A potentially cost-effective ion implanter for solar cells has become commercially available very recently. As the emitter dopant profiles differ from the standard diffusions, a combination of process simulation and device simulation is used to predict possible applications as front emitter. The simulations show that ion energies of 10 to 30 keV and doses in the range of 5×1014 to 7×1015 cm-2 are sufficient for reducing the phosphorus peak density and, hence, obtaining cell efficiency levels above 20%, if appropriate surface passivation and wafer materials are used. The simulations strongly indicate, however, that cell efficiency improves only marginally if the cell has a fully metallized rear Al-BSF and a boron-doped Cz base in the degraded state. Simulated cells with a local rear Al-BSF show an efficiency improvement of more than 0.3% absolute in the degraded state

Altermatt, Pietro P.

Chen, R.

Dunham, S.T.

Letay, G.

Ohrdes, Tobias

Steingrube, S.

Wagner, Hannes

Zechner, C.

AbstractA potentially cost-effective ion implanter for solar cells has become commercially available very recently. As the emitter dopant profiles differ from the standard diffusions, a combination of process simulation and device simulation is used to predict possible applications as front emitter. The simulations show that ion energies of 10 to 30 keV and doses in the range of 5×1014 to 7×1015cm-2 are sufficient for reducing the phosphorus peak density and, hence, obtaining cell efficiency levels above 20%, if appropriate surface passivation and wafer materials are used. The simulations strongly indicate, however, that cell efficiency improves only marginally if the cell has a fully metallized rear Al-BSF and a boron-doped Cz base in the degraded state. Simulated cells with a local rear Al-BSF show an efficiency improvement of more than 0.3% absolute in the degraded state

Ohrdes, T.

Wagner, H.

Altermatt, P.P.

Elsevier - Publisher Connector 

Available online at www.sciencedirect.com
* Corresponding author.  ohrdes@isfh.de. 
 
Silicon PV – 1st International Conference on Silicon Photovoltaics 
Solar cell emitter design with PV-tailored implantation 
T. Ohrdesa*, S. Steingrubeb, H. Wagnerb, C. Zechnerc, G. Letayc, R. Chend,  
S. T. Dunhamd, P. P. Altermattb 
a Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, 31860 Emmerthal, Germany
bLeibniz University of Hannover, Institute for Solid State Physics, Dep. Solar Energy, Appelstr. 2, 30167 Hannover, Germany  
c SYNOPSYS Switzerland LLC, Thurgauerstrasse 40, Zurich, Switzerland  
d Department of Electrical Engineering, University of Washington, Seattle, WA 98195, USA 
 
Abstract 
A potentially cost-effective ion implanter for solar cells has become commercially available very recently. As the 
emitter dopant profiles differ from the standard diffusions, a combination of process simulation and device simulation 
is used to predict possible applications as front emitter.  The simulations show that ion energies of 10 to 30 keV and 
doses in the range of 5u1014 to 7u1015 cm-2 are sufficient for reducing the phosphorus peak density and, hence, 
obtaining cell efficiency levels above 20%, if appropriate surface passivation and wafer materials are used. The 
simulations strongly indicate, however, that cell efficiency improves only marginally if the cell has a fully metallized 
rear Al-BSF and a boron-doped Cz base in the degraded state. Simulated cells with a local rear Al-BSF show an 
efficiency improvement of more than 0.3% absolute in the degraded state. 
 
