Brabec, C. J.

Fromherz, T.

Hummelen, J. C.

Padinger, F.

Sariciftci, N. S.

English

Large area photovoltaic devices based on interpenetrating networks of donor and acceptor molecules have been fabricated by using the doctor blade technique. Devices containing blends of poly (para phenylenevinylene, PPV) derivatives as donor materials with various C60 derivatives as acceptors are compared. It is shown that among the various combinations of materials short circuit currents are maximal for blends of PPV and a highly soluble fullerene derivative (PCBM monoadduct), whereas the open circuit potentials stay almost the same for all devices. For the best combination of donor - acceptor materials it has been shown that upscaling to 10×15 cm is possible by using the doctor blade technique without loosing efficiency

Proceedings - University of Groningen

   University of GroningenFabrication of large area photovoltaic devices containing various blends of polymer andfullerene derivatives by using the doctor blade techniquePadinger, F.; Brabec, C. J.; Fromherz, T.; Hummelen, J. C.; Sariciftci, N. S.Published in:Opto-Electronics ReviewIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Padinger, F., Brabec, C. J., Fromherz, T., Hummelen, J. C., & Sariciftci, N. S. (2000). Fabrication of largearea photovoltaic devices containing various blends of polymer and fullerene derivatives by using thedoctor blade technique. Opto-Electronics Review, 8(4), 280-283.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.OPTO-ELECTRONICS REVIEW 8(4), 280-283 (2000)Fabrication of large area photovoltaic devices containingvarious blends of polymer and fullerene derivativesby using the doctor blade techniqueF. PADINGER1*, C.J. BRABEC2, T. FROMHERZ1, J.C. HUMMELEN3,and N.S. SARICIFTCI21Quantum Solar Energy Linz (QSEL), A-4010 Linz, Austria2Christian Doppler Laboratory for Plastic Solar Cells, Physical Chemistry,Johannes Kepler University, A-4040 Linz, Austria3Stratingh Institute and Materials Science Center, University of Groningen,9747 AG Groningen, The NetherlandsLarge area photovoltaic devices based on interpenetrating networks of donor and acceptor molecules have beenfabricated by using the doctor blade technique. Devices containing blends of poly (para phenylenevinylene, PPV)derivatives as donor materials with various C60 derivatives as acceptors are compared. It is shown that among thevarious combinations of materials short circuit currents are maximal for blends of PPV and a highly soluble fullerenederivative (PCBM monoadduct), whereas the open circuit potentials stay almost the same for all devices. For the bestcombination of donor - acceptor materials it has been shown that upscaling to 10×15 cm is possible by using the doctorblade technique without loosing efficiency.Keywords: solar cells, poly (para phenylenevinylene) and derivatives, fullerenes and derivatives, bulk heterojunction,doctor blade technique.1.  IntroductionPolymer photovoltaics offer great technological potentialas a renewable, alternative source for electrical energy. Thedemand for inexpensive renewable energy sources is thedriving force to new approaches in the production of low-cost polymer photovoltaic devices. In the last couple ofyears, enhanced efforts have been put into the developmentof solar cells based on organic molecules and conjugatedpolymers [1-11].The technological advantages for the fabrication ofpolymer based organic solar cells, like roll to roll produc-tion of large areas, lead to a possible reduction of the pro-duction costs for large area production. The mechanicalflexibility as well as the tunability of the bandgap offer in-teresting perspectives of polymer based solar cells as com-pared to solar cells based on inorganic materials. Becauseof these advantages, the development of polymer solarcells would have a major impact, even if the efficiencies ofthese types of photovoltaic devices up to now are smallerthan the efficiencies achieved in inorganic solar cells.For the generation of electrical power by absorption ofphotons  it  is  necessary  to  spatially  separate  the  elec-tron-hole (e-h) pair generated by photoexcitation before re-combination processes can take place. In