ABSTRACT
Homoleptic zinc(II) complexes of di(phenylacetylene)azadipyrromethene (e.g., Zn(WS3)2) are potential non-fullerene electron acceptors for organic photovoltaics. To tune their properties, fluorination of Zn(WS3)2 at various positions was investigated. Three fluorinated azadipyrromethene-based ligands were synthesized with fluorine at the para-position of the proximal and distal phenyl groups, and at the pyrrolic phenylacetylene moieties. Additionally, a CF3 moiety was added to the pyrrolic phenyl positions to study the effects of a stronger electron withdrawing unit at that position. The four ligands were chelated with zinc(II) and BF2+ and the optical and electrochemical properties were studied. Fluorination had little effect on the optical properties of both the zinc(II) and BF2+ complexes, with λmax in solution around 755 nm and 785 nm, and high molar absorptivities of 100 × 103 M-1cm-1 and 50 × 103 M-1cm-1, respectively. Fluorination of Zn(WS3)2 raised the oxidation potentials by 0.04 V to 0.10 V, and the reduction potentials by 0.01 V to 0.10 V, depending on the position and type of substitution. The largest change was observed for fluorine substitution at the proximal phenyl groups and CF3 substitution at the pyrrolic phenylacetylene moieties. The later complexes are expected to be stronger electron acceptors than Zn(WS3)2, and may enable charge transfer from other conjugated polymer donors that have lower energy levels than poly(3-hexylthiophene) (P3HT).
ABSTRACT
In order to promote the development of solar cells with varying types of sensitizers including dyes and quantum dots, it is crucial to establish a general experimental analysis that accounts for all important optical and electrical losses resulting from interfacial phenomena. All of these varying types of solar cells share common features where a mesoporous scaffold is used as a sensitizer loading support as well as an electron transport material, which may result in light scattering. The loss of efficiency at interfaces of the sensitizer, the mesoporous TiO2 nanoparticle films, the FTO conductive layer, and the supportive glass substrate should be considered in addition to the photoinduced electron transport properties within a cell. On the basis of optical parameters, one can obtain the internal quantum efficiency (IQE) of a solar cell, an important parameter that cannot be directly measured but must be derived from several key experiments. By integrating an optical loss model with an electrical loss model, many solar cell parameters could be characterized from electro-optical observables including reflectance, transmittance, and absorptance of the dye sensitizer, the electron injection efficiency, and the charge collection efficiency. In this work, an integrated electro-optical approach has been applied to SiPc (Pc 61) dye-sensitized solar cells for evaluating the parameters affecting the overall power conversion efficiency. The absorptance results of the Pc 61 dye-sensitized solar cell provide evidence that the adsorbed Pc 61 forms noninjection layers on TiO2 surfaces when the dye immersion time exceeds 120 min, resulting in shading light from the active layer rather than an increase in photoelectric current efficiency.
