According to the materials used to prepare solar cells, thin film solar cells can be divided into the following categories: multi-element compound thin film solar cells, organic semiconductor thin film solar cells, dye-sensitized nano thin film solar cells, amorphous silicon thin film solar cells and polycrystalline silicon thin film solar cells. This article will introduce organic semiconductor thin film solar cells and amorphous silicon thin film solar cells.
Organic semiconductor thin film solar cell
Organic semiconductors have many special properties and can be used to manufacture many thin-film semiconductor devices, such as field-effect transistors, field-effect electro-optic modulators, light-emitting diodes, photovoltaic devices, etc. Organic semiconductors absorb photons to generate electron-hole pairs, with a binding energy of 0.2~1.0eV. The dissociation of electron-hole pairs at the interface between the P-type semiconductor material and the N-type semiconductor material results in efficient charge separation, forming a so-called heterojunction solar cell. Organic semiconductors used in photovoltaic devices are roughly divided into molecular organic semiconductors and polymer organic semiconductors. Later, double-layer organic semiconductor heterojunction solar cells appeared. Organic semiconductors can be divided into three categories according to their chemical properties: soluble, insoluble and liquid crystal; sometimes according to monomers, they can be divided into three categories: dyes, pigments and polymers. The doping of organic semiconductors uses other molecules and atoms, and electrochemical methods can also be used to oxidize them. Impurities that can make it into P-type include Cl2, Br2, I2, NO2, TCNQ CN-PPV, etc.; doping with alkali metals can make it into N-type.
Amorphous silicon thin film solar cell
Amorphous silicon is the earliest commercialized thin film battery. A typical amorphous silicon (α-Si) solar cell is to deposit a transparent conductive film (TCO) on a glass substrate, use plasma reaction to deposit P-type, I-type, and N-type three layers of α-Si, and then evaporate metal electrode Al/Ti on it. Light is emitted from the glass layer, and the battery current is drawn through the transparent conductive film and the metal electrode Al/Ti. Its structure is glass/TCO/P-I-N/AI/Ti, and the substrate can also be plastic film, stainless steel sheet, etc. After the introduction of a large amount of hydrogen (10%) in amorphous silicon, the band gap has increased from 1.1eV to 1.7eV, which has strong light absorption. In addition, a thick “intrinsic layer” is added between the thinner P and N layers to form a P-I-N structure. The I layer with less impurity defects is used as the main absorption layer to form an electric field in the region where the photo-generated carriers are generated, which enhances the collection effect of carriers. In order to reduce the loss caused by the large lateral resistance of the top thin doped layer, the upper electrode of the battery uses a transparent conductive film. In addition, a texture is prepared on the transparent conductive film to enhance light transmission. At present, the most used transparent conductive materials are SnO2 and ITO (a mixture of In2O3 and SnO2), and ZAO (aluminum-doped zinc oxide) is considered a new type of excellent transparent conductive material. Due to the wide energy distribution of sunlight, semiconductor materials can only absorb photons with energy higher than its energy gap value, and the rest of the photons will be converted into heat energy, and cannot be converted into effective electrical energy through the photo-generated carriers transferred to the load. Therefore, for a single-junction solar cell, even if it is made of crystalline materials, the theoretical limit of its conversion efficiency is only about 29%. In the past, amorphous silicon cells were mostly in the form of single-junction cells. Later, double-junction stacked cells were developed to collect photo-generated carriers more effectively. BP Solar uses Si-Ge alloy as the bottom cell material. Because the band gap of Si-Ge alloy is relatively narrow, it enhances the spectral response of the cell as the bottom cell material. Beckaert uses amorphous silicon with different Ge contents to make a triple-junction series cell with two bottom cells, creating the highest stable efficiency of 6.3% for amorphous silicon cell modules. Among thin-film solar cells, amorphous silicon cells were first commercialized. In 1980, Japan’s Sanyo Electric Company used α-Si solar cells to make pocket calculators. In 1981, it realized industrial production. The annual sales of α-Si cells once accounted for 40% of the world’s photovoltaic sales. As the performance of amorphous silicon cells continues to improve, their costs continue to decline, and their application areas continue to expand. Expanded from calculators to various consumer products and other fields such as solar radios, street lights, microwave relay stations, traffic crossing signal lights, weather monitoring and photovoltaic water pumps, household independent power supplies, and grid-connected power generation. This part will be discussed in detail in the following chapters.