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A screen-printed interdigitated back contact cell using a boron-source diffusion barrier
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 88 (2005) 119–127 www.elsevier.com/locate/solmat
Letter
A screen-printed interdigitated back contact cell using a boron-source diffusion barrier Peter Hacke, James M. Gee Advent Solar, Inc., 800 Bradbury Drive SE, Albuquerque, NM 87106, USA Received 26 October 2004; accepted 9 February 2005 Available online 19 April 2005
Abstract A low-cost approach to fabricating interdigitated back contact cells is carried out on the principle of screen-printing a material that serves both to dope the rear surface and as a diffusion barrier to the dopant species of the opposite polarity. With this technique, an interdigitated pattern of n+ and p+ regions is formed on the cell back. Shunt-free rear interdigitated junctions are achieved. This work produced a cell with confirmed conversion efficiency of 10.5%. Areas for further efficiency gains are discussed. r 2005 Elsevier B.V. All rights reserved. Keywords: Solar cell; Interdigitated back contact; Silicon; Diffusion barrier; Screen-printing
1. Introduction Screen-printing is currently favored by industry for low-cost mass production of solar cells, especially with processes such as metallization. It is also used for application of doping materials for diffusions [1]. Work on interdigitated back contact (IBC) cells started around 1977 for use in concentrator applications [2]. Most famously, high-performance IBC cells with point contacts were developed at Stanford University using photolithography in the process to define n- and p-type regions and contact vias on the cell rear. They reported efficiencies above 22% [3,4]. Corresponding author. Tel.: +1 404 661 3852.
E-mail address: [email protected] (P. Hacke). 0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2005.02.003
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P. Hacke, J.M. Gee / Solar Energy Materials & Solar Cells 88 (2005) 119–127
Using screen-printing on the other hand, EBARA Solar Corporation reported a 10.6% efficient 5.0 cm2 IBC cell using high lifetime 100 mm-thick n-type dendritic web Si [5]. IBC cells have a number of attractive features including no front metallization yielding no grid shadowing loss and improved aesthetics. Cell interconnection requires less material and is expected to be simpler. Some of the technological issues that need to be addressed in making IBC cells include achieving n and p regions on the rear surface that are free of shunt currents, whether by parasitic shunts associated with an inversion layer on the rear surface or tunneling through the junction. Similarly, metallization must be well registered over the base and emitter contacts so as to not short-circuit the device. Passivation of the front surface and the surfaces between the positive and negative metallization on the rear must be achieved to prevent surface recombination loss of minority carriers. It is beneficial to have long minority carrier diffusion lengths because photogenerated carriers over the base contact must travel laterally to be collected, for example. Screen-printed diffusion barriers have been used to delimit n and p regions of emitter wrap-through cells [6]. Diffusion barriers have also been investigated for use in buried contact cells to protect the emitter diffusion while performing the contact diffusion [7]. Boron has been included in CVD-deposited SiO2 to form back surface fields in emitter wrap-through cells made on p-type substrates [8], and an Al–P codiffusion process has also been reported for the fabrication of emitter wrap-though solar cells [9]. In this work we demonstrate the principle of making IBC cells using a group III element in a screen printable matrix that functions simultaneously as a diffusion barrier to phosphorus diffusion and to dope the underlying semiconductor to achieve a patterned emitter on the rear surface of solar cells. A number of group III or group V elements could be used in the screen-printed diffusion barrier/source, each with their benefits and disadvantages; however, the group III element is boron in this work.
2. Experimental High resistivity (800 O cm, 300 mm thick, 42 cm2 area) n+ float zone-refined wafers were cleaned in a H2SO4/H2O2 (Piranha) solution, etched back in an HF/HNO3 solution to 260 mm, and cleaned in a HCl/H2O2 solution. The boron diffusion barrier was printed in an interdigitated pattern, dried, and cured in oxygen ambient. Boron was then diffused from the diffusion barrier into the underlying Si to achieve sheet resistivity of about 120 O/&. The setpoint was decreased and POCl3 was flowed for n+ diffusion into the unprotected areas of the Si for a n+ layer sheet resistivity of about 35 O/&. We describe two basic IBC processes using the screen-printed diffusion barrier in this paper. In the first (Fig. 1 and sample S1), n+ front surfaces and segmented n+ and p+ regions were formed on n-type Si wafers with a barrier pattern of 2 mm pitch with alternating n+ and p+ regions, both of 1 mm width. After diffusion, the barrier and diffusion glasses were etched bare and a small amount of Si was etched back
ARTICLE IN PRESS P. Hacke, J.M. Gee / Solar Energy Materials & Solar Cells 88 (2005) 119–127
n+
After step 4
1 Clean Si 2 Screen print barrier with boron and cure 3 Boron diffuse to 120 Ω/ and phosphorus diffuse to 35 Ω / 4 Etch barrier, P-glass, and Si to 145 Ω / over p-Si and 45 Ω / over n-Si 5 Apply SiN (both sides) 6 Screen-print and fire Al p-contact 7 Screen-print and fire Ag n-contact
121
n- type Si diffusion barrier with boron
p+
After step 7
Al
Ag
Fig. 1. Process description for sample S1 in which the diffusion barrier is etched away.
