PEDOT:PSS Schottky junction on PET substrates

Optik - International Journal for Light and Electron Optics 181 (2019) 984–992

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Original research article

Flexible organic solar cells with graphene/PEDOT:PSS Schottky junction on PET substrates

T

Thokchom Jayenta Singha, Sumitra Singhb,c, , Sk Masiul Islamc,d, , Rubina Getb,e, Pramila Mahalaf, Khomdram Jolson Singha ⁎

⁎

a

Manipur Institute of Technology, Takyelpat, Imphal, 795001, Manipur, India Flexible and Non-Silicon Electronics, CSIR-Central Electronics Engineering Research Institute, Pilani, Rajasthan, 333031, India c Academy of Scientific and Innovative Research (AcSIR), CSIR-CEERI Campus, Pilani 333031, India d Optoelectronics and MOEMS, Council of Scientific and Industrial Research-Central Electronics Engineering Research Institute, Pilani, Rajasthan 333 031, India e Shri Jagdishprasad Jhabarmal Tibrewala University, Jhunjhunu, Churela, Rajasthan 333001, India f Dept. of Electrical and Electronics Engineering, Birla Institute of Technology and Science, Pilani, Rajasthan, 333031, India b

ARTICLE INFO

ABSTRACT

Keywords: Organic Schottky junction Solar cells Flexible substrate Charge transport Efficiency

In this article, a computational study pertaining to optimal design and simulations of organic Schottky junction solar cells containing poly(3,4ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) and graphene layers is undertaken using Silvaco TCAD Atlas tool. Polyethylene terephthalate (PET) is used as a flexible substrate, whereas graphene is taken as cathode for the devices with a structure graphene/PEDOT:PSS/PET. During design and simulation, thickness of the PEDOT:PSS layer is varied from 50 to 90 nm. The optimized device shows excellent photovoltaic characteristics under AM1.5 G illumination. The open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and energy conversion efficiency (η) is found to be 0.68 V, 0.68 mAcm−2, 60.34% and 2.87%, respectively. A band diagram is proposed to explain the carrier transport phenomena. The proposed structures do not contain any metallic nanoparticle in the polymer matrix and therefore eliminate the issues arising from the nanoparticle and polymer blend based solar cells where the phase separation between the nanoparticles and polymers lowers the device stability and lifetime.

1. Introduction Recently, energy harvesting devices based on organic solar cells (OSCs) have gained tremendous research interest due to their ease in solution processability, low-cost manufacturing methods, material abundance, large area deposition, light weight and high mechanical flexibility [1]. These promising features are suitable in scalable fabrication techniques to achieve a long-standing goal of economic and eco-friendly energy harvesting through solar power [1–10]. Organic solar cells are considered to be a potential inexpensive substitute to conventional inorganic solar cells because of their ease of processing and compatibility with flexible substrates and thus attracted great industrial and academic attention as a promising source of inexpensive renewable energy [11–19]. Tremendous research thrust to improve the power conversion efficiency (PCE) of the OSCs have been made by designing the novel

⁎ Corresponding authors at: Flexible and Non-Silicon Electronics, CSIR-Central Electronics Engineering Research Institute, Pilani, Rajasthan-, 333031, India. E-mail addresses: [email protected] (S. Singh), [email protected] (S. Masiul Islam).

https://doi.org/10.1016/j.ijleo.2018.12.179 Received 20 December 2018; Accepted 29 December 2018 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.

