Interlayer thickness dependence of photovoltaic properties of polycrystalline p-β-FeSi2(Al)/n-Si(100) heterojunctionsA. Bag, S. Mallik, and C. K. Maiti Citation: Journal of Renewable and Sustainable Energy 6, 023110 (2014); doi: 10.1063/1.4871285 View online: http://dx.doi.org/10.1063/1.4871285 View Table of Contents: http://scitation.aip.org/content/aip/journal/jrse/6/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Reduction of Cu-rich interfacial layer and improvement of bulk CuO property through two-step sputtering for p-CuO/n-Si heterojunction solar cell J. Appl. Phys. 116, 074501 (2014); 10.1063/1.4893321 Photovoltaic characteristics of a-Si/β-FeSi2/c-Si double heterojunction fabricated by magnetron sputtering J. Vac. Sci. Technol. A 29, 051202 (2011); 10.1116/1.3623739 Photovoltaic characteristics of p- β -FeSi 2 ( Al ) / n-Si ( 100 ) heterojunction solar cells and the effects ofinterfacial engineering Appl. Phys. Lett. 98, 013507 (2011); 10.1063/1.3536523 Microstructure and photovoltaic performance of polycrystalline silicon thin films on temperature-stable ZnO:Allayers J. Appl. Phys. 106, 084506 (2009); 10.1063/1.3240343 Fabrication and characterization of amorphous Si/crystalline Si heterojunction devices for photovoltaicapplications J. Vac. Sci. Technol. A 22, 1015 (2004); 10.1116/1.1701854

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Interlayer thickness dependence of photovoltaic propertiesof polycrystalline p-b-FeSi2(Al)/n-Si(100) heterojunctions

A. Bag,a) S. Mallik, and C. K. MaitiVLSI Engineering Laboratory, Department of Electronics and ECE,Indian Institute of Technology, Kharagpur 721302, India

(Received 28 December 2012; accepted 28 March 2014; published online 9 April 2014)

Interlayer thickness dependence of photovoltaic properties of solar cells

fabricated using aluminum (Al) alloyed polycrystalline p-type b-phase iron

disilicide [p-b-FeSi2(Al)]/n-type Si(100) bi-layer heterojunctions are reported.

p-b-FeSi2(Al) films of thickness �40 nm were deposited by radio frequency

magnetron-sputtering on n-Si(100) substrates with varying Al interlayer

thickness. Sharp film-substrate interfaces were confirmed by field emission

scanning electron microscopy. The formation of polycrystalline b-phase of FeSi2after rapid thermal annealing at �650 �C was confirmed by x-ray diffraction

analysis. The current density-voltage (J–V) and photo response characteristics were

measured at room temperature. Photovoltaic performance of the heterojunction solar

cells improved significantly after the introduction of an epitaxial Al-containing pþ-Si

layer of thickness �10 nm at p-b-FeSi2(Al)/ n-Si(100) interface. An enhancement in

the short-circuit current density and overall performance improvement, compared to

devices having �8 nm and �12 nm thick interlayer, has been observed. Under

air mass 1.5 illumination, the cells showed a maximum conversion efficiency of

3.00%. The short-circuit current density was found to be 26.01 mA/cm2 and the

open-circuit voltage was found to be 335 mV. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4871285]

I. INTRODUCTION

In the last few decades, ever increasing demand for alternative renewable clean energy

sources have stimulated scientific research in solar energy harvest. Especially, attempts are

being made to develop photovoltaic (PV) devices and materials that can deliver higher energy

conversion efficiency in a cheaper way to meet this need. Transition metal silicides have

attracted attention as cost effective alternate for electronic-grade Si.1 Among the transition

metal silicides, the semiconducting beta-phase iron disilicide (b-FeSi2) is considered as a prom-

ising PV material as it has a high absorption coefficient (a > 105 cm�1, at photon energies

higher than 1 eV), a direct energy band-gap of 0.85 eV, chemical stability at temperatures as

