In this review, the contribution of chemistry towards producing new and innovative hole-transporting materials for highly efficient perovskite solar cells is presented in a rational and systematic manner.

New star-shaped benzotrithiophene (BTT)-based hole-transporting materials (HTM) BTT-1, BTT-2 and BTT-3 have been obtained through a facile synthetic route by crosslinking triarylamine-based donor groups with a benzotrithiophene (BTT) core. The BTT HTMs were tested on solution-processed lead trihalide perovskite-based solar cells. Power conversion efficiencies in the range of 16 % to 18.2 % were achieved under AM 1.5 sun with the three derivatives. These values are comparable to those obtained with today s most commonly used HTM spiro-OMeTAD, which point them out as promising candidates to be used as readily available and cost-effective alternatives in perovskite solar cells (PSCs). SinceitsfirstuseaslightabsorberinasensitizedsolarcellbyMiyasaka and co-workers, [1] organic-inorganic methylammonium (MA) lead halide MAPbX 3 (X = I, Br) perovskites have experienced a scientific research blast for photovoltaic applications. [2][3][4][5][6][7] Organometal trihalide perovskites exhibit exceptional intrinsic properties such as light absorption from visible to near-infrared range, high extinction coefficient, long electron-hole diffusion lengths, a direct band gap as well as high charge carrier mobilities, among others. [8][9][10] Furthermore, the perovskite material is relatively versatile and its electronic properties can be widely tuned by cationic or anionic substitution. As an example, the formamidinium (FA) based perovskite FAPbI 3 shows excellent light harvesting properties due to its lower band gap energy (E g ), [11] whereas by using the methylammonium mixed halide perovskite MAPbI x Br 3Àx higher E g and open circuit voltage (V oc ) can be obtained. [12] Improved energy conversion efficiencies are also observed when combining the two perovskite materials in the compositional modification (FAPbI 3 ) 1Àx (MAPbBr 3 ) x that was recently presented by Seok et al., [13] reporting power conversion efficiencies (PCEs) above 20 %. The ambipolar behavior of the perovskite allows its combination either n-i-p or p-i-n configurations with electron transporting (ETMs) and/or with hole-transporting (HTMs) materials. These interface layers play an important role for transporting and blocking the charges. A wide number of HTMs have been synthesized and investigated in combination with perovskite absorber ranging from classical semiconducting polymers to small molecules.Focusing on the latter, different central cores have been used in the state-of-the-art photovoltaic devices, such as 9,9'-spirobifluorene, [14] spiro-liked derivatives, [15] thiophene derivatives, [16] triphenylamine, [17] bridged-triphenylamines, [18,19] pyrene, [20] 3,4-ethylenedioxythiophene, [21] linear p-conjugated, [22][23][24] triptycene, [25] tetraphenylethene, [26] silolothiophene or triazines, [27] all of them namely decorated with diarylamines, triarylamines and/or carbazole derivatives. With this approach, the best power conversion efficiency obtained to date, up to 20 %, [13] has been achieved by using the polymer poly[bis(4-phenyl)(2,4,6-t...

In this work, we assess the possible reasons for the differences observed in open circuit voltage (VOC) in mixed cation perovskite solar cells when comparing four different hole transport materials (HTMs), namely TAE-1, TAE-3, TAE-4 and spiro-OMeTAD.

On the way to ultraflat single-molecule junctions with transparent electrodes, we present a fabrication scheme based on epitaxial graphene nanoelectrodes. As a suitable molecule, we identified a molecular wire with fullerene anchor groups. With these two components, stable electrical characteristics could be recorded. Electrical measurements show that single-molecule junctions with graphene and with gold electrodes display a striking agreement. This motivated a hypothesis that the differential conductance spectra are rather insensitive to the electrode material. It is further corroborated by the assignment of asymmetries and spectral features to internal molecular degrees of freedom. The demonstrated open-access graphene electrodes and the electrode-insensitive molecules provide a model system that will allow for a thorough investigation of an individual single-molecule contact with additional probes.