Triplet-triplet annihilation in organic molecules increases the photocurrent
of thin-film solar cell

Lips, Klaus

Schmidt, T.

Schulze, T.

Institutional Repository of the Freie Universität Berlin

10.1117/2.1201403.005390Enhancing solar cells withphotochemical upconversionTim Schulze, Klaus Lips, and Timothy W. SchmidtTriplet-triplet annihilation in organic molecules increases the pho-tocurrent of thin-film solar cells.Photovoltaics have the potential to become a major contributorto future sustainable energy generation. To this end, device effi-ciency needs to be brought beyond present physical limits. Solarcells cannot absorb light with less energy than the bandgap (ab-sorption threshold of the photovoltaic absorber material), andconsequently this energy source is usually uncaptured. Upcon-version (UC) devices harvest those unused subthreshold pho-tons behind the solar cell, create one higher-energy photon outof (at least) two transmitted photons, and radiate upconvertedlight back toward the solar cell, thus expanding the usable so-lar spectrum (see Figure 1). Key requirements for UC units arebroad absorption and high UC quantum yield under even thelow-intensity incoherent illumination that is available to solarenergy conversion devices.Upconversion was proposed as a means to increase solarcell efficiency about a decade ago,1 and research has sincethen been primarily focused on UC processes occurring inlanthanide-doped glasses or nanoparticles.2 Although transi-tions between the rare earth 4f orbitals lie in an energy rangepromising for combination with common crystalline siliconsolar cells, UC systems based on erbium (Er) or ytterbium (Yb)suffer from weak absorption by Laporte-forbidden transitions, anarrow absorption bandwidth, and the resulting low efficiency.Recently, triplet-triplet annihilation (TTA) in organic moleculeshas been demonstrated to be an alternative UC mechanismhighly promising for solar energy application.3 It proceeds ef-ficiently even for low photon flux and displays highly tunableabsorption characteristics.The key to high quantum yield from TTA-UC lies in the com-bination of two different classes of organic molecules dissolvedtogether in organic solvents or in a polymeric host matrix.4 A so-called sensitizer species, which in many cases is a metalated por-phyrin, absorbs a low-energy photon (process 1 in Figure 2) andundergoes fast intersystem crossing (ISC, process 2), wherebyFigure 1. Principle of a solar cell assisted by upconversion (UC). Thecreation of usable electron-hole pairs in the photovoltaic absorber mate-rial (left) is augmented by light, usually transmitted unused, which isconverted to higher photon energies in the UC unit (right).the spin of the exciton changes from 0 (singlet, electron and holespin cancel) to 1 (triplet, electron and hole spin add). This changeof spin state is facilitated by the heavy metal in the porphyrincenter. The resulting triplet state cannot radiatively relax back tothe ground state and is therefore long-lived. Interaction with asecond class of molecule transfers the triplet energy to the so-called emitter species by triplet energy transfer (TET, process 3).Successive interaction of two emitters yields TTA (process 4),which combines the two triplet excitons to create one higher-energy singlet state. The singlet state quickly relaxes back to theground state under efficient emission of a high-energy photon(process 5).We recently explored the feasibility of TTA-UC for expand-ing the spectral harvest of thin-film solar cells. We combinedContinued on next page10.1117/2.1201403.005390 Page 2/3TTA-UC systems comprising tailored molecular sensitizerspecies5with amorphous silicon (a-Si) solar cells,6, 7with organicbulk heterojunction solar cells,8 and with dye-sensitized solarcells.9 We defined a figure of merit (FOM) for UC-assisted so-lar cells, which takes into account the inherent nonlinearity ofthe UC process in the benchmarking of different UC systems.Usually, UC systems are measured under highly concentratedillumination, which hinders a fair comparison of UC units mea-sured under different conditions. Our FOM traces the UC perfor-mance back to conditions equivalent to the standard operation ofa solar cell,6 and equals the photocurrent increase by UC under1-sun conditions.Using the FOM for guidance, we concentrated on increas-ing the performance of the solar cell/UC assembly. Shifting thesensitizer absorption into the IR by extending the -system ofthe palladated sensitizer porphyrin led to a fourfold increaseof the FOM6 compared with the original sensitizer species, us-ing 130nm-thick hydrogenated a-Si (a-Si:H) cells. This prelim-inary result already beats the state of the art for a-Si:H cellsaugmented by rare-earth UC by a factor of about 200.10 Com-bining the UC unit with a back-reflecting surface helps to multi-pass the incident light as well as to out-couple the upconvertedlight, and led to a consecutive increase of the FOM7 by anotherfactor of 3 to about 2–310 4mA/cm2. Roughly the same levelof photocurrent enhancement could be obtained for organic bulkFigure 