A method to derive the emitter saturation current density J0e with lateral resolution is applied to investigate selective emitter structures. The method uses PL lifetime imaging at several injection densities to laterally evaluate J0e by applying the method of Kane and Swanson [1] pixel by pixel. Samples with two-sided diffused emitters on lowlydoped Cz wafers were used to produce selective emitter structures by laser doping of the phosphorus-rich glass (LDSE). By comparison of experimental and numerical simulation results of J0e linescans, a limited resolution of a feature size of an inhomogeneous emitter is determined to be theoretically between 0.5-1.0 mm and experimentally about 2 mm. The method was successfully applied to investigate the dependence of J0e on the laser power of a selective emitter structure. The expected behaviour of a maximum J0e for medium laser intensities is observed. The method is suitable to evaluate the selective emitter process and its optimization.BMU/032520

Altermatt, Pietro P.

Fischer, G.

Müller, M.

Schlegel, K.

AbstractA method to derive the emitter saturation current density J0e with lateral resolution is applied to investigate selective emitter structures. The method uses PL lifetime imaging at several injection densities to laterally evaluate J0e by applying the method of Kane and Swanson [1] pixel by pixel. Samples with two-sided diffused emitters on lowly-doped Cz wafers were used to produce selective emitter structures by laser doping of the phosphorus-rich glass (LD-SE). By comparison of experimental and numerical simulation results of J0e linescans, a limited resolution of a feature size of an inhomogeneous emitter is determined to be theoretically between 0.5-1.0mm and experimentally about 2mm. The method was successfully applied to investigate the dependence of J0e on the laser power of a selective emitter structure. The expected behaviour of a maximum J0e for medium laser intensities is observed. The method is suitable to evaluate the selective emitter process and its optimization

Altermatt, P.P.

Elsevier - Publisher Connector 

 Energy Procedia  27 ( 2012 )  293 – 299 
1876-6102 © 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientifi c committee of the 
SiliconPV 2012 conference.
doi: 10.1016/j.egypro.2012.07.066 
SiliconPV: April 03-05, 2012, Leuven, Belgium 
Evaluating The Quality Of Selective Emitter Structures By 
Imaging The Emitter Saturation Current Density 
M. Müllera*, P.P. Altermattb, K. Schlegela, G. Fischera  
aSolarWorld Innovations GmbH, Berthelsdorferstr. 111A, 09599 Freiberg, Germany 
bDep. Solar Energy, Inst. Solid-State Physics, Leibniz University of Hannover, Appelstr. 2, 30167 Hannover, Germany 
 
Abstract  
A method to derive the emitter saturation current density J0e with lateral resolution is applied to investigate selective 
emitter structures. The method uses PL lifetime imaging at several injection densities to laterally evaluate J0e by 
applying the method of Kane and Swanson [1] pixel by pixel. Samples with two-sided diffused emitters on lowly-
doped Cz wafers were used to produce selective emitter structures by laser doping of the phosphorus-rich glass (LD-
SE). By comparison of experimental and numerical simulation results of J0e linescans, a limited resolution of a 
feature size of an inhomogeneous emitter is determined to be theoretically between 0.5-1.0 mm and experimentally 
about 2 mm. The method was successfully applied to investigate the dependence of J0e on the laser power of a 
selective emitter structure. The expected behaviour of a maximum J0e for medium laser intensities is observed.  The 
method is suitable to evaluate the selective emitter process and its optimization.  
 
© 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific 
committee of the SiliconPV 2012 conference 
 
Keywords: Emitter saturation current density; photoluminescence; selective emitter 
1. Introduction 
For the electrical characterization of silicon solar cells, it is common to quantify the saturation current 
density of the emitter J0e. In this paper, we investigate selective emitters for crystalline Si solar cells, 
meaning that the emitter is highly-doped at the front metal contacts and rather lowly doped elsewhere. By 
now, it has been common to measure the J0e of homogeneous samples, using the procedure of Kane and 
 
