Design of Contact Electrodes for Semiconductor Nanowire Solar

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Design of Contact Electrodes for Semiconductor Nanowire Solar Energy Harvesting Devices Tzu-ging Lin, Sarath Ramadurgam, and Chen Yang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04046 • Publication Date (Web): 23 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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Design of Contact Electrodes for Semiconductor Nanowire Solar Energy Harvesting Devices Tzuging Lin1, Sarath Ramadurgam1, Chen Yang1,2* 1

Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA

2

Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA

*

Corresponding Author. Email: [email protected]

Abstract: Transparent, low resistive contacts are critical for efficient solar energy harvesting devices. It is important to reconsider the material choices and electrode design as devices move from 2D films to 1D nanostructures. In this paper, we study the effectiveness of indium tin oxide (ITO) and metals, such as Ag and Cu, as contacts in 2D and 1D systems. Although ITO has been studied extensively and developed into an effective transparent contact for 2D devices, our results show that effectiveness does not translate to 1D systems. Particularly with consideration of resistance requirement, nanowires with metal shells as contacts enable better absorption within the semiconductor as compared to ITO. Furthermore, there is a strong dependence of contact performance on the semiconductor band-gap and diameter of nanowires. We found that metal contacts outperform ITO for nanowire devices, regardless of the sheet resistance constraint, in the regime of diameters less than 100 nm and band-gaps greater than 1 eV. These metal shells optimized for best absorption are significantly thinner than ITO, which enables for the design of devices with high nanowire number density and consequently higher device efficiencies.

Keywords: nanowires, transparent contact electrode, solar cells, anti-reflection coatings 1

ACS Paragon Plus Environment

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In the past decade, extensive studies have been reported on solar energy harvesting by semiconductor nanowire devices.1,

2, 3

Solar energy in different wavelength regime can be

utilized by semiconductors with different bandgaps according to Shockley–Queisser theory.4, 5, 6 For example, solar energy in the visible light regime can be utilized by Si or InP, which can effectively absorb solar light with wavelengths below 800 nm. Solar radiation in the nearinfrared regime can be utilized by small bandgap semiconductors,7 such as GaSb, particularly through thermophotovoltatic devices, which convert thermal radiation into electric energy more effectively than conventional Si photovoltaic (PV) devices.8,

9, 10

Additionally, mid to wide

bandgap semiconductor materials,11, 12, such as GaP,13, 14 have band gaps aligned with the 1.23 V electric potential required for water splitting

15

, therefore they are potential candidates for

photoelectrochemical cells. Among proposed device structures, semiconductor nanowires have drawn tremendous attention due to their larger surface area and stronger coupling with light compared with traditional thin film device structures. Specifically, nanowires confine photons in two transverse dimensions and allow photons propagating in the longitudinal dimension along the nanowire axis, leading to unique optical coupling properties for solar energy applications. Typically, in solar energy harvesting devices, it is required to have electrodes in contact with the absorbing semiconductor material. The contact electrodes often need to transmit light and conduct electric current simultaneously. Metal oxides, such as indium tin oxide (ITO) and aluminium zinc oxide (AZO), are widely used to form conductive electrodes on both thin-film and nanowire photovoltaic devices in laboratories and industries.16, 17, 18 Compared to metals, these conductive metal oxides exhibit good optical transparency and poor electrical conductivity. 16, 19, 20, 18

Although metal electrodes have good electric conductivity, they are not transparent 2


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leading to poorer absorption and in turn poor conversion efficiencies. In this work, we demonstrate that such limitation of metals can be circumvented when employing 1D nanowire solar cell devices. The optical

performance of metals can be designed to be better than

conductive oxides for 1D nanowire systems due to the dimension dependent light coupling of photonic structures.17, 21

Figure 1. Schematic of representative photovoltaic device designs based on (a) radial and (b) axial p-n nanowires, in which electric contact electrodes are the outer shell of nanowires. Figure 1 shows typical structures of semiconductor nanowire photovoltaics devices based on radial (figure 1a) and axial (figure 1b) p-n junction nanowire devices.3, 22 Reported studies have suggested that coaxial nanowires based on radial collection demonstrated a considerably larger efficiency compared to axial nanowires based on axial collection. 2, 23 24 For both designs, contact electrodes deposited on the nanowire will affect the absorption of solar energy in the semiconductor and transmission of electric energy converted by the semiconductor p-n junction from the incident light. To maximize the optical absorption and minimize electric resistive losses, the contact layers are expected to be highly optical transparent and electrically conductive. Studies have been reported on designing new contact electrodes with various sizes and 3


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geometries, such as metal nanowire,18, nanotube,19 and graphene.