n+ 1 Clean Si 2 Screen print barrier with boron and cure 3 Boron diffuse to 120 Ω / and phosphorus diffuse to 35 Ω/ 4 Etch P-glass 5 Apply SiN (both sides) 6 Screen-print and fire Al 7 Screen-print and fire Ag
After step 4 n- type Si
diffusion barrier with boron
p+ After step 7
Ag Al
Fig. 2. Process description for sample S2. The diffusion barrier remains in the completed cell.
before application of the passivation layers and metallization. In the second process scheme (Fig. 2 and sample S2), the diffusion barrier had slots through which phosphorus could diffuse and through which screen-printed Al could later be alloyed with the underlying Si, to form p+ contacts to the base by overdoping. In this case, the phosphorous glass was removed but the diffusion barrier was allowed to remain. High-frequency plasma-enhanced chemical vapor deposition of SiN on the top and bottom surfaces were applied for antireflection and passivation. Effective minority carrier lifetimes of the patterned wafers before metallization as determined by quasi-steady-state photo-conductance decay were 70 and 53 ms at 1 1016 cm 3 photogenerated carrier concentration for samples S1 and S2, respectively. There was evidence of majority carrier trapping because high lifetimes at lower injected carrier concentrations were observed [10]. Al metallization was screen printed to form positive contacts by alloying Al with the Si to form a local back surface field. Ag metallization was printed to form negative contacts. Firing of the metallization was performed in a belt furnace.
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3. Results and discussion Samples were tested by suns-Voc technique [11] to estimate I– V curve parameters void of series resistance losses—these are referred to as pseudo-parameters (Table 1). The obtained curves were fitted with the two diode model. S1 displays a favorably high shunt resistance. This success is attributed to the etching of the most highly doped Si under the diffusion source glasses resulting in the elimination of tunneling shunts. There is still room for improvement because the saturation current density representing recombination currents in the non-ideal diode, Jo2, is high such that the pseudo-fill factor (pseudo-FF) is less than optimum at 0.776. In sample S2, where the diffusion barrier is left remaining on the surface and the doped Si surface is not etched back, a slightly less favorable shunt resistance is measured. S2 also displays comparatively reduced Voc. Much of the reduction in Voc and pseudo-FF is associated with the significantly increased Jo2 term and to a much lesser extent the small increase in Jo1 associated with reduced effective lifetime of minority carriers in the cell. Unlike conventional n+/p/p+ cells, junction shunting by Ag of the top emitter grid contact into the cell base if fired at too high a temperature does not have an analogue in this cell design. On the other hand, excessive penetration of the base metallization through the rear n+ surface field will reduce base lifetime via increased surface recombination. After testing electrical properties of the cell, a portion of the cell S2 was stripped of metal and the surface etched to determine a bulk minority carrier lifetime of 140 ms at 1 1016 cm 3 injection which is significantly reduced from 3 ms, the starting lifetime of the Si. The occurrence of majority carrier trapping was concluded based on anomalously high lifetimes observed at lower injection levels. Best cell results are given in Fig. 3 as confirmed by Sandia National Laboratories under 1 sun, air mass 1.5 global conditions. Dark I– V curve fitting was performed and fitting parameters are shown; however, it should be recalled that current paths differ when driving carriers though the dark cell. The illuminated I– V curve was not well fitted with a two diode model. Examining the cell test data (Table 2), there is significant reduction in fill factor compared to the pseudo-FF measured by suns-Voc technique. The effect of series resistance on fill factor can be estimated from the difference between the two because the suns-Voc-determined pseudo-FF is essentially the cell fill factor void of series resistance losses. While the effect of contact resistance losses cannot be eliminated from consideration, contact resistance measured on test Table 1 Derived cell parameters from suns-Voc measurements including open circuit voltage, pseudo-fill factor (fill factor void of series resistance), saturation currents with a two diode model where ideality factor of the second diode is 2, and shunt resistance Cell Voc (V) Pseudo-FF Jsc S1 S2 a
0.591 0.573
0.776 0.747
a
29.8 30.0
(mA/cm2) Pseudo eff (%) Jo1 (pA/cm2) Jo2 (nA/cm2) Rsha (O/cm2) 13.01 12.76
2.3 2.6
From final cell test, used to calculate diode parameters in this table.
