombination rate constant comparisons). These examinations will have broader implications for further studies on the optimization of BHJ solar cell efficiencies with polymer donors and a wider range of nonfullerene acceptors.

[*] Prof. J. R. Durrant, Mr. A. Maurano, Dr. R. Hamilton, Dr. C. G. Shuttle, Dr. B. O’Regan, Dr. W. Zhang, Prof. I. McCulloch Departments of Chemistry, Imperial College London South Kensington SW7 2AZ (United Kingdom) E-mail: j.durrant@imperial.ac.uk Dr. A. M. Ballantyne, Prof. J. Nelson Departments of Physics, Imperial College London South Kensington SW7 2AZ (United Kingdom) Dr. H. Azimi, Dr. M. Morana, Prof. C. J. Brabec Konarka Austria, Altenbergerstrasse 69 A-4040 Linz (Austria) Dr. H. Azimi Christian Doppler Laboratory for Surface Optics Johannes Kepler University Linz (Austria)

We report magnetoconductance measurements on polymer:fullerene photovoltaic devices in the regime of high fields and low temperatures, where spin polarization of injected carriers plays an important role. The current either decreases or increases with magnetic field, depending on whether the interfacial charge-transfer state lies above or below the intramolecular triplet state in energy. Rapid transitions from triplet charge-transfer states to lower-lying triplet excitons constitute an important loss mechanism that is responsible for the negative magnetoconductance observed.

We present light intensity dependent measurements of the quantum efficiency of P3HT:PCBM photovoltaic devices. Unlike previous studies we focus on ultralow light intensities down to 10−3 mW/cm2. We find that although when the devices are excited at intensities close to 1 Sun they exhibit very little bias or light intensity dependence, this is clearly not the case for light intensities below 1 mW/cm2, where the cell’s efficiency becomes highly dependent on the bias and light intensity. Using a simple model for the device efficiency we can fit the experimental data across a wide range of parameters and thus separate the effects of generation efficiency (geminate recombination) and charge recombination. Our finding suggests that recombination through trap (charge transfer) states is an important loss mechanism and we are able to quantify the density and depth of these states.

We explore the interrelation between density of states, recombination kinetics, and device performance in effi cient poly(4,8-bis-(2-ethylhexyloxy)- benzo(1,2-b:4,5-b')dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one) thieno(3,4-b)thiophene-2,6-diyl):(6,6)-phenyl-C 71 -butyric acid methyl ester (PBDTTT-C:PC 71 BM) bulk-heterojunction organic solar cells. We modulate the active-layer density of states by varying the polymer:fullerene com- position over a small range around the ratio that leads to the maximum solar cell effi ciency (50-67 wt% PC 71 BM). Using transient and steady-state techniques, we fithat nongeminate recombination limits the device effi - ciency and, moreover, that increasing the PC 71 BM content simultaneously increases the carrier lifetime and drift mobility in contrast to the behavior expected for Langevin recombination. Changes in electronic properties with fullerene content are accompanied by a signifi cant change in the magnitude or energetic separation of the density of localized states. Our comprehensive approach to understanding device performance represents signifi cant progress in understanding what limits these high-effi ciency polymer:fullerene systems.

We demonstrate the use of a simple charge extraction measurement to determine the charge carrier densities n in annealed poly(3-hexylthiophene):methanofullerene solar cells under operating conditions. By applying charge extraction to the device under forward bias in the dark (Jdark), we find Jdark∝n2.6. This dependence on charge density is the same as that we find for bimolecular recombination losses observed in such devices under irradiation at open circuit, suggesting that the dark current originates from bimolecular recombination at the polymer/fullerene interface.

Transient mobility spectroscopy (TMS) is presented as a new tool to probe the charge carrier mobility of commonly employed organic and inorganic semiconductors over the relevant range of charge densities. The charge density dependence of the mobility of semiconductors used in hybrid and organic photovoltaics gives new insights into charge transport phenomena in solid state dye sensitized solar cells.

Transient absorption spectroscopy has been used to investigate charge photogeneration in organic solar cells based upon PCPDTBT/PC70BM (poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]/[6,6]-phenyl-C71-butyric acid methyl ester) photoactive layers as a function of externally applied bias. We find that the yield of dissociated polarons is independent of applied bias in these devices.

This paper in support of the grid resource management and scheduling techniques based on grid computing environment for use in single-processor Resource Advanced Reservation(AR) problem, given the rigid scheduling algorithms and slack scheduling algorithms of support resource AR, and the simulation results on the performance of the two algorithms were studied. The results show that rigid resource reservation operations to ensure the implementation of AR jobs, slack scheduling reduces the average waiting time and average slowdown factor of non-reserved job, improved the performance of non-reserved jobs.

