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Lecture R. Oesterbacka


12 June 2009 FWN-Building 5116.0116, Nijenborgh 4, 9747 AG, Groningen
Speaker: Prof. Dr. R. Oesterbacka
Affiliation: Dept. of Physics and Center for Functional Materials, Åbo Akademi University, Turku, Finland
Title: Reduced recombination in bulk-heterojunction solar cells: effect on organic magnetoresistance
Date: Fri Jun 12, 2009
Start: 11.00
Location: FWN-Building 5116.0116
Host: P.W.M. Blom
Telephone: +31 50 363 4376/8750 (secr)


In this work we clarify the difference in charge transport and recombination in bulk-heterojunction solar cells fabricated from a 1:2 mixture of regioregular poly(3-hexylthiophene) (RRPHT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methano-fullerene (PCBM). We have used a multitude of experimental techniques such as Time-of-Flight (TOF), Double Injection (DoI) and Charge Extraction by Linearly Increasing Voltage (CELIV) techniques to measure charge carrier transport and recombination [1].

We found that the charge carrier bimolecular recombination is present at very high carrier densities. However, in good solar cells the bimolecular recombination was found to be strongly reduced compared to Langevin-type recombination accompanied with electric field-independent quantum efficiency for charge generation [2]. In low-efficiency samples the recombination of charge carriers is close to Langevin-type with electric field-dependent quantum efficiency as a result. The charge carrier mobilities for both electrons and holes as a function of applied external electric field are measured and the results are presented for both types of solar cells.

Furthermore, we have used the bulk heterojunction solar cells to clarify the role of generation of coulombically bound electron-hole (e-h) pairs, precursor to exciton formation, on the magnetoresistance (MR) in organic thin film devices. We have systematically varied the e-h recombination coefficients, which are directly proportional to the probability for the charge carriers to meet in space, and found that a reduced probability of electrons and holes meeting in space lead to a vanishing MR. Our results clearly show that MR is a direct consequence of a high probability for e-h pair formation ratio and by reducing the e-h pair formation in polymer-fullerene bulk heterojunction solar cells, we have observed almost vanishing MR [3].

Last modified:22 October 2012 2.30 p.m.