Transport in organic solar-cell materials studied by time-resolved terahertz spectroscopy

This project is related to the following Fields, Subjects and Techniques:

Motivation

Blends of polymers with electron acceptors are promising materials for fabrication of inexpensive solar cells. Using time-resolved terahertz (THz) spectroscopy, we can directly probe the very early phases of the photo-initiated charge transport, and thus to contribute to the understanding of the basic photo-processes in solar cells.

Operation of organic solar cell

Scheme of basic processes occurring in organic solar cells. These include photo-generation of excitons, exciton dissociation into separated charges and transport of the separated charges to the electrodes.

Simplified picture of processes occurring in organic solar cells

Visit the site of the polymer solar cells project to see more details.

Studied materials

Chemical structure of polymers and of the electron acceptor used in our studies. The polymers have been synthesized in the Department of Biomolecular and Organic Electronics at Linköping University. [Appl. Phys. A 79, 31 (2004), Adv. Mat. 19, 3308 (2007)].

We investigate two types of polymers. The APFO-3 is a polyfluorene co-polymer. It has quite low band gap (1.9 eV) which allows collection of about 40 % of the incident solar spectrum energy. In conjunction with PCBM electron acceptor, it reaches power conversion efficiency of 2.6 % [Thin Solid Films 515, 3126 (2007)]. The LBPP-1 co-polymer has an even lower band gap (1.0 eV). However, its power conversion efficiency is much lower (0.38 %, [Adv. Mat. 19, 3308 (2007)]).

Information provided by THz spectroscopy

The fundamental output of time-resolved THz spectroscopy consists in transient conductivity spectra Δσ(f). From the shape of these spectra, it is possible to distinguish between the response of free and bound charge carriers. Response of free charges is characterized by non-zero real part of conductivity at the lowest frequencies. Response of localized charge carriers causes increase of the real part of conductivity with frequencies while the corresponding imaginary part is negative. The spectra in the following graph thus exhibit a response of both types of charge carriers.

Example of transient far-infrared conductivity spectra measured in an APFO-3:PCBM blend at various pump-probe delays. The weight fraction of PCBM in this example was 80 %. The points represent measured data while the curves are results of a fit to a theoretical model [Submitted to J. Phys. Chem. C].

The important information which can be extracted from the measured transient conductivity spectra is a product of quantum efficiency—with which free charge carriers are generated—and of their mobility. For example, in the above graph this product reaches 4.7 %·cm²V^–1s^–1, measured 2 ps after photo-excitation. The separated charge carriers thus necessarily exhibit much higher mobility as compared to measurements in a dc electrical field [Appl. Phys. Lett. 88, 082103 (2006)].

Collaborations

• Department of Biomolecular and Organic Electronics at Linköping University, Sweden