Simultaneous enhancement in open circuit voltage and short circuit current of hybrid organic-inorganic photovoltaics by inorganic interfacial modification

Here, we investigate the model poly(3-hexathiophene)/ZnO system and show that by introducing a caesium carbonate interlayer, a simultaneous increase in all photovoltaic parameters can be achieved.We kindly acknowledge Prof. Uwe Bunz and Prof. Annemarie Pucci for access to device fabrication facilities and AFM measurements, respectively. P.E.H. thanks the Excellence Initiative for Funding. A.A.B. is a Royal Society University Research Fellow.This is the author accepted manuscript. The final version is available from The Royal Society of Chemistry via http://dx.doi.org/10.1039/C5TC03206

Monolayer Organic Field-Effect Phototransistors: Photophysical Characterization and Modeling

Оrganic field-effect transistors (OFET) can combine photodetection and light amplification and, for example, work as phototransistors. Such organic phototransistors can be used in light-controlled switches and amplifiers, detection circuits, and sensors of ultrasensitive images. In this work, we present photophysical characterization of well-defined ultrathin organic field-effect devices with a semiconductive channel based on Langmuir-Blodgett monolayer film. We observe clear generation of photocurrent under illumination with a modulated laser at 405 nm. The increase of photocurrent with the optical modulation frequency indicates the presence of defect states serving as traps for photogenerated carriers and/or the saturation of charge concentration in the thin active layer. We also propose a simple one-dimensional numerical model of a photosensitive OFET. The model is based on the Poisson, current continuity and drift-diffusion equations allows future evaluation of the photocurrent generation mechanism in the studied systems.JZ wants to thank China Scholarship Council for supporting PhD studies. AAB is Royal Society University Research Fellow. The research was supported by UK-Russia International Collaborations Grant by Royal Society (IE150071) and Russian Foundation for Basic Research (15-53-10070 KO_а)

Morphology, Temperature, and Field Dependence of Charge Separation in High-Efficiency Solar Cells Based on Alternating Polyquinoxaline Copolymer

Charge separation and recombination are key processes determining the performance of organic optoelectronic devices. Here we combine photoluminescence and photovoltaic characterisation of organic solar cell devices with ultrafast multi-pulse photocurrent spectroscopy to investigate charge generation mechanisms in the organic photovoltaic devices based on a blend of an alternating polyquinoxaline copolymer with fullerene. The combined use of these techniques enables the determination of the contributions of geminate and bimolecular processes to the solar cell performance. We observe that charge separation is not a temperature-activated process in the studied materials. At the same time, the generation of free charges shows a clear external-field and morphology dependence. This indicates that the critical step of charge separation involves the non-equilibrium state that is formed at early times after photoexcitation, when the polaronic localisation is not yet complete. This work reveals new aspects of molecular level charge dynamics in the organic light-conversion systems.We thank Maxim Pschenichnikov for useful discussions, and Ergang Wang for providing TQ1. This work was supported by the Netherlands Organization for Scientific Research (NWO) through the “Stichting voor Fundamenteel Onderzoek der Materie” (FOM). A.A.B. also acknowledges a VENI grant from NWO. A.A.B. is currently a Royal Society University Research Fellow. Photovoltaics research at Linköping was supported by the Swedish Research Council (VR), the European Commission Marie Skłodowska-Curie actions, the Swedish Energy Agency, and the Knut and Alice Wallenberg foundation (KAW).This is the author accepted manuscript. The final version is available from the American Chemical Society via http://dx.doi.org/10.1021/acs.jpcc.5b1080

Improving charge separation across a hybrid oxide/polymer interface by Cs doping of the metal oxide

The process of photoinduced charge carrier separation in hybrid optoelectronics remains only partially understood, with the mechanism behind creation and dissociation of bound charge pairs (BCPs) being open questions. To investigate these processes, we employ the model hybrid ZnO/P3HT system and show that Cs doping of ZnO results in a decrease in the density of gap states at the metal oxide surface and in turn, a reduction in the yield of BCPs. This provides direct experimental evidence for a previously proposed model of BCP creation by electron trapping at the metal oxide surface states. Furthermore, Cs doping is found to substantially increase open circuit voltage in these devices without the negative effects on short circuit current that were observed in studies with other dopants. This offers new possibilities for hybrid photovoltaic devices with increased power conversion efficiencies and provides valuable insights on the charge separation processes in hybrid organic-inorganic photovoltaics.We kindly thank Prof. Annemarie Pucci and Prof. Uwe Bunz for providing access to the AFM and the device fabrication facilities, respectively. A. A. B. is a Royal Society University Research Fellow. P. E. H. and Y. V. thank the Excellence Initiative for funding.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1002/admi.20150061

Molecular doping of single-walled carbon nanotube transistors: optoelectronic study

