solarcell
Solar Energy Materials &Solar Cells 91(2007)1120–1126
Fabrication and characterization of 4-tricyanovinyl-N ,N -diethylaniline/
p-silicon hybrid organic–inorganic solar cells
M.M.El-Nahass a ,H.M.Zeyada b ,K.F.Abd-El-Rahman a ,A.A.A.Darwish a,?
a
Physics Department,Faculty of Education,Ain Shams University,Rorxy Square 11757,Cairo,Egypt
b
Physics Department,Faculty of Science at New Damietta,New Damietta 34517,Egypt Received 11February 2007;received in revised form 20March 2007;accepted 23March 2007
Abstract
Hybrid organic–inorganic solar cell was fabricated by thin ?lm of 4-tricyanovinyl-N ,N -diethylaniline deposited on p-Si substrates.The capacitance–voltage characteristics indicated that the junction is of abrupt nature.The dark forward current density-voltage characteristics indicated a tunneling conduction at relatively low voltages followed by a space-charge-limited-conduction mechanism at relatively high voltages.Under illumination,the cell exhibits photovoltaic characteristics with an open-circuit voltage of 0.70V,a short-circuit current density of 9.15mA cm à2,and a power conversion ef?ciency of 3.10%.The effect of g -rays irradiation (100kGy absorbed dose)on the characteristics of the cell was also investigated.The ?ll factor and the power conversion ef?ciency decrease by 20.9%and 39%of the original value,respectively.r 2007Elsevier B.V.All rights reserved.
Keywords:Organic semiconductors;Organic solar cell
1.Introduction
Recently,new functional materials with special optical and electrical properties are required for electronic and optoelectronic https://www.sodocs.net/doc/a011357467.html,anic layered semiconductor is one of such class of prospective layered materials,which have been used to fabricate organic–inorganic heterojunc-tions and organic optoelectronic devices [1,2].Therefore,organic thin ?lm devices such as organic light emitting diode,photo-detectors,and solar cells have made consis-tent progress in their performances [3–6].It is mainly because the organic thin ?lms have several clear advantages over inorganic thin ?lms such as large-area coverage,mechanical ?exibility,low-temperature process-capability,easy preparation,low cost,etc.
Investigations of organic–inorganic heterojunctions at-tract some interest due to both the unusual nature of these contacts as well as to the potential new devices that can be applied.Recently,different organic–inorganic heterojunc-tions such as pyronine-B/Si [7],b -carotene/Si [8],chitin/n-Si [9],and MEH-PPV/p-Si [10]have been fabricated and then the electronic parameters have been measured in each fabricated https://www.sodocs.net/doc/a011357467.html,anic–inorganic solar cells such as polythiophene/GaAs [11],CuPc/p-Si [12],ZnPc/Si [13,14],MgPc/n-Si [15],Dicyclopropyl fulgide/n-Si [16],NiPc/p-Si [17],TPP/n-Si [18],CuPc/n-GaAs [19],and H 2Pc/n-Si [20]have showed photoconversion ef?ciencies in the range of 0.74–3.5%.In such cells,most of the incident light is absorbed within the organic layer,which acts as the active layer.Rather high V oc can be obtained,but the photo-current is very poor due to low carrier mobilities,high resistivity,and high carrier recombination rate in the organic materials.
The recent reports on the interesting electrical and electronic properties of organic–inorganic heterojunction have motivated the researchers to investigate the electronic properties of this heterojunction with different organic materials.4-tricyanovinyl-N ,N -diethylaniline (TCVA)is a donor–acceptor disubstituted benzenes dye,which belongs to a class of organic compounds known as molecular rotors [21–23].It has a great importance as ?uorescence probes [22–24]and it is used for coloring synthetic polymer ?bers [25].In addition,it is applied as a sublimable dye in
https://www.sodocs.net/doc/a011357467.html,/locate/solmat
0927-0248/$-see front matter r 2007Elsevier B.V.All rights reserved.doi:10.1016/j.solmat.2007.03.016
?Corresponding author.
E-mail address:aaadarwish@https://www.sodocs.net/doc/a011357467.html, (A.A.A.Darwish).
heat-transfer recording materials[26]and in photoconduc-tive recording materials[27].
