Characterization of microspherical semi-transparent solar cells and modules

Solar Energy 81 (2007) 711–716 www.elsevier.com/locate/solener

Characterization of microspherical semi-transparent solar cells and modules M. Biancardo a,*, K. Taira b,*, N. Kogo b,1, H. Kikuchi b,1, N. Kumagai N. Kuratani b,1, I. Inagawa b,1, S. Imoto b,1, J. Nakata b,1 a

b,1

,

Risoe National Laboratories, The Danish Polymer Centre, Frederiksborgvej 399, Roskilde, Denmark b Kyosemi Corporation, 385-31 Toiso, Eniwa, Hokkaido 061-1405, Japan Received 10 January 2006; received in revised form 10 October 2006; accepted 17 October 2006 Available online 29 December 2006 Communicated by: Associate Editor Darren M. Bagnall

Abstract Microspherical solar cells and modules have been fabricated. The spherical nature of these semi-transparent devices allows the microspherical cells to harvest both directly incident and diffuse components of sunlight thereby improving the solar energy conversion efficiency. Indoor and outdoor characterizations of these three dimensional semi-transparent cells and modules are carried out using a Lambertian reflector in order to assess the maximum efficiency of the devices. In the absence of the reflector the cell efficiency is 13.5% under standard illumination (100 mW cm 2, A.M. 1.5, 25 C). However, this is significantly enhanced in the presence of the reflector. Microspherical modules with the reflector are directly compared to similar semi-transparent modules comprised of traditional planar devices, in outdoor tests at low light intensity (2.5–25 mW cm 2) to further demonstrate the benefits of the design particularly at low angle of incident radiation.  2006 Elsevier Ltd. All rights reserved. Keywords: Spherical solar cells; Lambertian reflector; Efficiency

1. Introduction Many applications of semi-transparent solar cells can be envisaged, e.g. sound barriers, building facades, and power-producing windows. These particular building integrations have been mostly explored by alternative-tosilicon solar cells such as dye sensitized solar cells (Biancardo et al., 2006; Dai et al., 2005; Toyoda et al., 2004) less common is the use of silicon based devices which fulfil the requirements of flexibility and semi-transparency (Fath et al., 2002). Semi-transparent devices need to absorb *

Corresponding authors. Tel.: +4546774718; fax: +4546774791 (M. Biancardo), Tel.: +81 123 34 2100; fax: +81 123 34 2110 (K. Taira). E-mail addresses: [email protected] (M. Biancardo), taira@ kyosemi.co.jp (K. Taira). 1 Tel.: +81 123 34 2100; fax: +81 123 34 2110. 0038-092X/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2006.10.009

as much of the global solar radiation that is incident on the devices as possible. They should, therefore, be designed to efficiently use the direct, reflected, and diffuse components of light that may be incident from all directions (including from behind the cell). Effective use of all components of incident light should lead to an increase in both the efficiency and the amount of hours of sunlight that can be utilized for electricity production. In general, planar silicon solar cells are very sensitive to the angle of incidence of refracted and reflected light and performance quickly drops as the angle of incident light is lowered. Certain 3D constructions have the advantage of increased operational time as three dimensionality allows harvesting of light with low incident angle. A spherical sunlight absorbing surface can capture sunlight three dimensionally, thus improving solar cell power generation capacity to a maximum and offering some benefits over

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Fig. 1. Sphelar module and a schematic of Sphelar cell (inset).

traditional cells that have only one planar surface to capitalize upon sunlight (Nakata, 2001, 2003; Kumagai et al., 2004). The combination of semi-transparency and 3D construction can also provide the possibility of further enhancement of the power generation by incorporating a reflecting surface behind the cell. In this paper we describe the concept of Sphelar cells (spherical solar photovoltaic cells) (Fig. 1). To overcome the fact that standard solar cell evaluation methods do not account for all the components of the solar radiation, we placed a Lambertian reflector behind the devices while measuring solar cell performance under standard conditions (A.M. 1.5, 100 mW cm 2, 25 C), in this way we are considering the performance of the semi-transparent devices under optimal conditions (as if a white blind had been placed behind the device while in use). Where traditional measurement standards underestimate the performance of non-traditional devices, the use of a diffuse reflector overestimates efficiency, in normal circumstances the actual performance will lie somewhere between the two extremes. 2. Experimental 2.1. Device preparation Both Sphelar cells and conventional planar cells have been produced. For the Sphelar cells, silicon spheres are

