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Improved electron transport in MAPbI3 perovskite solar cells based on dual doping graphdiyne
Author’s Accepted Manuscript Improved Electron Transport in MAPbI 3 Perovskite Solar Cells Based on Dual Doping Graphdiyne Jiangsheng Li, Tonggang Jiu, Chenghao Duan, Yao Wang, Hongna Zhang, Hongmei Jian, Yingjie Zhao, Ning Wang, Changshui Huang, Yuliang Li www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(18)30075-2 https://doi.org/10.1016/j.nanoen.2018.02.014 NANOEN2501
To appear in: Nano Energy Received date: 23 December 2017 Revised date: 5 February 2018 Accepted date: 5 February 2018 Cite this article as: Jiangsheng Li, Tonggang Jiu, Chenghao Duan, Yao Wang, Hongna Zhang, Hongmei Jian, Yingjie Zhao, Ning Wang, Changshui Huang and Yuliang Li, Improved Electron Transport in MAPbI 3 Perovskite Solar Cells Based on Dual Doping Graphdiyne, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.02.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improved Electron Transport in MAPbI3 Perovskite Solar Cells Based on Dual Doping Graphdiyne
Jiangsheng Lia, Tonggang Jiua,c*, Chenghao Duana, Yao Wanga, Hongna Zhanga, Hongmei Jiana, Yingjie Zhaod, Ning Wanga, Changshui Huanga,c, Yuliang Lib,c*
a
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of
Sciences, Qingdao, 266101, P. R. China. b
Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. c
University of Chinese Academy of Sciences, Beijing 100049, P.R. China.
d
Qingdao University of Science and Technology, Qingdao 266042, P. R. China.
E-mail: [email protected], [email protected]
ABSTRACT:
The properties of electron transport layers play a crucial role in determining the performance of perovskite solar cells. Here we reported that graphdiyne doped in both PCBM and ZnO films of perovskite solar cells with an inverted structure based on MAPbI3 for the first time. A high efficiency of 20.0% was achieved in MAPbI3 perovskite solar cells. Moreover, the J-V hysteresis and stability were significantly improved as well. It was found that dual doping graphdiyne not only increased electrical conductivity, electron mobility, and charge extraction ability in the electron
transport layers but also improved film morphology of the electron transport layers and reduced charge recombination which contribute to fill factor improvement. Therefore the results indicate that dual doping graphdiyne is a promising strategy to enhance perovskite solar cells performance. Keywords: graphdiyne, dual doping, electron transport layers, MAPbI3, perovskite solar cells.
1. Introduction Hybrid organic-inorganic perovskite solar cells (PSCs) based on MAPbI3 have attracted considerable attention due to their high absorption coefficient, long holeelectron diffusion length, excellent carrier transport, low-cost, and easy fabrication process at relative low-temperatures. Great progress has been made in the field of PSCs based on MAPbI3, which lead to the power conversion efficiency (PCE) of PSCs exceeding 20% in few years.1-3 It is an effective way to enhance the performance of PSCs by improving the electrons transport properties of electron transport layers (ETLs). Therefore, a series of ways have been adopted to engineer the ETLs of PSCs, such as self-assembly monolayers, hybrids with conjugated molecules, doping ETLs as well as surface modification of ETLs.4-7 Among these approaches, it is a simple and effective method by adding dopant in ETLs of PSCs to further improve the performance of PSCs.8-10 However, the study of effects and understanding of doping in ETLs in the field of PSCs is still lacking. Therefore, the requirement to introduce new approach of doping ETLs as well as further
understanding the electron transport mechanism for improving PSCs performance is urgent. Graphdiyne (GD), a new allotropic form of carbon family, possesses rigid carbon network and highly π-conjugated structure consisting of sp- and sp2-hybridized carbons, a natural band gap and uniformly distributed pores.11-17 Therefore, it is supposed to have a promising application in photoelectrics, electronic, and geometric characteristics.18-25 As reported in the literature, GD has been introduced into PSCs as a dopant into P3HT working as hole-transporting materials (HTMs) and PCBM working as ETLs.26,27 And both of them suggested that GD may basically provide better percolation paths and improve the electron transport efficiency to the device due to the relatively strong π-π stacking interaction between GD particles and P3HT or PCBM. But the related researches are still lacking in the field of PSCs. In the present work, we introduce a novel approach to enhance the property of ETLs for the further improvement of PSCs performance by dual doping GD in both PCBM and ZnO films of PSCs for the first time. This way led to the PCE increased from 16.6% to 20.0% based on MAPbI3 active layer. Moreover, the devices based on dual doping GD exhibited a remarkable improved stability and J-V hysteresis. To figure out the effects of dual doping GD on PSCs devices, the surface morphology, photoluminescence properties, electron mobility, device stability, electrochemical impedance spectroscopy as well as exciton generation rate and dissociation probability have been investigated in details. 2. Experimental Section
The details of material and characterizations, device fabrications, and measurements are provided in the Supplementary material.
3. Results and Discussion
Figure 1(a) The p-i-n device architecture. (b) GD dispersion solution in chlorobenzene. (c) The chemical structures of GD used for device fabrication. SEM Top view SEM images
of
(d)
ITO/P3CT-K/perovskite/PCBM,
(e)
ITO/P3CT-K/
perovskite/PCBM(GD), (f) ITO/P3CT-K/perovskite/PCBM/ZnO, (g) ITO/P3CT-K/ perovskite/PCBM(GD)/ZnO, (h) ITO/P3CT-K/perovskite/PCBM/ZnO(GD) and (i) ITO/P3CT-K/perovskite/PCBM(GD)/ZnO(GD). The p-i-n device architecture and chemical structures of GD are presented in Figure 1 (a). GD powder was dispersed in chlorobenzene with a concentration of 1.5 mg/mL by ultrasonic dispersion shown in Figure 1(b). As can be seen, because of the
good lipophilicity of unmodified carbon materials, the GD black suspension is quite stable without precipitation in several hours after 60 h ultrasonic dispersion. It is beneficial for the dispersing of GD after been doped in the PCBM and ZnO solution. As we all know, the morphology of perovskite layer and interlayer played an important role in achieving high performance of PSCs. So SEM measurements were performed, and the results were shown in Figure 1. After the perovskite layer was coated with the PCBM or PCBM(GD) layer, the surfaces morphologies displayed slightly differences as shown in Figure 1(d) and Figure 1(e). The rough perovskite layer became smoother when coated by PCBM(GD) film than that coated by pure PCBM film shown in Figure 1(d) and Figure 1(e). The graphdiyne may also help to passivate the grain-boundary of the perovskite and then reduce the trap states of the surface to eliminate the recombination.28 And after the PCBM or PCBM(GD) layer were coated with the ZnO or ZnO(GD) layer, the surfaces morphologies displayed slightly differences as well, as shown in Figure (f), (g), (h) and (i). The rough PCBM layer became smoother when coated by ZnO(GD) film than that coated by pure ZnO film. Some nanoparticles still can be seen in Figure 1(d) and (e) which probably come from PCBM aggregates. The condition can be improved by coating ZnO or ZnO(GD) film.
