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Optical and electrical properties of fullerene C70 for solar cell applications
Optical Materials 101 (2020) 109717
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Optical and electrical properties of fullerene C70 for solar cell applications Sheenam Sachdeva a, Devinder Singh a, b, S.K. Tripathi a, * a b
Department of Physics, Panjab University, Chandigarh, 160014, India Amity School of Applied Sciences, Amity University, Lucknow Campus, Lucknow, UP, 226028, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Fullerenes Vapor deposition Optical properties Defects Electrical properties
This research paper demonstrates the successful deposition of fullerene C70 for application as an acceptor ma terial in Schottky barrier organic solar cell (OSC) device with structure fluorine-doped-tin-oxide/ molybdenumtrioxide/fullerene/lithiumfluoride/aluminium (FTO/MoO3/C70/LiF/Al) using vapor thermal deposition technique. Morphology analysis using field emission scanning electron microscopy (FESEM) shows the mesoporous nature of highly cross-linked C70 molecules in deposited C70 thin film. But more uniform C70 film with less porosity has been deposited with high evaporation rate. We report the optical and electrical characterizations which reveal the thin film to be useful in photovoltaic applications. To study optical absorption spectra over visible energy range, prepared samples have been characterized by ultraviolet–visible (UV–vis) absorption spectroscopy. Optical dielectric parameters and dispersion properties of samples are studied. Constant Photocurrent Method (CPM) is employed to measure the mid gap absorption spectra of fabricated device. Defect density of states (DOS) distribution, Urbach energy and steepness parameter are also evaluated. Current-voltage characteristics of device in dark as well as under illumination surmise the behavior of device similar as Schottky barrier OSC.
1. Introduction Considerable attention on energy resources has spurred significant interest in the fabrication and characterization of organic solar cells (OSCs) [1]. Recently, fullerene C70 based Schottky OSCs have attained encouraging power conversion efficiencies (PCEs) [2]. However, this progress is quite slow in the field of OSCs as compared to their inorganic counterparts. Many facets of devices have been left to attain higher performance of these OSCs. To achieve higher efficiencies and remark able progress, detailed knowledge of transport properties of materials is required for optimization of Schottky OSCs [3–6]. Charge transport and recombination are the important bottlenecks for the major contributions of losses present in the OSCs that limit the efficiencies and stability of OSC devices. The optical and electrical properties of OSCs primarily depend on photoabsorption and density of states (DOS) distribution in the mid gap of the materials [7]. These properties play crucial role in photoconversion process of OSCs. Electronic defect states (intrinsic or extrinsic) are one of the foremost facets that significantly affect the transport phenomenon in the OSC devices, must be studied and char acterized. The existence of defect states directly impact the stability and lifetime of OSCs [8]. Evaluation of defect trap states opens an important
area of research to characterize and mitigate these defects for the sta bility of OSCs. Therefore, to improve photoabsorption and stability of Schottky OSCs, optical parameters and defect states are imperative to be deter mined for the further progress in this field of photovoltaic technology with better understanding of charge transport and electron-hole recombination in the device. It is reviewed that many authors have paid attention to increase the optical absorption in the field of thin-film technology and revealed the information about the optical properties and DOS in materials and OSCs. Most of the researchers have concen trated their study to determine the dispersion parameters and defect states distribution of thin films [9,10]. No attempt has been made yet to study the dispersion parameters and DOS distribution of C70 based Schottky OSC device. However, these are of utmost importance in fabricating a high efficient OSC. It is suggested that dispersion param eters and DOS distribution of defect states within mid gap of device has to be investigated. The present study pursuits the extensive study of optical characterization (mainly dispersion parameters) of fullerene C70 based thin films, DOS distribution and electrical characterization of C70 based Schottky OSC device. Thin film of organic semiconductor, such as, C70, is considered because it is drawing significant interest in OSCs due
* Corresponding author. E-mail addresses: [email protected], [email protected] (S.K. Tripathi). https://doi.org/10.1016/j.optmat.2020.109717 Received 27 September 2019; Received in revised form 22 November 2019; Accepted 21 January 2020 0925-3467/© 2020 Published by Elsevier B.V.
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to its potential for future photovoltaic absorber with optimal electronic and excellent optical properties and has exhibited incredible perfor mance in OSCs [11]. To elaborate dispersive behaviour and defect states distribution, layer C70 on glass substrates is deposited by vapor thermal deposition technique. The aim of this research work is to fabricate a device which finds application in developing a high efficient Schottky OSC, therefore, stacked layers, Molybdenum trioxide/C70/Lithium Fluoride, MCL (MoO3/C70/LiF) consisting C70 sandwich between anode buffer layer (ABL) and cathode buffer transport layer (CBL) are used for device fabrication with fluorine-doped tin oxide (FTO)-coated glass substrate as anode. In this work, firstly, we have described the evaluation of optical and dispersive transport by evaluating optical constants such as index of refraction and extinction coefficient (n,k), real and imaginary parts of dielectric constant (ε1 and ε2), dispersion properties like dispersion en ergies (E0 and ED), lattice dielectric constant (ε´), and maximum fre quency (ωP) of layers. Defect tail states are observed in sub band gap absorption coefficient (α) spectra of C70 layer. Constant photocurrent method (CPM) is employed to find the mid gap absorption spectra and defect DOS distribution of fabricated OSC which is determined using derivative method. The parameters like Urbach energy (EU) and steep ness parameter (S) are also evaluated. In addition, current-voltage (I-V) characteristics in dark as well as under illumination have been measured to confirm the type of synthesized device. Reported results justify the junction property of device which is consistent with Schottky barrier OSCs.
Fig. 1. Powder X-ray diffraction (XRD) pattern of fullerene C70 powder.
symmetry due to orientational disorder present in the structure of C70. This is attributed to the presence of mixture of two different (facecentered cubic, fcc and hexagonal close packing, hcp) structures. Most of the peaks are doubly indexed as hcp and fcc. A very few peaks are indexed independently as fcc with average lattice constant, a ¼ 1.416 nm and hcp with lattice constant a ¼ b ¼ 1.123 nm, c ¼ 1.722 nm and c/ a ¼ 1.53. This small c/a value for hcp phase of C70 suggests the orien tational disordered structure of C70 [14,15].
2. Experimental and characterization C70 (acceptor material), MoO3 (ABL), LiF (CBL) and FTO-coated glass substrates (13 Ω/sq) were purchased from Sigma–Aldrich. All chemicals with purity > 98% were used as received. Deionized water was used throughout the preparation. For thin film deposition, corning 7059 glass substrates and FTO-coated glass substrates were used. Thin films were deposited on pre-cleaned required substrates using vapor thermal deposition technique [vacuum coating unit HINDHIVAC, Model:VS-65D] after maintaining a pressure of approx. 2 � 10 5 mbar [12]. The deposited C70 layers were uniform and well-adherent to the substrates. Before taking optical and electrical measurements, layers deposited on both substrates are placed in deposition chamber for a day. Firstly, fullerene C70 powder is structurally characterized using powder X-ray Diffraction (XRD) technique. Surface morphologies of deposited thin films of C70 have been found out with the help of Field Emission Scanning Electron Microscopy (FESEM) (HITACHI SU8010 model). Optical absorption spectra of layers on glass substrates have been determined using UV/VIS/NIR spectrophotometer (Perkin Elmer LAMBDA 750) at room temperature. To determine α-values of fabricated device in sub band gap region CPM measurements have been taken using the experimental set up described elsewhere [3]. Thereafter, DOS dis tribution is evaluated for fabricated device. Electrical properties of OSC devices are studied by placing the respective sample in a well equipped stainless steel made sample holder set up [13]. For electrical measure ments, sample holder is evacuated to ~10 3 mbar pressure with the help of rotary pump. Two-probe method has been used for current-voltage measurements. I-V characteristics in dark and under illumination are performed using Keithley 6517A source unit and for photo excitation source, a tungsten bulb (150 W) is used.
