p-Si heterojunction

Microelectronic Engineering 116 (2014) 58–64

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Characterization and photovoltaic performance of organic device based on CoMTPP/p-Si heterojunction M.M. El-Nahass a, A.A. Atta a,b,⇑, E.F.M. El-Zaidia a, A.A. Hendi c a

Physics Department, Faculty of Education, Ain Shams University, Rorxy Square 11757, Cairo, Egypt Physics Department, Faculty of Science, Taif University, Taif, 888 Taif, Saudi Arabia c Physics Department, Sciences Faculty for Girls, King Abdulaziz University, Jeddah, Saudi Arabia b

a r t i c l e

i n f o

Article history: Received 15 March 2013 Received in revised form 21 September 2013 Accepted 14 October 2013 Available online 22 October 2013 Keywords: CoMTPP Nanocrystalline structure Conduction mechanism Capacitance–voltage Photovoltaic properties

a b s t r a c t Hybrid organic/inorganic heterojunction of nanocrystalline 5,10,15,20-Tetrakis(4-methoxyphenyl)21H,23H-porphine cobalt(II), (CoMTPP) and p-Si was fabricated by using the conventional thermal evaporation technique. The morphologies of the CoMTPP/p-Si were investigated by scanning electron microscopy (SEM). The dark current–voltage (I–V) characteristics of Au/p-CoMTPP/p-Si/Al heterojunction diode measured at different temperatures ranging from 298 to 423 K have been investigated. Analytical approaches involving the thermionic emission and space charge limited currents (SCLC) were used to explain the I–V behavior in the forward bias. On the other hand, the carrier generation–recombination process limits the reverse current. The dependence of capacitance–voltage (C2–V) for the device CoMTPP/p-Si was found to be almost linear which indicates that the junction behavior is abrupt nature and then the essential junction parameters were obtained. The performance of heterojunction showed a photovoltaic behavior with an open circuit voltage, Voc, of 0.283 V, short circuit photocurrent ISC, of 0.433 mA and power conversion efficiency, g of 3.6%. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Organic photovoltaic devices have been one of the most exciting frontiers of current renewable energy research because of their advantages such as low cost of fabrication, light weight and easy processing [1–4]. A broad range of solar cell technologies are currently being developed including dye-sensitized nanocrystalline photoelectrochemical solar cells, polymer/fullerene bulk heterojunctions, small molecule thin films and organic–inorganic hybrid devices [5]. In organic devices, the photovoltaic effect arises from the dissociation of excitons into charge carriers in the built-in field region (depletion region) of a rectifying contact [6]. Hybrid inorganic–organic heterojunction cells have been subject of increasing interest over the last few years as a promising candidate to overcome the efficiency limitation of purely organic solar cells [7]. Macrocyclic porphyrins dyes are among few organic semiconductors that have strong absorption in the visible part of the solar spectrum, large exciton diffusion length and easy molecular structure modification by chemical procedure and efficient photoactivity as photoconverters [8]. Porphyrins are of interest for optoelectronic and other device applications due to their synthetic versatility, thermal stability, large p-electron system, and photochemical ⇑ Corresponding author at: Physics Department, Faculty of Education, Ain Shams University, Rorxy Square 11757, Cairo, Egypt. E-mail address: [email protected] (A.A. Atta). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.10.008

properties. Owing to its unique opto-electrical property, porphyrins thin films offer the promise of widespread adoption in numerous technology areas, including electro-luminescent devices [9], photonic devices [10], solar energy conversion devices [11], photodynamic therapy and diagnosis of cancer using laser excitation [12], photochromic recording medium [13], catalysis [14], photoelectro-chemical cell [15], optoelectronic device fabrication [16] and gas sensors [17]. Our previous work on thermally evaporated (CoMTPP) films revealed that the films have a wide absorption range spectrum in UV–vis region, which is appropriate for photovoltaic application [18,19]. The present study deals with the rectification properties, the working conduction mechanism, and junction parameters of CoMTPP/p-Si heterojunction prepared by conventional thermal deposition technique. The dynamic capacitance – voltage (C–V) method was also applied to determining the carrier’s concentration, the built-in voltage and the type of the formed junction. Also the photovoltaic properties of this junction are investigated.

2. Experimental techniques The 5,10,15,20-Tetrakis(4-methoxyphenyl)-21H,23H-porphine cobalt(II), (CoMTPP) powder (purity 96%) used in this study is obtained from Aldrich Chem. Co. The molecular structure of CoMTPP is shown in Fig. 1.

