Selectively transparent and conducting photonic ... - OSA Publishing

Selectively transparent and conducting photonic crystal rear-contacts for thin-film silicon-based building integrated photovoltaics P. G. O’Brien,1 A. Chutinan,2 P. Mahtani,2 K. Leong,2 G. A. Ozin,3 and N. P. Kherani1,2* 1

Department of Materials Science and Engineering, University of Toronto, 184 College Street, room 140 Toronto, Ontario, M5S 3E4, Canada 2 Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road Toronto, Ontario, M5S 3G4, Canada 3 Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, M5S 3H6, Canada *[email protected]

Abstract: Wave-optics analysis is performed to show that selectively transparent and conducting photonic crystals (STCPCs) can be utilized as rear contacts to enhance the performance of building-integrated photovoltaics (BIPV). For instance, the current generated in an a-Si:H cell with an STCPC functioning as its rear contact is comparable to that of a similar cell with an optimized ZnO/Ag rear contact. However, the solar lumens (~3.5 klm/m2) and power (~430W/m2) transmitted through the cell with the STCPC rear contact can potentially provide indoor heating and lighting, respectively. Moreover, experimental results show that STCPC rear contacts could be used to control the color temperature of light transmitted through BIPV panels. ©2011 Optical Society of America OCIS codes: (050.5298) Photonic crystals; (350.6050) Solar energy; (040.5350) Photovoltaics; (310.0310) Thin films.

References and links 1.

D. Ginley, M. A. Green, and R. Collins, “Solar energy conversion toward 1 terawatt,” Mater. Res. Bull. 33(4), 355–364 (2008). 2. Electricity from Renewable Resources: Status, Prospects, and Impediments, American's Energy Future Panel on Electricity from Renewable Resources, National Academy of Sciences, National Academy of Engineering, National Research Council of the National Academies. (National Academies Press, 2010), Ch. 2. 3. T. T. Chow, “A review on photovoltaic/thermal hybrid solar technology,” Appl. Energy 87(2), 365–379 (2010). 4. H. A. Zondag, “Flat-plate PV-thermal collectors and systems: a review,” Renew. Sustain. Energy Rev. 12(4), 891–959 (2008). 5. H. Maurus, M. Schmid, B. Blersch, P. Lechner, and H. Schade, “PV for buildings: benefits and experiences with amorphous silicon in BIPV applications,” Refocus 5(6), 22–27 (2004). 6. M. Pagliaro, R. Ciriminna, and G. Palmisano, “BIPV: merging the photovoltaic with the construction industry,” Prog. Photovolt. Res. Appl. 18(1), 61–72 (2010). 7. D. L. Staebler and C. R. Wronski, “Reversible conductivity changes in discharge-produced amorphous Si,” Appl. Phys. Lett. 31(4), 292–294 (1977). 8. A. V. Shah, R. Platz, and H. Keppner, “Thin-film silicon solar cells: a review and selected trends,” Sol. Energy Mater. Sol. Cells 38(1–4), 501–520 (1995). 9. J. Meier, S. Dubail, R. Flückiger, D. Fischer, H. Keppner, and A. Shah, “Intrinsic microcrystalline silicon (µcSi:H)- a promising new thin film solar cell material,” in Proceedings of the 1st World Conference on Photovoltaic Energy Conversion (IEEE, New York, 1994), pp. 409–412. 10. J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Photonic Crystals: Molding the Flow of Light, 2nd ed. (Princeton University Press, 2008). 11. P. G. O’Brien, A. Chutinan, K. Leong, N. P. Kherani, G. A. Ozin, and S. Zukotynski, “Photonic crystal intermediate reflectors for micromorph solar cells: a comparative study,” Opt. Express 18(5), 4478–4490 (2010). 12. S. Fahr, C. Rockstuhl, and F. Lederer, “The interplay of intermediate reflectors and randomly textured surfaces in tandem solar cells,” Appl. Phys. Lett. 97(17), 173510 (2010).

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Received 27 Jun 2011; revised 29 Jul 2011; accepted 6 Aug 2011; published 16 Aug 2011

