improved diffusion profiles in back-contacted back-junction si solar cells with an overcompensated...

Phys. Status Solidi A 208, No. 12, 2871–2883 (2011) / DOI 10.1002/pssa.201127199 p s sa

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Improved diffusion profiles inback-contacted back-junction Si solar cellswith an overcompensated boron-doped emitter

Christian Reichel*, Martin Bivour, Filip Granek, Martin Hermle, and Stefan W. Glunz

applications and materials sciencewww.pss-a.comp

Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstrasse 2, 79110 Freiburg, Germany

Received 1 April 2011, revised 3 June 2011, accepted 6 June 2011

Published online 14 July 2011

Keywords back-contacted silicon solar cells, electrical shading, overcompensation of diffused junctions

*Corresponding author: e-mail [email protected], Phone: þ49 761 4588 5287, Fax: þ49 761 4588 9250

The performance of n-type back-contacted back-junction

silicon solar cells where the boron-doped emitter diffusion on

the rear side is locally overcompensated by a phosphorus-doped

base-type back surface field (BSF) diffusion has been analysed

theoretically and experimentally. By overcompensating the

emitter diffusion the noncollecting base-type region can be

reduced significantly allowing electrical shading losses to be

minimized. It has been found that for solar cells with a lowly

doped BSF diffusion the local external quantum efficiency and

the short-circuit current density Jsc could be improved

significantly. For reference solar cells with an undiffused gap

between emitter and BSF diffusion and a large noncollecting

base-type region, a maximum Jsc of 40.9mA/cm2 could be

achieved and for solar cells with a locally overcompensated

boron-doped emitter diffusion featuring a small noncollecting

base-type region a maximum Jsc of 41.4mA/cm2 has been

measured. The reduction of Jsc losses caused by free carrier

absorption (FCA) in highly doped silicon at near-infrared

wavelengths is also shown. Furthermore, theoretical inves-

tigations are performed by one-dimensional device simulations

and the influence of highly doped and lowly doped emitter and

BSF diffusions on the open-circuit voltage Voc is presented. For

solar cells with a locally overcompensated boron-doped emitter

diffusion Voc could be improved from 629 to 652mV when

lowly-doped diffusions and thermally grown SiO2 and antire-

flection plasma enhanced chemical vapour deposited (PECVD)

SiNx passivation stacks are applied. For the reference solar cells

with an undiffused gap between the lowly doped emitter and

BSF diffusions Voc of 693mV could be achieved for a plasma

enhanced atomic layer deposited (PEALD) Al2O3 passivation

layer.

2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction For interdigitated n-type back con-tacted back-junction silicon solar cells, the boron-dopedemitter and the phosphorus-doped base-type back surfacefield diffusions as well as the p- and n-metal fingers arelocated on the rear side. This leads to a higher photo-generated current density inside the solar cell since due to theabsence of the front side metallization grid, no opticalshading losses occur. The presence of the noncollectingbase-type region on the rear side minimizes the collectingemitter region so that photogenerated minority chargecarriers have to be transported vertically as well as laterallyto the collecting emitter on the rear side. Thus, especially fora solar cell with a large noncollecting base-type region, seeFig. 1A, the lateral distance of minority charge carriersgenerated above the noncollecting base-type region to thecollecting emitter is increased significantly. The increased

lateral distance in combination with an enhanced recombi-nation in the noncollecting base-type region leads to aninefficient collection of minority charge carriers. This effectis known as electrical shading and reduces the minoritycharge carrier collection probability, particularly in thenoncollecting base-type region, and thus the short-circuitcurrent density Jsc [1–5]. One possible approach to decreasethe lateral distances of minority charge carriers and thusminimize electrical shading losses is to reduce the non-collecting base-type region and to increase the collectingemitter region considerably.

However, since it is beneficial that the p- and n-metalfingers have the same width in order to avoid electrical seriesresistance losses, the increase of the collecting emitter regionand the reduction of the noncollecting base-type region willresult in an overlap of the n-metal finger and the p-type


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Figure 1 (online colour at: www.pss-a.com) Schematic cross-sec-tion of the symmetry element of the three different interdigitated n-type back-contacted back-junction silicon solar cell designs. For the‘reference’ solar cell concept (structure A) the boron-doped emitterand the phosphorus-doped base-typeBSFdiffusion are separated byan undiffused gap region. The ‘buried emitter’ solar cell concept(structure B) is characterized by a locally overcompensated boron-doped emitter diffusion with a phosphorus-doped base-type BSFdiffusion. In this case the phosphorus-doped base-type BSF is notseparated from the boron-doped emitter diffusion and effectively onthe same electrical potential as the n-type base. The boron-dopedemitterdiffusionandthephosphorus-dopedbase-typeBSFdiffusionof the ‘passivated emitter’ solar cell concept (structure C) are alsoseparated but the emitter is locally overcompensated and thuspassivated by a floating phosphorus-doped base-type diffusion.

emitter region. In this case the n-metal finger and the p-typeemitter region are only insulated by a dielectric passivationlayer on the rear side. A dielectric passivation layer withinsufficient electrical insulation properties, caused by cracksand/or pinholes, will increase the risk of shunting betweenthe n-metal finger and the p-type emitter region and hencewill reduce the solar cell efficiency significantly.

The ‘buried emitter’ solar cell concept, see Fig. 1B,developed by Harder et al. [6] and Granek et al. [7] is a solarcell design that meets the aforementioned requirementsand eliminates the shunting risk through the dielectricpassivation layer on the rear side. The solar cell design is


characterized by the local overcompensation of the boron-doped emitter diffusion on the rear side with a phosphorus-doped base-type BSF diffusion. In this case, the p–n junctionbetween the boron-doped emitter and the phosphorus-dopedbase-type BSF acts as an insulator and allows the geometryof the minority charge carrier collecting emitter regionon the rear side to be independent of the interdigitated p- andn-metal fingers. Furthermore, it has been shown that theovercompensation of a boron-doped emitter diffusion with asilicon oxide passivated shallow phosphorus-doped diffu-sion can improve the surface passivation quality of n-typesilicon solar cells with a boron-doped emitter [8, 9]. In thiscase, the p–n junction on the front side of the solar cell isfloating and has the same positive effect on the surfacepassivation as the so-called floating emitter [10]. However,as will be shown later, increased recombination losses areintroduced by this additional p–n junction on the rear side ofthe ‘buried emitter’ solar cell, leading to a lower open-circuitvoltage Voc potential.