 
Keywords: device simulation; process simulation; Si solar cells; implantation 
1876–6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011.
doi:10.1016/j.egypro.2011.06.119
Energy Procedia 8 (2011) 167–173
Open access under CC BY-NC-ND license.
© 2011 Published by Elsevier Ltd. 
Selection and/or peer-review under responsibility of SiliconPV 2011
Open access under CC BY-NC-ND license.
168  T. Ohrdes et al. / Energy Procedia 8 (2011) 167–173
1. Introduction 
An ion implanter has become commercially available specifically tailored to solar cells [1]. For 
forming emitters, rather low ion doses are necessary, which require such a short time for implantation that 
the mechanical handling of the wafers limits the throughput. Selective emitters can be implanted with 
masks. Costs may be saved because there is no edge isolation necessary, and the removal of a 
phosphosilicate glass layer is not required. A passivating oxide can be grown during annealing. 
In contrast to the IC industry, the excess carrier lifetimes are important in PV. Therefore, rapid thermal 
anneal (RTA), usually applied in the IC industry, is not feasible [2]. Instead, a furnace anneal is necessary 
with a thermal budget similar to the standard phosphorus diffusion. We model favorable conditions by a 
combination of implantation, annealing, and device simulations. 
2. Implantation models 
Emitter formation by ion implantation and subsequent thermal annealing offers a flexible emitter 
design. By selecting the implant energy and dose, one can determine the distribution of P atoms after 
implantation. During ion implantation, the surface region of the silicon wafer gets amorphized. This 
region recrystallizes by solid phase epitaxial regrowth (SPER) at the onset of thermal annealing. In the 
deeper non-amorphized regions, the implantation process creates a high concentration of point defects: 
silicon self interstitials (I) and vacancies (V). During thermal annealing, these point defects can 
recombine or diffuse to the surface or form larger clusters of defects, such as {311} defects or dislocation 
loops [3]. After sufficiently long diffusion time, these clusters dissolve again into point defects, which 
diffuse towards the wafer surface and recombine. Until all defects are dissolved, the concentration of I is 
above thermal equilibrium, and the P diffusion via P-I pairs is enhanced. For an overview on diffusion 
mechanisms in silicon, see for example [4,5]. For solar cell emitter formation, phosphorus ions are 
implanted into the Si wafer with energies in the range of 10’s of keV. The generated crystal defects can 
be annealed at furnace temperatures typical for solar cell phosphorus diffusion. 
For process modeling, it is necessary to account for (i) the spatial distribution of dopant atoms and 
point defects after implantation, (ii) amorphization of Si, (iii) formation and dissolution of crystal defects 
during the annealing, and (iv) I and V mediated dopant diffusion. We use the process simulator 
SENTAURUS PROCESS [6], which is widely used in IC industry, together with its standard models. As an 
illustration of the model accuracy, Fig. 1(a) compares our simulation result of a P emitter profile to SIMS 
data from Benick et al. [2]. 
Figure 1(b) shows simulation results of various phosphorus profiles. The higher dose of 7u1015 cm-2 is 
used for regions under the metal contacts, which require a high surface concentration of P. The device 
simulation below in Sec. 3 yields very similar cell efficiencies independently of annealing time in the 
range between 20 and 40 minutes at 890ºC. It is apparent that both a low surface density and a deep 
metallurgical junction depth can be attained with implantation. It was predicted already in Ref. [7] that 
such emitters have the lowest saturation currents J0 if an effective surface passivation can be achieved. 
Fig. 1(c) shows two improved homogeneous emitter designs achieved by standard POCl3 diffusions [8]. 
The homogeneous oxidized emitter will be used for comparison with the implanted, selective emitter 
below. 
 
 
 
 
 
T. Ohrdes et al. / Energy Procedia 8 (2011) 167–173 169
 T. Ohrdes / Energy Procedia 00 (2011) 000–000 
 
 
 
 
 