To best of our knowledge,no physical study has been reported on TCVA.This has given us the motivation to study its physical properties especially in thin?lm form. One of them is to study in detail the transport properties of TCVA/p-Si heterojunction cell.The purpose of this study is to obtain a heterojunction by depositing TCVA on top surface of the p-Si substrate and to investigate its electronic characteristic parameters using current density–voltage(J–V)and capacitance–voltage(C–V)https://www.sodocs.net/doc/a011357467.html,ually,J–V and C–V measurements give useful information about the properties of the heterojunction interface.In addition,we report the photovoltaic proper-ties necessary for solar cell parameters optimization and fabrication.The development of a new generation of highly ef?cient photovoltaic cells with a suitable radiation resistance is of great interest for space power applications. The radiative rays in space create defects inside the solar cell material.This paper,also,deals with the effect of gamma irradiation on the fabricated cell output para-meters.
2.Experimental details
The TCVA compound,whose molecular structure has been shown in Fig.1,was prepared by direct reaction between N,N-diethylaniline and tetracyanoethylene[28] and the synthesis was described as follows:
A solution of N,N-diethylaniline(10mmol)was dis-solved in N,N-dimethylformamide,DMF,(20ml)and cooled to273K.Tetracyanoethlene,TCNE,(10mmol) was also dissolved in DMF(10ml)and it was then added drop wise to the aromatic amine solution with stirring at 273K.After it has been completely added,the solution was then heated at363K for3h with continuous stirring.The solvent was removed under reduced pressure and the crude dye was dissolved in chloroform and chromatographeied on silica gel using a mixture of chloroform ethyl acetate (3:7)as eluent.Recrystallization from toluene gave TCVA as dark brown crystal in85%yield.1H-NMR(CDCL3):d 8.06(d,J?10Hz),6.73(d,2H,J?10Hz),3.53(q,4H, J?7Hz,CH2),1.29(q,6H,J?7Hz,CH3).Elemental analysis found C,71.85;H,5.72;N,22.31;calculated for C15H14N4:C,72.00;H,5.60;N,22.40.
The differential scanning calorimeter showed that this compound is stable up to423K.The optical absorption of thermally evaporated TCVA?lms showed that TCVA thin ?lm has a wide absorption range spectrum in UV–vis region,which is appropriate for photovoltaic application. Furthermore,the analysis of absorption coef?cient re-vealed indirect transitions with1.45eV as energy band gap. The samples have been prepared using a polished p-type Si wafer with(100)orientation and hole concentration of 1.6?1017cmà3with thickness of400m m.In order to remove the native oxide on surface on p-Si,the substrate was etched by CP4solution(HF:HNO3:CH3COOH in ratio 1:6:1)for10s,then rinsed with deionized water and dried. The p-Si substrates were coated from one side by TCVA thin?lm of thickness55nm using thermal evaporation technique to fabricate TCVA/p-Si heterojunction cell.The front contact of this heterojunction was made with gold mesh electrode.The active area of the fabricated hetero-junction was about0.20cm2.The back contact was made by depositing a relatively thick?lm of Al to the bottom of the p-Si substrate.Thus,an Au/TCVA/p-Si/Al cell was obtained.All evaporation processes were carried out in a vacuum coating unit(Edwards,E306A)at about10à4Pa. The fabricated TCVA/p-Si cell was annealed in air at373K for1h to complete the junction formation.Annealing of heterojunction has been the usual step in obtaining the best ef?ciency cells.This annealing might remove any channels, which could be raised during the fabrication.
The dark capacitance–voltage(C–V)characteristics for the fabricated cell were measured at1MHz and room temperature,using a computerized CV-410meter(Solid State Measurement,Inc.,Pittsburgh).The current densi-ty–voltage(J–V)characteristics of the fabricated cell were achieved by measuring the current corresponding to a certain potential difference across the junction,using a conventional circuit.The voltage across the junction and current passing through it were measured simultaneously using a high impedance electrometers(Keithley610,617, respectively).The dark J–V characteristics were obtained in a complete dark chamber at room temperature or inside a dark furnace in case of measurements at higher tempera-tures.Power characteristics under an80mW/cm2(AM1.5) white light source(halogen lamp)were measured.The intensity of incident light was measured using a digital lux meter(BCHA,model93408).
