Solar cells: this is how it works
If an electrical conductor is connected to the contacts of a solar cell, electrons flow from the negative pole to the positive pole: an electric current is produced, just like in a battery. In order to make this possible, electrons and mobile positive charge carriers – atoms that have emitted an electron – must initially be produced in the solar cell and then separated from each other in such a way that they accumulate at the contacts. The energy required for these processes comes from the particles of light captured by the solar cell.
Solar cells are based on certain materials referred to as semiconductors. Semiconductors such as silicon become electrically conductive when supplied with heat or light. By deliberately introducing impurities – scientists refer to this process as "doping" – the number of mobile charge carriers in the solar cell and therefore the cell’s current yield can be improved.
The schematic of a solar cell as currently studied at Jülich is shown here: two electrodes or contacts envelop a package made up of three different semiconductor layers. The upper electrode is Solar Cells – a Multi-Layered Construction transparent so that particles of light can penetrate into the semiconductor layers. The lower electrode can reflect particles of light that have overshot the target back into the semiconductor layers (reflecting back contact). All the incident sunlight is therefore utilized in an optimal way. The solar cell is protected by a layer of glass. The semiconductor sandwich inside the solar cell consists of an undoped i-layer in the centre that connects two differently doped layers with each other – the n-doped and p-doped layers. The researchers introduce impurity atoms that have more electrons than the pure material into the n-doped layer (n for negative) and impurity atoms with fewer electrons than their neighbours into the p-doped layer (p for positive). This layer conducts positive charges. The combination of the three semiconductor layers means that electrons and positive charge carriers are systematically separated from each other.