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Performance evaluation and material parameter perspective of eco-friendly highly efficient CsSnGeI3 perovskite solar cell
Superlattices and Microstructures 135 (2019) 106273
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Performance evaluation and material parameter perspective of eco-friendly highly efficient CsSnGeI3 perovskite solar cell Raghvendra *, Rashmi Ranjan Kumar, Saurabh Kumar Pandey Sensors & Optoelectronics Research Group (SORG), Discipline of Electrical Engineering Indian Institute of Technology Patna, Bihar, 801106, India
A R T I C L E I N F O
A B S T R A C T
Index terms: CsSnGeI3 Non-toxic Perovskite solar cell HTL Defects
Lead toxicity and stability are major hurdles in commercialization of the perovskite solar cell. We present the theoretical investigation and analysis of eco-friendly and stable CsSnGeI3 based solar cell. For better understanding of material parameter, various factors affecting the cell perfor mance such as thickness, doping concentration, defect density and doping density of charge transport layer have been rigorously investigated. From simulation results, we observed that the device performance extensively depends on defect density and doping concentration of perovskite absorber layer. We have proposed Cesium Tin–Germanium Tri-iodide (CsSnGeI3) as an efficient light absorber material as compared to lead counterparts. With optimized parameters of proposed architecture, we have achieved 13.29% efficiency. We have also done the comparative analysis of different hole transport layer (HTL) layers to replace spiro-OMeTAD. The results indicate that CsSnGeI3 can be a great prospective to be an absorber layer for high-efficiency perovskite solar cells.
1. Introduction In the past few years, Perovskite solar cells (PSCs) have generated significant interest among the scientific and photovoltaic community. Owing to excellent optical, electrical properties [1] and ease of fabrication steps, the efficacy of PSCs has increased rapidly from 3.8% in 2009 [2] to the latest efficiency of 23.7% [3]. This value closely resembles the performance of traditional commercial devices based on other compound materials such as GaAs, CdTe, etc. [4]. Apart from the solar cell, perovskite materials have many applications in other optoelectronic devices such as light-emitting diode [5–7], photodetector [8], and photodiode [9]. However, the lead toxicity [10,11], which has various health and environmental hazards is the major hurdle in commercial applications of perovskite solar cell [12]. Thus, efficient lead-free PSCs need to be considered and explored. In this context, Ming-Gang Ju et al. [13] performed theoretical modelling using density function theory to address lead-free perovskites. Elements like Sn, Bi and Ge take over Pb in PSCs [14]. Among these tin-based (CsSnI3) perovskite becomes the most promising candidate [14,15]. But due to self-oxidation of Sn from Sn2þ to Sn4þ and phase instability in the CsSnI3 perovskite prevent further application as a solar cell [16]. Recently Min Chen et al. have reported that alloying CsSnI3 with Ge (II) to form a CsSn0.5Ge0.5I3 gives highly stable and air tolerant perovskite solar cell [17]. Apart from this, the hole transport layer (HTL) plays a significant role in device performance, stability and contributes to charge transportation from perovskite to back contact [18,19]. In particular, Spiro-OMeTAD is routinely used for HTL in most of the PSCs [20,
* Corresponding author. E-mail addresses: [email protected] (Raghvendra), [email protected] (S.K. Pandey). https://doi.org/10.1016/j.spmi.2019.106273 Received 21 May 2019; Received in revised form 7 September 2019; Accepted 15 September 2019 Available online 19 September 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of planar perovskite solar cell with n-i-p configuration. Table 1 Parameters used in simulation of CsSn0.5Ge0.5I3 based perovskite solar cell. Parameters
FTO
PCBM
CsSn0.5Ge0.5I3
SpiroOMeTAD
p-CuSCN
p-NiO
p-CuI
p-Cu2O
Thickness (nm) Acceptor Concentration (cm 3) Donor Concentration (cm 3) Relative Permittivity Electron Affinity (eV) Bandgap (eV) Electron Mobility (cm2/V-s) Hole Mobility (cm2/V-s) Conduction Band Density of State (cm 3) Valance Band Density of State (cm 3) Electron & hole life time (ns) References
500 – 2 � 1019 9 4 3.5 20 20 2.2 � 1018
50 – 4 � 1018 3.9 3.9 2.1 1 � 10 3 2 � 10 3 2.2 � 1019
350 1 � 1014 – 28 3.9 1.5 974 213 3.1 � 1018
350 4 � 1018 – 3 2.1 3.2 2 � 10 4 2 � 10 4 2.5 � 1018
350 4 � 1018 – 10 1.7 3.6 100 25 2.2 � 1019
350 4 � 1018 – 10.7 1.46 3.8 12 2.8 2.8 � 1019
350 4 � 1018 – 6.5 2.1 3.1 100 43.9 2.8 � 1019
350 4 � 1018 – 7.11 3.2 2.17 200 80 2 � 1017
1.8 � 1019 1 [4]
2.2 � 1019 1 [4]
3.1 � 1018 10.92 [17]
1.8 � 1019 1 [28]
1.8 � 1018 1 [28]
1 � 1019 1 [4]
1 � 1019 1 [4]
1.1 � 1019 1 [29]
Defect type Capture Cross section of electron (cm3) Capture Cross section of hole (cm3) Energetic distribution Reference for defect energy level Defect concentration (cm 3)
Bulk Trap Distribution
Interface trap Density
