A direct comparison of monomeric vs. dimeric and non-annulated vs. N-annulated perylene diimide electron acceptors for organic photovoltaics

Article abstract of DOI:10.1039/C8NJ06491A

Herein we report on four perylene diimide (PDI) variants to study the effects that N-annulation and dimerization have on optoelectronic properties and organic solar cell device performance. The PDI compounds were synthesized following a scalable and efficient procedure and studied by cyclic voltammetry, solution and solid-state optical absorption spectroscopy, emission spectroscopy, and density functional theory analysis. The thin-film morphology was probed by optical and atomic force microscopy. Dimerization stabilized the frontier molecular orbital energy levels and reduced aggregation in the thin-films. N-Annulation resulted in a destabilization of the frontier molecular orbital energy levels in both the monomers and dimers and enabled the formation of smoother films with smaller domain sizes. Each PDI molecule was evaluated as a non-fullerene acceptor in bulk-heterojunction solar cells using the donor polymers PTB7-Th and PffBT4T-2OD. The dimers outperformed the monomers, while N-annulated PDIs led to devices with higher open-circuit voltages.

Full text of DOI:10.1039/C8NJ06491A

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A direct comparison of monomeric vs. dimeric
and non-annulated vs. N-annulated perylene
diimide electron acceptors for organic
photovoltaics†
Cite this:DOI: 10.1039/c8nj06491a
Maryam Nazari,
Gregory C. Welch
Edward Cieplechowicz,
*
Thomas A. Welsh
and
Herein we report on four perylene diimide (PDI) variants to study the effects that N-annulation and
dimerization have on optoelectronic properties and organic solar cell device performance. The PDI
compounds were synthesized following a scalable and efficient procedure and studied by cyclic
voltammetry, solution and solid-state optical absorption spectroscopy, emission spectroscopy, and
density functional theory analysis. The thin-film morphology was probed by optical and atomic force
microscopy. Dimerization stabilized the frontier molecular orbital energy levels and reduced aggregation
in the thin-films. N-Annulation resulted in a destabilization of the frontier molecular orbital energy levels
in both the monomers and dimers and enabled the formation of smoother films with smaller domain
sizes. Each PDI molecule was evaluated as a non-fullerene acceptor in bulk-heterojunction solar cells
using the donor polymers PTB7-Th and PffBT4T-2OD. The dimers outperformed the monomers, while
N-annulated PDIs led to devices with higher open-circuit voltages.
Received 24th December 2018,
Accepted 2nd March 2019
DOI: 10.1039/c8nj06491a
rsc.li/njc
non-fullerene acceptors (NFAs) have been extensively studied in
Introduction
8
,14,15,22–27
recent years.
Indacenodithiophene-based compounds
Functional organic p-conjugated molecules and polymers have (IDTs) have proved quite effective owing to strong low-energy
1
–4
been widely used as active materials in photovoltaic devices.
Ease of synthetic modification to the p-conjugated backbone leading to favorable phase separated BHJ morphologies.
and periphery functional groups allows for facile tuning of physical, Indeed IDT-based OSCs exhibit remarkably high fill factors
optical absorption and good miscibility with donor polymers
2
8–30
3
0–32
optical, and electronic properties, which enables tailored device (FF).
However, high performing IDT-based NFAs typically
5–8
properties that can lead to improved performance. Molecules and require multiple synthetic steps, which makes large scale production
9
,10
11–13
33
polymers based on thiophenes
and/or dye materials
more difficult. An alternative to IDT based NFAs are those based on
have been most commonly used in solution processed bulk- perylene diimides (PDIs), which are easily synthesized and modified
heterojunction (BHJ) organic solar cells (OSCs) owing to their via simple chemical reactions from cheap and abundant dyes and
13,34–37
optoelectronic properties being well suited to visible light pigments.
