Singlet Fission for Photovoltaics with 130 % Injection Efficiency

Abstract

A novel pentacene dimer (P2), and a structurally analogous monomer (P1), bearing a carboxylic acid were designed and synthesized for use in n‐type dye‐sensitized solar cells (DSSC). In P2, the triplet excited states that are formed via the rapid, spin‐allowed process singlet fission (SF) were expected to enable carrier multiplication in comparison to the slow, spin‐forbidden intersystem crossing (ISC) seen in P1. We opted for meta‐positioning of the two pentacenes and the carboxylate anchor in P2 to balance the intramolecular dynamics of singlet fission and electron injection. Electron injection from energetically low‐lying triplet excited states of pentacenes necessitated, however, the intrinsic and extrinsic lowering of the Fermi level of the semiconductor. Device analysis, in combination with incident photon‐to‐current efficiency assays as well as femtosecond transient absorption measurements, showed that indium‐zinc oxide (IZO) in the presence of Li+ emerged as the optimum choice for the photoelectrodes. We documented efficient electron injection from the triplet excited states of P1 and P2 with carrier multiplication of nearly 130%.