30 April 2015 | 09:00
By Jan Gorgol

The photovoltaic effect is the creation of a voltage in a material upon exposure to sunlight; it is how a solar cell converts sunlight into electricity. It was first observed by Becquerel in 1839. However it was not until the advent of Silicon technology’s in the 1950’s that photovoltaic cells were mass produced, exhibiting power conversion efficiencies greater than 10%.

Today, Si solar cells account for roughly 90% of solar cells manufactured for power generation. The conversion efficiency of commercial large-area modules is 13-18%. These Si photovoltaics (PVs) have been very successful and the industry is growing at approximately 50-60% per annum. However, global energy generation from solar power still amounts to less than 0.1 % of total energy usage. To generate a much larger fraction of our global power from solar energy, a radically new manufacturing technology is required.

Perovskite photovoltaics have the potential of providing solar energy at a much lower cost than traditional technologies, as they can be mass produced by electronic printing and are made from inexpensive, abundant materials.

In the last 2 years metal halide perovskite solar cell efficiencies at the laboratory scale have doubled to nearly 20%. Since PCE values over 20% are realistically anticipated with the use of cheap organometallic halide perovskite materials, perovskite solar cells are viewed as a very promising photovoltaic technology [1,2].

Additionally Perovskite photovoltaics can obtain voltages of over 1.1V, which are higher than other polycrystalline solar cells. This opens up the possibility of laminating perovskite devices on top of silicon or CIGS solar cells, creating multijunction solar cells with a power efficiency approaching 30% or even more.

The Perovskite structure is commonly understood as that of a compound material of stoichiometry AMX3, where X is an anion and A and M are cations of different sizes (A being larger than M). The majority of known Perovskite materials are fully inorganic.

Crystal structure of perovskite compounds

However, Solar cell applications feature another type of Perovskite material, the organo-metal halide Perovskites, where M is typically a divalent metal ion such as Pb(2+), Sn(2+) and Ge(2+) while X is a halide anion such as I(-), Cl(-) and Br(-) and A is an organic cation.

Some key attributes of these Perovskites include ease of manufacture, strong absorption, and low carrier recombination rates. An additional advantage is their ability to capitalize on over 20 years of development of related dye-sensitized and organic photovoltaic cells. One negative side of perovskites is the facts that Lead has been a major constituent of all highly performing perovskite cells to date, raising environmental toxicity issues during device fabrication, use, and disposal. Replacement of Pb by Sn is being examined.

Parallel to these developments, scientists have been active in studying the fundamental properties of these perovskite materials and solar cell operation including stability.

Today’s perovskite solar cells are not very stable or reliable. They generally undergo degradation (sometimes quite rapidly) on exposure to moisture and UV. In particular, the devices are highly sensitive to moisture (rapidly change colour) and can easily exhibit hysteresis. Improving the stability of the Perovskite to light, electro migration and moisture is now being studied for commercialization of this promising technology [3]. It has been found that Perovskites degrade in a high humidity environment. Hence, the stability of Perovskite under moisture is critical to enhance absolute performance whilst also improving understanding of the manufacture and fundamental processes involved.

Recently, a number of researchers are turning to the GenRH series of humidity generators to aid and accelerate their understanding of humidity effects in the manufacturability, storage and usability of these new higher efficiency cells. We welcome further enquires in this area.

Reference:

[1] Nam-Gyu Park, Perovskite solar cells: an emerging photovoltaic technology Materials today Volume 18, Issue 2, March 2015, Pages 65–72
[2] Martin A. Green, Anita Ho-Baillie and Henry J. Snaith The emergence of perovskite solar cells .NATURE PHOTONICS | VOL 8 | JULY 2014 | www.nature.com/naturephotonics
[3] Guangda Niu, Wenzhe Li, Fanqi Meng, Liduo Wang, Haopeng Dong ,Yong Qiu
Study on the stability of CH3NH3PbI3 films and the effect of post-modification by aluminum oxide in all-solid-state hybrid solar cells. J. Mater. Chem. A, 2014, 2, 705–710

About the author:

Jan Gorgol studied Physics at Bristol University followed by a Masters working on GeSe2 glasses at Brunel University while working with XPS & SEM at the Experimental Techniques Centre. After working extensively in surface science instrumentation globally he now is Product Manager for the GenRH series of humidity generation products at Surface Measurement Systems Ltd.