They exhibit strong light absorption in the blue-
absorption and charge transfer. Over the past few years, the green region of the visible spectrum, have deep electronic energy
power conversion efficiency (PCE) of OSCs has dramatically levels, and are good electron transport materials making them
1
4
improved with current records of 414% for singe-junction
and 415% for tandem devices.
compatible with nearly all low-band gap hole transporting polymers.
Indeed, polymer-based OSCs that use dimeric or tetrameric PDI
1
5,16
38
39
Many OSCs use fullerene acceptors owing to excellent electron NFAs have achieved 49% and 410% PCE, respectively.
transport properties and tendency to form organized nano-
The PDI chromophore is quite versatile as it can be easily
1
7–19
aggregates in BHJ films.
However, fullerenes suffer from functionalized at the bay, headland, and/or imide positions to
several disadvantages including poor visible light absorption, tune molecular and bulk properties (Fig. 1). A common method
lack of energy level tunability, photochemical instability, and of functionalization of PDI is to dimerize at the bay position. One
2
0,21
40
difficulty in synthesis and purification.
For these reasons, such PDI dimer has achieved PCE of up to 6% in OSC devices.
Another common functionalization is heteroatom annulation at
the bay position, which has been an effective strategy to control
the solubility, self-assembly, and optoelectronic properties of
PDI materials. For OSC applications, PDI dimers with S and Se
Department of Chemistry, University of Calgary, 2500 University Drive N.W.,
Calgary, Alberta, T2N 1N4, Canada. E-mail: gregory.welch@ucalgary.ca;
Tel: +1-403-210-7603
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj06491a
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Fig. 1 Functionalization points for the PDI chromophore and chemical structures of the PDI materials directly compared in this study.
annulation have been reported with PCEs above 7% and 8%, have been reported previously while compound 3 is new. Full
4
1,42
respectively.
PDI compounds,
In our group, we have focused on N-annulated synthetic details can be found in the ESI.†
4
3–46
which, in addition to tuning the opto-
electronic properties like S and Se annulation, provides an
additional handle for functionalization to tune solubility and
Optical properties
4
5,47–51
self-assembly.
OSC devices with PCEs above 7% have The solution and film optical absorption properties of compounds
5
2
been achieved using these N-annulated PDI NFAs.
1, 2, 3, and 4 were determined using UV-vis spectroscopy (Fig. 2).
While investigating the N-annulated PDI dimers over the The solution spectra of the monomers (1 and 2) exhibit character-
past few years, we noticed that there has never been a direct istic transitions for PDI based materials. They exhibit absorption
comparison between non-annulated PDI and N-annulated PDI from 400–550 nm with three distinct bands for the 0-0, 0-1 and 0-2
53
monomers and dimers for OSCs. It is important to fully study vibronic transitions. A transition from solution to film reveals
the impact of such structural changes to gain a greater under- differences between the two compounds. For 1 the onset of
standing of how these molecules function in OSC devices. This absorption is red-shifted by D72 nm to 627 nm, while 2 is only
study serves to fill that gap and provides a structure–property- red-shifted by D48 nm to 606 nm. The difference implies weaker
functional relationship study. The structures of PDI acceptors intermolecular coupling in films of 2 vs. 1, attributed to the
used in this study are shown in Fig. 1. Compounds 1, 2, and 4 presence of the N-alkyl functional group.
À1
Fig. 2 UV-visible spectra of PDI materials (A) 1, (B) 2, (C) 3, and (D) 4 in C
6
H
5
Cl solution (0.01 mg mL ) and as ‘as-cast’ films spin-coated from C
6
H
5
Cl
À1
solutions (10 mg mL ) onto glass substrates under identical conditions. Left y-axis = solution. Right y-axis = film.
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Table 1 Optical and electrochemical data for compounds 1, 2, 3, and 4 in spectra of 3 and 4 are only slightly red-shifted from the solution
solution and as thin-films
spectra, much less than their monomeric analogues, consistent
5
2
52
52
with less intermolecular coupling in these systems owing to the
Compound
1
2
3
4
5
6,57
non-planar geometries.
in Table 1.