100 290
10000 6000
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10.0 1.0
Amps
0.1 0.01 0.001 Rsh Jo_1 Jo_2 N Rs
1.0x10-4 1.0x10-5 0
0.10
0.20
(a)
IBC S1
0.30 0.40 Volts
05/10/04 1:18 PM
0.50
ohm-cm2 168000 pA/cm2 6 nA/cm2 50 2 2 4 ohm-cm
0.60
0.70
24.8 °C
1.4 PM 1.2
Amps
1.0 0.8 0.6 0.4 0.2 0.0 0.0
0.1
0.2
(b)
05/10/04 1:17 PM IBC S1
0.3
0.4
0.5
0.6
Volts 24.9 °C
42.0 cm2 1.0000 M* 584.2 Voc(mV) 1.0087 S* 416.1 Vmp(mV)
29.76 Jsc(mA/cm2) 1.250 Isc(A) 1.056 Imp(A)
0.602 FF 10.46 % Eff
AM1.5G 1.00 Suns
Fig. 3. Dark (a) and light (b) current–voltage curves of sample S1. Two diode model curve fitting parameters are given in (a) and are indicated by solid squares and a curve.
patterns processed along side this cell indicated negligible (7 mO cm2) series resistance losses. This suggests that a significant portion of the series resistance is associated with the high resistivity n-type base. While under illumination, the cell functions in a high injection regime. We must consider recombination and the redistribution of carriers due to electric fields may result in modulation of the number of carriers and the series resistance of the cell.
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Table 2 Cell test results Cell
Voc (V)
Jsc (mA/cm2)
FF
Eff (%)
Rs (O/cm2)
S1 S2
0.584 0.568
29.8 30.0
0.602 0.604
10.5 10.3
4.0 4.9
7
Rs (ohm cm2)
6
5
4
3 0
0.50
1.00
1.50
2.00
2.50
Suns Fig. 4. Series resistance as a function of illumination for sample S2. A fitting curve is drawn to guide the eye. Some scatter in the trend line is attributed to the sensitivity of the series resistance measurement technique to small variations in temperature. The results indicate the series resistance of the cell is modulated by the free carriers generated by illumination.
To explore this further, the series resistance of the cell was measured varying the illumination intensity from about 0.5 to 2 suns (Fig. 4). The series resistance measurement [12] involves shading the cell in the cell tester, examining the difference in voltage at around the maximum power point of the lighted curve and the shaded curve at a low current that is relatively void of series resistance losses. The shaded curve is likely to in fact contain some effect of series resistance when the series resistance is high and therefore introduce error. The results are nevertheless reasonable considering the extent of the fill factor loss measured at one sun and that the observed trend was consistent regardless of the fraction of light shaded during testing. Over the range of intensities measured, the trend indicates that the cell is operating in a regime where the series resistance is modulated by the photogenerated carrier concentration in the base.
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Loss of base conductivity in cells designed to operate in high injection mode was described by Schwartz, Lundstrom, and Nasby [13,14] to occur towards the back surface field in concentrator cells. This results from the Fermi level bending that occurs as current flows in the cell according to the differing mobilities of electrons and holes, the relative lack of photoionizing light in the cell rear, low doping level, and the field associated with the high low (n/n+) junction. Examining the rear of a p+/n/n+ structure in PC1D simulator [15] under one sun condition to approximate the rear n/n+ region of the current device suggests the same phenomenon occurs. Perturbation of the modeling parameters yield an identical conclusion to that reached by Schwartz and coworkers—using thinner cells and increasing the base doping density will improve the base conductivity. Reduced resistivity through doping of the base will be helpful for improving series resistance; however, such material will have reduced lifetime and could be more susceptible to further reduction in lifetime if impurities are introduced during the cell process as they evidently were in this work. We can easily conceive a number of areas for optimization. Reduced n+ doping of the front surface along with suitable high-quality surface passivation will benefit the current output of the cell via reduced Auger and Shockley Read Hall recombination. Surface texturing will decrease the reflected light and improve the current. It is expected that rear emitter area greater than the 50% coverage used in these tests will reduce the required distance for travel of minority carriers photogenerated over the base to reach the junction, thus improving current collection. To observe the extent this may be true, an area of sample S2 was examined with laser beam-induced current (Fig. 5). With nominally 50% of the cell back covered by the p+ emitter, about 45% of the front surface displays the maximum response to 633 nm light. The rear surface emitter coverage could be reasonably expanded to 80% with a corresponding increase in current collection. Similarly, reduction in cell thickness will increase the collection of minority carriers photogenerated at the cell front and decrease series resistance. While the screen-printing equipment used for printing the diffusion barrier was also used for metallization, dedicated, clean equipment and
Fig. 5. 633 nm HeNe laser beam-induced current scan of a 0.5 1.0 cm2 area at the edge of cell S2. The maximum intensity region (1.0, white) corresponds approximately to the area above the 1 mm-wide emitter. The fraction of collected current over the base contact is 0.69 and is dark grey. The black region at the left is beyond the cell edge.
