p-Si heterojunction

Synthetic Metals 161 (2011) 2253–2258

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Electrical and photovoltaic properties of FeTPPCl/p-Si heterojunction M.M. El-Nahass, H.S. Metwally, H.E.A. El-Sayed, A.M. Hassanien ∗ Physics Department, Faculty of Education, Ain Shams University, Roxy 11757, Cairo, Egypt

a r t i c l e

i n f o

Article history: Received 30 June 2011 Received in revised form 14 August 2011 Accepted 20 August 2011 Available online 21 September 2011 Keywords: FeTPPCl Organic/inorganic heterojunction Hybrid heterojunction Photovoltaic

a b s t r a c t Hybrid heterojunction cell based on thermally evaporated 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride (FeTPPCl) as the organic semiconductor and p-Si wafer as the inorganic semiconductor have been investigated. This device showed rectification behaviour like diode. The conduction mechanisms and the diode parameters have been studied using current–voltage (I–V) characteristics in the temperature range (298–373 K) and capacitance–voltage (C–V) characteristics at room temperature. This cell exhibited photovoltaic characteristics with a short-circuit current (Isc ) of 2.8 mA, an open-circuit voltage (Voc ) of 0.475 V, a fill factor FF = 32%. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Porphyrins are found in a large variety of biological systems (e.g., bacterial photosynthetic system, cytochrome P450, cytochrome C, and hemoglobin) [1]. 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 [2]. Porphyrins are promising components to be used in molecular electronics due to their rich electronic/photonic properties. Porphyrins and metalloporphyrins have been incorporated into a large number of electronand energy transfer model systems and have also been used to create artificial light-harvesting antenna [3], photosynthetic reaction centers, photonic wires [4], and redox switches [5]. Preparation of supramolecular architectures of porphyrins on solid surfaces would constitute a basis for further development towards molecular circuitry or other constructions for molecular electronics applications [6]. Porhyrins dyes offer the practical applications in photonics (new organic solar cells, molecular optoelectronics, non-linear optical materials and smart materials), photomedicine (photodynamic diagnosis, therapy of cancer and the simulation of proteins in enzyme catalysis as well as molecular recognition) and photosynthesis (in modeling of electron transfer and charge separation phenomena in reaction centers) [6–8]. 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 [9]. Agranovich et al.

∗ Corresponding author. E-mail address: [email protected] (A.M. Hassanien). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.08.030

[10] reported that the absorption coefficient of light of an organic layer in the region of its resonances can be quite large. This means that in this case a very thin layer of a thickness smaller than the exciton diffusion length is able to absorb the sunlight in the resonant region of its spectrum and to transfer this energy to semiconductor layers. For this reason we can consider the organic component of the hybrid structure as a special type of concentrator of the solar energy. Several hybrid organic–inorganic material systems based on Si substrate have already been investigated such as NiPc/p-Si [11], TPP/n-Si [12], ZnPc/p-Si [13], 4-tricyanovinyl-N,Ndiethylaniline/p-Si [14], 2-(2,3-dihydro-1,5-dimethyl-3-oxo-2phenyl-1H-pyrazol-4-ylimino)-2-(4-nitrophenyl)acetonitrile/p-Si [15], CoPc/p-Si [16], oxazine/n-Si [17] polyaniline/p-silicon [18] and Au/CuPc–PTCD/p-Si/Al [19]. Our previous work on thermally evaporated 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride (FeTPPCl) films revealed that the FeTPPCl film has a wide absorption range spectrum in UV–vis region, which is appropriate for photovoltaic application [20–22]. Furthermore, analysis of the absorption coefficient revealed indirect transitions with 1.50 eV as optical band gap energy. In addition, Tonezzer et al. [23] reported that FeTPPCl has promising features that leads to a useful sensing material for optical gas sensing applications. The schematic diagram of the molecular structure of FeTPPCl is shown in Fig. 1. The electrical properties of heterojunctions were characterized by measuring the current density–voltage (J–V) curve in the dark and under illumination. The J–V curve in the dark is the easy way to estimate the quality of the junction, grid and contact resistances. Also, the basic parameters including open-circuit voltage (Voc ), short-circuit current density (Jsc ), fill factor (FF) are usually determined through three points on the J–V curve under illumination. Analysis of the J–V curve gives not only the basic parameters,

