High efficiency silicon cells for luminescent solar concentrators

Solar Cells, 4 (1981) 37 - 46

37

HIGH EFFICIENCY SILICON CELLS F O R LUMINESCENT S O L A R CONCENTRATORS

C. M. GARNER, F. W. SEXTON and R. D. NASBY

Sandia National Laboratories, Albuquerque, N M 8 7115 (U.S.A.) (Received October 15, 1980;accepted November 27, 1980)

Summary Energy conversion efficiencies o f 18% and 19.4% at 0.1 W cm -2 and 1 W cm-2 respectively were measured for a luminescent solar concentrator cell under an air mass one solar spectrum. This cell was designed for operation under illumination of 600 nm wavelength and 1 W cm -2. From these results and spectral response data an efficiency of 26% can be projected for illumination at 600 nm. This agrees well with c o m p u t e r code calculations of 28%. The substrate doping level, cell grid pattern, minority carrier lifetime and antireflection coating properties were studied to optimize the cell performance for 600 nm and longer wavelengths.

1. Introduction A solar cell is designed for use in luminescent solar concentrator (LSC) systems at the wavelengths of the LSC and for a specified illumination level. Optimization of the cell design includes minimizing the reflectance at the LSC wavelength, selecting the proper silicon substrate doping concentration and optimizing the front electrode grid. Cell design was optimized using computer codes for an illumination intensity of 1.0 W cm -2 at wavelengths of 6 0 0 , 8 0 0 and 1000 nm. An efficiency of 19.4% was measured at 1 W cm-2 in an air mass one (AM1) spectrum for a cell designed for 600 nm illumination. The major potential for the LSC is in low cost concentration systems which do n o t require tracking of t h e sun. The LSC operates b y absorbing radiation at p h o t o n energies greater than a specific energy with a characteristic absorption spectrum and then re-emitting p h o t o n s of this characteristic longer wavelength [ 1, 2 ]. The radiation is then internally reflected through the LSC substrate to illuminate a photovoltaic cell with a possible net concentration of irradiance on the cell. The p h o t o n flux on the cell will be reduced from that incident on the LSC b y internal loss mechanisms which include a q u a n t u m efficiency of less than unity, reabsorption of emitted light b y the dye, poor capture of emitted light b y the substrate and substrate 0379-6787/81/0000-0000/$02.50

© Elsevier Sequoia/Printed in The Netherlands

38 internal reflectance of less than unity [3]. Present solar cells must be optimized for collecting current generated by photons over a broad wavelength range (from about 400 to 1000 nm), while cells for use with an LSC may be optimized for very narrow wavelength ranges. In some present concentrator cells a single-layer antireflection (AR) coating has an average reflectance loss of approximately 10% - 12% over the solar spectrum; however, a single-layer AR coating can easily be matched to the cell over a narrow wavelength band of light and the reflectance can be reduced to almost zero. In previous work the efficiencies of solar cells have been studied as functions of substrate doping concentration for carrier generations produced by the solar spectrum [4 - 6 ] . However, when a narrow band of wavelengths produces carriers in a cell, a very different choice of substrate doping may result. The dye emission wavelength affects the choice of base doping because of the variation in absorption depth with wavelength. Deeper absorption depths require longer minority carrier diffusion lengths in the base for efficient carrier collection. For wavelengths near 1000 nm, high base resistivities near 10 ~2 cm are needed to obtain the longer lifetimes and to maximize current collection. A third aspect of the cell also requires optimization. The grid pattern must be designed to minimize losses due to reflection and series resistance in the contacts and diffused layers of the cell. The pattern for the LSC cells was optimized for operation at 1 W cm-2 and is reported here. These aspects are discussed in the following sections. Cell designs for illumination at 6 0 0 , 8 0 0 and 1000 nm are discussed and experimental results on a cell designed for 600 nm are presented.