conjugated poly-mers, the stabilisation of the photoexcited e-h pair can beachieved by blending the polymer with an acceptor mole-cule, which has an electron affinity that is larger than theelectron affinity of the polymer but still smaller than itsionisation potential. In addition, the highest occupied mo-lecular orbital (HOMO) of the acceptor should be lowerthan the HOMO of the conjugated polymer. Under theseconditions it is energetically favourable for the photo-excited conjugated polymer to transfer an electron to theacceptor molecule. The hole remains in the polymer va-lence band, which is the lowest available energy state forthe hole.In our work this alignment of the energy levels is real-ised by blending poly(para-phenylenevinylene) (PPV) de-rivatives (acting as electron donors) with derivatives of C60(electron acceptor). It has been shown that in these systemsthe transfer of the photoexcited electrons onto thefullerenes occurs within approximately 200 fs after the ex-citation [12,13]. Since all other known competing relax-ation processes in conjugated polymers occur on timescales that are orders of magnitudes larger than 200 fs, thisultrafast charge transfer must have a quantum efficiency ofapproximately 1, i.e., nearly all photons absorbed by the_______________*e-mail: franz.padinger@jk.uni-linz.ai.at280 Opto-Electr., 8, no. 4, 2000 © 2000 COSiW SEP, WarsawFig. 1. Molecular structures of MDMO-PPV, C60 and PCBM monoadduct. PCBM multiadduct has the same functionalised side groups asPCBM monoadduct, but contrary to PCBM monoadduct it has between two and three side groups.conjugated polymer generate an electron on C60 and a posi-tive charge on the polymer. In addition, the recombinationof the charge separated excited state takes place on aµs-time scale [12], allowing the generation of high concen-trations of non-equilibrium electrons and holes.In the following we compare surface qualities of poly-mer layers containing different derivatives of C60. Usingdoctor blade technique uniform films with a thickness be-tween 100 and 150 nm are produced. This technique alsoshows that upscaling is possible for plastic solar cells usinga polymer substrate (roll to roll process) and also large cellsup to 10×15 cm are produced and compared to 6×6 cm so-lar cells in their current/voltage (I-V) behaviour.2.  ExperimentalFor a better smoothness of the substrate (ITO covered PETwith a resistance of 55 ohm/square) a film of poly-(3,4-ethylenedioxythiophene)-poly-styrenesulfonate (PEDOT)from Bayer AG was casted on the ITO by doctor blading.After drying of the PEDOT film, the solar active layer hasbeen casted by doctor blading from a solution of poly [2-methoxy, 5-(3’,7’-dimethyl-octyloxy)]-p-phenylenevinylene (MDMO-PPV) mixed with C60 as well as mono-and multiadduct of the solubilised C60 derivative [6,6]-Phenyl C61-butyric acid methyl ester (PCBM) in toluene.Devices were produced on 6×6 cm2 large transparent ITOcoated PET substrates by doctor blading technique. Withthis technique the polymer-fullerene solution is depositedin a narrow slit between the surface of the substrate and theblade of the doctor blade. Then the blade is dragged by amotor at a constant velocity over the substrate and thesubstrate is covered uniformly by the polymer-fullerene so-lution. All doctor blading activities have been performedunder room conditions. The mixing ratio of polymer tofullerene is 1:3 (weight/weight) for the mono- andmultiadduct PCBM and 1:2 (weight/weight) for C60 due toits lower solubility in organic solvents compared to PCBM.The enhanced solubility of PCBM compared to C60 allowsa high fullerene-conjugated polymer ratio and, therefore,strongly supports the formation of an interpenetrating net-work of donor and acceptor molecules (donor-acceptor bulkheterojunction). Although the mixing ratio between polymerand C60 is lower than the mixing ratio between the polymerand PCBM, the fullerene concentration is in all cases higherthan 17 vol% of fullerene, which has been found to be thepercolation threshold for a MDMO-PPV/PCBM solar cell[12,14,15]. The thickness of the polymer/fullerene layers isin a range of 100-150 nm [12]. In this blend the polymeracts as electron donor while C60 and