The fabricated cell was exposed to energetic gamma rays with photon energy of1.25MeV.The absorbed dose is 100kGy.Both J–V and C–V characteristics were measured in dark and under illumination before and after exposure to gamma irradiation.
3.Results and discussion
3.1.Current density–voltage characteristics
Fig.2illustrates the J–V characteristics of TCVA/p-Si cell at room temperature before and after g-irradiation.
The curves exhibited diode-like behavior,which can be understood by the formation of a barrier at the interface.It is also observed that g -irradiation increases the saturation and the leakage current.Leakage current is likely to be from some mid-gap states,which are formed by defects in the organic ?lm.The number of the defects may increase with g -ray radiation causing the current leakage to increase at the p–n heterojunction.The recti?cation ratio (RR),which is de?ned as the ratio of the forward current to the reverse current at a certain value of the applied voltage (RR ?J F /J R ),was calculated at 71V for as fabricated and g -irradiated cells and listed in Table 1.g -irradiation decreases the RR by 4times less than that for as fabricated cell.
Fitting the dark forward current–density according to the modi?ed ideal diode equation [29]
J ?J O exp q eV àJR s Tnk B T à1
tV àJR s
R sh ,(1)
where J o is the reverse saturation current density,n is
the diode quality factor,q is the electronic charge,R s is the series resistance,and R sh is the shunt resistance.The parameters J o and n can be determined from the curves shown in Fig.2together with Eq.(1).The values of J o and n were calculated at room temperature for the fresh and irradiated cells and listed in Table 1.The cell was non-ideal where the diode quality factor is greater than unity,which may be attributed to either recombination of electrons and
holes in the depletion region,and/or the increase of the diffusion current due to increasing applied voltage.g -irradiation increases the both J o and n .
The equation also includes the effects of parasitic series and parallel resistance,which can obscure the intrinsic parameters of the device.The series resistance,R s ,and the shunt resistance,R sh ,were determined from the diode junction resistance (R J ?q V /q I ),which can be determined from the current–voltage curves.Typical experimental results of R J against V are shown in Fig.3.At suf?ciently high forward bias the junction resistance approaches a constant value,which is the series resistance,R s .On the other hand,the junction resistance is also constant at suf?ciently high reverse bias,which is equal to the diode shunt resistance,R sh .However,junction series and shunt resistances are important factors in improving cell performance and design.Determination of its magnitude is necessary to continue device improve-ment,especially for devices exposed to high light inten-sities.The obtained values of R s and R sh were determined at room temperature for as fabricated and g -irradiated cells and listed in Table 1.It is clear that the series resistance increased with g -ray radiation.This behavior may be due to increasing the number of defects in the organic ?lm.
J –V characteristics of as fabricated cell were measured as a function of temperature in the range of 298–363K.Fig.4shows the temperature dependence of current in semi-logarithmic scale from which it can be seen that the junction exhibits a marked recti?cation characteristics for all the measuring temperatures.In addition,the forward current can be classi?ed into two regions according to the applied voltage.Firstly,the current increases exponentially with applied voltage and then deviates from the exponen-tial due to the effect of a series resistance on the system.This behavior indicates the presence of two conduction mechanisms,which may be described by the two-lumped
-1.5-1.0-0.5
0.00.5 1.0 1.5
10-6
10-5
10-4
10-3 10-2
J (A /c m 2)
V (volts)
Fig. 2.Dark J –V characteristics of TCVA/p-Si cell before and after exposure to g -rays.
Table 1
The junction parameters of TCVA/p-Si cell at room temperature
RR at 71V
J o (m A/cm 2)n R s (k O )R sh (M O )As fabricated cell
99.4 1.76 2.38 2.25 3.77g -Irradiated cell
25.7
4.23
4.3
3.10
0.16
-1.5-1.0-0.5
0.00.5 1.0 1.5
103
104105R j (Ω)
V (volts)
Fig.3.Junction resistance,R J ,versus V for TCVA/p-Si cell before and after exposure to g -rays.
