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14 September 2018 Frontside scattering structures for enhanced performance in flexible ultrathin crystalline silicon solar cells
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Abstract
We demonstrate that frontside scattering structures combining a metal nanomesh transparent electrode with dielectric nanosphere (NS) arrays may improve the performance of ultrathin crystalline silicon (c-Si) solar cells. The increased light scattering as characterized by increased haze from these structures leads to longer path lengths within the c-Si and thus, higher short-circuit current densities and improved power conversion efficiencies. We demonstrate a 69% improvement in power conversion efficiency with metal nanomesh/NS coatings compared to indium tin oxide. Furthermore, we demonstrate the ultrathin film c-Si solar cells are robust under repeated bending.

1.

Introduction

Ultrathin film crystalline silicon (c-Si) photovoltaics have advantages over conventional bulk c-Si solar cells, such as less material usage and lower cost. Conventional bulk c-Si solar cells are 200 to 300  μm in thickness. Not only do thin films use less material, but they enable the use of poorer quality material with shorter minority carrier diffusion lengths since carriers do not have to diffuse as far to be collected. Ultrathin c-Si may also enable flexible solar cells that are lightweight and bendable and may be incorporated into a variety of unique materials, such as paper, cloth, or plastics. Flexible solar cells are easy to transport and install and may be integrated into crative applications, such as clothing, curtains, paper, and furniture. A large number of efforts have focused on organic1,2 or amorphous silicon3,4 solar cells, but the poor minority carrier lifetime and low carrier mobility of these materials have limited their solar conversion efficiency.

One of the main challenges with ultrathin c-Si is that c-Si is not a strong absorber of sunlight in the near-infrared region. Additional structures must be incorporated into the Si to improve its absorption and efficiency. Many nanostructures have been demonstrated for subwavelength light trapping, such as nanowires,510 nanoholes,8,11 nanocones,1214 or photonic crystals.1518 Metal nanostructures, such as nanoparticle arrays1921 and nanogrooves,22 have also been studied. However, the additional Si/metal interfaces or Si surfaces from these other approaches result in higher internal quantum efficiency losses from increased surface recombination.

Another approach to light trapping is to increase the scattering of photons into the c-Si, which increases the light’s path length inside the c-Si and, hence, increases its absorption.23,24 Typically, solar cells utilize a quarter-wave thickness antireflection layer coating to reduce reflection losses at interfaces and have a top contact consisting of a metal busbar and fingers or a transparent conductive oxide.25 However, these structures are thin film layers that do not provide for any light scattering into the underlying active region. Recently, we demonstrated through simulations and experiments that a high index of refraction dielectric nanospheres (NS) on the frontside of c-Si thin film structures can improve power conversion efficiency (PCE) substantially by scattering incident light into the underlying absorber layer.26,27 We have also recently shown that metal nanomeshes may be engineered for light scattering or haze.28,29 In this work, we integrate both metal nanomeshes and NS coatings onto a c-Si ultrathin film solar cell and demonstrate improvements in efficiency as well as flexiblity. The PCE of c-Si solar cell with metal nanomesh is improved by 53% compared to that of c-Si solar cell with indium tin oxide (ITO). More importantly, the metal nanomesh has superior flexibility to ITO, which significantly enhances the flexibility and durability of the c-Si thin film solar cells. With the introduction of a dielectric NS monolayer array on top of the metal nanomesh, the scattering of light is further enhanced, and as a result, the PCE of the c-Si solar cell can be increased by an additional 11% or 69% compared to the original ITO.

2.

