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//-->.pos {position:absolute; z-index: 0; left: 0px; top: 0px;}JOURNAL OF APPLIED PHYSICS112,034517 (2012)Photon recycling across a ultraviolet-blocking layer by luminescencein polymer solar cells€€Sebastian Engmann, Marie Machalett, Vida Turkovic, Roland Rosch, Edda Radlein,Gerhard Gobsch, and Harald HoppeInstitute of Physics, Ilmenau University of Technology, Weimarer Str. 32, D-98693 Ilmenau, Germany(Received 23 April 2012; accepted 11 July 2012; published online 15 August 2012)UV-blocking layers can increase the long term stability of organic solar cell devices; however, theylimit the amount of light that can be utilized for energy conversion. We present photon recyclingand down-conversion via a luminescent layer across a UV-blocking TiO2layer. Our results showthat the use of an additional UV-blocking layer does not necessarily reduce the overall efciencyof organic solar cells, since the loss in photocurrent due to the UV-absorption loss can be partiallyCcompensated using high energy photon down-conversion via luminescence layers.V2012American Institute of Physics.[http://dx.doi.org/10.1063/1.4745016]I. INTRODUCTIONII. EXPERIMENTALOrganic photovoltaic currently receives great attentionfrom a large community, spanning from natural sciences toengineering. Impressive progress in the development of bet-ter materials and advanced cell architectures over the lastfew years resulted in continuous efciency increase to over9%.1This efciency boost promotes organic photovoltaicsto become more competitive with other thin lm technolo-gies and increases market opportunities. First products arelaunched into the market,2although the durability still needsto be improved further for long term applications.3One ofthe major driving factors for degradation is the interaction ofUV-light with the organic semiconductors. In combinationwith the ingress of oxygen and water into the solar celldevices, photoinduced oxidation severely limits the lifetimein organic photovoltaics (OPV).3–6One way to reduce deg-radation is to prevent water and oxygen diffusion into thedevice by encapsulation. In addition, getter materials can beintroduced to further reduce the inuence of radicals withinthe encapsulated device.7Finally, a good amount of photo-chemistry can be effectively hindered by introducing UV-blocking layers within the sealing of the device. However,cutting out the UV-part of the electromagnetic spectrum alsoleads to a loss in energy conversion due to the missing pho-tocurrent generation. To avoid the problem of decreasingdevice performance by stabilization with UV-blockinglayers, down-conversion of a part of the blocked UV-lightenables recycling of the photons otherwise dismissed.For an efcient recycling of the down-converted photonson the one hand, to achieve an efcient down-converted pho-tons recycling, the absorption region of the conversion layershould be well matched with the range of the blocked UV light.On the other hand, a proper overlap between the emission ofthe conversion layer and the absorption of the photoactive layershould be achieved as well. We have veried this benecialphoton recycling process by stacking a luminescent layer ontop of a UV-blocking layer at the outer side of a thin lm poly-mer solar cell. An increase of the external quantum efciencyin the UV region<380nm is demonstrated.0021-8979/2012/112(3)/034517/4/$30.00We employed a combination of titanium and silicon ox-ide to form a UV-blocking layer on top of the transparentside of the solar cell. This UV-blocking layer was built up inthree steps. First, a TiO2sol-gel based on titanethoxid andacetylacetone was deposited on top of the glass, in order topromote better adhesion. The second layer was a thick po-rous TiO2layer, formed from a colloidal solution purchasedby Evonik Degussa (AerodispVW 740X). To stabilize theTiO2layer, the lm was topped by a SiOx sol-gel. In orderto form a rigid network, the layer stack was thermally con-verted at 250C.Organic solar cells were prepared using poly(3-hexylth-iophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM) bulk-hetero junction devices. An approxi-mately 100 nm thick polymer/fullerene active layer was spincoated from a 2 wt. % chlorobenzene