e 
Fig. 1. (a): Experimental phosphorus profile after implantation and annealing from Benick et al. [2] (symbols) and our simulation 
(lines). (b): Our simulated phosphorus emitter profiles. (c): POCl3 profiles from Ref. [8], ECV measurements and simulations [9]. 
Table 1. A set of device simulation models and parameters that has been developed specifically for the simulation of Si solar cells. 
Device simulation  
Equations numerically solved Semiconductor drift-diffusion equations 
Temperature 300 K 
Free carrier statistics Fermi-Dirac 
Intrinsic carrier density ni = 9.65u109 cm-3  [10] 
 Obtained with setting Eg to a lower value in [11] 
Band gap narrowing model Schenk [12] 
Free carrier mobility Klaassen’s unified mobility model [13,14] 
Radiative recombination B = 4.73u10-15 cm3/s  [15] 
     Its doping and injection-dependence See [16] and [17] 
Auger recombination Dziewior and Schmid [18] 
     Its temperature dependence [19] C(n,p) = (A + B(T/T0) + C(T/T0)2)(1+He-(n,p)/N0) 
 A = 2.8u10-31, 7.91u10-32 cm6/s; B = 0, -1.239u10-32 cm6/s 
 C = 0, 3.231u10-32 cm6/s; H = 8, 8 
 N0 = 2.5u1017 cm-3, N0 = 2.5u1017 cm-3; T0 = 300K 
SRH bulk recombination Boron-doped Cz silicon: Refs. [20,21,22] 
 Undegraded state: equal lifetime parameters, midgap 
SRH surface recombination Phosphorus-diffused surfaces: Ref. [23] 
Optical simulation  
Solar spectrum Standard AM1.5g, i.e. calculated with SMARTS [24] 
Optical dispersion relations Silicon from Green [25], Al from Shiles [26] 
 SiNx measured at ISFH. 
3. Device simulation 
For our two-dimensional numerical device simulations [27], we use the device simulator SENTAURUS 
DEVICE [6], which is widely used in the IC and PV industry, with the device models listed in Table 1. 
4. Results 
Generally, implantation of the emitter leads to efficiency improvements only if the recombination 
losses in the remaining device parts are similar or lower than the losses in the emitter. To assess the 
(b) (a) (c) 
170  T. Ohrdes et al. / Energy Procedia 8 (2011) 167–173
benefits, we use as a reference the simulations of two experimentally fabricated cell structures of Ref. [9]: 
one cell structure is a standard industrial design but with a POCl3 emitter that has been oxidized for 15 
minutes at 900ºC, the other structure has the same emitter but the rear is contacted by Al-BSF fingers 
instead of being fully metalized. The emitter and the non-metallized parts of the rear are passivated with a 
SiO2/SiNx stack. In our simulations, we replace the front POCl3 emitter with an implanted selective 
emitter, but keep the same SiO2/SiNx stack as passivation layer. The resulting I-V parameters are shown 
in Fig. 2.  
It is apparent that the I-V parameters of a fully metalized Al-BSF cell do only marginally benefit from 
an implanted emitter. The recombination losses in the Al-BSF are rather high [9] and start dominating the 
total losses when the emitter is significantly improved. This is obtained assuming a non-degraded 2.5 
ȍcm boron-doped Cz base with a SRH lifetime of 280 μs. After degradation, the benefits from an 
implanted emitter are further reduced because the recombination losses in the base dominate. 
In contrast, the cell with the local Al-BSF shows an efficiency improvement by more than 0.3% 
absolute with implanting. This is due to an improvement in Voc and to some extent in Jsc, despite the 
reduction in FF due to the local rear contacts (the rear contact pattern and the corresponding base 
resistivity are not fully optimized). Again, a SRH lifetime of 280 μs is assumed. Degradation has a far 
larger impact on the I-V parameters compared to a cell with a fully metalized rear, for the following 
reasons. Degradation causes the SRH lifetime in the boron-doped Cz base to be strongly injection 
dependent. The lifetime improves considerably going from MPP to the open-circuit condition, and thus 
causes a low fill factor. See Ref. [9] for more details. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 2. Simulated I-V parameters. The reference cells with a POCl3 diffusion, shown in Fig. 1(c), is a reproduction of an experiment 
from [9] shown here as red dots. The adjacent bars indicate a simulation with the implanted emitter of Fig. 1(b) before and after 
degradation of the 2.5 ȍcm boron-doped Cz base with an interstitial oxygen density of 7u1017 cm-3 [20]. 
T. Ohrdes et al. / Energy Procedia 8 (2011) 167–173 171
Crucial for the benefits of an implanted emitter is an effective front surface passivation. By comparing 
fabricated with simulated cells, we found that the recombination velocity parameter Sp of our SiO2/SiNx 
stacks at the front surface can be approximately parameterized as 
 
Sp[cm/s] = 500(Ndon/1019 cm-3)0.6 + 60 (Ndon/1019 cm-3)3  .     (1) 
 
This parameterization is similar to an alnealed oxide in Ref. [23], probably because both the Al layer 
and the SiNx layer release similar amounts of hydrogen into the oxide during the firing step. Fig. 3 shows 
the influence of the surface passivation of the cell with the local BSF in the non-degraded state. A rather 
small reduction in efficiency is predicted for a tenfold increase in the SRH parameter Sp, but even higher 
Sp values reduce cell efficiency considerably. 
Finally, Fig. 4 depicts the I-V parameters as a function of the implanted dose. While a low dose creates 
an emitter with low recombination but high resistive losses, a high dose does results in the opposite, and 
hence an optimum dose exists. The profiles shown in Fig. 1(b) have a sheet resistivity of 165 ȍ/sq or 50 
ȍ/sq, respectively. The optimum depends on the specific passivation quality and the specific front 
metallization (e.g. finger distance and finger width) and should be simulated for each cell manufacturer 
individually. 
 
 
 
 
 
 
 
 
 
 
Fig. 3. Simulated I-V parameters as a function of the SRH parameter Sp at the emitter surface. 
 
