Organic Electronics 13 (2012) 3108–3117

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Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Effect of linker used in D–A–p–A metal free dyes with differentp-spacers for dye sensitized solar cells

Surya Prakash Singh b,⇑, M.S. Roy c, Anup Thomas b, K. Bhanuprakash b, G.D. Sharma a,⇑a R&D Center for Science and Engineering, Jaipur Engineering College, Kukas, Jaipur 303101, Rajasthan, Indiab Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500607, Indiac Defence Laboratory, Jodhpur 342011, Rajasthan, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 January 2012Received in revised form 5 March 2012Accepted 9 May 2012Available online 7 July 2012

Keywords:Dye sensitized solar cellsDiketo-pyrrolo-pyrroleMetal free dyeThiophene and phenyl p-conjugated bridgesElectrochemical impedance spectroscopy

1566-1199/$ - see front matter � 2012 Elsevier B.Vhttp://dx.doi.org/10.1016/j.orgel.2012.05.013

⇑ Corresponding authors.E-mail addresses: spsingh@iict.res.in (S.P. Sin

yahoo.com (G.D. Sharma).

Two novel D–A–p–A metal free dyes with triphenylamine as donor, dithiophene-diketo-pyrrolo-pyrrole as acceptor unit, thiophene and phenyl p-conjugated bridges and a cyano-acetic acid as electron acceptor (TDPP1 and TDPP2 were denoted for thiophene and phenylp-conjugated bridge, respectively) have been designed and used as sensitizers for DSSCs.Incorporation of dithiophene-diketo-pyrrolo-pyrrole, reduces the band gap significantly.The influence of p-conjugated bridge on optical and electrochemical properties were inves-tigated. Results demonstrated that the absorption band of TDPP with thiophene p-conju-gated bridge has red shifted due to the enhancement of electron donating ability of p-conjugated bridge. The DSSC based on TDPP1 shows prominent power conversion effi-ciency about 4.81%, which is higher that for TDPP2 (3.42%). The electrochemical impedancespectroscopy analysis reveal that the charge recombination resistance at the TiO2/dye/elec-trolyte interface for the DSSC based on TDPP1 is higher than that for TDPP2, whichimproves both Jsc and Voc. The PCE of the DSSC based on TDPP1 is further improved upto 6.34%, when deoxycholic acid (DCA) was employed as coadsorbant.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Dye sensitized solar cells (DSSCs) based on nano-porousTiO2 electrode sensitized to visible light with dye sensitiz-ers have been extensively studied because of their highpower conversion efficiency, low cost of production and re-garded as low cost next generation solar cells. Great pro-gresses have been made in their performance andstability since the pioneering work of O’Regan and Grätzelin 1991 [1]. The performances of DSSCs are mainly affectedby the photosensitizers, photoanode, Counter electrodeand electrolytes [2]. Among the factors that affect drasti-cally power conversion efficiency (PCE) of the DSSC, thedyes play a key role for achieving the high PCE and havebeen intensively studied. Until now, DSSCs based on

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gh), sharamagd_in@

ruthenium polypyridyl complex photosensitizers such asN3, N719 and the black dyes have shown a record PCE al-most 12% [3]. However, the large scale application ruthe-nium complexes have become a critical problem due tolimited resources and difficult steps for purification.

Recently, metal-free organic dyes have attractedincreasing attention due to their high structural flexibility,high molar extinction coefficient, low toxicity, and envi-ronmental friendliness. DSSCs employing organic dyes fea-turing an electron donor and acceptor moiety connected bya p-conjugation bridge [4] have reached about 10% PCEwith liquid electrolytes [5]. Generally, the highest occupiedmolecular orbital (HOMO) and lowest unoccupied molecu-lar orbital (LUMO) energy levels of the sensitizers used inDSSCs should match the corresponding energy levels ofI�3 =I� redox potential and conduction band edge of TiO2

electrode to ensure dye regeneration and efficient electroninjection [6]. The HOMO–LUMO band gap corresponds tothe optical wavelength. The HOMO level relates to the

N

N

N

O

OS

SAr

NCCOOH

TDPP1 TDPP2

SAr=

Scheme 1. Chemical structures of TDPP1 and TDPP2.