29 August 2011 / Vol. 19, No. 18 / OPTICS EXPRESS 17040

13. S. B. Mallick, N. P. Sergeant, M. Agrawal, J. Lee, and P. Peumans, “Coherent light trapping in thin-film photovoltaics,” Mater. Res. Bull. 36(6), 453–460 (2011). 14. F. Duerinckx, I. Kuzma-Filipek, K. Van Nieuwenhuysen, G. Beaucarne, and J. Poortmans, “Reorganized porous silicon Bragg reflectors for thin-film silicon solar cells,” IEEE Electron Device Lett. 27(10), 837–839 (2006). 15. X. Sheng, J. Liu, I. Kozinsky, A. M. Agarwal, J. Michel, and L. C. Kimerling, “Design and non-lithographic fabrication of light trapping structures for thin film silicon solar cells,” Adv. Mater. (Deerfield Beach Fla.) 23(7), 843–847 (2011). 16. P. G. O’Brien, D. P. Puzzo, A. Chutinan, L. D. Bonifacio, G. A. Ozin, and N. P. Kherani, “Selectively transparent and conducting photonic crystals,” Adv. Mater. (Deerfield Beach Fla.) 22(5), 611–616 (2010). 17. P. G. O’Brien, D. P. Puzzo, N. P. Kherani, G. A. Ozin, A. Chutinan, Z. Lu, and M. G. Helander, “Transparent conductive porous nanocomposites and methods of fabrication thereof,” Pat. No. WO/2011/044687, (April 21, 2011). 18. C. Z. O. Transparent, Basics and Applications in Thin Film Solar Cells, ed. R. Hull, J. Parisi, R.M. Osgood, and H. Warlimont, (Springer, 2008). 19. J. Springer, A. Poruba, L. Müllerova, M. Vanecek, O. Kluth, and B. Rech, “Absorption loss at nanorough silver back reflector of thin-film silicon solar cells,” J. Appl. Phys. 95(3), 1427–1429 (2004). 20. J. Springer, A. Poruba, and M. Vanecek, “Improved three-dimensional optical model for thin-film silicon solar cells,” J. Appl. Phys. 96(9), 5329–5337 (2004). 21. D. M. Whittaker and I. S. Culshaw, “Scattering-matrix treatment of patterned multilayer photonic structures,” Phys. Rev. B 60(4), 2610–2618 (1999). 22. M. Liscidini, D. Gerace, L. C. Andreani, and J. E. Sipe, “Scattering-matrix analysis of periodically patterned multilayers with asymmetric unit cells and birefringent media,” Phys. Rev. B 77(3), 035324 (2008). 23. ASTMG, 173–03, Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37 Degree Tilted Surface (ASTM International, 2005). 24. S. Adachi, Optical Constants of Crystalline and Amorphous Semiconductors: Numerical and Graphical Information, 1st edition, (Kluwer Acaemic Publishers, 1999). 25. F. David, Edwards, “Silicon (Si),” in Handbook of Optical Constants of Solids, E.D. Palik, ed. (Academic, 1985). 26. R. B. Stephens and G. D. Cody, “Optical reflectance and transmission of a textured surface,” Thin Solid Films 45(1), 19–29 (1977). 27. F. E. Schubert, Light-Emitting Diodes, 2nd ed. (Cambridge University Press, 2006), Ch.16. 28. A. Nabil and J. Mardaljevic, “Useful daylight illuminance: a new paradigm for assessing daylight in buildings,” Lighting Res. Tech. 37(1), 41–59 (2005). 29. S. Hegedus, “Thin film solar modules: the low cost, high throughput and versatile alternative to Si wafers,” Prog. Photovolt. Res. Appl. 14(5), 393–411 (2006). 30. Á. Ruiz, J. M. Salmerόn, F. Sánchez, R. González, and S. Álvarez, “A calculation model for trombe walls and its use as a passive cooling technique,” International Conference on Passive and Low Energy Cooling for the Built Environment, (Santorini, 2005), pp. 365–369.

1. Introduction Engineers and research scientists continue to work towards economizing renewable energy sources in order to avoid the increasing difficulties associated with using fossil fuels and the detrimental climatic changes being caused by their emissions. Harnessing solar power with photovoltaic cells and thermal collectors is a promising renewable energy alternative because only a very small fraction of the insolation on earth is required to meet the global energy demand [1]. For example, assuming 10% energy conversion, PV systems covering just 0.25% of the United States land mass would generate the 4.2 million GWh of electricity domestically generated in 2007 [2]. Building integrated photovolatics (BIPV) offers an elegant way of installing the required amount of PV systems without using additional land. In practice, BIPV panels are used to replace building materials such as rooftops, skylights and facades and convert a portion of the solar irradiance into electricity while transmitting the remaining portion of the solar irradiance into the building to provide lighting or to be used as a heat source. In general the requirements of a BIPV panel are design specific and depend on the lighting and heating requirements of the building under consideration. Thorough reviews covering the history of BIPV and PV/T as well as current research being conducted in these fields are provided in the literature [3,4]. Thin-film hydrogenated amorphous silicon (a-Si:H) cells, have numerous inherent advantages for BIPV applications [5]. For example, amorphous silicon is a readily available non-toxic material that can be processed at relatively low temperatures and is amenable to large area deposition on low-cost flat glass substrates. Also, a-Si:H has a very low