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materials and device architectures, however, is not even close to reach the commercial requirements. One of the major bottleneck of OSCs to achieve high efficiency is the low open-circuit voltage (VOC), which is limited by the energy offset between the highest occupied molecular orbital (HOMO) of the electron donor and the lowest unoccupied molecular orbital (LUMO) of the electron acceptor [4]. Moreover, the role of hole transporting buffer layer (HTBL) and electron transporting buffer (ETBL) layer is important not only to facilitate charge transport to their respective electrodes but also it improves the interfacial properties between electrode and active layer [1]. PEDOT:PSS is widely used as a hole transport layer (HTL) in organic and perovskite based optoelectronic devices because it provides a smooth surface on the electrode. Furthermore, PEDOT:PSS is formed by combining two ionomers and it has high optical transparency throughout the visible, near IR and UV electromagnetic spectrum. Again, owing to high electrical conductivity (14,000 S/m) and electron mobility (15,000 cm2 V−1 s−1) for graphene, it has been used extensively to fabricate OSCs [11]. Tandem or multijunction solar cells and thin film solar cells are found to be promising as the second generation of photovoltaic technology facilitating wide range of applications to reduce the cost of solar cells [20–23]. Organic photo detector and photovoltaic solar cells using PEDOT:PSS and conjugated polymer blends showed good external quantum efficiency (EQE) in the order of 20%, which make the material system attractive for optoelectronic device applications [24]. In a report, Wang et al. [25] studied the characteristics of hybrid Si nanocones/PEDOT:PSS based solar cells, which shows a power conversion efficiency of 7.1%. Report is available in the literature where PEDOT:PSS layer was used as a highly conductive layer by substituting ITO electrode to fabricate flexible inverted polymer solar cells with structure Ag-grid/HC-PEDOT:PSS/ZnO/photoactive layer/MoO3/Al [26]. Eom et al. [27] reported the solar cell efficiency of 3.16% using inkjet-printed PEDOT:PSS layer modified by specific surfactants to improve its surface morphology and conductivity. It may be mentioned here that incorporation of metallic nanoparticle into the polymer matrix facilitates phase separation between nanoparticle and polymer. This lowers the device stability and lifetime [11,19]. In the present study, we, thus, propose organic solar cells on flexible substrate using a combination of graphene and PEDOT:PSS polymer. Design and simulation of organic Schottky junction solar cells containing two different layers, viz. PEDOT:PSS and graphene on flexible polyethylene terephthalate (PET) substrate with a structure graphene/ PEDOT:PSS/PET was undertaken throughout the study. A comparative study has also been made on the photovoltaic performance of the devices by varying the thickness of the active PEDOT:PSS layer. 2. Modeling and simulation methodology The solar cell devices were designed on PET substrates of thickness 100 nm. Two types of devices with structures graphene/ PEDOT:PSS/PET are designed and simulated throughout the study. A Schottky Devices: In Schottky solar cells, semiconductor is sandwiched between two metals. However, in this study both the contacts are at the top surface. One makes the Schottky contact, whereas the other is Ohmic contact. PEDOT:PSS, a hole-rich (p-type, doping concentration 1 × 1019 cm−3) organic semiconductor, typically used as an active layer to form a Schottky contact (cathode) with graphene and Ohmic contact (anode) with platinum (Pt). Schematic diagrams of device A and device B are shown in Fig. 1(a) and (b), respectively. B Simulation Methods: Design and simulations of organic Schottky junction solar cells was undertaken using Silvaco TCAD Atlas tool. SRH (ShockleyRead-Hall) recombination, OPTR (Optical recombination) and AUGER recombination models were used during simulation. Graphene is included as a user defined metal conductor. Physical parameters used in simulation for Schottky junction solar cells are listed in Table 1. For both the devices, thickness of PEDOT:PSS layer was varied from 50 to 90 nm and their photovoltaic parameters such as open circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and power conversion efficiency (η) were performed under standard spectrum AM1.5 G illumination. 3. Numerical results and discussion Mechanism of the Schottky junction solar cell can be realized from the energy band diagram as shown in Fig. 2. Difference in energy levels between the Fermi level of metal and the conduction band of the semiconductor leads to abrupt potential difference and forms Schottky barrier height. Graphene and PEDOT:PSS junction facilitates the band bending, which is pertinent for the separation of charges. Due to the work function difference between the graphene and PEDOT:PSS, a built-in potential is generated in the PEDOT:PSS near the interface region. Light illumination above the bandgap, promotes the separation of photogenerated electrons (e−) and holes (h+) towards the graphene and PEDOT:PSS layer, respectively. Graphene-based Schottky junction solar cell favors to pass light through its Schottky electrode due to its high transparency and generate electron–hole pairs in the PEDOT:PSS, compared to its conventional counterpart. This, in turn, increases the optical to electrical energy conversion efficiency. Thickness of the PEDOT:PSS layer was varied from 50 to 90 nm for both device A and device B. The photovoltaic parameters of solar cells such as Jsc, Voc, FF, and η are determined from current density-voltage (J–V) characteristics under AM1.5 G as shown in Fig. 3. From Fig. 3, the current density and efficiency for both the devices are found to be maximum at a PEDOT:PSS thickness of 70 nm. Fill factor and opencircuit voltage of both the devices are observed to be moderately high when the thickness of PEDOT:PSS is 70 nm. Hence, the optimized thickness of PEDOT:PSS layer was determined as 70 nm to carry out simulations of all the devices. Typical J–V 985

