Classification and technical analysis of mainstream Solar cells.

This article collects and organizes the technical routes and data of mainstream solar cells. Not intended as investment advice.

The working principle of photovoltaic crystalline silicon cells

The basis of the working principle of solar cells is the photovoltaic effect of semiconductor PN junctions. The so-called photovoltaic effect is an effect that when the object is illuminated, the charge distribution state in the object changes to generate electromotive force and current. When sunlight or other light irradiates the PN junction of the semiconductor, a voltage will appear on both sides of the PN junction, which is called photovoltage.

When light is irradiated on the PN junction, electron-hole pairs are generated, and the carriers generated near the P-N junction inside the semiconductor are not recombined and reach the space charge region, attracted by the internal electric field, electrons flow into the N region, and the holes As a result, excess electrons are stored in the N area and excess holes in the P area. They form a photogenerated electric field opposite to the direction of the potential barrier near the P-N junction. In addition to partially offsetting the effect of the barrier electric field, the photogenerated electric field also makes the P region positively charged, and the N region is negatively charged, and the thin layer between the N region and the P region generates an electromotive force, which is the photovoltaic effect.

Classification of mainstream crystalline silicon cells

Distinguished from the regularity of the particle arrangement inside the crystal, crystalline silicon batteries are divided into single crystal batteries and polycrystalline batteries. At present, monocrystalline cells have become the absolute mainstream in the industry, roughly divided into monocrystalline P-PERC structures, N-TOPCon structures, N-HJT structures, IBC structures, etc., of which P-PERC accounts for about 85%.

P-PERC crystalline silicon cell

PERC (Passivated Emitter Rear Cell)——emitter and back passivation battery technology, the difference from conventional batteries lies in the back, PERC batteries use a passivation film to passivate the back, replacing the traditional all-aluminum back field, enhancing the light The internal back reflection of the silicon base reduces the recombination rate on the back, thereby increasing the efficiency of the cell by 0.5%-1%. At present, P-type monocrystalline cells have adopted PERC technology, with a thickness of about 180um. Generally, the front side is the negative electrode and the back side is the positive electrode. Taking the 22.8% efficiency battery as an example, the power of each 182 battery is about 7.53W, the open circuit voltage is about 687mV, the short circuit current is about 13.39A, the working voltage is 597mV, and the working current is 12.61A.

N-TOPCon crystalline silicon battery

TOPCon (Tunnel Oxide Passivated Contact). There is no essential difference between the front side and conventional N-type solar cells or N-PERT solar cells, and the core technology of the battery is the passivation contact on the back side. The back of the battery is composed of a layer of ultra-thin silicon oxide (1~2nm) and a layer of phosphorus-doped microcrystalline amorphous mixed Si film, which together form a passivation contact structure. The passivation performance is activated by the annealing process, and the crystallinity of the Si film changes during the annealing process, from microcrystalline amorphous mixed phase to polycrystalline. Annealed at an annealing temperature of 850°C, iVoc>710mV, J0 at 9-13fA/cm2, showing excellent passivation performance of passivated contact structures. This structure can prevent the recombination of minority carriers and holes, and increase the open circuit voltage and short circuit current of the battery. The ultra-thin oxide layer can allow many electrons to tunnel into the polysilicon layer while blocking the recombination of minority electrons and holes. The good passivation effect of ultra-thin silicon oxide and heavily doped silicon film makes the surface energy band of the silicon wafer bend, thus forming a field passivation As a result, the probability of electron tunneling is greatly increased, the contact resistance is reduced, and the open circuit voltage and short circuit current of the battery are increased, thereby improving the conversion efficiency of the battery. Currently, there are two technological routes of LPCVD and PEALD.