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temperature coefficient of just −0.1%/K compared to −0.4%/K for c-Si and copper indium gallium selenide CIGS cells. An aesthetically pleasing example wherein semi-transparent aSi:H PV panels are used to generate electricity while providing the proper balance of shelter and day lighting is New York’s Stillwell Avenue subway station [6]. One disadvantage of using a-Si:H cells is that their performance suffers from light induced degradation by means of the Staebler-Wronski effect [7]. This effect can be alleviated to some extent by thinning the cell to ensure a large internal electric field within its active region [8]. However, thinning the a-Si:H cell also causes a reduction in its absorption and consequently light-trapping strategies must be incorporated into thinned a-Si:H cells. The light-trapping problem in a-Si:H cells for BIPV applications is comparable to that in the micromorph cell [9]. In both cases the majority of incident photons of energy greater than the absorption edge of a-Si:H (~1.7 eV) must be confined and absorbed in the cell in order to boost its current while photons of lesser energy must be transmitted; in the micromorph cell the transmitted photons power the underlying mc-Si:H cell while in BIPV applications they reduce thermal and lighting energy requirements for buildings. Recently presented theoretical calculations show that highly transparent and conducting Bragg-reflectors, effectively onedimensional photonic crystals [10], are well-suited to function as intermediate reflectors in micromorph cells on account of their large photonic stop-gap and low absorption losses [11,12]. In this configuration the stop-gap of the one-dimensional PC can be set to reflect a large portion of the solar irradiance of energy greater than ~1.7 eV back into the a-Si:H cell while simultaneously transmitting photons of lesser energy. As described subsequently, in this work we extend this idea to enhance the performance of a-Si:H based BIPV facades. Over the last few years a number of research groups have focused their efforts on understanding and utilizing photonic crystal structures that enhance light-trapping in thin-film photovoltaics [13]. A relevant example is the integration of Bragg-reflectors into the rear-side of thin-film silicon-based photovoltaic cells that reduce transmission losses [14,15]. Herein, we present optical simulations and experiments that investigate the potential benefits of utilizing a new class of one-dimensional photonic crystals, namely selectively transparent and conducting photonic crystals (STCPCs) [16], as rear contacts that simultaneously function as a solar spectrum splitter to enhance light trapping in thin a-Si:H solar cells for BIPV applications. A cross-sectional SEM image of these STCPCs, which are comprised of alternating layers of sputtered ITO and spin-coated silica nanoparticle films, is shown as the inset in Fig. 1. A brief description of the techniques used to fabricate the silica nanoparticle films are provided in the appendix. Typically, silica nanoparticles are highly resistive. However, during the sputtering process ITO infiltrates and coats the pores of the silica nanoparticle film to create a continuous network of electrically conductive material to provide a pathway for electron transport throughout the STCPC. Further details on the electrical properties of STCPCs are available in the literature [17], while the work presented herein focuses solely on the optical aspects of integrating STCPC rear contacts into thin-film siliconbased BIPV panels. STCPCs comprising silica nanoparticle films are well-suited to function as rear contacts for a-Si:H cells in BIPV applications because they combine intense wavelength selective broadband reflectance with the transmissive and conductive properties of sputtered ITO. For example, one-dimensional STCPCs comprised of alternating layers of sputtered ITO and spincoated silica nanoparticle films can be made to exhibit Bragg-reflectance peaks in the visible spectrum as high as 95% and with a full width at half maximum that is greater than 200 nm. At the same time, their average transmittance over the remaining portion of the visible spectrum outside their stop-gap is greater than 80% [17]. Moreover, the spectral position of the stop-gap can be easily tuned through precise control of the thickness of their sputtered ITO layers. Also, unlike solar spectrum splitters made from more conventional but non-conducting optical materials such as silica or titania, STCPC rear-contacts can be used as a drop-in replacement for the rear contact of an a-Si:H cell.

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The details about the modeled a-Si:H cell structure with STCPCs are presented in the next Section while the modeling results are presented in Section 3. Section 4 discusses how STCPC rear contacts provide versatility in designing BIPV facades. Experimental steps towards realizing a-Si:H cells integrated with STCPC rear-contacts for BIPV applications are presented in Section 5 and some concluding remarks are provided in Section 6. 2. Modeling details of a-Si:H cells with STCPC rear contacts for BIPV applications A schematic of the a-Si:H cell modeled in this work is shown in Fig. 1 wherein the cell is divided into three regions; the first region comprises the glass substrate and front contact, the second region contains the a-Si:H cell, and the third region is the rear contact. The cell is fabricated in the superstrate configuration on a glass substrate which is assumed to have an index of refraction of 1.5 for all wavelengths. To simplify the calculations this substrate is assumed to be semi-infinite and the reflection from the top surface of the glass is neglected. Consequently, the results presented herein may overestimate the lumens and solar power transmitted through the cell by a few percent. It is assumed that a 400nm thick transparent conducting oxide (TCO) film has been deposited onto the rear side of the glass substrate to serve as an electrical contact. This TCO film must be sufficiently thick to conduct the current generated in the cell and also highly transparent to avoid absorption losses [18].

⨀E Region 1 Substrate and front TCO contact Region 2 p- i- n- a-Si:H cell Region 3 ZnO rear contact

Case 1

Region 3 STCPC rear contact Case 2 Region 3 ZnO/Ag rear contact

Case 3

Fig. 1. The a-Si:H superstrate cell modeled in this work. Three different cases are considered for the choice of the rear contact; a homogeneous ZnO film, a STCPC, and a ZnO/Ag composite film. A cross-sectional SEM image of an STCPC comprised of sputtered ITO and silica nanoparticle films is shown as the inset.

