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Solar Energy Materials & Solar Cells 92 (2008) 1043– 1046

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Solar Energy Materials & Solar Cells

0927-02

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Photovoltaic effect in single-layer organic solar cell devices fabricated withtwo new imidazolin-5-one molecules

Vibhor Jain a,b, Basanta Kumar Rajbongshi c, Arun Tej Mallajosyula a,b, Gitalee Bhattacharjya c, SSundar Kumar Iyer a,b,�, Gurunath Ramanathan c

a Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, Indiab Samtel Centre for Display Technologies, Indian Institute of Technology Kanpur, Kanpur 208016, Indiac Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India

a r t i c l e i n f o

Article history:

Received 6 August 2007

Received in revised form

9 February 2008

Accepted 28 February 2008Available online 29 April 2008

Keywords:

Imidazolin-5-ones

Organic solar cells

Green fluorescent proteins

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.solmat.2008.02.039

esponding author at: Department of Ele

e of Technology Kanpur, Kanpur 208016, In

512 259 0063.

ail address: sskiyer@iitk.ac.in (S.S. Kumar Iyer

a b s t r a c t

Organic solar cells were fabricated with two new imidazolin-5-one molecules as active layers. The use

of imidazolin-5-ones, derivatives of a biomolecule chromophore, for photovoltaic applications is

particularly attractive due to its biodegradable nature and tunable properties. Single-layer devices with

two analogues of imidazolin-5-ones were prepared and characterized. Devices fabricated with one of

the molecules as the active layer showed a maximum Jsc of 0.52 mA cm�2 and Voc of 0.68 V at an incident

power of 20.32 mW cm�2, while the other set of devices showed a maximum Jsc of 0.63 mA cm�2 and Voc

of 0.57 V at the same incident power.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Organic solar cells (OSCs) have undergone a rapid growth inthe past 20 years ever since the development of a two-layerorganic photovoltaic cell by Tang in 1985 [1]. Availability ofinexpensive and varied raw materials accompanied by an easyfabrication procedure and the ability to tune molecular propertieshas made organic photovoltaic an attractive proposition [2–4].However, OSCs still suffer from problems such as low efficiency,poor reliability and instability. Hence the quest for a material,which will solve these problems, is still an area of active researchtoday [5,6].

Imidazolin-5-one is the main chromophore, which is respon-sible for the high fluorescence property of green fluorescentproteins (GFPs) [7,8]. In the protein the chromophore, 4-(p-hydroxybenzylidene) imidazolin-5-one, is attached to the peptidebackbone through 1 and 2 positions of the ring [9]. It is formed viaa post-translational internal cyclization of the Ser65-Ty66-Gly67

tripeptide followed by 1,2-dehydrogenation of tyrosine [10]. Thesubstituents attached to imidazolin-5-one ring can be varied tomodulate the optical and electrical properties of the molecule thatcan thus be engineered for absorption in a desired frequency

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).

range [11]. This ability to tune imidazolin-5-ones provides thedesigner with flexibility to alter the molecular properties as perhis/her desire. In addition, the molecules score over most otherknown materials for solar cell application in terms of theirbiodegradable nature, thus making them environment and userfriendly.

In this paper, characteristics of the two set of devices using asingle layer of imidazolin-5-one molecules as the active layer havebeen discussed. The devices showed photovoltaic effect whenexposed to light and the current density–voltage (J– V) relations ofthe same have been studied.

2. Materials

In our work, we have investigated two molecules as potentialcandidates for solar cell applications. The chemical structure ofthese two molecules is depicted in Fig. 1(a) and their energy banddiagram is given in Fig. 1(b).

The two molecules are

Molecule A: (4Z)-4-(4-methoxybenzylidene)-2-((E)-4-methox-ystyryl)-1-phenyl-1,4-dihydro-5H-imidazolin-5-one.Molecule B: (4Z)-4-(4-N,N-dimethylaminobenzylidene) -1-methyl-2-phenyl-5H-imidazolin-5-one.

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Molecule BMolecule A

Vacuum Level (0 eV)

LUMO (1.99 eV) LUMO (1.81 eV)

HOMO (4.83 eV)

HOMO (5.40 eV)

E G (3.41 eV) E G (3.02 eV)

Molecule A Molecule B

Fig. 1. (a) Molecular structures of the two new imidazolin-5-one molecules

synthesized based on the chromophore of green fluorescent protein; (b) energy

level band diagram of the molecules denoting the HOMO–LUMO levels and the

energy gap drawn with respect to the vacuum level.

Fig. 2. Absorption spectra of the two imidazolin-5-one molecules (Molecules A

and B) in solution and thin film form. The broadening and red shift of absorption

spectrum in thin film form compared to the solution form can be observed.

Transparent Glass Substrate

Active Layer

ITO

Solar Cell DevicesPEDOT:PSS

Al

Fig. 3. Schematic cross-section and top view of the fabricated device structure.

Devices are formed at the intersection of the horizontal anode and vertical cathode

lines. Active layer denotes the imidazolin-5-one molecule layer, which can be a

single layer of Molecule A or B.

