LONGI GREEN ENERGY TECHNOLOGY CO., LTD. As a result of the agreement, any application of LONGi products that relates to Gallium-doped technology will be permitted legally on a global basis. Gallium-doped silicon can effectively solve LID (Light induced degradation) in a P-type PERC module. LONGi already addressed the problem of the high cost of Gallium-doped silicon through its own technological efforts and will therefore now be able to provide this for the whole PV industry. At the same time, LONGi commits to the price of Gallium-doped silicon being the same as that for Boron-doped silicon, which will further help PV become the most cost-effective method of power generation.
2.Literature review of Gallium-doped and Boron-doped silicon
Today's industry-standard Boron-doped monocrystalline silicon still suffers from LID over its lifetime. Industrial Czochralski (Cz) silicon contains significant amounts of interstitial oxygen which, in combination with Boron-doping, can result in LID and, in turn, affect cell efficiency. To our knowledge, the first observation of LID in non-particle-irradiated solar cells fabricated on boron-doped Cz-Si wafers was made by Fischer and Pschunder in 1973 [1]. They recorded a strong degradation of short-circuit current and open-circuit voltage during the first hours of illumination until a stable level was reached. Interestingly, the initial cell performance could be completely recovered by a low temperature anneal at only 200-degreeC. Additional photoconductance decay measurements indicated that the observed effect was due to a bulk carrier lifetime varying between two levels, corresponding to two different states of the material, A and B. State A is associated with a high lifetime and requires low-temperature annealing, while state B is associated with a low carrier lifetime and is caused by illumination. Both levels were found to have a tendency to saturate which can be reversed by applying the appropriate treatment.
Over the following few years, several attempts were made to develop a defect model which explained the mechanism of LID. Some of them proposed the mechanism of metallic impurities, but none of the models were capable of explaining the complete degradation/recovery cycle observed in Boron-doped Cz silicon. It was not until a complete defect reaction model proposed by
Therefore, two straightforward methods to eliminate the LID are either the reduction of interstitial oxygen content or substitution of the boron by a different dopant (e.g., by Gallium). Bianca from ISFH and
Fabricating industrial Boron-doped Cz silicon with low oxygen content (e.g., 2.6 ppm) is very difficult. Technologically, it can be achieved by magnetically-confined Czochralski (MCz) crystal growth. However, due to the application of a strong magnetic field, machine costs are significantly higher when compared to commonly used Cz pullers. Therefore, using Gallium as a dopant becomes a feasible way to solve the LID problem.
Peak efficiency of up to 21% has been achieved on 0.4 cm material. In the relatively broad resistivity range from 0.25 to 1.34 cm, cell efficiency can reach more than 97% of the peak value, suggesting that the resistivity of Gallium-doped silicon wafers should be based on this result in order to achieve maximum cell efficiency.
In recent years, light and elevated temperature induced degradation (LeTID) has been observed in multicrystalline, float-zone and Cz silicon. It involves an initial lifetime degradation, but typically recovers over time, with degradation and recovery rates depending on thermal history.
In the same annealing conditions, the degradation of a Gallium-doped PERC cell is lower than that of a Boron-doped equivalent.
According to a research report by the
The difficulty in application of Gallium-doped silicon wafers resides in the control of resistivity compared with Boron-doped silicon, since there is a significant difference between the segregation coefficient of Boron (0.75) and Gallium (0.008). LONGi's R&D focuses on the characteristics of Gallium-doped silicon, in order to achieve a reasonable resistivity range and higher doping accuracy. The objective is to improve the Gallium-doping process via an innovative model which controls the resistivity range from 0.3 to 1.5 .cm, which is almost the same as that for Boron-doped silicon.
LONGi can also provide corresponding resistivity products according to customer demand, in which it is able to reduce the probability of different brightness in EL images on cells. Moreover, the oxygen and carbon content and carrier lifetime of LONGi's Gallium-doped silicon wafers are basically the same as for Boron-doped products.
Compared with a Boron-doped silicon wafer, Gallium-doped silicon avoids the LID caused by a boron-oxygen defect. As long as the hydrogen content in the cell production process is controlled, LeTID can be inhibited and the total degradation of both cell and module will be very low.
4.The application of Gallium-doped silicon
LONGi's Gallium-doped silicon wafer has been verified by a number of cell manufacturers, its efficiency and anti-PID performance showing an improvement when compared to a Boron-doped cell.
Summary
In summary, Gallium-doped PERC cells have a higher efficiency and better anti-LID and anti-LeTID performance compared with Boron-doped cells. LONGi will promote its Gallium-doped silicon wafers for the whole industry chain, making a significant contribution to a reduction in initial module degradation, lowering equipment costs for cell manufacturers and increasing profits for the whole PV industry.
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