by Michael Crowley
Silicon photovoltaics are a proven sustainable energy solution and take up 90% of the solar cell market. Silicon cells have many practical advantages associated with them, including high cell efficiency, stability and longevity. They are also extremely cost effective. The cost of mass produced silicon cells has dropped below $1/W, in some cases as low as $0.3/W. At the same time, efficiency continues to increase to 20%. The biggest hindrance to further increases in efficiency are high rates of recombination at the surface of the cells, which is what Shinde et. al (2016) have been working on.
In order to lower the rates of recombination, the technique of passivation has been employed. SiNx and SiO2 compounds have been used for passivation in the past. Although the desired surface passivation is accomplished, these compounds require high process temperatures (300° – 1000° C). At these temperatures, the properties of the silicon crystalline structure are affected. If these temperatures are reduced, efficiency and longevity are expected to increase.
To combat high process temperatures, new techniques have been presented. It has been shown that passivation can be achieved by using Si-O and Si-H and organic passivation. Shinde et. al (2016), look at passivation of n-type emitter by organic cover layer Oleylamine (OLA). This passivation technique will increase efficiency and has the ability to be processed at room temperature.
When comparing basic, standard silicon cells to cells passivated with OLA, many interesting results were found. Uncoated emitter of standard cells register reflection values between 27 – 30% for the 300 – 800 nm range. When the same cells are coated with OLA, the reflection efficiency increases. This result shows that OLA is achieving the goal of serving as a passivating layer. It also rules at the possibility that this layer acts as an anti-reflection layer.
When analyzing cell efficiency, OLA coating proves successful as well. Without the OLA coating, cell efficiency was registered at 13.34%. With the OLA coating, that value shoots up to 15.20%. This again shows the ability for OLA coating to successfully passivate the cell surface. Finally, by looking at Raman measurements, bonding with the silicon is clearly achieved. Raman measurements observe vibrational, rotational and low frequency processes in a system.
All of these results are achieved at room temperature and this makes their finding very significant. OLA coating has a huge potential to significantly reduce thermal requirements for the fabrication of solar cells and potentially reduce payback time frames for photovoltaic systems.
Shinde, O.S., Funde, A.M., Agarwal, M. et al. (2016) Emitter passivation of silicon solar cell via organic coating at room temperature. J Mater Sci: Mater Electron 27: 12459. doi:10.1007/s10854-016-5706-8