S.P. Singh et al. / Organic Electronics 13 (2012) 3108–3117 3109

donor and p-spacer parts and LUMO level depends on theacceptor parts and can be easily modified to tune theirproperties for device performance optimization. One ofthe main factors limiting the performance of DSSCs withorganic dyes is the formation of dye aggregates on the sur-face of the semiconductor oxide, leading to quenching ofthe excited state of the dye molecules and lowering theoverall performance of the device [7]. In addition, efficientlight harvesting as well as inhibition of intermolecular p–paggregation and charge recombination can be realizedthrough effective molecular design in the sensitizer struc-tures [6a]. After many extensive studies on molecularmodification based on the donor–p–acceptor configura-tion, effective strategies for fine tuning of energy levels ofdyes to better match of solar spectrum are now developedand aggregation of the adsorbed dyes could be preventedby introducing bulky non-planer structures [8]. As a conse-quence to practical and commercial applications, there isstill much scope for molecular engineering to achieve high-er PCE as well as longer lifetimes for stability.

It has been reported that extending the p-conjugatedbonding bridges in the metal free organic dyes, effectivelyred shifted the absorption response. Tian and co-workershas reported a series of donor–donor–p–acceptor (D–D–p–A) structural organic dyes incorporating several donorgroups into the triphenylamine framework with starburstconfiguration, resulting in a redshift in absorption and sup-pression of charge recombination with electrolyte [9].However, the synthesis of these types of organic dyes iscomplicated and the dye adsorbed on the TiO2 surface islow.

Here we present introduction of diketo-pyrrolo-pyrrole(DPP) unit as acceptor in copolymer and small moleculeshave been widely utilized in organic solar cells [10]. In no-vel D–A–p–A based dyes, additional acceptor unit is ex-pected to function as an electron trap where electronseparation occurs, and facilitating the electron migrationdirection to anchoring unit. The TPA can improve the holetransporting ability of the material and the nonplanarstructure of the TPA also prevents the formation of aggre-gates. The incorporation of low band gap and strongaccepting unit of diketo-pyrrolo-pyrrole (DPP) adjacent toTPA donor, red shifts the absorption band towards thelonger wavelength region, which is beneficial for light har-vesting property of the dye.

Based on above consideration, we designed two novelmetal free dyes (TDPP1 and TDPP2) having donor–accep-tor–p–acceptor (D–A–p–A) configuration (chemical struc-ture shown in Scheme 1) with triphenylamine (TPA) asthe donor, DPP as acceptor and cyanoacrylic acid bothas acceptor and anchoring group. The effects of the p-con-jugated bridges on the photophysical, electrochemical andphotovoltaic properties of these dyes were investigated indetail. We found that the photoelectronic properties,especially power conversion efficiency (PCE) is quite sen-sitive to the structural variations of p-bridge. We haveachieved over all PCE about of 4.81% and 3.42% for theDSSCs based on TDPP1 and TDPP2, respectively. ThePCE of DSSC based on TDPP1 has been further improvedup to 6.34% when DCA coadsorbant is used in the dyesolution.

2. Experimental details

TDPP1 and TDDP2 dyes were synthesized as the routedescribed in literature [11,12] and detail in given in thesupplementary information.

2.1. Electrochemical measurements

The oxidation potentials of the dyes adsorbed on TiO2

films were measured in a three electrode electrochemicalcell. TiO2 films adsorbed with dye were used as workingelectrode, a platinum wire was used as counter electrodeand a Ag/AgCl electrode was used a reference electrode.The measurements were performed using a potentiostat/galvanostat. The supporting electrolyte was 0.1 M TBAPF6

(tetra-n-butylammonium hexafluorophosphate) in aceto-nitrile as the solvent.

2.2. Computational details

The ground state geometries for all the molecules in thisstudy were obtained at DFT/B3LYP/6-31G(d,p) level. Fre-quencies were evaluated at the same level of theory forthe optimized geometries to ensure the structure obtainedwas a minimum on the potential energy surface. All possi-ble conformers were optimized and only the relatively sta-ble one is reported here (see Supporting information). Thegeometric parameters for ground state are also listed in thesupporting information.

