Biljana Bozic-Weber

Hole-transport functionalized copper(I) dye sensitized solar cells

Ligands containing first and second generation hole-transport triphenylamino-dendrons have been evaluated as ancillary ligands in copper(I) DSCs yielding an optimal efficiency of 3.77% in unmasked cells. The effects of masking the DSCs on measured parameters are discussed.

Kelvin probe force microscopy of nanocrystalline TiO2 photoelectrodes

Summary Dye-sensitized solar cells (DSCs) provide a promising third-generation photovoltaic concept based on the spectral sensitization of a wide-bandgap metal oxide. Although the nanocrystalline TiO2 photoelectrode of a DSC consists of sintered nanoparticles, there are few studies on the nanoscale properties. We focus on the microscopic work function and surface photovoltage (SPV) determination of TiO2 photoelectrodes using Kelvin probe force microscopy in combination with a tunable illumination system. A comparison of the surface potentials for TiO2 photoelectrodes sensitized with two different dyes, i.e., the standard dye N719 and a copper(I) bis(imine) complex, reveals an inverse orientation of the surface dipole. A higher surface potential was determined for an N719 photoelectrode. The surface potential increase due to the surface dipole correlates with a higher DSC performance. Concluding from this, microscopic surface potential variations, attributed to the complex nanostructure of the photoelectrode, influence the DSC performance. For both bare and sensitized TiO2 photoelectrodes, the measurements reveal microscopic inhomogeneities of more than 100 mV in the work function and show recombination time differences at different locations. The bandgap of 3.2 eV, determined by SPV spectroscopy, remained constant throughout the TiO2 layer. The effect of the built-in potential on the DSC performance at the TiO2/SnO2:F interface, investigated on a nanometer scale by KPFM measurements under visible light illumination, has not been resolved so far.

Factors controlling the photoresponse of copper( i ) diimine dyes containing hole-transporting dendrons in dye-sensitized solar cells: substituent and solvent effects

Two series of 2,2′-bipyridine (bpy) ligands bearing different 6,6′-substituents (Me, nBu, isoBu, hexyl, Ph and 2-naphthyl) and carrying first-generation (ligands 1–6) or second-generation (ligands 7–12) hole transporting dendrons in the 4,4′-positions are reported. They have been incorporated into homoleptic copper(I) complexes [CuL2][PF6]. FTO/TiO2 electrodes functionalized with the anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid), 13, were dipped in either CH2Cl2 or acetone solutions of [CuL2][PF6] to produce two series of surface-bound heteroleptic dyes. Their performances in dye-sensitized solar cells (DSCs) are assessed. Solid-state absorption spectra of dye-functionalized electrodes show that dye uptake is greater if acetone is used in the dye-dipping cycle rather than CH2Cl2, and the DSCs made using acetone generally perform better than analogous DSCs made using CH2Cl2. Using acetone-dipping solutions, the best DSC efficiencies are obtained with the second-generation dyes [Cu(13)(L)]+ (L = 7–11 with Me, nBu, isoBu, hexyl, Ph groups); [Cu(13)(12)]+ (12 contains 2-naphthyl groups in the 6,6′-positions) and its first-generation analogue [Cu(13)(6)]+ perform poorly. When CH2Cl2 is used in the dipping cycle, DSCs with dyes [Cu(13)(1)]+ and [Cu(13)(7)]+ (6,6′-Me2-substituted) show the highest VOC, JSC and η values, and EQE spectra confirm electron injection over a wider energy range than for other dyes. For CH2Cl2 in the dipping cycle (but not for acetone), [Cu(13)(5)]+ (6,6′-Ph2-substituted) performs as well as [Cu(13)(1)]+. The overall results of the study indicate that a combination of small 6,6′-substituents and acetone in the dye-dipping cycle lead to the best performing dyes.

Modular synthesis of simple cycloruthenated complexes with state-of-the-art performance in p-type DSCs