Results and Discussion

Figure 1(a) shows a schematic of our ultrathin film c-Si solar cell structure that consists of a c-Si film between a Ag metal nanomesh front transparent electrode and a Ti/Ag back contact. The c-Si layer is doped n+, p, and p+ from top to bottom. The metal nanomesh is about 35-nm thick with a pitch of 1300 nm and hole diameter 1200 nm, which is fabricated by the microsphere lithography method.28 The metal nanomesh has a sheet resistance Rs10  Ω/sq compared to 30 to 40  Ω/sq for ITO with a thickness about 100 nm, which is the optimal thickness for JSC based on our measurements. On top of the metal nanomesh sits a closely packed hexagonal lattice monolayer of 800-nm-diameter polystyrene (PS) NSs with a refractive index of about 1.59, which were coated on top of the c-Si thin film solar cell. Figure 1(b) shows a scanning electron microscope (SEM) image of the top view of the PS NS hexagonal array. The NS layer is closely packed and uniform over the solar cell. Figure 1(c) shows a cross-section SEM image of the solar cell. The c-Si film has a thickness of 14.7  μm, which is slightly thinner than that in our previous paper where the c-Si was 15.0-μm thick.27 Our previous simulation results indicated that the ultimate efficiency is relatively insensitive to PS NS diameters higher than 800 nm.27 Larger diameter NSs are difficult to coat uniformly on substrates and may also exhibit higher parasitic absorption from impurities.

Fig. 1

(a) Schematic of the integrated solar cell with the metal nanomesh and dielectric NSs. The ultrathin film c-Si has a metal nanomesh as the front contact and Ti/Ag as the back contact. A closely packed hexagonal monolayer of 800-nm-diameter PS NSs lies on top of this solar cell. (b) Top view SEM image of the PS NSs on c-Si solar cell. (c) Cross-sectional view of the solar cell with PS NS monolayer on top.

JPE_8_3_030501_f001.png

Figure 2(a) plots the current density versus voltage (JV) curves of the solar cells measured under the illumination of an AM1.5 global solar simulator. The short-circuit current density Jsc of the solar cell with ITO as front contact is 16.2  mA/cm2. After the front contact was changed from ITO to metal nanomesh, Jsc increased to 21.2  mA/cm2. Jsc increased further to 23.4  mA/cm2 after the PS NS coating.

Fig. 2

JV characteristics of c-Si solar cells with different frontside structures.

JPE_8_3_030501_f002.png

Table 1 lists the Jsc, open-circuit voltage (Voc), fill factor (FF), and PCE of the three different types of solar cells. As a result of the lower sheet resistance of the metal nanomesh, the FF of the solar cell with metal nanomesh as transparent electrode improved to 75% compared to 64% with ITO as front contact. The PCE of the ultrathin c-Si solar cells improved from 4.9% with ITO to 7.5% with metal nanomesh, which is an improvement of 53%. After the solar cells were coated with PS NSs, the efficiency further improved to 8.3%, which represents an additional 11% improvement or a total improvement over the ITO solar cells of 69%. The Voc of the three cells is all about the same, but the main change is in the Jsc, which is indicative of enhanced light trapping without a degradation in surface recombination. Simulation results support our experimental observations. Finite-difference time-domain simulations were used to calculate the maximum short-circuit current density of the ultrathin c-Si solar cells under the assumption that all photons absorbed generate one electron–hole pair and that all carriers are collected without recombination (i.e., at 0 K). The calculated Jsc,max for the ultrathin solar cells is 28.5  mA/cm2 with ITO and 37.9  mA/cm2 with the metal nanomesh and PS array. The surface passivation and doping profiles may be further optimized to reduce surface recombination at the front and back surfaces. To further understand the results, we characterized both the total transmission and haze of the three different frontside structures. The same structures were fabricated on glass substrates, and then the total transmission and specular transmission were characterized using a spectrophotometer with and without an integrating sphere, respectively. The haze factor is defined as

Eq. (1)

H(λ)=Ttotal(λ)Tspec(λ)Ttotal(λ)×100%,
where λ is the free space wavelength, Ttotal is the total transmission, and Tspec is the specular transmission. Figure 3 shows the (a) total transmission and (b) haze factor spectra for the three different types of frontside structures.

Table 1

Photovoltaic properties of the ultrathin c-Si solar cells with different frontside structures.

Jsc (mA/cm2)Voc (Volts)FF (%)PCE (%)
ITO16.20.47644.9
Nanomesh21.20.47757.5
Nanomesh and NSs23.40.47758.3

Fig. 3

Measured (a) total transmission and (b) haze of three different frontside structures.

