2 research outputs found
A Versatile Approach to Organic Photovoltaics Evaluation Using White Light Pulse and Microwave Conductivity
State-of-the-art low band gap conjugated polymers have
been investigated
for application in organic photovoltaic cells (OPVs) to achieve efficient
conversion of the wide spectrum of sunlight into electricity. A remarkable
improvement in power conversion efficiency (PCE) has been achieved
through the use of innovative materials and device structures. However,
a reliable technique for the rapid screening of the materials and
processes is a prerequisite toward faster development in this area.
Here we report the realization of such a versatile evaluation technique
for bulk heterojunction OPVs by the combination of time-resolved microwave
conductivity (TRMC) and submicrosecond white light pulse from a Xe-flash
lamp. Xe-flash TRMC allows examination of the OPV active layer without
requiring fabrication of the actual device. The transient photoconductivity
maxima, involving information on generation efficiency, mobility,
and lifetime of charge carriers in four well-known low band gap polymers
blended with phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM),
were confirmed to universally correlate with the PCE divided by the
open circuit voltage (PCE/<i>V</i><sub>oc</sub>), offering
a facile way to predict photovoltaic performance without device fabrication
Beyond Fullerenes: Design of Nonfullerene Acceptors for Efficient Organic Photovoltaics
New
electron-acceptor materials are long sought to overcome the
small photovoltage, high-cost, poor photochemical stability, and other
limitations of fullerene-based organic photovoltaics. However, all
known nonfullerene acceptors have so far shown inferior photovoltaic
properties compared to fullerene benchmark [6,6]-phenyl-C<sub>60</sub>-butyric acid methyl ester (PC<sub>60</sub>BM), and there are as
yet no established design principles for realizing improved materials.
Herein we report a design strategy that has produced a novel multichromophoric,
large size, nonplanar three-dimensional (3D) organic molecule, DBFI-T,
whose π-conjugated framework occupies space comparable to an
aggregate of 9 [C<sub>60</sub>]-fullerene molecules. Comparative studies
of DBFI-T with its planar monomeric analogue (BFI-P2) and PC<sub>60</sub>BM in bulk heterojunction (BHJ) solar cells, by using a common thiazolothiazole-dithienosilole copolymer donor (PSEHTT), showed that DBFI-T has superior charge photogeneration
and photovoltaic properties; PSEHTT:DBFI-T solar cells combined a
high short-circuit current (10.14 mA/cm<sup>2</sup>) with a high open-circuit
voltage (0.86 V) to give a power conversion efficiency of 5.0%. The
external quantum efficiency spectrum of PSEHTT:DBFI-T devices had
peaks of 60–65% in the 380–620 nm range, demonstrating
that both hole transfer from photoexcited DBFI-T to PSEHTT and electron
transfer from photoexcited PSEHTT to DBFI-T contribute substantially
to charge photogeneration. The superior charge photogeneration and
electron-accepting properties of DBFI-T were further confirmed by
independent Xenon-flash time-resolved microwave conductivity measurements,
which correctly predict the relative magnitudes of the conversion
efficiencies of the BHJ solar cells: PSEHTT:DBFI-T > PSEHTT:PC<sub>60</sub>BM > PSEHTT:BFI-P2. The results demonstrate that the large
size, multichromophoric, nonplanar 3D molecular design is a promising
approach to more efficient organic photovoltaic materials