Ionics (28) 14:263 267 DOI 1.17/s11581-7-153-4 ORIGINAL PAPER Effect of C 6 on methyl red nd crystl violet dye-doped photovoltic device Ajnt Hldr & Suhsis Mity & N. B. Mnik Received: 25 June 27 /Revised: 29 July 27 /Accepted: 2 August 27 /Pulished online: 23 Octoer 27 # Springer-Verlg 27 Astrct The potentil of dye sensitiztion of orgnic photovoltic devices hs een investigted. The photoelectricl properties of such devices hve een studied. With the help of spin-coting method, single lyer nd doule lyer structures re prepred with the help of oth methyl red nd crystl violet dye t time. Methyl red nd crystl violet dye re dispersed in polyvinyl lcohol used s n inert polymer inder nd polyethylene oxide complexed with LiClO 4 ion slt s solid electrolyte. Ethylene cronte nd propylene cronte re used s plsticizers. A lyer of this lend is sndwitched etween two electrodes, one of which is indium tin oxide (ITO)-coted glss plte nd nother is Al electrodes. In this study, the use of C 6 lyer over the previously prepred lend is done. ITO-coted glss plte nd Al electrodes re tken s counter electrodes. Use of C 6 molecule over the polymer lend in heterojunction incresed the efficiency of photovoltic devices. In this type of device, the polymer lend cts s n electron donor to the second lyers, wheres C 6 plys the role of n electron cceptor. Keywords Photovoltic device. Methyl red. Crystl violet. C 6 Introduction A. Hldr (*) : S. Mity : N. B. Mnik Condensed Mtter Physics Reserch Centre, Deprtment of Physics, Jdvpur University, Clcutt, 732, West Bengl, Indi e-mil: hldr_ju23@yhoo.com Photovoltic devices sed on orgnic mterils hve ttrcted ttention in the recent yers due to the possiility of producing lrge re nd flexile devices. Severl pproches for efficiency improvement hve een proposed, especilly with respect to geometry nd mterils synthesis. Nowdys, severl reserch groups re le to otin good light/current conversion efficiencies, motivting even more the reserch in the field. As single polymer lyer device presents low efficiency due to the mechnism of chrge genertion nd trnsport, the use of C 6 (fullerene) lyer (tht hs high electroffinity vlue) onto the polymer in heterojunction or mixtured in the polymer film (lend) incresed drsticlly the efficiency of the photovoltic devices [1 5]. The low energy conversion efficiency of some photovoltic devices is merely due to poor photogenertion excitons seprtion nd low moilities of holes or electrons in the orgnic thin films. To improve the dissocition of excitons nd chrge trnsport to the electrodes, the orgnic mterils hve een ssocited to the electron cceptors like fullerene [6 8]. This donor cceptor network leds to n extremely rpid electron trnsfer from polymer to the C 6 tht induces high dissocition rte. To improve the photovoltic properties of n orgnic solr cell, dye sensitiztion cn e dopted. Addition of orgnic dyes could enhnce the light sorption of orgnic mterils. Thus, dye sensitiztion cn e used to enlrge the spectrl response of the photovoltic devices. In the cse of spin-coted films, the orgnic mterils nd dyes must e comptile in solution nd in the solid stte. In this pper, we present detiled study of the influence of C 6 lyer over oth ctionic nd nionic dye-sed photoelectrochemicl cell (PEC) structure. The PEC under study contins lend mde of crystl violet dye dispersed in trnsprent polyvinyl lcohol (PVA), polyethylene oxide (PEO) complexed with lithium perchlorte (LiClO 4 ), ethylene cronte (EC), nd propylene cronte (PC). Methyl red nd crystl violet dye re used s n opticl ctive mteril nd is dispersed in PVA, which cts s n inert inder.
