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9.4% Efficient Amorphous Silicon Solar Cell on High Aspect-Ratio Glass Microcones Jeehwan Kim,* Corsin Battaglia,* Mathieu Charrière, Augustin Hong, Wooshik Jung, Hongsik Park, Christophe Ballif, and Devendra Sadana
Strong light absorption and excellent carrier collection are essential for achieving solar cells with high power conversion efficiency. However, for hydrogenated amorphous silicon (a-Si:H), the absorption coefficient becomes small towards the near-infrared and the carrier lifetimes are not sufficient to increase the absorber thickness beyond 250 nm. Many studies have promoted the idea of going optically thick, to maximize the light absorption path, while staying electrically thin, to guarantee short carrier collection distances by folding the absorber layer on a substrate with modified topological features. However, most a-Si:H solar cells reported in the literature are deposited on substrates with typical aspect ratios far below 1 (called 2D hereafter),[1–8] including the current world record cell on pyramidally textured zinc oxide grown by chemical vapor deposition.[1] In fact only a handful studies have attempted topologies with aspect ratios >1 (3D),[9–14] but efficiencies have so far not been able to rival with the more planar 2D topologies with aspect ratios 60% compared to a planar control device. Scanning electron microscopy (SEM) images in Figure 1 show the process flow for forming high aspect-ratio 3D microcone solar cells. Tin (Sn) microspheres self-assembled on glass substrates were used as masks for reactive ion etching (RIE) to form glass microcones with aspect ratio of ∼2. Remaining Sn on
J. Kim,[+] A. Hong, W. Jung, H. Park, D. Sadana IBM T.J. Watson Research Center Yorktown Heights New York 10598, USA E-mail: [email protected] C. Battaglia,[+] M. Charrière, C. Ballif cole Polytechnique Fédérale de Lausanne (EPFL) Institute of Microengineering (IMT) Neuchâtel, Switzerland E-mail: [email protected] [+]Equally
contributed
DOI: 10.1002/adma.201400186
Adv. Mater. 2014, DOI: 10.1002/adma.201400186
top of the microcones was removed subsequently by wet chemical etching. An aluminum-doped zinc oxide (ZnO:Al) front electrode was then conformally sputtered on the glass microcones followed by the deposition of the p/i/n amorphous silicon stack by plasma-enhanced chemical vapor deposition and a sputtered 200 nm thick ZnO:Al back contact. In order to obtain a conformal p/i/n stack with typical thicknesses of 18 nm-180 nm-18 nm respectively, we aimed at fabricating microcones with a base diameter ∼1.5 µm by creating Sn microspheres of ∼1.5 µm prior to reactive ion etching. All 2D and 3D cells fabricated in this study were deposited at the same condition by a plasmaenhanced chemical vapor deposition system at 200 °C. Figure 2a and b show the cross-section SEM image of a complete p/i/n stack on the 3D glass microcones and a 2D pyramidally textured ZnO reference. The conical shape of the microcone substrate is important for obtaining a nice conformal coating of the p/i/n stack with uniform thicknesses.[3,11] At the same p/i/n deposition thicknesses of 18nm/180nm/18nm, a pronounced increase in short-circuit current density (JSC) of 2.0 mA/cm2 was observed in the current density-voltage (J–V) characteristic (Figure 2c and Table 1) of the 3D solar cell (microcone I)) resulting in an efficiency of 8.7% compared to the efficiency of 7.7% for a reference 2D solar cell (pyramid I) deposited on pyramidally textured ZnO grown by low-pressure chemical vapor deposition as in the world record cell.[1] The enhancement in the external quantum efficiency (EQE) at wavelengths above 500 nm (Figure 2d) together with the almost identical reflectance (R) (Figure 2e) of both structures suggests that the effective light absorption path is substantially increased on the high aspect-ratio microcones. Importantly, the fill factor (FF) for the 3D solar cell (microcone I) is comparable to that of the cell on the 2D pyramidal structure (pyramid I) although the surface area of the microcone solar cells is substantially higher (see Table 1). Good FFs of 68% are maintained because of a uniform deposition of the amorphous silicon p/i/n layers over the sharply tipped microcones with optimized spacing. The measured open-circuit voltages (VOC) and dark current (J0) values of the microcone and pyramid reference cells are also very similar with 903 mV vs 905 mV and 3.1 × 10−9 mA/cm2 vs 2.5 × 10−9 mA/cm2, respectively. This demonstrates that a careful design of the substrate topology enables good electrical performance even on high-aspect ratio structures. The reduced EQE in the blue region (Figure 2d) prevents the 3D solar cell (microcone I) from achieving even higher efficiency. To better understand the lower blue response we analyzed the carrier collection distance and the light absorption distance in the microcone solar cells. Whereas the carrier
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Figure 1. SEM images showing the fabrication process for 3D a-Si:H solar cells on high aspect-ratio glass microcones (a) as-deposited Sn films with thickness 180 nm (deposition rate = 1 Å/s), and (b) annealed 180-nm Sn films resulting in formation of Sn microspheres, (c) glass microcones formed by RIE through Sn microsphere masks, (d) glass microcones after removing Sn, (e) p/i/n amorphous silicon stacks on ZnO:Al front electrode sputtered on the glass microcones.
