Current matching using CdSe quantum dots to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells Ya-Ju Lee,1, * Yung-Chi Yao,1 Meng-Tsan Tsai,2 An-Fan Liu,1 Min-De Yang,3 and JiunTsuen Lai3 1
Institute of Electro-Optical Science and Technology, National Taiwan Normal University, 88, Sec.4, Ting-Chou Road, Taipei 116, Taiwan 2 Department of Electrical Engineering, Chang Gung University, Tao-Yuan 333, Taiwan 3 WIN Semiconductors Corp, No.358, Hwaya 2th Rd., Hwaya Technology Park, Tao-Yuan 333, Taiwan *[email protected]
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 OCIS codes: (310.1210) Antireflection coatings; (040.5350) Photovoltaic; (220.4241) Nanostructure fabrication.
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#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A953
10. J. Wallentin, N. Anttu, D. Asoli, M. Huffman, I. Åberg, M. H. Magnusson, G. Siefer, P. Fuss-Kailuweit, F. Dimroth, B. Witzigmann, H. Q. Xu, L. Samuelson, K. Deppert, and M. T. Borgström, “InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit,” Science 339(6123), 1057–1060 (2013). 11. F. Hetsch, X. Xu, H. Wang, S. V. Kershaw, and A. L. Rohach, “Semiconductor nanocrystal quantum dots as solar cell components and photosensitizers: material, charge transfer, and separation aspects of some device toplogies,” J. Phys. Chem. Lett. 2(15), 1879–1887 (2011). 12. M. S. Leite, R. L. Woo, J. N. Munday, W. D. Hong, S. Mesropian, D. C. Law, and H. A. Atwater, “Towards an optimized all lattice-matched InAlAs/InGaAsP/InGaAs multijunction solar cell with efficiency >50%,” Appl. Phys. Lett. 102(3), 033901 (2013). 13. Sharp Develops Concentrator Solar Cell with World's Highest Conversion Efficiency of 44.4%,” http://sharpworld.com/corporate/news/130614.html (2013) 14. J. Geisz, D. Friedman, J. Ward, A. Duda, W. Olavarria, T. Moriarty, J. Kiehl, M. Romero, A. Norman, and K. Jones, “40.8% efficient inverted triple-junction solar cell with two independently metamorphic junctions,” Appl. Phys. Lett. 93(12), 123505 (2008). 15. K. Tanabe, “A review of ultrahigh efficiency III-V semiconductor compound solar cells: multijunction tandem, lower dimensional, photonic up/down conversion and plasmonic nanometallic structures,” Energies 2(3), 504– 530 (2009). 16. L. A. Kosyachenko, Solar Cells-Silicon Wafer-Based Technologies (InTech, Rijeka, Croatia, 2011), p. 335–337. 17. W. Shockley and H. J. Queisser, “Detailed balance limit of efficiency of p-n junction solar cells,” J. Appl. Phys. 32(3), 510 (1961). 18. M. W. Wanlass, S. P. Ahrenkiel, R. K. Ahrenkiel, D. S. Albin, J. J. Carapella, A. Duda, J. F. Geisz, S. Kurtz, T. Moriarty, R. J. Wehrer, and B. Wernsman, “Lattice-mismatched approaches for high-performance, III-V photovoltaic energy converters,” in Proceedings of the 31th IEEE Photovoltaic Specialists Conference (Institute of Electrical and Electronics Engineers, New York, 2005), pp. 530–535. 19. R. R. King, M. Haddad, T. Isshiki, P. Colter, J. Ermer, H. Yoon, D. E. Joslin, and N. H. Karam, “Nextgeneration, high-efficiency III-V multijunction solar cells,” in Proceedings of the 28th IEEE Photovoltaic Specialists Conference (Institute of Electrical and Electronics Engineers, New York, 2000), pp. 998–1001. 20. F. Dimroth, U. Schubert, and A. W. Bett, “25.5% efficient Ga0.35In0.65P/Ga0.83In0.17 as tandem solar cells grown on GaAs substrates,” IEEE Electron Dev. 21(5), 209–211 (2000). 21. A. J. Nozik, “Quantum dot solar cells,” Physica E 14(1–2), 115–120 (2002). 22. R. D. Schaller and V. I. Klimov, “High efficiency carrier multiplication in PbSe nanocrystals: implications for solar energy conversion,” Phys. Rev. Lett. 92(18), 186601 (2004). 23. A. Franceschetti, J. M. An, and A. Zunger, “Impact ionization can explain carrier multiplication in PbSe quantum dots,” Nano Lett. 6(10), 2191–2195 (2006). 24. M. Wolf, R. Brendel, J. H. Werner, and H. J. Queisser, “Solar cell efficiency and carrier multiplication in Si1xGex alloys,” J. Appl. Phys. 83(8), 4213–4221 (1998). 25. C.-Y. Huang, D.-Y. Wang, C.-H. Wang, Y.-T. Chen, Y.-T. Wang, Y.-T. Jiang, Y.-J. Yang, C.