Excitation Dynamics in Low Band Gap Donor–Acceptor Copolymers and Blends |
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Authors: | Björn Gieseking Berthold Jäck Eduard Preis Stefan Jung Michael Forster Ullrich Scherf Carsten Deibel Vladimir Dyakonov |
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Affiliation: | 1. Experimental Physics VI, Julius‐Maximilian University of Würzburg, 97074 Würzburg, Germany;2. Macromolecular Chemistry and Institute for Polymer Technology Bergische University of Wuppertal, 42097 Wuppertal, Germany;3. Bavarian Center for Applied Energy Research e.V. (ZAE Bayern), 97074 Würzburg, Germany |
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Abstract: | Donor–acceptor (D–A) type copolymers show great potential for the application in the active layer of organic solar cells. Nevertheless the nature of the excited states, the coupling mechanism and the relaxation pathways following photoexcitation are yet to be clarified. We carried out comparative measurements of the steady state absorption and photoluminescence (PL) on the copolymer poly[N‐(1‐octylnonyl)‐2,7‐carbazole]‐alt‐5,5‐[4′,7′‐di(thien‐2‐yl)‐2′,1′,3′‐benzothiadiazole] (PCDTBT), its building blocks as well as on the newly synthesized N‐(1‐octylnonyl)‐2,7‐bis‐[(5‐phenyl)thien‐2‐yl)carbazole (BPT‐carbazole). The high‐energy absorption band (HEB) of PCDTBT was identified with absorption of carbazoles with adjacent thiophene rings while the low‐energy band (LEB) originates instead from the charge transfer (CT) state delocalized over the aforementioned unit with adjacent benzothiadiazole group. Photoexcitation of the HEB is followed by internal relaxation prior the radiative decay to the ground state. Adding PC70BM results in the efficient PL quenching within the first 50 ps after excitation. From the PL excitation experiments no evidence for a direct electron transfer from the HEB of PCDTBT towards the fullerene acceptor was found, therefore the internal relaxation mechanisms within PCDTBT can be assumed to precede. Our findings indicate that effective coupling between copolymer building blocks governs the photovoltaic performance of the blends. |
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Keywords: | bulk‐heterojunctions charge transfer copolymers excitation dynamics organic solar cells |
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