2. Antireflection coating design To reduce the reflectance o f the cells in a given wavelength range, an SiNx AR coating is deposited onto the cell. Before the SiNx layer is deposited, a thin layer of thermal oxide (about 100 A thick) is required to passivate the cell front surface. Thus the AR coating is a double-layer coating and the thickness of SiNx in this coating is different from t h a t of a single layer. To optimize the reflectance of this coating, a computer code [7] was used which optimizes the thickness of each layer of a particular AR coating for a given cell performance and incident spectrum. The code takes into account the divergences in the refractive index and absorption as functions of wavelength in all layers including the substrate and optimizes the current o u t p u t of the solar cell as a function of layer thicknesses. The optimized layer thicknesses as functions o f wavelength are shown in Table 1 with the SiO2 layer thickness held at 100 A. Since the refractive index of silicon diverges sharply in the UV region, it is impossible to obtain a reflectance null at 300 and 400 nm using SiNx ; however, these wavelengths are of little interest in LSC systems. In the visible and near-IR regions the reflectance is essentially zero at the desired wavelengths. Furthermore, at 6 0 0 , 8 0 0 and

39 TABLE 1 Minimum antireflection coating reflectivity as a function of wavelength

(nm)

SiN x layer thickness (1~)

SiO 2 layer thickness (A)

Reflectivity at k (%)

300 400 500 600 700 800 900 1000

320 390 505 625 750 870 985 1110

lOO 100 100 100 100 100 100 100

5.5 2.5 0.1 o.o 0.07 0.18 0.25 0.3

1 0 0 0 n m t h e r e f l e c t a n c e is v e r y l o w o v e r a b a n d w i d t h o f a b o u t 1 0 0 n m , as s h o w n in F i g . 1. T h u s a l o w r e f l e c t a n c e c a n e a s i l y b e o b t a i n e d f o r cells designed for the range 600 - 1000 nm using this double-layer AR coating. T h e m e a s u r e d r e f l e c t a n c e o f a cell d e s i g n e d f o r 6 0 0 n m w a s a p p r o x i m a t e l y zero at that wavelength. 50

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3. Cell m o d e l i n g T h e r e h a v e b e e n m a n y s t u d i e s o f t h e d e p e n d e n c e o f s o l a r cell e f f i c i e n c y o n s u b s t r a t e d o p i n g f o r t h e s o l a r s p e c t r u m . H o w e v e r , since t h e a b s o r p t i o n

40

depth varies drastically with wavelength, the o p t i m u m base dopant concentration for the LSC cell may be very different from that obtained in a study of conventional solar cells. The three wavelengths studied were 6 0 0 , 8 0 0 and 1 0 0 0 nm, and to select the best base doping concentration for these wavelengths a computer code [8] was used which solves the transport equations numerically in one dimension. This code treats band-gap-narrowing effects and Auger recombination. Also, since the minority carrier lifetime of a solar cell is often degraded in processing, the dependence of cell efficiency on minority carrier lifetime will be discussed. The one-dimensional numerical solutions of the transport equations for current collection efficiency, open-circuit voltage Voc and fill factor FF of a cell are shown in Fig. 2 as functions of dopant concentration for an incident photon flux of 1018 c m - 2 s-1 (about 0.3 W cm- 2 ) at a wavelength of 800 nm. For the m a x i m u m lifetime [9] o f the material the current collection efficiency decreases with increasing dopant concentration, while the fill factor and open-circuit voltage both increase with dopant concentration. As the minority carrier lifetime is degraded by a factor of 10, the fill factor is most severely degraded at low doping levels whereas the current collection efficiency and open-circuit voltage are reduced most at high dopant concentrations. The high current collection at low dopant concentrations is due to the dependences of the minority carrier lifetime r and the diffusion coefficient on the doping concentration (i.e. "r and the diffusion coefficient decrease with increasing dopant concentration). 1.00

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41 Because of the great increase in absorption depth in going from 600 to 1000 nm, the peak in efficiencies for 1000 nm light occurs at a much lower dopant concentration (owing to the increase in minority carrier lifetime with lower d o p a n t concentration) than with 600 nm light (Fig. 3). The efficiency of the cell with 600 nm light is n o t greatly dependent on the substrate dopant concentration for either lifetime with a broad peak of efficiency centered at 1017 - 1018 cm -3. As the LSC wavelength is increased to 800 nm, the peak of efficiency is shifted to 1016 - 1017 cm -3. The decrease in lifetime of a factor of 10 with 800 nm light reduces the efficiency most near the peak values. With a wavelength o f 1000 nm the efficiency is highest at 10 TM - 1015 cm -3 for b o t h lifetimes, and the decrease in lifetime reduces the efficiency most at higher dopant concentrations. Thus on using a longer wavelength dye, a substrate with a lower doping level must be used, whereas at shorter wavelengths the cell efficiency is less sensitive to the d o p a n t concentration. In all cases the decrease in lifetime of a factor o f 10 reduces the efficiency b y at least 2 percentage points and at higher doping concentrations much larger losses of efficiency were observed for longer wavelength light. The maximum efficiency with 600 nm illumination is 28%. As the wavelength of the light approaches the band gap of silicon, the m a x i m u m efficiency increases to 36% and 41% for wavelengths of 800 nm and 1000 nm respectively. These efficiencies are very high b u t would be degraded somewhat in practice b y metal shadowing and series resistance losses.