* Corresponding author. Tel:  +49-3731-301-1439; fax: +49-3731-301-1690 
E-mail address: matthias.mueller@sw-innovations.de 
Vailable online at www.sciencedirect.com
 12 Published by Elsevier Ltd. Sel ction and peer-revi w under r sponsibility of the sci ntifi c committee of 
the SiliconPV 2012 conference. Open acc ss under CC BY-NC-ND license.
Open access under CC BY-NC-ND license.
294   M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299 
Swanson [1] in combination with the quasi-steady state photoconductance (QSSPC) method [2]. In this 
paper, we apply our recently developed imaging method for measuring J0e with lateral resolution of 
inhomogeneous samples such as selective emitters [3]. We use photoluminescence (PL) measurements 
combined with photoconductance measurements to derive calibrated images of the effective excess 
carrier lifetime. At least two images under two different high-injection conditions are taken. The J0e 
image is then derived, applying the method of Kane and Swanson [1] for each pixel. As an application of 
the method, the J0e dependence on laser-doping with different laser intensities is shown. 
2. Experimental method 
In the method of Kane and Swanson [1], the effective lifetime of excess carriers Weff is measured as a 
function of the injection density 'n on a lightly doped wafer. The wafer is optimally [4,5] in high-level 
injection conditions because the recombination rate in the emitter is proportional to the square of 'n, and 
this recombination rate can then be separated from the linear recombination rate in the wafer. The inverse 
Weff can be expressed as [1]:  
 
Wiqn
ndopN
backeJfronteJ
SRHAueff
2
)(
)(0)(0
111 '
¹¸
·
©¨
§   WWW
,             (1) 
 
where Ndop is the wafer doping density, ni is the intrinsic carrier density, q is the elementary charge, and 
W is the wafer thickness. WAu may be calculated from [6]. It is not necessary to know the Shockley-Read-
Hall lifetime WSRH as long as it is not strongly injection-dependent, because to determine J0e only the slope 
of (1/τeff – 1/τAu) versus Δn is of interest. However, a value for ni needs to be decided for when converting 
the measured slope to J0e, and this value should be stated when publicizing J0e measurements.  We use ni 
= 8.31x109 cm-3 according to Ref. [7]. 
Our lateral determination of J0e is based on photoconductance calibrated PL lifetime images [9]. PL 
images show the radiative recombination rate of the sample under illumination, i.e. the PL intensity IPL 
can be expressed by [10] 
 
)( nNnBnpBRI dopradradradPL ''| v ,               (2) 
and a calibration function IPL(a,C) [10] 
²)( nCnanIPL '' ' ,                        (3) 
 
has to be found, where a and C are calibration constants, derived by comparing the area averaged PL 
intensity with the QSSPC signal over the coil at the same injection level, for several 'n values. For the 
calibration, a homogeneous part of the sample is used in order to reduce the uncertainty. The PL image is 
converted using (3) to an image of 'n, and Weff is then derived from 
 
G
n
eff
' W ,                 (4) 
 
where G is the average carrier-generation rate per volume. 
Two Weff images are taken under two distinct high-injection conditions. According to Eq. (1), the 
inverse lifetimes at each pixel of the two lifetime images are used, and J0e is calculated according to: 
 M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299 295
 
»
»
»
»
¼
º
«
«
«
«
¬
ª
''
¹¸
·
©¨
§ 
''
¹¸
·
©¨
§ 
 
),(),(
),(1),(1
),(),(
),(1),(1
2
2
),(0 yxlownyxhighn
low
yxAuyxeff
yxlownyxhighn
high
yxAuyxeffWiqnyxeJ
WWWW .          (5)  
 
The factor ½ is due to the symmetrical sample preparation, where two identical emitters are diffused 
at the front and back surfaces of the sample.  
Our calibrated lifetime images are derived using the LIS-R1 system from BT Imaging [11], which 
contains a Sinton Instruments photoconductance set-up [12]. The optics for the laser light allows for 
illumination levels of up to 10 suns. A schematic illustration of the set-up can be found for example in 
[9]. The optical reflectivity, required by the LIS-R1 system, is derived using a PerkinElmer UV/VIS 
spectrometer [13], while the wafer thickness is measured with a Käfer digital-dial gauge [14]. The emitter 
sheet resistance Rsheet is measured after phosphorus glass removal (PGR) using a four-point-probe setup. 
3. Simulation method 
As the measuring method has been developed only recently [3], we analyze the lateral behavior of J0e 
by means of numerical device simulations in two dimensions. For this purpose, we use TCAD (Dessis) 
from Synopsys [15] to solve the fully coupled set of semiconductor equations, and we apply the physical 
models and parameters described in Ref. [16].  
In device simulations, J0e is not an input but a result from choosing a dopant profile, a surface 
recombination velocity S, and models such as Auger recombination, mobility etc. There are two main 
ways to extract J0e from device simulations. One is to assume that all the current j that enters a dopant 
profile is caused to supply the recombination occurring within the dopant profile, i.e. j, p and n is probed 
at the base side of the space-charge region. Hence, there is 
 