solution on top of aPEDOT:PSS (Clevios PH) covered ITO substrates. The alu-minium cathode was evaporated in high vacuum at 2Â10À6mbar base pressure. All solar cell devices were annealed atelevated temperature for 5 min prior to the glass encapsula-tion. Absorption and photoluminescence measurements werecarried out in the spectral range between 300 and 800 nm.Solar cells were characterized using a class A solar simulatorwith an illumination intensity of 100 mW/cm2. Quantum ef-ciency (IPCE) measurements were performed in the rangebetween 300 and 800 nm. The material used as the lumines-cent layer is a UV-curing epoxy glue, containing an anthra-cene derivative used as the photo initiator enabling UVcuring of the epoxy matrix.RIII. RESULTS AND DISCUSSIONSolar cells with an active area of 0.5 cm2were preparedin conventional geometry9on ITO coated glass. On a part ofthe samples, a TiO2-SiOXUV-blocking layer was coated onthe outer side of the glass substrates. Figure1shows thetransmission spectra obtained prior to the solar cell prepara-tion. The transmission of high energy photons through theCV2012 American Institute of Physics112,034517-1034517-2100Engmannet al.2.0x105J. Appl. Phys.112,034517 (2012)Absorption coeff. P3HT:PCBM [ 1/cm]a)80ITO/glass1.5x105b)AluminiumP3HT/[60]PCBMClevios PHITOGlassTiOx-SiOxLuminescent layerTransmission [ %]60ITO/glass/TiOx1.0x1054020P3HT : PCBM1wt.% : 1wt.%5.0x104FIG. 1. Transmission spectra of ITO coated glass withand without UV-blocking layer. Additionally, theabsorption coefcient of P3HT:PCBM is shown(a). The layer stack including the cathode layers to com-plete a solar cell device is shown in (b).2003004005006007008000.0900Wavelength [ nm]substrate with UV-blocking layer is drastically reduced com-pared to the reference substrate. Since UV-radiation wasidentied as a source of degradation of organic solar cells,the introduction of an UV-blocking layer should result in anincreased life-time of the photovoltaic device, which wassuggested by Jïrgensen3and recently shown by Ryuet al.10The drawback of the UV-lter is that the current output ofthe solar cell, and thus the efciency, gets reduced, as theadditional absorption of the UV-blocking layer limits chargegeneration in the photoactive layer. However, a better spec-trally adapted UV-blocking layer will reduce these losses.Here, we discuss how the loss of UV-photons for currentgeneration can be in part recovered by addition of a lumines-cent layer as the outermost layer towards the light source. Itis to mention that since this luminescence layer absorbs apart of the UV light itself, a sufciently thick lm couldreplace the UV-blocking layer. However, in this study weuse an additional UV-blocking layer to ensure zero transmis-sion of the UV light into the solar cell, in order to investigatethe effect of the uorescence from the luminescence layer onthe photocurrent. By doing so, we do not have to distinguishbetween the effect of uorescence and residual light trans-mission through the luminescence layer.While the introduction of such a luminescence layer byphysical vapor deposition was recently reported by Wanget al.,8our goal was to implement a roll-to-roll compatibleprocess. On the other hand, the photo down-conversion layerused in the mentioned reference was N,N-bis(3-methyl-phenyl)-N,N-bis(phenyl)-benzidine (TPD) exhibiting astrong luminescence in the wavelength range between375 nm and 450 nm far below the absorption maximum ofP3HT/PCBM solar cell devices. The reported efciencyenhancement was mainly due to the deposition of an anti-reection coating on top of the solar cell devices and the pos-itive effect of the luminescence layer could in our opinionnot be proven by corresponding IPCE measurements.Two sets of devices were prepared: both of them containedthe UV-blocking layer and additionally one of them was coatedwith an approximately 50lmthick luminescence layer on topof the blocking layer facing towards the illumination source.The luminescence layer was deposited by doctor blading andcured by UV-irradiation, both of the processes easily done in aroll-to-roll process. After production, the solar cells were meas-ured under AM1.5 solar irradiation, as described in the experi-mental section. Because of the comparably