V. Jain et al. / Solar Energy Materials & Solar Cells 92 (2008) 1043–10461044

Highest occupied molecular orbital (HOMO) and lowest un-occupied molecular orbital (LUMO) levels were estimated usingthe Gaussian 98 programme [12], which makes use of the time-dependent density function theoretical calculations (TDDFTB3LYP) [13]. Cyclic voltametry (CV) [14] was also performed onthe molecules in dichloromethane solution and HOMO levelenergy was found to be in good agreement with the theoreticalvalues.

The absorption spectra (absorbance) of the materials insolution and thin film form are shown in Fig. 2. All the absorptionpeaks lie in the blue region.

The solutions were prepared by dissolving the compounds inchloroform. The thin films were prepared by heating powders ofthe compound in a metal boat kept inside a high vacuum(4�10�6 mbar pressure) chamber. The vapours of the compoundwere depositing on a glass substrate kept above the boat.

It can be observed from all the figures that the absorptionspectrum in thin film form is broader as compared to that insolution form. Also the absorption peak for thin film is red shiftedin comparison to the solution peak. These observations can beexplained by taking into account the intermolecular interactionsin thin film form, where the molecules are on an average closerwith a wider spread in their spacing [15].

3. Experimental

The structure of the fabricated devices consisted of thefollowing layers (Fig. 3):

Indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS)/imidazolin-5-one (MoleculeA or B)/aluminium.

ITO-coated glass substrates were first patterned using photo-lithography to obtain ITO strips of 3 mm width. The patternedsubstrates were RCA cleaned and dried to remove any moisturecontent. These substrates were further exposed to UV-ozone gasfor 15 min to improve the ITO work function. PEDOT:PSS solution(Baytron) was filtered and then spin-coated onto these substratesat 1000 rpm for 60 s and dried at 120 1C for 1 h. The thickness ofPEDOT:PSS layer was measured to be 120 nm using Alpha Stepper

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V. Jain et al. / Solar Energy Materials & Solar Cells 92 (2008) 1043–1046 1045

System from KLA Tencor. A 20-nm thick layer of imidazolin-5-onewas deposited on these substrates by vacuum evaporation at apressure of 4�10�6 mbar at an average rate of 0.05 nm s�1.Aluminium cathode was then deposited in a separate chamberusing vacuum evaporation at 6�10�6 mbar pressure. The thick-ness of aluminium layer was 200 nm. Deposition was donethrough a shadow mask with cathode metals deposited orthogo-nal to the ITO strips to obtain devices of 3 mm�3 mm dimen-sions. The devices were then encapsulated in a thick glass toinhibit the degradation rate. Encapsulation was done by applyingaraldite epoxy on the edges of the encapsulation glass and thendrying it in nitrogen atmosphere for 5–6 h.

4. Results and discussion

The band diagram of the fabricated device is shown in Fig. 4.The current–voltage characteristics of the devices were measuredusing a Keithley 4200 semiconductor characterization system andcurrent density–voltage (J– V) plots were determined. The deviceswere illuminated under a 150 W quartz tungsten halogen lamp(OSRAM). Both the devices showed photovoltaic effect.

J–V relations in semi-log scale of the single-layer devicefabricated with Molecule A as the active layer under differentconditions of illumination are shown in Fig. 5.

Short-circuit current density (Jsc) varies with incident lightintensity as [16]

JscaPainc (1)

Fig. 5. Current density–voltage (J–V) characteristics of the ITO/PEDOT:PSS/

Molecule A/Al device with araldite encapsulation in dark and under illumination

at different intensities.

ITO (4.8 eV)

Imidazolin-5-one Molecule A or B

HOMO Level

LUMO Level

Al (4.3 eV)

PEDOT (5.0 eV)

Fig. 4. Energy band diagram of the single-layer devices fabricated with ITO as

anode, PEDOT:PSS as buffer layer, Molecule A or B as the active layer and Al as

cathode. For Molecule A, the HUMO and LUMO levels are 5.40 and 1.99 eV, while

for Molecule B they are 4.83 and 1.81 eV, respectively.

where Pinc is the incident power. Generally, a lies between 0.85and 1.0. For devices fabricated using Molecule A, it is low at 0.55.Jsc is observed to vary from 0.24 to 0.63mA cm�2 at an incidentpower of 3.78 and 20.32 mW cm�2, respectively. The open-circuitvoltage (Voc) increases logarithmically at a rate of 273 mV perdecade increase in Pinc from 0.36 (at Pinc ¼ 3.78 mW cm�2) to0.57 V (at Pinc ¼ 20.32 mW cm�2).

The J– V relations in dark and in the presence of light for single-layer devices fabricated with Molecule B are shown in Fig. 6. Forthis device the exponent a as in (1) above, is found to be 0.44,which again is much less than the ideal expected value. Jsc variesfrom 0.24 to 0.52 mA cm�2 for Pinc varying from 3.78 to20.32 mW cm�2, respectively. The Voc increases from 0.54 (forPinc ¼ 3.78 mW cm�2) to 0.68 V (for Pinc ¼ 20.32 mW cm�2). Thiscorresponds to an increase in Voc of 197 mV for every decadeincrease in Pinc.