eNeutral 1 � 10 13 1 � 10 13 Gaussian From Mid Gap Varied
Donor (CsSnGeI3/ETL), Acceptor (CsSnGeI3/HTL) 1 � 10 8 Fit 1 � 10 8 Fit Gaussian Fit From Mid Gap Fit Varied
21]. But due to the high cost and morphological deformation behavior of spiro-OMeTAD, cell performance is sorely affected [22]. Thus there is a crucial requirement to explore new kinds of HTL materials. The use of inorganic HTM (Cu2O, CuSCN, NiO, CuI) offer high hole mobility as well as reduce the overall cost of solar cell [20]. The numerical simulation gives better insight into the device. Many researchers have done theoretical analysis for better under standing the influence of physical change on performance and the feasibility of new design concepts without actual fabrication [23–26]. In this perspective, theoretical analysis was performed using FTO/PCBM/CsSnGeI3/spiro-OMeTAD/Au device structure as shown in Fig. 1. The results were compared with the experimental parameters, which were initially proposed by Min Chen et al. [17] that validates our model. To the best of our knowledge, no analogous study for CsSnGeI3 based perovskites has been reported in the literature. This study aims to suggest possible optimization routes for efficiency improvements of the solar cell by analyzing various device parameters. Furthermore, we have compared different HTL layers to assess promising replacement of Spiro-OMeTAD for CsSnGeI3 PSCs. Our results provide a promising alternative in lead-free highly efficient solar absorber material for photovoltaic application. 2. Device modelling and simulation In this work, the proposed perovskite structure is simulated using SENTAURUS-TCAD software. All electrical simulations are performed using drift-diffusion transport model, in which electrostatic Poisson equation and carrier continuity equation for both electron and hole are solved iteratively. The transfer matrix method (TMM) was used to calculate the optical intensity and optical 2
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Fig. 2. Energy band diagram of n-i-p perovskite solar cell.
generation rate in planar layered media. For optical simulation, complex refractive index model and quantum yield model were used. In the quantum yield model, if photon energy is greater than or equal to band gap, then quantum yield is set to 1 otherwise zero. The absorbed photon density is computed by TMM solver from absorption coefficient, which is proportional to extinction coefficient. The TMM solver compute optical intensity by taking into account the interface effects due to standing waves. The J-V characteristics of illuminated solar cell were simulated by ramping anode voltage from 0 V to 1 V. Here for each anode voltage, the optical generation rate inside the device is calculated and therefore, the integrated optical generation rate over the device is computed as a sum of optical generation rate due to all the individual photon wavelength in the AM1.5G solar spectrum. All simulations were carried out at room temperature (300 K). Valance band and conduction band density of state are calculated from effective mass of electron and hole using following formula � �3 2πm*e kB T 2 NC ¼ 2 h2
(1)
� �3 2πm*h kB T 2 NV ¼ 2 h2
(2)
Here, NC ; NV ; m*e ; m*h ; kB ; h; T are conduction band effective density of state, valence band effective density of state, effective mass of electron; and effective mass of hole, Boltzmann constant, plank constant and temperature respectively [27]. The design of the n-i-p structure is shown in Fig. 1, which comprises p-type spiro-OMeTAD as the HTL, CsSnGeI3 perovskite as an absorber layer and n-type PCBM as the electron transport layer (ETL). Front and back contact were taken from fluorine-doped tin oxide (FTO) and gold respectively. Most of the material parameters employed for this study was from recent published theoretical and experimental research works [17,28,29] as shown in Table 1. To examine interface properties, interface traps were introduced at the CsSnGeI3/PCBM interface and the spiro-OMeTAD/CsSnGeI3 interface. The defect energy level is at the middle of the band gap with Gaussian distri bution. The relationship among diffusion length, carrier mobility and life-time is given by equation sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kT (3) L ¼ μe;h τ q where L is diffusion length, μe;h is electron and hole mobility, kT =q is thermal voltage and τ is carrier lifetime [27]. The relationship between lifetime and trap densities are given by equation
τ¼
1 Nt σ vth
(4)
Where Nt is trap density, σ is capture cross section and vth is thermal velocity (107 cm/s) [27]. According to equations (3) and (4), diffusion length decreases with increase in trap density thus reducing the charge collection at contact.
3
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Fig. 3. Electric field distribution inside solar cell at FTO/PCBM (0.6 μm), PCBM/perovskite (0.62 μm) and perovskite/spiro-OMeTAD (0.97 μm).
Fig. 4. J-V characteristics of perovskite solar cell. The open symbols are experimental data [17] and solid line represent the simulation plot.
