Solution processed bulk heterojunction solar cells based on A-D-A small molecules with a dihydroindoloindole (DINI) central donor and different acceptor end groups

Article abstract of DOI:10.1039/c6tc00323k

Four acceptor-donor-acceptor (A-D-A) small molecules with dihydroindoloindole (DINI) as the central donor unit and different acceptor end groups such as dicyanovinylene (DCV), indenedione (IND), cyanoacrylate (CA) and rhodanine (Rho) linked through bithiophene as the π-linker, DINI-DCV, DINI-IND, DINI-CA and DINI-Rho, were designed and synthesized for the application as donor materials along with PC₇₁BM as an acceptor for solution processed organic bulk heterojunction solar cells. The effect of acceptor end groups on the photovoltaic performance was investigated. The organic solar cells (OSCs) based on as cast DINI-IND showed the highest power conversion efficiency of 3.04%, as cast DINI-CA showed the lowest PCE of 1.63% and the other two exhibit a PCE in between them. These results showed that acceptor end groups affect the overall performance of the cells. The PCE of OSCs has been further improved up to 7.04% and 6.16% employing two-step annealing (TSA) treated DIN-IND:PC₇₁BM (1:2) and DIN-CN:PC₇₁BM (1:2), respectively. The enhancement in the PCE of OSCs with TSA treated active layers is attributed to the better nanophase morphology, the increase in the crystalline nature and light harvesting efficiency and more balanced charge transport and charge collection probability.

Full text of DOI:10.1039/c6tc00323k

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Solution processed bulk heterojunction based on A-D-A small molecules with
dihydroindoloindole (DINI) central donor with different acceptor end groups
Jangkeun Sim,1 Hyejeoung Lee,1 Kihyoung Song,2 S. Biswas3, Abhishek Sharma4, Ganesh.
D. Sharma,*3 Jaejung Ko*1
1Department of New Material Chemistry, Korea University, Sejong, 339-700, Republic of
Korea
2Department of Chemistry, Korea National University of Education, Chungbuk, 363-791,
Republic of Korea
3Molecular Electronics and Optoelectronics Research Laboratory, Department of Physics,
LNMIIT (Deemed University), Jamdoli, Jaipur (Raj.) 302301, India
4Electronics Communication Engineering, LNMIIT (Deemed University), Jamdoli, Jaipur
(Raj.) 302301, India
Four acceptor –donor –acceptor (A-D-A) small molecules with dihydroindoloindole (DINI)
as central donor unit and different acceptor end groups such as dicyanovinylene (DCV),
indenedione (IND), cyanoacrylate (CA) and rhodanine (Rho) linked through bi-thiophene as
π-linkers, DINI-DCV, DINI-IND, DINI-CA and DINI-Rho were designed and synthesized for
the application as donor material along with PC71BM as acceptor for solution processed
organic bulk heterojunction solar cells. The effect of acceptor end groups on the photovoltaic
performance was investigated. The organic solar cells (OSCs) based on as cast DINI-IND
showed the highest power conversion efficiency of 3.04 % and as cast DINI-CA showed the
lowest PCE of 1.63 % and other two exhibits the PCE in between them. These results showed
that acceptor end group affects the overall performance of the cells. The PCE of OSCs has
been further improved up to 7.04 % and 6.16 % employing two step annealing (TSA) treated
DIN-IND:PC71BM (1:2) and DIN-CN:PC71BM (1:2), respectively. The enhancement in the
PCE of OSCs with TSA treated active layers is attributed to the better nanophase morphology,
the increase in crystalline nature, and light harvesting efficiency and more balanced charge
transport and charge collection probability.
Key words: A-D-A small molecules; different end groups; bulk heterojunction solar cells;
power conversion efficiency; two step annealing.
*corresponding authors