usually created by dripping molten silicon and allowing the surface tension of the Si droplets to mold them into single crystal silicon spheres during free-fall (Nakata et al., 2000). However, in this work, Si wafers have been used to build both Sphelar and more conventional planar cells. One-X-cm p-type thick Si wafers were diced to produce Si cubes that were rounded into spheres. First, a small segment on the sphere was formed by mechanical grinding to provide orientation for electrode deposition, this was followed by oxidation and patterning. Phosphorous was selectively diffused on the surface of a p-type silicon single crystal and a thin n-type layer was provided to form a p–n junction parallel to the surface. A conventional POCl3 thermal diffusion process was used to form the contact region. To produce the planar cells Si plates were first obtained by scribing 1 X-cm p-type Si wafers. The wafers were oxidized and POCl3 diffusion was allowed through a window for selective diffusion. To complete the Sphelar and planar device fabrication silver and aluminum electrodes were formed, on the top and bottom of the spheres and plates to produce links between emitter and base. The Sphelar cell was 1.8 mm in diameter and the flat cell was 2 · 2 mm2 (Fig. 2). The electrode arrangement makes the spherical cells non-directional and can realize an even distribution of generated current, facilitating a serial and/or parallel connection of the cell to other cells (US6, 204, 545 B1).

Fig. 2. Images of a 1.8 mm-/ spherical (a) and a 2 · 2 mm2 square flat solar cell (b), respectively.

M. Biancardo et al. / Solar Energy 81 (2007) 711–716

713

60 KHS Current density (mA/cm2)

Solar constant 575

250 mm Sphelar® 1.9 mm Spectralon®

w. reflector Sphelar

40 w/o reflector

Flat Planar

20

Fig. 3. Schematic of the measurement set up for testing Sphelar modules.

The modules tested were built with 361 individual Sphelar cells, embedded in a silicone matrix which allows semi-transparency over the visible range (only 8.7% of the module area is covered by the silicon beads). The samples tested labelled as Kyo_4, flexible module, and as Kyo_2, module sandwiched between two soda-lime glasses, were tested under A.M. 1.5, 100 mW cm 2 at 25 C. The silicon solar cell employed to compare the outdoor performance was a commercial thin film amorphous silicon photovoltaic modules provided by RS with 82.5 cm2 active area provided by 15 elements of 5.5 cm2 serially connected. 2.2. Device testing A solar simulator (KHS Solar Konstant 575 from Steuernagel Lichttechnik GmBH) was used to assess the modules under simulated sunlight. The power output of the lamp was set to be 100 mW cm 2 and calibrated using a bolometric precision pyranometer (Eppley Laboratories). The spectral distribution of the solar simulator was required to match A.M. 1.5 and this was monitored using an AvaSpec 2048 (Avantes). The homogeneity of the incident light intensity, evaluated using the pyranometer, was ±2%, while the spectral distribution was within 25% of the sun spectrum in the range between 400 and 700 nm thus corresponding to class A simulation under the ASTM E927 standard. J/U measurements were recorded in the interval 1 V/12 V. Lifetime measurements under simulated sunlight were performed under short circuit conditions. The electrical measurements were performed using Keithley 2400 sourcemeters. The reflector is a Lambertian reflector made of Spectralon. The reflectance is higher than 99% between 400 and 1500 nm and diffuses light almost perfectly. The cell measurement arrangement is shown in Fig. 3. Outdoor testing was performed during October 2005 in Roskilde (DK). 3. Results and discussion Individual 1.8-mm Ø Sphelar cells and 2 · 2 mm2 flat cells were prepared and tested under the same conditions

0

0

0.2

0.4 Voltage (V)

0.6

0.8

Fig. 4. J/U curves of planar and 3D Sphelar solar cells, with (w) and without (w/o) reflector under A.M. 1.5, 100 mW cm 2 and 25 C. The J/U curves (w) and (w/o) reflector overlap.