3. Results and discussion 3.1. Device parameters and energy band diagram Energy band diagram of simulated device is shown in Fig. 2. It clearly shows that large energy barrier at the CsSnGeI3/PCBM and spiro-OMeTAD/CsSnGeI3 interface will prevent photo-generated minority carrier transfer from perovskite to the charge selective region. From the electric field distribution shown in Fig. 3, it is observed that the perovskite active region was completely depleted and an electric field of about 3.043 V/μm was established because of the high carrier concentration in spiro-OMeTAD (p-type) and PCBM (n-type). The current voltage relationship (Fig. 4) and quantum efficiency (Fig. 5) of simulated and experimental results are compared with each other. Which show nearly identical behavior with short-circuit current density (JSC) of 18.45 mA/cm2, an open-circuit voltage (VOC) of 0.61 V, a fill factor (FF) of 69.5% and power-conversion efficiency (PCE) of 7.816%, which is in accordance with experimental results [17]. With developed device model we explain the effect of defect density, interface property, absorber thickness, doping on the performance of PSCs. 3.2. Influence of the absorber defect density Optical generation, charge transportation and recombination occurs inside the absorber layer so, quality of absorber layer significantly affects the device performance. This defect creates local site for generation and recombination of photo generated charge 4
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Fig. 5. Quantum efficiency of perovskite solar cell. The open symbols are experimental data [17] and solid line represent the simulation plot.
Fig. 6. Simulated J-V characteristics of the PSCs for FTO/PCBM/CsSnGeI3/spiro-OMeTAD/Au architecture as a function of defect densities in perovskite material.
carriers [19]. Influence of defect density can be explained by Shockley-Read-Hall (SRH) recombination model. RSRH ¼
p:n n2i τp ðn þ ni Þ þ τn ðp þ ni Þ
(5)
Where, RSRH is SRH recombination rate, p and n are hole and electron concentration, τn and τp are electron and hole life time of charge carrier and ni is intrinsic charge carrier concentration. In order to analyze effect of defect density, it was varied from 1014 cm 3 to 1018 cm 3. Current-density voltage curve at various defect level is shown in Fig. 6. It is noticed that for defect density greater than 1016 cm 3 performance of solar cell is more afflicted while defect density below 1015 cm 3, it remains almost unaffected. At optimized defect density of 1015 cm 3, Voc 0.72 V, Jsc 18.48 mA/cm2, FF 75.74% and PCE 10.18% parameters obtained. 3.3. Influence of the interface defect density The performances of planar PSCs are very sensitive to the interfaces properties, in particular the ETL/perovskite interface in the n-ip structure. This incident can be interpreted as: Because of large absorption coefficient of perovskite, charge carrier concentration at 5
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Fig. 7. Simulated J-V characteristics of solar cell as a function of Perovskite/PCBM interface trap density.
Fig. 8. Jsc, Voc, FF and PCE of perovskite solar cell simulated with absorber layer thickness.
PCBM/perovskite is higher compared to perovskite/spiro-OMeTAD interface [30]. Hence recombination increases with increase in carrier concentration. To investigate PCBM/perovskite interface, trap density is varied from 106 to 109 cm 2. J-V characteristics for various trap density is shown in Fig. 7. We found that for trap density below 107 cm 2 device performance remain almost constant. 6
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Fig. 9. Jsc, Voc, FF and PCE of perovskite solar cell simulated with doping concentration in absorber layer.
With appropriate interface engineering and surface passivation, we can reduce interface trap density and increase overall device performance. 3.4. Influence of the absorber thickness Absorber layer thickness plays important role in evaluating the performance of the device. The variation of solar cell parameters with absorber layer thickness (from 100 nm to 600 nm) is shown in Fig. 8. As the thickness of absorber layer increases, light absorption also increases [15]. This results in an increment of short circuit current up to 300 nm. For thick layer, charge carrier collection at respective electrode decreases due to longer transfer route of the photo-generated carrier enhancement the recombination that mainly effect open circuit voltage [4]. There is an increment in series resistance with thickness of absorber layer, thus the fill factor drops with absorber layer thickness. Power conversion efficiency depends on light absorption and transport of carrier. For both thin and thick layers, PCE decreases because in former absorption is low and for later recombination is high. Thus PCE increases steeply with layer thickness up to 200 nm, which goes up to thickness of 300 nm and then decrease afterwards. From Fig. 8 we deduced that absorber thickness of 300 nm is sufficient for optimum device performance. 3.5. Influence of the absorber doping concentration Doping of absorber layer is varied from 1014–1017 cm 3 and its effect on the performance of perovskite solar cell is reported. Fig. 9 shows the performance parameter of perovskite solar cell with acceptor concentration. It is observed that carriers are collected more efficiently for acceptor concentration of 4 � 1016 cm 3. As doping concentration increase the built-in electric field increases, resulting in separation of charge carriers and improve solar cell performance [31]. On other hand, for NA above 4 � 1016 cm 3 scattering in creases which results in decrement of short circuit current. Hence further increase in doping is not favorable. Thus optimum value of doping concentration is crucial parameter to enhance the performance of solar cell. The optimum performance i.e. Jsc (19.32 mA/cm2), Voc (0.654 V), FF (71.36%) and PCE (9.017%) is obtained at acceptor density of 4 � 1016 cm 3. 7
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Fig. 10. Simulated PCE of solar cell as a function of doping concentration in ETL (PCBM) layer.
