Post-deposition activation of latent hydrogen-bonding: A new paradigm for enhancing the performances of bulk heterojunction solar cells

Article abstract of DOI:10.1002/adfm.201400896

Small conjugated molecules (SM) are gaining momentum as an alternative to semiconducting polymers for the production of solution-processed bulk heterojunction (BHJ) solar cells. The major issue with SM-BHJs is the low carrier mobility due to the scarce control on the phase-segregation process and consequent lack of preferential percolative pathways for electrons and holes to the extraction electrodes. Here, a new paradigm for fine tuning the phase-segregation in SM-BHJs, based on the post-deposition exploitation of latent hydrogen bonding in binary mixtures of PCBM with suitably functionalized electron donor molecules, is demonstrated. The strategy consist in the chemical protection of the H-bond forming sites of the donor species with a thermo-labile functionality whose controlled thermal cleavage leads to the formation of stable, crystalline, phase-separated molecular aggregates. This approach allows the fine tuning of the nanoscale film connectivity and thereby to simultaneously optimize the generation of geminate carriers at the donor-acceptor interfaces and the extraction of free charges via ordered phase-separated domains. As a result, the PV efficiency undergoes an over twenty-fold increase with respect to control devices. This strategy, demonstrated here with mixtures of diketopyrrolopyrrole derivatives with PCBM can be extended to other molecular systems for achieving highly efficient SM-BHJ solar cells.

Full text of DOI:10.1002/adfm.201400896

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Post-deposition Activation of Latent Hydrogen-Bonding:
A New Paradigm for Enhancing the Performances of Bulk
Heterojunction Solar Cells
Francesco Bruni, Mauro Sassi, Marcello Campione, Umberto Giovanella, Riccardo Ruffo,
Silvia Luzzati, Francesco Meinardi, Luca Beverina,* and Sergio Brovelli*
1
. Introduction
Small conjugated molecules (SM) are gaining momentum as an alternative
to semiconducting polymers for the production of solution-processed bulk
heterojunction (BHJ) solar cells. The major issue with SM-BHJs is the low
carrier mobility due to the scarce control on the phase-segregation process
and consequent lack of preferential percolative pathways for electrons and
holes to the extraction electrodes. Here, a new paradigm for fine tuning the
phase-segregation in SM-BHJs, based on the post-deposition exploitation
of latent hydrogen bonding in binary mixtures of PCBM with suitably func-
tionalized electron donor molecules, is demonstrated. The strategy consist
in the chemical protection of the H-bond forming sites of the donor species
with a thermo-labile functionality whose controlled thermal cleavage leads to
the formation of stable, crystalline, phase-separated molecular aggregates.
This approach allows the fine tuning of the nanoscale film connectivity and
thereby to simultaneously optimize the generation of geminate carriers at
the donor–acceptor interfaces and the extraction of free charges via ordered
phase-separated domains. As a result, the PV efficiency undergoes an over
twenty-fold increase with respect to control devices. This strategy, demon-
strated here with mixtures of diketopyrrolopyrrole derivatives with PCBM can
be extended to other molecular systems for achieving highly efficient SM-BHJ
solar cells.
Over the last two decades, bulk-hetero-
junction solar cells based on binary mix-
tures of polymeric p-type electron donors
[
1]
and n-type fullerene acceptors
have
received enormous interest from both the
academic community and the industry
as promising candidates for replacing
silicon in photovoltaic (PV) devices fabri-
cated through cost-effective solution-based
methods on flexible or bendable sub-
strates. Advancements in terms of mor-
phological control and manipulation,
fundamental understanding of their
[
2]
[
3]
[4]
photophysical
ties as well as device architecture have
allowed to reach efficiencies up to 9%.
and transport proper-
[
5]
[6,7]
[
8,9]
However, solar cells based on polymeric
systems still suffer of intrinsic limitations
that hinder their scale up to mass produc-
tion. Specifically, chain length polydisper-
sity, structural defects and contamination
with polymerization precursors, byprod-
ucts and metal based catalysts result in
limited batch-to-batch reproducibility
and poor control over the device perfor-
mances.[10] For these reasons, increasing attention is recently
being diverted to so-called “small molecule” bulk-heterojunc-
tion (SM-BHJ) solar cells where the long-chain polymer donors
are replaced with conjugated molecular systems that can be
synthesized with well defined molecular structure tuned to
match the light harvesting capabilities and oxidation potential/
F. Bruni, Dr. M. Sassi, Prof. R. Ruffo,
Prof. F. Meinardi,
Prof. L. Beverina, Prof. S. Brovelli
Dipartimento di Scienza dei Materiali
Università degli Studi di Milano-Bicocca
via Cozzi 55, 20125, Milano, Italy
E-mail: luca.beverina@unimib.it;
sergio.brovelli@unimib.it
[
11]
electron affinity of the best performing polymeric materials.