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ln (I, A)

-8 -10 298 K 313 K 333 K 353 K 373 K

-12 -14 -16 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

V , volt Fig. 2. I–V–T characteristics for Au/FeTPPCl/p-Si/Al at different temperatures in forward and reverse conditions.

but also the extracted parameters such as reverse saturation current, Jo , series resistance, Rs , shunt resistance, Rsh , and ideality factor, n. These parameters can be applied in turn to improve the device quality during fabrication and the solar cell performance [24–27]. In this work the attention has been focused on fabricating a junction of Au/FeTPPCl/p-Si/Al and investigating the electronic characteristic parameters of it using current–voltage (I–V) and capacitance–voltage (C–V) measurements. Also the photovoltaic properties of this junction are investigated. 2. Experimental techniques To obtain an Au/FeTPPCl/p-Si/Al cell, the sample has been prepared by using a polished p-type Si wafer with (1 0 0) orientation and hole concentration of 1.6 × 1023 m−3 with thickness of 400 ␮m. In order to remove the native oxide on p-Si surface, the substrate was etched by CP4 solution (HF:HNO3 :CH3 COOH in ratio 1:6:1) for 10 s, then rinsed with deionized water and isopropyl alcohol and dried in oven. The p-Si substrates were coated from one side by FeTPPCl thin film of thickness 80 nm using thermal evaporation technique to fabricate FeTPPCl/p-Si heterojunction cell. Evaporation of the material is carried out with a quartz crucible heated by a tungsten coil. The evaporation rates as well as the film thickness of the evaporated films was controlled by using a quartz crystal monitor FTM6, the deposition rate was controlled at 2.5 nm/s. The front contact of this heterojunction was made with gold mesh grid electrode. The active area of the fabricated heterojunction was about 0.09 cm2 . The back contact was made by depositing a relatively thick film of Al to the bottom of the p-Si substrate. The device had the same structure as our previous published works (see Refs. [11–16]). The obtained cell was annealed in air at 373 K for 1 h to complete the junction formation. Annealing of heterojunction has been the usual step in obtaining the best efficiency cells. This annealing might remove any channels, which could be raised during the fabrication [16]. In addition, thermal annealing condition was found very important for improving short circuit current, fill factor, and therefore the efficiency of the device [28]. The dark capacitance–voltage (C–V) characteristics for the fabricated cell were measured at 1 MHz and at room temperature, using a computerized 410 C–V meter interface model 4108. The dark

3. Results and discussion 3.1. Dark current–voltage characteristics Fig. 2 shows the typical current–voltage (I–V) characteristic observed for Au/FeTPPCl/p-Si/Al heterojunction under the forward and reverse bias measured at different temperatures ranging from 298 to 373 K and in the voltage range −2 to 2 V. The device exhibits rectification behaviour whereby the current–voltage (I–V) characteristics are limited by the properties of the inorganic semiconductor substrate and the magnitude of the energy barriers at the hetero-interface. The rectification ratio (RR), which is defined as the ratio of the forward current to the reverse current at a certain applied voltage, is estimated to be 15 at ±1 V at room temperature. Fig. 3 shows the current–voltage (I–V) characteristic observed for the heterojunction under the forward bias at different temperatures. It is obvious that, there is a high series resistance effects at relatively high voltage region VF > 0.4.

-4

(II)

(I)

-6 -8

ln (I, A)

Fig. 1. The molecular structure of FeTPPCl.

current–voltage (I–V) characteristics of the fabricated cell were measured simultaneously using a high impedance electrometer (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 measurements at higher temperatures. The illuminated (I–V) characteristics are carried out by using (Keithley 6517 A) under intensity of light =80 mW/cm2 provided by a tungsten lamp. The intensity of the incident light was measured by using digital lux-meter (BCHA, model 93408).

