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.

The synthesis and characterization of a series of novel small-molecule hole-transporting materials (HTMs) based on an anthra [1,2-b:4,3-b′:5,6-b′′:8,7-b′′′]tetrathiophene (ATT) core are reported. The new compounds follow an easy synthetic route and have no need of expensive purification steps. The novel HTMs were tested in perovskite solar cells (PSCs) and power conversion efficiencies (PCE) of up to 18.1 % under 1 sun irradiation were 2 measured. This value is comparable with the 17.8 % efficiency obtained using spiroOMeTAD as a reference compound. Similarly, a significant quenching of the Photoluminescence in the first nanosecond is observed, indicative of effective hole transfer.Additionally, the influence of introducing aliphatic alkyl chains acting as solubilizers on the device performance of the ATT molecules is investigated. Replacing the methoxy groups on the triarylamine sites by butoxy-, hexoxy-or decoxy-substituents greatly improved the solubility of the compounds without changing the energy levels, yet at the same time significantly decreasing the conductivity as well as the PCE, 17.3 % for ATT-OBu, 15.7 % for ATT-OHex and 9.7 % for ATT-ODec.

Isomerism of benzotrithiophene-based hole transporting materials is shown to have a significant impact on conductance properties, affording photovoltaic efficiency values as high as 19%.

Three new star-shaped hole-transporting materials (HTMs) incorporating benzotripyrrole, benzotrifuran, and benzotriselenophene central cores endowed with three-armed triphenylamine moieties (BTP-1, BTF-1, and BTSe-1, respectively) are designed, synthesized, and implemented in perovskite solar cells (PSCs). The impact that the heteroatom-containing central scaffold has on the electrochemical and photophysical properties, as well as on the photovoltaic performance, is systematically investigated and compared with their sulfur-rich analogue (BTT-3). The new HTMs exhibit suitable highest-occupied molecular orbitals (HOMO) levels regarding the valence band of the perovskite, which ensure efficient hole extraction at the perovskite/HTM interface. The molecular structures of BTF-1, BTT-3, and BTSe-1 are fully elucidated by single-crystal X-ray crystallography as toluene solvates. The optimized (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15based perovskite solar cells employing the tailor-made, chalcogenide-based HTMs exhibit remarkable power conversion efficiencies up to 18.5%, which are comparable to the devices based on the benchmark spiro-OMeTAD. PSCs with BTP-1 exhibit a more limited power conversion efficiency of 15.5%, with noticeable hysteresis. This systematic study indicates that chalcogenide-based derivatives are promising HTM candidates to compete efficiently with spiro-OMeTAD.

Two novel homo and hetero three-dimensional nanographenes, NG1 and NG2, featuring a cyclooctatetraene core are designed, synthesized, and characterized. A concise and efficient bottom-up methodology was employed during which 24 new carbon−carbon bonds were formed. By means of a Scholl reaction nanographenes with 53 fused rings are realized, which exhibited good solubility in common organic solvents. The resulting saddle-like structures of NG1 and NG2 are electron-rich and show good chemical and electrochemical stability. Their molecular structures are fully elucidated by single-crystal X-ray crystallography. From their crystal structure analysis is concluded that both nanographenes are chiral and crystallize as a racemic mixture. Our work was rounded-off by excited state investigations such as electron and energy transfer with electron-acceptors and -donors.

Fused oligothiophene-based π-conjugated organic derivatives have been widely used in electronic devices. In particular, two-dimensional (2D) heteroarenes offer the possibility of broadening the scope by extending the π-conjugated framework, which endows enhanced charge transport properties due to the potential intermolecular π–π stacking. Here, the synthesis and characterization of two new small-molecule hole-transporting materials (HTMs) for perovskite solar cells (PSCs) are reported. The newly custom-made compounds are based on dibenzoquinquethiophene (DBQT) and dibenzosexithiophene (DBST) cores, which are covalently linked to triphenylamine moieties to successfully afford the four-armed tetrakistriphenylamine (TTPA) derivatives TTPA–DBQT and TTPA–DBST. The combination of these novel central scaffolds with the electron-donor TTPA units bestow the resulting HTMs with the appropriate energy levels and, therefore, good electronic contact with the perovskite for extracting the hole efficiently. TTPA–DBQT surpasses TTPA–DBST not only in terms of conductivity but also in light-to-energy conversion efficiency using conventional mesoscopic n–i–p perovskite devices, 18.1% and 14.3%, respectively. These results were systematically compared with the benchmark HTM, 2,2′,7,7′-tetrakis­(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). Additionally, scanning electron microscopy (SEM) hints that TTPA–DBQT forms high quality and fully homogeneous films, whereas TTPA–DBST leads to the formation of thinner films with pinholes, which explains its lower fill factor despite its better hole-extraction properties owing to its more planar π-extended scaffold.

A new HTM incorporating a saddle-like, thiophene-rich core endowed with four triphenylamine units is proposed. The new HTM leads to PSCs reaching efficiencies up to 16.3%.

Three novel donor–π‐bridge–donor (D‐π‐D) hole‐transporting materials (HTMs) featuring triazatruxene electron‐donating units bridged by different 3,4‐ethylenedioxythiophene (EDOT) π‐conjugated linkers have been synthesized, characterized, and implemented in mesoporous perovskite solar cells (PSCs). The optoelectronic properties of the new dumbbell‐shaped derivatives (DTTXs) are highly influenced by the chemical structure of the EDOT‐based linker. Red‐shifted absorption and emission and a stronger donor ability were observed in passing from DTTX‐1 to DTTX‐2 due to the extended π‐conjugation. DTTX‐3 featured an intramolecular charge transfer between the external triazatruxene units and the azomethine–EDOT central scaffold, resulting in a more pronounced redshift. The three new derivatives have been tested in combination with the state‐of‐the‐art triple‐cation perovskite [(FAPbI3)0.87(MAPbBr3)0.13]0.92[CsPbI3]0.08 in standard mesoporous PSCs. Remarkable power conversion efficiencies of 17.48 % and 18.30 % were measured for DTTX‐1 and DTTX‐2, respectively, close to that measured for the benchmarking HTM spiro‐OMeTAD (18.92 %), under 100 mA cm−2 AM 1.5G solar illumination. PSCs with DTTX‐3 reached a PCE value of 12.68 %, which is attributed to the poorer film formation in comparison to DTTX‐1 and DTTX‐2. These PCE values are in perfect agreement with the conductivity and hole mobility values determined for the new compounds and spiro‐OMeTAD. Steady‐state photoluminescence further confirmed the potential of DTTX‐1 and DTTX‐2 for hole‐transport applications as an alternative to spiro‐OMeTAD.