Table 1. Parameters of Optimized Devices Based on Different Electron Transport Layers. PCE (%) ETL
Voc(V)
Jsc(mA/cm2)
FF (%)
PCBM/ZnO
Best
Average
1.04
21.34
74.8
16.59
16.30
1.05
24.06
79.0
20.00
19.60
PCBM(GD)/ ZnO(GD)
Figure 2 (a) J-V characteristic curves of PCBM/ZnO and PCBM(GD)/ZnO(GD) based perovskite solar cells under AM 1.5G 100 mW/cm2 simulated solar light. (b) Corresponding external quantum efficiency (EQE) spectra of the PCBM/ZnO and PCBM(GD)/ZnO(GD) based devices. The distribution of (c) Jsc and (d) PCE of devices based on PCBM/ZnO and PCBM(GD)/ZnO(GD) as the ETLs. Currentvoltage data from the devices of (e) ITO/Al/PCBM/Al, ITO/Al/PCBM(GD)/Al, (f) ITO/Al/ZnO/Al and ITO/Al/ZnO(GD)/Al, plotted in the format ln(JL3/V2) versus (V/L)0.5, where J is the current density and L is the thickness of the PCBM,
PCBM(GD), ZnO or ZnO(GD) layer. The lines are the fit to the respective experimental points. To figure out the impact of dual doping GD on the performance of the PSCs, the current density-voltage (J-V) characteristics obtained under AM 1.5G irradiation (100 mW cm-2) are depicted in Figure 2(a) and the detailed parameters are summarized in Table 1. As can be seen, the reference device based on pure PCBM/ZnO showed a Voc of 1.04 V, a short circuit current (Jsc) of 21.34 mA/cm2, a fill factor (FF) of 74.8% and a resulted PCE of 16.59% (average PCE of 16.30%). After doping GD into both PCBM and ZnO films, the device performances were improved significantly. The device based on PCBM(GD)/ZnO(GD) exhibited a Voc of 1.05 V, a Jsc of 24.06 mA/cm2, a FF of 79.0% and a corresponding PCE of 20.00% (average PCE of 19.60%). Corresponding external quantum efficiency (EQE) spectra of this device and the reference device were displayed in Figure 2(b). It can be seen that the EQE spectrum of the device with GD dual doping was obviously increased from the wavelength range of 300-800 nm as compared with PCBM/ZnO based device, which agreed with the Jsc obtained from the J-V results shown above. The distribution of Jsc and PCE of these devices are shown in Figure 2(c) and (d). As can be seen, the values of Jsc and PCE were increased significantly from PCBM/ZnO based devices to dual doping based devices. Moreover, the statistical data of 15 devices fabricated with PCBM/ZnO and PCBM(GD)/ZnO(GD) electron transport layers were presented in the form of a standard box plot, respectively, as shown in Figure S1. To further confirm the effect of GD on the performance of the PSCs, the devices based on only doping of GD in PCBM or ZnO were fabricated. As can be seen in table S1, after only doping GD into PCBM or ZnO, the device performances were improved as well. The device based on PCBM/ZnO(GD) exhibited a Voc of 1.04 V, a Jsc of 22.1 mA/cm2, a
FF of 76.9% and a corresponding PCE of 17.63% (average PCE of 17.35%) and the device based on PCBM(GD)/ZnO exhibited a Voc of 1.02 V, a Jsc of 23.3 mA/cm2, a FF of 78.4% and a corresponding PCE of 18.61% (average PCE of 18.37%). The current density-voltage (J-V) characteristics and distribution of different parameters of devices are shown in Figure S2 and Figure S3, which consistent with the previous conclusion. To figure out the origin of Jsc increment, the electron mobility was approximated using space charge limited current (SCLC) method in which device structures were ITO/Al/PCBM/Al, ITO/Al/PCBM(GD)/Al, ITO/Al/ZnO/Al and ITO/Al/ZnO(GD)/Al. The results are plotted as ln (JL3/V2) vs (V/L)
0.5
, as shown in Figure 2(e) and Figure
2(f). Electron mobility was calculated from the intercept of the corresponding lines on the axis of ln (JL3/V2). It is easy to work out that the electron mobility of pure PCBM and ZnO based electron-only device was 1.05×10-6 cm2 V-1 s-1 and 2.87×10-8 cm2 V-1 s1
. After GD doping, the electron mobility of devices increased to 1.90×10-4 and
2.12×10-6 cm2 V-1 s-1, respectively. The result exhibited the obvious improvement after the doping of GD on electron transport mobility.29
Figure 3(a) The photo- and dark-currents of the PCBM(GD)/ZnO(GD) based device. (b) Obtained from forward bias to short circuit (FB-SC) and from short circuit to forward bias (SC-FB). (c) Obtained at forward scan direction and different scanning rate with delaying time ranging from 50, 100, 200, 300, 400 to 500 ms. (d) Stability tests of PCBM/ZnO and PCBM(GD)/ZnO(GD) as the ETL. All of devices were stored in a nitrogen filled glovebox without encapsulation. The highest efficiency device with photo- and dark-currents was shown in Figure 3(a) has a Jsc of 24.06 mA/cm2, a Voc of 1.054 V, a FF of 79.0% and a PCE of 20.00%. The J-V curves presented in Figure 3(b) were determined for one of the most efficient cells, measured with a 50 ms scanning delay in both reverse-scan and forward-scan modes under standard air-mass 1.5 global (AM 1.5 G) illumination. The J-V curves show no appreciable hysteresis between the two modes. The Jsc, Voc, and FF values obtained from the J-V curve in the reverse-scan mode were 24.08 mA/cm2, 1.053 V,
and 75%, respectively, yielding a PCE of 19.05% under standard AM 1.5 G conditions. The corresponding values obtained from the J-V curve in the forward-scan mode were 24.06 mA/cm2, 1.054 V and 79%, respectively, yielding a similar efficiency of 20.00%. The steady-state photocurrent represents the actual power output and should be used to accurately characterize the device efficiency. As shown in Figure S4, the photocurrent rose quickly to maximum, and the photocurrent was stable. In addition, the J-V curves of PCBM(GD)/ZnO(GD) based devices at different scanning rates with delay time ranging from 50, 100, 200, 300, 400 to 500 ms were also checked, as shown in Figure 3(c). The J-V curves at different scanning rates almost overlapped with each other. These results indicated that the photocurrent hysteresis could be reduced to a certain extent by dual doping GD in PCBM and ZnO layers. The weaker photocurrent hysteresis in PCBM(GD)/ZnO(GD) based devices may be ascribed to the improved electrical properties. The lifetime measurement of devices were performed and exhibited the stability of PSC devices with different ETLs in Figure 3(d). All devices were stored and measured in the nitrogen-filled glove box without any encapsulation. As can be seen, the PCE of devices based on dual doping GD remained 86% after the storage of 30 days which was much better than the one without GD (remaining 21%), which indicated that dual doping GD is an effective way to improve the device stability. To further confirm the effect of GD doping on stability of the devices, the lifetime measurements of devices with only doping of GD in PCBM or ZnO were performed, as shown in Figure S5, we can observe that after doping GD in PCBM, the PCE of devices remained about 70% after the storage of more than 15 days and the PCE of devices based on only doping GD in ZnO remained 60% after the storage of 14 days, indicating that the only doping of GD significantly improve the device stability as well, which consistent with the previous