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Introduction
Organic solar cells (OSCs) based on the bulk heterojunction (BHJ) active layer are
attracting a lot of attention to both academic and industrial communities due to their cost
effective and flexible solar energy conversion to electrical energy devices [1]. Significant
progress have been made in recent years and power conversion efficiencies (PCEs) over 9-
10 % using a single layer polymer BHJ solar cells have been reported [2] after the
optimization of donor –acceptor ratio, choice of solvent, solvent and thermal annealing and
interface engineering. In contrast to conjugated polymers, conjugated small molecules
possess advantages such as simple synthesis, purification, defined molecular structure and
molecular weight without batch to batch variation [3]. After the optimization of molecular
structure and morphology of BHJ active layer, the PCE of the OSCs has been improved from
0.03% to 9-10% [4]. The π-conjugated small molecules built up from alternating electron-rich
donor (D) and electron-poor acceptor (A) segments provide excellent materials with strong
charge transfer character, absorptions in the visible to near-infrared region, highest occupied
molecular orbital (HOMO)– lowest unoccupied molecular orbital (LUMO) energy gap, and
good charge carrier mobility [5]. Among them, acceptor-donor-acceptor (A-D-A) conjugated
small molecules were developed and used as donors for solution processed organic solar cells
[6]. The electronic levels and solubility of these small molecules can be controlled by proper
substitution and functionalization which not only leads to broad absorption of solar spectrum,
but also high open circuit voltage and excellent stability towards oxidation.
In the design of new small molecules as electron donor for OSCs, basic properties
such as energy levels, charge transport, and absorption are the basic parameters to be
considered. Aromatic fused rings have been extensively used to design the high mobility
organic semiconductors because the extended conjugation in fused rings is beneficial to form
effective interchain π-π overlaps and enhance the charge transport [7], leading to the high
charge carrier mobility [8]. Moreover, the extended π-conjugation also results in enhancing
the delocalization of π-electrons to attain effective π-conjugation length, reducing to the
bandgap [9].
Dihydroindoloindole (DINI) is an electron rich planar aromatic fused structure with
two nitrogen atoms, which can be easily substituted with alkyl chains for better solubility of
the small material. Owczarczyk et al. reported DINI based polymers and showed that the
DINI is an effective backbone for polymer solar cells (PSCs) applications [10]. Kranthiraja et
al have also reported the fabrication of BHJ PSCs employing DINI based D-A copolymer,

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achieving the PCE of 1.68% [11]. Although DINI has interesting characteristics, the
exploration of small molecules based on DINI has not been employed for small molecule
BHJ solar cells. We have synthesized a series of A-D-A small molecules with same
dihydroindoloindole (DINI) as central donor unit and different acceptor end groups such as
dicyanovinylene (DCV), indenedione (IND), cyanoacrylate (CA) and rhodanine (Rho) linked
through bi-thiophene linker and investigated their optical and electrochemical properties. We
have used these small molecules as donor layer along with PC71BM as acceptor and varied
the weight ratio of donor to acceptor and found that 1:2 weight ratio showed best
photovoltaic performance. The OSCs fabricated with optimized DINI-CA:PC71BM, DINI-
DCV:PC71BM, DINI-Rho:PC71BM and DINI-IND:PC71BM spin cast from DCB showed
the PCE of 1.63 %, 2.53 %, 2.13 % and 3.04 %, respectively. Therefore, different acceptor
end groups of these small molecules lead to variation in the PCE of OSCs. In order to
improve PCE further, we have employed a simple TSA (combined thermal annealing and
subsequent solvent vapor annealing) treatment of active layer method and the PCE has been
improved significantly (7.04 % and 6.16 % for DINI-IND:PC71BM and
DINI-
DCV:PC71BM, respectively) due to the better nanophase morphology, crystallinity, molecular
packing and light harvesting ability of active layers. These combined properties enhance the
exciton generation efficiency and relatively better balanced charge transport, leading to
improvement in Jsc, FF and PCE of resulted OSCs.
Experimental details
The synthesis of core and ligands employed for the synthesis of small molecules were
described in supplementary information.
2,2'-(5',5''-(5,10-dihexyl-5,10-dihydroindolo[3,2-b]indole-2,7-diyl)bis(3,4'-dihexyl-2,2'-
bithiophene-5',5-diyl))bis(methan-1-yl-1-ylidene)dimalononitrile(DINI-DCV):
Malononitrile (120 mmol) dissolved in CHCl3 (40 mL) was added to 5a (20 mmol) followed
by 2 drops of piperidine. The reaction was stirred for 2 h at room temperature. The mixture
was poured into water and extracted with dichloromethane. The organic layer was then dried
over anhydrous MgSO4. After evaporation of the solvent, the residue was purified by column
chromatography on a silica gel (ethylacetate : hexane = 1:5 ). The crude product was washed
with MeOH to afford a purple solid in 65% yield. 1H NMR (300 MHz, CD2Cl2) : δ 7.87 (d, J
= 8.4Hz, 1H), 7.73 (s, 1H), 7.57 (s, 1H), 7.53 (s, 1H), 7.33(s, 1H), 7.29 ( dd, J = 8.4Hz, 1H ),
4.53 - 4.57 (t, 2H), 2.88 – 2.94 (t, 2H), 2.79 – 2.84 (t, 2H), 2.03 - 2.05 (t, 2H), 1.72-1.77(t,
4H), 1.28 – 1.48 (m, 18H), 0.86 – 0.93 (m, 9H). 13C NMR ( 75 MHz, CDCl3) : δ 150.20,
145.08, 143.73, 142.14, 142.01, 140.97, 140.47, 139.75, 131.92, 131.36, 127.25, 127.07,