Although first results of the ‘buried emitter’ solar celldesign have been very promising, the Voc of about 630mVhas been identified to be the key factor limiting the solar cellperformance [6]. Numerical device simulations performedby Harder et al. [11] have shown that an optimization of theboron-doped emitter and the phosphorus-doped base-typeBSF diffusion profiles are essential for the improvement ofthe Voc and the achievement of higher efficiencies. Recentlypresented results with an increased Voc of 655mV, anexcellent Jsc of 41.4mA/cm2 and an efficiency of 21.8%have shown the potential of the ‘buried emitter’ solar cellconcept [12]. For a deeper insight into the device limitingfactors, especially those of the additional p–n junctionbetween the boron-doped emitter diffusion and the phos-phorus-doped base-type BSF diffusion, both the ‘buriedemitter’ solar cell and the ‘reference’ solar cell concept havebeen compared by our group [13]. It could be shown thatdue to an increased minority charge carrier collectionprobability of the ‘buried emitter’ solar cell featuring asmall noncollecting base-type region, Jsc could be improvedsignificantly. Nevertheless, the efficiencies have beenalmost in the same range for both solar cell concepts despitethe gain in Jsc of the ‘buried emitter’ solar cell. This hasbeen attributed to a lowerVoc of about 630mV for the ‘buriedemitter’ solar cell compared to a Voc of 660mV for the‘reference’ solar cell.

In a continuative step theoretical and experimentalinvestigations are performed in this study in order to furtherimprove the understanding of the reduction in the Voc ofthe ‘buried emitter’ solar cell concept. Numerical devicesimulations are performed that are similar to the ones shownbyHarder et al. [11]. However, the presented approach givesamore detailed description of the problems arising due to theapplication of highly doped diffusions on the rear side of the‘buried emitter’ solar cell design. The ‘buried emitter’ solarcell is compared to a solar cell concept where the boron-doped emitter diffusion is separated from the phosphorus-doped base-type BSF diffusion but where it is also locally

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Phys. Status Solidi A 208, No. 12 (2011) 2873

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overcompensated by floating phosphorus-doped base-typediffusion, see Fig. 1C. The latter solar cell concept is calledthe ‘passivated emitter’ solar cell and is only used because ofthe resemblance to the ‘buried emitter’ solar cell as well asfor the interpretation of the numerical device simulations.Experimental results obtained for the ‘reference’ solar celland the ‘buried emitter’ solar cell are compared and theimpact of lowly doped diffusions on Voc and Jsc arepresented. Besides the positive effects of lowly dopeddiffusions on theVoc, the increase in the Jsc due to a reductionof free carrier absorption (FCA) occurring in highly dopedsilicon at near-infrared wavelengths [14, 15] is investigatedin more detail by quantum efficiency measurements.

2 Theoretical investigations2.1 Simulation model The performance of the three

different solar cell designs are analysed by one-dimensionalnumerical device simulations using PC1D [16]. Figure 2shows the different models and the corresponding connec-tions of the positive and negative pole to the emitter andthe base and/or the base-type BSF diffusion. It is worthmentioning that due to the one-dimensional simulations ofthe different solar cell designs, two-dimensional transportphenomenas related to the noncollecting base-typeregion, consisting of the undiffused gap and base-type BSFdiffusion, are neglected. The ‘reference’ solar cell (structureA) is a rear side collecting solar cell with a p-type emitterdiffusionwhereas for the ‘buried emitter’ solar cell (structureB) and the ‘passivated emitter’ solar cell (structure C) theemitter diffusion on the rear side is additionally over-compensated by a n-type BSF diffusion. In contrast to the‘passivated emitter’ solar cell where the BSF diffusion on

n++ BSFn+ FSF p++ Emitter

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Jrec,frontJrec,fsf

Jrec,bulk

Jrec,emitter

Jrec,rear

Jrec,bsf

reference buried emitter

passivatedemitter

Figure 2 (onlinecolour at:www.pss-a.com)Modelof thedifferentsolar cell structures used for one-dimensional numerical devicesimulations. Model A) represents the ‘reference’ solar cell concept(structureA).Theemitterandthebasearecontactedseparatelybythepositive and negative pole. Model B) and model C) correspond tothe ‘buried emitter’ solar cell concept (structure B) and ‘passivatedemitter’ solarcell concept (structureC)where theemitterdiffusion isovercompensated by the base-type BSF diffusion. In contrast tomodel B), the base-type BSF diffusion of the ‘passivated emitter’solar cell concept is not directly contacted with the n-type base,so that in this case the base-type BSF is floating.

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the rear side is floating, the BSF diffusion of the ‘buriedemitter’ solar cell is contacted via a shunt element to the baseand hence to the negative pole. The n-type base has a dopingconcentration of about 5 1015 cm3 corresponding to a1V cm basematerial and an assumedminority charge carrierlifetime of 1200ms. Figure 3 shows the Gaussian-like p-typeemitter and the Erfc-like n-type BSF diffusion profiles usedfor the simulations. The peak doping concentration N0, thepeak position xpeak and the depth xdepth of the diffusionprofiles as well as the sheet resistance Rsheet of the highlydoped and the lowly doped p-type emitter and n-type BSFdiffusions are also presented in Table 1. For the ‘buriedemitter’ solar cell with lowly doped diffusion profiles, thenegative contact to the base and/or the BSF is located atx¼ 180mm (selected arbitrarily) and x¼ 200mm, see alsoFig. 3. The positive contact to the emitter is located atx¼ 199mm. The surface recombination velocity of thephosphorus-doped surfaces S0n and boron-doped surfacesS0p have been calculated by the empirical determinedrelation reported by Cuevas et al. for silicon dioxidepassivated surfaces with S0n¼ 70 cm/s (N0n/7 1017 cm3)for N0n> 7 1017 cm3 [17] and S0p¼ 500 cm/s (N0p/11016 cm3) 1/3for N0p> 1 1016 cm3 [18], respectively.