3.2. Micro-structural analysis The micro-structural properties of the deposited C70 thin films are studied using convenient FESEM technique. Fig. 2 gives the FESEM images of deposited C70 films on the glass substrate with different evaporation rate (18 Å/s, 25 Å/s, 32 Å/s) and magnifications (500 nm and 200 nm). It is observed from the micrographs, that thin films of C70 are uniform and well adhere to the glass substrates; the most uniform C70 layer has been deposited with evaporation rate 32 Å/s. Fig. 2(a–c) show that with the increase in the evaporation rate, grain size of C70 particles in films has been decreased and density of C70 particles gradually increases. It appears that melting of C70 powder during small evaporation rate results in heat generation which leads to agglomeration of particles. Moreover, Fig. 2(d–f) clearly depict that C70 thin films with some porosity are deposited on the substrates. There are pinholes found within the aggregates of C70 particles. This implies mesoporous C70 films have been synthesized. The highly cross-linked fullerene C70 molecules (25 hexagons, 12 pentagons) with mesostructures can develop numerous applications. The unique property of meso fullerene having randomly oriented pores shows an excellent electrical behaviour due to which meso materials are of considerable interest in many research areas for energy storage and conversion applications [16]. It is also noticed that with the increase in evaporation rate, pinholes are significantly reduced and thickness of thin films are raised from 113 nm to 194 nm. At high evaporation rate, thin films are more uniform with roughness 1.838 nm. Surface roughness of C70 film has been measured from the atomic force microscopy (AFM) image (Fig. 3). Therefore, to study the disorderness and defect states introduced by these pinholes in the C70 layers, thin film deposited with thinner C70 film is used for optical and electrical measurements due to the small exciton diffusion length of fullerene C70 [3].
3. Results and discussions 3.1. Structural analysis In order to get knowledge about the structural disorder and defects of fullerene C70, XRD spectrum has been taken. Fig. 1 shows the XRD spectrum of C70 powder. Many sharp diffraction peaks has been seen in XRD pattern of fullerene C70. It is observed that there is lack of
3.3. Optical characterization The optical properties of materials are a very useful source of 2
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Fig. 2. FESEM images of thermally evaporated mesoporous C70 thin films on glass substrate with different evaporation rate (a) 18 Å/s (b) 25 Å/s (c) 32 Å/s at 500 nm magnification, (d) 18 Å/s (e) 25 Å/s (f) 32 Å/s at 200 nm magnification.
the absorption in the visible region which is due to occurrence of optical transitions in C70 [19]. The fundamental absorption corresponds to the transition from extended valence band states to extended conduction band states. This shows that absorption coefficient increases with increasing hν. The absorption coefficient is evaluated to get information about the type of transition involved and value of optical band gap. The optical band gap energy (Eg) is determined using Tauc’s equation [20]. The band gap energy (Eg) is determined using Tauc equation (Eq. (1)): � ðαhνÞ1=2 ¼ B1 hν Eg (1) where Eg, hν, and B are band gap energy, energy of the incident light, and a constant, respectively. Fig. 4(c) shows the Tauc plot with photon en ergy. Optical band gap of C70 layers are found to be ranging between 1.46 and 1.65 eV by extrapolating the linear region of the plot [21]. This dictates that energy gap of C70 layers deposited with different evapo ration rate is dependent on the thickness of thin films. Increase in the thickness of thin films with evaporation rate (explained in section 3.2) has lowered the values of energy gap. Decrease in grain size of C70 particles has also been observed (Fig. 2) with increase in evaporation rate. The increase in the density of C70 particles due to decrease in grain size must increase the energy gap [22]. But in contrast, energy gap has been reduced. This is mainly attributed to the small absorption coeffi cient of C70 layer due to large thickness. The obtained Eg is in good agreement with the literature values [23]. Somewhat lesser value of Eg is due to the formation of localized tail states in layer [19]. Localized states are introduced in the layer after the exposure of thin film with air for few seconds during the removal of deposited C70 samples from vacuum deposition chamber. The additional levels in energy gap are responsible for the formation of localized trap states in sub band gap region. Therefore, this has resulted the broadening of band, formation of band tails and the reduction in the energy associated with the optical transi tion in C70. Aforementioned localized defect states behave like trapping
Fig. 3. AFM image of deposited fullerene thin film.
information about their structure. The optical properties of material enable us to relate the atomic structure, band structure and its electrical properties [17]. The optical properties of materials typically consist of optical band gap, absorption coefficient, α, refractive index, n, and their dispersion behaviour. 3.3.1. Absorption spectra Ultraviolet–visible absorption spectrum of C70 with wavelength range from 370 nm to 800 nm is shown in Fig. 4(a). It is noticed that there are two strong maxima at 385 nm and 523 nm having minima in the blue region lying within 370 nm–550 nm wavelengths. These are attributed to interband transitions among the π-orbitals of C70. Owing to complex electronic structure and lower symmetry of C70, the weak onset bands are also present in the spectrum above 550 nm and also reported in literature [18]. The optical transitions above 550 nm cannot be precisely measured from transmission measurements [19]. Therefore, CPM measurements have been adopted for these types of weak optical transitions. Fig. 4(b) shows α vs photon energy (hν) of C70 which clearly depicts 3
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Fig. 4. (a) Graph between absorbance and wavelength (λ), (b) Variation of absorption coefficient (α) vs photon energy (hν), and (c) Tauc plot of C70 thin film deposited on glass substrate.
centres and have influence on the optical absorption properties [24]. Defect states present in the material have been determined and described in section 3.4.
dielectric constant are obtained by the relation:
3.3.2. Optical dielectric constants Index of refraction, n is an important parameter for optical materials. Information regarding the propagation of incident radiations, local field and polarization of ions inside the films are determined from the spectral dependence of index of refraction. Spectral dependence of complex refractive index can be obtained by the measuring optical absorption and transmission measurements. Spectra of optical dielectric constants like (n,k) with wavelength in the visible region are given in Fig. 5(a). It is observed that optical spectra (n,k) of C70 follow the same trend as ab sorption spectra (Fig. 4(a)). Swanepoel’s method has been used to evaluate the values of n [25]. The k-values are determined using the equation:
ε2 ¼ 2nk
k ¼
αλ 4π
ε1 ¼ n2
k2
(3) (4)
Fig. 5(b) shows the plots of the dielectric constant (ε1 and ε2) versus photon energy. Similar variations in dielectric constants (ε1, ε2) have been noticed. The dielectric constant (ε1) has greater values as compared to imaginary part (ε2). These plots (Fig. 5(b)) confirm that photonelectron interactions occur in the energy region where n-values are increasing. These spectra corroborate the refractive index spectra. It is also noticed that dielectric constant values are positive for whole energy region. Hence there is always propagation of radiations occurred throughout the films in the energy region shown in Fig. 5(b). This description leads to a material specific conductivity. The optical con ductivity, σ opt, is calculated as using α-values as follows:
(2)
σ opt ¼
Two strong absorption bands at low wavelengths are observed in the spectra of n which depicts the similarity of incident radiations frequency and C70 electrons frequency. The decrease in n-values with the variation of wavelength shows the normal dispersive behaviour that leads to decoupling of C70 electrons from oscillating electric field of incident electromagnetic radiations. The n-values depend on the density and polarizability of valence electrons using Lorentz-Lorenz formula [26]. The decrease in k-values with increase in wavelength reveals the reduction in the loss of light by scattering and absorbance. The dielectric constant is determined by analyzing the obtained values of index of refraction. The real and imaginary (ε1, ε2) parts of
αnc 4π
(5)
with c as the velocity of light. Plot of σopt vs photon energy for C70 is shown in Fig. 5(c). This indicates that there is increase in the conduc tivity with increasing energy of photons due to the absorption in C70 layer. The dissipation loss (tanδ) is evaluated from the relation: tan δ ¼
ε2 ε1
(6)
Fig. 5(d) shows the variation of tanδ as a function of frequency for C70. It shows that dissipation loss is increased with increasing frequency. After discussing the results of spectra showing in Fig. 5, it is 4
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Fig. 5. (a) Plot showing the variation of refractive index (n) and extinction coefficient (k) with wavelength (λ) (b) Plot of real ε1 and imaginary ε2 parts of dielectric constant vs photon energy (hν), (c) Plot between optical conductivity (σopt) and photon energy (hν) (d) Plot of dissipation factor (tan δ) vs frequency (ν), of C70 thin film.
