Current Matching Using CdSe Quantum Dots to Enhance the Power

The past few years have witnessed an explosive growth in research that addresses different aspects of the use of semiconductor materials in varied configurations for photovoltaic applications. Among them, III-V compound tandem solar cells, which take

advantage of the bandgap tunability by elemental multi-junction compositions and of the high optical absorption by direct bandgap materials, have attracted increasing attention for their extremely high conversion efficiency [1.43]-[1.46]. Ideally, a calculated power conversion efficiency as high as η=50.1% (under AM1.5G, 1000 sun) is achievable for a series-connected InGaP/GaAs/Ge triple-junction solar cell, which is far beyond the theoretical limit of a single-junction solar cell estimated by the Shockley-Queisser’s calculation scheme [1.47], [1.48]. In practice, an appropriate alignment of the bandgap energy of multi-stacking layers that provides current matching between each subcell is the most challenging issue in this tandem architecture, which restricts the maximum power conversion efficiency of the device and the potential applications in the photovoltaic industry. More specifically, the GaAs middle subcell generally limits the overall photocurrent of a InGaP/GaAs/Ge tandem solar cell.

To overcome the issue of current mismatching, several approaches, such as the use of a quaternary AlGaInP top subcell and the substitution of the middle subcell with an InGaAs material,have been widely investigated [1.49]. However, an introduction of Al content into the InGaP top subcell causes a significant photocurrent droop due to the associated oxygen contamination on minority-carrier properties [1.50]. In addition, the substitution of a fraction of the gallium atoms with indium in the middle subcell accompanies a lattice mismatch and requires a complicated growth scheme such as the graded buffer layers to avoid a large dislocation density that also reduces the photocurrent of the device [1.51]. Hence, for InGaP/GaAs/Ge tandem solar cells, an approach that does not adversely affect the device’s performance and that is capable of resolving the current-mismatching issue is necessary. Recently, semiconductor nanoparticles, known as quantum dots (QDs), have been intensively studied and utilized to generate multiple carrier excitations from one incident photon by the

so-called impact ionization [1.52]-[1.54]. Such a nonlinear phenomenon can cause a solar cell’s quantum efficiency to be greater than 100%, primarily due to the discrete carrier density of states and the strong quantum confinement effect [1.55]. Additionally, as the electronic energy levels and the optical spectrum strongly depend on the QD’s dimension, its effective bandgap energy can be tunable. For the same reason, the semiconductor QDs are also adopted as downconverter materials to help harvest the ultraviolet regime of solar energy in silicon solar cells [1.56]. In this study, we recognize the photon conversion aspect of nanocrystal QDs and explore a novel strategy using CdSe QDs to tailor the incident spectrum of solar light to enhance the photocurrent of a limited subcell in InGaP/GaAs/Ge tandem solar cells and to enhance the overall power conversion efficiency of the cell. We demonstrate the ability of CdSe QDs to enhance the performance of the device, not only by theoretical calculations based on the fundamental of material optics but also by directly measuring the device’s electrical characteristic. The device exhibits an enhancement of 10.39% in the power conversion efficiency compared to the device’s counterpart without integrating QDs.

The theoretical and experimental results validate that the CdSe QDs have promising potential for efficient solar spectrum utilization in InGaP/GaAs/Ge tandem solar cells.

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