Highly Flexible Self-Powered Organolead Trihalide Perovskite

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Highly-Flexible Self-Powered Organolead Trihalide Perovskite Photodetectors with Gold Nanowire Networks as Transparent Electrodes Chunxiong Bao, Weidong Zhu, Jie Yang, Faming Li, Shuai Gu, Yangrunqian Wang, Tao Yu, Jia Zhu, Yong Zhou, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08318 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 25, 2016

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ACS Applied Materials & Interfaces

Highly-Flexible Self-Powered Organolead Trihalide Perovskite Photodetectors with Gold Nanowire Networks as Transparent Electrodes Chunxiong Bao,†‡* Weidong Zhu,†‡ Jie Yang, †‡ Faming Li,†‡ Shuai Gu,†§ Yangrunqian Wang,†‡ Tao Yu,†‡ †

‖┴ *

Jia Zhu, †§ Yong Zhou†‡ and Zhigang Zou†‡

‖┴

National Laboratory of Solid State Microstructures, Nanjing University, Nanjing

210093, P. R. China ‡

Ecomaterials and Renewable Energy Research Center (ERERC), Department of

Physics, Nanjing University, Nanjing 210093, P. R. China §

Collage of Engineering and Applied Science, Nanjing University, Nanjing 210093,

P. R. China ‖

Collaborative Innovation Center of Advanced Microstructures, Nanjing University,

Nanjing 210093, P. R. China ┴

Jiangsu Key Laboratory for Nano Technology, Nanjing 210093, P. R. China

*Address correspondence to (C. Bao) [email protected], (T. Yu) [email protected] ABSTRACT Organolead trihalide perovskites (OTPs) such as CH3NH3PbI3 (MAPbI3) have attracted much attention as the absorbing layer in solar cells and photodetectors (PDs).

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Flexible OTP devices have also been developed. Transparent electrodes (TEs) with higher conductivity, stability and flexibility are necessary to improve the performance and flexibility of flexible OTP devices. In this work, patterned Au nanowire (AuNW) networks with high conductivity and stability are prepared and used as TEs in self-powered flexible MAPbI3 PDs. These flexible PDs show peak external quantum efficiency and responsivity of 60% and 321 mA/W, which are comparable to those of MAPbI3 PDs based on ITO TEs. The linear dynamic range and response time of the AuNW based flexible PDs reach ~84 dB and ~4 µs, respectively. Moreover, they show higher flexibility than ITO based devices, around 90%, and 60% of the initial photocurrent can be retained for the AuNW based flexible PDs when bent to radii of 2.5 and 1.5 mm. This work suggests a high-performance, highly flexible and stable TE for OTP flexible devices.

KEYWORDS: perovskite, photodetector, flexibility, metal nanowire network, transparent electrode.

1. Introduction Transparent electrodes (TEs) are essential components of many optoelectronic devices such as light-emitting diodes (LEDs), flat-panel displays, touchscreens, and solar cells.1 With the development of portable and wearable optoelectronic devices, there is a growing demand for high-performance TEs with light weight and high flexibility. Nowadays, Sn doped In2O3 (ITO) is the most commonly used commercial 2


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TE for its high conductivity and transmittance. However, the brittleness limits its applications in flexible devices.2-4 In recent years, conductive polymer,5,6 graphene,7,8 carbon nanotube (CNT)

9,10

and metal nanowire networks11-15 have been widely

studied as alternatives of ITO when used as flexible TEs. Among them, metal nanowire networks show tremendous competiveness due to their high conductivity, outstanding flexibility, and compatibility for large area and scalable fabrication. Organolead trihalide perovskite (OTP) (CH3NH3PbX3, X=Cl, Br or I) solar cells have attracted tremendous attention since its first report in 2009.16 Remarkable progress has been achieved, reaching 22.1% in the power conversion efficiency of OTP solar cells. 17 With outstanding optical absorption property, high carrier mobility, long carrier lifetime, ambipolar charge transport property and facile fabrication processes, OTPs not only show attractive prospect in solar cells, but also in a variety of other optoelectronic devices, such as LEDs, 18-20 lasers, 21,22 and photodetectors (PDs).

