2 research outputs found
Contorted Hexabenzocoronenes with Extended Heterocyclic Moieties Improve Visible-Light Absorption and Performance in Organic Solar Cells
The large band gaps of existing contorted
hexabenzocoronene derivatives
severely limit visible-light absorption, restricting the photocurrents
generated by solar cells utilizing contorted hexabenzocoronene (cHBC).
To decrease the band gap and improve the light-harvesting properties,
we synthesized cHBC derivatives having extended heterocyclic moieties
as peripheral substituents. Tetrabenzofuranyldibenzocoronene (cTBFDBC)
and tetrabenzothienodibenzocoronene (cTBTDBC) both exhibit broader
absorption of the solar spectrum compared to cHBC, with peak absorbances
on the order of 10<sup>5</sup> cm<sup>–1</sup> in the near-ultraviolet
and in the visible. Planar-heterojunction organic solar cells comprising
cTBFDBC or cTBTDBC as the donor and C<sub>70</sub> as the acceptor
surpass those having cHBC in photocurrent generation and power-conversion
efficiency. Interestingly, devices containing cTBFDBC/C<sub>70</sub> exhibit the highest photocurrents despite cTBTDBC having the smallest
band gap of the three cHBC derivatives. X-ray reflectivity of the
active layers indicates a rougher donor–acceptor interface
when cTBFDBC is employed instead of the other two donors. Consistent
with this observation, internal quantum efficiency spectra suggest
improved charge transfer at the donor–acceptor interface when
cTBFDBCî—¸as opposed to cTBTDBC or cHBCî—¸is used as the
donor
Low-Temperature Synthesis of a TiO<sub>2</sub>/Si Heterojunction
The classical SiO<sub>2</sub>/Si
interface, which is the basis
of integrated circuit technology, is prepared by thermal oxidation
followed by high temperature (>800 °C) annealing. Here we
show
that an interface synthesized between titanium dioxide (TiO<sub>2</sub>) and hydrogen-terminated silicon (H:Si) is a highly efficient solar
cell heterojunction that can be prepared under typical laboratory
conditions from a simple organometallic precursor. A thin film of
TiO<sub>2</sub> is grown on the surface of H:Si through a sequence
of vapor deposition of titanium tetraÂ(<i>tert</i>-butoxide)
(<b>1</b>) and heating to 100 °C. The TiO<sub>2</sub> film
serves as a hole-blocking layer in a TiO<sub>2</sub>/Si heterojunction
solar cell. Further heating to 250 °C and then treating with
a dilute solution of <b>1</b> yields a hole surface recombination
velocity of 16 cm/s, which is comparable to the best values reported
for the classical SiO<sub>2</sub>/Si interface. The outstanding performance
of this heterojunction is attributed to Si–O–Ti bonding
at the TiO<sub>2</sub>/Si interface, which was probed by angle-resolved
X-ray photoelectron spectroscopy. Attenuated total reflectance Fourier
transform infrared spectroscopy (ATR-FTIR) showed that Si–H
bonds remain even after annealing at 250 °C. The ease and scalability
of the synthetic route employed and the quality of the interface it
provides suggest that this surface chemistry has the potential to
enable fundamentally new, efficient silicon solar cell devices