Abstract
Semiconductor nanowires are a promising candidate for nextgeneration solar cells. However, the optical response of nanowires is, due to diffraction effects, complicated to optimize. Here, we optimize through optical modeling the absorption in a dualjunction nanowirearray solar cell in terms of the ShockleyQuessier detailed balance efficiency limit. We identify efficiency maxima that originate from resonant absorption of photons through the HE11 and the HE12 waveguide modes in the top cell. An efficiency limit above 40% is reached in the band gap optimized Al_{0.10}Ga_{0.90}As/In_{0.34}Ga_{0.66}As system when we allow for different diameter for the top and the bottom nanowire subcell. However, for experiments, equal diameter for the top and the bottom cell might be easier to realize. In this case, we find in our modeling a modest 1–2% drop in the efficiency limit. In the Ga_{0.51}In_{0.49}P/InP system, an efficiency limit of η = 37.3% could be reached. These efficiencies, which include reflection losses and suboptimal absorption, are well above the 31.0% limit of a perfectlyabsorbing, idealized singlejunction bulk cell and close to the 42.0% limit of the idealized dualjunction bulk cell. Our results offer guidance in the choice of materials and dimensions for nanowires with potential for high efficiency tandem solar cells.
Introduction
The use of IIIV nanowires for pin junction solar cells is an emerging avenue for photovoltaics^{1,2,3,4,5,6}. Both single wire^{4,6,7} and largearea nanowire array^{5,8,9} devices show promise for next generation solar cells. Already for single nanowire systems, diffraction of light can lead to resonant coupling of light into the nanowire with several absorption peaks as a function of wavelength^{10,11}. Optimization of the geometry of the singlenanowire geometry is necessary to obtain maximum photocurrent^{12} and open circuit voltage^{4,12}. The resonances can lead to a 20 times stronger absorption per volume semiconductor material in a IIIV nanowire as compared to a bulk sample^{13}.
An array of nanowires gives in turn access to largearea devices when higher output power is needed. For such arrays, an efficiency of 13.8% has been demonstrated using InP nanowires with a single pin junction in the axial direction^{5} and an efficiency of 15.3% has been reached with GaAs nanowires^{9}. However, the use of a single material gives an upper limit for the amount of sun light that can be converted into electrical energy^{14}, due to two reasons. First, the energy of photons with energy below the band gap energy of the semiconductor cannot be utilized since those lowenergy photons cannot be absorbed. Second, a large part of the energy of absorbed highenergy photons is wasted due to thermalization. In this thermalization process, the photogenerated electrons and holes relax in energy to their respective band edges.
To reach higher efficiencies in solar cells, an avenue is to use multiple semiconductors, epitaxially grown on top of each other^{15}. See Fig. 1 for a system with two different semiconductor materials, where one material is used in the top cell and a different material in the bottom cell. The idea in such a tandem device is to absorb high energy photons in a high band gap top cell. In that top cell, the thermalization loss of the high energy photons is decreased compared to the single junction cell. The lower energy photons continue to the bottom cell where they are absorbed. Due to the lower band gap of the bottom cell than in the single junction cell, more photons are absorbed than in the single junction cell. In this way, the tandem cell can absorb more photons than the single junction cell, while at the same time having reduced thermalization losses. However, in planar cells, the crystal lattice constant between materials in adjacent subcells/layers should be matched to yield highquality materials without performance limiting dislocations. Such requirements on crystallattice matching limit strongly the choice of materials for tandem cells.
Nanowire structures offer a clear benefit for multijunction solar cells compared with planar cells. Efficient strain relaxation in nanowires allows for the fabrication and combination of dislocationfree, highly latticemismatched materials^{16,17,18,19}. Furthermore, IIIV semiconductor nanowire arrays can in principle be fabricated on top of a Si substrate^{19}, giving the prospect of using the Si substrate as the bottom cell^{2,20,21,22,23}.
Thus, nanowires offer freedom for the material choice in multijunction solar cells, making it easy to reach optimum material combinations to match the solar spectrum. Furthermore, the resonant absorption by designing the nanowire geometry holds the prospect of lower material usage than in thinfilms^{13}. Therefore, to enable highefficiency nanowire tandem solar cells, we need to understand the optimum choice of materials for the subcells as well as the optimum nanowire geometry to have the best absorption characteristics for photovoltaics. Already for single junction nanowirearray cells, we know that both the array pitch and the nanowire diameter need to be optimized simultaneously. At the same time, the optimum diameter depends on the band gap of the solar cell, that is, on the material choice^{8}.
