Emerging Hole-Selective Monolayers for Optoelectronic Applications

Established organic hole-transporting materials (HTMs) have been used for a variety of optoelectronic devices, such as light-emitting diodes, solar cells, and field-effect transistors. However, the typical hole mobility and conductivity values for such materials are relatively low. For this reason, organic HTMs either need to be doped, or the formed films should be very thin (often less than 10 nm). Unfortunately, doping could lead to reduced long-term stability, while the formation of very thin films can often be challenging.<br/>To overcome this limitation, we have recently come up with a new concept for the formation of the hole-selective contacts, by utilizing self-assembly of certain organic compounds. Due to its self-limiting nature, the process of the film formation stops at the thickness of the single molecule, while the robust anchoring phosphonic acid group is providing a strong affinity to the oxide-based substrates.<br/>Proof-of-concept was demonstrated in the perovskite solar cells.[1] While competitive efficiency was achieved, it was still behind that of the traditional polymeric HTM PTAA. The breakthrough was achieved, when the materials called <b>2PACz</b> ([2-(9H-carbazol-9-yl)ethyl]phosphonic acid) was introduced in the tandem perovskite/CIGS solar cells, leading to the record efficiency.[2] It was possible due to the good interface with the perovskite, as well as the ability of the self-assembled monolayers (SAMs) to conformally cover the rough surface of the bottom CIGS cell. As a follow-up of this work, several important results were achieved, including record efficiency of the Si/perovskite tandem solar cell,[3] and one of the highest reported efficiency of the organic solar cell.[4]<br/>In my presentation, I am going to discuss the opportunities, provided by the concept of the SAM-based selective contacts, as well as its limitations. In addition, possible structural changes will be discussed, that could be used for the design of the novel SAMs with specific applications.