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ments at higher temperatures. Illumination characteristics for the same samples under 10 mW/cm2 white light source (tungsten lamp) were measured. The intensity of the incident light was measured using a digital lux meter (BCHA, model 93408). 3. Results and discussion

Exothermic Endothermic

Thermal analysis was carried out by DSC using a Shimadzu DSC50 apparatus. The DSC scan was obtained for the sample by heating accurately weighed sample (4.36 mg) in N2 atmosphere at a constant heating rate of 10 K min1. The temperature range covered in DSC was from room temperature to 650 K. The topography of CoMTPP thin films on p-Si was investigated by using a scanning electron microscope (SEM) model (JEOL JSMT200). The p-type Si wafer with (1 0 0) orientation and hole concentration of 1022 m3 samples is obtained from Nippon Mining Co. In order to remove the native oxide on the surface of p-Si, the substrate was etched by CP4 solution (HF:HNO3:CH3COOH) in ratio (1:6:1) for 10 s, after etching, the Si wafers were washed with distilled water and then with ethyl alcohol. The p-Si substrates were coated from one side by CoMTPP thin film of thickness 100 nm using thermal evaporation technique to fabricate CoMTPP/p-Si heterojunction cell. The front contact of this heterojunction was made with gold mesh electrode to be used as ohmic electrode. The back contact was made by depositing a relatively thick film of Al to the bottom of the p-Si substrate. Thus, an Au/CoMTPP/p-Si/Al cell was obtained. A schematic diagram of the heterojunction device is given in Fig. 2. The fabricated CoMTPP/p-Si cell was annealed in air at 383 K for 1 h to complete the junction formation. Annealing of heterojunction is the usual step for obtaining the best efficiency 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 at 1 MHz and at room temperature, using a computerized CV-410 meter (Solid State Measurement, Inc., Pittsburgh). The current–voltage (I–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 high impedance electrometers (Keithley 617). The dark I–V characteristics were obtained in a complete dark chamber at room temperature or inside a dark furnace in case of measure-

Heat Flow (mW)

Fig. 1. Molecular structure of CoMTPP.

The differential scanning calorimeter (DSC) analysis is used to evaluate the thermal stability of the organic materials. Fig. 3 shows DSC curve of CoMTPP at a heating rate of 10 K/min. These curves reveal that the CoMTPP has a thermal stability up to approximately 650 K, increasing the temperature above this degree causes a sublimation of the material into gas vapor molecules. This study indicates that the compound could be used in its solid form for any application up to 650 K. Fig. 4 shows the SEM image of the as-deposited CoMTPP film on glass at room temperature. It is obvious that the average grains in the range of 380 nm. The SEM micrograph of the as-deposited CoMTPP on p-Si substrate is shown in Fig. 5. It is clear that the particles have leaves-shaped with nanoparticle of average size 120 nm. So the type of substrate may be influenzas on the crystallite size of as-deposited CoMTPP. To study the formation of junction, the current–voltage (I–V) characteristics of Au/p-CoMTPP/p-Si/Al have been measured at different temperatures ranging from (298 to 373 K) and in voltage range (–1.4 to 1.4 V) for a thickness of 80 nm as shown in Fig. 6. The results show that curves have the same I–V behavior, for the same applied voltage, current increases with increasing temperature, indicating a negative temperature coefficient for the resistivity. The results show also existence of leakage current in reverse bias direction. The device clearly exhibits the rectification behavior in dark which is enhanced by increasing the temperatures. The results exhibited diode-like behavior, which can be understood by the formation of a barrier at the interface. The rectification ratio (RR) is defined as the ratio of the forward current to the reverse current at a certain value of the applied voltage. The rectification ratio has been estimated to be 50 at bias potential of 1 V. The series resistance, Rs, and the shunt resistance, Rsh, are important parameters in determining cell performance. The series resistance can be estimated from the inverse slope at positive voltages where the I–V curve becomes linear. The Rsh can be obtained by taking the inverse slope at reverse where the I–V curve becomes linear. The obtained values of the series resistance (Rs) and the shunt resistance (Rsh) at room temperature were

300

350

400

450

500

550

600

650

700

Temperature (K) Fig. 2. Schematic diagram of Au/CoMTPP/p-Si/Al solar cell.

Fig. 3. DSC thermogram of CoMTPP compound at a heating rate of 10 K/min.

