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Current matching using CdSe quantum dots to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells

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Abstract

A III-V multi-junction tandem solar cell is the most efficient photovoltaic structure that offers an extremely high power conversion efficiency. Current mismatching between each subcell of the device, however, is a significant challenge that causes the experimental value of the power conversion efficiency to deviate from the theoretical value. In this work, we explore a promising strategy using CdSe quantum dots (QDs) to enhance the photocurrent of the limited subcell to match with those of the other subcells and to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells. The underlying mechanism of the enhancement can be attributed to the QD’s unique capacity for photon conversion that tailors the incident spectrum of solar light; the enhanced efficiency of the device is therefore strongly dependent on the QD’s dimensions. As a result, by appropriately selecting and spreading 7 mg/mL of CdSe QDs with diameters of 4.2 nm upon the InGaP/GaAs/Ge solar cell, the power conversion efficiency shows an enhancement of 10.39% compared to the cell’s counterpart without integrating CdSe QDs.

© 2013 Optical Society of America

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Figures (7)

Fig. 1
Fig. 1 (a) Schematic plot of an InGaP/GaAs/Ge triple-junction solar cell with CdSe QDs spread on the top surface to tailor the solar spectrum and enhance the photocurrent of the GaAs middle subcell. (b) Simplified structure of the tandem solar cell to facilitate optical calculations.
Fig. 2
Fig. 2 (a) Absorption and (b) photoluminescence spectra of CdSe QDs of different sizes in toluene.
Fig. 3
Fig. 3 Calculated light intensity of the solar spectrum I(1,2,3)i(λ) distributed on each subcell for the device (a) without and (b) with CdSe QDs with diameters of D = 2.1 nm. The quantum efficiency QE(1,2,3)(λ) of each subcell is also plotted in the figure.
Fig. 4
Fig. 4 Calculated J-V characteristics of each subcell by Eq. (8) for the device (a) without and (b) with CdSe QDs with diameters of D = 2.1 nm.
Fig. 5
Fig. 5 (a) Calculated short-circuit current density (JSC) of each subcell and (b) the overall power conversion efficiency (PCE) of the device as a function of the CdSe QD’s diameter under 1-sun illumination. The enhancement ratio of the PCE compared to the device without CdSe QDs is also labeled in the figure. The black dash-line indicates the current-matching condition of InGaP and GaAs subcells.
Fig. 6
Fig. 6 (a) Electrical performance of the bare tandem solar cell after dispersing 7 mg/mL of CdSe QDs with diameters of 4.2 nm compared to the one without CdSe QDs under AM1.5G sunlight illumination. The electrical performances of devices with a traditional antireflection coating (ARC, 100 nm SiNx) and CdSe QDs on top of the SiNx ARC are also plotted for comparison. Inset: photograph of the actual devices with dimensions of 1 cm × 1 cm. (b) J-V characteristics with different concentration of CdSe QDs. A summary of JSC as a function of the QD concentration is also plotted and inserted into the figure.
Fig. 7
Fig. 7 Reflectance of the devices spin-casting CdSe QDs with various concentrations of 5 mg/mL, 7 mg/mL, and 9 mg/mL. The reflectance of the bare tandem solar cell is also plotted in the figure.

Equations (8)

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A=log( I 0i I 0t )
α 0 = (ln10)A L 0
I 0i = I AM1.5G (1 R 0 ) R 0 = ( n air n QD n air + n QD ) 2
I 1i = I 0t (1 R 1 )+ I PL = I 0i 10 A (1 R 1 )+ I PL
I 1t = I 1i exp[( α 1 L 1 )] α 1 = 4π κ 1 λ
I 2i = I 1t (1 R 2 )= I 1i exp[( α 1 L 1 )](1 R 2 ) I 3i = I 2t (1 R 3 )= I 2i exp[( α 2 L 2 )](1 R 3 )
A (1,2,3) (λ)= I (1,2,3)i I (1,2,3)t = I (1,2,3)i [1exp( α (1,2,3) L (1,2,3) )]
J (1,2,3) (V)= q hc 0 λ I (1,2,3)i { 1exp[( α (1,2,3) L (1,2,3) )] }Q E (1,2,3) (λ)dλ q( n (1,2,3) 2 +1) E g(1,2,3) 2 kT 4π 3 c 2 exp( eV E g(1,2,3) kT )
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