Vasilis Nikolaou

Recent advances and insights in dye-sensitized NiO photocathodes for photovoltaic devices

Solar energy is undoubtedly one of the most exploitable energy sources providing a potential solution to address the environmental issues deriving from the excessive use of fossil fuels. Over the last few years p-type dye sensitized solar cells (p-DSCs) have attracted substantial attention, since their incorporation with n-type DSCs could potentially lead to more efficient tandem cell pn-DSCs. Moreover, new research has been devoted to dye-sensitized photoelectrochemical cells (DSPECs), in which solar energy is utilized to generate hydrogen via water splitting. This article provides a summary of recent sensitizers employed in dye-sensitized NiO photocathodes for DSC and DSPEC, discussing approaches to enhance their overall performance. In particular, we intend to provide new directions through molecular design of new dyes and stimulate additional research development in the fields of DSCs and DSPECs.

Pyridyl vs. bipyridyl anchoring groups of porphyrin sensitizers for dye sensitized solar cells

The synthesis of two porphyrins with donor–π–acceptor (D–π–A) molecular architecture is described, namely Znpor-py (3a) and Znpor-bpy (3b), which consist of a 4-tert-butyl-phenyl group as donor and either a pyridine or a bipyridine group as acceptor at opposite positions of the porphyrin macrocycle. Photophysical and electrochemical properties of the two compounds, as well as theoretical DFT calculation results suggest that the two porphyrins have the potential to act as sensitizers in dye-sensitized solar cells (DSSCs). Both dyes contain N(pyridyl) atoms able to act as anchors onto the acid sites of TiO2. A Znpor-py (3a) sensitized solar cell was found to exhibit power conversion efficiency (PCE) of 3.57%, while the corresponding Znpor-bpy (3b)-functionalized solar cell showed a higher PCE of 5.08%. The enhanced short circuit current Jsc and open circuit voltage Voc parameters are the main factors responsible for the improved photovoltaic performance of the latter solar cell. These are attributed to its faster charge injection into the TiO2 photoanode and its reduced charge recombination at the electrode/electrolyte interface, which result from the stronger binding and coordination geometry of the bipyridine anchoring group on TiO2. Electrochemical impedance spectra (EIS) of the two solar cells further support these assumptions, revealing a higher charge recombination resistance Rrec and a longer electron lifetime τe for the Znpor-bpy (3b) sensitized solar cell.

A supramolecular assembling of zinc porphyrin with a π-conjugated oligo(phenylenevinylene) (oPPV) molecular wire for dye sensitized solar cell

A novel π-conjugated oligo(phenylenevinylene) (oPPV) (or LC) was prepared, as a new organic dye for dye-sensitized solar cells (DSSC), which contains a cyanoacrylic acid group on one end and a pyridyl group on the other. Solar cells sensitized by LC were fabricated, and were found to exhibit a power conversion efficiency (PCE) value of 2.45%. Furthermore, we describe the formation of a supramolecular dyad (ZnTPP–LC) via a metal–ligand bond between LC, since its pyridyl group allows it to interact with several metal centers, and zinc-tetraphenyl-porphyrin (ZnTPP) onto the photoelectrodes TiO2 surface of the solar cell. More specifically, LC was bound at first onto TiO2 with its cyanoacrylic acid anchoring group, and then a metal–ligand supramolecular bond was formed, with the addition of a porphyrinic solution, between the nitrogen atom of LCs pyridyl group and the zinc. ZnTPP–LC solar cell was then fabricated resulting in a record PCE value of 5.27% concerning the supramolecular DSSCs. As shown by photovoltaic measurements (J–V curves) and incident photon to current conversion efficiency (IPCE) spectra of the two solar cells, the higher PCE value of the supramolecular one can be attributed to its enhanced photovoltaic parameters, and particularly its enhanced short circuit current (Jsc). This Jsc improvement is due to ZnTPP–LCs higher light-harvesting efficiency and the larger electron injection of both ZnTPP and LC into TiO2s conduction band (CB) of the corresponding solar cell. These results are in accordance with electrochemical impedance spectra (EIS) of the DSSCs, which revealed longer electron lifetime, higher charge recombination resistance and shorter electron transport time for the solar cell based on ZnTPP–LC as compared to the one sensitized by LC.

