at SciVerse ScienceDirect

Renewable Energy 42 (2012) 95e98

Contents lists available

Renewable Energy

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

Exceeding 19% efficient 6 inch screen printed crystalline silicon solar cells withselective emitter

Eunjoo Leea,*, Kyeongyeon Choa, Dongjoon Oha, Jimyung Shima, Hyunwoo Leea, Junyoung Choia,Jisun Kima, Jeongeun Shina, Soohong Leeb, Haeseok Leea

aR&D center, Solar cell division, Shinsung Solar Energy, 404-1 Baekhyeon-dong, Bundang-gu, Seongnam-si, Gyeonggi-do 463-420, Republic of KoreabDepartment of Electronic Engineering, Sejong University, Seoul 143-747, Republic of Korea

a r t i c l e i n f o

Article history:Received 10 December 2010Accepted 8 September 2011Available online 25 September 2011

Keywords:Silicon solar cellHigh efficiencySelective emitterScreen printing

* Corresponding author. Tel.: þ82 31 7889 312; faxE-mail address: ejlee@shinsung.co.kr (E. Lee).

0960-1481/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.renene.2011.09.010

a b s t r a c t

The process conditions for high efficiency industrial crystalline Si solar cells with selective emitter wereoptimized. In the screen printed solar cells, the sheet resistance must be 50e60 U/sq. because of metalcontact resistance. But the low sheet resistance causes the increase of the recombination and blueresponse at the short wavelength. Therefore, the screen printed solar cells with homogeneous emitterhave limitations of efficiency, and this means that the selective emitter must be used to improve cellefficiency. This work demonstrates the feasibility of a commercially available selective emitter process,based on screen printing and conventional diffusion process. Previous work, we announced about 18.5%efficient selective emitter solar cell by variation of heavy emitter pattern width. Now, we improved cellefficiency from 18.5% to 19% by transition of heavy emitter pattern and shallow emitter doping condi-tion. A maximum cell efficiency of 19.05% is obtained on a 156 mm � 156 mm crystalline silicon solarcell.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Selective emitter is a prospective technique to improve thecrystalline Si(c-Si) solar cell efficiency without a complex change ofconventional manufacturing process for screen printed c-Si [1]. Thisstructure combines the advantages of shallow and heavy emittersimultaneously in c-Si solar cells [2]. The shallow emitter hasa good surface passivation and blue response at the short wave-length, but a high contact resistance. On the other hands, the heavyemitter has a good contact resistance [3].

Until now, a few papers have reported on high efficient selectiveemitter c-Si solar cells, where they have focused on the new processto form the selective emitter structure, and new contact materials.Most of the selective emitter processes require expensive extramasking, etching steps, and a double diffusion process makingselective emitters not cost effective [4]. Previous work, we studiedabout screen printed solar cell with selective emitter over 18%efficiency [5,6]. In this paper, we have fabricated the selectiveemitter c-Si solar cells with an area of 156 mm � 156 mm by usinga conventional diffusion process (POCl3) and screenprinted contact.Here, to improve Voc of c-Si solar cells, the correlations of heavy

: þ82 31 7889 584.

All rights reserved.

emitter area and shallow emitter doping concentration are clari-fied, and consequently a high efficient, hw19%, c-Si solar cells aredemonstrated.

2. Experiments

2.1. Cell process

The substrate is boron doped p-type solar grade CZ-monocrystalline silicon wafer. Its resistivity is 0.5e3 U�cm, size156 mm � 156 mm (pseudo square) and thickness 200 mm. Thefabrication process is shown in Fig. 1. After surface texturing byalkali solution, the oxide layer is grown on 1050 �C for masking ofphosphorous heavy diffusion. To form the selective emitter struc-ture, the etch paste(Solar Etch BRS type 40, Merck corp.) is appliedand patterned for phosphorous heavy diffusion. The pattern(110 mmgrid line openingwidth of screenmask) is formed by screenprintingmethod. POCl3 is diffused heavily on the surface (20e30 U/Eff.) andthen masking oxide totally removed. Then POCl3 diffused lightlyabout 80e100 U/Eff. on the surface. A sheet resistance is estimatedby4-point probe. Following diffusion, the phosphorous silicate glassis etched off. SiNx passivation is deposited on the front side byplasma enhanced chemical vapor deposition (PECVD). Metal

Fig. 1. Process sequence.

Fig. 3. The Sun-Voc results as a function of the heavy emitter width.

E. Lee et al. / Renewable Energy 42 (2012) 95e9896

contacts (Ag-front contact, Al-back contact) are screen-printed andco-fired.

