Microelectronic Engineering 110 (2013) 418–421

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Microelectronic Engineering

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

Refractive index graded anti-reflection coating for solar cells basedon low cost reclaimed silicon

Yufei Liu a,⇑, Owen James Guy a, Jash Patel b, Huma Ashraf b, Nick Knight b

a College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, UKb SPTS Technologies Limited, Ringland Way, Newport NP18 2TA, UK

a r t i c l e i n f o

Article history:Available online 14 March 2013

Keywords:Anti-reflection coating (ARC)Graded refractive indexReclaimed siliconSolar cells

0167-9317/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.mee.2013.03.003

⇑ Corresponding author. Tel.: +44 1792 602816; faxE-mail address: Y.Liu@swansea.ac.uk (Y. Liu).

a b s t r a c t

To meet the demands of the high efficiency and low cost of the photovoltaic industry, it has focusedattention on more cost effective silicon cells and cells with a significantly lower energy payback time.In this study, silicon substrates, reclaimed from the integrated circuit industry via polishing and etchingtreatments, are used after the wet etching process for the surface texturing. An advanced refractive index(RI) graded anti-reflection coating (ARC) is developed based on the stacked depositions of a group of multisilicon nitride nano layers, using an SPTS Plasma Enhanced Chemical Vapor Deposition (PECVD) system.Compared to the cell with single RI ARC, the reclaimed silicon wafers with the novel graded RI ARC haveshown a significantly enhanced cell performance. The short circuit current, open circuit voltage, max out-put power, and fill factor were increased by 7.16%, 4.17%, 30.1% and 15.6%, respectively, based on the lowcost reclaimed silicon substrates.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Since the first crystalline silicon solar cell, with a solar-energyconversion efficiency of 6%, was developed at Bell laboratories in1950s [1], the conversion efficiency of the solar cell has nowreached over 41.1% with current-matched triple-junction units un-der concentrated sunlight. This has been further increased bySHARP to 43.5% by 2012 [2,3]. Based on the huge demands forrenewable energy, the photovoltaic market is expected to growrapidly within the next 20 years. Today’s solar cells market annu-ally exceeds $9 billion worldwide and silicon cells account for90% of today’s solar electricity [1,2].

Growth of photovoltaic industry has focused attention on morecost effective silicon cells and cells with a significantly lower en-ergy payback time. An 8-in. silicon wafer, reclaimed from the inte-grated circuit industry via polishing and etching treatments,provides an ideal substrate for solar cell products and could meetthe requirements of both high performance and low-cost. An anal-ysis of the energy payback time for a range of PV systems is shownin Fig. 1.

Reclaimed silicon wafers, supplied by Pure Wafer Plc, UK, weretreated with the low cost wet etching process for the surface tex-turing, the technology of which could reduce the surface reflectionby giving the light multiple chances to enter the cell while enhanc-ing the average path length for capturing the light within cell by

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total internal reflection [4]. Due to the different etch rates on crys-tal silicon planes of (100) and (111) in etch solutions, such as KOH,randomly distributed pyramids of intersecting (111) planes wererevealed on the wafer surface. This allowed a larger portion ofthe incident light to be coupled into the wafer, thus the total reflec-tance from the surface is strongly reduced [5]. Fig. 2 shows a scan-ning electron microscope (SEM) image of the textured surface of areclaimed silicon substrate.

2. Refractive index graded ARC

SPTS PECVD system (SPTS Technologies Ltd., UK) is used to de-posit a wide range of inorganic and organic, doped and un-dopedfilms for a wide range of applications in Photonics, CompoundSemiconductors, MEMS and Advanced Packaging applications. Itoffers independent adjustment of plasma chemistry, RF frequency,substrate bias and wafer temperature for precise control of filmproperties such as refractive index, hardness and internal stress.The process gas showerhead has been optimised to offer market-leading deposition rates and cross-wafer uniformity.

Graded RI silicon nitride (SiN) ARCs were deposited on texturedreclaimed silicon substrates, using an SPTS PECVD system. The RIwas adjusted by varying the ratio of ammonia (NH3) and silane(SiH4) process gases. Graded RI SiN ARC multilayers were depos-ited in situ using a multi-step deposition process. Fig. 3 showsthe SEM image of the textured silicon surface after the stackedSiN ARC deposition. Photovoltaic cells were fabricated using theARC coated silicon wafers by printing a silver grid contact on the

Fig. 1. Energy payback time for a range of PV systems.

Y. Liu et al. / Microelectronic Engineering 110 (2013) 418–421 419

ARC coated cell surface and printing an Al contact on the back sideof the silicon substrate.

The film properties of the deposited SiN ARCs have also beenstudied, including the deposition rate, uniformities, RI and RIrange, stress, and the Fourier Transform Infrared (FTIR) for thequalitative studies of the Si–H bond content.

