dients in the photoactive layer, [ 1b , 2 ] which make inverted device architectures an attractive approach for all types of PSCs. Reversing the polarity of charge-collecting electrodes in i-PSCs can be achieved by utilizing appropriate electron-transporting and hole-transporting interlayers at the indium tin oxide (ITO)/active layer and active layer/metal interfaces, respectively.As of today, transition metal oxides (MOs) constitute one of the most promising classes of interlayer materials due to their relatively high mobility, environmental stability, and high transparency. Significant progress has been reported not only for electron-transporting layer MOs, such as Cs 2 CO 3 , TiO x , and zinc oxide (ZnO), [ 3 ] but also for various hole-transporting MOs, including MoO 3 , V 2 O 5 , NiO, Sb 2 O 3 , and WO 3 , [ 4 ] which have been effective in improving power conversion effi ciencies (PCEs) in i-PSCs. However, in principle, the polar surfaces of MOs may have poor interfacial contact with hydrophobic organic active layer, inhibiting electron extraction in i-PSCs.To overcome phase compatibility issues at the hybrid organic/inorganic interfaces in i-PSCs, we have introduced a variable amount of phosphonate-terminated nonionic side chain substitution into an isoindigo (IIG)-based model polymer. The polymers were designed with the following considerations: (i) The phosphonate groups have good compatibility with MOs due to their polar and hydrophilic character, which is expected to improve interfacial contact via interfacial dipole formation, improving charge transport and collection. (ii) The incorporation of phosphonate side chains can both increase the dielectric constant of the donor polymer and suppress nongeminate recombination losses in i-PSCs, allowing the possibility of longer carrier lifetimes and improved short-circuit current ( J SC ). [ 5 ] (iii) A further aspect of the nonionic phosphonates is the possibility to avoid electrical fi eld-induced redistribution of ions/electrode interfaces; a mechanism which leads to instability when conjugated polyelectrolytes with ionized functionalities are used in the same context. [ 6 ] In this work, we have examined the effects that phosphonate side chain substitution has on the semiconducting properties of IIG-based polymers. Several factors were taken into account in Considering that a high compatibility at hybrid organic/inorganic interfaces can be achieved using polar and hydrophilic functionalities, this approach is used to improve inverted polymer solar cell performance by introducing nonionic phosphonate side chains (at 0%, 5%, 15%, and 30% substitution levels) into a series of isoindigo-based polymers (PIIGDT-P n ). This approach led to ≈20% improvement in power conversion effi ciency compared to a nonmodifi ed control polymer, via an increased short-circuit current ( J SC ). This enhancement is believed to stem from reduced nongerminate recombination and improved charge carried extraction when the level of phosphonate substitution is optimized. These res...