HIT cell profile
HIT is the abbreviation of Heterojunction with Intrinsic Thin-layer, meaning intrinsic film heterojunction. This type of solar cell was first developed by Japan Sanyo company in 1990, and the conversion efficiency can reach 14.5% (4mm2 cell). Later in the continuous improvement of Sanyo, HIT cell conversion efficiency can reach 24.7% in 2014 and 25.6% in 2015. The following table shows the development process of Sanyo HIT cells.
In 2008, the company started film development, and 97 microns n-type crystalline silicon is adopted as substrate in the subsequent development and production; cell capacity reached 600 mw in 2010. In 2015, the Sanyo HIT patent protection came to the end and the technical barriers are eliminated. It is a good time for China to develop and promote HIT technology.
the structure and principle of the HIT cell
Sanyo HIT cell structure which was successfully prepared at first is as shown in Figure 1. After nearly 25 years of research and development, the current cell structure is developed as shown in Figure 2. To reduce the surface of the optical loss and further improve the conversion efficiency, Sanyo combined HIT and IBC cell technology to develop HJBC cell that keeps the current world record of 25.6%. The structure of the cell is shown in Figure 3.
The key of HIT cell is the structure, that is, inserting i-a-Si between a-Si / c-Si. i-a-Si / c-Si interface state is lower than the a-Si / c-Si, and this structure can significantly reduce surface recombination of c-Si to obtain a high open-circuit voltage. In 2013, the open circuit of voltage with cell efficiency of 24.7% can reach up to 750 mV.
Characteristics and advantages of the HIT cell
(1) No PID phenomenon
The upper surface of the cell is TCO and the charge won't happen polarization phenomenon on TCO of the surface of the cell, so there is no PID phenomenon. Moreover, measured data also confirms this.
（2）Low processing temperature
In all working process, HIT cell processing temperatures are lower than 250 ℃, so as to avoid the low productivity and high cost of high-temperature diffusion junction. and the low temperature process will make the optical band gap, deposition rate, absorption coefficient and hydrogen content of a-Si thin film get more precise control, also avoid adverse effects due to thermal stress caused by high temperatures.
HIT cell always refreshes the world record for cell conversion efficiency of the mass production. HIT cell efficiency is 1-2% higher than P type monocrystalline silicon cells, and the difference is increasing slowly with time.
（4）High light stability
The Staebler-Wronski effect which occurs commonly in amorphous silicon solar cell won't appear in HIT solar cells. While N-type silicon is used by HIT cells, and phosphorus is used as dopant, so there is almost no light attenuation phenomenon.
（5）Development towards thinness
HIT cell process temperature is low, the upper and lower surfaces of the structure are assembled, and there is no mechanical stress so thin-type can be smoothly achieved. Through research, for the N-type silicon substrate with higher minority carrier lifetime (SRV <100cm / s) ,the thinner the film is, the higher open circuit voltage will be. As shown in Figure 4.
（6）Low temperature conversion efficiency coefficient
The typical temperature of HIT cell - conversion efficiency factor is -0.29% which is far lower than conventional crystalline silicon cells of -0.45%, thanks to the typical cell structure and high open-circuit voltage. In the application of power station, low temperature coefficient can get a higher generation benefits.
Current situation and future development prospects of HIT cell
Panasonic HIT cell keeps the current world record of 25.6% for conversion efficiency, the efficiency of its cell components has reached 22.5%. Panasonic is currently the mainstream of HIT cell and components supplier on the market.
At present, other HIT vendors with more mature technology are mainly included: KANEKA (thanks to years of hard work in the field of amorphous silicon thin film and technology accumulation, the conversion efficiency of 25.1%), SUNPREME (using Tandem tandem structure and optimization anti-reflective material such as ITO, the efficiency in printing silver wire and copper respectively are 22% and 22.5%), Roth & Rau (magnetron sputtering, total aluminum back electrode, copper plating grid line, twice printing, high aspect ratio, the conversion efficiency up to 22.8%).
These technology providers developed a series of new technology from improving efficiency and reducing cost. The large-scale industrialization of HIT efficient cell now is in advance step by step. According to another report, Shanxi Jin Jinneng Group established the resolution to set up HIT cell line at the end of last year.While other domestic enterprises have already started HIT cell research and development.
In the near future, HIT cell technology, because of its high efficiency, high reliability and relatively simple low-temperature process technology and other features, is expected to become the mainstream of the market.