The conventional commercial solar cells are made of p-type single crystalline substrates. It is required to work out, in this forum, the causes behind that. Could n-type substrates be equally as good as the p-type substrate?
In fact the use of p-type substrate has historical reasons: the minority carrier diffusion lenght in n-type substrate is more affected by material damage due to high energy particles (this is indeed closely related to the lower mobility). Since solar cells were made nearly exclusively for satellites, stability against such particles and gamma radiation from natural sources and nuclear bombs, testet in the higher atmosphere, was crucial. For terrestrial applications, n-type substrate offers some advantages over p-type substrate: the minority carrier diffusion length is less affected by various metal impurities, contained in industrial available material. Current Cz material reaches higher diffusion length on n-type than on p-type material. Additionaly, there is nearly no degradation due to boron-oxygen defects, a problem for highly efficient p-type solar cells. Nowadays, one of the best crystalline technology is on n-type substrate (Sunpower Corp.) and others are starting (e.g. Yingli) but there is still a big headstart for p-type technologies, developed in the last decades. Nevertheless the PV-industry expects an increasing share of n-type substrate as you can see in the ITRPV roadmap on p.24
http://www.itrpv.net/Reports/Downloads/
For a detailed overview there should be some publications in the proceedings of various PV-conferences of the last years.
The answer is simple: with N substrate the thin upper side of junction is P where the minority carrier is higher mobility electron. Therefore the series resistance can be lower 2.5 times making a solar cell having smaller internal resistance and higher efficiency. The upper side needs to be thin design due to the short absorption length of bule side light.
You can use P substrate then you need to make grid pattern of electrode to reduce the current path thus the internal series resistance.
In fact the use of p-type substrate has historical reasons: the minority carrier diffusion lenght in n-type substrate is more affected by material damage due to high energy particles (this is indeed closely related to the lower mobility). Since solar cells were made nearly exclusively for satellites, stability against such particles and gamma radiation from natural sources and nuclear bombs, testet in the higher atmosphere, was crucial. For terrestrial applications, n-type substrate offers some advantages over p-type substrate: the minority carrier diffusion length is less affected by various metal impurities, contained in industrial available material. Current Cz material reaches higher diffusion length on n-type than on p-type material. Additionaly, there is nearly no degradation due to boron-oxygen defects, a problem for highly efficient p-type solar cells. Nowadays, one of the best crystalline technology is on n-type substrate (Sunpower Corp.) and others are starting (e.g. Yingli) but there is still a big headstart for p-type technologies, developed in the last decades. Nevertheless the PV-industry expects an increasing share of n-type substrate as you can see in the ITRPV roadmap on p.24
http://www.itrpv.net/Reports/Downloads/
For a detailed overview there should be some publications in the proceedings of various PV-conferences of the last years.
As the purpose of making solar cells was for space application at the beginning, people preferred p-type substrates as they are less sensitive to high energy emissions (e. g. cosmic rays), particles, etc.
But according to the recent International Technology Roadmap for Photovoltaics (ITRPV), it looks like the use of n-type substrates is going to be increased.
In addition we take into account, that the market share for p-Type cells does have a lot to do with the feasibility and cost of mass production:
- p-type cells can work with a phosphorus diffusion (for pn-junction formation ) which is currently still easier to handle in industry than a boron diffusion process. In addition the phosphorus gettering process facilitates the use of less expensive production environment.
- p-type cells currently mostly feature an aluminium-back-surface field which is easily fabricated by using a screen printed low cost aluminium paste. Implementing PERL structures is also possible with this approach.
- The boron-oxygen complex is a severe hurdle for efficiency improvement in p-type, but we also see progress here and new materials as 'quasi mono' which are close to be monocrystalline and feature less Oxygen.
Thus I believe the advantages of n-type are clearly present but p-type is also moving quickly and currently a bit more cost effective. So I guess the biggest challenge will be to develop cost effective fabrication processes to increase the market share of this n-type material. I see exciting approaches here and believe we will see a lot of progress in the next years, it is a good topic to work on.
- Badges
- Science topic