Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Evaluation of the performance of stilbene-based hole transport materials with an emphasis on their configuration for use 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: Efficient hole transport materials Stilbene derivatives Cost-effective DFT/TD-DFT Perovskite solar cells ABSTRACT This paper reports a series of new hole transport materials (HTMs) based on the two conformers of α,β-dicya- nostilbene (SCN) and α,β-dialstilbene (SCO) with an emphasis on their optical and electrochemical properties. The calculations have been performed by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) along with the Marcus theory. The frontier orbitals, Stokes shift, stability, reorganization en- ergy, and hole mobility of the proposed materials have been obtained and discussed. The calculations show that the hole reorganization energy is much lower than the electron reorganization energy for our structures, in- dicating that this kind of materials can act as hole transport materials more efficiently than electron transport materials. All the designed structures have the ability to transfer holes and contribute to the photocurrent by the absorption of additional photons. This contribution is due to the fact that the absorption of these structures occurs in the 665.65–712.47 nm region where the perovskite absorption is very low. The obtained results for both conformers of SCN-NMe 2 DPA are really surprising. Accordingly, the hole mobility of the cis and trans conformers of this structure has the value of 1.75 × 10 -2 and 2.18 × 10 -2 cm 2 v -1 s -1 , respectively which is about 7 and 9 times larger than that of spiro-OMeTAD as the most commonly used HTM. Moreover, the HOMO level of this structure (-5.22 eV) is 0.05 eV deeper than that of spiro-OMeTAD, making it more desirable for efficient usages. Based on these outcomes, it is expected that a device encompassing SCN-NMe 2 DPA as the hole transport material has larger open-circuit voltage (V oc ) and short-circuit current density (J sc ) than the device with spiro-OMeTAD. 1. Introduction Perovskite solar cells (PSCs) are of great importance owing to their several advantages, such as low cost, easy process, and strong light absorption that lead to high photovoltaic performance. So far, many efforts have been made to increase the efficiency and stability of PSCs. One of the reasons for the decrease in the efficiency of PSCs is the unwanted recombination of charges. Recently, p-type semiconductors are used as hole transport materials (HTMs) to prevent the re- combination charges in PSCs. Up until now, the highest performance reported for a HTM used in PSCs belongs to spiro-OMeTAD, which shows efficiency of more than 20% (Bi et al., 2016). However, the high cost and rather complicated synthesis of the core of spiro-OMeTAD limit its wide applications in PSCs (Bandara and Weerasinghe, 2005; Xu et al., 2013). Accordingly, design and synthesis of cost-effective, stable alternative HTMs with high hole mobility and with no competition with the perovskite in the absorbing light seem essential. One of the most effective strategies for this purpose is to replace the spiro core of spiro- OMeTAD by other groups. In this way, some efforts have been recently made by changing the spiro core with pyrene (Ashassi-Sorkhabi et al., 2019a; Jeon et al., 2013), tetraphenylethene (Wu et al., 2016), carba- zole (Xu et al., 2014), tetraazafulvalene (Ashassi-Sorkhabi and Salehi- Abar, 2018a), swivel-cruciform thiophene (Krishnamoorthy et al., 2014), and thiophene (Li et al., 2014a) cores. These materials have been explored as a new, low-cost HTM in PSCs, exhibiting promising efficiencies. Moreover, new structures are synthesized by introducing various substituents for cost-effective cores to develop efficient HTMs (Choi et al., 2015; Do Sung et al., 2014; Nishimura et al., 2015; Rojas et al., 2018; Wu et al., 2018). Thus far, no efforts have been reported for the use of stilbene derivatives as the core of HTMs in PSCs. Accordingly, in this work, we introduce four novel, efficient HTMs with the core of α,β-dialstilbene (SCO-) and α,β-dicyanostilbene (SCN-) in two config- urations of cis and trans (see Fig. 1). The designed HTMs are defined as trans-SCN-NMe 2 DPA, cis-SCN-NMe 2 DPA, trans-SCO-NMe 2 DPA, and cis- SCO-NMe 2 DPA. The optical and electrochemical properties of the pro- posed HTMs are evaluated by combining TD-DFT and DFT with the https://doi.org/10.1016/j.solener.2019.06.076 Received 16 March 2019; Received in revised form 2 June 2019; Accepted 28 June 2019 ⁎ Corresponding author. E-mail address: habib.ashassi@gmail.com (H. Ashassi-Sorkhabi). Solar Energy 188 (2019) 951–957 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T