Efficient and highly light stable planar perovskite solar cells with graphene quantum dots doped PCBM electron transport layer

Author’s Accepted Manuscript Efficient and Highly Light Stable Planar Perovskite Solar Cells with Graphene Quantum Dots Doped PCBM Electron Transport Layer Zhengrui Yang, Jiangsheng Xie, V. Arivazhagan, Ke Xiao, Yaping Qiang, Kun Huang, Ming Hu, Xuegong Yu, Deren Yang www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(17)30477-9 http://dx.doi.org/10.1016/j.nanoen.2017.08.008 NANOEN2122

To appear in: Nano Energy Received date: 20 June 2017 Revised date: 4 August 2017 Accepted date: 6 August 2017 Cite this article as: Zhengrui Yang, Jiangsheng Xie, V. Arivazhagan, Ke Xiao, Yaping Qiang, Kun Huang, Ming Hu, Xuegong Yu and Deren Yang, Efficient and Highly Light Stable Planar Perovskite Solar Cells with Graphene Quantum Dots Doped PCBM Electron Transport Layer, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.08.008 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.

Efficient and Highly Light Stable Planar Perovskite Solar Cells with Graphene Quantum Dots Doped PCBM Electron Transport Layer Zhengrui Yanga, Jiangsheng Xiea, V. Arivazhagana, Ke Xiaob, Yaping Qiangb, Kun Huanga, Ming Hua, Xuegong Yua*, Deren Yanga* a

State Key Laboratory of Silicon Materials and Department of Materials Science and

Engineering, Zhejiang University, Hangzhou 310027, China b

Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang

Sci-Tech University, Hangzhou 310018, China

: [email protected], [email protected]

*

Corresponding Author.

Abstract Organic-inorganic hybrid perovskite solar cells (PSCs) have triggered a great deal of research on organic electron transport layers, such as phenyl C61 butyric acid methyl ester (PCBM), due to their potential application as a strong contender in photovoltaic industry with simple fabrication process and low cost. However, the low electrical conductivity and electron mobility of PCBM hinder the promotion of PSCs. Here we report a successful case of graphene quantum dots (GQDs) doping into PCBM electron transport layer (ETL) of planar N-I-P PSCs, resulting in an obvious increase in PCBM conductivity together with the enhanced charge extraction and reduced the trap state density of perovskite films. A low doping ratio (0.5 wt%) would be efficient to boost the Voc, Jsc and FF, a PCE of 17.56% is achieved. More importantly, the light stability of PSCs with PCBM: GQDs was improved: the unpackaged cells can keep >80% of the initial PCE under simulated sunlight with the full UV component present after 300 h, in contrast to the reference device that dropped <50% during the same period of time.

Graphcial Abstract

Keywords: Perovskite solar cell, Light stability, Graphene quantum dots, PCBM, Electrical conductivity

Introduction Organic-inorganic hybrid perovskite solar cells (PSCs) play progressively more important roles in photovoltaic (PV) devices because of their high power conversion efficiency (PCE) and low fabrication cost [1,2,3,4,5]. Organic inorganic hybrid lead halide perovskite as a promising material shows some excellent properties, such as strong light harvesting ability, high carrier mobility and long diffusion length [6,7]. The PSCs are usually designed as N-I-P (regular) or P-I-N (inverted) structures, depending on the direction of transfer of electron [8]. The PCE of PSCs has been rapidly improved by dint of morphology control, device architecture optimization and