its derivatives act aselectron acceptors. The chemical structure of thecompounds is shown in Fig 1.The uniform donor acceptor layer of the solar cell issandwiched between two electrodes with different workfunctions, in our case aluminium (Φm = 4.3 eV) and ITO(Φm = 4.7 eV) are used.As a top electrode aluminium was deposited by vacuumevaporation. For the 6×6 cm2 substrates 4 Al-electrodes witha size of 4 cm2 each were chosen, on the 10×15 cm2substrates 2 Al-electrodes with an active area of 50 cm2each were vacuum evaporated. For all deposition steps thevacuum was below 10-5 mbar.The I-V characteristics were recorded under an illumina-tion intensity of 6 mW/cm2 white light (luminescence tube)by a Keithley SMU 2400 Source Meter (illuminationthrough the transparent ITO side), whereas the surface prop-erties were monitored by atomic force microscopy (AFM).3.  Results and discussionThe quality and the homogeneity of the polymer-fullerenestrongly influences the efficiency of the solar cell. The doc-tor blading technique allows casting thin layers of a solutioncontaining the polymer and the fullerene uniformly on thesurface of the substrate. A thin, wet layer with a constantthickness, still containing a certain amount of the solvent, isleft on the substrate then. This residual solvent is removedby heating or under active vacuum pumping. During thisdrying process a separation of the two components withinthe otherwise homogeneous mixture may take placedepending on the different solubility of polymer andfullerene. For different derivatives of fullerenes the solubil-Opto-Electr. Rev., 8, no. 4, 2000 F. Padinger 281Fig. 2. AFM picture of: (a) MDMO-PPV blended with PCBM monoadduct (weight ratio 1:3), (b) PCBM multiadduct (weight ratio 1:3)and (c) C60 (weight ratio 1:2)ity in organic solvents is different, therefore influencingthe quality of the polymer-fullerene film. Inhomogeneousfilms with pin holes and/or large serial resistivities lead tosmall filling factors (FFs), lower rectification and de-creased open circuit voltages (Voc).Figure 2 shows AFM pictures of solar cells made fromsolutions of MDMO-PPV with PCBM monoadduct (Fig.2a), PCBM multiadduct (Fig. 2b) and C60 (Fig. 2c) respec-tively. The most homogeneous film is found for theMDMO-PPV/PCBM multiadduct device. PCBMmultiadduct has the same chemical structure than PCBMmonoadduct, but contrary to PCBM monoadduct (onefunctionalised side group) two or three functionalised sidegroups are on the fullerene. These functionalised sidegroups give a better solubility in organic solvents com-pared to C60 or PCBM monoadduct. Also with PCBMmonoadduct homogeneous films can be produced, but it isclearly seen that the surface is not as smooth as withPCBM multiadduct. This increasing roughness forMDMO-PPV/PCBM monoadduct might be due to theslightly lower solubility of PCBM monoadduct in organicsolvents compared to PCBM multiadduct. For the MDMO-PPV film containing C60 phase separation on a µm scalecan be observed. As all solutions were filtered by 0.2 µmsyringe filters before casting, these large particles musthave formed during the drying process of the doctorbladed films and are consequently assigned to C60 crystals.These crystals of C60 are up to several micrometers long.However, in spite of the enhanced surface roughness andphase separation no pin holes were detected on the surfaceof the doctor bladed films by AFM.In Fig. 3, the I-V characteristics of photovoltaicdevices made from blends of PPV with the three differentfullerenes are monitored under illumination with6 mW/cm2 white light from a luminescence tube. It isclearly shown that the highest short-circuit current is ob-served when PCBM monoadduct is used as electron accep-tor (|Isc| = 280 µA/cm2), whereas PCBM multiadduct de-vices  show  the  lowest  short  circuit  current  (|Isc|  =  50µA/cm2). Devices produced from blends containing C60give a short circuit current, which is in between the twoPCBM derivatives (|Isc| = 145 µA/cm2).The open circuit potentials Voc of the three devicesshown in Fig. 3 are around Voc = 0.72 V, independent ofthe molecule used as acceptor. It is worth noting that themeasured open circuit voltage is significantly higher thanone would expect