Metal-free Dye-sensitized Solar Cells Research Articles

Improved efficiency and stability for acridine orange sensitizers by adding electron donating/accepting π-linker moieties

Shortcomings associated with metal-free dye-sensitized solar cells can be overcome with dye modification. Using Density Functional Theory (DFT) and Time Dependent (TD-DFT), acridine orange dyes (main Donors) are modified with molecular moieties (2-Hydroxy-4-propoxyphen)(phenyl)methanone (UV absorber), 4,4′-Dimethoxydiphenylamine (Donor 1), 2-(Thiophen-2-yl)-1,3,4-oxadiazole (π-bridge), anisole (Donor 2), cyanoacetic-acid and carboxylic-acid functional groups (Acceptors 1 and 2), to form new UV Absorber-D-(A)2 and D’- π -D-(A)2 dyes with least computational costs, where acridine derivatives act as main donors. Absorption spectra, highest-occupied molecular orbitals, and lowest-unoccupied molecular orbitals are predicted by DFT and TD-DFT calculations. Ionization potential, electron affinity, electronegativity and light-harvesting efficiency are predicted with frontier-molecular orbital energies by Koopmans' theorem, using TiO2 conduction-band edge (ECB −4.0 eV) and [I-/I3-] redox-potential (-4.80 eV). Absorption redshifts with expanded range of 700 nm, and improved energy-alignment with TiO2, charge-transfer processes, redox-couple compatibility, cell stability and higher performance are all achieved by molecular level tuning of dyes.

Bi-anchoring organic sensitizers of type D-(π-A)2 comprising thiophene-2-acetonitrile as π-spacer and malonic acid as electron acceptor for dye sensitized solar cell applications

Two new bi-anchoring organic sensitizers of type D-(π-A)2 comprising the identical π-spacer (thiophene-2-acetonitrile) and electron acceptor (malonic acid) but different aryl amine as electron donors (diphenylamine and carbazole) were synthesized, characterized and fabricated metal free dye-sensitized solar cell devices. The intra molecular charge transfer property and electrochemical property of these dyes were investigated by molecular absorption, emission, cyclic voltammetric experiments and in addition, quantum chemical calculation studies were performed to provide sufficient driving force for the electron injection into the conduction band of TiO2 which leads to efficient charge collection. Among the fabricated devices, carbazole based device exhibits high current conversion efficiency (η=4.7%) with a short circuit current density (JSC) 15.3mA/cm2, an open circuit photo voltage (VOC) of 0.59V and a fill factor of 0.44 under AM 1.5 illumination (85mW/cm2) compared to diphenylamine based device.

Phosphorus-doped reduced graphene oxide as an electrocatalyst counter electrode in dye-sensitized solar cells

In this study, phosphorus (P) atoms were doped into reduced graphene oxide (rGO) with two bonding states of P–C and P–O by using annealing treatment. For the first time, the P-doped rGO (PrGO) was employed as the counter electrode (CE) for dye-sensitized solar cell (DSSC). The electrochemical studies reveal that the P doping can effectively enhance the electrocatalytic activity of rGO, and P–C structures have higher electrocatalytic activity than P–O ones for I−/I3− redox reaction. More significantly, PrGO shows comparable electrocatalytic ability to Pt as CEs. It therefore seems promising for PrGO to be widely used in metal-free DSSCs with low cost and high efficiency.

Effects of multi anchoring groups of catecholamine polymer dyes on the electrical characteristics of metal free dye-sensitized solar cells: A comparison study

Poly epinephrine and poly dopamine were synthesized through a new manner in an alkaline solution containing tris(hydroxymethyl)aminomethane (THAM) buffer with pH=8.5 under nitrogen atmosphere to make the dyes with very strong adhesion and efficient electron injection by direct dye-to-TiO2 charge transfer. These dyes were used for fabrication of metal free dye sensitized solar cells. Photovoltaic characteristics were obtained for cells made by utilization of two dyes and it was established that the poly epinephrine dye exhibited larger current density and energy conversion efficiency than the poly dopamine dye. This is attributed to the strong attachment of the poly epinephrine dye to the surface of the TiO2 nanoparticles due to the presence of three anchoring OH groups on the back bone of the polymer. Under simulated AM 1.5G solar light (100mWcm−2), a poly epinephrine-sensitized cell exhibited a short circuit current density of 3.63mAcm−2, and overall power conversion efficiency of 0.41%, which is almost 60% larger than that of the poly dopamine dye-sensitized cell (0.26%). UV–Vis absorption spectra of the dyes were recorded and it was shown that absorption by poly epinephrine has extended over a broad range of wavelengths from UV up to mid-range of visible region and has a peak centered at 420nm. However the range of absorption wavelengths for poly dopamine is limited to the initial part of visible region and the peak wavelength occurs at 257nm. Moreover, optical band gaps of the dyes were obtained from wavelengths of the absorption edges of the respecting UV–Vis absorption spectra (2.25 and 3.32eV for poly epinephrine and poly dopamine respectively). HOMO and LUMO energy levels of the dyes were estimated from cyclic voltammograms. Electrochemical impedance spectroscopy was applied to study of the electrical properties of the cells including poly epinephrine and poly dopamine dyes.