Studies of Highly Regioregular Poly(3‐hexylselenophene) for Photovoltaic Applications

Abstract

Solar cells based on conjugated polymers are attracting increasing interest due to their potential to enable a renewable energy technology based on simple and low cost manufacture.[1] Devices made from blends of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) have been widely studied and are consistently reported to produce efficiencies over 3%. [2,3] Regioregular P3HT is known to be a good hole transporting polymer with relatively long wavelength absorption, producing films with a highly crystalline morphology and good charge mobility (10 -4 cm 2 V -1 s -1 ). [4,5] According to a model presented by Scharber et al., [6] polymer:PCBM devices could be improved to produce energy conversion efficiencies of up to 11% by replacing P3HT with an electron donating polymer that has better optimised electronic energy levels. In particular, by decreasing the LUMO level of the donor while keeping the HOMO at the same level as P3HT, the reduced band gap would allow a greater light harvesting ability without compromising the open circuit voltage (V oc ).In this paper, we report on a study of a selenium analogue of P3HT, regioregular poly(3-hexylselenophene) (P3HS). The synthesis of this material has recently been reported by Heeney et al. [7] The polymer was shown to have a smaller optical gap (1.6 eV) than regioregular P3HT (1.9 eV) and because both polymers were shown to have the same HOMO level (4.8 eV), this decrease in band gap was assigned to a drop in the P3HS LUMO level. Taking into account the extended absorption range (up to 760 nm) and the similar field-effect transistor (FET) charge mobility to regioregular P3HT, [7] it was anticipated that P3HS might be a good candidate for polymer solar cells. The propensity for P3HT films to form crystalline structures is important for generating the interpenetrating nanomorphology that facilitates charge separation and charge transport in P3HT:PCBM blend devices.[4] X-ray diffraction (XRD) measurements were made of pristine P3HS films, before and after thermal annealing at 150°C, to investigate whether P3HS shares this crystalline nature (Fig. 1a). Since P3HS solutions have a tendency to gel at low temperatures, all P3HS and P3HS blend films reported in this paper were deposited by spin-coating from a hot (80°C) solution on to a substrate of the same temperature. This heating of the substrate and solution allowed for the preparation of much smoother films than if prepared at room temperature. Whilst as spun films showed no XRD peaks, a sharp diffraction peak (diffraction angle 2h = 3.

Polytellurophenes

“…For example, a recent report on poly(3-hexylselenophene) (P3HS) solar cells has shown that this materials has a better short-circuit current (due to better spectral response), yet poorer fill factor (due to poorer charge transport in a P3HS-electron acceptor blend) than (P3HT). [59] The result is a slightly decreased efficiency (2.7% for P3HS and 3.0% for P3HT) under analogous conditions. Researchers are cautioned not to draw conclusions about these polymers prematurely.…”

Linear optical properties and electronic structures of poly(3-hexylthiophene) and poly(3-hexylselenophene) crystals from first principles

“…[36] Their chemical structure is almost the same as that of rr-P3HT, but the sulfur atoms in the backbones are substituted by selenium atoms. It has been expected that the narrower band gap of rr-P3HS may lead to a more pronounced absorption in the BHJ device.…”

Controlling Electrical Properties of Conjugated Polymers via a Solution‐Based p‐Type Doping

“…The HOMO level of these polymers ranges from À4.8 eV (P3HT) to À5.9 eV (F8BT). [13][14][15] F 4 TCNQ is found to be dissolved in wide range of organic solvents including acetonitrile, toluene, chloroform, chlorobenzene, thiophene and xylene, to produce a concentration of 2 mg mL…”