Efficiency Limits In Wide-Bandgap Ge-Containing Donor Polymer:Nonfullerene Acceptor Bulk Heterojunction Solar Cells

A precise picture of the photophysics that determine the efficiency of nonfullerene acceptor (NFA) bulk heterojunction (BHJ) organic solar cells is still missing, yet needed to mitigate the remaining loss channels. Herein, charge carrier generation and recombination dynamics in blends of a novel wide-bandgap germanium-containing donor polymer, namely, PEHGeNDT-BT, paired with either O-IDTBR or O-IDTBCN as NFA in BHJ solar cells are investigated by (ultrafast) transient spectroscopy and time-delayed collection field (TDCF) experiments. Photovoltaic devices yield moderate power conversion efficiencies (PCEs) of 5.3% when using O-IDTBCN as the acceptor, and only about 2% when using O-IDTBR as the acceptor, the latter severely limited by its low photocurrent and moderate fill factor (FF) of approximate to 43%. Time-resolved photoluminescence experiments reveal limited exciton quenching as one loss channel in O-IDTBR-based blends, accompanied by significant geminate recombination further reducing the photocurrent, while field-dependent charge generation is identified as the origin of the low FF. Geminate recombination is less in the O-IDTBCN blend and charge generation is field-independent, leading to improved photocurrent and FF. Carrier drift-diffusion simulations of the devices' current-voltage (J-V) characteristics confirm that the experimentally determined kinetic parameters and process yields can reproduce the measured J-V curves under steady-state solar illumination.