Fig. 11. Simulated PCE of solar cell as a function of doping concentration in HTL (Spiro-OMeTAD) layer. Table 2 Solar cell PCE for different HTL materials. Different HTL materials
Valance band offset(VBO) VBO ¼ (Valance band maximum)absorber – (Valance band maximum)HTL
Hole mobility (cm2/Vs)
Spiro-OMeTAD Cu2O CuSCN NiO CuI
0.1 eV 0.03 eV 0.1 eV 0.14 eV 0.2 eV
2 � 10 200 100 12 100
4
Efficiency η (%) 7.776 8.052 7.96 7.818 7.616
3.6. Influence of doping in the HTL and ETL layer In our simulation, the donor and acceptor concentration in ETL and HTL are varied from 1016 to 1019 cm 3. The performances of device with varied concentration in PCBM and spiro-OMeTAD films are depicted in Figs. 10 and 11. The high doping concentration will lead to increased conductivity and faster charge collection due to enhanced electric field that result in improved cell performance.
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Fig. 12. (a) Energy band alignment (b) PCE of perovskite solar cell for different HTL materials.
Fig. 13. Simulated J-V characteristics for optimized perovskite solar cell. Table 3 Comparative analysis of cell parameters for lead-free perovskite solar cell. Absorber
VOC (V)
JSC (mA/cm2)
FF (%)
Efficiency η (%)
Ref.
CsSn0.5Ge0.5I3 CsSn0.5Ge0.5I3 FA0.75MA0.25Sn0.95Ge0.05I3 FA0.75MA0.25SnI3 FASnI3
0.797 0.63 0.42 0.61 0.58
20.43 18.61 19.5 21.2 17.5
81.63 60.6 55 62.7 66.3
13.29 7.11 4.48 8.12 6.75
This Work [17] [32] [33] [34]
3.7. Effect of different HTL layers We have studied the effect of different types of HTLs to obtain better performance a perovskite solar cell. The material parameters of CuI, CuSCN, Cu2O spiro-OMeTAD and NiO are taken from reported literatures [28,29] which are summarized in Table 1. Table 2 show clearly that the device including Cu2O as hole transport material has the highest performance among all HTLs. HTL with small valence band offset should be considered suitable as an alternate HTL due to best band alignment illustrated in Fig. 12. Secondly, high hole mobility of Cu2O result in fast hole collection at contact. An optimized device (FTO/PCBM/CsSnGeI3/spiro-OMeTAD/Au) with optimum trap densities gives power conversion efficiency of 13.29%, with short circuit current density of 20.43 mA/cm2 and open circuit voltage of 0.797 V has been shown in Fig. 13. Table 3 shows the solar cell parameters showing superior agreement with experimental work for different lead-free perovskites. 9
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We demonstrated cesium tin-germanium tri-iodide (CsSnGeI3) based n-i-p solar cell using device simulation software. The influ ence of doping, thickness, trap densities, HTL and ETL doping concentration on solar cell have been discussed. The result suggest absorber thickness 300 nm and bulk trap density of less than 1015 cm 3 is recommended for better solar cell performance. Additionally reducing interface defect densities below 107 cm 2 can effectively improve cell performance. Mitigation of interface trap by passiv ation of traps and by increasing crystallinity of absorber layer is suggested to increase the device PCE up to 13.29%. Cu2O constitutes a promising hole transport material that offers the possibility to enhance further the efficiency of perovskite based solar cells. This work will provide future prospects for lead-free stable perovskite materials for PSC designers. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2019.106273.
