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V2O5 -PEDOT: PSS bilayer as hole transport layer for highly efficient and stable perovskite solar cells
Organic Electronics 53 (2018) 66–73
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V2O5 -PEDOT: PSS bilayer as hole transport layer for highly efficient and stable perovskite solar cells
T
Dian Wang, Naveen Kumar Elumalai∗, Md Arafat Mahmud, Matthew Wright, Mushfika Baishakhi Upama, Kah Howe Chan, Cheng Xu, Faiazul Haque, Gavin Conibeer, Ashraf Uddin∗∗ School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, 2052, Sydney, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Vanadium oxide Charge transport Stability Interface Impedance spectroscopy
Hybrid halide perovskite solar cells (PSCs) have emerged as a strong candidate for low cost photovoltaics, owing to ease of processing and material abundance. The stability and performance of these devices are contingent on the quality of the interfaces. In this work, we report the novel interface engineered hole transport layer (HTL), incorporating Vanadium Pentoxide (V2O5) and Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) bilayer (PVO) for efficient charge transport in the devices. The devices incorporating the PVO bilayer HTL exhibits 20% higher power conversion efficiency (PCE) than conventional PEDOT only devices. The PSCs incorporating the PVO bilayer HTL demonstrated superior electronic properties as evaluated using impedance spectroscopy measurements. The recombination resistance (RRec) of the bilayer based devices are 57% higher than the reference cells. In addition to high charge selectivity, the bilayer PSCs exhibit low interfacial capacitance originating from electrode polarization and almost zero hysteresis. Furthermore, the bilayer based devices are more stable than PEDOT only devices; retaining 95% of their initial PCE even after 18 days of testing. The mechanism behind superior charge transport in PVO bilayer HTL and its role in stability enhancement are also discussed.
1. Introduction Perovskite solar cells (PSCs) are emerging as a promising photovoltaic technology in last few years owing to their superior electronic properties, such as high absorption coefficient, direct bandgap, and long exciton diffusion length [1–3]. The power conversion efficiency (PCE) of PSCs has increased rapidly from 3.8% in 2009 to over 22% in 2016 [1]. Two major types of device architectures are employed: normal [4,5] and inverted structure [6,7]. The inverted devices are simple to fabricate as they generally employs as planar structure – which avoids the pore filling problem and high temperature processes. Unlike the normal structure where the electron transport layer (ETL) is deposited on the substrate, the inverted devices have a hole transport layer (HTL). Poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) is widely used as the HTL in perovskite solar cells due to its high optical transparency, low redox potential [8]and ease of processing. However, the acidic nature of PEDOT: PSS leads to corrosion of the ITO. Moreover, PEDOT: PSS is highly hygroscopic and absorbs moisture which could cause decomposition of the perovskite
∗
absorber layer [9]. Therefore, other HTL materials such as metal oxides [10], graphene oxide [11], and PbS [12] have been investigated in PSCs. Seok et al. used pulsed laser deposition technique to incorporate NiOx as HTL in an inverted device, which resulted in a PCE of ∼17% [10], Ye et al. used electro deposition to build a CuSCN based inverted device with 16.6% [13]. However, such method is not compatible for large scale fabrication. Vanadium pentoxide (V2O5) is a promising HTL candidate for perovskite solar cells due to its high transparency to visible light and high carrier mobility with energy levels well suited for hole transport in perovskite solar cells [14]. The energy levels of V2O5 and PEDOT are well studied and reported in several literatures. The valence band (VB) of the V2O5 is about 4.7 eV [15,16], closer to the HOMO level of PEDOT:PSS which is about 5.0–5.2eV [15–17]. Together, they form an electrically barrier free cascading contact for hole transport across the interface. Recently, Peng et al. reported a solution processed n-type V2Ox in PSCs with an efficiency of 17.5%, however, the device structure still uses PEDOT on top of ITO which could lead to significant device degradation in the long-term [18]. A solution processed p-type VOx
Corresponding author. Corresponding author. E-mail addresses: [email protected] (N.K. Elumalai), [email protected] (A. Uddin).
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https://doi.org/10.1016/j.orgel.2017.10.034 Received 31 May 2017; Received in revised form 26 September 2017; Accepted 25 October 2017 Available online 31 October 2017 1566-1199/ © 2017 Elsevier B.V. All rights reserved.
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clearly indicates the presence of spatial features that reflect the underlying surface morphology of the ITO substrate. Fig. 1(A) shows SEM images of the V2O5 layers on ITO substrates deposited from saturated V2O5 solution. The surface morphology has a large number of spatial features that represent surface irregularities (per unit area) similar to the ITO surface (Fig. S1), which is a relatively higher number compared to those observed in pristine PEDOT: PSS layer and PVO bilayer as shown in Fig. 1(D) and (F) respectively. The PVO bilayer is densely packed and it could serve as a good interfacial layer adjacent to the perovskite film which is to be deposited subsequently. It is well known that the perovskite film morphology is influenced considerably by the surface properties of the adjacent interfacial layer [22]. The details are discussed in the sections below. The AFM images of the V2O5, pristine PEDOT and PVO layer deposited on the ITO substrate is shown in Fig. 2, while the AFM of ITO is given in Fig. S2. The RMS roughness values are 3.41(V2O5), 1.19(PEDOT), 1.15 (PVO) and 1.75nm(ITO) respectively. After applying the V2O5, the substrate becomes even more rough which identifies the existence of the thin layer of V2O5. The surface irregularities of the V2O5 only device are higher than the other two HTL layers. PEDOT being polymeric material results in a smooth surface and it also reduces the surface roughness from 3.41 in V2O5 HTL to 1.15 nm in PVO HTL. The top view of the AFM images 2(A) and 2(E) corresponding to the V2O5 and PVO devices clearly indicates a significant improvement in surface morphology, where V2O5 HTL closely resembles the ITO surface and PVO HTL is relatively much smoother. The smooth surface is also expected to positively influence the morphology of the adjacent perovskite layer [22]. After the formation of the HTL layers on ITO substrate, the optical properties are investigated using absorbance, transmittance and reflectance measurement. The absorbance (A) of the HTL layer can be expressed in terms of the Beer-Lambert's law.
