Synthesis of D-D-A-type small organic molecules with an enlarged linker system towards organic solar cells and the effect of co-adsorbents on cell performance

Article abstract of DOI:10.1039/c5nj02270c

The synthesis of two D-D-A type novel small molecules bearing N,N-dimethyl/ethyl aniline as a donor and a methine unit connected with cyano and carboxyl groups as an acceptor linked with conjugated π-linkers has been performed. The obtained compounds were

Full text of DOI:10.1039/c5nj02270c

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Synthesis of D–D–A-type small organic molecules
with an enlarged linker system towards organic
solar cells and the effect of co-adsorbents on cell
performance†
Cite this:DOI: 10.1039/c5nj02270c
Priyanka P. Kumavat,a Prashant K. Baviskar,b Babasaheb R. Sankapalc and
Dipak S. Dalal*a
The synthesis of two D–D–A type novel small molecules bearing N,N-dimethyl/ethyl aniline as a donor
and a methine unit connected with cyano and carboxyl groups as an acceptor linked with conjugated
p-linkers has been performed. The obtained compounds were characterized by using FT-IR, 1H-NMR,
13C-NMR, LC-HR-MS, TGA, FE-SEM and UV-Vis spectroscopic techniques. These designed small organic
molecules have been utilized for the fabrication of novel organic solar cells on high surface
area titanium dioxide films consisting of interconnected nanoparticles with the device configuration
FTO/TiO2/organic compound/electrolyte/platinum. The effect of cholic acid and dodecylamine as
co-adsorbents with an organic compound has been studied towards the device performances and the
results are incorporated herein.
Received (in Montpellier, France)
27th August 2015,
Accepted 9th November 2015
DOI: 10.1039/c5nj02270c
www.rsc.org/njc
using poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0]dithiophene-
2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-
1. Introduction
Over the past couple of decades, the search for better and clean diyl]] (PTB7) as a sensitizer has also been reported.9 A systematic
renewable energy resources has been intensified due to the analysis of relevant parameters for the production of blade coated
increase in energy consumption. Solar energy is one of the several solar cells using a low band gap/fullerene blend was conducted
promising renewable energy resources that could contribute to a and efficiencies of upto 3.6% for coated and 1.6% for printed
sustainable energy supply.1,2 Moreover, organic solar cells (OSCs) cells on a flexible substrate was obtained.10 Inverted solar cells
has gained considerable interest as future renewable alternative consisted of a blade-coated poly(N-90-heptadecanyl-2,7-carbazole-
energy resources due to their added advantages such as low- alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)) (PCDTBT) and
cost fabrication, lightweight, and good mechanical flexibility.3–6 [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (1 : 4 by
Hence, a broad range of distinct device technologies based on weight) active layer sandwiched between a ZnO electron extrac-
organic molecules are being developed very rapidly.
tion layer and a MoO3/Ag anode has been reported with a power
Two novel organic dyes containing julolidine as the electron conversion efficiency (PCE) of around 5%.11 High open-circuit
donor and cyanoacetic acid or rhodamine-3-acetic acid as the voltage (1.03 V) has been achieved by solution-processed organic
electron acceptor bridged by the bithiophene unit were synthe- solar cells based on benzothiadiazole-triphenylamine small mole-
sized which resulted in 2.6–6.7% efficiencies.7 Benzimidazole cules incorporating p-linkage.12 A series of solution-processable,
based metal free organic dyes with and without cyano vinyl units p-type, low-optical-gap semiconductors based on penta- and hexa-
have been designed and synthesized via one step processes and thiophenes asymmetrically endcapped with a triarylamine or
tested as sensitizers in dye sensitized solar cells.8 A significant triarylamino-substituted dendron and dicyanovinyl groups has
enhancement of ternary blend organic solar cell efficiency been reported for a PCE ranging from 0.47 to 1.72%.13 Bulk
heterojunction solar cells have been prepared by using donor
a School of Chemical Sciences, North Maharashtra University, Jalgaon – 425 001,
M.S., India. E-mail: dsdalal2007@gmail.com; Fax: +91-257-2258403;
Tel: +91-257-2257432
materials 2,5-bis[4-(N,N-diphenylamino)styryl]thiophene and
2,5-bis[4-(N,N-diphenylamino)styryl]-2,20-bithiophene and resulted
in PCE values of 0.23 and 0.34%, respectively.14 A PCE of 1.42%
has been achieved by oligomer FPTTPA as an efficient electron
donor in bulk heterojunction organic solar cells.15 A series of
oligomers constituting alternating thiophene and fluorenone
b Department of Physics, School of Physical Sciences, North Maharashtra University,
Jalgaon – 425 001, M.S., India
c Nano Material and Device Laboratory, Department of Applied Physics,
Visvesvaraya National Institute of Technology, Nagpur – 440010, M.S., India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj02270c residues, designed to obtain efficiency of around 0.7%.16 A new
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family of soluble extended arylacetylene optoelectronic materials 2.2 Synthesis of organic compounds
has been reported with a highest PCE of around 1.3%.17 Also
Scheme 1 presents the synthetic route for the preparation of
compound 1 and compound 2.
