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Origin of gap states in the electron transport layer of organic solar cells

The mechanism of the formation of gap states in bathocuproine (BCP) used as an electron transport layer (ETL) in poly(3-hexylthiophene) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) based solar cell is investigated. The photo-generated electrons in the active layer are extracted to a cathode electrode through the gap states of BCP located near the lowest unoccupied molecular orbital (LUMO) level of the electron acceptors. In the most extensively used BCP, the origin of the formation of gap states is not well understood. We show that gap states in BCP are induced by out-diffusion of P3HT molecules into the BCP layer. A Secondary Ion Mass Spectrometry (SIMS) depth profile reveals that the out-diffusion of P3HT occurs during the deposition of BCP on the P3HT:PCBM active layer. The mixing of P3HT molecules with the BCP layer induces a chemical reaction between BCP and P3HT to produce the N–S bonds (between the nitrogen from BCP and the sulfur from P3HT), which acts as gap states in BCP. The power conversion efficiency increased from 0.03% to 3.0% as the deposition rate is reduced from 5.0 A s−1 to 0.1 A s−1. This increase originates from the change in the diffusion length of P3HT with the deposition rate of BCP, leading to an increase in conductivity as well as the alignment of the Fermi level through the gap states.