layer has also been reported by Sun et al. with PCE∼14% in inverted PSCs but the efficiency is still lower than the n-type vanadium oxides [19]. In this study, we demonstrate solution processed inverted perovskite solar cells with the device structure ITO/V2O5/PEDOT/Perovskite/ PC71BM/Ag with a high PCE of ∼15% using low temperature processing methods. The V2O5 incorporated devices (PVO) are compared with the PEDOT only devices in terms of performance efficiency and degradation stability. The modified devices (PVO) exhibited 20% higher PCE than PEDOT only devices. Impedance spectroscopy analysis is performed to investigate the superior photovoltaic properties of PVO devices. The recombination resistance and electrode polarization of the two devices are also measured and correlated with the device performance. The mechanism of superior charge transport across the PVO devices are ascertained. The stabilities of both devices are also studied for 18 days and the factors behind the enhancement are also discussed. 2. Experimental methods 2.1. Device fabrication Patterned ITO glass substrates were used for all devices. First, the substrates were ultrasonicated with Hellmanex III, DI water, acetone and isopropanol, each for 10 min. V2O5 (99.99%, Sigma Aldrich) was dissolved in DI water. It was sonicated for 10 min, and then stirred overnight at room temperature until an orange solution was formed. This solution was filtered with a 0.45μm PTFE filter. The solubility in water is poor; it has a solubility limit of 0.7 mg/mL in water at room temperature [20]. The perovskite precursor solution was formed by mixing PbI2 (99.999%, Lumtec) with methyl ammonium iodide (Dyesol), in a 1:1 ratio in DMF (Sigma Aldrich) with a 1M concentration. This solution with was stirred overnight. The saturated V2O5 solution was first spin coated on the ITO substrate at 2000 rpm for 30s and then subsequently annealed at 120 °C for 5 min in air to remove the water. Next, PEDOT:PSS (Al4083) was spin coated on the substrate, the glass was heated for another 15 min at 120 °C. The substrates were then transferred into a N2 purged glovebox. The perovskite precursor was spin coated at 2500 rpm for 30s. Nitrogen flow was applied to the substrate to form a flat film [21]. The substrates were then annealed at 100 °C for 10 min. PC71BM (1-Material) was then spin coated on the substrate at 3000 rpm for 30s. A 100 nm Ag electrode was evaporated (at 1e−5 mbar) on the substrate to form the full devices.
I A = log ⎛ 0 ⎞ = εlc ⎝I⎠ where, Io I, l, ε and c denote reference light intensity, sample light intensity, length of optical light path material, dielectric constant and precursor concentration respectively. The transmission spectra of the V2O5, PEDOT and PVO layers are shown in Fig. 3. V2O5 has high transparency to visible light due to its wide band gap. The PEDOT and PVO layers exhibited relatively high transmittance at wavelengths of 350–500 nm. The improvement in transmittance at these wavelengths is attributed to the formation of large PEDOT rich particles during annealing after the deposition, where the number of interfaces between the PEDOT:PSS rich particles and PSS lamellae are significantly reduced [23]. The incorporation of PEDOT both as a pristine and in a PVO layer also improves the optical field distribution of the devices as observed in the inverted device geometry [24,25]. It is also expected that the refractive index at the ITO/V2O5 and ITO/PVO interfaces to be different, and the latter is much favourable for light transmission with less reflection. Therefore, incident light is transmitted efficiently across the PVO layer compared to that of V2O5 only devices. The percolation of PEDOT into the surface irregularities of the V2O5 in the PVO devices results in a compact layer, where the light transmission characteristics is dominated by the PEDOT resulting in less light scattering at the interface. The overall transmittance of the PEDOT and PVO devices are above 80% all throughout the scanned wavelength range as shown in Fig. 3, which would serve as a beneficial aspect in terms of improving the light absorption in the device. The absorptions with perovskite layers are given in Fig. S3 for both PEDOT and PVO devices. The SEM images of the as-deposited MAPbI3 perovskite layer on top of the PEDOT and PVO layer is shown in Fig. 4 (A) and (B) respectively. The grain size of the perovskite film deposited on top of the PEDOT is larger than that of the one deposited on the PVO layer. The perovskite film on PVO is also characterized by the presence of a large number of
2.2. Characterization Current-Voltage (I–V) characterization was carried out in PVmeasurement IV test with a Keithley 2400, the light intensity was calibrated to 1 SUN intensity with an AM1.5 filter. The surface topology and device cross sectional view were captured by Carl Zeiss AURIGA Cross Beam SEM (Scanning Electron Microscopy) and the surface roughness was measured with Bruker Dimension ICON SPM AFM (Atomic Force Microscopy) machine. The impedance analysis was conducted with an Autolab PGSTAT-30 equipped with a frequency analyzer module in the frequency range from 1 MHz to 10 Hz. External quantum efficiency (EQE) measurements were performed using a QEX10 spectral response system from PV measurements Inc. Transmission and absorption was measured using a Perkin Elmer UV–Vis spectrometer (Lambda 1050). The stability of the device performance was measured 1,2,3, and 5 weeks after the fabrication, the devices are stored inside a glovebox. 3. Result and discussion The SEM micrographs of the various HTL layers deposited on top of the ITO substrate are shown in Fig. 1. Each SEM image of the HTL layer 67
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Fig. 1. Scanning Electron Microscopy (SEM) images of (A)–(B) V2O5 and (C)–(D) PEDOT:PSS (E)–(F) V2O5 - PEDOT: PSS (PVO bilayer) on top ITO/glass substrate.