M.M.El-Nahass et al./Solar Energy Materials &Solar Cells 91(2007)1120–1126
1122
diode model as
J ?J o1exp
q eV àJR s T
n 1k B T à1
tJ o2exp q eV àJR s Tn 2k B T
t
V àJR s
R sh
,e2T
where the diode quality factor and the reverse saturation currents–density are n 1and J 01for the ?rst region and n 2and J 02for the second region.
In the ?rst region (low voltage),the dark forward J –V characteristics are shown in Fig.5.It was found that the gradient of the forward current was almost constant for various temperatures and found that the quality factor decrease as the temperature of the cell increases.These results suggest that the tunneling mechanism may well be the predominant current transport mechanism in this region and the voltage dependence of the junction current can be expressed as [30]J ?J o1?exp ea V T ,
(3)
where a is a constant independent of temperature.These results suggest that the forward currents at V p 0.3V are mainly dominated by tunneling mechanisms.The data of Fig.5were analyzed using Eq.(3)and the parameter a was found to be 16V à1.On the other hand,the reverse saturation current density J o1was obtained by extrapolat-ing the forward current curves to zero voltage.According to multi-step tunneling mechanisms,J o1should change exponentially with temperature and given by [31]J o 1?J oo exp eb T T,
(4)
where b is constant independent of applied voltage.The inset of Fig.5shows the plot of ln J o as a function of temperature and a straight line was obtained indicating a multi-step tunneling mechanism.The slope of the obtained straight line gives the value of b ,which was found to be 0.036K à1.The ratio of a /b can be used to determine the
change of built-in voltage (V bi )with respect to the absolute
temperature (T )that is given by [31]d V bi =d T ?àb =a .(5)
The calculated value of d V bi /d T is equal to à2.25?10à3V/K.The number of steps (R )required for traversing an electron through the depletion region can be calculated using the following relationship [32]:a ?e8=3h Tem ? 1=RN 1T1=2,
(6)
where h is Planck’s constant,N 1is the acceptor concentra-tion in p-Si,e 1is the dielectric constant of p-Si,and m *is the effective mass of electron (m *?0.56m e ).It was found that R is about 2.11?105steps.
In the second region (higher voltage),other conduction mechanism seems to be predominant.It is observed that the current shows a power dependence of voltage with slope of log J –log V of about 2as seen from plot in Fig.6.This power dependence suggests that the dark current is a space-charge-limited current (SCLC)dominated by a single trap level.According to Lampert theory,the relation for the current density in this case is given by [33]J SCLC
?98 2my V 2d
3,(7)
where d is the thickness of the TCVA ?lm,e 2is the
permittivity of TCVA,m is the mobility of holes,and y is the trapping factor,which is de?ned by the ratio of the free charge to trapped charge and given by
y ?N v N t es Texp àE t
k B T
,(8)
where k B is Boltzmann’s constant,N v is the effective density of states in the valence band edge,and N t(s)is the total trap concentration at energy level E t above the valence band edge.
-1.5
-1.0
-0.5
0.00.5
1.0
1.5
10-6
10-5
10-4
10-3
10-2
10-1
J (A /c m 2)
V (volts)
Fig.4.Dark J –V characteristics of the fresh TCVA/p-Si cell at different temperatures in both forward and reverse bias.
0.00
0.05
0.10
0.150.200.250.30
-19-18-17-16-15-14
-13
-12-11-10
-9
l n [ J (A /c m 2) ]
V (volts)
Fig.5.Variation of ln J with V of TCVA/p-Si cell at low voltage and temperature dependence of reverse saturation current-density for TCVA/p-Si cell.
M.M.El-Nahass et al./Solar Energy Materials &Solar Cells 91(2007)1120–1126
1123
The inset of Fig.6shows the dependence of log J on 1/T in the SCLC region,which yields a straight line.The total trap concentration,N t(s),may be determined from the intercept on the current density axis and the slope gives the single traps level,E t .The values of N t(s)?4.16?1020cm à3and E t ?0.34eV were obtained assuming N v ?1021cm à3[34]and using e ?2.69?10à13F cm à1and m ?2.1?10à2cm 2V à1s à1,which is measured by us in different work and under https://www.sodocs.net/doc/a011357467.html,ing Eq.(8)and the data from Fig.6,a value for the trapping factor,y ,of 4.13?10à6was obtained.
