Solar Energy Materials & Solar Cells 95 (2011) 1151–1156

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Large area 15.8% n-type mc-Si screen-printed solar cellwith screen printed Al-alloyed emitter

Sukhvinder Singh a,n, Frederic Dross a, Niels E. Posthuma a, Robert Mertens a,b

a IMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgiumb KU Leuven/ESAT, B-3001 Leuven, Belgium

a r t i c l e i n f o

Article history:

Received 23 August 2010

Received in revised form

18 December 2010

Accepted 23 December 2010Available online 4 February 2011

Keywords:

n-type mc-Si

Si solar cells

Al-alloyed emitter

Rear-junction

Screen-printed

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2010.12.043

esponding author.

ail addresses: Sukhvinder.Singh@imec.be, ssp

a b s t r a c t

A record efficiency of 15.8% (independently confirmed at Fraunhofer ISE calibration laboratory) is

reported on large area (120 cm2) n-type mc-Si rear junction Si solar cell. Minor modifications to the

industrial process for p-type, such as optimization of Al-alloyed screen-printed emitter and phosphorus

front surface field, led to an improvement in cell properties. Large improvement in short-circuit current

of the cell was possible by decreasing the cell thickness to 130 mm.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

In view of the large diffusion lengths achievable in n-typesilicon, owing to the minority carrier lifetimes up to the milli-second range, n-type silicon solar cells have attracted great atten-tion recently [1–4]. Other advantages of n-type silicon as comparedto p-type include the absence of light-induced degradation, due toabsence of boron–oxygen pairing, and its higher tolerance tocommon impurities and high-temperature processing [5–7]. Inaddition to the above advantages, the industrial trend of reducingcell cost by going to thinner wafers allows the use of the cost-effective, highly doped n-type multi-crystalline (mc) wafers forrear junction cells. In spite of these advantages, n-type cells are notcommonly produced in industry. The high segregation coefficientmakes it difficult to stabilize the industrial fabrication of n-typematerial; but the main reason reported for this dominance ofp-type substrates is the historical developments of p-type solarcells in space applications which were found to be more resistantto radiation [2,8]. This knowledge of p-type space cells led to laterdevelopments in p-type industrial cell structures for terrestrialapplications. Recently, the superior behavior of n-type materialwas beneficially used and excellent cell parameters were obtainedon lab-scale [9] as well as on large area using BBr3 diffusionsources [10]. The most efficient industrial cells and panels so farare using n-type material [11,12]. The main obstacle for industrial

ll rights reserved.

ruthi@gmail.com (S. Singh).

implementation of n-type cells is the unavailability of n-typewafers of good quality. Within the European Project Lab2Line, aEuropean private–public consortium has worked on the develop-ment of an industrial process for the production of large arean-type multi-crystalline cells (4100 cm2) with high efficiency. Asa part of this project, a process involving minimal modifications toa standard p-type baseline process has been investigated. Thenecessary changes to the standard p-type baseline process includea pre-gettering step, an optimization of the firing profile, the front-surface-field diffusion, the paste quantity and the content for frontand rear contacts. However, in rear-junction technologies in gen-eral and in n-type Al-emitter cells in particular, the materiallifetime limits the short-circuit current unless the diffusion lengthis longer than 3 times the thickness of the substrate. This meansthat either the wafers with sufficiently high lifetime or very thinwafers should be used for rear-junction solar cells. In an earlierwork, this limitation has been reported on thick substrates [13].

In this paper, the process optimizations to achieve increased solarcell conversion efficiencies for rear-junction cells on n-type mc-Sisubstrates, such as pre-gettering and optimization of Al emitter willbe described. First, the experimental details will be outlined, followedby a detailed discussion of the obtained results. Finally, the mostimportant conclusions will be summarized in the last paragraph.

2. Experimental details

In this work, an industrially applicable process for screen-printed, rear-junction, n-type mc-Si cells has been developed.

Ag screen printed contacts

n+ FSF SiNx coatingn-type mc Si base

p+ Al emitterp+ Al emitter

Sputtered / evaporated + Screen printed Al-contact

Fig. 1. Schematic cross-section of a screen-printed n-type Al-emitter rear junction

solar cell on multi-crystalline silicon.

Fig. 2. Efficiency of the cells versus the screen-printed Al paste quantity at

different firing temperatures.

Fig. 3. (a) Short circuit current and open circuit voltage of the cells and (b) fill

factor and efficiency of the cells versus the screen-printed Al paste quantity for

cells fired at 990 1C.

