Simultaneous transient absorption and transient electrical measurements on operating dye-sensitized solar cells: Elucidating the intermediates in iodide oxidation

Simultaneously acquired transient absorption, photocurrent, and photovoltage signals are used to characterize operational dye-sensitized solar cells (DSSCs) under 1 sun illumination. At open circuit, the biphasic decay of both photovoltage and 980 nm absorption have identical decay profiles. The data show that, in devices incorporating N719 [(Bu₄N) ₂[Ru(dcbpyH)₂(NCS)₂] (dcbpy = 4, 4′-dicarboxy-2, 2′-bipyridyl) as the sensitizer, the 950-1020 nm absorption can be assigned to electrons alone. This differs from some previous assignments. At short circuit, the comparison of the integrated photocurrent transient, electron absorption, and the absorption of the oxidized dye (830 nm) can be used to show relative loss of photocurrent to oxidized dyes. In addition, we show that, under operational conditions, there is no evidence for long-lived di-iodine radicals (I₂^•-). Further, we show that the decay of the transient bleach at 560 nm, previously assigned to the bleach of the groundstate dye, contains additional long-lived bleach components that correlate with the electron transient. This calls into question the use of the bleach transients for kinetic studies of dye regeneration. The use of simultaneous transient absorption and electrical signals significantly increases the information available from transient absorption studies, at little extra cost in time or equipment.