In reverse-bias direction,it is observed that the current has a small dependence on voltage,although it increases rapidly with temperature indicating that the reverse current should be limited by another transport mechanism.The leakage current occurs via generation-recombination of carriers in the depletion region and is given by the relationship [29]J R ?const :V 1=2.
(9)
Fig.7is a graphical representation of J R versus V 1/2at constant temperature.A linear relationship was obtained indicating that the reverse current is controlled by the generation recombination of carriers in the depletion region [29,35].The reverse current resulting from genera-tion recombination of carriers is thermally activated [29,36]and is controlled by J R $exp eàD E =k B T T,
(10)
where D E is the carrier activation energy.The temperature dependence of ln J on 1/T is shown in the inset of Fig.7and activation energy of D E ?0.54eV was obtained.This is approximately one half the band gap of Si (where E g ?1.12eV),which suggests that the dark current dose indeed originates in the substrate.Thus,both the temperature and voltage dependence of the reverse dark current indicates that the dominant mechanism for leakage
current over a wide range of temperatures is generation and recombination of carrier in Si bulk rather than at the organic–inorganic interface or in the organic ?lm itself.3.2.Capacitance–voltage characteristics
The capacitance of the TCVA/p-Si cell was measured at a high frequency of 1MHz in dark and at room temperature.Fig.8shows a typical C –V characteristic obtained from as fabricated and g -irradiated cells.As observed from this ?gure,1/C 2–V variation is linear in the voltage range studied indicating that the junction is of abrupt nature.For abrupt junction,the junction capaci-tance as a function of reverse-bias potential is given by [37]C 2?
qN 1N 2 1 22e 1N 1t 2N 2T1
eV bi àV T
,
(11)
0.00.20.40.60.8 1.0 1.2 1.4 1.6
5
10
1520
J R (10-6 A /c m 2)
V 1/2(volts)1/2
Fig.7.Reverse J –V characteristics of TCVA/p-Si cell at room tempera-ture and plot of ln(J R )versus 1000/T for TCVA/p-Si cell.
-1.0-0.8-0.6-0.4-0.20.00.2
0.4
0.6
0.8
1.0
0.0
0.51.01.52.02.53.03.54.0C -2 (1019 F -2)
V (volts)
Fig.8.C à2–V characteristic for TCVA/p-Si cell before and after exposure to g -rays.
-0.6
-0.5
-0.4
-0.3-0.2-0.10.00.10.2
-7.0
-6.5-6.0-5.5-5.0-4.5-4.0
-3.5-3.0-2.5
l o g [ J (A /c m 2) ]
log [ V (volts) ]
Fig.6.Variation of log(J )with log(V )at higher forward voltage bias for TCVA/p-Si cell and variation of ln(J )with 1000/T in SCLC for TCVA/p-Si cell.
M.M.El-Nahass et al./Solar Energy Materials &Solar Cells 91(2007)1120–1126
1124
where N2is the donor density in TCVA,and V bi is the built-in potential.
The built-in voltages were calculated by extrapolating the1/C2curve to V?0and the slope of the straight lines gives the donor concentration in TCVA.The built-in voltage,donor concentration,capacitance of the device, C o,at zero bias were estimated and listed in Table2.A reduction in the built-in voltage and an increase in the interface states at the organic–inorganic interface were obtained after exposure to g-radiation.
