The light on the surface of the semiconductor will absorb it. Photon absorption produces a majority carrier and a minority carrier at the same time.
In many photovoltaic applications, the number of photogenerated carriers is far less than the majority carriers produced by doping that exist in solar cells.
As a result, the number of majority carriers in the semiconductor remains basically unchanged when illuminated, but the minority carriers increase significantly. The number of photogenerated minority carriers far exceeds the minority carriers originally existing in solar cells under dark conditions, so the number of minority carriers in illuminated solar cells can be approximated to the number of photogenerated carriers.
The lifetime of photogenerated carriers
The lifetime of photogenerated carriers mainly depends on the recombination in the semiconductor, which can be roughly divided into three types:
(1)Shoekley One Read compound;
(2) Radiation compound;
(3) Auger compound. Since the semiconductor has a large absorption coefficient for photons with energy greater than the band gap, the photons emitted by the radiation recombination can always be absorbed. From the photoelectric conversion efficiency of the solar cell, the influence of the radiation recombination can be ignored; the Auger recombination life is mainly determined by the band gap. And thermal equilibrium carrier concentration.
The photo-generated carrier generation mechanism of organic solar cells
Although organic solar cells, like inorganic solar cells, directly convert sunlight photons into Electron holes are devices that generate electricity, but their specific working principles are quite different. There is a debate in academia on how to generate photo-generated carriers in organic solar cells. The organic semiconductor materials (including conjugated polymers and conjugated small molecules) that make up organic solar cells usually have a relative dielectric constant, which is much lower than that of inorganic semiconductors (>10), resulting in far less shielding effects between charges than inorganic semiconductors. Therefore, the mainstream theory believes that the generation mechanism of photo-generated carriers in organic solar cells is as follows (referred to as exciton theory): a photon will not directly form a free electron and a free hole like an inorganic semiconductor after being absorbed by an organic semiconductor material. Instead, an electron-hole pair bound together by the Coulomb force is first formed, called an exciton. After a certain period of time, the exciton diffuses to the donor-acceptor interface and passes through the energy level difference (mainly the difference between the lowest occupied molecular orbital (LUMO) energy level of the donor and the acceptor) for charge separation and finally generates electron-hole pairs ( Academia generally believes that there are no strictly free electrons and free holes in organic semiconductors, only positive polaron and negative polaron.
Recent studies have shown that excitons diffuse At the donor-acceptor interface, it will not be directly separated into electron-hole pairs, but will first be transformed into a charge transfer state (Charge Transfer State, abbreviated as CT state). The CT state is metastable. Energy level is essentially the electron-hole pair bound on the interface. It can continue to complete charge separation to generate photogenerated carriers. It may also cause the loss of carriers due to the direct recombination of electrons and holes on the interface. Transfer to other energy levels with lower energy, such as the triplet energy level of donor or acceptor materials.
Analysis of photo-generated carrier attenuation characteristics
When semiconductor materials are subjected to When light is excited, electrons are excited to transition to the conduction band, forming conduction band electrons and valence band holes, commonly known as photogenerated carriers. The movement process of carriers has a very important influence on the performance of semiconductors, which is the analysis of semiconductor microscopic dynamics. Important concepts. In semiconductors, some energy levels are generated in the forbidden band due to impurities, vacancies, interstitial atoms or dislocations. These energy levels may act as traps or recombination centers, depending on impurities, temperature and other doping. It depends on the situation.
The microwave absorption method was first used to measure the attenuation process of photogenerated carriers on semiconductor wafers. Microwave measurement technology is particularly suitable for measuring nanocrystalline luminescent materials, nanophotochemical materials, nanofilm materials, etc. The movement process of the carriers is of great significance for revealing its luminescence mechanism.
By studying the decay dynamics of free photoelectrons and shallow bound electrons, it is helpful to better understand the energy band structure of semiconductor crystal materials As well as the luminescence dynamics mechanism, it provides a scientific basis for the research and development of semiconductor materials with high luminous efficiency.
Different microstructures of materials form different concentrations and types of defects, corresponding to different photogenerated carrier trapping centers Defect states. Different defect states may form shallow-level traps, deep-level traps, radiative recombination centers, non-radiative recombination centers, etc. Because Si C radiation recombination time is very short (picosecond to nanosecond order), and light Luminescence efficiency is low, so radiation recombination is the main reason for the attenuation of photogenerated carriers.
The behavior of photogenerated carriers in the trap is reversible, that is, there are both trapping and anti-trapping processes. The depth determines the speed of the photogenerated carrier decay. In the photogenerated carrier decay process, the early time behavior mainly reflects the role of the shallow trap. Then, the photogenerated carrier relaxation effect of the deep trap and the shallow trap will work together. In the end, the behavior of deep traps dominates. This view has been used in amorphous and microcrystalline silicon films. It is confirmed in the transient photoconductivity measurement.