264 Ionics (28) 14:263 267 The ilyer [19, 2] structure is more dvntgeous thn the single lyer structure for severl resons: Exciton splitting is enhnced y the D A interfce, the ctive region is extended to oth the donor nd the cceptor sides of the junction, nd the trnsport of electrons nd holes is seprted into different mterils reducing the recomintion losses. In ddition to this, y using two different semiconductors, the nd gps cn in principle e tuned to mtch etter the solr spectrum. Experimentl Smple preprtion The structure of the dye, methyl red (Fluk), hving the sorption pek t 44 nm tht corresponds to n opticl nd gp of the order of 2.82 ev, nd crystl violet (BDH, 1.4 1.2 Fig. 1 Structure of crystl violet dye nd methyl red LiClO 4 is mixed with solid polymer mtrix PEO to form the solid-stte ionic conductor. The ionic conductivity of PEO/ LiClO 4 complexes re very low [9, 1]. The use of plsticizer is common technique to enhnce the ionic conductivity [11]. In this system, we hve used EC tht ws dissolved in PC s plsticizers to enhnce the moility of the chrge crriers. One solid film of this lend is sndwiched etween trnsprent indium tin oxide (ITO)-coted glss plte nd Al plte, which cts s two contct electrodes. Upon illumintion from light source, dye molecules sor light nd photocrriers re generted. These photocrriers re then seprted y the externl field generted t the contct of the electrodes. It is expected tht the internl field produced y the redistriution of the ion species within the PEC [12 16] enhnces the migrtion process of these photocrriers. The rrier potentil in contcts of ITO nd polymer mterils is lowered [17, 18] due to the ccumultion of these ion species ner the respective electrodes leding to enhncement of chrge injection through the metl polymer interfce lyer. In this work, drk I V chrcteristics nd the photovoltic currents with different intensity hve een mesured for comprison purpose. Improvement of photovoltic prmeters such s open circuit voltge, V oc, short circuit current, I sc, fill fctor, FF, nd conversion efficiency, η, hs een oserved, wheres ITO-coted glss plte nd Al electrode re used s two contct electrodes. Asornce As Unit 1..8.6.4 1.4 1.2 1..8.6.4.2. 2 4 6 8 1 12 Wvelength (nm) 2 3 4 5 6 7 8 Wve Length (nm) Fig. 2 Asorption spectr of MR (curve A) ndcv(curve B), mixture of CV nd MR
Ionics (28) 14:263 267 265 4 3 2 1 2 4 6 8 1 12 14 18 Voltge (mv) 14 12 Fig. 3 Structure of photoelectrochemicl cell. Setup for photovoltic mesurement 1 8 6 Englnd), hving the sorption pek t 59 nm tht hs n opticl nd gp of the order 2.1 ev, re shown in Fig. 1 nd. Asorption spectr of two dyes [21, 22] re lso shown in Fig. 2 nd. In clened test tue, 1 g of PVA ws mixed with 1 l of doule distilled wter, wrmed gently, nd stirred to mke trnsprent viscous solution of PVA (S. D. Fine Chem., Boisr; M.W. 125). Two milligrms of methyl red (nionic dye) nd 2 mg of crystl violet (ctionic dye) re mixed with this solution. A solid electrolyte ws prepred in seprte clened eker y mixing PEO (BDH, Englnd; M. W. 6) LiClO 4 (Fluk, 99.5% pure) EC (Fluk, 99.5% pure) nd PC (Fluk, 99.5% pure). The complex of PEO LiClO 4 EC PC (3.6:3.6:19.6:46.2% y weight) were mixed, stirred, nd heted round temperture 6 C for 4 h. This gel-like solid electrolyte is mixed with the previously prepred dye PVA solution to form the lend. This lend