Effect of Additives on the Morphology of PCPDTBT:PC71BM Thin Films for Organic Photovoltaics

Figure 1. GISAXS patterns of PCPDTBT:PC71BM thin films (1:2.7) fabricated a) without and b) the use of 3 vol% ODT. The figure originally published in the ACS AuthorChoice open access publication referenced below.

Figure 2. Effect of ODT on the nanometer-scaled morphology of PCPDTBT:PC71BM thin films for organic photovoltaics. a) The presence of ODT leads to enhanced lamellar polymer crystallization. B) Therefore, fullerene molecules must be expulsed from the film; a fullerene rich topping layer forms. The figure originally published in the ACS AuthorChoice open access publication referenced below.
One very successful way to realize polymer based organic solar cells makes use of a thin organic blend film consisting of a semi-conducting polymer and a fullerene derivative. These films allow not only for efficient large-scale application via printing or spraying techniques on flexible substrates but also allow for manufacturing semitransparent devices. Thus, light-weight, cost-efficient and optically tunable solar cells are available which open a vast range of applications such as in mobile devices or architecture. The most important issues yet to overcome are degradation, to enhance long-term stability and limited power conversion efficiency, to increase the device performance.

A way to improve the photovoltaic efficiency is to add processing additives in the mutual polymer-fullerene solution before film application. Typically, high boiling point solvents are thereby added that influence the demixing process of the two materials which leads to the formation of an inter-penetrating domain network, a so-called bulk-heterojunction (BHJ), within the active layer. In order to systematically improve OPV performance and to understand degradation effects, it is necessary to understand the exact mechanism of how the solvent additive (SA) changes the BHJ morphology.

In the present study, the exact mechanism of how the addition of 3 vol% SA in the processing solvent influences the nanometer-scaled BHJ morphology of films with different mixing ratios. Therefore, PCPDTBT:PC71BM thin films have been made from a chlorobenzene-based mutual solution with and without use of 3 vol% 1,8-octanedithiol (ODT) with different polymer-fullerene mixing ratios between 1:1.5 and 1:2.7. A combination of grazing incidence small and wide angle X-ray scattering (GISAXS and GIWAXS), X-ray reflectivity (XRR) and optical spectroscopy has been used to probe the vertical and lateral morphologies, whereby GISAXS and GIWAXS investigations were performed at the Austrian SAXS beamline of Elettra Sincrotrone de Trieste. Figure 1 shows exemplary GISAXS patterns of a film with a composition of 1:2.7 without and with use of ODT. 

When SA is used, a clear lateral scattering pattern appears around the polymer Yoneda region. Detailed analysis reveals that this pattern arises from enhanced phase separation, causing also a doubling of small polymer domains on a length scale of a few ten nanometers. The phase separation is found to be driven by polymer crystallization as seen in the GIWAXS investigation. Since, by the crystallization process, fullerene molecules must be pushed out of the polymer matrix, a fullerene-rich topping layer forms on the blend films which is investigated using XRR. As a result, the mixing ratio between polymer and small molecule becomes independent of the blend composition within the bulk of the film.

Combining these results, a full image of how ODT improves the power conversion capabilities of PCPDTBT:PC71BM thin films can be drawn as illustrated in Figure 2: The presence of solvent additive leads to a decrease of the solubility limit of fullerene molecules in the polymer matrix which leads to enhanced polymer crystallization and fullerene expulsion. Thereby polymer domains on a length scale of a few tens of nanometers form. These lead to both, improved charge carrier transport characteristics and enhanced light absorption, due to an enhanced degree of crystallinity. Furthermore, a fullerene-rich top layer forms. This layer acts as hole-blocking layer which decreases charge carrier recombination. All these effects contribute to an enhanced photovoltaic performance.

Retrieve article

Effect of Blend Composition and Additives on the Morphology of PCPDTBT:PC71BM Thin Films for Organic Photovoltaics;
Christoph J. Schaffer, Johannes Schlipf, Efi Dwi Indari, Bo Su, Sigrid Bernstorff, and Peter Müller-Buschbaum;
ACS Appl. Mater. Interfaces 7 (38), 21347-21355 (2015). doi: 10.1021/acsami.5b05939

Last Updated on Tuesday, 14 May 2019 17:05