conclusion. GD doping into the PCBM would fill the electron traps originated from dimerization of PCBM and then alleviate the negative effect of PCBM dimerization.30 In addition, GD doping may enhance the fracture resistance of the PCBM, which lead to a more stable electron transport layer. Thus, the stabilization of devices is improved. On the other hand, the suppressed interfacial recombination could account for the superior stability in planar perovskite solar cells.31 GD doping suppressed the interfacial recombination significantly proved by following discussion such as maximum exciton generation rate and electrochemical impedance spectroscopy which also contribute to the device stabilization.
Figure 4(a) The photoluminescence (PL) spectra of various thin films. (b) Timeresolved photoluminescence of PCBM or PCBM(GD) as the interlayer and (c) timeresolved photoluminescence of ZnO or ZnO(GD) as the interlayer. (d) Plots of photocurrent density (Jph) with respect to the effective bias (Veff) in the devices with
PCBM/ZnO and PCBM(GD)/ZnO(GD) as the ETLs. (e) The impedance spectra of the PCBM/ZnO and PCBM(GD)/ZnO(GD) based devices (measured at 0.7 V bias voltage under dark condition), and (f) bode spectra of the PCBM/ZnO and PCBM(GD)/ZnO(GD) based devices. (measured at 0.7 V bias voltage under dark condition). (g) Rct, (h) τ, (i) capacitances obtained by EIS. In order to investigate if PCBM(GD) film efficiently extract photogenerated carriers from the perovskite absorber, PL spectra were measured as well, as shown in Figure 4(a). In our devices, P3CT-K, as hole transport layer, and PCBM(GD), as electron transport layer, can extract photogenerated carriers. The PL of P3CTK/pervoskite/PCBM, and P3CT-K/pervoskite/PCBM(GD) on ITO substrates was measured. The excitation light entered the sample from the glass substrate side with excitation at 600 nm, and PL spectra were collected. We observed a significant quenching effect when the perovskite layer was coated with either the PCBM layer or the PCBM(GD) layer. After doping the PL spectrum of perovskite film was quenched further. It indicated that the GD doping enhanced electron transport from perovskite to PCBM and reduce the electron accumulations usually occurred in the interlayer of perovskite device due to the relatively strong π-π stacking interaction between GD and PCBM so as to improve the performance of device.32-34 To further understand the electron transport processes, the time-resolved photoluminescence has been measured and the results are shown in Figure 4(b) and (c). We can yields the lifetime of carriers by fitting the data with two exponential decay curves. As a consequence, we can obtain that the carrier lifetime of pure PCBM and PCBM(GD) film were 25.95 and 3.11 ns. The shorter τave indicates more rapid electron transfer from perovskite to PCBM (GD) layer. When doping GD in ZnO films, the carrier lifetime drops from 45.00 to 3.20 ns, respectively, which exhibited
that the electron transport property of ZnO film was obviously improved. The faster charge transfer can lead to improved Jsc and PCE, consistent with the previous conclusions.35-38 To further understand the influence of the dual doping of GD on the PSCs devices, the maximum exciton generation rate (Gmax) and exciton dissociation probability (P) in the devices was plotted in Figure 4(d). As can be seen, the effect of the bilayer structure was illustrated on the photocurrent density (Jph) with respect to the effective voltage (Veff). Jph is describe as Jph = JL − JD, where JL and JD are the current density under illumination and in the dark, respectively. Veff is calculated as Veff = V0 – Va, where V0 is the voltage at which JL=JD and Va is the applied voltage In the low effective field region (Veff ＜ 0.1 V), the photocurrents in three devices increased sharply with effective voltage. At large reverse bias (Veff＞1 V), the Jph gradually reached a saturated value.39-41 From Figure 4(d), with the increase of effective
voltage,
the
saturation
photocurrent
(Jsat)
in
the
device
with
PCBM(GD)/ZnO(GD) reached earlier than that with PCBM/ZnO based devices. Generally, the saturated photocurrent correlated to the maximum exciton generation rate (Gmax), which is mainly determined by the light absorption, and is given by Jsat = qGmaxL, where q is the electronic charge and L is the thickness of active layer (300 nm). The values of Gmax for the PCBM/ZnO and PCBM(GD)/ZnO(GD) based devices are 1.45×1028 m-3s-1 and 2.24×1028 m-3 s-1 ,respectively. An enhancement of Gmax occurred after the dual doping of GD in PCBM and ZnO films. And it is noticed that the exciton dissociation probability (P) obtained from the normalized photocurrent density Jph/Jsat is obvious different among three devices, as shown in Figure 4(d). For example, at 0.2 V of Veff, P value is 68% for the device based on PCBM/ZnO only interlayer while it is 89% for the devices with the PCBM(GD)/ZnO(GD) interlayers,