3. Test results and discussions

200 mm reclaimed silicon wafer substrates have been used forthis study. With the standard SPTS high frequency (13.56 MHz)SiN deposition process, the ARC film was obtained while the depo-sition rate is approximately 17.5 nm/min, RI is about 2.04, thedeposition uniformity is about ±1.9% and the tensile stress is about281 MPa. Table 1 compares standard single RI SiN layer (Sample A)and the novel graded RI ARCs (Sample B and C), which significantlyenhanced the cell performance. With Sample C, the short circuitcurrent, open circuit voltage, max output power, and fill factor

Fig. 2. SEM image of the textured surface of reclaimed silicon substrate.

were increased by 7.16%, 4.17%, 30.1% and 15.6%, respectively. Itshould be noted that doping and carrier mobility in the reclaimedsilicon wafers is not optimised, and thus the cell performance islimited by these factors. Nevertheless, it is clear that cells usingthe RI graded ARC perform better than those using a standard sin-gle RI ARC.

The deposited SiN ARCs have been further studied using a Ther-ma-Wave Opti-Probe wafer inspection tool. The properties, includ-ing deposition rate, uniformities, RI and RI range, have beenstudied with 49 points mapping measurements cross each wafer.Stress was calculated with pre and post wafer bow measurementsusing a Tencor Flexus 2320 dual wave length thin film stresssystem.

Details of the test results are shown in Table 2. Cross waferthickness uniformity is defined as

U% ¼ �100� max�min2�mean

� �

Fig. 3. SEM image of textured silicon after deposition of silicon nitride ARC.

Table 1Comparison test results of refractive index graded anti-reflection coatings.

Sample ID Nitride type Isc (mA) Voc (V) Pmax (mW) FF

A Normal SiN ARC 2639 0.48 571 0.45B RI graded ARC 1 2716 0.48 676 0.51C RI graded ARC 2 2828 0.5 743 0.52

420 Y. Liu et al. / Microelectronic Engineering 110 (2013) 418–421

Within-wafer RI (RI ½ range), was calculated as

RI12

Range ¼ �max�min2

The effective refractive indices of the SiN ARCs are important to theanti reflective performance, which is determined by Fresnel equa-tions. When the light meets the interface normal incidence (perpen-dicularly to the surface), the intensity of light reflected is given bythe reflection coefficient, R:

R ¼ n0 � ns

n0 þ ns

� �2

where n0 and nS are the refractive indices of the first and secondmedia, respectively. An inter layer could help to reduce the light

Fig. 4. (a) Qualitative FTIR results of the Si–H bond content and (b) qualitative relation

Table 2Film properties of refractive index graded anti-reflection coatings.

Sample ID Nitride type Dep rate (nm/min)

A Normal SiN ARC 17.5B RI graded ARC 1 19.1C RI graded ARC 2 14.7

reflection and the optimum value is given by the geometric meanof the two surrounding indices

n1 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffin0 � nsp

For the example of silicon (nsilicon � 3.50) and air (nair � 1.0), the the-oretically refractive index of the best ARC is n1 � 1.871, [6,7]. Fromtest results between Tables 1 and 2, it has been found that the RI ofSample C is about 1.865, which is similar to the theoretical optimumrefractive index of 1.871, and offered the best photoelectric conver-sion performance. Furthermore, it also performed the best film uni-formity and lowest film stress.The Si–H bond content in thedeposited SiN ARCs have been studied using a Nicolet Magna 560IR Spectrometer. The qualitative results of the Si–H bond content isshown in Fig. 4(a), and the qualitative relation between the photo-electric conversion performance (maximum power output: PMAX)and the qualitative Si–H bond content is shown in Fig. 4(b). The re-sults show that the lower Si–H bond content offered the better pho-toelectric conversion performance, which is because that Si–H bondsare responsible for the release of hydrogen that passivates the under-lying silicon, and the release of hydrogen from the silicon nitride layerduring the thermal process of firing contacts remains disputed as theimprovement mechanism for the carrier lifetime [8–10].

between the photoelectric conversion performance (PMAX) and Si–H bond content.

U% RI RI range Tensile stress (MPa)

±3.42 1.9903 ±0.0022 +243.6±6.16 2.1805 ±0.0056 +677.3±2.08 1.8652 ±0.0043 +210.0

Y. Liu et al. / Microelectronic Engineering 110 (2013) 418–421 421

4. Conclusions

A novel multi stacked SiN ARC is successfully developed withan in situ multi-step deposition process, using an SPTS PECVDsystem. The performances of the fabricated solar cells show that,with the graded RI SiN ARC, the short circuit current, open cir-cuit voltage, max output power, and fill factor were successfullyincreased by 7.16%, 4.17%, 30.1% and 15.6%, respectively. Usingthe low cost silicon substrates, reclaimed from the integratedcircuit industry, may save up to 30% wafer cost of the total solarcell modules.

Acknowledgments

This project was supported by Knowledge Transfer Partnershipsbetween Swansea University and SPTS Technologies Limited (UK).

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