-C. Chen, and Y.F. Chen, “Efficient light harvesting by photon downconversion and light trapping in hybrid ZnS nanoparticles/Si nanotips solar cells,” ACS Nano 4(10), 5849–5854 (2010). 26. Y.-J. Lee, C.-J. Lee, and C.-M. Cheng, “Enhancing the conversion efficiency of red emission by spin-coating CdSe quantum dots on the green nanorod light-emitting diode,” Opt. Express 18(S4), A554–A561 (2010). 27. H. Kato, S. Adachi, H. Nakanishi, and K. Ohtsuka, “Optical properties of (AlxGa1-x)0.5In0.5P quaternary alloys,” Jpn. J. Appl. Phys. 33(1A), 186–192 (1994). 28. M. Bass, C. DeCusatis, J. Enoch, V. Lakshminarayanan, G. Li, C. MacDonald, V. Mahajan, and E. V. Stryland, Handbook of Optics, Third Edition Volume IV: Optical Properties of Materials, Nonlinear Optics, Quantum Optics (set) (McGraw Hill Professional, New York, 2009). 29. D. Souri and K. Shomalian, “Band gap determination by absorption spectrum fitting method (ASF) and structural properties of different compositions of (60−x) V2O5–40TeO2–xSb2O3 glasses,” J. Non-Cryst. Solids 355(31–33), 1597–1601 (2009). 30. ASTMG173–03, Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37 degree Tilted Surface (ASTM International, West Conshohocken, Pennsylvania, 2005). 31. P. Bermel, C. Luo, L. Zeng, L. C. Kimerling, and J. D. Joannopoulos, “Improving thin-film crystalline silicon solar cell efficiencies with photonic crystals,” Opt. Express 15(25), 16986–17000 (2007).
1. Introduction 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 [1–11]. 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 [12–15]. 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
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A954
single-junction solar cell estimated by the Shockley-Queisser’s calculation scheme [16,17]. 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 [18]. 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 [19]. 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 [20]. 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 [21–23]. 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 [24]. 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 [25]. 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. 2. Experiment and simulation Figure 1(a) shows a schematic configuration of the proposed structure. The three latticematched subcells of the triple-junction solar cell from top to bottom in order are the InGaP, GaAs, and Ge subcells, grown on the p-type Ge substrate by low-pressure metal-organic chemical vapor deposition (MOCVD). To form a QD suspension, CdSe QDs were dispersed onto the top AlInP-window layer by spin-casting that covered the metal electrode (further details of the CdSe QDs synthesis procedure is reported in our previous work [26]). Onto the sample was dropped a fixed volume (~125 μL) of colloid QDs at various concentrations in toluene solution; it was then spin-cast at 2500 r.p.m for 10s to disperse CdSe QDs uniformly. The sample was then placed under a fume hood, where it stood for one minute to evaporate off toluene to enable light J-V measurements to be made. Trimethyl sources of aluminum, gallium and indium were used as group-III precursors, and arsine and phosphine were used for the group-V reaction agents. Silane (SiH4) and diethylzinc (DEZn) were used as the ntype and p-type dopant sources, respectively. The InGaP and GaAs subcells are connected to each other by a p-AlGaAs (p = 4 × 1020 cm−3, 20 nm)/n-InGaP (n = 1 × 1020 cm−3, 20 nm) tunnel junction, whereas the GaAs middle subcell is connected to the Ge bottom subcell by a p-GaAs (p = 6 × 1019 cm−3, 30 nm)/n-GaAs (n = 1 × 1020 cm−3, 30 nm) tunnel junction. Silver was chosen as the metal electrodes for both the front and backside contacts. The chip size of
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A955
the individual cell is designed to be 1 cm × 1 cm. The device’s performance was characterized by an Oriel Sol3A solar simulator. The current density vs. voltage (J-V) characteristic was obtained using a Keithley 2400 multi-meter in four-wire sensing mode to eliminate the resistance contribution from the probes and the contact resistances.
