Analysis of factors affecting the conversion efficiency of Silicon solar cells

The mechanism of conversion efficiency loss of crystalline silicon solar cells

The conversion efficiency of solar cells is limited by light absorption, carrier transport, and carrier collection. For monocrystalline Silicon solar cells, the theoretical maximum conversion efficiency is 28 % because the excess energy of the upper band gap is transmitted to the lower band gap photon. Only by minimizing losses can solar cells be developed that are efficient enough.

The reasons that affect the conversion efficiency of crystalline silicon solar cells mainly come from two aspects:

(1) Optical loss, including loss of reflection on the front surface of the battery, loss of shadow on the contact grid and loss of non-absorption in the long band.

(2) Electrical loss, which includes loss of photocurrent composites on semiconductor surfaces and in vivo, contact resistance of semiconductors and metal grids, and contact resistance of metals and semiconductors. The most important of these is to reduce the composite of photocarriers, which directly affects the open-circuit voltage of solar cells. The composite of photocarriers is mainly due to the introduction of a large number of composite centers on the front surface of a high concentration diffusion layer. In addition, when the diffusion length of the minority carrier is equal to or more than the thickness of the Silicon wafer, the effect of the composite speed of the back surface on the characteristics of the solar cell is also obvious.

Method to improve the conversion efficiency of crystalline silicon solar cells

(1) Light trap structure. In general, high-efficiency monocrystalline Silicon cells use chemical corrosion to make velvet technology, and the reflectivity of the velvet surface can reach less than 10 %. At present, the more advanced cashmere production technology is reactive plasma etching technology(RIE). The advantage of this technology is that it has nothing to do with the crystal direction of crystalline silicon. It is suitable for thinner silicon wafers and usually uses SF6/O2 mixed gas during etching. The F free radicals chemically etch Silicon to form volatile SiF4, and the O free radicals form SixOyFz to passivation the side walls to form a velvet structure. At present, the equipment used by the Korean company Zhouxing can produce a velvet reflectivity of less than 2 % to 20 %.

(2) Reduced reflector film. Its basic principle is a membrane located on the surface of the medium and the battery with a certain refractive index, which can completely offset the interference between the various levels of reflection generated by the incident light. Monocrystalline Silicon cells can generally use TiO2, SiO2, SnO2, ZnS, MgF2 monolayer or double-layer reduced reflector membranes. The reflectivity can be reduced to about 2 % after steaming off the reflective film on the surface of the velvet surface.

(3) Passivation layer: The passivation process can effectively reduce the composition of photocarriers in certain areas. In general, high-efficiency solar batteries can be passivated by thermal oxygen passivation, atomic hydrogen passivation, or the surface diffusion of phosphorus, boron, and aluminum. Thermooxygen passivation is the formation of Silicon oxide films on the front and back of the battery, which can effectively prevent the carrier from Compounding on the surface. Atomic hydrogen passivation is due to the large number of suspension bonds on the surface of Silicon. These suspension bonds are the effective composite centers of carriers, and atomic hydrogen can neutralize suspension bonds, thus weakening the composite.

(4) Added back field: For example, in a P-type battery, add a P + dense doping layer on the back to form a P + / P structure, At the interface of P + / P, a built-in electric field pointing from P region to P + is generated. Due to the accumulation of photocarriers separated from the built-in power site, a photogenic voltage with a positive P + end and a negative P end is formed. This photogenic voltage is the same as the photogenic voltage at both ends of the PN junction of the battery structure itself. The open circuit voltage Voc was increased. At the same time, due to the existence of the back electric field, the photocurrent carrier is accelerated. This can also be seen as increasing the effective diffusion length of the carrier, thus increasing the collection probability of this part of the minority, and the short-circuit current Jsc is also improved.

(5) Improvement of substrate materials: The selection of high-quality Silicon materials, such as N-type Silicon with long carrier life, small borooxygen-reaction after knot preparation, good conductivity, low saturation current.

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