N type solar cells, the way of the future

Quality N-Type Solar Panels

 Solar panels – N Type Silicon

A lot of research is being conducted on n-type silicon-based photovoltaic technologies. The results look promising and it should take a large slice of the market pie in the years to come.

More and more companies are considering n-type solar cells because of the higher efficiency potential of these concepts. Most recently Trina Solar has applied allot of their research and development budget in this technology. Hopefully making commercial solar power even more affordable into the future

N Type Silicon has increased cell efficiency of Solar Panels

The aim is to increase the efficiency of the cells as in the case of the Panda. Yingli has had the Panda product on the market for over 18 months now and has a proven history in the field.We at Velocity Solar have personally installed many Yingli Panda systems over the last 2 Years with greater yields.

Why research N-Type Silicon wafers

The main objective of technology development in the photovoltaics sector is to reduce costs in order to achieve grid parity. Reaching grid parity is becoming more urgent as the incentives in various leading PV markets start getting greatly reduced with the target of being completely eliminated within three to four years. Provided that the additional production costs related to more complex cell and module processes can be limited, targeting high solar cell efficiencies will be a very effective way to strongly reduce the cost of PV modules in terms of dollars per kilowatt peak. Savings that are made in the cost of balance of systems lead to reduced electricity generation costs ($/kWh).

In 2011, around 84 percent of PV module production was based on p-type crystalline silicon (Si) technology. N-type monocrystalline Si had a market share of around four percent. The remaining 12 percent was occupied by thin film PV (CdTe, a-Si, etc.). The p-type versus n-type Si technology scenario has historical reasons. The very first solar cell – fabricated in 1954 in the Bell-Labs – was made of a monocrystalline n-type Si wafer.

Until the 1980s, the main industrial application of PV was for space applications in the form of satellites and so on. P-type Si proved to be less sensitive to degradation caused by exposure to cosmic rays (high-energy particles such as protons and electrons). Thus for decades, all industrial PV cell development was based on p-type silicon.

Consequently, on an industrial level the key processes such as emitter diffusion and metallization were available only for p-type Si wafer substrates. In the last decade, a lot of research has been done in the field of n-type Si-based PV. The results have proven its potential to outperform compared with the standard p-type Si PV in terms of efficiency. As a consequence, there is a growing interest in the development and the industrial implementation of n-type Si based cell and module technologies. According to the latest edition of the International Technology Roadmap for Photovoltaics (ITRPV 03/2012), its share could reach around 30 percent of the monocrystalline silicon solar module market by 2015.

 

Panda Efficiency progress
Panda Efficiency progress

N-type silicon material

The starting material (Si feedstock) for producing n-type silicon crystals is the same type of polysilicon as that used for p-type Si crystals (based on the Siemens process). The difference is in the doping process during crystallization: while for p-type Si usually boron is used as a dopant, for n-type Si crystals usually phosphorus is added to the Si melt.

In the past 10 years, a lot of scientific research has been carried out on n-type (mainly phosphorus-doped) Si material and related cell processes. This confirmed that compared to standard p-type (boron-doped) Si solar cells, n-type silicon cells feature two important advantages. First, they do not suffer from light induced degradation (LID) caused by the simultaneous presence of boron and oxygen in the wafers, a phenomenon that in standard p-type silicon solar cells leads to a reduction of the module power output by usually two to three percent within the first weeks of installation. Second, n-type Si wafers are less sensitive to impurities that are usually present in silicon feedstock; consequently, less efforts have to be made to obtain n-type Si wafers with a high electronic quality. Accordingly, n-type wafers featuring high solar cell efficiency potential can be produced more cost effectively than high quality p-type wafers. This is in spite of the fact that n-type multicrystalline Si has not yet been studied exhaustively.

Greater Efficiency gained from N Type solar panels

p-type solar cell with a homogeneous emitter and aluminum back surface field (Al-BSF) has an efficiency limit of about 19 percent with the current passivation and metallization concepts. If selective emitters are applied, 19.5 percent is feasible and if additional changes are made on the rear side (so called passivated emitter and rear cell, or PERC concept), 20 percent can be reached, as centrotherm has shown in their Centaurus concept. Using open metal rear side (H-pattern) cells with passivated regions between fingers, such as in the Panda n-type approach, rear side reflection and the passivation ability increase the potential of such cells to 21 percent. Additionally, such cells can be used for bifacial installations, which can boost the energy yield (kWh/kWpeak) by up to 25 percent. The cells with the highest efficiency potential are the Heterojunction with Intrinsic Thin layer (HIT) and Interdigitated Back Contact (IBC) concepts reaching efficiencies above 23 percent. However, these cell concepts from Sanyo and SunPower are of a very high complexity resulting in high costs per watt peak.

Most of the solar cells produced currently are p-type based, processed with a standard selective emitter and with the tendency to move towards the PERC concept. However, more and more companies are considering n-type solar cells because of the higher efficiency potential of these concepts. The following n-type solar cell concepts can be found on the PV market currently:

Additionally, many PV companies’ R&D departments are working on all the aforementioned concepts, with the idea of bringing these technologies into pilot production soon. Examples are:

  • BOSCH, Trina and others
  • Sunways and Suniva (Phostop) and others
  • Silfab (ZEBRA), Siliken, Trina

Of course all cell concepts have their own advantages and challenges, not only with regards to the cell but also at the substrate and module levels. However, this article will focus on the cell level issues only.

For an indepth look at the above mentioned information please follow link below

N-Type Silicon Technology