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materials optimized for pre-diffusion wafer processing will benefit minority lifetime, and in turn, improve efficiency greatly. Estimating these gains, improvements in the contact geometry and lifetime such that the region generating maximum current was increased from 50% to 80% would increase Jsc to 35.5 mA/cm2, addition of optimized texturing should reduce reflectivity from 8% to about 3% leading to improvement in Jsc to 37.4 mA/cm2. A fill factor of 0.74 could be obtained with reduced base resistivity and thickness associated with a total specific series resistance of 0.8 O cm2, a specific resistance which is typical for good quality solar cells with screen-printed metallization. Improvements in Jo2 should increase the fill factor significantly further. Optimized front surface passivation of n+/n/p+ structures followed by a high-temperature sequential Al and Ag firing steps has been found to yield open circuit voltage of 620 mV [1]. Assuming this value, an IBC cell exceeding 17% efficiency should be readily obtainable with this process, though optimized single step belt-furnace firing metallization should lead to improved retention of surface passivation and higher open circuit voltage. It is not clear to what extent junction recombination at the back surface can be improved with this diffusion barrier process such that Jo2 will decrease and the fill factor will increase, but some improvement can be expected through optimization of the diffusions. These factors aside, the recombination losses associated with relatively large area screen-printed metallization that is fired thought the dielectric will be the limiter of the cell performance compared to previously published high-efficiency IBC cell structures.
4. Conclusion Cell processes using screen-printing to form interdigitated emitters and metallization for fabricating interdigitated back contact (IBC) cells are demonstrated. An interdigitated emitter pattern is achieved by screen-printing a group III-containing diffusion barrier that (1) provides a p-type diffusion into the underlying Si and creates a junction when applied to an n-type wafer and (2) locally prevents diffusion so that a n-type doping species may be diffused into the uncovered areas. Negligible shunt losses are readily achievable; however, base and junction saturation currents require reduction by means of clean processing and optimization of the diffusions, respectively. Series resistance losses that occur with a high resistivity base in high injection mode devices can be understood considering previously reported models describing carrier redistribution in the base. Numerous areas for performance increase to achieve low-cost IBC solar cells remain to be exploited.
Acknowledgements The authors would like to acknowledge A. Rohatgi and A. Ebong. Sandia National Laboratories provided assistance in cell characterization. We also benefited from discussions with A.F. Carroll, J. Salami, C.B. Honsberg, and S. Bowden.