30, 18, 19, 31

25

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metal grid,18 metal thin film,26,

27, 28, 29

carbon

However, there are few studies on design guides with a

focus on how to optimize the material and dimensions of the contact electrodes with respect to the specific dimensions of the nanowire photovoltaics devices.

32

Particularly, it is highly

beneficial to design thin conformal contact electrode shells that also circumvent the need for complex patterning or material deposition. In this paper, we systematically study the optical absorption in photovoltaics devices through simulating the optical absorption of 1D nanowire and 2D thin film semiconductor devices coated with ITO and metal contact layers under solar illumination. Specifically, the absorption in the semiconductor core in the nanowires and the semiconductor thin film is compared for different electrode materials and thicknesses. Under the constraint of the electrical conductivity requirement, we found that metal contact electrodes are promising alternatives to ITO. Specifically, ITO shell thickness of 400 and 50 nm are chosen as benchmarks for comparing metal electrodes. The 400 nm thickness is a benchmark for achieving low sheet resistance of 10 Ω. The 50 nm thickness is a second benchmark as it acts as an anti-reflective coating and enhances absorption in nanowires. This is also a typical contact electrode thickness used in solar devices irrespective of the charge collection architecture. Additionally, to understand how the contact electrodes affect solar energy harvesting in different wavelength regimes, a simulation of optical absorption within semiconductor nanowires have been performed when considering different band gaps and different contact electrodes. Specifically, we found that the contact electrodes affect the optical absorption by semiconductors with different bandgaps via different ways, which provides a guideline for future design of solar cell devices.

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METHOD The absorption in semiconductor in nanowire and thin film structures was simulated employing Mie scattering theory12,

33, 34

and transfer matrix theory,35,

36

respectively. For the

individual nanowire, the optical absorption efficiency (ηi where i=1, 2 corresponds to the core and the shell respectively) under transverse magnetic (TM) or transverse electric (TE) illumination was obtained by solving Maxwell’s equations following Mie-formalism

as a

function of wavelength:37

(/) =

(/)

(/)

∬ () (

) + (

(/)

) + (

)

(1)

where, is the total radius of the nanowire, is the wave-number of the incident light, () is the dielectric function, is the amplitude of the incident light and (/) is the transverse electric (TE) or transverse magnetic (TM) field obtained from Maxwell’s equations. The absorption efficiency for unpolarized incident light ( ) was obtained by averaging and . We also simulated the ideal photocurrent for nanowires based on equation 212 and used it as a direct measure for integrated absorption. We assume air mass 1.5 global (AM 1.5G; 1-sun) illumination incident perpendicular to the NW axis. '()*+(, $

J = " #- ).

(4) 4 %& /0.23

(2)

Here q is charge of an electron, 4 is wavelength, h is Planck constant, c is speed of light, and /0.23 is the intensity of AM 1.5G illumination. The integration range is from 300 nm to the bandgap of the semiconductor material, where solar spectrum and absorption range of the semiconductor overlap. A nanoHUB tool developed by Yang Group provided more detailed description and is available to public to compute the optical properties of coated nanowires.38 5


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For semiconductor thin films, the ideal photocurrent (i.e. the integrated absorption) was calculated based on the excitation generating rate of electron-hole pairs ( 5 ) at normal incidence given by solving Maxwell’s equations based on transfer matrix theory.36 Following transform matrix formalism, time average absorbed power in a semiconductor thin film as a function of depth x was obtained by carefully considering the contribution of incident light, reflected light, and interference of incident light and reflected light in the layered structure.35 Excitation generating rate of electron-hole pairs ( 5 ) was then calculated by multiplying time average absorbed power by

$

%&

assuming ideal internal quantum efficiency. Therefore, the ideal

photocurrent is given by J = " #

6%& )788

'()*+(,

#- ). 5 4 9

(3)

where, i=1, 2 corresponds to the bottom and top layers, respectively. Assuming ideal charge transport, the calculated absorption in the presence of a contact electrode becomes a direct metric for qualitatively comparing device efficiency RESULTS AND DISCUSSION 2

