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An Additive Opens New Visions in Solar Cells Energy Science

X-ray scattering insight and an associated mechanistic interpretation and a correlation with
an enhanced performance are presented.

ACTIVITY REPORT 2016
bulk heterojunction (BHJ) polymer solar cell (PSC) active layer prepared with SH-na and DIO additives
A is regarded as a promising renewable-energy are significantly greater than that prepared without
resource with the advantages of low cost, flexible additive by factors 2.0 and 1.3, respectively. For the
devices and solution processing of a large area. The active layers prepared without additive after a dip-
device performance of PSC has been improved great- ping treatment, the (100) signal also significantly
ly via molecular design and device structure architec- enhanced by factor 1.56, implying that a dipping
ture in recent years. Low-bandgap polymers served to treatment can substantially improve the crystallinity
extend the light absorption for increased short-circuit of PTB7.
current (J sc) and open-circuit voltage (V oc) through a
modulated highest occupied molecular-orbital level, The GIWAXS profiles of the blended films also exhib-
hence further enhancing the efficiency of power con- ited the same ordering signals of PC 71BM aggregates
version (PCE). Among low-bandgap polymers, fluo- of a pure PC 71BM spin-cast film, revealing a similar
rinated-thieno thiophene-based PTB7 is one of the local packing structure in these films from the various
most widely used materials in PSC with a large PCE. routes. In contrast, GISAXS 2D patterns (Fig. 1(b))
and 1D in-plane profiles (Fig. 1(c)) show distinct
In this reported work of Show-An Chen (National Ts- features in the observed q-range. The GISAXS 2D
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ing Hua University), U-Ser Jeng (NSRRC), Chun-Jen Su patterns of the active layer without additive exhibit
(NSRRC), Wei-Ru Wu (NSRRC) and Jey-Jau Lee (NSR- much richer scattering features than those of other
RC), novel additive 1-naphthalenethiol (SH-na) was processing routes with several oblique side signals
proposed to process the PSC active layer of PTB7:P- oriented about ≈ 10° from the surface normal direc-
C 71BM. Spin-casting with SH-na as additive achieved tion; these features correspond to a faceted surface
a large PCE. Dipping of the active layer in a methanol morphology of the active layer processed without
solution of a critical SH-na concentration increased additive. The most distinct features in all GISAXS pat-
the PCE further. They cooperatively performed syn- terns are the strong vertical scattering stripes located
chrotron-based X-ray-scattering techniques at TLS near q y ≈ 0.002 Å , uniquely observed with the active
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23A1 to clarify the correlation between morphology layer prepared without additive. Such a scattering
optimization and an improved PCE. feature indicates the existence of phase-separated
PC 71BM-rich domains with a large mean spacing, (d) ≈
The crystallization of PTB7 and the aggregation of 314 nm, along the in-plane direction of the film.
PC 71BM in the active layers of varied processing routes
(with SH-na, DIO or a dipping treatment) were inves- Figure 1(c) shows characteristic in-plane GISAXS
tigated with grazing incidence wide- and small-angle profiles for the active layers without additive and
X-ray scattering (GIWAXS and GISAXS). These GI- that subject to the methanol/SH-na solution dip-
WAXS profiles were extracted respectively along the ping treatment. The large dimension, 260 nm, of the
in-plane (q y) (Fig. 1(a)) from the corresponding 2D oblate aggregates is modeled to be preferentially
GIWAXS patterns, revealing a marginally preferable oriented along the in-plane direction. An additional
face-on orientation of PTB7 crystallites; which might in-plane profile I (q y) is extracted at q z = 0.038 Å (cir-
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be beneficial to enhance the vertical hole transport cumventing the beamstop-blocked scattering zone,
and thus improve the device performance. The PTB7 marked with blue squares in Fig. 1(b)) to illustrate
(100) reflections (corresponding to ordered packing the interference signal at q y ≈ 0.002 Å ; the profile
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along the polymer side chains) for the active layers was adequately fitted (Fig. 1(c)) with oblate aggre-
processed without additive, with DIO and with SH- gates of the same size with mean in-plane spacing
na, locate respectively at scattering vectors q = 0.351, 312 nm. In contrast, the GISAXS profile for an active
0.357 and 0.355 Å , corresponding to characteristic layer subject to a dipping treatment was best fitted
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layer spacings 17.9, 17.6 and 17.7 Å, respectively. The (Fig. 1(c)) with bimodal size distributions, which
result indicates that these additives might facilitate comprise smaller prolate aggregates and small glob-
a tightening of the chain packing of PTB7. Moreover, ular PC 71BM aggregates to account for the scattering
the integrated intensities of the (100) signal of the hump at q y ≈ 0.025 Å .
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