In Fig. 4 the current–voltage characteristics of the threedifferent solar cell designs under illumination are illustratedwhen lowly doped and highly doped diffusions are applied. Itcan be seen that the performance of the ‘reference’ solar cellswith lowly doped and highly doped diffusions are the best.Please notice that the effect of electrical shading and thus achange in Jsc is not included in these simulations. The‘passivated emitter’ solar cell with highly doped diffusionshas a marginally lower Voc than the ‘reference’ solar cellwhich can be attributed to the additional highly doped base-type BSF. In this case, the additional recombination in thebulk and at the surface of the overcompensating BSFinfluences Voc negatively. However, in contrast to the

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Figure 3 (online colour at: www.pss-a.com) Profiles of the lowlydoped(dashed lines)and thehighlydoped(solid lines)p-typeemitterandn-typeBSF.Theconnectionsof thepositive(þ)andnegative()pole for the ‘buried emitter’ solar cell concept are indicated.



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Table 1 Parameters for the profiles of the highly doped and thelowly doped p-type emitter and n-type BSF diffusions used inPC1D.

N0

(cm3)xpeak(mm)

xdepth(mm)

Rsheet

(V/&)

highly doped diffusionsp-type Gaussian 5 1019 199.0 4.0 13.3n-type Erfc 5 1020 200.0 0.7 14.2lowly doped diffusionsp-type Gaussian 1 1019 199.5 2.0 80.5n-type Erfc 5 1019 200.0 0.3 201.5

n+ FSF n Base p++ Emitter n++ BSF

‘reference’ solar cell and the ‘passivated emitter’ solar cell itcan be observed that Voc of the ‘buried emitter’ solar cell issignificantly lower. For the ‘passivated emitter’ and the‘buried emitter’ solar cell designs with lowly dopeddiffusions the current–voltage curves reveal that Voc isincreased and nearly in the same range.

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Figure 4 (online colour at: www.pss-a.com) Simulated current–voltage characteristics of the different solar cell designs with highlydoped (top) and lowly doped diffusion profiles (bottom) underillumination. The voltage of interest is 525mV and marked with across.


2.2 Highly doped diffusions For an applied voltageof 525mV, see cross in Fig. 4 (top), the external currentdensity of the ‘buried emitter’ solar cell is 33.6mA/cm2

which is significantly lower than for the ‘passivated emitter’solar cell where the external current density is 39.0mA/cm2.The difference in the external current density is 5.4mA/cm2

and since the external current density equals the generationcurrent density minimized by the recombination currentdensity Jext(V)¼ JgenJrec(V), a higher recombinationcurrent density in the ‘buried emitter’ solar cell with highlydoped diffusions exists. For numerical device simulation theoverall internal solar cell parameters can be traced back tothe different device regions of the solar cell to get a deeperinsight into the device limiting factors. This is done for the‘passivated emitter’ and the ‘buried emitter’ solar cells withhighly doped diffusions, as can be seen in Fig. 5 and Fig. 6.

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structure C) structure B)J 39.0 mA/cm2 33.6 mA/cm2

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Figure 5 (online colour at: www.pss-a.com) Conduction andvalencebandedgesaswell as electronandholequasi-Fermienergies(top) and electron and hole charge carrier concentrations (bottom)of the ‘buried emitter’ solar cell (structure B) and the ‘passivatedemitter’ solar cell (structure C) with highly doped diffusions for avoltage of 525mV. The corresponding external current density foreach structure is also shown.

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Figure 6 (online colour at: www.pss-a.com) Electron and holecurrent densities (top) and cumulative recombination rates (bottom)of the ‘buried emitter’ solar cell (structure B) and the ‘passivatedemitter’ solar cell (structure C) with highly doped diffusions for avoltage of 525mV. The calculated generation and recombinationcurrent densities of the ‘passivated emitter’ and the ‘buried emitter’solar cell and the corresponding external current density for eachstructure are also shown.

In Fig. 5 (top) it can be observed that the ‘buried emitter’solar cell exhibits a lower band bending DE between theemitter and the BSF than the ‘passivated emitter’ solar cell atthe same applied voltage. This lower energy barrier heightcan be attributed to the fact that in contrast to the ‘passivatedemitter’ solar cell, the BSF diffusion on the rear side of the‘buried emitter’ solar cell is not isolated by the emitter andthus not floating, but rather on the same electrical potential asthe base, see also model B in Fig. 2B. Therefore, the bandbending between the base and the emitter and between theemitter and BSF of the ‘buried emitter’ solar cell is reducedby the applied voltage compared to equilibrium conditions.For the ‘passivated emitter’ solar cell this is only true forthe band bending between the base and the emitter, seeFig. 5 (top). Under equilibrium conditions the difference in

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the band bending between the emitter and BSF is notobserved.

In contrast to the ‘buried emitter’ solar cell, the bandbending between the emitter and the BSF of the ‘passivatedemitter’ solar cell is a result of the transport of electrons fromthe base through the emitter into the non-contacted andtherefore floating BSF. Since the electrons have to overcomethe energy barrier between the base and the emitter, only afew space charges are compensated in the BSF of the‘passivated emitter’ solar cell. Thus, a higher band bendingbetween the emitter and the BSF remains compared to the‘buried emitter‘ solar cell where the contact between the baseand theBSF resulting in an electron current flowvia the shuntelement (5.5mA/cm2) from the base into the BSF. This leadsto a compensation of many space charges and a reduction ofthe band bending between the emitter and the BSF.

The lower band bending between the emitter and theBSF of the ‘buried emitter’ solar cell leads to an increaseddiffusion of holes from the emitter into the highlyrecombinative BSF and of electrons from the BSF into theemitter. Therefore, the minority charge carrier concentrationin the emitter and in the BSF is increased compared to the‘passivated emitter’ solar cell, see Fig. 5 (bottom). In Fig. 6(top), the hole current density from the emitter into theBSF isindicated by a positive value for x> 199mmand the electroncurrent from the BSF into the emitter is depicted by thepositive electron current for x> 199mm. In contrast, themajority charge carrier flow from the emitter into the BSFand vice versa of the ’’passivated emitter’’ solar cell wherethe BSF is floating, is nearly zero for x> 199mm. Thisresults in a lower hole concentration in the BSF and emittercompared to the ‘buried emitter’ solar cell.