Here, we perform optical modeling to calculate and optimize the absorption of light in a dual junction tandem nanowire solar cell (Fig. 1) with the scattering matrix method^{1,8,13,24,25}. This modeling allows us to perform a ShockleyQueisser detailed balance analysis to study and optimize the efficiency potential of the nanowire solar cell as a function of material choice and geometrical design of the nanowires. We show that an efficiency limit above 40% can be reached in the band gap optimized Al_{0.10}Ga_{0.90}As/In_{0.34}Ga_{0.66}As system when we allow for different diameter D_{top} and D_{bot} for the top and the bottom subcell. However, for experiments, the case of D_{top} = D_{bot} might be easier to realize. In this case, we find a 1–2% drop in the efficiency. In the experimentally relevant Ga_{0.51}In_{0.49}P/InP system, an efficiency limit of η = 37.3% is reached for a nanowire length of 13 μm when using equal diameters of D_{top} = D_{bot} = 160 nm and a pitch P = 380 nm (we analyze also the effect of varying nanowire length, with results summarized in Table 1). These efficiencies for nanowire tandem cells are well above the 31.0% limit of an idealized, perfectly absorbing singlejunction bulk cell and close to the 42.0% limit of the idealized, band gap optimized dualjunction bulk cell.
Material choice for the top and the bottom cell in a nanowire tandem solar cell
To choose the materials for the top and the bottom nanowire subcell, we perform the wellknown ShockleyQueisser detailed balance analysis^{14} assuming first perfect absorption of above band gap photons in each subcell^{22,26}. This analysis corresponds to the case when each subcell absorbs optimally, without reflection losses. The specific assumption and technical details of the analysis can be found in the Supplementary Information.
Importantly, we assume that cell 1 absorbs all photons of energy above E_{1}, the band gap energy of cell 1. Cell 2 absorbs in turn all photons with energies between E_{1} and E_{2}, the band gap energy of cell 2. We assume that A_{1}(λ) = 1 for λ < λ_{1,bg} and A_{1}(λ) = 0 otherwise. Here, A_{1}(λ) [A_{2}(λ)] is the absorption spectrum of cell 1 (2), that is, the fraction of incident light of wavelength λ absorbed in cell 1 (2). Similarly, we assume that A_{2}(λ) = 1 for λ_{1,bg} < λ < λ_{2,bg} and A_{2}(λ) = 0 otherwise. Here, λ_{1,bg} = 2πћc/E_{1} and λ_{2,bg} = 2πћc/E_{2}. In this way, we find the materials that maximize the efficiency limit of the nanowire solar cell when the geometry is designed for optimum absorption (Fig. 2). Note that below, in the section Geometry Design, when considering the effect of the nanowire geometry on the absorption, we model the absorption spectra A_{1}(λ) ≤ 1 and A_{2}(λ) ≤ 1 for each choice of the geometry, which includes suboptimal absorption and varying reflection losses.
ShockleyQueisser detailed balance efficiency as a function of material band gaps for perfectly absorbing subcells.
The maximum efficiency η = 42.0% shows up at E_{1} = 1.58 eV and E_{2} = 0.95 eV for the top and the bottom cell band gap, respectively. Notice that in this analysis for the perfectly absorbing subcells, for E_{1} < E_{2} the bottom cell (cell 2) does not absorb any photons. Thus, j_{2} = 0 and consecutively the current through this currentmatched seriesconnected solar cell is zero, leading to η = 0. The inset shows the efficiency limit for varying top cell band gap for the case of an InP bottom cell, that is, when E_{2} = 1.34 eV.
We find a maximum efficiency of 42.0% when the band gaps of the top and the bottom cell are E_{1} = 1.58 eV and E_{2} = 0.95 eV respectively (Fig. 2). Note that the results in Fig. 2 are in good agreement with previous detailedbalance calculations of multijunction bulk cells^{22,26}. In our case, for the modeling of the emission to the substrate, we use a refractive index of n = 3.5 to represent the InP substrate. We note that the emission of photons into this highrefractive index substrate has caused a 3% decrease in this maximum efficiency.