As shown in the second region in Fig. 1 the effects of texturing the surface of the TCO in a triangular pattern are also considered. The texture height is scanned from 0nm to 300nm. Thus, including the textured surface, the total height of the ZnO front contact ranges from 400 to 700nm. It is assumed that a 100nm thick homogeneous a-Si:H cell is deposited such that it forms a conformal coating on the top ZnO contact. The thickness of a-Si:H cells is typically closer to 200nm. However, extremely thin a-Si:H cells are of particular interest for BIPV applications because these cells transmit the greatest amount of solar power and lumens. As previously mentioned, it is also desirable for the thickness of the a-Si:H cells to be kept to a minimum in order to mitigate the effects of light induced degradation by means of the Staebler-Wronski effect. The triangular pattern beneath the a-Si:H cell in region two is also occupied by ZnO.

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Three different types of rear contacts for the a-Si:H cell are considered in Fig. 1. In the first case the rear contact is a 400nm thick homogeneous ZnO film and in the second case the rear contact is a one-dimensional STCPC. As previously mentioned the sputtered TCO films within the STCPC shown in Fig. 1 are ITO, however, to maintain consistency these TCO films are assumed to be ZnO in the modeled structure. In the calculations the peak position of the stop-gap of the STCPC is scanned from 400nm to 700nm. In all instances a quarter-wave stack is assumed wherein the optical thickness of both the ZnO and the silica nanoparticle films were set equal to one-fourth of the peak wavelength. That is, tZnO· nZnO = tnp· nnp = ¼ ߣp, where tZnO and tnp are the respective thicknesses and nZnO and nnp are the respective indices of refraction of the ZnO and silica nanoparticle films at the Bragg-peak position, ߣp. In the third case the rear contact consists of a ZnO film backed by a 100nm thick Ag reflector. In practice, ZnO films are deposited between the cell and the rear metallic reflector to reduce the amount of parasitic absorption that occurs in the rear contact [19,20]. The calculations presented herein use the scattering matrix method [21,22] to evaluate the performance of the three different cell structures depicted in Fig. 1 upon exposure to the ASTM AM1.5 (Global tilt) solar spectrum at normal incidence [23]. A brief description of the scattering matrix method has been provided in previous work [11]. Herein the method is used to determine the amount of light absorbed in the a-Si:H cell, the transmitted solar irradiance, the reflection and the absorption losses occurring in the rear contact (region three). These results are determined from the difference between the Poynting vectors calculated at the boundaries of the three regions shown in Fig. 1. The calculation is performed over the wavelength range 280-2530nm and the number of calculated sampling (wavelength) points is 450. Furthermore, the optical constants for ZnO and Ag are taken from the literature [24,25]. The optical constants of the silica nanoparticle and a-Si:H films are determined from spectroscopic ellipsometry measurements and the results are plotted in Figs. 8 and 9, respectively, in the appendix. Also, some simplifying assumptions were made in carrying out the calculations. Firstly, the incident light is assumed to be E-polarized where, as shown in Fig. 1, E is perpendicular to the page normal. Also, each part of regions one and two, with the exception of the a-Si:H cell, is assumed to be non-absorbing. Finally, the optical parameters of the p- and n- regions in the cell are neglected, that is the optical parameters of the entire cell are assumed to be that of intrinsic a-Si:H. Under these assumptions the photo-induced current generated in the a-Si:H cell can be calculated using Eq. (1):

J a − Si:H = q ∫ A ( ℏω )S ( ℏω ) d ℏω

(1)

where in this equations q is the electronic charge, A(ћω) is the absorption in the upper a-Si:H cell, S(ћω) represents the AM1.5 solar spectrum, ћ is Planck’s constant divided by 2π, and ω is the angular frequency. It should be noted that the photo-induced current calculated using equation one provides a good approximation to the short circuit current under the assumption of loss-less charge collection and thus strongly overestimates the current that would be generated in the cell under normal operating conditions. Nevertheless, the photo-induced current calculated from equation one provides a good measure to compare the performance of the different rear-contacts considered in Fig. 1. 3. Performance of a-Si:H cells with STCPC rear contacts for BIPV applications The transmitted solar power and current generated by the a-Si:H cell for case 1, as shown in Fig. 1, wherein the rear contact is a homogeneous ZnO film, are plotted as a function of the texture height in Fig. 2. Without texturing the power transmitted is ~490 W/m2. As the texture height is increased from zero the transmitted power increases until it reaches a maximum value of ~540W/m2 at approximately 90nm. The fact that the transmitted power increases and displays a maximum as the texture height increases from zero can be explained with reference to the work of Stephens and Cody [26]. The reflectance of a flat surface can be decreased by

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540

16 15

520

14 13

500

12 11

480

10 9

460

8 0

50

100

150

200

Current Generated [mA/cm2]

Transmitted Power [W/m2]

applying a shallow texture, with feature sizes that are small in comparison to the wavelength of the incident light. In this case the surface appears as an effective medium with a gradually varying index of refraction and functions as an anti-reflection coating (ARC). However, for larger texture feature sizes, comparable to the wavelength of incoming light, the surface effectively scatters the incident light into off-normal directions causing a reduction in the transmission. The current generated in a non-textured cell is 8.4mA/cm2 while a maximum current of 13.2mA/cm2 is generated when the texture height is about 200nm. The generated current decreases for a texture height of greater than 200nm due to increased reflection losses. The current generated in the cell can be further enhanced by incorporating an Ag film into the rear contact, which also functions as a highly efficient back-reflector. The current generated in the a-Si:H cell with a ZnO/Ag rear contact is also plotted in Fig. 2 and has a maximum value of 14.8mA/cm2. The ZnO film was 150nm thick in the optimized ZnO/Ag rear contact and the texture height of the a-Si:H cell was also about 200nm. Although the addition of the Ag rear contact boosts the current generated in the a-Si:H cell, no power is transmitted in this case.