large thickness, nointerference effects have to be considered when evaluating thetransmission through this layer.Figure2shows the IV-characteristics of the best deviceswith and without luminescence layer. The comparably smallshort circuit currentJscis due to the strong absorbance of theTiO2-SiOXlayer in the UV-vis region, compare with Figure1.TableIshows the device parameters of the best performing so-lar cell. While the ll factorFF,as well as the open circuit volt-ageVoc, are very comparable in both devices, the generatedphoto current is about 15% higher in the device with the lumi-nescence layer compared to the device without.The relative efciency increase of the device containingthe luminescent layer is caused by two effects. First, since therefractive index of the TiOx layer is comparably high and thesurface is rough due to the use of colloidal nano-particles, theluminescence layer, based on a low refractive index material,acts as an anti-reection coating. This effect is especially bene-cial in the visible wavelength region near the emission maxi-mum of the sun, and therefore this effect results in the biggestFIG. 2. Device characteristics of the de-vice with and without luminescent layer.The device structure is (luminescencelayer)/UV-blockinglayer/glass/ITO/PEDOT:PSS/P3HT:PCBM/Al.034517-3Engmannet al.J. Appl. Phys.112,034517 (2012)TABLE I. Solar cell parameters derived from IV measurements underAM1.5.Jsc(mA/cm2)Without luminescent layerWith luminescent layer4.695.40Voc(mV)639647FF(%)52.451.4Efficiency(%)1.571.79efciency increase between the two devices. Nevertheless, thesecond effect is due to the increase in photocurrent based onthe re-emission of photons by luminescence into the absorptionmaximum of the device. UV light which would be lost eitherby absorption of the UV-blocking layer or simply by the glasssubstrate is absorbed by the anthracene derivative inside thenearly transparent epoxy matrix and emitted near the absorptionmaximum of the photoactive layer.In order to quantify the increase in photocurrent over therelevant spectral region, we conducted external quantum ef-ciency (incident photon to charge carrier efciency/IPCE)measurements showing the spectral response of the device.Figure3shows the photocurrent response of a device withand without luminescence layer, as well as the absorbance andthe photoluminescence of this layer. With the luminescentlayer, the current generation is extended far into the UVregion. Due to the large lm thickness and strong lumines-cence near the overall IPCE maximum, a considerableincrease in the current generation can be observed. Due to thenon-ideal spectral overlap between the luminescent and theUV-blocking layer a part of the absorption is still lost around400 nm as compared to the control device. However, this canbe overcome in the future by maximizing the correspondingspectral overlap.Another topic which has to be addressed in the future isthe long-term stability of the organic luminescence layer.Due to the organic nature of this layer, photo-induced degra-dation may occur. In order to investigate the stability underillumination, transmission and photoluminescence spectra ofthe luminescence layer deposited on glass were measured.Figure4shows the initial photoluminescence spectra to-gether with those after exposure to light from a metal halideFIG. 4. Absorbance and photoluminescence spectra of the organic lumines-cence layer deposited on glass. The luminescence layer was exposed to lightfrom a metal halide lamp (intensity 1000 W/m2) under ambient conditions.lamp (intensity 1000 W/m2) under ambient conditions for1 day and for 1 week. Due to photoreactions, the luminescencelayer slowly degrades which manifests in polymer yellowing.Furthermore, the peak photoluminescence signal gets reducedto about 50% of its initial value within 1 week of continuousillumination. However, we are convinced that degradationeffects can be overcome in the future. First, the polymer ma-trix for the uorophore, in this study an epoxy material, couldbe exchanged with a carrier matrix exhibiting lower yellowingduring UV illumination. Second, sol-gel or conventionallyprocessed oxygen and water barrier layers can be applied, inorder to prevent photo-oxidation of the uorophore. Thesebarrier layers will become necessary in exible organic elec-tronics, in order to prevent water and oxygen permeationthrough the substrate, which may cause oxidation or photo-oxidative processes. Once these ultra-barrier layers allow asufcient UV-light transmission, the concept of energy trans-fer (and thus photon recycling) over a UV-barrier becomes aviable option in the future OPV production.IV. CONCLUSIONSFIG. 