Photovoltaic Performance of 4,8-Bis(2′-ethylhexylthiophene)thieno[2,3-<i>f</i>]benzofuran-Based Dyes Fabricated with Different Donors in Dye-Sensitized Solar Cells

Abstract

Thieno[2,3-<i>f</i>]benzofuran (BDF) has the advantages of a highly planarized structure, strong electron-donating ability, high hole mobility, good conjugation, and a wide spectral response range. In recent years, BDF has been widely used in organic solar cells, especially in bulk-heterojunction (BHJ) organic solar cells. In this work, a model molecule <b>PSB-1</b> was synthesized based on this highly planar fragment and used as a photosensitizer in dye-sensitized solar cells (DSCs), then different aromatic amine donors such as triphenylamine (TPA), carbazole (CZ), and phenothiazine (PTZ) were introduced to the end of <b>PSB-1</b>, and a series of dyes <b>PSB-2</b>, <b>PSB-3</b>, and <b>PSB-4</b> were designed and synthesized. After that, the relationship among the molecular structure, energy level, and photovoltaic performance of the benzo-[1,2-<i>b</i>:4,5-<i>b</i>']dithiophene (BDT) dye was studied by theoretical calculations, photophysics, electrochemistry, and photovoltaic properties. The results show that the introduction of a strong donor can effectively improve the energy level, absorption spectrum, and photovoltaic performance of <b>PSB-1</b>. Through the preliminary test, we found that the energy conversion efficiency (photovoltaic conversion efficiency-PCE) of <b>PSB-4</b> is up to 5.5%, which is nearly 90% higher than that of <b>PSB-1</b> (PCE = 2.9%), while the introduction of a weak donor greatly weakens the effect, in which the PCE of <b>PSB-3</b> is 3.5%, which is only 20% higher than that of the model molecule. By an analysis of the molecular frontier orbital distribution using theoretical calculations, we found that the electron cloud of the highest occupied orbital level (highest occupied molecular orbital-HOMO) of <b>PSB-3</b> is mainly distributed on the BDF group so that the electron transfer of excited-state molecules mainly occurs from the BDF to the receptor (CA).