Energetics of Excited States in the Conjugated Polymer Poly(3-hexylthiophene) Carsten Deibel, a Daniel Mack, and Julien Gorenflot Experimental Physics VI, Julius-Maximilians-University of W¨urzburg, 97074 W¨urzburg, Germany Achim Sch¨ oll, Stefan Krause, and Friedrich Reinert Gemeinschaftslabor f¨ur Nanoanalytik, Forschungszentrum Karlsruhe, 76021 Karlsruhe, and Julius-Maximilians-University of W¨urzburg, 97074 W¨urzburg, Germany Daniel Rauh and Vladimir Dyakonov Experimental Physics VI, Julius-Maximilians-University of W¨urzburg, 97074 W¨urzburg, Germany and Bavarian Centre for Applied Energy Research (ZAE Bayern), 97074 W¨urzburg, Germany (Dated: January 13, 2010) There is an enormous potential in applying conjugated polymers in novel organic opto-electronic devices such as light emitting diodes and solar cells. Although prototypes and first products exist, a comprehensive understanding of the fundamental processes and energetics involved during photoexcitation is still lacking and limits further device optimisations. Here we report on a unique analysis of the excited states involved in charge generation by photoexcitation. On the model system poly(3-hexylthiophene) (P3HT), we demonstrate the gen- eral applicability of our novel approach. From photoemission spectroscopy of occupied and unoccupied states we determine the transport gap to 2.6 eV, which we show to be in agreement with the onset of photoconductivity by spectrally resolved photocurrent measurements. For photogenerated singlet exciton at the absorption edge, 0.7 eV of excess energy are required to overcome the binding energy; the intermediate charge transfer state is situated only 0.3 eV above the singlet exciton. Our results give direct evidence of energy levels involved in the photogeneration and charge transport within conjugated polymers. PACS numbers: 71.23.Cq; 79.60.Ht; 73.50.Pz; 78.55.Qr; 73.61.Ph Keywords: organic semiconductors; polymers; energy levels; photoelectron spectroscopy; photoluminescence; photoconduc- tivity energy in conjugated polymers. 4 The exciton binding energy has been measured for a range of conjugated polymers, but the single particle gap has not been determined. 11–13 The reason for the non-comprehensive determination of energy levels in previous works is the difficulty to find a suitable, complemen- tary choice of experimental techniques relevant to photoexci- tation and the generation of free charges. In bulk heterojunction solar cells, 1,2 which have already show about 6% power conversion efficiency, 3 the light is mainly absorbed in the conjugated polymer, generating singlet excitons, which are strongly bound due to the weak screen- ing in organic semiconductors. 4 The exciton dissociation can be induced by acceptor molecules, often fullerene derivatives, and depends on the difference between the energy of the sin- glet exciton and the energy of the donor–acceptor polaron pair. If the latter state is energetically favourable, the excitons are dissociated by an ultrafast and very efficient electron trans- fer from the donor polymer. 5 Unfortunately, the energy differ- ence between exciton and polaron pair state is lost, leading to a lowering of the open circuit voltage, an important parameter determining the photovoltaic power conversion efficiency. 6,7 This example illustrates clearly why the energy levels of the utilized materials are of crucial importance for the fundamen- tal understanding and, consequently, for the systematic opti- mization of organic solar cells. In this article we report on a novel approach which is capa- ble of providing this data by applying several complementary experimental techniques. The investigation was performed systematically by photoelectron spectroscopy (PES), inverse photoelectron spectroscopy (IPES), optical absorption, field- induced photoluminescence quenching (PL( F )), and wave- length sensitive external quantum efficiency measurements (EQE) on the same or comparable samples of the model sys- tem poly(3-hexylthiophene) (P3HT). By this unique combina- tion, we can provide conclusive energy levels for the positive and negative polarons, the transport gap, the absorption gap, the exciton binding energy, as well as a value for the polaron pair binding energy. Fig. 1 schematically shows the processes occuring during photoexcitation and charge transport for a typical polymeric semiconductor, as well as the respective energy levels. If charge is transported, the molecule will adapt to the (single) positive or negative charge in the HOMO or LUMO level, re- spectively, resulting in positive or negative polaronic levels (HOMO + and LUMO $^{−}$). They are separated by the transport gap, also called single particle gap E$_{g}$$_{,}$$_{s}$ . In contrast, the ab- sorption of a photon results in the generation of a correlated electron–hole pair, a singlet exciton. The separation of the re- The magnitude of the binding energies of excited states in organic semiconductors, in particular for polaron pairs, is a controversial topic. 4,8 Lately, the notion that the binding en- ergy of the primary excitation can be overcome by the thermal energy 9 is becoming less accepted, 10 being replaced by a con- sensus that binding energies are much larger than the thermal arXiv:1001.2330v1 [cond-mat.mtrl-sci] 13 Jan 2010 $^{a}$deibel@physik.uni-wuerzburg.de I. INTRODUCTION