contact of the ITO/PEDOT layer in the PEDOT only device is expected to be smooth resulting in a smaller leakage current. The overall PCE of the PVO device is 20% higher than the PEDOT device owing to high JSC and good fill factor. Moreover, neither the devices exhibited any hysteresis, as shown in Fig. S5. The removal of hysteresis in the PEDOT device is entirely expected [31], but the absence of hysteresis in the PVO device which incorporates a metal oxide layer (V2O5) is an interesting result, and reflects the low interfacial capacitance at the HTL/ Perovskite interface which will be discussed in a later section [32]. The stabilized efficiency output is also measured where the current density was tracked at the max power point of the J-V curve as shown in Fig. S6. The graph clearly shows that the PVO devices exhibit high current density than the reference devices. The PVO device stabilizes at 14.1%, while the reference device stabilizes at 12.7%, similar to the trends observed in previous reports [33,34]. The efficiencies measured under stabilized conditions are in agreement with those observed in I–V characteristics. The external quantum efficiency (EQE) spectra observed for the PEDOT and PVO devices are shown in Fig. 6. The PVO devices have higher spectral response in the whole spectrum. The PVO devices exhibit a strong EQE peak at 600 nm with a quantum efficiency of about 90%. The calculated JSC values from the EQE measurements are slightly lower compared to the JSC values obtained from the J-V characterization; the minor differences between the JSC values from EQE and J-V measurements are consistent with reports in previous literatures [35–37]. The EQE valley of the PVO device is red shifted to 675 nm
grain boundaries. Reports have shown that the grain boundaries could play a beneficial role in charge separation and collection across the perovskite layer [26–28]. The RMS values obtained from the AFM analysis are 5.49 nm and 5.38 nm for PEDOT and PVO devices respectively. The corresponding AFM images are shown in Fig. S4. The surface roughness is almost the same in both cases. The current –voltage characteristics observed for the PEDOT and PVO devices are shown in Fig. 5 (A), the corresponding I–V parameters are listed in Table 1, and the device structure is shown in Fig. 5 (B). The short circuit current density (JSC) for the PVO device is 22.69 mA/cm2 which is ∼20% higher than the PEDOT device with JSC of 18.86 mA/ cm2. The VOC of the PEDOT device at 895.8 mV is slightly higher than the PVO device at 884.48 mV. The small difference in the VOC ∼10 mV between the two devices is expected to arise from the incorporation of V2O5 which is in contact with ITO and the associated work function difference between them. Nevertheless, the fill factor is similar in both devices with PEDOT device (FF∼ 74.08%) as compared to PVO device (FF ∼74.70%). The similarity in the fill factor can be attributed to the series and shunt resistance interplay between the two devices. The series resistance in the PVO device (Rs - 4.1 Ωcm2) is lower than the PEDOT device (Rs - 4.7 Ωcm2), which is attributed to the incorporation of V2O5 which has better charge mobility than PEDOT [29,30]. On the other hand, the shunt resistance is higher for the PEDOT device (Rsh – 984 Ωcm2) than the PVO device (Rsh – 317 Ωcm2) which is attributed to the quality of the interface and work function difference between the ITO and the V2O5 layer in the PVO device, whereas the interfacial 68
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Fig. 2. Two and Three dimensional Atomic Force Microscopy (AFM) images of (A)–(B) V2O5 and (C)–(D) PEDOT:PSS (E)–(F) V2O5 - PEDOT: PSS (PVO bilayer) on top ITO/glass substrate.
The high frequency arc is associated with carrier transport in the HTL while the lower frequency arc is related to recombination at each interface of the perovskite layer [38]. The semicircle related to recombination is more prominently observed in our study than has been reported before in other studies [39,40]. The impedance spectra of PEDOT and PVO devices are shown in Fig. 7. The IS spectrum of PVO device shows a larger semicircle than the PEDOT devices. To fit the impedance spectra, an equivalent circuit is used as shown in Fig. 7, which has been reported previously in the literature [39–41]. Briefly,
compared to the EQE valley for the PEDOT device at 650 nm. The observed increase in the EQE peak as well as the red shift for the PVO device is expected to be due to the enhanced collection characteristics of the V2O5 incorporated HTL layer in the PVO devices. The reason behind the improved JSC and charge collection properties of the PVO devices are further investigated using impedance spectroscopy analysis. In order to understand the internal electrical characteristics of the devices, impedance spectroscopy (IS) is utilized. Generally, the Nyquist plots show the combination of semicircles of the complex plane. 69
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Fig. 3. Transmittance of the V2O5, PEDOT, V2O5/PEDOT (PVO bilayer) on ITO substrate.
RS is associated with series resistance, while RHTL and RREC are the contact resistance and recombination resistance respectively. CBULK refers to the chemical or bulk capacitance and CHTL is the geometric capacitance. In planar perovskite solar cells, it is postulated that CHTL dominates over CBULK, or in other words, the effects of charge storage in the contacts outweigh that for charges associated with photogenerated electrons and holes in the perovskite layer [42]. The modelled results from fitting with the equivalent circuit have been presented in Table 2. The series resistance RS is relatively lower in PVO device (RS - 12.6 Ω) than that of with PEDOT only (RS – 13.5 Ω). The lower RS in the PVO devices can be attributed to the high carrier mobility of V2O5 layer, which is around two or threefold higher than the PEDOT [29,30]. However, the contact resistance RHTL of PVO device is 29% higher, which can be attributed to the interfacial barrier between the ITO and V2O5, due to the difference of the work function [14]. In the case of PEDOT device, an Ohmic contact is formed. The recombination resistance RREC is 60.3Ω for the PVO device, whereas the reference device exhibits low RREC of 38.5 Ω. The difference corresponds to the diameters of the semicircles of the IS spectra as shown in Fig. 7. The RREC in the PVO device is ∼57% higher than the PEDOT device. The modified HTL layer in the PVO extracts the charge carriers more efficiently than the PEDOT layer, due to the high mobility of the V2O5 layer incorporated between ITO and PEDOT. Therefore, the charge carriers generated from the perovskite layer are collected before recombination, which explains the high JSC observed in the PVO device. The superior charge collection property and high charge selectivity of HTL in the PVO device also explains the lower value of bulk capacitance (CBULK ∼17nF) than that of PEDOT (CBULK ∼19nF). For planar perovskite solar cells, the fast transport of carriers through the perovskite film and the associated time constant may be beyond the capabilities of impedance spectroscopy to measure. Also, the recombination lifetime is not obtained, but instead the time constant (τREC) for presumably the
Fig. 5. (A) The device structure diagram. (B) Current-Voltage (I–V) curves of PEDOT and PVO devices.
Table 1 Photovoltaic parameters of the PVO and PEDOT devices.
PVO PEDOT
Jsc (mA/cm2)
Voc (mV)
FF (%)
PCE (%)
Rseries (Ω·cm2)
Rshunt (Ω·cm2)
22.69 18.86
884.07 895.8
74.70 74.08
15 12.52
4.1 4.7
317 984
recombination process for charge storage at contacts, can be found as the product of Rrec and CHTL [42]. From the fitted results, we found that τREC improved from 2.2 μs of the reference device to 3.7 μs for device with the modified HTL. The longer lifetime translates to improved carrier extraction at the electrodes, and thus increased JSC. It is also noteworthy to mention that the size and distribution of grain Fig. 4. SEM images of perovskite film. (A) on PEDOT film; (B) on PVO HTL. The scale represents 400 nm.