In case of both the ‘passivated emitter’ and the ‘buriedemitter’ solar cells with highly doped diffusions, the bulk ofthe emitter as well as the bulk and the surface of the BSF arehighly recombinative regions. Hence, an increased minoritycharge carrier concentration in the emitter and the BSF leadsto an increased recombination rate, especially in the BSF ofthe ‘buried emitter’ solar cell, as shown in Fig. 6 (bottom) bythe cumulative recombination rates. The generation currentdensity is 40.6mA/cm2. The total recombination currentdensity of the ‘passivated emitter’ solar cell is 1.6mA/cm2

whereas for the ‘buried emitter’ solar cell the totalrecombination current density is 7.0mA/cm2 for an appliedvoltage of 525mV. The recombination current density Jrec ineach individual region of the ‘passivated emitter’ and the‘buried emitter’ solar cell has been determined by thedifference of the cumulative recombination rates of eachregion multiplied by the elementary charge q, see Table 2and Fig. 2 (right hand-side).

The recombination current density at the front side and inthe bulk of the front surface field of the ‘passivated emitter’solar cell and the ‘buried emitter’ solar cell are the same.Furthermore, it can bee seen that the recombination currentdensity in the bulk of the emitter is small and the samefor both solar cell designs. Nevertheless, for the ‘buriedemitter’ solar cell, a high recombination current density of



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Table 2 Generation and recombination current density at the front side Jrec,front, in the bulk of the FSF Jrec,fsf, in the base Jrec,bulk, in theemitter Jrec,emitter, in the BSF Jrec,bsf and at the rear side Jrec,rear of the ‘reference’ solar cell (structure A), the ‘buried emitter’ solar cell(structure B) and the ‘passivated emitter’ solar cell (structure C) for an applied voltage Vapp of 525mV and for open-circuit voltage Voc.

Vapp

(mV)Jgen(A/cm2)

Jrec,total(A/cm2)

Jrec,front(A/cm2)

Jrec,fsf(A/cm2)

Jrec,bulk(A/cm2)

Jrec,emitter

(A/cm2)Jrec,bsf(A/cm2)

Jrec,rear(A/cm2)

highly doped diffusionsstructure A 525 40.59 1.50 0.72 0.13 0.52 0.09 – 0.03structure B 525 40.59 7.00 0.72 0.13 0.52 0.09 5.06 0.47structure C 525 40.59 1.63 0.72 0.13 0.52 0.05 0.19 0.02lowly doped diffusionsstructure A 525 40.93 1.47 0.73 0.13 0.54 0.01 – 0.06structure B 525 40.93 1.65 0.76 0.14 0.53 0.01 0.06 0.15structure C 525 40.93 1.55 0.73 0.14 0.53 0.00 0.04 0.11

Voc

(mV)Jgen(A/cm2)

Jrec,total(A/cm2)

Jrec,front(A/cm2)

Jrec,fsf(A/cm2)

Jrec,bulk(A/cm2)

Jrec,emitter

(A/cm2)Jrec,bsf(A/cm2)

Jrec,rear(A/cm2)

highly doped diffusionsstructure A 670 40.71 40.70 6.21 1.14 5.03 22.26 – 6.06structure B 575 40.59 40.58 0.84 0.16 0.68 0.61 35.04 3.25structure C 663 40.59 40.58 4.92 0.92 4.19 8.81 19.93 1.81lowly doped diffusionsstructure A 679 40.94 40.93 8.33 1.53 6.35 2.37 – 22.35structure B 655 40.93 40.92 3.83 0.71 3.45 0.80 8.53 23.60structure C 662 40.93 40.93 4.73 0.88 4.09 0.88 8.06 22.29

5.1mA/cm2 exists in the highly doped BSF. In contrast, forthe ‘passivated emitter’ solar cell, this recombination currentdensity is only 0.2mA/cm2. The recombination currentdensity at the surface of the BSF of both structures is alsodifferent. For the ‘buried emitter’ solar cell, the recombina-tion current density is 0.5mA/cm2 whereas almost nothingrecombines at the surface on the rear side of the ‘passivatedemitter’ solar cell. Hence, the external current density of the‘buried emitter’ solar cell, see also Fig. 4 (top), is reduced by5.4mA/cm2 compared to the ‘passivated emitter’ solar cell.The limitation of the Voc is consequently attributed to thehigh recombination rates in the bulk and at the surface of thehighly doped BSF. The same conclusions can also be drawnfrom the simulations performed by Harder et al. [11].

2.3 Lowly doped diffusions By adapting thediffusion profiles the aforementioned recombination lossescan be reduced significantly for the ‘buried emitter’ solarcell. This is indicated by the decrease in the totalrecombination current densities at a voltage of 525mVand under open-circuit voltage conditions, see Table 2 andFig. 4 (bottom). The application of lowly doped diffusionscauses a lower space charge concentration under equilibriumconditions and thus a lower band bending.Nevertheless, onlya minor difference in the bend bending between the emitterand the BSF can be observed for the ‘buried emitter’ solarcell with highly doped and lowly doped diffusions, see Fig. 5(top) and Fig. 7 (top). Although the energy barrier heightbetween the emitter and the BSF is comparable to the ‘buriedemitter’ solar cell with highly doped diffusions, the diffusionof majority charge carriers from the emitter into the BSF


and vice versa is considerably lower for the ‘buried emitter’solar cell with lowly doped diffusions, see Fig. 8 (top). Thisis a result of a reduced recombination rate especially in theBSF. Thus, a lower gradient for the majority charge carriertransport exists, indicated by the nearly zero electron andhole current density for x> 199mm, see also Fig. 8 (top). Therecombination current density for the ‘buried emitter’ solarcell is consequently reduced by 5.0mA/cm2 in the BSF andby 0.3mA/cm2 on the rear side, also shown in Table 2.