A modular approach based on Suzuki–Miyaura cross coupling and Miyaura borylation has been used to prepare two cyclometallated [Ru(N⁁N)2(C⁁N)]+ complexes which possess either a carboxylic or phosphonic acid group attached via a phenylene spacer to the 4-position of the pyridine ring in the C⁁N ligand. The key intermediate in the synthetic pathway is [Ru(bpy)2(1)]+ where bpy = 2,2′-bipyridine and H1 is 4-chloro-2-phenylpyridine. The crystal structure of [Ru(bpy)2(1)][PF6] is presented. Reaction of [Ru(bpy)2(1)][PF6] with 4-carboxyphenylboronic acid leads to [Ru(bpy)2(H6)][PF6], while the phosphonic acid analogue is isolated as the zwitterion [Ru(bpy)2(H5)]. The cyclometallated complexes have been characterized by mass spectrometry, multinuclear NMR spectroscopy, absorption spectroscopy and electrochemistry. [Ru(bpy)2(5)] adsorbs onto NiO FTO/NiO electrodes (confirmed by solid-state absorption spectroscopy) and its performance in p-type dye-sensitized solar cells (DSCs) has been compared to that of the standard dye P1; two-screen printed layers of NiO give better DSC performances than one layer. Duplicate DSCs containing [Ru(bpy)2(H5)] achieve short-circuit current densities (JSC) of 3.38 and 3.34 mA cm−2 and photoconversion efficiencies (η) of 0.116 and 0.109%, respectively, compared to values of JSC = 1.84 and 1.96 mA cm−2 and η = 0.057 and 0.051% for P1. Despite its simple dye structure, the performance of [Ru(bpy)2(H5)] parallels the best-performing cyclometallated ruthenium(II) dye in p-type DSCs reported previously (He et al., J. Phys. Chem. C, 2014, 118, 16518) and confirms the effectiveness of a phosphonic acid anchor in the dye and the attachment of the anchoring unit to the pyridine (rather than phenyl) ring of the cyclometallating ligand.

Understanding why replacing I3–/I– by cobalt(II)/(III) electrolytes in bis(diimine)copper(I)-based dye-sensitized solar cells improves performance

The performances of dye-sensitized solar cells (DSCs) comprising heteroleptic bis(diimine)copper(I) based dyes combined with either [Co(bpy)3]2+/3+, [Co(phen)3]2+/3+ or I3−/I− redox mediators (bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline) have been evaluated. The copper(I) dyes contain the anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid), 1, and an ancillary ligand (2, 3 or 4) with a 2,9-dimethyl-1,10-phenanthroline metal-binding domain. Ligands 2 and 3 include imidazole 2′-functionalities with 4-(diphenylamino)phenyl (2) or 4-(bis(4-n-butoxy)phenylamino)phenyl (3) domains; in 4, the phen unit is substituted in the 4,7-positions with hole-transporting 4-(diphenylamino)phenyl groups. The photoconversion efficiency, η, of each of [Cu(1)(2)]+, [Cu(1)(3)]+ and [Cu(1)(4)]+ considerably improves by replacing the I3−/I− electrolyte by [Co(bpy)3]2+/3+ or [Co(phen)3]2+/3+, and after a change of electrolyte solvent (MeCN to 3-methoxypropionitrile). Due to the faster charge transfer kinetics and more positive redox potential, the cobalt-based electrolytes are superior to the I3−/I− electrolyte in terms of open-circuit voltage (VOC), short-circuit current (JSC) and η; values of VOC = 594 mV, JSC = 9.58 mA cm−2 and η = 3.69% (relative to η = 7.12% for N719) are achieved for the best performing DSC which contains [Cu(1)(4)]+ and [Co(bpy)3]2+/3+. Corresponding values for [Cu(1)(4)]+ and I3−/I− DSCs are 570–580 mV, 5.98–6.37 mA cm−2 and 2.43–2.62%. Electrochemical impedance spectroscopy (EIS) has been used to study DSCs with [Cu(1)(4)]+ and the three electrolytes. EIS shows that the DSC with I3−/I− has the highest recombination resistance, whereas the [Co(phen)3]2+/3+ electrolyte gives the highest chemical capacitance and VOC and, between [Co(bpy)3]2+/3+ and [Co(phen)3]2+/3+, the higher recombination resistance. The [Co(phen)3]2+/3+ electrolyte exhibits the highest mass transport restrictions which result in a lower JSC and DSC efficiency compared to the [Co(bpy)3]2+/3+ electrolyte.

Evaluation of polynuclear dendrons as photosensitizers for dye-sensitized solar cells

Triruthenium dendrons bearing carboxylic acid substituents at the apex have been evaluated as photosensitizers in dye-sensitized solar cells. The disadvantages of a single carboxylate binding site are partially compensated by the higher charges of the trinuclear dendrons. However, the trinuclear units prove to be ineffective photosenstizers compared to mononuclear model compounds.

