Theoretical description of the geometric and electronic structures of organic-organic interfaces in organic solar cells: a brief review (original) (raw)
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Journal of Physical Chemistry C, 2016
The individual steps of the light-to-energy conversion process in the vicinity of the interfaces of organic solar cells are investigated with kinetic Monte Carlo simulations employing Marcus hopping rates obtained from quantum-chemical calculations. A chemically diverse set of ptype semiconducting molecules in heterojunction with fullerene C 60 is used. Starting with exciton diffusion, exciton dissociation, charge generation, and charge separation are modeled on an atomistic level. Numerous aspects were already analyzed, but comprehensive simulations including all three processes in amorphous model interface systems and a comparison of various different molecular p-type semiconductors seem to be missing. Our investigation identifies several important kinetic effects that could limit device efficiencies, such as the strong reduction of charge transport rates in the vicinity of the interface due to Coulomb interactions between the charges, the importance of adjusting the relative rates of exciton transfer and dissociation, and the impact of morphology. Charge drift velocities and hole mobilities obtained from the simulations compare well with experimental values indicating that the main effects are covered by the simulations. A correlation between experimental short-circuit currents and simulated charge drift velocities suggests that slow charge-transfer processes could represent a major efficiency-limiting parameter in organic solar cells.
Journal of Electron Spectroscopy and Related Phenomena, 2013
In this short review, we will give examples on how photoelectron spectroscopy (PES) assisted by models on interface energetics can be used to study properties important to bulk heterojunction type organic photovoltaic devices focusing on the well-known bulk heterojunction blend of poly(3hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and its model system P3HT:C 60 . We also will discuss some of the limitations of PES as applied to organic semiconductors (OS) and photovoltaic devices and finish with reviewing recent theoretical advances that now enable calculation of relevant parameters at (hybrid) interfaces measured by PES.
ACS applied materials & interfaces, 2017
We investigate the impact of electronic polarization, charge delocalization, and energetic disorder on the charge-transfer (CT) states formed at a planar C60/pentacene interface. The ability to examine large complexes containing up to seven pentacene molecules and three C60 molecules allows us to take explicitly into account the electronic polarization effects. These complexes are extracted from a bilayer architecture modeled by molecular dynamics simulations and evaluated by means of electronic-structure calculations based on long-range-separated functionals (ωB97XD and BNL) with optimized range-separation parameters. The energies of the lowest charge-transfer states derived for the large complexes are in very good agreement with the experimentally reported values. The average singlet-triplet energy splittings of the lowest CT states are calculated not to exceed 10 meV. The rates of geminate recombination as well as of dissociation of the triplet excitons are also evaluated. In lin...
EPJ Web of Conferences, 2012
This paper contains a brief discussion of the role of detailed structural and computational studies, within the general field of organic photovoltaics. We review some of our recent work on poly(3-butylthiophene) (P3BT) and on [6,6]phenyl-C 61butyric acid methyl ester (PCBM). The first is a prototypical hole-transporting material, whose crystal forms I' and II were solved by us through the combined use of powder Xray diffraction, electron diffraction and molecular modelling. PCBM is a widely used fullerene derivative with electron-transporting properties. It has a rich polymorphism, which to date remains largely unexplored. Our molecular dynamics simulations have revealed interesting features of its solid-state organization, including that in the amorphous phase.
Mapping the energy level alignment at donor/acceptor interfaces in non-fullerene organic solar cells
Nature Communications, 2022
Energy level alignment (ELA) at donor (D)-acceptor (A) heterojunctions is essential for understanding the charge generation and recombination process in organic photovoltaic devices. However, the ELA at the D-A interfaces is largely underdetermined, resulting in debates on the fundamental operating mechanisms of high-efficiency non-fullerene organic solar cells. Here, we systematically investigate ELA and its depth-dependent variation of a range of donor/non-fullerene-acceptor interfaces by fabricating and characterizing D-A quasi bilayers and planar bilayers. In contrast to previous assumptions, we observe significant vacuum level (VL) shifts existing at the D-A interfaces, which are demonstrated to be abrupt, extending over only 1-2 layers at the heterojunctions, and are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of donor-non-fullerene-acceptor systems.
Photoconductivity measurements of the electronic structure of organic solar cells
Physical Review B, 2011
Experimental and theoretical studies of the electronic structure of bulk heterojunction (BHJ) organic solar cells are reported. The photoconductivity spectral response of the solar cells has a weak absorption band extending from the band-gap energy down to <1 eV due to charge-transfer optical excitation at the interface between the polymer and the fullerene. The low-energy absorption indicates an exponential band tail of localized states and an absorption model based on the one-electron joint density of electronic states accounts for the data. Transient photoconductivity measurements of the carrier mobility exhibit a temperature-dependent carrier dispersion. Data analysis for the particular case of transport in the BHJ structure is developed. A multiple trapping model of the dispersive transport is consistent with localized band tail states having a comparable density-of-states distribution to those observed by optical absorption. Theoretical calculations of the density of states including disorder in the π-π spacing of the polymer chains also shows exponential band tailing. A density-of-states model is developed from the data and is discussed.
The journal of physical chemistry. B, 2015
The observation that in efficient organic solar cells almost all electron-hole pairs generated at the donor-acceptor interface escape from their mutual coulomb potential remains to be a conceptual challenge. It has been argued that it is the excess energy dissipated in the course of electron or hole transfer at the interface that assists this escape process. The current work demonstrates that this concept is unnecessary to explain the field dependence of electron-hole dissociation. It is based upon the formalism developed by Arkhipov and co-workers as well as Baranovskii and co-workers. The key idea is that the binding energy of the dissociating "cold" charge-transfer state is reduced by delocalization of the hole along the polymer chain, quantified in terms of an "effective mass", as well as the fractional strength of dipoles existent at the interface in the dark. By covering a broad parameter space, we determine the conditions for efficient electron-hole dissoc...
Organic solar cells based on the combination of squaraine dyes (as electron donors) and fullerenes (as electron acceptors) have recently garnered much attention. Here, molecular dynamics simulations are carried out to investigate the evolution of a squaraine–C 60 bilayer interface as a function of the orientation and order of the underlying squaraine layer. Electronic couplings between the main electronic states involved in exciton dissociation and charge (polaron pair) recombination are derived for donor–acceptor complexes extracted from the simulations. The results of the combined molecular- dynamics−quantum-mechanics approach provide insight into how the degree of molecular order and the dynamics at the interface impact the key processes involved in the photovoltaic effect.