Publication date: 1st July 2014
Beyond the more common application of quantum dots (QDs) as broadband tunable absorbers in photovoltaics (PV), QDs also exhibit many interesting photo physical processes that may serve useful for the design and optimization of various types of solar cells.A mismatched band offset is one of the fundamental origins for photovoltage loss in photovoltaic devices. Here, we show that such over potentials between the absorber and the selective contacts can be reduced by exploiting the photo induced dipole (PID) phenomenon. This is achieved by using type-II QDs on top of another semiconductor serving as a sensitizer and a TiO2 electron conductor. The type-II QDs offer an efficient spatial charge separation that allows for the accumulation of negative charges in the TiO2 and positive charges in the QDs cores across the sensitizer layer that serves here also as a dielectric layer, thus creating a significant photodipole. The generated PID, negatively shift the TiO2 conduction band in respect to the electrolyte, significantly increasing the solar cell open circuit voltage (Voc).
Here we investigate the PID effect in detail by using a flat TiO2/CdSe electrode configuration cell, which has several clear advantages. First, the PID effect can be “turned on” (and thus quantified in a direct manner) at will by controlling the excitation color. Second, it enables to quantify the charge density per QD. Further, to better understand the dynamics of the system, we also use ZnSe:Te/CdS (Te doped) QDs which have a deeper confined hole state. The effect was studied by charge extraction measurements combined with transient photovoltage. The Te doped ZnSe/CdS QDs give rise to a nearly 150mV increase in the open circuit voltage of the cell, leading to a maximal Voc of nearly 800mV despite the use of a polysulfide electrolyte.
With a limited choice of hole conducting materials, the flexibility of the PID concept introduces a new design strategy toward the development of additional kinds of high voltage PV cells such as solid state QDSSCs, oxide PVs, organic solar cells (OPV), hetrojunction cells, and perovskite based solar cells. In addition, implementing similar concepts may enable other efficiency improvements such as by use of the QDs in an intermediate band solar cell.
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