There is a discontinuity observed in the characteristics fordevice under illumination in Figs. 5 and 6 at V ¼ 0. This is becausethe characteristics for positive and negative bias were measuredafter a time lag. The delay in measurement resulted in somedevice degradation, lowering the current measured for the reversebias under illumination.

The dark characteristics of the devices fabricated withMolecule A were analysed to understand the electrical propertiesof the molecule. Molecule A has a molecular weight of 410 g mol�1

and a density of 1.1 g cm�3. Thus, assuming one HOMO and LUMOstate per molecule, it has the density of states [17] Nv as1.62�1021 cm�3. The barrier for hole injection is very small forthis device as can be observed from the band diagrams and hencewe can safely assume the current to be bulk limited. Thus, thevalues of mobility can be computed using trap-limited currentmodel, which assumes an exponential trap density [18]. Themobility value obtained using this method is greater than2�10�10 cm2 V�1 s�1.

The dark current characteristics for the devices with MoleculeB were also analysed. Estimating Nv as 2.17�1021 cm�3 forMolecule B, the mobility value is estimated to be of the order of10�11 cm2 V�1 s�1.

Weak dependence of Jsc on Pinc has been observed for both thedevices. This could be due to increased recombination rates in thedevice as the intensity of incident light increases. Hence, not all

Fig. 6. Current density–voltage (J–V) characteristics of the ITO/PEDOT:PSS/

Molecule B/Al device with araldite encapsulation in dark and under illumination

at different intensities.

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Table 1Comparison of the solar cell performance parameters for the two devices fabricated

Intensity (mW cm�2) Device with molecule A Device with molecule B

Jsc (mA cm�2) Voc (V) FF Z (�10�4%) Jsc(mA cm�2) Voc (V) FF Z (�10�4%)

3.64 0.24 0.36 0.23 5.19 0.24 0.54 0.23 8.07

5.58 0.31 0.42 0.16 3.44 0.27 0.57 0.23 5.80

9.60 0.41 0.52 0.17 3.52 0.37 0.62 0.22 4.85

20.32 0.63 0.57 0.12 1.95 0.52 0.68 0.20 3.12

V. Jain et al. / Solar Energy Materials & Solar Cells 92 (2008) 1043–10461046

the electron–hole pairs generated by the increased incidentphotons are able to reach the electrodes. This results in a lowerJsc. It is also possible that the built in electric field is not sufficientfor complete charge transportation from the point of chargegeneration to the electrodes. Bimolecular charge transport in thesingle-layer device facilitates carrier recombination, thus loweringthe value of a (1). Recombination rates can be minimised withimproved film and interface morphology. The built-in fields can beimproved with better choice of electrodes or by reducing thethickness of the active layer.

The fill-factor (FF) decreases with intensity for both MoleculeA- and B-based devices. This could again be due to increase inrecombination in the presence of higher carrier densities at higherintensities of light. However, the FF values for the case of MoleculeB are higher compared to those for Molecule A for all intensities.

Due to non-ideal increase of Jsc and fall in FF with intensity, thepower conversion efficiency (Z) of both the type of devicesdecreases with increase in intensity.

A comparison of the two solar cells in terms of theirperformance parameters (Jsc, Voc and FF) is given in Table 1.Although Jsc is higher in the case of Molecule A, devices based onMolecule B show higher Voc and FF, and therefore, higher-powerconversion efficiency.

The power conversion efficiencies for both the single-layerdevices fabricated using the imidazolin-5-one molecules, how-ever, is low compared to that reported for other organic solar cells.Besides the fact that single-layer devices inherently have lowerefficiency, low carrier mobility, poor interface morphology andlack of optimisation of fabrication procedure are the main causefor this.

5. Conclusion

Due to the need for large-area solar cells to meet the growingenergy requirements, it is essential to have biodegradable andenvironment-friendly molecules. The use of imidazolin-5-ones,derivatives of a biomolecule chromophore, for photovoltaicapplications is particularly attractive due to its biodegradablenature. Besides, they have been reported to have anticonvulsantanalgesic properties [19].

The photovoltaic response demonstrated for single-layerorganic solar cell devices with the two molecules of imidazolin-5-one family has shown the potential of these molecules inplaying a role in generating clean and inexpensive electricalpower from the abundant energy of the sun.

Acknowledgements

The authors would like to thank the Indian Space ResearchOrganisation (ISRO) for financial support. V.J. wishes to thank

Prof. B. Mazhari, Dept of Electrical Engineering, for his invaluableguidance. B.K.R. and G.B. thank the Council of Scientific andIndustrial Research (CSIR) for junior and senior research fellow-ship. S.S.K.I. acknowledges financial support from the SwissNational Science Foundation (]20720–109486), Zurich and theDepartment of Science and Technology (DST), New Delhi.

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