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T. Jayenta Singh, et al.

Fig. 1. Schematic diagram of flexible organic solar cells (a) device A, and (b) device B. Table 1 Different physical parameters of PEDOT:PSS and graphene. Symbols

Parameters

PEDOT:PSS [28]

Graphene [29]

Eg χ ε μe μh Nv Nc ϕ Na

energy bandgap, eV electron affinity, eV dielectric constant, Fm−1 electron mobility, cm2 v−1 s−1 hole mobility, cm2 v−1 s−1 density of state in valence band, cm−3 density of state in conduction band, cm−3 work-function, eV acceptor concentration, cm−3

1.6 3.6 2.2 1 40 2 × 1021 2 × 1021 5.1 1 × 1019

0 – 25 943 943 – – 4.9 –

Fig. 2. Schematic energy band diagram of graphene/p-PEDOT:PSS Schottky junction.

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Fig. 3. Variation of (a) open-circuit voltage (b) short circuit current density (c) fill factor and (d) efficiency for device A and device B.

characteristics of flexible organic solar cells based on graphene/p-PEDOT:PSS Schottky junction under dark and illumination condition is shown in Fig. 4(a) and (b), respectively. Initially, device behaves as a diode with rectifying behavior as shown in the inset of Fig. 4(a). It may be mentioned here that ideality factor (n) is an important parameter in describing the performance of a device, which generally lies in the range of 1 to 2 [30]. It also indicates the occurrence of low recombination of the carriers in the junctions. From Fig. 4(a), the ideality factor (n) is calculated to be 1.12 in the voltage ranging from 0.42 to 0.65 V. The ideality factor (n) of the graphene/p-PEDOT:PSS Schottky junction solar cell was calculated using the following equation [31].

Fig. 4. Typical I–V characteristics of flexible organic Schottky junction solar cells for device A and B under (a) dark and (b) illumination (AM1.5 G) condition. 987

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n=

q d (V ) kT d ( ln I )

(1)

where q is the electronic charge, k is the Boltzmann constant and T is the absolute temperature in K. J–V characteristics of the solar cell can be explained by the thermionic emission which can be mathematically expressed as follows [31–33].

I = I0 exp

q (V IRs ) nkT

1

exp

q (V

IRs ) (2)

kT

where V is the applied voltage across the contact, n is the ideality factor, Rs is the series resistance, T is the absolute temperature, k is the Boltzmann constant, q is the electronic charge, and I0 is the reverse saturation current expressed as

q B kT

I0 = A T 2 exp

(3)

**

where α is the cell area, A is the effective Richardson constant (depends on the current-transport mechanism). ΦB is the zero-biased barrier height which can be mathematically expressed as follows [34]. B

=

kT I0 log q AA T

(4) −14

2

The value of the reverse saturation current is found to be 1.63 × 10 A/cm . The low value of the reverse saturation current can be attributed to the less thermal losses in the device. The barrier height was determined to be 0.5 eV, which finds its consistency with the earlier reported results [34,35]. It is noted that lateral configuration causes lateral current injection which need both contacts on the same surface. Owing to etched surface in device A, current tends to crowd at the etched region located near to the Ohmic contact. Since device B does not employ any etching process, the current spreading effect becomes significant. Therefore, the short-circuit current density in device B is higher than device A as shown in Fig. 4(b). It is noted that open-circuit voltage (Voc) and the recombination rate are related to each other and is defined by the following expression [36].