JPE_8_3_030501_f003.png

Table 2 lists the solar transmission, Tsol, and solar haze, Hsol, for the three different types of frontside structures. The solar integration transmission is calculated from

Eq. (2)

Tsol=b(λ)Ttotal(λ)dλb(λ)dλ,
where b(λ) is the photon flux density and Ttotal(λ) is the total optical transmission at wavelength λ. The solar haze is calculated in the same manner.

Table 2

The total solar transmission, Tsol (%), and solar haze factor, Hsol (%), of three different types of contacts.

Tsol (%)Hsol (%)
ITO77.15.7
Nanomesh70.638.5
Nanomesh and NSs63.070.0

The solar transmission for the ITO is indeed the best, while the metal nanomesh with PS NSs are increasingly worse. However, the ITO has the lowest haze, followed by the metal nanomesh, and then the metal nanomesh with PS NSs. This behavior was similarly found in our evaluation of the performance limits of metal nanomeshes where transmission and haze tend to have a strong negative correlation.29 We find that the haze factor is much more important in determining the overall performance of the ultrathin c-Si solar cells. While the metal nanomesh with PS NSs has the lowest transparency, the efficiency in ultrathin c-Si solar cells is improved the most with these frontside structures due to increased scattering into the underlying c-Si.

Finally, we assessed the flexible of the solar cell. A 10-μm-thick c-Si solar cell with the Ag metal nanomesh with a thickness of 35 nm, pitch of 1300 nm, and hole diameter of 1200 nm was fabricated and bent around a steel rod with 1-cm diameter. As shown in Fig. 4, the short-circuit density, Voc, FF, and PCE do not change significantly after 1200 cycles of bending. In contrast, ITO is known to form cracks after bending, and the resistivity of ITO has been shown to increase by up to 30 times after 25 cycles of bending.30

Fig. 4

PCE against the number of bending cycles on a 10-μm c-Si thin film solar cell with metal nanomesh with a geometry of t=35  nm, a=1300  nm, and w=100  nm.

JPE_8_3_030501_f004.png

3.

Methods

The ultrathin c-Si film was fabricated from double-side polished p-type (100) c-Si wafers (100-mm diameter, 10 to 20  Ωcm, 475- to 525-μm thickness). The wafer was immersed in 25% KOH solution at 90°C for about 2.5 h to obtain about 14.7-μm-thick c-Si films.31 The top side of the free standing ultrathin c-Si film was doped n+ via phosphorus spin-on dopant (SOD) (P8545, Honeywell Accuspin), and the back side of the c-Si film was doped p+ via boron SOD (B40, Honeywell Accuspin). The dopants were diffused into the ultrathin c-Si film by rapid thermal annealing at 900°C for 15 min. After doping, the dopant residue was removed by buffered oxide enchant solution, and then the sample was washed with deionized water thoroughly.

4.

Conclusions

In conclusion, we have studied several frontside structures for increased scattering and PCE in ultrathin c-Si solar cells. We find that metal nanomesh contacts with NS coatings enhance the short-circuit current of the solar cells due to increase scattering into the underlying c-Si as characterized by the increased haze factor. We have demonstrated an efficiency enhancement of 69% compared to ITO structures as well as no degradation in performance after bending tests over 1200 cycles. Frontside scattering structures have the potential to significantly improve the performance of solar cells with poor absorption, such as ultrathin c-Si or organic solar cells.

Acknowledgments

This work was supported by the National Science Foundation under Grant No. 1552712.

References

1. 

M. Kaltenbrunner et al., “Ultrathin and lightweight organic solar cells with high flexibility,” Nat. Commun., 3 770 (2012). https://doi.org/10.1038/ncomms1772 NCAOBW 2041-1723 Google Scholar

2. 

C. Lungenschmied et al., “Flexible, long-lived, large-area, organic solar cells,” Sol. Energy Mater. Sol. Cells, 91 (5), 379 –384 (2007). https://doi.org/10.1016/j.solmat.2006.10.013 SEMCEQ 0927-0248 Google Scholar

3. 

T. Soderstrom et al., “Optimization of amorphous silicon thin film solar cells for flexible photovoltaics,” J. Appl. Phys., 103 114509 (2008). https://doi.org/10.1063/1.2938839 JAPIAU 0021-8979 Google Scholar

4. 

