depletion region or depletion layer.There is an electric field in the depletion region pointing toward p-type region from the n-type region.This electric field is called built-in electric field. Holes in the n-type region near the PN-junction spread to the PN interface are pulled to the p-type region by the built-in electric field.Due to the same principle,electrons in the p-type region near the junction are pulled to the n-type region.The electrons and holes in the junction region move towards the n-and p-type region respectively.If the external circuit is open, photo-induced electrons and holes can accumulate near the PN junction,which builds up a stable potential difference,called optical electromotive force.This phenomenon is known as the photovoltaic effect(abbreviated as PV). 2.Photocell The SPC is a silicon-based device that converts the photo energy to the electricity utilizing photovoltaic effect.The key part of SPC is a PN junction with very large area.Because the PN junction area in SPC is much larger than that in normal diode,the photoelectric current of is very large.For example,exposed to 1000 luminous light,the SPC(made in china,model 2DU10) used in our experiment can created the 300mV of open-circuit voltage or 50mA short-circuit current 1)Structure of SPC 灵敏面积 The typical structure of a SPC is shown P扩散 in the Fig.1. n-型硅体积 耗尽区 2)The illumination characteristics of SPC n扩散 (1)short-circuit current and illumination 金属接触 When the SPC is under illumination,there is a photo-induced current /ph from the N area Figure 1 Typical P-N structure of silicon photocell to P area.As a diode,there is a forward diode current /p from P area to the N area,which is opposite to the Iph.As a result,the total current should be (1) V is the junction voltage.lo means inverse saturation current without illumination.I Ph is proportional to the illumination intensity.The factor of proportionality is related to the load and -2-- 2 - depletion region or depletion layer. There is an electric field in the depletion region pointing toward p-type region from the n-type region. This electric field is called built-in electric field. Holes in the n-type region near the PN-junction spread to the PN interface are pulled to the p-type region by the built-in electric field. Due to the same principle, electrons in the p-type region near the junction are pulled to the n-type region. The electrons and holes in the junction region move towards the n-and p-type region respectively. If the external circuit is open, photo-induced electrons and holes can accumulate near the PN junction, which builds up a stable potential difference, called optical electromotive force. This phenomenon is known as the photovoltaic effect (abbreviated as PV). 2. Photocell The SPC is a silicon-based device that converts the photo energy to the electricity utilizing photovoltaic effect. The key part of SPC is a PN junction with very large area. Because the PN junction area in SPC is much larger than that in normal diode, the photoelectric current of is very large. For example, exposed to 1000 luminous light, the SPC (made in china, model 2DU10) used in our experiment can created the 300mV of open-circuit voltage or 50mA short-circuit current 1) Structure of SPC The typical structure of a SPC is shown in the Fig. 1. 2) The illumination characteristics of SPC (1) short- circuit current and illumination When the SPC is under illumination, there is a photo-induced current IPh from the N area to P area. As a diode, there is a forward diode current ID from P area to the N area, which is opposite to the IPh. As a result, the total current should be ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ −⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = − = − 1 n 0 exp k T qV I I I I I B Ph D Ph （1） V is the junction voltage. I0 means inverse saturation current without illumination. I Ph is proportional to the illumination intensity. The factor of proportionality is related to the load and Figure 1 Typical P-N structure of silicon photocell