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0.01

Region (I)

1E-3

Region (II)

IF (A)

1E-4

298K 313K

1E-5

333K 353K 373K

1E-6

1E-7

Fig. 4. The SEM micrograph for CoMTPP deposited on glass.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

V (V) Fig. 7. Semilogarithmic plots of the forward bias of I–V characteristics at different temperature.

transport mechanisms controlling the conduction process in the studied device. As shown in Fig. 7 the dark forward current voltage (IF–V) characteristics for the device are consisting of two distinct regions. The current increases exponentially with applied voltage and current deviates from the exponential due to the effect of a series resistance. The system behavior indicates the presence of two conduction mechanisms which may be described by the two-lumped diode model as [20]:

    qðV  IRs Þ qðV  IRs Þ V  IRs I ¼ I01 exp  1 þ I02 exp 1 þ c1 kB T c 2 kB T Rsh

Fig. 5. The SEM micrograph for CoMTPP deposited on p-Si.

0.01

1E-3

I (A)

1E-4

298K 313K 333K 353K 373K

1E-5

1E-6

1E-7 -1.5

-1.2

-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9

1.2

1.5

V(V) Fig. 6. I–V characteristics of CoMTPP/p-Si heterojunction at different temperatures in forward and reverse bias potential.

determined to be 250 X and 30  103 X, respectively. The analysis of the I–V characteristics is also extremely useful to identify the

ð1Þ

where I0 is the reverse saturation current, q is the electronic charge, V is the applied voltage, kB is the Boltzmann constant, T is the temperature and c is the ideality factor, Rs and Rsh are series and shunt resistances, respectively. The current–voltage characteristics of the device are largely dependent on the series and shunt resistance. A lower value of Rs means that higher current will flow through the device. Rsh corresponds to fewer leaks in the device. The ideal device would have a series and parallel resistances approaching to zero and infinity, respectively. The subscripts 1 and 2 indicate that two possible contributions to the diode current could be present. The information about the conduction mechanism can be obtained from the I–V characteristics at different temperatures. In region I (V < 0.35 V), the exponential behavior of I–V characteristics depends on the property of active material used in the investigated device. The slope of I–V characteristics in the exponential region depends on two parameters, i.e. ideality factor, c and the reverse saturation current Io. The ideality factor gives information about the recombination process taking place in the device and shape of the interfaces, i.e. the internal bulk morphology for the organic devices [21]. The other parameter that affects the exponential part of I–V characteristics is the saturation current density which gives the number of charges able to overcome the energetic barrier in the reverse bias [22,23]. The present device shows an exponential behavior in region (I) Fig. 8. This exponential dependence at this voltage range can be attributed to the formation of depletion region between CoMTPP and p-Si. In the region (I), the forward current increases exponentially. Thus, the voltage dependence of the junction current can be expressed as [24]:

I ¼ Io exp



qV c kB T

 ð2Þ

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voltage which clearly shows a linear behavior. From the slope of this straight line, the potential barrier height ub, is determined and equals to 0.40 eV. At relatively high forward voltages in the region (II) (0.35 6 V 6 1.4 V), as shown in Fig. 7. A different mechanism is dominant. The obtained IF–V data show in Fig. 7 is replotted in terms of log (IF)–log (V) and is shown in Fig. 10. As observed in this figure, the current shows a power law exponent of the form I a Vm with value of the exponent m greater than 2, slope decreasing slightly from 2.75 to 2.58 upon increasing temperature as seen from plot in Fig. 10. Which means that the conduction in the device is controlled by the space charge limited characteristic for a p-type semiconductor with an exponential distribution of trapping levels is given by [26]

1E-10

IO / T2 (A/ K2)

1E-11

1E-12

J ¼ qAlh NV 1E-13 2.6

2.7

2.8

2.9

3.0

3.1

3.2

3.3

3.4

1000/ T (K-1) Fig. 8. Semilogarithmic plots of Io/T2 versus 1000/T for CoMTPP/p-Si heterojunction.

where Io is the saturation current and can be obtained by extrapolation of the linear portion of log (IF)–V to the log (IF) axis, at zero voltage, q is electronic charge, V is the applied voltage across the device, kB is the Boltzmann constant, T is the absolute temperature and c is the diode quality factor. The parameters Io and c have been determined from the curves shown in Fig. 7 together with Eq. (2). The calculated values of Io and c at room temperature are 6.3  108 A and 1.76 ± 0.02, respectively. The deviation of ideality factor c from unity may be attributed to either recombination of electrons and holes in depletion region and/or the increase of the diffusion current due to increasing applied voltage [24]. The diode quality factor is almost found to be constant at different temperatures. This behavior is in accordance with the thermionic emission mechanism. According to the thermionic conduction, the saturation current is given by [25]:

Io ¼ AA T 2 exp

  qub kB T

ð3Þ

where A is the effective area of the device, A⁄⁄ is the Richardson constant and ub is the barrier height. According to Eq. (3), the plot of Io/T2 and IF/T2 should yield a straight line. Fig. 8 represents the semi-logarithmic variation of Io/T2 vs. 1000/T and Fig. 9 represents the semi-logarithmic variation of IF/T2 vs. 1000/T at different

1E-10

qP  kB T t

 PðEÞ ¼ P0 exp

V lþ1 d

ð4Þ

2lþ1

E kB T t

 ð5Þ

where P(E) is the concentration of traps per unit energy range above the valence band edge. The total trapping concentration Nt may be obtained by performing an integration over the distribution is given by Gould and Rahman [28] as the following equation:

Nt ¼ P  kB T t

ð6Þ

The variation of log IF with 1000/T for p-CoMTPP/p-Si/Al heterojunction diode at different bias potentials is shown in Fig. 11, The slopes of these lines are given by [29]:

Slope ¼

  Dðln IÞ 2 ¼ T t log eV=qd Nt Dð1=TÞ

ð7Þ

And the intercept on the log J axis is given by [30]:

Log J o ¼ logðqlNv V=dÞ

ð8Þ

-1.5

298K 313K 333K 353K 373K

-2.0

Log [ (IF) (A)]

2

2

IF /T (A /K )

1E-9

l

e

where lh is the mobility of charge carrier, Nv is effective density of states at the valence band edge, e is permittivity of the CoMTPP thin film, Po is trap concentration per unit energy range at the valance band edge, and d is film thickness. This expression predicts a power-law dependence of I on V with the exponent m = l + 1 ) Tt = (m  1) T [27], where m = 2.75 and l = 1.75 represents the ratio Tt/T. Tt is a temperature parameter characterizing the exponential trap distribution which is given by:

1E-8

0.1V 0.2V 0.3V



-2.5

-3.0

-3.5

1E-11

-4.0

-4.5 1E-12 2.6

-0.4 2.7

2.8

2.9

3.0

3.1

3.2

3.3

3.4

1000/T (K-1) Fig. 9. Semilogarithmic plots of IF/T2 versus 1000/T for different voltage.

-0.3

-0.2

-0.1

0.0

0.1

0.2

Log[(V) (V)] Fig. 10. Variation of log I with log V at higher forward voltage bias for CoMTPP/p-Si heterojunction.

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10-2

IR ( A )

IF (A)

0.7 V 1V

1E-3

0.5V .7V 1V

10-3

1E-4

1E-5

10-4 2.6

2.7

2.8

2.9

3.0

3.1

3.2

3.3

3.4

3.5

2.6

1000/T (K-1)

2.7

2.8

2.9

3.0

3.1

3.2

3.3

3.4

3.5

1000/ T (K-1)

Fig. 11. Variation of log IF with 1000/T for p-CoMTPP/p-Si/Al heterojunction diode at different bias potentials.

The value of dielectric constant equals to 3.65 was evaluated, from optical properties [18,19] and the NV is the effective density of sates at the valence band edge was taken as 1027 m3 [31] substituting these value in Eq. (7), the total trap concentration, Nt, was estimated at room temperature: Tt  521.5 K as 1.2  1025, 1.52  1025 and 2.15  1025 m3 at applied voltages 0.5, 0.7 and 1Volt, respectively. By substituting the values of Nt, and Tt the following values of trap parameters were derived at room temperature: Po = 1.67  1045 J1 m3, Po = 2.11  1045 J1 m3 and Po = 3  1045 J1 m3 at 0.5, 0.7 and 1Volt applied voltages, respectively. The values of Nt and Po seem to be reasonable in that they are of the same order as those obtained by Zeyada et al. [32] for Au/p-DOPNA/p-Si/Al heterojunction. The measured reverse-bias characteristics of CoMTPP/p-Si heterojunctions at different ambient temperatures are shown in Fig. 12. It is observed that the current has a weak dependence on voltage indicating that the reverse current should be limited by generation–recombination [33–36]. The leakage current resulting from generation–recombination of carriers is thermally activated process [33,35] where the value of the current increases with increasing the temperature as given by the relationship [33]