high as 937 �C, high thermoelectric power (Seebeck coefficient of k � 10�4/K) and high resist-

ance against the humidity, chemical attacks and oxidization. Moreover, b-FeSi2 is an environ-

mentally friendly semiconductor since the Fe and Si elements are ecofriendly and highly

abundant.2–8 It is a novel photovoltaic material with reported theoretical energy conversion effi-

ciency about 16%–23%.9 However, up to now; there are only a few reports on b-FeSi2 solar

cells reporting a maximum conversion efficiency of 3.7%.10 In general, poor features of

b-FeSi2 solar cells might be due to the low quality of b-FeSi2 films and rough interface

between the b-FeSi2 films and the Si substrate, which is mainly induced by the significant

inter-diffusion of Fe and Si atoms during b-FeSi2 formation. Several reports suggest that high

quality of both film and heterojunction interface is essential to achieve higher efficiency. To

a)Author to whom correspondence should be addressed. Electronic mail: abag@iitkgp.ac.in. Tel.: þ91-3222-281475. Fax:

þ91-3222-255303.

1941-7012/2014/6(2)/023110/6/$30.00 VC 2014 AIP Publishing LLC6, 023110-1

JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 6, 023110 (2014)

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achieve quality films, few dedicated systems (PLD, FTDCS, etc) and several intermediate pro-

cess steps like substrate heating, high temperature and longer annealing are required. But a less

complex process would be beneficial to facilitate the mass production of b-FeSi2 as a photovol-

taic material.10–12 In this work, incorporating Al into b-FeSi2 films using conventional magne-

tron sputtering and rapid thermal processing (RTP), we have studied the effects of Al interlayer

thickness on the performance of ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) heterojunction solar cells.

II. EXPERIMENTAL

All the photovoltaic devices were fabricated on n-type Si(100) substrates (q � 5 X-cm).

Before device fabrication, Si substrates were cleaned using Piranha solution followed by dip-

ping in 1% hydrofluoric (HF) acid for 1 min to remove the surface native oxide layer. After

cleaning, the substrates were immediately loaded into a magnetron sputtering chamber. The

chamber was then evacuated to a base pressure of 4.0 � 10�6 millibars. A layer of �40 nm

thick Al-containing amorphous FeSi2 was deposited at room temperature by co-sputtering of

stoichiometric FeSi2 target and pure Al target in Ar ambient. A thin Al interlayer of three dif-

ferent thicknesses (8, 10, and 12 nm) were sputter deposited from pure Al target prior to depo-

sition of amorphous-FeSi2(Al) layer. The samples were then subjected to RTA in N2 at 650 �Cfor 2 min (ULVAC-RIKO, MILA-3000). Finally, heterojunction solar cells were fabricated by

sputter deposition of ITO (�100 nm thick) at room temperature on p-b-FeSi2(Al) as top elec-

trode and by evaporating Al layer at the back-side of n-Si substrate. Current density-voltage

(J-V) characteristics of the devices in dark and under AM 1.5 illumination (XES-151S) with a

power density of 100 mW/cm2 were measured using Agilent B1500A semiconductor device an-

alyzer. The polycrystalline b-phase formation after RTA at �650 �C was confirmed by x-ray

diffraction (results not shown). Sharp film-substrate interfaces and film thickness were con-

firmed by field emission scanning electron microscopy (FESEM).

III. RESULTS AND DISCUSSION

The cross-sectional FESEM image of FeSi2(Al) film of thickness of �40 nm with �10 nm

thick Al interface layer is shown in Fig. 1. The basic structure of ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100)

FIG. 1. Cross-sectional FESEM image of FeSi2(Al)/Al film deposited on n-Si(100) substrate. Inset: Schematic diagram of

ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) heterojunction solar cell with Al back electrode.

