18.4 % Organic Solar Cells Using a High Ionization Energy Self‐Assembled Monolayer as Hole‐Extraction Interlayer. Issue 17 (17th May 2021) - Lin, Yuanbao
Abstract: Self‐assembled monolayers (SAMs) based on Br‐2PACz ([2‐(3, 6‐dibromo‐9 H ‐carbazol‐9‐yl)ethyl]phosphonic acid) 2PACz ([2‐(9H‐Carbazol‐9‐yl)ethyl]phosphonic acid) and MeO‐2PACz ([2‐(3, 6‐dimethoxy‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid) molecules were investigated as hole‐extracting interlayers in organic photovoltaics (OPVs). The highest occupied molecular orbital (HOMO) energies of these SAMs were measured at −6.01 and −5.30 eV for Br‐2PACz and MeO‐2PACz, respectively, and found to induce significant changes in the work function (WF) of indium‐tin‐oxide (ITO) electrodes upon chemical functionalization. OPV cells based on PM6 (poly[(2, 6‐(4, 8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo[1, 2‐b:4, 5‐b']dithiophene))‐alt‐(5, 5‐(1', 3'‐di‐2‐thienyl‐5', 7'‐bis(2‐ethylhexyl)benzo[1', 2'‐c:4', 5'‐c']dithiophene‐4, 8‐dione)]) : BTP‐eC9 : PC71 BM ([6, 6]‐phenyl‐C71‐butyric acid methyl ester) using ITO/Br‐2PACz anodes exhibited a maximum power conversion efficiency (PCE) of 18.4 %, outperforming devices with ITO/MeO‐2PACz (14.5 %) and ITO/poly(3, 4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT : PSS) (17.5 %). The higher PCE was found to originate from the much higher WF of ITO/Br‐2PACz (−5.81 eV) compared to ITO/MeO‐2PACz (4.58 eV) and ITO/PEDOT : PSS (4.9 eV), resulting in lower interface resistance, improved hole transport/extraction, lower trap‐assisted recombination, and longer carrier lifetimes. Importantly, the ITO/Br‐2PACz electrode was chemically stable, and after removal of the SAM it could be recycled and reused to construct fresh OPVs with equally impressive performance. Abstract : Super SAM : Two self‐assembled monolayers (SAMs; Br‐2PACz and MeO‐2PACz) are investigated as hole‐extracting interlayer in organic photovoltaics and compared against the widely used PEDOT : PSS. Cells based on the ternary bulk‐heterojunction blend PM6 : BTP‐eC9 : PC71 BM and ITO/Br‐2PACz as the anode exhibit the highest power conversion efficiency of 18.4 %, outperforming devices with ITO/MeO‐2PACz (14.5 %) and even ITO/PEDOT : PSS (17.5 %).