In contrast, for the ‘passivated emitter’ solar cell withlowly doped diffusions, no significant reduction of theoverall recombination losses can be observed. In Fig. 7 (top)it can bee seen that the band bending between the emitter andthe BSF of the ‘passivated emitter’ solar cell is only slightlyhigher than for the ‘buried emitter’ solar cell with lowlydoped diffusions. Thismeans that for the ‘passivated emitter’solar cell with a floating BSF, the compensation of the spacecharges is as efficient as for the ‘buried emitter’ solar cellwith lowly doped diffusions. The lower energy barrier heightbetween the base and the emitter and the transport of holesfrom the emitter into the floating BSF results in a higherrecombination current density of 0.1mA/cm2 at the rear sidewhen lowly doped diffusions are applied. However, despitethe fact that for both the ‘buried emitter’ and the ‘passivatedemitter’ solar cell a considerable reduction of the recombi-nation rate in the emitter and especially in the BSF isachieved, an increased recombination rate on the surface ofthe rear side can be observed under open-circuit voltageconditions, see Table 2. This is due to the fact that the surfacerecombination velocity of the BSF given by the empiricaldetermined relation reported by Cuevas et al. [17] for a

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Figure 7 (online colour at: www.pss-a.com) Conduction andvalencebandedgesaswell as electronandholequasi-Fermienergies(top) and electron andhole charge carrier concentrations (bottom)ofthe ‘buried emitter’ solar cell (structure B) and the ‘passivatedemitter’ solar cell (structure C) with lowly doped diffusions forthe same applied voltage as shown in Fig. 5.

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Figure 8 (online colour at: www.pss-a.com) Electron and holecurrent densities (top) and cumulative recombination rates (bottom)of the ‘buried emitter’ solar cell (structure B) and the ‘passivatedemitter’ solar cell (structure C) with lowly doped diffusions for thesame applied voltage as shown in Fig. 6.

silicon oxide passivation is relatively high for the ‘buriedemitter’ and the ‘passivated emitter’ solar cell with lowlydoped diffusions. Hence, a passivation layer allowing lowersurface recombination velocity on the rear side of the BSFhas to be applied in order to overcome the limitation in Voc

for both solar cell designs.Please notice that the calculated generation current

density is about 40.9mA/cm2 and more than 0.3mA/cm2

higher than the generation current density of the ‘passivatedemitter’ and the ‘buried emitter‘ solar cells with highlydoped diffusions. This is a result of the FCA in highly dopedsilicon at near-infrared wavelengths [14, 15] which is alsoconsidered in PC1D by an implemented model [19].

3 Experimental investigations3.1 Solar cell fabrication The 2 2 cm2 large back-

contacted back-junction silicon ‘reference’ and ‘buried

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emitter’ solar cells have been fabricated on n-type float-zonegrown 180mm thick silicon wafers with base resistivities of5 and 10V cm. The structuring of the front and the rear sideof the solar cells has been performed by photolithography.The front side has been textured with inverted pyramids andpassivated by a shallow 350V/& nþ-type front surface field(FSF) diffusion as well as a thin layer of a thermally grownsilicon oxide 10 nm thick SiO2 and a 70 nm thick antireflec-tion coating of plasma enhanced chemical vapour deposited(PECVD) SiNx. The passivation on the rear side consists ofa stack of a 10 nm thin thermal SiO2 and a 70 nm thickantireflection PECVD SiNx and has been applied for the5 and 10V cm base material. A 100 nm thick plasmaenhanced atomic layer deposited (PEALD) Al2O3 passiva-tion and optical layer on the rear side has also beeninvestigated for the 5V cm base material. The passivationon the rear side has been opened locally in order to contact



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"reference" solar cell (structure A) T82_21b (lowly-doped diffusions, 5 Ωcm) T35_5d (highly-doped diffusions, 10 Ωcm)

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the diffused regions with electron beam evaporated p- andn-metal fingers. On the basis of the results of the numericaldevice simulations shown in this study, different boron-doped pþþ-type emitter and shallow phosphorus-dopednþþ-type BSF diffusions have been investigated in prelimi-nary experiments. The highly doped emitter and BSFdiffusions of the solar cells have a sheet resistance of about10V/& and about 20V/&, respectively. The adapted lowlydoped emitter and BSF diffusions have a sheet resistanceof about 110 V/& and about 225 V/&, respectively. Incontrast to the ‘reference’ solar cell, the boron-doped emitterdiffusion of the ‘buried emitter’ solar cell was locallyovercompensated by the shallow phosphorus-doped base-type BSF diffusion, see Fig. 9. The boron-doped and thephosphorus-doped diffusion profiles are realized in a tubefurnace using POCl3 and BBr3, respectively. The metalliza-tion consists of anAl, Ti, Pd, andAg stackwith a thickness ofabout 3mm. The interdigitated metallization grid on the rearside has been accomplished by the lift-off technique. In orderto realize an excellent surface passivation and an appropriatemetal-semiconductor contact, the solar cells with a stack of athin thermal SiO2 and an antireflection PECVD SiNx

passivation, as well as the PEALD Al2O3 passivation wereannealed in room atmosphere at a temperature of about350 8C for 15min and at 400 8C for 15min, respectively.

The width of the symmetry element of the ‘reference’solar cell and the ‘buried emitter’ solar cell with a locallyovercompensated boron-doped emitter diffusion is1100mm, corresponding to a pitch distance of 2200mm.The boron-doped emitter diffusion width and the phos-phorus-doped BSF diffusion width of the symmetry elementof the ‘reference’ solar cell, depicted in Fig. 1A, are 800mmand 150mm, respectively. In this case the undiffused gap is150mm wide. The boron-doped emitter diffusion width of

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

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highly-doped lowly-doped n-type diffusion p-type diffusion

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Figure 9 (onlinecolourat:www.pss-a.com)Dopingconcentrationof the boron-doped p-type emitter diffusion overcompensated bythe phosphorus-doped n-type BSF diffusion on the rear side of thesolar cells (region of the overcompensation width in Fig. 1C).The doping profiles have been determined by electrochemicalcapacitance voltage measurements.


the ‘buried emitter’ solar cell is 1050mm wide and locallyovercompensated by a 550mm wide phosphorus-dopedbase-type BSF diffusion, see Fig. 1C, resulting in an virtualemitter coverage on the rear side of 95%.

3.2 Local and global quantum efficiency Theimpact of highly doped and lowly doped base-type BSFdiffusions on electrical shading losses especially in thenoncollecting base-type region of both solar cell designs isinvestigated by spectrally resolved laser beam inducedcurrent (SR-LBIC) measurements. Hence, the local externalquantum efficiency EQE can be determined perpendicularto the interdigitated diffusions and metal fingers on the rearside of the ‘reference’ solar cell, see Fig. 10 (top), and of the‘buried emitter’ solar cell, see Fig. 10 (bottom).

