Pi-Conjugated polymers for photovoltaics

Abstract

Polymer solar cells employ a nanoscopic phase separation or bulk heterojunction (BHJ) between two complementary molecular based p and n-type organic semiconductors to convert sunlight directly into electricity. The operational principle involves a complex sequence of events, starting with the absorption of light, followed by creation, separation, transport, and collection of charges. The most widely used and studied conjugated p-type polymers have an optical band gap in the visible range of the optical spectrum (~2.0 eV). Efficiencies up to 4-5% have been reported for poly(3-hexylthiophene) (P3HT) as a p-type polymer blended with a C60 fullerene ([60]PCBM) as n-type material. However, for commercial applications to become economically feasible, higher power conversion efficiencies are required. The research described in this thesis focuses on (fine)tuning the semiconducting polymer properties with respect to PCBM to improve power conversion efficiency. Appropriately matching the energy levels of the polymer to PCBM should allow a high open circuit voltage (Voc) while maintaining efficient electron transfer from the LUMO (lowest unoccupied molecular orbital) level of the polymer to the LUMO level of the PCBM. Furthermore, reducing the band gap of the p-conjugated polymer, allocating a good overlap between the polymer absorption and the solar emission spectrum, potentially increases the number of absorbed photons and hence photovoltaic performance. By varying the chemical nature of the building blocks we aim to control the position and separation of the energy levels, intended to lead to efficient bulk heterojunction solar cells. Chapter two involves the modification of P3HT by replacing n-hexyl with nbutoxymethyl side chains. The inductive electron withdrawing effect of n-butoxymethyl should increase the polymer oxidation potential and consequently enhance the open circuit voltage due to an improved energy level alignment with PCBM. Although poly[3-(n-butoxymethyl)- thiophene] was obtained in high regioregularity (>95%), the absorption data in the film point to a lesser degree of 3D ordering compared to P3HT. The Voc was indeed enhanced by ~0.1 V to 0.71 V in BHJ solar cells with [60]PCBM, but the lower 3D order in the solid state limited the short circuit current resulting in a power conversion efficiency of 1.7%. Chapter three describes small band gap polymers consisting of electron-rich dithiophene (donor) and electron-deficient thienopyrazine or bisquinoxaline units (acceptor). In the thin solid films the optical band gaps (Eg) vary from 1.38 eV for the bisquinoxaline to 1.21 eV for thienopyrazine containing polymers. All polymers provide a photovoltaic response, when blended with PCBM, in the near infrared region up to the optical band gap. Furthermore it is shown that a LUMO-LUMO offset of