 
 
  
 
 
 
 
 
 
Fig. 4. Simulated I-V parameters as a function of the implanted dose. With increasing dose, there are decreasing resistive losses and 
increasing recombination losses. The optimum depends on the front metallization geometry and the front surface passivation. 
172  T. Ohrdes et al. / Energy Procedia 8 (2011) 167–173
5. Conclusions 
Simulations show that phosphorus ion energies of 10 – 30 keV and implantation doses near 5u1014 cm-
2 for the emitter between the fingers and doses of 7u1015 cm-2 under the front metal fingers are suitable for 
reducing the phosphorus peak density at the surface. Cell efficiency levels above 20% can only be 
achieved if appropriate surface passivation and wafer materials are used. The simulations strongly 
indicate that cell efficiency improves only marginally if the cell has a fully metallized rear Al-BSF and a 
boron-doped Cz base in a degraded state. Simulated cells with a local rear Al-BSF show an efficiency 
improvement due to an implanted selective emitter design by more than 0.3% absolute. 
T. Ohrdes et al. / Energy Procedia 8 (2011) 167–173 173
References 
                                                          
[1]  Varian, Gloucester, MA. 
[2]  Benick J, Bateman N, Hermle M. Very low saturation current densities on ion implanted boron emitters. 25th EU PV Solar 
Energy Conf. Valencia, Spain, 2010, p. 1169. 
[3]  Cristiano F et al. Thermal Evolution of Extrinsic Defects in Ion Implanted Silicon: Current Understanding and Modelling.  
Mat. Res. Soc. Symp. Proc. 717. Silicon Front-End Junction Formation Technologies. 2002;C5.7.1–C5.7.12. 
[4] Plummer JD, Deal MD, Griffin PB. Silicon VLSI technology. New York: Prentice Hall; 2000. 
[5]  Pichler P. Intrinsic Point Defects, Impurities and Their Diffusion in Silicon. Venna: Springer; 2004. 
[6] Sentaurus Process User Guide, Version E-2010.12, Synopsys, Inc., Mountain View, CA 
[7] Cuevas A, Balbuena M. Thick-emitter silicon solar cells. 20th IEEE Specialists Conf., Las Vegas, 1988, p.429 
[8]  S. Gatz, H. Hannebauer, R. Hesse, F. Werner, A. Schmidt1, T. Dullweber, J. Schmidt, K. Bothe, R. Brendel. 19.4%-efficient 
large-area fully screen-printed silicon solar cells. Phys. Status Solidi RRL 2011;5:147. 
[9] See the contribution from S. Steingrube et al. at this conference. 
[10]  Altermatt PP, Schenk A, Geelhaar G, Heiser G. Reassessment of the intrinsic carrier density in crystalline silicon in view of 
band-gap narrowing. Journal of Applied Physics 2003;93:1598. 
[11] Green MA. Intrinsic Concentration, Effective Densities of States, and Effective Mass in Silicon. Journal of Applied Physics 
1990;67:2944. 
[12] Schenk A. Finite-temperature full random-phase approximation model of band gap narrowing for silicon device simulation. 
Journal of Applied Physics 1998;84:3684.  
[13] Klaassen DBM. A Unified Mobility Model for Device Simulation – I. Model Equations and Concentration Dependence. Solid-
State Electronics 1992;35:953.  
[14] Klaassen DBM. A Unified Mobility Model for Device Simulation – II. Temperature Dependence of Carrier Mobility and 
Lifetime. Solid-State Electronics 1992;35:961. 
[15] Trupke T, Green MA, Würfel P, Altermatt PP, Wang A, Zhao J, Corkish R. Temperature dependence of the radiative 
recombination coefficient of intrinsic crystalline silicon. Journal of Applied Physics 2003;94:4930. 
[16]  Altermatt PP, Geelhaar F, Trupke T, Day X, Neisser A, Daub E. Injection dependence of spontaneous radiative recombination 
in c-Si: experiment, theoretical analysis, and simulation. Proc. 5th International Conference on Numerical Simulation of 
Optoelectronic Devices (NUSOD05), IEEE, NY, 2005, p. 47. 
[17]  Altermatt PP, Geelhaar F, Trupke T, Dai X, Neisser A, Daub E. Injection dependence of spontaneous radiative recombination 
in crystalline silicon: Experimental verification and theoretical analysis. Applied Physics Letters 2006;88:261901. 
[18]  Dziewior J, Schmid W. Auger coefficients for highly doped and highly excited silicon. Applied Physics Letters 1977;31:346. 
[19]  Altermatt PP, Schmidt J, Heiser G, Aberle AG. Assessment and Parameterisation of Coulomb-Enhanced Auger Recombination 
Coefficients in Lowly Injected Crystalline Silicon. Journal of Applied Physics 1997;82:4938. 
[20]  Bothe K, Sinton R, Schmidt J. Fundamental Boron-Oxygen-related Carrier Lifetime Limit in Mono- and Multicrystalline 
Silicon. Progress in PV 2005;13:287. 
[21]  Schmidt J,Cuevas A. Electronic properties of light-induced recombination centers in boron-doped Czochralski silicon. Journal
of Applied Physics 1999;86;3175. 
[22]  Rein S, Glunz SW. Electronic properties of the metastable defect in boron-doped Czochralski silicon: Unambiguous 
determination by advanced lifetime spectroscopy. Journal of Applied Physics 2003;82:1054. 
[23]  Altermatt PP, Schumacher JO, Cuevas A, Kerr MJ, Glunz SW, King RR, Heiser G.Numerical modeling of highly doped Si:P 
emitters based on Fermi–Dirac statistics and self-consistent material parameters. Journal of Applied Physics 2002;92:3187. 
[24]  Gueymard C. Parameterized Transmittance Model for Direct Beam and Circumsolar Spectral Irradiance. Solar Energy 
2001;71:325. 
[25]  Green MA. Self-consistent optical parameters of intrinsic silicon at 300 K including temperature coefficients. Solar Energy 
Materials and Solar Cells 2008;92:1305. 
[26]  Shiles E, Sasaki T, Inokuti M, Smith DY. Self-consistency and sum-rule tests in the Kramers-Kronig analysis of optical data: 
Application to aluminium. Physical Review B 1980:22:1612. 
[27]  Altermatt PP, Dreissigacker S, Yang Y, Sprodowski C, Dezhdar T, Koc S, Veith B, Herrman S, Bock R, Bothe K, Schmidt J, 
Brendel R. Highly predictive modelling of entire Si solar cells for industrial applications. 24th European PV Solar Energy 
Conference, Hamburg, Germany, (WIP, Munich), 2009, p. 901 


