16:10 - 16:35InvitedID: 279
CHARGE TRANSFER STATES IN ORGANIC SOLAR CELLS
Soft Matter Optoelectronics, University of Bayreuth, Bayreuth, Germany
The generation of charges in organic solar cells occurs by the dissociation of interfacial charge transfer (CT) states. It is therefore crucial to understand the properties of these states. Spectroscopy of CT state emission and absorption (inferred from the photocurrent) can yield pertinent information, provided one knows how to extract it.
In this talk I shall first demonstrate that the interpretation of CT spectra in thin films requires due consideration of the inhomogeneously broadened density of states. If energetic disorder is included, the spectra can be analysed in a conventional Franck– Condon or a Marcus–Levich–Jortner type picture, with the Stokes’ shift between the maxima of absorption and emission reflecting the reorganization energy of the donor–acceptor pair. This advances beyond the currently applied method, where a classical Marcus-type approach is used without consideration of the energetic inhomogeneity of the film, and where the Stokes’ shift is associated predominantly with the intramolecular reorganization energy of the donor. In contrast to the classical Marcus-type approach, our model correctly reproduces the spectra over the entire temperature range from 5 to 300 K. Conceptually, it implies that the CT state emission or absorption can be described as a process dominated by quantum mechanical tunnelling rather than by strong thermal activation.
I shall then consider that the high efficiencies reported for organic solar cells, and reports on an almost negligible thermal activation for the photogeneration of charge carriers have called into question whether photoinduced interfacial charge transfer states are bound by a significant coulomb attraction, and how this can be reconciled with very low activation energies. We have addressed this question here in a combined experimental and theoretical approach. We determined the interfacial binding energy of a charge-transfer state in a blend of MeLPPP:PCBM, using energy resolved electrochemical impedance spectroscopy and find it to be about 0.5 eV. Temperature-dependent photocurrent measurements on the same films, however, give an activation energy that is about one order of magnitude lower. Using analytical calculations and MC simulation we illustrate how (i) interfacial energetics and (ii) topology of transport reduce the activation energy required to separate the interfacial electron-hole pair, with about equal contributions.
1) F.-J. Kahle, A. Rudnick, H. Bässler, A. Köhler
How to interpret absorption and fluorescence spectra of charge transfer states in an organic solar cell; Mater. Horiz. 5, 837 (2018).
2) S. Athanasopoulos, F. Schauer, V Nádaždy, M. Weiß, F.-J. Kahle, U. Scherf, A. Köhler, H. Bässler,
Revealing the spin-vibronic coupling mechanism of thermally activated delayed fluorescence. Adv. Energy Mater. in press
16:35 - 16:50ID: 115
POLYMER DIELECTRIC MIRRORS TO BOOST PERFORMANCES OF LUMINESCENCE SOLAR CONCENTRATORS
1Dipartimento di Chimica e Chimica Industriale, Università di Genova, Genova, Italy; 2Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Pisa, Italy; 3Dipartimento di Fisica, Università di Pavia, Pavia, Italy
Polymer dielectric mirrors have been demonstrated very efficient for several applications in photonics including the control of spontaneous emission, lasing, and sensing.1 In this work, we report on their use to improve the optical efficiency of luminescent solar concentrators (LSC).2
LSCs are transparent, planar waveguides doped with high quantum yield fluorophores. The slabs have a refractive index higher than their surroundings favoring total internal reflection of dispersed dye fluorescence. Such photons are then guided to the sides, where standard solar cells collect them. Notwithstanding their simplicity, several processes limit the LSC device efficiency like the matching between the fluorophore absorption and the solar spectrum, the fluorophore efficiency, the efficiency of the lateral solar cells, the waveguiding properties and the light trapping within the slab.
We propose to use flexible polymer distributed Bragg reflectors to enhance the trapping efficiency by recycling photons leaving the slab within the escape cone (about 25%) without affecting the propagation of light within the waveguide.2
The Bragg stacks are obtained by alternating layers of cellulose acetate and both polyvinylcarbazole or a new stable and ad hoc synthesized solution processable hydrated titania–poly(vinyl alcohol) nanocomposite (HyTiPVA) with a refractive index greater than 1.8 over a broad spectral range. The large dielectric contrast for such polymer structures allows a wide reflection band. We demonstrate that the application of the Bragg stacks with photonic band-gap tuned to the low energy side of the dye emission spectrum induces a 10% enhancement of optical efficiency. Upon doubling the size of the LSC and by using polymer DBRs as a mosaic mirror, the efficiency improvement is retained demonstrating that scaling-up of our approach is possible.