is heted out temperture 6 C nd stirred properly to mix them well for out 2 h. This viscous gel-like solution is then sndwiched etween two electrodes. Thus, single lyer smple is prepred. The ctive re of the cell ws.16 cm 2. To prepre doule lyer film, C 6 solution in dichloroenzene lyer is deposited over the single lyer of viscous gel-like solution tht ws sndwitched etween the previously sid sme contct electrodes. The ctive re of the cell ws 1.8 cm 2. The electrodes were clened in chloroform solution nd dried under vcuum out 2 h efore 4 2 5 1 15 2 25 3 35 Voltge (mv) Fig. 4 I V chrcteristics t 1 Sun incident-illumintion (without C 6 lyer). I V chrcteristics t 1 Sun incident-illumintion (with C 6 lyer) use. The two electricl leds re tken out from the two ends of the electrodes. The complete cell is vcuum dried for out 6 h t round 6 C efore the finl mesurement. The structure of the cell is shown in Fig. 3. 4 3 2 1 41.22mW/cm 2 51.68mW/cm 2 58.14mW/cm 2 64.6mW/cm 2 1.mW/cm 2 5 1 15 2 25 Time (sec) Fig. 5 Photocurrent t different intensity of incident illumintion in cse of single lyer structure (without C 6 lyer)
266 Ionics (28) 14:263 267 14 12 58.14mW/cm 2 1mW/cm 2 14 1 8 6 4 12 1 8 2 6 2 4 6 8 1 12 Time (sec) Fig. 6 Photocurrent t different intensity of incident illumintion in cse of doule lyer structure (with C 6 lyer) Mesurements 4 324 4 5 6 7 8 9 1 For opticl mesurement, tungsten lmp of 2 W is used. Light is llowed to incident on the cell. By vrying the intensity of incident rdition voltge drops, hence, the photocurrent cross the sensing resistnce is mesured. The current flowing through the device ws estimted y me- V oc (mv) 322 32 318 316 4. 314 3.5 4 5 6 7 8 9 1 3. 2.5 2. 1.5 Fig. 8 Short circuit current vs intensity curve (with C 6 lyer). Open circuit voltge vs intensity curve (with C 6 lyer) 1..5. 4 5 6 7 8 9 1 suring the voltge drop (mesured y Agilent dt cquisition unit, Model No: 3497A) cross 56 KΩ sensing resistnce. The intensity is mesured y clirted lux meter (Kyoritsu Electricl Instruments Works, Tokyo, model 52). Photocurrent is mesured y vrying the intensity of light. 163 Results nd discussion 162 Photovoltic mesurements V oc (mv) 161 159 4 5 6 7 8 9 1 Fig. 7 Short circuit current vs intensity curve (without C 6 lyer). Open circuit voltge vs intensity curve (without C 6 lyer) Setup of photovoltic mesurement is shown in Fig. 3. From Fig. 3, it is cler tht mmeter is not required when mesurement of open circuit voltge (V oc ) is done. In the similr wy, voltmeter does not ply ny role t the time of short circuit current (I sc ) mesurement. By chnging vrile resistnce (R), Fig. 4 nd re drwn. Current voltge chrcteristics under 1Sun incident illumintion re shown in Fig. 4 nd. Mesurement of I sc nd V oc is done under different intensity of illumintion. The vrition of short circuit current (I sc ) with time (t) for different intensity illumintion is