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119.97, 114.71, 114.08, 113.78, 110.75, 75.49, 45.69, 31.73, 31.25, 30.49, 30.23, 29.45, 29.30,
29.09, 27.06, 26.33, 22.68, 14.20. MS: m/z 1190.61 [M+]. Anal. calcd for C74H90N6S4 : C,
74.58; H, 7.61. Found: C, 74.18; H, 7.58.
(2E,2'E)-dihexyl3,3'-(5',5''-(5,10-dihexyl-5,10-dihydroindolo[3,2-b]indole-2,7-
diyl)bis(3,4'-dihexyl-2,2'-bithiophene-5',5-diyl))bis(2-cyanoacrylate) (DINI-CA): Hexyl
2-cyanoacetate (120 mmol) dissolved in CHCl3 (40 mL) was added to 5a (20 mmol) followed
by 2 drops of piperidine. The reaction was stirred overnight at room temperature. The mixture
was poured into water and extracted with dichloromethane. The organic layer was then dried
over anhydrous MgSO4. After evaporation of the solvent, the residue was purified by column
chromatography on a silica gel (dichloromethane : hexane = 1:1 ). The crude product was
washed with hexane and MeOH to afford a deep red solid in 53% yield. 1H NMR (300 MHz,
acetone) δ 8.35(s, 1H), 7.99 (d, J = 7.5Hz, 1H), 7.86 (s, 1H), 7.68 (s, 1H), 7.40 (s, 1H),
7.31( dd, J = 7.5Hz, 1H), 4.66 - 4.71 (t, 2H), 4.26 - 4.31 (t, 2H), 2.93 – 2.98 (t, 2H), 2.85 –
2.90 (t, 2H), 1.76 - 1.78 (m, 8H), 1.33-1.49(m, 24 H), 0.85 – 0.96 (m, 12H). 13C NMR ( 75
MHz, CDCl3) : δ 177.99, 177.42, 163.45, 159.13, 156.47, 147.06, 146.28, 143.02, 142.64,
140.90, 140.16, 139.45, 132.60, 132.47, 127.43, 126.97, 120.88, 116.33, 114.01, 97.26, 78.71,
66.66, 31.78, 31.55, 30.51, 29.49, 29.35, 29.12, 28.71, 27.08, 25.64, 22.78, 14.28. MS: m/z
1396.79 [M+]. Anal. calcd for C86H116N4O4S4 : C, 73.88; H, 8.36. Found: C, 73.56; H, 8.30.
2,2'-(5',5''-(5,10-dihexyl-5,10-dihydroindolo[3,2-b]indole-2,7-diyl)bis(3,4'-dihexyl-2,2'-
bithiophene-5',5-diyl))bis(methan-1-yl-1-ylidene)bis(1H-indene-1,3(2H)-dione)
(DINI-Ind): 1,3-Indandione (120 mmol) dissolved in CHCl3 (40 mL) was added to 5a (20
mmol) followed by 2 drops of piperidine. The reaction was stirred at room temperature. The
mixture was poured into water and extracted with dichloromethane. The organic layer was
then dried over anhydrous MgSO4 . After evaporation of the solvent, the residue was purified
by column chromatography on a silica gel (ethylacetate : hexane = 1:5 ). The crude product
1
was washed with MeOH to afford a purple solid in 62% yield. H NMR (300 MHz, THF) δ
7.99(s, 1H), 7.87 - 7.95 (m, 4H), 7.77 - 7.85 (m, 2H), 7.61 (s, 1H), 7.39 (s, 1H), 7.28 (d, J =
7.8Hz, 1H), 4.60 - 4.62 (t, 2H), 2.91 - 2.96 (t, 2H), 2.82 – 2.88 (t, 2H), 1.80 - 2.04 (m, 4H),
1.67 - 1.80 (m, 2H), 1.40-1.46(m, 18H), 0.90 – 0.96 (m, 9H). 13C NMR ( 400 MHz, THF) : δ
190.00, 189.79, 146.49, 143.33, 142.12, 141.79, 141.23, 140.36, 140.04, 135.81, 135.74,
134.10, 131.30, 125.13, 123.51, 120.76, 118.92, 115.12, 111.56, 46.11, 32.89, 32.19, 32.13,
31.51, 31.28, 30.81, 30.47, 30.23, 29.93, 27.91, 26.01, 25.88, 25.61, 25.29, 25.09, 23.74,
23.66, 14.62, 1.53. MS: m/z 1350.64 [M+]. Anal. calcd for C86H98N2O4S4 : C, 76.40; H, 7.31.
Found: C, 76.33; H, 7.27.