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charge region around the GB [21,27,45]. The built-in-field thus formed around GB, attracts the photo-generated electrons towards the GB and serves as a low energy barrier for photo-induced charge carriers. Here, the photo-generated electrons move along the GBs and subsequently transported to electron transport layer while the photo-induced holes are transported along the GC and subsequently collected by the hole transport layer [21,27,43]. Thus, the GBs in perovskite act as efficient charge separation interface and serve as channels for charge transfer rather than acting as recombination sites [21,27,43,45]. Having said that, we can clearly comprehend that the hole selective contact in the PVO device extracts the carriers much efficiently than the reference device due to superior charge transport properties. Since both the device structure has same device structure and similar perovskite film, the difference in performance could be attributed to the enhanced functionality of the modified HTL layer in the PVO device. Recent work has shown that two capacitance steps exist in frequency-dependent capacitance measurements. The intermediate frequency capacitance step is due to dipole polarization or chemical capacitance in the perovskite layer, while the low frequency capacitance step originates from electrode polarization [46]. Trapping of electronic carriers or ionic charges can result in charge accumulation at the contact interfaces, which may alter the local electric field. Consequently, electrode polarization can lead to thin space charge regions forming near the contacts [47]. The capacitance frequency (C-F) spectra of the PVO and PEDOT devices are shown in Fig. 8. The capacitance corresponding to electrode polarization of the PVO device at very low frequency at around 1 Hz is ∼100 μ F/cm2, which is about one-two order of magnitude lower than that of PEDOT. This clearly indicates low charge accumulation at the HTL/perovskite interface for the PVO device, which could be attributed to the superior charger transport and collection property of the PVO layer facilitated by the incorporation of V2O5. Furthermore, the capacitance value of the PVO device reaches a plateau much faster than the reference device. A characteristic shift of inflection point from ∼100 Hz in PVO device to ∼5000 Hz in the PEDOT device is observed in Fig. 8; which indicates that the chemical stability of the HTL/Perovskite interface in PVO is higher with reduced chemical interactions between the selective contacts and ions in perovskite [46]. This is achieved by a redistribution of the localized electric field in the vicinity of the HTL/perovskite interface. The schematic depicting the interfacial charge transport characteristics in the PEDOT and PVO devices are shown in Fig. 9. The PVO interlayer promotes efficient charge collection at the ITO interface once the holes reach the V2O5 layer, owing to the high carrier mobility of
Fig. 6. External Quantum Efficiency (EQE) of PEDOT and PVO devices.
Fig. 7. Nyquist plots of PEDOT and PVO devices at bias of 900 mV under dark, the equivalent circuit used for fitting is shown in the inset.
Table 2 The fitted parameters of Nyquist plots from EIS under dark conditions for PVO and PEDOT devices measured under 0.9V bias.
PVO PEDOT
RS (Ω)
RC (Ω)
RREC (Ω)
CHTL (nF)
CBULK (nF)
τREC (μs)
12.6 13.5
16.9 13.1
60.3 38.5
61.2 57.7
17.2 19
3.7 2.2
boundaries of perovskites in both devices are similar and plays a benign role in terms of charge carrier separation and extraction. The benign or detrimental nature of the grain boundaries are quite debated and it is highly depended on the nature of processing of the perovskite films. In our work, we employed gas assisted perovskite film processing [21], which has been reported to result in grain boundaries being favourable for charge separation and transport [21,27]. Similar kind of beneficial role of GBs has been observed in some of our recent reports along with detailed explanation [43,44]. Briefly, the grain boundaries in perovskites become electrically charged with the presence of impurities or vacancies. The unbalanced energy state between the charged GB and the grain core (GC) causes a downward band bending in the space
Fig. 8. Capacitance-Frequency (C–F) plot measured at zero applied bias for PEDOT and PVO devices. PVO devices exhibit low interfacial capacitance.
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Fig. 9. Schematic of the carrier extraction pathways in (A) PVO and (B) PEDOT devices. The PVO layer (V2O5-PEDOT bilayer) provides faster carrier extraction pathways for holes.
reported in OPV and perovskite devices as being due to the acidic property of the PEDOT film that corrodes the ITO layer; the PEDOT is also hydrophilic and absorbs moisture to damage the active layer [9]. The V2O5 layer insulates the ITO from direct contact with the PEDOT layer thereby reducing the corrosive degradation of the ITO and hence contributing to the improved stability of the PVO device.
V2O5 compared to that of PEDOT. The influence of superior charge extraction can be correlated with high recombination resistance (RRec) observed in the PVO devices. The enhanced change transport characteristics of the PVO layer are also expected to arise from the efficient fermi-level pinning of the PVO interlayer with the ITO. On the other hand, the charge recombination is relatively higher in PEDOT only devices, as it has low recombination resistance and high electrode polarization compared to the PVO devices. The stability of the PVO device and PEDOT is also investigated. The PVO devices show better stability than the PEDOT device, as shown in Fig. 10. A FF increase in the second day of the test is due to the metallization of Silver paste on the anode. It takes two days to form fully dried ohmic contact, which decreases the series resistance and improves the FF. After the 18 days test, the PVO device still retains 95% of its initial PCE, which is 40% more than that of PEDOT. Degradation of the devices in terms of FF or increase of the series resistance, given in Fig. S7. The PVO devices retains 95% of their initial FF even after 18 days of measurement, whereas the PEDOT devices only have 65% of the FF left. The degradation mechanism of the PEDOT has been previously
4. Conclusion The IS study investigated the optical and electronic properties of V2O5/PEDOT:PSS bilayer (PVO) films and their efficacy as hole transporting interlayers (HTL) in perovskite solar cells. The PVO devices exhibits 20% higher power conversion efficiency (PCE) than conventional PEDOT only devices. The PVO based devices exhibited JSC enhancement primarily due to enhanced charge transport properties rather than improvement in light absorption. Moreover, the PVO devices exhibited low bulk and geometric capacitance, consequently leading to efficient redistribution of the electric field at the interface, thereby resulting in enhanced charge extraction from the light absorbing perovskite layer. In addition, the impedance measurements revealed that the recombination resistance (RRec) of the PVO devices is 57% higher than the PEDOT only devices. Furthermore, electrode polarization is significantly lower in PVO devices; consequently the devices are hysteresis-free. Furthermore, the bilayer based PSCs (PVO) have higher stability than PEDOT only devices; retaining 95% of its initial PCE even after 18 days of testing. The reason for stability enhancement is attributed to superior charge collection properties of the PVO HTL and degradation stability of the V2O5 nanoparticles. Acknowledgements The authors gratefully acknowledge the financial support provided by Future Solar Technologies Pty. Ltd. for this research work. The authors would like to thank the Australian Centre for Advanced Photovoltaics, SPREE staff and technicians for their kind support. We are also grateful to members in the OPV group for their feedback, helpful discussions and support whilst completing this work. Appendix A. Supplementary data
Fig. 10. Normalised stability trends depicting the performance (PCE) of the PVO and PEDOT devices stored in glovebox and tested in ambient conditions with no encapsulation.