All optical absorption data are listed
Solution Abs max (nm)
Solution Abs onset (nm)
Molar absorptivity (M cm
Solution Em max (nm)
Solution quantum yield (nm)
Film Abs max (nm)
528
555
531
558
535
605
537
577
À1
À1
)
84 000 100 000 69 000 147 000
578
91
507
627
615
n/a
n/a
n/a
À1.15
À1.06
3.65
The photoluminescence (PL) spectra of all compounds were
measured in solution and film (Table 1 and Fig. S7, ESI†).
Solutions of compounds 1, 2, 3, and 4 show emission maxima
at 578, 580, 612, and 588 nm, respectively, and have shapes
typical of PDI materials. Both the monomers 1 and 2 have two
emission bands while the dimers have only one. Quantum
yields were measured to be 91, 64, 10, and 60% for compounds
580
64
497
606
612
10
535
621
n/a
n/a
n/a
588
60
537
617
Film Abs onset (nm)
Film Em max (nm)
n/a
633
E1/2 oxidation (V)
1.09
1.01
5.89
À1.30
À1.21
3.50
1.14
1.03
5.94
À1.29
À1.16
3.51
Oxidation onset (V)
Ionization potential (eV)
n/a
E1/2 1st reduction (V)
À1.04
À1.02
3.76
1
, 2, 3, and 4 solutions in C H Cl, respectively. Compound 3
6 5
Reduction onset (V)
Electron affinity (eV)
has the largest Stokes shift and lowest quantum yield indicating
significant non-radiative energy loss, an unusual phenomenon.
The solution spectra of the dimers (3 and 4) are different
from their monomeric analogues. While compound 3 exhibits
similar absorption from 400–500 nm with three distinct transitions,
Electrochemical properties
the absorption band is broadened and the absorption peaks less The electrochemical properties of the PDI materials were
sharp. The onset of absorption is also red-shifted compared to 1. determined using solution cyclic voltammetry (Fig. 3 and Fig. S8,
The features of compound 3 are nearly identical to a similar PDI ESI†). All compounds exhibit fully reversible reduction waves. The
54,55
dimer with longer alkyl chains off the imide N-atoms.
The monomers 1 and 2 exhibit two reduction waves while the dimers 3
solution spectra of 4 is the most different with only two distinct and 4 exhibit four reduction waves, which is typical for PDI
vibronic transitions, with the 0-0 (ca. 537 nm) transition the most monomers and dimers, as the PDI chromophore is stable as
4
1,52,54
intense. Such differences can be attributed to the more isotropic a radical dianion.
The N-annulated compounds 2 and 4
shape of the dimeric compounds leading to different intra and exhibited two reversible oxidation waves while compounds 1 and
52,54
intermolecular electronic interactions. Of note the solution spectra 3 did not reversibly oxidize, consistent with the literature.
of each compound do not change upon heating implying no Oxidation occurs at the electron rich pyrrole ring at the bay
significant aggregation in solution (Fig. S6, ESI†). The thin-film position in 2 and 4. The E1/2 oxidation and reduction potentials
Fig. 3 Cyclic voltammograms (with ferrocene internal standard) of PDI materials (A) 1, (B) 2, (C) 3, and (D) 4 with E1/2 potentials shown.
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for each compound were used to experimentally determine the fibril like features of several hundred nanometers in size, while
ionization potential (IP) and electron affinity (EA) energies, the film of 4 is nearly feature less. These results confirm the
respectively, which correspond to theoretical HOMO and LUMO long-standing notion that dimerization of PDI chromophores
energy levels (Table 1). A conversion factor of 4.8 eV (accepted reduces aggregation and that the introduction of an N-hexyl
58
HOMO energy level of ferrocene) was used.
chain further reduces the tendency of the PDI chromophores to
N-Annulation has a large effect on the reduction potentials form aggregated solid-state morphologies with large features.