This article reviews some of our recent progress on materials design guidelines for photoinduced charge generation in bulk-heterojunction organic solar cells. Over the last 7 years, our group has employed transient absorption measurement to determine the relative quantum yields of long-lived polaron pairs for over 300 different organic Donor/Acceptor blend films. We have shown that this optical assay of charge separation can be a strong indicator of photocurrent generation efficiency in complete devices. In this review, we consider the lessons that can be drawn from these studies concerning the parameters that determine efficiency of this photoinduced charge separation in such solar cells. We consistently find, from studies of several materials series, that the energy offset driving charge separation is a key determinant of the efficiency of this charge generation, and thereby photocurrent generation. Moreover, we find that the magnitude of the energy offset required to drive charge separation, and the st...

This article presents a critical discussion of the various physical processes occurring in organic bulk heterojunction (BHJ) solar cells based on recent experimental results. The investigations span from photoexcitation to charge separation, recombination, and sweep‐out to the electrodes. Exciton formation and relaxation in poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and poly‐3(hexylthiophene) (P3HT) are discussed based on a fluorescence up‐conversion study. The commonly accepted paradigm describing the conversion of incident photons into charge carriers in the BHJ material is re‐examined in light of these femtosecond time‐resolved measurements. Transient photoconductivity, time‐delayed collection field, and time‐delayed dual pulse experiments carried out on BHJ solar cells demonstrate the competition between carrier sweep‐out by the internal field and the loss of photogenerated carriers by recombination. Finally, an emerging hypothesis is discussed: that bimolecular recombination accounts for the majority of recombination from short circuit to open circuit in optimized solar cells, and that bimolecular recombination is bias‐ and charge‐density‐dependent. The study of recombination loss processes in organic solar cells leads to insights into what must be accomplished to achieve the “ideal” solar cell.

This Perspective discusses the physics, implementation and findings of Kinetic Monte Carlo (KMC) models applied to organic photovoltaic devices (OPVs). It is shown that KMC models can relate morphology and energy levels on the nano-scale to geminate and non-geminate recombination, charge transport, charge injection and charge extraction measured in macro-scale OPVs. In particular, KMC investigations probing the circumstances under which geminate recombination is, and is not, a significant loss mechanism in OPVs are reviewed. Furthermore, the mechanisms which yield non-geminate (bimolecular) recombination in OPVs that is both slower than the predictions of Langevin, and charge density dependent, are discussed. It is also shown how KMC models can predict average mobility, as well as spatial heterogeneity and temporal dispersion around this average value in disordered bulk heterojunctions. Finally, it is shown how KMC can be used to quantify the effect of non-ideal electrodes, interlayers and surface wetting layers on OPV performance.

There are numerous theoretical approaches to estimating the power conversion efficiency (PCE) of organic solar cells (OSCs), ranging from the empirical approach to calculations based on general considerations of thermodynamics. Depending on the level of abstraction and model assumptions, the accuracy of PCE estimation and complexity of the calculation can change dramatically. In particular, PCE estimation with a drift-diffusion approach (widely investigated in the literature), strongly depends on the assumptions made for the physical models and optoelectrical properties of semiconducting materials. This has led to a huge deviation as well as complications in the analysis of simulated results aiming to understand the factors limiting the performance of OSCs. In this work, we intend to highlight the complex relation between mobility, exciton dynamics, nanoscale dimension, and loss mechanisms in one framework. Our systematic analysis represents key information on the sensitivity of the drift-diffusion approach, to estimate how physical parameters and physical processes bind the PCE of the device under the influence of structure, contact, and material layer properties. The obtained results ultimately led to recommendations for putting effort into certain properties to get the most out of avoidable losses, presented the impact and importance of modification of material properties, and in particular, recommended to what degree the design of new material could improve OSC performance.

The ultrafast charge generation process in organic solar cell devices is investigated by transient reflection spectroscopy on five state-of-the-art bulk heterojunction systems. The charge generation process in operating devices is found to be a combination of an ultrafast generation mechanism over several hundred femto-seconds and a slow process from pico-seconds to nanoseconds, limited by exciton diffusion dynamics. In addition, the lack of electric field dependence in the charge dynamics rules out geminate recombination as an important loss mechanism.

The surface plasmonic effect and scattering effect of gold nanorods (AuNRs) on the performance of bulk heterojunction photovoltaic devices based on the blend of polythiophene and fullerene are investigated. AuNRs enhance the excitation since the plasmonic effect increases the electric field, mainly in the area near the interface between the active layer and AuNRs. The results show that the incident photo-to-electron conversion efficiency (IPCE) obviously increases for the device with a layer of gold nanorods, resulting from the plasmonic effect of AuNRs in the range of 500–670 nm and the scattering effect in the range of 370–410 nm. The power conversion efficiency (PCE) is increased by 7.6% due to the near field effect of the localized surface plasmons (LSP) of AuNRs and the scattering effect. The short circuit current density is also increased by 9.1% owing to the introduction of AuNRs. However, AuNRs can cause a little deterioration in open circuit voltage.

