Optical properties of Poly-[2-methoxy-5-(2-ethyl-hexyloxy)-phenylene vinylene and its application in photovoltaic cells

A conjugated polymer poly[2-methoxy-5-(2-ethyl-hexyloxy)-phenylene vinylene] (MEH-PPV) exhibits unique absorption band in the visible region due to electron transitions between nonlocalized bands and emits light in three different wavelength regions. Doping of iodine increases absorption in the visible region of polymer. MEH-PPV is used as a hole-conductor and a sensitizer in titana based solid-state photovoltaic cells. Maximum photocurrent of 1.3 mAcm and voltage of about 543 mV are observed for a photovoltaic cell with the polymer sensitizing layer. However, slightly higher photocurrent (2.4 mAcm) with a decrement of voltage (465 mV) is observed for a solid-state cell with a configuration of TiO2|dye|MEHPPV|I2, under AM 1.5 conditions. Incident light to power conversion efficiency of these cells is about 0.6 %.


INTRODUCTION
Conjugated polymers have aroused blooming attraction due to their peculiar properties of higher absorption co-efficient, luminescent and sufficiently fast carrier mobility. For examples, poly phenylene vinylene (PPV) and its derivatives exhibit luminescence at red region of the visible spectrum [1,2]. An efficient separation of excitons is observed in combination of MEH-PPV with a material which has low electron affinity function [3]. In * Corresponding author: psirimanne@hotmail.com Institute of Physics -Sri Lanka P. M. Sirimanne and E. V. A. Premalal / Sri Lankan Journal of Physics, Vol. 8 (2007) [29][30][31][32][33][34][35][36][37] 30 addition, sufficient fast hole conductivity is observed in MEH-PPV [4,5]. These properties suggest capability of utilization of PPV and its derivatives in solid-state devices. We have made an attempt to utilize MEH-PPV as a sensitizer and a hole-conductor, in titana based photovoltaic cells. Photo-properties of TiO 2 |MEH-PPV|electrolyte and TiO 2 |dye|MEH-PPV|I 2 |graphite cells are studied and our primary observations are discussed.

EXPERIMENTAL
Meso-porous TiO 2 films with thickness of ~ 10 µm were prepared by applying a colloidal solution of hydrolyzed titanium isopropoxide which contains TiO 2 powder (P-25 Degussa, Japan) on preheated conducting glass plates (FTO) as descried elsewhere [6].

Fabrication of photo-voltaic cells with MEH-PPV sensitizing layer
MEH-PPV was embedded in porous matrix of TiO 2 by a slow solvent evaporation technique. MEH-PPV in CHCl 3 was used as the coating solution. TiO 2 |MEH-PPV electrodes were allowed to dry at room temperature. An FTO electrode was attached to TiO 2 |MEH-PPV electrode as the counter electrode. The electrolyte was spread in between two electrodes by capillary action. Standard I -|I 3 -(E o = 0.53 V vs NHE, [7]) redox-couple in acetonitrile was used as the electrolyte.

RESULTS AND DISCUSSION
MEH-PPV absorbs visible light in the wavelength region of 400-575 nm with an optimal at 500 nm (curve a, Fig. 1). Electron transitions between non-localized bands (π→π*) are responsible for this unique absorption band [8,9]. The onset of absorption mimics that band gap of MEH-PPV is 2.2 eV. This value agrees with Daoud et al′s observations [5]. A clear enhancement of absorbance is observed by doping iodine in the polymer (curve b). Almost similar shapes in absorption spectra reveal that light doping does not make any significant influence in disordering of polymer chain. Similar result has been observed by introducing polystyrene in MEH-PPV [9]. It is known that MEH-PPV exhibits a strong emission band in the red portion of the visible spectrum. Luminescence spectrum of a solution of MEH-PPV (10 -3 M) is also shown in Fig. 1 (curve c). This luminescence band is associated with single-chain excitons [10]. In addition, two other emission bands (with maxima at 550 and 660 nm) are observed by diluting the stock solution (10 -3 M) from CHCl 3 (curve d).
Luminescence band with the maximum at 550 nm is associated with reduction of interaction between adjacent chains caused due to diluting effect [11]. Vibrational modes are responsible for the remaining band at 660 nm [12]. Only this luminescence band is slightly red shifted compared to previously reported luminescence bands by Yan et al and Lui et al [10,13]. Excimer species in the polymer are responsible for this red shift [9].

Characteristics of photo-voltaic cells with MEH-PPV as a hole conductor
Negligible photo-properties are observed in solid-state TiO 2 |N3|MEH-PPV device, under illumination. However, significant enhancement in the photo-voltage is observed after exposing electrodes to iodine vapor. The resulted current is rather low and has to be improved by several fractions. It is known that applying a thin layer of graphite on polymer layer increases the electron collection efficiency of the cell toward the FTO layer [16].
Therefore, a thin layer of graphite was applied on the electrode followed by doping iodine. conditions. However, thickness of the MEH-PPV film was maintained not to exceed the excitation diffusion length, since this type of cells produces much higher performance when thickness of the film is equal to the magnitude of diffusion length [5]. A drastic decrement in the photocurrent of about 75% was observed in the first 20 seconds in the absence of MEH-PPV film in the cell. This result suggests that enrolment of MEH-PPV layer as an efficient electron donor in the regeneration of dye molecules. However, TiO 2 |N3|MEH-PPV|I 2 |graphite cell produces moderate photo-performance compared to other dye sensitized systems [6] and much higher performance than other solar cells composed of polymer hole-conducting layer [17]. Photo-properties of hetero-junctions made of polymer-TiO 2 composites or blends have been reported. In these studies, MEH-PPV and other polymers have been used as light harvesting analog or hole conductor it self [3,4]. However, we have not observed any significant performance in the cell in the absence of a sensitizer. It seems that the charge generation of this device takes place via injection of excited dye molecules. TiO 2 |N3|MEH-PPV|I 2 |graphite cell exhibits maximum IPCE of 35% at 500 nm (inset of Fig. 5). Fig. 6 illustrates the electron-transfer process of TiO 2 |N3|MEH-PPV|I 2 |graphite cell. Photo-performance of the cell could be further improved by increasing conductivity of the polymer.