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.
To systematically analyze the dependence of the QD’s dimension on the device’s efficiency, we developed a simple model based on the fundamentals of material optics that simulates the energy distribution of the solar spectrum converted by CdSe QDs on each individual subcell. As a result, the power conversion efficiency of the device with CdSe QDs of different sizes can be quantitatively determined and compared. The solar cell device [Fig. 1(a)] was simplified as an InGaP/GaAs/Ge multi-stacking layer to facilitate the optical calculation, as shown in Fig. 1(b). The p-type and n-type active regions are combined as a single layer, and the metal electrode on the top surface is neglected. The solar light was normally incident from the top surface down through the whole device. The optical dispersion is also considered by applying the wavelength-dependent refractive index and the extinction coefficient on each subcell [27, 28]. As the energy bandgap of the QDs is well known to be dominated by the diameters of the QDs, the incident spectrum and radiative intensity of the solar light inside the device can be tuned. In Fig. 1(b), the solar light is incident onto the CdSe QDs, and the solar light with high energy was partially absorbed by the QDs and re-emitted as radiation with photon energies that equaled the bandgap of the CdSe QDs. Hence, when CdSe QDs of the necessary diameters are applied to the top of a tandem solar cell, with the radiation of the CdSe QDs effectively generates more photocurrent that is supplied to the current-limiting subcell which has the lowest photocurrent of the three subcells, promoting the overall power conversion efficiency of the device. The formula for the incident intensity I0i and the transmitted light intensity I0t can be expressed as follows: I A = log 0i I 0t
(1)
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A956
Here, A denotes the absorbance of the CdSe QDs, which can essentially be determined by measuring the absorption spectrum [29], and I0i and I0t indicate the intensity inside the CdSe QDs. The CdSe QDs’ absorbance can also be defined as follows:
α0 =
(ln10) ⋅ A L0
(2)
where α0 and L0 are the absorption coefficient and the layer thickness of the CdSe QDs, respectively. By considering the optical reflection at the interface between the air and the CdSe QDs, the incident intensity of solar light inside the top of the CdSe QDs is given by the following: I 0i = I AM 1.5G ⋅ (1 − R0 ) nair − nQD R0 = n +n QD air
2
(3)
where IAM1.5G denotes the sun’s light intensity given by the ASTM AM 1.5 solar spectrum [30] and nair and nQD are the refractive indices of the air and the CdSe QDs, respectively. Restated, the CdSe QDs absorb the high-energy regime of the incident solar light and re-emit radiation with an energy intensity IPL that is equivalent to the QDs’ band-gap energy, which can be directly acquired by the photoluminescence (PL) measurement. Therefore, by considering the optical conversion of the CdSe QDs and the optical reflection at the dotInGaP interface, the incident intensity of the solar light inside the top surface of the InGaP subcell can be described as follows:
I1i = I 0t ⋅ (1 − R1 ) + I PL = I 0i ⋅10− A ⋅ (1 − R1 ) + I PL
(4)
Again, R1 is the optical reflection at the dot-InGaP interface and is defined as R1 = ( (nQD − nInGaP ) / ((nQD + nInGaP ) ) , where nInGaP is the refractive index of the InGaP 2