23-26

Meanwhile, the low-temperature solution processability makes OTP a

promising absorber in flexible device and suitable to low-cost roll-to-roll production. 27-29

Considerable efforts have been devoted to the fabrication of flexible OTP devices.

Some transparent conductive materials such as ITO, PEDOT:PSS and CNT have been introduced as TEs of the flexible OTP devices. 27,28,30 However, they seem unable to function well in flexible OTP devices. For example, the brittleness of ITO limits the flexibility of the devices. The relatively low conductivity of CNT and PEDOT:PSS limits the performance of devices, especially for large-area devices. Solution 3



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processed Ag nanowire (AgNW) network satisfies the requirements of both flexibility and conductivity, but they face obvious obstacles when used in OTP devices. Halogen atoms in OTP react with Ag and produce silver halides, which will significantly decrease the conductivity of Ag, and then significantly reduce the device’s stability. 31-33

The additional introduced interfacial blocking layer can partially overcome this

issue, yet it will significantly confine the flexibility of the devices. On the other hand, the roughness of the solution processed AgNW networks is usually greater than 100 nm, 34 which can reduce the quality of the subsequent layers and lead to the formation of leakage pathways in the device, thereby lead to poor device performance.

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Therefore, it is desirable but challenging to design excellent TEs for stable and efficient flexible OTP optoelectronic devices. In situ patterned metal nanowire networks possess the excellent conductivity and small surface roughness. Moreover, various other metals can be prepared into this kind of network besides Ag. 36 It suggests a possible route to realize the desirable TEs for flexible OTP optoelectronic devices. In this work, patterned Au nanowire (AuNW) networks with small surface roughness were prepared. We demonstrated that AuNW networks show good stability to CH3NH3PbI3 (MAPbI3) films. Furthermore, we prepared high-performance flexible MAPbI3 PDs based on the AuNW networks taking advantages of the networks’ small surface roughness, high flexibility and stability to MAPbI3.

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Figure 1. (a) Schematic of the fabrication process of patterned AuNW network transparent electrode via in situ etching method. (b) Photograph of a flexible AuNW transparent electrode. (c) SEM image of a patterned AuNW network on PEN substrate (PEN/AuNW). (d) Higher magnification SEM image of PEN/AuNW. (e) Transmittance spectra of PEN/AuNW with sheet resistances of 15 and 30 Ω sq-1 respectively, as well as ITO on PET (PET/ITO) with a sheet resistance of 12 Ω sq-1. AFM images of (f) bare AuNW network and (g) AuNW network covered with PEDOT:PSS layer. The cross-sectional profiles denoted by the dash line were shown in the right panels. The root mean square roughness (Rq) obtained from the AFM images are 20.3 nm for bare AuNW network and 7.8 nm for PEDOT:PSS covered AuNW network.

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2. Results and Discussion The patterned AuNW network was in situ fabricated on transparent substrate via etching method as schematically shown in Figure 1a. Briefly, polymer fiber network was fabricated on a metal frame and then transfered to the surface of a Au film deposited on a transparent substrate. AuNW network TE was obtained after the polymer fiber network covered Au film was ethched by Ar ions and the polymer fiber was removed. Figure 1b shows the conductivity of the AuNW network on flexible PEN substrate. Figure 1c and d show the lower and higher-magnification scanning electron microscope (SEM) images of the fabricated AuNW network. The lower-magnification one shows that the network is composed of ultra-long nanowires. The length of the nanowire is greater than the size of the view (~500 µm), which is much greater than the length of the reported solution processed AgNWs (typically less than 10 µm). 35 Actually, the length of the patterned nanowire can reach several to tens centimeters, which depends on the length of the electrospun polymer nanofiber of the mask and the size of the electrode. The higher-magnification SEM image in Figure 1d shows that the width of the nanowire is about 450 nm (the width distribution is shown in supporting information Figure S1), from which we can estimate the aspect ratio (a ratio of length to diameter) of the nanowire to be greater than 104, which is much larger than that of solution processed AgNW (100-800). 37 A larger aspect ratio is significantly conducive to the higher conductivity of the percolation network. 6