IR / expðDEt =kB TÞ

Fig. 13. Semilogarithmic plots of IR versus 1/T for CoMTPP/p-Si heterojunction at different voltage.

where DEt is the carrier activation energy. The semi logarithmic of the reverse-current (IR) was plotted as a function of 1000/T at different reverse-bias voltages as shown in Fig. 13. The activation energy is determined from the slope of the straight line, which has a value of 0.46 eV. The obtained activation energy is approximately equal to half the band gap of Si; (where Eg = 1.11 eV). The dark capacitance–voltage characteristics of Au/CoMTPP/ p-Si/Al heterojunction were measured as a function of reverse applied bias (V) at I MHz. Measurement of the depletion region capacitance under forward bias is difficult because the diode is conducting and the capacitance is shunted by a large conductance. However, the capacitance can be easily measured as a function of the reverse bias. As the frequency increases, the interface capacitance contribution to the device capacitance decreases [37]. Fig. 14 shows the relation between 1/C2 vs. V of Au/p-CoMTPP/p-Si/Al heterojunction at room temperature. The linearity of this relation indicates that the junction is considered as a one sided abrupt heterojunction. For one-sided abrupt junctions, the capacitance per unit area at equilibrium is given by [33]:

ð9Þ C¼

dQ c dðqNWÞ eeo ¼ ¼ 2 dV W d½qðN=2eeo W

ð10Þ

1E-3

1.4

298K 313K 333K 353K 373K

1E-6 0.0

23

1E-5

1.0

2

IR ( A )

-2

1/ C (10 F )

1.2

1E-4

0.8

0.6

0.4 0.2

0.4

0.6

0.8

1.0

1.2

1.4

V (V) Fig. 12. Semi logarithmic plots of IR against for CoMTPP/p-Si heterojunction.

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

V (volt) Fig. 14. 1/C2–V characteristics for CoMTPP/p-Si heterojunction.

1.0

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where

Table 1 Photovoltaic parameters of some organic solar cells.

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2eeo V b W¼ qN

ð11Þ

where N is the ionized traps, Vb is the built-in potential, q is the electronic charge, eo is the permittivity of free space, W is the width of the depletion region and e is the relative permittivity of the CoMTPP thin films (e = 3.65) [18,19]. When a voltage is applied to the junction, the total electrostatic potential variation across the junction is given by (Vb + V) for the reverse biasing. So, in the reverse case;

 W¼

2eeo ðV b þ VÞ qN

Solar cell

Jsc (mA/ cm2)

Voc (V)

Fill factor

Efficiency (%)

Ref.

Au/CoMTPP/p-Si/Al

4.33

0.283

0.3

3.6

Au/Tetraphenyl porphyrin/n-Si/Al Al/TPP/ZnTPP/Au Au/p-DOPNA/p-Si/Al Au/4-tricyanovinyl-N,N diethyl aniline/p-Si/Al Al/ ZnTPP/MC/Au

2.76

0.25

0.37

2.45

Present work [40]

– 4.83 9.15

0.51 0.29 0.7

0.38 0.33 0.39

2.7 5.74 3.1

[41] [42] [43]

–

0.66

0.25

0.19

[44]

1=2 ð12Þ

If the measurements are carried out at sufficiently high frequencies, the charge at the interface states cannot follow an AC signal. This will occur when the time constant is too long to permit the charge carriers to move in and out of the states in response to an applied signal [33,38]. Thus the depletion layer capacitance can be expressed as:

1 C2

¼

2 qN a eeo A2

ðV b þ VÞ

ð13Þ

The built-in voltages of the junctions were calculated by extrapolating the 1/C2 curve to V = 0 and from the slope of the straight lines the values of Na can be determined. The carrier concentration Na was found to be 0.43  1015 m3 and from the intercept the value of built-in-potential Vbi for the diode was estimated of 0.38 V. The capacitance of the device, Co, at zero bias was measured and found to be 3.9 pF. The (I–V) characteristic of Au/p-CoMTPP/p-Si/Al with active area of 0.1 cm2 and under illumination of power of 10 mW/cm2 is illustrated in Fig. 15. The current value at a given voltage for this device under illumination is higher than in the dark. This indicates that the absorption of light by the active layer generates carriers contributing photocurrent due to the production of excitons and their subsequent dissociation into the free-charge carriers at the barrier, i.e. CoMTPP/p-Si interface. As it is observed from Fig. 15 the device shows photovoltaic characteristics with short-circuit photocurrent (Isc) (The current flowing freely through an external circuit that has no load resistance; the possible maximum current of 0.433 mA and open circuit voltage (Voc) (the difference of electrical potential between two terminals of a device when there is no external load connected) of 0.283 V. The plot of electric output power vs. voltage is also shown in Fig. 15 the electrical power