²
²0 ii
e nnpn
jJ  ,                               (6) 
 
Please note that the local j is not accessible to experiment [17]. Thus we use a second simulation 
method, that reproduces Kane and Swansons’s experiment [2], which means that the photo-generation 
density G is ramped while 
 
G
n
R
n
tot
eff
' ' W                                (7) 
 
is monitored, where Rtot = RSRH+RAu+Rrad, i.e. the sum of all recombination included in the simulation. 
The lateral derivation of J0e in the simulation is done by averaging all values over an array with widths of 
50 Pm. No optical simulation is carried out of rays according to the spatial distribution of Rrad. This 
implies that all rays are detected where they are generated. 
 
 
296   M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299 
4. Sample preparation 
To determine J0e, wafers which are diffused and passivated identically on both sides, have a high 
resistivity ρ and a high τeff, are used, so that high-injection condition can be achieved. We use saw-
damage etched p-type 6’’ Cz material with a resistivity ρ = 6 – 12 Ωcm. The wafers receive a HF-dip 
followed by POCl3 diffusion for forming a homogeneous n+-region. To obtain a laser-doped selective 
emitter (LD-SE), the phosphorus glass is heated by a laser, leading to additional local phosphorous 
diffusion, and thus forming the n++ region. The processing is finished by applying phosphorus glass 
removal (PGR) and SiN deposition by PECVD on both sides of the wafers. 
Two different test structures where produced, which are schematically shown in Fig. 1. The first has 
four square-shaped n++ regions with a size of 40 mm by 40 mm symmetrically on the front and back side. 
The second structure investigates the J0e dependence on the laser intensity for the laser-doping process. 
For that, rectangular-shaped n++ regions with a size of 10 mm by 30 mm are produced on one wafer with 
different laser intensities, but just on one side. The latter sample is investigated before and after firing in a 
standard industrial belt furnace for screen printing. These structures were produced by laser-doping, 
where laser stripes were placed very close to each other, possibly with overlap due to positioning 
accuracy, causing double diffusions. 
 
Fig. 1. Test structures: (a) symmetrical four square-shaped n++ regions with a size of 40 mm by 40 mm, (b) one-sided rectangular-
shaped n++ regions with a size of 10 mm by 30 mm with different laser intensities. 
5. Results 
To calculate the experimental J0e image, two calibrated lifetime images with a resolution of 403 pixels 
by 363 pixels are taken at approximately 4 and 8 suns. This corresponds to an injection level of about 
4×1015 cm-3 and 8×1015 cm-3 in the n+ region, denoted as “2” in Fig. 2 (a).  
Also in Fig. 2 (a), two small parts of square-shaped n++ structures with a size of 40 mm by 40 mm are 
partly visible on the left and right edges of the image as bright areas. In the area “2” of Fig. 2 (a), an area-
averaged J0e of 238±15 fA/cm² is derived for the homogeneous n+ emitter. For the area “1”, it is 419±19 
fA/cm², which is the n++ region. The uncertainty is the standard deviations of the area-average. We 
deliberately made the n++ regions larger than the coil of the QSSPC in order to directly compare a QSSPC 
measurement of a n++ region with the J0e image. We observe a deviation in J0e between the two 
measurement techniques that is smaller than 10% in relative terms. 
Some artefacts are visible in Fig. 2 (a) and Fig. 3 (a). The round structures in the centre are due to the 
difference between the optical reflectivity of the QSSPC coil and the rest of the QSSPC stage. The 
brighter stripes at the edges of the images are due to effects in the optical system. 
 M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299 297
  
Fig. 2. (a) Emitter saturation current density image where two selective emitter structures on the left and right side are visible (J0e in 
fA/cm²). The round structures are artefacts. (b) Comparison of experimental (from area “1” to “2”) and simulated (τeff method) 
linescans of J0e. 
 