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References [1] P. Hacke, J. Moschner, S. Yamanaka, D.L. Meier, Proceedingsof the 19th European Photovoltaic Solar Energy Conference, Paris, 2004, 2CV.3.53. [2] M.D. Lammert, R.J. Schwartz, IEEE Trans. Electron. Devices 24 (1977) 337–342. [3] R.A Sinton, Y. Kwark, P. Gruenbaum, R.M. Swanson, Proceedings of the 18th IEEE Photovoltaic Specialists Conference, 1985, pp. 61–65. [4] R.R. King, R.A. Sinton, R.M. Swanson, Appl. Phys. Lett. 54 (1989) 1460–1462. [5] D.L. Meier, H.P. Davis, P. Hacke, R.A. Garcia, S. Yamanaka, J. Salami, J.A. Jessup, Proceedings of the 17th European Photovoltaic Solar Energy Conference, Munich, 2001, pp. 1323–1326. [6] W. Neu, A. Kress, W. Jooss, P. Fath, E. Bucher, Sol. Energy Mater. Sol. Cells 74 (2002) 139–145. [7] B.S. Richards, Prog. Photovoltaic Res. Appl. 12 (2004) 253–281. [8] D.D. Smith, J.M. Gee, M.D. Bode, J.C. Jimeno, IEEE Trans. Electron. Devices 46 (1999) 1993–1999. [9] K. Faika, M. Wagner, P. Fath, E. Bucher, 28th IEEE Photovoltaic Specialists Conference, 2000, pp. 260–263. [10] D. Macdonald, A. Cuevas, Appl. Phys. Lett. 74 (1999) 1710–1712. [11] R.A. Sinton, A. Cuevas, Proceedings of the 16th European Photovoltaic Solar Energy Conference, Glasgow, Scotland, 2000, pp. 1152–1155. [12] S. Bowden, A. Rohatgi, Proceedings of the 17th European Photovoltaic Solar Energy Conference, Munich, 2001, pp. 1803–1806. [13] R.J. Schwartz, M.S. Lundstrom, R.D. Nasby, IEEE Trans. Electron. Devices 28 (1981) 264–269. [14] R.J. Schwartz, M.S. Lundstrom, Proceedings of the 14th IEEE Photovoltaic Specialists Conference, 1980, pp. 1392–1393. [15] A. Basore, D.A. Clugston, 25th IEEE Photovoltaic Specialists Conference, Washington, 1996.
Solar Energy Materials & Solar Cells 88 (2005) 119–127 www.elsevier.com/locate/solmat
Letter
A screen-printed interdigitated back contact cell using a boron-source diffusion barrier Peter Hacke, James M. Gee Advent Solar, Inc., 800 Bradbury Drive SE, Albuquerque, NM 87106, USA Received 26 October 2004; accepted 9 February 2005 Available online 19 April 2005
Abstract A low-cost approach to fabricating interdigitated back contact cells is carried out on the principle of screen-printing a material that serves both to dope the rear surface and as a diffusion barrier to the dopant species of the opposite polarity. With this technique, an interdigitated pattern of n+ and p+ regions is formed on the cell back. Shunt-free rear interdigitated junctions are achieved. This work produced a cell with confirmed conversion efficiency of 10.5%. Areas for further efficiency gains are discussed. r 2005 Elsevier B.V. All rights reserved. Keywords: Solar cell; Interdigitated back contact; Silicon; Diffusion barrier; Screen-printing
1. Introduction Screen-printing is currently favored by industry for low-cost mass production of solar cells, especially with processes such as metallization. It is also used for application of doping materials for diffusions [1]. Work on interdigitated back contact (IBC) cells started around 1977 for use in concentrator applications [2]. Most famously, high-performance IBC cells with point contacts were developed at Stanford University using photolithography in the process to define n- and p-type regions and contact vias on the cell rear. They reported efficiencies above 22% [3,4]. Corresponding author. Tel.: +1 404 661 3852.
E-mail address: [email protected] (P. Hacke). 0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2005.02.003
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Using screen-printing on the other hand, EBARA Solar Corporation reported a 10.6% efficient 5.0 cm2 IBC cell using high lifetime 100 mm-thick n-type dendritic web Si [5]. IBC cells have a number of attractive features including no front metallization yielding no grid shadowing loss and improved aesthetics. Cell interconnection requires less material and is expected to be simpler. Some of the technological issues that need to be addressed in making IBC cells include achieving n and p regions on the rear surface that are free of shunt currents, whether by parasitic shunts associated with an inversion layer on the rear surface or tunneling through the junction. Similarly, metallization must be well registered over the base and emitter contacts so as to not short-circuit the device. Passivation of the front surface and the surfaces between the positive and negative metallization on the rear must be achieved to prevent surface recombination loss of minority carriers. It is beneficial to have long minority carrier diffusion lengths because photogenerated carriers over the base contact must travel laterally to be collected, for example. Screen-printed diffusion barriers have been used to delimit n and p regions of emitter wrap-through cells [6]. Diffusion barriers have also been investigated for use in buried contact cells to protect the emitter diffusion while performing the contact diffusion [7]. Boron has been included in CVD-deposited SiO2 to form back surface fields in emitter wrap-through cells made on p-type substrates [8], and an Al–P codiffusion process has also been reported for the fabrication of emitter wrap-though solar cells [9]. In this work we demonstrate the principle of making IBC cells using a group III element in a screen printable matrix that functions simultaneously as a diffusion barrier to phosphorus diffusion and to dope the underlying semiconductor to achieve a patterned emitter on the rear surface of solar cells. A number of group III or group V elements could be used in the screen-printed diffusion barrier/source, each with their benefits and disadvantages; however, the group III element is boron in this work.
