Two-Step Photon Absorption in an Expanded Perovskite Analog with a Molecular Intermediate Band

Intermediate-band solar cells (IBSCs) introduce an intermediate band (IB) within the absorber’s band gap, allowing consecutive sub-band-gap two-step photon absorption (TSPA) to supplement band-gap absorption, enabling a 50% increase in the fundamental device efficiency limit. Current IBSC absorbers, however, introduce the IB as impurities in trace concentrations, affording weak sub-band-gap absorption. We demonstrate a novel material design for TSPA using (dmpz)Pb2I6 (dmpz2+ = N,N′-dimethylpyrazinium) containing a molecular IB. Notably, the organic–inorganic hybridization evident in electronic structure calculations, where electronic states from the iodide appear in the dmpz-based bands, and large thermal fluctuations revealed by molecular dynamics trajectories enhance subgap absorption at 300 K. Transient electronic signatures of photoreduced dmpz•+ radicals verify the charge-transfer nature of the sub-band-gap absorption, whereas coherent high-frequency vibrations reveal vibronic coupling between dmpz and the inorganic framework. Proof-of-concept solar cells with (dmpz)Pb2I6 generate increased photocurrent upon subgap IR excitation and display quadratic-to-linear power dependence. This first demonstration of TSPA with an intrinsic organic IB offers a new material class for IBSCs.