Study of carrier charge transport in organic solar cells

• Grant number: 14/02575-3
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate
• Start date: April 01, 2014
• End date: August 22, 2014
• Field of knowledge: Physical Sciences and Mathematics - Physics - Condensed Matter Physics
• Principal Investigator: Roberto Mendonça Faria
• Grantee: Douglas José Coutinho
• Supervisor: Heinz von Seggern
• Host Institution: Instituto de Física de São Carlos (IFSC). Universidade de São Paulo (USP). São Carlos , SP, Brazil
• Institution abroad: Technische Universität Darmstadt (TU Darmstadt), Germany
• Associated to the scholarship: 11/17143-3 - Hybrid photovoltaic cells: Architectures based on photonic and plasmonic effects, BP.DR

Abstract

One of the most successful active materials for organic solar cells is the regioregular poly(3-alkylthiophene) (rr-P3HT), which has also been used as active layer in thin film electronic and optoelectronic devices. However, despite the scientific and technological progress within the last two decades, polythiophene devices still have to overcome some obstacles in order to reach the market. One of those obstacles is the control over the interaction between oxygen molecules and the polymer and its electrical implications. In particular, the oxygen action on the charge transport properties of organic light-emitting diodes and organic field-effect transistors was subject of intensive investigation in the beginning of the last decade. Recently, this topic has regained relevance due to the strong influence of oxygen environment on the performance of organic solar cells, not only because the degradation caused by photo-oxidation, but also due to reversible changes in the electrical device properties. It is well documented that the action of oxygen molecules in P3HT:PCBM bulk heterojunctions (BHJ) (PCBM - phenyl-C61-butyric acid methyl ester), which is one of the most studied organic solar cell structure, results in doping effects, generation of trap levels and considerable changes in the electrical properties. The oxygen molecules thereby form of a reversible charge transfer complex in P3HT, which has been considered to be the reason for the increase of charge carrier concentration and a decreased charge carrier mobility in P3HT: PCBM films. In the present understanding, electrons are transferred from the P3HT HOMO to the oxygen molecules, which form charge transfer complexes with the thiophene rings thereby acting as electron traps. The remaining holes in the HOMO are the reason for a p-doping of P3HT and are responsible for an increase in conductivity and a change in charge transport. Thereby charge transport in conjugated polymer films is strongly affected by traps, which can be either intrinsically generated by polarization effects, e.g. defects along the chain backbones or at amorphous/crystalline interfaces, or from impurities. Using fractional thermal stimulated current techniques, Nikitenko et al. showed that intrinsic rr-P3HT thin films exhibited a density of states centered at 0.5 eV above the HOMO which act as deep hole traps. With the same technique, Schafferhans et al. showed that when in contact with oxygen P3HT films also presents a Gaussian distribution of trap states centered at 0.05 eV which are much shallower traps. In this project a detail investigation of the effect of oxygen molecules on charge transport in solar cell structures based on thin film blends of a nanostructured biphase of rr-P3HT and PCBM is proposed. Two main experimental techniques will be used: the fractional thermally stimulated current technique (F-TSC) and the current extraction by linearly increasing voltage technique (CELIV). From F-TSC one expects to identify trap levels of holes and electrons and to derive the energy distribution of the density of occupied states. CELIV is used to extract charge transport parameters, like charge carrier mobility, related to the dynamics of the charge transport under increasing electric field. Both techniques will be carried out keeping the device oxygen free and oxygen exposed. (AU)