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[28] F. Azri, A. Meftah, N. Sengouga, A. Meftah, Electron and hole transport layers optimization by numerical simulation of a perovskite solar cell, Sol. Energy 181 (Mar. 2019) 372–378, https://doi.org/10.1016/j.solener.2019.02.017. [29] M.I. Hossain, F.H. Alharbi, N. Tabet, Copper oxide as inorganic hole transport material for lead halide perovskite based solar cells, Sol. Energy 120 (Oct. 2015) 370–380, https://doi.org/10.1016/j.solener.2015.07.040. [30] Z. Yang, J. Dou, M. Wang, Interface engineering in n-i-p metal halide perovskite solar cells, Sol. RRL 2 (12) (Sep 2018), 1800177, https://doi.org/10.1002/ solr.201800177. [31] H.J. Du, W.C. Wang, J.Z. Zhu, Device simulation of lead-free CH3NH3SnI3 perovskite solar cells with high efficiency, Chin. Phys. B 25 (10) (Aug. 2016), 108802, https://doi.org/10.1088/1674-1056/25/10/108802. [32] N. Ito, M.A. Kamarudin, D. Hirotani, Y. Zhang, Q. Shen, Y. Ogomi, S. Likubo, T. Minemoto, K. Yoshino, S. Hayase, Mixed Sn-Ge perovskite for enhanced perovskite solar cell performance in air, J. Phys. Chem. Lett. 9 (7) (Mar. 2018) 1682–1688, https://doi.org/10.1021/acs.jpclett.8b00275. [33] Z. Zhao, F. Gu, Y. Li, W. Sun, S. Ye, H. Rao, Z. Liu, Z. Bian, C. Huang, Mixed-Organic-cation tin iodide for lead-free perovskite solar cells with an efficiency of 8.12%, Adv. Sci. 4 (11) (Jul. 2017), 1700204 https://doi.org/10.1002/advs.201700204. [34] F. Gu, S. Ye, Z. Zhao, H. Rao, Z. Liu, Z. Bian, C. Huang, Improving performance of lead-free formamidinium tin triiodide perovskite solar cells by tin source purification, Sol. RRL 2 (10) (Aug. 2018), 1800136, https://doi.org/10.1002/solr.201800136.
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Performance evaluation and material parameter perspective of eco-friendly highly efficient CsSnGeI3 perovskite solar cell Raghvendra *, Rashmi Ranjan Kumar, Saurabh Kumar Pandey Sensors & Optoelectronics Research Group (SORG), Discipline of Electrical Engineering Indian Institute of Technology Patna, Bihar, 801106, India
A R T I C L E I N F O
A B S T R A C T
Index terms: CsSnGeI3 Non-toxic Perovskite solar cell HTL Defects
Lead toxicity and stability are major hurdles in commercialization of the perovskite solar cell. We present the theoretical investigation and analysis of eco-friendly and stable CsSnGeI3 based solar cell. For better understanding of material parameter, various factors affecting the cell perfor mance such as thickness, doping concentration, defect density and doping density of charge transport layer have been rigorously investigated. From simulation results, we observed that the device performance extensively depends on defect density and doping concentration of perovskite absorber layer. We have proposed Cesium Tin–Germanium Tri-iodide (CsSnGeI3) as an efficient light absorber material as compared to lead counterparts. With optimized parameters of proposed architecture, we have achieved 13.29% efficiency. We have also done the comparative analysis of different hole transport layer (HTL) layers to replace spiro-OMeTAD. The results indicate that CsSnGeI3 can be a great prospective to be an absorber layer for high-efficiency perovskite solar cells.
1. Introduction In the past few years, Perovskite solar cells (PSCs) have generated significant interest among the scientific and photovoltaic community. Owing to excellent optical, electrical properties [1] and ease of fabrication steps, the efficacy of PSCs has increased rapidly from 3.8% in 2009 [2] to the latest efficiency of 23.7% [3]. This value closely resembles the performance of traditional commercial devices based on other compound materials such as GaAs, CdTe, etc. [4]. Apart from the solar cell, perovskite materials have many applications in other optoelectronic devices such as light-emitting diode [5–7], photodetector [8], and photodiode [9]. However, the lead toxicity [10,11], which has various health and environmental hazards is the major hurdle in commercial applications of perovskite solar cell [12]. Thus, efficient lead-free PSCs need to be considered and explored. In this context, Ming-Gang Ju et al. [13] performed theoretical modelling using density function theory to address lead-free perovskites. Elements like Sn, Bi and Ge take over Pb in PSCs [14]. Among these tin-based (CsSnI3) perovskite becomes the most promising candidate [14,15]. But due to self-oxidation of Sn from Sn2þ to Sn4þ and phase instability in the CsSnI3 perovskite prevent further application as a solar cell [16]. Recently Min Chen et al. have reported that alloying CsSnI3 with Ge (II) to form a CsSn0.5Ge0.5I3 gives highly stable and air tolerant perovskite solar cell [17]. Apart from this, the hole transport layer (HTL) plays a significant role in device performance, stability and contributes to charge transportation from perovskite to back contact [18,19]. In particular, Spiro-OMeTAD is routinely used for HTL in most of the PSCs [20,
* Corresponding author. E-mail addresses: [email protected] (Raghvendra), [email protected] (S.K. Pandey). https://doi.org/10.1016/j.spmi.2019.106273 Received 21 May 2019; Received in revised form 7 September 2019; Accepted 15 September 2019 Available online 19 September 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of planar perovskite solar cell with n-i-p configuration. Table 1 Parameters used in simulation of CsSn0.5Ge0.5I3 based perovskite solar cell. Parameters
FTO
PCBM
CsSn0.5Ge0.5I3
SpiroOMeTAD
p-CuSCN
p-NiO
p-CuI
p-Cu2O
