Perovskyte solar cells: an emerging technology that promises high efficiency and low manufacture cost.

Over the last 10 year a perovskite based solar photo-voltaic cells have emerged as a promising technology that could offer high energy conversion efficiency and low manufacture cost. Compared to other emerging PV technologies the efficiecy of perovskite solar cells has increased much more rapidly. The first perovskite solar cell in 2006 had an efficiency of 2.6%, while this year efficiency of 20.1 % have already been reported (Nrel.gov, 2015). Other technologies have taken decades to reach 20% efficiency. 

Record PV cell efficiency over time (click to enlarge). Note the rapid increase for perovskyte cells (yellow circle with red border) compared tt other technologies (Nrel.gov, 2015).  

This has made perovskites one of the hottest topics in PV research. While on 2009 a total of 50 publications where made on this topic, on 2014 more than 400 publications were made and as of May 2015 more than 300 publications were made (Ossila.com, 2015). 

What exactly is a perovskite solar cell?

Perovskite solar cells are any cell with a perovskite material light absorbing layer. Perovskite materials are any material with the crystal structure of calcium titanium oxide (CaTiO3). The basic composition is given by the formula ABX3 :

–A = An organic cation  (eg. methylammonium (CH3NH3)+)

–B = A big inorganic cation  (usually lead(II) (Pb2+))

–X3= A slightly smaller halogen anion (usually chloride (Cl-) or iodide (I-))

The perovskyte crystal structure, note that both unitary cells are equivalent (Ossila.com, 2015).

For perovskite PV cells the two materials that have proven to be most successful are: 

-Methylammonium lead trihalide (CH3NH3PbX3) (Bandgap 2.3 eV - 1.6 eV) and, 

-Formamidinum lead trihalide (H2NCHNH2PbX3) (Bandgap 2.2 eV - 1.5 eV).

So why have these material drawn so much attention?

Perovskites have several advantages that make then suitable for high efficiency solar cells. First of all their light absorption coefficient is very high and it increases with temperature making them good light absorbents (Green et al., 2014). This light absorption spectra can also be tuned by changing the halide content of the perovskite which is a big advantage to tune the material for specific applications (Jun Hong, 2013). 

Variable absorption coefficient for different composition perovskites (Eperon et al., 2014) 

They exhibit large carrier diffusion lengths, making them suitable for thin film applications (Hodes,  2013). Their open circuit voltage is also high, resulting in cells with high efficiency (Ball et al., 2014). Finally the manufacture methods for perovskites are simple wet chemistry methods, such as spin coating or vapor deposition, that require only common lab equipment and are easy to scale. This makes the fabrication process simpler and lower in cost. 

Flexible perovskite solar cell fabricated by low temperature deposition (http://www.materialsviews.com/new-processing-methods-flexible-perovskite-solar-cells/)

However, there are some challenges that perovskite cells still need to overcome. Perovskite cells are unstable and degrade rapidly under moisture and UV radiation conditions. This means that full encapsulation of the cell is required (D’Innocenzo et al., 2014). Another drawback is that the composition of the perovskite has lead in it, a toxic metal. There have been attempts to replace the lead with other elements but they have not been successful and lead still offer materials that produce the highest efficiency cells (Baker, 2014). 

Summarizing we can say that perovskites are a promising material that has showed rapid development and efficiency increase. Perovskites combine high performance parameters (diffusion length, absorption coefficient, Voc) and potential for low cost manufacture. However challenges like the use of lead and rapid degradation still need to be overcome for commercialisation of this technology. 

References 

1.Baker, Joel R. 2014. “Perovskite Solar Cells.”

2.Ball, James M., Michael M. Lee, Andrew Hey, and Henry J. Snaith. 2013. “Low-Temperature Processed Meso-Superstructured to Thin-Film Perovskite Solar Cells.” Energy & Environmental Science 6: 1739. doi:10.1039/c3ee40810h.

3.Collavini, Silvia, Sebastian F Vçlker, and Juan Luis Delgado. 2015. “Understanding the Outstanding Power Conversion Efficiency of Perovskite-Based Solar Cells,” 9757–59. doi:10.1038/srep00591.

4.D’Innocenzo, Valerio, Giulia Grancini, Marcelo J P Alcocer, Ajay Ram Srimath Kandada, Samuel D Stranks, Michael M Lee, Guglielmo Lanzani, Henry J Snaith, and Annamaria Petrozza. 2014. “Excitons versus Free Charges in Organo-Lead Tri-Halide Perovskites.” Nature Communications 5: 3586. doi:10.1038/ncomms4586.

5.Eperon, Giles E., Samuel D. Stranks, Christopher Menelaou, Michael B. Johnston, Laura M. Herz, and Henry J. Snaith. 2014. “Formamidinium Lead Trihalide: A Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells.” Energy & Environmental Science 7 (3): 982. doi:10.1039/c3ee43822h.

6.Green, Martin A, Anita Ho-baillie, and Henry J Snaith. 2014. “The Emergence of Perovskite Solar Cells” 8 (July). Nature Publishing Group. doi:10.1038/nphoton.2014.134.

7.Hodes, Gary. 2013. “Perovskite-Based Solar Cells” 342 (October): 317–19.

8.Nrel.gov, 'NREL: National Center for Photovoltaics Home Page', 2015. [Online]. Available: http://www.nrel.gov/ncpv/

9.Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells Nano Letters (2013), 13(4), 1764-1769 CODEN: NALEFD; ISSN: 1530-6984;