Lithium‐Ion‐Based Conjugated Polyelectrolyte as an Interface Material for Efficient and Stable Non‐Fullerene Organic Solar Cells

Abstract

An eco‐friendly n‐type water/alcohol‐soluble conjugated polyelectrolyte PFEOSO3Li was synthesized and applied as a cathode interfacial layer in organic solar cells. The π‐delocalized polyfluorene backbone has an intimate connection with the hydrophobic active layer, and the side chain with lithium ion may move toward the ZnO layer through the self‐assembly property of conjugated polyelectrolytes. UV photoelectron spectroscopy indicated that modification with PFEOSO3Li dramatically lowers the work function of indium‐doped tin oxide (ITO)/ZnO and may form strong interfacial dipoles between ZnO and the active layer. Meanwhile, introduction of lithium ions as spectator cations may contribute to reduction of the intrinsic surface defects of ZnO. The green emission in the photoluminescence spectrum of ZnO disappeared after modification with PFEOSO3Li. In addition, the roughness of ZnO barely changed after coating with PFEOSO3Li, and the surface became more hydrophobic, which demonstrates that the thin conjugated polyelectrolyte layer exhibits good adhesion with both ZnO and the active layer. These phenomena indicate that the introduction of PFEOSO3Li makes ITO/ZnO an efficient cathode. As a result, inverted organic solar cell devices with ZnO/PFEOSO3Li double‐interlayers exhibit high efficiencies of 11.7 and 10.6 % for PBDB‐T:IT‐M and PBDB‐T:ITIC blend systems, respectively.

Self‐Assembled Monomolecular Layer Modified ZnO for Efficient Inverted Polymer Solar Cells with 11.53% Efficiency

“…These results demonstrate that the APTES molecule is introduced via covalent bonding on the ZnO interface, as schemed in Figure a. The probably covalent bonding at ZnO and APTES is believed to form interfacial dipoles at their interfaces, which is also beneficial to form an excellent contact between the CIL and the photoactive layer for efficient charge transport. Figure c displays the XPS O 1 s peaks of ZnO and ZnO‐APTES films.…”

“…Cathode interfacial materials, as one of the key materials of PSCs, require high light transmittance, high electron mobility, and matching energy levels with organic semiconductors . Many cathode interfacial materials have been successfully applied in the inverted PSCs, including metal salts such as Cs 2 CO 3 , SrF 2 , and titanium chelate and conjugated or non‐conjugated polyelectrolytes . Among various cathode interface materials, transition metal oxidation such as SnO 2 and ZnO has received extensive attention for its excellent optical transparency, high reflection constant, and thermal and chemical stability .…”

Solution‐Processable 2D α‐In2Se3 as an Efficient Hole Transport Layer for High‐Performance and Stable Polymer Solar Cells

“…The correlation between V oc and P is determined by the formula V oc ∝ nkTq −1 ln( P ), where k is Boltzmann's constant, n is the ideality factor, T is the temperature, and q is the elemental charge. When bimolecular recombination is the dominant mechanism in the devices, the slope of V oc versus ln( P ) is equal to 1( kT / q ), whereas the slope is close to 2( kT / q ), implying that geminate recombination is dominant and trap‐assisted recombination becomes severe . As shown in Figure b, the slope values are 1.48 ( kT / q ), 1.37 ( kT / q ), 1.29 ( kT / q ), 1.16 ( kT / q ), and 1.28 ( kT / q ) for devices with 0, 5, 8, 13, and 16 nm α‐In 2 Se 3 , respectively.…”

“…The low J sat (12.46 mA cm −2 ) and J ph / J sat (87.68% and 66.89%) values are obtained in the devices without HTL. Assuming that all photogenerated excitons can be dissociated into free charges and collected at a high V eff (>2 eV), J sat is only determined by the total number of absorbed photons . The highest J sat of the α‐In 2 Se 3 device (13 nm) indicates that more photons are harvested by the active layer.…”

Progress in Materials, Solution Processes, and Long‐Term Stability for Large‐Area Organic Photovoltaics

“…[ 134 ] To suppress the degradation reaction of the nonfullerene acceptor caused by the photocatalytic activity of ZnO, insertion of a thin interlayer at the interface or surface modification of the ZnO ETL in inverted OPV devices has been extensively studied using lithium doping, [ 135 ] aluminum doping, [ 136 ] magnesium doping, [ 137 ] C 60 self‐assembled monolayers, [ 119 ] pyrene‐bodipy dye, [ 138 ] and the water‐soluble conjugated polyelectrolyte PF EO SO 3 Li. [ 139 ] Surface modification of the ZnO ETL reduces interface trap‐assisted recombination losses and has enhanced their long‐term stability, resulting in extrapolated T80 lifetimes longer than 20 years.…”