Dyes and Pigments 175 (2020) 108140 Available online 17 December 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved. New pyran-based molecules as both n- and p-type sensitizers in semi-transparent Dye Sensitized Solar Cells Matteo Bonomo a, b, * , Antonio Carella c, ** , Fabio Borbone c , Luisa Rosato c , Danilo Dini a , Lorenzo Gontrani d, e a Dept. of Chemistry, University of Rome LA SAPIENZA, p.le Aldo Moro 5, 00185, Rome, Italy b Dept. of Chemistry and NIS Interdepartmental Centre and INSTM Reference Centre, University of Turin, via Pietro Giuria 7, 10125, Turin, Italy c Dept. of Chemical Sciences, University of Naples “Federico II”, Complesso Universitario Monte Sant’Angelo, 80126, Via Cintia, Naples, Italy d Dept. of Industrial Engineering, University of Rome “Tor Vergata”, Viale Degli Ingegneri, I-00133, Rome, Italy e Dept. of Chemistry “Giacomo Ciamician”, University of Bologna, Via F. Selmi 2, I-40126, Bologna, Italy A R T I C L E INFO Dedicated to the birth of Edoardo. Keywords: Pyran-based sensitizers Dye sensitized solar cells DFT Electrolyte engineering ABSTRACT We report here on the synthesis of four novel pyran-based DSSC photosensitizers. The dyes are constituted of a pyran core functionalized with different electron acceptor groups and with peripheral cyano-acrylic groups. The molecules were carefully characterized for what concerns optical and electrochemical properties and a clear dependence from the strength of the electron-acceptor group functionalizing the pyran core was observed. A detailed DFT analysis on the dyes gave a better insight of the electronic properties and showed a good accord with the experimental properties. The dyes were used as photosensitizers both in n-type and p-type DSSCs. In the frst case, the stabilization of the LUMO achieved partially prevented a fast kinetics of the electron injection. To overcome this, we used an I-based electrolyte containing an excess of LiI: the photoconversion effciency signifcantly increases for all the dye-sensitized devices, up to a maximum of 2.1%. The dyes were successfully employed the as photosensitizers in p-type DSSC, obtaining PCEs approaching 0.1%, a promising result in the feld on NiO-based devices. More remarkably, as far as we are aware, this is the frst paper in which a single series of dyes is effectively employed as sensitizers in both n and p-type DSSCs. 1. Introduction Dye Sensitized Solar Cells (DSSCs) have attracted scientists’ atten- tion since their frst discovery by Gr€ atzel and O’Regan in 1991 [1]. The heart of this type of device is a wide band gap n-type (and less likely p-type) semiconductor [2,3] whose surface is properly functionalized (sensitized) with an organic or metalorganic sensitizer in order to catch the visible/infrared region of the solar spectrum. The sensitized semi- conductor acts as electrode in a multilayer structure and, together with a counter-electrode, sandwiches an electrolyte solution containing a proper redox mediator [4–7]. The optimization of each part of these multicomponent devices, both individually and in their mutual re- lationships, has allowed a signifcant increase of the effciency and stability across the years [8–11]. Concerning n-type DSSC, typically based on Titanium (IV) Oxide semiconductor, the most exploited class of sensitizers is based, so far, on metalorganic dyes because they assure a broad absorption over the entire solar spectrum, fast and effcient charge injection and a good thermal, photophysical and photochemical stability [12–14]. On the other hand, they usually embody rare-earth elements as metal core (e.g. Ru, Ir,…) [15,16] making them quite expensive. Furthermore, both Ruthenium and Iridium possess low recycling rate and they have been recently defned as Critical Raw Material (CRM) by European Commission. This issue has been pushing a growing interest toward fully organic, rare-earth metal free, dyes [17–20]: in addition this class of sensitizers are characterized, as compared to metalorganic counterpart, by less complicated synthesis and purifcation procedures and by higher molar absorption coeffcients. The classical molecular structure of this class of dyes is based on a push-pull structure in which an electron donor is bonded through a π-bridge to an electron acceptor (and the whole structure typically covalently decorated by hydrophobic tails). Benefting from the previous optimization of the design rules for metalorganic sensitizers, the effciency of DSSC based on metal-free dyes * Corresponding author. Dept. of Chemistry, University of Rome LA SAPIENZA, p.le Aldo Moro 5, 00185, Rome, Italy. ** Corresponding author. E-mail addresses: matteo.bonomo@unito.it (M. Bonomo), antonio.carella@unina.it (A. Carella). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig https://doi.org/10.1016/j.dyepig.2019.108140 Received 7 October 2019; Received in revised form 5 November 2019; Accepted 12 December 2019