To get a deeper insight into the absorption spectrum,excitation energies were calculated using time dependent(TD) DFT under RPA approximation [13]. The basis set6-31+G(d,p) was used for TDDFT calculations. Dichloro-methane solvent effects were also included in the TDDFTcalculations using the PCM solvation model. We estimatedthe excitation energies by carrying out single point TDDFTcalculations using both the B3LYP and M06-2X DFT func-tional. We report only the M06-2X TDDFT results as theB3LYP functional yielded results which did not match theexperiment (shown in Supporting information) [13]. Allthe calculations performed here were obtained using theGAUSSIAN 09 software [14].

3110 S.P. Singh et al. / Organic Electronics 13 (2012) 3108–3117

2.3. Preparation of DSSCs

The dye sensitized TiO2 electrode was prepared asfollow:

TiO2 paste was prepared by mixing 1 g of TiO2 powder(P25, Degussa), 0.2 mL of acetic acid, 1 mL of water. Then60 mL of ethanol was slowly added while sonicating themixture for 3 h. Finally, Triton X-100 was added and a welldispersed colloidal paste was obtained (TiO2). The wholeprocedure is slow under vigorous stirring. The mixturewas stirred vigorously for 2–4 h at room temperature andthen stirred for 4 h at 100 �C to form a transparent colloidalpaste. The TiO2 paste was deposited on the F-doped tinoxide (FTO) coated glass substrates by the doctor bladetechnique. The TiO2 coated FTO films were sintered at450 �C for 30 min. Before immersing the TiO2 photoanodesinto dye solution, these films were soaked in the 0.2 Maqueous TiCl4 solution for 12 h and washed with deionizedwater and fully rinsed with ethanol, the films were againheated at temperature of 450 �C, followed by cooling at50 �C and dipping into 5 � 10�4 M solution of dyes inTHF for 12 h at room temperature. The thickness of theTiO2 photoanode is about 15 lm. For coadsorption deoxy-cholic acid (DCA) was added in the dye solution.

3. Results and discussion

3.1. Synthesis of dyes

Synthesis of the both dyes were shown in Scheme 2.

3.1.1. Optical and electrochemical propertiesThe normalized absorption spectra of the TDPP1 and

TDDP2 dyes in diluted solution of dichloromethane(DCM) solution and adsorbed on TiO2 films are shown inFig. 1a and b, respectively. As can be seen from these fig-ures that both dyes exhibits two major prominent bands,appearing at 410–420 nm and 610–640 nm respectively.The bands around 410–420 was due to the p–p⁄ transitionof diketo-pyrrolo-pyrrole (DPP) conjugated molecule withp-spacer. The bands observed in higher wavelength regioncan be attributed to intramolecular charge transfer (ICT)between the TPA donor and acceptor, in which DPP showsstrong visible band having absorption maxima around 636and 610 nm for TDPP1 and TDPP2, respectively, owing tothe introduction of DPP group in TDPP2 and TDPP1,respectively, which could favor the light harvesting inDSSC. The absorption peak of TDPP1 was largely shiftedto a longer wavelength than TDPP2, because of the thio-phene instead of phenyl in the p-conjugated linker part.The absorption band of both dyes in longer wavelength re-gion is red shifted to the DPP based dyes, reported recentlyby Qu et al. [15]. This can be attributed to the enhanced ICTdue to the presence of thiophene unit in the TDPP1 dye[16]. The red-shifted phenomena of absorption peak inthe visible region can be ascribed to the different electrondonating ability of the unit used in the p-bridge in D–A–p–A structure. The thiophene bridge has more strong donat-ing ability than phenyl bridge, which enhances the ICTbetween the donating and acceptor units. Compared to

the absorption spectra in DCM solution, the absorptionspectra of the two dyes adsorbed to the TiO2 film werebroadened significantly and nearly 35 nm red shift of themaxima absorption peak was observed, which can beattributed to the formation of a J type aggregate or elec-tronic coupling of dyes on TiO2 surface. The molar extinc-tion coefficient of both dye adsorbed on the TiO2 isalmost same. The interaction between the carboxylategroup and surface of Ti4+ ions may lead to increased delo-calization of the p⁄ orbital of the conjugated frame work.The energy of the p⁄ energy level is decreased by the delo-calization, which is responsible for red shift in the absorp-tion band in longer wavelength region.

