Perovskite Solar Cells - an Introduction

Mixed organic-inorganic halide perovskites is a technology that provides higher energy conversion efficiency and significantly lower processing costs as compared to silicon.  Other advantages include ease of fabrication, strong solar absorption, and low non-radiative carrier recombination rates in these materials which are easily prepared.  In addition, it can depend on over 20 years of research development of related dye-sensitised and organic PhotoVoltaic (PV) cells.

An important property for some perovskite architecture is having reasonably high carrier mobility.  So is the formation of a range of properties based on mixed compounds using suitable perovskite architectural materials.  However, one of the major constituents of perovskite architecture is Lead (Pb), which is toxic and unsafe.  In addition, perovskites usually, and sometimes rapidly, degrade when exposed to moisture and ultraviolet radiation.  

The structure of perovskites is described by the formula ABX₃.  X is an anion, A is a larger cation, and B is a smaller cation.  Under present applications, cation A is organic and can be either methylammonium, ethylammonium, or formamidinium to give good results.  Methylammonium (CH₃NH₃) is often used.  The anion X is a halogen, which is either iodine (I), bromine (Br), or chlorine (Cl).  The cation B has been Lead (Pb) for efficiency of solar cells.  Tin (Sn) forms similar compounds with lower bandgaps, which are theoretically more ideal.  However, there is usually lower stability due to ease of oxidation of Tin.

Hence, the standard perovskite compound is methylammonium lead triiodide.  Mixed halides are also important.  

The crystallographic stability and probable structure of perovskites can be guessed by using a tolerance factor t and octahedral factor μ, where

t = (R_A + R_X) / [√2 (R_B + R_X)]
μ = R_B/R_X

R_A, R_B and R_X are the ionic radii of the corresponding ions.  The equation of t arises from the ratio of the distance A-X to the distance B-X in an idealised solid sphere model.

Usually, 0.81 < t < 1.11 and 0.44 < mu < 0.90.  If 0.89 < t < 1.0, a cubic structure for perovskites is likely.  If t values are lower, there will be less symmetric tetragonal or orthorhombic structures.  However, transitions between these perovskite structures occur frequently on heating.  Usually, the high temperature phase is cubic.



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