concluded that increase in photon-electron interactions enhances the optical conductivity of C70 films with increasing energy of light photons which is due to decrease in the dissipation loss in that energy region. Thereby, refractive index starts decreasing with the increase in wave length beyond 500 nm upon falling light photons on the sample.
relationship [27], the expression for index of refraction dispersion relation is given by: n2
1
�
1
¼
E0 ED
1 ðhνÞ2 E0 ED
(7)
with E0 as single-oscillator energy, ED as energy of dispersion. Fig. 6(a) shows the plot of (n2-1) 1 vs (hν)2. The values of E0 and ED are evaluated from the slope and intercept of graph by extrapolation of lines in low energy region. The values of E0 and ED for pristine C70 are 2.65 eV and 7.98 eV, respectively. It is observed that the single oscillator energy and
3.3.3. Energy dispersion parameters For the applications in optical devices, evaluation of energy disper sive parameters is needed. These significant factors are found out from refractive index dispersion relation. Using Wemple–DiDomenico
Fig. 6. (a) Graph between (n2-1)
1
and (hν)2, and (b) Graph between n2 and λ2, of C70 thin film deposited on glass substrate. 5
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the dispersion energy are high for pristine C70 which could be attributed to the presence of number of scattering centres and impurity levels [28]. Due to absorption by excitons in the high energy region, non-linear (negative) curvature is noticed. Non-linear (positive) curvature is also observed at low energies which are attributed to lattice vibrations [27]. The static refractive index n´, is given by relation: rffiffiffiffiffiffiffiffiffiffiffiffiffiffi ED 0 n ¼ 1þ (8) E0 Using the values of E0 and ED calculated from Wemple–DiDomenico relationship, the value of n´ is determined and its value is found to be 0 2.00. To determine lattice (high frequency) dielectric constant (εL), ε ¼ 2 2 n versus λ is plotted in Fig. 6(b). The value of εL is determined using relation [10]: � 2 �� � e N 2 0 λ ε ¼ εL (9) m* 4πc2 ε0 with N as free carrier concentration, ε´ as real part of dielectric constant. The symbols c, ε0, m* and λ have their usual meanings. At higher wavelengths, ε´ is linearly dependent on n2. The value 4.67 of εL for C70 is evaluated by extrapolating the linear region of the plot (Fig. 6(b)) to λ→0. The obtained value of N/m* is 2.81 � 1047 cm 3 g 1. Drude’s theory of dielectrics is used to determine concentration of optically generated charge carriers, at low energies. The formula that has been used is given as follows:
ε1 ðℏωÞ ¼ n2
k2 ¼ εopt
ðℏωD Þ2 ðℏωÞ2
Fig. 7. Device structure (FTO/MoO3/C70/LiF/Al).
intensity of light while changing the energy of incident light photons through monochromator [7,25]. Spectral dependence of α by measuring number of incident photons at various photon energies is plotted in Fig. 8(a). The exponential region in the absorption spectra that gives us the information about the excitation of valance band tail states electrons to the conduction band extended states is named as Urbach tail which determine the structural disorder. The Urbach region involves the transitions between deep localized defect states. These defect states may be due to the surface contamination of C70 thin film by impurities due to exposure of film to air for few seconds. Urbach tail is expressed as: � � hν (13) α ¼ α0 exp EU
(10)
with εopt, ℏω, ℏωD as residual dielectric constant, photon energy, screened plasma energy, respectively. To find εopt, graph between (n2 k2) and (ℏω)2 is plotted. The evaluated value of εopt is 4.33. The maximum (plasma) frequency, ωP is defined as: pffiffiffiffiffiffiffi (11) ℏωD ¼ ℏωP εopt
with α0 as a factor that is determined experimentally, hν as photon en ergy and EU as Urbach energy. The steepness parameter S of the defect states is evaluated from the equation:
The determined value of ℏωP is 1.05 eV. The value of Nopt is found out using the expression: Nopt ¼
ε0 εopt m*e ω2P e2
(12)
S¼
kB T EU
(14)
with kB and T as the Boltzmann constant and absolute temperature, respectively. The Urbach energy and steepness parameter are obtained using exponential fit (Eq. (13)) of the absorption coefficient spectra. The value of EU comes out to be 49.3 meV which is comparable to 55 meV of C70 thin film [19] and steepness parameter, S for this sample comes out to be 0.53. Urbach energy (EU) is the characteristics parameter of dis order present in the samples. To obtain the DOS distribution in mid gap region, derivative method [7] is employed. The absorption coefficient for the transitions between valence band tail states Ni(ε) to the conduction band extended states Nf(εþhν) is given by convolution integral as [30]: Z ∞ B αðhνÞ ¼ Ni ðεÞf ðεÞf1 f ðε þ hνÞgNf ðε þ hνÞdε (15) hν ∞
with ε0, me*, e as permittivity of free space, effective mass of electron and elementary charge, respectively. Its value is 1.01 � 1027 m 3 for C70 thin film and is in good agreement with literature value of carrier concentration in semiconductors [29]. The evaluation of optical constants is of considerable importance for applications in integrated optical devices and OSCs. These numerable terms of the materials would be very helpful in designing an efficient device. These are the key parameter for device design. To use C70 in circuits, it is necessary that their dielectric and energy losses be under stood so as they have appropriate value ranges. 3.4. Constant photocurrent method For the device fabrication, stacked layers (MCL) are used. For MCL deposition, FTO-coated glass substrates were used. Layers of HTL, acceptor and ETL materials are deposited in sequence on FTO coated glass substrates using vapor thermal deposition technique Al cathode is deposited on the stacked layers deposited on FTO substrates using masks of desired active cell area (0.09 cm2). Device structure of fabricated OSC is shown in Fig. 7. To get physical insight of localized tail states and defect states in the material, mid gap absorption spectra is studied. The absorption spectra in the mid gap region is obtained using CPM technique. In CPM, photocurrent is generated in the fabricated device (Fig. 7) with incident monochromatic light. Photocurrent is kept constant with the variation of
with f(ε) as Fermi-Dirac distribution, B as transition matrix element and has a constant value. The integral can be deconvoluted using various methods. Hata et al. [31] have proposed a simple method to deconvolute the DOS. The electronic transitions from occupied states of valence band (VB) to conduction band (CB) are detected by CPM. So, CB tail states are not involved. After approximation by step function and using Heaviside function θ(ε þ hν þ EC), the above equation becomes: Z ∞ αCPM ffi Ni ðεÞf ðεÞθðε þ hν þ EC Þdε (16) ∞
6
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Fig. 8. (a) Graph between absorption coefficient (α) extracted from CPM measurements and energy of incident photons (hν), (b) Density of states (DOS) distribution of device vs photon energy (hν).
with EC as conduction band mobility edge and � 0; ε < EC hν θðε þ hν þ EC Þ ¼ 1; ε � EC hν Then, equation (16) becomes � � 1 dαðhνÞ Ni ðεÞf ðεÞ ¼ BNC dðhνÞ hν¼EC ε
high conversion efficiencies [2]. We have investigated the Schottky junction type OSC device. The DC current conduction process and photovoltaic behavior of Schottky OSC are investigated by taking current-voltage characteristics in dark and under illumination, respectively. Current-voltage (electrical) charac teristics of devices have to be fully analyzed to get complete insight of electrical behaviour.