Dye-sensitized solar cells with hole-stabilizing surfaces: "inorganic" versus "organic" strategies†

Two 2,2′:6′,2′′-terpyridine ligands (9 and 10) incorporating second-generation diphenylamino-dendrons have been synthesized and characterized; one ligand contains chromophoric benzothiadiazole domains. Using the ‘surface-as-ligand, surface-as-complex’ strategy, zinc(II)-containing sensitizers [Zn(Lanchor)(Lancillary)]2+ with carboxylic or phosphonic acid anchors (1 and 2, respectively) have been assembled and tested in n-type DSCs. The solid-state absorption spectra of dye-functionalized electrodes show a broad spectral response for all the dyes with enhanced intensity for those containing the benzothiadiazole units. However, the [Zn(Lanchor)(Lancillary)]2+ dyes perform poorly, exhibiting very low values of the short-circuit current density (JSC) and open-circuit voltage (VOC). The external quantum efficiency (EQE) spectra confirm that electron injection occurs, but EQEmax is ≤3%. Non-optimal positioning of the thiadiazole domain in the dye probably contributes to the poor performances. Screening of DSCs containing FTO/TiO2 photoanodes without adsorbed dye shows that they generate small short-circuit current densities and open-circuit voltages which contribute significantly to parameters reported for badly performing dyes. An organic dye 11, structurally similar to 10 and containing a 2-cyanoacrylic acid anchor, is also reported. This exhibits a broad and intense spectral response between 300 and 600 nm, and shows efficient electron injection over a broad wavelength range. DSCs containing 11 are stable over a 17 day period and show global efficiencies of 3.93–4.57% (ca. 70% with respect to N719 set at 100%). Ground state DFT calculations reveal that the HOMO in each of [Zn(1)(9)]2+, [Zn(2)(9)]2+, [Zn(1)(10)]2+, [Zn(2)(10)]2+ and 11 is localized on the peripheral diphenylamino units, allowing for hole-transfer to the reduced electrolyte. In 11, a major contribution from the 2-cyanoacrylic acid anchoring group appears in the LUMO manifold; however, while the LUMO in each zinc(II) dye is localized on anchoring ligand 1 or 2, it is concentrated close to the metal centre which may contribute to poor electron injection.

Heteroleptic copper(I) sensitizers with one versus two hole-transporting units in functionalized 2,9-dimethyl-1,10-phenanthroline ancillary ligands

A series of homoleptic [Cu(L)2][PF6] complexes in which L is a 2,9-dimethyl-1,10-phenanthroline fused at the 5,6-positions with a 2′-functionalized imidazole (ligands 1–4), or substituted at the 4,7-positions with electron-donating 4-(diphenylamino)phenyl groups (ligand 5) is described; the imidazole 2′-functionality in 1 is 4-bromophenyl, in 2 is 4-(diphenylamino)phenyl, in 3 is 4-(bis(4-n-butoxy)phenylamino)phenyl, and in 4 is 4-(carbazol-9-yl)phenyl. The copper complexes were characterized by mass spectrometry, NMR and absorption spectroscopies and cyclic voltammetry; the single crystal structure of ligand 4 has been determined. Compared to the solution absorption spectra of [Cu(1)2][PF6], [Cu(2)2][PF6], [Cu(3)2][PF6] and [Cu(4)2][PF6], that of [Cu(5)2][PF6] shows increased absorbance at wavelengths >375 nm. An on-surface strategy was used to assemble heteroleptic [Cu(6)(L)]+ dyes on TiO2 electrodes where 6 is ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid); solid-state absorption spectra confirmed enhanced light-harvesting between 375 and 600 nm for [Cu(6)(5)]+ with respect to [Cu(6)(1)]+, [Cu(6)(2)]+, [Cu(6)(3)]+ and [Cu(6)(4)]+. Comparison of the performances of dye-sensitized solar cells (DSCs) containing [Cu(6)(2)]+, [Cu(6)(3)]+ and [Cu(6)(4)]+ with those with [Cu(6)(1)]+ indicate only a marginal influence of the diphenylamine or carbazole hole-transporting domains in 5,6-substituted phenanthroline dyes. The introduction of the 4-(diphenylamino)phenyl hole-transporting units in the 4- and 7-positions of the phen unit in 5 proves to be beneficial, with DSCs containing [Cu(6)(5)]+ performing better than those with the other four dyes; duplicate DSCs were tested for each dye to validate the results. While the values of the maximum external quantum efficiencies (EQEmax) for [Cu(6)(1)]+ and [Cu(6)(4)]+ are greater than for [Cu(6)(5)]+, the extension of the EQE spectrum for [Cu(6)(5)]+ to longer wavelengths results in higher short-circuit current densities (JSC) compared to DSCs with [Cu(6)(1)]+, [Cu(6)(2)]+, [Cu(6)(3)]+ and [Cu(6)(4)]+.