a new series of thiophene-based monodispersed molecular
materials has shown how the photovoltaic properties of a bulk
heterojunction device can be controlled by suitably tailoring the
properties of the donor molecule and efficiencies ranging from
0.2 to 1.0%.18 Improved PCE with 3% of cyanine solar cells with
polyaniline anodes has also been reported.19 A novel molecule
(D(CATBTzT)BDT) has been synthesized with a maximum PCE of
3.61%.20 Oligomers containing dithieno[3,2-b:2,3-d]pyrrole and
thieno[2,3-c]pyrrole-4,6-dione units for solution-processed OSCs
with PCEs ranging from 0.3 to 2.2% have been reported.21 Efficien-
cies of 0.62 and 0.64% with improved stability has been achieved
by inserting an ultra-thin layer of gold at the PEDOT:PSS/CuPc
interface in a planar CuPc/C60 based organic solar cell.22
In this work, an attempt has been made to synthesize N,N-
dimethylaniline as a donor and a methine (–CHQCH–) unit
connecting with cyano (–CN) and carboxyl (–COOH) groups as
an acceptor linked with conjugated p-linkers. These materials
are used as active materials on a high surface area TiO2 layer
coated on a fluorine doped tin oxide coated glass substrate and
used to fabricate solar cells. The influence of cholic acid and
dodecylamine in combination with formed materials has been
studied towards the improvement of the device performance.
First, the starting material 2-cyano-3-p-tolybut-2enolic acid
(i) was synthesized from 1.34 mL of 4-methylacetophenone
(10 mmol) and 0.85 g of cyanoacetic acid (10 mmol), 0.192 g
of ammonium acetate (5 mmol) and 1.71 mL of acetic acid
(5 mmol), stirred well for 12 h at 100 1C in toluene. This reaction
mixture was poured in water and extracted in ethyl acetate.
The obtained starting material (i) was purified by hexane:ethyl
acetate (9 : 1). Then 1 g of 2-cyano-3-p-tolybut-2enolic acid (i)
(5 mmol), 0.74 g of 4-(dimethylamino)benzaldehyde (5 mmol)
for compound 1 and 0.88 g of 4-(diethylamino)benzaldehyde
(5 mmol) for compound 2, respectively, with 10% NaOH were
stirred at room temperature for 8–10 h in ethanol. This reaction
mixture was poured in water and neutralized with 1 : 1 HCl for
compound 1 and compound 2. Obtained precipitates were filtered
and washed with distilled water, the yellow coloured compounds
were obtained as crude products. These crude products were
purified by silica-gel column chromatography with ethyl acetate :
toluene (1 : 9) and then purified products were recrystallized
with aqueous ethanol.
2.3 Characterization data (detailed spectra are given in S1,
ESI†)
2-Cyano-3-(p-tolyl)but-2-enoic acid (compound i). Yield:
1.66 g, 83%, white colour.