for PDI. Compounds 2 (ca. À1.30 V) and 4 (ca. À1.29 V) have For OSCs, smaller, less aggregated domains usually lead to best
significantly more negative E1/2 reduction potentials than in performance. Thus, the N-annulated dimer is expected to yield
their counterparts 1 (ca. À1.15 V) and 3 (ca. À1.04 V). As a the best photovoltaic devices.
result, the EA of 2 and 4 is decreased compared to 1 and 3. This
is a powerful example of how N-annulation of PDI can influence
and tune the electrochemical properties by increasing the
Organic solar cells
electron density within the polycyclic aromatic core. This has
a consequence for OSC devices in terms of increased open-
efficiency of the device, OSC devices were fabricated under
circuit voltages. The differences in electronic energy levels were
confirmed using DFT and TD-DFT calculations (Fig. S10 and
Table S1, ESI†).
Due to the critical role of donor/acceptor pairings in the
identical conditions with compounds 1, 2, 3, and 4 using two
different polymer donors (Fig. 5). PTB7-Th and PffBT4T-2OD,
also referred to as PCE10 and PCE11, respectively, were selected as
they have complementary optical absorption (ca. 400–800 nm),
appropriately offset electronic energy levels, and are proven to yield
Thin-film morphology
4
5,60,61
good PCE devices with PDI based NFAs.
Due to ease of
6
2
The thin-film morphology of compounds 1, 2, 3, and 4 were fabrication and greater environmental stability, OSCs were
investigated using polarized optical microscopy (POM) and fabricated using an air-processed and air-tested inverted geo-
6
3
atomic force microscopy (AFM) (Fig. 4). All films were spin- metry: ITO/ZnO/BHJ/MoO
x
/Ag.
À1
coated from 10 mg mL
C
6
H
5
Cl solutions onto cleaned glass
BHJ films using the donor polymers PTB7-Th and PffBT4T-
À1
substrates under identical conditions. All compounds formed 2OD were fabricated from C
6
H
5
Cl solutions (10 mg mL ) using a
uniform films but with very different roughness and domain 1 : 1 polymer : PDI weight ratio. The active layers were spin-cast in
sizes. The non-annulated PDI monomer 1 formed highly aggre- the air, with room temperature solutions and substrates, without
gated films with large domains upwards of 1000 nm in width additives or any post-deposition treatments. Controlled processing
(Fig. 4A). The N-annulated PDI monomer 2 also formed aggre- conditions were used to ensure a direct comparison of the PDI
gated films but with smaller more fibril like domains ranging materials. We fully recognize that solar cell performance can be
from 300–500 nm in width. These aggregated films are consis- improved though processing optimization but that was beyond the
tent with literature findings for monomeric PDIs which are well scope of this study. Tabulated data for all devices is given in Table 2.
known to self-assemble into organized nanostructures in the Current–voltage curves and graphical metric comparison for PTB7-
5
9
solid-state via strong p–p interactions. A major difference is Th:PDI and PffBT4T-2OD:PDI based OSCs are shown in Fig. 6.
seen with the PDI dimers. Both compounds 3 and 4 form very The optical absorption spectra of PTB7-Th:PDI films (Fig. S11A,
smooth films with small domain sizes. The film of 3 shows ESI†) exhibit absorption from 400–800 nm whereas those of
Fig. 4 Polarized optical microscopy (POM) and atomic force microscopy (AFM) images of PDI materials (A) 1, (B) 2, (C) 3, (D) 4 as ‘as-cast’ films spin-
coated from C H Cl solutions.