In only a few years, SM-BHJ solar cells have held their promise
Prof. M. Campione
Dipartimento di Scienze dell’Ambiente e del Territorio e di
Scienze della Terra
Università degli Studi di Milano-Bicocca
Piazza della Scienza 1, 20126, Milano, Italy
[12]
and reached efficiencies up to over 10%. Recently, Mitsubishi
Chemical announced word record efficiency of 11.7% using a
[
13]
binary mixture of benzoporphyrins and fullerene derivatives.
With respect to traditional silicon solar cells where free
charges are generated directly upon photoexcitation, splitting
of excitons in organic semiconductors requires to overcome a
Dr. U. Giovanella, Dr. S. Luzzati
Istituto per lo Studio delle Macromolecole
Consiglio Nazionale delle Ricerche
[14]
larger potential barrier (exciton binding energy >50 meV)
(ISMac-CNR)
owing to the lower dielectric constant of the medium. In BHJ
solar cells, this is typically achieved by mixing materials with
different electron affinity and oxidation potential, so as to form
Via Bassini 15, 20133, Milano, Italy
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distributed interfaces where excitons are split in geminate
electron-hole pairs by ultrafast charge transfer processes.
functionalized donor molecules blended with PCBM through
thermally induced decomposition of labile protecting groups
that prevent the H-bonding from occurring in the as-deposited
films.
[
15]
Because of the typical homopolar conductivity of organic mate-
rials, the transport of electrons and holes to the respective
electrodes requires preferential percolation pathways of p- and
n-type character that are formed by separating the donor and
the acceptor moieties in phase segregated domains. In order
to optimize the photovoltaic efficiency it is therefore pivotal to
finely balance the degree of mixing: on the one hand, a fine
interpenetration assures that the size of segregated domains
does not exceed the exciton diffusion length, which would sup-
press exciton splitting into geminate e-h pairs at the donor/
acceptor interface; on the other hand, phase separation into
highly conductive crystalline domains promotes efficient long-
This concept was initially introduced for increasing the
[
27,28]
solution processability of molecular pigments
and, more
recently, it has been applied to increase the charge mobility
in ambipolar organic transistors through the formation of
[
29]
highly ordered molecular arrangements. The key aspect of
the approach, schematically depicted in Figure 1a, is the tem-
porary ‘deactivation’ of the H-bond-forming sites of the donor
molecule by its protection with a thermally labile function-
ality. This makes the donor miscible with PCBM in common
solvents, allowing for the use of conventional solution-based
deposition methods on transparent conductive substrates. The
formation of phase segregated domains is successively finely
tuned by thermal cleavage of the protective group and conse-
quent activation of the H-bonding sites. Through this approach,
we can control the blend morphology from the initial kinetic
structures formed during film deposition (i.e., evaporation of
the solvent during spin-coating) all the way to thermodynamic
macrophase-separated materials and thereby access interme-
diate morphologies that simultaneously optimize the charge
separation efficiency at the donor/acceptor interfaces and
charge mobility through the formation of interpenetrated phase
separated percolation pathways.
[
16,17]
range geminate charge separation and transport.
In addi-
tion, molecular orientation[ and interdiffusion of molecular
species between phase separated domains have been recently
shown to significantly affect the performances of BHJ solar
cells,[ thereby further highlighting the relevance of devel-
oping means to fine tune the self-assembly and segregation
processes in molecular blends.
18]
19]
In polymer-fullerene blends, phase segregation happens
spontaneously due to their unfavorable entropy of mixing,
yielding highly interpenetrated phase separated domains
20]
(
10–20 nm in size)[ whose extent can be further controlled
through post processing techniques, such as thermal or sol-
vent vapor annealing.[ In contrast, phase separation in SM-
BHJs lacks of entropic contribution and therefore requires an
additional driving force to occur. In order to address this issue,
several groups have adopted various approaches including the
exploitation of different solubility of the blend constituents in
21]
2
. Results and Discussion
In our work we used a functionalized diketopyrrolopyrrole
(DPP) derivative as electron donor blended with a conventional
PCBM acceptor. DPP derivatives have recently received consid-
erable attention as efficient donor materials thanks to their high
polarizability and good self-assembly properties arising from
their planar conjugated bi-cyclic structure that promotes π–π
the processing solvents,[ the insertion of additives and the
adoption of post-processing procedures such as solvent vapor
and/or thermal annealing.[ Phase segregation can be further
promoted by increasing the dipolar character of the donor spe-
cies or by exploiting π–π interactions between neighboring
conjugated moieties that result in intermolecular aggrega-
22]
[23]
24]
[
30]
interaction in the solid state. Furthermore, the electron with-
drawing DPP core can be functionalized with electron rich sub-
stituents, which confer to the molecule a donor–acceptor-donor
(D-A-D) character. The resulting intramolecular charge transfer
process provides an additional mean to tune the absorption
[
25]
tion and the formation of close packed crystalline domains.
Even when successful in inducing phase-separation, these
approaches typically introduce no cohesive forces capable of
stabilizing the processed blend in a desired optimized mor-
phology. The resulting phase separation is therefore inherently
unstable and subject to changes due to temperature or to the
exposure to environmental agents, such as moisture or sol-
vents. To date, fine control of phase separation represents the
most critical challenge for boosting the efficiency of SM-BHJ
solar cells. Specifically, in contrast to polymer BHJs where
the film morphology and segregation can be finely controlled
through simple annealing procedures owing to the low molec-
ular mobility of long polymeric chains, thermal annealing of
small-molecule BHJs leads to rapid and therefore less control-
lable segregation that may results in suppression of exciton
[
31]
spectrum so as to cover the whole visible spectral region.