interface engineering. A typical N-I-P structure comprising of high temperature sintered (>450℃) mesoporous TiO2 (mp-TiO2) layer as electron transport layer (ETL) has achieved the top PCE of 22.1% [9]. However the high temperature processing is not favorable in the fabrication of devices and hampers the development of flexible modules [10]. To surmount these obstacles, researchers have focused on planar devices using low-temperature solution-processed transport layers. Metal oxide materials such as TiO2 [11], ZnO [12], Zn1-xMgxO [13], SnO2 [14], Zn2SnO4 [15] synthesized at low temperatures have been used as ETLs in planar PSCs. However most of the devices show hysteresis and low stability probably due to the undesirable charge accumulation/recombination at the ETL/perovskite interface. Recently, efficient and high stability PSCs have been achieved using a chlorine-capped TiO2 colloidal nanocrystal film via solution processing at low temperatures (<150°C) [2]. Exceeding 20% certified efficiency PSCs with almost free of hysteresis has been achieved using low-temperature solution-processed SnO2 nanoparticles as an ETL [16]. But the stability of these planar PSCs under the full spectrum of light has yet to be researched. Meanwhile, organic electron transport layers such as [6, 6]-phenyl C61 butyric acid methyl ester (PCBM [17]), fullerene (C60 [18]) and PCBDAN [19] have been successfully applied in N-I-P PSCs. These organic transport layers have attracted the most attention, owing to their simple and low-temperature fabrication process. It has been confirmed that fullerene and its derivatives can reduce the trap density and passivate grain boundaries of perovskite films, as a result, the photocurrent hysteresis is eliminated [20]. Besides, our previous studies have demonstrated that these devices also show higher UV-light stability in contrast to the devices with mp-TiO2 [21,22]. However the intrinsic low electrical conductivity and electron mobility of PCBM still hinder the promotion of PSCs [23]. One of the solutions is to use a doping technology with

some

additives

such

as

graphene

oxide

[24],

Graphdiyne

[25],

1H-benzoimidazole derivatives [23] and DMBI [26], which have been reported to improve electrical properties of PCBM, thus enhancing the performances of P-I-N PSCs. However an efficient dopant with low doping ratios in the PCBM for high

performance N-I-P PSCs is not yet well researched. We noticed that graphene quantum dots (GQDs) are single- or few-layer graphene but have a tiny size of only several nanometers with special quantum-confinement effects and edge effects, making them distinct from both conventional quantum dots and graphene [27]. It is well known that GQDs exhibit long electron lifetime and ultrafast electron extraction [28], and have been successfully employed as an interlayer between TiO2 and perovskite to enhance device performance [29,30]. In this work, we demonstrate for the first time the utilization of GQDs as an additive to PCBM ETL of planar forward PSCs. As a consequence, the addition of GQDs in the PCBM is not only dramatically to increase the PCE of PSCs, but also to improve the light stability of devices.

Results and Discussion The high resolution transmission electron microscopy (HRTEM) image of GQDs (Fig.1a) show the diameters of GQDs are approximately 5 nm, and the space between the lattice fringes are about 0.21 nm. The optical absorption of GQDs shows the band at ca. 400 nm (Fig. 1b). The average height of the GQDs is about 1.2 nm as observed from the AFM image and corresponding line profile (Figs. 1c and 1d), indicating that most of the GQDs are single layered or bi-layered [31]. We

fabricated

the

PSCs

devices

with

the

structure

ITO/PCBM/MAPbI3/Spiro-OMeTAD/Au using GQDs as a dopant in the PCBM (Fig. 2a). In order to certain the optimal doping concentration, the PCBM: GQDs ETL was prepared by adding GQDs with different mass percent (0, 0.1, 0.5 and 1.0 wt%) into the PCBM. The device with the doping concentration of 0.5 wt% shows the best performance, as shown in Fig. S1. Thus, further test and profiling was conducted in the devices incorporating the PCBM: GQDs with the optimum concentration of GQDs (0.5 wt%), in a straight comparison with the pristine one. Both the PCBM and PCBM: GQDs films deposited on ITO exhibit a low root-mean-square (RMS) roughness of 0.983 nm and 0.954 nm respectively, as shown in Fig. S2. A uniform

