temperature and light dependent electrical properties of graphene

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Temperature and light dependent electrical propertiesof Graphene/n-SiCH3-terminated solar cells

V.V. Brus a,b,, M.A. Gluba a, X. Zhang a, K. Hinrichs c, J. Rappich a, N.H. Nickel a

a Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH, Institut fur Silizium-Photovoltaik, Kekulestr. 5, 12489 Berlin, Germanyb Chernivtsi National University, Department of Electronics and Energy Engineering, Kotsubinsky Str. 2, 58012 Chernivtsi, Ukraine

c Leibniz-Institute fur Analytische Wissenschaften ISAS e. V., Department Berlin, Albert-Einstein-Strasse 9, 12489 Berlin, Germany

Received 11 December 2013; received in revised form 23 March 2014; accepted 9 May 2014Available online 25 June 2014

Communicated by: Associate Editor Sam-Shajing Sun

Abstract

The charge transport in Schottky-type solar cells fabricated from graphene/methyl-passivated silicon heterojunctions is studied indetail. The electrical device characteristics are affected by ambient temperature and illumination conditions. Moreover, the presenceof deep and shallow interface states influences the current across the junction at forward and reverse bias. In the dark, thermionic emis-sion over the potential barrier is clearly affected by the recombination via interface states, while under illumination those states becomeelectrically inactive. 2014 Elsevier Ltd. All rights reserved.

Keywords: Graphene; Methyl group; Surface passivation; Solar cell

1. Introduction

Nowadays photovoltaics is a dynamic and complexinterdisciplinary field of materials science and semiconduc-tor industry. The development and application of newmaterials with unique properties and different surface mod-ifications are under great demand.

Graphene is one of the new materials, which is very pro-

spective for application in photovoltaic devices due to itshigh transparency, conductivity and chemical resistance(Singh et al., 2011; Falkovsky, 2008; Castro Neto et al.,2009). Therefore, the novel graphene based solar cells are

very interesting for both fundamental and applied research(Wang et al., 2008; Liu et al., 2010; Choi et al., 2012; Kwonet al., 2013; Li et al., 2010; Phan et al., 2012 ).

There are a number of recent publications on electricalproperties of G/Si rectifying junctions, which are prospec-tive for application in optoelectronics and photovoltaics(Phan et al., 2012; Liu et al., 2012; Chauhan et al., 2012;Tongay et al., 2012; Miao et al., 2012; Ye and Dai, 2012;

An et al., 2013a; Fan et al., 2011; An et al., 2013b ).To form a graphene/Si contact the deposition of graph-

ene is commonly carried out onto the fresh H-terminatedSi surface. However, this surface is neither stable in airnor compatible to the aqueous chemistry needed to processlarge-area graphene. This deficiency results in the degrada-tion of the G/Si solar cells due to increased surface recombi-nation via interface states and pronounced series resistanceof the unavoidable silicon oxide under the graphene layer(Miao et al., 2012; Cui et al., 2013; Sutter et al., 2010).

http://dx.doi.org/10.1016/j.solener.2014.05.021

0038-092X/2014 Elsevier Ltd. All rights reserved.

Corresponding author at: Chernivtsi National University, Departmentof Electronics and Energy Engineering, Kotsubinsky Str. 2, 58012Chernivtsi, Ukraine. Tel.: +380 66 4608294.

E-mail addresses:[email protected], [email protected] (V.V. Brus).

www.elsevier.com/locate/solener

Available online at www.sciencedirect.com

ScienceDirect

Solar Energy 107 (2014) 7481
http://dx.doi.org/10.1016/j.solener.2014.05.021mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.solener.2014.05.021http://crossmark.crossref.org/dialog/?doi=10.1016/j.solener.2014.05.021&domain=pdfhttp://dx.doi.org/10.1016/j.solener.2014.05.021mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.solener.2014.05.021