Two new bulky substituted Zn porphyrins bearing carboxylate anchoring groups as promising dyes for DSSCs

Two novel zinc-metallated porphyrins (ZnP3C and ZnP6C), bearing three and six long alkoxy chains at the periphery of each porphyrin and a terminal carboxylic acid group, have been synthesized and fully characterized. Photophysical and electrochemical measurements demonstrated that the two sensitizers possess suitable frontier orbital energy levels for their use as sensitizers in DSSCs. The solar cells sensitized by ZnP3C and ZnP6C were fabricated resulting in power conversion efficiencies (PCEs) of 5.03 and 6.09%, respectively. As shown by photovoltaic measurements (J–V curves) and incident photon-to-current conversion efficiency (IPCE) spectra of the two solar cells, the higher PCE value of the latter is attributed to its enhanced photovoltaic parameters, and particularly its enhanced short circuit current (Jsc). This is related to the stronger absorption profile of the sensitizing dye ZnP6C and the higher dye loading of the corresponding solar cell. Furthermore, electrochemical impedance spectroscopy (EIS) showed that the DSSC based on ZnP6C exhibits a longer electron lifetime (τe) and more effective charge recombination resistance between the injected electrons and the electrolyte.

Axially Assembled Photosynthetic Antenna-Reaction Center Mimics Composed of Boron Dipyrromethenes, Aluminum Porphyrin, and Fullerene Derivatives

Sequential photoinduced energy transfer followed by electron transfer leading to the formation of charge separated states in a newly assembled series of supramolecular triads comprised of boron dipyrromethenes (BODIPY or BDP), aluminum porphyrin (AlTPP) and C60 is demonstrated. In the present strategy, the energy donor (BDP) and electron acceptor (C60) were axially positioned to the plane of AlTPP via the central metal. The structural integrity of the newly synthesized compounds and self-assembled systems were fully established using spectral, electrochemical and computational methods. Thermodynamic feasibility of energy transfer from 1BDP* to AlTPP and subsequent electron transfer from 1AlTPP* to generate BDP-AlTPP•+-C60•- charge separated states was derived from free-energy calculations. Occurrence of ultrafast energy transfer from 1BDP* to AlTPP was established from studies involving steady-state and time-resolved emission, as well as femtosecond transient spectroscopic techniques. The BDP-AlTPP•+-C60•- charge separated states persisted for several nanoseconds prior returning to the ground state.

Engineering of Porphyrin Molecules for Use as Effective Cathode Interfacial Modifiers in Organic Solar Cells of Enhanced Efficiency and Stability

In the present work, we effectively modify the TiO2 electron transport layer of organic solar cells with an inverted architecture using appropriately engineered porphyrin molecules. The results show that the optimized porphyrin modifier bearing two carboxylic acids as the anchoring groups and a triazine electron-withdrawing spacer significantly reduces the work function of TiO2, thereby reducing the electron extraction barrier. Moreover, the lower surface energy of the porphyrin-modified substrate results in better physical compatibility between the latter and the photoactive blend. Upon employing porphyrin-modified TiO2 electron transport layers in PTB7:PC71BM-based organic solar cells we obtained an improved average power conversion efficiency up to 8.73%. Importantly, porphyrin modification significantly increased the lifetime of the devices, which retained 80% of their initial efficiency after 500 h of storage in the dark. Because of its simplicity and efficacy, this approach should give tantalizing glimpses and generate an impact into the potential of porphyrins to facilitate electron transfer in organic solar cells and related devices.