3.3.Photovoltaic response of the TCVA/p-Si heterojunction To estimate the photovoltaic response,the TCVA/p-Si cell under illumination through the Au electrode was connected to a load resistance variable from zero to in?nity.The current through the load and the voltage across the junction were measured at room temperature and plotted as shown in Fig.9for as fabricated and g-irradiated cells.The incident light produced a short-circuit photocurrent-density J sc and an open-circuit voltage V oc.The solar cell parameters are obtained from the following equations[38],where:
The experimental ef?ciency of the solar cell is given by Z%?P M=P in?eFF V oc J sc=P inT?100,(12) where P in is illumination intensity impinging on the cell that was estimated by80mW cm2and FF is the?lling
factor given by
FF?V M J M=V oc J sc(13)
where V M and J M are potential and current-density at maximum power point,therefore the maximum power obtained from the cell is given by
P M?V M J M?FF V oc J sc.(14)
The solar cell parameters of as fabricated and g-irradiated cells have been calculated and listed in Table 3.The solar cell ef?ciency was obtained without correcting for re?ection or electrode absorption losses.It was about 3.1%,which is in order of the value obtained for polythiophene/GaAs[11]and it is greater than that reported for CuPc/p-Si[12],ZnPc/Si[13,14],MgPc/n-Si [15],Dicyclopropyl fulgide/n-Si[16],NiPc/p-Si[17],TPP/ n-Si[18],CuPc/n-GaAs[19],and H2Pc/n-Si[20].
The TCVA/p-Si cell was so sensitive to the light where they exhibit a high photocurrent level even under zero bias. Under illumination,it has been assumed that the absorp-tion of light by TCVA creates excitons and by Si creates electron/hole pairs.The photocurrents observed are attributed to the transport of charges formed at the organic–inorganic interface as a result of charge separation from the excitons and electron/hole pairs at the interface. When the light is incident,large number of photo-carriers are generated at organic–inorganic interface and subse-quently dissociation takes place under the in?uence of the electric?eld at this interface.Once dissociation occurs, electrons are immediately transferred in Si by the built-in ?eld in organic–inorganic heterojunction and are collected by the Al electrode,while holes are transported in the TCVA layer toward to Au electrode.
It is obvious that the photocurrent in the irradiated cell was reduced and the power conversion ef?ciency decreased. After exposure to100kGy absorbed dose,?ll factor and the power conversion ef?ciency decrease by20.9%and 39%,respectively,of the original value.The above behavior can be understood by classifying the radiation-induced defects into the bulk or the surface of the cell.The degradation could be due to the creation of a radiation defect,which compensates the positive charges at the organic–inorganic interface and consequently leads to a reduction in the cell output parameters[39].
4.Conclusion
The fabricated TCVA/p-Si hybrid organic–inorganic solar cell was characterized by J–V and C–V methods.At
Table2
The values of built-in voltage,donor concentration,capacitance of the device at zero bias of TCVA/p-Si cell at room temperature
C o(pF)V bi(volts)N2(1012cmà3) As fabricated cell2540.68 2.04
g-Irradiated cell2790.57 2.11
0.00.10.20.30.40.50.60.70.8
2
4
6
8
10
J
(
m
A
/
c
m
2
)
V (volts)
Fig.9.Load J–V characteristics under80mW cmà2illumination for TCVA/p-Si cell before and after exposure to g-rays.
Table3
The photovoltaic parameters of TCVA/p-Si cell at room temperature
J sc
(mA cmà2)
V oc
(V)
J M
(mA cmà2)
V M
(V)
FF Z%
As fabricated
cell
9.150.70 5.400.460.3883.10 g-Irradiated cell8.230.60 5.060.300.3071.89
M.M.El-Nahass et al./Solar Energy Materials&Solar Cells91(2007)1120–11261125
relatively low voltages,the dark forward current density is dominated by the multi-step tunneling model with 2.1?105steps required for traversing through the deple-tion region.While at high-voltage,a space-charge-limited-conduction mechanism is observed with trap concentration of4.16?1020cmà3.The operating transport mechanism in reverse-bias direction is generation recombination of carriers,which is a thermally activated process.The C–V measurements showed that the junction is of abrupt nature with built-in voltage of0.68.Under illumination,the TCVA/p-Si cell showed promising photovoltaic properties and had solar conversion ef?ciency of 3.1%.The g-radiation does not in?uence the operating transport mechanisms in the cell,but it harmfully decreases the cell parameters.
Acknowledgments
The authors would deeply like to thank Prof.Abdullah M.Asiri(Chemistry Department,Faculty of Science,King Abdul Aziz University,Saudi Arabia)for his help with the synthesis of the material.One of authors,Ahmed A. Darwish,wishes to thank Sana’a University,Yemen,for the award of a scholarship for PhD at Ain Shams University,Egypt.
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