023110-2 Bag, Mallik, and Maiti J. Renewable Sustainable Energy 6, 023110 (2014)

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heterojunction solar cells with Al back electrode is shown in the inset of Fig. 1. The film is

continuous and almost uniform in thickness. A sharp interface between the FeSi2(Al) film, Al

layer and the Si substrate is observed from the FESEM image. During RTA, the Al interlayer

got mostly dissolved into FeSi2 to replace Si, which resulted in the formation of the heavily

Al-doped epitaxial-Si (pþ-Si) interfacial layer by the expelled Si.13

A comparison of the J-V characteristics of ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) hetero-

junction solar cells (having different Al interlayer thickness) under dark condition are shown

in Fig. 2. The dark J-V characteristics of the 8 nm Al interlayer device is almost symmetrical

for forward as well as reverse bias, indicating that it behaves like a resistor rather than a

diode; probably due to the poor interface quality. Similarly forward current density for devi-

ces with 12 nm thick Al interlayer is only 0.725 orders of magnitude higher than reverse.

The devices having 10 nm Al interlayer exhibit only typical rectifying diode characteristics

as expected from heterojunctions. In case of 10 nm Al interlayer devices, the reverse bias

current is typically 2–3 orders of magnitude lower than the forward bias current at room

temperature which suggests that a good p–n heterojunction has been formed. The J–V char-

acteristics under AM 1.5 (100 mW/cm2) simulated solar irradiation conditions of all devices

are shown in Fig. 3. Devices with 8 nm thick Al interlayer exhibited negligible Jsc and show

no variation in current density with change in bias, indicating that it behaves like a

photo-resistor. A maximum 3% power conversion efficiency was achieved for the cells hav-

ing �10 nm Al interlayer between n-Si and p-b-FeSi2(Al) layer. The key parameters of the

devices showing photovoltaic action with different Al interlayer thickness are listed in Table

I. The fill factor (FF) and the power conversion efficiency (g) of the device was measured

using the relations

FF ¼ Im � Vm

Isc � Voc

� �; (1)

and

g ¼ Voc � Isc � FF

Pin

� �; (2)

FIG. 2. Dark current density-voltage (J–V) characteristics of ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) heterojunction solar cells

with varying Al interlayer thickness.

023110-3 Bag, Mallik, and Maiti J. Renewable Sustainable Energy 6, 023110 (2014)

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where Voc is the open-circuit voltage, Isc is the short-circuit current, FF is the fill factor, g is

the power conversion efficiency, Pin is the incident light power density, and Im and Vm are the

current and voltage at the maximum power point.

Figure 4 shows power conversion efficiency (g) and short-circuit current density (Jsc) under

AM 1.5 (100 mW/cm2) simulated solar irradiation for ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) heter-

ojunction solar cells with different Al interlayer thickness. Maximum Jsc and g values are found

for devices with �10 nm Al inter layer. In the solar cells with similar type of heterojunction

structures, Liew et al.14 have reported the presence of an amorphous layer in a

p-b-FeSi2(Al)/n-Si(100) heterojunctions without an Al interlayer. This amorphous layer, prob-

ably SiOx due to presence of residual oxygen during sputtering and anneal process, could have

induced a Fermi level pinning which results in a low effective built-in potential and conse-

quently a low Voc. On the other hand, devices with Al interlayer may have suppressed the for-

mation of SiOx at the FeSi2(Al)/Si interface leading to an improved built-in potential over that

of the solar cell without Al interlayer.14 In our experiments, devices with 10 nm Al interlayer

shows a higher Jsc values. It may be concluded that the 10 nm thick Al interlayer may have

completely suppressed the formation of amorphous interlayer at the FeSi2(Al)/Si interface lead-

ing to improved Jsc. But the device with 12 nm Al interlayer shows a lower Jsc values which

may be due to the significant reduction of electron collection for the heavily Al-doped epitaxial

pþ-Si layer. The fill factor and Voc with different Al interlayer thicknesses of

ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) heterojunction solar cells are shown in Fig. 5. The variation

in the thickness of the interlayer of the solar cell also affects the open circuit voltage (Voc) to

some extent. A maximum Voc of �335 mV was found for the devices having �10 nm Al inter-

layer and is smaller compared to theoretically expected value. The reason for such a low Voc

may be due to the diffusion of iron atoms in the depletion region. The diffused iron atoms also

act as trap centers for the photo-generated carriers, which reduce not only the Voc but also

FIG. 3. Current density–voltage (J–V) characteristics of ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) heterojunction solar cell under

AM 1.5 (100 mW/cm2) simulated solar irradiation condition with varying Al interlayer thickness.

