Perovskite Solar Cells - a first history as of early 2014

In 2006, it was reported that CH₃NH₃PbBr₃ cells have an efficiency of 2.2%.  In 2009, efficiency was raised to 3.8% by replacing bromine with iodine.  However, all devices were unstable and the hole transporting medium (HTM) was not solid state.

In 2011, an efficiency of 6.5% was achieved by applying TiO₂ surface treatment before deposition of perovskite as sparsely spaced hemispherical nanoparticles.  Although perovskite nanoparticles have better absorption when compared with standard N719 dye sensitizers, they dissolve in the electrolyte, so performance quickly degraded.

In 2012, a solid state HTM was finally used.  Spiro-MeOTAD was used to penetrate nanoporous TiO₂ when dissolved in an organic solvent.  After evaporation, only solute molecules are left.  Spiro-MeOTAD improved the stability and reported efficiency to 9.7%.

In 2012, another 4 developments occurred.  Spiro-MeOTAD was used with mixed halide CH₃NH₃PbI_3-xCl_x.  This provided better stability and carrier transport than using only iodine as the halogen.  The second development was to coat nanoporous TiO₂ surfaces with a thin perovskite layer.  This created extremely thin absorber (ETA) cells.  The third development was to use a non-conducting Al₂O₃ network/scaffolding instead of the conducting nanoporous TiO₂.  This improved the V_oc and efficiency rose to 10.9%.  The fourth development utilise perovskite cells without the scaffolding, demonstrating ambipolar transport with simple planar cells (see diagram below for the scaffolding).

In 2013, reported efficiency rose to 12.0% using both optional layers in the diagram.  A solid layer made of nanoporous TiO₂ infiltrated by perovskite is the scaffolding.  The best HTM used was to be poly-triarylamine.  The reported efficiency increased further to 12.3% when similar structures and mixed halide CH₃NH₃PbI_3-xBr_x is used.  Low Br content of <10% provided the best starting efficiency due to a lower bandgap.  Higher Br content of >20% gave better stability against humidity.  There is a correlation between this and a structural transition from tetragonal to pseudo-cubic.  This is due to a higher tolerance factor t due to the smaller ionic radius of Br.

In 2013, 2 further developments were made in May.  The first development improved the morphology by using TiO₂ scaffolding and a 2 step iodide deposition.  It included an independent measurement of efficiency amounting to 14.1%.  The 2nd development also improved the morphology and resulted in an efficiency of 15.4%.  It used simple but very different planar solar cells without scaffolding.  It used the 2 source thermal evaporation for CH₃NH₃PbI_3-xCl_x deposition.

Towards the end of 2013, an independently confirmed efficiency of 16.2% was reached by using mixed halide CH₃NH₃PbI_3-xBr_x with 10% to 15% Br, and a poly-triarylamine HTM.  Both optional layers shown in the diagram above were used.  The main factor for the improvement is the thickness of the perovskite-TiO₂ scaffolding relative to the continuous perovskite layer.  There are 2 unconfirmed increases in efficiencies in early 2014 - 17.9% and 19.3%.



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