Integrated Solar Absorption (mA/cm )

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70 Thin Film GaSb InP Si GaP

60 50

Nanowire GaSb InP Si GaP

d

40 30 20 10 0

0

50

100

150

200

250

300

Semiconductor Feature Size d (nm)

Figure 2. Simulated integrated solar absorption of GaSb, Si, InP, and GaP nanowires and thin 6


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films as functions of the semiconductor feature size d, assuming devices are illuminated by AM1.5G solar radiation. To understand the impact of contacts on the optical performance of 1D and 2D photovoltaic devices under solar radiation illumination, we first theoretically investigate the integrated solar energy absorption of representative semiconductors considering both nanowire and thin film structures. Figure 2 plots the integrated absorption as a function of the semiconductor feature size for GaSb (red), InP (blue), Si (black) and GaP (green). The feature sizes here refer to the thickness and diameter for the thin film and the nanowire structure, respectively. We specifically chose GaSb9, 39, Si,2, 40 InP,3, 40-41 and GaP,14, 42 as they represent semiconductors harvesting solar energy in different wavelengths. GaSb (red) has a wide absorption range resulting from its small bandgap (0.73 eV) making it ideal for thermophotovoltaics8, 9, 10. Other semiconductor materials with smaller bandgap, such as InSb and InAs, have wider absorption range than GaSb, however their photovoltage is smaller resulting lower solar energy conversion efficiencies.4 The optical absorptions of InP (blue) and Si (black) are significantly smaller than GaSb (red) due to their larger bandgaps compared to that of GaSb. The efficiency of single junction solar cell made of Si or InP are demonstrated to be significantly higher than that of GaSb by Shockley–Queisser theory,4 making them ideal for single junction devices. The absorption of GaP (green) is the lowest among these materials due to its large bandgap (2.26 eV). However, GaP can provide sufficient photovoltage and proper band edge alignment to drive the reduction reaction of carbon oxide43, 44 as well as the hydrogen production via water splitting.11, 13, 14 More significantly, figure 2 also shows that the simulated absorption of semiconductor nanowires (solid line) are higher than that of thin films (dashed line) since Mie resonance in nanowires is stronger than Fabry Perot resonance in thin films with the same feature size.45, 17, 21 7


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Figure 3. Simulated integrated solar absorption of Si thin films (a, b) and nanowire cores (c, d) coated with ITO (red) with Sn doping concentration equal to 6.1 ×1020 cm-3, Ag (grey) and Cu (blue). The semiconductor feature sizes are 20 nm in (a) and (c), and 280 nm in (b) and (d) as indicated in the figure. 8


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To evaluate the effect of the contact electrode layers on the optical absorption of semiconductors in relevant PV device structures, we simulated the integrated solar absorption in the semiconductor nanowires and the semiconductor thin films as a function of contact electrode thickness for Ag, Cu and ITO with incident light perpendicular to the nanowire axis and thin film surface, respectively. We present the results for Si in figure 3 for two feature sizes, 20 nm and 280 nm, as these feature sizes correspond to the absorption peaks of Si nanowires under AM1.5 G solar spectrum shown in figure 2. Here the absorption plotted is in the semiconductor cores and in the semiconductor thin film only. The absorption within the metal/ITO electrodes was not considered. For Si thin films coated with Ag and Cu, the integrated absorption of solar energy by Si decreases drastically with increase of the contact thickness (grey and blue curves in Figure 3a and b). Utilizing ITO contact significantly enhances the integrated solar energy absorption in Si thin films (red curves in Figure 3a and b) mainly attributing to the quarter wavelength antireflection effect contributing by ITO layer.32 The absorption peaks for both Si films happen at ITO thickness of around 50 nm. This ITO thickness results in destructive interference between incident and reflected light at a wavelength of 400 nm which reduces reflection and consequently improves absorption. Significantly, compared to metal electrodes Ag and Cu, the Si thin films with ITO contact demonstrated greater absorption regardless thickness of the contact layer studied, consistent with the choice of ITO for conventional thin film solar devices. To minimize resistive losses in the contact, typically, the sheet resistance is required to be less than 10 Ω.

