Photoelectron Spectroscopy of the Contact between the Cathode and the Active Layers in Plastic Solar Cells: The Role of LiF (original) (raw)
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The European Physical Journal Applied Physics, 2011
By engineering the interface between the intermediate photoactive layer and the cathode aluminum (Al) electrode, through the introduction of ultra-thin layers of various materials, in a standard bulk heterojunction (BHJ) polymer solar cell (PSC) fabricated of regioregular poly(3-hexylthiophene) (rr-P3HT) and phenyl-C 61-butyric acid methyl ester (PCBM), the power conversion efficiency (PCE) has been effectively improved. The devices fabricated using individual single interlayer of bathocuproine (BCP), lithium fluoride (LiF) and Buckminster fullerene C60 have shown optimal efficiencies of ∼2.40%, ∼3.21% and ∼1.92% respectively. Further improvement of the photovoltaic efficiency was achieved by introducing a composite bilayer made of LiF in combination with BCP as well as with C60 at the BHJ/cathode interface. The best results were obtained by interposing a 9 nm of C60 interlayer in combination with a 0.9 nm thick LiF layer, with the PCE of the PV cells being effectively increased up to 3.94% which represents an improvement of 23% as compared to the standard device with LiF interlayer alone. The photocurrent density (J sc) versus voltage (V ) characteristic curves shows that the increase of the efficiency is essentially due to an increase in Jsc. Moreover, all the sets of devices fabricated using various interlayers over a certain range of thickness exhibit an optimum PCE that is inversely proportional to the series resistance of the PV cells. We presume that interposing a C60/LiF layer at the interface could repair the poor contact at the electron acceptor/cathode interface and improve the charge career extraction from the BHJ.
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Harnessing solar energy with solar cells based on organic materials (in particular polymeric solar cells) is an attractive alternative to silicon-based solar cells due to the advantages of lower weight, flexibility, lower manufacturing costs, easier integration with other products, low environmental impact during manufacturing and operations and short energy payback times. However, even with the latest efficiencies reported up to 17%, the reproducibility of these efficiencies is not up to par, with a significant variation in the efficiencies reported across the literature. Since these devices are based on ultrathin multilayer organic films, interfaces play a major role in their operation and performance. This review gives a concise account of the major interfacial issues that are responsible for influencing the device performance, with emphasis on their physical mechanisms. After an introduction to the basic principles of polymeric solar cells, it briefly discusses charge generation and recombination occurring at the donor-acceptor bulk heterojunction interface. It then discusses interfacial morphology for the active layer and how it affects the performance and stability of these devices. Next, the formation of injection and extraction barriers and their role in the device performance is discussed. Finally, it addresses the most common approaches to change these barriers for improving the solar cell efficiency, including the use of interface dipoles. These issues are interrelated to each other and give a clear and concise understanding of the problem of the underperformance due to interfacial phenomena occurring within the device. This review not only discusses some of the implemented approaches that have been adopted in order to address these problems, but also highlights interfacial issues that are yet to be fully understood in organic solar cells.
Current Applied Physics, 2001
We report on the valence orbital structure of poly(para-phenylenevinylene) (PPV)-like oligomers. We studied these molecules as isolated oligomers in the gas phase, as well as in thin films deposited on metal substrates. We use a simple model based on a previously reported Hamiltonian that accurately describes the development of the low lying electronic excitations as a function of the number of repeating units. In the study on the thin organic films we report on the energy level alignment at metal / organic and organic / organic interfaces, where the organic layer is either a PPV-like oligomer or C 60 . The results are important for understanding organic photovoltaic devices.
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Applied Physics Letters, 2012
The indium-tin-oxide/active layer interface is critical to the performance of organic solar cell devices. In this study, submonolayer films of LiF nanoparticles are deposited on the electrode surface with the assistance of polymeric micelle reactors, with controlled nanoscale surface coverage. Incorporation of the solution-processed bi-layer electrodes into a conventional poly(3-hexyl-thiophene): [6,6]-phenyl C 61-butyric acid methyl ester device shows significant improvement in device performance, especially when used in combination with a poly(3,4-ethylenedioxythiophene: poly(styrene sulfonate) layer. The nearly 5Â improvement in the short circuit current and decrease in the contact resistance is mostly likely related to the increase in surface work function from the use of LiF nanoparticles. The results strongly indicate that engineering of the interfaces is a useful tool for future device optimization. V
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, 2015
Polyviologen (PV) derivatives are known materials used for adjusting the work function (WF) of cathodes by reducing the electron injection/collection barrier at the cathode interface. To tune and improve device performance, we introduce different types of counteranions (CAs), such as bromide, tetrafluoroborate, and tetraphenylborate, to a PV derivative. The effective WF of the Al cathode is shown to depend on the size of the CA, indicating that a Schottky barrier can be modulated by the size of the CA. Through the increased size of the CA from bromide to tetraphenylborate, the effective WF of the Al cathode is gradually decreased, indicating a decreased Schottky barrier at the cathode interface. In addition, the change of the power conversion efficiency (PCE) and the short circuit current (J sc) value show good correlation with the change of the WF of the cathode, signifying the typical transition from a Schottky to an Ohmic contact. The turn-on electric field of the electron-only device without PV was 0.21 MV/cm, which is dramatically higher than those of devices with PV-X (0.07 MV/cm for PV-Br, 0.06 MV/cm for PV-BF 4 , and 0.05 MV/cm for PV-BPh 4) This is also coincident with a decrease in the Schottky barrier at the cathode interface. The device ITO/PEDOT/P3HT:PCBM/PV/Al, with a thin layer of PV derivative and tetraphenylborate CA as the cathode buffer layer, has the highest PCE of 4.02%, an open circuit voltage of 0.64 V, a J sc of 11.6 mA/cm 2 , and a fill factor of 53.0%. Our results show that it is possible to improve the performance of polymer solar cells by choosing different types of CAs in PV derivatives without complicated synthesis and to refine the electron injection/collection barrier height at the cathode interface.
Photovoltaic and photoconductive properties of aluminum/poly(p-phenylene vinylene) interfaces
Synthetic Metals, 1994
Photoconductive and photovoltaic properties of AI/PPV/ITO sandwich devices were investigated by measuring steady-state photocurrents resulting from illumination through the Al electrode. A built-in potential (Vh~) was detected at the AI/PPV interface. The voltage dependence of the photocurrent in the vicinity of Vb~ was measured at 1 mW/cm 2 of incident illumination to give an open-circuit voltage and a short-circuit current of 1.2 V and 6 x 10-7 A/cm 2, respectively. The dependences of the short-circuit current on excitation wavelength and illumination intensity are presented and the C-V characteristics of the AI/PPV interface are analyzed. The quantum collection efficiency decreased from 5% to 1% as the intensity of illumination increased from 10-s to 1 mW/cm 2. The photovoltaic conversion power efficiency was about 0.07% for intensities approaching 1 mW/cm 2. The Vh~ value was accounted for by surface band-bending at the Al/PPV interface.