Exploring new p-type semiconductor nanoparticles alternative to the commonly used NiO is crucial for p-type dye-sensitized solar cells (p-DSSCs) to achieve higher open-circuit voltages (Voc). Here we report the first application of delafossite CuGaO2 nanoplates for p-DSSCs with high photovoltages. In contrast to the dark color of NiO, our CuGaO2 nanoplates are white. Therefore, the porous films made of these nanoplates barely compete with the dye sensitizers for visible light absorption. This presents an attractive advantage over the NiO films commonly used in p-DSSCs. We have measured the dependence of Voc on the illumination intensity to estimate the maximum obtainable Voc from the CuGaO2-based p-DSSCs. Excitingly, a saturation photovoltage of 464 mV has been observed when a polypyridyl Co(3+/2+)(dtb-bpy) electrolyte was used. Under 1 Sun AM 1.5 illumination, a Voc of 357 mV has been achieved. These are among the highest values that have been reported for p-DSSCs.

We report the first application of cyclometalated ruthenium complexes of the type Ru[(N∧N)2(C∧N)]+ as sensitizers for p-type NiO dye-sensitized solar cells (NiO p-DSCs). These dyes exhibit broad absorption in the visible region. The carboxylic anchoring group is attached to the phenylpyridine ligand, which results in efficient hole injection. Moreover, the distance between the Ru[(N∧N)2(C∧N)]+ core and the carboxylic anchoring group is systematically varied by inserting rigid phenylene linkers. Femtosecond transient absorption (TA) studies reveal that the interfacial charge recombination rate between reduced sensitizers and holes in the valence band of NiO decreases as the number of phenylene linkers increases across the series. As a result, the solar cell made of the dye with the longest spacer (O12) exhibits the highest efficiency with both increased short-circuit current (J sc) and open-circuit voltage (V oc). The incident photon-to-current conversion efficiency (IPCE) spectra match well with the absorption spectra of sensitizers, suggesting the observed cathodic current is generated from the dye sensitization. In addition, the absorbed photon-to-current conversion efficiencies (APCEs) display an increment across the series. We further studied the interfacial charge recombination of our solar cells by electrochemical impedance spectroscopy (EIS). The results reveal an enhanced hole lifetime as the number of phenylene linkers increases. This study opens up opportunities of using cyclometalated Ru complexes for p-DSCs.

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p-Type dye-sensitized solar cells (p-DSCs) have attracted increasing attention recently, but they suffer from low fill factors (FFs) and unsatisfactory efficiencies. A full comprehension of the hole transport and recombination processes in the NiO p-DSC is of paramount importance for both the fundamental study and the practical device optimization. In this article, NiO p-DSCs were systematically probed under various bias and illumination conditions using electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent spectroscopy (IMPS), and intensity modulated photovoltage spectroscopy (IMVS). Under the constant 1 sun illumination, the recombination resistance (R rec ) of the cell deviates from an exponential relationship with the potential and saturates at ∼130 Ω cm 2 under the short circuit condition, which is ascribed to the overwhelming recombination with the reduced dye anions. Such a small R rec results in the small dc resistance, which decreases the "flatness" of the J−V curve. The quantitative analysis demonstrates that the FF value is largely attenuated by the recombination of holes in NiO with the reduced dyes. Our analysis also shows that if this recombination can be eliminated, then an FF value of 0.6 can be reached, which agrees with the theoretical calculation with a V oc of 160 mV.

Three organic donor-acceptor dyes with different bridging ligands are reported for p-type NiO dye-sensitized solar cells (DSCs). The 3,4-ethoxythiophene linker outperforms thiophene and phenyl groups giving the best solar cell performance with J sc ¼ 1.74 mA cm À2 , V oc ¼ 90 mV, FF ¼ 0.38, and an efficiency of 0.060%.Tandem DSCs, where both cathode and anode are sensitized with dye molecules, hold great promise for higher efficiencies than the conventional n-type DSCs. 1-5 However, the development of tandem DSCs is limited by the low efficiency of p-type DSCs. Currently, NiO is the semiconductor support used in p-DSCs. It limits the energy conversion efficiency due to its low transparency to the visible light and also its valence band edge. Many dye molecules for p-DSCs suffer from the fast charge recombination between holes in NiO and the reduced dye or I À in the electrolyte. [6][7][8][9] Therefore, developing new p-type semiconductors and sensitizers is the grand challenge in this promising area.Lindquist and co-workers first demonstrated the generation of cathodic photocurrent with an erythrosine-sensitized NiO cathode. 1 Since then, a few other photosensitizers including coumarin, 7,8,10 perylene monoimide 6,11 and porphyrin 9 derivatives have been synthesized. Sun, Hagfeldt and co-workers first reported push-pull dyes with anchoring group on the triphenylamine donor. Solar cells sensitized with those dyes gave high incident photon-to-electron conversion efficiencies (IPCEs). 12-14 More recently, efficient sensitizers consisting of oligothiophene units have been reported, which exhibit long-lived charge separated excited states and have produced the most efficient p-type DSCs so far. 15 In the design of a donor-acceptor dye, the linker group between the donor and the acceptor plays an important role in terms of optical absorption and charge transfer properties, and thus has been widely studied for n-type DSCs. 3,[16][17][18] However, in terms of p-type DSCs, the studies are limited. 12,14 Sun et al. have shown that the phenyl linker in their organic chromophores provides higher IPCE but narrower spectral response than the thiophene linker. 12 Nattestad et al. have varied the length of the oligothiophene bridge in order to control the spatial separation of the photo-generated charge carriers. 15 Among the various linkers, 3,4-ethoxythiophene (EDOT) has been proved to be an excellent bridging ligand in the sensitizers designed for n-type DSCs. 16 The planarity of the dye molecule can facilitate the electron transfer from donor to acceptor. However, to our best knowledge, EDOT has not been reported as a linker in donoracceptor dyes for p-type DSCs. Herein we report three new organic donor-acceptor dyes with different linker groups designed for NiObased p-type DSCs (Scheme 1). The dyes are comprised of triphenylamine as an electron donor and dicyanovinyl as an electron acceptor. We have compared three linkers: phenyl, thienyl, and EDOT groups, and demonstrated that EDOT outperforms the other two linkers. This i...