respectively. The raise of P value means the reduction of the exciton recombination rate. The results suggests that Gmax was increased and the exciton recombinations was suppressed when dual doping GD in PCBM and ZnO films thus lead to better Jsc and FF values. To analyze the electron transport and interfacial properties in PSCs devices, electrochemical impedance spectroscopy (EIS) measurement was performed at 0.7 V bias voltage under dark condition. The EIS results of devices measured in dark are shown in Figure 4(e) and Figure 4(f). As we all know, in the Nyquist plots, the diameter of the semicircle presents the charge transfer resistance (Rct) related to the current leakage and recombination of electrons in ETLs as well as active layer. Larger Rct means the less current leakage and electron recombined occurred at the surface.42,43 As can be seen, the diameters of the semicircles increased significantly from PCBM/ZnO based device to PCBM(GD)/ZnO(GD) based device, which meant that after dual doping GD in PCBM and ZnO films, the interfacial contact between perovskite and ETLs was improved dramatically consequently reduced the current leakage and charge recombination occurred at interface.44-46 To further confirm the conclusion, EIS measurement under bias voltage from 0.6 V to 1.1 V (between anode and cathode) was performed under dark condition. As shown in Figure 4(g), the Rct decreases evidently as bias voltage increases. And the Rct of the PCBM(GD)/ZnO(GD) based devices is still higher than that of PCBM/ZnO at various bias voltages. The higher Rct for the PCBM(GD)/ZnO(GD) based devices compared with PCBM/ZnO is attributed to the less charge recombination occurred at interface. We think the main reason is that the way of dual doping GD passivated the surface defects of PCBM and ZnO films, which reduced the current leakage and charge recombination occurred at
interface. Therefore FF were enhanced after the dual doping of GD. The result is consistent with the improved FF shown in J-V characteristics. Bode EIS spectra performed at 0.7 V bias voltage under dark condition are shown in Figure 4(f) to determine the dependence of electron lifetime (τ) on PCBM/ZnO and PCBM(GD)/ZnO(GD) based devices. The electron lifetime (τ) can be obtained by the following equation:
τ=1/2πfp Where fp is the peak frequency corresponding to electrochemical reaction of chargetransfer process at PCBM/ZnO or PCBM(GD)/ZnO(GD) interface. As a consequently, we can obtain the values of τ for PCBM(GD)/ZnO(GD) and PCBM/ZnO based devices are 8.96μs and 6.09μs, respectively. To further confirm the conclusion, the τ of PCBM(GD)/ZnO(GD) and PCBM/ZnO based devices under bias voltage from 0.6 V to 1.1 V (between anode and cathode) were calculated. As shown in Figure 4(h), the τ of the PCBM(GD)/ZnO(GD) based devices is higher than that of PCBM/ZnO at various bias voltages as well, which can be attributed to the high electron density and the rapid electron transfer from PCBM(GD)/ZnO(GD) interface. Accordingly, it can be concluded that the dual doping of GD in PCBM and ZnO films is an effective way to accelerating electron transfer, and enhancing photovoltaic performance of PSCs, which consistented with the time-resolved photoluminescence and electron mobility measurements of PCBM and ZnO films. In order to explore the effect of photogenerated dipoles on the internal photovoltaic processes in PSCs, capacitance-voltage (C-V) measurement on PSCs with PCBM(GD)/ZnO(GD) and PCBM/ZnO as the ETLs. The C-V characteristics, measured under bias voltage from 0.6 V to 1.1 V under dark condition, can provide
the information on charge accumulation as well as geometry capacitance in solar cells. According to the literature, the Vpeak value can reflect the amount of charge accumulation at electrode interfaces. The larger and smaller Vpeak values correspond to more and less interfacial accumulation of photogenerated charge carriers at electrode interfaces,
respectively.
As
shown
in
Figure
4(i),
the
Vpeak
value
of
PCBM(GD)/ZnO(GD) based devices is smaller than that of PCBM/ZnO based devices.47,48 The results indicate that the device with PCBM(GD)/ZnO(GD) has less charge accumulations at the electrode interfaces, which can be explained that GD may basically provide better percolation paths and improve the electron transport efficiency to the device due to the relatively strong π-π stacking interaction between GD particles and PCBM or ZnO films. As we know, the less charge accumulations at the electrode interfaces can reduce the J-V hysteresis, which indicated that the photocurrent hysteresis could be reduced by introducing GD dual doping in PCBM and ZnO layers. The conclusion is consistented with the measure of the J-V curves with different scanning directions, as shown in Figure S6, J-V hysteresis is significantly reduced by the GD dual doping.
Figure 5 The schematic diagram of effect of GD dual doping. The mechanism of dual doping GD in PCBM and ZnO interfaces is proposed according to the aforementioned characterizations. Figure 5 showed that pristine PCBM and ZnO films have various surface defects which act as recombination centers for photogenerated charges so as to hamper the electron transfer and decrease fill factor of the device, which can be proved by time-resolved photoluminescence spectra, steady photoluminescence spectra and EIS. After GD dual doping, the surface defects are significantly passivated, which suppresses the charge recombination and increases the electron transfer rate as well as the charge transport mobility. All of these lead to an enhanced Jsc, and FF consequently result in the improvement of device performance for PSCs.
4. Conclusion In summary, we successfully dual doping GD in both PCBM and ZnO films to fabricate the highly efficient inverted PSCs. It was found that dual doping GD in ETLs obtained increased electrical conductivity, electron mobility and improved films morphology proved by SEM, SCLC, EIS, PL quenching measurements. Photoluminescence spectra revealed that dual doping GD can passivate the traps of PCBM and ZnO films, which decreases the carrier recombination at interface and consequently improves FF. Simutaneously, the devices based on dual doping GD exhibited a remarkable reduced J-V hysteresis. Capacitance-voltage measurement revealed that dual doping GD can reduced J-V hysteresis by decreasing charge accumulations at the electrode interfaces. Moreover the stability was improved as well. All of these demonstrated that dual doping GD is a promising strategy to increase PSCs performance and present large potential for optoelectronic applications as well. Acknowledgments The Project is supported by Natural Science Foundation of China (51672288), Youth Innovation Promotion Association of Chinese Academy of Sciences and Scientific Research Staring Foundation of Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences. This study was also supported by Major Basic Research Program of Shandong Natural Science Foundation (ZR2017ZB0313). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.nanoen.
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Graphical abstract
Highlights
1.
Graphdiyne was doped in both PCBM and ZnO films of perovskite solar cells.
