1. Photovoltaic effect:
The basis of solar cell energy conversion is the photovoltaic effect of the semiconductor PN junction. When light is incident on the semiconductor photovoltaic device, photons having energy greater than the silicon forbidden band width pass through the anti-reflection film into the silicon, and photogenerated electron-hole pairs are excited in the N region, the depletion region, and the P region.
Depletion zone: Photogenerated electrons - The hole pairs are immediately separated by the built-in electric field after they are generated in the depletion region, the photogenerated electrons are sent to the N region, and the photogenerated holes are pushed into the P region. According to the depletion approximation condition, the carrier concentration at the boundary of the depletion region is approximately 0, that is, p = n = 0.
In the N region: photogenerated electrons - after the hole pair is generated, the photogenerated hole diffuses to the boundary of the pn junction. Once it reaches the boundary of the PN junction, it is immediately subjected to the built-in electric field, and is dragged by the electric field force to make it drift. The area enters the P area, and the photogenerated electrons (multiple sub-) are left in the N area.
In the P region: the photogenerated electrons (small children) first enter the N region because of diffusion, and then drift due to drift, and photogenerated holes (multiple sub-) remain in the P region. Thus, an accumulation of positive and negative charges is formed on both sides of the p-n junction, so that the N region stores excess electrons, and the P region has excess holes. Thereby, a photo-generated electric field having a built-in electric field in the opposite direction is formed.
1. In addition to partially canceling the effect of the barrier electric field, the photogenerated electric field also makes the P region positively charged, the N region negatively charged, and the thin layer between the N region and the P region generates an electromotive force, which is the photovoltaic effect. When the battery is connected to a load, the photocurrent flows from the P-zone load to the N-zone, and the power output is obtained in the load.
2. If the P-N junction is open at both ends, the electromotive force can be measured, which is called the open circuit voltage Uoc. The typical open circuit voltage for crystalline silicon cells is 0.5 to 0.6V.
3. If the external circuit is short-circuited, a photocurrent proportional to the incident light energy flows through the external circuit. This current is called the path current Isc.
Factors affecting photocurrent:
1. The more electron-hole pairs generated by the light in the interface layer, the greater the current.
2. The more light energy absorbed by the interface layer, the larger the interface area, ie, the larger the battery area, the greater the current formed in the solar cell.
3. The N zone, the depletion zone and the P zone of the solar cell can generate photogenerated carriers;
4. The photo-generated carriers in each zone must pass through the depletion zone before recombination to contribute to the photocurrent. Therefore, the actual photo-generated current must take into account various factors such as generation and recombination, diffusion and drift in each zone.