The syntheses of five homoleptic copper(I) complexes [CuL2][PF6] are described in which L is a 4,4'-di(4-bromophenyl)-6,6'-dialkyl-2,2'-bipyridine ligand (compounds 1-4 with methyl, (n)butyl, (iso)butyl and hexyl substituents, respectively) or 4,4'-di(4-bromophenyl)-6,6'-diphenyl-2,2'-bipyridine (5). The new ligands 2-5 and copper(I) complexes [CuL2][PF6] (L = 1-5) have been fully characterized. The single crystal structures of 2{[Cu(1)2][PF6]}·3Me2CO, [Cu(2)2][PF6], 2{[Cu(3)2][PF6]}·Et2O and [Cu(5)2][PF6]·CH2Cl2 have been determined. The first three structures show similar distorted tetrahedral environments for the Cu(+) ions with angles between the least squares planes of the bpy domains of 85.6, 86.4 and 82.9°, respectively; in contrast, the Cu(+) ion in [Cu(5)2][PF6]·CH2Cl2 is in a flattened coordinate environment due to intra-cation face-to-face π-interactions. The solution absorption spectra of the complexes with ligands 1-4 are virtually identical with an MLCT band with values of λmax = 481-488 nm. In contrast, the absorption spectrum of [Cu(5)2][PF6] shows two broad bands in the visible region. Cyclic voltammetric data show that oxidation of the copper(I) centre occurs at a more positive potential in [Cu(2)2][PF6], [Cu(3)2][PF6] and [Cu(4)2][PF6] than in [Cu(1)2][PF6] or [Cu(5)2][PF6] with the latter being oxidized at the lowest potential. The complexes have been used to prepare dye-sensitized solar cells (DSCs) incorporating heteroleptic dyes of type [Cu(L)(Lanchor)](+) where L is 1-5 and Lanchor is a 6,6'-dimethyl-2,2'-bipyridine functionalized in the 4- and 4'-positions with phosphonic acid groups with (Lanchor = 7) and without (Lanchor = 6) a spacer between the metal-binding and anchoring domains. The presence of the spacer results in enhanced performances of the dyes, and the highest energy conversion efficiencies are observed for the dyes [Cu(3)(7)](+) (η = 2.43% compared to 5.96% for standard dye N719) and [Cu(5)(7)](+) (η = 2.89% compared to 5.96% for N719). Measurements taken periodically over the course of a week indicate that the cells undergo a ripening process (most clearly seen for [Cu(5)(6)](+) and [Cu(5)(7)](+)) before their optimum performances are achieved. IPCE (EQE) data are presented and confirm that, although the photo-to-current conversions are promising (37-49% for λmax≈ 480 nm), the copper(I) dyes do not realize the broad spectral response exhibited by N719.

To date, the most efficient perovskite solar cells (PSCs) employ an n–i–p device architecture that uses a 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) hole‐transporting material (HTM), which achieves optimum conductivity with the addition of lithium bis(trifluoromethane)sulfonimide (LiTFSI) and air exposure. However, this additive along with its oxidation process leads to poor reproducibility and is detrimental to stability. Herein, a dicationic salt spiro‐OMeTAD(TFSI)2, is employed as an effective p‐dopant to achieve power conversion efficiencies of 19.3% and 18.3% (apertures of 0.16 and 1.00 cm2) with excellent reproducibility in the absence of LiTFSI and air exposure. As far as it is known, these are the highest‐performing n–i–p PSCs without LiTFSI or air exposure. Comprehensive analysis demonstrates that precise control of the proportion of [spiro‐OMeTAD]+ directly provides high conductivity in HTM films with low series resistance, fast hole extraction, and lower interfacial charge recombination. Moreover, the spiro‐OMeTAD(TFSI)2‐doped devices show improved stability, benefitting from well‐retained HTM morphology without forming aggregates or voids when tested under an ambient atmosphere. A facile approach is presented to fabricate highly efficient PSCs by replacing LiTFSI with spiro‐OMeTAD(TFSI)2. Furthermore, this study provides an insight into the relationship between device performance and the HTM doping level.

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.

The search for lead‐free alternatives to lead‐halide perovskite photovoltaic materials resulted in the discovery of copper(I)‐silver(I)‐bismuth(III) halides exhibiting promising properties for optoelectronic applications. The present work demonstrates a solution‐based synthesis of uniform CuxAgBiI4+x thin films and scrutinizes the effects of x on the phase composition, dimensionality, optoelectronic properties, and photovoltaic performance. Formation of pure 3D CuAgBiI5 at x = 1, 2D Cu2AgBiI6 at x = 2, and a mix of the two at 1 < x < 2 is demonstrated. Despite lower structural dimensionality, Cu2AgBiI6 has broader optical absorption with a direct bandgap of 1.89 ± 0.05 eV, a valence band level at ‐5.25 eV, improved carrier lifetime, and higher recombination resistance as compared to CuAgBiI5. These differences are mirrored in the power conversion efficiencies of the CuAgBiI5 and Cu2AgBiI6 solar cells under 1 sun of 1.01 ± 0.06% and 2.39 ± 0.05%, respectively. The latter value is the highest reported for this class of materials owing to the favorable film morphology provided by the hot‐casting method. Future performance improvements might emerge from the optimization of the Cu2AgBiI6 layer thickness to match the carrier diffusion length of ≈40–50 nm. Nonencapsulated Cu2AgBiI6 solar cells display storage stability over 240 days.

Copper redox mediators have enabled open‐circuit voltages (VOC) of over 1.0 V in dye‐sensitized solar cells (DSCs) and have helped to establish DSCs as the most promising solar cell technology in low‐light conditions. The addition of additives such as 4‐tert‐butylpyridine (tBP) to these electrolytes has helped in achieving high solar cell performances. However, emerging evidence suggests that tBP coordinates to the Cu(II) species and limits the performance of these electrolytes. To date, the implications of this coordination are poorly understood. Here, the importance of Lewis base additives for the successful implementation of copper complexes as redox mediators in DSCs is demonstrated. Two redox couples, [Cu(dmp)2]+/2+ and [Cu(dpp)2]+/2+ (with dmp = 2,9‐dimethyl‐1,10‐phenanthroline and dpp = 2,9‐diphenyl‐1,10‐phenanthroline) in combination with three different Lewis bases, TFMP (4‐(trifluoromethyl)pyridine), tBP, and NMBI (1‐methyl‐benzimidazole), are considered. Through single‐crystal X‐ray diffraction analysis, absorption, and 1H‐NMR spectroscopies, the coordination of Lewis bases to the Cu(II) centers are studied. This coordination efficiently suppresses recombination losses and is crucial for high performing solar cells. If, however, the coordination involves a ligand exchange, as is the case for [Cu(dpp)2]+/2+, the redox mediator regeneration at the counter electrode is significantly retarded and the solar cells show current limitations.