The physical origin of the open‐circuit voltage in bulk heterojunction solar cells is still not well understood. While significant evidence exists to indicate that the open‐circuit voltage is limited by the molecular orbital energies of the heterojunction components, it is clear that this picture is not sufficient to explain the significant variations which often occur between cells fabricated from the same heterojunction components. We present here an analysis of the variation in open‐circuit voltage between 0.4–0.65 V observed for a range of P3HT/PCBM solar cells where device deposition conditions, electrode structure, active‐layer thickness and device polarity are varied. The analysis quantifies non‐geminate recombination losses of dissociated carriers in these cells, measured under device operating conditions. It is found that at open‐circuit, losses due to non‐geminate recombination are sufficiently large that other loss pathways may effectively be neglected. Variations in open‐circuit voltage between different devices are shown to arise from differences in the rate coefficient for non‐geminate recombination, and from differences in the charge densities in the photoactive layer of the device. The origin of these differences is discussed, particularly with regard to variations in film microstructure. By separately quantifying these differences in rate coefficient and charge density, and by application of a simple physical model based upon the assumption that open‐circuit is reached when the flux of charge photogeneration is matched by the flux of non‐geminate recombination, we are able to calculate correctly the open‐circuit voltage for all the cells studied to within an accuracy of ±5 mV.

The output open-circuit voltage of bulk heterojuntion polymer-fullerene solar cells exhibts an offset of Δ≈0.3 V with the difference between the donor highest-occupied molecular orbital (HOMO) and the acceptor lowest-unoccupied molecular orbital (LUMO), as discussed by Scharber et al. [Adv. Mater. (Weinheim, Ger.) 18, 789 (2006)]. We show that the energetic disorder of the electronic density-of-states (DOS) of acceptor and donor materials reduces the splitting of the electron and hole Fermi levels in bulk heterojunction solar cells, by restricting the accumulation of carriers into the tails of the DOS. A simple model based on the kinetic balance between photogeneration and recombination fluxes provides a lower limit to the energy offset Δ>(σn2+σp2)/kBT in terms of the disorder parameters.

The non-geminate recombination of charge carriers in polymer-fullerene solar cells has been modeled in the last few years with a trap-assisted recombination model, which states that the apparent recombination order depends on the concentration of trapped charges tailing into the band gap. Higher concentrations of trapped charges lead to higher apparent recombination orders. In this work, the mass fraction f of highly crystalline nanofibrillar P3HT to the total P3HT content in P3HT:PCBM solar cells is consistently varied, controlling the temperature of a nanofibers-P3HT casting dispersion. A systematic study of the apparent recombination order, measured with a transient photovoltage technique, as a function of f is presented. A correlation is shown between the apparent recombination order, the P3HT crystallinity, and the trap concentration in the band gap measured with an admittance spectroscopy technique

The morphology of the active layer in polymer:fullerene solar cells is a key parameter in determining their performance. In this study we monitor the charge carrier dynamics in bilayers of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (fabricated by sequential spin coating and vacuum deposition) before and after thermal annealing, and compare these against conventional solution processed bulk heterojunction (BHJ) blend films. Transmission electron microscopy images, supported by field effect mobility data show that while not-annealed P3HT/PC61BM bilayers possess a sharp interface, intermixing proceeds upon annealing. Transient absorption studies indicate that the not-annealed bilayer yields fewer, but longer lived, charge carriers compared to the BHJ. Monte Carlo (MC) simulations further suggest that the difference in non-geminate recombination dynamics observed for the BHJ and bilayer films could be related to the confinement of charge carriers to the interface, with the lower dimensionality for the flat interface bilayer films relative to the intercalated donor-acceptor network BHJ films leading to lower recombination.

The interfacial charge transfer recombination processes under working conditions that limit the device performance in quantum dot/polymer bulk heterojunction solar cells have been investigated. We have employed spectroscopic techniques as charge extraction (CE), laser transient absorption spectroscopy (L-TAS) and transient photovoltage (TPV). From the CE technique, we have observed an exponential charge accumulation in the device when light intensity is increased, as it has been related to conventional molecular photovoltaic devices. The L-TAS showed that recombination dynamics, in thin films, are dominated by the presence of trap in both, the nanocrystals and the polymer materials. Finally, from the TPV experiments we observed a strong carrier lifetime, in complete devices, upon charge density. This strong dependency of charge concentration vs charge lifetime has been found to be higher than in organic photovoltaics based on polymer and fullerenes. The comparison between the recombination dynamics from TPV and LTAS, performed with and without electrodes, respectively, allow us to exclude any process related with the metal contacts in the strong dependency of the recombination with the charge density.