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42

4. Grid design To optimize the grid structure, the width and separation of the contacts must be varied to minimize the losses due to metal shadowing and to resistance in the contacts and diffused layer. At a given current density, as the contacts are moved further apart, the shadowing losses decrease whereas the resistance losses increase in both the metal contacts and the diffused layer. Thus a m i n i m u m in the total losses will occur as the distance between contacts is varied for a given grid linewidth. The trade-off between these losses has been discussed previously [ 10] and will not be discussed further here. An LSC cell designed at Sandia National Laboratories is shown in Fig. 4. This cell has been optimized for an illumination intensity of 1.0 W cm -2 corresponding to a current density of roughly 0.3 A cm 2.

Fig. 4. T h e LSC cells w h i c h we f a b r i c a t e d have c o n t a c t w i d t h s o f 25 p m a n d c o n t a c t separations of 850 pm.

5. Cell fabrication The LSC concentrator cells were fabricated using 0.2 - 0.4 ~ cm (100) n-type float-zoned wafers. All dopants were introduced with gaseous source diffusions. Two of the most important steps in this process are a phosphorus back-surface diffusion and an oxidation to passivate the front surface. The back-surface phosphorus diffusion, which also serves as a gettering step, is carried o u t first and then all subsequent processes are performed at 1000 °C or less to maintain a long minority carrier lifetime [ 11, 12 ]. A thermal oxide passivates the front surface and reduces the dark current [ 11 ], resulting in an increase in Voc. The AR coating is a plasma-assisted chemically vapourdeposited SiNx layer (670 A thick) deposited at 300 °C onto the 100 A thermal oxide. This coating is optimized for 600 nm illumination. The cell metallization is T i - P d - A g .

6. Results The efficiency, Voc and F F of a dye concentrator cell (optimized for illumination at about 600 n m and 1 W cm -2 ) are shown as functions of

43

sunlight concentration (the AM1 solar spectrum) in Fig. 5. The efficiency at an illumination of 1 sun is greater than 18% and the peak efficiency is 19.4% which occurs at an illumination of 10 suns. These efficiencies are the highest reported to date in sunlight at these low concentrations. The characteristics of this cell which combine to yield these high efficiencies are high Voc (638 mV at I sun illumination), a high current density at 1 sun illumination (34.4 mA cm -2 at 100 W cm -2 ) and a high FF (0.82 at 1 sun illumination). The decrease in efficiency at illumination levels above 10 suns is due to the degradation in FF which is caused by resistance losses in the diffused layer and metallization. From the Voc, F F and spectral response the efficiency v e r s u s concentration at 600 nm can be projected. A current of 0.50 A W -1 of optical power (632.8 nm) was measured with a 5 mW H e - N e laser beam focused between the contacts. Since the m a x i m u m possible current at this wavelength is 0.51 A W -1 , the cell w i t h o u t metal shadowing is collecting about 98% of possible photogenerated current. At 600 nm the cell w i t h o u t metal shadowing should have the collected current reduced from a m a x i m u m possible of 0.484 to 0.474 A W -1 . When the 3% shadowing loss is taken into account, the cell would collect 0.46 A W -1 of the light at 600 nm. From the product of the current collection at 600 nm, Voc and FF, the efficiency at 600 nm can be projected. The projected efficiency at 600 nm of this cell and the efficiency calculated from the code are shown in Fig. 6 as functions of current density. As is seen in Fig. 6 the projected efficiency of the cell at current densities below 0.33 A cm- 2 is within 1 - 2 percentage points of the calculated efficiency. Furthermore, the peak projected efficiency of the cell with 600 nm illumination is 26%,

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45

95% current collection of the cell at 800 nm and the Voc and FF of the cell at the m a x i m u m efficiency of the cell with AM1 illumination (Fig. 5)) would be 34% if the AR coating of the cell were optimized at 800 nm. This compares with the code efficiency of a b o u t 37% at the same current density. Thus this base doping yields a cell which would be efficient for both 600 and 800 nm. In addition to the quantum efficiency information, information can be obtained a b o u t the effectiveness of the processing. Since the normalized quantum efficiency at 400 nm is greater than 90% the front-surface passivation has reduced the surface recombination velocity.