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(5E,5'E)-5,5'-(5',5''-(5,10-dihexyl-5,10-dihydroindolo[3,2-b]indole-2,7-diyl)bis(3,4'-
dihexyl-2,2'-bithiophene-5',5-diyl))bis(methan-1-yl-1-ylidene)bis(3-ethyl-2-
thioxothiazolidin-4-one)(DINI-Rho): 3-Ethylrhodanine(100 mmol) and AcONH4)dissolved
in AcOH (40 mL) was added to 5a (5 mmol). The reaction was refluxed overnight. Then the
product were isolated by filtration and washed with H2O and MeOH. The deep red product
was dissolved in CHCl3 and dried over anhydrous Na2SO4 . After evaporation of the solvent,
the residue was purified by column chromatography on a silica gel (dichloromethane : hexane
= 2 : 3 ). The crude product was washed with hexane and MeOH to afford a red-purple solid
1
in 74% yield. H NMR (300 MHz, CD2Cl2) δ 7.88(d, J = 7.5 Hz, 1H), 7.81 (s, 1H), 7.56 (s,
1H), 7.30 - 7.32 (m, 2H), 7.24(s, 1H), 4.54 - 4.56 (t, 2H), 4.14 - 4.21 (q, 2H), 2.86 – 2.91 (t,
2H), 2.78 - 2.84 (t, 2H), 2.00 - 2.03 (t, 2H), 1.70 - 1.74 (m, 4H), 1.27-1.41(m, 18H), 0.85 –
0.94 (m, 12H). 13C NMR (400 MHz, CDCl3) :192.41, 185.87, 184.43, 168.22, 150.46, 138.90,
130.61, 125.97, 40.12, 31.95, 31.85, 30.52, 29.92, 29.58, 29.44, 27.14, 22.87, 22.84, 22.78,
14.31, 14.24, 12.52. MS: m/z 1380.56 [M+]. Anal. calcd for C78H100N4O2S8 : C, 67.78; H,
7.29. Found: C, 67.53; H, 7.22.
Device fabrication and characterization
The small molecule BHJ OSCs were fabricated with configuration of ITO coated
glass /PEDOT:PSS/donors:PC71BM/Al using a conventional solution spin coating method.
The indium tin oxide (ITO) coated glass substrates were cleaned subsequently by ultrasonic
treatment in detergent, deionized water, acetone and isopropyl alcohol for 20 min. A layer of
PEDOT:PSS was spin coated (3500 rpm, ~40 nm) onto ITO coated glass substrate and baked
at 120 °C for 20 min. The active layer was spin coated onto the top of PEDOT:PSS layer
from the different weight ratios of donors and PC71BM in dichlorobenzene (DCB) at 1500
rpm for 25 s. For two step annealing (TSA) treatment, the optimized active layer was
thermally annealed by placing the active layer on hot plate at 110 °C for 5 min and then
cooled up to room temperature. The subsequent solvent vapor annealing (SVA) and the active
layer was place in a glass Petri dish containing 200 ꢀL THF. The thickness of the active
layers is about 90 ±5 nm. Finally 50 nm aluminum (Al) layer was deposited onto the top of
active layer under the high vacuum by a shadow mask. The effective area of each device is
about 20 mm2. The hole-only and electron-only devices with ITO/PEODT:PSS/active
layer/Au and ITO/Al/active layer:PC71BM/Al architectures were also fabricated in an
analogous way in order to measure the hole and electron mobility, respectively. The current–
voltage characteristics of BHJ organic solar cells were measured using a computer controlled

