Enhanced Structural Order of Polymer Photovoltaic Devices Deposited on Hydrogenated Metal Oxide Surfaces

Dimitris Davazoglou^a, Panagiotis Argitis^a, Marinos Tountas^a, Georgios Papadimitropoulos^a, Ermioni Polydorou^a, Maria Vasilopoulou^a, Anastasia Soultati^a, Dimitra Georgiadou^a

^aNational Center for Scientific Research Demokritos, Terma Patriarchou Grigoriou, Athens, 15354, Greece

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Dimitris Davazoglou, 037
Publication date: 5th February 2015

In organic photovoltaic (OPV) devices the polymer morphology and chain alignment need to be controlled simultaneously and favorably in order to obtain high mobilities and enhanced exciton dissociation. In the commonly used blend of regioregular P3HT and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) the most effective strategy toward constructing the nanoscale donor/acceptor percolated networks with well-optimized morphology is the application of a post annealing treatment at high temperature. However, in contrast to OPVs based on P3HT as donor in the BHJ, which require thermal annealing to achieve their highest efficiency, devices based on the new generation donor materials and exhibiting small band gap to offer a more efficient light harvesting of the solar spectrum, such as PCDTBT, are better performed without the post thermal annealing step. It was found that π-πstacking of the PCDTBT molecules is reduced upon annealing, resulting in simultaneous reduction of hole mobility and device performance. As a consequence, the ubiquitous post annealing step applied in the P3HT-based OPVs is considered prohibitive for devices using new donor materials as the PCDTBT copolymer. Hence, the optimization of polymer morphology and chain alignment in OPV devices not allowed to be heated is still a challenge. In this work, hydrogenation of metal oxides, which are widely used at bottom electrodes to enhance carrier extraction rates (i.e. tungsten, molybdenum, zinc and titanium oxide), is proposed to improve polymer crystallinity/order and enhance organic photovoltaic (OPV) device performance. Organic active layers based on blends using different polymers, in particular poly(3-hexythiophene) (P3HT) and poly[(9-(1-octylnonyl)- 9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT), which normally differ in both morphology and electronic structure, were both benefitted from deposition in hydrogenated metal oxide substrates as they exhibited enhanced π-πinteraction of their chains resulting in better edge-on and, especially, face on orientation, as it was revealed from X-ray diffraction measurements. Open circuit voltage (V_oc), short-circuit current (J_sc) and fill factor (FF) of OPVs using these hydrogenated metal oxides at bottom electrodes were all improved regardless of the polymer donor in the photoactive layer. This is explained as a result of the increased crystallinity/order and favorable morphology of the polymer, as the metal oxides’ surface becomes hydrogen rich, followed also by reduced recombination losses and increased carriers lifetime, as was verified by transient photoluminescence measurements.

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