30

Therefore, for ITO, decreasing contact thickness is not always a valid strategy to

improve the absorption, as larger sheet resistance lead to significantly poor overall efficiency. For sub-10 Ω sheet resistance, the thickness of ITO layer is expected to be above 394 nm, 9


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estimated based on heavily doped ITO layer with Sn concentration of 6.1 × 1020 cm-3 and estimated resistivity of 3.94 ×103 nm Ω.46. For Ag/Cu contact layer, the sheet resistance was reported to be 10 Ω as the thickness reaches 10 nm. This is larger than the sheet resistance expected from bulk resistivity of Ag and Cu due to the increased boundary scattering of electrons as the thickness of Ag/Cu decreases.28, 29 It is necessary to consider the absorption in Si thin films again within the context of this sheet resistance requirement. For both 20 nm and 280 nm thick Si thin films coated with 394 nm thick ITO contact layers, the optical absorption of Si are nearly the same as bare Si thin films (red curves in Fig. 3a and b). The 20 nm thick Si thin film coated with 10 nm Ag (or Cu) contact layers shows approximately 90 % optical absorption of that in the bare Si thin film. The absorption by 280 nm thick Si thin films coated with 10 nm Ag and Cu contact reduce to approximately 83 % and 73 % of that in bare Si thin films, respectively, as shown in Figure 3b. These results confirm again that ITO contact electrodes enable better solar energy absorption in semiconductor thin films than metal contact electrodes do. Different from results for Si thin films, figure 3c and d show the integrated absorption in Si cores substantially decreased when increasing the thickness of ITO shells. These suggest that thick ITO contact layer alters the Mie resonance inside Si NWs,32 resulting in the suppression of solar energy absorbed by Si cores. Additionally, this feature is more prominent as the diameter of nanowires reduces, as indicated by comparison between the ITO results (red) in figure 3c and 3d. Specifically, for 20 nm and 280 nm diameter Si nanowires with 394 nm thick ITO contact layers, the optical absorption of Si is reduced to nearly 2 % and 38 % of the absorption by bare Si nanowires, respectively. Figure 3c shows that for 20 nm diameter Si NWs coated with 10 nm thick Ag and Cu, the optical absorption by Si is found to be 41 % and 28 % of that in bare Si NWs. Meanwhile, for 280 nm diameter Si NWs coated with 10 nm thick Ag and Cu the optical 10


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absorption in Si is 82 % and 65 % of that in bare Si NWs. Furthermore, tilting the incident angle of light into the Si NW coated with Ag/Cu contact will lead to less optical absorption due to the decreasing optical cross section (Figure S1). Collectively, the absorption in Si NWs coated with 10 nm thick Ag and Cu is significantly higher than that coated with 394 nm thick ITO. Additionally, for both thin film and nanowire cases, Ag contact exhibits a slightly better transparency than Cu contact. Such observation is attributed to the fact that Cu has a larger imaginary part of dielectric constant in the 300 nm to 600 nm range than that of Ag, resulting in more solar energy absorbed by Cu than Ag. Collectively, our results show that under the constraint of sheet resistance, Ag and Cu contact layers can preserve the solar energy more effectively, i.e. be more transparent, than ITO contact in Si nanowire solar harvesting devices,

AM 1.5G Solar Spectrum 2 (mW/cm )

particularly for devices based on thin Si nanowires. 2.0 1.5

a

1.0 0.5 0.0 1.5

Absorption Efficiency (a.u.)

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b

1.0 0.5 0.0 1.5

c

Si core-diameter = 20 nm Bare Si NW Si NW / 10 nm Ag shell Si NW / 10 nm Cu shell Si NW/ 50 nm ITO shell Si NW / 394 nm ITO shell 3 (ρ=3.94 x 10 (nmΩ)) Si core-diameter = 280 nm

1.0 0.5 0.0 400

600 800 Wavelength (nm)

1000

Figure 4. (a) AM 1.5 G solar spectrum and (b, c) simulated core absorption efficiency in Si/ITO, 11