To judge the feasibility of electron transfer from excitedstate (LUMO) of the dye into the conduction band of TiO2,the redox potentials of these two dyes were investigatedby cyclic voltammetry (CV). The oxidation potential of bothTDPP1 and TDPP2 was measured to be 0.80 V vs NHE. Theidentical value of HOMO level of both DPP indicates thatthe HOMO energy level is exclusively determined by donorunit in the D–A–p–A dye [17]. The reduction potential forTDPP1 and TDPP2, calculated from the difference of oxida-tion potential and Eo-o determined from the absorption on-set are �1.06 V and �0.95 V vs NHE, respectively. TheHOMO levels of these dyes are much more positive thanI�3 =I� redox potential (�0.4 V), ensuring that there is en-ough driving force for the efficient dye regenerationthrough the recapture of injected electrons from I� bythe radical ions. The excited state of these dyes i.e. LUMOare much negative than conduction band edge of TiO2

(�0.5 V vs NHE), indicating that the electron injection pro-cess from the excited dye molecule to conduction band isenergetically favorable. The difference in the energy differ-ence between the reduction potential i.e. LUMO level andthe conduction band edge of TiO2 is more for the TDPP1than that for TDPP2 indicates enhanced electron injectionefficiency for DSSC based of TDPP1.

To get a deeper understanding of the above discussedoptical properties obtained from the experiment, resultsof the computational studies are analyzed. The TDDFTcomputed vertical excitation energies (in nm) the oscilla-tor strength and the CI coefficients of major transitionsusing M06-2x functional and 6-31+G(d,p) are listed in Ta-ble 1. There is good agreement with the experimentally ob-served values. The long wave-length intense transition isdominated by the HOMO–LUMO transition (77% for both).Minor contributions from other excitations are also seen.In order to understand the nature of transitions we haveplotted the molecular orbitals in Fig. 2. From the figure itis clear the singlet–singlet transition is dominated in bothmolecules by charge transfer from the donor side groups tothe cyanoacrylic acceptor part. The B3LYP/6-31+G(d,p)//6-31G(d,p) calculated HOMO, LUMO values (Fig. 2) are also inagreement with the trend observed for the oxidationpotentials obtained from cyclic voltammetry.

3.1.2. Photovoltaic properties of DSSCsFig. 3 shows the incident monochromatic photon-to-

current conversion efficiency (IPCE) for the DSSCs basedon TDPP1 and TDDP2. The DSSCs based on these dyesexhibit high and broad IPCE values in the range of

N

BOH

OH

N

N

O

OS

S

Br

Br

N

N

N

O

OS

SAr

NCCOOH

NC COOH

TDPP1

TDPP2

S

S

B ArHO

HOCHO

N

N

N

O

OS

SAr CHO

N

N

N

O

OS

SBr

+Pd(PPh3)4, Na2CO3

THF, H2O

Pd(PPh3)4, Na2CO3

THF, H2O

piperidine, THF

Ar=

Ar=

Ar=

1 23

4a, 4b

Scheme 2. Synthesis of TDPP1 and TDPP2 metal free dyes.

S.P. Singh et al. / Organic Electronics 13 (2012) 3108–3117 3111

520–680 nm and with highest value of 72% at 662 nm and63% at 638 nm for TDPP1 and TDPP2, respectively. TheIPCE spectra of DSSCs are in good agreement with the opti-cal absorption spectra dyed TiO2 film. The IPCE value at anywavelength is expressed as

IPCEðkÞ ¼ LHEðkÞx/injxgc ¼ LHEðkÞx/ðkÞET

where LHE(k) is the light harvesting efficiency of the pho-toanode, /inj is the electron injection efficiency from theexcited state of dye into the conduction band of TiO2, gc

is the efficiency of injected charge collection at the backcontact, and /(k)ET is defined as an electron transfer yield,which is the product of the electron injection efficiencyand charge collection efficiency. The LHE depends on theamount of dye adsorbed on the surface of photoanodeand absorption band of sensitizer. We have measured theamount of dye adsorbed on the photoanode and found thatit is almost same for both dyes. The molar extinction coef-ficient of the dye adsorbed on the TiO2 surface is same asshown in the optical absorption spectra (Fig. 1a and b).Therefore, we consider the similar value of LHE for bothdyes, higher values of IPCE values of the DSSCs sensitizedby the dye with thiophene as p-spacer means that theDSSC sensitized by dye thiophene linker have higher elec-tron transfer yield /(k)ET than the dye with phenyl. There-fore, the introduction of thiophene linker to DPP based dyestructure i.e. TDPP1 has a positive effect on IPCE.

