2 research outputs found
Understanding the Apparent Charge Density Dependence of Mobility and Lifetime in Organic Bulk Heterojunction Solar Cells
Energetic disorder in organic semiconductors
leads to strong dependence
of recombination kinetics and mobility on charge density. However,
observed mobilities and reaction orders are normally interpreted assuming
uniform charge carrier distributions. In this paper, we explore the
effect of the spatial distribution of charge on the determination
of mobility and recombination rate as a function of average charge
density. Since the spatial gradient changes when the thickness of
a device is varied, we study thickness series of two different polymer:fullerene
systems and measure the charge density dependence of mobility and
lifetime. Using simulations, we can show that the high apparent reaction
orders frequently observed in the literature result from the spatial
gradient of charge density at open circuit. However, the mobilities,
measured at short circuit, are less affected by the gradients and
therefore may show substantially different apparent charge density
dependence than the recombination constants, especially for small
device thicknesses
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
donor–acceptor domains versus the dynamics <i>between</i> phase-segregated domains. This paper addresses this issue by studying
blends and devices of the amorphous silicon–indacenodithiophene
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 polymer–fullerene
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 3–5 nm. At high fullerene
compositions (≥67%), where the intermixed domains are 1–3
nm and the pure fullerene phases reach ∼4 nm, the geminate
recombination is suppressed by the reduction of the intermixed phase,
making the fullerene domains accessible for electron escape