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Si/Ag, and Si/Cu nanowire core/shell structures. Si core diameter is 20 nm in (b) and 280 nm in (c), respectively. To understand how the contact electrode affects the optical absorption of Si cores in different wavelength regime in the solar spectrum, we simulated the absorption efficiency as a function of wavelength for Si cores with 20 nm and 280 nm core-diameter nanowires coated with Ag, Cu, and ITO contact shells. Thickness of metals and ITO were chosen to be 10 nm and 394 nm based on the sheet resistance requirement discussed above. In addition, we also consider 50 nm thick ITO layer that corresponds to the absorption peak in figure 3. It is important to note here that the 50 nm ITO layer does not meet the sheet resistance constraint. For the bare 20 nm diameter Si nanowire (black, figure 4b), the core absorption maximum appears at 350 nm. This peak decreases to about 49 % or 36 % of its original value once the Ag or Cu contact shell is depositing on the Si core respectively. When ITO is used for contact, the 20 nm diameter of Si core is too small to observe any Mie resonances in the visible range,32 thus the optical absorption of Si core decreases drastically with increase in the thickness of ITO shells (red curves in figure 4b). Compared to 20 nm diameter bare Si nanowires, 280 nm diameter nanowires have larger volume enabling multiple optical resonances in a wider wavelength regime from 300 nm to 900 nm (figure 4c). Particularly, the larger diameter enables visible range Mie resonances for ITO shells and in turn increases absorption. Furthermore, the absorption peak of the Si core with the 50 nm ITO shell appears at 390 nm, consistent with the quarter wavelength antireflection by the 50 nm ITO shell. Comparison between the Si cores with metal contact shells (grey and blue) and ITO shell (dashed red) confirmed that under the constraint of 10 Ω sheet resistance requirement, metal contact electrodes enable greater optical absorption in Si in the nanowire device structures 12


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than the ITO. In addition, these results also demonstrate that the effectiveness of optimizing the thickness of ITO contact to the quarter wavelength of incident light to improve the absorption is highly dependent on the core diameters. The 50 nm ITO shells is effective for the 280 nm core but not the 20 nm Si core.

b

a 1.4

GaP-core 10 nm Cu shell 10 nm Ag shell 1.2 50 nm ITO shell 394 nm ITO shell 1.0

Ratio

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c

Si-core

GaSb-core

0.8 0.6 0.4 0.2 0.0

0

100

200

300 0

100

200

300 0

100

200

300

semiconductor core diameter (nm)

Figure 5. Ratio between the calculated integrated solar absorption of (a) GaP, (b) Si and (c) GaSb nanowire cores coated with contact layers and that in bare semiconductor nanowires as a function of semiconductor core diameters. Note that the absorption within the semiconductor core is presented here. Contacts include 50 nm thick ITO (red solid), 394 nm thick ITO (red dash), 10 nm thick Ag (grey) and 10 nm Cu (blue). In order to evaluate the impact of contact materials to the absorption of semiconductors beyond Si, we calculated and compared the absorption of the semiconductor cores in semiconductor-core contact -shell nanowire structures as a function of the semiconductor core 13


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diameter for GaP, Si and GaSb. Figure 5 plots the ratio between the calculated integrated absorption of the cores in the core shell structures and the calculated absorption of the bare nanowires. We use the ratio here as a metric to compare the performance of different materials. Ideally, this ratio should be close to 1 for a good ‘transparent’ shell and greater than 1 for shells that act as ‘anti-reflective’ coatings. In general, the ratio is always less than 1 for semiconductor core diameters less than 100 nm. For small diameter applications, any coating affects the predicted semiconductor optical performance. For instance, ITO coating in Si nanowires (40 nm diameter) results in significant reduction of the electric field inside in the core as compared to a metal shell (see Figure S3). This leads to the lower absorption in ITO coated nanowire cores when compared to metal coated cores. It is therefore critical to consider the absorption in coated structures and carefully choose the shell material and thickness. At larger diameters, however, multiple optical modes contribute towards absorption particularly at higher wavelengths where metal shells absorb significantly. Hence, ITO shells perform better for larger diameter nanowires and smaller band-gap absorbers. Contact shells considered in Figure 5 include 10 nm Ag, 10 nm Cu, 394 nm ITO and 50 nm ITO for the rationale discussed previously. Results demonstrate the following several key features. There is a strong dependence for ratio of absorption with and without shells on contact materials, semiconductor bang-gap and nanowire diameters. For GaP, cores diameters greater than 50 nm (figure 5a), the Ag layer performs better than Cu and thick ITO (394 nm), and is even comparable to 50 nm thick ITO. For diameters smaller than 50 nm, both metals are substantially better than ITO. In the case of Si nanowires (figure 5b), the performance of metals at smaller diameters, like for GaP, is comparable or better than ITO. However, at larger diameters, an optimized ITO shell of 50 nm performs better than metals. For materials with small bandgaps 14


