Photovoltaic materials are substances that convert sunlight directly into electricity through the photovoltaic effect, where photons in sunlight excite electrons in the material, generating an electric current. These materials form the basis of solar cells, which are essential for solar technology, one of the most promising renewable energy sources. The landscape of photovoltaic materials is evolving rapidly. While traditional silicon-based solar cells dominate the market due to their mature efficiency, stability, and strong manufacturing infrastructure, the field has expanded to include a variety of materials such as thin-film technologies (e.g., cadmium telluride and copper indium gallium selenide), organic photovoltaics, and perovskite materials. Perovskites have attracted much attention due to their high efficiency and tunable band gap, although challenges such as long-term stability and toxicity remain.

The materials used in photovoltaic devices are typically silicon (single crystal, multicrystalline, or amorphous), gallium arsenide, metal chalcogenides, and organometallics. Recently, mesoscopic solar cells have made some progress in the commercial market. Crystalline silicon is the most commonly used material in solar cells. Crystalline silicon cells have a service life of more than 25 years and do not age, making them ideal for industrial solar power generation.

Organic solar cells have become a hot topic of industrial research, as solution-processable conjugated organic materials have the potential to enable the simple fabrication of low-cost, mechanically flexible, and large-area photovoltaic devices, thereby harnessing the sun's sustainable clean energy. Significant efforts have been devoted to improving the power conversion efficiency of such solar cells. A major breakthrough has been achieved by using a bulk heterojunction structure, in which the active layer is spin-coated from a mixed solution of donor and acceptor materials. The resulting partially separated hybrid structure allows efficient exciton ionization over a large interfacial area, while also maintaining adequate charge transport and extraction via a bicontinuous donor and acceptor phase.

Organic solar cells are the only photovoltaic devices that exploit the absorption of photons by molecules and convert them into charges, without the need for intermolecular transport or electronic excitation. It is also the only solar cell that separates the two functions of light collection and charge carrier transport, while conventional photovoltaic devices perform both operations simultaneously. This places stringent requirements on the optical and electronic properties of the semiconductor, namely its bandgap and band position, as well as charge carrier mobility and recombination time of photogenerated charges, greatly limiting the choice of suitable materials that can be used as efficient photovoltaic converters.

Molecular sensitizers or semiconductor quantum dots are located at the interface between electron (n) and hole (p) conducting materials. The former are usually broadband semiconductor oxides such as TiO2, ZnO or SnO2, while the latter are redox electrolytes or p-type semiconductors. Under light excitation, the sensitizer injects electrons in the conduction band of the oxide and regenerates them by injecting holes in the electrolyte or p-type conductor. Since the sensitizer injects electrons in the n-type and holes in the p-type collector, only majority carriers are generated. These charges move in their respective transport media to the front and back contacts of the photocell, where they are collected as current. The electric field near the junction separates the positive and negative charges generated under illumination, attracting electrons to the n-doped material and holes to the p-doped material. Very pure materials are required to give the photogenerated electron-hole pairs a sufficiently long lifetime.

A good "next generation" solar cell must carry out a series of processes efficiently. These processes include:

absorption of photons in the active layer,

migration of excitons to the interface and charge transfer at the interface,

separation of charges by Coulomb mutual attraction, and

transport of separated charges to the electrodes.

Each of these processes can be monitored spectroscopically, making them powerful tools for measuring device performance. Optical probes are very valuable tools for studying the nature of excited states and the kinetics of charge transfer and recombination processes in donor-acceptor systems. Excited states are generated by an excitation laser pulse and their absorbance is measured by recording the transmission of a second pulse. The second laser pulse arrives at a controllable time after the excitation pulse.

Devices using new semiconductors will eventually become available and significantly reduce production costs. To avoid charge carrier recombination inside the cell, the exciton transport path must be short and exciton separation must be promoted. Translated to device architecture, this means that the area of each donor or acceptor material is small and the interface area between them is large - both of which can be achieved by controlling the cell design at the nanoscale. Further understanding of charge recombination processes, especially their dependence on nanoscale morphology, is needed to further improve efficiency, extend high efficiency to larger area devices, and reduce reliance on trial and error for structural and device optimization.

Perovskite solar cells based on organic-inorganic lead halides are the most efficient solution-processed solar cells to date, challenging DSCs, thin-film and multicrystalline silicon solar cells. Perovskite solar cells with transparent contacts can be used to compensate for the heat losses of silicon solar cells in tandem devices, thus providing a way to overcome the efficiency limitations of other types of solar cells. However, the perovskite top cell in the tandem structure requires a contact layer with high conductivity and optimal transparency. Despite the intensive research in this field, there are concerns about the long-term stability and toxicity of lead in the typical perovskite CH3NH3PbI3. Although the search for lead-free perovskites has naturally turned to other transition metal cations and formulations that replace the organic part, the efficiency of these alternatives is still much lower than that of lead-based perovskites. The perovskite family offers a rich crystal structure and substituents with the potential to discover new and exciting photophysical phenomena, which are expected to improve the efficiency of solar cells.