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Graphdiyne-Doped P3CT‑K as an Efficient Hole-Transport Layer for MAPbI3 Perovskite Solar Cells Jiangsheng Li,† Min Zhao,† Chengjie Zhao,† Hongmei Jian,† Ning Wang,† Lili Yao,† Changshui Huang,† Yingjie Zhao,§ and Tonggang Jiu*,†,‡ †
Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT), Chinese Academy of Sciences, Qingdao 266101, P. R. China Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences, Dalian 116011, P. R. China § Qingdao University of Science and Technology, Qingdao 266042, P. R. China ‡
S Supporting Information *
ABSTRACT: Here we reported the doping of graphdiyne in P3CT-K in MAPbI3 perovskite solar cells as hole-transport materials. The doping could improve the surface wettability of P3CT-K, and the resulting perovskite morphology was improved with homogeneous coverage and reduced grain boundaries. Simultaneously, it increased the hole-extraction mobility and reduced the recombination as well as improved the performance of devices. Therefore, a high efficiency of 19.5% was achieved based on improved short-circuit current and fill factor. In addition, hysteresis of the J−V curve was also obviously reduced. This work paves the way for the development of highly efficient perovskite solar cells. KEYWORDS: graphdiyne, doping, P3CT-K, MAPbI3, perovskite solar cells cells have been reported.22,23 All of them indicated that GD could demonstrate a better route and enhance charge transport because of its highly π-conjugated structure. In this paper, we apply a new method to engineer a P3CT-K film with doping of GD to improve the performance of the PSCs. The PCE of devices with P3CT-K(GD) as HTLs was increased from 16.8% to 19.5%. Furthermore, the devices with P3CT-K(GD) as HTLs showed the negligible hysteresis of current density−voltage (J−V) curves. In order to explore the influence of GD doping on PSC devices, the surface morphology, photoluminescence properties, electrochemical impedance spectroscopy (EIS), exciton generation rate, and dissociation probability have been investigated in detail.
1. INTRODUCTION Perovskite solar cells (PSCs) with inverted architecture have received great attention for their advantages of excellent optoelectronic properties and solution processes at low temperature.1−3 In this device configuration, the bottom transparent electrodes are modified by interlayer materials, serving as hole-transport layers (HTLs), which is crucial for achieving high power conversion efficiency (PCE).4,5 Among the various materials of the HTLs, conjugated polyelectrolytes (CPEs), which include π-delocalized backbones with charged pendant groups, have been employed diffusely in PSCs because of their relatively good wettability and ease of processing at low temperature. P3CT as one of the most used layers in PSCs possesses excellent stability and relatively high hole mobility.6−8 As reported in the literature, the performance of PSCs based on P3CT can be enhanced by controlling the aggregation of the CPEs,9 whereas a relative study about engineering P3CT is still lacking in the field of PSCs. So, seeking a new method to engineer P3CT as a HTL is important to improve the performance of PSCs. Graphdiyne (GD), a 2D carbon material comprising benzene rings bonding together with diacetylenic linkages, has been successfully synthesized by Li’s group.10−14 Compared to graphene and other carbon materials, GD shows a characteristic electronic structure of sp- and sp2-hybridized carbons with a delocalized π system, which bestows GD with fascinating properties, such as excellent semiconducting properties, superior electrical properties, high stability, etc.15−21 So far, only a few works correlated with the application of GD in solar © XXXX American Chemical Society
2. RESULTS AND DISCUSSION The PSC architecture and the structure of GD are exhibited in Figure 1a. GD was dispersed in methanol by ultrasonication (0.5 mg mL−1). The dispersion process of GD is depicted in Figure 1b. As shown, after ultrasonication for 48 h, GD dispersion is very stable, and there is no sediment in a few hours. The solutions of P3CT-K and P3CT-K(GD) are Special Issue: Graphdiyne Materials: Preparation, Structure, and Function Received: February 12, 2018 Accepted: April 24, 2018
A
DOI: 10.1021/acsami.8b02611 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. (a) PSC architecture and the structure of GD. (b) Dispersion process of GD in methanol and solutions of P3CT-K without and with GD doping.