-10

298 K 313 K 333 K 353 K 373 K

-12 -14 -16 -18 0.0

0.5

1.0

1.5

2.0

V, volt Fig. 3. Dark I–V characteristics of Au/FeTPPCl/p-Si/Al at different temperatures in forward bias.

2.6

0.55

2.4

0.54

2.2

0.53

2.0

0.52

1.8

0.51

1.6

0.50

1.4 280

300

320

340

2255

Φb0 (eV)

n

M.M. El-Nahass et al. / Synthetic Metals 161 (2011) 2253–2258

0.49 380

360

T, (K) Fig. 4. Temperature dependence of the ideality factor, n , and the barrier height, ˚bo , for FeTPPCl/p-Si.

Fig. 5. Plot of [dV/d(ln I)] vs. I at different temperatures for FeTPPCl/p-Si.

When an organic film is inserted between metal and semiconductor, the current–voltage characteristics can be analyzed using the following equation [29,30]: I = Io exp

 q(V − IR )   s

nkB T

 q˚  bo

Io = AA∗ T 2 exp −

kB T

 q(V − IR )  s

1 − exp − ,

kB T

,

(1) (2)

where Io is the reverse saturation current, q is the elementary charge, V is the applied voltage, n is the diode ideality factor, kB is the Boltzmann’s constant, T is the temperature in Kelvin, Rs is the series resistance, A is the effective area and A* is the effective Richardson constant that takes the value 32 A/cm2 K2 for p-Si [30] and ˚bo is the zero-bias barrier height which is expressed as: ˚bo

kB T = ln q



AA∗ T 2 Io



.

(3)

The value of the ideality factor, n, is calculated from the slope of the linear portion of forward bias of I–V characteristic by using the following equation: n=

q kB T

 dV

d(ln I)



− IRs .

n=

kB T

d(ln I)

.

states and the series resistance, Rs , associated with bulk Si and the ohmic contacts. When the applied voltage is sufficiently large, the ideality factor and the series resistance were evaluated using a method developed by Cheung and Cheung [34]. The Cheung’s method is achieved by using Eq. (1) in the following form [30]:

(4)

Under relatively low forward bias condition (VF < 0.4 V), voltage across the series resistance (IRs ) can be neglected, then, Eq. (4) is simplified to the following equation:

 dV  q

Fig. 6. Plot of H(I) and I at different temperatures for FeTPPCl/p-Si.

V = IRs + n˚bo + n



I AA∗ T 2

dV nkB T + IRs , = q d(ln I)

(5)

By using Eqs. (3) and (5), the experimental values of the ˚bo and the ideality factor, n, for each temperature are determined from the intercept and slope of the relation ln(I) vs. V for forward bias VF < 0.4 V. It should be noted that the barrier height, ˚bo , is the contact barrier that exist at the interface between the organic and the inorganic layers. The heterojunction barrier height controls the injection of charge from the metal/organic contact into the inorganic semiconductor substrate [31]. Fig. 4 shows the temperature dependence of n and ˚bo for Au/FeTPPCl/p-Si/Al heterojunction, from which it can be seen that n decreases while ˚bo increases by increasing the temperature. Boyarbay et al. [18] reported that although the standard thermionic emission assumes the barrier height is independent of the temperature and that is why it fails to explain these results. Temperature dependence of the ideality factor and the barrier height can be elucidated on the basis of inhomogeneity barrier height model. The fluctuations in the barrier height are invariably present even in the most carefully fabricated diodes [32,33]. In the relative higher voltage region, VF > 0.5 V, the concave deviation from straight line is caused by the presence of the interface

kB T ln q

H(I) = V − n

kB T ln q



 ,

(6)

(7)

I AA∗ T 2



H(I) = IRs + n˚bo .