Single-walled carbon nanotubes (SWCNT) are a promising material for future optoelectronic applications, including flexible electrodes and field-effect transistors. Molecular doping of carbon nanotube surface can be an effective way to control the electronic structure and charge dynamics of these material systems. Herein, two organic semiconductors with different energy level alignment in respect to SWCNT are used to dope the channel of the SWCNT-based transistor. The effects of doping on the device performance are studied with a set of optoelectronic measurements. For the studied system, we observed an opposite change in photo-resistance, depending on the type (electron donor vs electron acceptor) of the dopants. We attribute this effect to interplay between two effects: (i) the change in the carrier concentration and (ii) the formation of trapping states at the SWCNT surface. We also observed a modest ~4 pA photocurrent generation in the doped systems, which indicates that the studied system could be used as a platform for multi-pulse optoelectronic experiments with photocurrent detection.JZ wants to thank China Scholarship Council for supporting PhD studies. AVE and IIB acknowledge Russian Science Foundation under grant №14-19-01308 for kindly support

The effect of ionic composition on acoustic phonon speeds in hybrid perovskites from Brillouin spectroscopy and density functional theory

© The Royal Society of Chemistry 2018. Hybrid organic-inorganic perovskites (HOIPs) have recently emerged as highly promising solution-processable materials for photovoltaic (PV) and other optoelectronic devices. HOIPs represent a broad family of materials with properties highly tuneable by the ions that make up the perovskite structure as well as their multiple combinations. Interestingly, recent high-efficiency PV devices using HOIPs with substantially improved long-term stability have used combinations of different ionic compositions. The structural dynamics of these systems are unique for semiconducting materials and are currently argued to be central to HOIPs stability and charge-transport properties. Here, we studied the impact of ionic composition on phonon speeds of HOIPs from Brillouin spectroscopy experiments and density functional theory calculations for FAPbBr3, MAPbBr3, MAPbCl3, and the mixed halide MAPbBr1.25Cl1.75. Our results show that the acoustic phonon speeds can be strongly modified by ionic composition, which we explain by analysing the lead-halide sublattice in detail. The vibrational properties of HOIPs are therefore tuneable by using targeted ionic compositions in the perovskite structure. This tuning can be rationalized by non-trivial effects, for example, considering the influence of the shape and dipole moment of organic cations. This has an important implications for further improvements in the stability and charge-transport properties of these systems

Water Infiltration in Methylammonium Lead Iodide Perovskite: Fast and Inconspicuous

While the susceptibility of CH3NH3PbI3 to water is well documented, water influence on device performance is not well understood. Herein we use infrared spectroscopy to show that water infiltration into CH3NH3PbI3 occurs much faster and at much lower humidity than previously thought. We propose a molecular model where water molecules have a strong effect on the hydrogen bonding between the methylammonium cations and the Pb-I cage. Furthermore, the exposure of CH3NH3PbI3 to ambient environment increases the photocurrent of films in lateral devices by more than one order of magnitude. The observed slow component in the photocurrent buildup indicates that the effect is associated with enhanced proton conduction when light is combined with water and oxygen exposure.C.M. and M.S. acknowledge support by the Heidelberg Graduate School of Fundamental Physics. A.A.B. is a Royal Society University Research Fellow.This is the author accepted manuscript. The final version is available from the American Chemical Society via http://dx.doi.org/10.1021/acs.chemmater.5b0388

Impact of Marginal Exciton-Charge-Transfer State Offset on Charge Generation and Recombination in Polymer:Fullerene Solar Cells

The energetic offset between the initial photoexcited state and charge-transfer (CT) state in organic heterojunction solar cells influences both charge generation and open-circuit voltage (Voc). Here, we use time-resolved spectroscopy and voltage loss measurements to analyze the effect of the exciton–CT state offset on charge transfer, separation, and recombination processes in blends of a low-band-gap polymer (INDT-S) with fullerene derivatives of different electron affinity (PCBM and KL). For the lower exciton–CT state offset blend (INDT-S:PCBM), both photocurrent generation and nonradiative voltage losses are lower. The INDT-S:PCBM blend shows different excited-state dynamics depending on whether the donor or acceptor is photoexcited. Surprisingly, the charge recombination dynamics in INDT-S:PCBM are distinctly faster than those in INDT-S:KL upon excitation of the donor. We reconcile these observations using a kinetic model and by considering hybridization between the lowest excitonic and CT states. The modeling results show that this hybridization can significantly reduce Voc losses while still allowing reasonable charge generation efficiency

On the energetics of bound charge-transfer states in organic photovoltaics

Using temperature-dependent optical-control spectroscopy, we show that the binding energy of localised charge-transfer state is about 90 meV in a range of organic photovoltaic systems.China Scholarship Council; Winton Programme for the Physics of Sustainability; the Engineering and Physical Sciences Research Council; the University of Cambridge (CHESS); Royal Society University Research Fellow; St John’s College, Cambridg