A naphthalene diimide side-chain polymer as an electron-extraction layer for stable perovskite solar cells†
Poly(N-(5-(5-norbornene-2-carbonyl)oxy)pentyl)-N′-n-hexyl-naphthalene-1,8:4,5-bis(dicarboximide) has been synthesized by esterification of (N-(5-hydroxypentyl)-N′-n-hexyl-naphthalene-1,8:4,5-bis(dicarboximide)) with exo-5-norbornene-2-carboxylic acid, and has been polymerized using the first-generation Grubbs initiator. This side-chain polymer exhibits good transparency throughout the visible (absorption onset at ca. 400 nm), good solubility in common low- and medium-polarity organic solvents, good resistance to dimethylformamide, and appropriate electron affinity for use as an electron-extraction layer in lead-halide perovskite solar cells. The performance of this polymer in n-i-p perovskite solar cells was compared to that of several small-molecule naphthalene-1,8:4,5-bis(dicarboximide) derivatives and of SnO2. Solar cells using the polymer exhibited open-circuit voltages of up to 1.02 V, short-circuit currents of over 21 mA cm−2, and power conversion efficiencies (PCE) reaching 14% which stabilize at 13.8% upon 90 s of illumination. Meanwhile control SnO2 devices exhibited a PCE of ca. 16%, and small-molecule devices gave PCE values of less than 10%. The devices employing the polymer exhibited improved long-term stability relative to the SnO2 control devices under continuous illumination.