collection distance corresponds to the physical thickness of the intrinsic amorphous silicon layer in both 2D and 3D solar cell (180 nm), the light absorption distance in the 3D cell corresponds, in a very simplified picture, to the much increased vertical thickness through which the incident sunlight travels (see Figure 2a). The area-averaged light absorption thickness in the full p/i/n stack of the 3D cell (microcone I) in the vertical direction estimated from SEM cross sections is 730 nm. The calculated effective absorption thicknesses of the individual p/i/n layers are consequently 65 nm-600 nm-65 nm, whereas the physical layer thicknesses are 18 nm-180 nm-18 nm. We therefore attribute the significantly enhanced red response observed for the 3D cell (microcone I) in comparison with the 2D cell (pyramid I) to the enhanced absorption thickness (600 nm). The FF of the 3D cell remains as high as for the 2D cell as the effective collection length (180 nm) remains small. However, due to the large effective absorption thickness of the p-layer (65 nm) significant losses in the blue part of the solar spectrum occur. Consequently we reduced the p-layer thickness from 18 nm to 6 nm, which markedly improves the blue response as seen in Figure 2d and consequently the JSC by 0.7 mA/cm2 and cell efficiency to 9% (microcone II) (see Table 1). To reach even higher JSC, we added a back-reflector (BR) on the back of the solar cells. Initially, a ZnO:Al/Ag BR was used on both microcone and pyramid solar cells and their JSCs were measured from corresponding EQE curves. The significant JSC enhancement (>20%) typically obtained on 2D cells made on crater-shaped ZnO:Al textured by HCl treatment was not observed for 2D pyramid cells (2%, see Figure 3a).[15] For the 3D microcone solar cell, JSC even declined by a massive 2 mA/cm2 (-13%, Figure 3b). As can be concluded from the
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practically identical reflectance spectra for those cells with and without Ag BR (insets of Figures 3 and 3b), this reduction in JSC of the 3D cell is unambiguously due to parasitic plasmonic absorption in the rough Ag BR. Previous studies have shown that Ag structures with small radii of curvature are indeed prone to strong absorption losses.[16–20] From plasmonics, it is also well known that small Ag particles possess a strong absorption cross section, which dominates over their scattering cross section. At larger particle sizes of about 100 nm however, the scattering cross section dominates over the absorption cross section.[21] Although parasitic plasmonic absorption is unavoidable on sharply tipped Ag BR structures, this effect could be somewhat reduced by having an optimal ZnO:Al thickness.[16] In this study, we have used 200-nm thick ZnO:Al which could be deviated from a desired thickness. In order to eliminate the parasitic plasmonic absorption in Ag, we employed non-plasmonic TiO2 nanoparticles (Figure 3c) as a BR on the back of the ZnO:Al. This improves the JSC of the microcone and pyramid solar cell by +0.7 and +0.6 mA/ cm2, respectively (Figure 3a and b). It should be noted that the difference in JSCs of the microcone cells with the two different BRs is 2.5 mA/cm2, whereas that of the pyramid cells is only 0.3 mA/cm2. This result clearly underlines the importance of a critical performance reassessment of all functional cell elements when going from 2D to 3D. Figure 3d and e show J–V and EQE curves of fully-optimized 3D (microcone IV) and 2D (pyramid II, pyramid III) cells. The deposited i-layer thickness for all these cells is 180 nm. The microcone IV and pyramid II cells have comparable light absorption losses in the p-type layer as absorption or vertical p-layer thickness is for both ∼20 nm (see Table 1). Nevertheless,