2.2. Heavy emitter pattern formation

The heavy emitter area is patterned and opened about 1.3 times,1.8 times and 2.2 times as comparedwith themetal width by screenprinting method. Fig. 2 (a), (b), (c) are 1.3 times, 1.8 times and 2.2times indicate the patterned heavy emitter width in comparisonwith front metal width, respectively. The used finger width ofscreen mask is 110 mm, 150 mm and 200 mm, respectively.

3. Results and discussion

We optimize the heavy emitter pattern and shallow emittersheet resistance condition.

3.1. Heavy emitter pattern optimization

The percentage of the heavy emitter area of the surface of waferis about 11.7%, 15.2% and 17.5% in (a), (b), and (c) respectively. Theportion of the metal area relative to the surface of the wafer is 9.4%.Actually, the exposed heavy emitter area on the surface of thewaferis respectively about 2.3%, 5.8% and 8.1%. Each percentage of theheavy emitter on the surface of the wafer relates to an increase ofrecombination, and with it variations in Voc occur.

Fig. 2. Heavy emitter pattern with

To confirm the accuracy of the Voc data as a function of a heavyemitter area, the Sun-Voc measurement was carried out. The Sun-Voc method measures the Voc by using the illumination intensitywithout using the Jsc of the solar cell. By using a flash lampwhich isturned off slowly, a Sun-Voc measure of Voc, as a variation of illu-mination intensity, is obtained [7].

Fig. 3 is the Sun-Voc measurement results as a function ofa heavy emitter width. This shows that the Voc is 635 mV at the11.7% heavy emitter portion, and it means an improvement of 9 mVof Voc, in comparison with the reference cell (homogeneousemitter cell: RSheetw50 U/Eff.). In addition, the 5 mV of Voc hasbeen improved as a function of the heavy eimtter area. The Vocincreases linearly as the heavy emitter portion decreases from 17.5%to 11.7%, and this means that the Voc can be improved with theshallow emitter portion being considered as a series resistance ofthe selective emitter c-Si solar cell.

As the shallow emitter area increases, a decrease in therecombination rate increases absorption of the blue light on thesurface of the solar cells. The blue light has a short wavelength,a high absorption coefficient and is absorbed close to the frontsurface. If the front surface has many highly doped regions, thegenerated minority carrier cannot be effectively collected at thefront surface by recombination and it leads to a decrease of Isc.

Fig. 4 shows the IQE and reflectance measurements as a func-tion of the heavy emitter area. This shows the variation in thequantum efficiency in the short wavelength region. First, theselective emitter solar cells indicate an outstanding quantumefficiency in the short wavelength in comparison with the refer-ence cell (the homogeneous emitter solar cell). Despite the lowrecombination, the solar cell which has the 11.7% heavy emitterportion, has a lower quantum efficiency than the other selectiveemitter solar cells at the 400e600 wavelength. The reason for thisis the wafer texture size. As shown by the reflectance, since thereference and the 11.7% heavy emitter portion have a largerpyramid size than the other selective emitter solar cells, they havea higher reflectance and with the higher reflectance comesa decrease in the quantum efficiency. But when comparing thereflectance difference and the quantum efficiency difference, if thesolar cell which has the 11.7% heavy emitter portion has a similar

compared with metal width.

Fig. 4. IQE (Internal Quantum Efficiency) and Reflectance measurements as a functionof the heavy emitter area.

Table 1IeV characteristics of screen printed silicon solar cells with selective emitterstructure.

Cell type Voc(mV)

Jsc(mA/cm2)

FF (%) Eff (%)

(a) 11.7% heavy 635.78 37.03 78.35 18.45(b) 15.2% heavy 633.22 37.02 78.28 18.35(c) 17.5% heavy 632.07 36.92 78.38 18.29Reference (Homogeneous cell) 627.77 36.46 78.39 17.94

Fig. 5. Life time mapping image.

E. Lee et al. / Renewable Energy 42 (2012) 95e98 97

texture size as the other selective emitter solar cells, it showsa higher quantum efficiency than the other selective emitter solarcells at the 400e600 wavelength.

The developed solar cell with selective emitter has achieved Voc635.78 mV, Jsc 37.03 mA/cm2, FF 78.35% and Eff 18.45% at the 11.7%heavy emitter portion solar cell. Table 1 shows the efficiency andparameters (Voc, Jsc and FF) of the reference cell (homogeneousemitter cell) and the selective emitter solar cells as a function of theheavy emitter area. The measurements condition is AM 1.5G,100mW/㎠and 25 �C. As shown in Table 1, although the solar cellswhich have the 17.5% heavy emitter portions are easy to align withthe metal contact in the heavy emitter region, this brings needlessdegradation of Voc. As shown, the 11.7% heavy emitter portion givesan improvement of Voc 8 mV, Jsc 0.57 mA/cm2, and Eff 0.5%, in

Table 2IeV characteristics of screen printed silicon solar cells with selective emitterstructure.

