Mapping the energy level alignment at donor/acceptor interfaces in non-fullerene organic solar cells (original) (raw)
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Energy Level Tuning of Non-Fullerene Acceptors in Organic Solar Cells
Journal of the American Chemical Society, 2015
The use of non-fullerene acceptors in organic photovoltaic devices could lead to enhanced efficiencies due to increased open-circuit voltages (VOC) and improved absorption of solar light. Here we systematically investigate planar heterojunction devices comprising peripherally substituted subphthalocyanines as acceptor, and correlate device performance with heterojunction energetics. Due to a balance between VOC and photocurrent, tuning of the interface energy gap is necessary to optimize power conversion efficiency in these devices. In addition, we explore the role of the charge transport layers in the device architecture. It is found that non-fullerene acceptors require adjusted buffer layers with aligned electron transport levels to enable efficient charge extraction, while the insertion of an exciton blocking layer at the anode interface further boosts photocurrent generation. These adjustments result in a planar heterojunction OPV device with 6.9% efficiency and a VOC above 1 V.
Applied Physics Letters, 2020
Introduction of interface layers can maximize the performance of certain organic solar cells. We demonstrate that high efficiency non-fullerene acceptor based solar cells can be further improved with the insertion of PC 70 BM as an interlayer between the electron transport layer and the active layer. The combination of ZnO and PC 70 BM layers between a cathode and a bulk heterojunction active layer appears to serve as a better selective contact by reducing charge transport barrier and recombination. The enhanced short-circuit current density (J SC) is characterized by a low series-resistance (<2 X cm 2), improved charge collection efficiency, and power conversion efficiency. These features are reflected in impedance spectroscopy and electrical noise measurements and provide a route for large-area organic solar cells.
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
ACS Omega, 2022
In the present work, five novel non-fullerene acceptor molecules are represented to explore the significance of organic solar cells (OSCs). The electro-optical properties of the designed A−D−A-type molecules rely on the central core donor moiety associated with different halogen families such as fluorine, chlorine, and bromine atoms and acyl, nitrile, and nitro groups as acceptor moieties. Among these, M1 exhibits the maximum absorption (λ max) at 728 nm in a chloroform solvent as M1 has nitro and nitrile groups in the terminal acceptor, which is responsible for the red shift in the absorption coefficient as compared to R (716 nm). M1 also shows the lowest value of the energy band gap (2.07 eV) with uniform binding energy in the range of 0.50 eV for all the molecules. The transition density matrix results reveal that easy dissociation of the exciton is possible in M1. The highest value of the dipole moment (4.6 D) indicates the significance of M4 and M2 in OSCs as it reduces the chance of charge recombination. The low value of λ e is given by our designed molecules concerning reference molecules, indicating their enhanced electron mobility. Thus, these molecules can serve as the most economically efficient material. Hence, all newly designed non-fullerene acceptors provide an overview for further development in the performance of OSCs.
The Journal of Physical Chemistry Letters, 2012
We investigate the causes for obtaining higher open-circuit voltage in solar cells that use a fullerene with a smaller electron affinity. Using impedance spectroscopy technique, we show that the change of fullerene LUMO energy has very little influence on the kinetic rate of charge transfer across the interface. In terms of the Marcus theory, large reorganization energy values govern the recombination kinetic rate, which is only slightly dependent on the fullerene LUMO energy, and also depends weakly on the energy location of recombining carriers within the broad density of states. Since the recombination rate is very similar in the different devices, we conclude that the larger open-circuit voltage is due to the larger donor HOMO/acceptor LUMO offset.
Spectroscopy of Charge-Transfer States in Non-fullerene Acceptor Organic Solar Cells
2019
Spectroscopy of Charge-Transfer States in Non-fullerene Acceptor Organic Solar Cells Wejdan Alsufyani The performance of non-fullerene acceptor (NFA)based organic solar cells (OSC) has shown continuous increase in recent years, reaching power‐conversion efficiencies up to 17% through the design and synthesis of efficient acceptor materials. Recent research is directed towards achieving higher efficiency of OSC, which is limited by the open-circuit voltage (Voc) which is lower than the Voc values achieved in inorganic or perovskites solar cells with comparable bandgaps. In this work, voltage losses in NFA based OSC were calculated by investigating charge‐transfer state energy (ECT) using electroluminescence spectroscopy and sensitive external quantum efficiency in three polymer:nonfullerene bulk heterojunction solar cells. PCE10:ITIC device acquired the highest ECT with a Voc of 0.82V, and a a power conversion efficiency (PCE) of 7.91%. While PCE10:O-IDTBR obtained the highest Voc of...
Dichotomous Role of Exciting the Donor or the Acceptor on Charge Generation in Organic Solar Cells
Journal of the American Chemical Society, 2016
In organic solar cells, photoexcitation of the donor or acceptor phase can result in different efficiencies for charge generation. We investigate this difference for four different 2-pyridyl diketopyrrolopyrrole (DPP) polymer-fullerene solar cells. By comparing the external quantum efficiency spectra of the polymer solar cells fabricated with either [60]PCBM or [70]PCBM fullerene derivatives as acceptor, the efficiency of charge generation via donor excitation and acceptor excitation can both be quantified. Surprisingly, we find that to make charge transfer efficient, the offset in energy between the HOMO levels of donor and acceptor that govern charge transfer after excitation of the acceptor must be larger by ∼0.3 eV than the offset between the corresponding two LUMO levels when the donor is excited. As a consequence, the driving force required for efficient charge generation is significantly higher for excitation of the acceptor than for excitation of the donor. By comparing char...
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