,

(8) (9)

Fig. 5 shows the plot of [dV/d(ln I)] vs. I. The slope of the linear fit gives the series resistance while its intercept gives the ideality factor. The temperature dependence of the series resistance and the ideality factor values are given inset Fig. 5. Using Eq. (7), the values of ˚bo and Rs were calculated at different temperatures from the relation between H(I) and I and given inset Fig. 6. Furthermore, Nord proposed an alternative method to determine the series resistance and the barrier height. The following functions have been defined in the modified Nord’s method [30,35] which applied to the full forward bias region of the I–V characteristics of the junction: F(V ) =

V kB T −  q



I AA∗ T 2

 ,

(10)

where  is the first integer (dimensionless) greater than n which obtained from I–V measurements, here it has been taken as 3. In this

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M.M. El-Nahass et al. / Synthetic Metals 161 (2011) 2253–2258 6.8 298 K 313 K 333 K 353 K 373 K

6.6 6.4 6.2 2

ln (J, A/m )

6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 -0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

ln (V, volt)

Fig. 7. Voltage dependence of F(V) at different temperatures for FeTPPCl/p-Si.

Fig. 8. Variation of ln(J) with ln(V) at higher forward voltage bias for FeTPPCl/p-Si and the temperature dependence of ln(J) inset it.

function the barrier height can be obtained by using the following equation [30,35]:

-6.0

Vo kB T − , b = F(Vo ) +  q

-7.0

(11)

kB T ( − n) . qI

(12)

V2 9 ε 3 , 8 d

(13)

where d is the thickness of the FeTPPCl film, ε is the permittivity of FeTPPCl,  is the mobility of charge carrier and  is the trapping factor, which is defined by the ratio of free charge to trapped charge and given by: =

Nv exp Nt

 −E  t

kB T

-8.0 -8.5 -9.0 -9.5 -10.0

The values of the barrier height and the series resistance at different temperatures were obtained using Eqs. (9) and (10) and listed inset Fig. 7. The discrepancy between the values of the series resistance and the barrier height obtained from Cheung’s functions and Nord’s functions may be due to the Cheung’s function applied only for the nonlinear region of forward bias while Nord’s function are applied for the whole forward bias region of I–V curve [30]. At the relatively higher voltage, the careful analysis of the I–V characteristics indicates that the increase in current diminish from the exponential behaviour to follow the power law. In this range of the I–V characteristics are influenced by the transport properties of the organic material. The transport through organic materials is governed by space-charge limited currents (SCLC) and follows Child’s law for a semiconductor with a moderate density of shallow traps [36]: J=

0.5 V 0.7 V

-7.5

ln ( IR)

where F(Vo ) is the minimum F(V) value of Nord’s function of F(V) vs. V graph and Vo is the corresponding voltage. Fig. 7 shows the voltage dependence of F(V) at different temperatures. Also, the series resistance, Rs , can be calculated from Nord’s functions as [30,35]: Rs =

-6.5

,

-10.5 -11.0 2.6

2.7

2.8

2.9

3.0

3.1

3.2

3.3

3.4

1000/T, (K-1) Fig. 9. Temperature dependence of ln(IR ) for FeTPPCl/p-Si.

another transport mechanism. Under reverse bias, the junction leakage was primarily dominated by the generation and recombination of charges within the bulk of Si substrate [14,37]. The reverse current resulting from generation recombination of carriers is thermally activated and is governed by [38]:

 E 

IR = IoR exp −

kB T

,

(15)

where E is the carriers activation energy. The temperature dependence of IR is shown in Fig. 9, from the slope of these straight lines the activation energy of 0.45 eV was obtained. This value is nearly equal one half of the energy band of Si substrate (Eg (Si) = 1.1 eV), which confirms that the temperature and voltage dependence of the dark reverse current is governed by generation and recombination of charge carriers in Si substrate rather than at organic–inorganic interface or organic material.

