Abstract
Electron transfer dynamics in dye sensitized solar cells (DSSCs) employing triphenylamine Y123 dye were investigated by means of femtosecond broadband transient absorption spectroscopy in the visible and mid-IR range of detection. The electron injection process to the titania conduction band was found to appear biphasically with the time constant of the first component within 350 fs and that of the second component between 80 and 95 ps. Subsequently, the effects of continuous irradiation on the ultrafast and fast electron transfer processes were studied in the systems comprising Y123 dye or carbazole MK2 dye in combination with cobalt- or copper-based redox mediators: [Co(bpy)3](B(CN)4)2/3 (bpy = 2,2′-bipyridine) or [Cu(tmby)2](TFSI)1/2 (tmby = 4,4′,6,6′ tetramethyl-2,2′-bipyridine, TFSI = bis(trifluoromethane)sulfonamide). We have found that the steady-state illumination led to acceleration of the electron injection process due to the lowering of titania conduction band edge energy. Moreover, we have observed that the back electron transfer to the oxidized dye was suppressed. These changes in the initial (up to 3 ns) charge separation efficiency were directly correlated with the photocurrent enhancement.
Highlights
Dye sensitized solar cells (DSSCs) have been much improved since the breakthrough achieved by O’Regan and Graetzel in 1991 [1]
To get insight into the dynamics of excited state of Y123, the transient absorption (TA) measurements were performed using three different pump wavelengths in the range of the dye’s operation spectra. This strategy was chosen taking into account the hypothesis that that different electron injection processes might occur in the system with different contributions, depending on the excitation wavelengths
The ratio of the residual and maximal signal values of the recombination kinetics (SRES parameter), can be a good representation of the charge separation efficiency in a short time scale as we have shown in our previous studies of DSSCs exploiting organic sensitizers [41,43,44,46,52]
Summary
Dye sensitized solar cells (DSSCs) have been much improved since the breakthrough achieved by O’Regan and Graetzel in 1991 [1]. DSSC have been recently outperformed by other emerging photovoltaic technologies (including perovskite [2], quantum dot solar cells [3], and organic photovoltaics [4]) in terms of power conversion efficiency (PCE) in full sunlight conditions, they are experiencing a renaissance and have attracted interest again, thanks to their superior performance in low ambient light conditions [5,6,7]. The introduction of fully organic D-π-A dyes [8,9,10,11] replacing ruthenium complexes [12] permitted bypassing a problem of low abundance and high price of this element as well as the issue of relatively low extinction coefficients of its coordination compounds Another milestone achievement has been the replacement of iodide/triiodide (I− /I3 − ) redox couple by new generation of coordination compounds of cobalt (Co3+ /Co2+ ) [9] and more environmentally friendly copper (Cu2+ /Cu+ ) [13,14,15], which allowed achieving over 1.1 V open circuit voltages (VOC ) in highly efficient DSSCs
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