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such as GaSb (figure 5c), metals perform only slightly better than ITO at lower diameters and significantly worse than ITO at higher diameters. The diameter dependence is due to the absence of visible range Mie resonances for small diameters, which leads to metals performing better than ITO as explained earlier. At larger diameters, 50 nm ITO shells result in strong Mie resonances within the core thereby improving the absorption. Hence, the performance of ITO is better. In addition, the absorption of light modeled in single semiconductor core-shell nanowire is mainly attributed to the cylindrical optical resonance in the nanowire (2: = ;4) and the single pass absorption by the semiconductor material. The bandgap of GaP is 2.23eV, making it only capable of absorbing light with wavelength less than 550 nm and leading to less single pass absorption of light in long wavelength regime. Therefore, the absorption ratio spectrum of GaP nanowire coated with 10 nm thick Ag and Cu contact has pronounced oscillation features as a function of nanowire core diameter, attributing to dominating cylindrical optical absorption. Unlike GaP nanowires, Si and GaSb nanowires have wider bandgaps, thus capable of having more single pass absorption of light than GaP in long wavelength regime, resulting in less pronounced oscillation in absorption ratio spectrum. More significantly, the absorption range of the contact electrode dictates the band-gap dependence of contact performance. For nanowires with core diameters of 300 nm, we found the light absorption ratios of Si, GaP, and GaSb-core coated with 50 nm ITO shell are slightly larger than 1, nearly equal to 1, and less than 1, respectively. This observation can be attributed to difference in light absorption regimes of GaP, Si, GaSb, and ITO. Note that 50 nm ITO acts as an anti-reflective coating only when ITO is transparent to the incident light. Si optical absorption is below 1120 nm. In this range, ITO is transparent and therefore enables absorption ratio larger 15


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than 1. For GaP, a small bandgap semiconductor, the optical absorption is below 560 nm. In this narrow absorption range, the anti-reflective enhancement with ITO is not significant. Therefore, the integrated solar absorption ratio of thick GaP nanowires coated with 50 nm ITO shell is close to 1. For GaSb, another small bandgap semiconductor, in addition to an ineffective anti-reflective property in ITO in the corresponding near infrared regime, there is an increase in photon absorption by free electrons in ITO shell,46 leading to further less optical absorption by GaSb cores than bare GaSb (Figure S2). In core-shell structures, metal shells absorb more strongly at longer wavelengths while ITO bandgap prevents any absorption beyond ~400 nm. This leads to the better performance of metals for semiconductors with larger band-gaps such as GaP and Si as compared to GaSb. Furthermore, the absorption of Cu is higher than Ag in the 300-600 nm range, which leads to the poorer performance. Besides, the absorption of TE and TM polarized light will be both suppressed by employing Ag/Cu contact layers on GaP, Si, and GaSb NWs (Figure S4). These trends provide a guideline for the choice of metal contact based on the semiconductor band-gap and diameter. Notably, the absorption as a function of the incident angle exhibits a gradual monotonic decay with a metal shell (Figure S1) especially for smaller diameter wires. This is particularly helpful for designing randomly oriented arrays that provide consistent photovoltage over a large range of angles for the incident light. Furthermore, the absorption of metal coated NWs under TE and TM polarized light follows a similar trend as the bare NWs (Figure S4). It is critical to note here that 50 nm ITO shells imply that the surface-to-surface nanowire separation is greater than 100 nm in order to preserve the Mie resonances i.e. enhanced absorption. This leads to a stringent limit on the number density of nanowires and does not satisfy the 10Ω sheet-resistance requirement. On the other hand, metal layers are substantially 16