Our molecule of choice is di-tert-butyl3,6-di(biphenyl-
4-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate
(hereafter referred to as DPP ), a D-A-D functionalized DPP
H
whose lactamic nitrogens are protected by tert-butoxycarbonyl
(t-BOC) groups (highlighted in red in Figure 1b). The decom-
position of the t-BOC functionalities into gaseous species (CO2
and isobutene) triggers the formation of H-bonds between
neighboring DPPH molecules leading to the desired phase
segregation. Morphological and structural analyses reveal the
gradual evolution of the amorphous and uniform pristine film
into a mosaic of segregated crystalline domains. The separa-
tion of donor and acceptor species is confirmed by spectro-
scopic measurements that show the progressive increase of
the DPPH luminescence in thermally treated (“deprotected”)
films, index of slightly reduced ultrafast exciton splitting that
instead completely quenches the emission of the pristine ‘pro-
tected’ blend. Despite the partially suppressed exciton splitting
[
26]
splitting and consequent reduction of photovoltaic activity
see Supporting Information).
Here we demonstrate a new paradigm for external fine con-
(
trol of phase segregation in SM-BHJs that raises the device
efficiency of over twenty times with respect to unprocessed
devices. Our strategy consists in the post-deposition activa-
tion of a network of latent hydrogen-bonds (H-bonds) between
2
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Figure 1. Concept of latent H-Bonding activated phase segregation in SM-BHJs. a) Schematic representation of the latent H-bonding strategy in
the fabrication of SM-BHJ solar cells. A solution of protected donor and acceptor molecules is deposited on transparent conductive substrates via a
simple solution process (i.e., spin coating, roll-to-roll/ink-jet printing). The post-deposition thermal treatment removes the thermolabile protective
functionalities thereby activating the latent H-bonding sites. As a result, the donor moieties network, leading to a fine controlled phase segregated
polycrystalline film. Deposition of the extraction electrodes is carried out by standard procedures. b) Activation of latent H-bonding in di-tert-butyl3,6-
di([1,1′-biphenyl]-4-yl)-1,4-dioxopyrrolo[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate. At 170 °C the ter-butoxycarbonyl (t-BOC) protection group (highlighted
in red) is converted in carbon dioxide (CO ) and isobutene (C H ).
2
4
10
in phase segregated films, the improved charge mobility
leads to a dramatic increase of the short circuit current, and
thereby boosts the power conversion efficiency up to twenty
times that of pristine devices. In order to unambiguously
ascribe the observed improvement of the PV performances to
the activation of latent H-bonding network, we synthesized a
control molecule, 3,6-di(biphenyl-4-yl)-2,5-bis(2-ethylhexyl)-2,5-
The thermogram provides an indication of the thermal
budget necessary to convert DPPH into its deprotected form.
In order to fine tune the thermal activation of hydrogen net-
working, we chose to provide the required thermal budget by
heating the samples at 170 °C, which is slightly below the peak
deprotection temperature of ≈200 °C. This allows us to slow
down the hydrogen networking process and thereby to control
its extent through the duration of the thermal treatment, here-
after expressed in terms of thermal treatment time (tTT).
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (referred to as DPP , see
C
Supporting Figure S1) with similar structure to DPPH except
for the protection of the lactamic nitrogens which, in this case,
is provided by 2-ethylhexyl alkyl chains that are stable at the
processing temperature and thus unaffected by the thermal
treatment. Thermal annealing of the control blends leads to
minor changes in the film structure that remains amorphous
and to the disruption of the photovoltaic activity, most likely
Thermal deprotection and consequent activation of
H-bonding sites is confirmed by Fourier transform infrared (FT-
IR) transmission measurements that allow us to monitor the
evolution of the functional groups in the film at different stages
of the thermal treatment (Figure 2b) through the intensity of
their characteristic vibrational modes. Upon annealing, the
characteristic signal of the C = O stretching mode at 1740 cm−
is reduced to below 50% of its initial intensity, indicating the
successful removal of the t-BOC protective groups (the residual
signal is due to the C = O bond of PCBM). Most importantly,
the characteristic signal of the N-H stretching mode between
1
due to uncontrollable complete segregation of DPP molecules
C
in nearly pure amorphous domains.
2
.1. Thermal Activation of Intermolecular Aggregation
−1
−1
3
000 cm and 3200 cm concomitantly grows, confirming the
We start our study by providing evidence of controlled thermal
decomposition of the t-BOC protective functionality and con-
sequent formation of intermolecular DPPH aggregates. In
Figure 2a we report the thermogravimetric analysis (TGA)
of DPPH powder and the respective mass release curves as
obtained through coupled mass spectroscopy measurements on
the exhaust gases. At 200 °C, a first 29% weight loss is observed
with concomitant emission of CO and C H , as expected for
activation of H-bonding sites.
