This review summarizes the current designs
and development of new types of organic small molecules as a hole-transporting
material (HTM) in a meso-structured perovskite solar cell (PSC). The roles of
each layer in the meso-structured perovskite device architecture are elaborated
and the employment of new types of organic HTMs in the device is compared with
the commercially available HTM spiro-OMeTAD in terms of the properties, device
performance and stability. The studies found that nearly half of the new
synthesized and pristine HTMs have comparable or better photovoltaic properties
than those of doped spiro-OMeTAD. These HTMs have the characteristics of a
fused planar core structure with extended p-conjugated lengths and
electron-donating functional groups, which are believed to contribute to their
high intrinsic conductivity and help make them an alternative to spiro-OMeTAD
as a better HTM in meso-structured PSCs. Some of the devices based on the new
synthesized HTMs even have longer device lifetimes than their
spiro-OMeTAD-based PSC counterparts. Moreover, studies found that the cost per
gram (Cg) and cost-per-peak Watt (Cw) of synthesized HTMs can be reduced via
minimizing the number of synthesis steps and by optimization of the starting
materials in order to yield low-cost HTMs for meso-structured PSC applications.