(17)
(18)
3.5.1. Dark I-V characteristics In view of application in OSCs, the Schottky device is electrically characterized using dark I-V characteristics. These are electrically characterized to understand the electrical behavior between metal contacts. Fig. 9(a) shows the forward and reverse bias I-V characteristics of fabricated device in dark, at room temperature. It reveals that the fabricated device follows the Schottky diode characteristics in forward and reverse bias both. Semi-logarithmic plot of dark I-V characteristics is shown in Fig. 9(b) which depicts the linear behavior at low forward bias and the non-linear (rectifying) behavior of diode at higher forward bias voltages. Downward curvature of diode is attributed to leakage current, series resistance, and shunt resistance present in the diode. The mathematical expression for I-V characteristics of a Schottky diode is given by relation [39]: � �� � �� qV qV 1 exp (20) I ¼ I0 exp rkT rkT
This expression reveals that occupied DOS are related to the first derivative of absorption coefficient w.r.t. energy of photons. The DOS distribution is shown in Fig. 8(b). Firstly, there is exponential decrease in DOS then DOS almost flattens with slight increase at certain incident energy. These energies correspond to discrete defect levels [32]. These occupied gap states are measured by CPM which provides useful infor mation about the device quality. But more analysis is required to confirm the results. For quantitatively estimation of density of defect states, Urbach tail in the CPM absorption coefficient spectra is consid ered. In this low energy region, density surface defect states (Nt) are assumed to be same as the Urbach tail states (NU) [30]. The number of defect tail states in valence band is given by expression: � � ðε EV Þ (19) Nt � NU � NV exp EU with NV as density of states in valence band, EV as valence band mobility edge. Using NV � 1021 cm 3 and (ε-EV) � (0.2–0.3 eV), calculated defect DOS are 9.77 � 1019 cm 3. This is the lower limit of DOS.
with � I0 ¼ AA* T 2 exp
3.5. Electrical characterization
qϕB � kT
(21)
as reverse saturation current, A as contact area of device, A* as Richardson constant, T as temperature, q as electronic charge, ϕB as potential barrier, k as Boltzmann constant and r as ideality factor. The rvalue of device is 3.77 and obtained with the help of expression given by:
OSCs performance has been rapidly rising due to the development of new organic materials and devices. Recently, small molecule (SM) OSCs have attained encouraging PCEs [33–36]. In SM-OSCs, with few ex ceptions, the acceptor material is usually a fullerene-based material viz. C70 while many types of materials (absorbing and non-absorbing) are used as the donor component [37]. Nevertheless, the conversion effi ciencies of SM-OSCs are lower than their inorganic counterparts. Reason behind lesser PCEs of OSCs is the low open-circuit voltage (VOC) of the devices. This low VOC of devices prevents OSCs to attain higher effi ciencies [1]. In order to enhance the PCE of OSCs for commercialization, an attractive step put forward is to increase the Voc of devices with no change in other figures of merit [38]. For this purpose, Schottky junction type of OSCs are the good option to be adapted which results high Voc. N-type organic semiconductor, such as fullerene (C70) as photoactive layer having Schottky junction with MoO3 is considered as promising candidate. Schottky junction OSCs device structure has been continu ously the area of intense research due to low cost, easy availability, and
r¼
q dV kT dðLnIÞ
(22)
The r-value greater than one attributes to high density of localized states at metal and organic molecule interface. Potential barrier ϕB of a diode has great influence on the performance of OSC [40]. The value of ϕB is 0.88 eV for device. It is known that the device resistance affects the electrical parameters [41]. The values of series resistance have been determined with the help of Cheung’s functions H(I) and dV/dLnI [42]. The plots of H(I) versus I and dV/dLnI versus I for diode is shown in Fig. 9(c) and these are linear in nature. Fig. 9(c) shows that both the plots are compatible to each other. Series resistance Rs has been 7
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Fig. 9. (a) Dark current and voltage characteristics, (b) Semi-logarithmic plot of current and voltage characteristics, and (c) Plots of dV/dLnI vs I and H(I) vs I, of FTO/MoO3/C70/Al device.
extracted from the slope of graphs. The values of Rs are 15.5 Ωcm2 and 16.3 Ωcm2 (H(I) versus I and dV/dLnI versus I, respectively). The pres ence of Rs reflects the inhomogeneities and electron-hole recombination in device. The value of ϕB is measured from the intercept of plots. The value of ϕB are 0.89 eV and 0.88 eV (H(I) versus I and dV/dLnI versus I, respectively) for device. These evaluated values are almost similar to the value of ϕB determined by simple I-V plot.
Fabricated device exhibits a jsc of 1.09 mAcm 2, a Voc of 1.7 V with a fill factor of 0.41, and PCE of 0.76%. The value of Pm is 0.77 mWcm 2 (Fig. 10(b)). Nevertheless the obtained PCE of device is lower than the reported values in literature. But the determined large Voc of device is highly appreciated. The obtained Voc is greater than the band gap of fullerene C70 due to anomalous photovoltaic effect [43]. This effect is occurred because of the presence of two different structures (fcc and hcp) of fullerene C70 as explained in section 3.1. This disordered structure of C70 results in the addition of Voc generated by two phases separately as individual Schottky barrier solar cell.
3.5.2. Light I-V characteristics Light I-V characteristics and power density voltage curves of fabri cated Schottky barrier solar cell are given in Fig. 10(a and b). The solar cell parameters like short circuit current density (jsc), Voc, maximum voltage and current (Vm, jm) where output power density generated Pm (¼ Vm � jm) is maximum, are evaluated from the curve (Fig. 10(a)).
4. Conclusions In order to design a high efficient OSC, charge transport behaviour in
Fig. 10. Current density - voltage characteristics under illumination, (b) Power density - voltage curves of fabricated FTO/MoO3/C70/LiF/Al device. 8
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fullerene (C70) based OSC is studied by evaluating the optical dispersion parameters and defect tail states distribution in mid gap region. The energy band gap of C70 comes out to be 1.57 eV. The optical constants (n,k) have been determined using Swanpoel’s method. The optical constants increase with the decreasing wavelength. The increase in index of refraction with decrease in wavelength tells that C70 shows normal dispersion behavior. Spectral dependence of dielectric constants (ε1,ε2) reveals that there is decrease in the values of both constants with decreasing energy of photons. This shows that, C70 yielded significant interactions with incident photons. The dissipation factor (tan δ) and optical conductivity (σopt) increases with increasing energy of photons. Dispersion energies E0 and ED, lattice dielectric constant εL, N/m* and Nopt have also been evaluated. Using CPM, defect states DOS distribution in mid gap region is determined. In addition, device is electrically characterized by I-V characteristics in dark as well as under illumination which confirm the electrical behavior of device as a Schottky barrier OSC.
[12] [13] [14] [15]
[16]
[17]
[18]
Authors contribution section Sheenam Sachdeva has carried out experimental findings of this work. Prof. S.K. Tripathi and Dr. Devinder Singh supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.
[19] [20]
Declaration of competing interest
[21]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[22] [23]
Acknowledgements
[24]
This work is financed by UGC-CAS grant. Sheenam Sachdeva is grateful to UGC-BSR, New Delhi for providing the fellowship. Dr. Devinder Singh is thankful to DST, New Delhi under Inspire faculty research grant [IFA-12-PH39].