 
For validation of the derived J0e image, we compare a linescan going from the area “1” to “2” in Fig. 
2(a), with numerical simulations in Fig. 2(b). In experiment, a saturation of J0e is achieved within 2 mm, 
while the simulation (τeff method) shows a saturation within 1mm under a generation rate of G = 5x1019 
cm-3 similar to the experiment. We assume that this is so because the detected rays are generated at 
various lateral positions due to different optical paths caused by reflection at the rear surface, sample 
stage and scattering at the front surface. The reason for the rather wide J0e transition between the abrupt 
change from n++ to n+ is mainly due to lateral currents in the base, causing a blurring of the electron and 
hole density n and p in the base, which is measured by PL. We found that at G as high as 5x1020 cm-3, the 
simulations show a lateral resolution of about 0.5 mm for the abrupt transition from n++ to n+, which is 
due to the reduction of the diffusion length at higher injection levels. 
By means of the J0e imaging method, we analyze the J0e dependence on laser-doping with different 
laser intensities. For that, J0e of the rectangles of the second structure in Fig. 1 (b) are determined with the 
described method on a single wafer as exemplarily shown in Fig. 3 (a) for three rectangles. In Fig. 3 (b), 
the derived J0e and the Rsheet for each square is shown depending on the relative laser intensity before and 
after firing the sample in a belt furnace. For increasing laser intensities the J0e first increases up to a 
relative laser intensity of 42% and then decreases, while the Rsheet decreases from about 95 Ω/□ to 26 Ω/□. 
Qualitatively similar results can be found in [18], which confirms the applicability of the method. Note 
that the benefit of the increased SiN passivation quality due to firing in the belt furnace is higher for the 
homogeneous emitter than for the laser-doped emitters with a high Rsheet. E.g. the J0e of the homogenous 
emitter drops from 234±14 fA/cm² to 83±7 fA/cm², while J0e of the selective emitter of Rsheet = 26 Ω/□ 
drops from 240±13 fA/cm² to 136±6 fA/cm². The second structure is suitable for investigating the 
recombination activity of a laser-doped emitter, which can be used for the optimization of selective 
emitter solar cells. However, the influence of the metallization step is not considered, i.e. additionally 
introduced recombination and the contact resistance RC of the contact formation are not taken account of. 
298   M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299 
 
Fig. 3. (a) Example J0e image of the second structure in fA/cm². (b) Dependence of J0e and Rsheet on the relative laser intensity of the 
laser-doping process. J0e is shown before/after firing in a belt furnace 
6. Conclusions 
The method of imaging J0e is investigated experimentally and by numerical simulations. By 
comparison, a limited resolution of a feature size of an inhomogeneous emitter is determined to be 
theoretically between 0.5-1.0 mm and experimentally about 2 mm. It is found that the reason is the lateral 
current in the base blurring the charge carrier densities n and p, which are experimentally observed by PL 
measurements. The successful application of the method used to investigate the influence of laser power 
to squared selective emitter structures shows that the Rsheet decreases with increasing laser power, while 
the recombination activity of the emitter characterized by J0e first increases and then decreases as 
expected. 
Acknowledgements 
This work was supported by the German Ministry for the Environment (BMU) under Contract No. 
0325204 (project SIMPSONS). The authors would like to thank the characterization group at SolarWorld 
Innovations for their support.  
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Mar. 2011. 