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COMMUNICATION Figure 2. Optimization of 3D a-Si:H solar cells. Cross sectional SEM of a-Si:H p/i/n stacks on (a) a 3D microcone and (b) 2D pyramid, (c) and (d) J–V and EQE curves of 2D pyramid I, 3D microcone I and, 3D microcone II, and (e) R of 2D pyramid I and 3D microcone II
the 3D microcones result in a strong enhancement in the light absorption over the full wavelength range resulting in a marked 2.7 mA/cm2 increase in JSC (Figure 3e, Table 1) and 16% efficiency improvement. Even when compared to a 2D cell with a thinner p-layer (pyramid III), which improves its blue response, the strong red response of the 3D cell (microcone IV) dominates the performance, which results still in a 11% efficiency improvement. Furthermore, the 3D cell exhibits a nearly 60% efficiency enhancement over its counterpart 2D cell on a
untextured flat substrate. In addition, it is interesting to note that the JSCs obtained from microcone IV and pyramid IV are comparable although the absorber deposition thicknesses for microcone IV and pyramid IV are 180 nm and 600 nm, respectively. This is because they both share the same equivalent light absorption (vertical) thickness of 600 nm. The conversion efficiency of 9.4% for the optimized 3D microcone cell is the highest so far reported for a solar cell with high aspect-ratio light trapping structures.
Table 1. Summary of characteristics of 2D pyramid and 3D microcone a-Si:H solar cells fabricated in this study. Substrate
2D Pyramid I
BR
p/i/n electrical ( = deposition) thickness (nm)
p/i/n vertical ( = absorption) thickness (nm)
Efficiency (%)
FF (%)
VOC (mV)
JSC (mA/cm2)
None
18/180/18
18/180/18
7.7
70.9
903
12.0
2D Pyramid II
TiO2
18/180/18
18/180/18
8.1
70.9
903
12.7
2D Pyramid III
TiO2
6/180/18
6/180/18
8.5
69.8
910
13.4
2D Pyramid IV
TiO2
18/600/18
18/600/18
7.7
55.5
905
15.3
2D Flat
TiO2
18/180/18
18/180/18
5.9
71.1
905
9.1
3D Microcone I
None
18/180/18
65/600/65
8.7
68.5
905
14.0
3D Microcone II
None
6/180/18
21/600/65
9.0
66.6
913
14.7
3D Microcone III
Ag
6/180/18
21/600/65
7.7
65.5
913
12.9
3D Microcone IV
TiO2
6/180/18
21/600/65
9.4
66.6
913
15.4
Adv. Mater. 2014, DOI: 10.1002/adma.201400186
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Figure 3. Effect of back-reflectors (BR) on the performances of 3D and 2D a-Si:H solar cells (a) EQE curves of a-Si:H solar cells on pyramids with different BRs, R curves in inset, (b) EQE curves of a-Si:H solar cells on microcones with different BRs, R curves in inset (c) SEM micrograph of TiO2 BRs on 3D microcone cells, (d) J–V curves of a-Si:H solar cells with a 180-nm i-layer on 3D microcones with a TiO2 BR (3D microcone IV), and 2D pyramid with a TiO2 BR (2D pyramid II and pyramid III), and (e) EQE of 3D microcone IV, 2D pyramid II and pyramid III.