S. Singh et al. / Solar Energy Materials & Solar Cells 95 (2011) 1151–11561152

Square 125 mm�125 mm, 325 mm-thick n-type multi-crystallinewafers with a resistivity of 1–3 O cm were used. The wafers weremade available to the Lab2Line consortium from an ingotsupplied by ENI Technology. A schematic illustration of the cellstructure of such a rear junction solar cell is shown in Fig. 1.

The process sequence starts with acidic texturing. It isfollowed by diffusion mask deposition, POCl3 diffusion for frontsurface field, diffusion mask removal, PECVD SiNx antireflectivecoating and screen-printing and subsequent co-firing of front andrear contacts. It is noticeable that the process flow is particularlysimilar to that of standard p-type industrial silicon solar cellprocessing. An additional phosphorus-gettering step was alsoapplied, just after texturing on some of the wafers, to improvethe minority carrier lifetime. This step consists of POCl3 diffusionat 845 1C with a total process time of 1 h.

To optimize the quality of the Al emitter, three types of screenswith different mesh sizes have been used to screen-print variousquantities (4.5–8 mg/cm2) of aluminium paste. In order to printeven higher quantities, up to 16 mg/cm2, the printing was carriedout twice. The printed paste was dried in a belt furnace at300 1C [14]. The wafers were weighed before printing of Al andafter paste drying for careful assessment of the past quantity. Thecells were subsequently fired at temperatures ranging from 950to 990 1C at two different belt speeds. The emitter formation athigher quantity of Al paste (49 mg/cm2) was studied in detail bypreparing cells with 9.5 and 16 mg/cm2 of Al paste. CommercialAg and Al screen-printing pastes from DuPont, PV146 and PV333were used in the experiments, respectively.

The bulk lifetime of wafers was measured using Quasi SteadyState Photoconductance (QSSPC) technique [15]. For these mea-surements, the wafers were passivated using PECVD SiNx depositedon both sides of the wafers. The bulk lifetime of the wafers wasmeasured assuming negligible surface recombination velocity. TheI–V curves of the cells were tested under one-sun illumination witha light intensity of 100 mW/cm2. The solar simulator was cali-brated using the reference cells tested at European Solar TestInstallation (ESTI) laboratories of the Joint Research Centre of theEuropean Commission, sited in Ispra (Italy).

Detailed characterization of the emitter structure has beendone by inspection using a scanning electron microscope (SEM).The samples for SEM were prepared by dicing 10 mm-wide stripsfrom the edge of the samples. Each strip was studied at twoplaces: at the edge and in the middle of the samples.

A few rear-junction cells have been realized on 130 mm-thinn-type mc-Si wafers with a resistivity of 2.5 O cm, obtained bymechanical polishing of the 330 mm-thick substrates.

3. Results and discussion

In Fig. 2, the effect of quantity of screen-printed aluminiumand firing temperature on the cell efficiency is shown. The exact

properties of the aluminium emitter are strongly related to theapplied quantity of Al paste and the drying and firing conditions.An increase in firing temperature results in increased cell effi-ciency for all quantities of printed aluminium paste. In addition,the efficiency increases monotonically with increasing quantity ofprinted paste from 4.5 to�9.5 mg/cm2. However, with furtherincrease in the quantity of printed paste up to 16 mg/cm2 a smalldecrease in efficiency is observed. The detailed effect of quantityof the printed paste on I–V parameters of the cells for one of thefiring temperatures is shown in Fig. 3. Fig. 3(a) shows the short-circuit current density (Jsc) and open-circuit voltage (Voc) as

Fig. 5. Cross-sectional SEM pictures of the Al-alloyed junction of the samples with Al qu

(g) and (h) 16 mg/cm2.

Fig. 4. Shunt resistance of the cells versus the screen-printed Al paste quantity at

different firing temperatures.

S. Singh et al. / Solar Energy Materials & Solar Cells 95 (2011) 1151–1156 1153

functions of the quantity of aluminium paste for the cells fired attemperature of 990 1C. The short-circuit current density shows asmall decrease with increasing Al paste quantity. Meanwhile, alarge increase of �80 mV in Voc is seen with increasing quantityof printed Al paste from 4.5 to 9.5 mg/cm2. At higher quantities ofprinted Al paste, Voc nearly saturates. The fill factor (FF) of thecells (depicted in Fig. 3(b)) also shows a very strong dependenceon the Al paste quantity. The FF nearly increases linearly from�53% to 75% with increasing Al paste quantity from 4.5 to9.5 mg/cm2. With a further increase in Al paste to 16 mg/cm2,the FF decreased to�71%. As seen from Fig. 3(b), the efficiencyfollows a trend similar to that of the fill factor. Interestingly, theshunt resistances of the corresponding samples, plotted in Fig. 4also displayed a similar trend and are considered to be the maindriver for the evolution of efficiency.