Supplementary data related to this article can be found at http://dx. 72
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
V2O5 -PEDOT: PSS bilayer as hole transport layer for highly efficient and stable perovskite solar cells
T
Dian Wang, Naveen Kumar Elumalai∗, Md Arafat Mahmud, Matthew Wright, Mushfika Baishakhi Upama, Kah Howe Chan, Cheng Xu, Faiazul Haque, Gavin Conibeer, Ashraf Uddin∗∗ School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, 2052, Sydney, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Vanadium oxide Charge transport Stability Interface Impedance spectroscopy
Hybrid halide perovskite solar cells (PSCs) have emerged as a strong candidate for low cost photovoltaics, owing to ease of processing and material abundance. The stability and performance of these devices are contingent on the quality of the interfaces. In this work, we report the novel interface engineered hole transport layer (HTL), incorporating Vanadium Pentoxide (V2O5) and Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) bilayer (PVO) for efficient charge transport in the devices. The devices incorporating the PVO bilayer HTL exhibits 20% higher power conversion efficiency (PCE) than conventional PEDOT only devices. The PSCs incorporating the PVO bilayer HTL demonstrated superior electronic properties as evaluated using impedance spectroscopy measurements. The recombination resistance (RRec) of the bilayer based devices are 57% higher than the reference cells. In addition to high charge selectivity, the bilayer PSCs exhibit low interfacial capacitance originating from electrode polarization and almost zero hysteresis. Furthermore, the bilayer based devices are more stable than PEDOT only devices; retaining 95% of their initial PCE even after 18 days of testing. The mechanism behind superior charge transport in PVO bilayer HTL and its role in stability enhancement are also discussed.
1. Introduction Perovskite solar cells (PSCs) are emerging as a promising photovoltaic technology in last few years owing to their superior electronic properties, such as high absorption coefficient, direct bandgap, and long exciton diffusion length [1–3]. The power conversion efficiency (PCE) of PSCs has increased rapidly from 3.8% in 2009 to over 22% in 2016 [1]. Two major types of device architectures are employed: normal [4,5] and inverted structure [6,7]. The inverted devices are simple to fabricate as they generally employs as planar structure – which avoids the pore filling problem and high temperature processes. Unlike the normal structure where the electron transport layer (ETL) is deposited on the substrate, the inverted devices have a hole transport layer (HTL). Poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) is widely used as the HTL in perovskite solar cells due to its high optical transparency, low redox potential [8]and ease of processing. However, the acidic nature of PEDOT: PSS leads to corrosion of the ITO. Moreover, PEDOT: PSS is highly hygroscopic and absorbs moisture which could cause decomposition of the perovskite
∗
absorber layer [9]. Therefore, other HTL materials such as metal oxides [10], graphene oxide [11], and PbS [12] have been investigated in PSCs. Seok et al. used pulsed laser deposition technique to incorporate NiOx as HTL in an inverted device, which resulted in a PCE of ∼17% [10], Ye et al. used electro deposition to build a CuSCN based inverted device with 16.6% [13]. However, such method is not compatible for large scale fabrication. Vanadium pentoxide (V2O5) is a promising HTL candidate for perovskite solar cells due to its high transparency to visible light and high carrier mobility with energy levels well suited for hole transport in perovskite solar cells [14]. The energy levels of V2O5 and PEDOT are well studied and reported in several literatures. The valence band (VB) of the V2O5 is about 4.7 eV [15,16], closer to the HOMO level of PEDOT:PSS which is about 5.0–5.2eV [15–17]. Together, they form an electrically barrier free cascading contact for hole transport across the interface. Recently, Peng et al. reported a solution processed n-type V2Ox in PSCs with an efficiency of 17.5%, however, the device structure still uses PEDOT on top of ITO which could lead to significant device degradation in the long-term [18]. A solution processed p-type VOx
Corresponding author. Corresponding author. E-mail addresses: [email protected] (N.K. Elumalai), [email protected] (A. Uddin).
∗∗
https://doi.org/10.1016/j.orgel.2017.10.034 Received 31 May 2017; Received in revised form 26 September 2017; Accepted 25 October 2017 Available online 31 October 2017 1566-1199/ © 2017 Elsevier B.V. All rights reserved.
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clearly indicates the presence of spatial features that reflect the underlying surface morphology of the ITO substrate. Fig. 1(A) shows SEM images of the V2O5 layers on ITO substrates deposited from saturated V2O5 solution. The surface morphology has a large number of spatial features that represent surface irregularities (per unit area) similar to the ITO surface (Fig. S1), which is a relatively higher number compared to those observed in pristine PEDOT: PSS layer and PVO bilayer as shown in Fig. 1(D) and (F) respectively. The PVO bilayer is densely packed and it could serve as a good interfacial layer adjacent to the perovskite film which is to be deposited subsequently. It is well known that the perovskite film morphology is influenced considerably by the surface properties of the adjacent interfacial layer [22]. The details are discussed in the sections below. The AFM images of the V2O5, pristine PEDOT and PVO layer deposited on the ITO substrate is shown in Fig. 2, while the AFM of ITO is given in Fig. S2. The RMS roughness values are 3.41(V2O5), 1.19(PEDOT), 1.15 (PVO) and 1.75nm(ITO) respectively. After applying the V2O5, the substrate becomes even more rough which identifies the existence of the thin layer of V2O5. The surface irregularities of the V2O5 only device are higher than the other two HTL layers. PEDOT being polymeric material results in a smooth surface and it also reduces the surface roughness from 3.41 in V2O5 HTL to 1.15 nm in PVO HTL. The top view of the AFM images 2(A) and 2(E) corresponding to the V2O5 and PVO devices clearly indicates a significant improvement in surface morphology, where V2O5 HTL closely resembles the ITO surface and PVO HTL is relatively much smoother. The smooth surface is also expected to positively influence the morphology of the adjacent perovskite layer [22]. After the formation of the HTL layers on ITO substrate, the optical properties are investigated using absorbance, transmittance and reflectance measurement. The absorbance (A) of the HTL layer can be expressed in terms of the Beer-Lambert's law.
