Perovskite Solar Cells (PSC) - a first history as of early 2014 - summarising the developments

The later few developments in the historical recounting were based on the different ways of structuring the PV device - either one or both optional layers (see diagram above).  They also include using 3 different mixed halide perovskites and 2 different HTM.  Similarities with OPV technology, especially the ease of fabrication, has increased the amount of research done in PSC.

There are other important developments.  The first was depositing PbI₂ from solution, with on site conversion to perovskite using vapour phase CH₃NH₃I reaction.  The second was the use of different organic cations: CH₃NH₃⁺, CH₃CH₂NH₃⁺ (ethylammonium), and NH₂CH=NH₂⁺ (formamidinium).  The latter 2 cations with larger ionic radii will increase the tolerance factor t, so crystal structures are more symmetrically cubic.  In addition to varying the organic cations, the inorganic cations and halide anions proportions can also be varied in mixed PSC, thereby adjusting their properties.  The most common adjustment is through halides.

The third important developments pertains to HTM and ETM (Electron Transport Media).  The FTO and compact TiO₂ combination has been replaced by another TCO (transparent conducting oxide) called ITO (indium tin oxide) with another thin ZnO nanoparticle layer (25nm).  This resulted in an efficiency of 15.7% for planar cells on glass.  It is also quite effective to use low temperature processing on the flexible polyethylene terephthalate.  Similarly, it is also quite effective to use inorganic HTM: CuI and CuSCN.  OPV that are quite effective are the use of (6,6)-phenyl C₆₁-butyric acid methyl ester as ETM, and the use of poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulphonate) as HTM.  The efficiencies are up to 12%.

These different technologies improve the potential uses, eg. flexible solar cells only need low processing temperatures < 150°C.  Compact TiO₂ will need 500°C to process.  The use of graphene nanoflakes in compact TiO₂ layer reduces the processing temperature.  A PSC structure with all optional layers and Al₂O₃ scaffolding, which is 0.6% graphene/TiO₂ by weight, achieved an efficiency of 15.6%.  Another PSC structure with scaffolding that combines TiO₂ nanoparticles with a titanium diisopropoxide bis(acetylacetonate) binder achieved an efficiency of 15.9%.

Another important development is the use of non-uniformity in solution deposition to produce semi-transparent PSC with neutral colour.  This produces small invisible islands instead of continuous perovskite, and should be less expensive than using lasers to fabricate semi-transparent a-Si solar cells.



Reference:

"The emergence of perovskite solar cells" by Martin A. Green, Anita Ho-Baillie and Henry J. Snaith, published online 27 June 2014, https://www.researchgate.net/publication/280388277