solar cell with a thin and legible PCBM: GQDs layer atop the perovskite layer can be observed from the cross sectional scanning electron microscopy (SEM) image (Fig. 2b). Fig. 3a shows the current density-voltage (J-V) curves of the best PSCs with doping the 0.5 wt% concentration of GQDs and without doping. It can be seen that the additive of GQDs in PCBM can enhance all the parameters of the devices. The Jsc increases from 20.57 to 22.03 mA cm-2, Voc from 1.058 to 1.093 V, and the FF from 68.05% to 73.14%. As a consequence, ≈20% enhancement in PCE from 14.68% to 17.56% was obtained (Table s1). Furthermore, the J-V curves of PSC with the PCBM: GQDs ETL measured under different scan directions (forward and reverse voltage scan) and various dwelling time (from 5 ms to 500 ms at 20 mv per step) show identical results (Fig. S3), indicating the absence of the hysteresis effect. To ensure the J-V measurement credible, steady state PCEs of the devices were measured (Fig. 3b). The device with the PCBM: GQDs reaches the maximum PCE instantly and stabilize at 17.5% for more than 100 s, much higher than the reference device that reaches the PCE at 14.2%. Fig. 3c shows the external quantum efficiency (EQE) spectra of the optimal PSCs with GQDs and reference one. The former shows a maximum EQE exceeding 90% in the range from 350 nm to 750 nm in contrast with the latter. The integrated EQE current densities of 21.3 and 19.8 mA cm-2 are consistent within 5% of the corresponding J-V measurements of 22.03 and 20.57 mA cm-2, respectively. It is well known that perovskite films show strong photoluminescence (PL) quenching upon combined with charge transport layer, owing to the efficient carrier transfer from the perovskite layer to the transport layer [32]. PL spectra of the perovskite films on glass, PCBM and PCBM: GQDs are shown in Fig. 4a. A greater extent PL quenching can be observed upon the PCBM: GQDs film, inferring that more effective chargers transfer from the perovskite to the ETL [33,34]. In order to study the charge-transfer kinetics from the perovskite to the ETL, the time-resolved photoluminescence (TRPL) was done, adopting the same structures as used for the PL measurements. All the curves shown in Fig. 4b are fitted with the following two-component exponential equation in accordance to the previous report [35]:

𝑓(𝑡) = 𝐴1 𝑒 −𝑡/𝜏1 + 𝐴2 𝑒 −𝑡/𝜏2 + 𝐵

(1)

Where A1, A2 are the decay amplitude, B is constant for the base-line offset and  is the decay time. The fast decay component 1 reflects the quenching of free carriers in the interface between the perovskite and ETL. The long decay component 2 could be due to recombination of free carriers in radiative channels [36]. The PL decay of the perovskite deposited on the PCBM: GQDs film exhibits a time constant of 1＝1.2 ns, whereas

1 ＝ 1.5 ns for the reference one (see Table s2 for the full fitting

parameters ). The above results demonstrate that the PCBM: GQDs have a higher PL quenching efficiency and quenching rate, in other words, doping GQDs into PCBM can strengthen the charge extraction ability at the ETL/perovskite interface. Therefore the Jsc and FF values of PSCs are obviously improved, due to the introduction of GQDs in PCBM. To better elucidate the origin of device performance and characterize the influence of GQDs-doping on the properties of PCBM, we made sandwich structure devices composed of ITO/ETL/Ag, with PCBM or PCBM: GQDs served as ETL. Fig. 4c shows the I-V characteristics of the devices. The direct current conductivity ( 0) can be calculated from the I-V plots, using the formula I=0Ad-1V, where d is the thickness of samples and A is the area of samples [20,24]. The calculation results revealed that the conductivity of PCBM exhibit a significant increase from 0.151 mS cm-1 to 0.422 mS cm-1 upon GQDs doping. The enhancement of conductivity due to the conductivity of GQDs is about 1.8 mS cm-1 which is one order of magnitude higher than PCBM [29]. Electron-only devices composed by ITO/ZnO/PCBM or PCBM: GQDs/Ag were also prepared to investigate the impact of GQDs on electron mobility. A significant increase of electron current density in PCBM: GQDs is associated with the improvement in the electron mobility of PCBM, as shown in Fig. 4d. The higher conductivity and electron mobility of PCBM: GQDs ETL validates the observed PL quenching, which originates from the enhanced charge extraction from perovskite towards the ETL. Dark current test is an effective way to assess charge carrier loss through the recombination and the leakage pathways. Thus, the J-V characteristics under dark