Y. Ichikawa et al., “Production technology for amorphous silicon-based flexible solar cells,” Sol. Energy Mater. Sol. Cells, 66 (1), 107 –115 (2001). https://doi.org/10.1016/S0927-0248(00)00163-X SEMCEQ 0927-0248 Google Scholar

5. 

V. Sivakov et al., “Silicon nanowire-based solar cells on glass: synthesis, optical properties, and cell parameters,” Nano Lett., 9 1549 –1554 (2009). https://doi.org/10.1021/nl803641f NALEFD 1530-6984 Google Scholar

6. 

L. Tsakalakos et al., “Silicon nanowire solar cells,” Appl. Phys. Lett., 91 233117 (2007). https://doi.org/10.1063/1.2821113 APPLAB 0003-6951 Google Scholar

7. 

D. R. Abujetas, R. Paniagua-Domnguez and J. A. Snchez-Gil, “Unraveling the Janus role of Mie resonances and leaky/guided modes in semiconductor nanowire absorption for enhanced light harvesting,” ACS Photonics, 2 921 –929 (2015). https://doi.org/10.1021/acsphotonics.5b00112 Google Scholar

8. 

S. Aixue et al., “Nanowire and nanohole silicon solar cells: a thorough optoelectronic evaluation,” Prog. Photovoltaics, 23 1734 –1741 (2015). https://doi.org/10.1002/pip.2613 PPHOED 1062-7995 Google Scholar

9. 

B. Hua et al., “Rational geometrical design of multi-diameter nanopillars for efficient light harvesting,” Nano Energy, 2 (5), 951 –957 (2013). https://doi.org/10.1016/j.nanoen.2013.03.016 Google Scholar

10. 

M. I. Kayes and P. W. Leu, “Comparative study of absorption in tilted silicon nanowire arrays for photovoltaics,” Nanoscale Res. Lett., 9 (1), 620 (2014). https://doi.org/10.1186/1556-276X-9-620 NRLAAD 1556-276X Google Scholar

11. 

K.-Q. Peng et al., “High-performance silicon nanohole solar cells,” J. Am. Chem. Soc., 132 6872 –6873 (2010). https://doi.org/10.1021/ja910082y JACSAT 0002-7863 Google Scholar

12. 

Z. Y. Wang et al., “Broadband optical absorption by tunable Mie resonances in silicon nanocone arrays,” Sci. Rep., 5 7810 (2015). https://doi.org/10.1038/srep07810 SRCEC3 2045-2322 Google Scholar

13. 

B. Wang and P. W. Leu, “Enhanced absorption in silicon nanocone arrays for photovoltaics,” Nanotechnology, 23 (19), 194003 (2012). https://doi.org/10.1088/0957-4484/23/19/194003 NNOTER 0957-4484 Google Scholar

14. 

C.-M. Hsu et al., “Wafer-scale silicon nanopillars and nanocones by Langmuir–Blodgett assembly and etching,” Appl. Phys. Lett., 93 133109 (2008). https://doi.org/10.1063/1.2988893 APPLAB 0003-6951 Google Scholar

15. 

B. Wang, K. P. Chen and P. W. Leu, “Engineering inverse woodpile and woodpile photonic crystal solar cells for light trapping,” Nanotechnology, 27 (22), 225404 (2016). https://doi.org/10.1088/0957-4484/27/22/225404 NNOTER 0957-4484 Google Scholar

16. 

H. Sai et al., “High-efficiency microcrystalline silicon solar cells on honeycomb textured substrates grown with high-rate VHF plasma-enhanced chemical vapor deposition,” Jpn. J. Appl. Phys., 54 (8S1), 08KB05 (2015). https://doi.org/10.7567/JJAP.54.08KB05 Google Scholar

17. 

R. Biswas et al., “Enhanced nanocrystalline silicon solar cell with a photonic crystal back-reflector,” Sol. Energy Mater. Sol. Cells, 94 (12), 2337 –2342 (2010). https://doi.org/10.1016/j.solmat.2010.08.007 SEMCEQ 0927-0248 Google Scholar

18. 