Thickness (nm) Acceptor Concentration (cm 3) Donor Concentration (cm 3) Relative Permittivity Electron Affinity (eV) Bandgap (eV) Electron Mobility (cm2/V-s) Hole Mobility (cm2/V-s) Conduction Band Density of State (cm 3) Valance Band Density of State (cm 3) Electron & hole life time (ns) References
500 – 2 � 1019 9 4 3.5 20 20 2.2 � 1018
50 – 4 � 1018 3.9 3.9 2.1 1 � 10 3 2 � 10 3 2.2 � 1019
350 1 � 1014 – 28 3.9 1.5 974 213 3.1 � 1018
350 4 � 1018 – 3 2.1 3.2 2 � 10 4 2 � 10 4 2.5 � 1018
350 4 � 1018 – 10 1.7 3.6 100 25 2.2 � 1019
350 4 � 1018 – 10.7 1.46 3.8 12 2.8 2.8 � 1019
350 4 � 1018 – 6.5 2.1 3.1 100 43.9 2.8 � 1019
350 4 � 1018 – 7.11 3.2 2.17 200 80 2 � 1017
1.8 � 1019 1 [4]
2.2 � 1019 1 [4]
3.1 � 1018 10.92 [17]
1.8 � 1019 1 [28]
1.8 � 1018 1 [28]
1 � 1019 1 [4]
1 � 1019 1 [4]
1.1 � 1019 1 [29]
Defect type Capture Cross section of electron (cm3) Capture Cross section of hole (cm3) Energetic distribution Reference for defect energy level Defect concentration (cm 3)
Bulk Trap Distribution
Interface trap Density
eNeutral 1 � 10 13 1 � 10 13 Gaussian From Mid Gap Varied
Donor (CsSnGeI3/ETL), Acceptor (CsSnGeI3/HTL) 1 � 10 8 Fit 1 � 10 8 Fit Gaussian Fit From Mid Gap Fit Varied
21]. But due to the high cost and morphological deformation behavior of spiro-OMeTAD, cell performance is sorely affected [22]. Thus there is a crucial requirement to explore new kinds of HTL materials. The use of inorganic HTM (Cu2O, CuSCN, NiO, CuI) offer high hole mobility as well as reduce the overall cost of solar cell [20]. The numerical simulation gives better insight into the device. Many researchers have done theoretical analysis for better under standing the influence of physical change on performance and the feasibility of new design concepts without actual fabrication [23–26]. In this perspective, theoretical analysis was performed using FTO/PCBM/CsSnGeI3/spiro-OMeTAD/Au device structure as shown in Fig. 1. The results were compared with the experimental parameters, which were initially proposed by Min Chen et al. [17] that validates our model. To the best of our knowledge, no analogous study for CsSnGeI3 based perovskites has been reported in the literature. This study aims to suggest possible optimization routes for efficiency improvements of the solar cell by analyzing various device parameters. Furthermore, we have compared different HTL layers to assess promising replacement of Spiro-OMeTAD for CsSnGeI3 PSCs. Our results provide a promising alternative in lead-free highly efficient solar absorber material for photovoltaic application. 2. Device modelling and simulation In this work, the proposed perovskite structure is simulated using SENTAURUS-TCAD software. All electrical simulations are performed using drift-diffusion transport model, in which electrostatic Poisson equation and carrier continuity equation for both electron and hole are solved iteratively. The transfer matrix method (TMM) was used to calculate the optical intensity and optical 2
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Fig. 2. Energy band diagram of n-i-p perovskite solar cell.
generation rate in planar layered media. For optical simulation, complex refractive index model and quantum yield model were used. In the quantum yield model, if photon energy is greater than or equal to band gap, then quantum yield is set to 1 otherwise zero. The absorbed photon density is computed by TMM solver from absorption coefficient, which is proportional to extinction coefficient. The TMM solver compute optical intensity by taking into account the interface effects due to standing waves. The J-V characteristics of illuminated solar cell were simulated by ramping anode voltage from 0 V to 1 V. Here for each anode voltage, the optical generation rate inside the device is calculated and therefore, the integrated optical generation rate over the device is computed as a sum of optical generation rate due to all the individual photon wavelength in the AM1.5G solar spectrum. All simulations were carried out at room temperature (300 K). Valance band and conduction band density of state are calculated from effective mass of electron and hole using following formula � �3 2πm*e kB T 2 NC ¼ 2 h2
(1)
� �3 2πm*h kB T 2 NV ¼ 2 h2
(2)
Here, NC ; NV ; m*e ; m*h ; kB ; h; T are conduction band effective density of state, valence band effective density of state, effective mass of electron; and effective mass of hole, Boltzmann constant, plank constant and temperature respectively [27]. The design of the n-i-p structure is shown in Fig. 1, which comprises p-type spiro-OMeTAD as the HTL, CsSnGeI3 perovskite as an absorber layer and n-type PCBM as the electron transport layer (ETL). Front and back contact were taken from fluorine-doped tin oxide (FTO) and gold respectively. Most of the material parameters employed for this study was from recent published theoretical and experimental research works [17,28,29] as shown in Table 1. To examine interface properties, interface traps were introduced at the CsSnGeI3/PCBM interface and the spiro-OMeTAD/CsSnGeI3 interface. The defect energy level is at the middle of the band gap with Gaussian distri bution. The relationship among diffusion length, carrier mobility and life-time is given by equation sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kT (3) L ¼ μe;h τ q where L is diffusion length, μe;h is electron and hole mobility, kT =q is thermal voltage and τ is carrier lifetime [27]. The relationship between lifetime and trap densities are given by equation
τ¼
1 Nt σ vth
(4)
Where Nt is trap density, σ is capture cross section and vth is thermal velocity (107 cm/s) [27]. According to equations (3) and (4), diffusion length decreases with increase in trap density thus reducing the charge collection at contact.
