On the Dissociation Efficiency of Charge Transfer Excitons and Frenkel Excitons in Organic Solar Cells: A Luminescence Quenching Study Kristofer Tvingstedt,* Koen Vandewal, Fengling Zhang, and Olle Ingana ¨s Biomolecular and Organic Electronics, Center of Organic Electronics (COE), Department of Physics, Chemistry and Biology, Linko ¨ping UniVersity, 58183 Linko ¨ping, Sweden ReceiVed: August 11, 2010; ReVised Manuscript ReceiVed: October 15, 2010 The field dependence of photocurrent found in many organic solar cells is a significant and detrimental setback for internal quantum efficiency. In this work we study the important contribution to this field dependence due to the dissociation efficiency of the weakly bound interfacial charge transfer (CT) state, crucial for organic bulk heterojunction solar cells. Three different donor polymers and two different acceptors are examined, and their respective dissociation characteristics are evaluated by photoluminescence (PL) quenching, both for Frenkel excitons and for the intermolecular charge transfer excitons. We observe that while the field-dependent photocurrent for pure polymers does correlate well with quenching efficiency, the CT exciton quenching from the blend generally displays a less pronounced correlation with extracted photocurrent. We further note that while the electroluminescence and photoluminescence of the pure polymer are identical, we observe a red shift for the blend electroluminescence. This indicates that lower energetic states, not visible in PL, are available in the blend. The emissive state of the blends probed by PL is therefore proposed to originate from sites that are involved in photocurrent generation to a lesser extent. Introduction Organic solar cells based on conjugated polymers and fullerenes show great potential as an inexpensive alternative to directly convert the abundant photonic energy available from our nearest star to useful electricity. 1,2 As power conversion efficiencies are improving, the organic photovoltaic alternative seems even more promising. New potential polymers with different optical and electronic properties are continuously emerging, and general agreement is increasing on that the main progress is expected to be found in the synthesis of novel donor and acceptor materials. 3 Although the general operational principle is rather well understood, a clear and accepted model that accurately describes the electric field dependence of the photocurrent in all quadrants of the I-V curve is presently missing 4 and the origin of the relevant factors limiting the internal quantum efficiency (IQE) is still debated. We however do believe that there currently is a satisfactory understand- ing of the origin of the open-circuit voltage (V OC ). Our recent studies of absorption and electroluminescence (EL) from a multitude of blends of organic donor and acceptor materials concludes that the V OC can be accurately and fully determined by the energy of the interfacial CT state and the recombination rate through it. 5-7 This electron-hole-pair interfacial state, with many names in the current literature, may not only be of high importance for the V OC but also for the extracted photocurrent. 4 The interfacial polymer-fullerene CT state has previously been observed by various techniques such as photoluminescence, 8-10 electroluminescence, 6 and sensitive photocurrent spectroscopy. 5 It is generally assumed that CT photoluminescence is a good probe of geminate recombination 10-12 of the bound charge pair and further that this recombination does constitute a significant limitation for IQE. 13,14 A general and important question is whether or not the CT state observable via PL is also an actual precursor for the free charge carriers available for extraction. The very important characteristic of the electric field depen- dence of photocurrent extraction has for a long time been seen in many, but not all, organic photovoltaic solar cells. This distinct feature has been attributed to several properties, such as a light-intensity-dependent shunt resistance, 15 a free carrier transport-limited collection efficiency (Schubweg limitation) by either trap recombination or bimolecular recombination, 16-20 as well as a geminate pair (CT state) field-dependent dominated dissociation efficiency (Onsager-Braun model). 10,12-14,21-25 Identifying the principal recombination mechanism that limits the amount of extracted photogenerated charge carriers at the electrodes would contribute to a better understanding of the processes that limits the IQE and governs the operational principle of the solar cells. 4 In this paper we approach these issues through the study of PL quenching by the application of an electric field. By studying the field dependence of the radiative part of the recombination through the interfacial CT state we seek to obtain information about one of the limiting recombination steps. We will here focus on the field dependence of dissociation of charge pairs in photovoltaic blends and at this time do not attend to carrier transport issues. A possible correlation of the PL quenching ratio and the amount of extracted photocurrent is observed and evaluated. Two different types of PL recombination routes, i.e., for Frenkel excitons and CT excitons, are discerned, and their respective dissociation field dependence is investigated. Similar field-dependent quenching studies do exist 12,25,26 but for very different photovoltaic blends without fullerenes and with rather diverging results from our present study. The very recent paper by Inal 27 that deals with a polymer/small molecule (M3EH- PPV/HV-BT) blend however makes a somewhat similar obser- vation as we do. With the aim to elucidate if the CT state observed in PL truly is a precursor for the photocurrent, we further compare EL vs PL of both Frenkel excitons and CT * To whom correspondence should be addressed. E-mail: kritv@ ifm.liu.se. J. Phys. Chem. C 2010, 114, 21824–21832 21824 10.1021/jp107587h  2010 American Chemical Society Published on Web 11/22/2010