1) P. Lova et al., Adv. Optical Mater. 2018, 6, 1800730.
2) G. Iasilli et al., Chem. Front. 2019, DOI:10.1039/c8qm00595h.
16:50 - 17:05ID: 184
PHOTO-PHYSICS IN NON-FULLERENE ELECTRON ACCEPTORS: INVESTIGATING CHARGE-PHOTOGENERATION DYNAMICS
The University of Queensland, Brisbane, Australia
Non-fullerene electron acceptors are attractive for organic solar cells due to the relative ease with which their physical and optical properties can be tuned to complement those of the donor material. In this study, the photo-physical properties of a series of high electron affinity small molecules with fluorene and benzothiadiazole moieties are investigated. We consider donor-acceptor (D-A), D-A-D and D-A-A-D structures and compare their physical and photo-physical properties.
Broadband transient absorption spectroscopy from picosecond to microsecond timescales, time-resolved photoluminescence measurements and photoluminescence quantum yields were employed to probe the evolution of the excited state population of the electron acceptors in solution and the solid-state. Global target analysis was used to extract each kinetically independent spectral feature in the transient absorption data.
Neat films of the D-A compound showed formation of the intramolecular singlet exciton upon photo-excitation with triplet exciton formation occurring at later times, while no intermolecular excited state species was observed. In contrast, the D-A-D and D-A-A-D structures exhibited an evolution of the photo-generated singlet exciton to form a long-lived intermolecular excited state species on a sub-nanosecond timescale. In the case of the D-A-D compound this could lead to the formation of a triplet exciton but for the D-A-A-D compound a long-lived polaron population was also observed.
The results show that the intermolecular interactions play an important role in the photo-physical properties of electron acceptors. In particular, the formation of intermolecular excited species was found to be conducive to polaron formation, providing an additional channel for charge generation in non-fullerene solar cells.
17:05 - 17:20ID: 170
FLUCTUATIONS IN EMISSION POLARIZATION & SPECTRUM IN SINGLE CHAINS OF THE ORGANIC PHOTOVOLTAIC POLYMER PTB7
1School of Chemistry, University of Glasgow, Glasgow, United Kingdom; 2Institut für Experimentelle und Angewandte Physik, Universität Regensburg, Regensburg, Germany; 3Department Chemie, Ludwig-Maximilians-Universität München, München, Germany
Understanding the nanoscopic arrangement of disordered conjugated polymer chains used in high performance organic photovoltaic cells is important if one wants to understand the spatial landscape within which the crucial processes of exciton diffusion, charge separation and extraction occur. Using single molecule spectroscopy we can isolate and observe the emission wavelength, lifetime and polarization of single chains and disordered single aggregates of the high performance photovoltaic polymer PTB7. We observe fast (millisecond) and slow (seconds) fluctuations in these observables over a 30 second measurement window. Aggregates of PTB7 are a disordered collection of chain lengths. As individual chromophores on the chain or aggregate activate and deactivate as a function of time a quantifiable dynamical change in the overall emission wavelength and transition dipole orientation is measured. These can take the form of sudden “jumps” of the observables, high frequency fluctuations with many emitting sites, or lower frequency oscillations between two or three favoured selections of exciton sites. Temporary or permanent bleaching of individual chromophores is responsible for the dynamical nature of these fluctuations. Overall, this information can then be used to determine the orientations and energies of different chromophores within a single nanoscale disordered aggregate. This is because we are in effect sampling multiple different combinations of chromophoric sites as a function of the measurement time, giving valuable information on the spatial and energetic landscape in disordered aggregates of PTB7.