In addition to achieving higher initial efficiency, the 3D concept further has the benefit of improving cell stability by minimizing light induced degradation (LID) of cell performance due to the Staebler-Wronski effect[22] thanks to the shorter carrier collection length. 3D solar cells with an i-layer of 180 nm and 2D solar cells with i-layers of 180 and 600 nm were degraded under 1.5 sun for 5 days. The relative LID in the 3D solar cells with a 180-nm thick i-layer is nearly identical to that in the 180 nm 2D solar cells, which is expected as the collection length is equivalent (see Table 2). ∼18% LID is within the range of reported LID values for cells with ∼200 nm thick i-layers.[23] However, the resulting stable efficiency of the 3D
solar cell is significantly higher than that of the 2D cell because of the much higher JSC. The degradation of the 2D solar cell with the 600 nm thick i-layer is 2x higher than that in 3D solar cells. These results clearly stress the importance of reducing the absorber layer thickness in order to obtain high stabilized efficiencies. Finally, we point out that our 3D microcone cell design benefits from the same tolerance known for the 2D cells with random pyramids with respect to the angular distribution of incident light, as the arrangement of microcones is equally random. In addition, light incident onto the flat air/glass interface of both cell designs is refracted towards the surface normal
Table 2. Light induced degradation performances of 3D and 2D a-Si:H solar cells. Substrate 3D Microcone
2D pyramid
2D pyramid
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Absorber thickness (nm) 180
180
600
VOC (mV)
JSC (mA/cm2)
Degradation (%)
66.6
913
15.4
18.3
58.8
880
15.2
State
Efficiency (%)
FF (%)
Initial
9.4
Stable
7.9
Initial
8.1
70.9
903
12.7
Stable
6.5
60.8
866
12.4
Initial
7.7
55.5
905
15.3
Stable
5.6
43.8
854
14.9
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
19.2
37.3
Adv. Mater. 2014, DOI: 10.1002/adma.201400186
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Received: January 13, 2014 Revised: February 28, 2014 Published online: [1] S. Benagli, D. Borrello, E. Vallat-Sauvain, J. Meier, U. Kroll, J. Hötzel, J. Bailat, J. Steinhauser, M. Marmelo, G. Monteduro, L. Castens, Proceedings of the 24th European Photovoltaic Solar Energy Conference, Hamburg, 2009. [2] C. Battaglia, C.-M. Hsu, K. Söderström, J. Escarré, F.-J. Haug, M. Charrière, M. Boccard, M. Despeisse, D. T. L. Alexander, M. Cantoni, Y. Cui, C. Ballif, ACS Nano. 2012, 6, 2790. [3] C.-M. Hsu, C. Battaglia, C. Pahud, Z. Ruan, F.-J. Haug, S. Fan, C. Ballif, Y. Cui, Adv. Energy Mater. 2012, 2, 628. [4] M. Meier, U. W. Paetzold, M. Prömpers, T. Merdzhanova, R. Carius, A. Gordijn, Prog. Photovolt: Res. and Appl. doi:10.1002/pip.2382. [5] M. Vanecek, O. Babchenko, A. Purkrt, J. Holovsky, N. Neykova, A. Poruba, Z. Remes, H. Meier, U. Kroll, Appl. Phys. Lett. 2011, 98, 163503. [6] H. Sai, K. Saito, M. Kondo, Appl. Phys. Lett. 2012, 101, 173901.
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rendering the photocurrent relatively insensitive to the light incidence. In summary, we have shown that optically-thick (equivalent optical thickness of 600 nm) and electrically-thin (180 nm) 3D solar cells can yield enhanced JSC while maintaining a high FF that is typically obtained from a 2D solar cell. This results in a respectable initial efficiency of 9.4% for a 3D a-Si:H solar cell. While this value remains below the performance benchmark provided by state-of-the-art 2D a-Si:H cells, our work represents an important milestones towards 3D a-Si:H cells operating beyond the 4n2 limit.
