The forward biasing of the device corresponds to the positive voltage at the ITO electrode with respect to the metallic electrode. From the plot we find that the ...
Chapter 3
Physical Interpretation of Device Characterization
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3.1. Introduction To co-relate the device fabricated using performance with its basic structural, electrical and optical properties, the device is to be characterized using different characterization techniques. Essential characterization techniques implemented on our devices has already been discussed in the previous chapter. Observation recorded in various characterization procedures are then required to be interpreted for understanding various mechanisms taking places within the device. Moreover, these observations provide input for calculating various significant device parameters and extend factual support for explaining its physical and chemical properties. Though there are many different interpretations possible as per the particular behaviour of a device but the basic theoretical understanding, for interpreting electrical and optical characterization results, is undoubtedly significant and should be essentially explored to go further. This chapter deals with the basic theories of optical and electrical characterization of the devices based on organic solar cell.
3.2. Optical Absorption Spectroscopy Optical absorption spectra are obtained from a uv-vis Spectro photometer. The absorption spectrum of the material gives important information about the band gap of the material The UV–Visible spectra of Rosebengal recorded in Dimethyl Formamide (DMF) (Fig.3.1), shows characteristics absorption band extending from 650 nm to 800 nm.
Fig.3.1. Absorption spectra of Rosebengal dye in DMF
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The band gap Eg(opt) (eV) can be determined by extrapolating the linear region down to zero absorption in longer wavelength region and is found to be 1.98 eV using the equation (3.1) E g (opt ) (eV ) =
1239.95 λ (nm)
(3.1)
The energy gap (Eg(opt)) of the materials, measured from the optical absorption curves, can be related to the absorbance (α), by following equation (3.2) [196].
αhυ = A(hυ − Eg (opt ) ) 2 1
(3.2)
where hν is the energy of the photon and A is the proportionality constant. In this study, same equations have been used to determine the optical band gap of the other materials.
3.3. Cyclic Voltametry Efficient charge transfer from donor to acceptor component, effective charge transport and charge collection at the electrodes are important parameters for designing and optimization of organic bulk hetero-junction photovoltaic devices. In this regard, the electrochemical data gives valuable information and allow the estimation of relative position of HOMO/LUMO levels of the materials used for device fabrication.
Fig.3.2. Cyclic voltammogram of Rosebengal 38
The knowledge of these levels is required for finding suitable donor–acceptor combination for the efficient bulk hetero-junction photovoltaic device based on organic materials. Several ways to evaluate the HOMO and LUMO energy levels from the onset oxidation and reduction potentials have been proposed in literatures [197, 198]. We have investigated the optical band gap of the materials employing both optical absorption and cyclic voltammetry (CV) measurements separately. The position of HOMO and LUMO level has been determined from the analysis of redox potential behavior observed in cyclic voltammogram curves as shown in Fig.3.2, using the expressions (3.3) and (3.4). Energy gap in this material is found to be 2.15 eV.
EHOMO = − Eoxonset − 4.75 (eV ) onset ELUMO = − Ered − 4.75 (eV )
(3.3) (3.4)
3.4. Current Voltage (J-V) Characteristics in Dark Current-Voltage (J-V) measurements refer to d.c. characterization of the devices for the purpose of performance analysis and parameter extraction. The current voltage characteristics of the fabricated devices are measured. A typical characteristics of ITO/RB:TiO2/Ag device is shown in Fig.3.3.
Fig.3.3. J-V Characteristics of a bulk heterojunction device in dark
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The forward biasing of the device corresponds to the positive voltage at the ITO electrode with respect to the metallic electrode. From the plot we find that the electric current increases exponentially at forward applied voltage whereas negligible current flows under reverse bias condition. J-V characteristics are symmetrical and exhibit rectification effect. This behavior can be explained by the small work function of the metallic electrode and by n-type and p-type conductivity of the organic layer.
Fig.3.4.Semilog plot of the J-V Characteristics at different temperatures
The rectifying J-V behavior of the devices follows the standard thermionic emission theory for conduction across the junction [199]. Applying this theory to our devices, J-V characteristics can be given by the Shockley equation [200]
J = J 0 [exp (qV / nKT ) − 1]
(3.5)
where J0 is the reverse saturation current density, q is the electric charge, V is the applied voltage, T is the ambient temperature, K is the Boltzmann constant and n is the diode ideality factor. The reverse saturation current density (J0) is given by J 0 = A*T 2 exp (φb / KT )
(3.6)
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фb is the barrier height and A* is the Richardson constant, given as
A * = 4 π qm * K
2
/ h3
(3.7)
Now in order to obtain different parameters, these J-V characteristics are re-plotted as semi-logic ln(J)- V curves shown in Fig.3.4, which shows an exponential increase in current with applied voltage. This exponential dependence can be attributed to the formation of depletion region at either of the interfaces (Semiconductor /ITO or semiconductor/ metal interface or semiconductor/ semiconductor interface). This exponential behavior disappears above a particular voltage, after which the current is mainly due the bulk resistance of the material. At low voltage region, the plots of ln (J)-V are linear and the current is due to the shunt (leakage) resistance of the barrier while at higher voltages, the current is limited by the series resistance of the organic layer. The intercept of the curve with the current axis corresponds to the J0 and the slope gives us the ideality factor (n). The values of ideality factor for these devices is greater than unity which can be attributed to (a) the presence of the trap levels due to impurity in the band gap which acts as the recombination center and (b) the occurrence of tunneling conduction [201]. Using these values of J0 and ideality factor (n), the potential barrier height (фb) can be calculated from equation (3.6).