Voc =

qrg B + J0 (1 + rR ) kT ln q J0 (1 + rR )

(5)

where rg is the generation enhancement ratio (rg=ϕA/ϕB), rR is the radiative enhancement ratio. Since the recombination rate is in the same order, hence there is no significant variation in Voc. For both the devices, the series and shunt resistance are calculated in 0.7–0.51 V and 0-0.19 V, respectively. The series and shunt resistance are determined according to the slope of the J–V curve at Voc and Jsc, respectively. The value of series and shunt resistance for device A was found to be 4 GΩ and 206 GΩ, whereas those values are obtained to be 2 GΩ and 97 GΩ for device B, respectively. Low value of series resistance in device B is attributed to the high current density. The FF and η of the devices can be calculated using the following expressions [37,38]. High value of shunt resistance causes high FF in device A.

FF = FFs 1

Voc + 0.7FFs Voc rsh

while FFs = FFo (1

and FF0 =

Voc

1.1rs ) +

(6)

rs2 5.4

(7)

ln(Voc + 0.72) Voc + 1

(8)

where FF0 is the fill factor of ideal solar cells (with no Rs and Rsh), FFs is the fill factor of solar cells that have Rs only, Voc is the normalized open circuit voltage, rs and rsh are the normalized resistance obtained by dividing actual resistance Rs and Rsh by Voc/Jsc. Power conversion efficiency can be obtained using the following expression [39].

=

Pmax V I FF Pmax [W ] × 100 = oc sc = Pin Pin 1000[Wm 2] × CellArea [m2]

(9)

Where Pmax is the maximum output power of the solar cell and Pin is the input power applied to solar cell with illumination of AM1.5 G. Efficiency of device B is higher (2.87%) compared to device A (2.06%) due to low current crowding, low thermal losses and low recombination rate as discussed above. To investigate the physical phenomena behind this, further study was also carried out in terms of photo-generation rate, recombination rate, electric field and carrier concentration for both the devices. Table 2 demonstrates the electrical and photovoltaic parameters of graphene/p-PEDOT:PSS based solar cells. Fig. 5(a) and (b)shows the rate of photogeneration for both the devices. Photogeneration rate is high at the graphene/PEDOT:PSS interface and gradually decreases towards the PEDOT:PSS/PET junctions. It is found to be slightly high for device B, however, almost comparable to device A. Near the cathode region for both devices, the photo generation rate was observed to be ˜1022. On the other hand, the photo generation rate near the anode junction was found to be 10 times higher in device A compared to device B. Photo generation becomes profound throughout the device B, whereas for device A it was limited to 10 nm only. The photo-generation rate can be obtained using the following expression [36]. 988

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Table 2 Electrical and photovoltaic parameters of the flexible organic solar cells. Device Structure

Photovoltaic characteristics

Graphene/p-PEDOT:PSS/ PET (Device A) Graphene/p-PEDOT:PSS/ PET (Device B)

Series resistance, GΩ

Shunt resistance, GΩ

Ideality factor (n)

Saturation current, A cm−2

Jsc (mA cm−2)

Voc(V)

FF (%)

η (%)

0.47

0.70

62.06

2.06

4

206

–

–

0.68

0.68

60.34

2.87

2

97

1.12

1.63 × 10−14

Fig. 5. The photo-generation rate near (a) cathode and (b) anode in graphene/p-PEDOT:PSS Schottky junction solar cells.

G=

0

P e hc

y

(10)