[7] V. Ferry, M. A. Verschuuren, H. B. T. Li, R. E. I. Schropp, H. A. Atwater, A. Polman, Appl. Phys. Lett. 2009, 95, 183503. [8] T. G. Chen, P. Yu, Y. L. Tsai, C. H. Shen, J. M. Shieh, M. A. Tsai, H. C. Kuo, Opt. Exp. 2012, 20, A412. [9] M. J. Naughton, K. Kempa, Z. F. Ren, Y. Gao, J. Rybczynski, N. Argenti, W. Gao, Y. Wang, Y. Peng1, J. R. Naughton, G. McMahon, T. Paudel, Y. C. Lan, M. J. Burns, A. Shepard, M. Clary, C. Ballif, F.-J. Haug, T. Söderström, O. Cubero, C. Eminian, Phys. Status Solidi 2010, 4, 181. [10] Y. Kuang, H. M. Karine, K. H. van der Werf, Z. S. Houweling, R. E. I. Schropp, Appl. Phys. Lett. 2011, 98, 113111. [11] J. Kim, A. J. Hong, J.-W. Nah, B. Shin, F. M. Ross, D. K. Sadana, ACS Nano. 2012, 6, 265. [12] J. Cho, B. O'Donnell, L. Yu, K. Kim, I. Ngo, P. R. Cabarrocas, Prog. Photovolt: Res. Appl. 2012, 21, 77. [13] S. Misra, L. Yu, M. Foldyna, P. Roca i Cabarrocas, Sol. Ene. Mat. & Sol. Cells 2013, 118, 90–95. [14] M. M. Adachi, M. P. Anantram, K. S. Karim, Scientific Reports 2013, 3, 1546. [15] J. Yoo, J. Lee, S. Kim, K. Yoon, I. J. Park, S. K. Dhungel, B. Karunagaran, D. Mangalaraj, J. Yi, Phys. Stat. Sol. (c) 2005, 2, 1228. [16] F.-J. Haug, T. Söderström, O. Cubero, V. Terrazzoni-Daudrix, C. Ballif, J. Appl. Phys. 2009, 106, 044502. [17] V. E. Ferry, A. Polman, H. A. Atwater, ACS Nano. 2010, 5, 10055. [18] P. Cuony, PhD Thesis, EPFL, Lausanne, 2011. [19] J. Springer, A. Poruba, L. Müllerova, M. Vanecek, O. Kluth, B. Rech, J. Appl. Phys. 2004, 95, 1427. [20] U. W. Paetzold, F. Hallermann, B. E. Pieters, U. Rau, R. Carius, G. von Plessen, Proceedings of the SPIE 2010, 7725, 772517. [21] K. Catchpole, A. Polman, Opt. Exp. 2008, 16, 21793. [22] D. L. Staebler, C. R. Wronski, J. Appl. Phys. 1980, 51, 3262. [23] M. Stuckelberger, M. Despeisse, G. Bugnon, J.-W. Schüttauf, F.-J. Haug, C. Ballif, J. Appl. Phys. 2013, 114, 154509.
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9.4% Efficient Amorphous Silicon Solar Cell on High Aspect-Ratio Glass Microcones Jeehwan Kim,* Corsin Battaglia,* Mathieu Charrière, Augustin Hong, Wooshik Jung, Hongsik Park, Christophe Ballif, and Devendra Sadana
Strong light absorption and excellent carrier collection are essential for achieving solar cells with high power conversion efficiency. However, for hydrogenated amorphous silicon (a-Si:H), the absorption coefficient becomes small towards the near-infrared and the carrier lifetimes are not sufficient to increase the absorber thickness beyond 250 nm. Many studies have promoted the idea of going optically thick, to maximize the light absorption path, while staying electrically thin, to guarantee short carrier collection distances by folding the absorber layer on a substrate with modified topological features. However, most a-Si:H solar cells reported in the literature are deposited on substrates with typical aspect ratios far below 1 (called 2D hereafter),[1–8] including the current world record cell on pyramidally textured zinc oxide grown by chemical vapor deposition.[1] In fact only a handful studies have attempted topologies with aspect ratios >1 (3D),[9–14] but efficiencies have so far not been able to rival with the more planar 2D topologies with aspect ratios 60% compared to a planar control device. Scanning electron microscopy (SEM) images in Figure 1 show the process flow for forming high aspect-ratio 3D microcone solar cells. Tin (Sn) microspheres self-assembled on glass substrates were used as masks for reactive ion etching (RIE) to form glass microcones with aspect ratio of ∼2. Remaining Sn on