Figure 2. AFM (2 × 2 μm) topographical images for the film of (a) ITO/P3CT-K and (b) ITO/P3CT-K(GD). Contact angle (4:1 DMF/DMSO) images of (c) P3CT-K and (d) P3CT-K(GD) on the ITO substrates.
decreased from 12° to 4° when GD was doped into the P3CTK solution. The decrease of the contact angle would be beneficial to the flow and spread of the perovskite precursor [in a 4:1 (v/v) DMF/DMSO cosolvent] on the P3CT-K(GD) layer, which was in favor of the crystallinity of perovskite.24,25 Figure 3 exhibits the SEM images of the top morphology of perovskite films based on indium−tin oxide (ITO)/P3CT-K and ITO/P3CT-K(GD) substrates. As illustrated in Figure 3a,d, some pinholes exist on the perovskite film based on P3CT-K, while no pinholes are spotted on the samples based on P3CT-K(GD). When the magnification was increased to 50K, as shown in Figure 3b,e, the perovskite film based on P3CT-K(GD) was homogeneous and the grain boundaries became perpendicular to the substrate after GD doping, facilitating a reduction in the surface area of the grain boundaries. Next, when the magnification was increased to 100K, we could observe from Figure 3c,f that the samples based
exhibited in Figure 1b. It is observed that both of them are deep-orange and clear. Parts a and b of Figure 2 display the surface morphologies of P3CT-K and P3CT-K(GD) films obtained by atomic force microscopy (AFM). The root-mean-square (RMS) roughness values of the P3CT-K and P3CT-K(GD) films are 1.82 and 1.04 nm, respectively. Both of them present good film morphology. After the doping of GD, the RMS of the P3CTK film was reduced, which indicated that a smoother surface was obtained. The favorable morphology could improve the coverage of the transporting layer and interfacial contact, consequently reducing current leakage and increasing efficient hole transport. The surface wettability of the HTL plays a vital role in building the perovskite films. Therefore, the contact angle [4:1 N,N-dimethylformide (DMF)/dimethyl sulfoxide (DMSO)] measurement was performed to study the surface wettability. As displayed in Figure 2c,d, the contact angle B
DOI: 10.1021/acsami.8b02611 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. SEM images from the films of (a) ITO/HTL/perovskite (20K×), (b) ITO/HTL/perovskite (50K×), (c) ITO/HTL/perovskite (100K×), (d) ITO/HTL(GD)/perovskite (20K×), (e) ITO/HTL(GD)/perovskite (50K×), and (f) ITO/HTL(GD)/perovskite (100K×).
Figure 4. (a) J−V scan curves and (b) EQE spectrum of devices with P3CT-K and P3CT-K(GD) as HTLs. (c) J−V scan curves of hole-only devices based on P3CT-K or P3CT-K(GD) HTLs. The device structure is ITO/P3CT-K or P3CT-K(GD)/MoO3/Ag. (d) TRPL spectra of P3CT-K and P3CT-K(GD) as HTLs. (e) Jph−Veff curves for the devices with P3CT-K or P3CT-K(GD) as HTLs. (f) Jph/Jsat−Veff curves for the two devices.
