Organic solar cell are made of thin films of carbon-based materials with a thickness of 10 to 100 nm. These materials have the characteristics of p-type or n-type semiconductor materials. They can be composed of polymers or small molecules. Polymer solar cells are made by spin coating or inkjet printing, while small molecule solar cells are made by vacuum evaporation. The third type of organic solar cell is a dye-sensitized solar cell based on titanium dioxide. Organic solar cells can be assembled on a variety of flexible substrates, such as plastics, etc., making it more application areas.
The original organic solar cell had a homogeneous structure with very low efficiency, less than 1%. Later, in the mid-1970s, a small molecule-based heterojunction solar cell appeared with an efficiency of 1%. The active area of this cell is a bilayer electron donor layer, PPV or CuPc, which is produced after absorbing photons. Electrons, electrons will not be transferred to the acceptor layer. C60 fullerene and its derivatives are the most commonly used electron acceptors, and most heterojunction batteries are generally made based on polymers.
Regardless of whether it is a homojunction or a heterojunction battery, the active layer is sandwiched between two electrodes with different functions like a sandwich. The upper surface is a transparent electrode made of ITO material, so that light is incident into the battery; the lower surface is an electrode made of Al material, so that the incident light is reflected back into the battery.
Improving the efficiency of organic solar cells can be achieved by adding adjacent materials in the active layer, such as an electron transport layer and a hole transport layer, which can more effectively promote the flow of photo-generated carriers to the electrode.
The ability of organic solar cell to transmit current comes from the sp2 hybrid orbital of carbon atoms. Pz for each sp2.
Orbital hybridized carbon atoms and adjacent pz, sp2 hybridized carbon atoms of electrons form a π boundary. The overlap of the pz orbitals will cause adjacent electrons to conduct each other.
The conductive properties of organic materials are different from those of inorganic semiconductor materials. In organic materials, there is no concept of conduction band and valence band. Instead, the highest occupied molecular orbital (HOMO) and the lowest vacant molecular orbital (LUMO) are used. But similar to inorganic semiconductor materials, there is a band gap between HOMO and LUMO. After the electron absorbs a photon of sufficient energy, it will transition from HOMO to LUMO, forming holes in the HOMO layer.
This pair of charges are called excitons. If they are effectively separated and reach the electrodes, a current is formed. The holes reach the anode (usually ITO) and the electrons reach the cathode (usually Al).
The basic difference between organic and inorganic solar cells is the separation of the carrier after the photon is absorbed. In inorganic solar cell, the pn junctions of wafers or thin-film solar cells form a built-in electric field so that electrons and holes move in different directions. In organic solar cells, there is no built-in electric field to separate electrons and holes. In a heterojunction battery, excitons (electron-hole pairs) need to diffuse to the interface. In this way, there is energy consumption in the process of effectively separating excitons between the two materials.
Organic materials that absorb light in solar cells are called donors. Excitons (electron-hole pairs) are generated during the transition of electrons from the HOMO layer to the LUMO layer. In order to separate the electron-hole pairs at the interface and the electrons can efficiently transition from the donor to the adjacent acceptor organic material, the LUMO layer of the acceptor should be lower than the LUMO layer of the donor to facilitate the efficient transfer of electrons to the acceptor. The current efficiency of organic solar cells is relatively low (the maximum is about 5%), and their long-term stability is also limited.