Info IT: Perovskite Solar Cells Some Recent Developments

Thursday, February 16, 2017

Perovskite Solar Cells Some Recent Developments

I have written the following article, which will be published later in the year in the journal Science Progress of which I am an editor. However, given the importance of its subject and the wide interest in perovskite solar cells, I am posting it here as a preview.

Introduction.

Perovskite solar cells were the subject of a current commentary, previously published in this journal¹ These are solar cells whose structure utilises a perovskite-type compound as the light-harvesting active layer.¹ The term perovskite solar cell originates from the ABX₃ stoichiometry of the absorber materials, which translates to a perovskite crystal structure (Figure 1). The most extensively studied absorber material is methylammonium lead trihalide (CH₃NH₃PbX₃; where X is a Cl, Br or I atom), with a bandgap energy in the range of 1.5—2.3 eV. The related material, formamidinum lead trihalide (H₂NCHNH₂PbX₃) is also promising, with bandgaps that occupy the range 1.5—2.2 eV. Indeed, since the smallest bandgap approaches more closely the optimum value for a single-junction cell than does methylammonium lead trihalide, it is expected that higher efficiencies might be attained using these materials². It has been shown that the bandgap in the methylammonium lead halides can be tuned by varying the halide content, and^2,3that the diffusion lengths for both holes and electrons in these materials are greater than 1 micron⁴. The latter feature means that charges can be transported over long distances in the perovskite, and the materials are effective in thin-film architectures. The charges per se have been demonstrated to comprise predominantly free electrons and holes, and are not bound excitons. This is a consequence of the fact that the binding energy for excitons is sufficiently low that charge separation can occur at room temperature⁵.

The rise in power conversion efficiencies of solar cells based on perovksite materials has been little short of meteoric, considering that just 3.8% was achieved in 2009 ⁶but this had climbed to 22.1% in early 2016 ⁷(Figure 2). Working on the premise that even higher efficiencies should be attainable and that the production costs of perovskite solar cells are relatively low, they have attracted sufficient commercial interest that a number of start-up companies have expressed their intention to be selling actual photovoltaic modules by 2017 ^8,9. In view of the potential widespread importance of perovskite solar cells in the future, it is most appropriate to include mention of them in undergraduate courses. To this end, a very timely account has been published which demonstrates how a simple such cell can be constructed for the purposes of practical demonstration¹⁰.

Production of perovskite solar cells.

The processing of and fabrication of perovskite solar cells is considerably less demanding than is the case for the more usual silicon-based cells, for which costly multistep processes are necessary, needing to be carried out in a clean room, involving high vacuum facilities, and temperatures in excess of 1000 °C ¹¹. In contrast, organic-inorganic perovskites can be produced using wet chemistry techniques under standard laboratory conditions. Thus, it has proved possible to synthesise the above mentioned methylammonium and formamidinium lead trihalides by means of various solvent-based and vapour deposition methods, with the indication that future production on the large scale should be possible^12,13. Despite the attractive simplicity of the method, solution processing leaves voids, platelets, and other defects in the perovskite layer, which might act to impede the photovoltaic efficiency of the cell. To avoid such problems, a method has been presented¹⁴that produces perovskite films with both a large crystal dimension and a smooth structure, with more advantageous light conversion efficiencies. An alternative approach, which involves solvent-solvent extraction at ambient temperature, enables the generation of crystalline films which cover an area of several square centimetres, in a controlled fashion, down to a thickness of 20 nanometers, and without creating pin-holes.. An alternative method¹⁵ involves annealing the spin coated, or exfoliated lead halide in the presence of methylammonium iodide vapour at temperatures close to 150 °C. This offers certain advantages over solution processing, since it may enable the creation of multi-stacked thin films over larger areas¹⁶, as might be a useful feature when it is desired to make multiple-junction cells. As a general point, we note that vapour deposition methods yield more uniform thicknesses than are created by simple solution processing. Both solution based and vapour deposition approaches have their own advantages, and should prove scalable at the manufacturing level, with much reduced costs and complexity in comparison with making silicon photovoltaic cells. Solution methods are cheaper than vapour based approaches, although the latter naturally eliminate some of the need for solvents and having to deal with solvent residues. Planar thin film layers can be created using both approaches, and either find application in mesoscopic designs, for example to place coatings on a metal oxide scaffold, as is typical for the perovskite or dye-sensitized solar cells that are currently made. However, a major uncertainty over the use of perovskite solar cells concerns their stability over the longer time, in view of the observation that such materials undergo degradation under typical environmental conditions, with concomitant losses in light conversion efficiencies.

Estimating the efficiency limit for perovskite solar cells.

The Shockley–Queisser limit (detailed balance)¹⁷for radiative efficiency is about 31% under an AM1.5G solar spectrum at 1000 W/m², as applied to a Perovskite with a bandgap of 1.55 eV¹⁸. This is somewhat less than the radiative limit of gallium arsenide (bandgap 1.42 eV) which can reach a radiative efficiency of 33%. A detailed balance model for the maximal output power of a solar cell that can be achieved on the proviso that certain conditions are met¹⁷:

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