A novel method for crystalline silicon solar cells with low contact resistance and antireflection coating by an oxidized Mg layer
© Lee et al; licensee Springer. 2012
Received: 10 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
One of the key issues in the solar industry is lowering dopant concentration of emitter for high-efficiency crystalline solar cells. However, it is well known that a low surface concentration of dopants results in poor contact formation between the front Ag electrode and the n-layer of Si. In this paper, an evaporated Mg layer is used to reduce series resistance of c-Si solar cells. A layer of Mg metal is deposited on a lightly doped n-type Si emitter by evaporation. Ag electrode is screen printed to collect the generated electrons. Small work function difference between Mg and n-type silicon reduces the contact resistance. During a co-firing process, Mg is oxidized, and the oxidized layer serves as an antireflection layer. The measurement of an Ag/Mg/n-Si solar cell shows that Voc, Jsc, FF, and efficiency are 602 mV, 36.9 mA/cm2, 80.1%, and 17.75%, respectively. It can be applied to the manufacturing of low-cost, simple, and high-efficiency solar cells.
The main objective of the current single crystalline silicon [c-Si] photovoltaic research is to enhance the efficiency of the solar cell without a too complicated process. One of the ways to increase the efficiency is to lower the front surface recombination using a low surface doping concentration. C-Si solar cells with a lowly doped emitter have high short-circuit current, and the absorption in the blue response region is better. It has been reported that the recombination velocity of the emitter layer with a sheet resistance of 100 Ω/sq is 60,000 cm/s while that of the emitter with a 45-Ω/sq sheet resistance is 180,000 cm/s .
The quality of metal contact can also affect the solar cell efficiency. For commercial c-Si solar cells, the front contact metallization is fabricated by the screen printing of Ag paste in grid patterns [2, 3]. The screen printed Ag electrodes may reduce the efficiency when operated at concentration levels of more than 1 sun due to high-series resistance. A small front metal contact area [4, 5] and a low metal density in Ag-screen printed lines contribute to the high resistance . The presence of a glass frit mostly contributed by SiO2 in the Ag paste also results in high metal-semiconductor contact resistivity . Increased resistive power loss occurs under light and eventually degrades the solar cell efficiency . To reduce the contact resistance, a selective emitter method is introduced recently. It consists of a lightly doped emitter to enhance the blue response of solar cells and a heavily doped emitter underneath the contact to reduce the contact resistance. Although the performance of the selective emitter solar cell is fairly good, its fabrication process is very complicated, making it difficult for mass production . To have similar effects without using the selective emitter method, a barrier material can be introduced between the Ag and Si since it is difficult to get ideal ohmic contacts with an n-type silicon substrate doped with phosphorus. The contact resistance is decided by the doping density of impurity and the work function difference between Si and the metal. To achieve decreased contact resistance, it is important to have a low work function difference between Si and the metal. One of the good candidates for the barrier is Mg . In this paper, Mg is inserted between Ag and Si. The effects of Mg on the contact resistance and the reflectance are investigated.
Results and discussion
The cross section through the contact shows that the screen printed Ag electrode is in contact with the pure Mg metal layer on Si, resulting in low contact resistance. The rest of the Mg part undergoes an oxidation process becoming MgO, acting as an ARC layer. The passivation capabilities of MgO layers formed under various experimental conditions are investigated.
Series resistance, shunt resistance, first ideality factor, and second ideality factor of processed solar cells
Rs (Ω cm)
Sun-Voc parameters of the processed solar cells with different Mg thicknesses without an oxidation step
Generally, the quantum efficiency is reduced by recombination. The same mechanisms which affect the collection probability also affect the quantum efficiency. Since blue light is absorbed very close to the surface, high front surface recombination will affect the blue portion of the quantum efficiency. Thus, a good front surface passivation is important. Green light is absorbed in the bulk of a solar cell, and a low diffusion length reduces the quantum efficiency in the green portion of the spectrum. The quantum efficiency can be viewed as the collection probability due to the generation profile of a single wavelength, integrated over the device thickness and normalized to the incident number of photons. From the IQE graph, not presented here, from a wavelength of λ = 400 to 1,100 nm, the best blue response is seen for the cell with a 200-Å-thick Mg layer, suggesting that it has the most decreased recombination velocity at the surface.
In conventional solar cells, a screen printed Ag paste is often used for front metallization materials. It induces the increase of contact resistance due to high barrier height between Ag and n-type Si. To solve this problem, Mg metal is evaporated before the formation of Ag electrode. After the Ag electrode is screen printed, a firing step is taken. As a result, a low contact resistance is obtained at the Ag/Mg/Si electrode. Since the exposed Mg metal goes through oxidation during firing, it becomes MgO serving as an ARC layer. Since the whole emitter is lightly doped (100 Ω/sq), the surface recombination velocity is reduced by the simple and low-cost fabrication process.
Optimum thickness of Mg has been investigated. It is found that the lifetime is 32.27 μs and the reflectance is 8.75% when the Mg thickness is 200 Å. This condition is applied to a 100-Ω/sq c-Si solar cell process to have low contact resistance with good ARC. The measurement of an Ag/Mg/n-Si solar cell shows that Voc, Jsc, FF, and efficiency are 602 mV, 36.9 mA/cm2, 80.1%, and 17.75%, respectively.
This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0018397). This work was supported financially by the National Research Laboratory (NRL-ROA-2007-000-1002).
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