As a result of thermal deprotection, DPPH moieties progres-
sively aggregate both in pure and blended films. Figures 2c
and 2d show the absorption spectra of a pure DPPH fi lm and
of a 1:1 DPP :PCBM blend as a function of the thermal treat-
H
ment time, tTT = 0–1000 s. The pristine DPPH fi lm (Figure 2c)
shows a first absorption band at about 2.7 eV that well resem-
bles the absorption profile of the diluted solution (Supporting
Information Figure S2) and is therefore ascribed to non-inter-
2
4
10
the thermal cleavage of the protective t-BOC group. DPPH is
[
26]
otherwise stable up to 500 °C where a further 40% mass loss
acting DPPH molecules. The thermal deprotection induces
a progressive drop of the 2.7 eV absorption feature with
and corresponding CO emission indicates the degradation of
2
the DPP core.
increasing t , accompanied by the growth of a lower energy
TT
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Figure 2. Thermogravimetric, UV–Vis absorption and FT-IR analyses. a) Thermogravimetric analysis (TGA) of DPPH powder performed under nitrogen
atmosphere (upper panel) and related mass release curves (lower panel) identifying the emission of CO (red line), C H (green line) and C H O
2
4
10
5
10
2
(
(
black line). The green curve has been shifted upwards by 10 times for clarity. b) IR transmission spectra of pristine (red line) and fully converted
tTT = 1000 s, blue line) DPPH:PCBM films (1:1 wt%) showing the drop of the C = O stretching mode upon thermal deprotection. Inset: IR transmis-
−1
sion spectra in the 2800–3300 cm region characteristic of N–H stretching mode. Absorption spectra of c) a pure DPPH film and of d) a DPPH:PCBM
(1:1 wt%) blend for increasing treatment time, tTT (indicated by the arrow). The photoluminescence spectra of the pure DPPH film excited at 400 nm
are shown in the inset (tTT = 0 s, red line; tTT = 30 s, purple line; tTT = 480 s, blue line;) together with the photographs of the samples before (yellow
films) and after 1000 s thermal treatment at 170 °C (brown films). e) Normalized absorption at 2.2 eV (circles) and at 2.7 eV (triangles) as a function
of t extracted from (c,d) for e) the pure DPPH film and f) the DPP :PCBM blend. The same color code applies throughout panels (b–f).
TT
H
4
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band at 2.2 eV ascribed to molecular aggregates, with a clear
isosbestic point at 2.5 eV. Accordingly, the DPPH photolumines-
cence (PL) is progressively quenched (inset of Figure 2c), pos-
sibly due to increased exciton mobility in the aggregated film
that allows excitons to rapidly reach non-radiative quenching
sites. Similar evolution of the absorption spectrum is observed
for the blended film where the absorption profile of DPPH is
overlapped to the absorption spectrum of PCBM. Also in this
case, the thermal treatment leads to progressive growth of the
absorption band at 2.2 eV, thus confirming that DPPH moie-
ties aggregate following deprotection also in the blended film.
To semi-quantitatively monitor the combined deprotection/
aggregation process, in Figure 2e,f we report the intensity of
the characteristic absorption bands as a function of tTT for both
films. In both cases, the evolution of the absorption features
at 2.2 eV and 2.7 eV is anti-correlated, indicating a progres-
sive conversion of isolated DPPH moieties into intermolecular
aggregates of deprotected species. We note that the growth of
the aggregate band is faster in the blend than in the pure DPPH
film, which suggests a role of PCBM in promoting the forma-
tion of DPP aggregates and/or the cleavage of the protecting
group. In contrast, UV–Vis absorption measurements on con-
broader with respect to shorter treatments, which indicates the
polycrystalline nature of the film. Further annealing has a det-
rimental effect on the surface morphology as the XRD signal
broadens and drops in intensity accompanied by a substantial
blurring of the morphologic features. Literature reports cases
of surface induced polymorphysm in DPP films, which might
play a role in the further evolution of the annealed crystalline
[
32]
domains. The whole process is in any case fully reproduc-
ible and thus controllable at any required treatment timescale.
Interestingly, the maximum crystallinity is observed for tTT
=
270 s (Figure 3c), corresponding to the most interpenetrated
morphology evidenced by the AFM images. The same evo-
lution of the film structure is observed for pure DPPH fi lms
while pure PCBM films are unaffected by the thermal treat-
ment (Supporting Information Figure S4,S5). This suggests
that the presence of PCBM does not sensibly interfere with
the thermal activation of the H-network, in agreement with the
optical absorption measurements reported in Figure 2.
Importantly the film morphology and crystallinity are pre-
served for shelf times up to 6 months. As shown in Figure 4a,
the XRD pattern of a representative DPP :PCBM film stored
H
in air under ambient illumination for over 6 months shows an
intense diffraction peak at 2θ = 7.6° and surface morphology as
a blend measured immediately after the thermal deprotection
(Figure 3). This further emphasises the ability of the thermally
activated H-network to stabilize the blend nanoarchitecture for
extended periods of time in lighting conditions at room tem-
perature. Furthermore, as a result of the strong cohesive forces
introduced by the H-bonds, the processed blend becomes
essentially insoluble in common solvents (Figure 4b), simi-
larly to what observed with latent pigments used in inks and
trol films of pure DPPC and DPP :PCBM 1:1 blend show no
C
aggregate band at any tTT (Supporting Information Figure S3),
thus confirming the key role of H-networking activated by
thermal removal of the t-BOC protection as a driving force for
intermolecular aggregation in the solid state.
