A photovoltaic cell is essentially p-n junction diode, but it is a little bit different from a simple p-n diode. In the construction of the solar cell, a thin layer of p - type semiconductor is grown on a thick n - type semiconductor which is grown metal on sheet or glass substrate. We make few finger electrodes or metal contacts (silver paint is used for metal contact) on the top of the p - type semiconductor layer and at the bottom of the n - type layer for better conduction electric current. These finger electrodes do not block the light to reach the thin p - type layer. There is a junction just below the p - type layer. The entire cell shielded by the thin glass to protect the solar cell from any environmental hazards. The figure below showed the growth of CZTS solar cell in the thin film research lab (Physics Department Utah University Research Lab). Yellowish color layer shown in the image is n- type CdS semiconductor on which thin layer of p- type (CZTS) semiconductor ( black in color) were grown.
Physics of Solar Cells
When solar radiation incident the on the diode, the photons of light can easily diffuse into the junction, through very thin p-type layer. The photons of light having sufficient energy incident on the depletion layer and split the layer which is formed at the junction. Hence a large number of electron-hole pairs is created at the junction.These free electrons- holes pairs move to their respective side i.e. electrons moves towards n side and holes towards p side of the junction under the influence of barrier field. Once, these newly produced free electrons enter to the n-type side, cannot further cross the junction because of junction barrier field. Similarly, the newly created holes once enter to the p-type side cannot further cross the junction because of same barrier field of the junction. Thefore population of electrons becomes more in n-type side of the junction and population of holes becomes more in the p-type side of the junction, the p-n junction diode will behave like a small battery cell. A Potential difference or voltage is developed across the diode which is known as photovoltage. When small a small load or bulb connected across the junction, then current will start to flow through the load or bulb.