Solar cell emitter design with PV-tailored implantation 

Institutionelles Repositorium der Leibniz Universität Hannover

Available online at www.sciencedirect.com* Corresponding author.  ohrdes@isfh.de.  Silicon PV – 1st International Conference on Silicon Photovoltaics Solar cell emitter design with PV-tailored implantation T. Ohrdesa*, S. Steingrubeb, H. Wagnerb, C. Zechnerc, G. Letayc, R. Chend,  S. T. Dunhamd, P. P. Altermattb a Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, 31860 Emmerthal, GermanybLeibniz University of Hannover, Institute for Solid State Physics, Dep. Solar Energy, Appelstr. 2, 30167 Hannover, Germany  c SYNOPSYS Switzerland LLC, Thurgauerstrasse 40, Zurich, Switzerland  d Department of Electrical Engineering, University of Washington, Seattle, WA 98195, USA  Abstract A potentially cost-effective ion implanter for solar cells has become commercially available very recently. As the emitter dopant profiles differ from the standard diffusions, a combination of process simulation and device simulation is used to predict possible applications as front emitter.  The simulations show that ion energies of 10 to 30 keV and doses in the range of 5u1014 to 7u1015 cm-2 are sufficient for reducing the phosphorus peak density and, hence, obtaining cell efficiency levels above 20%, if appropriate surface passivation and wafer materials are used. The simulations strongly indicate, however, that cell efficiency improves only marginally if the cell has a fully metallized rear Al-BSF and a boron-doped Cz base in the degraded state. Simulated cells with a local rear Al-BSF show an efficiency improvement of more than 0.3% absolute in the degraded state.   Keywords: device simulation; process simulation; Si solar cells; implantation 1876–6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011.doi:10.1016/j.egypro.2011.06.119Energy Procedia 8 (2011) 167–173Open access under CC BY-NC-ND license.© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011Open access under CC BY-NC-ND license.168  T. Ohrdes et al. / Energy Procedia 8 (2011) 167–1731. Introduction An ion implanter has become commercially available specifically tailored to solar cells [1]. For forming emitters, rather low ion doses are necessary, which require such a short time for implantation that the mechanical handling of the wafers limits the throughput. Selective emitters can be implanted with masks. Costs may be saved because there is no edge isolation necessary, and the removal of a phosphosilicate glass layer is not required. A passivating oxide can be grown during annealing. In contrast to the IC industry, the excess carrier lifetimes are important in PV. Therefore, rapid thermal anneal (RTA), usually applied in the IC industry, is not feasible [2]. Instead, a furnace anneal is necessary with a thermal budget similar to the standard phosphorus diffusion. We model favorable conditions by a combination of implantation, annealing, and device simulations. 2. Implantation models Emitter formation by ion implantation and subsequent thermal annealing offers a flexible emitter design. By selecting the implant energy and dose, one can determine the distribution of P atoms after implantation. During ion implantation, the surface region of the silicon wafer gets amorphized. This region recrystallizes by solid phase epitaxial regrowth (SPER) at the onset of thermal annealing. In the deeper non-amorphized regions, the implantation process creates a high concentration of point defects: silicon self interstitials (I) and vacancies (V). During thermal annealing, these point defects can recombine or diffuse to the surface or form larger clusters of defects, such as {311} defects or dislocation loops [3]. After sufficiently long diffusion time, these clusters dissolve again into point defects, which diffuse towards the wafer surface and recombine. Until all defects are dissolved, the concentration of I is above thermal equilibrium, and the P diffusion via P-I pairs is enhanced. For an overview on diffusion mechanisms in silicon, see for example [4,5]. For solar cell emitter formation, phosphorus ions are implanted into the Si wafer with energies in the range of 10’s of keV. The generated crystal defects can be annealed at furnace temperatures typical for solar cell phosphorus diffusion. For process modeling, it is necessary to account for (i) the spatial