Solar Energy Materials & Solar Cells 91 (2007) 1775–1781 AM1 molecular screening of novel porphyrin analogues as dye-sensitized solar cells Mannix P. Balanay a , Carl Vincent P. Dipaling a , Sang Hee Lee a , Dong Hee Kim a,Ã , Kee Hag Lee b a Department of Chemistry, Kunsan National University, Kunsan 573-701, South Korea b Department of Chemistry, Research Institute of Basic Sciences, and BK21, Wonkwang University, Iksan 570-749, South Korea Received 23 March 2007; received in revised form 21 May 2007; accepted 14 June 2007 Available online 20 July 2007 Abstract AM1 calculations were used to study the charge-separated state of porphyrin analogues as sensitizers. Initial calculations were performed on the donor and acceptor moieties to determine the molecular orbital (MO) energy levels independently. The analogues were modeled by combining the donor and acceptor moieties. The charge-separated state of the porphyrin analogues was analyzed from the MO energy levels of the donor and acceptor moieties, and from the porphyrin analogues (donor–acceptor pairs). MO spatial orientations were also used to elucidate the charge-separated state of the analogues. Our results revealed that the relative positions of the lowest unoccupied molecular orbital (LUMO) levels of the donor and acceptor moieties could predict the MO spatial orientations of the porphyrin analogues, i.e., the donor–acceptor pairs. The LUMO is localized in the acceptor and the highest occupied molecular orbital (HOMO) in the donor region when the difference between LUMO donor and LUMO acceptor is positive, which indicates a charge-separated state. Of the molecular models, (Zn-5G) had the smallest HOMO–LUMO gap and had a relative difference of 0.16 eV compared to (Zn-1A), which has a known cell efficiency of 4.8%. r 2007 Elsevier B.V. All rights reserved. Keywords: Porphyrin; Calculations; DSSC; Charge separation; HOMO–LUMO gap 1. Introduction Dye-sensitized solar cells (DSSCs) are the most promis- ing alternative to conventional silicon-based solar cells and have thus attracted much interest [1–6]. They offer several advantages compared to conventional solar cells, such as easy fabrication, low production cost, environment friend- liness, and a photovoltage that is significantly less sensitive to light intensity variation. DSSC uses mostly nanocrystal- line-TiO 2 film and organic or organometallic complex photosensitizers, on which its solar-energy-to-electric con- version efficiency depends [1–3]. O’Regan and Gra¨tzel [1] first reported dye-sensitized colloidal TiO 2 films based on a cis-dithiocyanato-bis(2,2 00 - bipyridyl-4,4 0 -dicarboxylate) ruthenium (II) complex (N3 dye) with an overall light-to-electric energy conversion yield of 7.1–7.9% in simulated solar light. However, Ross and Nozik [4] proved that the energy conversion yield could reach 66% theoretically, with a hot-carrier flat-plate device operated under typical terrestrial conditions. Numerous studies have sought to find suitable photo- sensitizers for DSSC, most of which are based on organo-Ru complexes, such as N3 dye [1], bis(tetrabutylammonium) trithiocyanato(4,4 0 ,4 00 -tricarboxy-2,2 0 :6 0 ,2 00 -terpyridine) ruthe- nium (II) (black dye) [5], and bis(tetrabutylammonium)- cis-(dithiocyanato)-N,N 0 -bis(4-carboxylato-4 0 -carboxylic acid- 2,2 0 -bipyridine) ruthenium (II) (N719) [6]. However, ruthe- nium is not readily available. Presently, several studies are focused on complexes that are derived from metals that are common in nature, such as organo-Zn complexes [7–11], and free-metal compounds, such as organic [12–17], natural [18–23], and porphyrin dyes [24–28]. Among the alternative dyes, porphyrin has attracted a great deal of attention because of its natural role in photosynthesis and the relative ease with which functional ARTICLE IN PRESS www.elsevier.com/locate/solmat 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.06.004 Ã Corresponding author. Tel.: +82 63 469 4576; fax: +82 63 469 4571. E-mail address: dhkim@kunsan.ac.kr (D.H. Kim).