2
.2. Morphological and Structural Analyses
[
27]
As mentioned above, the key advantage of the proposed
strategy is the ability to externally tune the film morphology
and phase segregation through the thermal activation of latent
H-bonds. To demonstrate the control over film morphology
and crystal structure we performed atomic force microscopy
varnishes.
Specifically, in Figure 4b, we report the optical
absorption spectra of pristine and deprotected blends before
and after washing in water, ethanol and acetone. Remarkably,
while the pristine films are dissolved by ethanol and acetone,
their deprotected analogues are unaffected by the washing
procedure.
(
AFM) and X-ray diffraction (XRD) measurements on pure
DPPH and blended films at different stages of thermal treat-
Finally, we performed AFM and XRD analyses on 1:1
ment. Figure 3a,b report the AFM images and XRD patterns
DPP :PCBM control blends (Supporting Information
C
of a 100 nm DPP :PCBM (1:1) film on a Si(100) substrate both
Figure S6,S7). The morphology of the pristine control film is
smooth and shows no mesoscopic phase segregation. In strong
H
as-spun and for tTT up to 510 s (the same analysis for a pure
DPPH fi lm is reported in Supporting Figure S4). The pristine
film shows uniform morphology with roughness (rms) below
contrast with the DPP -based films, the surface smoothness is
H
preserves up to tTT = 270 s, when isolated cap-shaped aggre-
gates emerge. XRD measurements show that the film remains
completely amorphous throughout the whole range of tTT inves-
tigated (tTT = 0 s to tTT = 510 s), thus indicating that eventual
phase-separation in the absence of H-network is not accompa-
nied by preferential structural ordering.
1
nm and no evident diffraction signal, indicating a substan-
tially amorphous state. After a 30 s thermal treatment, the film
surface becomes structured with the formation of elongated
lamellar domains with thickness of about 1 nm. The sur-
face roughness undergoes a three-fold increase with respect
to its initial value (see Supporting Information Table S1).
The XRD pattern shows a sharp diffraction peak at 2θ = 7.6°
(
with replicas up to the fifth order) which are indicative of
2.3. Hydrogen-networked SM-BHJ Solar Cells
the development of a crystalline phase with the highest out-
of-plane periodicity with spacing of 1.12 nm, consistent with
the morphological features observed in the respective AFM
image. Extended treatments up to tTT = 270 s induce a fur-
ther increase in crystallinity and the formation of 10–20 nm
three-dimensional crystalline domains. We note that, while
the volume fraction of the crystalline phase grows, resulting in
more intense XRD signal, the diffraction peak is substantially
To experimentally validate the concept of post deposition acti-
vation of latent H-bonding for improving the performances
of SM-BHJ solar cells, we fabricated and tested simple proof-
of-principle devices consisting of a 100 nm DPP :PCBM
H
(70:30 wt%) film at different stages of thermal treatment
sandwiched between an ITO/PEDOT:PSS anode and a LiF/Al
cathode (1.5 nm LiF, 90 nm Al). The electrical response of the
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Figure 3. H-networked SM-BHJ morphology and structure. Development of crystalline phase segregated domains through activation of latent H-bonds
in DPP :PCBM blends. a) AFM surface topography of DPP:PCBM (1:1 wt%) films on Si(100) substrates as measured at room temperature after thermal
H
treatments performed for the time reported on the top-right of each panel. The white scale bar corresponds to 400 nm. The corresponding surface
roughness is reported in Supporting Information Table S1. b) X-ray diffraction patterns for the same films as in ‘a’ showing the XRD peak corresponding
to the out-of-plane spacing d = 1.12 nm of the crystal phase of unprotected DPP . c) Evolution of the integrated intensity of the peak with thermal
H
treatment time. The same color code applies throughout the figure.
devices under AM 1.5G illumination is reported in Figure 5a,
showing negligible dependence of the open circuit voltage (VOC
that these proof-of-principle devices have not been optimized
for high absolute efficiency in terms of the active layer thick-
ness, electrodes work functions and donor/acceptor ratio.
Improved performances can potentially be achieved by tuning
the DPP energy levels and optical gap, so as to better cover
the solar spectrum and improve charge separation. In fact, as
shown by the energy levels diagram in Figure 5b, the highest
occupied molecular orbital (HOMO) level of DPPH is slightly
lower than the HOMO of PCBM (–6.22 eV vs –6.1 eV) which
results in poor charge separation at the hetero interface. Sev-
eral examples of BHJ solar cells based on DPP derivatives
have been reported achieving conversion efficiencies up to
)
on t , a further evidence that the conjugated structure of the
TT
DPPH core is preserved upon thermal deprotection. Remark-
ably, the short circuit current density (JSC) shows a 30-fold
increase upon annealing for tTT = 270 s, resulting in the con-
comitant growth of the power conversion efficiency (PCE) to
≈
0.15%, which is over 20 times larger than the efficiency of the
pristine device (Figure 5b). The electrical characteristics of the
best performing device of each set are reported in Supporting
Information Table S2.