(14)

where Nv is the effective density of states at the valence band edge and Nt is the total trap concentration at energy level Et above the valence band edge. Fig. 8 shows the relation of ln(J) vs. ln(V) at different temperatures. This relation shows a power law dependence with order ≈2 indicating that the predominant operating current is SCLC with a single trap level. The inset of Fig. 8 shows the temperature dependence of ln(J) at 1.5 V, which gives a straight line. The trap level Et is determined from the slope of line fitting and has the value of 0.1 eV. In reverse bias direction, it is observed that the current has a small dependence on voltage, although it increases with temperature indicating that the reverse current should be limited by

3.2. Capacitance–voltage characteristics The dark capacitance–voltage characteristics of Au/FeTPPCl/pSi/Al heterojunction were measured at 1 MHz. High frequency (1 MHz) is used to investigate the device capacitance, because the data obtained from the C–V measurement in the low frequencies range represents the sum of the space charge capacitance and the interface capacitance. As the frequency increases, the interface capacitance contribution to the device capacitance decreases [18]. Fig. 10 shows the relation between 1/C2 vs. V of Au/FeTPPCl/pSi/Al heterojunction at room temperature. The linearity of this relation indicates that the junction is considered as an abrupt

M.M. El-Nahass et al. / Synthetic Metals 161 (2011) 2253–2258

2257

-5

1.2x10

-5

1.0x10

C-2 (pF -2)

-6

8.0x10

-6

6.0x10

-6

4.0x10

-6

2.0x10

0.0 -2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

V, (volt) Fig. 10. Plot of 1/C2 –V characteristics for FeTPPCl/p-Si.

Fig. 11. A proposed energy level diagram of a FeTPPCl/p-Si organic/inorganic heterojunction under thermal equilibrium.

heterojunction. The junction capacitance as a function of reversebias potential is given by [30,39]: 3.0

where εS is the dielectric constant of p-Si and Vdo is the diffusion potential at zero bias determined from the extrapolation of the linear parts of (1/C2 –V) plot to the V axis as shown in Fig. 10. The barrier height can be determined from the relation: ˚bo (C − V ) = Vdo + Vp ,

70

(16)

2.5

60

2.0

50

1.5

40

(17)

30

where Vp is the potential difference between the top of the valence band in the neutral region of p-Si and the Fermi level. The value of Vp has been calculated as 0.228 eV [30]. The diffusion potential value of 0.6 eV and doping concentration value 4.07 × 1017 cm−3 were calculated using Eq. (16). Therefore, the barrier height of value 0.828 eV was calculated using Eq. (17). The discrepancy between the barrier heights obtained from the (I–V) and that obtained from (C–V) can be due to the barrier height inhomogeneity [30]. At the end of this part of paper it is better to explain the origin of the rectification behaviour of the Au/FeTPPCl/p-Si/Al device. The excellent paper by Forrest [37] demonstrated that the early observations of the organic/inorganic contact suggested that it could be explained by using a Schottky-like metal–semiconductor junction model, but this picture did not explain the ability to rectify on both p- or n-type semiconductor substrates. Hence the Schottky model was replaced by the picture of the organic/inorganic contact as a heterojunction between an undoped (nearly intrinsic) organic semiconductor contacting a doped inorganic semiconductor substrate. The rectifying junction is then formed by the energy offset between the frontier orbitals of the organic and the conduction or valence band minima of the inorganic semiconductor. Given that the organic materials contained a very small free-charge density, however, an additional aspect to this model was included. That is, unlike conventional semiconductor heterojunction, the Fermi energy of the organic was almost completely determined by the density of charge injected from the metal contact on its surface, or across the organic/inorganic heterojunction. The significant movement of the quasi-Fermi energy from its mid-energy gap position could also result in a change in the energy level offset at the organic/inorganic heterojunction interface – possibly allowed to shift its position by an intervening oxide layer residing on the inorganic semiconductor surface [37,40]. A proposed energy level diagram of the fabricated device at thermal equilibrium is shown in Fig. 11. El-Nahass et al. showed that FeTPPCl has band gap of 2.43, and there are excitation bands available inside the bandgap at various energy levels along with the localized energy states [21]. At thermal equilibrium one ay assume that the hole current density from the Si to the organic semiconductor equals the current den-

20 10 300

1.0

Photocurrent (a.u)

qεS A2 Na , 2(Vdo − V )

Abs. (a.u.)