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Keithley 238 source meter under simulated AM1.5G, 100 mW/cm2. A xenon light source
coupled with optical filter was used to give the stimulated irradiance at the surface of the
devices. The incident photon to current efficiency (IPCE) of the devices was measured at
illuminating the device through the light source and monochromator and the resulting current
was measured using a Keithley electrometer under short circuit condition.
Results and discussion
Optical and electrochemical properties
The normalized absorption spectra of the four small molecules in dilute DCB solution
and thin films are shown in Figure 1a and 1b, respectively. The corresponding data are
summarized in Table 1. All the small molecules showed two absorption bands covering the
wavelength range from 350 to 650 nm. The absorption band in high energy and low energy
wavelength regions are attributed to localized π-π* transitions and intramolecular charge
transfer (ICT) transitions between central donor and terminal acceptor units, respectively. It
can be seen from the Figure 1a that the absorption peak assigned to high energy band for all
small molecules is almost same, but the absorption peak assigned to ICT bands are different
due to the different electron accepting ability of end group units. The bathochromic shift
following the trend DINI-IND>DINI-DCV>DINI-Rho>DINI-CA is in agreement with the
electron accepting ability of IND>DCV>Rho>CA. This indicates that the small molecule
having stronger electron withdrawing moiety exhibits a red-shift absorption spectra due to its
stronger ICT [12]. The absorption spectra of all small molecules in thin film show broad and
bathochromic shift of ICT absorption band compared to their corresponding solution spectra,
indicating that these molecules follow J aggregates in film state and increased intermolecular
interactions and backbone planarity in the solid state [13]. Moreover, all the small molecules
showed a vibronic shoulder, indicating the existence of strong π-π stacking between the
molecule backbones. The optical bandgap of these small molecules were estimated from the
absorption edges in thin film spectra of corresponding small molecules to be 1.78 eV, 1.95 eV,
1.74 eV, and 1.88 eV for DINI-DCV, DINI-CA, DINI-IND and DINI-Rho, respectively.
The highest occupied molecular orbital (HOMO) energy level of SMs was estimated
from the onset oxidation potential of corresponding cyclic voltammogram (shown in Figure
S13, supplementary information) with the assumption that the energy level of ferrocene (Fc)
was 4.8 eV below the vacuum level. The LUMO energy level of the small molecules was
estimated according to ELUMO=EHOMO + Eo-o, where Eo-o is determined from the intersection of
absorption and emission spectra in solution (as shown in Figure S14, supplementary

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information). The estimated HOMO/LUMO energy levels for small molecules are compiled
in Table 1. It is well known that the HOMO and LUMO energy levels are affected by the
electron donating and withdrawing units in D-A type compounds, respectively. Similar value
of LUMO energy level for DINI-DCV and DINI-IND indicates that DCV and IND have
almost same electron withdrawing ability. The LUMO energy level of these small molecules
is sufficiently higher than the LUMO of PC71BM (-4.1 eV), indicating that the efficient
photoinduced charge separation is possible and can be used as donor for BHJ organic solar
cells.
Figure 2 shows the optimized structure of DINI-CA, DINI-Rho, DINI-DCV and
DINI-IND, which are calculated by the time dependent density functional theory (TD-DFT)
using B3LYP functional/3-21 G* basis set. The orbital density of the HOMO of these small
molecule are similarly observed in both DINI and thiophene bridge. However, the LUMO is
located predominantly in the different electron accepting end group units and thiophene
bridge, showing a general orbital distribution of D-A small molecules.
Photovoltaic properties
In order to achieve the high PCE of organic BHJ solar cells, there should be a balance
between the absorbance and charge transport ability of active layer. The weight ratio of small
molecule donors and PC71BM were varied from 1:05 to 1:2.5 and devices with the ratio 1:2
showed the maximum PCE. The current-voltage (J-V) characteristics of the BHJ organic
solar cells based on DINI-DCV, DINI-CA, DINI-Rho and DINI-IND as donor and PC71BM
acceptor with optimized D/A weight ratio (1:2) are shown in Figure 3a. The corresponding
photovoltaic parameters are compiled in Table 2. The device based on DINI-IND:PC71BM
showed highest PCE of 3.04 % with Jsc of 7.20 mA/cm2, Voc of 0.88 V and FF of 0.48,
whereas the device based on DINI-CA:PC71BM showed the lowest PCE of 1.63 % with Jsc of
4.98 mA/cm2, Voc of 0.78 V and FF of 0.42. It can be seen from the table 2 that the device
based on DINI-DCV showed highest Voc, due to the its deeper HOMO level compared to
other, since the Voc of the BHJ OSC is directly related to the difference between the LUMO
of acceptor and HOMO of donor used in the active layer. The variation of Jsc of the OSCs is
consistent with the absorption spectra of active layer and IPCE spectra of the devices (Figure
4). The Jsc estimated from the integration of the IPCE spectra are compiled in Table 2 and
matched with the Jsc obtained from J-V characteristics measurements.
The low values of PCE based on these small molecules is mainly attributed to the low
values of both Jsc and FF and related to the poor morphology of active layer, since the
nanoscale phase separation is needed for the efficient exciton dissociation and charge

