Journal of Photochemistry and Photobiology A: Chemistry 132 (2000) 1–17 Review Solar cells based on dyes Asok K. Jana Department of Chemistry, Mahatma Gandhi Government Arts College, Mahe, Pondicherry 673311, India Received 24 September 1999; accepted 4 December 1999 Abstract From early developments to recently developed stages of solar cells have been discussed in details, considering the effect of different parameters. In this paper, primitive photoelectrochemical (PEC) cell and PEC cell with Fe–thionine and different dyes-reducing agent have been reviewed. Solar cell with phenazine dyes have also been pointed out separately since these dyes give more output in solar cell. The use of inorganic semiconductor electrode (e.g. SnO2,In2O3 and ZnO) and utilization of semiconducting properties of organic dyes in PEC cell have been mentioned. Furthermore, utilization of surfactant media for storage of solar energy and production of hydrogen from solar cell using dye are also included. ©2000 Elsevier Science S.A. All rights reserved. Keywords: Dyes; Photochemical cells; Photopotential; Photocurrent 1. Introduction A list of abbreviations and symbols is mentioned at the end of this paper as a large numbers of chemical systems The search for renewable sources of energy has led to have been cited here. an increasing interest in photochemical cells because of their possible role as transducers of solar to electrical en- 1.1. A primitive PEC cell ergy. The photoeffects in electrochemical systems were first observed by Becquerel [1] in his investigation on the Anciently a photochemical cell is composed with inert solar illumination on metal electrodes in 1839. Later it was metal electrodes immersed in two redox couple [i.e. A/B observed by Moser [2] and Rigollot [3] that the sensitivity (light sensitive) and Y/Z] and the best performance is ob- of silver/silver halide and copper/copper oxide electrode tained if one couple (e.g. A/B) is highly reversible and the could be increased by coating them with a dye stuff. Thomp- other (Y/Z) is highly irreversible [9]. As a result, the reac- son [4] and Stora [5] reported that pure metal electrodes tion occurs in the light and dark are as follows: were also sensitive to light when coated with a dye or im- Light mersed in a dye solution. The result of the first 100 years A + Z B + Y (1) had been reviewed by Copeland and co-workers [6]. A Dark summary of the properties of photoelectrochemical (PEC) Furthermore, optimum output of power depends on the cells described in the literature upto 1965 was compiled by cell length, the concentration of A, Y and Z, the kinetics of Kuwana [7] and later work has been reviewed by Archer the homogeneous reaction B+Y, the extinction coefficient [8]. of A, and the cell load [10,11]. This review paper is comprised of evolution of solar cell In this comprehensive mode cell, five major types from the very beginning stage and it includes vast field of re- of process [12] occurs which are (i) the intermolecular search in PEC cell in which dyes are used for photoeffected oxidation–reduction reactions in the dark of the solution and electron-transfer reaction. The basic principles of PEC cell at the electrode, (ii) the electrodic charge transfer reactions, are not discussed here. PEC cell are of three types i.e. pho- (iii) the steps for the conversion of bulk species to surface togalvanic (PG), photovoltaic (PV) and photogalvanovoltaic species with or without the possibility of adsorption, (iv) (PGV). So, these three types of PEC cell are considered in direct photochemical pathways, and (v) the photochemical different sections of this paper. side reactions. 1010-6030/00/$ – see front matter ©2000 Elsevier Science S.A. All rights reserved. PII: S1010-6030(99)00251-8 2 A.K. Jana / Journal of Photochemistry and Photobiology A: Chemistry 132 (2000) 1–17 1.2. PEC cell with dyes 1.2.1. Iron–thionine system A particularly striking example of an oxidation–reduction Generally inorganic elements possess redox properties be- system, in which light is converted to a marked proportion cause of large electronegativity difference between bonded into chemical energy, is the iron–thionine system observed elements. The organo-metallic and heterocompounds (e.g. by Weber [13] firstly. The reaction was reversible in the dark, dye) like inorganic element also show redox property al- but could be driven in an irreversible manner by precipitation though they are covalent in nature. So dye molecules, hav- of the simultaneously formed ferric ions (Weiss) [14]. The ing redox property as well as light sensitivity, can be used in PEC effect of this Fe(II)–thionine system was first seriously comprehensive solar cell as a redox couple among the two considered by Rabinowitch [15]. which is mentioned in model primitive cell. The structure The cell characteristics of Fe(II)–thionine PG cell are and absorption wavelength maxima of some dyes are listed reported by many authors, few of them are mentioned in in Table 