2. Experimental
2.1 Chemicals and Instrumentations
IR (cmꢀ1): 3097, 2990, 2231, 1695, 1566, 1271, 914.
1H NMR (400 MHz, CDCl3): 2.36 (s, 3H), 2.60 (s, 3H), 7.29
(d, 2H, J = 7.9), 7.42 (d, 2H, J = 8.0).
4-(Dimethylamino)benzaldehyde, cyanoacetic acid, ammonium
acetate, tetra-n-butyl ammonium iodide, and propylene carbonate
were purchased from sd-fine chem limited, India. 4-(Diethylamino)-
benzaldehyde was purchased from Sigma Aldrich and 4-methyl-
acetophenone from Spectrochem chemicals. Chromatographic
separations were carried out on silica gel (60–120 mesh). All
chemicals used were of AR grade.
13C NMR (100 MHz, CDCl3): 20.84, 22.80, 105.05, 116.90,
125.50, 126.87, 127.25, 128.59, 129.03, 129.10, 137.48, 140.05,
163.53, 171.03.
MS: calculated for C12H11NO2 201; found for ([C12H11NO2] +
H) 202.1.
1H-NMR and 13C-NMR spectra were recorded on a Bruker
Advance II spectrometer operating at 400 and 100 MHz, respectively.
Liquid Chromatography-High resolution mass (LC-HR-MS) spectra
were recorded on a 6550 iFunnel QTOF LC-MS/MS Make-Agilent
Technologies 1290 Infinity Binary Pump. The thermogravimetric
analyses (TGA) were carried out on a Perkin Elmer 4000 Instrument
under purified nitrogen gas flow with a 10 1C minꢀ1 heating rate.
Surface roughness and morphology of thin films were characterized
by Field Emission-Scanning Electron Microscopy (FE-SEM) on an
S-4800 instrument from Hitachi, Japan, operated at 10 kV (Kilo Volt)
and elemental analysis was performed by the EDS unit coupled
with the FE-SEM unit. The optical absorption spectra were recorded
(i.e. in between 300 and 800 nm wavelength ranges) using a UV-Vis
spectrophotometer (Shimadzu 2450) with a 1.0 cm quartz cell. Film
thicknesses were measured by using a thickness profiler DEKTAK-
150 profilometer for area 1000 nm. The current density–voltage
( J–V) characteristics of the organic solar cells (OSCs) were measured
under AM 1.5G (100 mW cmꢀ2) illumination which was provided
by a 3A grade solar simulator (Newport, USA, 94043A, calibrated
with a standard crystalline silicon solar cell).
Scheme 1 Synthetic route for (a) compound 1 and (b) compound 2.
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2-Cyano-5-(4-(dimethylamino)phenyl)-3-(4-(4-(dimethylamino)-
Paper
styryl)phenyl)penta-2,4-dienoic acid (compound 1). Yield: 1.72 g,
75.10%, yellow colour.
IR (cmꢀ1): 2886, 2811, 2218, 1654, 1537, 1179, 936, 813.
1H NMR (400 MHz, CDCl3): 2.98 (brs, 6H), 3.04 (brs, 6H),
6.66 (d, 2H, J = 8.8 Hz), 6.69 (d, 2H, J = 9.0 Hz), 7.27 (d, 2H,
J = 8.0 Hz), 7.44 (d, 2H, J = 15.36 Hz), 7.56 (d, 2H, J = 8.8 Hz),
7.64 (d, 2H, J = 15.36 Hz), 7.85 (d, 2H, J = 8.0 Hz), 7.92 (d, 2H,
J = 8.1 Hz), 7.97 (s, 1H).
13C NMR (100 MHz, CDCl3): 21.15, 39.91, 40.12, 93.96,
111.27, 111.49, 115.95, 117.61, 118.50, 121.92, 128.06, 128.91,
130.27, 133.29, 135.76, 142.45, 144.66, 151.66, 153.14, 153.51,
164.84, 188.27.
LC-HR-MS: calculated for C30H29N3O2 463.2279; found for
([C30H29N3O2]–H) 462.2206.
