Abstract
Since the introduction of Perovskite Solar Cells, their photovoltaic efficiencies have grown impressively, reaching over 25%. Besides the exceptional efficiencies, those solar cells need to be improved to overcome some concerns, such as their intrinsic instability when exposed to humidity. In this respect, the development of new and stable Hole Transporting Materials (HTMs) rose as a new hot topic. Since the doping agents for common HTM are hygroscopic, they bring water in contact with the perovskite layer, thus deteriorating it. In the last years, the research focused on “dopant-free” HTMs, which are inherently conductive without any addition of dopants. Dopant-free HTMs, being small molecules or polymers, have still been a relatively small set of compounds until now. This review collects almost all the relevant organic dopant-free small-molecule HTMs known so far. A general classification of HTMs is proposed, and structure analysis is used to identify structure–property relationships, to help researchers to build better-performing materials.
Highlights
The discovery that organic molecules, normally considered as insulating, were able to conduct electricity was a real breakthrough in the field of organic chemistry [1,2,3], and organic molecules’ conductivity became a hot topic [2,3,4,5]
The application of conductive polymers grew rapidly, along with the comprehension of the conductivity phenomenon and charge recombination [2,6]. Another breakthrough was obtained by recognizing that electrical conductivity was not a property merely related to polymers and to small organic molecules, which found great application in organic electronics [7]
The design of a proper Hole Transporting Materials (HTMs) (the same is valid for Electron Transporting Material (ETM)) should consider that, when the charge extractor material is deposited onto the perovskite layer, it could slightly modify the energetic level of its frontier molecular orbital via the band-bending effect
Summary
The discovery that organic molecules, normally considered as insulating, were able to conduct electricity was a real breakthrough in the field of organic chemistry [1,2,3], and organic molecules’ conductivity became a hot topic [2,3,4,5]. The application of conductive polymers grew rapidly, along with the comprehension of the conductivity phenomenon and charge recombination [2,6]. Another breakthrough was obtained by recognizing that electrical conductivity was not a property merely related to polymers and to small organic molecules, which found great application in organic electronics [7]. PSCs always require the presence of both an Electron Transporting Layer (ETL) to drain efficiently electrons and a Hole Transporting Layer (HTL) able to lose electrons efficiently, accepting holes In this manner, both electrons and holes are drained from the photoactive layer (i.e., the perovskite film) to the respective electrode, limiting the recombination phenomena.
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