To choose the IIIV materials for the nanowire subcells, we calculated first the band gap for varying ternary compounds^{27}. After this, we investigated which ternary compounds have tabulated, experimentally determined, reliable refractive index values available for the optics modelling. Among the ternaries for which such refractive index data were readily available, we identified Al_{0.10}Ga_{0.90}As (band gap of 1.55 eV^{27} and refractive index from ref. [28]) for the top cell and In_{0.34}Ga_{0.66}As (band gap of 0.95 eV^{27} and refractive index from ref. [29]) for the bottom cell as a good material combination with an efficiency limit of 40.7%.
However, we could imagine that the fabrication of a dualjunction nanowire solar cell could benefit from the knowledge and control of the fabrication of singlejunction nanowire solar cells. In this case, the natural candidates are the wellperforming InP^{5} and GaAs^{9}. The band gaps of both these materials work well for the bottom cell (see inset in Fig. 2 for an InP bottom cell and Supplementary Figure S1 for a GaAs bottom cell). However, the surface recombination velocity of unpassivated GaAs can be five orders of magnitude higher than that of unpassivated InP^{30}. Therefore, GaAs nanowires need dedicated surface passivation schemes,^{9} whereas the requirement on surface passivation is relieved for InP nanowires^{5}. Therefore, we chose to concentrate on an InP bottom cell. Here, a maximum efficiency of 38.6% is found with a top cell band gap energy of 1.86 eV (inset of Fig. 2). We note that Ga_{0.51}In_{0.49}P, for which refractive index data is available^{31}, has a band gap energy of 1.85 eV^{31}, giving an efficiency limit of 38.5% in the tandem configuration with InP. Depending on the surface properties of the GaInP, this GaInP/InP system could perhaps even provide the prospect of high efficiency without dedicated surface passivation schemes. Thus, we study the efficiency limit of both the AlGaAs/InGaAs and the GaInP/InP system.
Geometry Design
After choosing the materials for the top and the bottom cell as described above (the nearly band gap optimized Al_{0.10}Ga_{0.90}As/In_{0.34}Ga_{0.66}As system as well as the technologically relevant Ga_{0.51}In_{0.49}P/InP system), we turn to consider the geometry of the nanowire subcells (Fig. 1). There are five geometry parameters: the length of each subcell (L_{top} and L_{bot}), the diameter of each subcell (D_{top} and D_{bot}) and the pitch (P) of the square array, which need to be optimized with respect to the absorption (A_{1}(λ) and A_{2}(λ)) of light in each subcell.
Different computational methods, such as the finiteelement method (FEM)^{3,32,33}, the rigorous coupled wave analysis (RCWA)^{4,6} and the scattering matrix method^{1,8,13,24,25}, have been used for studying the diffraction and absorption of light in nanostructures through the solution of the Maxwell equations, which give results in good agreement with experiments^{13}. We chose to employ the scattering matrix method to solve the Maxwell equations for normally incident light in order to calculate the absorption spectrum A_{1(2)}(λ) of the nanowire top and bottom cells. We use tabulated refractive index values n(λ) for the Al_{0.10}Ga_{0.90}As^{28}, In_{0.34}Ga_{0.66}As^{29}, Ga_{0.51}In_{0.49}P^{31} and InP^{34}. We then calculate the ShockleyQueisser detailed balance efficiency (see Supplementary Information Eqs. (S1)–(S6) for technical details).
Note that the optics modeling is done with the nanowires on top of an InP substrate (see Fig. 1). However, absorption of light in the substrate does not contribute to the current or voltage of the solar cell in our analysis. Thus, the substrate functions optically merely to partially reflect the light that reaches the substrate. Therefore, a change to a different substrate, like the lessexpensive Si^{19}, with similar n ≈ 3.5 as the InP would give very similar absorption spectra.
We start by considering the case of Al_{0.10}Ga_{0.90}As (band gap of E_{1} = 1.55 eV) for the top cell and In_{0.34}Ga_{0.66}As (band gap of E_{2} = 0.95 eV) for the bottom cell, which was found to be a good band gap combination with efficiency limit of 40.7% for perfectly absorbing subcells.
It is known that the nanowire diameter affects strongly the absorption of light^{1,5,7,8,13,35}. Therefore, to study the effect of the nanowire diameter on the absorption in the tandem cell, we fix P = 530 nm, L_{top} = 2000 nm and L_{bot} = 2900 nm [Fig. 3(a)]. As a main feature: the efficiency appears to be a function of just D_{top} when D_{bot} is large enough (typically when D_{bot} > 250 nm). In this case of large D_{bot}, two local maxima show up in the efficiency as a function of D_{top}. To show these maxima clearly, we set D_{bot} to a fix value of 470 nm. Here, these two efficiency peaks show up at a top cell diameter of D_{top} = 150 nm and D_{top} = 345 nm, respectively [Fig. 3(b)].