Evaluating The Quality of Selective Emitter Structures by Imaging the Emitter Saturation Current Density 

Institutionelles Repositorium der Leibniz Universität Hannover

 Energy Procedia  27 ( 2012 )  293 – 299 1876-6102 © 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientifi c committee of the SiliconPV 2012 conference.doi: 10.1016/j.egypro.2012.07.066 SiliconPV: April 03-05, 2012, Leuven, Belgium Evaluating The Quality Of Selective Emitter Structures By Imaging The Emitter Saturation Current Density M. Müllera*, P.P. Altermattb, K. Schlegela, G. Fischera  aSolarWorld Innovations GmbH, Berthelsdorferstr. 111A, 09599 Freiberg, Germany bDep. Solar Energy, Inst. Solid-State Physics, Leibniz University of Hannover, Appelstr. 2, 30167 Hannover, Germany  Abstract  A method to derive the emitter saturation current density J0e with lateral resolution is applied to investigate selective emitter structures. The method uses PL lifetime imaging at several injection densities to laterally evaluate J0e by applying the method of Kane and Swanson [1] pixel by pixel. Samples with two-sided diffused emitters on lowly-doped Cz wafers were used to produce selective emitter structures by laser doping of the phosphorus-rich glass (LD-SE). By comparison of experimental and numerical simulation results of J0e linescans, a limited resolution of a feature size of an inhomogeneous emitter is determined to be theoretically between 0.5-1.0 mm and experimentally about 2 mm. The method was successfully applied to investigate the dependence of J0e on the laser power of a selective emitter structure. The expected behaviour of a maximum J0e for medium laser intensities is observed.  The method is suitable to evaluate the selective emitter process and its optimization.   © 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the SiliconPV 2012 conference  Keywords: Emitter saturation current density; photoluminescence; selective emitter 1. Introduction For the electrical characterization of silicon solar cells, it is common to quantify the saturation current density of the emitter J0e. In this paper, we investigate selective emitters for crystalline Si solar cells, meaning that the emitter is highly-doped at the front metal contacts and rather lowly doped elsewhere. By now, it has been common to measure the J0e of homogeneous samples, using the procedure of Kane and  * Corresponding author. Tel:  +49-3731-301-1439; fax: +49-3731-301-1690 E-mail address: matthias.mueller@sw-innovations.de Vailable online at www.sciencedirect.com 12 Published by Elsevier Ltd. Sel ction and peer-revi w under r sponsibility of the sci ntifi c committee of the SiliconPV 2012 conference. Open acc ss under CC BY-NC-ND license.Open access under CC BY-NC-ND license.294   M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299 Swanson [1] in combination with the quasi-steady state photoconductance (QSSPC) method [2]. In this paper, we apply our recently developed imaging method for measuring J0e with lateral resolution of inhomogeneous samples such as selective emitters [3]. We use photoluminescence (PL) measurements combined with photoconductance measurements to derive calibrated images of the effective excess carrier lifetime. At least two images under two different high-injection conditions are taken. The J0e image is then derived, applying the method of Kane and Swanson [1] for each pixel. As an application of the method, the J0e dependence on laser-doping with different laser intensities is shown. 2. Experimental method In the method of Kane and Swanson [1], the effective lifetime of excess carriers Weff is measured as a function of the injection density 'n on a lightly doped wafer. The wafer is optimally [4,5] in high-level injection conditions because the recombination rate in the emitter is proportional to the square of 'n, and this recombination rate can then be separated from the linear recombination rate in the wafer. The inverse Weff can be expressed as [1]:   WiqnndopNbackeJfronteJSRHAueff2)()(0)(0111 '¹¸·©¨§   WWW,             (1)  where Ndop is the wafer doping density, ni is the intrinsic carrier density, q is the elementary charge, and W is the wafer thickness. WAu may be calculated from [6]. It is not necessary to know the Shockley-Read-Hall lifetime WSRH as long as it is not strongly injection-dependent, because to determine J0e only the slope of (1/τeff – 1/τAu) versus Δn is of interest. However, a value for ni needs to be decided for when converting the measured slope to J0e, and this value should be stated when publicizing J0e measurements.  