subcell. The incident intensity of the solar light I1i is attenuated to I1t as the light travels through the InGaP subcell with a thickness of L1: I1t = I1i ⋅ exp[−(α1 L1 )]
α1 =
4πκ1
(5)
λ
where α1 and κ1 represent the absorption and extinction coefficients of the InGaP subcell, respectively. In the same way that we derived Eqs. (3) and (5), the incident intensities I2i and I3i inside the top surface of the GaAs and Ge subcells can be written as follows: I 2i = I1t ⋅ (1 − R2 ) = I1i ⋅ exp[−(α1 L1 )] ⋅ (1 − R2 ) I 3i = I 2t ⋅ (1 − R3 ) = I 2i ⋅ exp[−(α 2 L2 )] ⋅ (1 − R3 )
(6)
where R2 and R3 are the optical reflections at the InGaP/GaAs and GaAs/Ge interfaces, respectively, and can also be determined by their wavelength-dependent refractive indices, as illustrated in Eq. (3). α2 and L2 are the absorption coefficient and the cell thickness of the GaAs subcell, respectively. Consequently, the optical absorption A(1,2,3) (λ ) of each subcell can be determined by the difference between the incident and transmitted light intensities inside the subcell: A(1,2,3) (λ ) = I (1,2,3)i − I (1,2,3) t = I (1,2,3) i ⋅ [1 − exp(−α (1,2,3) L(1,2,3) )]
(7)
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A957
where the subscripts 1, 2, and 3 stand for the InGaP, GaAs, and Ge subcells, respectively. To calculate the power generation efficiency of each subcell, we assume that each absorbed photon with an energy larger than the subcell’s band-gap energy generates an electron-hole pair and consider the quantum efficiency of each subcell QE(1,2,3) (λ ) . Therefore, the shortcircuit current density J (1,2,3) versus applied voltage of each subcell is expressed by the sum of the photon-generated current minus the intrinsic current generated by radiative recombination as follow [31]: J (1, 2 , 3 ) (V ) =
q
hc
∞
0
λ ⋅ I (1, 2 , 3 ) i ⋅ {1 − exp[ − (α (1, 2 , 3 ) L(1, 2 ,3 ) )]} ⋅ QE(1, 2 , 3 ) ( λ ) d λ −
q ( n(1, 2 ,3 ) + 1) E g (1, 2 ,3 ) kT 2
2
4π c 3
2
exp(
eV − E g (1, 2 , 3 ) kT
)
(8) where Eg (1,2,3) is the band-gap energy of each subcell, kT is the thermal energy at the operating temperature T in Kelvin unit, and n(1,2,3) is the refractive index of each subcell. By substituting the proper values of I (1,2,3)i and QE(1,2,3) (λ ) , we can immediately estimate the J-V characteristics of each subcell. Hence, the dependence of the CdSe QD’s size on the power conversion efficiency of the InGaP/GaAs/Ge triple-junction solar cell can also be determined. This calculation also helps us to determine the dimension of the CdSe QD that optimizes the power conversion efficiency. 3. Results and discussion
Figure 2 shows the measured (a) absorbance and (b) PL spectra of CdSe QDs of different sizes in toluene. Accordingly, the reduced dimensionality of the CdSe QDs exhibits quantization of electronic energy levels, and consequently, a blue shift of the optical absorption edge occurs. The optical absorption edge shifts from ~700 nm to ~500 nm as the diameters of the CdSe QDs decrease from D = 6.6 nm to D = 2.1 nm. At a given diameter, the optical absorption increases as the emitting wavelength of the excitation source decreases. Similarly, the main peak of the CdSe QD emission is blue-shifted as the diameters of the CdSe QDs decrease. The main PL peaks of λpeak = 640 nm, λpeak = 610 nm, λpeak = 590 nm, λpeak = 560 nm, λpeak = 520 nm, and λpeak = 480 nm were observed at CdSe QD diameters of D = 6.6 nm, D = 5.0 nm, D = 4.2 nm, D = 3.3 nm, D = 2.5 nm, and D = 2.1 nm, respectively. Additionally,
Fig. 2. (a) Absorption and (b) photoluminescence spectra of CdSe QDs of different sizes in toluene.