The same study performed on control solar cells based on
[
30,33]
a DPP :PCBM (70:30 wt) blend shows that, while the pristine
4%.
We therefore expect significant improvements by
C
device has similar PCE to its DPP :PCBM counterpart, the
applying the proposed strategy to optimized molecular sys-
tems. The latent pigment strategy can also be extended to
other classes of high performance pigments such as quina-
H
device treated for tTT = 270 s exhibits no PV activity (Sup-
porting Information Figure S8 and Table S3). This unambigu-
ously demonstrates that the deprotection of the DPPH mole-
cules and the consequent H-networking play a key role in
improving the PV performances of our solar cells. We note
[
34]
cridone and isoindigo.
achieved through the implementation of more complex device
Further optimization can also be
[
6,8]
architectures.
6
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Figure 4. Stability. a) Diffraction patterns of DPP :PCBM (1:1 wt%) films on silica substrates at tTT = 270 s after a shelf time of 6 months (pristine –
H
black line, treated for tTT = 270 s – red line). The inset shows the AFM surface topography of the same sample. The scale bar corresponds to 400 nm.
b) UV–Vis absorption spectra of pristine (red) and treated (tTT = 270 s, blue) DPP :PCBM films before (continuous line) and after (dotted line)
H
washing in various solvents. The spectra of the films washed in water and ethanol have been shifted up for clarity. ‘Deprotected’ blends are essentially
insoluble in any of the solvents tested due to H-networking that introduces strong cohesive forces which dramatically improve the stability of the film
with respect to common untreated films.
To gather a deeper insight into the role of the molecular
aggregates in deprotected crystalline films on the production
of photocurrent, we measured the spectrally resolved incident
photon-to-current efficiency (IPCE) of the same solar cells as in
Figure 5a,b. In Figure 5c,d we report the optical absorption and
the IPCE spectrum of a pristine device and of a cell treated for
Figure 5. H-networked SM-BHJ solar cells a) Current density–voltage (JV) responses of SM-BHJ solar cells incorporating a 100 nm thick DPP :PCBM
H
2
(
70:30 wt%) film treated for tTT = 0, 30, 60,150, 270, 300 s under AM 1.5G illumination (100 mW/cm ). Inset, zoomed-in JV response of unprocessed
device. b) evolution of the short circuit current density (triangles) and power conversion efficiency (PCE, squares) with t . Symbols are the mean
TT
values averaged over 10 devices and error bars represent the respective standard deviation. Schematic diagram of the energy levels of DPPH and PCBM
are reported in the inset. Absorption spectrum (continuous line) and IPCE (dashed line) spectra for c) a SM-BHJ solar cell incorporating a pristine
DPP :PCBM (70:30 wt%) film (red lines) and d) the same blend treated for 270 s (blue lines). The same color code applies throughout the figure.
H
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DOI: 10.1002/adfm.201400896
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tTT = 270 s. As expected, the IPCE spectrum of the “protected”
device resembles well its absorption spectrum with no evident
contribution of intramolecular aggregates at 2.2 eV. On the
other hand, the IPCE spectrum of the ‘deprotected’ cell shows
a marked band at 2.2 eV that is significantly more intense than
the corresponding optical absorbance, indicating that intermo-
lecular aggregates more efficiently convert absorbed light into
photocurrent with respect to isolated molecules.
lead to the formation of amorphous phase-segregated domains
rich in DPPH and PCBM (see Figure 3). In this film mor-
phology, DPPH excitons are photogenerated sufficiently far
from PCBM to be able to recombine radiatively before reaching
the domain interfaces where they are split by ultrafast electron
transfer. As a result, the film becomes progressively more emis-
sive (zone “1” in Figure 6b) and the PL decay curve shows the
typical stretched exponential behavior that reflects the distri-
bution of distances between the photoexcitation spot and the
quenching site (i.e., the DPPH/PCBM interface) characteristic
[
17,35]
2
.4. Photophysical Investigation of Thermally Induced
of the diffusion limited recombination regime (Figure 6c).
H-Networking
The increase of crystallinity observed in XRD analyses for
extended treatments enhances the exciton mobility in the
Finally, we use time-resolved photoluminescence experi-
ments to probe the phase separation on a molecular scale and
thereby to unravel the photophysical processes underpinning
the improved efficiency in H-networked SM-BHJ solar cells.
Figure 6a–c shows the time-integrated PL spectra and the PL
decay curves of a DPP :PCBM blend at increasing t . The
corresponding contour plot of PL decay traces versus tTT is
reported in Figure 6d. A schematic representation of the photo-
physical processes occurring in the film at different stages
of thermal treatment is depicted in Figure 6e. In the as-spun
DPP -rich domains leading to faster exciton diffusion and con-
H
sequent quenching of the PL by electron transfer to PCBM or
III
by nonradiative decay in defect sites (zone “2” in Figure 6b,e ).