J. Kim,[+] A. Hong, W. Jung, H. Park, D. Sadana IBM T.J. Watson Research Center Yorktown Heights New York 10598, USA E-mail: [email protected] C. Battaglia,[+] M. Charrière, C. Ballif cole Polytechnique Fédérale de Lausanne (EPFL) Institute of Microengineering (IMT) Neuchâtel, Switzerland E-mail: [email protected] [+]Equally
contributed
DOI: 10.1002/adma.201400186
Adv. Mater. 2014, DOI: 10.1002/adma.201400186
top of the microcones was removed subsequently by wet chemical etching. An aluminum-doped zinc oxide (ZnO:Al) front electrode was then conformally sputtered on the glass microcones followed by the deposition of the p/i/n amorphous silicon stack by plasma-enhanced chemical vapor deposition and a sputtered 200 nm thick ZnO:Al back contact. In order to obtain a conformal p/i/n stack with typical thicknesses of 18 nm-180 nm-18 nm respectively, we aimed at fabricating microcones with a base diameter ∼1.5 µm by creating Sn microspheres of ∼1.5 µm prior to reactive ion etching. All 2D and 3D cells fabricated in this study were deposited at the same condition by a plasmaenhanced chemical vapor deposition system at 200 °C. Figure 2a and b show the cross-section SEM image of a complete p/i/n stack on the 3D glass microcones and a 2D pyramidally textured ZnO reference. The conical shape of the microcone substrate is important for obtaining a nice conformal coating of the p/i/n stack with uniform thicknesses.[3,11] At the same p/i/n deposition thicknesses of 18nm/180nm/18nm, a pronounced increase in short-circuit current density (JSC) of 2.0 mA/cm2 was observed in the current density-voltage (J–V) characteristic (Figure 2c and Table 1) of the 3D solar cell (microcone I)) resulting in an efficiency of 8.7% compared to the efficiency of 7.7% for a reference 2D solar cell (pyramid I) deposited on pyramidally textured ZnO grown by low-pressure chemical vapor deposition as in the world record cell.[1] The enhancement in the external quantum efficiency (EQE) at wavelengths above 500 nm (Figure 2d) together with the almost identical reflectance (R) (Figure 2e) of both structures suggests that the effective light absorption path is substantially increased on the high aspect-ratio microcones. Importantly, the fill factor (FF) for the 3D solar cell (microcone I) is comparable to that of the cell on the 2D pyramidal structure (pyramid I) although the surface area of the microcone solar cells is substantially higher (see Table 1). Good FFs of 68% are maintained because of a uniform deposition of the amorphous silicon p/i/n layers over the sharply tipped microcones with optimized spacing. The measured open-circuit voltages (VOC) and dark current (J0) values of the microcone and pyramid reference cells are also very similar with 903 mV vs 905 mV and 3.1 × 10−9 mA/cm2 vs 2.5 × 10−9 mA/cm2, respectively. This demonstrates that a careful design of the substrate topology enables good electrical performance even on high-aspect ratio structures. The reduced EQE in the blue region (Figure 2d) prevents the 3D solar cell (microcone I) from achieving even higher efficiency. To better understand the lower blue response we analyzed the carrier collection distance and the light absorption distance in the microcone solar cells. Whereas the carrier
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Figure 1. SEM images showing the fabrication process for 3D a-Si:H solar cells on high aspect-ratio glass microcones (a) as-deposited Sn films with thickness 180 nm (deposition rate = 1 Å/s), and (b) annealed 180-nm Sn films resulting in formation of Sn microspheres, (c) glass microcones formed by RIE through Sn microsphere masks, (d) glass microcones after removing Sn, (e) p/i/n amorphous silicon stacks on ZnO:Al front electrode sputtered on the glass microcones.