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transport. Two step annealing (TSA) approach that combines thermal and solvent vapor
annealing has been adopted to tune the morphology of the active layer using polymers and
small molecules as donor [14] and found that the PCE has been significantly improved. We
have examined the effect of TSA for devices employing all the small molecules and found
that all the devices showed the increase in PCE (see the table 2), but DINI-DCV and DINI-
IND showed the higher PCE than other two molecules. Therefore, we have discussed the
effect of TSA only for the devices based on DINI-DCV and DINI-IND in detail and the J-V
characteristics of these devices under illumination intensity of 100 mW/cm2 are shown in
Figure 5a. The PCE was improved up to 7.04 % (Jsc=12.18 mA/cm2, Voc= 0.85 V and FF=
0.68) and 6.16 % (11.74 mA/cm2, Voc = 0.86 V and FF= 0.61) for DINI-IND and DINI-DCV,
respectively, with remarkably enhanced Jsc and FF. The significant improved PCE originates
from the favorable morphological evolution in the active layer induced by the combined
effect of thermal and solvent vapor annealing. The IPCE spectra of the devices are shown in
Figure 5b. The IPCE peak of both DINI-DCV and DINI-IND was significantly enhanced
after the TSA treatment (see Figure 3b and 5b), contributing to higher Jsc. Moreover, the IPCE
spectra of the devices based on the TSA treated active layer shows broader response and red-
shift as compared to the devices without TSA, which is consistent with their absorption
spectra (Figure 4). The increase in the absorption intensity and broadening of absorption
profile towards longer wavelength region of blended film upon TSA treatment may be related
to the improved the π-π stacking and planarity of donor.
To get information about the reason behind the improvement in the PCE of the
devices by the TSA treatment, X-ray diffraction (XRD) measurement was applied to explore
the molecular packing in these active layers (Figure 6). As shown in Figure 6, as cast DINI-
DCV:PC71BM and DINI-IND:PC71BM film show a weak (100) diffraction peak located at
2θ=3.96° and 4.22°, corresponding to the d-spacing of 2.33 nm and 2.18 nm, respectively,
between the molecular layers. This suggests that DINI-IND forms a denser molecular
packing than DINI-DCV counterpart may induce a better nano-phase separation
between .donor and acceptor in the as cast blend film, may be responsible for the higher PCE
for device based on DINI-IND. After TSA, the (100) peak shifts to 2θ=4.26° and 4.45° for
DINI-DCV and DINI-IND, respectively and also becomes much stronger, indicating an
improved layer packing molecules existing in the active layer. The increased in the diffraction
intensity for TSA treated blended films indicates that the degree of crystallinity was
significantly improved as compared to as cast counterpart. The combined effect of thermal

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annealing and subsequent solvent vapor annealing promotes the nanoscale morphology in the
active layer for efficient charge transfer and transportation, resulting in an improvement in Jsc
and FF.
The morphological change in the active layer with TSA treatment has been further
investigated by transmission electron microscopy (TEM). Figure 7 shows the TEM images of
DIN-DCV:PC71BM and DIN-IND:PC71BM blended films with and without TSA treatment.
The bright and dark regions correspond to donors and PC71BM domains, respectively [15].
The blended film cast from CB showed very uniform and homogeneous morphology, owing
to the good miscibility of donors and PC71BM. Desirable nano-phase with separated donor
and acceptor domains is not observed, resulting in low Jsc and FF and overall PCE. However,
upon TSA treatment of active layer, the domain size of both donor and PC71BM becomes
bigger and a nanoscale phase separation is developed as shown in Figure 7, consequently
leading to a significant improvement in the photovoltaic performance of devices.
On organic solar cells using BHJ active layer, hole and electron mobilities are
important parameters to ensure the efficient charge carrier transport towards the electrodes
and to suppress the photocurrent loss by competing charge recombination. Moreover, balance
between the hole and electron mobilities in the active layer is also important for improving Jsc
and FF of the OSCs. The hole mobilities of the blended films with and without TSA were
measured by the space charge limited current method [16], with hole only device structure
ITO/PEDOT:PSS/active layer/Au, as shown in Figure 8. The hole mobilities are 0.98 x10-5
and 1.45 x10-5 cm2/Vs for as cast DINI-DCV and DIN-IND, respectively. After the TSA, the
mobilities increase to 8.98 x10-5 and 1.16 x10-4 cm2/Vs for DINI-DCV and DIN-IND,
respectively. We have also measured the electron mobilities using the electron only device
structure ITO/Al/active layer/Al using same SCLC method and found that electron mobilities
is in the range of 2.42-2.48 x10-4 cm2/Vs for both as cast and TSA treated blended films. The
relatively unbalanced carrier mobilities (ratio of electron to hole mobilities are 24.69 and 16.7
for DINI-DCV and DIN-IND, respectively) for as cast films can be ascribed to the poor
morphology which results in the lower photovoltaic performance. The increase in hole
mobility in the blend film with the TSA treatment leads to the more balanced charge transport
(electron to hole mobilities are 2.76 and 2.14 for DINI-DCV and DIN-IND, respectively),
enhancing the FF and overall PCE. The high hole mobility and better charge transport for the
devices processed with TSA treated active layers is consistent with the increase in
crystallinity of the active layer, and molecular packing as illustrated by XRD and TEM
measurements. Therefore TSA treatment resulted much enhanced hole mobility together with

