3D ORGANIZATION OF THIN FILMS FOR THREE COMPONENTS ACTIVE LAYER IN PHOTOVOLTAIC DEVICES

Abstract

Polymer-fullerene or polymer-polymer based bulk heterojunction (BHJ) solar cells can be fabricated by using low-cost manufacturing methods. However, because of the low mobility of organic materials, there is a competition between separation and recombination of the photogenerated carriers within the thin BHJ film. Thus, there is a need to develop strategies to increase light harvesting in the films without increasing the film thickness. Nanoparticles (NPs) have been receiving a lot of interest for exhibiting interesting optical, electrical, and magnetic properties. These novel properties can be exploited in nanotechnology by forming compact and ordered architectures of nanoparticles within of BHJ film. The addition of metal NPs 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] and have been functionalized both with 2-naphthalenethiol and alkanethiol having different length. Such particles in combination with poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) have been used to fabricate two different BHJ (P3HT:PCBM and P3HT:F8BT) with three components thin films. In particular we have realized planar and bulk heterojunction thin films by the 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, Au-NPs and PCBM or F8BT 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/donor or donor/acceptor interfaces as well as at the top of the acceptor layer. These structures and their components have been studied by 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 P3HT/PCBM 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.