Noise Characteristic of Solar Cells under Various Illumination

Research of stationary, transport and noise characteristics of different type of PN junctions shows that illumination of PN junction can have influence not only on origin photocurrent and photovoltage, but also to noticeable changes of space charge regions quality. This phenomenon was observation so only on heterojunction based on AB compounds [1]-[2]. Simultaneously no one influence of illumination has been found on space-charge regions characteristics of a number of homo and heterojunction based on AB compounds [3]. Behaviour of low-frequency signal noise in cooled InSb photodiodes show variety of features also in this case predicating occuring changes in space charge region [4]. Monitoring of noise characteristics makes possible to better analyse qualities and reliability of solar cells [5-7]. In this work was undertaken admeasurement of 1/f noise silicon solar cells of area about 104 cm. Experiments were carried out on PN junction silicon solar cells prepared by boron diffusion in p-type Si wafers of area about 104 cm. These measurements were taken at room temperature T = 300 K. The source of illumination is a Halogen Lamp (55 W) connected to d.c. stabilized power source to obtain different level of illumination intensities (L) related to the lamp current shown in figure 2. The dark and illuminated I-V characteristics for a typical Si solar cell npp PESC of area about 104 cm (for a chosen sample No. 20/2) in reverse direction are shown in figure 1. The illuminated curves were obtained with a halogen lamp (55W) In reverse direction as L the photocurrent increases until it reaches its saturation value (the short circuit Isc). The changes of the numbers of electron-hole pairs produced current with increasing illumination intensity L indicates that the illumination increases the photocarriers passing the depletion region layer. Experiment results and evaluation Fig. 1 shows the current versus reverse voltage plot for No. 34V2 solar cell PN junction for various illumination levels and an ambient temperature of 300 K. Fig. 2 shows the reverse current, as measured at UR = 2V, to grow with the PN junction incident light intensity, for samples No. 34V2 a VS11. Fig. 3 illustrates the noise voltage spectral density versus forward voltage plots, for sample 34V2, both in the dark and when lighted with L = 260 lx. The noise voltage was picked up across a load resistance RL = 100Ω , at a pass band mean frequency of 1 kHz and a bandwidth of 20 Hz. It is seen that the maximum value of SUM is lower under illumination than in the dark and the maximum value position is shifted to higher forward voltage values. This is connected with the light-induced drop in the junction differential resistance. It is evident from Fig. 4 that the samples are generating a G-R noise with a relaxation frequency of about 800 Hz when in the dark. At lower frequencies, 1/f noise is predominating. If the sample is illuminated with light of intensity L = 260 lx, G-R noise is observed throughout the frequency band under 10 Hz. In this case, the relaxation frequency is below 1 Hz and, in regard of the Unipan 233 selective nanovoltmeter measuring range available, it could not be observed. Thermal noise of the experiment setup is predominant at frequencies above 10 Hz. Fig. 1. Dependence of current versus voltage solar cell No. 34V2 in various illumination levels and temperature 300 K. Fig. 2. Dependence of reverse current versus illumination of solar cell No. 34V2 and VS11 and temperature 300 K. Fig. 3. Dependens of noise voltage spectral density versus forward voltage of solar cell No 34V2 without illumination and with illumination 260 Lx. Fig. 4. Dependense noise voltage versus frequence solar cell No. 34V2 for diferent illumination