250

Texture Height (nm) Fig. 2. The photo-induced current generated (right axis) as a function of texture height for the a-Si:H cell with the homogeneous ZnO contact shown as case 1 in Fig. 1 (dashed line) and the a-Si:H cell with the ZnO/Ag rear contact shown as case 3 in Fig. 1 (solid black line). The transmitted solar power (left axis) is also plotted for the cell with the homogeneous ZnO contact (red line).

An appropriately designed STCPC rear contact can be incorporated into the a-Si:H cell in order to boost its generated current while simultaneously transmitting a large portion of the solar irradiance for BIPV applications. The contour plot shown in Fig. 3(a) shows the current generated in the a-Si:H cell for case 2, as shown in Fig. 1, as a function of the texture height and Bragg-peak position of the STCPC rear contact. A maximum current of 14.6mA/cm2 is generated in an a-Si:H cell that has a texture height of about 200nm and a Bragg-peak position of 600nm. Figure 3(b) shows a contour plot of the transmitted solar power as a function of the STCPC Bragg-peak position and the texture height. For the cell with the optimized generated current, with a texture height of 200nm and an STCPC Bragg-peak position of 600nm, the solar power transmitted through the cell is 430W/m2, or 43.0%. The contour plot of the lumens transmitted through the cell is shown as Fig. 3(c) and the a-Si:H cell with a texture height of 200nm and an STCPC Bragg-peak position of 600nm transmits 3,400 lm/m2.

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(c)

(b)

(a)

Fig. 3. (a) Contour plot of the photo-induced current density for case 2 (see Fig. 1) as a function of the texture height (ordinate) and the Bragg-peak position of the STCPC (abscissa) (b) Contour plot of the transmitted solar irradiance. (c) Contour plot of the transmitted lumens.

The absorption in the a-Si:H cell for the various configurations shown in Fig. 1 is plotted as a function of wavelength in Fig. 4. The optimal texture height of 200nm is assumed for all cases. For the configuration shown as case 2 the Bragg-peak position of the STCPC was set to an optimal value of 600nm while the thickness of the ZnO film was set to 150nm for the configuration shown as case 3. The absorption in an untextured a-Si:H cell shown as case 1 in Fig. 1 is also plotted as a reference case in Fig. 4 for comparison. Moreover, the reflection from a bare STCPC with a peak position of 600nm is also shown as the dotted grey line and in comparing the plots for cases one and two it can be seen that the absorption in the a-Si:H cell is primarily enhanced within the stop-gap of the STCPC. Reference Cell

1

Absorption/Reflection

Case 1 (ZnO)

Case 32 (STCPC)

0.8

Case 23 (ZnO/Ag) STCPC Reflection

0.6 0.4 0.2 0 300

400

500

600

700

800

900

Wavelength (nm) Fig. 4. The absorption spectra for the a-Si:H cell shown in Fig. 1 for the reference case wherein the cell is not textured (solid grey line), for case 1 with a homogeneous ZnO contact (dotted black line), for case 2 wherein the rear contact is an STCPC with a Bragg-peak centered at 600nm (dashed black line), and for case 3 where the rear contact is a ZnO film backed by a thin Ag film. For all cases, other than the reference case, the texture height is 200nm. The reflection from a free-standing STCPC surrounded by air with a Bragg-peak centered at 600nm is shown as the dotted grey line.

The transmitted solar irradiance for the a-Si:H reference cell and for the a-Si:H cells with a homogeneous ZnO rear contact (case1) and with an STCPC rear contact (case 2) are plotted alongside the AM1.5 solar spectrum in Fig. 5(a). For each of these cases the transmitted solar irradiance is multiplied with the CIE 1978 eye sensitivity function [27] to attain the amount of lumens transmitted which are plotted in Fig. 5(b). The 110.3 klm/m2 available from the AM1.5 solar irradiance are also plotted in Fig. 5(b) where it is shown that the majority of this visible irradiance is not transmitted through an a-Si:H cell. For example, 30.4 klm are transmitted through the reference cell. For the optimal case of the a-Si:H cell shown in Fig. 1 with a homogeneous ZnO back-reflector and a texture height of 200nm, 20.2klm are #149944 - $15.00 USD

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transmitted through the cell. For the optimal instance of case 2, where the a-Si:H cell has a texture height of 200nm and a STCPC rear contact with Bragg reflectance peak centered at 600nm, just 3.4 klm are transmitted. The generated photo-current as well as the transmitted lumens and solar power for the various cell configurations considered in this work are summarized in Table 1 along with the corresponding absorption and reflection losses. 1000 W/ m2 (AM1.5) 492 W/ m2 (Ref Case) 517 W/ m2 (Case 1: ZnO) 430 W/m2 (Case 2: STCPC)