16.80% (average 16.01%), an open-circuit voltage (Voc) of 1.05 V, a short-circuit current (Jsc) of 20.6 mA cm−2, and a fill factor (FF) of 77.9%. After the doping of GD into the P3CT-K films, the device showed a PCE of 19.50% (average 18.88%), Voc of 1.06 V, Jsc of 22.8 mA cm−2, and FF of 80.8%. The increases of Jsc and FF are mainly responsible for the improvement of the PSC performance. Also, the details of devices based on P3CTK(GD) with different concentrations of doped GD are reported in Table S1. The external quantum efficiency (EQE) spectra based on different HTLs are shown in Figure 4b. We can observe that there was a notable increment of the EQE, with a wide wavelength range in the P3CT-K(GD)-based devices. To understand the origin of the increment of Jsc, J−V curves of hole-only PSCs with structures of ITO/P3CT-K/MoO3/Ag and P3CT-K(GD)/MoO3/Ag were measured to test their holeextraction behaviors. It is observed from Figure 4c that the J−V curves of a GD-doped P3CT-K film exhibit higher current density at the same forward bias than the undoped one, indicating a better capability of hole extraction of P3CTK(GD).28,29
on P3CT-K(GD) showed a morphological enhancement by improving the coverage and uniformity of the perovskite films. Hence, it is proposed that the doping of GD can realize heterogeneous nucleation uniformly and facilitate the perovskite growth in a better way by improving the surface wettability of HTLs. As is known, the morphology of the perovskite layer plays a crucial role in determining the performance of PSCs.24−27 To understand the role of GD doping in PSCs, the J−V curves are presented in Figure 4a and the details are reported in Table 1. The reference device with P3CT-K showed a PCE of Table 1. Parameters of PSCs with P3CT-K or P3CT-K(GD) as HTLs PCE (%) −2
ETL
Voc (V)
Jsc(mA cm )
FF (%)
best
average
P3CT-K P3CT-K(GD)
1.05 1.06
20.6 22.8
77.9 80.8
16.80 19.50
16.01 18.88 C
DOI: 10.1021/acsami.8b02611 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. (a) Impedance spectra of PSCs based on P3CT-K and P3CT-K(GD). (b) Bode curves of the devices based on P3CT-K and P3CTK(GD). Plots of (c) Rrec, (d) τ, and (e) capacitances.
interface was suppressed after the doping of GD. As a result, it is beneficial for the improvement of FF. To understand the influence of GD doping on the charge lifetime (τ). Bode spectra was performed in the dark at 0.7 V bias voltage, as shown in Figure 5d. The charge lifetime (τ) was determined by the equation
To confirm hole-extraction processes, the measured TRPL spectra are exhibited in Figure 4d. The decay lifetimes of P3CT-K/perovskite and P3CT-K(GD)/perovskite were 18.4 and 10.8 ns, respectively. As is known, the shorter τave means quicker hole extraction from the active layer to HTLs.30−32 The results indicated a more effective charge extraction by P3CTK(GD) from the perovskite active layer, which is beneficial to improving Jsc and ultimately leading to the enhancement of PCE. The relationship between the photocurrent density (Jph) and effective voltage (Veff) for the devices with different HTLs was plotted to figure out the effect of GD doping on PSCs,33−36 as shown in Figure 4e,f. As shown in Figure 4e, Jsat of the device based on P3CT-K(GD) was reached earlier than that based on P3CT-K. We can obtain that the Gmax values of the PSCs based on P3CT-K and P3CT-K(GD) are 4.17 × 1027 and 4.58 × 1027 m−3 s−1. An improvement of Gmax can be observed with GD doping. It is noticed that the exciton dissociation probability (P) value of devices based on P3CT-K(GD) is obviously different from the one based on P3CT-K, as shown in Figure 4f. For instance, when Veff is 0.1 V, the value of P is 77% based on P3CT-K, while it is 84% for P3CT-K(GD)-based devices. The increase of P indicated a decrease of the exciton recombination rate. It suggests that the exciton recombination rate decreased after doping in P3CT-K. Therefore, Jsc and FF were improved. EIS was used to investigate the influence of GD doping on the charge-transport properties of PSCs. The EIS results measured at 0.7 V bias voltage under dark conditions are exhibited in Figure 5a,b. As shown in Figure 5a, the semicircular diameter of the device based on P3CT-K increased obviously compared to that based on P3CT-K(GD), which suggested that the doping of GD in P3CT-K dramatically reduced charge recombination occurring at the interface.36−40 Moreover, EIS measurement was carried out under bias voltages from 0.6 to 1.0 V in the dark. As exhibited in Figure 5c, an obvious decrease of Rrec was witnessed with the increases of the bias voltage. In addition, Rrec of the devices based on P3CT-K(GD) is larger than that with P3CT-K under bias voltages from 0.6 to 1.0 V, which indicated that the charge recombination occurring at the