2.
The J-V hysteresis and stability were significantly improved.
3.
Electrical conductivity, electron mobility and charge extraction ability were increased.
4.
Film morphology of the electron transport layers was improved.
PII: DOI: Reference:
S2211-2855(18)30075-2 https://doi.org/10.1016/j.nanoen.2018.02.014 NANOEN2501
To appear in: Nano Energy Received date: 23 December 2017 Revised date: 5 February 2018 Accepted date: 5 February 2018 Cite this article as: Jiangsheng Li, Tonggang Jiu, Chenghao Duan, Yao Wang, Hongna Zhang, Hongmei Jian, Yingjie Zhao, Ning Wang, Changshui Huang and Yuliang Li, Improved Electron Transport in MAPbI 3 Perovskite Solar Cells Based on Dual Doping Graphdiyne, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.02.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improved Electron Transport in MAPbI3 Perovskite Solar Cells Based on Dual Doping Graphdiyne
Jiangsheng Lia, Tonggang Jiua,c*, Chenghao Duana, Yao Wanga, Hongna Zhanga, Hongmei Jiana, Yingjie Zhaod, Ning Wanga, Changshui Huanga,c, Yuliang Lib,c*
a
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of
Sciences, Qingdao, 266101, P. R. China. b
Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. c
University of Chinese Academy of Sciences, Beijing 100049, P.R. China.
d
Qingdao University of Science and Technology, Qingdao 266042, P. R. China.
E-mail: [email protected], [email protected]
ABSTRACT:
The properties of electron transport layers play a crucial role in determining the performance of perovskite solar cells. Here we reported that graphdiyne doped in both PCBM and ZnO films of perovskite solar cells with an inverted structure based on MAPbI3 for the first time. A high efficiency of 20.0% was achieved in MAPbI3 perovskite solar cells. Moreover, the J-V hysteresis and stability were significantly improved as well. It was found that dual doping graphdiyne not only increased electrical conductivity, electron mobility, and charge extraction ability in the electron
transport layers but also improved film morphology of the electron transport layers and reduced charge recombination which contribute to fill factor improvement. Therefore the results indicate that dual doping graphdiyne is a promising strategy to enhance perovskite solar cells performance. Keywords: graphdiyne, dual doping, electron transport layers, MAPbI3, perovskite solar cells.
1. Introduction Hybrid organic-inorganic perovskite solar cells (PSCs) based on MAPbI3 have attracted considerable attention due to their high absorption coefficient, long holeelectron diffusion length, excellent carrier transport, low-cost, and easy fabrication process at relative low-temperatures. Great progress has been made in the field of PSCs based on MAPbI3, which lead to the power conversion efficiency (PCE) of PSCs exceeding 20% in few years.1-3 It is an effective way to enhance the performance of PSCs by improving the electrons transport properties of electron transport layers (ETLs). Therefore, a series of ways have been adopted to engineer the ETLs of PSCs, such as self-assembly monolayers, hybrids with conjugated molecules, doping ETLs as well as surface modification of ETLs.4-7 Among these approaches, it is a simple and effective method by adding dopant in ETLs of PSCs to further improve the performance of PSCs.8-10 However, the study of effects and understanding of doping in ETLs in the field of PSCs is still lacking. Therefore, the requirement to introduce new approach of doping ETLs as well as further
understanding the electron transport mechanism for improving PSCs performance is urgent. Graphdiyne (GD), a new allotropic form of carbon family, possesses rigid carbon network and highly π-conjugated structure consisting of sp- and sp2-hybridized carbons, a natural band gap and uniformly distributed pores.11-17 Therefore, it is supposed to have a promising application in photoelectrics, electronic, and geometric characteristics.18-25 As reported in the literature, GD has been introduced into PSCs as a dopant into P3HT working as hole-transporting materials (HTMs) and PCBM working as ETLs.26,27 And both of them suggested that GD may basically provide better percolation paths and improve the electron transport efficiency to the device due to the relatively strong π-π stacking interaction between GD particles and P3HT or PCBM. But the related researches are still lacking in the field of PSCs. In the present work, we introduce a novel approach to enhance the property of ETLs for the further improvement of PSCs performance by dual doping GD in both PCBM and ZnO films of PSCs for the first time. This way led to the PCE increased from 16.6% to 20.0% based on MAPbI3 active layer. Moreover, the devices based on dual doping GD exhibited a remarkable improved stability and J-V hysteresis. To figure out the effects of dual doping GD on PSCs devices, the surface morphology, photoluminescence properties, electron mobility, device stability, electrochemical impedance spectroscopy as well as exciton generation rate and dissociation probability have been investigated in details. 2. Experimental Section
The details of material and characterizations, device fabrications, and measurements are provided in the Supplementary material.
3. Results and Discussion
Figure 1(a) The p-i-n device architecture. (b) GD dispersion solution in chlorobenzene. (c) The chemical structures of GD used for device fabrication. SEM Top view SEM images
of
(d)
ITO/P3CT-K/perovskite/PCBM,
(e)
ITO/P3CT-K/
perovskite/PCBM(GD), (f) ITO/P3CT-K/perovskite/PCBM/ZnO, (g) ITO/P3CT-K/ perovskite/PCBM(GD)/ZnO, (h) ITO/P3CT-K/perovskite/PCBM/ZnO(GD) and (i) ITO/P3CT-K/perovskite/PCBM(GD)/ZnO(GD). The p-i-n device architecture and chemical structures of GD are presented in Figure 1 (a). GD powder was dispersed in chlorobenzene with a concentration of 1.5 mg/mL by ultrasonic dispersion shown in Figure 1(b). As can be seen, because of the
good lipophilicity of unmodified carbon materials, the GD black suspension is quite stable without precipitation in several hours after 60 h ultrasonic dispersion. It is beneficial for the dispersing of GD after been doped in the PCBM and ZnO solution. As we all know, the morphology of perovskite layer and interlayer played an important role in achieving high performance of PSCs. So SEM measurements were performed, and the results were shown in Figure 1. After the perovskite layer was coated with the PCBM or PCBM(GD) layer, the surfaces morphologies displayed slightly differences as shown in Figure 1(d) and Figure 1(e). The rough perovskite layer became smoother when coated by PCBM(GD) film than that coated by pure PCBM film shown in Figure 1(d) and Figure 1(e). The graphdiyne may also help to passivate the grain-boundary of the perovskite and then reduce the trap states of the surface to eliminate the recombination.28 And after the PCBM or PCBM(GD) layer were coated with the ZnO or ZnO(GD) layer, the surfaces morphologies displayed slightly differences as well, as shown in Figure (f), (g), (h) and (i). The rough PCBM layer became smoother when coated by ZnO(GD) film than that coated by pure ZnO film. Some nanoparticles still can be seen in Figure 1(d) and (e) which probably come from PCBM aggregates. The condition can be improved by coating ZnO or ZnO(GD) film.
