Harri Aarnio

Role of electron-hole pair formation in organic magnetoresistance

Magneto-electrical measurements were performed on diodes and bulk heterojunction solar cells (BHSCs) to clarify the role of formation of coulombically bound electron-hole (e-h) pairs on the magnetoresistance (MR) response in organic thin film devices. BHSCs are suitable model systems because they effectively quench excitons but the probability of forming e-h pairs in them can be tuned over orders of magnitude by the choice of material and solvent in the blend. 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 disappearance of the MR. Our results clearly show that MR is a direct consequence of e-h pair formation. We also found that the MR line shape follows a power law-dependence of B0.5 at higher fields.

Roll-to-Roll Fabrication of Bulk Heterojunction Plastic Solar Cells using the Reverse Gravure Coating Technique

The roll-to-roll reverse gravure (RG) coating technique was used to produce thin homogeneous films (∼100 nm) for organic bulk heterojunction solar cells. The conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and the active layer regioregular poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C 61 -butyric acid methyl ester (P3HT:PCBM) were successfully subsequently RG coated on an ITO covered plastic substrate in ambient air. Working solar cells were achieved after annealing and thermal evaporation of the top contact. The AM1.5 power conversion efficiency (PCE) of the RG coated organic solar cells was determined to 0.74% (at 100 mW/cm 2 ). This was very similar to the results of a reference device that was spin coated on a glass substrate in a nitrogen glove box.

Magnetoresistance in Poly (3-hexyl thiophene) Based Diodes and Bulk Heterojunction Solar Cells

We report on magnetotransport studies of regio regular poly (3-hexyl thiophene) (RRP3HT) based diodes and P3HT: 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]- methanofullerene (PCBM) bulk heterojunction solar cells. While the P3HT diodes with non-magnetic electrodes ITO and Al show up to 16% positive magnetoresistance (MR) under 300 mT magnetic field at room temperature, solar cells with much reduced electronhole pair (e-h) formation probability show almost negligible MR clearly suggesting that MR in organic semiconductors is strongly dependent on the probability of forming Coulombically bound e-h pairs. Voltage and temperature dependence of MR sign and magnitude is discussed for the diode devices which provides important insight of the scientifically puzzling yet technologically very promising magnetotransport properties of organic semiconductor based devices.