3. IPCE of P3HT:PCBM solar cells with and without luminescencelayer for photon recycling. For comparison, the absorbance (blue) and pho-toluminescence (green) of the luminescent layer on glass, as well as theAM1.5 solar spectrum (grey), are shown in the same spectral region.We have shown that the overall device performance ofpolymer solar cells can be increased by photon recycling viaintroduction of a luminescent layer on top of a UV-blockinglayer. Thus the absorption and the corresponding photocur-rent loss due to a UV-blocking layer, can in part be compen-sated by application of an anti-reection and luminescentlayer, if the luminescence is efciently matched to theabsorption of the photo active layer. Using this approach, wedemonstrate the current output of the solar device to beincreased by more than 10%. Luminescent layers based on auorophore inside a transparent matrix, as described here,can easily be produced by roll-to-roll techniques. In thefuture, such uorophore could be incorporated as additivesinto luminescence substrates. Since photon recycling byluminescence shows typically higher efciencies than photondown-conversion of one high energy photon into two lowerenergy photons, this seems to be an appropriate way toincrease the overall device performance of exible solarcells.034517-4Engmannet al.6J. Appl. Phys.112,034517 (2012)F. C. Krebs, J. E. Carl, N. Cruys-Bagger, M. Andersen, M. R. Lilliedal,eM. A. Hammond, and S. Hvidt, “Lifetimes of organic photovoltaics: Pho-tochemistry, atmosphere effects and barrier layers in ITO-MEHPPV:PCBM-aluminium devices,”Sol. Energy Mater. Sol. Cells,86,499–516 (2005).7T. Kuwabara, T. Nakayama, K. Uozumi, T. Yamaguchi, and K. Taka-hashi, “Highly durable inverted-type organic solar cell using amor-phous titanium oxide as electron collection electrode inserted betweenITO and organic layer,”Sol. Energy Mater. Sol. Cells92,1476–1482(2008).8F. Wang, Z. J. Chen, L. X. Xiao, B. Qu, and Q. H. Gong, “Enhancement ofthe power conversion efciency by expanding the absorption spectrumwith uorescence layers,”Opt. Express19,A361–A368 (2011).9J. A. Renz, T. Keller, M. Schneider, S. Shokhovets, K. D. Jandt, G.Gobsch, and H. Hoppe, “Multiparametric optimization of polymer solarcells: A route to reproducible high efciency,”Sol. Energy Mater. Sol.Cells93,508–513 (2009).10M. S. Ryu, H. J. Cha, and J. Jang, “Improvement of operation life-time for conjugated polymer: Fullerene organic solar cells by intro-ducing a UV absorbing lm,”Sol. Energy Mater. Sol. Cells94,152–156 (2010).ACKNOWLEDGMENTSFinancial support of the author by the "Bundesministe-rium f€r Bildung und Forschung," Germany within the proj-uects “SonnTex” (BMBF 03X3518G) and “EOS” (BMBF03X3516F) are gratefully acknowledged. We also thank the“Center for Micro- and Nanotechnologies” (ZMN) for pro-viding the facilities needed in sample preparation.1R. F. Service, “Outlook brightens for plastic solar cells,”Science332,293–293 (2011).2Seehttp://www.konarka.comfor Konarka Power Plastics.3M. Jïrgensen, K. Norrman, and F. C. Krebs, “Stability/degradation ofpolymer solar cells,”Sol. Energy Mater. Sol. Cells92,686–714 (2008).4S. Chambon, A. Rivaton, J.-L. Gardette, and M. Firon, “Durability ofMDMO-PPV and MDMO-PPV: PCBM blends under illumination in theabsence of oxygen,”Sol. Energy Mater. Sol. Cells92,785–792 (2008).5Y.-M. Chang, W.-F. Su, and L. Wang, “Inuence of photo-induced degra-dation on the optoelectronic properties of regioregular poly(3-hexylthiophene),”Sol. Energy Mater. Sol. Cells92,761–765 (2008).Journal of Applied Physics is copyrighted by the American Institute of Physics (AIP). Redistribution of journalmaterial is subject to the AIP online journal license and/or AIP copyright. For more information, seehttp://ojps.aip.org/japo/japcr/jspzanotowane.pl doc.pisz.pl pdf.pisz.pl hannaeva.xlx.pl