condition were measured, as shown in Fig. S4. The dark J-V curves show typical diode characteristics, with a leakage contribution at low voltages and an exponential regime at medium voltages, with a transition to the space-charge-limited regime. The slope of the dark J-V curves in the exponential region is steeper for PCBM: GQDs than for PCBM, delivering a lower saturation current density (J0) and lower ideality factor (), according to the Shockley diode equation [37,19]. It infers that the introduction of GQDs in PCBM can effectively reduce the charge recombination loss. Therefore the Voc and FF values of devices are significantly improved. Except low power conversion efficiency, the poor stability of PSCs is another barrier for the development of practical solid-state solar cells [38,39]. We therefore monitored the stability of devices under continuous solar illumination in the glove box without encapsulation. The devices performances were measured in air with ~ 45% humidity. The normalized PCE, Voc, Jsc and FF are plotted independently in Fig. 5 to reveal the discrepancy in the stability of both the devices clearly. The broken line of normalized Jsc, Voc versus time shows a small difference within 10%, but the change of normalized FF over time shows the prominent difference: the GQDs doped device shows a minor alteration, by contrast, the reference device degrades dramatically. Finally, for the GQDs doped device, the PCE maintains beyond 80% after 300 h of continuous solar illumination while the PCE of the reference device drops down to <50%. It is noteworthy to state that that the stability of PSCs were tested under A.M. 1.5 G as a light source that includes UV irradiation, meaning that our study also paves a way to overcome the ultraviolet light instability of PSCs. The stability of devices in air environment at 25℃ and 45% humidity were also tested for 30 days. As shown in Fig. S5, the plots of normalized parameters as a function of time show identical results, which confirm that the PCBM: GQDs based PSCs is more stable than the PCBM based PSCs. We believe that there are likely to be many factors that result in the enhanced stability. It is well known that molecule like PCBM and fullerene can dimerize due to light exposure causing a sever burn-in observed in the solar cell performance during aging [40,41]. One of the ways to conquer the dimerization is filling electron traps

with higher electron density materials. Snaith et al. recently showed less sever burn-in by suppressing dimerization in fullerene with n-doping, thereby enhancing the stability of planar PCSs [42]. Our results here could also be explained by this. The addition of GQDs in the PCBM would fill the electron traps originated from dimerization of PCBM upon light exposure and then alleviate the negative effect of PCBM dimerization. On top of that, the addition of GQDs may enhance the fracture resistance of the PCBM. Thus, it can be concluded that doping of PCBM with GQDs give rise to a more stable ETL, in turn to improve stabilization of PSCs.

Conclusion In summary, we have proposed an effective way to enhance the performance of planar PSCs with PCBM: GQDs electron transport layer. The GQDs additive provides a higher electron mobility and conductivity. We observed rapid and intense PL quenching from the perovskite layer coated on PCBM: GQDs layer, indicating the reduced charge recombination loss at the ETL/perovskite interface and the enhanced charge extraction. Due to these remarkable advancements, a device with 17.56% power conversion efficiency and free of hysteresis was fabricated with all the device component layers deposited at room temperature and treated below 100℃. More importantly, the device with PCBM: GQDs maintains about 80% of its original highest value under continuous full spectrum sunlight for a period of over 300 h and shows almost no attenuation in an air environment for about 30 days. We believe that the introduction of GQDs in PCBM is a simple and effective way for realizing and upscaling efficient and highly light stable planar PSCs.

Experimental Section Materials and equipment Chlorobenzene (CB, 99.8%), acetonitrile (99%) were purchased from Aladdin. Dimethyl

formamide

(DMF,

99%),

dimethylsulfoxide

(DMSO,

99.9%),

4-tert-butylpyridine (96%), bis (trifluoromethane) sulfonamide (lithium salt, 99.95%) were purchased from Sigma-Aldrich. PCBM (99%), Spiro-OMeTAD (99.8%), CH3NH3I (MAI, 99.5%), PbI2 (99.99%) were purchased from Xi’an Polymer Light Technology Corp. Graphene quantum dots (GQDs) was purchased from Nanjing XFNANO Materials Tech Co., Ltd. Diethyl ether was purchased from Sinopharm Chemical Reagent Co., Ltd. The performances of the devices were measured under AM 1.5 G condition at an illumination intensity of 100 mW cm–2 by a Keithley 2400 source meter with a solar simulator (94022A, Newport), calibrated by a standard Si solar cell (PVM937, Newport). The external quantum efficiency (EQE) of PSCs was measured by an EQE measurement system (Model QEX10, PV Measurements, Inc.) across a wavelength range of 300-850 nm. The photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra were measured with the PL spectrometer (Edinburgh Instruments, FLS 920). SEM imaging was carried out using S-4800 (Hitachi) field-emission scanning electron microscope. The morphologies of the films were measured by AFM, Dimension Edge, Bruker. The UV-Vis absorption was measured by a spectrophotometer (U-4100, Hitachi Limited). The high resolution transmission electron microscopy (HRTEM) images of GQDs were measured by FEI Titan G2 80-200 ChemiSTEM. The long-term stability measurements were carried out under AM 1.5 G condition at an illumination intensity of 100 mW cm–2 by CEL-HXUV300. Device fabrication The ITO glass was cleaned through a four-step cleaning process (deionized water, cleaning fluid, acetone and ethanol), and then dried by nitrogen gas. Before the deposition of the ETL, the ITO glass was treated with UV-Ozone for 20 min. The GQDs was dispersed in mixed solvents of water: isopropanol = 1: 10 with ~1 mg/mL concentration. Then different mass percent of GQDs (0%, 0.1%, 0.5%, and 1wt%) were added into the PCBM, having an initial concentration of 12.5 mg/ml and stirred for 12 h at room temperature. The ETL solutions were spin-coated at 1500 rpm for 35

