New Conjugated Polymers for Organic Solar Cells

Title: Indacenodithiophene and Quinoxaline-Based Conjugated Polymers for Highly Efficient Polymer Solar Cells

Authors: Yong Zhang, Jingyu Zou, Hin-Lap Yip, Kung-Shih Chen, David F. Zeigler, Ying Sun, and Alex K.-Y. Jen

Affiliation:  Department of Materials Science and Engineering, University of Washington, Seattle, Washington

Journal: Chemistry of Materials

Organic solar cells are studied extensively for their potential as solution-processable, light-weight, low-cost, large-area energy generators. The advantages of organic solar have yet to fully be achieved, however, due to the low power conversion efficiency (PCE) of the solar cells which must reach ~10% if organic solar is to become marketable on a large scale. In this paper the authors present two new conjugated polymer for organic electronics and solar applications, and show remarkably high PCEs for solar cells made from these polymers.

Synthesis of highly conjugated polymers diphQ and phanQ

The polymers of interest were synthesized through Stille coupling of two highly conjugated monomers: indacenodithiophene and quinoxaline (see Scheme 1). Two quinoxaline derivatives were used: one in which the pendant phenyl groups are linked together (phanQ polymer) and one in which they are separate (diphQ polymer). The authors speculate that by linking the phenyl groups, the polymer will have a longer conjugation length. The conjugation length is the distance over which the polymer has an uninterrupted stretch of alternating double and single bonds. The longer the conjugation length, the lower the band gap (difference between HOMO and LUMO levels of the polymer), and the lower the band gap the more red photons can be absorbed and converted to useful energy, which is desirable for solar cells because much of the integrated solar spectrum is in the red/infrared.

In this paper the authors measured a number of properties of the polymers of interest, including band gap, HOMO/LUMO levels, charge carrier mobility, and photovoltaic device performance. The band gap was measured from optical properties of the polymers (absorption onset), and the HOMO level was measured by cyclic voltammetry. The authors found that a thin film of the diphQ polymer had a higher energy absorption band edge (corresponding to a larger band gap) than a solution of the same polymer, which they attribute to poor crystal packing in the solid state which means the effective conjugation length for the polymer is shorter. PhanQ, on the other hand, had a very similar absorption profile in the solid state thin film compared to solution, and the absorption features were red-shifted compared to the diphQ polymer, both of which suggest it has a longer conjugation length.

The hole mobility (how much area a positive charge carrier explores in a given time with a given applied voltage) of the two polymers were measured using field-effect transistors. The hole mobilities were relatively high for conjugated polymers, at around 10-2 cm2 V-1 s-1. Thermal annealing improved the hole mobility for diphQ because it allowed the chains to reconfigure into a better packed crystal, but annealing had little effect on the measured mobility for phanQ, presumably because the spin-cast film had already achieved good crystal packing.

Solar cells made from these polymers showed a high PCE of ~6%, which is quite high for organic photovoltaics. This PCE was achieved because the polymers have (1) a relatively small band gap, (2) good alignment with the electronic energy levels of the acceptor molecule, the fullerene PCBM, which allows for efficient charge separation without too much loss of energy. Overall, this work shows progress towards organic electronic materials that may allow the realization of the promise of low-cost, high-efficiency, large-area solar cells.

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