layer has also been reported by Sun et al. with PCE∼14% in inverted PSCs but the efficiency is still lower than the n-type vanadium oxides [19]. In this study, we demonstrate solution processed inverted perovskite solar cells with the device structure ITO/V2O5/PEDOT/Perovskite/ PC71BM/Ag with a high PCE of ∼15% using low temperature processing methods. The V2O5 incorporated devices (PVO) are compared with the PEDOT only devices in terms of performance efficiency and degradation stability. The modified devices (PVO) exhibited 20% higher PCE than PEDOT only devices. Impedance spectroscopy analysis is performed to investigate the superior photovoltaic properties of PVO devices. The recombination resistance and electrode polarization of the two devices are also measured and correlated with the device performance. The mechanism of superior charge transport across the PVO devices are ascertained. The stabilities of both devices are also studied for 18 days and the factors behind the enhancement are also discussed. 2. Experimental methods 2.1. Device fabrication Patterned ITO glass substrates were used for all devices. First, the substrates were ultrasonicated with Hellmanex III, DI water, acetone and isopropanol, each for 10 min. V2O5 (99.99%, Sigma Aldrich) was dissolved in DI water. It was sonicated for 10 min, and then stirred overnight at room temperature until an orange solution was formed. This solution was filtered with a 0.45μm PTFE filter. The solubility in water is poor; it has a solubility limit of 0.7 mg/mL in water at room temperature [20]. The perovskite precursor solution was formed by mixing PbI2 (99.999%, Lumtec) with methyl ammonium iodide (Dyesol), in a 1:1 ratio in DMF (Sigma Aldrich) with a 1M concentration. This solution with was stirred overnight. The saturated V2O5 solution was first spin coated on the ITO substrate at 2000 rpm for 30s and then subsequently annealed at 120 °C for 5 min in air to remove the water. Next, PEDOT:PSS (Al4083) was spin coated on the substrate, the glass was heated for another 15 min at 120 °C. The substrates were then transferred into a N2 purged glovebox. The perovskite precursor was spin coated at 2500 rpm for 30s. Nitrogen flow was applied to the substrate to form a flat film [21]. The substrates were then annealed at 100 °C for 10 min. PC71BM (1-Material) was then spin coated on the substrate at 3000 rpm for 30s. A 100 nm Ag electrode was evaporated (at 1e−5 mbar) on the substrate to form the full devices.
I A = log ⎛ 0 ⎞ = εlc ⎝I⎠ where, Io I, l, ε and c denote reference light intensity, sample light intensity, length of optical light path material, dielectric constant and precursor concentration respectively. The transmission spectra of the V2O5, PEDOT and PVO layers are shown in Fig. 3. V2O5 has high transparency to visible light due to its wide band gap. The PEDOT and PVO layers exhibited relatively high transmittance at wavelengths of 350–500 nm. The improvement in transmittance at these wavelengths is attributed to the formation of large PEDOT rich particles during annealing after the deposition, where the number of interfaces between the PEDOT:PSS rich particles and PSS lamellae are significantly reduced [23]. The incorporation of PEDOT both as a pristine and in a PVO layer also improves the optical field distribution of the devices as observed in the inverted device geometry [24,25]. It is also expected that the refractive index at the ITO/V2O5 and ITO/PVO interfaces to be different, and the latter is much favourable for light transmission with less reflection. Therefore, incident light is transmitted efficiently across the PVO layer compared to that of V2O5 only devices. The percolation of PEDOT into the surface irregularities of the V2O5 in the PVO devices results in a compact layer, where the light transmission characteristics is dominated by the PEDOT resulting in less light scattering at the interface. The overall transmittance of the PEDOT and PVO devices are above 80% all throughout the scanned wavelength range as shown in Fig. 3, which would serve as a beneficial aspect in terms of improving the light absorption in the device. The absorptions with perovskite layers are given in Fig. S3 for both PEDOT and PVO devices. The SEM images of the as-deposited MAPbI3 perovskite layer on top of the PEDOT and PVO layer is shown in Fig. 4 (A) and (B) respectively. The grain size of the perovskite film deposited on top of the PEDOT is larger than that of the one deposited on the PVO layer. The perovskite film on PVO is also characterized by the presence of a large number of
2.2. Characterization Current-Voltage (I–V) characterization was carried out in PVmeasurement IV test with a Keithley 2400, the light intensity was calibrated to 1 SUN intensity with an AM1.5 filter. The surface topology and device cross sectional view were captured by Carl Zeiss AURIGA Cross Beam SEM (Scanning Electron Microscopy) and the surface roughness was measured with Bruker Dimension ICON SPM AFM (Atomic Force Microscopy) machine. The impedance analysis was conducted with an Autolab PGSTAT-30 equipped with a frequency analyzer module in the frequency range from 1 MHz to 10 Hz. External quantum efficiency (EQE) measurements were performed using a QEX10 spectral response system from PV measurements Inc. Transmission and absorption was measured using a Perkin Elmer UV–Vis spectrometer (Lambda 1050). The stability of the device performance was measured 1,2,3, and 5 weeks after the fabrication, the devices are stored inside a glovebox. 3. Result and discussion The SEM micrographs of the various HTL layers deposited on top of the ITO substrate are shown in Fig. 1. Each SEM image of the HTL layer 67
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Fig. 1. Scanning Electron Microscopy (SEM) images of (A)–(B) V2O5 and (C)–(D) PEDOT:PSS (E)–(F) V2O5 - PEDOT: PSS (PVO bilayer) on top ITO/glass substrate.