s and the films were annealed at 100 ℃ for 10 min. The average thickness of the later was 40 nm. When the substrates were cooled down to the room temperature, The 159 mg of CH3NH3I, 461 mg of PbI2 and 78 mg of DMSO (molar ration 1:1:1) were mixed in 600 mg of DMF solution. The perovskite solution was then spin-coated onto ETL substrate by a spin coating process at 1000 rpm for 10 seconds and 5000 rpm for another 20 seconds. About 5 s after the initiation of the second spin coating step, diethyl ether (0.6 ml) was dropped onto the substrates. Then, the films were heated at 70 ℃ for 1 min and 100 ℃ for 10 min on a hotplate respectively. The substrates were again cooled down to room temperature. Afterward, HTL solution was spin-coated on the top of perovskite films at 3000 rpm for 35 s in a nitrogen filled glove box. The HTL solution was prepared by dissolving 72.25 mg Spiro-OMeTAD, 17.5 L lithium salt solution (520 mg/mL dissolved in acetonitrile) and 28.8 L 4-tert-Butylpyridine in 1 mL chlorobenzene. Finally, 100 nm of gold was deposited through a shadow mask by thermal evaporation. For all the devices, the active area is 4 mm2 and measured in air with ~ 45% humidity.

Author information

Present Addresses a

State Key Laboratory of Silicon Materials and Department of Materials Science and

Engineering, Zhejiang University, Hangzhou 310027, China b

Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang

Sci-Tech University, Hangzhou 310018, China

Supporting Information Supplementary data associated with this article can be found in the online version

Acknowledgements The authors would like to thank for financial support from the Natural Science Foundation of China (No. 61422404), Program for Innovative Research Team in University of Ministry of Education of China (IRT13R54) and Science Challenge Project (No. JCKY2016212A503).

