Energy-Level Alignment at Metal–Organic and Organic–Organic Interfaces in Bulk-Heterojunction Solar Cells (original) (raw)
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IEEE Journal of Selected Topics in Quantum Electronics, 2010
Ultra violet photoelectron spectroscopy measurements in combination with the Integer Charge Transfer model is used to obtain the energy level alignment diagrams for two common types of bulk heterojunction solar cell devices based on poly(3-hexylthiophene) or poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene) as the donor polymer and (6,6)phenyl-C61-butric-acid as the acceptor molecule. A ground state interface dipole at the donor/acceptor heterojunction is present for both systems but the origin of the interface dipole differs, quadrupole-induced in the case of poly(2-methoxy-5-(3',7'dimethyl-octyloxy)-1,4-phenylene vinylene) and integer charge transfer state based for poly(3-hexylthiophene). The presence of bound electron-hole charge carriers (charge transfer states) and/or interface dipoles is expected to enhance exciton dissociation into free charge carriers, reducing the probability that the charges become trapped by Coulomb forces at the interface followed by recombination.
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 Nano, 2012
I n organic photovoltaic devices, outer interface structures play a significant role in establishing optimal contact conditions for efficient extraction (or blocking) of charge carriers. Buffer layers of different nature are currently employed to enhance both power conversion efficiency (PCE) and cell stability by improving contact performance. Several materials have been explored to enhance the electron selectivity of the cathode contact: alkali metal compounds (LiF, etc.), metal oxides (TiO x , ZnO, etc.), and low molecular weight organic compounds have been reported to contribute to the overall PCE and solar cell lifetime, as reviewed in recent reports. 1À3 Among those approaches, the effect of the dipole moment associated with self-assembled monolayers (SAM) attached to the interface, which alter the energy level alignment between the cathode metal and the bulk of the blend, 4 is particularly interesting, as well as the inclusion of conjugated polyelectrolyte interlayers. 5 In all of these cases, the energy shift induced by the charge dipole built up at interface layers enables the use of air-stable high work function metals. It is then inferred that electrostatic mechanisms occurring at the nanometer scale, both in the active layer bulk and at interfaces, have a great influence on the overall device operation. Interface dipole layers are regarded as a determining ingredient of the metal/organic contact equilibration. 8À12 Several models have been proposed to account for the energy level alignment at interfaces, depending on the degree of interaction between the metal contact and the deposited organic layer. When the chemical interaction between the metal and contacting conjugated molecules or polymers is not negligible, it is expected that molecules attached to the metal surface undergo both a shift and a broadening of their molecular energy levels. Energy distribution of the attached molecules should be modeled by a specific interfacial density of states (IDOS) which differs from that encountered in the bulk of the organic layer. The situation is ABSTRACT Electronic equilibration at the metalÀorganic interface, leading to equalization of the Fermi levels, is a key process in organic optoelectronic devices. How the energy levels are set across the interface determines carrier extraction at the contact and also limits the achievable open-circuit voltage under illumination. Here, we report an extensive investigation of the cathode energy equilibration of organic bulk-heterojunction solar cells. We show that the potential to balance the mismatch between the cathode metal and the organic layer Fermi levels is divided into two contributions: spatially extended band bending in the organic bulk and voltage drop at the interface dipole layer caused by a net charge transfer. We scan the operation of the cathode under a varied set of conditions, using metals of different work functions in the range of ∼2 eV, different fullerene acceptors, and several cathode interlayers. The measurements allow us to locate the charge-neutrality level within the interface density of sates and calculate the corresponding dipole layer strength. The dipole layer
Energy level alignment at metal-organic and organic-organic interfaces with Alq3 and NTCDA
2011
Ultra violet photoelectron spectroscopy measurements in combination with the Integer Charge Transfer model is used to obtain the energy level alignment diagrams for two common types of bulk heterojunction solar cell devices based on poly(3-hexylthiophene) or poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene) as the donor polymer and (6,6)phenyl-C61-butric-acid as the acceptor molecule. A ground state interface dipole at the donor/acceptor heterojunction is present for both systems but the origin of the interface dipole differs, quadrupole-induced in the case of poly(2-methoxy-5-(3',7'dimethyl-octyloxy)-1,4-phenylene vinylene) and integer charge transfer state based for poly(3-hexylthiophene). The presence of bound electron-hole charge carriers (charge transfer states) and/or interface dipoles is expected to enhance exciton dissociation into free charge carriers, reducing the probability that the charges become trapped by Coulomb forces at the interface followed by recombination.
