1. Photoelectric characteristics of solar cells
When light shines on the solar cell, currents IR and IJ flow through the battery load resistance R and inside the battery respectively, where IJ is the forward current passing through the PN junction. When the light is constant, the photocurrent IP=IR+IJ is also constant. The flow of photocurrent inside and outside the solar cell can be represented by an equivalent circuit. The terminal voltage VJ of the solar cell should be equal to the voltage VR on the resistor. The current IJ of the photocell has an exponential relationship with the change of VJ.
IJ =IP[（expqVJ）/（A×K×T）-1] （1-1）
In the formula: q is the electronic charge, q=1.6×10-10C; T is the absolute temperature, K; K is Boltzmann’s constant, K=1.380×10-23J/K or 0.86×10-4eV;
A is the effective area of the battery, mm2.
The relationship between the current and voltage on the load resistance R is
From the formula (1-2), the relationship between the current and voltage on the load resistance is R=Vj/I, the voltage drop of the load resistance is equal to the junction voltage, and the electric power obtained on the load resistance R is I×Vj . For solar cells to obtain high conversion efficiency, they must output as much power I×Vj as possible under a certain amount of solar radiation.
2. The spectral characteristics of solar cells
The spectral characteristics refer to the relationship of solar cells with the same energy but different wavelengths of light emitted by people. In solar cells, only those photons with energy greater than the “forbidden band” width of the material can produce electron-hole pairs in the photovoltaic material when absorbed. And those photons with energy less than the width of the “forbidden band” cannot generate electron-hole pairs even if they are absorbed (they can only heat the photovoltaic material). Photovoltaic materials have a cut-off wavelength for light absorption. Theoretical analysis shows that as far as sunlight is concerned, the photovoltaic material that can get the best performance should have a “band gap” of 1.5eV. When the width of the “band gap” increases, the total solar energy absorbed by the photovoltaic material will become less and less.
Each solar cell has its own spectral response curve to sunlight, which indicates the sensitivity of the solar cell to light of different wavelengths (photoelectric conversion ability), the cut-off wavelength of photovoltaic material characteristics and the absorption efficiency of solar energy.
When the sun shines on the solar cell, the light of a certain wavelength and the spectral sensitivity of the solar cell at that wavelength determine the photocurrent value of that wavelength, and the total photocurrent value is the sum of the photocurrent values of each wavelength.