(a) Efficiency limit as a function of D_{top} and D_{bot} for P = 530 nm, L_{top} = 2000 nm and L_{bot} = 2900 nm. Here, the top cell is of Al_{0.10}Ga_{0.9}As and the bottom cell of In_{0.34}Ga_{0.66}As. (b) Efficiency limit as a function of top cell diameter as extracted from the dashed black line in (a). (c,d) Absorption spectra (red and green lines) for the diameters marked by the vertical lines in (b). Here, the diameter increases in the order of dashed, solid and dashed dotted line. We show also the normalized number of available incident photons as a function of wavelength (blue line). (e,f) Photogenerated current j_{ph1(ph2)} in (e) the top cell and (f) the bottom cell, respectively.
To understand the origin of these two efficiency maxima, we study the number of incident photons as a function of wavelength [blue line in Fig. 3(c,d)]. In the region 600 nm to 800 nm, the solar spectrum shows the highest number of incident photons as a function of wavelength. Since we assume that each absorbed photon contributes one charge carrier to the photogenerated current, strong absorption in this wavelength region is very important for j and consequently to the efficiency.
Therefore, we study the absorption spectrum in the top cell as a function of the diameter in the top cell around D_{top} = 150 nm and D_{top} = 345 nm, respectively, where the two local maxima in η show up. In Fig. 3(c), when the diameter increases from 120 nm to 150 nm, we find an absorption peak in the spectrum and it moves from about 600 nm to 700 nm^{8,36,37}. This peak can be explained as resonant coupling of incident light into the HE_{11} waveguide mode of the individual nanowires. This resonant coupling leads to enhanced absorption in nanowire arrays^{36}. When D_{top} increases further to 180 nm [red dotted line in Fig. 3(b)], the absorption peak has started to vanish since it redshifts beyond the bandgap wavelength. This shifting and disappearance of the absorption peak leads consequently to a small decrease in the efficiency as D_{top} increases from 150 nm to 180 nm.
Similarly, in the case of D_{top} = 345 nm we find again an absorption peak at λ ≈ 700 nm [Fig. 3(d)]. This time, the absorption peak originates from the higher order HE_{12} waveguide mode. This absorption peak has redshifted beyond the band gap wavelength when D_{top} has increased to 375 nm [Fig. 3(d)], leading to a slight decrease in the efficiency. Thus, we find an efficiency maximum for the nanowire tandem solar cell [Fig. 3(a)] when D_{top} is optimized to place the HE_{11} or the HE_{12} absorption peak just below the band gap wavelength. Very similar results have been reported for the diameter optimization of a single junction InP nanowire solar cell^{8}.
To understand why the efficiency does not noticeably depend on D_{bot} for D_{bot} > 250 nm [Fig. 3(a)], we study the photogeneration of charges in the top cell (j_{ph1}) and the bottom cell (j_{ph2}) [Fig. 3(e,f)]. Since j = j_{1} = j_{2} and j_{1} ≤ j_{ph1} and j_{2} ≤ j_{ph2} (see Supplementary Information for details), the smaller one of j_{ph1} and j_{ph2} is expected to limit the solar cell efficiency [Fig. 3(a)]. When D_{top} < 100 nm, the total current of the tandem cell is strongly limited by j_{ph1}. As the diameter of the top cell increases, j_{ph1} can increase to about 20 mA/cm^{2}. However, when the bottom cell diameter is larger than 250 nm, j_{ph2} > 20 mA/cm^{2}. Thus, for D_{bot} > 250 nm, j_{ph2} > j_{ph1} and the efficiency follows the absorption properties of the currentlimiting top cell and therefore depends mainly on D_{top} and only very weakly on D_{bot}.
We note that for the bottom cell, we find a pronounced maximum in j_{ph2} as a function of D_{bot} for D_{bot} ≈ 250 nm when D_{top} ≈ 0. We assign this maximum in j_{ph2} to the HE_{11} resonance in the bottom cell. We notice that in Fig. 3(f), that maximum is to a large degree overshadowed for D_{top} > 0 by the strong dependence of j_{ph2} on D_{top}. When we study the dependence of the efficiency on D_{bot} for a fixed D_{top} (see Supplementary Figure S2), we find that the maximum at D_{bot} ≈ 250 nm shows up also for D_{top} > 0 and broadens with increasing D_{top}.