1.6 1.2 0.8 0.4 0

(b) Lumens [klm/m^2]

Photon Flux [W/m^2]

(a)

110.3 klm/m2 (AM1.5) 30.4 klm/m2 (Ref Case) 20.2 klm/m2 (Case 1: ZnO) 3.4 klm/m2 (Case 2: STCPC)

5 4 3 2 1 0

280

780

1280

1780

2280

380

Wavelength (nm)

480

580

680

780

Wavelength (nm)

Fig. 5. (a) The AM1.5 solar irradiance is plotted as the yellow region. The solar photon flux transmitted through the a-Si:H cell, shown in Fig. 1, for the reference case wherein the cell is not textured and the rear contact is a homogeneous ZnO film is shown as the brown region. The transmitted solar irradiance for case 1 is shown as the light blue region overlaying the brown region. The orange region, overlaying the blue region, shows the solar irradiance transmitted through the cell for case 2. (b) The lumens available from the AM1.5 solar irradiance are plotted as the yellow shaded area. The brown region represents the lumens transmitted through the reference cell, the light blue region overlaying brown region represents the lumens transmitted for the cell shown as case 1. The orange region overlaying the light blue region, shows the lumens transmitted for case 2. It is noted that the transmitted photon flux for the case 3 wherein the rear ZnO contact is coated with an Ag film the transmission through the cell is negligible.

4. Designing BIPV panels with STCPC rear contacts There are many variables involved in optimizing a BIPV system such as available roof-top space and façade area that can accommodate PV panels. The orientation of these surfaces with respect to the sun is also of great importance. One must also consider how the climate varies throughout the year and the heating and cooling demands for the building in consideration. Furthermore, semi-transparent PV panels are generally not a highly efficient source of indoor lighting because they typically do not provide uniform illumination across deep rooms. Nabil et al., have suggested a new paradigm called “useful daylight illuminance” or UDI [28]. It is suggested that UDI lies within the range of 100-2000 lux. For illumination conditions less than 500 lux occupants may require supplementary lighting and they often complain of glare and draw the blinds when the illumination exceeds 2000 lux. Moreover, manufacturers of aSi:H based semitransparent BIPV panels for windows suggest that a transmission of about 10% is desirable for comfortable indoor lighting conditions [29]. Due to the inevitable variability on the demand in BIPV systems from one instance to the next it is not possible to design an optimal rear contact that can be used generically for all a-Si:H based BIPV panels. However, as discussed in the following paragraphs, STCPCs can be used as rear contacts that alter the performance of a-Si:H based cells in a unique fashion that is desirable for BIPV systems under many circumstances. As previously mentioned, a 100nm thick a-Si:H cell with an STCPC functioning as its rear contact can be optimized to generate a photo-induced current of 14.6mA/cm2. In comparison, the photo-induced current generated in an a-Si:H cell of the same thickness with an optimized ZnO/Ag rear contact is 14.8mA/cm2. Thus, one may expect a reduction of 0.2mA/cm2, or ~1.5%, in the short circuit current for a-Si:H cells with STCPC rear contacts. However, the benefits of the a-Si:H cell with the STCPC back-reflector are that it transmits ~430W/m2 and #149944 - $15.00 USD

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3.5Klm/m2 while the amount of solar irradiance transmitted through the ZnO/Ag rear contact is negligible. This transmitted solar irradiance could potentially be collected in a trombe wall to provide heating during the winter and ventilation and/or passive cooling during the warmer summer months [5,30]. Regarding indoor lighting 3.5Klm/m2 provides some UDI, however, it does not meet the 10%, or ~11Klm, suggested by manufactures. STCPC rear contacts can be used to optimize the performance of PV panels under the design constraint that ~10% of the solar luminance must be transmitted. A contour plot of the lumens transmitted through a 100nm a-Si:H cell as a function of its texture height and the Bragg-peak position of its STCPC rear contact is shown as Fig. 3(c). This figure shows that ~11Klm are transmitted through the cell when the STCPC Bragg peak is positioned at about 500nm. Figure 3(a) shows that for an STCPC Bragg-peak position of 500nm the photoinduced current is maximized when the texture height of the a-Si:H cell is about 200nm. As reported in Table 1 (see case 2b, denoted as the design case), the lumens and solar power transmitted in an a-Si:H cell with this texture height and STCPC rear contact are 10.9klm and 507 W/m2, respectively. The photo-induced current generated in this cell is 13.8mA/cm2. This represents an increase of 4.5% in comparison to the 13.2 mA/cm2 generated in the a-Si:H cell with a homogeneous ZnO rear contact (case 1). Table 1. Summary of Results for the Optimal Cases of the Various Cell Configurations Considered as Well as the Design Case Cell configuration

Generated Transmitted Transmitted Reflection Rear contact photo-current lumens solar power losses absorption losses [mA/cm2] [klm/m2] [W/m2] [W/m2] [W/m2] Reference Cell (no texture) 8.4 30.4 492 248 36 Case 1: ZnO 13.2 20.2 517 118 33 *Case 2: STCPC (600nm) 14.6 3.4 430 166 38 *Case 2b: STCPC (500nm) 13.8 10.9 507 106 35 Case 3: ZnO + Ag 14.8 0 0 577 52 *Case 2 and 2b (design case) have STCPC rear-contacts with stop-gaps centered at 600nm and 500nm, respectively.