collection distance corresponds to the physical thickness of the intrinsic amorphous silicon layer in both 2D and 3D solar cell (180 nm), the light absorption distance in the 3D cell corresponds, in a very simplified picture, to the much increased vertical thickness through which the incident sunlight travels (see Figure 2a). The area-averaged light absorption thickness in the full p/i/n stack of the 3D cell (microcone I) in the vertical direction estimated from SEM cross sections is 730 nm. The calculated effective absorption thicknesses of the individual p/i/n layers are consequently 65 nm-600 nm-65 nm, whereas the physical layer thicknesses are 18 nm-180 nm-18 nm. We therefore attribute the significantly enhanced red response observed for the 3D cell (microcone I) in comparison with the 2D cell (pyramid I) to the enhanced absorption thickness (600 nm). The FF of the 3D cell remains as high as for the 2D cell as the effective collection length (180 nm) remains small. However, due to the large effective absorption thickness of the p-layer (65 nm) significant losses in the blue part of the solar spectrum occur. Consequently we reduced the p-layer thickness from 18 nm to 6 nm, which markedly improves the blue response as seen in Figure 2d and consequently the JSC by 0.7 mA/cm2 and cell efficiency to 9% (microcone II) (see Table 1). To reach even higher JSC, we added a back-reflector (BR) on the back of the solar cells. Initially, a ZnO:Al/Ag BR was used on both microcone and pyramid solar cells and their JSCs were measured from corresponding EQE curves. The significant JSC enhancement (>20%) typically obtained on 2D cells made on crater-shaped ZnO:Al textured by HCl treatment was not observed for 2D pyramid cells (2%, see Figure 3a).[15] For the 3D microcone solar cell, JSC even declined by a massive 2 mA/cm2 (-13%, Figure 3b). As can be concluded from the
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practically identical reflectance spectra for those cells with and without Ag BR (insets of Figures 3 and 3b), this reduction in JSC of the 3D cell is unambiguously due to parasitic plasmonic absorption in the rough Ag BR. Previous studies have shown that Ag structures with small radii of curvature are indeed prone to strong absorption losses.[16–20] From plasmonics, it is also well known that small Ag particles possess a strong absorption cross section, which dominates over their scattering cross section. At larger particle sizes of about 100 nm however, the scattering cross section dominates over the absorption cross section.[21] Although parasitic plasmonic absorption is unavoidable on sharply tipped Ag BR structures, this effect could be somewhat reduced by having an optimal ZnO:Al thickness.[16] In this study, we have used 200-nm thick ZnO:Al which could be deviated from a desired thickness. In order to eliminate the parasitic plasmonic absorption in Ag, we employed non-plasmonic TiO2 nanoparticles (Figure 3c) as a BR on the back of the ZnO:Al. This improves the JSC of the microcone and pyramid solar cell by +0.7 and +0.6 mA/ cm2, respectively (Figure 3a and b). It should be noted that the difference in JSCs of the microcone cells with the two different BRs is 2.5 mA/cm2, whereas that of the pyramid cells is only 0.3 mA/cm2. This result clearly underlines the importance of a critical performance reassessment of all functional cell elements when going from 2D to 3D. Figure 3d and e show J–V and EQE curves of fully-optimized 3D (microcone IV) and 2D (pyramid II, pyramid III) cells. The deposited i-layer thickness for all these cells is 180 nm. The microcone IV and pyramid II cells have comparable light absorption losses in the p-type layer as absorption or vertical p-layer thickness is for both ∼20 nm (see Table 1). Nevertheless,
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201400186
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COMMUNICATION Figure 2. Optimization of 3D a-Si:H solar cells. Cross sectional SEM of a-Si:H p/i/n stacks on (a) a 3D microcone and (b) 2D pyramid, (c) and (d) J–V and EQE curves of 2D pyramid I, 3D microcone I and, 3D microcone II, and (e) R of 2D pyramid I and 3D microcone II
the 3D microcones result in a strong enhancement in the light absorption over the full wavelength range resulting in a marked 2.7 mA/cm2 increase in JSC (Figure 3e, Table 1) and 16% efficiency improvement. Even when compared to a 2D cell with a thinner p-layer (pyramid III), which improves its blue response, the strong red response of the 3D cell (microcone IV) dominates the performance, which results still in a 11% efficiency improvement. Furthermore, the 3D cell exhibits a nearly 60% efficiency enhancement over its counterpart 2D cell on a
untextured flat substrate. In addition, it is interesting to note that the JSCs obtained from microcone IV and pyramid IV are comparable although the absorber deposition thicknesses for microcone IV and pyramid IV are 180 nm and 600 nm, respectively. This is because they both share the same equivalent light absorption (vertical) thickness of 600 nm. The conversion efficiency of 9.4% for the optimized 3D microcone cell is the highest so far reported for a solar cell with high aspect-ratio light trapping structures.