Thus, we have found above two clear local maxima for η, one for D_{top} = 150 nm and one for D_{top} = 345 nm that originate, respectively, from resonant absorption through the HE_{11} and HE_{12} modes in the top cell. However, the results above were derived for a fixed L_{top}, L_{bot} and P. Next, we optimize the efficiency limit for all these five parameters (D_{top}, D_{bot}, L_{top}, L_{bot} and P) simultaneously. To make the optimization numerically feasible, we introduced a numerically efficient iteration process (See Supplementary Information for details). We choose to show the results in Fig. 4 as a function of top cell length L_{top}. For tabulated values of the optimized geometry, see Supplementary Information Table S1. For a more complete dependence of the efficiency on the geometrical parameters, see Supplementary Figures S3–S14. Notably, with proper design, an efficiency limit above 40% can be reached by the use of Al_{0.10}Ga_{0.90}As for the top cell and In_{0.34}Ga_{0.66}As for the bottom cell [blue line, when L_{top} > 6 μm, in Fig. 4(a)].
In this optimization, we can identify maxima in η to originate from the above discussed HE_{11} and HE_{12} resonances in the top cell [Fig. 4(a)]. In the region of L_{top} > 600 nm, the HE_{11} resonance of the top cell leads to a higher efficiency limit than that of the HE_{12} resonance. These results are in agreement with those for a single junction nanowire array solar cell where the HE_{11} resonance usually leads to the highest efficiency^{8}. For the dual junction cell here, we call these maxima for brevity the HE_{11} and HE_{12} maxima/optima.
For a single junction nanowire cell^{8}, rough values for the optimum diameter were estimated as
Here, is the real part of the refractive index (at the band gap wavelength) and D_{HE11(HE12)} is the diameter that optimizes the wavelength position of the HE_{11} and HE_{12} resonance in order to maximize η. The value for the constant c_{HE11(12)} can be extracted from the work on the singlejunction nanowire solar cells^{8}.
The diameter for the HE_{11} (HE_{12}) resonance of the top cell in Fig. 4(b) is D_{top} ≈ 150 nm (D_{top} ≈ 345 nm) in qualitative agreement with values from Equation (1) [about 169 nm for HE_{11} and 394 nm for HE_{12} resonance]. We find that D_{bot} fluctuates only slightly when D_{top} ≈ 150 nm to yield the HE_{11} maximum (blue dotted line in Fig. 4(b)). In contrast, D_{bot} fluctuates more at the HE_{12} maximum (green dotted line in Fig. 4(b)). This fluctuation in D_{bot} is understood from the fact that for the HE_{12} maximum at D_{top} ≈ 345 nm, the efficiency shows a very broad maximum in D_{bot} (black dashed line in Fig. 3a and Supplementary Figure S2), which allows for large variations in D_{bot} when L_{top}, L_{bot} and P are optimized.
Similarly as for the single nanowire case^{8}, we find that the optimum pitch P [solid lines in Fig. 4(b)] tends to increase with increasing nanowire length, that is, with increasing L_{top} and L_{bot}. This behavior can be understood as a competition between increased absorption and increased reflection with decreasing P^{8}. With increasing nanowire length, the absorption increases and we can allow for a larger P to decrease reflection losses.
In our results, we find that L_{bot} > L_{top} [Fig. 4(f)]. However, the efficiency tends to increase as a function of L_{bot} (see Supplementary Figures S3–S14) and therefore whether we end up in the case of L_{bot} > L_{top} or in the case of L_{bot} < L_{top} depends on how heavily we maximize the efficiency η at the cost of increasing L_{bot}. We allowed the optimization to stop with respect to L_{bot} when we reached a value of dη/dL_{bot} < 0.001 μm^{−1} in our geometry optimization (see Supplementary Information). In this case, for all the considered L_{top}, the optimized value for L_{bot} ended up slightly larger than L_{top}.
For fabrication purposes, it could be a benefit to consider D_{top} = D_{bot}, that is, nanowires of a single diameter D throughout (see the red dashed line in Fig. 4a for the resulting efficiency). We found in this case large fluctuations in the optimum value of L_{bot} when L_{top} is increasing (the fluctuation in L_{bot} could be larger than the value of L_{top}). To be able to analyze this case as a function of L_{top}, we set an upper limit of L_{top}+1000 nm for L_{bot}.
