Solid-State Mesostructured Perovskite CH₃NH₃PbI₃ Solar Cells: Charge Transport, Recombination, and Diffusion Length

We report on the effect of TiO₂ film thickness on charge transport and recombination in solid-state mesostructured perovskite CH₃NH₃PbI₃ (via one-step coating) solar cells using spiro-MeOTAD as the hole conductor. Intensity-modulated photocurrent/photovoltage spectroscopies show that the transport and recombination properties of solid-state mesostructured perovskite solar cells are similar to those of solid-state dye-sensitized solar cells. Charge transport in perovskite cells is dominated by electron conduction within the mesoporous TiO₂ network rather than from the perovskite layer. Although no significant film-thickness dependence is found for transport and recombination, the efficiency of perovskite cells increases with TiO₂ film thickness from 240 nm to about 650–850 nm owing primarily to the enhanced light harvesting. Further increasing film thickness reduces cell efficiency associated with decreased fill factor or photocurrent density. The electron diffusion length in mesostructured perovskite cells is longer than 1 μm for over four orders of magnitude of light intensity

5,10-Dihydroindolo[3,2‑<i>b</i>]indole-Based Copolymers with Alternating Donor and Acceptor Moieties for Organic Photovoltaics

A series of new donor–acceptor π-conjugated copolymers incorporating 5,10-dihydroindolo­[3,2-<i>b</i>]­indole (DINI) as an electron donating unit have been designed, synthesized, and explored in bulk heterojunction solar cells with diketopyrrolopyrrole and thienopyrroledione as the electron accepting units. A significant effect of the size and shape of the pendant alkyl substituents attached to the DINI unit on the optical and electronic properties of the copolymers is described. Our study reveals a good correlation between the theoretical calculations performed on the selected materials and the experimental HOMO, LUMO, absorption spectra, and band gap energies of the corresponding copolymers. The band gaps of the conjugated copolymers can be tailored over 0.4 eV by the electron-withdrawing nature of the different acceptor units to provide better overlap with the solar spectrum, and the energy levels can be tuned ∼0.2 eV depending on the alkyl substituents employed. For the polymers in this study, a nonoptimized power conversion efficiency as high as 3% was observed