Polymers for Application in Photovoltaics

Organic Photovoltaics (OPV)

One potential alternative to crystalline silicon photovoltaic (PV) cells is cells made from thin films (<1 micometer) of conjugated (semiconducting) and photoactive polymers, which can easily be cast onto flexible substrates over a large area using wet-processing techniques. These photoactive polymers are attractive semiconductors for photovoltaic cells because they are strong absorbers and can be deposited on flexible substrates at low cost. Cells made with a single polymer and two electrodes (see figure 1a) tend to be inefficient because the photogenerated excitons are usually not split by the built-in electric field, which arises from differences in the electrode work functions. The efficiency can be increased by splitting the excitons at an interface between two semiconductors with offset energy levels (see figure 1b). The exciton diffusion length in several different conjugated polymers has subsequently been measured to be 4-20 nm. To address the problem of limited exciton diffusion length in conjugated polymers, two conjugated polymers with offset energy levels are mixed so that all excitons would be formed near an interface, as depicted in figure 1c [Appl. Phys. 1995, 78, 4510]. This device structure, called a bulk heterojunction (BHJ), provided a route by which nearly all photogenerated excitons in the film could be split into free carriers. To allow the transport of the free carriers to the electrodes a bi-continuous blend structure is most advantageous. The active layer, consisting of two interpenetrating sub-networks with donor, respectively acceptor character, is sandwiched between two charge carrier-selective contacts [Chem. Mater. 2004, 16, 4533-4542].

In the BHJ devices based on two interpenetrating sub-networks, the conjugated polymer and electron acceptor have been randomly interspersed throughout the film. In the case of polymer-PCBM and polymer-CdSe nanorod devices, the random distribution of electron acceptors can lead to electron trapping on isolated acceptors unless a large weight fraction of acceptors is used. Moreover, the randomly distributed interface between the two semiconductors can lead to incomplete PL quenching in the conjugated polymer in regions of the polymer that are more than an exciton diffusion length away from an acceptor. Thus, a well-ordered conjugated polymer-electron acceptor film can be understood as an ideal device structure, as shown schematically in figure 1d. In the ideal device every exciton formed on the conjugated polymer will be within a diffusion length of an electron acceptor, although quantitative modelling has pointed out that some light emission will still occur in the polymer even if this is the case.

Central aim of the project is to gain a fundamental understanding of the correlation between the installed morphology and the observed photophysical properties in photoactive polymer films. The performance of organic photovoltaics (PVs) is severely limited by poor exciton dissociation and charge transport due in part to high rates of exciton recombination and low charge mobilities in polymers. This challenge can be partially overcome through the use of blended and layered heterojunctions. Such morphologies offer multiple exciton dissociation sites and separate charge pathways, thus limiting exciton recombination, and allowing for thicker, more absorbing, polymer films. The best device performance does not necessarily correlate with the excited state lifetime, however. Morphological differences, such as charge pathways that enable efficient charge transport, often outweigh the effect of charge transfer. Suggestions for improvement of nanoscale morphology are still lacking due to the complexity of the structural changes from the simple nanostructure which needs to be transferred into a PV device (see figure 2 for a simplified PV device geometry).

The poly(phenylenevinylene) (PPV) family of polymers serves as a prototypical conjugated polymer class for application as well as for fundamental understanding of the electronic processes in conjugated polymers. By a suitable modification of the chemical structure, the goal is to achieve electroluminescence that spans the visible and near-infrared regions. In order to improve the processability of PPV, flexible side chains are introduced on the polymer back-bone resulting in PPV derivatives such as MEH-PPV and others.
A donor material getting a lot of attention at present is poly(3-hexylthiophene) (P3HT), because of its lower bandgap as compared to Poly-para-(phenylene-vinylene). P3HT is studied because it has a high absorption coefficient close to the maximum photon flux in the solar spectrum. P3HT is also known in the field of organic electronics as a high-mobility material. Since the solar cell's active-layer thickness is a trade-off between enough light absorption and a decent charge-carrier extraction, the charge-carrier mobility is of considerable importance. The efficiency of P3HT based cells is strongly influenced by thermal treatment, generally ascribed to ordering of the P3HT due to the regioregularity of its structure. During this annealing step the P3HT is able to recrystallize. This has a positive influence on the solar cell's absorption spectrum and the mobility of the charges inside the P3HT.

So far, it has been shown that different types of BHJ solar cells can be made in a laboratory environment with relatively simple and new low temperature deposition techniques using a conjugated polymer as the donor material. Most research activities are focused on the characterization of the opto-electronic properties. Structural characterization at is limited to imaging techniques such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM). We focus to the field of polymer:polymer based BHJ PV in an approach combining our expertise in thin polymer films and in the use of advance scattering experiments (GISAXS and GIUSAXS). Expected results are the detection of the internal structure of the BHJ, which is a typical determination of a thin film morphology, and its changes due to an increase in device integration. From these results we will be able to address both challenges, an improvement in the understanding of many of the fundamental physical processes involved in the operation of these cells, as well as possible ways to optimize materials and structure.

Selected Publications:

  • 1. M.A.Ruderer, E.Metwalli, W.Wang, G.Kaune, S.V.Roth, P.Müller-Buschbaum
    Thin Films of Photoactive Polymer Blends;
    Chem.Phys.Chem. 10, 664-671 (2009) link
  • 2. C.R.McNeill, A.Abrusci, I.Hwang, M.A.Ruderer, P.Müller-Buschbaum, N.C.Greenham
    Photophysics and photocurrent generation in polythiophene/polyfluorene co-polymer blends;
    Adv.Funct.Mat. 19, 3103-3111 (2009) link
  • 3. M.A.Ruderer, M.Hirzinger, P.Müller-Buschbaum
    Photoactive nanostructures of polypyrrole;
    Chem.Phys.Chem. 10, 2692-2697 (2009) link
  • 4. G.Kaune, P.Müller-Buschbaum
    Gradient-doping of a conductive polymer film with a layer-by-layer approach;
    phys.stat.sol. (RRL) 4, 52-54 (2010) link