2-Cyano-5-(4-(diethylamino)phenyl)-3-(4-(4-(diethylamino)styryl)-
phenyl)penta-2,4-dienoic acid (compound 2). Yield: 1.8 g, 69.76%,
yellow colour.
IR (cmꢀ1): 3074, 2975, 2222, 1686, 1528, 1176, 935, 814.
1H NMR (400 MHz, CDCl3): 1.18–1.25 (m, 12H), 3.39–3.49
(m, 8H), 6.65–6.69 (m, 4H), 7.28 (d, 4H, J = 16.04 Hz), 7.52
(d, 4H, J = 8.88 Hz), 7.93 (dd, 4H, J = 8.48 & 4.32 Hz), 8.0858 (s, 1H).
13C NMR (100 MHz, CDCl3): 12.16, 43.98, 44.16, 93.18,
110.83, 117.64, 118.0, 127.93, 128.84, 130.52, 133.67, 145.35,
151.06, 153.35, 165.01.
Fig. 1 Schematic illustration of the constructed organic solar cell (OSC).
was composed of 0.5 M tetra-n-propyl ammonium iodide, 0.1 M
I2 in ethylene carbonate/acetonitrile (2 : 8 volumes).31 The apparent
area of OSCs was 0.2 cm2 and performances of OSCs were
studied by J–V measurements.
LC-HR-MS: C34H37N3O2 519.2656; found for ([C34H37N3O2] + H)
520.2713.
3. Results and discussion
2.4 Fabrication of solar cell devices
3.1 Synthetic route of organic compounds
Solar cells were fabricated on fluorine doped tin oxide (FTO) In the recent past, researchers have been concentrating on the
coated glass substrates. The FTO substrates were cleaned development of newer small organic molecules containing
ultrasonically with acetone, toluene, methanol, and isopropyl donor–acceptor moieties linked with p-linkers because such
alcohol subsequently. On the FTO substrate, a layer of TiO2 film small molecules have shown good applications in photovoltaic
with a maximum thickness of 3.5 mm and a roughness of cells. In literature, the synthesis of two D–A–D donors involving
310 nm was coated by the dip coating method using acetone isoindigo (IsI) as a central acceptor block and benzofuran (BF) or
dispersion. For this, TiO2 powder was dispersed in acetone dithienopyrrole (DTP) as side donor units and their preliminary
(4.0% w/v) and dispersed conditions were maintained by sonica- evaluation in organic photovoltaic cells has been reported.32
tion. To this dispersion pre-cleaned FTO substrate was dipped Moreover, four A–D–A type small molecules using 4,4,9,9-
for 10 seconds and removed for drying. This dipping and drying tetrakis(4-hexylphenyl)-indaceno[1,2-b:5,6-b]dithiophene as central
cycle were repeated 10 times. After 10 successive cycles these building blocks, bithiophene or terthiophene as p-bridges,
TiO2 coated FTO films were first dried at 100 1C for 30 min alkyl cyanoacetate or rhodanine as end acceptor groups were
and then annealed at 500 1C for 1 h in air.23 Then different synthesized and investigated as electron donors in solution-
solutions containing different combinations of synthesised processed OSCs;33 furan-bridged thiazolo[5,4-d]thiazole based
organic compounds (10 mM compound 1 and compound 2), D–p–A–p–D type linear chromophores for solution-processed
5 mM cholic acid and 5 mM dodecylamine, were prepared bulk-heterojunction organic solar cells;34 indole and triisopropyl
separately in ethanol and coated on FTO/TiO2 films by a simple phenyl as capping units for a diketopyrrolopyrrole (DPP) acceptor
dipping method at 60 1C for 10 minutes. Cholic acid24–28 central unit: an efficient D–A–D type small molecule for organic
and dodecylamine29,30 were added to organic compounds as solar cells.35
co-adsorbents in order to study their effect on cell efficiency.
Fig. 1 shows the graphical representation of prepared OSCs.