3
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Fig. 3. Electric field distribution inside solar cell at FTO/PCBM (0.6 μm), PCBM/perovskite (0.62 μm) and perovskite/spiro-OMeTAD (0.97 μm).
Fig. 4. J-V characteristics of perovskite solar cell. The open symbols are experimental data [17] and solid line represent the simulation plot.
3. Results and discussion 3.1. Device parameters and energy band diagram Energy band diagram of simulated device is shown in Fig. 2. It clearly shows that large energy barrier at the CsSnGeI3/PCBM and spiro-OMeTAD/CsSnGeI3 interface will prevent photo-generated minority carrier transfer from perovskite to the charge selective region. From the electric field distribution shown in Fig. 3, it is observed that the perovskite active region was completely depleted and an electric field of about 3.043 V/μm was established because of the high carrier concentration in spiro-OMeTAD (p-type) and PCBM (n-type). The current voltage relationship (Fig. 4) and quantum efficiency (Fig. 5) of simulated and experimental results are compared with each other. Which show nearly identical behavior with short-circuit current density (JSC) of 18.45 mA/cm2, an open-circuit voltage (VOC) of 0.61 V, a fill factor (FF) of 69.5% and power-conversion efficiency (PCE) of 7.816%, which is in accordance with experimental results [17]. With developed device model we explain the effect of defect density, interface property, absorber thickness, doping on the performance of PSCs. 3.2. Influence of the absorber defect density Optical generation, charge transportation and recombination occurs inside the absorber layer so, quality of absorber layer significantly affects the device performance. This defect creates local site for generation and recombination of photo generated charge 4
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Fig. 5. Quantum efficiency of perovskite solar cell. The open symbols are experimental data [17] and solid line represent the simulation plot.
Fig. 6. Simulated J-V characteristics of the PSCs for FTO/PCBM/CsSnGeI3/spiro-OMeTAD/Au architecture as a function of defect densities in perovskite material.
carriers [19]. Influence of defect density can be explained by Shockley-Read-Hall (SRH) recombination model. RSRH ¼
p:n n2i τp ðn þ ni Þ þ τn ðp þ ni Þ
(5)
Where, RSRH is SRH recombination rate, p and n are hole and electron concentration, τn and τp are electron and hole life time of charge carrier and ni is intrinsic charge carrier concentration. In order to analyze effect of defect density, it was varied from 1014 cm 3 to 1018 cm 3. Current-density voltage curve at various defect level is shown in Fig. 6. It is noticed that for defect density greater than 1016 cm 3 performance of solar cell is more afflicted while defect density below 1015 cm 3, it remains almost unaffected. At optimized defect density of 1015 cm 3, Voc 0.72 V, Jsc 18.48 mA/cm2, FF 75.74% and PCE 10.18% parameters obtained. 3.3. Influence of the interface defect density The performances of planar PSCs are very sensitive to the interfaces properties, in particular the ETL/perovskite interface in the n-ip structure. This incident can be interpreted as: Because of large absorption coefficient of perovskite, charge carrier concentration at 5
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Fig. 7. Simulated J-V characteristics of solar cell as a function of Perovskite/PCBM interface trap density.
Fig. 8. Jsc, Voc, FF and PCE of perovskite solar cell simulated with absorber layer thickness.
PCBM/perovskite is higher compared to perovskite/spiro-OMeTAD interface [30]. Hence recombination increases with increase in carrier concentration. To investigate PCBM/perovskite interface, trap density is varied from 106 to 109 cm 2. J-V characteristics for various trap density is shown in Fig. 7. We found that for trap density below 107 cm 2 device performance remain almost constant. 6
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Fig. 9. Jsc, Voc, FF and PCE of perovskite solar cell simulated with doping concentration in absorber layer.
