In recent years, some new types of mesoscopic solar cells, such as dye-sensitized solar cells, have caught a lot of attention. Besides, MSC technologies not only represent an efficient strategy to reduce processing costs and achieve high energy conversion efficiency but also offer a tantalizing prospect for wide applications because of its simpler manufacturing process without high energy consumption and earth-abundant raw materials. Since being reported by M. Grätzel’s group in 1991 (figure 1a), DSSC has been paid intensive attention in the last twenty years. Up to now, the certified highest PCE of DSSCs has reached 11.9%, presenting excellent market competitiveness and commercial prospect. However, such DSSCs employ a liquid-state electrolyte containing highly volatile solvent, which not only affects the long-term stability of the device but also limits its large-scale manufacture and application. Thus, replacing the liquid-state electrolyte with solid-state intermediate became one of the most important research areas for DSSCs

In 1998, M. Grätzel’s group firstly reported a solid-state organic hole-transporting-material spiro-OMeTAD and developed a solid-state DSSC (figure 1b). Their work were published on Nature in the same year. After a series of optimization, the PCE of ss-DSSC increased steadily from 0.74% to 7.2%. Until 2012, a breakthrough was eventually achieved. N.-G. Park’s group and M. Grätzel’s group used CH3NH3PbI3 (MAPbI3) as the light absorber to fabricate a solid-state mesoscopic perovskite solar cell (MPSC) with the PCE of up to 9.7 (figure 1c). In October 2012, H. J. Snaith’s group adopt a mesoporous Al2O3 as an inert scaffold and successfully enhanced the efficiency up to 10.9% (figure 1d). And then, M. Grätzel’s group invented a sequential two-step deposition method for perovskite layer. With this method, they fabricated a solid-state mesoscopic solar cell that achieved a certified PCE of 14.1%, which is based on (CH3NH3)PbI3, HTM spiro-OMeTAD and Au counter electrode(figure 1e). Several months later, S. I. Soek’s group enhanced the certified PCE of MPSC from 14.1% to 16.2%. One year later, A world champion efficiency of PSCs obtained by S. I. Soek’s group with a PCE of 20.1%. However, the HTM spiro-OMeTAD they used is very expensive, whose price is 10 times higher than Au or Pt. This is obviously difficult to be commercialized. In fact, The HTM-free MPSC was firstly reported by L. Etgar et al. in August 2012 with a PCE of 5.5%(figure 1f). Then in April 2014, the group improved the efficiency of such HTM-free MPSCs to 10.85%, but the counter electrode was still Au which is one of precious metals. It is obvious that the Au counter electrode is not only expensive but also difficult to fabricated which need special devices providing high vacuum and high energy consumption environment. Such process would do damage to the structure of solar cells so short circuit and worse repeatability come about. All in all, how to invent low-cost and HTM-free PSCs is the main challenge before PSC can be applied universally.

In 2013, H. W. Han’s group firstly developed a HTM-free fully printable MPSC based on mesoscopic triple-layer architecture and carbon counter electrode, which evolved from monolithic DSSCs. And We achieved a PCE of 6.67%. Then in 2014, H. W. Han’s group employed 5-ammoniumvaleric acid (5-AVA) into the conventional MAPbI3, and developed a mixed-cation perovskite (5-AVA)x(MA)1- x PbI3 crystals that applied in the printable MPSC. Finally, we got a certified PCE of 12.84% and the MPSC was stable for 1008 hours in ambient air under light soaking.