N-HJT

HJT (Heterojunction with Intrinsic Thin-film) - Intrinsic thin film heterojunction battery. It has a symmetrical double-sided battery structure with N-type crystalline silicon in the middle. The intrinsic amorphous silicon film and the P-type amorphous silicon film are deposited sequentially on the front side to form a P-N junction. On the back side, an intrinsic amorphous silicon film and an N-type amorphous silicon film are deposited sequentially to form a back surface field. In view of the relatively poor conductivity of amorphous silicon, transparent conductive films (TCO) are deposited on both sides of the battery for conduction, and finally screen printing technology is used to form double-sided electrodes. It is mainly due to the double passivation effect of the N-type silicon substrate and the amorphous silicon on the surface defects of the substrate. At present, the mass production efficiency is generally above 24%; the technical route of more than 25% has been very clear, that is, the use of doped nanocrystalline silicon, doped microcrystalline silicon, doped microcrystalline silicon oxide, and doped microcrystalline carbonization on the front and rear surfaces Silicon replaces the existing doping; the conversion efficiency of HJT superimposed IBC and perovskite in the future may increase to more than 30%. Since the HJT battery substrate is usually N-type single crystal silicon, and N-type single crystal silicon is doped with phosphorus, there is no boron-oxygen recombination, boron-iron recombination, etc. in P-type crystalline silicon, so HJT batteries are immune to the LID effect . The surface of the HJT battery is deposited with a TCO film without an insulating layer, so there is no chance of charging the surface layer, and structurally avoids the occurrence of PID. The attenuation of HJT battery is 1-2% in the first year, and attenuation is 0.25% per year thereafter, which is far lower than the attenuation of gallium-doped PERC battery (attenuation of 2% in the first year, attenuation of 0.45% per year thereafter), so the whole life cycle of HJT battery per W The power generation is about 1.9%-2.9% higher than that of double-sided PERC cells.

N-IBC crystalline silicon battery

IBC (Interdigitated Back Contact) - interdigitated back contact battery technology. Make the contact electrodes of P/N junction, substrate and emitter region on the back of the battery in the shape of interdigitated fingers. Core technology: how to prepare p-regions and n-regions with good quality and arranged in interdigitated intervals on the back of the battery. By printing a boron-containing interdigitated diffusion mask layer on the back of the battery, the boron on the mask layer enters the N-type substrate to form a p+ region after diffusion, and the area where the mask layer is not printed is formed after phosphorus diffusion n+ area. Pyramid-shaped suede is prepared on the front surface to enhance light absorption, and at the same time, a front surface field (FSF) is formed on the front surface. The use of ion implantation technology can obtain p-region and n-region with good uniformity and precise controllable junction depth. There is no grid line shielding on the front of the cell, which can eliminate the shading current loss of the metal electrode and realize the maximum utilization of incident photons. Compared with conventional solar cells The short-circuit current can be increased by about 7%; due to the back contact structure, there is no need to consider the shielding of the grid line, and the proportion of the grid line can be appropriately widened, thereby reducing the series resistance and having a high fill factor; the surface passivation and surface light trapping structure can be carried out The optimized design can obtain lower front surface recombination rate and surface reflection, thereby improving Voc and Jsc; the appearance is beautiful, especially suitable for photovoltaic building integration; but the cost of IBC batteries is high and has not been industrialized, and the process of IBC batteries is complicated , using semiconductor technologies such as masks and photolithography many times, the cost is almost twice that of conventional batteries.

P-IBC crystalline silicon battery

As early as 16-17, TNO promoted the P-type IBC structure. P-IBC adds LPCVD and others are compatible with PERC. The laser is a little different, 90% compatible. P-IBC technology is based on P-type silicon wafers. The efficiency of the front side has an advantage. At present, it is still biased towards single-sided, and the double-sided rate is less than 50%. It is positioned as a distributed product. The opportunity cost of P-IBC is close to that of PERC, and the efficiency is 24.5%-25%, achieving a cost gap of 0.01-0.03 RMB/W.

Collected/written on 2023/1/10. Author: Lambert Liu.

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