Fast migration to the donor–acceptor interfaces and consequent
shorter PL lifetime in films subject to treatments between 200 s
and 300 s is confirmed by both the more pronounced fast PL
decay component in Figure 6c and by the accumulation of iso-
lines in the contour plot in Figure 6d. Finally, longer thermal
treatments induce the progressive amorphization of the film
H
TT
IV
which suppresses exciton mobility (Figure 6e ) and leads to
DPP :PCBM blend, donor and acceptor molecules are finely
the slow recover of the emission yield (zone “3” in Figure 6b).
As a result of hindered exciton diffusion, the decay kinetics
becomes single exponential with lifetime ≈800 ps comparable
to that of pure DPPH fi lm (Supporting Information Figure S9),
H
mixed and the recombination of DPPH excitons is dominated
I
by ultrafast electron transfer to PCBM (Figure 6e ). Accordingly,
the film is completely non emitting. Brief thermal treatments
Figure 6. Photophysical study of H-networked SM-BHJs a) PL spectra, b) integrated PL intensity And c) PL decays of a DPP :PCBM (1:1 wt%) film for
H
different treatment time. The same color code applies throughout panels (a–c). d) Contour plot of the PL decays as a function of tTT (logarithmic scale).
The dashed lines highlights the region corresponding to the highest exciton mobility. e) Schematic representation of the photophysical processes occur-
I
ring in the film at the different stages of the thermal deprotection: e ) In the as spun film, DPPH donors (D, blue squares) and PCBM acceptors (A, white
II
squares) are finely mixed. Photogenerated excitons are rapidly split by ultrafast charge transfer (red arrow) and no PL is observed; e ) The progressive
growth of DPP -rich domains due to the ongoing phase segregation process makes radiative exciton relaxation the dominant recombination process,
H
III
leading to progressively more intense PL (yellow arrow, corresponding to zone “1” in panel (b)); e ) Crystallization of the phase-separated domains
(highlighted in striped blue/green) favors the mobility of excitons in the film and, consequently, their diffusion (white arrows) to the donor–acceptor
interfaces. As a result, rapid electron transfer competes with radiative exciton recombination leading to the gradual drop of the PL intensity observed in
IV
zone “2” of panel (b); e ) Amorphization of the film for prolonged thermal annealing suppresses exciton diffusion leading to the progressive recover
of the PL (corresponding to zone “3” in panel “b”).
8
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indicating that the dominant decay process is now the radiative
pure product as a yellow solid (1.186 g, 1.948 mmol, yield 85.8%).
Characterizations are in agreement with those previously reported.
Electrochemical Analysis: 20 mg of t-BOC protected DPPH were
[
37]
recombination of excitons in the large DPP -rich domains.
H
Importantly, the PL drop due to increased exciton mobility
observed for tTT ≈ 270 s correlates remarkably well with the
maximum crystallinity and the higher power conversion effi-
ciency shown in Figure 3, 5. It is also worth noting that higher
PCE are obtained despite the fact that the phase segregation
process induced by thermal activated H-networking reduces the
exciton splitting efficiency by increasing the size of the DPPH-
and PCBM-rich domains, as shown by the higher zero delay PL
intensity in deprotected films for tTT up to 75 s, with respect to
their pristine form. This, together with the higher current den-
sity flowing through the device, further demonstrates that the
improvement of PV performances is mainly due to the larger
charge mobility in the H-networked blends, which overcom-
pensates the slightly lower number of free carrier generated in
the phase-separated film.
dispersed in 1 mL of CH Cl . The suspension was filtered to obtain a
2
2
saturated solution and casted on the top of a well polished glassy carbon
2
tip working electrode (surface area 0.071 cm ). A thin molecular film of
DDPH was then obtained by solvent evaporation at room temperature
and thermal treatment (20 min at 170 °C). The glassy carbon
supported film was characterized by Differential Pulsed Voltammetry
−1
in tetrabutylammonium perchlorate 10 M in anhydrous acetonitrile at
0 mV/s. The counter, and pseudoreference electrodes were a Pt flag and
2
a Ag/AgCl wire, respectively. The Ag/AgCl pseudoreference electrode was
−3
externally calibrated by adding ferrocene (10 M) to the electrolyte (E1/2
0
=
.39 V, ΔE = 62 mV). Measurements were carried out in a glove box filled
with argon ([O2] < 1 ppm). LUMO energies was calculated by the DPV
peak potential value considering the Fc vacuum level at –5.23 eV.
Structural and Morphological Analyses: X-ray diffraction specular scans
on single crystals and films were performed with a PANalytical X’Pert Pro
powder diffractometer with Cu Kα radiation. AFM images were obtained
with a Nanoscope V MultiMode atomic force microscope (Veeco) using
single-beam silicon cantilevers (force constant of 40 N/m) operating
in intermittent contact (tapping) mode. Image analysis was performed
with the program WSXM.
