The basic structure of a semiconductor solar cell device is a PN junction diode. When the P-type and N-type semiconductors contact to form a PN junction, there is a huge difference in the carrier concentration at both ends of the PN junction. The neutral property is destroyed, and a space charge zone (depletion zone) is formed at the junction; a built-in electric field is generated, and the minority carriers are affected by the built-in electric field to move, forming a drift current. When the drift current of the carrier reaches equilibrium, the net carrier current is zero and the system returns to the thermal equilibrium state. What happens when a photon with an energy greater than the energy gap is injected from one end of the PN junction structure?
First, if the two ends of the PN junction are connected together, the electron-hole pairs generated by the light in the depletion zone will be affected by the built-in electric field. The electrons will drift to the N-type semiconductor region, and the holes will go to the P-type semiconductor region. Drift, resulting in a drift current flowing from the N-type to the P-type. As for the electron-hole pairs generated by illumination in the N-type and P-type semiconductor regions outside the depletion region, due to the lack of a built-in electric field, and the majority carrier concentration is basically not affected by the effect of light, it is obvious. Change (under the hypothesis of a low injection of the solar spectrum), so only a minority carrier diffusion current will be generated. Taking the P-type semiconductor region as an example, since the electrons in the depletion region near the P-type end region continue to flow to the N-type semiconductor region, the electron concentration at the edge of the depletion region is low, so the P-type
The electrons generated by light in the semiconductor region will diffuse into the depletion region, and then flow into the N-type semiconductor region; that is, the illumination effect will generate minority carrier diffusion currents in the N-type and P-type semiconductor regions outside the depletion region, and the electrons are caused by The P-type semiconductor region flows to the N-type semiconductor region, and the holes flow from the N-type semiconductor region to the P-type semiconductor region. Therefore, the sum of the drift current in the depletion region, the electron diffusion current generated by the P-type semiconductor region, and the hole diffusion current generated by the N-type semiconductor region is the so-called photocurrent, that is, the short-circuit current, which flows to the PN junction. The current of the tube under forward bias is opposite.
When a load resistor is connected at the two ends of the PN junction, the photocurrent generated by the illumination effect flows out of the P pole and flows through the load resistance, resulting in a potential difference between the two ends of the load resistor. The direction of this potential difference is like a forward bias, resulting in a PN junction. The built-up potential in the depletion region decreases, so the majority carrier diffusion current increases, which cancels part of the photocurrent.
If the two ends of the PN junction are open (not connected), it means that when the photocurrent generated by the illumination effect flows to the surface of the two ends of the PN junction, it cannot be discharged, and negative charges (electrons) will accumulate on the end surface of the N-type semiconductor region at the same time. Positive charges (holes) are on the surface of the end of the P-type semiconductor region, causing a parallel plate capacitance effect. When the voltage generated by the accumulated charge suppresses the built-in voltage in the depletion region, the majority of carriers are easily diffused into the depletion region, and the light is minor. The carrier diffusion current and the drift current in the depletion region recombine, and the net current will approach zero. The voltage at this time is the so-called open circuit voltage. The terminal potential of the P-type semiconductor region is higher than the terminal potential of the N-type semiconductor region, which is the so-called forward bias.
PN junction diode
The so-called PN junction is the junction formed by contacting the N-type semiconductor and the P-type semiconductor. The most important characteristic of the PN junction is that it has rectify properties, that is, when a positive bias is applied to the P-type semiconductor terminal ( Called forward bias), current can easily flow from the P-type semiconductor terminal to the N-type semiconductor terminal; on the contrary, if a positive bias is applied to the N-type semiconductor terminal (called reverse bias), the current cannot Flow from the N-type semiconductor terminal to the P-type semiconductor terminal. Figure 1.1 is the current-voltage characteristics of a typical silicon semiconductor PN junction. The abscissa represents the voltage applied to the P-type semiconductor terminal (in V), and the ordinate represents the current that flows from the P-type to the N-type semiconductor terminal (in mA). It can be found from the figure that when the operation is forward biased (the voltage is positive), the current-starts to be almost zero, and as the voltage continues to increase to about 0.TV, the current starts to increase rapidly, that is, the forward conduction starts. . When operating in reverse bias (the voltage is negative), the current is almost zero, and does not change with the increase in voltage, until it reaches a maximum critical voltage (VB), the current suddenly increases rapidly, this phenomenon It is called junction breakdown, and its critical voltage depends on the semiconductor material, doping concentration and the structure of the junction and other parameters, which can range from several volts to several thousand volts.
Figure 1.1 Current and voltage characteristics of a typical silicon semiconductor PN junction
To understand the reasons for the above-mentioned current-voltage characteristics, we must start with the discussion of the combination of two different doping types of semiconductors. Figure 1.2(a) shows uniformly doped and separated P-type and N-type semiconductor materials and their corresponding energy band diagrams. The majority carriers in the P-type semiconductor are holes, and the minority carriers are electrons, and the Fermi level is close to the top of the valence band; on the contrary, the majority carriers in the N-type semiconductor are electrons, and the minority carriers are electrons. It is empty six, and its Fermi level is close to the bottom of the conduction band.
When the P-type and N-type semiconductors are tightly combined together [Figure 1.2(b)], a carrier concentration gradient will immediately form at the junction, causing the majority of the carrier holes at the P-type semiconductor end to diffuse into the N-type semiconductor, and at the same time, The majority carrier electrons of N-ming semiconductors also diffuse into P-type semiconductors. Therefore, the holes in the P-type semiconductor near the junction region either diffuse into the N-type semiconductor, or recombine with the electrons from the N-type semiconductor and disappear, resulting in the negatively charged acceptor impurity ions (N); while the N-type semiconductor is near the junction The electrons in the region either diffuse into the P-type semiconductor, or recombine with the holes from the P-type semiconductor and disappear, leaving positively charged donor impurity ions. Therefore, a negative space charge is formed at the P-type semiconductor terminal near the junction, and a positive space charge is formed at the N-type semiconductor terminal near the junction, and a N-type semiconductor is directed to the P-type semiconductor at the junction. This electric field will drive the minority carrier electrons of the P-type semiconductor terminal to drift to the N-type semiconductor terminal, and at the same time, it will also drive the minority carrier holes of the N-type semiconductor terminal to drift to the P-type semiconductor terminal.
When the PN junction reaches a state of thermal equilibrium, a fixed-width carrier-depleted region is formed at the junction, which is called a depletion region, also called a space charge region. At this time, the diffusion current caused by the concentration gradient and the drift current caused by the built-in electric field of the space charge will completely cancel out [Figure 1.2(c)].
Figure 1.2 (a) Uniformly doped and separated P-type and N-type semiconductors and their corresponding energy band diagrams; (b) When the P-type and N-type semiconductors are connected together, the majority carriers at both ends begin to diffuse to the junction , Recombination occurs; (c) When the thermal equilibrium state is reached, a depletion zone and a built-in electric field will be formed at the junction, and a drift electron hole flow will be generated to counteract the diffusion electron hole flow.