Cell type Emitter sheetresistance(U/Eff.)

Uniformity(%)

(d) 80e85 2.1(e) 90e95 3.2(f) 100e105 4.4(g) 110e115 7.0

comparison with the reference cell and gives an improvement ofVoc 3.7 mV, and Eff 0.1% as a function of heavy emitter size withoutincreasing the series resistance.

3.2. Shallow emitter optimization

To form the shallow emitter, a conventional POCl3 diffusion wasused. Table 2 shows the variation of sheet resistance of thephosphorous-diffused emitter. The emitter sheet resistance of80e120 U/Eff. was regarded as a suitable condition for achievinga high-efficiency silicon solar cell.The emitter sheet resistance wasmeasured by the four point probe method and uniformity wascalculated. As the emitter sheet resistance increases, dopingconcentration of surface was non-uniform. Further optimization ofthe uniformity of doping concentration increases the efficiencyimprovement.

To compare the variations in life time as a function of theshallow emitter doping concentration, the effective carrier life timemappings were carried out and are presented in Fig. 5. Themappings were carried out after the shallow emitter diffusion andthe SiNx passivation layer deposition.

The average life times for the (d), (e), (f) and (g) samples are,respectively, 15.66 ㎲, 17.01 ㎲, 24.05 ㎲ and 19.38 ㎲. The distri-butions of life time are 15e25 ㎲, and the life time of the homo-geneous emitter is 4.35㎲. In the absence of a passivation layer, the

Table 3IeV characteristics of screen printed silicon solar cells with selective emitterstructure.

Celltype

Voc(mV)

Jsc(mA/cm2)

FF (%) Eff (%)

(d) 636 38.0 77.03 18.65(e) 637 38.5 77.66 19.05(f) 638 38.1 76.72 18.66(g) 638 38.0 77.33 18.79

E. Lee et al. / Renewable Energy 42 (2012) 95e9898

more the shallow emitter area decreases, the greater the increase inthe life time.

The developed solar cell with selective emitter has achieved Voc637 mV, Jsc 38.5 mA/cm2, FF 77.66% and Eff 19.05% at cell with the90e95 U/Eff. shallow emitter doping concentration. Table 3 showsthe efficiency and parameters (Voc, Jsc and FF) of the selectiveemitter solar cells as a function of the shallow emitter dopingconcentration.

4. Conclusions

The selective emitter solar cell process for industrial productionline was optimized. Until now, a few papers have reported on highefficient selective emitter c-Si solar cells, where they have focusedon the new process to form the selective emitter structure, and newcontact materials. In this paper, we have fabricated the 6-inchselective emitter c-Si solar cells by using a conventional diffusionprocess (POCl3) and screen printed contact. Experimentally,shallow doped sheet resistances have been varied between 80 and120 U/Eff. Here, to improve efficiency of c-Si solar cells, the heavyemitter pattern and shallow emitter doping conditions are clarified,and consequently a high efficient, Eff.w19%, c-Si solar cells are

demonstrated. Furthermore, this technique has an advantage ofmass production application.

Acknowledgement

This work was partially supported by Establishment of Footholdin Green-semiconductor Industry under the Chungcheong LeadingIndustry Office.

References

[1] Harald Kerp, Jalal Salami, Gorka Bueno, Knuth Albertsen, Juan Carlos Jimeno,Aziz Shaikh, et al. 15th International Photovoltaic Solar Energy Conference;2005.

[2] Misiakos K, Lindholm FA. Solar Cells 1986;17:29.[3] Bähr M, Dauwe S, Mittelstädt L, Schmidt J, Gobsch G. 19th EU-PVSEC; 2004.[4] Munzer KA, Holdermann KT, Schlosser RE, Sterk S. Proceedings of 16th EPVSEC,

Glasgow, 2000. pp. 1185e1188.[5] Eunjoo Lee, Hyunwoo Lee, Ilhwan Kim, Junyoung Choi, Dongjun Oh, Jimyung

Shim, et al. 19th PVSEC; 2009.[6] Kyeong-Yeon Cho, Il-Hwan Kim, Dong-Joon Oh, Ji-Myung Shim, Eun-Joo Lee,

Hyun-Woo Lee, et al. 35th IEEE PVSC; 2010.[7] Andres Cuevas, Jason Tan. Solar Energy Materials & Solar Cells 2009;93:

958e60.