distribution of dopant atoms and point defects after implantation, (ii) amorphization of Si, (iii) formation and dissolution of crystal defects during the annealing, and (iv) I and V mediated dopant diffusion. We use the process simulator SENTAURUS PROCESS [6], which is widely used in IC industry, together with its standard models. As an illustration of the model accuracy, Fig. 1(a) compares our simulation result of a P emitter profile to SIMS data from Benick et al. [2]. Figure 1(b) shows simulation results of various phosphorus profiles. The higher dose of 7u1015 cm-2 is used for regions under the metal contacts, which require a high surface concentration of P. The device simulation below in Sec. 3 yields very similar cell efficiencies independently of annealing time in the range between 20 and 40 minutes at 890ºC. It is apparent that both a low surface density and a deep metallurgical junction depth can be attained with implantation. It was predicted already in Ref. [7] that such emitters have the lowest saturation currents J0 if an effective surface passivation can be achieved. Fig. 1(c) shows two improved homogeneous emitter designs achieved by standard POCl3 diffusions [8]. The homogeneous oxidized emitter will be used for comparison with the implanted, selective emitter below.      T. Ohrdes et al. / Energy Procedia 8 (2011) 167–173 169 T. Ohrdes / Energy Procedia 00 (2011) 000–000      e Fig. 1. (a): Experimental phosphorus profile after implantation and annealing from Benick et al. [2] (symbols) and our simulation (lines). (b): Our simulated phosphorus emitter profiles. (c): POCl3 profiles from Ref. [8], ECV measurements and simulations [9]. Table 1. A set of device simulation models and parameters that has been developed specifically for the simulation of Si solar cells. Device simulation  Equations numerically solved Semiconductor drift-diffusion equations Temperature 300 K Free carrier statistics Fermi-Dirac Intrinsic carrier density ni = 9.65u109 cm-3  [10]  Obtained with setting Eg to a lower value in [11] Band gap narrowing model Schenk [12] Free carrier mobility Klaassen’s unified mobility model [13,14] Radiative recombination B = 4.73u10-15 cm3/s  [15]      Its doping and injection-dependence See [16] and [17] Auger recombination Dziewior and Schmid [18]      Its temperature dependence [19] C(n,p) = (A + B(T/T0) + C(T/T0)2)(1+He-(n,p)/N0)  A = 2.8u10-31, 7.91u10-32 cm6/s; B = 0, -1.239u10-32 cm6/s  C = 0, 3.231u10-32 cm6/s; H = 8, 8  N0 = 2.5u1017 cm-3, N0 = 2.5u1017 cm-3; T0 = 300K SRH bulk recombination Boron-doped Cz silicon: Refs. [20,21,22]  Undegraded state: equal lifetime parameters, midgap SRH surface recombination Phosphorus-diffused surfaces: Ref. [23] Optical simulation  Solar spectrum Standard AM1.5g, i.e. calculated with SMARTS [24] Optical dispersion relations Silicon from Green [25], Al from Shiles [26]  SiNx measured at ISFH. 3. Device simulation For our two-dimensional numerical device simulations [27], we use the device simulator SENTAURUS DEVICE [6], which is widely used in the IC and PV industry, with the device models listed in Table 1. 4. Results Generally, implantation of the emitter leads to efficiency improvements only if the recombination losses in the remaining device parts are similar or lower than the losses in the emitter. To assess the (b) (a) (c) 170  T. Ohrdes et al. / Energy Procedia 8 (2011) 167–173benefits, we use as a reference the simulations of two experimentally fabricated cell structures of Ref. [9]: one cell structure is a standard industrial design but with a POCl3 emitter that has been oxidized for 15 minutes at 900ºC, the other structure has the same emitter but the rear is contacted by Al-BSF fingers instead of being fully metalized. The emitter and the non-metallized parts of the rear are passivated with a SiO2/SiNx stack. In our simulations, we replace the front POCl3 emitter with an implanted selective emitter, but keep the same SiO2/SiNx stack as passivation layer. The resulting I-V parameters are shown in Fig. 2.  