Table 1. Parameters of Optimized Devices Based on Different Electron Transport Layers. PCE (%) ETL
Voc(V)
Jsc(mA/cm2)
FF (%)
PCBM/ZnO
Best
Average
1.04
21.34
74.8
16.59
16.30
1.05
24.06
79.0
20.00
19.60
PCBM(GD)/ ZnO(GD)
Figure 2 (a) J-V characteristic curves of PCBM/ZnO and PCBM(GD)/ZnO(GD) based perovskite solar cells under AM 1.5G 100 mW/cm2 simulated solar light. (b) Corresponding external quantum efficiency (EQE) spectra of the PCBM/ZnO and PCBM(GD)/ZnO(GD) based devices. The distribution of (c) Jsc and (d) PCE of devices based on PCBM/ZnO and PCBM(GD)/ZnO(GD) as the ETLs. Currentvoltage data from the devices of (e) ITO/Al/PCBM/Al, ITO/Al/PCBM(GD)/Al, (f) ITO/Al/ZnO/Al and ITO/Al/ZnO(GD)/Al, plotted in the format ln(JL3/V2) versus (V/L)0.5, where J is the current density and L is the thickness of the PCBM,
PCBM(GD), ZnO or ZnO(GD) layer. The lines are the fit to the respective experimental points. To figure out the impact of dual doping GD on the performance of the PSCs, the current density-voltage (J-V) characteristics obtained under AM 1.5G irradiation (100 mW cm-2) are depicted in Figure 2(a) and the detailed parameters are summarized in Table 1. As can be seen, the reference device based on pure PCBM/ZnO showed a Voc of 1.04 V, a short circuit current (Jsc) of 21.34 mA/cm2, a fill factor (FF) of 74.8% and a resulted PCE of 16.59% (average PCE of 16.30%). After doping GD into both PCBM and ZnO films, the device performances were improved significantly. The device based on PCBM(GD)/ZnO(GD) exhibited a Voc of 1.05 V, a Jsc of 24.06 mA/cm2, a FF of 79.0% and a corresponding PCE of 20.00% (average PCE of 19.60%). Corresponding external quantum efficiency (EQE) spectra of this device and the reference device were displayed in Figure 2(b). It can be seen that the EQE spectrum of the device with GD dual doping was obviously increased from the wavelength range of 300-800 nm as compared with PCBM/ZnO based device, which agreed with the Jsc obtained from the J-V results shown above. The distribution of Jsc and PCE of these devices are shown in Figure 2(c) and (d). As can be seen, the values of Jsc and PCE were increased significantly from PCBM/ZnO based devices to dual doping based devices. Moreover, the statistical data of 15 devices fabricated with PCBM/ZnO and PCBM(GD)/ZnO(GD) electron transport layers were presented in the form of a standard box plot, respectively, as shown in Figure S1. To further confirm the effect of GD on the performance of the PSCs, the devices based on only doping of GD in PCBM or ZnO were fabricated. As can be seen in table S1, after only doping GD into PCBM or ZnO, the device performances were improved as well. The device based on PCBM/ZnO(GD) exhibited a Voc of 1.04 V, a Jsc of 22.1 mA/cm2, a
FF of 76.9% and a corresponding PCE of 17.63% (average PCE of 17.35%) and the device based on PCBM(GD)/ZnO exhibited a Voc of 1.02 V, a Jsc of 23.3 mA/cm2, a FF of 78.4% and a corresponding PCE of 18.61% (average PCE of 18.37%). The current density-voltage (J-V) characteristics and distribution of different parameters of devices are shown in Figure S2 and Figure S3, which consistent with the previous conclusion. To figure out the origin of Jsc increment, the electron mobility was approximated using space charge limited current (SCLC) method in which device structures were ITO/Al/PCBM/Al, ITO/Al/PCBM(GD)/Al, ITO/Al/ZnO/Al and ITO/Al/ZnO(GD)/Al. The results are plotted as ln (JL3/V2) vs (V/L)
0.5
, as shown in Figure 2(e) and Figure
2(f). Electron mobility was calculated from the intercept of the corresponding lines on the axis of ln (JL3/V2). It is easy to work out that the electron mobility of pure PCBM and ZnO based electron-only device was 1.05×10-6 cm2 V-1 s-1 and 2.87×10-8 cm2 V-1 s1
. After GD doping, the electron mobility of devices increased to 1.90×10-4 and
2.12×10-6 cm2 V-1 s-1, respectively. The result exhibited the obvious improvement after the doping of GD on electron transport mobility.29
Figure 3(a) The photo- and dark-currents of the PCBM(GD)/ZnO(GD) based device. (b) Obtained from forward bias to short circuit (FB-SC) and from short circuit to forward bias (SC-FB). (c) Obtained at forward scan direction and different scanning rate with delaying time ranging from 50, 100, 200, 300, 400 to 500 ms. (d) Stability tests of PCBM/ZnO and PCBM(GD)/ZnO(GD) as the ETL. All of devices were stored in a nitrogen filled glovebox without encapsulation. The highest efficiency device with photo- and dark-currents was shown in Figure 3(a) has a Jsc of 24.06 mA/cm2, a Voc of 1.054 V, a FF of 79.0% and a PCE of 20.00%. The J-V curves presented in Figure 3(b) were determined for one of the most efficient cells, measured with a 50 ms scanning delay in both reverse-scan and forward-scan modes under standard air-mass 1.5 global (AM 1.5 G) illumination. The J-V curves show no appreciable hysteresis between the two modes. The Jsc, Voc, and FF values obtained from the J-V curve in the reverse-scan mode were 24.08 mA/cm2, 1.053 V,
and 75%, respectively, yielding a PCE of 19.05% under standard AM 1.5 G conditions. The corresponding values obtained from the J-V curve in the forward-scan mode were 24.06 mA/cm2, 1.054 V and 79%, respectively, yielding a similar efficiency of 20.00%. The steady-state photocurrent represents the actual power output and should be used to accurately characterize the device efficiency. As shown in Figure S4, the photocurrent rose quickly to maximum, and the photocurrent was stable. In addition, the J-V curves of PCBM(GD)/ZnO(GD) based devices at different scanning rates with delay time ranging from 50, 100, 200, 300, 400 to 500 ms were also checked, as shown in Figure 3(c). The J-V curves at different scanning rates almost overlapped with each other. These results indicated that the photocurrent hysteresis could be reduced to a certain extent by dual doping GD in PCBM and ZnO layers. The weaker photocurrent hysteresis in PCBM(GD)/ZnO(GD) based devices may be ascribed to the improved electrical properties. The lifetime measurement of devices were performed and exhibited the stability of PSC devices with different ETLs in Figure 3(d). All devices were stored and measured in the nitrogen-filled glove box without any encapsulation. As can be seen, the PCE of devices based on dual doping GD remained 86% after the storage of 30 days which was much better than the one without GD (remaining 21%), which indicated that dual doping GD is an effective way to improve the device stability. To further confirm the effect of GD doping on stability of the devices, the lifetime measurements of devices with only doping of GD in PCBM or ZnO were performed, as shown in Figure S5, we can observe that after doping GD in PCBM, the PCE of devices remained about 70% after the storage of more than 15 days and the PCE of devices based on only doping GD in ZnO remained 60% after the storage of 14 days, indicating that the only doping of GD significantly improve the device stability as well, which consistent with the previous