With appropriate interface engineering and surface passivation, we can reduce interface trap density and increase overall device performance. 3.4. Influence of the absorber thickness Absorber layer thickness plays important role in evaluating the performance of the device. The variation of solar cell parameters with absorber layer thickness (from 100 nm to 600 nm) is shown in Fig. 8. As the thickness of absorber layer increases, light absorption also increases [15]. This results in an increment of short circuit current up to 300 nm. For thick layer, charge carrier collection at respective electrode decreases due to longer transfer route of the photo-generated carrier enhancement the recombination that mainly effect open circuit voltage [4]. There is an increment in series resistance with thickness of absorber layer, thus the fill factor drops with absorber layer thickness. Power conversion efficiency depends on light absorption and transport of carrier. For both thin and thick layers, PCE decreases because in former absorption is low and for later recombination is high. Thus PCE increases steeply with layer thickness up to 200 nm, which goes up to thickness of 300 nm and then decrease afterwards. From Fig. 8 we deduced that absorber thickness of 300 nm is sufficient for optimum device performance. 3.5. Influence of the absorber doping concentration Doping of absorber layer is varied from 1014–1017 cm 3 and its effect on the performance of perovskite solar cell is reported. Fig. 9 shows the performance parameter of perovskite solar cell with acceptor concentration. It is observed that carriers are collected more efficiently for acceptor concentration of 4 � 1016 cm 3. As doping concentration increase the built-in electric field increases, resulting in separation of charge carriers and improve solar cell performance [31]. On other hand, for NA above 4 � 1016 cm 3 scattering in creases which results in decrement of short circuit current. Hence further increase in doping is not favorable. Thus optimum value of doping concentration is crucial parameter to enhance the performance of solar cell. The optimum performance i.e. Jsc (19.32 mA/cm2), Voc (0.654 V), FF (71.36%) and PCE (9.017%) is obtained at acceptor density of 4 � 1016 cm 3. 7
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Fig. 10. Simulated PCE of solar cell as a function of doping concentration in ETL (PCBM) layer.
Fig. 11. Simulated PCE of solar cell as a function of doping concentration in HTL (Spiro-OMeTAD) layer. Table 2 Solar cell PCE for different HTL materials. Different HTL materials
Valance band offset(VBO) VBO ¼ (Valance band maximum)absorber – (Valance band maximum)HTL
Hole mobility (cm2/Vs)
Spiro-OMeTAD Cu2O CuSCN NiO CuI
0.1 eV 0.03 eV 0.1 eV 0.14 eV 0.2 eV
2 � 10 200 100 12 100
4
Efficiency η (%) 7.776 8.052 7.96 7.818 7.616
3.6. Influence of doping in the HTL and ETL layer In our simulation, the donor and acceptor concentration in ETL and HTL are varied from 1016 to 1019 cm 3. The performances of device with varied concentration in PCBM and spiro-OMeTAD films are depicted in Figs. 10 and 11. The high doping concentration will lead to increased conductivity and faster charge collection due to enhanced electric field that result in improved cell performance.
8
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Fig. 12. (a) Energy band alignment (b) PCE of perovskite solar cell for different HTL materials.
Fig. 13. Simulated J-V characteristics for optimized perovskite solar cell. Table 3 Comparative analysis of cell parameters for lead-free perovskite solar cell. Absorber
VOC (V)
JSC (mA/cm2)
FF (%)
Efficiency η (%)
Ref.
CsSn0.5Ge0.5I3 CsSn0.5Ge0.5I3 FA0.75MA0.25Sn0.95Ge0.05I3 FA0.75MA0.25SnI3 FASnI3
0.797 0.63 0.42 0.61 0.58
20.43 18.61 19.5 21.2 17.5
81.63 60.6 55 62.7 66.3
13.29 7.11 4.48 8.12 6.75
This Work [17] [32] [33] [34]
3.7. Effect of different HTL layers We have studied the effect of different types of HTLs to obtain better performance a perovskite solar cell. The material parameters of CuI, CuSCN, Cu2O spiro-OMeTAD and NiO are taken from reported literatures [28,29] which are summarized in Table 1. Table 2 show clearly that the device including Cu2O as hole transport material has the highest performance among all HTLs. HTL with small valence band offset should be considered suitable as an alternate HTL due to best band alignment illustrated in Fig. 12. Secondly, high hole mobility of Cu2O result in fast hole collection at contact. An optimized device (FTO/PCBM/CsSnGeI3/spiro-OMeTAD/Au) with optimum trap densities gives power conversion efficiency of 13.29%, with short circuit current density of 20.43 mA/cm2 and open circuit voltage of 0.797 V has been shown in Fig. 13. Table 3 shows the solar cell parameters showing superior agreement with experimental work for different lead-free perovskites. 9
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We demonstrated cesium tin-germanium tri-iodide (CsSnGeI3) based n-i-p solar cell using device simulation software. The influ ence of doping, thickness, trap densities, HTL and ETL doping concentration on solar cell have been discussed. The result suggest absorber thickness 300 nm and bulk trap density of less than 1015 cm 3 is recommended for better solar cell performance. Additionally reducing interface defect densities below 107 cm 2 can effectively improve cell performance. Mitigation of interface trap by passiv ation of traps and by increasing crystallinity of absorber layer is suggested to increase the device PCE up to 13.29%. Cu2O constitutes a promising hole transport material that offers the possibility to enhance further the efficiency of perovskite based solar cells. This work will provide future prospects for lead-free stable perovskite materials for PSC designers. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2019.106273.
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