0.07 0.4

0.05

I(mA)

0.3

0.04 0.2

0.03 0.02

0.1

Output power (mW)

0.06

0.01 0.0 0.00

0.05

0.10

0.15

0.20

0.25

0.00 0.30

V(volt) Fig. 15. Current–voltage characteristics and output power under illumination.

Fig. 16. Dynamic I–V characteristics in dark and under illumination condition.

increases with bias voltage and reaches a maximum value. The maximum point is called the maximum power point with coordinate (PM = IM  VM). Here IM and VM are maximum current and voltage values of maximum power point, respectively. This maximum value represents the condition, where the solar cell can deliver its maximum power to an external load. The VM and IM values for Au/CoMTPP/p-Si/Al device were found to be 160 mV and 0.225 mA, respectively. The fill factor FF, is given by equation [39]

FF ¼

IM V M Isc V oc

ð14Þ

where VM and IM are the potential and current at maximum power point, Fill Factor, FF is found to be 0.30 this value depends on the solar cell device and structure. The value of the fill factor is low in comparison to those of solar cells based on inorganic materials. The main reason of this effect is generally found to be the field dependent nature of the charge photogeneration process or high series resistance of the organic layer. Which is found to be 0.30; this value depends on the solar cell device and structure. The value of the fill factor is low in comparison to those of solar cells based on inorganic materials. The main reason of this effect is generally found to be the field dependent nature of the charge photogeneration process or high series resistance of the organic layer. The maximum power obtained from the cell

PM ¼ V M IM ¼ FFV oc Isc

ð15Þ

The power conversion efficiency, g, of the cell is given by the following equation [39]:

64

g¼

M.M. El-Nahass et al. / Microelectronic Engineering 116 (2014) 58–64

PM J V oc ¼ FF sc  100 Pin Pin

ð16Þ

where Pin is the incident light intensity, that was previously estimated by 10 mW/cm2 and Jsc (=Isc/A) is short circuit. These parameters lead to power conversion efficiency (g) of 3.6%. The determined photovoltaic parameters for Au/CoMTPP/p-Si/Al solar cell are compared with other published organic/inorganic solar cell [40–44] as tabulated in Table 1. A reasonable characteristic is obtained for CoMTPP/p-Si solar cell. Fig. 16 shows the current–voltage characteristics of p-CoMTPP/p-Si heterojunction solar cell at room temperature under a dark and illumination conditions using an oscilloscope. A good diode with rectification behavior can be seen for the investigated CoMTPP/p-Si heterojunction under dark condition as shown in Fig. 16. The dynamic I–V characteristics support the rectification and the photovoltaic performance and characterization of the investigated device under a dark and illumination condition. 4. Conclusion Organic–inorganic heterojunction is fabricated using thermally evaporated CoMTPP on p-type Si wafer. The SEM shows the asdeposited CoMTPP on p-Si has leaves-shaped with nanoparticle of average size 120 nm .The dark current–voltage measurements induct that the rectification behavior of p-CoMTPP/p-Si heterojunction. The thermionic emission and space charge limited current (SCLC) are used to explain the dark forward (I–V) characteristics. The basic diode parameters such as the ideality factor, series resistance and the barrier height were extracted from the I–V measurement of Au/CoMTPP/p-Si/Al heterojunction rectifying contacts. Under reverse bias, the junction leakage was primarily dominated by the generation and recombination of charges within the bulk of Si substrate [36]. The investigation of C–V measurements at 1 MHz shows a linear behavior which indicates that the junction is abrupt and then the essential junction parameters were obtained. A short–circuit current of 0.433 mA, an open–circuit voltage of 0.283 V and a fill factor of 30% were extracted from (I–V) characteristics under illumination and the power conversion efficiency (g) of 3.6%. References [1] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [2] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. [3] K. Kim, J. Liu, M.A.G. Namboothiry, D.L. Carrolla, Appl. Phys. Lett. 90 (2007) 163511.

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