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pitch = 2200 µm

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Figure 10 (onlinecolourat:www.pss-a.com)Localexternalquan-tumefficiencyEQEmeasuredbySR-LBIC for a laserwavelength of790 nm corresponding to an absorption depth of 11mm. In contrastto the ‘reference’ solar cell (top), no reduced EQE is observed inthe noncollecting base-type region for the ‘buried emitter’ solar cell(bottom). The rear side has been passivated with SiO2 and PECVDSiNx. Themeasured short-circuit current density of structure A) andstructure B) are also shown for the 5V cm basematerials with lowlydoped diffusions and for the 10V cm base materials with highlydoped diffusions.

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Figure 11 (online colour at: www.pss-a.com) Internal quantumefficiencyIQEandreflectionR for the‘reference’solarcell (structureA)(top)and the‘buriedemitter’ solarcell (structureB)(bottom)withhighly dopeddiffusions on10V cmbasematerial (full symbols) andlowly doped diffusions on 5V cm base material (open symbols).

For the ‘reference’ solar cell with the highly doped BSFdiffusions it can be observed that the high recombinationin the noncollecting base-type region causes a significantdecrease in EQE to about 84% at x¼ 1.1mm. However,when the BSF diffusion profile is adapted, it can be seen thatEQE in the noncollecting base-type region is increased toabout 94% at x¼ 1.1mm. The significantly higher EQE inthe noncollecting base-type region of the ‘reference’ solarcell with the lowly dopedBSF diffusions is caused by a lowerrecombination in the bulk of the BSF, as well as the excellentsurface passivation with a stack of a thin thermal SiO2 andan antireflection PECVD SiNx. Hence, a lower effectivesurface recombination velocity and an increased minoritycharge carrier collection probability in the noncollectingbase-type region can be achieved for both base resistivities.For the ‘reference’ solar cell with the highly doped and lowlydoped BSF diffusions, it can also be observed that in thecollecting emitter region at x¼ 0mm only a marginaldifference between the 5V cm and the 10V cm basematerial exists. Theoretical investigations have shownthat higher bulk minority charge carrier lifetimes andlower effective surface recombination velocities at thehigh-low junction on the front side leads to a higher minoritycharge carrier collection probability and thus to a higherEQE when a 10V cm base material is used [20]. However,the minority charge carrier collection probability in thecollecting emitter region can be strongly influenced by therecombination in the noncollecting base-type region [21].This leads to a lower EQE in the collecting emitter region forthe ‘reference’ solar cell on the 10V cm base material andwith the highly doped and highly recombinative BSFdiffusions.

In contrast to the ‘reference’ solar cell with a largenoncollecting base-type region, the distance for the transportof theminority charge carriers to the collecting emitter of the‘buried emitter’ solar cell is much smaller, especially forminority charge carriers generated above the base-type BSF.Thus, no significant electrical shading losses are observed inthe noncollecting base-type region. Therefore, the EQE inthe noncollecting base-type region at x¼ 1.1mm is almostthe same as in the collecting emitter region at x¼ 0mm forthe ‘buried emitter’ solar cell with lowly doped and highlydoped BSF diffusions. Furthermore, it can be observed thatthe difference between the ‘buried emitter’ solar cell, withhighly doped and lowly doped diffusions is nearly 1%.In the case of the ‘buried emitter’ solar cell the minoritycharge carrier collection probability seems to be moreeffective for the 10V cm base material than for the 5V cmbase material. This can be attributed to the higher minoritycharge carrier lifetime and the reduced effective surfacerecombination velocity especially at the high-low junction atthe front side when the 10V cm base material is used [21].Nevertheless, the measured Jsc of the ‘buried emitter’ solarcell on 10V cm base material and with highly dopeddiffusions is only 40.7mA/cm2 whereas it is 41.4mA/cm2

for the ‘buried emitter’ solar cell on 5V cm base materialwith lowly doped diffusions. This can be explained by the

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reduced FCA losses at near-infrared wavelengths whenlowly doped instead of highly doped diffusions areapplied on the rear side (see also simulation results). Thecorresponding global internal quantum efficiency IQEand the reflection of the ‘reference’ solar cell and of the‘buried emitter’ solar cell with lowly doped and highly dopeddiffusions are shown in Fig. 11. It can be observed that forwavelengths in the near-infrared region, the IQE is stronglyincreased due to the reduced FCA and the higher reflectionat the rear side.

The comparison of the local EQE reveals thatelectrical shading losses can be reduced significantly notonly by increasing the emitter fraction and reducing thenoncollecting base-type region on the rear side but alsoby adapted BSF diffusion profiles. In the latter case,this occurs even when the noncollecting base-type regionwidth on the rear side of the ‘reference’ solar cell, seebase width in Fig. 1A, is much larger than that of the‘buried emitter’ solar cell. Furthermore, the highest Jsccan only be realized when, besides the reduction of



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electrical shading losses, also losses associated withFCA are minimized.

3.3 Open-circuit voltage limit The Voc potential ofthe ‘reference’ solar cell is analysed for lowly doped andhighly doped diffusions and when a passivation of a stack ofa thin thermal SiO2 and an antireflection PECVD SiNx aswell as a PEALD Al2O3 passivation layer [22] are applied tothe rear side. The Voc limit has been estimated by means ofminority charge carrier lifetime measurements of symmetri-cal test structures with the quasi-steady state photoconduc-tance method [23]. From these measurements, the saturationcurrent density of the front surface field, the emitter and thebase-type BSF diffusions as well as for the undiffused gapregion, can be extracted and when area-weighted, the Voc

limit of the ‘reference’ solar cell can be determined. Forthe ‘buried emitter’ solar cell the Voc limit has not beendetermined because of the fact that the symmetrical teststructure is one-dimensional and that in this case thephosphorus-doped base-type BSF diffusion overcompensat-ing the boron-doped emitter diffusion is floating and notcontacted to the n-type base. Hence, the determined Voc limitwould correspond to the ‘passivated emitter’ solar cell ratherthan to the ‘buried emitter’ solar cell. In Table 3, thesaturation current density of each region is presented forlowly doped and highly doped emitter and base-type BSFdiffusions.