2. Top: Working principle of triplet-triplet annihilation upcon-version (TTA-UC) shown on a schematic energy diagram. A key roleis played by the long-lived triplet states Tx of the organic molecules,which cannot decay quickly due to quantum-mechanical laws. Bot-tom: Picture of the TTA-UC effect. ISC: Intersystem crossing. Sx : Sin-glet states. TET: Triplet energy transfer.heterojunction solar cells8 as well as for dye-sensitized solar cellswith a fully device-integrated UC unit.9 The latter result wasmeasured under illumination conditions very similar to deviceoperation, reaching concentration factors as low as 3 suns. Ourcurrent record FOM stands at 7:6 10 4mA/cm2, obtained withan a-Si:H solar cell optimized for low parasitic absorption.8In summary, we have shown cumulative FOM improve-ments of roughly three orders of magnitude compared withlanthanide-based UC and a first move toward device-integratedUC units. However, the photocurrent increase under 1 sunachieved to date is still far from commercially relevant. We aretherefore now working to increase the UC yield under low-illumination conditions. We are following two strategies in par-allel and have recently obtained initial, promising results. First,we are increasing the dye concentration by tethering the UC-active molecules onto nanosized support structures to increasethe steady-state triplet density and therefore the UC yield.11 Sec-ond, we are focusing the incident light locally in the UC layer us-ing micro-optical structures such as parabolic back-reflectors.12In combination, these strategies will enable us to exploit near-field optical effects such as plasmonic resonances or wave-guidemodes once characteristic absorption lengths in the 10–100nmrange have been reached, which should make further increasesin the UC yield possible. We are confident that TTA-UC willcontribute significant photocurrent to high-gap solar cells andphotoelectrochemical devices for fuel production in the nearfuture.T. F. S. acknowledges a Feodor-Lynen fellowship from the Alexandervon Humboldt Foundation. T.W. S. acknowledges Australian ResearchCouncil funding FT130100177, DP110103300, and project fundingfrom the Australian Renewable Energy Agency.Author InformationTim Schulze and Klaus LipsHelmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbHBerlin, GermanyTimothy W. SchmidtThe University of New South WalesSydney, AustraliaReferences1. T. Trupke, M. A. Green, and P. Wu¨rfel, Improving solar cell efficiencies by up-conversion of sub-band-gap light, J. Appl. Phys. 92, pp. 4117–4122, 2002.2. M. Haase and H. Scha¨fer, Upconverting nanoparticles, Angew. Chem. Int’l Ed. 50(26), pp. 5808–5829, 2011.Continued on next page10.1117/2.1201403.005390 Page 3/33. S. Baluschev, T. Miteva, V. Yakutkin, G. Nelles, A. Yasuda, and G. Wegner,Up-conversion fluorescence: noncoherent excitation by sunlight, Phys. Rev. Lett. 97,p. 143903, 2006.4. Y. C. Simon and C. Weder, Low-power photon upconversion through triplet-tripletannihilation in polymers, J. Mater. Chem. 22, p. 20817, 2012.5. T. Khoury and M. J. Crossley, A strategy for the stepwise ring annulation of all fourpyrrolic rings of a porphyrin, Chem. Commun. 46, pp. 4851–4853, 2007.6. Y. Y. Cheng, B. Fu¨ckel, R. W. MacQueen, T. Khoury, R. G. C. R. Clady, T. F.Schulze, N. J. Ekins-Daukes, et al., Improving the light-harvesting of amorphous siliconsolar cells with photochemical upconversion, Energy Environ. Sci. 5, pp. 6953–6959,2012.7. T. F. Schulze, Y. Y. Cheng, B. Fu¨ckel, R. W.MacQueen, A. Danos, N. J. L. K. Davis,M. J. Y. Tayebjee, et al., Photochemical upconversion enhanced solar cells: effect of a backreflector, Aust. J. Chem. 65, pp. 480–485, 2012.8. T. F. Schulze, J. Czolk, Y.-Y. Cheng, B. Fu¨ckel, R. W. MacQueen, T. Khoury, M.J. Crossley, et al., Efficiency enhancement of organic and thin-film silicon solar cells withphotochemical upconversion, J. Phys. Chem. C 116 (43), pp. 22794–22801, 2012.9. A. Nattestad, Y. Y. Cheng, R. W. MacQueen, T. F. Schulze, F. W. Thompson, A. J.Mozer, B. Fu¨ckel, et al., Dye-sensitized solar cell with integrated triplet-triplet annihila-tion upconversion system, J. Phys. Chem. Lett. 4, pp. 2073–2078, 2013.10. J. de Wild, J. K. Rath, A. Meijerink, W. G. J. H. M. van Sark, and R. E. I. Schropp,Enhanced near-infrared response of a-Si:H solar cells with ˇ -NaYF4:Yb3C (18%), Er3C(2%) upconversion phosphors, Sol. Energy Mater. Sol. Cells 94, pp. 2395–2398, 2010.11. R. W. MacQueen, T. F. Schulze, T. Khoury, Y. Y. Cheng, B. Stannowski, K. Lips,M. J. Crossley, and T. Schmidt, Nanostructured upconverters for improved solar cellperformance, Proc. SPIE 8824, p. 882408, 2013. doi:10.1117/12.202690712. T. F. Schulze, Y. Y. Cheng, T. Khoury, M. J. Crossley, B. Stannowski, K. Lips,and T. W. Schmidt,Micro-optical design of photochemical upconverters for thin-film solarcells, J. Photon. Energy 3 (1), p. 034598, 2013.c 2014 SPIE

Enhancing solar cells with photochemical upconversion

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Enhancing solar cells with photochemical upconversion

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