In continuation of our work on the synthesis of organic
molecules36–40 and their application in organic photovoltaic
The typical maximum thickness of the fabricated device layers cells,41–44 herewith we are reporting the synthesis of two small
in the FTO/TiO2/organic compound photoanode was 4.9 mm organic molecules namely 2-cyano-5-(4-(dimethylamino)phenyl)-
along with a roughness of 336 nm. This photoanode was 3-(4-(4-(dimethylamino)styryl)phenyl)penta-2,4-dienoic acid
sandwiched together with platinum coated FTO as the counter (compound 1) and 2-cyano-5-(4-(diethylamino)phenyl)-3-(4-
electrode (cathode). Then the electrolyte was introduced into (4-(diethylamino)styryl)phenyl)penta-2,4-dienoic acid (Compound 2)
the photoanode and cathode by using capillary action, which (the synthesis route is shown in Scheme 1). Initially, the starting
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material 2-cyano-3-p-tolybut-2enolic acid (i) was synthesized by cyanoacetic acid by reflux in toluene using ammonium acetate
the knoevenagel condensation of 4-methylacetophenone with and acetic acid. Then the product (i) was subjected to condensa-
tion with 4-(dimethylamino)benzaldehyde or 4-(diethylamino)-
benzaldehyde with the removal of a water molecule in the
presence of 10% NaOH using ethanol as a solvent to give
compound 1 and compound 2 respectively. The synthesis of
both these compounds has been confirmed by different char-
acterisation techniques (detailed spectra are given in S2, ESI†).
3.2 Thermal stability of organic compounds
The thermal properties of both compounds are investigated by
thermogravimetric analysis (TGA). Both compounds exhibit good
thermal stability with a decomposition temperature (5% weight
loss) at 221 1C and 192 1C under nitrogen (Fig. 2). The high
thermal stability of the compounds prevents the degradation of
the active layer in cells which ultimately results in high stability
of the cells.
Fig. 2 TGA of synthesised organic compounds: (a) compound 1 (b)
compound 2.
3.3 Optical properties
Fig. 3 shows the normalized UV-Vis absorption spectra under
different conditions. The UV-Vis optical spectrum of compound 1
and compound 2 in ethanol without a co-adsorbent shows
absorption peaks at 415 and 424 nm respectively, whereas with
cholic acid the absorption peaks shifted to 452 and 429 nm,
respectively. The addition of cholic acid to both compounds
resulted in a slight shift in the maximum wavelength towards
the more visible range. Moreover the addition of dodecylamine
to both compounds resulted in a shift of the wavelength towards
the UV region. TiO2 coated on the FTO plate shows lower
absorbance in the visible region as compared to absorbance
for OSCs. Thin films of both compounds with cholic acid coated
Fig. 3 Normalized UV-Vis spectrum of (a) compound
1 in ethanol, on FTO/TiO2 showed a broad absorption band throughout the
(b) compound 2 in ethanol, (c) compound 1 with cholic acid (2 : 1) in
ethanol, (d) compound 2 with cholic acid (2 : 1) in ethanol, (e) compound 1
with dodecylamine (2 : 1) in ethanol, (f) compound 2 with dodecylamine
(2 : 1) in ethanol, (g) compound 1 with cholic acid and dodecylamine (2 : 1 : 1)
in ethanol, (h) compound 2 with cholic acid and dodecylamine (2 : 1 : 1)
visible region (400–800 nm). Compared to that in solution, the
absorption spectrum of both films suggests that strong inter-
molecular interaction and aggregation exist in the solid state.
The optical band gap of TiO2 is estimated to be 3.26 eV by the
in ethanol, (i) FTO/TiO2, (j) FTO/TiO2/compound 1 : cholic acid and (k) FTO/ absorption onset at 324 nm of the thin film (the detailed plot is
TiO2/compound 2 : cholic acid.
given in S2, ESI†).
Fig. 4 Cyclic voltammetry of compound 1 and 2 in 1000 ppm tetra-n-butylammonium-hexafluorophosphate (TBAPF6) in acetonitrile solution at a scan
rate of 100 mV sꢀ1
.