We use ni = 8.31x109 cm-3 according to Ref. [7]. Our lateral determination of J0e is based on photoconductance calibrated PL lifetime images [9]. PL images show the radiative recombination rate of the sample under illumination, i.e. the PL intensity IPL can be expressed by [10]  )( nNnBnpBRI dopradradradPL ''| v ,               (2) and a calibration function IPL(a,C) [10] ²)( nCnanIPL '' ' ,                        (3)  has to be found, where a and C are calibration constants, derived by comparing the area averaged PL intensity with the QSSPC signal over the coil at the same injection level, for several 'n values. For the calibration, a homogeneous part of the sample is used in order to reduce the uncertainty. The PL image is converted using (3) to an image of 'n, and Weff is then derived from  Gneff' W ,                 (4)  where G is the average carrier-generation rate per volume. Two Weff images are taken under two distinct high-injection conditions. According to Eq. (1), the inverse lifetimes at each pixel of the two lifetime images are used, and J0e is calculated according to:  M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299 295 »»»»¼º««««¬ª''¹¸·©¨§ ''¹¸·©¨§  ),(),(),(1),(1),(),(),(1),(122),(0 yxlownyxhighnlowyxAuyxeffyxlownyxhighnhighyxAuyxeffWiqnyxeJWWWW .          (5)   The factor ½ is due to the symmetrical sample preparation, where two identical emitters are diffused at the front and back surfaces of the sample.  Our calibrated lifetime images are derived using the LIS-R1 system from BT Imaging [11], which contains a Sinton Instruments photoconductance set-up [12]. The optics for the laser light allows for illumination levels of up to 10 suns. A schematic illustration of the set-up can be found for example in [9]. The optical reflectivity, required by the LIS-R1 system, is derived using a PerkinElmer UV/VIS spectrometer [13], while the wafer thickness is measured with a Käfer digital-dial gauge [14]. The emitter sheet resistance Rsheet is measured after phosphorus glass removal (PGR) using a four-point-probe setup. 3. Simulation method As the measuring method has been developed only recently [3], we analyze the lateral behavior of J0e by means of numerical device simulations in two dimensions. For this purpose, we use TCAD (Dessis) from Synopsys [15] to solve the fully coupled set of semiconductor equations, and we apply the physical models and parameters described in Ref. [16].  In device simulations, J0e is not an input but a result from choosing a dopant profile, a surface recombination velocity S, and models such as Auger recombination, mobility etc. There are two main ways to extract J0e from device simulations. One is to assume that all the current j that enters a dopant profile is caused to supply the recombination occurring within the dopant profile, i.e. j, p and n is probed at the base side of the space-charge region. Hence, there is  ²²0 iie nnpnjJ  ,                               (6)  Please note that the local j is not accessible to experiment [17]. Thus we use a second simulation method, that reproduces Kane and Swansons’s experiment [2], which means that the photo-generation density G is ramped while  GnRntoteff' ' W                                (7)  is monitored, where Rtot = RSRH+RAu+Rrad, i.e. the sum of all recombination included in the simulation. The lateral derivation of J0e in the simulation is done by averaging all values over an array with widths of 50 Pm. No optical simulation is carried out of rays according to the spatial distribution of Rrad. This implies that all rays are detected where they are generated.   296   M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299 4. Sample preparation To determine J0e, wafers which are diffused and passivated identically on both sides, have a high resistivity ρ and a high τeff, are used, so that high-injection condition can be achieved. We use saw-damage etched p-type 6’’ Cz material with a resistivity ρ = 6 – 12 Ωcm. The wafers receive a HF-dip followed by POCl3 diffusion for forming a homogeneous n+-region. To obtain a laser-doped selective emitter (LD-SE), the phosphorus glass is heated by a laser, leading to additional local phosphorous diffusion, and thus forming the n++ region. The processing is finished by applying phosphorus glass removal (PGR) and SiN deposition by PECVD on both sides of the wafers. Two different test structures where produced, which are schematically shown in Fig. 1. The first has four square-shaped n++ regions with a size of 40 mm by 40 mm symmetrically on the front and back side. The second structure investigates the J0e dependence on the laser intensity for the laser-doping process. For that, rectangular-shaped n++ regions with a size of 10 mm by 30 mm are produced on one wafer with different laser intensities, but just on one side. The latter sample is investigated before and after firing in a standard industrial belt furnace for screen printing. These structures were produced by laser-doping, where laser stripes were placed very close to each other, possibly with overlap due to positioning accuracy, causing double diffusions.  