Quantum ratchet intermediate band solar cells

Renewable energy sources are becoming increasingly more vital as the supply of fossil fuels is depleted, and awareness of the impact of non-sustainable fuel sources on the environment is increasing. A solar cell is an example of one such renewable energy source, which converts sunlight into electricity; providing an eternal, clean energy supply. The concept of converting solar energy into electricity has been around since the late 1800’s [1]; but it wasn’t until well after Einstein’s landmark paper on a completely new quantum theory of light in 1905 [2] that a modern single-junction solar cell was made using a semiconducting p-n junction, as is used today [3]. This type of solar cell, although used almost exclusively in the world today, has a fun- damental efficiency limit of 31% [4]; so despite falling costs of materials and manufacturing, efficiencies have remained relatively low. Intermediate band solar cells, the focus of this project, potentially increase this limit to 47% [5] by attempting to absorb a greater portion of the solar spectrum, without loss of voltage over an equivalent single-junction cell. Examples have been made using quantum dots, however poor efficiencies are reported due to the low absorption cross section of quantum dots and short lived intermediate states, which prevents efficient absorption of low energy photons. This project however, aims to experimentally demonstrate a modification to the inter- mediate band cell, called a “Quantum Ratchet” intermediate band solar cell. This uses the coupled subbands of a quantum well superlattice as the “Quantum Ratchet” state, to increase the absorption cross section, and spatially separate excited electrons from their cor- responding holes to extend the lifetime of the carriers in this state, allowing more efficient absorption of a second photon. The goal of the project is therefore to experimentally demonstrate a photocurrent due to a sequential absorption of two below-bandgap photons, and show an extended lifetime of carriers in the “Quantum Ratchet” state. Presented here will be the theory behind intermediate band solar cells, along with experimental results from a specifically designed “Quantum Ratchet” device which have been submitted for publishing. Also presented are the experimental results from an ‘upconverting’ sample - a process which shows promise as a ‘natural ratchet’, the results of which have been published in Physical Review B [60]. Open Access