We find an interesting behavior for the optimized diameter D for these singlediameter nanowires [red dashed line in Fig. 4(b)]. For the smallest considered L_{top} of 500 nm, D starts close to the D_{top} ≈ D_{HE11} ≈ 150 nm of the HE_{11} maximum for the case in which we allow for D_{top} ≠ D_{bot}. When L_{top} increases toward the largest considered value of 8000 nm, D increases toward the value of the D_{bot} ≈ 200 nm which optimizes the HE_{11} maximum in the D_{top} ≠ D_{bot} case. This behavior can be understood as follows. When L_{top} is small, the absorption in the top cell is weak in relative terms and photons also in the short wavelength region can reach the bottom cell due to insufficient absorption in the top cell. As a result, the current and therefore the efficiency, of the solar cell is limited by absorption in the top cell. As a consequence, the optimum D occurs when the absorption in the top cell is optimized for, which happens at D ≈ D_{HE11}. In contrast, when L_{top} is large, the absorption in the top cell is instead strong and the performance of the solar cell becomes limited by the currentgeneration in the bottom cell, which is optimized for D in a similar way as when D_{top} ≠ D_{bot}. Thus, for large L_{top}, D goes toward the D_{bot} that optimizes the HE_{11} maximum.
Since we find the optimum for D close to the diameters found for the HE_{11} maximum in the D_{top} ≠ D_{bot} case, we find, not completely surprisingly, values for P close to those of the HE_{11} case of D_{top} ≠ D_{bot}. As an end result, we find that the efficiency for this case of D = D_{top} = D_{bot} is typically 1 to 2% lower than when we allow for D_{top} ≠ D_{bot} [Fig. 4(a)].
We have also studied the efficiency of the InP based Ga_{0.51}In_{0.49}P/InP nanowire tandem system [Fig. 4(d–f)], with maximum efficiency of 38.5% for perfectly absorbing subcells, which should be set in relation to the limit of 42.0% for the idealized, perfectly absorbing, band gap optimized dualjunction bulk cell. Also for this material choice we reach an efficiency within 2% of this maximum, with L_{top} > 6 μm and L_{bot} > 7 μm, when we allow for D_{top} ≠ D_{bot}. Also here, an additional drop by about 1% occurs with the constraint D_{top} = D_{bot}. With Ga_{0.51}In_{0.49}P and InP as the material and D_{top} = D_{bot}, we reach η = 35.5% when L_{top} = 2000 nm and L_{bot} = 3000 nm, considerably higher than the maximum 31.0% possible in the single junction bulk solar cell case. To aid the reader, we show in Table 1 the values extracted from Fig. 4 for this case of D_{top} = D_{bot} (for the HE_{11} and HE_{12} maximum, we refer the reader to Supplementary Information Table S1).
Conclusion
We performed electromagnetic modeling to investigate theoretically the absorption properties of a dual junction nanowire array solar cell. We used then the ShockleyQueisser efficiency limit as a metric for optimizing the materials and geometry of the nanowires. The optimized geometries are presented in Fig. 4, Table 1 and Supplementary Information Table S1. The drop in efficiency limit when moving away from such an optimized geometry is presented in Supplementary Information Figures S3–S14. These results present a guideline for choosing a nanowire geometry that has promise for optimized absorption in a dualjunction nanowire array solar cell. In this way, our results can be used as a starting point for theoretical studies on the optimization of the electrical properties of dualjunction nanowire array solar cells. Our results can also guide in the choice of materials and dimensions for the fabrication of nanowires aimed for tandem solar cells.
Additional Information
How to cite this article: Chen, Y. et al. Design for strong absorption in a nanowire array tandem solar cell. Sci. Rep. 6, 32349; doi: 10.1038/srep32349 (2016).
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Acknowledgements
This work was performed within NanoLund and received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7People2013ITN) under REA grant agreement No. 608153, PhD4Energy and the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 641023, NanoTandem. This article reflects only the author’s view and the Funding Agency is not responsible for any use that may be made of the information it contains.
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Y.C. performed the simulations, analyzed and interpreted the results and wrote the paper. M.E.P. and N.A. initiated and supervised the study and interpreted the results. All authors contributed to writing and editing the manuscript.
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Chen, Y., Pistol, ME. & Anttu, N. Design for strong absorption in a nanowire array tandem solar cell. Sci Rep 6, 32349 (2016). https://doi.org/10.1038/srep32349
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DOI: https://doi.org/10.1038/srep32349
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