As mentioned before, in order to understand the featuresdescribed above, the cross-sections of the samples were studiedby scanning electron microscopy (SEM). The SEM images ofAl-alloyed junctions of the samples with Al quantity of 4.5, 7.5,9.5 and 16 mg/cm2 are shown in Fig. 5. The correspondingthicknesses of printed paste for these samples seen from low

antity of (a) and (b) 4.5 mg/cm2, (c) and (d) 7.5 mg/cm2, (e) and (f) 9.5 mg/cm2 and

S. Singh et al. / Solar Energy Materials & Solar Cells 95 (2011) 1151–11561154

magnification SEM images (not shown) were measured to beapproximately 30, 50, 70 and 110 mm, respectively. The SEMimages of the Al junction for very small Al paste quantities(4.5 mg/cm2) are shown in Fig. 5(a) and (b). The junction depthfor this case is 3–7.5 mm with large variations. It is suspected thatdiscontinuities in the junction lead to shunt paths, which werealso seen from the degradation of the shunt resistance (Fig. 4).Such a low quantity of printed Al (4.5 mg/cm2) is probably notsufficient for making a uniform continuous junction. Similareffects have been reported for p-type cells with screen-printedAl as back surface field (BSF). It has been reported that to achievea uniform BSF, it is necessary to have a continuous liquid Al–Silayer on the surface at peak firing temperature, usually requiringa minimum of 6 mg/cm2 deposited amount of aluminumpaste [16]. This explains the very low values of open circuit

Fig. 6. Electrical parameters of cells fired at peak firing temperature of 990 1C and

belt speeds of 100 and 120 IPM.

Fig. 7. Cross-sectional SEM pictures of the Al-alloyed junction of the samples with

voltage and fill factor measured on the cells with such lowAl-paste quantity. Fig. 5(c) and (d) shows the SEM images of thesample with 8 mg/cm2 (�50-mm-thick) printed Al. Although thejunction depth for this case is increased (approximately6–11 mm), large non-uniformities in junction depth can still beobserved. Nonetheless, compared to the previous case, the cellsshow large improvements in fill factor and open-circuit voltage.The SEM images for the cell with 9.5 mg/cm2 (70 mm) of screen-printed Al are shown in Fig. 5(e) and (f). It is observed that thishigh quantity of printed Al leads to highly uniform, flat and deepjunction formation. The junction depth in this case varies approxi-mately within 9–11 mm. The deep and uniform junction, corre-lates well with the highest fill factor and open-circuit voltage. TheSEM images of the junction for the highest Al paste quantity(16 mg/cm2) show a decrease in junction depth. The junctiondepth in this case is between 4.5 and 8 mm. Although the junctionin this case still remains flat, the decrease in junction depthindicates that a reduced amount of Al paste took part in thealloying process, as explained in [16]. This results in a smalldecrease in the efficiency of the cell, mainly due to the decrease infill factor (Fig. 3). The reason for this lower interaction at higherpaste quantity deserves investigation. It has been reported inliterature that high paste quantities need special care whendrying, otherwise cavities can form in the paste resulting inshunted cells [14,16]. To investigate the correlation betweenquantity of printed paste and the firing conditions, the cells withoptimum quantity (9.5 mg/cm2) and higher quantity (16 mg/cm2)of Al paste were prepared. Keeping the other processing para-meters the same, these cells were fired at two different beltspeeds. The I–V data of these cells are shown in Fig. 6(a)–(d). Forthe cells with 9.5 mg/cm2 printed Al, both Jsc and Voc show highestvalues for highest belt speed. However, cells with higher quantityof paste show best performances when fired at lower belt speed.The fill factor of the cells is (slightly) higher in both cases at lowerbelt speed. The efficiency of the cells also follows similar trend asthose of Jsc and Voc. With a lower belt speed, the cells have moretime at moderate temperature to dry effectively, before reachingthe high-temperature zone, and reaching the critical temperatureof Si–Al alloying. As already discussed, it has been observed that anon-sufficient drying resulted in the formation of cavities insidethe paste during firing and shunting of the cell [14,16]. A smalldecrease in efficiency observed for higher quantity of printedpaste for high belt speeds could be recovered by firing the cells atreduced belt speed. This was also confirmed by the SEM images ofthe Al junction formed by printing 16 mg/cm2 of aluminium pasteand firing at lower belt speed, as shown in Fig. 7. These imagesshow a highly uniform, flat and deep junction with a junctiondepth of around 10 mm, similar to the images of the junctionformed by using 9.5 mg/cm2 Al paste and fired at high speed(Fig. 5(e) and (f)).

