Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat The effect of molecular structure and ultrafast electron injection dynamics on the efficiency of BODIPY sensitized solar cells Elif Akhuseyin Yildiz a , Gokhan Sevinc b , H. Gul Yaglioglu a,* , Mustafa Hayvali c,** a Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100, Beşevler, Ankara, Turkey b Department of Chemistry, Science and Literature Faculty, Bilecik Şeyh Edebali University, 11230, Bilecik, Turkey c Department of Chemistry, Faculty of Science, Ankara University, 06100, Beşevler, Ankara, Turkey ARTICLE INFO Keywords: Azomethine BODIPY dyes Ultrafast charge injection DSSC Power conversion efficiency ABSTRACT In an attempt to investigate the effect of charge transfer dynamics on the photovoltaic performance of the dye- sensitized solar cells (DSSCs), a new series of 4,4-difluoro-3a,4a-diaza-s-indacene (BODIPY) compounds were designed and sensitized. Charge transfer dynamics of the studied compounds were investigated by using fem- tosecond transient absorption (TA) spectroscopy technique. TA spectroscopy results revealed that excited state lifetime of the sensitized dyes can be controlled by altering the position of the anchoring group as well as the conjugation length of the studied compounds. Photovoltaic performances of the DSSCs produced with the sensitized dyes were evaluated by using incident photon to current efficiency (IPCE) spectra as well as IeV measurements. It was found that the dye with longer conjugation length showed efficient electron injection to the conduction band of the semiconductor (TiO 2 ) depending on the anchoring group position. In addition to that, the adsorption of the sensitizer on the TiO 2 was affected by the position of OH moieties. Therefore, the results of IPCE and J-V measurements suggest that, in order to improve the photovoltaic performance, the anchoring group should be bound to the positions which do not prevent the molecular rotation. This work could be useful for developing new strategies towards molecular engineering for DSSC applications. 1. Introduction Dye sensitized solar cells (DSSCs) have received considerable at- tention due to renewable and low cost energy technologies for light to electrical energy conversion [1]. The cell has a sandwich structure that consists of dye-coated titanium dioxide nanoparticle electrode, a redox mediator and a metal coated counter electrode [1]. In an ideal DSSC, the effective redox regeneration and slow charge recombination process are desired in order to get high power conversion efficiency (PCE) [2]. Upon excitation of the sensitizer, the electron localized in LUMO energy level of the dye molecule is injected into the conduction band of semiconductor oxide layer. The charge diffusing through the semi- conductor oxide layer reaches the counter electrode via outer cycle. Electron transferred from counter electrode to electrolyte mediator, reduces the electrolyte and then recombine with the ground state hole. The previous studies of this technology have utilized many Ruthenium- based dyes as a sensitizer that has high PCE [2–6] and long lived excited state lifetimes [4,7,8]. However, ruthenium dyes are expensive, rare and their preparation includes lengthy purification steps [9,10]. Therefore, novel low cost and metal free organic dyes are required to developed DSSCs with high performance. The metal free organic sen- sitizer such as porphyrins [11], phthalocyanines, coumarines [12], cy- anines [13], rhodamines [14], phenothiazine [15], pyran [16] and benzothiadiazole based [17,18] dyes have been investigated for DSSC applications. New strategies towards molecular engineering are needed to develop new dyes for this application. Therefore, designing, syn- thesizing new organic dyes and investigating their photovoltaic per- formances has been an appealing research topic [3,6,19]. Borondipyrromethene (BODIPY) photosensitizers are novel fluor- ophores with high absorption coefficients, fluorescence quantum yields, good solubility and excellent photostability [20,21]. In addition to these features, tetramethyl substituted BODIPY derivatives include sharp (full width half maximum ∼25 nm) absorption band around 500 nm and have long excited state lifetimes (calculated lifetimes are about 5 ns for boron-dipyrrin dyes) [22]. Owing to these features, new BODIPYfluorophores have been developed for light harvesting and transferring antenna applications [23–31]. Recent studies have focused on the development of BODIPY dyes with different modification such as https://doi.org/10.1016/j.optmat.2019.02.025 Received 29 November 2018; Received in revised form 15 February 2019; Accepted 18 February 2019 * Corresponding author. ** Corresponding author. E-mail addresses: yoglu@eng.ankara.edu.tr (H.G. Yaglioglu), hayvali@science.ankara.edu.tr (M. Hayvali). Optical Materials 91 (2019) 50–57 0925-3467/ © 2019 Elsevier B.V. All rights reserved. T