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The sample structure studied in this paper differs fromthe previously analyzed electric junctions since we have adirect contact of the SiCH3-terminated surface withgraphene in the contrary to oxidized Si. Methyl groupscan terminate every Si dangling bond on the Si(111) sur-face, providing complete chemical passivation and the best

resistance to oxidation of the modified Si surface (Shenet al., 2010; Yang et al., 2012). We deal with oneSiCH3/G interface instead of previously analyzed systemswith two interfaces Si/SiOxand SiOx/G. Therefore, methylpassivation may significantly affect the electrical and pho-toelectrical properties of a G/Si heterojunction. Thisassumption is supported by a recent report on increasedefficiency of G/SiCH3-terminated solar cells in compari-son to cells with native oxide. This effect is ascribed tothe decrease of surface recombination velocity and theoptimized energy offset at the methylated Si surface (Xieet al., 2013). However, the mechanism of this improvementis not entirely clear since any detailed investigation of the

electrical properties of G/n-SiCH3-terminated solar cellshas not yet been carried out.

This paper reports on the current transport throughG/n-SiCH3-terminated solar cells at different tempera-tures as well as under different light conditions. From theelectrical characteristics of the heterojunctions we deducethe dominating current transport mechanism and the influ-ence of interface states in the dark and under illumination.

2. Experimental part

Single crystal n-Si(11 1) substrates (q= 0.7 X cm, n=6.41015 cm3) were used for the fabrication of theG/n-SiCH3terminated solar cells. The Si substrates wereultrasonically cleaned in isopropanol and rinsed in deion-ized water. The successive etching in piranha solution (concH2SO4:conc H2O21:1 in volume) and 5% HF solution werecarried out for 5 min each step in order to eliminate theorganic contaminations and native oxide, respectively.Afterwards the substrates were rinsed in deionized waterand dried under a steam of dry nitrogen.

The H-terminated Si substrates were immediately trans-ferred to a nitrogen-purged glovebox (O2, H2O


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IFS 55 Fourier transform spectrometer using a mercurycadmiumtelluride detector.

The morphology and structural properties of the surfaceof the G/n-SiCH3solar cell were inspected by means of ascanning electron microscope (SEM) Hitachi S 4100 andRaman spectroscopy (LabRAM micro Raman, Dilor, exci-

tation wavelength 632.82 nm).The photoluminescent (PL) spectra of the Si(111) sur-face under different conditions were obtained with a dyelaser pumped by a nitrogen laser used as an excitation lightsource to emit single pulses at a wavelength 743 nm(7.5 lJ/pulse, 0.6 ns pulse).

Currentvoltage (IV) characteristics of the G/n-SiCH3solar cells were measured by a high current source measur-ing unit Keithley 238 at different temperatures in the darkand under white light illumination (20 mW/cm2, measuredby a calibrating Si solar cell (Fraunhofer ISE CalLab PVCells)). The temperature control was carried out by a ther-mal inducing vacuum platform ThermoChuck TP0315B.

The light source was a solar simulator (Steuernagel).The capacitance spectroscopy of the solar cells under

investigation was measured by an electrochemical interfaceand impedance analyzer CompactStat (Ivium Technolo-gies) within the frequency range from 1 Hz to 1 MHz.The small amplitude AC signal (10 mV) was applied inorder to prevent any influence of the AC signal on the mea-sured capacitance.

3. Results and discussions

3.1. Graphene/n-SiCH3-terminated surface

The presence of the methyl groups bonded to the Si sur-face was proven by means of the infrared spectroscopicellipsometry. The tanw spectra of the electrochemicallyCH3-terminated and the former H-terminated Si(111) sur-face in the 1200/1300 cm1 wavenumber region is pre-sented in Fig. 2. The symmetric bending mode of themethyl groups (umbrelladeformation, dCH3) can be wellseen and has an absorption maximum at about 1255 cm1.

For more details on the experiment see Ref. (Yang et al.,2012).