Fig. 4 shows the current–voltage (J–V) characteristics ofDSSCs based on TDPP1 and TDPP2 under illuminationintensity of 100 mW/cm2. Table 2 summaries the short cir-cuit current (Jsc), open circuit voltage (Voc), fill factor (FF)and over all power conversion efficiency (PCE) of theDSSCs. The DSSC sensitized with TDPP1 gave Jsc of10.4 mA/cm2, Voc of 0.68 V and a FF of 0.68, correspondingto overall PCE of 4.81%. Under same conditions, the TDPP2sensitized DSSC gave a Jsc of 8.8 mA/cm2, Voc of 0.64 V andFF of 0.60, corresponding to an overall PCE of 3.42%. TheDSSC based on TDPP1 dye has the higher PCE than TDPP2due to the higher electron injection efficiency as evidencedby the higher driving force for electron injection. We canalso see from Fig. 4 and Table 2 that the TDPP1 has thehigher Voc than TDPP2, this can be ascribed to less aggrega-tion occurring for TDPP1 by the decrease of co-planaritybetween DPP moiety and thiophene spacer. Compared toTDPP2, TDPP1 based DSSC has higher Jsc, indicating thatthiophene may have even better charge carrier mobilitythan phenyl moiety.

Electrochemical impedance spectroscopy (EIS) analysis[18] was performed to study the interfacial charge transferprocesses in the DSSC based on different dyes. Fig. 5showed the electrochemical spectra for the DSSCs basedon TDPP1 and TDPP2 under forward bias of 0.65 V in thedark. The Nyquist plots (Fig. 5a) showed two semicircleslocated in the high and middle frequency regions. Another

Fig. 1. Optical absorption spectra of (a) TDPP1 and (b) TDPP2 in DCM solution and absorbed on TiO2 film.

Table 1Computed vertical transitions, the oscillator strength and the CI coefficients of major transitions. a

kmax

(nm)Oscillatorstrength (f)

CI

TDPP1 635 2.093 HOMO ? LUMO (0.62) HOMO � 1 ? LUMO (�0.24) HOMO ? LUMO + 1 (0.18) HOMO � 2 ? LUMO + 1(0.10)

423 0.140 HOMO � 1 ? LUMO (0.39) HOMO ? LUMO + 1 (�0.37) HOMO � 1 ? LUMO + 1 (0.36) HOMO ? LUMO(0.25)

381 0.728 HOMO � 2 ? LUMO (0.58) HOMO ? LUMO + 1 (�0.21) HOMO�3 ? LUMO (�0.17) HOMO ? LUMO + 2(0.15) HOMO ? LUMO (0.12)

TDPP2 605 2.064 HOMO ? LUMO (0.62) HOMO � 1 ? LUMO (0.24) HOMO ? LUMO + 1 (�0.22)408 0.152 HOMO � 1 ? LUMO (0.47) HOMO � 1 ? LUMO + 1 (0.34) HOMO ? LUMO (0.26) HOMO � 1 ? LUMO

(�0.25)356 0.495 HOMO � 5 ? LUMO (0.43) HOMO�2 ? LUMO (�0.38) HOMO � 5 ? LUMO + 1 (�0.25) HOMO � 3 ? LUMO

(�0.18)350 0.645 HOMO � 5 ? LUMO (0.39) HOMO � 2 ? LUMO (0.33) HOMO � 3 ? LUMO (0.23) HOMO � 5 ? LUMO + 1

(�0.22)

a Calculated at M06-2x/6-31+G(d,p)//B3lyp/6-31G(d,p).

