Synthesis of new 2-(5-(4-alkyl-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)thiophen-2-yl)methylene)malononitrile: Dopant free hole transporting materials for perovskite solar cells with high power conversion efficiency

Two multi-chromophoric hybrid small molecule organic materials (ICTH1 and ICTH2) with D-pi-A architecture were synthesized and their applicability as hole transporting materials for perovskite solar cells was explored. The strong electron-donating dithieno[3,2-b:2',3'-d]pyrrole (D) and electron-accepting dicyanovinylene (A) moieties impart high intramolecular charge transfer nature to these materials and eliminates the need of additives or dopants during the fabrication of solar cell devices. Both ICTH1 and ICTH2 have deep HOMO level compared to spiro-OMeTAD and is advantageous to produce high Voc values. Hole transporting ability of ICTH1 and ICTH2 is ascertained by fabricating OFETs. Compared to spiro-OMeTAD, both ICTH1 and ICTH2 showed one order higher hole mobility. Perovskite solar cells (PSCs) with conventional architecture of FTO/Meso-TiO2/MAPbI(3)/HTM/Ag are fabricated using ICTH1 and ICTH2 as hole transporting and the simplest MAPbI(3) as perovskite materials. PSCs with ICTH2 and ICTH1 HTMs, showed power conversion efficiency (PCE) of 18.75% and 17.91% with active area of 2.04 and 2.18 cm(2), respectively. Under the similar conditions, PSCs with spiro-OMeTAD HTM showed 14.74% PCE with active area of 1.84 cm(2). Even after 40 days, solar cell devices with ICTH2 showed superior performance (PCE of 16.08%) over the spiro-OMeTAD devices (PCE of 6.77%). Apart from the superior device performance, other prominent factors like synthetic simplicity, dopant-free nature and low production cost make ICTH1 and ICTH2 as potential alternatives to spiro-OMeTAD for using as HTM in PSCs.