2. Experimental High resistivity (800 O cm, 300 mm thick, 42 cm2 area) n+ float zone-refined wafers were cleaned in a H2SO4/H2O2 (Piranha) solution, etched back in an HF/HNO3 solution to 260 mm, and cleaned in a HCl/H2O2 solution. The boron diffusion barrier was printed in an interdigitated pattern, dried, and cured in oxygen ambient. Boron was then diffused from the diffusion barrier into the underlying Si to achieve sheet resistivity of about 120 O/&. The setpoint was decreased and POCl3 was flowed for n+ diffusion into the unprotected areas of the Si for a n+ layer sheet resistivity of about 35 O/&. We describe two basic IBC processes using the screen-printed diffusion barrier in this paper. In the first (Fig. 1 and sample S1), n+ front surfaces and segmented n+ and p+ regions were formed on n-type Si wafers with a barrier pattern of 2 mm pitch with alternating n+ and p+ regions, both of 1 mm width. After diffusion, the barrier and diffusion glasses were etched bare and a small amount of Si was etched back
ARTICLE IN PRESS P. Hacke, J.M. Gee / Solar Energy Materials & Solar Cells 88 (2005) 119–127
n+
After step 4
1 Clean Si 2 Screen print barrier with boron and cure 3 Boron diffuse to 120 Ω/ and phosphorus diffuse to 35 Ω / 4 Etch barrier, P-glass, and Si to 145 Ω / over p-Si and 45 Ω / over n-Si 5 Apply SiN (both sides) 6 Screen-print and fire Al p-contact 7 Screen-print and fire Ag n-contact
121
n- type Si diffusion barrier with boron
p+
After step 7
Al
Ag
Fig. 1. Process description for sample S1 in which the diffusion barrier is etched away.
n+ 1 Clean Si 2 Screen print barrier with boron and cure 3 Boron diffuse to 120 Ω / and phosphorus diffuse to 35 Ω/ 4 Etch P-glass 5 Apply SiN (both sides) 6 Screen-print and fire Al 7 Screen-print and fire Ag
After step 4 n- type Si
diffusion barrier with boron
p+ After step 7
Ag Al
Fig. 2. Process description for sample S2. The diffusion barrier remains in the completed cell.
before application of the passivation layers and metallization. In the second process scheme (Fig. 2 and sample S2), the diffusion barrier had slots through which phosphorus could diffuse and through which screen-printed Al could later be alloyed with the underlying Si, to form p+ contacts to the base by overdoping. In this case, the phosphorous glass was removed but the diffusion barrier was allowed to remain. High-frequency plasma-enhanced chemical vapor deposition of SiN on the top and bottom surfaces were applied for antireflection and passivation. Effective minority carrier lifetimes of the patterned wafers before metallization as determined by quasi-steady-state photo-conductance decay were 70 and 53 ms at 1 1016 cm 3 photogenerated carrier concentration for samples S1 and S2, respectively. There was evidence of majority carrier trapping because high lifetimes at lower injected carrier concentrations were observed [10]. Al metallization was screen printed to form positive contacts by alloying Al with the Si to form a local back surface field. Ag metallization was printed to form negative contacts. Firing of the metallization was performed in a belt furnace.
