Controlling the concentration gradient in sequentially deposited bilayer organic solar cells via rubbing and annealing (original) (raw)
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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.
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.
Advances in OptoElectronics, 2011
We have fabricated bulk heterojunction organic solar cells using coumarin 6 (C6) as a small organic dye, for light harvesting and electron donation, with fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), acting as an electron acceptor, by spin-coating technique. We have investigated thermal annealing and blend concentration effects on light harvesting, photocurrent, and performance parameters of the solar cells. In this work, we introduced an experimental method by which someone can easily detect the variation in the contact between active layer and cathode due to thermal annealing after cathode deposition. We have showed, in this work, unusual behavior of solar cell composed of small organic molecules under the influence of thermal annealing at different conditions. This behavior seemed uncommon for polymer solar cells. We try from this work to understand device physics and to locate a relationship between production parameters and performance parameters of th...
ACS Nano, 2013
Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces have an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques as variable-angle spectroscopic ellipsometry and capacitance-voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in four donor/acceptor systems: PCPDTBT, P3HT, PCDTBT and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene rich interface at the cathode is a prerequisite to enhance contact selectivity, and consequently power conversion efficiency.
Chemistry of Materials, 2015
A critically important question that must be answered to understand how organic solar 3 cells operate and should be improved is how the orientation of the donor and acceptor 4 molecules at the interface influences exciton diffusion, exciton dissociation by electron transfer and recombination. It is exceedingly difficult to probe the orientation 6 in bulk heterojunctions because there are many interfaces and they are arranged with 7 varying angles with respect to the substrate. One of the best ways to study the 8 interface is to make bilayer solar cells with just one donor-acceptor interface. Zinc 9 phthalocyanine is particularly interesting to study because its orientation can be 10 adjusted by using a 2-nm-thick copper iodide seed layer before it is deposited. 11 Previous studies have claimed that solar cells in which fullerene acceptor molecules 12
Solar Energy, 2019
We fabricated organic solar cells (OSCs) based on bulk-heterojunction photoactive layers with fullerene (PTB7:PC 71 BM) and non-fullerene (PBDB-T:ITIC) systems by introducing 1,8-diiodooctane (DIO) and 4-fluorobenzaldehyde (4-FBA) as co-additives, respectively. Introduction of the co-additives led to a uniform surface morphology of photoactive layer and formed well distributed interpenetrating networks between donors and acceptors. Carrier recombination were reduced due to the favorable structure for charge transport. Consequently, the devices with co-additives achieved an enhanced performance with a PCE of 8.5% (J SC = 16.4 mA/cm 2 and FF = 68.3%) in the fullerene system, and a PCE of 10.1% (J SC = 16.9 mA/cm 2 and FF = 67.8%) in the non-fullerene system. In addition, the devices with co-additives showed improved stabilities compared to those with single additives. Correspondingly, the reduction ratio of PCE at ambient atmosphere conditions decreased from 17.07% to 10.59% in the fullerene system, and 34.02% to 24.75% in the non-fullerene system.
Lateral Alternating Donor/Acceptor Multilayered Junction for Organic Solar Cells
ACS Applied Energy Materials, 2019
Blended junctions are indispensable for organic solar cells; however, the fabrication of electron and hole transport routes in blended cells remains quite challenging. Herein, a lateral alternating multilayered junction using a high-mobility organic semiconductor is proposed and demonstrated. A total of 93% of the photogenerated electrons and holes are laterally collected over a long distance (0.14 mm). The exciton-collection efficiency reaches 75% in a lateral alternating multilayered junction with a layer thickness of 10 nm. A lateral organic alternating multilayered junction that completely collects both excitons and carriers can be an alternative blended junction for organic solar cells.
Self-Assembly of Selective Interfaces in Organic Photovoltaics
Advanced Functional Materials, 2013
The composition of polymer-fullerene blends is a critical parameter for achieving high efficiencies in bulkheterojunction (BHJ) organic photovoltaics. Achieving the "right" materials distribution is crucial for device optimization as it greatly influences charge-carrier mobility. The effect of the vertical concentration profile of materials in spin-coated BHJs on device properties has stirred particularly vigorous debate. Despite available literature on this subject, the results are often contradictory and inconsistent, likely due to differences in sample preparation and experimental considerations. We attempt to reconcile published results by studying the influence of heating, surface energy, and solvent additives on vertical segregation and doping in polymer-fullerene BHJ organic photovoltaics using neutron reflectometry and near edge x-ray absorption fine structure spectroscopy. We show that surface energies and solvent additives greatly impact heat-induced vertical segregation. We also find that interface charging due to Fermi level mismatch increases PCBM-enrichment at the BHJ-cathode interface. Current-voltage measurements show that self-assembly of interfaces affects the open circuit voltage, resulting in clear changes to the power conversion efficiency.