Carbon-based Perovskite Solar Cells
Solar cells are currently the most promising photoelectric conversion devices. Since the Bell Laboratory in the United States produced monocrystalline silicon solar cells in 1954, with people's contiguous exploration of the new energy field, the types of solar cells have gradually enriched. Throughout the development history of the solar cell, there were roughly three generations so far. The first generation was a silicon-based solar cell. After long-term exploration, its manufacturing process is now mature and has been successfully used commercially. The second generation was a thin-film solar cell. Although its efficiency is relatively high, it has the problems of environmental toxicity and scarcity of raw materials, making it difficult to promote and apply. The third generation was a new type of solar cell, including quantum dot sensitized solar cells (QDSsCs), dye-sensitized solar cells (DSSCs), organic polymer solar cells (OPSCs) and Perovskite solar cells (PSCs). Among them, perovskite solar cells are the current research's hotspots, which are evolved from dye-sensitized solar cells.Because DSSCs in the form of liquid media are difficult to keep, CH3NH3PbI3 perovskite is adopted as light absorption materials for cells on the basis of DSSCs, and all solid-state PSCs are produced for the first time, reaching photoelectric conversion efficiency of 9.7%. At present, the highest certified photoelectric conversion efficiency of all solid-state PSCs using metal electrodes has reached 25.2%, indicating that perovskite solar cells have very good application prospects. The structure of this new type of PSCs is composed of the conductive glass substrate, electron transport layer, perovskite light absorption layer, hole transport layer and metal counter electrode layer. Commonly used hole transport materials are organic materials such as Spiro-OMeTAD, and precious metal materials such as gold and silver are mainly used for the counter electrode. These materials are extremely expensive and the manufacturing process is complicated. Organic materials will also corrode perovskite materials, which is not conducive to the commercialization of PSCs. In order to further reduce costs, simplify manufacturing processes and improve stability of the battery, using carbon materials with low prices, stable chemical properties and good conductivity as the counter electrode layer of PSCs without hole transport layers has become a researching hotspot for researchers.
Structures and working principles of the carbon-based perovskite solar cell C-PSCs device
Carbon materials have a work function (-5.0eV) that matches perovskite materials, which have good electrical conductivity and hydrophobicity. They have become an ideal alternative material for the traditional hole transport layer and precious metal counter electrode of perovskite solar cells. As shown in the Figure 1, the device structures of PSCs without a hole transport layer based on using carbon materials as the counter electrode are usually: the conductive glass substrate, electron transport layer, perovskite light absorption layer and carbon material counter electrode layer. The perovskite material can act as both a light absorbing layer and a hole transport layer due to its bipolar characteristics, so carbon-based PSCs without a hole transport layer can be produced. The photovoltaic effect has the same basic working principle as carbon-based perovskite solar cells. After perovskite is excited by light, electrons and holes are generated in the conduction band and valence band respectively. Then, free electrons in the conduction band of the perovskite material are injected into the electron transport layer material in the conduction band; the electron transport layer accelerates the extraction of light-generated electrons and prevents hole transport, and finally the electrons are transported to the FTO conductive glass. The bottom of the valence band of the perovskite material is lower than the Fermi energy of the carbon material, and the holes in the perovskite valence band are easily collected by the carbon counter electrode's material. Commonly used materials for the carbon counter electrode are graphite, carbon black, carbon nanotubes, graphene and other carbon materials. The main function is to link with external circuits to form a complete current cycle. However, not all the electrons and holes generated by perovskite can be fully utilized, and some other forms of carrier loss will occur, which will affect the efficiency of the cell. In the carbon electrode device without hole transport layers, the main carrier loss occurs at the interface of perovskite or carbon electrode. Therefore, researchers have made a lot of efforts to improve the carrier transmission's efficiency between the perovskite and carbon electrode. The efficiency of carbon materials used for the counter electrode of perovskite solar cells is also rising.
Fig.1 The structure and schematic diagram of perovskite solar cells with carbon electrode
