Diketopyrrolopyrrole-Based Cation as a Semiconducting Spacer in Layered Perovskite

Incorporating π-conjugated organic spacers in layered "2D" perovskites in order to extend solar light harvesting has remained a challenge, as typically-incorporated spacers (e.g. phenethylammonium, PEA⁺) do not absorb deep into the visible spectrum. Here, we introduce a visible-light-absorbing π-conjugated dithiophene-diketopyrrolopyrrole-based spacer cation that forms a pure iodide layered perovskite that exhibits complementary organic-inorganic absorption and a type-II nano-heterojunction electronic structure. By chain length engineering, a dihexyl-substituted diketopyrrolopyrrole cation (DPP-dH²⁺) is identified as the optimal length required to form an ordered layered structure (DPP-dH)PbI₄. Transient absorption spectroscopy reveals bidirectional charge transfer, with hole transfer from the inorganic [PbI₄]^2- slabs to the organic spacer, and electron transfer in the reverse direction upon excitation at longer wavelengths. Time-resolved microwave conductivity and space-charge-limited current measurements demonstrate reduced trap density and electron mobilities up to 1.3 × 10^-3 cm²V^-1s^-1. Leveraging its extended visible absorption, the resulting layered perovskite exhibits a marked improvement in photovoltaic power conversion efficiency compared to (PEA)₂PbI₄ as well as an extended incident photon harvesting reaching 650 nm (1.9 eV), establishing diketopyrrolopyrrole spacers as a promising platform for next-generation optoelectronics.