 G.J. Hedley, F. Steiner, J. Vogelsang & J.M. Lupton, J. Phys. Chem. Lett., 2017, 8 (15), 3494–3499, DOI: 10.1021/acs.jpclett.7b01363
 G.J. Hedley, F. Steiner, J. Vogelsang & J.M. Lupton, Small, 2018, 14 (51), 1804312, DOI: 10.1002/smll.201804312
17:20 - 17:35ID: 208
FROM TRANSIENT SPECTROSCOPY ON FILMS TO STEADY-STATE SOLAR CELL DEVICE PERFOMRANCE
1KAUST Solar Center (KSC), Physical Sciences and Engineering Division (PSE), Material Science and Engineering Program (MSE), King Abdullah University of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia; 2Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics, Center of Organic Electronics (COE), Linköping University, Linköping, Sweden
Time-resolved spectroscopy is an unrivalled tool to study photophysical phenomena involving both neutral and charged excited species, for instance the processes involved in photocurrent generation and those limiting the device efficiency. However, it is not straightforward to extrapolate (transient) spectroscopy results acquired after pulsed laser excitation with high photon density to devices operating under steady-state one sun illumination conditions. In order to bridge that gap, we quantified the rates of the different photophysical processes, which we identified by transient absorption spectroscopy, and used them to simulate operating solar cell devices. The simulations indeed reproduce the experimentally-measured performances and thus explain the losses during photocurrent generation. Finally, our simulations enable us to gain information which are experimentally not accessible such as the density of neutral excited states, namely singlet and triplet excitons, in an operating organic solar cell. This paves the way to a better understanding of the impact of those states, their formation and recombination processes on the device performance.
S. Karuthedath, J. Gorenflot, A. Melianas, Z. Kan, M. Kemerink, F. Laquai - Density Dependence of Charge Carrier Recombination and Triplet State Formation in TQ1:PC71BM Solar Cells - in preparation.
17:35 - 17:50ID: 203
DISSIMILAR RECOMBINATION RATES OF SINGLET AND TRIPLET CHARGE TRANSFER STATES IN ORGANIC PHOTOVOLTAICS
University of St Andrews, St Andrews, United Kingdom
Non-geminate charge carrier recombination is a major loss mechanism in bulk heterojunction solar cells. It is still not fully understood how recombination pathways and the rates depend on materials energetics, blend morphology and carrier mobility. Here we study carrier recombination in efficient photovoltaic blends of PTB7:PC71BM over a wide carrier density range using broadband transient absorption and time-resolved fluorescence spectroscopies. We find that free carriers recombine to the PTB7 triplet state via a triplet charge transfer state 3CT and also to the ground state via a singlet charge transfer state 1CT. By modelling recombination kinetics using the reported time-dependent carrier mobilities we find that the encounter pairs dissociate many times before recombining in the optimized blend. The recombination rate of 3CT is about eight times higher than that of 1CT and about 96% of charges recombine to the polymer triplet state. Recombination of free electrons with trapped holes is identified as the main loss mechanism at carrier densities typical for solar cells. Our results suggest that non-geminate recombination can be suppressed by tuning the energies of 3CT and local triplet states and by reducing the density of hole traps to make improved solar cells.
17:50 - 18:05ID: 188
TEMPERATURE DEPENDENT CHARGE CARRIER GENERATION IN ORGANIC BLENDS AND ITS RELATION TO THE OPEN CIRCUIT VOLTAGE
Åbo Akademi University, Åbo, Finland
We study temperature dependent processes in an organic polymer:fullerene solar cell blend. A temperature dependent charge carrier generation obtained from continous-wave Photoinduced Absorption (cwPA) is compared to measurements of the open-circuit voltage (Voc). We show that the decreasing generation obtained from cwPA is insufficient to explain the loss of Voc at low temperatures and discuss other explanations for this phenomenon.
When measuring the temperature dependence of Voc in P3HT:ICBA (poly(3-hexylthiophene):indene-C60 bisadduct) we observe a decrease with decreasing temperature at low temperatures, which could be caused by a decrease in generation efficiency. Measurements of Voc are, however, influenced by and ultimately limited by contact effects and affected by several other temperature dependent factors, such as mobility and recombination. As cwPA is a contactless experiment performed at open circuit conditions it is a good complementary tool to study the processes that influence Voc.The cwPA method is an optical pump-probe technique that gives information of long-lived charge carrier dynamics, such as polarons. The results from cwPA show that the generation increases and then saturates (around 180 K) with increasing temperature. From cwPA we also obtain information about the long-lived charged species recombination dynamics, which combined with the generation can be used to predict the temperature dependence of Voc. Doing this shows a clear difference between the measured behavior and the predicted one, leading to the conclusion that the loss of Voc is not an effect originating in the interface between donor and acceptor in the active material.