Exploring InAs/GaAs quantum dots for next generation solar cells

Reid, William
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University of Delaware
Due to the current economics surrounding energy production, it is imperative that we increase the efficiency of solar cells if we hope to lessen our dependency on fossil fuels. Current device structures waste much of the sun’s energy, losing energy as heat as apposed to converting it into electricity. Quantum Dot’s (QDs) unique material properties, such as tunable band gaps and discrete energy states, make them an ideal building block to engineer novel photovoltaic device structures. Three different QD based solar cell designs are discussed. The first, a quantum dot based intermediate band solar cell, is a design in which the QD’s are incorporated into the band gap of an existing solar cell. In theory this allows for a two-photon process that utilizes photons that would generally be wasted in a traditional solar cell. Theory predicts that for this idea to work efficiently the quantum dots must form delocalized bands in the band gap of the solar cell. A theoretical calculation is performed to determine if this delocalized band is possible. It is determined that at least three orders of magnitude of improvement in the homogeneity of quantum dots is needed in order create a delocalized band. For this reason the likelihood of creating an efficient IBSC based on QDs is deemed to be unlikely and two other ideas are proposed. The quantum dot cluster intermediate band idea is very similar to the IBSC, except that the requirement that the intermediate band is totally delocalized is relaxed.It is proposed that an electron wavefunction that is delocalized over only a few quantum dots might be enough to decrease relaxation rates from the conduction band to the intermediate band and therefore may result in an improved efficiency. The second idea, photon up-conversion, electrically separates the quantum dots from the solar cells. The quantum dots are coupled to a graded potential. Two low energy photons could be used to excite electrons into the first and higher excited states of a QD and then the electron could tunnel through the triangular barrier created by the graded potential in order to recombine with the hole at an energy approximately equal to the sum of the two photons used in the excitation process. The success of all three ideas is dependent on the device structure, including the thickness of tunneling barriers, the barrier height of tunneling barriers, and the homogeneity of quantum dots. In order to determine the optimal device structure for any of these three ideas, it is vital that we understand the effect of these device structures on the electron lifetime and charge transport dynamics. A method for examining the electron lifetime as a function of device structure using time-resolved photoluminescence is discussed.