3112 S.P. Singh et al. / Organic Electronics 13 (2012) 3108–3117

small semicircle, which should have appeared at the lowfrequency region, is overlapped by the middle frequencylarge semicircle. The large semicircle in the Nyquist plotslocated in middle is attributed to the dark reaction imped-ance caused by charge transportation at the TiO2/dye/elec-trolyte interface, and other two small semicircles locatedin the low and high frequency regions are assigned to thecharge transfer at counter electrode and diffusion of I�3 ionsin the electrolyte, respectively [18c,19]. The charge recom-bination resistance (Rrec) at the TiO2 surface can be de-duced by fitting curves from the range of middle

frequency using a Z-view software. Rrec is related to thecharge recombination rate between injected electron andI�3 ion in electrolyte, estimating by the large semicirclewidth. A large Rrec means the small charge recombinationrate and vice versa. The Rrec values for the DSSC based onTDPP1 and TDPP2 were estimated to be 115 and 94 ohm,respectively. The higher value of Rrec for DSSC based onTDPP1 as compared to that for TDPP2, indicating thatthe electron recombination resistance augments fromTDPP2 to TDPP1. The electron lifetime values derived fromthe curve fitting are 13.6 and 18.9 ms for TDPP2 and

Fig. 2. The pictorial representations of molecular orbital plots along with the energy Eigen values in eV obtained at B3LYP/6-31+G(d,p)//6-31G(d,p).

Fig. 3. IPCE spectra of DSSCs sensitized with TDPP1 and TDPP2 dyes.

S.P. Singh et al. / Organic Electronics 13 (2012) 3108–3117 3113

TDPP1 sensitized DSSC, respectively. The higher electronlifetime observed with TDPP1 sensitized DSSC indicatedthe back reaction of the injection electron with I�3 in theelectrolyte is effectively suppressed. This is also reflectedfrom the improvement in the Voc, yielding substantially en-hanced PCE of DSSC. We can conclude that the sensitizerwith thiophene p-spacer may be more efficient for sup-pression of the back reaction of the injected electron withI�3 ions in the electrolyte. The Bode plots as shown in Fig. 5b

also support the difference in the electron lifetime forDSSCs based on TDPP1 and TDPP2. The two peaks in figurelocated at the high frequency (right) and middle frequency(left) respectively corresponds to the small semicircle (left)and large semicircle (right) in the Nyquist plots (Fig. 5a).The reciprocal of the peak frequency for the middle fre-quency peak is regarded as the electron lifetime since itrepresents the charge transfer process at the TiO2/dye/electrolyte interface. It is evident that the electron life-

Fig. 4. Current–voltage (J–V) characteristics of DSSCs based on TDPP1, TDPP2 and TDPP1 with DCA coadsorbant.

Table 2Photovoltaic data of DSSCs based on TDPP1 and TDPP2 dyes.

Dye Jsc (mA/cm2) Voc (V) FF PCE (%)

TDPP1a 10.4 0.68 0.68 4.81TDPP2a 8.9 0.64 0.60 3.42TDPP1b 12.14 0.73 0.72 6.34

a Dye bath: DCM solution (2 � 10�4 M).b Dye bath: DCM solution (2 � 10�4 M) with saturated DCA as

coadsorbant.

3114 S.P. Singh et al. / Organic Electronics 13 (2012) 3108–3117

times of the DSSC based on TDPP1 is larger than that basedon TDPP2, thereby explaining the higher value of Voc forTDPP1 based DSSC.

The PCE of the DSSC based on these two DPP dyes is stilllow in comparison of that for Ru based dyes and other me-tal free dyes. In DSSCs, upon light absorption, initial chargeseparation occurs in the adsorbed dye molecules and elec-trons are injected from their excited states into the con-duction band (CB) of the semiconductor. The formedoxidized dye molecules are regenerated to their originalneutral states by the donor (i.e. I� ions) in the electrolyte.After flowing to the counter electrode through an externalcircuit, the electrons are accepted by the acceptor (i.e. I�3ions) in the electrolyte, which reduced back to I� ions.However, upon visible light absorption by the adsorbeddye molecules excited electrons are not only injected intothe trap states and/or recombine with the oxidized dyemolecules. Moreover, it is possible for the free electronson semiconductor surface to recombine with acceptorssuch as I�3 ions. Such charge recombination leads to lossesin both Jsc and Voc, resulting in decrease in overall PCE. Toreduce the possible charge recombination pathways occur-ring at the semiconductor/dye/electrolyte interface, sev-eral kinds of additives have been introduced to co-adsorbonto the semiconductor surface to improve the photovol-taic performance of DSSCs. Coadsorption of deoxycholicacid (DCA) with dye molecules, which suffer from aggrega-tion has been proven to improve the PCE of DSSCs, signifi-cantly [20]. Since the DSSC based TDPP1 dye exhibits

higher PCE value than that for TDPP2, we focus the effectof coadsorbant only for the DSSC based on TDPP1.

