Herein, we studied the use of two different Self Assembled Monolayers (SAMs) made of semiconductor hole transport organic molecules to replace the most common p-type contact, PEDOT:PSS, in PiN methyl ammonium lead iodide perovskite solar cells (PSCs).

Over hundreds of new organic semiconductor molecules have been synthesized as hole transport materials (HTMs) for perovskite solar cells. However, to date, the well-known N, N, N, N, N, N, N, octakis-(4-methoxyphenyl)-9,9-spirobi-[9,9'-spirobi[9 H-fluorene]-2,2',7,7'-tetramine (spiro-OMeTAD) is still the best choice for the best perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term stable devices, and its market price makes its use in large-scale production costly. Novel synthetic routes for new HTMs have to be sought that can be carried out in fewer synthetic steps and can be easily scaled up for commercial purposes. On the one hand, synthetic chemists have taken, as a first approach, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the spiro-OMeTAD molecule as a reference to synthesize molecules with similar energy levels, although these HOMO and LUMO energy levels often have been measured indirectly in solution using cyclic voltammetry. On the other hand, the "spiro" chemical core has also been studied as a structural motif for novel HTMs. However, only a few molecules incorporated as HTMs in complete functional perovskite solar cells have been capable of matching the performance of the best-performing perovskite solar cells made using spiro-OMeTAD. In this Account, we describe the advances in the synthesis of HTMs that have been tested in perovskite solar cells. The comparison of solar cell efficiencies is of course very challenging because the solar cell preparation conditions may differ from laboratory to laboratory. To extract valuable information about the HTM molecular structure-device function relationship, we describe those examples that always have used spiro-OMeTAD as a control device and have always used identical experimental conditions (e.g., the use of the same chemical dopant for the HTM or the lack of it). The pioneering work was focused on well-understood organic semiconductor moieties such as arylamine, carbazole, and thiophene. Those chemical structures have been largely employed and studied as HTMs, for instance, in organic light-emitting devices. Interestingly, most research groups have reported the hole mobility values for their novel HTMs. However, only a few examples have been found that have measured the HOMO and LUMO energy levels using advanced spectroscopic techniques to determine these reference energy values directly. Moreover, it has been shown that those molecules, upon interacting with the perovskite layer, often have different HOMO and LUMO energies than the values estimated indirectly using solution-based electrochemical methods. Last but not least, porphyrins and phthalocyanines have also been synthesized as potential HTMs for perovskite solar cells. Their optical and physical properties, such as high absorption and good energy transfer capabilities, open new possibilities for HTMs in perovskite solar cells.

Four hole transport materials (HTMs) based on a benzothiadiazole (BT) central core have been synthesized and successfully employed in triple-cation mixed-halide perovskite solar cells (PSCs), reaching 18.05% solar to energy conversion efficiency. The synthesis of these HTMs follows the push-and-pull approach to modulate the HOMO energy level by combining the BT group as an electron acceptor and diphenyl- and triphenyl-amines as electron donors. Here we show that despite adjusting the HOMO energy level to that of the perovskite is a believed requisite to achieve efficient interfacial hole transfer, additional factors must be taken into account to design novel and efficient HTMs, such as a high hole mobility, solubility in organic solvents, and thermal stability.

Electrochromism/electrofluorochromism is the reversible tuning of the absorption/photoluminescence by redox processes. Dual functional electrochromic/electrofluorochromic materials can be exploited in, for example, smart windows, encryption/anticounterfeiting technologies, displays, and biomedical sensors. While electrofluorochromic materials showing tunable fluorescence intensity are relatively widespread, those with tunable color emission are not. Moreover, electrofluorochromic materials with tunable emission in the near‐infrared (NIR) range are seldom encountered. Here, this work shows a new molecular approach to afford both the above functionalities, in addition to red–NIR electrochromism, by combining the properties of highly stable arylamine radical cation mixed valence species, with those of donor–acceptor systems exhibiting the photoinduced twisted intramolecular charge transfer mechanism.