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Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Optical and electrical properties of fullerene C70 for solar cell applications Sheenam Sachdeva a, Devinder Singh a, b, S.K. Tripathi a, * a b
Department of Physics, Panjab University, Chandigarh, 160014, India Amity School of Applied Sciences, Amity University, Lucknow Campus, Lucknow, UP, 226028, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Fullerenes Vapor deposition Optical properties Defects Electrical properties
This research paper demonstrates the successful deposition of fullerene C70 for application as an acceptor ma terial in Schottky barrier organic solar cell (OSC) device with structure fluorine-doped-tin-oxide/ molybdenumtrioxide/fullerene/lithiumfluoride/aluminium (FTO/MoO3/C70/LiF/Al) using vapor thermal deposition technique. Morphology analysis using field emission scanning electron microscopy (FESEM) shows the mesoporous nature of highly cross-linked C70 molecules in deposited C70 thin film. But more uniform C70 film with less porosity has been deposited with high evaporation rate. We report the optical and electrical characterizations which reveal the thin film to be useful in photovoltaic applications. To study optical absorption spectra over visible energy range, prepared samples have been characterized by ultraviolet–visible (UV–vis) absorption spectroscopy. Optical dielectric parameters and dispersion properties of samples are studied. Constant Photocurrent Method (CPM) is employed to measure the mid gap absorption spectra of fabricated device. Defect density of states (DOS) distribution, Urbach energy and steepness parameter are also evaluated. Current-voltage characteristics of device in dark as well as under illumination surmise the behavior of device similar as Schottky barrier OSC.
1. Introduction Considerable attention on energy resources has spurred significant interest in the fabrication and characterization of organic solar cells (OSCs) [1]. Recently, fullerene C70 based Schottky OSCs have attained encouraging power conversion efficiencies (PCEs) [2]. However, this progress is quite slow in the field of OSCs as compared to their inorganic counterparts. Many facets of devices have been left to attain higher performance of these OSCs. To achieve higher efficiencies and remark able progress, detailed knowledge of transport properties of materials is required for optimization of Schottky OSCs [3–6]. Charge transport and recombination are the important bottlenecks for the major contributions of losses present in the OSCs that limit the efficiencies and stability of OSC devices. The optical and electrical properties of OSCs primarily depend on photoabsorption and density of states (DOS) distribution in the mid gap of the materials [7]. These properties play crucial role in photoconversion process of OSCs. Electronic defect states (intrinsic or extrinsic) are one of the foremost facets that significantly affect the transport phenomenon in the OSC devices, must be studied and char acterized. The existence of defect states directly impact the stability and lifetime of OSCs [8]. Evaluation of defect trap states opens an important
area of research to characterize and mitigate these defects for the sta bility of OSCs. Therefore, to improve photoabsorption and stability of Schottky OSCs, optical parameters and defect states are imperative to be deter mined for the further progress in this field of photovoltaic technology with better understanding of charge transport and electron-hole recombination in the device. It is reviewed that many authors have paid attention to increase the optical absorption in the field of thin-film technology and revealed the information about the optical properties and DOS in materials and OSCs. Most of the researchers have concen trated their study to determine the dispersion parameters and defect states distribution of thin films [9,10]. No attempt has been made yet to study the dispersion parameters and DOS distribution of C70 based Schottky OSC device. However, these are of utmost importance in fabricating a high efficient OSC. It is suggested that dispersion param eters and DOS distribution of defect states within mid gap of device has to be investigated. The present study pursuits the extensive study of optical characterization (mainly dispersion parameters) of fullerene C70 based thin films, DOS distribution and electrical characterization of C70 based Schottky OSC device. Thin film of organic semiconductor, such as, C70, is considered because it is drawing significant interest in OSCs due
* Corresponding author. E-mail addresses: [email protected], [email protected] (S.K. Tripathi). https://doi.org/10.1016/j.optmat.2020.109717 Received 27 September 2019; Received in revised form 22 November 2019; Accepted 21 January 2020 0925-3467/© 2020 Published by Elsevier B.V.
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to its potential for future photovoltaic absorber with optimal electronic and excellent optical properties and has exhibited incredible perfor mance in OSCs [11]. To elaborate dispersive behaviour and defect states distribution, layer C70 on glass substrates is deposited by vapor thermal deposition technique. The aim of this research work is to fabricate a device which finds application in developing a high efficient Schottky OSC, therefore, stacked layers, Molybdenum trioxide/C70/Lithium Fluoride, MCL (MoO3/C70/LiF) consisting C70 sandwich between anode buffer layer (ABL) and cathode buffer transport layer (CBL) are used for device fabrication with fluorine-doped tin oxide (FTO)-coated glass substrate as anode. In this work, firstly, we have described the evaluation of optical and dispersive transport by evaluating optical constants such as index of refraction and extinction coefficient (n,k), real and imaginary parts of dielectric constant (ε1 and ε2), dispersion properties like dispersion en ergies (E0 and ED), lattice dielectric constant (ε´), and maximum fre quency (ωP) of layers. Defect tail states are observed in sub band gap absorption coefficient (α) spectra of C70 layer. Constant photocurrent method (CPM) is employed to find the mid gap absorption spectra and defect DOS distribution of fabricated OSC which is determined using derivative method. The parameters like Urbach energy (EU) and steep ness parameter (S) are also evaluated. In addition, current-voltage (I-V) characteristics in dark as well as under illumination have been measured to confirm the type of synthesized device. Reported results justify the junction property of device which is consistent with Schottky barrier OSCs.
Fig. 1. Powder X-ray diffraction (XRD) pattern of fullerene C70 powder.
symmetry due to orientational disorder present in the structure of C70. This is attributed to the presence of mixture of two different (facecentered cubic, fcc and hexagonal close packing, hcp) structures. Most of the peaks are doubly indexed as hcp and fcc. A very few peaks are indexed independently as fcc with average lattice constant, a ¼ 1.416 nm and hcp with lattice constant a ¼ b ¼ 1.123 nm, c ¼ 1.722 nm and c/ a ¼ 1.53. This small c/a value for hcp phase of C70 suggests the orien tational disordered structure of C70 [14,15].
2. Experimental and characterization C70 (acceptor material), MoO3 (ABL), LiF (CBL) and FTO-coated glass substrates (13 Ω/sq) were purchased from Sigma–Aldrich. All chemicals with purity > 98% were used as received. Deionized water was used throughout the preparation. For thin film deposition, corning 7059 glass substrates and FTO-coated glass substrates were used. Thin films were deposited on pre-cleaned required substrates using vapor thermal deposition technique [vacuum coating unit HINDHIVAC, Model:VS-65D] after maintaining a pressure of approx. 2 � 10 5 mbar [12]. The deposited C70 layers were uniform and well-adherent to the substrates. Before taking optical and electrical measurements, layers deposited on both substrates are placed in deposition chamber for a day. Firstly, fullerene C70 powder is structurally characterized using powder X-ray Diffraction (XRD) technique. Surface morphologies of deposited thin films of C70 have been found out with the help of Field Emission Scanning Electron Microscopy (FESEM) (HITACHI SU8010 model). Optical absorption spectra of layers on glass substrates have been determined using UV/VIS/NIR spectrophotometer (Perkin Elmer LAMBDA 750) at room temperature. To determine α-values of fabricated device in sub band gap region CPM measurements have been taken using the experimental set up described elsewhere [3]. Thereafter, DOS dis tribution is evaluated for fabricated device. Electrical properties of OSC devices are studied by placing the respective sample in a well equipped stainless steel made sample holder set up [13]. For electrical measure ments, sample holder is evacuated to ~10 3 mbar pressure with the help of rotary pump. Two-probe method has been used for current-voltage measurements. I-V characteristics in dark and under illumination are performed using Keithley 6517A source unit and for photo excitation source, a tungsten bulb (150 W) is used.
