The materials, called perovskites, are particularly good at light absorbing and has stunned the photovoltaic community. Solar cells made of this materials, namely the currently termed perovskite solar cells (PSCs) as evolved from dye-sensitized solar cells (DSSCs), was recognized as one of Science’s 10 breakthroughs in 2013. However, only by an innovative strategy we developed to make the highest quality single-crystal perovskites was us able to study the photovoltaic merits of this materials in their purest form – perfect single crystals [1]. By using a combination of laser-based techniques to track down the rapid motion of charge carriers in the material, we determined the mobility—how fast the carriers can move through the material as well as the diffusion length—how far carriers can travel without getting trapped by imperfections in the material. Our work identifies the bar for the ultimate solar energy-harvesting potential of perovskites.

On the other hand, despite intense research efforts, the performance of spiro-OMeTAD as the most commonly used hole-transporting layer (HTL) in PSCs and DSSCs has remained stagnant, creating a major bottleneck for improving solar cell efficiency. Thinking that the material has given all it has to offer, many researchers have begun investigating alternative materials to replace spiro-OMeTAD in future solar cells. But in a new study published in Science Advances [2], we have taken a closer look at spiro-OMeTAD and found that it still has a great deal of untapped potential. For the first time, we have grown single crystals of the pure material, and in doing so, we have made the surprising discovery that spiro-OMeTAD's single-crystal structure has a hole mobility that is three orders of magnitude greater than that of its thin-film form (which is currently used in solar cells). This work reports a major breakthrough for the fields of perovskite and solid-state dye-sensitized solar cells by finally clarifying the potential performance of the material and showing that improving the crystallinity of the hole transport layer is the key strategy for further breakthroughs in device engineering of these solar cells. The findings suggest that, at least in the short term, the time-consuming process of designing and synthesizing radically new organic hole conductors as replacements to spiro-OMeTAD may not be necessary.

References

[1] D. Shi, et al., Science 347, 519 – 522 (2015).

[2] D. Shi, et al., Science Advances 2, e1501491 (2016).