REVIEW ARTICLE CURRENT SCIENCE, VOL. 104, NO. 7, 10 APRIL 2013 847 *For correspondence. (e-mail: giribabu@iict.res.in) Are porphyrins an alternative to ruthenium(II) sensitizers for dye-sensitized solar cells? Lingamallu Giribabu* and Ravi Kumar Kanaparthi Inorganic and Physical Chemistry Division, CSIR–Indian Institute of Chemical Technology, Hyderabad 500 607, India This review article reports recent advances in porphy- rin-based sensitizers for dye-sensitized solar cell (DSSC) applications. The sensitizer is known to be one of the key components of the DSSC device, which plays a vital role in achieving high efficiency and du- rability. So far, ruthenium(II) polypyridyl complexes have been extensively used as sensitizers achieving more than 11% efficiency. The major technical dis- advantages with these complexes are expensive due to rarity of the metal, tedious purification process and lack of absorption in the red region of the visible spec- trum, where the light harvesting process is maximum. For this reason, porphyrins are found to be probable alternative sensitizers based on their thermal, elec- tronic and photochemical properties. A great variety of porphyrins have been used as sensitizers in DSSC applications for the last three decades and recently, the efficiency of porphyrin-based sensitizers has crossed 12%. Keywords: Porphyrin, pyrrole, ruthenium, sensitizers, solar cell. DURING the last two decades, with the development of nanocrystalline films of very high surface area, the photo- sensitization of wide band-gap semiconductors such as TiO 2 by adsorbed dyes has become more realistic for dye-sensitized solar cell (DSSC) applications 1–6 . In a porous thin film consisting of nanometre-sized TiO 2 par- ticles, the effective surface area can be enhanced 1000- fold, thus making light absorption more efficient even though there is only a monolayer of dye on each nanopar- ticle 7 . DSSCs have attracted significant attention because of environmentally pleasant, easy to fabricate and low- cost alternatives to conventional solid-state photovoltaic devices. Historically the DSSC concept was started in 1972 with chlorophyll-sensitized zinc oxide (ZnO) elec- trode 8 . By using these two chlorophyll systems, photons are converted into electric current by charge injection of excited chlorophyll molecules into ZnO electrode. Since then many efforts have been made to improve the power conversion efficiency. But, a major breakthrough appeared in 1991, when O’Regan and Grätzel 1 reported a DSSC device with 7.1% efficiency. Typically, the device composed of a porous layer of TiO 2 nanoparticles covered with a molecular dye absorbs sunlight like chlorophyll in green leaves. The TiO 2 is immersed in an electrolyte solution, above which a plati- num-based catalyst is placed. Similar to a conventional alkaline battery, an anode (the titanium dioxide) and a cathode (the platinum) are placed on either side of a liquid conductor (the electrolyte). Detailed device fabri- cation and the working principle of a DSSC are well documented in our earlier reports 5,6 . Since 1991 many efforts have been paid to improve the power conversion efficiency. Moreover, it did not take much time to prove that DSSCs are good a alternative for the conventional first and second generation silicon and other thin-film so- lar cells. Interestingly, the DSSC technology works well even in the diffused light conditions, unlike in the first and second generations of photovoltaic devices. Among various components of the DSSC device, the sensitizer is one of the key components in achieving high efficiency and durability. The most successful charge transfer sensitizers employed so far in DSSC are cis- dithiocyanatobis-(2,2′-bipyridyl-4,4′-dicarboxylate)ruthe- nium(II) (together with its various protonated forms), its modified forms (N3 and N719) and trithiocyanato 4,4′4″- tricaboxy-2,2′ : 6′,2″-terpyridine ruthenium(II) (the black dye), which yield conversion efficiencies up to 11% under air mass (AM) 1.5 solar conditions with liquid redox electrolyte 1,9 . Nevertheless, studies are still needed to fill the gap between today’s benchmark conversion efficiency of 32% (Shockley–Queiser 10 limit predicted for a single junction cell). This can be achieved only through proper molecular designing of the sensitizer. A great variety of ruthenium(II) complexes have been reported in the literature in order to further improve the efficiency and durability of the device 11–14 . Even though the ruthenium(II) polypyridyl complexes are more domi- nant in today’s DSSC research, they are expensive due to rarity of the metal. Moreover, they are less durable due to the presence of two or three –NCS groups, present in ru- thenium(II) sensitizers. Another important drawback is that these complexes lack absorption in the red region of the visible spectrum and also have relatively low molar extinction coefficients above 600 nm. The next challenge is that the metal complex based sensitizers involve care- ful synthesis and tricky purification steps. Considering these drawbacks of ruthenium(II) sensitizers, metal-free