where, P is the cumulative effect of reflection, transmission and loss due to the absorption over the path, y is the relative distance from the given ray, h is the Planck's constant, λ is the wavelength, c is the speed of light and α is the absorption coefficient, η0 is the internal quantum efficiency which represents the number of carriers generated per photon per pair. Fig. 6(a) and (b) represents the total recombination rate for device A and B as a function of distance near the cathode and anode region. During the simulation of the device SRH (Shockley-Read-Hall) recombination, OPTR (optical recombination) and AUGER recombination were used to determine the recombination rate of the devices. Recombination rate of device B is found to be smaller than device A near the cathode and anode region as shown in Fig. 6(a) and (b). This implies that device B has longer carrier lifetimes, thereby better charge collection of the device. Therefore, device B exhibits higher efficiencies than device A. Since Schottky junction is formed at the cathode region, the devices have a higher electric field near the cathode region compared to the anode region. Again, the photo-generation rate of the devices is observed to be higher near the cathode region. So, from Fig. 7(a) and (b), it was evident that device B have a higher electric field near the cathode region, whereas device A have a higher electric field near the anode region. These findings gives an clear indication that devices B have higher potential at the graphene/PEDOT:PSS interface, which in turn leads to better collection of charge than device A. Current conduction in the devices is primarily influenced by the majority carrier holes owing to formation of Schottky junction in the devices. Fig. 8(a) and (b) represent the concentration of holes and electrons near the cathode and anode region for both the devices. Concentration of holes and electrons are observed to be higher for device A. However, the efficiency of device A was obtained to be low compared to device B. It is attributed to the high recombination rate in device A compared to device B, as demonstrated in the Fig. 6(a) and (b). It may be mentioned here that high concentration of holes and electrons for device A have 989

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Fig. 6. The recombination rate near (a) cathode and (b) anode in graphene/p-PEDOT:PSS Schottky junction solar cells.

Fig. 7. The profile of the electric field in graphene/PEDOT:PSS based solar cells near (a) cathode and (b) anode region.

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Fig. 8. Concentration of (a) holes and (b) electrons in the flexible organic solar cells.

shorter lifetimes and thus recombined quickly compared with device B. As a result, collection of charges becomes low for device A. Therefore, device B exhibits higher current density and efficiencies compared to device A. 4. Conclusion In conclusion, flexible organic solar cells based on graphene/p-PEDOT:PSS Schottky junction was modeled and numerically simulated using Silvaco TCAD Atlas tool. Thickness of the PEDOT:PSS layer was varied from 50 to 90 nm for all the devices. The performance of the solar cells having PEDOT:PSS thickness of 70 nm is found to be superior to others, which is attributed to the high electric field and low recombination rate. However, the open-circuit voltage (Voc), short-circuit current density (Jsc), fill-factor (FF) and energy conversion efficiency (η) is found to be 0.68 V, 0.68 mAcm−2, 60.34% and 2.87%, respectively. These simulation results reveal the effects of device design strategy and material properties on the device performance. Therefore, this study can pave a way for an optimal design of high performance and low-cost solar cells. Thus, with its simple structure coupled with reasonably good solar output characteristics comparable to the state-of-the-art devices, it has great potential in the family of flexible organic solar cells. Acknowledgments One of the authors (Pramila Mahala) gratefully acknowledges SERB-National Postdoctoral Fellowship (PDF/2017/002648) to carry out this research. The work has also been supported by the Council of Scientific and Industrial Research (CSIR), New Delhi, India through a project (MLP-2101). The authors are thankful to Director, CSIR-CEERI, Pilani for his constant support and encouragement. References [1] J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.C. Chen, J. Gao, Li G, Y. Yang, A polymer tandem solar cell with 10.6% power conversion efficiency, Nat. Commun. 4 (2013) 1446. [2] Q. Zhang, B. Kan, F. Liu, G. Long, X. Wan, X. Chen, Y. Zuo, W. Ni, H. Zhang, M. Li, Z. Hu, F. Huang, Y. Cao, Z. Liang, M. Zhang, T.P. Russell, Y. Chen, Smallmolecule solar cells with efficiency over 9%, Nat. Photonics 9 (2014) 35–41. [3] M. Zhang, H. Irfan, Y. Ding, C.W. Gao, Tang, Organic Schottky barrier photovoltaic cells based on MoOx/C60, Appl. Phys. Lett. 96 (2010) 183301. [4] L.-M. Chen, Z. Xu, Z. Hong, Y. Yang, Interface investigation and engineering – achieving high performance polymer photovoltaic devices, J. Mater. Chem. 20 (2010) 2575. [5] M. Zhang, H. Wang, H. Tian, Y. Geng, C.W. Tang, Bulk heterojunction photovoltaic cells with low donor concentration, Adv. Mater. 23 (2011) 4960–4964. [6] X. Xiao, K.J. Bergemann, J.D. Zimmerman, K. Lee, S.R. Forrest, Small-molecule planar-mixed heterojunction photovoltaic cells with fullerene-based electron

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