7. Conclusions A cell designed for a luminescent solar concentrator was fabricated which yielded efficiencies of 18% and 19.4% at 0.1 W c m - 2 and 1.0 W c m - 2 respectively under a solar spectrum. This cell has the highest reported efficiency of a silicon cell to date at these low concentrations. From spectral response results the projected peak efficiency of this cell at 600 nm is 26% compared with a calculated efficiency of 28%. The difference b e t w e e n these t w o efficiencies is due to contact shadowing and series resistance losses in the diffused layer and metal contacts. To achieve the high cell efficiencies reported here, front-surface passivation and phosphorus gettering were used. Since the projected performance of the cell at 600 nm is close to the efficiency predicted b y the code calculations, the efficiency predictions of the code at other wavelengths should be realistic. Efficiencies of 35.7% and 40.6% at 800 nm and 1000 nm respectively are calculated from the code. Furthermore, it was shown that the reflectance of a cell can be reduced nearly to zero at a chosen wavelength between 500 and 1000 nm b y changing the AR coating thickness. At longer wavelengths the code shows that silicon of low d o p a n t concentration (i.e. of high resistivity) should be used to achieve high current collection, whereas at shorter wavelengths more highly d o p e d silicon (i.e. lower resistivity) can be used.

Acknowledgment This work was supported b y the U.S. Department of Energy.

References I C.F. Rapp and N. L. Boling, Luminescent solar concentrators, Proc. 13th Photovoltaic Specialists' Conf., Washington, DC, June 5 - 8, 1978, IEEE, New York, 1978, p. 690. 2 A. Goetzberger and W. Grenbel, Solar energy conversion with fluorescent collectors, Appl. Phys., 14 (1977) 123.

46 3 V. Wittwer, K. Heidler, A. Zastrow and A. Goetzberger, Efficiency and stability of experimental fluorescent planar concentrators (FPCs), Proc. 14th Photovoltaic Specialists' Conf., San Diego, CA, January 7 - 10, 1980, IEEE, New York, 1980, p. 760. 4 J. R. Hauser and P. M. Dunbar, Performance limitations of silicon solar cells, IEEE Trans. Electron Devices, 24 (1977) 305. 5 P. Lauwers, J. Van Meerbergen, P. Bulteel, R. Mertens and R. Van Overstraeten, Influence of bandgap narrowing on the performance of silicon n - p solar cells, SolidState Electron., 21 (1978) 747. 6 H. J. Hovel, Semiconductors and Semimetals, Vol. 11, Solar Cells, Academic Press, New York, 1975, p. 76. 7 F. W. Sexton, Optimization of multilayer AR coatings, Proc. 155th Meet. o f the Electrochemical Society, Boston, MA, Spring 1979, p. 188. 8 H. T. Weaver, Comparison of solar cell performance to calculations using different energy bandgap models, Appl. Phys. Lett., 37 (1980) 1009. 9 D. Kendall, Proc. Conf. on the Physics Applications o f Lithium-diffused Silicon, Goddard Research Center, December 1969, National Aeronautics and Space Adminis tration, Washington, DC, 1969. 10 C. M. Garner, Extrinsic losses in solar cells for linear focus systems, S A N D Tech. R e p 1781, 1979 (Sandia National Laboratories). 11 R. D. Nasby and J. G. Fossom, Characterization of p+nn + BSF silicon concentrator solar cells, Proc. 14th Photovoltaic Specialists' Conf., San Diego, CA, January 7 - 10, 1980, IEEE, New York, 1980, p. 419. 12 H. T. Weaver and R. D. Nasby, Minority carrier lifetimes in silicon solar cells determined from spectral and transient measurements, Solid-State Electron., 22 (1979) 687.