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Because the width of the AuNWs is much larger than that of solution processed AgNWs, in order to keep a high transmittance of the AuNW network, the gap between AuNWs should be smaller than that of solution processed AgNWs. The conductivity and transmittance of the as-prepared patterned metal nanowire networks with differnet wire gaps were demonstrated in our prevours work.36 Figure 1e shows the transmittance spectra of the AuNW networks-coated polyethylene naphthalate (PEN/AuNW) with different sheet resistances and the ITO-coated polyethylene terephthalate (PET/ITO) with a sheet resistance of 12 Ω sq-1 for comparison. For the PEN/AuNWs, a very high and broad plateau of transmittance in the visible range can be observed, which is different from that of the PET/ITO. That is because the transmitted light through the mesh of the network without selectively absorption. The low transmittance of the PEN/AuNW in UV range is attributed to the strong absorption of PEN substrate to UV light (Figure S2). In this work, the AuNW based devices were fabricated with PEN/AuNW TEs with performance (Rs~15 Ω sq-1, T~78%) comparable to that (Rs~12 Ω sq-1, T~80%) of PET/ITO TEs. The surface morphology and roughness of the AuNW networks were investigated with atomic force microscope (AFM), and the AFM images are presented in Figure 1f and g. Figure 1f is the surface morphology of the bare AuNW network, the height difference of the surface is around 50 nm, which depends on the thickness of the Au film. The root mean square roughness (Rq) of the bare AuNW network is around 20.3 7



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nm, which is much smaller than that (~110 nm) of solution processed AgNW network. 34

The height difference and Rq of the surface can be further reduced to 30 nm and 7.8

nm after spin coating a poly(3,4- ethylenedioxythiophene):polystyrenesulphonate (PEDOT:PSS) layer as hole conductor on the network (Figure 1g). A smooth surface of the TE is indispensable when used in OTP planar devices because a rough surface of the electrode would degenerate the quality of the HTM and perovskite films and then degenerate the performance of the device, which can be confirmed by the device yields of MAPbI3 PDs with 50 nm thick (97%) and 100 nm thick (0%) AuNW networks as TEs (supporting information, Table S1).

Figure 2. (a) Schematic of monitoring resistance change of the Ag and Au nanowire networks after CH3NH3PbI3 film deposition. (b) Resistance changes of bare and 8


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PEDOT:PSS covered Ag and Au nanowire networks after MAPbI3 film deposition. SEM image of the 60-day aged AgNW (c) and AuNW (d) after removing the MAPbI3 film.

Chemical durability to halogen contained in OTP is a key problem of TEs when used in OTP devices. The chemical durability was investigated by monitoring the resistance changes of the bare and PEDOT:PSS covered AuNW networks after the MAPbI3 film deposition on them (Figure 2a). For comparison, the resistance changes of the bare and PEDOT:PSS covered patterned AgNW networks were also measured. As shown in Figure 2b, the resistances of the bare and PEDOT:PSS covered AgNW network increased around 1000 times right after the MAPbI3 film was deposited, and the resistances increased further with the ageing time. The results indicate that the chemical durability of AgNW to the MAPbI3 film and its precursor solution is poor, which agrees with the results of the literature. 33 Nevertheless, the resistance of the bare AuNW network increased slightly (~15%) after the MAPbI3 film deposition and ageing for 60 days, indicating the good chemical durability of AuNW to the MAPbI3 film and its precursor solution. The stability of hole conductor-free perovskite solar cells with Au as electrodes can also confirm the good durability of Au to MAPbI3. 38 After spin coating a PEDOT:PSS layer, AuNW network showed even better chemical durability to MAPbI3. The resistance of PEDOT:PSS covered AuNW network displayed little change after MAPbI3 deposition and ageing for 60 days. The morphologies of the AgNW and AuNW network were characterized after ageing for 60 days with MAPbI3 film. Figure 2c and d show the SEM images of the 60-day 9



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ageing AgNW and AuNW network after dissolving the MAPbI3 film. It can be seen that the AgNW network was seriously corroded by the MAPbI3 film, almost no AgNW was left on the substrate after the MAPbI3 film was removed (Figure 2c), while the AuNW network can maintain comparable intact network morphology (Figure 2d), which confirm the good chemical durability of AuNW compared to AgNW. The good chemical durability of AuNW network to OTP makes it possible to be used as TEs in OTP devices.