Fig. 1. Test structures: (a) symmetrical four square-shaped n++ regions with a size of 40 mm by 40 mm, (b) one-sided rectangular-shaped n++ regions with a size of 10 mm by 30 mm with different laser intensities. 5. Results To calculate the experimental J0e image, two calibrated lifetime images with a resolution of 403 pixels by 363 pixels are taken at approximately 4 and 8 suns. This corresponds to an injection level of about 4×1015 cm-3 and 8×1015 cm-3 in the n+ region, denoted as “2” in Fig. 2 (a).  Also in Fig. 2 (a), two small parts of square-shaped n++ structures with a size of 40 mm by 40 mm are partly visible on the left and right edges of the image as bright areas. In the area “2” of Fig. 2 (a), an area-averaged J0e of 238±15 fA/cm² is derived for the homogeneous n+ emitter. For the area “1”, it is 419±19 fA/cm², which is the n++ region. The uncertainty is the standard deviations of the area-average. We deliberately made the n++ regions larger than the coil of the QSSPC in order to directly compare a QSSPC measurement of a n++ region with the J0e image. We observe a deviation in J0e between the two measurement techniques that is smaller than 10% in relative terms. Some artefacts are visible in Fig. 2 (a) and Fig. 3 (a). The round structures in the centre are due to the difference between the optical reflectivity of the QSSPC coil and the rest of the QSSPC stage. The brighter stripes at the edges of the images are due to effects in the optical system.  M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299 297  Fig. 2. (a) Emitter saturation current density image where two selective emitter structures on the left and right side are visible (J0e in fA/cm²). The round structures are artefacts. (b) Comparison of experimental (from area “1” to “2”) and simulated (τeff method) linescans of J0e.   For validation of the derived J0e image, we compare a linescan going from the area “1” to “2” in Fig. 2(a), with numerical simulations in Fig. 2(b). In experiment, a saturation of J0e is achieved within 2 mm, while the simulation (τeff method) shows a saturation within 1mm under a generation rate of G = 5x1019 cm-3 similar to the experiment. We assume that this is so because the detected rays are generated at various lateral positions due to different optical paths caused by reflection at the rear surface, sample stage and scattering at the front surface. The reason for the rather wide J0e transition between the abrupt change from n++ to n+ is mainly due to lateral currents in the base, causing a blurring of the electron and hole density n and p in the base, which is measured by PL. We found that at G as high as 5x1020 cm-3, the simulations show a lateral resolution of about 0.5 mm for the abrupt transition from n++ to n+, which is due to the reduction of the diffusion length at higher injection levels. By means of the J0e imaging method, we analyze the J0e dependence on laser-doping with different laser intensities. For that, J0e of the rectangles of the second structure in Fig. 1 (b) are determined with the described method on a single wafer as exemplarily shown in Fig. 3 (a) for three rectangles. In Fig. 3 (b), the derived J0e and the Rsheet for each square is shown depending on the relative laser intensity before and after firing the sample in a belt furnace. For increasing laser intensities the J0e first increases up to a relative laser intensity of 42% and then decreases, while the Rsheet decreases from about 95 Ω/□ to 26 Ω/□. Qualitatively similar results can be found in [18], which confirms the applicability of the method. Note that the benefit of the increased SiN passivation quality due to firing in the belt furnace is higher for the homogeneous emitter than for the laser-doped emitters with a high Rsheet. E.g. the J0e of the homogenous emitter drops from 234±14 fA/cm² to 83±7 fA/cm², while J0e of the selective emitter of Rsheet = 26 Ω/□ drops from 240±13 fA/cm² to 136±6 fA/cm². The second structure is suitable for investigating the recombination activity of a laser-doped emitter, which can be used for the optimization of selective emitter solar cells. However, the influence of the metallization step is not considered, i.e. additionally introduced recombination and the contact resistance RC of the contact formation are not taken account of. 298   M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299  Fig. 3. (a) Example