Fig.3.5. Ln (J)- Ln (V) plots of the J-V Characteristics at different temperatures
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The same J-V characteristics are also plotted as ln (J)- ln (V) plots at different temperatures as shown in Fig.3.5, which gives us the detailed information about the charge transport mechanism. These curves show two distinct regions at each temperature. At low voltage, the slopes of the ln(J)- ln(V) plots are approximately unity, corresponding to the ohmic conduction. While at higher voltage above a well- defined transition voltage, the slopes are approximately greater than two corresponding to the space charge limited conductivity (SCLC) region controlled by the exponential distribution of traps. This is a common feature in most of the semiconductors with low mobility and high resistivity. At low voltage region, there is ohmic conduction that can be described by Ohm’s law [202]. J = qno µ (V / d )
(3.8)
where µ is the mobility, V is the applied voltage, d is the thickness of the organic layer and n0 (p0 for p-type) is the concentration of free electrons (for n- type) in conduction band (holes for p- type in valance band) which is given as [203]. For n- type no = N c exp[− ( Ec − E f ) / KT )]
(3.9)
For p- type po = N v exp[− ( E f − Ev ) / KT )]
(3.10)
where Nc is the effective density of states in conduction band for n-type while Nv is the effective density of states in valance band in the case of p- type. (Ec- Ef) is the separation of the conduction band edge from the Fermi level, while (Ef- Ev) is the separation of the Fermi level from the valance band edge, K is the Boltzman constant and T is the absolute temperature. Substituting the values Eq. (3.9) and Eq. (3.10) in Eq. (3.8) J = qµ (V / d ) N c exp[ ( Ec − E f ) / KT )]
for n-type
(3.11)
J = qµ (V / d ) N c exp[ ( E f − Ev ) / KT )]
for p-type
(3.12)
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The plot of log(J) against 1000/T shown in Fig.3.6 at a particular voltages, both for ohmic and SCLC region.
Fig.3.6. Variation of log (J) with 1000/T for the device (a) 0.6 V in Ohmic region and (b) 3.8 V in SCLC region
The slope of the line and intercept with current axis yields the values of Ef- Ec and Nt. For a given value of the thickness of the film (d), the straight line for SCLC region, the current density for the type of conduction is given as [204]. For n-type
J = ( 98 )εµ ( N o / N t ) (V 2 d 3 ) exp[ − ( Ec − E f ) / KT )]
(3.13)
For P-type
J = ( 98 )εµ ( N o / N t ) (V 2 d 3 ) exp[ − ( E f − Ev ) / KT )] where ε is the permittivity of the material which is generally taken as 3×10
(3.14) -11
F/m almost
for all organic semiconductors, Nt is the trap concentration. The exponential distribution of traps may be described in terms of Tc as P ( E ) = Po exp (− E / KT )
(3.15)
Where P(E) is the trap concentration per unit energy range at an energy E below the conduction band edge. The total concentration of traps is given as
N t = Po KTc
(3.16) 43
Fig.3.7. log (J) vs V1/2 plot of the device.
Fig.3.7. shows log (J) vs √V characteristics in low voltage region for the device in different temperatures. These curves show the linear dependence, which can be interpreted either in terms of the Schottky effect (field lowering of the interfacial barrier at the blocking electrode) or by Poole-Frenkel effect (field assisted thermal detrapping of the carriers). The J-V characteristics for these mechanisms can be described by [205]. For Schottky effect 1
1
J s = A*T 2 exp (−φb / KT ) × exp (qβ sV 2 / KTW 2 )
(3.17)
and for Poole Frenkel effect 1
1
J pf = J pfo exp ( qβ pf V 2 / KTd 2 )
(3.18)
where A* is the Richardson constant, фb is the Schottky barrier height, βs is the Schottky coefficient, βpf is the Poole Frenkel coefficient and W is the width of the depletion layer. The theoretical value of these coefficient is given by 2 βs= βpf= (q/πε)1/2. For ε= 3×10-11 F/m, the values of βs and βpf are 2.05x 10-5 and 4.1x10-5 respectively. From the slope of the straight line of the log (J) vs V1/2, for d=600 nm. The value of the coefficient can be obtained using the equations (3.17) and (3.18). If the so obtained practical value of the coefficient is near to βs then we conclude that Schottkey effect is dominant while if the practical value is equal to βpf, we say that the Poole Frenkel effect is dominant. The value
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of the constant is not a constant for all the temperatures, showing that the depletion layer width varies with the temperature. To summarize, using the current- voltage characteristics (J-V plot) we can determine the nature of the material, ideality factor (n), mobility (µ) potential barrier (φb) and carrier concentration
3.5. Current- Voltage (J-V) Characteristics under illumination The electric current that a photovoltaic solar cell delivers corresponds to the number of created charges that are collected at the electrodes. This number depends on the fraction of photons absorbed (ηabs), the fraction of electron-hole pairs that are dissociated (ηdiss), and finally the fraction of (separated) charges that reach the electrodes (ηout) determining the overall photocurrent efficiency (ηj)
η j = η abs ×η diss ×η out
(3.19)
The fraction of absorbed photons is a function of the absorption spectrum, the absorption coefficient, the absorbing layer thickness, and of internal multiple reflections at electrodes. The fraction of dissociated electron-hole pairs on the other hand is determined by whether they diffuse into a region where charge separation occurs and on the charge separation probability there [206]. To reach the electrodes, the charge carriers need a net driving force, which generally results from a gradient in the electrochemical potentials of electrons and holes. The two “forces” contribute to this are the internal electric fields and concentration gradients of the respective charge carrier species. The first leads to a field induced drift and the other to a diffusion current. Though a detailed analysis requires the knowledge of charge carrier distributions over film depth, thin film devices (