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A958
all of the PL intensity exhibits a similar full width at half maximum of FWHM = 32 nm. The above observations are direct evidence that CdSe QDs demonstrate wavelength conversion for incident photons and that the conversion interval is mainly dominated by the diameters of the QDs. By substituting the absorbance and the PL intensity measured above into Eqs. (4)-(6), Fig. 3 plots the calculated light intensity of the solar spectrum I(1,2,3)i(λ) distributed on each individual subcell for the device (a) without and (b) with CdSe QDs with diameters of D = 2.1 nm. The quantum efficiency of each subcell QE(1,2,3)(λ) was also plotted in the figure and will be employed with I(1,2,3)i(λ) to derive the J-V characteristic of the device by Eq. (8). In this case, the CdSe QDs mainly convert the ultraviolet light into visible light, and hence, the peak intensity of I1i (λpeak = 480 nm) is even stronger than that of the original solar spectrum. Additionally, the InGaP subcell exhibits the highest QE response to the incident wavelength of ~500 nm. We therefore expect that applying CdSe QDs with diameters of D = 2.1 nm primarily enhances the photocurrent of the InGaP subcell. The light intensities of the solar spectra on the GaAs and Ge subcells are also enhanced because the CdSe QDs provide an additional functionality as an antireflection coating for light trapping of the solar light [25].
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.
Figure 4 plots the 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. As expected, due to the unoptimized dimensions of the CdSe QDs, the enhancement of the photocurrent is mainly observed in the InGaP subcell. By applying CdSe QDs, the calculated short-circuit current density of the InGaP subcell is considerably enhanced from JSC = 9.60 mA to JSC = 12.01 mA. However, as in the previous discussion of Fig. 3, the enhancement of the solar intensity in the GaAs subcell is insufficient; therefore, the short-circuit current density of that subcell is only slightly boosted from JSC = 9.38 mA to JSC = 10.19 mA, and the overall power conversion efficiency of the device increases from η = 25.12% to η = 26.75% (blue dashline). Restated, the enhanced photocurrents of the GaAs and Ge subcells are mainly attributed to the reduction of optical reflection because the CdSe QDs also serve as an antireflection coating for the long spectral regime.
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A959
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.
Next, we are going to investigate the influence of the CdSe QDs’ dimensions on the device’s performance. The absorbance and PL spectra of the CdSe QDs of different dimensions measured in Fig. 2 are substituting into Eq. (8), and the results are summarized in Fig. 5. Figure 5 shows (a) the short-circuit current density of each subcell and (b) the power conversion efficiency of the device as a function of the CdSe QD’s diameter. Accordingly, the JSC of the Ge subcell is slightly boosted to JSC ~12.70 mA/cm2, and is barely affected by variations of the QD’s diameter, as the photon conversion effect of the CdSe QDs that is used in this study occurs mainly for solar light with an incident wavelength of λ
Institute of Electro-Optical Science and Technology, National Taiwan Normal University, 88, Sec.4, Ting-Chou Road, Taipei 116, Taiwan 2 Department of Electrical Engineering, Chang Gung University, Tao-Yuan 333, Taiwan 3 WIN Semiconductors Corp, No.358, Hwaya 2th Rd., Hwaya Technology Park, Tao-Yuan 333, Taiwan *[email protected]
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 OCIS codes: (310.1210) Antireflection coatings; (040.5350) Photovoltaic; (220.4241) Nanostructure fabrication.