It can clearly be recognized that for a SiO2 andan antireflection PECVD SiNx passivation stack thesaturation current density of the base-type BSF diffusionJ0,bsf is 76 fA/cm2 and much smaller than the 322 fA/cm2

that was determined for highly doped diffusions. Evenfor a PEALD Al2O3 passivation layer, the value for J0,bsf isin the same range as for the SiO2 and the antireflectionPECVD SiNx passivation stack on lowly doped BSFdiffusions.

In contrast, the passivation quality of lowly dopedemitter diffusions with the SiO2 and the antireflectionPECVD SiNx passivation stack is worse than for the highly

able 3 Saturation current density of each region and upperoundary of the open-circuit voltage limit Voc,limit of the ‘refer-nce’ solar cell (structure A) with different rear side passivationyers (bulk recombination is neglected). For the calculations antrinsic charge carrier concentration of 8.79 109 cm3 has beensed [27, 28].

highly dopeddiffusions(SiO2þ SiNx)

lowly dopeddiffusions(SiO2þ SiNx)

lowly dopeddiffusions(Al2O3)

base (V cm) 10 5 5

0,fsf (fA/cm2) 28 24 24

0,emitter (fA/cm2) 234 387 11

0,bsf (fA/cm2) 322 76 85

0,gap (fA/cm2) 12 6 4

rea-weighted

oc,limit (mV) 664 657 706

Table 4 Results of the best back-contacted back-junction siliconsolar cells with highly doped and lowly doped diffusions anddifferent rear side passivation layers (designated area measure-ments with AM1.5G spectrum).

passivation(rear side)

rbase(V cm)

Voc

(mV)Jsc(mA/cm2)

FF(%)

h(%)

reference – highly doped diffusionsSiO2þ SiNx 10 660 39.9 77.8 20.5buried emitter – highly doped diffusionsSiO2þ SiNx 10 629 40.9 77.4 19.9reference – lowly doped diffusionsSiO2þ SiNx 5 645 40.8 77.1 20.3Al2O3 5 693 40.7 71.2 20.1buried emitter – lowly doped diffusionsSiO2þ SiNx 5 652 40.9 76.4 20.4Al2O3 5 649 41.1 77.0 20.5

Tbelainu

rJJJJaV


doped emitter diffusions. Theoretically, the Auger recombi-nation rate in the bulk of the lowly doped emitter should besmaller than for the highly doped emitter, but it seemsthat due to the lower peak doping concentration atthe surface and the insufficient passivation quality of athermally grown SiO2 on boron-doped surfaces [24], therecombination at the surface of the emitter diffusion isincreased significantly. Thus, the saturation current densityof the emitter diffusion J0,emitter is 387 fA/cm

2 for a lowlydoped emitter and 234 fA/cm2 for a highly doped emitter.The application of a PEALD Al2O3 passivation layer tothe lowly doped emitter diffusions results in a J0,emitter of11 fA/cm2.

An excellent surface passivation of the n-type basewithout diffusions could also be achieved for both the SiO2

and PECVD SiNx passivation stack, as shown by Graneket al. [25], and for PEALD Al2O3 passivation layers asalso reported by Hoex et al. [26]. Different values havebeen determined for the saturation current density of theundiffused gap J0,gap for the n-type base with a resistivity of5 and 10V cm. For the ‘reference’ solar cell with the SiO2

and the antireflection PECVD SiNx passivation stack andlowly doped diffusions, a Voc limit of 657mV has beencalculated, whereas the application of an PEALD Al2O3

passivation layer leads to a Voc limit of 706mV. For the‘reference’ solar cell with highly doped diffusions and theSiO2 and PECVD SiNx passivation stack, a Voc limit of664mV has been determined.

3.4 Solar cell results In Table 4 the parameters of thecurrent–voltage curves of the ‘reference’ solar cell withlowly doped and highly doped diffusions are presented. Itcan be seen that the measured Voc of the 10V cm basematerial is in good agreement with the estimated Voc limit.For solar cells fabricated on the 5V cm base material, themeasured Voc is about 10mV lower than the estimated Voc

limit. However, an extraordinarily high Voc of 693mV couldbe achieved when a PEALD Al2O3 passivation layer isapplied on the rear side. It could also be observed that due

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to the adaptation of the base-type BSF and the emitterdiffusion, electrical shading losses, and losses associatedwith FCA could be minimized significantly. Therefore, anexcellent Jsc of about 40.8mA/cm2 could be achieved for the‘reference’ solar cell when a SiO2 and an antireflectionPECVD SiNx passivation stack are applied on the rear side.In this case, the Jsc gain is almost 1mA/cm2 when comparedthe ‘reference’ solar cell with highly doped diffusions. Thequite low fill factor of only 77.1% for the ‘reference’ solarcell with a SiO2 and an antireflection PECVD SiNx

passivation stack limits the efficiency of the solar cell designto 20.3%. The low fill factor can be attributed to seriesresistance losses. The series resistance has been determinedby the comparison of the suns-Voc curve [29] and theilluminated current–voltage curve of the solar cell [30]and is about 0.7V cm2 for the ‘reference’ solar cell withlowly doped diffusions and a SiO2 and an antireflectionPECVD SiNx passivation stack. The series resistance of the‘reference’ solar cell with highly doped diffusions is about0.5V cm2 and mostly dominated by the base resistance dueto the lateral transport of majority charge carriers to the BSFdiffusions [4, 5]. For the solar cells with lowly dopeddiffusions the lateral transport of majority charge carriersstill seem to play an important role although the baseresistivity is higher than for the solar cell with highly dopeddiffusions.