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considerably better balanced hole and electron mobility which led to a much higher charge
carrier extraction efficiency and thus responsible for higher FF [17]. DINI-IND:PC71BM
based device achieved FF value of 0.68, which may be associated with its high and balanced
charge carrier mobility and well as lower charge recombination losses [18].
The relationship between the photocurrent (Jph) and effective voltage (Veff) was
investigated for the devices with and without TSA treated DINI-DCV:PC71BM and DINI-
IND:PC71BM layers and shown in Figure 9. Here, Jph is defined as Jph =JL-JD, where JL and JD
are the current densities under illumination and in the dark, respectively, Veff = Vo-Vappl
,
where Vo is the voltage at which Jph=0 and Vappl is applied voltage [19]. In the devices based
on the TSA treated active layer, Jph shows a linear dependence at the low Veff and saturated at
a high Veff. The devices based on the without TSA treated active layers also showed a linear
Jph- Veff dependence at low values of Veff, but does not fully saturate even at high voltage.
The Veff value corresponds to an internal electric field to sweep out the photogenerated
charge carriers in the active layer towards the electrodes. It means that much stronger electric
field is required to seep out photogenerated charges and suggest relative poor charge transport
within the active layer. It is consistent with the absence of ordered domains to provide
effective percolation pathways for charge carriers in the active layer. The saturation current
density (Jsat) is higher for the devices prepared with TSA treated active layers which is higher
than corresponding as cast counterparts. It means that the TSA active layer based device has a
higher maximum exciton generation rate (Gmax). Gmax is given by Jsat=qGmaxL, where L is the
thickness of the active layer. The higher Gmax reflects the higher photon absorption which is
consistent with the absorption profile of TSA treated active layer (Figure 4). We have also
estimated the charge collection probability (Pc) in the devices processed with as cast and TSA
treated active layers, Pc= Jsc/Jphsat [20] and the values are 0.54, 0.56 0.82, and 0.84 for as cast
DINI-DCV:PC71BM, DINI-IND:PC71BM, TSA treated DINI-DCV:PC71BM and DINI-
IND:PC71BM based devices, respectively. Therefore, the enhanced Jsc in the TSA treated
active layer based OSC is attributable to the enhanced light absorption and light harvesting in
the active layer and efficient charge collection.
Conclusions
We have designed and synthesized four A-D-A small molecules with common central
dihydroindoloindole (DINI) donor unit, different acceptor end groups such as
dicyanovinylene (DCV), indenedione (IND), cyanoacrylate (CA) and rhodanine (Rho) linked