6 5
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Fig. 5 (A) Chemical structures of donor polymers PTB7-Th and PffBT4T-2OD used to make BHJ organic solar cells. R = 2-ethylhexyl. (B) Optical
À1
absorption spectra of PTB7-Th and PffBT4T-2OD films spin-cast from 5 mg mL under identical conditions. The lower absorption intensity of PTB7-Th
reflects its lower solution viscosity. (C) Energy level plot of the polymer donors and PDI acceptors. Values estimated from CV data obtained in our labs
(
Fig. S8, ESI†). HOMO energy levels of 1 and 3 could not be determined by CV.
a
b
c
Table 2 Photovoltaic parameters for PTB7-Th:1,2,3,4 and PffBT4T-2OD:1,2,3,4 based organic solar cells
À2
Parameters
VOC (V) avg. (best)
J
sc (mA cm ) avg. (best)
FF (%) avg. (best)
PCE (%) avg. (best)
PTB7-Th:1
PTB7-Th:2
PTB7-Th:3
PTB7-Th:4
PffBT4T-2OD:1
PffBT4T-2OD:2
PffBT4T-2OD:3
PffBT4T-2OD:4
0.81 (0.81)
0.98 (1.00)
0.73 (0.75)
0.97 (0.99)
0.78 (0.80)
0.95 (0.98)
0.78 (0.79)
1.02 (1.03)
10.3 (11.5)
7.0 (7.4)
10.3 (10.4)
12.1 (12.4)
3.8 (4.5)
3.7 (4.0)
9.6 (9.8)
40.8 (43.2)
33.3 (35.1)
51.6 (52.3)
46.9 (46.9)
41.6 (44.3)
41.1 (43.0)
47.7 (47.4)
40.9 (42.1)
3.40 (4.00)
2.29 (2.60)
3.86 (4.07)
5.51 (5.72)
1.25 (1.59)
1.45 (1.69)
3.59 (3.66)
3.06 (3.59)
7.3 (8.3)
a
À1
b
Active layers were spin cast at 1500 rpm from 10 mg mL solutions in CB (1 : 1 blend ratio). Active layers were spin cast at 1200 rpm from
À1
c
1
x
0 mg mL solutions in CB. (1 : 1 blend ratio). Device architecture: ITO/ZnO/BHJ/MoO /Ag.
PffBT4T-2OD:PDI films (Fig. S12A, ESI†) exhibit absorption monomer 2 gave slightly higher efficiency (ca. 1.45%) due to the
from 350–750 nm, both suitable for solar photon harvesting. For higher VOC (0.95 V vs. 0.78 V). Use of the dimeric PDI acceptors
all films (expect PffBT4T-2OD:1) the emission of the polymer donor led to higher PCEs of 3.59% and 3.06%, respectively, for
was efficiently quenched by the PDI acceptor indicating electron PffBT4T-2OD:3 and PffBT4T-2OD:4 based OSCs. Although devices
transfer occurs in the BHJ films. PL spectra of blended films are based on N-annulated PDI dimer 4 gave slightly lower efficiency
À1
given in the ESI† (Fig. S11B and S12B). The lack of PL quenching in due to the lower short-circuit current and FF ( JSC = 7.34 mA cm
,
the film of PffBT4T-2OD:1 is explained by the large phase separated FF = 40.31%), using compound 4 lead to significantly higher VOC
surface seen in the AFM image (Fig. 13A, ESI†). Lack of donor– (0.78 V for PffBT4T-2OD:3 films vs. 1.02 V for PffBT4T-2OD:4
acceptor interface interaction inhibits electron transfer.
films).
In general, use of the N-annulated PDI materials (2 and 4)
OSCs with PTB7-Th:1 active layer had average PCE of 3.4%.
This performance is on par with related solar cells using lead to solar cells with significantly higher VOC, over 200 mV on
64,65
monomeric PDIs.