Table 1 Photovoltaic performances of Sphelar with (w) and without (w/o) reflector in comparison to a planar cell under standard illumination (100.7 mW cm 2, A.M. 1.5, 25 C) Sample

Jsc (mA cm 2)

Voc (mV)

FF%

g%

Sphelar (w) Sphelar (w/o) Planar

51.96 30.42 26.21

0.585 0.571 0.594

77.4 77.9 77.9

23.54 13.54 12.10

both with and without a reflector. J/U curves show that, in the absence of the reflector, the current densities for the two cells are comparable, while in the presence of the reflector the operational current of Sphelar cell is enhanced by 70% (Fig. 4). The results are reported in Table 1 using the sphere cross sections as active area for the spherical solar cell. This demonstrates the possibility of a high increase in the efficiency of spherical solar cells by employing a back reflector plate, an advantage that can only be obtained using a 3D construction. All subsequent measurements for the Sphelar modules were tested in the presence of Lambertian reflector. The glass sandwiched Sphelar module (Kyo_2) shows an efficiency of 17.4% with a max power of 17.52 mW cm 2 under A.M. 1.5 standard illumination conditions. Spherical solar cell efficiencies are calculated using an active area equal to the sphere cross section (0.09 · 0.09 · p · 361 = 9.18 cm2). Since the actual surface occupied by a spherical solar cell is its cross section, we believe that this should be the correct way to evaluate spherical solar cell power generation especially when they are considered as PV elements within a semi-transparent window where there is a trade-off between transparency and efficiency. As a rough estimate, the maximum efficiency of a module using close-packed Sphelar cells would be around 14%.

M. Biancardo et al. / Solar Energy 81 (2007) 711–716

As expected, the module’s Jsc, Voc, Pmax and g% have demonstrated clear light intensity dependence (Figs. 5–7), while the variation on FF% with different light intensity is not as strongly affected (Fig. 6). No saturation was found with over 1.5 Sun of irradiation. The Wronsky and Staebler effect (Staebler and Wronski, 1977) was not observed after a long illumination duration and after temperature controlled experiments. This effect, expected in amorphous silicon photovoltaics, is due to the tendency of devices to lose efficiency upon initial exposure to light due to the creation of metastable defects in the a-Si:H. The defects act as additional recombination centres and reduce the carrier lifetime and hence the photoconductivity. The absence of this effect confirms the high crystallinity of the silicon used in the Sphelar cells. Jsc and Voc, remain constant (0.62%, 1.1% difference in Jsc and Voc) and Pmax and g% show no exponential decrease after 1000 h of illumination (3.7% and 3.3% difference in Pmax and g%). The flexible module, Kyo_4, tested under the same A.M. 1.5 conditions provides better performance than Kyo_2.

70 10.8

60

10.6

40 10.4

30

U (V)

J (mA/cm2)

50

20 10.2 10 0

0

500

1000

1500

10 2000

W/m2

30

82

25

80

20

78

15

76

10

74

5

72

0 0

500

1000

1500

19.0 18.5 18.0 17.5 17.0 16.5 16.0 15.5

0

500

1000

1500

2000

W/m2 Fig. 7. Kyo_2 efficiency showing linear dependence on light intensity.

This behaviour is most probably due to the absence of the glass filter that acts as a light intensity and cut-off filter reducing the intensity and the spectral range of the radiation that reaches the module. At 100.7 mW cm 2 the efficiency was 17.8% with a Pmax of 17.80 mW cm 2 under A.M. 1.5 standard illumination condition. The FF% of the module was also found to be very high reaching a value of 79.5%. Lifetime studies show constant performance over time as reported in Table 2 and Fig. 8. Outdoor testing was performed on the Sphelar module (Kyo_4) and the commercial amorphous silicon solar cells having different active areas and different performances (Table 3). The comparison between the two cells aims to evaluate the perceived advantages of the three dimensionality of Sphelar devices when compared to a planar sample in real use. Previous indoor laboratory tests have shown that, in spherical solar modules, the current output decline with reducing angle of incident light is reduced when compared to conventional flat solar modules (Nakata, 2003). This suggests that a correct comparison between flat and spherical solar cells modules should be based on the total power generation throughout a day rather than by conversion efficiency under direct sun simulator irradiation perTable 2 Lifetime profile of Kyo_4 performance under A.M. 1.5 standard, 25 C, 100 mW cm 2

FF%

Power (mW/cm2)

Fig. 5. J ( ) and U ( ) dependence on the light intensity of Kyo_2, Jsc calculated over 0.483 cm2 active area.

19.5

Efficiency (%)

714

70 2000

W/m2 Fig. 6. Kyo_2 light intensity influence on power output ( ) and FF% ( ) of Kyo_2. Power (mW cm 2) was calculated using the full active area (9.18 cm2).