Figure 3. (a) Schematic structure, (b) energy level diagram and (c) photograph of the flexible CH3NH3PbI3 photodetector based on PEN/AuNW transparent electrode. (d) Top-view SEM image of the CH3NH3PbI3 film fabricated on PEN/AuNW transparent electrode. (e) Cross-sectional SEM image of the flexible CH3NH3PbI3 photodetector based on PEN/AuNW transparent electrode.

As an example of the application of AuNW network in flexible OTP devices, we primitively fabricated the flexible perovskite PDs with the above AuNW networks. 10


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The device structure of the PDs is schematically shown in Figure 3a. In situ patterned AuNW network on PEN was used as the TE of the device. A PEDOT:PSS layer with a thickness of about 30 nm was spin-coated onto the AuNW network substrates as the hole transport layer. A 400 nm thick MAPbI3 perovskite layer was deposited as the absorber layer. The [6,6]- phenyl-C61-butyic acid methyl ester (PCBM) (~100 nm) and Al (~100 nm) layer were deposited as electron transport layer and contact electrode, respectively. Figure 3b shows the energy level diagram of the device. It can be seen that the suitable work function (5.1 eV) of Au makes the AuNW network electrode a good hole collecting electrode in the device. A photograph of the device shown in Figure 3c illustrates the bendability of the device. Figure 3d shows the top-view SEM image of the MAPbI3 film fabricated on the PEN/AuNW/PEDOT:PSS substrate, demonstrating the MAPbI3 film is pin-hole-free and possesses a flat-surface, which is significant to the device performance.The cross-sectional SEM image of the flexible device shown in Figure 3e confirms the thickness of each layer. Figure 4a shows the typical current density-voltage (J-V) curves for this device in dark and illuminated with monochromatic lights of different wavelength (300 nm, 700 nm and 800 nm). Owing to the charge-selective contacts, the dark current is small (6.5×10-4 mA cm-2 at -100 mV). When illuminated by the incident monochromatic lights, the current of the device at around 0 V bias increased significantly. Notably, the device exhibits remarkable response to the visible light (700 nm) but weaker response to UV (300 nm) and near IR (800 nm) light. Moreover, a short circuit 11



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current density (JSC) and open circuit voltage (VOC) can be obtained from the J-V curves of device under illumination, indicating that the device can also work as a photovoltaic device. That is because there exist built-in electric fields in the heterojunction of the device, which can force the photogenerated carriers in the absorber to be separated and collected by different electrodes. Then a very small (or even zero) bias voltage was needed to extract the carriers, that is to say, the PD can also be self-powered and work at 0 V bias.

Figure 4. (a) Current density-voltage (J-V) curves of the flexible MAPbI3 photodetector based on PEN/AuNW transparent electrode under dark condition and illuminated by monochrome light with different wavelengths. (b) External quantum efficiencies (EQE) and responsivity (Rλ) of flexible MAPbI3 photodetectors based on PEN/AuNW (PD1) and PET/ITO (PD2) transparent electrodes at 0 V bias. (c) Photocurrent density of the PEN/AuNW based flexible MAPbI3 photodetector under different incident light intensities measured at 0 V bias. The linear dynamic range (LDR) of the device is 84 dB. (d) Transient response at a pulse frequency of 30 kHz 12


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of the PEN/AuNW based flexible MAPbI3 photodetector (PD1) with an active area of 0.09 cm2 and a Si photodiode with an active area of 0.25 cm2.

Responsivity (Rλ) is one of the key figure-of-merits of PDs, which indicates how efficiently the PD responds to the incident light with different intensities: 39

= ⁄ = ∙ ⁄1240 /

(1)

Where Jph is the photocurrent density of the PD, Llight is the incident light intensity, EQE is the external quantum efficiency of the PD to the wavelength (λ, nm). The EQE and the Rλ of AuNW (PD1) and ITO (PD2) based flexible MAPbI3 PDs at 0 V bias versus wavelength are displayed in Figure 4b. Both of the two devices show high response to the light with wavelength ranging from 400 to 750 nm. The peak EQE of AuNW and ITO based devices are 60% and 65%, respectively. The higher peak EQE of ITO based device could be ascribed to the higher carrier collection efficiency of the continuous ITO film compared to that of the AuNW network. The AuNW based device (PD1) shows lower EQE at the shorter wavelength range (

Highly Flexible Self-Powered Organolead Trihalide Perovskite

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