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3.4 Electrochemical properties
known oxidation potential of +4.8 eV. The HOMO and LUMO
energy levels and the electrochemical band gaps (Eegc) of both
molecules were calculated from the onset oxidation potentials
(Eoxi) and onset reduction potentials (Ered) according to the
following equations:45,46
In order to determine the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
of the synthesized molecules, cyclic voltammetry (CV) was
employed in on a glassy carbon working electrode in 1000 ppm
tetra-n-butylammonium-hexafluorophosphate (TBAPF6) in aceto-
nitrile solution as the supporting electrolyte at a scan rate of
100 mV sꢀ1 under a nitrogen atmosphere at room temperature.
The potential of the Ag/AgCl reference electrode was calibrated
by using a ferrocene/ferrocenium redox couple which has the
HOMO = (Eoxi ꢀ Eferrocene1/2 + 4.8) eV
LUMO = (Ered ꢀ Eferrocene1/2 + 4.8) eV
Eegc = LUMO ꢀ HOMO
where Eoxi and Ered are the measured onset potentials relative to
Ag/Ag+. The redox potential of the Fc/Fc+ internal reference was
0.40 V vs. SCE.
Table 1 Optical and electrochemical properties of compound 1 and
compound 2
Material
Eoxi (V) Ered (V) HOMO (eV) LUMO (eV) Eegc (eV) Egopt (eV)
The onset oxidation and reduction potential of compound 1 is
1.37 V and ꢀ1.38 V respectively, thus the corresponding HOMO
and LUMO energy level is ꢀ5.77 eV and ꢀ3.02 eV. For compound 2,
onset oxidation and reduction potential of Compound 1 is 1.35 V
and ꢀ1.38 V respectively, and the corresponding HOMO and
Compound 1 1.37
Compound 2 1.35
ꢀ1.38 ꢀ5.77
ꢀ1.38 ꢀ5.75
ꢀ3.02
ꢀ3.02
2.75
2.73
2.76
2.74
Fig. 6 J–V characteristics of the different devices under AM 1.5G illumination
at 100 mW cmꢀ2 for different devices as (a) FTO/TiO2/compound 1:cholic
acid (2: 1)/electrolyte/Pt electrode, (b) FTO/TiO2/compound 2 : cholic acid
(2: 1)/electrolyte/Pt electrode, (c) FTO/TiO2/compound 1/electrolyte/
Pt electrode, (d) FTO/TiO2/compound 2/electrolyte/Pt electrode, (e) FTO/
TiO2/compound 1: dodecylamine (2 : 1)/electrolyte/Pt electrode, (f) FTO/TiO2/
compound 2 : dodecylamine (2 : 1)/electrolyte/Pt electrode, (g) FTO/TiO2/
compound 1: cholic acid : dodecylamine (2 : 1 : 1)/electrolyte/Pt electrode and
(h) FTO/TiO2/compound 2 : cholic acid: dodecylamine (2: 1 : 1)/electrolyte/
Pt electrode.
Fig. 5 FE-SEM images of (a) TiO2 film, (b) compound 1 without cholic acid
coated on TiO2 film, (c) compound 2 without cholic acid coated on TiO2 film,
(d) compound 1 with cholic acid coated on TiO2 film and (e) compound 2
with cholic acid coated on TiO2 film.
Table 2 Summary of device performance for different devices
Max. wavelengtha
(nm)
Voc
(V)
Efficiency
(%)
Device Cell structure
Jsc (mA cmꢀ2
)
FF
a
b
c
d
e
f
FTO/TiO2/compound 1/electrolyte/Pt electrode
FTO/TiO2/compound 2/electrolyte/Pt electrode
415
424
452
429
411
411
408
0.272
0.233
0.439
0.538
0.398
0.272
0.394
0.265
0.029
0.632
1.158
0.178
0.045
0.257
0.0435
0.3194
0.4688
0.4438
0.372
0.2836
0.4452
0.003
0.002
0.130
0.277
0.026
0.003
0.045
FTO/TiO2/compound 1 : cholic acid (2 : 1)/electrolyte/Pt electrode
FTO/TiO2/compound 2 : cholic acid (2 : 1)/electrolyte/Pt electrode
FTO/TiO2/compound 1 : dodecylamine (2 : 1)/electrolyte/Pt electrode
FTO/TiO2/compound 2 : dodecylamine (2 : 1)/electrolyte/Pt electrode
FTO/TiO2/compound 1 : cholic acid : dodecylamine
(2 : 1 : 1)/electrolyte/Pt electrode
g
h
FTO/TiO2/compound 2 : cholic acid : dodecylamine
(2 : 1 : 1)/electrolyte/Pt electrode
389
0.535
0.261
0.4685
0.065
a
Absorption spectra recorded in ethanol.