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122
3. Results and discussion Samples were tested by suns-Voc technique [11] to estimate I– V curve parameters void of series resistance losses—these are referred to as pseudo-parameters (Table 1). The obtained curves were fitted with the two diode model. S1 displays a favorably high shunt resistance. This success is attributed to the etching of the most highly doped Si under the diffusion source glasses resulting in the elimination of tunneling shunts. There is still room for improvement because the saturation current density representing recombination currents in the non-ideal diode, Jo2, is high such that the pseudo-fill factor (pseudo-FF) is less than optimum at 0.776. In sample S2, where the diffusion barrier is left remaining on the surface and the doped Si surface is not etched back, a slightly less favorable shunt resistance is measured. S2 also displays comparatively reduced Voc. Much of the reduction in Voc and pseudo-FF is associated with the significantly increased Jo2 term and to a much lesser extent the small increase in Jo1 associated with reduced effective lifetime of minority carriers in the cell. Unlike conventional n+/p/p+ cells, junction shunting by Ag of the top emitter grid contact into the cell base if fired at too high a temperature does not have an analogue in this cell design. On the other hand, excessive penetration of the base metallization through the rear n+ surface field will reduce base lifetime via increased surface recombination. After testing electrical properties of the cell, a portion of the cell S2 was stripped of metal and the surface etched to determine a bulk minority carrier lifetime of 140 ms at 1 1016 cm 3 injection which is significantly reduced from 3 ms, the starting lifetime of the Si. The occurrence of majority carrier trapping was concluded based on anomalously high lifetimes observed at lower injection levels. Best cell results are given in Fig. 3 as confirmed by Sandia National Laboratories under 1 sun, air mass 1.5 global conditions. Dark I– V curve fitting was performed and fitting parameters are shown; however, it should be recalled that current paths differ when driving carriers though the dark cell. The illuminated I– V curve was not well fitted with a two diode model. Examining the cell test data (Table 2), there is significant reduction in fill factor compared to the pseudo-FF measured by suns-Voc technique. The effect of series resistance on fill factor can be estimated from the difference between the two because the suns-Voc-determined pseudo-FF is essentially the cell fill factor void of series resistance losses. While the effect of contact resistance losses cannot be eliminated from consideration, contact resistance measured on test Table 1 Derived cell parameters from suns-Voc measurements including open circuit voltage, pseudo-fill factor (fill factor void of series resistance), saturation currents with a two diode model where ideality factor of the second diode is 2, and shunt resistance Cell Voc (V) Pseudo-FF Jsc S1 S2 a
0.591 0.573
0.776 0.747
a
29.8 30.0
(mA/cm2) Pseudo eff (%) Jo1 (pA/cm2) Jo2 (nA/cm2) Rsha (O/cm2) 13.01 12.76
2.3 2.6
From final cell test, used to calculate diode parameters in this table.
100 290
10000 6000
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10.0 1.0
Amps
0.1 0.01 0.001 Rsh Jo_1 Jo_2 N Rs
1.0x10-4 1.0x10-5 0
0.10
0.20
(a)
IBC S1
0.30 0.40 Volts
05/10/04 1:18 PM
0.50
ohm-cm2 168000 pA/cm2 6 nA/cm2 50 2 2 4 ohm-cm
0.60
0.70
24.8 °C
1.4 PM 1.2
Amps
1.0 0.8 0.6 0.4 0.2 0.0 0.0
0.1
0.2
(b)
05/10/04 1:17 PM IBC S1
0.3
0.4
0.5
0.6
Volts 24.9 °C
42.0 cm2 1.0000 M* 584.2 Voc(mV) 1.0087 S* 416.1 Vmp(mV)
29.76 Jsc(mA/cm2) 1.250 Isc(A) 1.056 Imp(A)
0.602 FF 10.46 % Eff
AM1.5G 1.00 Suns
Fig. 3. Dark (a) and light (b) current–voltage curves of sample S1. Two diode model curve fitting parameters are given in (a) and are indicated by solid squares and a curve.
patterns processed along side this cell indicated negligible (7 mO cm2) series resistance losses. This suggests that a significant portion of the series resistance is associated with the high resistivity n-type base. While under illumination, the cell functions in a high injection regime. We must consider recombination and the redistribution of carriers due to electric fields may result in modulation of the number of carriers and the series resistance of the cell.
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Table 2 Cell test results Cell
Voc (V)
Jsc (mA/cm2)
FF
Eff (%)
Rs (O/cm2)
S1 S2
0.584 0.568
29.8 30.0
0.602 0.604
10.5 10.3
4.0 4.9
7
Rs (ohm cm2)
6
5
4
3 0
0.50
1.00
1.50
2.00
2.50
Suns Fig. 4. Series resistance as a function of illumination for sample S2. A fitting curve is drawn to guide the eye. Some scatter in the trend line is attributed to the sensitivity of the series resistance measurement technique to small variations in temperature. The results indicate the series resistance of the cell is modulated by the free carriers generated by illumination.
To explore this further, the series resistance of the cell was measured varying the illumination intensity from about 0.5 to 2 suns (Fig. 4). The series resistance measurement [12] involves shading the cell in the cell tester, examining the difference in voltage at around the maximum power point of the lighted curve and the shaded curve at a low current that is relatively void of series resistance losses. The shaded curve is likely to in fact contain some effect of series resistance when the series resistance is high and therefore introduce error. The results are nevertheless reasonable considering the extent of the fill factor loss measured at one sun and that the observed trend was consistent regardless of the fraction of light shaded during testing. Over the range of intensities measured, the trend indicates that the cell is operating in a regime where the series resistance is modulated by the photogenerated carrier concentration in the base.