TABLE I. Parameters of ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) heterojunction solar cells under AM 1.5 simulated solar irradi-

ation.a

Interlayer thickness (nm) Jsc (mA/cm2) Voc (mV) FF (%) g (%)

10 26.01 335 34.44 3.000

12 9.57 335 25.21 0.808

aAll cells have area of 5 � 6 mm2.

023110-4 Bag, Mallik, and Maiti J. Renewable Sustainable Energy 6, 023110 (2014)

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reduce the Jsc.15 Figure 5 shows the variation of fill factor where the device having maximum ef-

ficiency, the maximum FF is 0.34. The small FF is attributed to the high series resistance of the

solar cells.16,17 To investigate the probable reason of small FF we have calculated both the series

and shunt resistances for the best device showing maximum efficiency of 3%. From the slope of

the I–V curve at I ¼ Isc (V ¼ 0), the shunt resistance (Rsh) has been extracted using the model

proposed by Ishibashi et al.18 The device having 10 nm Al interlayer shows a shunt resistance of

761 X and a high series resistance (Rs) of 103.95 X which confirms the reason of small fill factor

of the devices. The detail technique of calculation of Rs and Rsh has been reported.19 Since the

device efficiency is proportional to the product Jsc � Voc � FF, most of the increase in the device

efficiency results from the increased Jsc and FF, rather than Voc. Hence the degradation of FF of

the device is mainly due to the higher Rs value which appears due to poor contact between ITO

and FeSi2(Al). Due to corrosion or peeling of contacts Rs increases, which cause the reduction in

g of the solar cells. Rsh arises due to presence of defects. Possibly due to metal electro-migration

Rsh decreases, which further degrade the FF of the solar cell. Thus performance of the

ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) heterojunction solar cells can be improved by minimizing

the value of Rs. Even though the FF of the devices is not high enough, the present study shows

the importance of Al interlayer at p-b-FeSi2(Al)/n-Si(100) interface to form high quality p-n junc-

tion. Observed efficiency of 3% in case of 10 nm Al interlayer devices is significant compared to

reported maximum efficiency of 3.7% in the literature.10

FIG. 4. Power conversion efficiency (g) and short-circuit current density (Jsc) under AM 1.5 (100 mW/cm2) simulated solar

irradiation for ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) heterojunction solar cells with different Al interlayer thickness.

FIG. 5. FF and open-circuit voltage (Voc) under AM 1.5 (100 mW/cm2) simulated solar irradiation for

ITO/p-b-FeSi2(Al)/pþ-Si/n-Si(100) heterojunction solar cells with different Al interlayer thickness.

023110-5 Bag, Mallik, and Maiti J. Renewable Sustainable Energy 6, 023110 (2014)

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IV. CONCLUSIONS

Photovoltaic properties of Al-alloyed polycrystalline p-type b-iron disilicide heterojunction

solar cells are investigated. It is shown that the efficiency of the cell depends on the thickness

of the Al interlayer. Power conversion efficiency and short-circuit current density were found to

be maximum for the devices with 10 nm thick Al interlayer. Open circuit voltage was found to

be small compared to theoretically expected value due to iron diffusion in the depletion region.

Further improvement in PV efficiency may be obtained by introducing an electron-blocking

anti-reflection layer over active layer.

ACKNOWLEDGMENTS

The authors acknowledge DST, New Delhi (DST/TM/SERI/2k11/100/(G)) for the financial

support.

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