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#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A953
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1. Introduction 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 [1–11]. 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 [12–15]. 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
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A954
single-junction solar cell estimated by the Shockley-Queisser’s calculation scheme [16,17]. 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 [18]. 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 [19]. 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 [20]. 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 [21–23]. 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 [24]. 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 [25]. 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. 2. Experiment and simulation Figure 1(a) shows a schematic configuration of the proposed structure. The three latticematched subcells of the triple-junction solar cell from top to bottom in order are the InGaP, GaAs, and Ge subcells, grown on the p-type Ge substrate by low-pressure metal-organic chemical vapor deposition (MOCVD). To form a QD suspension, CdSe QDs were dispersed onto the top AlInP-window layer by spin-casting that covered the metal electrode (further details of the CdSe QDs synthesis procedure is reported in our previous work [26]). Onto the sample was dropped a fixed volume (~125 μL) of colloid QDs at various concentrations in toluene solution; it was then spin-cast at 2500 r.p.m for 10s to disperse CdSe QDs uniformly. The sample was then placed under a fume hood, where it stood for one minute to evaporate off toluene to enable light J-V measurements to be made. Trimethyl sources of aluminum, gallium and indium were used as group-III precursors, and arsine and phosphine were used for the group-V reaction agents. Silane (SiH4) and diethylzinc (DEZn) were used as the ntype and p-type dopant sources, respectively. The InGaP and GaAs subcells are connected to each other by a p-AlGaAs (p = 4 × 1020 cm−3, 20 nm)/n-InGaP (n = 1 × 1020 cm−3, 20 nm) tunnel junction, whereas the GaAs middle subcell is connected to the Ge bottom subcell by a p-GaAs (p = 6 × 1019 cm−3, 30 nm)/n-GaAs (n = 1 × 1020 cm−3, 30 nm) tunnel junction. Silver was chosen as the metal electrodes for both the front and backside contacts. The chip size of
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A955
the individual cell is designed to be 1 cm × 1 cm. The device’s performance was characterized by an Oriel Sol3A solar simulator. The current density vs. voltage (J-V) characteristic was obtained using a Keithley 2400 multi-meter in four-wire sensing mode to eliminate the resistance contribution from the probes and the contact resistances.
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.
To systematically analyze the dependence of the QD’s dimension on the device’s efficiency, we developed a simple model based on the fundamentals of material optics that simulates the energy distribution of the solar spectrum converted by CdSe QDs on each individual subcell. As a result, the power conversion efficiency of the device with CdSe QDs of different sizes can be quantitatively determined and compared. The solar cell device [Fig. 1(a)] was simplified as an InGaP/GaAs/Ge multi-stacking layer to facilitate the optical calculation, as shown in Fig. 1(b). The p-type and n-type active regions are combined as a single layer, and the metal electrode on the top surface is neglected. The solar light was normally incident from the top surface down through the whole device. The optical dispersion is also considered by applying the wavelength-dependent refractive index and the extinction coefficient on each subcell [27, 28]. As the energy bandgap of the QDs is well known to be dominated by the diameters of the QDs, the incident spectrum and radiative intensity of the solar light inside the device can be tuned. In Fig. 1(b), the solar light is incident onto the CdSe QDs, and the solar light with high energy was partially absorbed by the QDs and re-emitted as radiation with photon energies that equaled the bandgap of the CdSe QDs. Hence, when CdSe QDs of the necessary diameters are applied to the top of a tandem solar cell, with the radiation of the CdSe QDs effectively generates more photocurrent that is supplied to the current-limiting subcell which has the lowest photocurrent of the three subcells, promoting the overall power conversion efficiency of the device. The formula for the incident intensity I0i and the transmitted light intensity I0t can be expressed as follows: I A = log 0i I 0t
(1)
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A956
Here, A denotes the absorbance of the CdSe QDs, which can essentially be determined by measuring the absorption spectrum [29], and I0i and I0t indicate the intensity inside the CdSe QDs. The CdSe QDs’ absorbance can also be defined as follows:
α0 =
(ln10) ⋅ A L0
(2)
where α0 and L0 are the absorption coefficient and the layer thickness of the CdSe QDs, respectively. By considering the optical reflection at the interface between the air and the CdSe QDs, the incident intensity of solar light inside the top of the CdSe QDs is given by the following: I 0i = I AM 1.5G ⋅ (1 − R0 ) nair − nQD R0 = n +n QD air
2
(3)
where IAM1.5G denotes the sun’s light intensity given by the ASTM AM 1.5 solar spectrum [30] and nair and nQD are the refractive indices of the air and the CdSe QDs, respectively. Restated, the CdSe QDs absorb the high-energy regime of the incident solar light and re-emit radiation with an energy intensity IPL that is equivalent to the QDs’ band-gap energy, which can be directly acquired by the photoluminescence (PL) measurement. Therefore, by considering the optical conversion of the CdSe QDs and the optical reflection at the dotInGaP interface, the incident intensity of the solar light inside the top surface of the InGaP subcell can be described as follows:
I1i = I 0t ⋅ (1 − R1 ) + I PL = I 0i ⋅10− A ⋅ (1 − R1 ) + I PL
(4)
Again, R1 is the optical reflection at the dot-InGaP interface and is defined as R1 = ( (nQD − nInGaP ) / ((nQD + nInGaP ) ) , where nInGaP is the refractive index of the InGaP 2
subcell. The incident intensity of the solar light I1i is attenuated to I1t as the light travels through the InGaP subcell with a thickness of L1: I1t = I1i ⋅ exp[−(α1 L1 )]
α1 =
4πκ1
(5)
λ
where α1 and κ1 represent the absorption and extinction coefficients of the InGaP subcell, respectively. In the same way that we derived Eqs. (3) and (5), the incident intensities I2i and I3i inside the top surface of the GaAs and Ge subcells can be written as follows: I 2i = I1t ⋅ (1 − R2 ) = I1i ⋅ exp[−(α1 L1 )] ⋅ (1 − R2 ) I 3i = I 2t ⋅ (1 − R3 ) = I 2i ⋅ exp[−(α 2 L2 )] ⋅ (1 − R3 )
(6)
where R2 and R3 are the optical reflections at the InGaP/GaAs and GaAs/Ge interfaces, respectively, and can also be determined by their wavelength-dependent refractive indices, as illustrated in Eq. (3). α2 and L2 are the absorption coefficient and the cell thickness of the GaAs subcell, respectively. Consequently, the optical absorption A(1,2,3) (λ ) of each subcell can be determined by the difference between the incident and transmitted light intensities inside the subcell: A(1,2,3) (λ ) = I (1,2,3)i − I (1,2,3) t = I (1,2,3) i ⋅ [1 − exp(−α (1,2,3) L(1,2,3) )]
(7)
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A957
where the subscripts 1, 2, and 3 stand for the InGaP, GaAs, and Ge subcells, respectively. To calculate the power generation efficiency of each subcell, we assume that each absorbed photon with an energy larger than the subcell’s band-gap energy generates an electron-hole pair and consider the quantum efficiency of each subcell QE(1,2,3) (λ ) . Therefore, the shortcircuit current density J (1,2,3) versus applied voltage of each subcell is expressed by the sum of the photon-generated current minus the intrinsic current generated by radiative recombination as follow [31]: J (1, 2 , 3 ) (V ) =
q
hc
∞
0
λ ⋅ I (1, 2 , 3 ) i ⋅ {1 − exp[ − (α (1, 2 , 3 ) L(1, 2 ,3 ) )]} ⋅ QE(1, 2 , 3 ) ( λ ) d λ −
q ( n(1, 2 ,3 ) + 1) E g (1, 2 ,3 ) kT 2
2
4π c 3
2
exp(
eV − E g (1, 2 , 3 ) kT
)
(8) where Eg (1,2,3) is the band-gap energy of each subcell, kT is the thermal energy at the operating temperature T in Kelvin unit, and n(1,2,3) is the refractive index of each subcell. By substituting the proper values of I (1,2,3)i and QE(1,2,3) (λ ) , we can immediately estimate the J-V characteristics of each subcell. Hence, the dependence of the CdSe QD’s size on the power conversion efficiency of the InGaP/GaAs/Ge triple-junction solar cell can also be determined. This calculation also helps us to determine the dimension of the CdSe QD that optimizes the power conversion efficiency. 3. Results and discussion
Figure 2 shows the measured (a) absorbance and (b) PL spectra of CdSe QDs of different sizes in toluene. Accordingly, the reduced dimensionality of the CdSe QDs exhibits quantization of electronic energy levels, and consequently, a blue shift of the optical absorption edge occurs. The optical absorption edge shifts from ~700 nm to ~500 nm as the diameters of the CdSe QDs decrease from D = 6.6 nm to D = 2.1 nm. At a given diameter, the optical absorption increases as the emitting wavelength of the excitation source decreases. Similarly, the main peak of the CdSe QD emission is blue-shifted as the diameters of the CdSe QDs decrease. The main PL peaks of λpeak = 640 nm, λpeak = 610 nm, λpeak = 590 nm, λpeak = 560 nm, λpeak = 520 nm, and λpeak = 480 nm were observed at CdSe QD diameters of D = 6.6 nm, D = 5.0 nm, D = 4.2 nm, D = 3.3 nm, D = 2.5 nm, and D = 2.1 nm, respectively. Additionally,
Fig. 2. (a) Absorption and (b) photoluminescence spectra of CdSe QDs of different sizes in toluene.
