Abstract
An investigation of the function of an indolene-based organic dye, termed D149, incorporated in to solid-state dye-sensitized solar cells using 2,2',7,7'- tetrakis(N,N-di-p-methoxypheny-amine)-9,9' - spirobifluorene (spiro-OMeTAD) as the hole transport material is reported. Solar cell performance characteristics are unprecedented. under low light levels, with the solar cells delivering up to 70% incident photon-to-current efficiency (IPCE) and over 6% power conversion efficiency as measured under simulated air mass (AM) 1.5 sun light at 1 and 10 mW cm(-2). However, a considerable nonlinearity. in the photocurrent as intensities approach "full sun" conditions is observed and the devices deliver up to 4.2% power conversion efficiency under simulated sun light of 100 mW cm(-2). The influence of dye-loading upon solar cell operation is investigated and the thin films are probed via photoinduced absorption (PIA) spectroscopy, tune-correlated single-photon counting (TCSPC), and photoluminescence quantum efficiency (PLQE) measurements in order to deduce the cause for the non ideal solar cell performance. The data ! suggest that electron transfer from the photoexcited sensitizer into the TiO2 is only between 10 to 50% efficient and that ionization of the photo excited dye via hole transfer directly to spiro-OMeTAD dominates the charge generation process: A persistent dye bleaching signal is also observed and assigned to a remarkably high density of electrons "trapped" within the dye phase, equivalent to 1.8 x 10(17) cm(-3) under full sun illumination. it is believed that this localized space charge build-up upon the sensitizer is responsible for the non-linearity of photocurrent with intensity and nonoptimum solar. cell performance under full sun conditions.