3.2. Micro-structural analysis The micro-structural properties of the deposited C70 thin films are studied using convenient FESEM technique. Fig. 2 gives the FESEM images of deposited C70 films on the glass substrate with different evaporation rate (18 Å/s, 25 Å/s, 32 Å/s) and magnifications (500 nm and 200 nm). It is observed from the micrographs, that thin films of C70 are uniform and well adhere to the glass substrates; the most uniform C70 layer has been deposited with evaporation rate 32 Å/s. Fig. 2(a–c) show that with the increase in the evaporation rate, grain size of C70 particles in films has been decreased and density of C70 particles gradually increases. It appears that melting of C70 powder during small evaporation rate results in heat generation which leads to agglomeration of particles. Moreover, Fig. 2(d–f) clearly depict that C70 thin films with some porosity are deposited on the substrates. There are pinholes found within the aggregates of C70 particles. This implies mesoporous C70 films have been synthesized. The highly cross-linked fullerene C70 molecules (25 hexagons, 12 pentagons) with mesostructures can develop numerous applications. The unique property of meso fullerene having randomly oriented pores shows an excellent electrical behaviour due to which meso materials are of considerable interest in many research areas for energy storage and conversion applications [16]. It is also noticed that with the increase in evaporation rate, pinholes are significantly reduced and thickness of thin films are raised from 113 nm to 194 nm. At high evaporation rate, thin films are more uniform with roughness 1.838 nm. Surface roughness of C70 film has been measured from the atomic force microscopy (AFM) image (Fig. 3). Therefore, to study the disorderness and defect states introduced by these pinholes in the C70 layers, thin film deposited with thinner C70 film is used for optical and electrical measurements due to the small exciton diffusion length of fullerene C70 [3].
3. Results and discussions 3.1. Structural analysis In order to get knowledge about the structural disorder and defects of fullerene C70, XRD spectrum has been taken. Fig. 1 shows the XRD spectrum of C70 powder. Many sharp diffraction peaks has been seen in XRD pattern of fullerene C70. It is observed that there is lack of
3.3. Optical characterization The optical properties of materials are a very useful source of 2
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Fig. 2. FESEM images of thermally evaporated mesoporous C70 thin films on glass substrate with different evaporation rate (a) 18 Å/s (b) 25 Å/s (c) 32 Å/s at 500 nm magnification, (d) 18 Å/s (e) 25 Å/s (f) 32 Å/s at 200 nm magnification.
the absorption in the visible region which is due to occurrence of optical transitions in C70 [19]. The fundamental absorption corresponds to the transition from extended valence band states to extended conduction band states. This shows that absorption coefficient increases with increasing hν. The absorption coefficient is evaluated to get information about the type of transition involved and value of optical band gap. The optical band gap energy (Eg) is determined using Tauc’s equation [20]. The band gap energy (Eg) is determined using Tauc equation (Eq. (1)): � ðαhνÞ1=2 ¼ B1 hν Eg (1) where Eg, hν, and B are band gap energy, energy of the incident light, and a constant, respectively. Fig. 4(c) shows the Tauc plot with photon en ergy. Optical band gap of C70 layers are found to be ranging between 1.46 and 1.65 eV by extrapolating the linear region of the plot [21]. This dictates that energy gap of C70 layers deposited with different evapo ration rate is dependent on the thickness of thin films. Increase in the thickness of thin films with evaporation rate (explained in section 3.2) has lowered the values of energy gap. Decrease in grain size of C70 particles has also been observed (Fig. 2) with increase in evaporation rate. The increase in the density of C70 particles due to decrease in grain size must increase the energy gap [22]. But in contrast, energy gap has been reduced. This is mainly attributed to the small absorption coeffi cient of C70 layer due to large thickness. The obtained Eg is in good agreement with the literature values [23]. Somewhat lesser value of Eg is due to the formation of localized tail states in layer [19]. Localized states are introduced in the layer after the exposure of thin film with air for few seconds during the removal of deposited C70 samples from vacuum deposition chamber. The additional levels in energy gap are responsible for the formation of localized trap states in sub band gap region. Therefore, this has resulted the broadening of band, formation of band tails and the reduction in the energy associated with the optical transi tion in C70. Aforementioned localized defect states behave like trapping
Fig. 3. AFM image of deposited fullerene thin film.
information about their structure. The optical properties of material enable us to relate the atomic structure, band structure and its electrical properties [17]. The optical properties of materials typically consist of optical band gap, absorption coefficient, α, refractive index, n, and their dispersion behaviour. 3.3.1. Absorption spectra Ultraviolet–visible absorption spectrum of C70 with wavelength range from 370 nm to 800 nm is shown in Fig. 4(a). It is noticed that there are two strong maxima at 385 nm and 523 nm having minima in the blue region lying within 370 nm–550 nm wavelengths. These are attributed to interband transitions among the π-orbitals of C70. Owing to complex electronic structure and lower symmetry of C70, the weak onset bands are also present in the spectrum above 550 nm and also reported in literature [18]. The optical transitions above 550 nm cannot be precisely measured from transmission measurements [19]. Therefore, CPM measurements have been adopted for these types of weak optical transitions. Fig. 4(b) shows α vs photon energy (hν) of C70 which clearly depicts 3
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Fig. 4. (a) Graph between absorbance and wavelength (λ), (b) Variation of absorption coefficient (α) vs photon energy (hν), and (c) Tauc plot of C70 thin film deposited on glass substrate.
centres and have influence on the optical absorption properties [24]. Defect states present in the material have been determined and described in section 3.4.
dielectric constant are obtained by the relation:
3.3.2. Optical dielectric constants Index of refraction, n is an important parameter for optical materials. Information regarding the propagation of incident radiations, local field and polarization of ions inside the films are determined from the spectral dependence of index of refraction. Spectral dependence of complex refractive index can be obtained by the measuring optical absorption and transmission measurements. Spectra of optical dielectric constants like (n,k) with wavelength in the visible region are given in Fig. 5(a). It is observed that optical spectra (n,k) of C70 follow the same trend as ab sorption spectra (Fig. 4(a)). Swanepoel’s method has been used to evaluate the values of n [25]. The k-values are determined using the equation:
ε2 ¼ 2nk
k ¼
αλ 4π
ε1 ¼ n2
k2
(3) (4)
Fig. 5(b) shows the plots of the dielectric constant (ε1 and ε2) versus photon energy. Similar variations in dielectric constants (ε1, ε2) have been noticed. The dielectric constant (ε1) has greater values as compared to imaginary part (ε2). These plots (Fig. 5(b)) confirm that photonelectron interactions occur in the energy region where n-values are increasing. These spectra corroborate the refractive index spectra. It is also noticed that dielectric constant values are positive for whole energy region. Hence there is always propagation of radiations occurred throughout the films in the energy region shown in Fig. 5(b). This description leads to a material specific conductivity. The optical con ductivity, σ opt, is calculated as using α-values as follows:
(2)
σ opt ¼
Two strong absorption bands at low wavelengths are observed in the spectra of n which depicts the similarity of incident radiations frequency and C70 electrons frequency. The decrease in n-values with the variation of wavelength shows the normal dispersive behaviour that leads to decoupling of C70 electrons from oscillating electric field of incident electromagnetic radiations. The n-values depend on the density and polarizability of valence electrons using Lorentz-Lorenz formula [26]. The decrease in k-values with increase in wavelength reveals the reduction in the loss of light by scattering and absorbance. The dielectric constant is determined by analyzing the obtained values of index of refraction. The real and imaginary (ε1, ε2) parts of
αnc 4π
(5)
with c as the velocity of light. Plot of σopt vs photon energy for C70 is shown in Fig. 5(c). This indicates that there is increase in the conduc tivity with increasing energy of photons due to the absorption in C70 layer. The dissipation loss (tanδ) is evaluated from the relation: tan δ ¼
ε2 ε1
(6)
Fig. 5(d) shows the variation of tanδ as a function of frequency for C70. It shows that dissipation loss is increased with increasing frequency. After discussing the results of spectra showing in Fig. 5, it is 4
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Optical Materials 101 (2020) 109717
Fig. 5. (a) Plot showing the variation of refractive index (n) and extinction coefficient (k) with wavelength (λ) (b) Plot of real ε1 and imaginary ε2 parts of dielectric constant vs photon energy (hν), (c) Plot between optical conductivity (σopt) and photon energy (hν) (d) Plot of dissipation factor (tan δ) vs frequency (ν), of C70 thin film.