As seen from the above, the efficiency of the cells has beenlimited due to low short circuit current of the cells. This is mainly

Al quantity of 16 mg/cm2 fired at 990 1C at belt lower belt speed (100 IPM).

S. Singh et al. / Solar Energy Materials & Solar Cells 95 (2011) 1151–1156 1155

due to the relatively thick wafers (325 mm) and the relatively lowsubstrate quality, with a resistivity of around 1.5 O cm, showing abulk lifetime of approximately 150 ms, as determined by QSSPCmeasurements. In order to investigate the cell parameters onsubstrates with increased resistivity and bulk lifetime, first, a fewcells were realized on higher quality wafers (2.8 O cm, withbulk lifetimes of 300 ms). The best electrical characteristics ofa cell processed under optimized conditions are listed in Table 1.A large improvement in short-circuit current of the cells isobserved as compared to the cells processed with low-qualitywafers.

Since the thickness of the wafers still limits the short circuitcurrent of the cells, a few 2.5 O cm wafers were mechanicallyground to reduce the starting wafer thickness from 325 to around130 mm. Further, a POCl3-gettering step was carried out to improvethe lifetime of the wafers before processing. Since, these mc-Siwafers were polished in the process of mechanical grinding whichwas done to reduce the wafer thickness, the cells were plasmatextured at the front [17], instead of the wet-chemical texturingprocess normally applied. An Al paste quantity of 16 mg/cm2 hasbeen printed on the rear. In view of the much lower substratethickness, the cell was fired at a lower peak firing temperature of880 1C, as compared to 990 1C for 325 mm cells. The I–V data of thebest cell processed on these 130 mm, 2.5 O cm wafers, are also listedin Table 1. The cell shows a large improvement in short-circuitcurrent, proving that the thickness of the substrate (i.e. the bulkrecombinations) was indeed limiting the short-circuit current. Thisstructure achieved an independently confirmed efficiency of 15.8%.This efficiency is, to the best of our knowledge, the highest efficiencyreported on large-area (121 cm2) mc-Si with an aluminum-alloyedemitter. Importantly, this record efficiency has been achieved onmoderately doped wafers (2.5 O cm) as compared to very lowlydoped (and therefore high quality) wafers (410 O cm) reported inliterature [18,19]. The second best efficiency reported on similar cell

Table 1Best cell results.

Wafer

thickness (mm)

Wafer resistivity

(O cm)

Cell area

(cm2)

Jsc

(mA/

cm2)

Voc

(mV)

FF

(%)

Z (%)

325 2.8 156 28.0 613.5 76.9 13.2

130 2.5 120.03 32.9 621.0 77.1 15.75a

a independently confirmed at Fraunhofer ISE calibration laboratory

Fig. 8. Spectral response and reflectance of the 15.8% efficient cell.

structure in literature is from Buck et al. [18], with an efficiency of14.8% on a 25 cm2 wafer (200 mm-thick) and with a bulk resistivityof 50 O cm. External quantum efficiency (EQE), internal quantumefficiency (IQE) and reflectance of the above cell are plotted in Fig. 8.It may be noted, that further improvement in the short circuitcurrent is possible by improving the blue response of the cell. Ashallower front-surface field, together with lower surface concen-tration are options to reduce the front-surface recombinationvelocity. This will require investigation of better screen-printingpastes to enable contact formation on lowly doped surfaces.

The cells realized on thin wafers show more bowing, ascompared to the cells prepared on 330 mm wafers. This bowingcan be possibly reduced by either reducing the amount of Alscreen-printed paste or, if the mechanical stability of the siliconwafer permit it, by applying cooling thermal shock to the finishedcell as reported in literature [20].

4. Conclusions

An industrial process with minimal changes to baseline p-typeprocess has been developed for n-type mc-Si cells. Improving theuniformity of the Al-emitter, by optimizing the Al paste quantity,results in large improvements in fill factor and open-circuitvoltage of the realized cells. Improvement in short circuit currentwas made possible by decreasing the cell thickness, resulting in arecord cell efficiency of 15.8%. These results clearly show thepotential of the process developed for industrial applicability ton-type mc-Si cells, which is also supported by the trend ofdecreasing cell thickness to reduce cell costs. Further improve-ments in the cell properties are possible by improving the short-circuit current of the cells. A shallower front surface field isexpected to result in a further increase in short circuit currentdensity of these n-type multi-crystalline rear-junction solar cells.

Acknowledgements

The authors acknowledge the help of Srisaran Venkatachalamfor SEM pictures and Didier Dehertoghe and Andre Janssens fortheir help during processing. This work is partially supportedby the European Commision in the Sixth Framework Program‘‘lab2line’’ project.

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