Reference [1] Nie, W. Y.; Tsai, H. H.; Asadpour, R.; Blancon, J. C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; Wang, H. L.; Mohite, A. D. Science 347 (2015) 522-525. [2] Tan, H. R.; Jain, A.; Voznyy, O.; Lan, X. Z.; de Arquer, F. P. G.; Fan, J. Z.; Quintero-Bermudez, R.; Yuan, M. J.; Zhang, B.; Zhao, Y. C.; Fan, F. J.; Li, P. C.; Quan, L. N.; Zhao, Y. B.; Lu, Z. H.; Yang, Z. Y.; Hoogland, S.; Sargent, E. H. Science 355 (2017) 722-726. [3] Dong, Q. F.; Fang, Y. J.; Shao, Y. C.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. S. Science 347 (2015) 967-970. [4] Xie, F. X.; Zhang, D.; Su, H. M.; Ren, X. G.; Wong, K. S.; Gratzel, M.; Choy, W. C. H. Acs Nano 9 (2015) 639-646. [5] Filip, M. R.; Eperon, G. E.; Snaith, H. J.; Giustino, F. Nat Commun 5 (2014) 5757. [6] Xing, G. C.; Mathews, N.; Sun, S. Y.; Lim, S. S.; Lam, Y. M.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Science 342 (2013) 344-347. [7] Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J Am Chem Soc 131 (2009) 6050-6051. [8] Green, M. A.; Ho-Baillie, A.; Snaith, H. J. Nat Photonics 8 (2014) 506-514. [9] NREL, Best research-cell efficiencies www.nrel.gov/ncpv/images/efficiency_chart.jpg. (accessed 12.03.16) [10] Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Science 348 (2015) 1234-1237. [11] Wojciechowski, K.; Saliba, M.; Leijtens, T.; Abate, A.; Snaith, H. J. Energ Environ Sci 7 (2014) 1142-1147. [12] Liu, D. Y.; Kelly, T. L. Nat Photonics 8 (2014) 133-138. [13] Song, J. X.; Zheng, E. Q.; Liu, L. J.; Wang, X. F.; Chen, G.; Tian, W. J.; Miyasaka, T. Chemsuschem 9 (2016) 2640-2647. [14] Anaraki, E. H.; Kermanpur, A.; Steier, L.; Domanski, K.; Matsui, T.; Tress, W.; Saliba, M.; Abate, A.; Gratzel, M.; Hagfeldt, A.; Correa-Baena, J. P. Energ Environ Sci 9 (2016) 3128-3134. [15] Shin, S. S.; Yang, W. S.; Noh, J. H.; Suk, J. H.; Jeon, N. J.; Park, J. H.; Kim, J. S.; Seong, W. M.; Il Seok, S. Nat Commun 6 (2015) 7410. [16] Jiang, Q.; Zhang, L. Q.; Wang, H. L.; Yang, X. L.; Meng, J. H.; Liu, H.; Yin, Z. G.; Wu, J. L.; Zhang, X. W.; You, J. B. Nat Energy 2 (2016) 16177. [17] Kim, J. H.; Chueh, C. C.; Williams, S. T.; Jen, A. K. Y. Nanoscale 7 (2015) 17343-17349. [18] Wojciechowski, K.; Leijtens, T.; Siprova, S.; Schlueter, C.; Horantner, M. T.; Wang, J. T. W.; Li, C. Z.; Jen, A. K. Y.; Lee, T. L.; Snaith, H. J. J Phys Chem Lett 6 (2015) 2399-2405. [19] Xie, J. S.; Yu, X. G.; Sun, X.; Huang, J. B.; Zhang, Y. H.; Lei, M.; Huang, K.; Xu, D. K.; Tang, Z. G.; Cui, C.; Yang, D. R. Nano Energy 28 (2016) 330-337. [20] Shao, Y. H.; Xiao, Z. G.; Bi, C.; Yuan, Y. B.; Huang, J. S. Nat Commun 5 (2014)