concluded that increase in photon-electron interactions enhances the optical conductivity of C70 films with increasing energy of light photons which is due to decrease in the dissipation loss in that energy region. Thereby, refractive index starts decreasing with the increase in wave length beyond 500 nm upon falling light photons on the sample.
relationship [27], the expression for index of refraction dispersion relation is given by: n2
1
�
1
¼
E0 ED
1 ðhνÞ2 E0 ED
(7)
with E0 as single-oscillator energy, ED as energy of dispersion. Fig. 6(a) shows the plot of (n2-1) 1 vs (hν)2. The values of E0 and ED are evaluated from the slope and intercept of graph by extrapolation of lines in low energy region. The values of E0 and ED for pristine C70 are 2.65 eV and 7.98 eV, respectively. It is observed that the single oscillator energy and
3.3.3. Energy dispersion parameters For the applications in optical devices, evaluation of energy disper sive parameters is needed. These significant factors are found out from refractive index dispersion relation. Using Wemple–DiDomenico
Fig. 6. (a) Graph between (n2-1)
1
and (hν)2, and (b) Graph between n2 and λ2, of C70 thin film deposited on glass substrate. 5
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the dispersion energy are high for pristine C70 which could be attributed to the presence of number of scattering centres and impurity levels [28]. Due to absorption by excitons in the high energy region, non-linear (negative) curvature is noticed. Non-linear (positive) curvature is also observed at low energies which are attributed to lattice vibrations [27]. The static refractive index n´, is given by relation: rffiffiffiffiffiffiffiffiffiffiffiffiffiffi ED 0 n ¼ 1þ (8) E0 Using the values of E0 and ED calculated from Wemple–DiDomenico relationship, the value of n´ is determined and its value is found to be 0 2.00. To determine lattice (high frequency) dielectric constant (εL), ε ¼ 2 2 n versus λ is plotted in Fig. 6(b). The value of εL is determined using relation [10]: � 2 �� � e N 2 0 λ ε ¼ εL (9) m* 4πc2 ε0 with N as free carrier concentration, ε´ as real part of dielectric constant. The symbols c, ε0, m* and λ have their usual meanings. At higher wavelengths, ε´ is linearly dependent on n2. The value 4.67 of εL for C70 is evaluated by extrapolating the linear region of the plot (Fig. 6(b)) to λ→0. The obtained value of N/m* is 2.81 � 1047 cm 3 g 1. Drude’s theory of dielectrics is used to determine concentration of optically generated charge carriers, at low energies. The formula that has been used is given as follows:
ε1 ðℏωÞ ¼ n2
k2 ¼ εopt
ðℏωD Þ2 ðℏωÞ2
Fig. 7. Device structure (FTO/MoO3/C70/LiF/Al).
intensity of light while changing the energy of incident light photons through monochromator [7,25]. Spectral dependence of α by measuring number of incident photons at various photon energies is plotted in Fig. 8(a). The exponential region in the absorption spectra that gives us the information about the excitation of valance band tail states electrons to the conduction band extended states is named as Urbach tail which determine the structural disorder. The Urbach region involves the transitions between deep localized defect states. These defect states may be due to the surface contamination of C70 thin film by impurities due to exposure of film to air for few seconds. Urbach tail is expressed as: � � hν (13) α ¼ α0 exp EU
(10)
with εopt, ℏω, ℏωD as residual dielectric constant, photon energy, screened plasma energy, respectively. To find εopt, graph between (n2 k2) and (ℏω)2 is plotted. The evaluated value of εopt is 4.33. The maximum (plasma) frequency, ωP is defined as: pffiffiffiffiffiffiffi (11) ℏωD ¼ ℏωP εopt
with α0 as a factor that is determined experimentally, hν as photon en ergy and EU as Urbach energy. The steepness parameter S of the defect states is evaluated from the equation:
The determined value of ℏωP is 1.05 eV. The value of Nopt is found out using the expression: Nopt ¼
ε0 εopt m*e ω2P e2
(12)
S¼
kB T EU
(14)
with kB and T as the Boltzmann constant and absolute temperature, respectively. The Urbach energy and steepness parameter are obtained using exponential fit (Eq. (13)) of the absorption coefficient spectra. The value of EU comes out to be 49.3 meV which is comparable to 55 meV of C70 thin film [19] and steepness parameter, S for this sample comes out to be 0.53. Urbach energy (EU) is the characteristics parameter of dis order present in the samples. To obtain the DOS distribution in mid gap region, derivative method [7] is employed. The absorption coefficient for the transitions between valence band tail states Ni(ε) to the conduction band extended states Nf(εþhν) is given by convolution integral as [30]: Z ∞ B αðhνÞ ¼ Ni ðεÞf ðεÞf1 f ðε þ hνÞgNf ðε þ hνÞdε (15) hν ∞
with ε0, me*, e as permittivity of free space, effective mass of electron and elementary charge, respectively. Its value is 1.01 � 1027 m 3 for C70 thin film and is in good agreement with literature value of carrier concentration in semiconductors [29]. The evaluation of optical constants is of considerable importance for applications in integrated optical devices and OSCs. These numerable terms of the materials would be very helpful in designing an efficient device. These are the key parameter for device design. To use C70 in circuits, it is necessary that their dielectric and energy losses be under stood so as they have appropriate value ranges. 3.4. Constant photocurrent method For the device fabrication, stacked layers (MCL) are used. For MCL deposition, FTO-coated glass substrates were used. Layers of HTL, acceptor and ETL materials are deposited in sequence on FTO coated glass substrates using vapor thermal deposition technique Al cathode is deposited on the stacked layers deposited on FTO substrates using masks of desired active cell area (0.09 cm2). Device structure of fabricated OSC is shown in Fig. 7. To get physical insight of localized tail states and defect states in the material, mid gap absorption spectra is studied. The absorption spectra in the mid gap region is obtained using CPM technique. In CPM, photocurrent is generated in the fabricated device (Fig. 7) with incident monochromatic light. Photocurrent is kept constant with the variation of
with f(ε) as Fermi-Dirac distribution, B as transition matrix element and has a constant value. The integral can be deconvoluted using various methods. Hata et al. [31] have proposed a simple method to deconvolute the DOS. The electronic transitions from occupied states of valence band (VB) to conduction band (CB) are detected by CPM. So, CB tail states are not involved. After approximation by step function and using Heaviside function θ(ε þ hν þ EC), the above equation becomes: Z ∞ αCPM ffi Ni ðεÞf ðεÞθðε þ hν þ EC Þdε (16) ∞
6
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Fig. 8. (a) Graph between absorption coefficient (α) extracted from CPM measurements and energy of incident photons (hν), (b) Density of states (DOS) distribution of device vs photon energy (hν).
with EC as conduction band mobility edge and � 0; ε < EC hν θðε þ hν þ EC Þ ¼ 1; ε � EC hν Then, equation (16) becomes � � 1 dαðhνÞ Ni ðεÞf ðεÞ ¼ BNC dðhνÞ hν¼EC ε
high conversion efficiencies [2]. We have investigated the Schottky junction type OSC device. The DC current conduction process and photovoltaic behavior of Schottky OSC are investigated by taking current-voltage characteristics in dark and under illumination, respectively. Current-voltage (electrical) charac teristics of devices have to be fully analyzed to get complete insight of electrical behaviour.