5. Experimental results towards BIPV panels with STCPC rear contacts Experiments were also performed in order to study the solar power and lumens transmitted through a-Si:H films with STCPC rear contacts. Specifically, STCPCs comprising five bilayers of sputtered ITO and SiO2 nanoparticle films were prepared on Corning 1737 glass substrates. Three samples were prepared with differing Bragg-peak positions that were located at approximately λc = 500nm, 570nm and 660nm. A schematic of these samples is presented as Fig. 6(a). A reference sample, wherein the rear contact was a 500nm thick ITO film deposited onto Corning 1737, was also prepared. A 100nm thick a-Si:H film was then deposited onto each of the four aforementioned samples. As a final step a 65nm thick ITO layer was sputtered onto these a-Si:H films to serve as an ARC. The transmission through each sample was then measured from 280 to 2500nm using a Lambda 1050 UV/VIS/NIR spectrometer. For reference, the transmission through the STCPCs before the a-Si:H film was deposited onto their surface is shown in Fig. 6(b). The transmittance spectra measured from the complete samples with the a-Si:H was then multiplied with the AM1.5 solar irradiance in order to determine the transmitted solar photon flux, which is plotted in Fig. 6(c). This transmitted solar photon flux, integrated over 280 to 2500nm, is used to determine the solar power transmitted through the samples which are listed in Table 2. The product of the transmitted solar photon flux and the CIE1978 eye sensitivity function is plotted in Fig. 6(d). It can also be noted that the solar lumens transmitted through each sample, also tabulated in Table 2, are greater than those listed in Table 1 because these experimental samples were not textured.

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Fig. 6. (a) A schematic diagram of the samples prepared for experimental investigation. (b) The transmittance spectra of STCPCs (without a-Si:H films deposited onto them) with differing Bragg-peak positions as indicated in the legend. (c) The solar irradiance and (d) the solar lumens transmitted through 100nm thick a-Si:H films with STCPC rear contacts that have different Bragg-peak positions. Table 2. Experimentally Measured Transmitted Power and Lumens through 100nm Thick a-Si:H Films with Different Rear Contacts Rear Contact 500nm ITO film STCPC (λc ~500nm) STCPC (λc ~570nm) STCPC (λc ~660nm)

Transmitted Power [W/m^2] 408 401 350 325

Transmitted Lumens [klm/m2] 40.6 24.8 12.2 18.3

It is also important that BIPV systems are aesthetically pleasing to ensure their public acceptance and marketability. In this context, STCPC rear contacts offer an appealing look and expand the range of materials and color available to architects. Photographs of a coin taken through the 100nm thick a-Si:H films deposited onto the STCPCs with approximate Bragg-peak positions of 500nm, 570nm, and 660nm, are shown in Fig. 9 in the appendix. These photographs, along with the spectral distribution of the transmitted lumens shown in Fig. 6(d), highlight the fact that that STCPC rear contacts in BIPV panels could also be used to set the solar irradiance entering a building to a desirable color temperature. For example, the color temperature of the solar irradiance transmitted through the a-Si:H film with the STCPC rear contact with Bragg-peak position centered at λc = 500nm has a cooler color temperature than the AM1.5 solar spectrum; light with a lower color temperature is referred to as “warm” and is often used in public areas to promote relaxation. Thus, BIPV panels with STCPC rear contacts may be well suited for public areas such as malls, glass stairwells, and subway stations. 6. Concluding remarks In this work we have presented an optical study of the potential for STCPC rear contacts to enhance the performance of BIPV panels. The advantage of using STCPCs instead of other

