Organic compounds are chemical compounds based on carbon, with the exception of carbon oxides, carbonic acid, carbonates and bicarbonates. They can also contain virtually all other chemical elements (the most common are hydrogen, oxygen and nitrogen, sulphur, phosphorus and halogens).
The first organic cells were based on anthracene [1] and politiofen [2]. Among conductive organic materials, there are two subtypes: polymeric materials and organic small-molecule materials.
The most groundbreaking feature in organic photovoltaics is the different form of p-n junction. An organic cell is made up of layers - between the electrodes (one of which is transparent) there is usually an active layer - a mixture of two organic materials: a donor and an acceptor. The donor material is the material that supplies electrons during the process. The acceptor material is the material receiving electrons.
We refer to this structure as a bulk heterojunction. Sometimes additional layers are added between the active layer and the electrodes to support hole and electron transport.

Other architectures (e.g. with independent donor and acceptor layers) have also been explored over the years. However, the volume heterojunction proved to be the most efficient. In the sandwich system shown in Fig. 1. The p-n junction occurs only at the donor-acceptor interface. In a mixture of donor and acceptor, the p-n junction occurs basically at the entire molecule interface. This is a very big advantage of organic photovoltaics - the efficiency of converting absorbed light into excitons is higher than in other cells. The other parts of the energy conversion process are not as efficient as in inorganic semiconductors, so the final efficiency of organic cells is lower than traditional silicon or thin-film cells.

The operation of a cell consists of several steps. The first and most important is the absorption of light. As a result of absorption, an electron is excited from the highest occupied molecular orbital (HOMO) of the donor material to the lowest unoccupied molecular orbital (LUMO). The excited electron and the remaining hole form an exciton - a quasiparticle bound by the coulombic interaction. The exciton diffuses (average exciton diffusion path is 10 nm) until the charge separation takes place. Charge separation usually takes place at the donor-acceptor interface (the point of contact between donor and acceptor molecules), but it also happens at impurities (oxygen traps, all non-intentional dopants). The last steps of the photovoltaic process are the charge transport and its collection on the electrodes. The energy diagram of the cell is shown in Fig. 2, and subsequent action steps on Fig. 3.

Loss mechanisms are associated with each step of the process: light may be reflected; an exciton may not be formed or an already formed exciton may recombine; hole and electron transport limitations may occur. In the case of transport between the active layer and the electrode, a potential barrier may occur at the electrode preventing charge transfer to the electrode (the barrier occurs when the work of the electrode output is mismatched with the level from which the charge is transported).

In the picture Fig. 4, the blue colour is the n-type area and the red colour is the p-type area. The boundary between them is the junction.
When comparing organic cells to silicon cells, it should be recalled that in semiconductors we are dealing with a crystal lattice and an energy gap. In organic cells, there is no crystalline structure, nor do we have the typical transport of holes and electrons in layers. The junction regions are scattered throughout the volume of the active layer. Instead, we consider a diffusing exciton and then charges transported along the HOMO and LUMO levels.

A comparison of energies as a function of density of states in silicon and organic cells is shown in Fig. 5 (based on [3]).

The materials making up the active layer are divided into donor and acceptor materials. To simplify, we can consider an acceptor to be equivalent to an n-type semiconductor - and a donor to be equivalent to a p-type semiconductor. However, whether a material is a donor or an acceptor in an organic cell, specifically the active layer, depends on the material that co-creates the active layer. The donor is the material that supplies the electrons.
Some of the most commonly used materials are poly(3-hexylthiophene), abbreviated as P3HT, and 6,6-phenyl-C₆₁-butyric acid methyl ester (PCBM). New compounds are constantly being synthesised and studied and this group is invariably being expanded. To be used in organic cells, materials must exhibit certain properties. The materials must be able to absorb in the solar spectrum, thermally and photochemically stable, conductive, and form a stable excited state (so that an exciton can be formed by excitation of an electron). It is important to select HOMO and LUMO levels and electrode exit works to facilitate charge transport.
Such materials are fullerene derivatives, polymers based on carbazole and Indole \( \left [ 3.2-b \right ] \) carbazole, fluorine and silol [4], [5].

The disadvantage of organic cells is their still low efficiency [6]. Year by year with the synthesis of new materials, it is increased, but it is possible that it will never match the efficiency of silicon cells. Unfortunately, over time the cells change their properties (efficiency decreases) due to degradation of polymers by interaction with external factors (e.g. water, oxygen). The advantage of organic cells is the low cost of materials and many simple and cheap methods of manufacturing such cells. The greatest advantage is the ability to apply organic cells to any surface. Flexible substrates are most commonly used. Clothes with built-in photovoltaic cells have appeared on the commercial market to provide quick recharging of, for example, a smartphone or portable player.