τ=
1 πf 2 p
f p is the frequency of the peak. The charge lifetime (τ) values for the devices based on P3CT-K(GD) and P3CT-K are 5.0 and 1.1 μs, respectively. Furthermore, the τ of devices based on P3CT-K(GD) and P3CT-K at various bias voltages were calculated. As shown in Figure 5d, the τ based on P3CTK(GD) is still larger than that based on P3CT-K under bias voltage from 0.6 to 1.0 V; that is, a more rapid charge transfer is performed after the doping of GD. Therefore, it can be considered to be an efficient way to facilitate charge transfer, resulting in device performance enhancement by doping GD into P3CT-K films,36 which agrees with the measurements of the hole-extraction mobility and TRPL of P3CT-K(GD) films. The capacitance−voltage (C−V) measured under bias voltage from 0.6 to 1.0 V in the dark was performed to further study the effect of GD doping on internal electrical processes in PSC devices. As reported in the literature, the amount of charge accumulation at electrode interfaces can be reflected by the capacitance value.41,42 The more photogenerated charge carriers accumulate at the interfaces, the larger the capacitance value becomes. We can see from Figure 5e that the capacitance value of devices with P3CT-K(GD) is smaller than that with P3CT-K, which revealed that less charge accumulation exists on the interfaces of the electrode in the PSC devices based on P3CT-K(GD). It is proposed that GD could create faster paths so as to enhance the charge-transport efficiency of the devices because of the highly π-conjugated structure interaction with the P3CT-K layer. According to the literature,36 J−V hysteresis can be improved by decreasing the charge accumulations at the electrode interfaces. Figure S1 exihibits an improvement of the hysteresis of J−V curves via GD doping. D
DOI: 10.1021/acsami.8b02611 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 6. Standard box plot of device parameters based on P3CT-K and P3CT-K(GD): (a) Voc; (b) Jsc; (c) FF; (d) PCE.
ORCID
To investigate the reproducibility of the PSCs, the standard box plot of devices based on P3CT-K and P3CT-K(GD) as HTLs was performed. As exhibited in Figure 6, after the doping of GD, a remarkable enhancement of the values of Jsc, FF, and PCE was spotted from the devices based on P3CT-K compared with that based on P3CT-K(GD). This means that the experiment accidental errors can be excluded and the devices with P3CT-K(GD) as HTLs can demonstrate excellent reproducibility and favorable performance.
Changshui Huang: 0000-0001-5169-0855 Yingjie Zhao: 0000-0002-2668-3722 Tonggang Jiu: 0000-0001-9608-4429 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support of the Major Basic Research Program of Shandong Natural Science Foundation (Grant ZR2017ZB0313) and Natural Science Foundation of China (Grant 51672288) for this project is highly appreciated. We also extend our gratitude to DICP and QIBEBT (Grant UN201705), Dalian National Laboratory for Clean Energy and Youth Innovation Promotion Association of Chinese Academy of Sciences.
3. CONCLUSION In conclusion, we propose a method to engineer the P3CT-K film by doping GD for further enhancement of the PSC performance. This way could improve the surface wettability of HTLs, and the resulting perovskite morphology was remarkably improved with homogeneous coverage and reduced grain boundaries. The doping of GD in HTLs could also increase the hole-extraction mobility and reduce the recombination, consequently improving the performance of the devices. Simultaneously, the devices with the doping of GD demonstrated a significantly improved J−V hysteresis. This work indicated that it is a quite efficient and simple method to improve the performance of PSCs by the doping of GD and provided a promising strategy for improvements of the optoelectronic devices.
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4. EXPERIMENTAL SECTION The details of experiment materials, device fabrications, and measurements are given in the Supporting Information.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02611. Reverse and forward J−V scans of the devices and experimental section (PDF)
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REFERENCES
AUTHOR INFORMATION
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DOI: 10.1021/acsami.8b02611 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.8b02611 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