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types of Bragg-reflectors that may be integrated into the rear side of BIPV panels is that their conductive properties allow for them to be utilized as a drop-in replacement for the rearcontact of the a-Si:H cell. Also, the sputtered ITO and spin-coated silica nanoparticle films within the STCPC rear contacts have low extinction coefficients and provide the large dielectric contrast required to fabricate Bragg-reflectors with broad and intense reflection peaks [17]. With regards to reducing the thermal load in buildings, STCPC rear contacts can potentially be used to fabricate thin-film silicon-based cells that transmit a large amount of solar power without significantly compromising the amount of current they generate. For example, the photo-current generated in an optimized 100nm thick a-Si:H cell with a ZnO rear contact is 13.2 mA/cm2. Comparatively, the photo-current generated in a similar a-Si:H cell with an optimized STCPC rear contact is 14.6 mA/cm2, representing a 10.6% relative increase in efficiency. It is noted that the photo-current generated in the same a-Si:H cell is 14.8 mA/cm2 when an optimized ZnO/Ag contact is used, representing a relative increase in efficiency of 12.1%. However, no solar power is transmitted through this cell whereas 430 W/m2 are transmitted through the a-Si:H cell with the optimized STCPC rear contact. Regarding their ability to supplement indoor lighting for buildings, STCPC rear-contacts offer a modest performance enhancement when integrated as rear-contacts in BIPV cells. For example, as discussed in Section 4, STCPC rear contacts increase in the photo-current generated in a thin a-Si:H cell by 4.5% under the design constraint that 10% of the solar lumens are to be transmitted. In this context STCPCs provide only a modest enhancement because there is a trade-off between the current generated in the cell and the amount of lumens it transmits since both applications use solar photons from the same spectral region. In fact, the texture height of the a-Si:H cell and the Bragg-peak position of the STCPC that maximize the current generated in the cell (Fig. 3(a)) also minimize the amount of lumens it transmits (Fig. 3(c)). Nevertheless, as discussed in Section 5, STCPC rear-contacts can be used to control the color temperature of the light transmitted by the BIPV panel and could potentially be used to provide background lighting in public areas. As a final point, the nanoparticle films within the STCPCs presented in this work were deposited using spin-coating technology, which would be cumbersome for fabricating largearea BIPV panels. However, nanoparticle films may be deposited using alternative technologies such as spray-coating and dip-coating,which are amenable to large-area deposition. 7. Appendix Preparation of silica nanoparticle films The nanoparticle films within the STCPCs presented in this work are prepared from a solution of SiO2 nanoparticles, acquired from Aldrich (Ludox SM-30, 30 wt%) and diluted in deionised water (3:1 deionized water/Ludox). At the dispensing stage in the spin-coating process, the dispersion was filtered through a 0.2 µm pore Nylon syringe filter. Dispersions were spin-coated at a speed of 3000 rpm for 20 seconds. Their effective index of refraction and extinction coefficient are plotted below in Fig. 7 over the spectral region spanning from 300nm to 1000nm. These plots were extrapolated to attain the optical constants of the silica nanoparticle films throughout the entire solar spectrum.

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2.0E-03

1.35

1.5E-03

←n 1.3

1.0E-03

k→

1.25

5.0E-04

1.2

0.0E+00

300

500

700

900

Extinction Coefficient (k)

index of refraction (n)

1.4

1100

Wavelength (nm) Fig. 7. The effective index of refraction and extinction coefficient for the silica nanoparticle films within the STCPC. In order to attain these optical parameters the ellipsometric parameters a and b were measured for the SiO2 nanoparticle film at an incident angle of 56°. A regression analysis employing a Levenberg-Marquardt algorithm was carried out using Winelli II software in order to fit the dispersion relations of these reference films, and thus determining their indices of refraction (n) and extinction coefficients (κ). The optical constants of the SiO2 nanoparticle films were fit to a Cauchy model and the statistical measure of the quality of fit attained for the ellipsometric measurements was fairly high (R2 > 0.97).

Preparation of hydrogenated amorphous silicon films The intrinsic hydrogenated amorphous silicon (a-Si:H) films presented in Section 5 were grown using Radio Frequency Plasma Enhanced Chemical Vapor Deposition (RF PECVD). A parallel plate system with an anode-cathode distance of 34mm was used. Films were grown at a substrate temperature of 200°C using silane gas (SiH4) maintained at a pressure of 175mTorr with a flow rate of 30sccm. The RF power and film growth rate were 3W and 0.28Å/s, respectively. The index of refraction and extinction coefficient of the films, as determined from ellipsometry measurements, are presented below in Fig. 8. Index of refraction/ Extinction Coefficient

6

n k

5 4 3 2 1 0 200

400

600

800

1000

Wavelength (nm) Fig. 8. Index of refraction and extinction coefficient for the a-Si:H films investigated in Section 5. The ellipsometric parameters α and β were measured for an a-Si:H reference film on Corning glass at an incident angle of 56°. A regression analysis was performed on the data attained from these ellipsometry measurements in order to fit the dispersion relations to a Tauc-Lorentz model, thus determining their effective indices of refraction (n) and extinction coefficients (k). The Bruggeman mixing theory was also used to model a graded surface roughness layer (50% a-Si:H + 50% Void). The regression analysis employed a Levenberg-Marquardt algorithm and was carried out using Winelli II software. The modeled thickness of the a-Si:H film and its surface roughness layer was 102.5nm and 1nm, respectively. The statistical measure of the quality of fit attained for the ellipsometric measurements was R2 = 0.995.

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Photographs of a coin taken through a-Si:H films deposited onto STCPCs

Fig. 9. A coin photographed through a 100nm thick a-Si:H film that was deposited onto an STCPC with a Bragg-peak position of (a) ~500nm, (b) ~570nm and (c) ~660nm. Each STCPC is comprised of 5 bilayers of spin-coalted SiO2 nanoparticle films and sputtered ITO films. In each case a 65nm thick ITO film was deposited onto the a-Si:H film to function as an ARC. The dime was placed on a table about a foot beneath a fluorescent lighting source. For each picture, the a-Si:H film/STCPC sample was held ~10cm away from the dime at a ~45° angle as it was brought into focus.

Acknowledgments This work was supported through grants from the Natural Sciences and Engineering Research Council of Canada and the Ontario Research Fund – Research Excellence program in collaboration with Arise Technologies.

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