The morphology of the front surface of the G/n-SiCH3solar cell is shown inFig. 3. It is characterized by nanoscalewrinkles formed after the transfer of graphene onto theSi(111)CH3-terminated surface. The graphene layer is

free of visible pinholes and residues from the transfer.Fig. 4shows the Raman spectrum of the G/n-SiCH3-terminated surface. First three Raman bands at 305, 522and 944 cm1 originate from the Si substrate (Brus et al.,2013a). The Raman bands at 1333, 1590 and 2661 cm1

are due to the D, G and 2D modes of graphene, respec-tively (Ferrari, 2007; Malard et al., 2009). The relativelylow ratio of the intensity of the large 2D mode to that ofthe G mode I2D/IG= 1.38 may result from the mechanicaltension due to the deformations, shown inFig. 3, as well asfrom the effect of the SiCH3-terminated surface.

Fig. 5shows the PL spectra of the oxidized and modifiedSi(11 1) surfaces measured at 295 K. The energy position of

the maxima of the PL spectra is well correlated with the Siband gap. The high PL intensity of the SiCH3-terminatedsurface comparing with that of the native oxide (34 nmthick) passivated Si surface, indicates the high quality elec-trochemical surface passivation by the methyl groups.However, the presence of a graphene layer on SiCH3-ter-minated surface decreases the PL intensity due to theincrease of the density of interface states at SiCH3/Ginterface, which act as non-radiative recombination cen-ters. Therefore, the presence of electrically active surfacestates at the SiCH3/G interface should be taken intoaccount during the analysis of electrical and photoelectrical

properties of the G/n-SiCH3solar cells.

3.2. Electrical properties under dark conditions

The G/n-SiCH3 solar cells possess a high rectificationratio RR= 5.2103 at 1 V and 283 K as can be seen inFig. 6. This value is evident for the formation of a highquality electric junction between graphene and n-SiCH3-terminated surface. There are three distinct regions

Fig. 2. Infrared tan w spectra of Si(111) surfaces after H- and electro-chemical CH3-termination from CH3MgBr in the regime of the CH3

bending mode vibration. Fig. 3. SEM image of the graphene layer on SiCH3-terminated surface.

76 V.V. Brus et al./ Solar Energy 107 (2014) 7481


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in the IV characteristics: (i) In contrast to commonSchottky diode characteristics, the reverse current doesnot saturate at high reverse bias. (ii) On the other side, athigh forward bias theIVcurve deviates from the exponen-tial trend. (iii) The expected exponential behavior isobserved only for moderate forward bias. The lack ofreverse saturation will be discussed later. To furtherelucidate effects (ii) and (iii) we replot the electrical charac-teristics under forward bias.

Fig. 7shows the forward branches of the IVcharacter-

istics of the G/n-SiCH3-terminated solar cells, measured

at different temperatures under dark conditions. There isa linear relation at low forward bias that results from the

exponential IVdependence, which can be analyzed usingthe following equation:

II0exp qV IRs

nkT

: 1

HereI0is the saturation current, nis the ideality factor,kisthe Boltzmann constant, Tis the absolute temperature andRs denotes the series resistance.

The ideality factor n is equal to 2.29 at 283 K. A value ofn= 2is commonlyattributed to the generationrecombinationmechanism within the space charge region via deep energylevels (Sze and Ng, 2007; Kosyachenko et al., 2004). How-ever, we used high quality Si single crystals (float-zone Si)in our study so that no deep energy levels are present. More-over, the strong temperature dependence of the ideality fac-tor is observed (see the inset in Fig. 7) that does not correlatewith the generationrecombination model. The idealitycoefficient decreases from 2.29 at 283 K to 1.7 at 328 K indi-cating that the dominating current transport mechanism isgoverned by the interface state assisted thermionic emission.

According to this model the dark forward currentthrough the G/n-SiCH3solar cells consists of a main emis-sion component Ie and an additional recombination com-ponent Ir, resulting from the recombination of chargecarriers via interface states at the G/SiCH3 interface. At

elevated temperatures the height of the potential barrierdecreases (Brus et al., 2013a,b) and leads to an increasedcontribution of the emission current. This results in thedecrease of the ideality coefficient, as observed in ourexperiment (see inset inFig. 7).

