The latest progress and development trend of high efficiency battery with efficiency over 25%introduction
In recent years, energy crisis and environmental pressure promote the rapid development of solar cell research and industry. At present, crystalline silicon solar cells are the most mature and widely used solar cells, accounting for more than 90% of the photovoltaic market, and will occupy a dominant position for a long time in the future. Among them, the crystal structure of monocrystal silicon is perfect, the band gap width is only 1.12eV, and the raw materials in nature are rich, especially the n-type monocrystal silicon has the advantages of less impurities, high purity, high minority carrier life, no grain boundary dislocation defects and easy control of resistivity, which is the ideal material for realizing high efficiency solar cells.
How to improve the conversion efficiency is the core of solar cell research. In 1954, Bell laboratory in the United States first prepared monocrystalline silicon solar cells with an efficiency of 6%. Since then, research institutions around the world have begun to explore new materials, technologies and device structures. In 1999, the University of New South Wales in Australia announced that the conversion efficiency of monocrystalline silicon solar cells reached 24.7% and 25% after the solar spectrum was revised in 2009, becoming a milestone in the research of monocrystalline silicon solar cells. The 25% conversion efficiency record achieved by the University of New South Wales has been maintained for 15 years. Until 2014, Panasonic in Japan and SunPower in the United States reported 25.6% and 25.2% efficiency successively. Since then, Japanese Kaneka company, German Fraunhofer research center and German Hamelin solar energy research institute have reported monocrystalline silicon solar cells with an efficiency of more than 25%, and the specific parameters are shown in Table 1.
Theoretical efficiency of single crystal silicon solar cell
For homojunction monocrystalline silicon solar cells, in 2004, Shockley and queisser theoretically calculated the ultimate efficiency of monocrystalline silicon solar cells up to 33%, also known as Shockley queisser (SQ) efficiency, but this efficiency only considers radiation recombination, ignoring non radiation recombination and intrinsic absorption loss (such as Auger recombination and parasitic absorption). In 2013, Richter et al. Proposed a novel and accurate method to calculate the ultimate efficiency of monocrystalline silicon solar cells, taking into account the solar spectrum, optical properties of silicon wafer, free carrier absorption parameters and the influence of carrier recombination and band gap narrowing of the new standard. When the thickness of silicon wafer is 110 μ m, the theoretical efficiency of monocrystalline silicon solar cells is 29.43%. The simulation results of silicon heterojunction (SHJ) solar cells show that the best back field structure can improve both VOC and JSC, as well as the significance of silicon thickness to the performance of the cells. The theoretical limit efficiency of SHJ cells with symmetrical structure is 27.02%. In 2013, Wen et al. Analyzed that the interface defects, band gap compensation and work function of transparent conductive oxide (TCO) all affect the interface transmission performance of a-Si: H (P) / n-czsi, and thus simulated the theoretical limit efficiency of 27.37%. In 2015, Liu Jian and others further proposed that appropriate thickness, doping concentration and back field structure of a-Si: H could improve the carrier transfer performance of a-Si: H / c-Si heterojunction solar cells, and the theoretical limit efficiency was 27.07%. All of the above studies show that the best back field can improve the transport of carriers and reduce the loss of carriers in the PN junction. It is also pointed out that carrier migration performance is an important condition for improving the conversion efficiency of SHJ batteries.
For a new type of undoped silicon heterojunction battery, in 2014, Islam and others used metal oxide as a new carrier selective passivation contact layer, which reduced the carrier loss in the "PN junction", and improved the voltage drop loss in contact with the metal. The ultimate efficiency of the simulation reached 27.98%. Table 2 summarizes the theoretical limiting efficiency of monocrystalline silicon solar cells under ideal conditions.
Analysis on the structure and characteristics of 2 high efficiency monocrystalline silicon solar cell
Martin Green analyzed the reasons for the loss of battery efficiency, including five possible ways as shown in Figure 1: (1) photons with energy less than the band gap width of the absorption layer of the battery can not excite and produce electron hole pairs, which will directly penetrate out. (2) Photons with energy larger than the band gap width of the absorption layer of the battery are absorbed. The generated electron hole pairs are excited to the high-energy states of the conduction band and the valence band respectively. The excess energy is released in the form of phonons. The electron hole of the high-energy state falls back to the bottom of the conduction band and the top of the valence band, resulting in energy loss. (3) Loss in PN junction due to charge separation and transport of photocarriers. (4) The contact between semiconductor material and metal electrode causes voltage drop loss. (5) Composite loss caused by material defects in the transport process of photocarriers.
The ways of energy loss can be summarized as optical loss (including (1), (2) and (3)) and electrical loss (including (3), (4) and (5)). In order to improve the efficiency of solar cells, it is necessary to reduce both optical loss and electrical loss. The effective measures to reduce the optical loss include the low refractive index antireflection film on the front surface, the suede structure on the front surface, the high reflection on the back and other light trapping structures and technologies, while the full back contact technology without metal electrode shielding on the front surface can maximize the utilization of incident light. To reduce the electrical loss, we need to improve the quality of silicon wafer, improve the formation technology of PN junction (such as ion implantation, etc.), new passivation materials and technologies (such as TOPCON, polo, etc.), metal contact technology, etc. In order to reduce the optical loss and electrical loss, a variety of monocrystalline silicon solar cells are proposed. At present, there are six kinds of monocrystalline silicon solar cells whose conversion efficiency is more than 25%.
2.1 passive emitter back field contact (PERC) battery family
A team led by Martin Green of the University of New South Wales (UNSW) proposed the perc structure of monocrystalline silicon solar cells, which achieved a high conversion efficiency of 22.8% on the p-type FZ Silicon chip, and its basic structure is shown in Figure 2A. In 1999, UNSW's team again announced that its Perl