contact of the ITO/PEDOT layer in the PEDOT only device is expected to be smooth resulting in a smaller leakage current. The overall PCE of the PVO device is 20% higher than the PEDOT device owing to high JSC and good fill factor. Moreover, neither the devices exhibited any hysteresis, as shown in Fig. S5. The removal of hysteresis in the PEDOT device is entirely expected [31], but the absence of hysteresis in the PVO device which incorporates a metal oxide layer (V2O5) is an interesting result, and reflects the low interfacial capacitance at the HTL/ Perovskite interface which will be discussed in a later section [32]. The stabilized efficiency output is also measured where the current density was tracked at the max power point of the J-V curve as shown in Fig. S6. The graph clearly shows that the PVO devices exhibit high current density than the reference devices. The PVO device stabilizes at 14.1%, while the reference device stabilizes at 12.7%, similar to the trends observed in previous reports [33,34]. The efficiencies measured under stabilized conditions are in agreement with those observed in I–V characteristics. The external quantum efficiency (EQE) spectra observed for the PEDOT and PVO devices are shown in Fig. 6. The PVO devices have higher spectral response in the whole spectrum. The PVO devices exhibit a strong EQE peak at 600 nm with a quantum efficiency of about 90%. The calculated JSC values from the EQE measurements are slightly lower compared to the JSC values obtained from the J-V characterization; the minor differences between the JSC values from EQE and J-V measurements are consistent with reports in previous literatures [35–37]. The EQE valley of the PVO device is red shifted to 675 nm
grain boundaries. Reports have shown that the grain boundaries could play a beneficial role in charge separation and collection across the perovskite layer [26–28]. The RMS values obtained from the AFM analysis are 5.49 nm and 5.38 nm for PEDOT and PVO devices respectively. The corresponding AFM images are shown in Fig. S4. The surface roughness is almost the same in both cases. The current –voltage characteristics observed for the PEDOT and PVO devices are shown in Fig. 5 (A), the corresponding I–V parameters are listed in Table 1, and the device structure is shown in Fig. 5 (B). The short circuit current density (JSC) for the PVO device is 22.69 mA/cm2 which is ∼20% higher than the PEDOT device with JSC of 18.86 mA/ cm2. The VOC of the PEDOT device at 895.8 mV is slightly higher than the PVO device at 884.48 mV. The small difference in the VOC ∼10 mV between the two devices is expected to arise from the incorporation of V2O5 which is in contact with ITO and the associated work function difference between them. Nevertheless, the fill factor is similar in both devices with PEDOT device (FF∼ 74.08%) as compared to PVO device (FF ∼74.70%). The similarity in the fill factor can be attributed to the series and shunt resistance interplay between the two devices. The series resistance in the PVO device (Rs - 4.1 Ωcm2) is lower than the PEDOT device (Rs - 4.7 Ωcm2), which is attributed to the incorporation of V2O5 which has better charge mobility than PEDOT [29,30]. On the other hand, the shunt resistance is higher for the PEDOT device (Rsh – 984 Ωcm2) than the PVO device (Rsh – 317 Ωcm2) which is attributed to the quality of the interface and work function difference between the ITO and the V2O5 layer in the PVO device, whereas the interfacial 68
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Fig. 2. Two and Three dimensional Atomic Force Microscopy (AFM) images of (A)–(B) V2O5 and (C)–(D) PEDOT:PSS (E)–(F) V2O5 - PEDOT: PSS (PVO bilayer) on top ITO/glass substrate.
The high frequency arc is associated with carrier transport in the HTL while the lower frequency arc is related to recombination at each interface of the perovskite layer [38]. The semicircle related to recombination is more prominently observed in our study than has been reported before in other studies [39,40]. The impedance spectra of PEDOT and PVO devices are shown in Fig. 7. The IS spectrum of PVO device shows a larger semicircle than the PEDOT devices. To fit the impedance spectra, an equivalent circuit is used as shown in Fig. 7, which has been reported previously in the literature [39–41]. Briefly,
compared to the EQE valley for the PEDOT device at 650 nm. The observed increase in the EQE peak as well as the red shift for the PVO device is expected to be due to the enhanced collection characteristics of the V2O5 incorporated HTL layer in the PVO devices. The reason behind the improved JSC and charge collection properties of the PVO devices are further investigated using impedance spectroscopy analysis. In order to understand the internal electrical characteristics of the devices, impedance spectroscopy (IS) is utilized. Generally, the Nyquist plots show the combination of semicircles of the complex plane. 69
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Fig. 3. Transmittance of the V2O5, PEDOT, V2O5/PEDOT (PVO bilayer) on ITO substrate.
RS is associated with series resistance, while RHTL and RREC are the contact resistance and recombination resistance respectively. CBULK refers to the chemical or bulk capacitance and CHTL is the geometric capacitance. In planar perovskite solar cells, it is postulated that CHTL dominates over CBULK, or in other words, the effects of charge storage in the contacts outweigh that for charges associated with photogenerated electrons and holes in the perovskite layer [42]. The modelled results from fitting with the equivalent circuit have been presented in Table 2. The series resistance RS is relatively lower in PVO device (RS - 12.6 Ω) than that of with PEDOT only (RS – 13.5 Ω). The lower RS in the PVO devices can be attributed to the high carrier mobility of V2O5 layer, which is around two or threefold higher than the PEDOT [29,30]. However, the contact resistance RHTL of PVO device is 29% higher, which can be attributed to the interfacial barrier between the ITO and V2O5, due to the difference of the work function [14]. In the case of PEDOT device, an Ohmic contact is formed. The recombination resistance RREC is 60.3Ω for the PVO device, whereas the reference device exhibits low RREC of 38.5 Ω. The difference corresponds to the diameters of the semicircles of the IS spectra as shown in Fig. 7. The RREC in the PVO device is ∼57% higher than the PEDOT device. The modified HTL layer in the PVO extracts the charge carriers more efficiently than the PEDOT layer, due to the high mobility of the V2O5 layer incorporated between ITO and PEDOT. Therefore, the charge carriers generated from the perovskite layer are collected before recombination, which explains the high JSC observed in the PVO device. The superior charge collection property and high charge selectivity of HTL in the PVO device also explains the lower value of bulk capacitance (CBULK ∼17nF) than that of PEDOT (CBULK ∼19nF). For planar perovskite solar cells, the fast transport of carriers through the perovskite film and the associated time constant may be beyond the capabilities of impedance spectroscopy to measure. Also, the recombination lifetime is not obtained, but instead the time constant (τREC) for presumably the
Fig. 5. (A) The device structure diagram. (B) Current-Voltage (I–V) curves of PEDOT and PVO devices.
Table 1 Photovoltaic parameters of the PVO and PEDOT devices.
PVO PEDOT
Jsc (mA/cm2)
Voc (mV)
FF (%)
PCE (%)
Rseries (Ω·cm2)
Rshunt (Ω·cm2)
22.69 18.86
884.07 895.8
74.70 74.08
15 12.52
4.1 4.7
317 984
recombination process for charge storage at contacts, can be found as the product of Rrec and CHTL [42]. From the fitted results, we found that τREC improved from 2.2 μs of the reference device to 3.7 μs for device with the modified HTL. The longer lifetime translates to improved carrier extraction at the electrodes, and thus increased JSC. It is also noteworthy to mention that the size and distribution of grain Fig. 4. SEM images of perovskite film. (A) on PEDOT film; (B) on PVO HTL. The scale represents 400 nm.