Advanced Functional Materials, 2014
1 wileyonlinelibrary.com optimize and improve the effi ciencies so as to enable their successful commercialization, signifi cant efforts are made to increase two particular photovoltaic parameters: short circuit current density ( J sc ) and open circuit voltage ( V oc ). The energy difference between the holetransporting level of the donor and the electron-transporting level of the acceptor heavily infl uences the V oc and can be seen as an upper limit to what can be achieved in the device. Strategies to increase the V oc typically have focused on synthesis of new polymers and/or new acceptor/fullerene derivatives so as to achieve optimal donor (D)-acceptor (A) energy level offsets. More recently, a signifi cant infl uence of photogenerated donor-acceptor charge transfer (CT) complexes on V oc has been demonstrated, but strategies for V oc (and overall effi ciency) improvement based on this effect are less explored. In general, the V oc is found to be proportional to the incoming light intensity I such that ( ) ∝ ln oc s eV n kT I , where n s is a prefactor (sometimes referred to as the light ideality factor), usually 1 < n s < 2. Ultraviolet photoelectron spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and cyclic voltammetry (CV) are typically used to measure the energies of the hole-and electron transporting levels, with CV being most commonly used due to its relative simplicity and low cost. Knowledge of the (bulk) transport levels does not enable the determination of electrode and BHJ energetics, however, as a potential step is often formed at metal/organic and organic/organic interfaces modifying the relative position of the energy levels at either side of the interface, even for weakly interacting physisorbed interfaces such as those typically found in a BHJ solar cell. It's proposed that the energy level alignment at weakly interacting metal/organic and organic/organic interfaces and in multilayer stacks can be predicted by the Integer Charge Transfer (ICT) model where the relation between the original Fermi level of a surface and the so-called pinning energies ( E ICT+,− ) of the organic semiconductor (OS) overlayer plays a key role. The E ICT+ ( E ICT-) energy of the positive (negative) ICT state relates to the smallest energy required to take away one electron (the largest energy gained from adding one electron) from (to) the OS molecule at an interface producing a fully relaxed state, where screening from the environment and the Coulombic interaction with the opposite charge across the Organic photovoltaics are under intense development and signifi cant focus has been placed on tuning the donor ionization potential and acceptor electron affi nity to optimize open circuit voltage. Here, it is shown that for a series of regioregular-poly(3-hexylthiophene):fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affi nity energy difference at the donor-acceptor interface, even though there is a large variation in electron affi nity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.
Effect of Contacts in Organic Bulk Heterojunction Solar Cells
Physical Review Applied, 2014
Interfaces play an important role in emerging organic electronic applications. In order to optimize and control the performance in organic devices such as organic solar cells, a comprehensive understanding of the contacts is essential. However, despite the vast progress made, a fundamental theory of the physical processes taking place at the contacts is still lacking. In this work, a numerical device model is used to clarify the effect of imperfect contacts in organic bulk heterojunction solar cells. The effect of increased injection barriers, reduced surface recombination, interfacial minority carrier doping, and traps for majority carriers at the electrodes causing reduced efficiencies is simulated. Two distinctly different underlying mechanisms leading to different S-shaped features are found, both leading to an effective shift of the builtin voltage. In the case of an extraction barrier to majority carriers at the contact, such as reduced surface recombination, the S kink is due to an induced diffusion potential. In the case of interfacial doping or traps, the S kink results from band bending caused by the fixed or trapped space charge. We derive analytical expressions describing the effective reduction of the built-in voltage and the (effective) open-circuit voltage providing means to quantify and distinguish the mechanisms. We show how to experimentally differentiate between these effects and provide tools to extract the relevant physical parameters.
Physical Review B, 2013
Efficient exciton dissociation at a donor-acceptor interface is the crucial, yet not fully understood, step for obtaining high efficiency organic solar cells. Recent theoretical work suggested an influence of polymer conjugation length and of interfacial dipoles on the exciton dissociation yield. This necessitates experimental verification. To this end, we measured the dissociation yield of several polymer/C 60 planar heterojunction solar cells up to high electric fields. The results indeed prove that the yield of exciton dissociation depends strongly on the conjugation length of the polymers. Complementary photoemission experiments were carried out to assess the importance of dipoles at the donor-acceptor interfaces. Comparison of exciton dissociation models with experimental data shows that the widely used Onsager-Braun approach is unsuitable to explain photodissociation in polymer/C 60 cells. Better agreement can be obtained using "effective mass" models that incorporate conjugation length effects by considering a reduced effective mass of the hole on the polymer and that include dielectric screening effects by interfacial dipoles. However, successful modeling of the photocurrent field dependence over a broad field range, in particular for less efficient solar cell compounds, requires that the dissociation at localized acceptor sites is also taken into account.
Science China-Chemistry, 2014
We review some of the computational methodologies used in our research group to develop a better understanding of the geometric and electronic structures of organic-organic interfaces present in the active layer of organic solar cells. We focus in particular on the exciton-dissociation and charge-transfer processes at the pentacene-fullerene interface. We also discuss the local morphology at this interface on the basis of molecular dynamics simulations. organic photovoltaics, organic-organic interface, multi-scale simulations
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...