①The basic principle of photoelectric conversion
(1) The photo-electric conversion phenomenon of semiconductors. The energy absorption coefficient of semiconductors is relatively large, generally above 105cm-1, which can fully absorb light energy and enable electrons with lower energy in the material to jump to higher energy levels. If the leap forward only occurs in the valence band or conduction band, no excess carriers are generated, but energy is exchanged with the crystal lattice, and finally light energy is converted into heat energy. If the absorbed light energy is greater than the forbidden band width of the semiconductor, electrons may jump from the valence band to the conduction band, thereby generating electron-hole pairs and generating unbalanced carriers. The greater the amount of unbalanced carriers, the greater the conductivity The larger, this is the photo-electric conversion phenomenon of semiconductors, see Figure 1.
(2) The photovoltaic effect of PN junction. Semiconductors have P type (holes) and N (electron) types, and the combination of the two forms a PN junction. When the light shines on the semiconductor PN junction and the light energy is greater than the forbidden band width of the semiconductor, electron-hole pairs are generated near the PN junction. Due to the existence of the built-in electric field, a photogenerated potential is generated. If it is connected to an external circuit, a current will appear. As shown in Figure 1(a), it is called the photovoltaic effect, which is the basis for the development of solar photovoltaic cells. The current I1 generated by the photovoltaic effect is
I1=qAG(Ln＋W＋Lp)
In the formula, q——the amount of electron charge;
G-PN junction electron-hole pair generation rate;
A——The area of PN junction;
W-the width of the space charge region;
Ln, Lp-the diffusion length of electrons and holes.
(3) Photovoltaic characteristics of PN junction. The photovoltaic characteristics of the PN junction are shown in Figure 1(c), and the expression is
Uoc=(kT/q)ln[(I1/I0)＋1]
In the formula, Uoc——the voltage when the load current is zero, that is, the open circuit voltage;
I1=lsc——The current when U is zero, the short-circuit current, and the short-circuit current of the PN junction when illuminated is equal to its photocurrent;
I0——Reverse saturation current, which rapidly increases to saturation with the increase of light intensity, while the increase of short-circuit current is proportional to the light intensity.
Important parameters in photovoltaic characteristics are short-circuit current Isc and open-circuit voltage Uoc. The influence of light intensity on photovoltaic characteristics is shown in Figure 1(b). It can be seen from the figure that Uoc increases exponentially with the light intensity until saturation, and the increase of the short-circuit current is proportional to the light intensity.
(4) Solar cell structure and equivalent circuit. The solar cell structure is shown in Figure 2, (a) is the battery structure principle, (b) is the ideal equivalent circuit diagram, (c) is the actual equivalent circuit diagram, and (d) is the U-I characteristic curve. The solar cell consists of a P-doped 0.25μm thick N-type Si layer as the emitter, and a B-doped 200μm thick host material P-type Si to form a PN junction, forming a pyramidal texture on the N-type Si layer to reduce Reflection increases the absorption rate of sunlight.
②Photoelectric conversion process and efficiency
(1) Photoelectric conversion process. The photoelectric conversion process is shown in Figure 2(a). Under light, the electrons in the negatively charged N zone move to the positively charged P zone, and the holes in the P zone move to the negatively charged N zone to form a built-in electric field. Zone and P zone produce measurable voltage (usually 0.5~0.6V) and current. Photoelectric conversion is effective near the interface. The photoelectric conversion rate depends on the ratio of light absorbed in the interface layer and transmitted to the crystal. Therefore, the thinner the incident surface, the more electron-hole pairs are generated through the PN junction interface when illuminated. The greater the current. If the electrodes are drawn on both sides, current will flow when the load is connected.
(2) Analysis of ideal equivalent circuit. The ideal equivalent circuit is shown in Figure 2(b). The constant current generated by the incident light, the diode saturation current and the load resistance form a loop. The ideal characteristics of this device are
I=Is(eqU/kT﹣1)﹣IL
Where IL——the intensity of a constant current source generated by incident light (from excess carriers excited by solar radiation);
Is——Diode saturation current;
g——electronic quantity;
k-Boltzmann’s constant;
T-absolute temperature.
(3) Actual equivalent circuit. The actual equivalent circuit is shown in Figure 2(c). There is also a parallel (leakage) resistance RF and series resistance Rs in the actual equivalent circuit. The characteristic curve of the U-I relationship is shown in Figure 2(d).
(4) Maximum output power: the area of the dotted square in the characteristic curve of Figure 2(d) is the maximum output power of the solar cell, and its expression is
Pm=ImUm≈IL[Uoc﹣(KT/q)ln(1＋qUm/KT)﹣KT/q]
(3) Conversion efficiency. The conversion efficiency is the ratio of the area ImUm of the diagonal square in the figure to the area of the solid line envelope, that is, the input power Pin
η=ImUm/Pin=IL{Uoc﹣(KT/q)ln[1＋(qUm/KT)]﹣KT/q} or η=FFILUoc/Pin
Where FF=ImUm/ILUoc=1﹣(KT/qUoc)ln[1＋(qUm/KT)]﹣KT/qUoc