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charge region around the GB [21,27,45]. The built-in-field thus formed around GB, attracts the photo-generated electrons towards the GB and serves as a low energy barrier for photo-induced charge carriers. Here, the photo-generated electrons move along the GBs and subsequently transported to electron transport layer while the photo-induced holes are transported along the GC and subsequently collected by the hole transport layer [21,27,43]. Thus, the GBs in perovskite act as efficient charge separation interface and serve as channels for charge transfer rather than acting as recombination sites [21,27,43,45]. Having said that, we can clearly comprehend that the hole selective contact in the PVO device extracts the carriers much efficiently than the reference device due to superior charge transport properties. Since both the device structure has same device structure and similar perovskite film, the difference in performance could be attributed to the enhanced functionality of the modified HTL layer in the PVO device. Recent work has shown that two capacitance steps exist in frequency-dependent capacitance measurements. The intermediate frequency capacitance step is due to dipole polarization or chemical capacitance in the perovskite layer, while the low frequency capacitance step originates from electrode polarization [46]. Trapping of electronic carriers or ionic charges can result in charge accumulation at the contact interfaces, which may alter the local electric field. Consequently, electrode polarization can lead to thin space charge regions forming near the contacts [47]. The capacitance frequency (C-F) spectra of the PVO and PEDOT devices are shown in Fig. 8. The capacitance corresponding to electrode polarization of the PVO device at very low frequency at around 1 Hz is ∼100 μ F/cm2, which is about one-two order of magnitude lower than that of PEDOT. This clearly indicates low charge accumulation at the HTL/perovskite interface for the PVO device, which could be attributed to the superior charger transport and collection property of the PVO layer facilitated by the incorporation of V2O5. Furthermore, the capacitance value of the PVO device reaches a plateau much faster than the reference device. A characteristic shift of inflection point from ∼100 Hz in PVO device to ∼5000 Hz in the PEDOT device is observed in Fig. 8; which indicates that the chemical stability of the HTL/Perovskite interface in PVO is higher with reduced chemical interactions between the selective contacts and ions in perovskite [46]. This is achieved by a redistribution of the localized electric field in the vicinity of the HTL/perovskite interface. The schematic depicting the interfacial charge transport characteristics in the PEDOT and PVO devices are shown in Fig. 9. The PVO interlayer promotes efficient charge collection at the ITO interface once the holes reach the V2O5 layer, owing to the high carrier mobility of
Fig. 6. External Quantum Efficiency (EQE) of PEDOT and PVO devices.
Fig. 7. Nyquist plots of PEDOT and PVO devices at bias of 900 mV under dark, the equivalent circuit used for fitting is shown in the inset.
Table 2 The fitted parameters of Nyquist plots from EIS under dark conditions for PVO and PEDOT devices measured under 0.9V bias.
PVO PEDOT
RS (Ω)
RC (Ω)
RREC (Ω)
CHTL (nF)
CBULK (nF)
τREC (μs)
12.6 13.5
16.9 13.1
60.3 38.5
61.2 57.7
17.2 19
3.7 2.2
boundaries of perovskites in both devices are similar and plays a benign role in terms of charge carrier separation and extraction. The benign or detrimental nature of the grain boundaries are quite debated and it is highly depended on the nature of processing of the perovskite films. In our work, we employed gas assisted perovskite film processing [21], which has been reported to result in grain boundaries being favourable for charge separation and transport [21,27]. Similar kind of beneficial role of GBs has been observed in some of our recent reports along with detailed explanation [43,44]. Briefly, the grain boundaries in perovskites become electrically charged with the presence of impurities or vacancies. The unbalanced energy state between the charged GB and the grain core (GC) causes a downward band bending in the space
Fig. 8. Capacitance-Frequency (C–F) plot measured at zero applied bias for PEDOT and PVO devices. PVO devices exhibit low interfacial capacitance.
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Fig. 9. Schematic of the carrier extraction pathways in (A) PVO and (B) PEDOT devices. The PVO layer (V2O5-PEDOT bilayer) provides faster carrier extraction pathways for holes.
reported in OPV and perovskite devices as being due to the acidic property of the PEDOT film that corrodes the ITO layer; the PEDOT is also hydrophilic and absorbs moisture to damage the active layer [9]. The V2O5 layer insulates the ITO from direct contact with the PEDOT layer thereby reducing the corrosive degradation of the ITO and hence contributing to the improved stability of the PVO device.
V2O5 compared to that of PEDOT. The influence of superior charge extraction can be correlated with high recombination resistance (RRec) observed in the PVO devices. The enhanced change transport characteristics of the PVO layer are also expected to arise from the efficient fermi-level pinning of the PVO interlayer with the ITO. On the other hand, the charge recombination is relatively higher in PEDOT only devices, as it has low recombination resistance and high electrode polarization compared to the PVO devices. The stability of the PVO device and PEDOT is also investigated. The PVO devices show better stability than the PEDOT device, as shown in Fig. 10. A FF increase in the second day of the test is due to the metallization of Silver paste on the anode. It takes two days to form fully dried ohmic contact, which decreases the series resistance and improves the FF. After the 18 days test, the PVO device still retains 95% of its initial PCE, which is 40% more than that of PEDOT. Degradation of the devices in terms of FF or increase of the series resistance, given in Fig. S7. The PVO devices retains 95% of their initial FF even after 18 days of measurement, whereas the PEDOT devices only have 65% of the FF left. The degradation mechanism of the PEDOT has been previously
4. Conclusion The IS study investigated the optical and electronic properties of V2O5/PEDOT:PSS bilayer (PVO) films and their efficacy as hole transporting interlayers (HTL) in perovskite solar cells. The PVO devices exhibits 20% higher power conversion efficiency (PCE) than conventional PEDOT only devices. The PVO based devices exhibited JSC enhancement primarily due to enhanced charge transport properties rather than improvement in light absorption. Moreover, the PVO devices exhibited low bulk and geometric capacitance, consequently leading to efficient redistribution of the electric field at the interface, thereby resulting in enhanced charge extraction from the light absorbing perovskite layer. In addition, the impedance measurements revealed that the recombination resistance (RRec) of the PVO devices is 57% higher than the PEDOT only devices. Furthermore, electrode polarization is significantly lower in PVO devices; consequently the devices are hysteresis-free. Furthermore, the bilayer based PSCs (PVO) have higher stability than PEDOT only devices; retaining 95% of its initial PCE even after 18 days of testing. The reason for stability enhancement is attributed to superior charge collection properties of the PVO HTL and degradation stability of the V2O5 nanoparticles. Acknowledgements The authors gratefully acknowledge the financial support provided by Future Solar Technologies Pty. Ltd. for this research work. The authors would like to thank the Australian Centre for Advanced Photovoltaics, SPREE staff and technicians for their kind support. We are also grateful to members in the OPV group for their feedback, helpful discussions and support whilst completing this work. Appendix A. Supplementary data
Fig. 10. Normalised stability trends depicting the performance (PCE) of the PVO and PEDOT devices stored in glovebox and tested in ambient conditions with no encapsulation.
Supplementary data related to this article can be found at http://dx. 72
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doi.org/10.1016/j.orgel.2017.10.034.
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