Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Design of two novel hole transport materials via replacing the core of spiro- OMeTAD with tetrathiafulvalene and tetraazafulvalene for application in perovskite solar cells Habib Ashassi-Sorkhabi ⁎ , Parvin Salehi-Abar Department of Physical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran ARTICLE INFO Keywords: Hole transport materials Tetrathiafulvalene Tetraazafulvalene Cost effective DFT/TD-DFT Perovskite solar cells ABSTRACT Two novel hole transport materials (HTMs) were designed by way of replacing the core of 2,2′,7,7′-tetrakis-(N,N- di-4-methoxyphenylamino)-9,9′spirobifluorene (spiro-OMeTAD) with tetrathiafulvalene (TTF) and tetra- azafulvalene (TAF). The frontier orbitals, stability and solubility, reorganization energy, and hole mobility of the new materials were investigated via density functional theory (DFT) calculations in combination with the Marcus hopping model. Time-dependent density functional theory (TD-DFT) was also used for evaluating the optical properties including the absorption and emission spectra. It was found that the HOMO level of the two proposed HTMs (-5.25 and -5.20 eV) is lower than that of spiro-OMeTAD (-5.17 eV). The results showed that the maximum absorption band of the new designed HTMs with the TTF and TAF cores is blue and red-shifted compared to spiro-OMeTAD. This may indicate that these materials have no competition with the perovskite in absorbing light. Additionally, the hole reorganization energy is less than the electron reorganization energy for our structures, revealing these materials can act as HTMs more efficiently than electron transport materials. The calculated hole mobility of TAF- and TTF- OMeTAD was obtained as 4.41 × 10 -4 and 1.15 × 10 -6 cm 2 v -1 s -1 , respectively. This result indicates that TAF-OMeTAD shows a great potential for use as an alternative HTM for expensive spiro-OMeTAD. 1. Introduction To date, perovskite solar cells (PSCs) as cost-effective devices have attracted comprehensive interest due to their low cost, facile synthesis, and superior power conversion efficiency (PCE) of > 20% (Bi et al., 2016). Hole transport materials (HTMs) with the ability to decrease the charge recombination are considered as one of the most important elements to achieve the high performance of perovskite solar cells. In recent years, significant investigations have been done experimentally (Galatopoulos et al., 2017; Magomedov et al., 2018; Ulfa et al., 2018a; Ulfa et al., 2018b; M.-D. Zhang et al., 2018) and theoretically (Gapol et al., 2017; Guo et al., 2017; Kirkpatrick and Nelson, 2005; Li et al., 2018; Wazzan et al., 2018; Y. Zhang et al., 2018; Z. Zhang et al., 2017) on various materials to design stable HTMs with higher hole mobility, low cost, and facile synthesis toward improved efficiency of PSC. The highest value of PCE reported for the perovskite solar cells is related to the use of spiro-OMeTAD as HTM. However, the application of spiro- OMeTAD in PSCs is limited by the high cost of synthesis of the spiro- bifluorene core. (Bandara and Weerasinghe, 2005; Xu et al., 2013) Replacing the spiro-bifluorene core by other groups with the cost-ef- fective synthetic process and high charge-carrier mobility could be a useful strategy for the commercialization of PSCs. Recently, Jeon el al. (Jeon et al., 2013) have replaced the spiro-bifluorene core of spiro- OMeTAD with a pyrene core to synthesize a new HTM and found that the device based on this novel structure shows a PCE comparable with spiro-OMeTAD. Xu el al. (Xu et al., 2014) designed and synthesized carbazole-based hole transport materials, denoted as X51, to apply in PSC. In another work, a new HTM was introduced and called as KTM3 by placing swivel-cruciform thiophene between the two phenyl groups of each fluorene unit in spiro-OMeTAD. (Krishnamoorthy et al., 2014) Li el al. (Li et al., 2014a) proposed two new HTMs based on thiophene cores, and Wu al el. (Wu et al., 2016) introduced a hole transport material by changing the spiro-bifluorene core with the tetra- phenylethene. Tetrathiafulvalene and tetraazafulvalene along with their derivatives have been widely studied as an electron-donating agent in the molecular conductors. However, the number of works re- ported on the application of tetrathiafulvalene as HTM in PSCs is so scarce. Liu et al. (Liu et al., 2014) have investigated the application of https://doi.org/10.1016/j.solener.2018.07.047 Received 16 June 2018; Received in revised form 16 July 2018; Accepted 18 July 2018 ⁎ Corresponding author. E-mail address: habib.ashassi@gmail.com (H. Ashassi-Sorkhabi). Solar Energy 173 (2018) 132–138 0038-092X/ © 2018 Elsevier Ltd. All rights reserved. T