5784. [21] Xie, J.; Yu, X.; Huang, J.; Sun, X.; Zhang, Y.; Yang, Z.; Lei, M.; Xu, L.; Tang, Z.; Cui, C.; Wang, P.; Yang, D. Advanced Science (2017) 1700018. [22] Zhang, Y.; Wang, P.; Yu, X.; Xie, J.; Sun, X.; Wang, H.; Huang, J.; Xu, L.; Cui, C.; Lei, M.; Yang, D. J. Mater. Chem. A 4 (2016) 18509-18515. [23] Bin, Z. Y.; Li, J. W.; Wang, L. D.; Duan, L. Energ Environ Sci 9 (2016) 3424-3428. [24] Kakavelakis, G.; Maksudov, T.; Konios, D.; Paradisanos, I.; Kioseoglou, G.; Stratakis, E.; Kymakis, E. Adv Energy Mater 7 (2017) 1602120. [25] Kuang, C. Y.; Tang, G.; Jiu, T. G.; Yang, H.; Liu, H. B.; Li, B. R.; Luo, W. N.; Li, X. D.; Zhang, W. J.; Lu, F. S.; Fang, J. F.; Li, Y. L. Nano Lett 15 (2015) 2756-2762. [26] Kim, S. S.; Bae, S.; Jo, W. H. Chem Commun 51 (2015) 17413-17416. [27] Yan, X.; Li, B. S.; Li, L. S. Accounts Chem Res 46 (2013) 2254-2262. [28] Williams, K. J.; Nelson, C. A.; Yan, X.; Li, L. S.; Zhu, X. Y. Acs Nano 7 (2013) 1388-1394. [29] Gao, P.; Ding, K.; Wang, Y.; Ruan, K. Q.; Diao, S. L.; Zhang, Q.; Sun, B. Q.; Jie, J. S. J Phys Chem C 118 (2014) 5164-5171. [30] Zhu, Z. L.; Ma, J. A.; Wang, Z. L.; Mu, C.; Fan, Z. T.; Du, L. L.; Bai, Y.; Fan, L. Z.; Yan, H.; Phillips, D. L.; Yang, S. H. J Am Chem Soc 136 (2014) 3760-3763. [31] Zhu, S. J.; Zhang, J. H.; Qiao, C. Y.; Tang, S. J.; Li, Y. F.; Yuan, W. J.; Li, B.; Tian, L.; Liu, F.; Hu, R.; Gao, H. N.; Wei, H. T.; Zhang, H.; Sun, H. C.; Yang, B. Chem Commun 47 (2011) 6858-6860. [32] You, J. B.; Meng, L.; Song, T. B.; Guo, T. F.; Yang, Y.; Chang, W. H.; Hong, Z. R.; Chen, H. J.; Zhou, H. P.; Chen, Q.; Liu, Y. S.; De Marco, N.; Yang, Y. Nat Nanotechnol 11 (2016) 75-81. [33] Zhu, Q. G.; Bao, X. C.; Yu, J. H.; Zhu, D. Q.; Qiu, M.; Yang, R. Q.; Dong, L. F. Acs Appl Mater Inter 8 (2016) 2652-2657. [34] Wu, Y. Z.; Yang, X. D.; Chen, W.; Yue, Y. F.; Cai, M. L.; Xie, F. X.; Bi, E. B.; Islam, A.; Han, L. Y. Nat Energy 1 (2016) 16148. [35] Pellet, N.; Gao, P.; Gregori, G.; Yang, T. Y.; Nazeeruddin, M. K.; Maier, J.; Gratzel, M. Angew Chem Int Edit 53 (2014) 3151-3157. [36] Ponseca, C. S.; Savenije, T. J.; Abdellah, M.; Zheng, K. B.; Yartsev, A.; Pascher, T.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A.; Wolf, J. P.; Sundstrom, V. J Am Chem Soc 136 (2014) 5189-5192. [37] Wetzelaer, G. J. A. H.; Scheepers, M.; Sempere, A. M.; Momblona, C.; Avila, J.; Bolink, H. J. Adv Mater 27 (2015) 1837-1841. [40] Yu, M.; Wang, Y.; Wang, H. Y.; Han, J.; Qin, Y. J.; Zhang, J. P.; Ai, X. C. Chem Phys Lett 662 (2016) 257-262. [38] Wang, D.; Wright, M.; Elumalai, N. K.; Uddin, A. Sol Energ Mat Sol C 147 (2016) 255-275. [39] Leijtens, T.; Eperon, G. E.; Pathak, S.; Abate, A.; Lee, M. M.; Snaith, H. J. Nat Commun 4 (2013) 2885. [40] Heumueller, T.; Mateker, W. R.; Distler, A.; Fritze, U. F.; Cheacharoen, R.; Nguyen, W. H.; Biele, M.; Salvador, M.; von Delius, M.; Egelhaaf, H. J.; McGehee, M. D.; Brabec, C. J. Energ Environ Sci 9 (2016) 247-256.

[41] Distler, A.; Sauermann, T.; Egelhaaf, H. J.; Rodman, S.; Waller, D.; Cheon, K. S.; Lee, M.; Drolet, N.; Guldi, D. M. Adv Energy Mater 4 (2014) 1300693. [42] Wang, Z.; McMeekin, D. P.; Sakai, N.; van Reenen, S.; Wojciechowski, K.; Patel, J. B.; Johnston, M. B.; Snaith, H. J. Adv Mater 29 (2017) 1604186.

Figure

Figure 1. (a) HRTEM image of GQDs (average size 5 nm); (b) UV-vis absorption spectrum of the GQDs; (c) 2D AFM image of the GQDs; (d) The height profile along the line in (c).

Figure 2. (a) Schematic device architecture of the fabricated planar perovskite solar cell; (b) Cross-section SEM image of the completed device, with PCBM: GQDs, CH3NH3PbI3, and Spiro-OMeTAD thicknesses of ~ 40, 430, and 200 nm respectively.

Figure 3. (a) J-V curves in forward bias and reverse bias for the best-performing PSC solar cell with the doping concentration of 0.5 wt% (PCBN:GQDs) and the reference one (PCBM); (b) Their photocurrent densities at the respective Vmp 0.915 and 0.818 V as a function of time; (c) Their external quantum efficiency and integrated short-circuit density curves.