Ionics (28) 14:263 267 267 shown in Figs. 5 nd 6 where ITO/dye-sensitized polymer lyer/al junction is illuminted through ITO side. Furthermore, experimentl evidences of this effect re presented in Figs. 7 nd,nd8 nd where the short circuit current (I sc ) nd the open circuit voltge (V oc ) re plotted s function of intensity of incident light. Vrious photovoltic prmeters such s V oc nd I sc re otined to e 163 mv nd 3.93 na, respectively, for 1 Sun intensity of illumintion. The power conversion efficiency of ech cell cn e estimted y using the eqution η% ¼ I sc V oc FF 1 ð1þ φ where φ is the incident intensity of light nd FF is defined y reltion FF ¼ V m I m ð2þ V oc I sc Where V m nd I m represent the voltge nd current density t mximum power point, respectively. The estimted vlues of FF nd power conversion efficiency re.33 nd.132% for 1 Sun rdition in cse of single lyer structure. The enhnced vlue of power conversion efficiency for the cell with C 6 is.153%. The photovoltic prmeters re shown in the following tle. Prmeters Cell without C 6 Cell with C 6 V oc (mv) 163 324 I sc (na) 3.93 156 FF.33.33 η (%).132.153 Fullerene lyer hs lso ttrcted gret del of interest, which enled to develop highly efficient opticl nd electricl devices. Bi-lyer devices cn e quite efficient in photoconversion, with the exciton dissocition occurring t the interfce of the photoctive mterils due to the difference of the electronffinity vlues. Enhncement of photovoltic prmeters in cse of ilyer/doule lyer in comprison to single lyer revels tht, here, polymer cts s n electron donor to the second lyers, wheres C 6 plys the role of n electron cceptor. Conclusions Reports on the photovoltic property of the device mde in PEC nd structure re not common. In this work, photovoltic properties of methyl red nd crystl violet dye-sensitized solid-stte PEC hve een descried. The film ws prepred y spin-coting technique. The cell without C 6 hs fill fctor of 33% nd power conversion efficiency of order.132% under 1 Sun rdition, which is modified in presence of C 6 lyer. The vlues of fill fctor nd power conversion efficiency in presence of dditionl C 6 lyer re 33 nd.153%, respectively. Our results indicte the possiility of using dye-sensitized PEC devices of different rchitecture s solr cells. Acknowledgements The uthor thnks Prof. A. N. Bsu, Deprtment of Physics nd Prof. S. C. Ber, Deprtment of Chemistry, Jdvpur University, Kolkt -32 for their vlule discussions. References 1. Yu G, Heeger AJ (1995) J Appl Phys 78:451 2. Romn LS, Mmmo W, Andersson MR, Ingns O (1997) Adv Mter 9:164 3. Brec CJ, Sriciftci NS, Hummewlen JC (21) Adv Func Mter 11:15 4. Romn LS, Mmmo W, Pettersson LAA, Andersson MR, Ingns O (1998) Adv Mter 1:774 5. Nodri FM, Koehler M, d Luz MGE, Romn LS (25) Microelectronics Journl 1 3 6. Koehler M, Romn LS, Ingns O, d Luz MGE (22) J Appl Phys 92:5575 7. Bhsikuttn AC, Shstri LV, Spre AV (21) J Photochem Photoiol A Chem 143:17 21 8. Go J, Hide F, Wng H (1997) Synthdic Metls 84:979 98 9. Bhttchry AJ, Bnerjee S, Middy TR, Trfdr S (1998) Frctls 6:285 1. Funhshi M, Hnn J-I (1999) Appl Phys Lett 74:2584 11. El-Nhss MM, Zeyd HM, Aziz MS, El-Grmz NA (25) Solid Stte Electronics 12. Hldr A (25) Indin J Phys 79:765 13. Hldr A (25) Ionics 11:315 14. Wtne T, Murkmi S, Mori K, Kshiw Y (1989) Mcromolecule 22:29 15. Koksng R, Olsen II, Shckle D (1994) Solid Stte Ionics 69:32 16. Yohnnes T, Ingns O (1996) J Electrochem Soc 143/7:2311 17. Funhshi M, Hnn J-I (1999) Appl Phys Lett 74:2584 18. Cmpell IH, Smith DL, Neef CJ, Ferrris JP (1998) Appl Phys Lett 72:2565 19. Yu G, Go J, Hummelen JC, Wudl F, Heeger A (1995) Science 27:1989 2. Peumns P, Ykimov A, Forrest SR (23) J Appl Phys 93: 3693 21. Gupt AK, Pl A, Shoo C (26) Dyes nd Pigments 69:224 332 22. Eren Sn S, Koysl O (23) Displys 24:29 212