conclusion. GD doping into the PCBM would fill the electron traps originated from dimerization of PCBM and then alleviate the negative effect of PCBM dimerization.30 In addition, GD doping may enhance the fracture resistance of the PCBM, which lead to a more stable electron transport layer. Thus, the stabilization of devices is improved. On the other hand, the suppressed interfacial recombination could account for the superior stability in planar perovskite solar cells.31 GD doping suppressed the interfacial recombination significantly proved by following discussion such as maximum exciton generation rate and electrochemical impedance spectroscopy which also contribute to the device stabilization.
Figure 4(a) The photoluminescence (PL) spectra of various thin films. (b) Timeresolved photoluminescence of PCBM or PCBM(GD) as the interlayer and (c) timeresolved photoluminescence of ZnO or ZnO(GD) as the interlayer. (d) Plots of photocurrent density (Jph) with respect to the effective bias (Veff) in the devices with
PCBM/ZnO and PCBM(GD)/ZnO(GD) as the ETLs. (e) The impedance spectra of the PCBM/ZnO and PCBM(GD)/ZnO(GD) based devices (measured at 0.7 V bias voltage under dark condition), and (f) bode spectra of the PCBM/ZnO and PCBM(GD)/ZnO(GD) based devices. (measured at 0.7 V bias voltage under dark condition). (g) Rct, (h) τ, (i) capacitances obtained by EIS. In order to investigate if PCBM(GD) film efficiently extract photogenerated carriers from the perovskite absorber, PL spectra were measured as well, as shown in Figure 4(a). In our devices, P3CT-K, as hole transport layer, and PCBM(GD), as electron transport layer, can extract photogenerated carriers. The PL of P3CTK/pervoskite/PCBM, and P3CT-K/pervoskite/PCBM(GD) on ITO substrates was measured. The excitation light entered the sample from the glass substrate side with excitation at 600 nm, and PL spectra were collected. We observed a significant quenching effect when the perovskite layer was coated with either the PCBM layer or the PCBM(GD) layer. After doping the PL spectrum of perovskite film was quenched further. It indicated that the GD doping enhanced electron transport from perovskite to PCBM and reduce the electron accumulations usually occurred in the interlayer of perovskite device due to the relatively strong π-π stacking interaction between GD and PCBM so as to improve the performance of device.32-34 To further understand the electron transport processes, the time-resolved photoluminescence has been measured and the results are shown in Figure 4(b) and (c). We can yields the lifetime of carriers by fitting the data with two exponential decay curves. As a consequence, we can obtain that the carrier lifetime of pure PCBM and PCBM(GD) film were 25.95 and 3.11 ns. The shorter τave indicates more rapid electron transfer from perovskite to PCBM (GD) layer. When doping GD in ZnO films, the carrier lifetime drops from 45.00 to 3.20 ns, respectively, which exhibited
that the electron transport property of ZnO film was obviously improved. The faster charge transfer can lead to improved Jsc and PCE, consistent with the previous conclusions.35-38 To further understand the influence of the dual doping of GD on the PSCs devices, the maximum exciton generation rate (Gmax) and exciton dissociation probability (P) in the devices was plotted in Figure 4(d). As can be seen, the effect of the bilayer structure was illustrated on the photocurrent density (Jph) with respect to the effective voltage (Veff). Jph is describe as Jph = JL − JD, where JL and JD are the current density under illumination and in the dark, respectively. Veff is calculated as Veff = V0 – Va, where V0 is the voltage at which JL=JD and Va is the applied voltage In the low effective field region (Veff ＜ 0.1 V), the photocurrents in three devices increased sharply with effective voltage. At large reverse bias (Veff＞1 V), the Jph gradually reached a saturated value.39-41 From Figure 4(d), with the increase of effective
voltage,
the
saturation
photocurrent
(Jsat)
in
the
device
with
PCBM(GD)/ZnO(GD) reached earlier than that with PCBM/ZnO based devices. Generally, the saturated photocurrent correlated to the maximum exciton generation rate (Gmax), which is mainly determined by the light absorption, and is given by Jsat = qGmaxL, where q is the electronic charge and L is the thickness of active layer (300 nm). The values of Gmax for the PCBM/ZnO and PCBM(GD)/ZnO(GD) based devices are 1.45×1028 m-3s-1 and 2.24×1028 m-3 s-1 ,respectively. An enhancement of Gmax occurred after the dual doping of GD in PCBM and ZnO films. And it is noticed that the exciton dissociation probability (P) obtained from the normalized photocurrent density Jph/Jsat is obvious different among three devices, as shown in Figure 4(d). For example, at 0.2 V of Veff, P value is 68% for the device based on PCBM/ZnO only interlayer while it is 89% for the devices with the PCBM(GD)/ZnO(GD) interlayers,
respectively. The raise of P value means the reduction of the exciton recombination rate. The results suggests that Gmax was increased and the exciton recombinations was suppressed when dual doping GD in PCBM and ZnO films thus lead to better Jsc and FF values. To analyze the electron transport and interfacial properties in PSCs devices, electrochemical impedance spectroscopy (EIS) measurement was performed at 0.7 V bias voltage under dark condition. The EIS results of devices measured in dark are shown in Figure 4(e) and Figure 4(f). As we all know, in the Nyquist plots, the diameter of the semicircle presents the charge transfer resistance (Rct) related to the current leakage and recombination of electrons in ETLs as well as active layer. Larger Rct means the less current leakage and electron recombined occurred at the surface.42,43 As can be seen, the diameters of the semicircles increased significantly from PCBM/ZnO based device to PCBM(GD)/ZnO(GD) based device, which meant that after dual doping GD in PCBM and ZnO films, the interfacial contact between perovskite and ETLs was improved dramatically consequently reduced the current leakage and charge recombination occurred at interface.44-46 To further confirm the conclusion, EIS measurement under bias voltage from 0.6 V to 1.1 V (between anode and cathode) was performed under dark condition. As shown in Figure 4(g), the Rct decreases evidently as bias voltage increases. And the Rct of the PCBM(GD)/ZnO(GD) based devices is still higher than that of PCBM/ZnO at various bias voltages. The higher Rct for the PCBM(GD)/ZnO(GD) based devices compared with PCBM/ZnO is attributed to the less charge recombination occurred at interface. We think the main reason is that the way of dual doping GD passivated the surface defects of PCBM and ZnO films, which reduced the current leakage and charge recombination occurred at
