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Research Papers

A Novel Solar Cell Shallow Emitter Formation Process by Ion-Implantation and Dopant Modulation Through Surface Chemical Etching

[+] Author and Article Information
Wei-Lin Yang

Institute of Photonics Technologies,
National Tsing Hua University,
Hsinchu 300, Taiwan
e-mail: s9866803@m98.nthu.edu.tw

Po-Hung Chen

Institute of Photonics Technologies,
National Tsing Hua University,
Hsinchu 300, Taiwan
e-mail: h85245690@yahoo.com.tw

Kun-Rui Wu

Institute of Photonics Technologies,
National Tsing Hua University,
Hsinchu 300, Taiwan
e-mail: coxph0100442@gmail.com

Likarn Wang

Institute of Photonics Technologies,
National Tsing Hua University,
Hsinchu 300, Taiwan
e-mail: lkwang@ee.nthu.edu.tw

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received August 26, 2016; final manuscript received June 5, 2017; published online August 31, 2017. Assoc. Editor: Wojciech Lipinski.

J. Sol. Energy Eng 139(6), 061001 (Aug 31, 2017) (6 pages) Paper No: SOL-16-1385; doi: 10.1115/1.4037378 History: Received August 26, 2016; Revised June 05, 2017

Ion-implantation is an advanced technology to inject dopants for shallow junction formation. Due to the ion-induced sputtering effect at low implant energy where dopants tend to accumulate at the silicon surface, the excess ion doses can be easily removed via a surface chemical wet etching process. By taking advantage of the dose limitation characteristic, we proposed a novel method to form shallow emitters with various dopant densities. Two integration flows have been investigated: (1) wet etch after implantation before junction anneal and (2) wet etch after implantation and junction anneal. The two integration flows observed a difference in the density of doping impurities during the thermal process, which is related to the substrate recombination rates. Selective emitter (SE) structures with the two types of integration flows were characterized. Comparing the blanket emitter and SE structures with two types of etching methods, the device with wet etch before annealing process achieved the best effective carrier lifetime of 53.05 μs, which leads to a higher short circuit current density. Hence, this SE cell demonstrated a better blue response and shows an improvement in the conversion efficiency.

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Figures

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Fig. 1

Diagram of SE formation where the lightly doped regions received a chemical etching step (group 1). To form the SE structure with group 2 process, an exchange of step 6 and step 7 was proceeded.

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Fig. 2

(a) Optical micrograph of solar cell with silver contact on top of the trench and (b) morphology of front side cell by scanning electron microscopy measurement

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Fig. 3

The variation of sheet resistance by different solutions of 10 mL HF, 1 L HNO3, and 2.2 L H2O etched before annealed (group 1) and 10 mL HF, 1 L HNO3, and 1.5 L H2O etched after annealed process (group 2)

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Fig. 4

Effective carrier lifetime versus excess carrier density for two groups of samples were etched before and after annealing process, respectively. Each group contains two values of sheet resistance 69.52 Ω/□ and 101.2 Ω/□.

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Fig. 5

Auger-corrected inverse effective lifetime versus excess carrier density for the samples of nonetched and etched before and after annealed process with sheet resistance of 63.72 Ω/□ and 102.59 Ω/□, respectively

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Fig. 6

SIMS profile of implant wafers at a dose of 5 × 1015 cm−2 with (solid line) and without etching treatment (dashed line) followed by the annealing process

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Fig. 7

Effective carrier lifetime of blanket emitter as heavily doped and SE as lightly doped by chemical etching

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Fig. 8

(a) EQE and (b) reflectance measurements as a function of blanket emitter and SE with two types of etching methods

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