Synthesis of new simple hole-transport materials bearing benzodithiazole based core for perovskite solar cells

Highlights

• We have synthesized two new HTMs BTT-PMe and BTT-2F by the simple synthetic route.

• These are fully characterized by ¹HNMR, FT-IR, UV-visible, Emission spectra, TCSPC and cyclic voltammetry studies.

• We have studied the effect of the fluorine atom on the optical properties.

• This work will give advances to develop simple and cost effective hole transport materials for perovskite solar cells.

Abstract

Benzo[c][1,2,5]thiadiazole(BT) core-based novel organic hole-transport materials (HTMs) BTT-PMe and BTT-2F are successfully synthesized for perovskite solar cells. The new HTMs are prepared by the simpler synthetic route with cost-effective purification steps. These HTMs are structurally confirmed by NMR, FT-IR and mass spectroscopy. The optical parameters are analysed using the UV–Vis spectrophotometer and cyclic voltammetry. To further confirmation of these properties we conducted the theoretical studies, which are fully matched with the experimental data. We have studied the effect of fluorine atom for its photo physical properties.

Introduction

Organic and inorganic perovskite solar cells (PSCs) have rapidly emerged as hottest area in the field of photovoltaic technologies, the breakthrough was done by Miyasaka et al., in 2009 (Kojima et al., 2009, Correa-Baena et al., 2017, Leijtens et al., 2017). These PSCs have reached power conversion efficiency (PCE) up to 24.2%. The PCE of PSCs is due to the proper aligned direct band gaps; broad and intensive light absorption with the high molar extinction coefficients; low exciton binding energy, long charge diffusion lengths (Ahmed et al., 2018, Chen et al., 2016) and superior charge carrier mobility of perovskites. Various PSCs have also been reported like super alkali perovskites and others to improve the photovoltaic properties (Xiaofeng et al., 2017, Tingwei et al., 2019, Tingwei et al., 2019, Tingwei et al., 2019).

The n-i-p PSCs constitutes of a conductive substrate, n-type semiconductor metal oxide, perovskite aslight-harvesting material, HTM and a metal electrode (Gao et al., 2014, Bi et al., 2016). The generated excitons in perovskite material after the incident of sunlight in PSCs diffuse to perovskite/HTM or electron transport material (ETM) interfaces and separated into holes and electrons. These separated holes and electrons move through HTM and ETM, collected by the respective electrodes. To prevent the charge recombination HTM and ETM plays a vital role. A good HTM should possess some desirable properties; high hole mobility, suitable energy levels, superior stability, good solubility, better solid-state morphology and favourable glass transition temperature (T_g). Till date 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) and PTAA (polytriarylamine) are the commercially available HTMs and showing over 22% PCE. However, the synthesis of spiro-OMeTAD involves multi-step, tedious synthetic routes, high cost and relatively low hole mobility in pristine form result in inferior PCE (Leijtens et al., 2012, Xu et al., 2017). Hence, the development of a new HTMs with simple synthetic protocol and less expensive is of high demand for efficient and low-cost production of PSCs. In this regard, several research groups are involved in developing cost-effective HTMs.

Till now a variety of HTMs are reported including p-type inorganic materials, such as CuSCN (Qin et al., 2014, Ye et al., 2015) or CuI (Sepalage et al., 2015) and small organic molecule HTMs (carbazole, triphenylamine, BT, fluorine, phenothiazine, S, N-heteropentacene, phenoxazine and pyrene core etc.,) (Cheng et al., 2016, Cheng et al., 2017, Cheng et al., 2018, Grisorio et al., 2017, Molina-Ontoria et al., 2016, Petrus et al., 2015, Rakstys et al., 2016). The Benzo[c] (Kojima et al., 2009, Correa-Baena et al., 2017, Chen et al., 2016) thiadiazole unit is well studied in bulk hetero-junction organic solar cells (BHJ) to improve the PCE. The central fluorinated benzene moiety HTM (DFTAB) was studied by Chen and co-workers in 2016. Linna Zhu et al. reported the BT and fluorinated BT core-based HTMs with a PCE of ~17.5 (Chen et al., 2016). Our research group is also involved in developing organic HTMs. One-step facile synthesis of a simple HTM for efficient PSCs has been reported to improve the PCE and low-cost PSCs. Recently, we have developed a very simple BT based HTMs, which have shown reasonable efficiency with suitable HOMO energy level (Swetha et al., 2018). In continuation of our work and to improve the photophysical properties, PCE and to know the effect of the fluorine atoms, we have developed two new HTMs. In this work, we have prepared BTT-PMe and BTT-2F HTMs, (Fig. 1) having 4,7-di(thiophen-2-yl)benzo[c] (Kojima et al., 2009, Correa-Baena et al., 2017, Chen et al., 2016)thiadiazole as core moiety with numerous aromatic entities. We have studied optical and electrochemical studies to know the nature of the fluorine atom.