Developing innovative composite thin films that can significantly increase both the efficiency of solar panels and the overall benefits of renewable energy

Abstract

Currently, silicon (Si) remains the material of choice for solar cells due to its abundance, non-toxicity and stability. Si itself has special chemical properties that allow it to convert light energy into electricity through the photovoltaic effect. The photovoltaic effect allows light to be absorbed into a material, causing the excitation of electrons to a higher energy state and the subsequent production of electric current. However, Si solar cells only have a theoretical maximum efficiency of 29 percent, meaning that of all of the sun’s energy absorbed, only 29 percent of it will ever be converted into usable electricity. Abe and his research team do not think this efficiency rate is high enough. Instead, they are interested in developing quantum dot solar cells that have a potential conversion efficiency of up to 66 percent. To achieve such high efficiency solar cells, the team first needs to synthesise composite thin films. The chemical composition of these films is critically important. Specifically, they must be made up of nanocrystals — extremely tiny crystalline particles that can emit light. They also must be semiconductors, materials such as Si, germanium (Ge), aluminium (Al) and copper (Cu) that can conduct electricity when their electrons are excited through external energy sources, usually temperature. In an atom, electrons orbit the nucleus in discrete shells. The outermost shell is called the ‘valence shell’ and in semiconductors, the electrons in this shell can become conductive only if they bridge the ‘band gap’. To do this, energy is required to move electrons from the valence band to the conductive band of the atom. But how are these composite thin films created? Using one-step physical synthesis, Abe and his team can place a semiconductor nanocrystal with a narrow band gap (that requires less energy to become conductive) inside a matrix of a semiconductor nanocrystal with a wide band gap (that requires more energy to become conductive). Wide-band gap semiconductors that include titanium dioxide (TiO2) and zinc selenide (ZnSe) are only capable of absorbing the ultraviolet part of the solar radiation spectrum. However, when combined with narrow-band gap semiconductors such as Ge and lead selenide (PbSe), the new composites exhibit a key reaction called the quantum confinement effect. The quantum confinement effect occurs when nanocrystal electrons are confined into a space smaller than they prefer, causing the electrons to move around more quickly, increasing their energy. This increase in energy then allows the electrons to pass the band gap and become conductive, thereby increasing their ability to convert all wavelengths of solar light into energy.