Expanding the spectral absorption breadth and efficiently harvesting excitons are crucial towards creating highly efficient polymer solar cells. Here we describe a strategy to realize broad-band light harvesting in poly(3-hexylthiophene) (P3HT)-based solar cells. We introduce the use of squaraine dye molecules that play a dual role towards improving P3HT-based solar cells. The first benefit is an increase in the spectral absorption in the near infrared region. The second advantage is the collection of excitons close to the interfacial heterojunctions via Förster resonance energy transfer (FRET). Unlike traditional multi-blend systems, where each donor works independently in separate spectral responses, FRET-based systems enable the effective use of multiple donors with significant improvements in light absorption and conversion. Ultrafast transient absorption experiments show that the excitation energy from P3HT can be transferred rapidly (within a few picoseconds) and efficiently (up to 96%) to the squaraine via FRET. As a result, the overall power conversion efficiency is improved. This architecture opens up a new paradigm towards transformative improvements of polymer solar cells.
The performance of the polymer/ZnO nanorod hybrid solar cells based on poly(3-hexylthiophene) and methanofullerenes
is improved with the enhanced optical absorption by increasing the thickness of the photoactive layer and introducing a
solution-processed interlayer. The dependence of the optical absorption on the thickness of the photoactive layer is
studied as a function of the spin-coating rate. With the slower
spin-coating rate, the photoactive layer is thicker, and the
polymer chains have longer time to self-organize and more effectively infiltrate into ZnO nanorod spacing. In addition, a
solution-processed fullerene interlayer is introduced to modify the ZnO nanorod surface. With this interlayer, the optical
absorption of the photoactive layer increases due to the better ordering of the photoactive layer. Our investigations show
that the power conversion efficiency (PCE) is improved from 1.6% to 2.6% with the thickness of the photoactive layer
from 240 nm to 350 nm by slowing the spin coating rate of the photoactive layer. Moreover, the PCE is also improved
by the fullerene interlayer. The slow-drying method and the
solution-processed fullerene interlayer both improve the
crystallinity of the polymer and light harvesting.
Sol-gel preparation of amorphous titanium oxide (TiOx) thin films with distinct morphological properties on the
hydrophobic substrate was obtained by solution spin coating method. The TiOx thin films were deposited by three
precursors using 2-methoxyethanol (2MOE), isopropanol (IPA) and mixture of 2MOE and hexane as solvents. We
demonstrate evidence that the morphology of TiOx thin film is strongly related to the employment of dissimilar solvent.
Among these three solvents, TiOx film obtained from 2MOE/hexane mixed solvent is a superior choice for the
preparation of TiOx thin film on the hydrophobic substrate because of its smooth surface morphology.