History, Progress and Perspective of Halide Perovskite Photovoltaics
Written by Arthur L. Robinson
In his Symposium X presentation, Nam-Gyu Park of Sungkyunkwan University placed perovskite photovoltaics squarely in the category of a disruptive technology, which he defined as one that replaces an existing technology. He cited the example of Kodak, which filed for bankruptcy in 2012 after dominating the analog film business for much of its 131 years of existence, owing to its unwillingness to pursue digital technology after its 1975 invention of the digital camera. Drawing on a 1962 theory of the diffusion of innovation, Park showed that there is a period of discontinuity marked by a transition state during which the old technology matures and then declines while the new one is in an innovation stage. In the auto industry, we are seeing the hybrid gasoline/battery vehicle as a transition to the electric vehicle, and in renewable energy, where the solar disruption has already begun, the replacement of silicon by perovskite photovoltaics, owing to superb performance and very low cost, will be marked by tandem silicon/perovskite solar cells.
Reviewing perovskite basics, Park first recalled the structure of an ABX3 perovskite (an isostructure of the orthorhombic MgTiO3 mineral found in the Earth’s lower mantle) then moved on to the organic-lead halide perovskites, APbI3, where in this talk the A is usually CH3NH3 (methylammonium or MA). Park cautioned that nanocrystals of these materials are vastly different from conventional semiconductor nanocrystals, such as CdSe and InP, in part because of the highly delocalized iodide cation. One difference is the defect tolerance of lead halide perovskites that is due to the bandgap being formed from two sets of antibonding orbitals, as opposed to bonding and antibonding states in conventional semiconductors, and therefore resulting in shallow, less damaging rather than damaging mid-gap states for defects.
Park then switched to a quick history of the perovskite solar cell (PSC), beginning with the 2009 report from Tsutomo Miyasaka’s group at Toin University of Yokohama of perovskite-sensitized solar cells consisting of MAPbBr3 or MAPbI3 nanocrystals coating an 8-µm-thickTiO2 layer, with the iodide version showing a power conversion efficiency of 3.8%. This was followed in 2011 by a report from Park’s group, which had raised the power conversion efficiency to 6.5%. One reason for the advance was a more concentrated precursor solution, which promoted the formation of favorable PbI6–4 and one-dimensional Pb2I4 colloids. A further development published by Park’s group in 2012 was based on decreasing the TiO2 film thickness to less than 1 µm and infiltrating the film pores with an organic hole-conducting polymer (Spiro meOTAD) to achieve a power conversion efficiency of 9.7%. This publication was followed by a rapid increase in the number of perovskite solar cell publications from a few to more than 2000 in 2016. Noting a similarity to the rapid increase in graphene publications that culminated in a Nobel Prize, Park humorously wondered what the future held in store for perovskite researchers. He also noted a subsequent increase in perovskite solar cell power conversion efficiencies, now at 22.7%. An analysis of publications and patents suggested five emerging research areas: perovskite fundamentals, perovskite solar cells, oxide perovskites, perovskite tandem photovoltaics, and perovskite light-emitting diodes (LEDs).
From here, Park looked at recent progress in perovskite solar cell research. In brief, among these were controlling the crystal size for high efficiency, use of an adduct synthesis approach (the addition of two or more distinct molecules resulting in a single reaction product containing all the atoms of all the components) for reproducibility, passivation of grain boundaries for longer carrier lifetime, substitution of formamidinium [HC(NH2)2] for methylammonium for high power conversion efficiency and photostability, the addition of a small amount of cesium to the formamidinium for resistance to humidity and reduced hysteresis in the I-V curve, and the synthesis of nanowires with improved lateral conductivity and carrier lifetime. Park also described a method for reduction in hysteresis and improving moisture stability by means of interfacial nanoengineering using a two-dimensional perovskite of the form A2PbI4, where A is phenethylamine or PEA, at the boundaries between the three-dimensional crystals.
Taking a wider view of possible perovskite applications, Park reported on efforts in his group to make LEDs, based on a nonstoichiometric adduct method and solvent-vacuum drying process. The researchers achieved an emission quantum efficiency of 8.2%. Park finished with an even bigger leap to x-ray imaging of humans, where repeated medical x-ray exposure can accumulate over time to reach cancer-causing doses. A direct imaging camera based on multicrystalline perovskite crystals was constructed and in tests achieved clear images at lower doses than existing amorphous selenium arrays.
Park summarized his talk with these points:
- Grain size with dominant radiative-recombination is important for high efficiency PSC.
- Grain boundary engineering is important not only for high efficiency but also for stability.
- Compositional engineering is important for structural stability and device stability.
- Understanding precursor solution chemistry is important for high quality perovskite films.
- Understanding defect chemistry and defect engineering are important for hysteresis-free PSC.
- Organic-inorganic lead halide perovskites are (and will be) game changers not only in photovoltaics but also in diverse optoelectronic applications.