OSCs using the N-annulated PDI 2 had a average, compared to the non-annulated PDI materials (1 and 3).
reduced average PCE of 2.3%. Despite the lower PCE, PTB7-Th:2 This result highlights the positive impact of N-annulation on the
OSCs had a higher VOC because of the raised LUMO level of electronic properties of the PDI that results in improved solar
compound 2 leading to a larger energy gap between the polymer cell device characteristics. Furthermore, use of the dimeric
HOMO energy level and acceptor LUMO energy level. PTB7-Th:3 compounds (3 and 4) resulted in better performing OSCs than
based OSCs had an average PCE of 3.9% while PTB7-Th:4 based those based on the monomeric analogues (1 and 2), emphasizing
OSCs had average PCE of 5.5%. Consistent with the comparison the long-standing notion that best PDI NFAs should be comprised
of 1 vs. 2, the OSCs based on 4 had higher VOC than those based of two or more PDI chromophores.
on 3. Again, a result of N-annulation destabilizing the PDIs
To help rationalize these results, the surface morphology
LUMO energy level. Although devices based on 4 had slightly was analysed using AFM. Phase and height images are shown in
lower fill factors compared to 3 based devices, higher PCEs were Fig. S12 and S13 (ESI†). BHJ blends using compound 1 had a
obtained owing to a higher VOC and JSC. The best PCE of 5.5% for high surface roughness (RMS = 7 nm for PTB7-Th blends and
4
6–48,52,66,67
PTB7-Th:4 OSCs is consistent with literature results.
RMS = 17 nm for PffBT4T-2OD blends) owing to the stronger
OSCs with PffBT4T-2OD:1 active layer gave an average PCE of aggregation tendencies of the planar PDI. Domain sizes were on
.25%. The OSCs with BHJ active layers using the N-annulated the order of microns, not suitable for efficient charge separation,
1
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Fig. 6 Current voltage-curves and metrical graphs for PTB7-Th:PDI (A and B) and PffBT4T-2OD:PDI (C and D) based organic solar cells.
thus the overall lower PCE. Blends using compound 2 had much were tested as non-fullerene acceptors with two different donor
smoother films (RMS o 3 nm) but appeared to be very intermixed polymers in inverted single-junction bulk heterojunction devices.
based on the uniform and featureless phase images. The lack of In all cases OSCs using the N-annulated PDI materials exhibited
order and phase separation may explain the poor FF leading to the higher open circuit voltages by B200 mV when compared to
low PCE. Thus, while the N-annulation is favourable for improving those OSC using the non-annulated PDI materials. This is a clear
the energetics of the blends (i.e. increased VOC), the additional result of the lower electron affinity of the PDI core afforded by the
N-hexyl group seems to impede phase-separation and molecular addition of the electron rich N-atom in the bay positions. The
organization lowering (i.e. lower JSC and FF), in the monomeric dimeric PDI materials gave OSCs with slightly better performance
systems. All blends using the dimeric PDI materials exhibited than those fabricated with monomeric PDIs (ca. B3–5% PCE vs.
smooth uniform films but with more defined features than that of B1–3% PCE). This was attributed to the fact that the dimeric PDI
compound 2. Blends films using 3 or 4 are very similar, and this materials lead to BHJ blends with smooth, well organized films.
reflects the similar JSC and FF seen on all OSCs using these OSCs based on the donor polymer PTB7-Th and the N-annulated
acceptors. Once again, the biggest difference in device performance PDI acceptor 4 gave best PCE of 5.5% and highlights the
is a result of the VOC and the relative electronic energy levels of the potential of this class of materials for the development of high
materials.
performance, fullerene-free OSCs.
Conclusion
Conflicts of interest
In this contribution we have compared four different PDI
molecular materials relevant to the field of organic photovoltaics.
The impact of N-annulation at the bay position of the PDI
chromophore and dimerization of the PDI chromophore were
evaluated. We have shown that N-annulation serves to both
There are no conflicts to declare.
Acknowledgements
decrease the electron affinity of the PDI and the tendency of the GCW acknowledges NSERC DG (435715-2013), CFI JELF
PDI to form large aggregates in the film. Dimerization of the PDI (34102), CRC, and the UofC. MN and TAW are grateful for UofC
chromophore reduces the tendency of the materials to aggregate Eyes High graduate scholarships. This research was undertaken
in the film with the N-annulated PDI dimer exhibiting the thanks in part to funding from the Canada First Research
smoothest films. In terms of OSC performance, all PDI materials Excellence Fund (CFREF).
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