It is worth to note that the determined values of the ide-ality coefficient close to the reported values for the originalG/n-Si solar cells without methyl passivation (Tongayet al., 2012; Shi et al., 2013; Cui et al., 2013). Thus, CH3-termination of Si(1 1 1) surface does not significantly reducethe mentioned recombination component Ir under darkconditions due to the formation of deep surface states at

the SiCH3/G interface during the fabrication process.

Fig. 4. Raman spectrum of the G/SiCH3-terminated surface.

Fig. 5. PL spectra of the Si(111) surface under different conditions.

Fig. 6. IV characteristics of the G/n-SiCH3-terminated solar cells atdifferent temperatures in semilogarithmic coordinates.

Fig. 7. Forward branches of the dark IV characteristics at differenttemperatures: 1 283 K, 2 298 K, 3 313 K, 4 328 K. The inset showsthe temperature dependence of the ideality factor, n.

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The deviation of the IV characteristics from lineardependences at large forward bias, shown in Fig. 7, resultsfrom the effect of series resistance Rsand small increase ofthe height of the potential barrier due to a downward shiftof the Fermi level EF of the positively charged graphene(Tongay et al., 2012). However, these factors can be

neglected at low forward bias.The authors of a recent work (Tongay et al., 2012) showthat the reverse current through a G/n-Si diode is not sat-urated, as follows from the thermionic emission model, dueto the increase of the Fermi level of graphene and, thus inturn, decrease of the height of the potential barrier atincreasing reverse bias. The reverse current, considered inthe scope of the proposed model, should be governed bythe following equation in the first approximation:

IrevV;T AT2 exp

q

kT UT

hvF

2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffipee0NDVbi Vrev

2qn0

s ! !;

2

whereA is the Richardson constant, qU(T) is the tempera-ture dependent height of the potential barrier, h is thePlanks constant, vF is the Fermi velocity, e0 is the permittiv-ity of free space, e is the dielectric constant of Si substrate,Vbi is the built-in voltage and n0 is the initial density ofcharge carriers in graphene.

Fig. 8shows the dark IVcharacteristics of the G/n-SiCH3solar cells at reverse bias. The reverse IVcharacter-istics are not saturated that correlates with the voltagedependent height of the potential barrier. The increase ofreverse current with the increase of temperature results

from the decrease of the height of the potential barrier.According to Eq. (2) the reverse IV characteristics

should be plotted as straight lines in semilogarithmical coor-dinates ln(Irev) vs. V

1=2rev . However, as shown in the inset of

Fig. 8the IVcharacteristics of the reverse biased G/n-SiCH3-terminated solar cells deviate from Eq. (2),proportionally to temperature. The deviation might be asso-ciated with the presence of shallow surface states at the

n-SiCH3/G interface, which, according to the SahNoy-ceShockley model (Sah et al., 1957; Kosyachenko et al.,2004), do not participate in recombination processes butact as traps of charge carriers.

The increase of the Fermi level of graphene at reversebias is caused by the induced density of charge carriers in

graphene by the electric field within depletion region(Tongay et al., 2012). However, the presence of shallowinterface traps, whose electric state (charged or discharged)depend on the location of Fermi level and, thus, on theapplied reverse bias should also be taken into account. Inthis case, the voltage dependence of the reverse currentthrough graphene based solar cells will deviate from Eq.(2)and also will depend on the energy distribution of theinterface trap density, which is, as usually, unknown anddepends on many parameters.

The temperature induced deviation of the reverse IVcharacteristics of the G/n-SiCH3 solar cells from Eq. (2)(see inset inFig. 8) is expectable in the scope of the consid-

ered model. It is known that efficient charge carriers trapsare created by shallow states (located close to the edges ofconduction or valance bands). Therefore, the trapping con-ditions of charge carriers are temperature sensitive (Sze andNg, 2007).