The optical absorption spectra of the DCA-TDPP1 ad-sorbed on TiO2 film is shown in Fig. 6 (for comparisonwe have also included the absorption spectra withoutDCA). After the addition of DCA the maximum absorptionpeak is shifted towards lower wavelength region (blueshift) and the maximum absorbance at maximum absorp-tion peak also reduces. We have measured the adsorbedamount of TDPP1 dye on the TiO2 electrodes with andwithout DCA. We have observed that when TiO2 film wasexposed to TDPP1 dye solution containing DCA, the dyeadsorption was reduced. The drop of TDPP1 dye adsorp-tion indicates that DCA competes for TiO2 surface siteswith the TDPP1 dye molecules. Strong interaction betweenthe adsorbed dye molecules and the oxide molecules in theTiO2 surface leads to aggregate formation and conse-quently broadening the absorption spectra was observedduring the dye absorption. We have also observed thatthe peak position in the absorption spectra of DCA-TDPP1in solution and DCA-TDPP1 adsorbed on TiO2 film does notchange. This is because protons and Ti4+ have similar elec-tron withdrawing ability.

The IPCE spectra of the DSSCs based on TDPP1 dye withand without DCA coadsorbant are shown in Fig. 7. It can beseen from Fig. 7 that the IPCE value has been improved,upon coadsorption with DCA. The enhancement in IPCE va-lue upon the DCA coadsorbant is attributed to the break upof dye aggregates. The J–V characteristics of the DSSC basedon TDPP1 with DCA coadsorbant is also shown in Fig. 4 andthe photovoltaic parameters are complied in Table 2. TheDSSC with DCA-TDPP1 dyes exhibits Jsc of 12.14 mA/cm2,Voc of 0.73 V and FF of 0.72, yielding an overall PCE of6.34%. The increased Jsc is due to the higher IPCE valuecaused by the higher injection efficiency resulting fromthe relatively independent dye molecules arraying onTiO2 surface. The increased Voc upon co-adsorption is dueto the retarded charge recombination as evidenced bythe dark current onset potential shift from the 0.50 V to0.56 V (Fig. 8).

Fig. 5. EIS (a) Nyquist plots and (b) Bode plots for DSSCs based on TDPP1 and TDPP2 dyes measured in dark under �0.65 V bias.

Fig. 6. Optical absorption spectra of with and without DCA coadsorbant TDPP1 adsorbed on TiO2 film.

S.P. Singh et al. / Organic Electronics 13 (2012) 3108–3117 3115

We have also measured the middle frequency peak inthe bode plot for the DSSC based on TDPP1 with DCAshifted to lower frequency relative to the TDPP1 devicewithout DCA, indicating the longer electron lifetime forformer species. We consider the addition of DCA will causethe dye forming a compact and monomolecular adsorptionof TiO2 surface due to the bulky DCA molecule, and thussuppressed the further back reaction.

4. Conclusions

In conclusions, we have used two metal free D–A–p–Aorganic dyes TDPP1 and TDPP2 containing a thiopheneand phenyl p-linker, respectively for the fabrication ofdye sensitized solar cells. The introduction of thiophenelinker shifts the absorption band towards the longer wave-length and also decreases the optical band gap. The DSSC

Fig. 7. IPCE spectra of DSSCs based on TDPP1 dye with and without DCA coadsorbant.

Fig. 8. Current–voltage characteristics of DSSCs based on TDPP1 with and without DCA coadsorbant, in dark.

3116 S.P. Singh et al. / Organic Electronics 13 (2012) 3108–3117

based on TDPP1 dye exhibits PCE of 4.81% which is higherthan that for DSSC based TDPP2 (3.42%). The higher valueof PCE for the DSSC based on TDPP1 dye has been attrib-uted to the higher charge recombination resistance in darkand longer lifetime as compared to that for TDPP2 dye. ThePCE has been further improved up to 6.34% for DSSC basedon TDPP1 when DCA is incorporation in the dye solution.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.orgel.2012.05.013.

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