Organic-inorganic perovskite solar cells (PSCs) have become one of the most promising solar cells, as the power conversion efficiency (PCE) has increased from less than 5% in 2009 to certiﬁed values of over 22%. In the typical PSC device architecture, hole transport materials that can effectively extract and transmit holes from the active layer to the counter electrode (HTMs) are indispensable. The well-known small molecule 2, 2', 7, 7'-tetrakis-(N, N-di-4-methoxy-phenyl amino)-9, 9'-spirobifluorene (spiro-OMeTAD) is the best choice for optimal perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term use in devices due to the sophisticated oxidation process associated with undesired ion migration/interactions. It has been found that spiro-OMeTAD can significantly contribute to the overall cost of materials required for the PSC manufacturing, thus its market price makes its use in large-scale production costly. Besides, another main drawback of spiro-OMeTAD is its poor reproducibility.
To engineer HTMs that are considerably cheaper and more reproducible than spiro-OMeTAD, shorter reaction schemes with simple purification procedures are required. Furthermore, HTMs must possess a number of other qualities, including excellent charge transporting properties, good energy matching with the perovskite, transparency to solar radiation, a large Stokes shift, good solubility in organic solvents, morphologically stable film formation, and others. To date, hundreds of new organic semiconductor molecules have been synthesized for use as HTMs in perovskite solar cells. Successful examples include azomethine derivatives, branched methoxydiphenylamine-substituted fluorine derivatives, enamine derivatives, and many others. Some of these have been incorporated as HTMs in complete, functional PSCs capable of matching the performance of the best-performing PSCs prepared using spiro-OMeTAD while showing even better stability.
In light of these results, we describe the advances made in the synthesis of HTMs that have been tested in perovskite solar cells, and give an overview of the molecular engineering of HTMs. Meanwhile, we highlight the effects of molecular structure on PCE and device stability of PSCs. This review is organized as follows. In the first part, we give a general introduction to the development of PSCs. In the second part, we focus on the introduction of the perovskite structure, device architecture, and relevant work principles in detail. In the third part, we discuss all kinds of molecular HTMs applied in PSCs. Special emphasis is placed on the relationship between HTM molecular structure and device function. Last but not least, we point out some existing challenges, suggest possible routes for further HTM design, and provide some conclusions.