The capacitance spectroscopy was used to provide anadditional independent experimental evidence of the pres-ence of interfaces states, distributed within the band gapof the CH3-terminated Si substrates covered by graphene.Fig. 9 shows the spectral dependences of the measuredcapacitance (corrected by the effect of the series resistance)of the G/n-SiCH3solar cells at different DC bias and room

temperature. When the small amplitude AC signal isapplied, the Fermi level moves up and down (10 meV)with respect to the interface state levels and change thecharge of the interface traps (charged/discharged). Thischange of the interface state charge increases the measuredcapacitance of the heterojunction (Sze and Ng, 2007; Brus,2013). Therefore, the measured capacitance decreases fromits maximum saturated level at low frequencies (all interfacestates follow the low frequency AC signal) to its minimumsaturated level at high frequencies (interface states cannot

Fig. 8. Reverse IV characteristics of G/n-SiCH3-terminated solar cellsat different temperatures: 1 283 K, 2 298 K, 3 313 K, 4 328 K. Theinset shows the reverse IVcharacteristics in semilogarithm coordinatesln (Irev) vs. V

1=2rev . The dashed lines are used in order to indicate the

deviation from linear dependence.

Fig. 9. Capacitance spectroscopy of the G/n-SiCH3solar cells, measured

at different DC bias under dark conditions at room temperature.



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follow the high frequency AC signal), which is determinedonly by the barrier capacitance (capacitance of the spacecharge region), since the effect of interface states has alreadybeen taken into account.

It is seen fromFig. 9that the contribution of the inter-face states to the measured capacitance of the G/n-SiCH3

solar cells depends on the applied DC bias at low frequen-cies, since the bias dependence of the barrier capacitance ismuch weaker (see the high frequency region inFig. 9). Thisis the experimental evidence of the presence of the energydistributed density of interface states Nss [cm

2 eV1](Sze and Ng, 2007). Therefore, the above mentioned sug-gestions regarding the presence of electrically active deepand shallow interface states in the G/n-SiCH3heterojunc-tions under dark conditions are physically based. Thedetail analysis of the AC characteristics of the G/n-SiCH3solar cells deserves additional investigation but it goesbeyond the scope of this paper.

3.3. Electrical properties under illumination

Hereafter theIVcharacteristics of the illuminated G/n-SiCH3 solar cells will be analyzed only within the forthquadrant, since it provides all photoelectrical parametersof a solar cell. The IVcharacteristics of the illuminated(white light illumination of 20 mW/cm2) G/n-SiCH3solarcells, measured at different temperatures, are shown inFig. 10.

In the presence of series and shunt resistanceRsand Rsh,respectively, one can extend(1) to the following expression

for the IV characteristics under illumination:

IIphI0 exp qV IRs

nkT

1

V IRs

Rsh: 3

Here, Iph is the photocurrent generated in the cell.Analyzing Eq. (3) under short circuit and open circuit

conditions when the inequality exp[q(V+ IRs)/nkT]/exp[qIscRs/nkT] = A1 is true the following equationcan be derived (Brus, 2012; Brus et al., 2012):

Isc IRs Rsh V

RshI0 exp

qV IRs

nkT

: 4

The actual values of the series Rs and shunt Rsh resis-tance of the illuminated G/n-SiCH3solar cells at different

temperatures can be easily determined from the appropri-ateIVcharacteristics (Fig. 10) (Schroder, 2006). It is seenthat Eq.(4) describes a liner dependence in semiclogarith-mical coordinates ln[{(Isc I))(Rs+ Rsh)V}/Rsh] vs.(V+ IRs) as observed experimentally and shown inFig. 11 in case of the illuminated G/n-SiCH3 solar cells.The deviation from the linear dependences in Fig. 11 atlow forward bias results from the failure of the inequalityA1 (see inset a) in Fig. 11).

One can calculate the values of the ideality coefficient nfrom the slope of the linear dependences in Fig. 11(Brus,2012). The ideality coefficient is equal to unity and doesnot depend on temperature (Table 1) indicating that thecurrent transport through the illuminated G/n-SiCH3solar cells is entirely determined by the thermionic emission(Sze and Ng, 2007). The recombination via interface statesat the SiCH3/G interface becomes negligibly small underillumination, opposed to those in dark. It is naturally toassume that the decrease of the surface recombinationvelocity under illumination comparing to that under darkconditions is caused by the release of charge carriers fromdeep lying surface states at the n-SiCH3/G interface thatexcludes them from the participation in recombination pro-cesses (Goossens and Schoonman, 1990).

