Polymer-Nanocrystal Hybrid Solar Cells Using P3HT and Pyrite FeS2

Polymer-Nanocrystal Hybrid Solar Cells Using P3HT and Pyrite FeS₂

Beau Richardson, John Bae, Leize Zhu, and Qiuming Yu

Department of Chemical Engineering, University of Washington, Seattle, WA 98195

qyu@uw.edu

In order to reduce dependence on exhaustible energy sources and reduce carbon emissions, much scientific effort has been directed towards reducing the cost of renewable energy sources to make them competitive with conventional sources.  Solar cells have great potential as a low-carbon, sustainable energy source but are still too expensive to compete.  Searching for new materials to design low-cost and more efficient solar cells can reduce the cost per kilowatt-hour (kWh) of energy produced from these devices and bring them closer to grid parity.  Specifically, pyrite FeS₂ nanocrystals have several advantages for making solar cells but its use in thin-film, bulk-heterojunction type devices has not been thoroughly studied.

Recently, our group has demonstrated controllable synthesis of pyrite nanocrystals in octahedral and cubic shapes.  We are incorporating these nanocrystals into a hybrid, bulk heterojunction device with the semiconducting polymer Poly (3-hexylthiophene-2,5-diyl) (P3HT) and studying how the different nanocrystal sizes and shapes affect charge transport, light absorption, and overall device performance.  In conjunction with the fabrication of these hybrid devices, we are also fabricating organic devices with P3HT:PCBM active layers under the same conditions for direct comparison.  The typical device is fabricated on an ITO coated glass substrate with a ~40 nm PEDOT:PSS hole conducting layer, followed by a ~100-200 nm P3HT/pyrite active layer and an ~85 nm Al electrode. The film thickness and the surface morphology of the hybrid thin films are measured by AFM. The optical properties of the film are studied using ellipsometry and UV-vis-NIR absorption spectroscopy.  The power conversion efficiency (PCE) is determined by measuring the I_sc and V_oc under AM 1.5 Global Spectrum at 100 mW/cm².  A time-correlated single photon counting (TCSPC) system is used to analyze the relaxation of electrons in these films from an excited state to a lower energy state.  By employing TCSPC to study P3HT, pyrite nanocrystals, PCBM, P3HT:PCBM, and hybrid P3HT:pyrite nanocrystal films, we can elucidate the charge transport characteristics between the n- and p-type materials.  The effects of the ratio of P3HT:pyrite nanocrystals, solvent, film thickness and annealing conditions on the device performance will be discussed.