When two types of semiconductors are joined together, a p-n junction ( Fig. 1 ) with specific energy characteristics is produced. This is due to the excess of free electrons in the n-type semiconductor and holes in the p-type semiconductor. When they are combined, the electrons move to the p-type semiconductor and the holes move to the n-type semiconductor, causing a charge imbalance and creating a potential difference.

The delivery of energy in the form of radiation quanta to the atoms of the crystal lattice (depending on the energy gap) can move electrons into the conduction band [1]. For this process to occur, radiation quantum \( h\nu \) has to have an energy level greater than the energy gap \( E_{g} \) ( \( h\nu >E_{g} \)) ( \( h \) – Planck's constant, \( \nu \) – frequency).
An exciton can dissociate into charges when exposed to temperature or an electric field. The exciton can travel through the semiconductor until it hits a region where an electric field is present. The electric field present at a p-n junction allows charges to separate. Positive charges accumulate in the p-type semiconductor and negative charges accumulate in the n-type semiconductor.

Light absorption can occur in the n-type region and the p-type region. The charges created by the break-up of the exciton move toward the electrodes. When a receiver is connected to the electrodes, an electric current will flow through it. The current-voltage characteristics of such a semiconductor junction are shown in Fig. 2. If such a junction is not illuminated, the characteristic I(U) will be obtained, the so-called dark characteristic, without illumination (one can say an ordinary diode characteristic). If the p-n junction is illuminated, the I(U) characteristic of the illuminated solar cell is bright.

The amount of light energy absorbed depends, among other things, on the size of the energy gap ( Fig. 3 ). The quantities shown in the table are the maximum amount of solar energy (in \( \% \)), that can be absorbed at a given energy gap. If the gap is greater than 4 eV, no part of the solar radiation reaching the Earth will be absorbed because the solar radiation quantum has an energy lower than 4 eV (see solar spectrum figure).

The assumption here is that solar radiation in the region from 300 nm to 2500 nm represents \( 100\% \) of the radiation. For example, a semiconductor with an energy gap of 1.03 eV can maximally absorb \( 81\% \) of the solar energy reaching the cell.

Conversion of sunlight energy to electricity is accomplished by several physical processes shown in Fig. 4. After light is absorbed, an exciton is created, which then breaks into charges. The charges move toward the electrodes where they accumulate.
Each of these processes is subject to losses due to imperfections in the device [2].
The incoming light radiation quanta are partially absorbed by the semiconductor material, some of the radiation is reflected from the diode surface, and the remaining unabsorbed radiation undergoes transmission.
The absorbed radiation quantum results in the formation of an exciton ( Fig. 4 ), which can travel through the semiconductor material but can also annihilate, turning into heat. The exciton breaks to form free positive and negative charges. The charges travel to the electrodes. During this process, there are losses associated with charge recombination, mobility limitation, and resistance at the semiconductor-electrode interface.

Some of the losses can be eliminated during the construction of a photovoltaic cell, therefore work is still underway to increase the efficiency of solar radiation energy conversion. Materials for photovoltaic cells should have a number of properties such as:

• the widest possible spectral region (table in Fig. 4 ),
• creation of the highest possible number of excitons,
• high mobility of electric charges,
• easy transport of charges to electrodes.

Data on the efficiency gains of photovoltaic panels produced with new or modified materials are published annually by the NREL (see chapter: 5.2 Changes in conversion efficiency ).