3
. Conclusions
Solar Cells Fabrication and Characterization: For the preparation of
solar cells, indium tin oxide (ITO)-coated glass substrates were sonicated
for 10 min in isopropanole and treated in a nitrogen plasma chamber
for 10 min. A layer of poly(3,4-ethylene dioxythiophene/poly (styrene –
sulfonate) (PEDOT:PSS, CleviosP VP.AI 4083) was spin coated from
aqueous solution. The film was annealed under nitrogen atmosphere at
150 °C for 15 min. Profilometric measurements of the PEDOT:PSS layer
reported a thickness of 40 nm. A 20 mg/mL chloroform solution of DPP
and [60]PCBM (Solenne B.V., The Netherlands) 70:30 wt. was stirred
overnight and then filtered through a 0.22 µm poly (tetrafluoroethylene)
PTFE filters. The solution was spin coated on the ITO PEDOT:PSS
substrates at 1500 rpm for 60 s which yielded a film ≈100 nm thick. A
lithium fluoride layer (1.5 nm) coated with aluminium (100 nm) was
thermally evaporated using a shadow mask. Current versus voltage curves
(I–V) characteristics were measured with a Keithley 2602 Digital Source
Meter. The cells were illuminated from the glass side with a 100 mW/cm2
ABET Technologies Sun 2000 Solar Simulator. No mask was used in the
I–V measurements. An average of ten devices per treatment time were
characterized. Spectrally resolved incident photon-to-current efficiency
In conclusion we demonstrated a novel approach to fine con-
trol the phase segregation of solution processed SM-BHJ solar
cells based on the exploitation of latent H-bonding via thermal
deprotection of electron donor DPP derivatives blended with
PCBM. As shown by AFM and XRD analyses, the thermally
induced H-networking of the DPP molecules provides the nec-
essary driving force for the formation of interpenetrated crys-
talline domains rich in donor and acceptor moieties, resulting
in over 20-fold increase of the PV efficiency. The improvement
of the PV performances in the SM-BHJ solar cells is due to a
larger short-circuit current resulting from the higher carrier
mobility in the crystalline aggregates of deprotected DPP mol-
ecules, in agreement with the higher exciton mobility high-
lighted by time-resolved photoluminescence experiments. The
demonstrated approach to controlled post-deposition phase
segregation in SM-BHJ solar cells, demonstrated here for DPP
derivatives blended with PCBM, could be extended to smaller
band gap molecular semiconductors for achieving a better
match with the spectrum of solar radiation and thereby strongly
boost the PV efficiency of small-molecule solar cells.
(IPCE) was recorded by illuminating the solar cells with calibrated
monochromatic light from a Xe lamp coupled to a monochromator. All
fabrications and characterizations were performed under nitrogen.
Spectroscopic Studies: Spectroscopic measurements were performed
on diluted chloroform solution and spin coated films on silica substrates.
IR spectroscopy was performed with a µ-FTIR Nicolet iN 10. Absorption
spectra were collected with a Varian Cary 50 spectrophotometer. Time-
resolved PL measurements were carried out using 410 nm (3.02 eV)
excitation from a doubled Ti:Sapphire laser (Coherent Mira 900),
and detecting the emitted light with an Hamamatsu C4742–95 streak
camera.
4
. Experimental Section
Materials: Both DPPH and DPPC were prepared starting from the
corresponding
,4(2H,5H)-dione (1). Derivative DPPC was obtained according to the
common procedure of alkylation of DPP derivatives by reacting 1 with
pigment
3,6-di(biphenyl-4-yl)pyrrolo[3,4-c]pyrrole-
1
[
36]
2
-ethylhexyliodide.
DPPH was prepared by a modification of the
Acknowledgements
literature procedure.
Synthesis of di-tert-butyl3,6-di(biphenyl-4-yl)-1,4-dioxopyrrolo[3,4-c]
pyrrole-2,5(1H,4H)-dicarboxylate (DPP ) : In an Ar filled glove box, a
S.B., F.M., and U.G. acknowledge support from Cariplo Foundation
(2012–0844). L.B and M.S. acknowledge support from Cariplo
H
Foundation (2010–1832). S.B. wishes to thank the European
Community’s Seventh Framework Programme (FP7/2007–2013) under
grant agreement N. 324603 for financial support (EDONHIST). The
authors thank Dr. A. Colombo for technical assistance in TGA and mass
spectrometry experiments.
suspension of 1 (1.000 g, 2.270 mmol) and DMAP (693 mg, 5.672 mmol)
in anhydrous THF (20 mL) is prepared and stirred at room temperature
for 15 min. A solution of di-tert-butyl dicarbonate (2.480 g, 11.36 mmol)
in THF (8 mL) is added and the mixture is stirred at room temperature
for 46h. A yellow suspension is obtained. Light petroleum ether (30 mL)
is added and the mixture is stirred for 30 min and filtered obtaining a
yellow solid. Product is dissolved in CH Cl and filtered through a short
Received: March 19, 2014
Revised: June 13, 2014
2
2
silica plug. Solvent is evaporated under reduced pressure obtaining
Published online:
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DOI: 10.1002/adfm.201400896
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