Furthermore, it can be seen that for the ‘reference’ solarcell with a PEALD Al2O3 passivation layer on the rear side,the fill factor is even lower and has a value of only 71.2%.Therefore, the efficiency of the ‘reference’ solar cell with aPEALD Al2O3 passivation layer is 20.1%, limiting thepotential of the solar cell, although an extraordinarily highVoc and Jsc of 40.7mA/cm2 could be achieved with aPEALD Al2O3 passivation layer on the rear side. Sinceonly the passivation on the rear side is different, the fillfactor losses can solely be attributed to the application ofthe PEALD Al2O3 passivation layer. Suns-Voc measure-ments of the ‘reference’ solar cell with a PEALD Al2O3

passivation layer have shown that the pseudo fill factor isonly 76.4%. This indicates that the solar cell is parasiticallyshunted due to the floating junction [31] introduced bythe highly negatively charged dielectric Al2O3 layer [32]on the rear side of the n-type base. In this case thefloating junction seems not to be sufficiently isolated fromthe n-metal contact of the BSF. The effect of parasiticshunting is however not observed for the ‘reference’ solarcells with a SiO2 and an antireflection PECVD SiNx

passivation stack. In this case no inversion layer is inducedon the rear side of the n-type base as in case of the PEALDAl2O3 passivation layer.

For the ‘buried emitter’ solar cell with a PEALD Al2O3

passivation layer on the rear side, the fill factor is increasedfrom 71.2 to 77.0% compared to the ‘reference’ solar cell.In this case no floating junction is induced on the rear sidedue to the absence of an n-type base region, i.e. undiffusedgap. Thus, parasitic shunting for this solar cell design iseliminated.

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3.5 Discussion The findings gained by the numericaldevice simulations performed in this study show that theadaptation of the emitter diffusion and especially the base-type BSF diffusion for the ‘buried emitter’ solar cell areessential for improving Voc. The correlation between highlydoped and lowly doped diffusions is also observedexperimentally as presented in Table 4. For the ‘buriedemitter’ solar cell both the SiO2 and the antireflectionPECVD SiNx passivation stack and the PEALD Al2O3

passivation layer provide an excellent passivation ofthe base-type BSF diffusion. Therefore, the Voc could beincreased from 629 to 652mV for the ‘buried emitter’ solarcell with lowly doped diffusions and a SiO2 and anantireflection PECVD SiNx passivation stack. Also, with aPEALD Al2O3 passivation layer on the rear side the Voc

could be improved considerably. However, the high Voc

potential of the ‘reference’ solar cell with lowly dopeddiffusion profiles and an PEALD Al2O3 passivation layer onthe rear side could not be shown by the ‘buried emitter’ solarcell concept. It has been shown by the performed numericaldevice simulations that under open-circuit voltage con-ditions the surface passivation of the phosphorus-dopedbase-type BSF on the rear side of the ‘buried emitter’ solarcell is the key factor limiting Voc, see also Table 2. Hence,a further improvement in Voc can only be realized bysignificantly reducing the recombination rates on therear side of base-type BSF diffusion. The application of aPEALD Al2O3 passivation layer on boron-doped emitterdiffusions instead of overcompensating the emitter with ashallow phosphorus-doped diffusion has also been favouredby Benick et al. [33]. For solar cells with a PEALD Al2O3

passivation layer on boron-doped emitter diffusions, anextraordinary Voc of 703mV has been reported in the latterwork.

In conclusion, it can be stated that the adaptation of theemitter diffusion, and especially the base-type BSF diffusionfor the ‘buried emitter’ solar cell is essential in order tominimize the diffusion of majority charge carriers from theemitter into the base-type BSF and vice versa. However, dueto the presence of the base-type BSF on the rear side ofthe ‘buried emitter’ and the ‘passivated emitter’ solar celland the additional recombination in the bulk and at thesurface, lower Voc values have to be expected than forthe ‘reference’ solar cell when the emitter and the BSFdiffusions are separated and when a PEALD Al2O3

passivation layers is applied on the rear side. The Jsc of thebest ‘buried emitter’ solar cell with lowly doped diffusionson 5V cm base material is in the same range as for thebest ‘buried emitter’ solar cell with highly doped diffusionprofiles on 10V cm base material. The quite low fill factor ofonly 77.0 and 76.4% for the ‘buried emitter’ solar cell with aSiO2 and an antireflection PECVD SiNx passivation stackand a PEALD Al2O3 passivation layer limit the efficiency ofthe solar cells to only 20.4 and 20.5%, respectively. Furtherinvestigations have to be performed in order to increasethe fill factors for the ‘reference’ solar cell and the ‘buriedemitter’ solar cell with lowly doped diffusions.



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4 Conclusions The performance of n-type back-contacted back-junction silicon solar cells where theboron-doped emitter diffusion on the rear side is locallyovercompensated by a phosphorus-doped base-type BSFdiffusion with highly doped and lowly doped diffusionprofiles have been analysed theoretically and experimen-tally. The results show that adapted diffusion profiles andreduced surface recombination velocities are essential inorder to increase the open-circuit voltage Voc and furtherimprove the short-circuit current density Jsc. Hence, adapteddiffusion profiles with different rear side passivation layersconsisting of a double layer of a thermally grown SiO2 and anantireflection PECVD SiNx as well as a single layer PEALDAl2O3 have been applied. The Voc of the solar cells with anovercompensated boron-doped emitter diffusion could beimproved experimentally from 629 to 652mV when lowlydoped diffusions are applied. For back-contacted back-junction silicon solar cells with an undiffused gap betweenemitter and BSF, an Voc of 693mV could be achieved for aPEALD Al2O3 passivation layer and lowly doped diffusionson the rear side. Electrical shading losses of solar cells with alowly doped base-type BSF diffusion could be minimizedand a significant reduction of losses caused by free chargecarrier absorption (FCA) in the highly doped emitter andBSF regions have been observed. It has been found thatfor solar cells with adapted diffusion profiles, not only theVoc but also the Jsc could be improved significantly, leadingto 40.9mA/cm2 for solar cells with a large noncollectingbase-type region and 41.4mA/cm2 for solar cells with anovercompensated boron-doped emitter diffusion and asmall noncollecting base-type region. For the latter solarcell concept electrical shading losses are minimizedsignificantly, allowing higher Jsc values to be obtained.

Acknowledgements The authors would like to thank allmembers of the Department Solar Cells – Development andCharacterization for their support and encouragement. A.Leimenstoll, F. Schatzle, S. Seitz, N. Konig and N. Kohn arerecognized for technical assistance and E. Schaffer is appreciatedfor solar cellmeasurements.W.Dopkins is gratefully acknowledgedfor carefully proofreading the manuscript. This work has beensupported by the German Federal Ministry for the Environment,Nature Conservation and Nuclear Safety under contract number0329849A ‘Th-ETA’.

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