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thinner allowing for high number density of nanowires, which in turn can boost the overall efficiency. Hence, there exists a tradeoff between the nanowire diameters, number density and shell thickness that should be optimized for best efficiencies in the target solar harvesting device. Based on our results, metal contact electrodes are more favorable to ITO for nanowire devices with small diameters (< 100 nm), large band-gap absorber (> 1 eV) and high number density. It is important to acknowledge that the presence of metal contact often results in the contact resistance of the metal-semiconductor junction. For III-V material, such as GaSb and GaP, the contact resistance can be successfully reduced by increasing the doping concentration near the contact. Taking GaSb as an example, a typical design of GaSb thermophotovoltaic devices usually used a heavily Zn doped (doping concentration larger than 1020 cm-3) GaSb layer as an emitter on top of the devices.10, 47, 9 Therefore the specific contact resistance of Ag and Cu on heavily doped GaSb could be below 10-8 Ω cm2,48, 49 corresponding to a total contact resistance less than 1 Ω for a 280 nm diameter, 10 µm long GaSb nanowire. GaP has smaller electron affinity than GaSb, leading to a larger Schottky barrier than GaSb. Besides, the levels of electrical activation of impurities in GaP is typically below 1019 cm-3. These factors combining together give rise to a larger specific contact resistance in GaP-metal interface (>10-5 Ω cm2) than GaSb.50 For conventional Si–metal junction, specific contact resistance around 10-6 Ω cm2 can be achieved by heavily doped Si above 1020 cm-3.51 In addition, contact resistance of metalSi junctions can be further reduced by forming the metal silicide compound, resulting in the diminishing the height of Schottky barrier .52 Although semiconductor nanowire devices with allaround metal contact tend to have larger depletion region than thin film devices, this problem can be solved by heavily doping the semiconductor near the metal-semiconductor interface, resulting a tremendously reduced contact resistance.51b 17


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In conclusion, this work presents a detailed study of the effect of electrode material, thickness, as well as nanowire diameter, materials and band gap on the semiconductor absorption in geometry of core-shell nanowires. Specifically, design of contact electrodes is a key factor contributing to the performance of solar cells and our work is the first study on the perspective of contact in nanowire solar energy device. Particularly, the use of a metal electrode layer in thin films strongly limits the total efficiency, while our work elucidates that no such fundamental limitation exists with nanowire devices coated with metal electrode shells. We have demonstrated that metals can be excellent alternatives to ITO as ‘transparent’ contact electrodes for nanowire based solar energy harvesting devices. This is due to the unique optical coupling for 1D nanostructures as compared to 2D thin-film devices and the substantially lower sheet resistance of metals leading to thinner electrode layers. Significantly, the anti-reflective properties of ITO electrodes in thin films is less prominent and even vanishing in semiconductorITO core-shell nanowires as the core diameter decreases. In addition, this work provides a quantitative analysis on optimizing contact electrodes for nanowires. We investigated the effect of semiconductor band-gap on the performance of the contact electrode. Our results demonstrate strong dependence of contact performance on the semiconductor band gap, which provides a guideline for designing contact electrodes suitable for the target solar energy-harvesting device. Finally, for practical implementation, a thinner electrode enables devices with higher number density of nanowires and consequently enhances overall device efficiency. The contacts considered here are thin uniform shells, which avoids the need for any complex patterning or multiple fabrications processes thereby enabling cheap scalable devices. Here, the absorption within single nanowires is considered. The absorption in an array can be different from a single nanowire. Specifically, the position, orientation, diameter and height distribution in arrays can 18


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significantly change the total absorption and hence, the exact electrode thickness would need to be further optimized. This work, however, aims at providing a fundamental understanding of the absorption within the nanowire core when coated with various electrodes. We expect the qualitative understanding gained here to apply to arrays. ASSOCIATED CONTENT

Supporting Information. Integrated solar absorption of silicon NWs with different core diameters and Ag and Cu contacts. Simulated core absorption efficiency in GaP, Si, GaSb nanowires with 50 nm ITO shells. The spatial contour plots of the electric field magnitude at 550 nm under TM and TE illumination for bare Si NW, Si–Ag core-shell nanowire, and Si–ITO coreshell nanowire. Calculated integrated solar absorption of TE and TM polarized light of GaP, Si and GaSb nanowire cores coated with Ag/Cu contact layers. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Contributions S.R and C.Y conceived the idea and provided guidance throughout the project. T.L and S.R performed calculation and analysis. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interests. Acknowledgement S. R. thanks the partial support from National Science Foundation ECCS 1118934. 19


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TOC

a 1.4

GaP-core 10 nm Cu shell 10 nm Ag shell 1.2 50 nm ITO shell 394 nm ITO shell 1.0

Ratio

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b

c

Si-core

GaSb-core

0.8 0.6 0.4 0.2 0.0

0

100

200

300 0

100

200

300 0

100

semiconductor core diameter (nm)

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Design of Contact Electrodes for Semiconductor Nanowire Solar

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