Since the ideality factor does not depend on tempera-

ture, the saturated current I0 can be determined by theextrapolation of the linear dependence toward the intercep-tion with the current axis at zero voltage ( Table 1).

Considering a linear temperature dependence of theheight of the potential barrierqU =qU0 bUT, the expres-sion for the saturation current I0, in first approximation,can be given as:

Fig. 10. IV characteristics of the illuminated G/n-SiCH3-terminatedsolar cells at different temperatures. The inset shows the equivalent circuit

of the solar cells considering the presence of series and shunt resistance.

Fig. 11. The determination of the dominating current transport mecha-nism through the illuminated G/n-SiCH3-terminated solar cells atdifferent temperatures: 1 283 K; 2 298 K; 3 313 K, 4 328 K. Theinsets show: (a) the voltage dependence of the ratio A; (b) the saturation

current ln(I0) vs. reciprocal temperature 103/T.

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I0T AT2 exp

qUT

kT

AT2 exp qU0 bUT

kT

AT2 exp qbU

k

exp

qU0

kT

; 5

where A is the Richardson constant, qU0 is the height ofthe potential barrier atT= 0 K,bUis the temperature coef-

ficient of the height of potential barrier. The value of theheight of the potential barrier at zero temperatureqU0 = 0.82 eV of the illuminated G/n-SiCH3-terminatedsolar cells was determined from the slope of the dependenceln (I0) vs. 10

3/T(see inset b) inFig. 11). It should be notedthat the determined value of qU0 might be different fromthe value in dark due to the change of the SiCH3/G inter-faces charge state under illumination. However, the heightof the potential barrier cannot be determined using Eq.(5)under dark conditions due to the effect of surface recombi-nation (temperature dependent ideality coefficient n (seeinset inFig. 7)).

It is known that the photoelectric performance of thesolar cells depends on the diode (injection) current, whichis quantitatively described by the ideality factorn (the effi-ciency decreases with increasing values ofn)(Sze and Ng,2007). Since the value of the ideality factor n of the G/n-SiCH3 solar cells decreases under illumination comparingto its value in the dark, this is the evidence of the lightinduced increase of the photoelectric performance.

4. Conclusions

The G/n-SiCH3-terminated solar cells were fabricationby the formation of an optical contact between CVD-

grown graphene with electrochemically methyl passivatedSi(111) single crystal substrates.The photoluminescence study of the bare and graphene

coated SiCH3-terminated surfaces and frequency depen-dent capacitance of the solar cells provide evidence onthe formation of electrically active surface states at theSiCH3/G interface, which are responsible for the increaseof the non-radiative recombination.

The dominating current transport mechanism throughthe G/n-SiCH3solar cells was established to be the inter-face state assisted thermionic emission under dark condi-tions. The reverse IV characteristics of the solar cellsunder investigation are not saturated due to the voltage

dependent height of the potential barrier. The increase ofthe Fermi level of graphene and, thus, the decrease of theheight of the potential barrier at reverse bias is caused bythe induced density of charge carriers in graphene by theelectric field within depletion region as well as by the charg-ing/discharging of shallow surface traps at the n-SiCH3/G

interface.The ideality coefficient of the illuminated G/n-SiCH3-terminated solar cells is equal to unity and does not dependon temperature indicating that the current transport istotally determined by the thermionic emission. The recom-bination of injected charge carriers via interface states atthe SiCH3/G interface becomes negligibly small underillumination, as opposed to that in the dark.

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Table 1Photoelectrical parameters of the G/n-SiCH3solar cells under white light20 mW/cm2.

T, K Isc,mA Voc, V n I0109, mA Rs, X Rsh, kX

283 0.1605 0.47 0.99 0.463 1690 20.9298 0.1613 0.444 0.98 2.8 1580 19.8313 0.1619 0.417 0.99 1.08 1480 17.3

328 0.1627 0.398 0.99 4.99 1390 15.7

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