(17)
(18)
3.5.1. Dark I-V characteristics In view of application in OSCs, the Schottky device is electrically characterized using dark I-V characteristics. These are electrically characterized to understand the electrical behavior between metal contacts. Fig. 9(a) shows the forward and reverse bias I-V characteristics of fabricated device in dark, at room temperature. It reveals that the fabricated device follows the Schottky diode characteristics in forward and reverse bias both. Semi-logarithmic plot of dark I-V characteristics is shown in Fig. 9(b) which depicts the linear behavior at low forward bias and the non-linear (rectifying) behavior of diode at higher forward bias voltages. Downward curvature of diode is attributed to leakage current, series resistance, and shunt resistance present in the diode. The mathematical expression for I-V characteristics of a Schottky diode is given by relation [39]: � �� � �� qV qV 1 exp (20) I ¼ I0 exp rkT rkT
This expression reveals that occupied DOS are related to the first derivative of absorption coefficient w.r.t. energy of photons. The DOS distribution is shown in Fig. 8(b). Firstly, there is exponential decrease in DOS then DOS almost flattens with slight increase at certain incident energy. These energies correspond to discrete defect levels [32]. These occupied gap states are measured by CPM which provides useful infor mation about the device quality. But more analysis is required to confirm the results. For quantitatively estimation of density of defect states, Urbach tail in the CPM absorption coefficient spectra is consid ered. In this low energy region, density surface defect states (Nt) are assumed to be same as the Urbach tail states (NU) [30]. The number of defect tail states in valence band is given by expression: � � ðε EV Þ (19) Nt � NU � NV exp EU with NV as density of states in valence band, EV as valence band mobility edge. Using NV � 1021 cm 3 and (ε-EV) � (0.2–0.3 eV), calculated defect DOS are 9.77 � 1019 cm 3. This is the lower limit of DOS.
with � I0 ¼ AA* T 2 exp
3.5. Electrical characterization
qϕB � kT
(21)
as reverse saturation current, A as contact area of device, A* as Richardson constant, T as temperature, q as electronic charge, ϕB as potential barrier, k as Boltzmann constant and r as ideality factor. The rvalue of device is 3.77 and obtained with the help of expression given by:
OSCs performance has been rapidly rising due to the development of new organic materials and devices. Recently, small molecule (SM) OSCs have attained encouraging PCEs [33–36]. In SM-OSCs, with few ex ceptions, the acceptor material is usually a fullerene-based material viz. C70 while many types of materials (absorbing and non-absorbing) are used as the donor component [37]. Nevertheless, the conversion effi ciencies of SM-OSCs are lower than their inorganic counterparts. Reason behind lesser PCEs of OSCs is the low open-circuit voltage (VOC) of the devices. This low VOC of devices prevents OSCs to attain higher effi ciencies [1]. In order to enhance the PCE of OSCs for commercialization, an attractive step put forward is to increase the Voc of devices with no change in other figures of merit [38]. For this purpose, Schottky junction type of OSCs are the good option to be adapted which results high Voc. N-type organic semiconductor, such as fullerene (C70) as photoactive layer having Schottky junction with MoO3 is considered as promising candidate. Schottky junction OSCs device structure has been continu ously the area of intense research due to low cost, easy availability, and
r¼
q dV kT dðLnIÞ
(22)
The r-value greater than one attributes to high density of localized states at metal and organic molecule interface. Potential barrier ϕB of a diode has great influence on the performance of OSC [40]. The value of ϕB is 0.88 eV for device. It is known that the device resistance affects the electrical parameters [41]. The values of series resistance have been determined with the help of Cheung’s functions H(I) and dV/dLnI [42]. The plots of H(I) versus I and dV/dLnI versus I for diode is shown in Fig. 9(c) and these are linear in nature. Fig. 9(c) shows that both the plots are compatible to each other. Series resistance Rs has been 7
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Optical Materials 101 (2020) 109717
Fig. 9. (a) Dark current and voltage characteristics, (b) Semi-logarithmic plot of current and voltage characteristics, and (c) Plots of dV/dLnI vs I and H(I) vs I, of FTO/MoO3/C70/Al device.
extracted from the slope of graphs. The values of Rs are 15.5 Ωcm2 and 16.3 Ωcm2 (H(I) versus I and dV/dLnI versus I, respectively). The pres ence of Rs reflects the inhomogeneities and electron-hole recombination in device. The value of ϕB is measured from the intercept of plots. The value of ϕB are 0.89 eV and 0.88 eV (H(I) versus I and dV/dLnI versus I, respectively) for device. These evaluated values are almost similar to the value of ϕB determined by simple I-V plot.
Fabricated device exhibits a jsc of 1.09 mAcm 2, a Voc of 1.7 V with a fill factor of 0.41, and PCE of 0.76%. The value of Pm is 0.77 mWcm 2 (Fig. 10(b)). Nevertheless the obtained PCE of device is lower than the reported values in literature. But the determined large Voc of device is highly appreciated. The obtained Voc is greater than the band gap of fullerene C70 due to anomalous photovoltaic effect [43]. This effect is occurred because of the presence of two different structures (fcc and hcp) of fullerene C70 as explained in section 3.1. This disordered structure of C70 results in the addition of Voc generated by two phases separately as individual Schottky barrier solar cell.
3.5.2. Light I-V characteristics Light I-V characteristics and power density voltage curves of fabri cated Schottky barrier solar cell are given in Fig. 10(a and b). The solar cell parameters like short circuit current density (jsc), Voc, maximum voltage and current (Vm, jm) where output power density generated Pm (¼ Vm � jm) is maximum, are evaluated from the curve (Fig. 10(a)).
4. Conclusions In order to design a high efficient OSC, charge transport behaviour in
Fig. 10. Current density - voltage characteristics under illumination, (b) Power density - voltage curves of fabricated FTO/MoO3/C70/LiF/Al device. 8
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Optical Materials 101 (2020) 109717
fullerene (C70) based OSC is studied by evaluating the optical dispersion parameters and defect tail states distribution in mid gap region. The energy band gap of C70 comes out to be 1.57 eV. The optical constants (n,k) have been determined using Swanpoel’s method. The optical constants increase with the decreasing wavelength. The increase in index of refraction with decrease in wavelength tells that C70 shows normal dispersion behavior. Spectral dependence of dielectric constants (ε1,ε2) reveals that there is decrease in the values of both constants with decreasing energy of photons. This shows that, C70 yielded significant interactions with incident photons. The dissipation factor (tan δ) and optical conductivity (σopt) increases with increasing energy of photons. Dispersion energies E0 and ED, lattice dielectric constant εL, N/m* and Nopt have also been evaluated. Using CPM, defect states DOS distribution in mid gap region is determined. In addition, device is electrically characterized by I-V characteristics in dark as well as under illumination which confirm the electrical behavior of device as a Schottky barrier OSC.
[12] [13] [14] [15]
[16]
[17]
[18]
Authors contribution section Sheenam Sachdeva has carried out experimental findings of this work. Prof. S.K. Tripathi and Dr. Devinder Singh supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.
[19] [20]
Declaration of competing interest
[21]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[22] [23]
Acknowledgements
[24]
This work is financed by UGC-CAS grant. Sheenam Sachdeva is grateful to UGC-BSR, New Delhi for providing the fellowship. Dr. Devinder Singh is thankful to DST, New Delhi under Inspire faculty research grant [IFA-12-PH39].
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