1. They are commonly used in this paper. Table 2. Table 1 The structure and absorption maxima of five classes of dyes Dye Class Structure λmax (nm) Thionine (TH+) Thiazines 596 Toluidine blue (Tb+) Thiazines 630 Methylene blue (MB) Thiazines 665 New methylene blue Thiazines 650 Azure A Thiazines 635 Azure B Thiazines 647 Azure C Thiazines 620 Phenosafranin (PSF) Phenazines 520 Safranin-O (Saf-O/SO) Phenazines 520 Safranin-T (Saf-T/ST) Phenazines 520 Neutral red (NR) Phenazines 534 Fluorescein Xanthenes 490 A.K. Jana / Journal of Photochemistry and Photobiology A: Chemistry 132 (2000) 1–17 3 Table 1. (Continued) Dye Class Structure λmax (nm) Erythrosin Xanthenes 530 Erythrosin B Xanthenes 525 Rhodamine B (Rh. B) Xanthenes 551 Rose bengal Xanthenes 550 Pyronine Y (PY) Xanthenes 545 Eosin Xanthenes 514 Rhodamine 6G Xanthenes 524 Acridine orange (AO) Acridines 492 Proflavin (PF) Acridines 444 Acridine yellow (AY) Acridines 442 Fuchsin Triphenyl methane derivatives 545 Crystal violet Triphenyl methane derivatives 578 Malachite green Triphenyl methane derivatives 423, 625 Methyl violet Triphenyl methane derivatives 580 4 A.K. Jana / Journal of Photochemistry and Photobiology A: Chemistry 132 (2000) 1–17 Table 2 Cell characteristics of PG cell with Fe(II)–thionine system Photovoltage (mV) Short-circuit current (ISC)(␮A) Solar energy efficiency (SEE) % Reference 10–240 – – [15] 185 – 3×10−4 [16] – – 30-fold of [16] [17] 230 (pH 4.0) – – [18] – – 2.36×10−1 [19] 138 26.3 – [20] 900 80.0 – [21] – – 1.1×10−2 [22] The thionine–Fe(II) aqueous PG system was studied ex- P/D=33. Yamad et al. [40] also observed greater photo- tensively [23–30] but the basic mechanism of the PG effect potential with thionine-poly(imino-ethylene) compared was unknown initially. However, Hatchard et al. [31] and to thionine-monoamine. Even thionine coated electrode Albery et al. [32] established the sequence of reactions in [41–44] favours irreversible oxidation process over reduc- Fe–thionine cell which are as follows: tion process but Harrison et al. [45] reported 19 times reduced power conversion efficiency with thionine coated + hν 1 + 3 + TH → TH → TH gold electrode compared to uncoated electrode. Kamat [46] − + + + also reported very low efficiency i.e. 10 4% with thionine 3TH + H → 3TH 2 2 dye incorporated Pt clay modified electrode. 3 2+ + + TH2 → TH + H (ISC to ground state) Lichtin et al. [47] developed the totally illuminated thin layer (TITL) iron–thionine PG cell and studied the elec- trodic phenomena at the anode of this cell. Albery [48] also 3 2+ + 2+ → • + + 3+ TH2 Fe TH2 Fe discussed theoretical principles regarding kinetics, electrode (Semi-thionine radical formation) selectivity and efficiency of the TITL cell. Natarajan et al. [49,50] approached the electrode coated method to TITL • + + + + + system using polymer bounded thionine coated on SnO TH + Fe3 → TH + Fe2 + H 2 2 electrode and clean Pt electrode, and obtained maximum ( ) Back thermal reaction power output compared to earlier study [39]. They proposed the following mechanism • + • + + + at the cathode: TH2 + TH2 TH + TH3 (Disproportionation reaction) + 2+ + hν 4+ P − TH + Fe + H → [P–TH2–Fe] 4+ + − → − • + + 2+ • + + + − [P–TH2–Fe] e P TH2 Fe TH2 → TH + H + e (Anode reaction at illuminated electrode) at the anode: + − + + − + Fe3 + e → Fe2 (Dark electrode). Fe2 − e → Fe3 . Thionine was represented by TH+, since in the dark Mechanism shows that the oxidation of the thionine in the + 4+ thionine is in the oxidized form TH . complex [P–TH2–Fe] present near the electrode, occurs Still, the photoenergy conversion is very low mainly due slowly for which efficiency of cell increases. The photocur- to back thermal reaction between reduced dye and Fe(III) rent of TITL system [51] with bare and thionine coated Pt [33,34] and formation of local cells. Last factor can be re- electrode increases to some extent by the addition of F− ion. duced if the Fe(II)–thionine system is studied in heteroge- The Kinetics of iron–thionine system have been studied neous cell. Kamat et al. [35–38] reported the better power by the steady state illumination [23,24,26,52–54] and flash output of heterogeneous cell than homogeneous cell which photolysis techniques [55,56]. Employing a cross beam il- are 0.022 and 0.001%, respectively. lumination kinetic spectrophotometry technique, Kamat et In order to decrease the back reaction of this sys- al. [57] studied the oxidation of leuco-thionine by Fe(III) in tem, Natarajan et al. [39] constructed a PG cell with aqueous solution i.e. the kinetics of photobleaching recov- Fe(II) and thionine coated with poly(N-methyl acry- ery. The photobleaching recovery follows pseudo first-order 2+ lamide), and observed that open-circuit voltage (VOC) kinetics through complex formation between TH4 and and ISC of this system increased with increase in ra- Fe(III), and the rate constant increases with increase in pH, tio of P/D (polymer/dye) and reached maxima with light intensity and temperature.