Controlled 3D Interfacing of Three Components Thin Films for Photovoltaics

One of the main problems related to the low performance of the organic photovoltaic (OPV) devices, concerns the low mobility of the materials forming the heterojunction. For this reason, there is competition between the sweep-out and recombination of the photogenerated carriers within the thin film bulk heterojunction (BHJ). To overcome this problem, it is usual to operate by reducing the thickness of the active layer, so that the recombination of charge carriers is inhibited. This choice, however, also translates into a lower absorption of light by the active film itself. Plasmonic structures allow to reduce the "physical" thickness of heterojunction, maintaining constant the "optical" thickness. The addition of metal nanoparticles in the BHJ films could ensure a greater absorption and an enhanced photogeneration of mobile carriers[1]. In this work, we present a study of the effect of thiol-capped Au nanoparticles (AuNPs), within an organic device, by controlling on the nanoscale the position of the nanoparticles in the different interfaces of the device. Au-NPs have been obtained by laser ablation in liquid solution (LASiS) [2], have been functionalized both with 2-naphthalenethiol and alkanethiol having different length. Such systems in combination with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) have been used to fabricate planar and bulk heterojunction thin films by Langmuir-Blodgett (LB) apparatus, which offers a simple method of producing ultra-thin films with fine control over thickness. We prepared at first planar heterojunction (PHJ) structures consisting of layers of P3HT, PCBM and Au-NPs by the horizontal lifting (Langmuir-Schaefer LS) technique. Then we induced a transition from PHJ to BHJ by thermal annealing to mix the layers. To study the effect of the position of the nanoparticles, different structures have been realized, which differ in the position of the Au-NPs layer, i.e. at substrate/P3HT or P3HT/PCBM interfaces as well as at the top of the PCBM layer. These films and their components have been studied by a number of microscopy and spectroscopy surface tools and by electrochemistry to investigate the energy levels. I/V curves and importantly fluorescence quenching analysis showed that 2-naphthalenethiol-capped Au-NPs film incorporated at the donor/acceptor interface ensures a more efficient charge transfer if compared to the same heterojunction without AuNPs. The beneficial effect of nanoparticles seems to be lost after the transition from PHJ to BHJ, where there is any control over the particles location. Other important advantages on the efficiency of these three components devices are discussed.