Table 1. Summary of characteristics of 2D pyramid and 3D microcone a-Si:H solar cells fabricated in this study. Substrate
2D Pyramid I
BR
p/i/n electrical ( = deposition) thickness (nm)
p/i/n vertical ( = absorption) thickness (nm)
Efficiency (%)
FF (%)
VOC (mV)
JSC (mA/cm2)
None
18/180/18
18/180/18
7.7
70.9
903
12.0
2D Pyramid II
TiO2
18/180/18
18/180/18
8.1
70.9
903
12.7
2D Pyramid III
TiO2
6/180/18
6/180/18
8.5
69.8
910
13.4
2D Pyramid IV
TiO2
18/600/18
18/600/18
7.7
55.5
905
15.3
2D Flat
TiO2
18/180/18
18/180/18
5.9
71.1
905
9.1
3D Microcone I
None
18/180/18
65/600/65
8.7
68.5
905
14.0
3D Microcone II
None
6/180/18
21/600/65
9.0
66.6
913
14.7
3D Microcone III
Ag
6/180/18
21/600/65
7.7
65.5
913
12.9
3D Microcone IV
TiO2
6/180/18
21/600/65
9.4
66.6
913
15.4
Adv. Mater. 2014, DOI: 10.1002/adma.201400186
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Figure 3. Effect of back-reflectors (BR) on the performances of 3D and 2D a-Si:H solar cells (a) EQE curves of a-Si:H solar cells on pyramids with different BRs, R curves in inset, (b) EQE curves of a-Si:H solar cells on microcones with different BRs, R curves in inset (c) SEM micrograph of TiO2 BRs on 3D microcone cells, (d) J–V curves of a-Si:H solar cells with a 180-nm i-layer on 3D microcones with a TiO2 BR (3D microcone IV), and 2D pyramid with a TiO2 BR (2D pyramid II and pyramid III), and (e) EQE of 3D microcone IV, 2D pyramid II and pyramid III.
In addition to achieving higher initial efficiency, the 3D concept further has the benefit of improving cell stability by minimizing light induced degradation (LID) of cell performance due to the Staebler-Wronski effect[22] thanks to the shorter carrier collection length. 3D solar cells with an i-layer of 180 nm and 2D solar cells with i-layers of 180 and 600 nm were degraded under 1.5 sun for 5 days. The relative LID in the 3D solar cells with a 180-nm thick i-layer is nearly identical to that in the 180 nm 2D solar cells, which is expected as the collection length is equivalent (see Table 2). ∼18% LID is within the range of reported LID values for cells with ∼200 nm thick i-layers.[23] However, the resulting stable efficiency of the 3D
solar cell is significantly higher than that of the 2D cell because of the much higher JSC. The degradation of the 2D solar cell with the 600 nm thick i-layer is 2x higher than that in 3D solar cells. These results clearly stress the importance of reducing the absorber layer thickness in order to obtain high stabilized efficiencies. Finally, we point out that our 3D microcone cell design benefits from the same tolerance known for the 2D cells with random pyramids with respect to the angular distribution of incident light, as the arrangement of microcones is equally random. In addition, light incident onto the flat air/glass interface of both cell designs is refracted towards the surface normal
Table 2. Light induced degradation performances of 3D and 2D a-Si:H solar cells. Substrate 3D Microcone
2D pyramid
2D pyramid
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Absorber thickness (nm) 180
180
600
VOC (mV)
JSC (mA/cm2)
Degradation (%)
66.6
913
15.4
18.3
58.8
880
15.2
State
Efficiency (%)
FF (%)
Initial
9.4
Stable
7.9
Initial
8.1
70.9
903
12.7
Stable
6.5
60.8
866
12.4
Initial
7.7
55.5
905
15.3
Stable
5.6
43.8
854
14.9
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
19.2
37.3
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rendering the photocurrent relatively insensitive to the light incidence. In summary, we have shown that optically-thick (equivalent optical thickness of 600 nm) and electrically-thin (180 nm) 3D solar cells can yield enhanced JSC while maintaining a high FF that is typically obtained from a 2D solar cell. This results in a respectable initial efficiency of 9.4% for a 3D a-Si:H solar cell. While this value remains below the performance benchmark provided by state-of-the-art 2D a-Si:H cells, our work represents an important milestones towards 3D a-Si:H cells operating beyond the 4n2 limit.
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