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Table 3 The influence of the TiO2 thickness
Dipping cycles
Thickness (mm)
Roughness (nm)
Voc (V)
Jsc (mA cmꢀ2
)
FF
Efficiency (%)
10
20
30
3.5
5.9
11.3
310
316
289
0.538
0.716
0.803
1.158
0.892
0.618
0.4438
0.3963
0.4461
0.277
0.253
0.223
LUMO energy level is ꢀ5.75 eV and ꢀ3.02 eV. Band gaps using the device (Table 2) as it has maximum % efficiency as
calculated from the HOMO and LUMO are 2.75 eV and 2.73 eV compared to other devices studied. Table 3 shows that the
for compound 1 and compound 2. respectively, which are consis- open-circuit voltage (Voc) of OSCs increases as the thickness
tent with the optical band gaps (Eogpt) 2.76 eV and 2.74 eV calculated increases, whereas short circuit photocurrent ( Jsc) has shown
from the UV-Vis absorption onset (the detailed plot is given in S3, moderate changes (the detailed J–V curve is given in S6, ESI†).
ESI†). The CV curves of these materials are shown in Fig. 4 and The study revealed that 10 dipping cycles has maximum cell
the electrochemical energy levels are summarized in Table 1. The efficiency as compared to 20 and 30 cycles (Table 3). Therefore,
HOMO and LUMO energy levels of the two synthesised organic 10 dipping cycles were used for further study.
molecules showed that they were suitable as donors in organic
solar cells with TiO2 as the acceptor.
4. Conclusions
3.5 Surface characterization
In conclusion, we have developed and characterized two small
FE-SEM was utilized to study the morphology of TiO2 and TiO2/
organic compounds as novel electron transporting chromophores
organic compound films. For this purpose, active layers were
towards photovoltaic applications. The study of the bilayer hetero-
junction organic solar cells with the incorporation of two N,N-
dimethyl/ethylaniline as a donor groups and a methine unit
coated on glass plates by using the fabrication process described
in the ‘Fabrication of solar cell devices’ section and the morphology
is shown in Fig. 5 (detailed reports are given in S4, ESI†).
connecting with cyano and carboxyl as accepting groups and
FE-SEM images revealed that the addition of cholic acid in
titanium dioxide as an acceptor material showed an efficiency
organic compounds increases the smoothness and homogeneity
up to 0.277% with an open circuit voltage of 0.538 V. The study of
of the surface over TiO2 particles showing an amorphous pattern
co-adsorbents revealed that the presence of cholic acid increases
which minimizes the chance of void formation in OSCs and
cell efficiency, whereas dodecylamine has shown adverse effects
ultimately results in moderate efficiency as compared to organic
on cell efficiency.
compounds without cholic acid. EDAX analysis (detailed spectra
are given in S5, ESI†) shows the elemental composition of respective
films and EDAX spectra confirm the presence of respective elements
in films.
Acknowledgements
One of the authors (PPK) acknowledges UGC, New Delhi, for
SAP (DSA-I) fellowship under the scheme ‘Research Fellowship
in Sciences for Meritorious Students’.
3.6 Photovoltaic properties
3.6.1 Effect of the co-adsorbent. OSCs are prepared with a
FTO/TiO2/compound/electrolyte/platinum device structure, and
tested under simulated AM 1.5G (100 mW cmꢀ2) illumination.
Different compositions of photoactive layers (sensitizer) are
used to optimize the device performance and their current
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