J0e image of the second structure in fA/cm². (b) Dependence of J0e and Rsheet on the relative laser intensity of the laser-doping process. J0e is shown before/after firing in a belt furnace 6. Conclusions The method of imaging J0e is investigated experimentally and by numerical simulations. By comparison, a limited resolution of a feature size of an inhomogeneous emitter is determined to be theoretically between 0.5-1.0 mm and experimentally about 2 mm. It is found that the reason is the lateral current in the base blurring the charge carrier densities n and p, which are experimentally observed by PL measurements. The successful application of the method used to investigate the influence of laser power to squared selective emitter structures shows that the Rsheet decreases with increasing laser power, while the recombination activity of the emitter characterized by J0e first increases and then decreases as expected. Acknowledgements This work was supported by the German Ministry for the Environment (BMU) under Contract No. 0325204 (project SIMPSONS). The authors would like to thank the characterization group at SolarWorld Innovations for their support.  References [1 ]D. E. Kane and R. M. Swanson. Measurement of the emitter saturation current by a contactless photoconductivity method. Proc. 18th IEEE PV Specialists Conf.,1985, pp. 578–581. [2] R. A. Sinton and A. Cuevas. Contactless determination of current–voltage characteristics and minority carrier lifetimes in semiconductors from quasi-steady-state photoconductance data., Appl. Phys. Lett., vol. 69, pp. 2510–2512, Oct. 1996. [3] M. Müller, P. P. Altermatt, K. Schlegel, G. Fischer. A method for imaging the emitter saturation current with lateral resolution. Submitted to IEEE Journal of Photovoltaics.   M. Müller et al. /  Energy Procedia  27 ( 2012 )  293 – 299 299[4] A. Cuevas. The effect of emitter recombination on the effective lifetime of silicon wafers. Solar Energy Materials & Solar Cells, vol. 57, pp. 277-290, Aug. 1999. [5] C. Reichel, F. Granek, J. Benick, O. Schultz-Wittmann, and S.W. Glunz. Comparison of emitter saturation current densities determined by injection-dependent lifetime spectroscopy in high and low injection regimes. Progr. in PV (2010, May), DOI: 10.1002/pip.942. [6] M. J. Kerr and A. Cuevas. General parameterization of Auger recombination in crystalline silicon. J. Appl. Phys. vol. 91, pp. 2473–2480, Feb. 2008. [7] K. Misiakos and D. Tsamakis. Accurate measurements of the silicon intrinsic carrier density from 78 to 340 K. J. Appl. Phys. vol. 74, pp. 3293–3297, Sep. 1993. [8] P. P. Altermatt, A. Schenk, F. Geelhaar, and G. Heiser. Reassessment of the intrinsic carrier density in crystalline silicon in view of band-gap narrowing. J. Appl. Phys. vol. 93, pp. 1598–1604, Feb 2003. [9] S. Herlufsen, J. Schmidt, D. Hinken, K. Bothe, and R. Brendel. Photoconductance-calibrated photoluminescence lifetime imaging of crystalline silicon. Phys. Stat. Sol. (RRL), vol. 2, 245–247, Sep. 2008. [10] S. Herlufsen, J. Schmidt, D. Hinken, K. Bothe, and R. Brendel. Camera-based photoluminescence lifetime imaging of crystalline silicon wafers. Proc. 24th EU PVSE Conf., 2AO.1.3, 2009. [11] BT Imaging, Sydney, Australia, http://www.btimaging.com/ [12] Sinton Consulting, Boulder, CO, http://www.sintoninstruments.com/ [13] PerkinElmer, Waltham , MA, http://www.perkinelmer.com/ [14] Käfer Messuhrenfabrik, Villingen-Schwenningen, Germany, http:// www.kaefer-messuhren.de/ [15] TCAD, Version 2011.09, Synopsys, Mountain View, CA, http://www.synopsys.com/ P. P. Altermatt. Models for numerical device simulations of crystalline silicon solar cells—a review. J. Computational Electronics; vol. 10, p. 314, 2011. [16] H. Mäckel and K. Varner, On the determination of the emitter saturation current density from lifetime measurements of silicon devices, Progress in PV (2012), DOI: 10.1002/pip.2167. [17] B. Paviet-Salomon, S. Gall, R. Monna, S. Manuel, A. Slaoui. Experimental and analytical study of saturation current density of laser-doped phosphorus emitters for silicon solar cells. Solar Energy Materials & Solar Cells, vol. 95, pp. 2536-2539, Mar. 2011. 

Evaluating the quality of selective emitter structures by imaging the emitter saturation current density

M. Müller

P.P. Altermatt

K. Schlegel

G. Fischer

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Energy Procedia

Evaluating The Quality of Selective Emitter Structures by Imaging the Emitter Saturation Current Density

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Evaluating the quality of selective emitter structures by imaging the emitter saturation current density

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