It is apparent that the I-V parameters of a fully metalized Al-BSF cell do only marginally benefit from an implanted emitter. The recombination losses in the Al-BSF are rather high [9] and start dominating the total losses when the emitter is significantly improved. This is obtained assuming a non-degraded 2.5 ȍcm boron-doped Cz base with a SRH lifetime of 280 μs. After degradation, the benefits from an implanted emitter are further reduced because the recombination losses in the base dominate. In contrast, the cell with the local Al-BSF shows an efficiency improvement by more than 0.3% absolute with implanting. This is due to an improvement in Voc and to some extent in Jsc, despite the reduction in FF due to the local rear contacts (the rear contact pattern and the corresponding base resistivity are not fully optimized). Again, a SRH lifetime of 280 μs is assumed. Degradation has a far larger impact on the I-V parameters compared to a cell with a fully metalized rear, for the following reasons. Degradation causes the SRH lifetime in the boron-doped Cz base to be strongly injection dependent. The lifetime improves considerably going from MPP to the open-circuit condition, and thus causes a low fill factor. See Ref. [9] for more details.                   Fig. 2. Simulated I-V parameters. The reference cells with a POCl3 diffusion, shown in Fig. 1(c), is a reproduction of an experiment from [9] shown here as red dots. The adjacent bars indicate a simulation with the implanted emitter of Fig. 1(b) before and after degradation of the 2.5 ȍcm boron-doped Cz base with an interstitial oxygen density of 7u1017 cm-3 [20]. T. Ohrdes et al. / Energy Procedia 8 (2011) 167–173 171Crucial for the benefits of an implanted emitter is an effective front surface passivation. By comparing fabricated with simulated cells, we found that the recombination velocity parameter Sp of our SiO2/SiNx stacks at the front surface can be approximately parameterized as  Sp[cm/s] = 500(Ndon/1019 cm-3)0.6 + 60 (Ndon/1019 cm-3)3  .     (1)  This parameterization is similar to an alnealed oxide in Ref. [23], probably because both the Al layer and the SiNx layer release similar amounts of hydrogen into the oxide during the firing step. Fig. 3 shows the influence of the surface passivation of the cell with the local BSF in the non-degraded state. A rather small reduction in efficiency is predicted for a tenfold increase in the SRH parameter Sp, but even higher Sp values reduce cell efficiency considerably. Finally, Fig. 4 depicts the I-V parameters as a function of the implanted dose. While a low dose creates an emitter with low recombination but high resistive losses, a high dose does results in the opposite, and hence an optimum dose exists. The profiles shown in Fig. 1(b) have a sheet resistivity of 165 ȍ/sq or 50 ȍ/sq, respectively. The optimum depends on the specific passivation quality and the specific front metallization (e.g. finger distance and finger width) and should be simulated for each cell manufacturer individually.           Fig. 3. Simulated I-V parameters as a function of the SRH parameter Sp at the emitter surface.            Fig. 4. Simulated I-V parameters as a function of the implanted dose. With increasing dose, there are decreasing resistive losses and increasing recombination losses. The optimum depends on the front metallization geometry and the front surface passivation. 172  T. Ohrdes et al. / Energy Procedia 8 (2011) 167–1735. Conclusions Simulations show that phosphorus ion energies of 10 – 30 keV and implantation doses near 5u1014 cm-2 for the emitter between the fingers and doses of 7u1015 cm-2 under the front metal fingers are suitable for reducing the phosphorus peak density at the surface. Cell efficiency levels above 20% can only be achieved if appropriate surface passivation and wafer materials are used. The simulations strongly indicate that cell efficiency improves only marginally if the cell has a fully metallized rear Al-BSF and a boron-doped Cz base in a degraded state. Simulated cells with a local rear Al-BSF show an efficiency improvement due to an implanted selective emitter design by more than 0.3% absolute. T. Ohrdes et al. / Energy Procedia 8 (2011) 167–173 173References                                                           [1]  Varian, Gloucester, MA. [2]  Benick J, Bateman N, Hermle M. Very low saturation current densities on ion implanted boron emitters. 25th EU PV Solar Energy Conf. Valencia, Spain, 2010, p. 1169. [3]  Cristiano F et al. Thermal Evolution of Extrinsic Defects in Ion Implanted Silicon: Current Understanding and Modelling.  Mat. Res. Soc. Symp. Proc. 717. Silicon Front-End Junction Formation Technologies. 