3 research outputs found
Evidence for âSlowâ Electron Injection in Commercially Relevant Dye-Sensitized Solar Cells by visâNIR and IR PumpâProbe Spectroscopy
We
present femtosecond to nanosecond transient absorption (TA)
data on electron injection in dye-sensitized solar cells (DSSCs) fabricated
with low volatility, commercially relevant electrolytes, with and
without added lithium. Results are shown over an extended time range
(300 fsâ6.3 ns) and extended wavelength range (800â1400
nm) for both N719 and C106 dyes. Kinetics were measured on both TiO<sub>2</sub> and noninjecting ZrO<sub>2</sub>. Using the latter, we have
determined the spectra and absorption coefficient of N719* across
the wavelength range. We find an isosbestic point in the TA spectra
on TiO<sub>2</sub> near 900 nm for all cells, existing from <1
ps to >1 ns. We show how measurements near this isosbestic point
can
give a false impression of uniformly femtosecond injection dynamics
in DSSCs. Comparison of dynamics measured at 1200 nm with mid-IR transient
absorption measured at 5100 nm confirms a majority proportion of slow
(>10 ps) electron injection in these commercially relevant cells.
We also comment on a recent publication which appears to directly
contradict the results we present
Charge Separation in Intermixed Polymer:PC<sub>70</sub>BM Photovoltaic Blends: Correlating Structural and Photophysical Length Scales as a Function of Blend Composition
A key challenge in achieving control
over photocurrent generation
by bulk-heterojunction organic solar cells is understanding how the
morphology of the active layer impacts charge separation and in particular
the separation dynamics <i>within</i> molecularly intermixed
donorâacceptor domains versus the dynamics <i>between</i> phase-segregated domains. This paper addresses this issue by studying
blends and devices of the amorphous siliconâindacenodithiophene
polymer SiIDT-DTBT and the acceptor PC<sub>70</sub>BM. By changing
the blend composition, we modulate the size and density of the pure
and intermixed domains on the nanometer length scale. Laser spectroscopic
studies show that these changes in morphology correlate quantitatively
with the changes in charge separation dynamics on the nanosecond time
scale and with device photocurrent densities. At low fullerene compositions,
where only a single, molecularly intermixed polymerâfullerene
phase is observed, photoexcitation results in a ⌠30% charge
loss from geminate polaron pair recombination, which is further studied
via light intensity experiments showing that the radius of the polaron
pairs in the intermixed phase is 3â5 nm. At high fullerene
compositions (â„67%), where the intermixed domains are 1â3
nm and the pure fullerene phases reach âŒ4 nm, the geminate
recombination is suppressed by the reduction of the intermixed phase,
making the fullerene domains accessible for electron escape
Isostructural, Deeper Highest Occupied Molecular Orbital Analogues of Poly(3-hexylthiophene) for High-Open Circuit Voltage Organic Solar Cells
We present the synthesis and characterization
of two novel thiazole-containing
conjugated polymers (<b>PTTTz</b> and <b>PTTz</b>) that
are isostructural to polyÂ(3-hexylthiophene) (P3HT). The novel materials
demonstrate optical and morphological properties almost identical
to those of P3HT but with HOMO and LUMO levels that are up to 0.45
eV deeper. An intramolecular planarizing nitrogenâsulfur nonbonding
interaction is observed, and its magnitude and origin are discussed.
Both materials demonstrate significantly greater open circuit voltages
than P3HT in bulk heterojunction solar cells. <b>PTTTz</b> is
shown to be an extremely versatile donor polymer that can be used
with a wide variety of fullerene acceptors with device efficiencies
of up to 4.5%. It is anticipated that this material could be used
as a high-open circuit voltage alternative to P3HT in organic solar
cells