interface. Therefore FF were enhanced after the dual doping of GD. The result is consistent with the improved FF shown in J-V characteristics. Bode EIS spectra performed at 0.7 V bias voltage under dark condition are shown in Figure 4(f) to determine the dependence of electron lifetime (τ) on PCBM/ZnO and PCBM(GD)/ZnO(GD) based devices. The electron lifetime (τ) can be obtained by the following equation:
τ=1/2πfp Where fp is the peak frequency corresponding to electrochemical reaction of chargetransfer process at PCBM/ZnO or PCBM(GD)/ZnO(GD) interface. As a consequently, we can obtain the values of τ for PCBM(GD)/ZnO(GD) and PCBM/ZnO based devices are 8.96μs and 6.09μs, respectively. To further confirm the conclusion, the τ of PCBM(GD)/ZnO(GD) and PCBM/ZnO based devices under bias voltage from 0.6 V to 1.1 V (between anode and cathode) were calculated. As shown in Figure 4(h), the τ of the PCBM(GD)/ZnO(GD) based devices is higher than that of PCBM/ZnO at various bias voltages as well, which can be attributed to the high electron density and the rapid electron transfer from PCBM(GD)/ZnO(GD) interface. Accordingly, it can be concluded that the dual doping of GD in PCBM and ZnO films is an effective way to accelerating electron transfer, and enhancing photovoltaic performance of PSCs, which consistented with the time-resolved photoluminescence and electron mobility measurements of PCBM and ZnO films. In order to explore the effect of photogenerated dipoles on the internal photovoltaic processes in PSCs, capacitance-voltage (C-V) measurement on PSCs with PCBM(GD)/ZnO(GD) and PCBM/ZnO as the ETLs. The C-V characteristics, measured under bias voltage from 0.6 V to 1.1 V under dark condition, can provide
the information on charge accumulation as well as geometry capacitance in solar cells. According to the literature, the Vpeak value can reflect the amount of charge accumulation at electrode interfaces. The larger and smaller Vpeak values correspond to more and less interfacial accumulation of photogenerated charge carriers at electrode interfaces,
respectively.
As
shown
in
Figure
4(i),
the
Vpeak
value
of
PCBM(GD)/ZnO(GD) based devices is smaller than that of PCBM/ZnO based devices.47,48 The results indicate that the device with PCBM(GD)/ZnO(GD) has less charge accumulations at the electrode interfaces, which can be explained that GD may basically provide better percolation paths and improve the electron transport efficiency to the device due to the relatively strong π-π stacking interaction between GD particles and PCBM or ZnO films. As we know, the less charge accumulations at the electrode interfaces can reduce the J-V hysteresis, which indicated that the photocurrent hysteresis could be reduced by introducing GD dual doping in PCBM and ZnO layers. The conclusion is consistented with the measure of the J-V curves with different scanning directions, as shown in Figure S6, J-V hysteresis is significantly reduced by the GD dual doping.
Figure 5 The schematic diagram of effect of GD dual doping. The mechanism of dual doping GD in PCBM and ZnO interfaces is proposed according to the aforementioned characterizations. Figure 5 showed that pristine PCBM and ZnO films have various surface defects which act as recombination centers for photogenerated charges so as to hamper the electron transfer and decrease fill factor of the device, which can be proved by time-resolved photoluminescence spectra, steady photoluminescence spectra and EIS. After GD dual doping, the surface defects are significantly passivated, which suppresses the charge recombination and increases the electron transfer rate as well as the charge transport mobility. All of these lead to an enhanced Jsc, and FF consequently result in the improvement of device performance for PSCs.
4. Conclusion In summary, we successfully dual doping GD in both PCBM and ZnO films to fabricate the highly efficient inverted PSCs. It was found that dual doping GD in ETLs obtained increased electrical conductivity, electron mobility and improved films morphology proved by SEM, SCLC, EIS, PL quenching measurements. Photoluminescence spectra revealed that dual doping GD can passivate the traps of PCBM and ZnO films, which decreases the carrier recombination at interface and consequently improves FF. Simutaneously, the devices based on dual doping GD exhibited a remarkable reduced J-V hysteresis. Capacitance-voltage measurement revealed that dual doping GD can reduced J-V hysteresis by decreasing charge accumulations at the electrode interfaces. Moreover the stability was improved as well. All of these demonstrated that dual doping GD is a promising strategy to increase PSCs performance and present large potential for optoelectronic applications as well. Acknowledgments The Project is supported by Natural Science Foundation of China (51672288), Youth Innovation Promotion Association of Chinese Academy of Sciences and Scientific Research Staring Foundation of Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences. This study was also supported by Major Basic Research Program of Shandong Natural Science Foundation (ZR2017ZB0313). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.nanoen.
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Graphical abstract
Highlights
1.
Graphdiyne was doped in both PCBM and ZnO films of perovskite solar cells.
2.
The J-V hysteresis and stability were significantly improved.
3.
Electrical conductivity, electron mobility and charge extraction ability were increased.
4.
Film morphology of the electron transport layers was improved.