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A958
all of the PL intensity exhibits a similar full width at half maximum of FWHM = 32 nm. The above observations are direct evidence that CdSe QDs demonstrate wavelength conversion for incident photons and that the conversion interval is mainly dominated by the diameters of the QDs. By substituting the absorbance and the PL intensity measured above into Eqs. (4)-(6), Fig. 3 plots the calculated light intensity of the solar spectrum I(1,2,3)i(λ) distributed on each individual subcell for the device (a) without and (b) with CdSe QDs with diameters of D = 2.1 nm. The quantum efficiency of each subcell QE(1,2,3)(λ) was also plotted in the figure and will be employed with I(1,2,3)i(λ) to derive the J-V characteristic of the device by Eq. (8). In this case, the CdSe QDs mainly convert the ultraviolet light into visible light, and hence, the peak intensity of I1i (λpeak = 480 nm) is even stronger than that of the original solar spectrum. Additionally, the InGaP subcell exhibits the highest QE response to the incident wavelength of ~500 nm. We therefore expect that applying CdSe QDs with diameters of D = 2.1 nm primarily enhances the photocurrent of the InGaP subcell. The light intensities of the solar spectra on the GaAs and Ge subcells are also enhanced because the CdSe QDs provide an additional functionality as an antireflection coating for light trapping of the solar light [25].
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.
Figure 4 plots the 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. As expected, due to the unoptimized dimensions of the CdSe QDs, the enhancement of the photocurrent is mainly observed in the InGaP subcell. By applying CdSe QDs, the calculated short-circuit current density of the InGaP subcell is considerably enhanced from JSC = 9.60 mA to JSC = 12.01 mA. However, as in the previous discussion of Fig. 3, the enhancement of the solar intensity in the GaAs subcell is insufficient; therefore, the short-circuit current density of that subcell is only slightly boosted from JSC = 9.38 mA to JSC = 10.19 mA, and the overall power conversion efficiency of the device increases from η = 25.12% to η = 26.75% (blue dashline). Restated, the enhanced photocurrents of the GaAs and Ge subcells are mainly attributed to the reduction of optical reflection because the CdSe QDs also serve as an antireflection coating for the long spectral regime.
#194254 - $15.00 USD Received 22 Jul 2013; revised 13 Sep 2013; accepted 15 Sep 2013; published 20 Sep 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. S6 | DOI:10.1364/OE.21.00A953 | OPTICS EXPRESS A959
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.
Next, we are going to investigate the influence of the CdSe QDs’ dimensions on the device’s performance. The absorbance and PL spectra of the CdSe QDs of different dimensions measured in Fig. 2 are substituting into Eq. (8), and the results are summarized in Fig. 5. Figure 5 shows (a) the short-circuit current density of each subcell and (b) the power conversion efficiency of the device as a function of the CdSe QD’s diameter. Accordingly, the JSC of the Ge subcell is slightly boosted to JSC ~12.70 mA/cm2, and is barely affected by variations of the QD’s diameter, as the photon conversion effect of the CdSe QDs that is used in this study occurs mainly for solar light with an incident wavelength of λ