C2 =

0.5

400

500

600

700

800

0.0 900

λ (nm) Fig. 12. Spectral dependence of the photoaction spectrum for FeTPPCl/p-Si.

sity from organic to the Si, but with the positive potential to the Si relative to the FeTPPCl the observed magnitude of the hole current density from the Si to the organic semiconductor greater than the current density from organic to the Si. The application of positive potential to the Si steers the holes from the Si to the FeTPPCl. As the result, it is assumed that the bending profile of the valance band moves upward from the Si side indication the lowering of the barrier [40].

3.3. Current–voltage characteristics under illumination Fig. 12 shows the photocurrent action spectrum for the FeTPPCl/p-Si heterojunction of the absorbance of FeTPPCl with thickness 190 nm at room temperature. This figure indicates that the photocurrent has a maximum response peak at about 600 nm while the absorbance curve has maxima at 430 nm. The action spectrum follows the absorption spectrum of FeTPPCl in the Q bands region [20–22]. Similar behaviour between the action spectra and the absorbance was obtained by Shimizua et al. [41] for a symmetrical sandwich type (ITO/C15 TPPH2 /ITO) cell. Fig. 13 shows the I–V characteristics of Au/FeTPPCl/p-Si/Al under illumination. The device of active area (0.09 cm2 ) exhibits a shortcircuit current (Isc ) of 2.8 mA, an open-circuit voltage (Voc ) of 0.475 V, then the fill factor can be calculated as [42]:

FF =

Jm Vm , Jsc Voc

(18)

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M.M. El-Nahass et al. / Synthetic Metals 161 (2011) 2253–2258

Table 1 Parameters of some organic solar cells based on porphyrin system. Solar cell construction

Jsc (mA/cm2 )

Voc (V)

Fill factor

Efficiency (%)

Refs.

Al/TPP/Au Al/TPP/MC/Au Al/TPP/MC(COOH)/Au Al/TPP/ZnTPP/Au Au/TPP/n-Si/Al Au/FeTPPCl/p-Si/Al

– – – – 2.76 2.8

0.74 0.99 0.75 0.51 0.25 0.475

0.18 0.27 0.27 0.38 0.37 0.32

0.06 0.33 0.60 2.7 2.45 5.63

[43] [43] [43] [43] [12] Present work

3.0

References active area = 2 mm*4.5 mm=.09 cm

2

2.5

I (mA)

2.0

1.5

1.0

0.5

V OC =0.475 V I SC = 2.8 mA P max = 1.65 mA *0.26 V FF=0.32

0.0 0.0

0.1

0.2

0.3

0.4

0.5

V (volt) Fig. 13. I–V characteristics under an illumination (80 mW/cm2 ) for FeTPPCl/p-Si.

where the product Jm Vm corresponds to the maximum power point. The calculated value of the FF = 32%. Then, the power conversion efficiency, , is: =

Jm Vm Jsc Voc FF = , Po Po

(19)

where Po is the incident light intensity ≈80 mW/cm2 . These parameters lead to power conversion efficiency () of 5.63%. Parameters of some organic solar cells based on porphyrin system are shown in Table 1. 4. Conclusion Organic–inorganic heterojunction is fabricated using thermally evaporated 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride (FeTPPCl) and p-type Si wafer. I–V characteristics demonstrated a rectification behaviour of p–p isotype heterojunction. Analytical approaches involving the thermionic emission and SCLC current were used to explain the I–V behaviour in the forward bias. The basic diode parameters such as the ideality factor, series resistance and the barrier height were extracted from the I–V measurement of Au/FeTPPCl/p-Si/Al rectifying contacts. The ideality factors were seen to increase and barrier heights decrease by decreasing temperature. These observations have been ascribed to barrier inhomogeneities at the metal semiconductor interface. The investigation of C–V measurements at 1 MHz points out that the junction is of abrupt nature with diffusion potential value of 0.6 eV. The dominant operating mechanism in the reverse bias is generation recombination of carriers, which is a thermally activated process. A short-circuit current of 2.8 mA, an open-circuit voltage of 0.475 V and a fill factor of 32% were extracted from (I–V) characteristics under illumination of =80 mW/cm2 .

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