Introduction: Solar cell efficiency and routes beyond current limits

The energy carried by sunlight has an enormous potential to provide humanity with all the energy it needs while limiting the catastrophic consequences of climate change caused to a large extent by current combustion of fossil fuels. A convenient way to harvest solar energy is by making use of the photovoltaic effect of certain semiconducting materials. When a solar photon interacts with such a material, its energy can be converted into electronic energy, which can be extracted to an external electrical circuit, providing both a photovoltage and a photocurrent. The material absorbing the sunlight imposes certain boundary conditions on its electrons which allow electronic states to have only specific distinct energies. Importantly, there will always be a certain minimum energy needed to promote a “resting” electron in a material to a higher electronic state. Therefore, all materials used for the absorption of sunlight will have a requirement on the minimal energy that must be carried by a photon in order for that photon to be absorbed by the material. Photons with energies lower than this absorption threshold will be transmitted by the material. In a solar cell, this portion of solar energy will be lost. This loss of solar photons, known as a transmission loss, restricts the photocurrent achievable from sunlight (see Fig. 1.1a). In a hypothetical solar cell with an optimal absorption threshold (also known as band gap) of 1.3 eV, the fraction of incident solar energy lost by transmission is ~25%.