Surface Photovoltage Spectroscopy Resolves Interfacial Charge Separation Efficiencies in ZnO Dye-Sensitized Solar Cells

Abstract

Any optimization of dye-sensitized solar cells (DSSCs) must consider the energetics and charge transfer kinetics of the dye, substrate, and redox couple. Here, we use surface photovoltage spectroscopy to probe the energetics and photochemical charge transfer efficiency in fluorenyl-thiophene dye (OD-8)-sensitized ZnO films. Discrete photochemical charge transfer events at the dye–ZnO interface and at the dye– I–/I3 – or [Co­(2,2′-bipyridyl)3]3+/2+ interfaces can be observed as negative photovoltage under dye excitation at 1.7 eV (460 nm). Without a redox couple, charge separation at the ZnO/dye interface is only 4% effective, likely due to the short electron hole separation distance. In the presence of the redox couples, charge separation approaches 26–54% of the theoretical limit, emphasizing the importance of the dye regeneration reaction via the redox couple. On the basis of the open circuit voltage, charge separation in fully assembled DSSCs is 100% efficient with iodide, but only 61% efficient with the cobalt redox couple. This suggests that device improvements are possible by optimizing the dye regeneration reaction with the cobalt redox couple.

Cyanoborates

“…[42c], In particular, Co(II)/Co(III) complexes with the TCB ion have drawn attention as redox shuttle for DSSCs in recent years. [56f], [57d], [57g], The application of IL‐based electrolytes in photovoltaic devices was reviewed, recently. [55v], [57j], [60h], Further information on selected properties and applications of tetracyanoborate ionic liquids can be found in recent review articles[52a], [53g], [58b], and a monograph…”

Photochemical Charge Separation and Dye Self-Oxidation Control Performance of Fluorescein, Rose Bengal, and Triphenylamine Dye-Sensitized Solar Cells

“…As can be seen in Figure , the conduction band edges of both SrTiO 3 and TiO 2 lie above the conduction band edges of FTO and ITO, allowing photochemical charge transfer from the ETL to the TCO. From the band edge offset, the theoretical photovoltage can be calculated, as shown previously. , This yields −0.7 V (TiO 2 /FTO) and −0.46 V (TiO 2 /ITO), slightly larger than the experimental values of −0.3 to −0.4 V for TiO 2 /FTO and −0.25 to −0.3 V for TiO 2 /ITO in Figures , S2, and S3. For SrTiO 3 , the theoretical photovoltage values (−1.5 V for SrTiO 3 /FTO and −1.26 V for SrTiO 3 /ITO) are somewhat smaller than the experimental values (−1.5 to −2.2 V for SrTiO 3 /FTO and −1.0 to −1.7 V for SrTiO 3 /ITO).…”

“…The former is a measure of the experimental electron transfer, while the latter is a measure of the thermodynamic driving force for electron transfer. Thus, the ratio between these two parameters allows a comparison of the electron transfer efficiency of the dyes, independent of the driving force …”

“…While KFM uses fixed illumination, in SPS, the wavelength of the illumination is varied. The measured surface photovoltage (ΔCPD under illumination) then provides information about the effective band gap of the absorber, defect states, the majority carrier type, and the efficiency of charge transfer. − For DSSCs derived from the organic fluorenyl-thiophene dye (OD-8), we recently used SPS to confirm the importance of the dye regeneration reaction for achieving good charge separation at dye. Additionally, the formation of Zn 2+ –dye aggregates was observed in ZnO-sensitized films .…”

Nanoscale Surface Photovoltage Mapping of 2D Materials and Heterostructures by Illuminated Kelvin Probe Force Microscopy

“…Surface photovoltage (SPV) spectroscopy is a powerful technique for understanding the influence of surface electronic states on the photoresponse of materials , and has been used extensively to understand charge dynamics in a variety of semiconductor materials. − This technique involves illuminating a semiconductor material and measuring the resulting surface photovoltage, typically using a metal–insulator–semiconductor configuration. A limitation of this technique, however, is its inability to measure spatial variation, which makes it impractical for studying individual nanostructures.…”