from a simple model describing the de-vice as a metal-insulator-metal (MIM) diode. In such amodel, the difference of the work functions of the electrodematerials defines the upper limit of the open circuit voltage.Although the work function of ITO is a poorly definedquantity (values ranging from 5.5 eV [16] to 4.1 eV [17]are reported in literature), recent estimates seem to con-verge into the range between 4.4 eV [18] and 4.7 eV [19].Using these values for the ITO work functions, one wouldexpect the open circuit voltage of an ITO/polymer: C60/Aldevice to be not larger than 0.4 V. Experimentally, how-ever, an open circuit voltage of 0.72 V is observed. Up tonow, the origin of the high open circuit voltage is not fullyFig. 3. I-V-behaviour of MDMO-PPV solar cells containingdifferent fullerene derivatives: PCBM monoadduct (open squares),PCBM multiadduct (small open circles) and C60 (solid line).282 Opto-Electr., 8, no. 4, 2000 © 2000 COSiW SEP, WarsawFig. 4. I-V-behaviour of a 10×15 cm2 MDMO-PPV/(PCBMmonoadduct) solar cell with an active area of 50 cm2 under light(solid squares) and in the dark (open squares). Illumination isprovided by white light from a fluorescence tube with an intensityof 6 mW/cm2.understood, it might be due to a formation of a dipol layer atthe electrode/polymer interface [20].To demonstrate the upscaling potential of the plastic so-lar cells by using the doctor blade technique, solar cells witha substrate area of 10×15 cm2 were fabricated. AgainPEDOT/ITO covered PET foil was used. As solar activelayer a thin film of MDMO-PPV/PCBM monoadduct wasdeposited by doctor blading. As top electrode aluminum wasevaporated. The active area of the solar cell is 2×50 cm2.Figure 4 shows the I-V behaviour of such a large area solarcell under illumination with 6 mW/cm2 white light from afluorescence tube and under dark conditions. The large areasolar cells show an open circuit voltage of 700 mV, what isonly slightly lower than the open circuit voltage of the 6×6cm2 solar cells. The short circuit current for the 10×15 cmsolar cell is |Isc| = 276 µA/cm2, which is almost the same asfor the smaller solar cell containing MDMO-PPV andPCBM monoadduct (|Isc| = 280 µA/cm2). The filling factorfor the large area solar cell, calculated after equation FF =VmaxImax/VocIsc is 0.48. The rectification of the large areasolar cell in the dark is around 102 (for -1.5 V to 1.5 V),whereas under illumination it is still around 10. At higherforward bias, the dark current and the photocurrent are quitesimilar, indicating that photoconductivity does not play animportant role in these devices.4.  ConclusionsFor photovoltaic devices fabricated from several blends ofMDMO-PPV as donor material and different fullerene de-rivatives (C60, PCBM mono and multiadducts) as acceptormaterials, the highest open circuit voltage and short circuitcurrents were obtained with a 1:3 (weight/weight) blend ofMDMO-PPV and PCBM monoadduct. Although it is shownthat the PCBM multiadduct, which achieved the best filmquality, shows the lowest short circuit current.It has also been demonstrated that upscaling of plasticsolar cells is possible without decreasing the short circuitcurrent or the open circuit voltage. Using the doctor bladetechnique solar cells of 10×15 cm2 with an active area of2×50 cm2 were fabricated with efficiencies directlycomparable to 6×6 cm4 cells with an active area of 4×4cm2.References1. J. Simon and J.J. Andre, Molecular Semiconductors,Spring-Verlag, Berlin, Heidelberg, 1985.2. M. Kaneko, Handbook of Organic Conductive Moleculesand Polymers, Vol. 4, p. 661, edited by H.S. 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Strafström and J.L. Bredas, ConjugatedPolymer Surfaces and Interfaces, Cambridge UniversityPress, Cambridge, 1996.Opto-Electr. Rev., 8, no. 4, 2000 F. Padinger 283

Fabrication of large area photovoltaic devices containing various blends of polymer and fullerene derivatives by using the doctor blade technique

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Fabrication of large area photovoltaic devices containing various blends of polymer and fullerene derivatives by using the doctor blade technique

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