M. D. Zoysa et al., “Enhanced efficiency of ultrathin (500 nm)-film microcrystalline silicon photonic crystal solar cells,” Appl. Phys. Express, 10 (1), 012302 (2017). https://doi.org/10.7567/APEX.10.012302 APEPC4 1882-0778 Google Scholar

19. 

B. Ding et al., “Synergistic effect of surface plasmonic particles in PbS/TiO2 heterojunction solar cells,” Sol. Energy Mater. Sol. Cells, 128 386 –393 (2014). https://doi.org/10.1016/j.solmat.2014.06.001 SEMCEQ 0927-0248 Google Scholar

20. 

T. Gao et al., “Designing metal hemispheres on silicon ultrathin film solar cells for plasmonic light trapping,” Opt. Lett., 39 4647 –4650 (2014). https://doi.org/10.1364/OL.39.004647 OPLEDP 0146-9592 Google Scholar

21. 

L.-B. Luo et al., “Surface plasmon resonance enhanced highly efficient planar silicon solar cell,” Nano Energy, 9 112 –120 (2014). https://doi.org/10.1016/j.nanoen.2014.07.003 Google Scholar

22. 

V. E. Ferry et al., “Plasmonic nanostructure design for efficient light coupling into solar cells,” Nano Lett., 8 (12), 4391 –4397 (2008). https://doi.org/10.1021/nl8022548 NALEFD 1530-6984 Google Scholar

23. 

G. Han et al., “Towards high efficiency thin film solar cells,” Progr. Mater. Sci., 87 246 –291 (2017). https://doi.org/10.1016/j.pmatsci.2017.02.003 PRMSAQ 0079-6425 Google Scholar

24. 

A. Zitzler-Kunkel et al., “NIR-absorbing merocyanine dyes for BHJ solar cells,” Chem. Mater., 26 (16), 4856 –4866 (2014). https://doi.org/10.1021/cm502302s CMATEX 0897-4756 Google Scholar

25. 

R. B. Wehrspohn, U. Rau and A. Gombert, Photon Management in Solar Cells, John Wiley & Sons, Weinheim, Germany (2015). Google Scholar

26. 

B. Wang and P. W. Leu, “High index of refraction nanosphere coatings for light trapping in crystalline silicon thin film solar cells,” Nano Energy, 13 226 –232 (2015). https://doi.org/10.1016/j.nanoen.2014.10.040 Google Scholar

27. 

B. Wang, T. Gao and P. W. Leu, “Broadband light absorption enhancement in ultrathin film crystalline silicon solar cells with high index of refraction nanosphere arrays,” Nano Energy, 19 471 –475 (2016). https://doi.org/10.1016/j.nanoen.2015.10.039 Google Scholar

28. 

T. Gao et al., “Uniform and ordered copper nanomeshes by microsphere lithography for transparent electrodes,” Nano Lett., 14 (4), 2105 –2110 (2014). https://doi.org/10.1021/nl5003075 NALEFD 1530-6984 Google Scholar

29. 

G. Tongchuan et al., “Fundamental performance limits and haze evaluation of metal nanomesh transparent conductors,” Adv. Opt. Mater., 6 (9), 1700829 (2018). https://doi.org/10.1002/adom.v6.9 2195-1071 Google Scholar

30. 

X. He et al., “A highly conductive, flexible, transparent composite electrode based on the lamination of silver nanowires and polyvinyl alcohol,” J. Mater. Chem. C, 2 9737 –9745 (2014). https://doi.org/10.1039/C4TC01484G Google Scholar

31. 

C. Wang et al., “Carbon nanotube electronics—moving forward,” Chem. Soc. Rev., 42 2592 –2609 (2013). https://doi.org/10.1039/C2CS35325C CSRVBR 0306-0012 Google Scholar
© The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
Baomin Wang, Tongchuan Gao, Ziyu Zhou, Bradley Pafchek, and Paul W. Leu "Frontside scattering structures for enhanced performance in flexible ultrathin crystalline silicon solar cells," Journal of Photonics for Energy 8(3), 030501 (14 September 2018). https://doi.org/10.1117/1.JPE.8.030501
Received: 7 June 2018; Accepted: 27 August 2018; Published: 14 September 2018
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