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Loss of base conductivity in cells designed to operate in high injection mode was described by Schwartz, Lundstrom, and Nasby [13,14] to occur towards the back surface field in concentrator cells. This results from the Fermi level bending that occurs as current flows in the cell according to the differing mobilities of electrons and holes, the relative lack of photoionizing light in the cell rear, low doping level, and the field associated with the high low (n/n+) junction. Examining the rear of a p+/n/n+ structure in PC1D simulator [15] under one sun condition to approximate the rear n/n+ region of the current device suggests the same phenomenon occurs. Perturbation of the modeling parameters yield an identical conclusion to that reached by Schwartz and coworkers—using thinner cells and increasing the base doping density will improve the base conductivity. Reduced resistivity through doping of the base will be helpful for improving series resistance; however, such material will have reduced lifetime and could be more susceptible to further reduction in lifetime if impurities are introduced during the cell process as they evidently were in this work. We can easily conceive a number of areas for optimization. Reduced n+ doping of the front surface along with suitable high-quality surface passivation will benefit the current output of the cell via reduced Auger and Shockley Read Hall recombination. Surface texturing will decrease the reflected light and improve the current. It is expected that rear emitter area greater than the 50% coverage used in these tests will reduce the required distance for travel of minority carriers photogenerated over the base to reach the junction, thus improving current collection. To observe the extent this may be true, an area of sample S2 was examined with laser beam-induced current (Fig. 5). With nominally 50% of the cell back covered by the p+ emitter, about 45% of the front surface displays the maximum response to 633 nm light. The rear surface emitter coverage could be reasonably expanded to 80% with a corresponding increase in current collection. Similarly, reduction in cell thickness will increase the collection of minority carriers photogenerated at the cell front and decrease series resistance. While the screen-printing equipment used for printing the diffusion barrier was also used for metallization, dedicated, clean equipment and
Fig. 5. 633 nm HeNe laser beam-induced current scan of a 0.5 1.0 cm2 area at the edge of cell S2. The maximum intensity region (1.0, white) corresponds approximately to the area above the 1 mm-wide emitter. The fraction of collected current over the base contact is 0.69 and is dark grey. The black region at the left is beyond the cell edge.
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materials optimized for pre-diffusion wafer processing will benefit minority lifetime, and in turn, improve efficiency greatly. Estimating these gains, improvements in the contact geometry and lifetime such that the region generating maximum current was increased from 50% to 80% would increase Jsc to 35.5 mA/cm2, addition of optimized texturing should reduce reflectivity from 8% to about 3% leading to improvement in Jsc to 37.4 mA/cm2. A fill factor of 0.74 could be obtained with reduced base resistivity and thickness associated with a total specific series resistance of 0.8 O cm2, a specific resistance which is typical for good quality solar cells with screen-printed metallization. Improvements in Jo2 should increase the fill factor significantly further. Optimized front surface passivation of n+/n/p+ structures followed by a high-temperature sequential Al and Ag firing steps has been found to yield open circuit voltage of 620 mV [1]. Assuming this value, an IBC cell exceeding 17% efficiency should be readily obtainable with this process, though optimized single step belt-furnace firing metallization should lead to improved retention of surface passivation and higher open circuit voltage. It is not clear to what extent junction recombination at the back surface can be improved with this diffusion barrier process such that Jo2 will decrease and the fill factor will increase, but some improvement can be expected through optimization of the diffusions. These factors aside, the recombination losses associated with relatively large area screen-printed metallization that is fired thought the dielectric will be the limiter of the cell performance compared to previously published high-efficiency IBC cell structures.
4. Conclusion Cell processes using screen-printing to form interdigitated emitters and metallization for fabricating interdigitated back contact (IBC) cells are demonstrated. An interdigitated emitter pattern is achieved by screen-printing a group III-containing diffusion barrier that (1) provides a p-type diffusion into the underlying Si and creates a junction when applied to an n-type wafer and (2) locally prevents diffusion so that a n-type doping species may be diffused into the uncovered areas. Negligible shunt losses are readily achievable; however, base and junction saturation currents require reduction by means of clean processing and optimization of the diffusions, respectively. Series resistance losses that occur with a high resistivity base in high injection mode devices can be understood considering previously reported models describing carrier redistribution in the base. Numerous areas for performance increase to achieve low-cost IBC solar cells remain to be exploited.
Acknowledgements The authors would like to acknowledge A. Rohatgi and A. Ebong. Sandia National Laboratories provided assistance in cell characterization. We also benefited from discussions with A.F. Carroll, J. Salami, C.B. Honsberg, and S. Bowden.
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