Time (h)

Pmax (mW cm 2)

Jsc (mA cm 2)a

Voc (V)

FF%

g%b

0 46 191 270 901 945 968 1034

17.57 17.83 17.60 17.17 17.22 17.38 17.46 17.62

42.03 42.23 42.44 40.91 40.38 42.10 39.71 41.05

10.48 10.43 10.43 10.39 10.40 10.44 10.36 10.46

75.8 76.9 75.5 76.8 77.9 75.1 80.6 77.1

17.4 17.7 17.5 17.1 17.1 17.3 17.3 17.5

a

Calculated over an active area of 1 row (19 cells = 0.483 cm2). The calculation of efficiency was obtained using an active area equal to the sphere cross section 0.09 · 0.09 · p · 361 = 9.18 cm2. Using half of the Si spherical cells as surface active area reduces the reported efficiency by a factor of 2. b

M. Biancardo et al. / Solar Energy 81 (2007) 711–716

715

30 25

I (mA)

20 15 10 5 0 0

200

400

600

800

1000

1200

Time (hrs) 2

Fig. 8. Kyo_4 lifetime tests under A.M. 1.5, 100 mW cm sun simulator light irradiation. The peculiar drop in cell performances after 312 h of illumination was due to a failure in the cooling system that suddenly increased the temperature with a consequent drop in Voc and Jsc.

Table 3 Indoor performance of RS and Kyo_2 under A.M. 1.5, 100 mW cm 2, 25 C Sample

Type

Total active area (cm2)

Number of elements

Light power (mW cm 2)

Jsc (mA cm 2)

Voc (mV)

RS

Amorphous Si Sphelar

82.5

15 serially connected 361 interconnected

108.4

11.25a

10 594

3.34

40.6

100.7

42.03b

10 475

17.57

75.8

Kyo_2 a b

9.18

Pmax (mW cm 2)

FF%

Calculated over the active area of 1 element 5.5 cm2. Calculated over the active area of 1 row (19 cells = 0.483 cm2).

20 0.6 current density (mA cm-2)

18 current density (mA cm-2)

16 14 12 10 8

Kyo_2

6 4

0.5 0.4 0.3 0.2 0.1 0.0 17

19

21 time (hrs)

23

25

RS

2 0 0

5

10

15

20

25

time (hrs)

Fig. 9. Outdoor test of commercial amorphous solar module (blue) and Sphelar module (Kyo_2) (black). The test was started on the 24 October 2005 in Roskilde (DK) at 11 a.m. The highly variable current behaviour is due to cloud passage over the cells. At sunrise (highly diffuse and low angle light; 8 a.m.) it can be seen that Sphelar has a better response to the light irradiation (inset). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)

pendicular to the module. Outdoor tests were performed during winter 2005 at Risø National Laboratory. The cell performance was recorded with time of the day. The outdoor sunlight intensity was very low during the measurements (at 11 a.m., 2.5 mW cm 2; at 2 p.m., max of the

daylight irradiation, 25 mW cm 2). The outdoor tests confirmed the higher current generation by spherical solar cells at low light intensity at very low-angle of irradiation. This can be observed in particular in Fig. 9 where 24 h of outdoor test is shown. The steep increase of current for

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Kyo_4 in the first hours of the morning sun indicates a better response to low angles of incident irradiation. Lowangle light is often diffuse due to clouds, haze or smog and its intensity is reduced. The linear fitting of the data during the first hour of the morning sun light shows a difference of one order of magnitude between the Kyo_4 and commercial samples (3 · 10 5 A/h vs. 2 · 10 4 A/h, respectively). The lower response to light irradiation of the planar samples indicates fewer effective working hours of such modules. 4. Conclusion Evaluation of 3D Sphelar solar cells and modules in the presence of a Lambertian reflector introduces 3D solar cells as an interesting flexible silicon based alternative to traditional flat panels. Spherical solar modules demonstrate a lower decline of current output at small angles of incident light compared to flat type solar modules. Outdoor measurements confirmed a higher current generation by spherical solar cells at low light intensity and at very low angles of irradiation corresponding to early morning sun irradiation. We believe that the realization of highly transparent and stable solar cells employing a solid state flexible construction will open new interesting paths for a future commercialisation of power producing fac¸ades, windows and semi-transparent roofs. Acknowledgements This work was supported by the Danish Technical Research Council (FTP 274-05-0053), and Eltra (PSO 103032 FU 3301).

One of the authors thanks Drs. F.C. Krebs and K. West for fruitful discussions and helpful suggestions.

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