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through bi-thiophene as π-linkers denoted as DINI-DCV, DINI-IND, DINI-CA and DINI-
Rho and their optical and electrochemical properties were investigated. These small
molecules were used as electron donors along with PC71BM as acceptor of solution
processed small molecule OSCs. After the optimization of donor to acceptor weight ratio,
spinning speed and concentration, the 1:2 weight ratio showed the best photovoltaic
performance for all small molecules. The OSCs based on DINI-CA:PC71BM, DINI-
DCV:PC71BM, DINI-Rho:PC71BM and DINI-IND:PC71BM spin cast from DCB showed
the PCE of 1.63 %, 2.53 %, 2.13 % and 3.04 %, respectively, indicating that acceptor end
group has impact on the overall PCE. In order to improve PCE further, we have employed a
simple TSA (combined thermal annealing and subsequent solvent vapor annealing) treatment
of active layer method and the PCE has been improved significantly (7.04 % and 6.16 % for
DINI-IND:PC71BM and DINI-DCV:PC71BM, respectively) which is ascribed to the better
nanophase morphology, crystallinity, molecular packing and light harvesting ability of active
layers, induced by the TSA treatment. These combined properties enhance the exciton
generation efficiency and relatively better balanced charge transport, leading to the
improvement in Jsc, FF and PCE of resulted OSCs.
Acknowledgments
This work was supported by the Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education(2016, R1415802)
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Table 1 Optical and redox parameters of the compounds.
Compounds λabs[a]/nm (ε/M-
λPL[a]/nm Eo-o (eV)
EHOMO
(eV)b
ELUMO
(eV)c
Egopt (eV)
λabs(nm)
(film)
1cm-1)
400 (31 899)
514 (46 703)
572
1.74
736
2.03
-5.14
-3.11
DINI-DCV
395 (44 325)
488 (61 865)
534
595
1.95
1.68
722
736
2.04
2.00
-4.96
-5.12
-2.92
-3.12
DINI-CA
397 (28 536)
528 (50 068)
DINI-IND
399 (38 089)
503 (60 243)
562
1.88
720
2.02
-5.08
-3.06
DINI-Rho
[a] UV-vis absorption spectra and fluorescence spectra were measured in Chlorobenzene
solution. [b] Redox potential of the compounds were measured in CH2Cl2 with 0.1M (n-
C4H9)4NPF6 with a scan rate of 100 mVs-1 (vs. Fc/Fc+). [c] ELUMO= EHOMO + Egap [d] Egap was
calculated from the intersection of absorption and emission spectra in solution
Table 2 Photovoltaic parameters of organic solar cells based on DINI-DCV, DINI-CA, DINI-
Rho and DINI-Ind as donor and PC71BM (optimized weight ratio 1:2)
Active layer
Jsc
(mA/cm2)
Jsc
Voc
FF
PCE (%)
(PCEaverage)
(mA/cm (V)
2)c
DINI-CA:PC71BMa
4.98
4.91
6.04
5.55
7.26
7.23
0.78
0.90
0.86
0.88
0.75
0.42
0.46
0.44
0.48
0.54
1.63 (1.59)
2.53(2.48)
2.13 (2.07)
3.04(3.98)
2.95(2.86)
DINI-DCV:PC71BMa 6.12
DINI-Rho:PC71BMa
DINI-IND:PC71BMa
DINI-CA:PC71BMb
5.64
7.20
7.30
DINI-DCV:PC71BMb 11.74
DINI-Rho:PC71BMb
11.08
11.65
0.86
0.61
6.16(6.12)
10.98
12.23
0.82
0.85
0.58
0.68
5.23 (5.18)
7.04(6.98)
DINI-IND:PC71BMb 12.18
aAs cast
bTSA treated active layer
cEstimated from IPCE spectra

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Scheme 1 Synthesis of DINI-DCV, DINI-CA, DINI-IND and DINI-Rho small molecules

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Figure 1 Normalized absorption spectra of DINI-DCV, DINI-CA, DINI-IND and DINI-Rho
(a) dilute DCB solution and (b) thin film cast from DCB solution.

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Figure 2 Isodensity surface plots of (a) DINI-DCV and (b) DINI-CA and (c) DINI-Ind and
(d) DINI-Rho, calculated by the time dependent-density functional theory (TD-DFT) using
the B3LYP functional/6-31G* basis set.

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Figure 3 (a) Current –voltage characteristics under illumination and (b) IPCE spectra of
devices based on optimized active layers of small molecule donors and PC71BM acceptor cast
from CB solution.

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Figure 4 Normalized absorption spectra of as cast (CB) and TSA treated DINI-DCV:PC71BM
and DINI-IND :PC71BM and TSA treated active layers

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Figure 5(a) Current –voltage characteristics under illumination and (b) IPCE spectra of the
OSCs based on TSA treated DINI-DCV:PC71BM and DINI-IND:PC71BM blends.

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Figure 6 XRD patterns of the as cast and TSA treated blended active layers

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Figure 7 TEM images (a) DINI-DCV:PC71BM (as cast) (b) DINI-DCV:PC71BM (TSA), (c)
DINI-IND:PC71BM (as cast) and (d) DINI-IND:PC71BM (TSA), scan bar is 200 nm

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Figure 8 Current- voltage characteristics of hole only devices to estimate the hole mobilities
in the as cast and TSA treated active layers
Figure 9 Variation of Jph with effective voltage (Veff) for the OSCs based on as cast and TSA
treated active layers

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The optimized device after TSA treatment showed PCE of 7.04 % and 6.16 % DIN-IND:PC71BM
(1:2) and DIN-CN:PC71BM (1:2), respectively