2002;C5.7.1–C5.7.12. [4] Plummer JD, Deal MD, Griffin PB. Silicon VLSI technology. New York: Prentice Hall; 2000. [5]  Pichler P. Intrinsic Point Defects, Impurities and Their Diffusion in Silicon. Venna: Springer; 2004. [6] Sentaurus Process User Guide, Version E-2010.12, Synopsys, Inc., Mountain View, CA [7] Cuevas A, Balbuena M. Thick-emitter silicon solar cells. 20th IEEE Specialists Conf., Las Vegas, 1988, p.429 [8]  S. Gatz, H. Hannebauer, R. Hesse, F. Werner, A. Schmidt1, T. Dullweber, J. Schmidt, K. Bothe, R. Brendel. 19.4%-efficient large-area fully screen-printed silicon solar cells. Phys. Status Solidi RRL 2011;5:147. [9] See the contribution from S. Steingrube et al. at this conference. [10]  Altermatt PP, Schenk A, Geelhaar G, Heiser G. Reassessment of the intrinsic carrier density in crystalline silicon in view of band-gap narrowing. Journal of Applied Physics 2003;93:1598. [11] Green MA. Intrinsic Concentration, Effective Densities of States, and Effective Mass in Silicon. Journal of Applied Physics 1990;67:2944. [12] Schenk A. Finite-temperature full random-phase approximation model of band gap narrowing for silicon device simulation. Journal of Applied Physics 1998;84:3684.  [13] Klaassen DBM. A Unified Mobility Model for Device Simulation – I. Model Equations and Concentration Dependence. Solid-State Electronics 1992;35:953.  [14] Klaassen DBM. A Unified Mobility Model for Device Simulation – II. Temperature Dependence of Carrier Mobility and Lifetime. Solid-State Electronics 1992;35:961. [15] Trupke T, Green MA, Würfel P, Altermatt PP, Wang A, Zhao J, Corkish R. Temperature dependence of the radiative recombination coefficient of intrinsic crystalline silicon. Journal of Applied Physics 2003;94:4930. [16]  Altermatt PP, Geelhaar F, Trupke T, Day X, Neisser A, Daub E. Injection dependence of spontaneous radiative recombination in c-Si: experiment, theoretical analysis, and simulation. Proc. 5th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD05), IEEE, NY, 2005, p. 47. [17]  Altermatt PP, Geelhaar F, Trupke T, Dai X, Neisser A, Daub E. Injection dependence of spontaneous radiative recombination in crystalline silicon: Experimental verification and theoretical analysis. Applied Physics Letters 2006;88:261901. [18]  Dziewior J, Schmid W. Auger coefficients for highly doped and highly excited silicon. Applied Physics Letters 1977;31:346. [19]  Altermatt PP, Schmidt J, Heiser G, Aberle AG. Assessment and Parameterisation of Coulomb-Enhanced Auger Recombination Coefficients in Lowly Injected Crystalline Silicon. Journal of Applied Physics 1997;82:4938. [20]  Bothe K, Sinton R, Schmidt J. Fundamental Boron-Oxygen-related Carrier Lifetime Limit in Mono- and Multicrystalline Silicon. Progress in PV 2005;13:287. [21]  Schmidt J,Cuevas A. Electronic properties of light-induced recombination centers in boron-doped Czochralski silicon. Journalof Applied Physics 1999;86;3175. [22]  Rein S, Glunz SW. Electronic properties of the metastable defect in boron-doped Czochralski silicon: Unambiguous determination by advanced lifetime spectroscopy. Journal of Applied Physics 2003;82:1054. [23]  Altermatt PP, Schumacher JO, Cuevas A, Kerr MJ, Glunz SW, King RR, Heiser G.Numerical modeling of highly doped Si:P emitters based on Fermi–Dirac statistics and self-consistent material parameters. Journal of Applied Physics 2002;92:3187. [24]  Gueymard C. Parameterized Transmittance Model for Direct Beam and Circumsolar Spectral Irradiance. Solar Energy 2001;71:325. [25]  Green MA. Self-consistent optical parameters of intrinsic silicon at 300 K including temperature coefficients. Solar Energy Materials and Solar Cells 2008;92:1305. [26]  Shiles E, Sasaki T, Inokuti M, Smith DY. Self-consistency and sum-rule tests in the Kramers-Kronig analysis of optical data: Application to aluminium. Physical Review B 1980:22:1612. [27]  Altermatt PP, Dreissigacker S, Yang Y, Sprodowski C, Dezhdar T, Koc S, Veith B, Herrman S, Bock R, Bothe K, Schmidt J, Brendel R. Highly predictive modelling of entire Si solar cells for industrial applications. 24th European PV Solar Energy Conference, Hamburg, Germany, (WIP, Munich), 2009, p. 901 

Solar cell emitter design with PV-tailored implantation

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Solar cell emitter design with PV-tailored implantation

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