Figure 4. (a) PL and (b) TRPL of CH3NH3PbI3 contacted with different interfaces: glass, PCBM, PCBM: GQDs; (c) I-V characteristics of the ITO/PCBM: GQDs/Au (red spheres) and ITO/PCBM/Au (black squares); (d) J-V curves of the electron-only devices with the structure of ITO/ZnO/PCBM or PCBM: GQDs/Ag.

Figure 5. Performances of the devices with PCBM and PCBM: GQDs ETL under continuous solar illumination in the glove box and measured in air (~45% humidity). Normalized a) PCE; b) Voc; c) Jsc; d) FF.

Figure S1. Photovoltaic metrics statistics of devices using the PCBM and PCBM: GQDs (0.1, 0.5, 1 wt%) as ETLs, respectively. a) PCE; b) Voc; c) Jsc; d) FF.

Figure S2. AFM images (size: 10x10 µm) of PCBM (a) and PCBM: GQDs (b) deposited on ITO. RMS is the root-mean-square roughness values which are estimated from the Nano-Scope Analysis software.

Figure S3. Current density-voltage curves of the typical PSC measured under different scan directions and scan rates.

Figure S4. Dark J–V characteristics of the devices with various ETLs.

Figure S5. Device performances of the devices with PCBM and PCBM: GQDs ETL in air environment at 25℃ and 45% humidity. Normalized a) PCE; b) Voc; c) Jsc; d) FF.

Table S1. Photovoltaic parameters of the PSCs with PCBM: GQDs and PCBM as ETLs.

Table S2. TRPL bi-exponential fitting results of CH3NH3PbI3 contacted with PCBM and PCBM: GQDs.

Photo and Biosketch

Zhengrui Yang obtained his Bachelor’s Degree in Metallurgy and Environment from Central South University in 2015. He is now a Master student at the Department of Materials Science and Engineering, Zhejiang University. His research focuses on organic –inorganic halide perovskite solar cells.

Jiangsheng Xie is a Ph.D. in State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University. He received his B.S. degree from University of Science and Technology Beijing. His current research focuses on organic–inorganic halide perovskites photovoltaic devices.

V. Arivazhagan is a postdoctoral researcher at State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University. He received his Ph.D. degree in Physics from Karunya University, India, in 2014. He worked at University of Bergen, Norway as a postdoctoral researcher for 2 years. He had been as Yggdrasil mobility fellow at Norwegian University of Science and Technology, Norway in 2012. His current research focusses on vacuum deposition of perovskite photovoltaic devices.

Ke Xiao received B.S. at Zhejiang Sci-Tech University in School of science (2011– 2015), M.S. at Zhejiang Sci-Tech University in School of Science (2015-present). His research interests include dye-sensitized solar cells and hybrid solar cells.

Yaping Qiang received B.S. at Shangqiu Normal University in College of Chemistry and Chemical Engineering (2011-2015),M.S. at Zhejiang Sci-Tech University in school of science (2015-present).Her research interests include 2D-pervoskite and perovskite light-emitting device.

Kun Huang received his undergraduate degree from Zhejiang University in 2014. Currently he is half way through his Ph.D. under the supervision of Prof. Xuegong Yu at State Key Laboratory of Silicon Materials, Zhejiang University. His research is mainly focused on growth of 2D materials and their photovoltaic devices.

Ming Hu is a master in State Key Laboratory of Silicon Materials and School of

Materials Science and Engineering, Zhejiang University, the same place where he got B.S. degree. His current research focuses on graphene-based photoelectric devices.

Xuegong Yu is a professor in Zhejiang University. He received his Ph.D. degree in Department of Material Science and Engineering at Zhejiang University in 2004. He had worked in North Carolina State University and Brandenburg Technology University for 5 years. His current research interest focuses on the fabrication and properties of optoelectronic device and related materials.

Deren Yang, a Cheung Kong Professor, is the director of the State Key Lab of Silicon Materials, and the director of the Institute for Semiconductor Materials at Zhejiang University in China. He received his Ph. D. degree in 1991 from the State Key Lab of Silicon materials at Zhejiang University, and then has worked there. In 1990s, he worked in Japan, Germany and Sweden for several years as a visiting researcher. He has engaged in the research of silicon materials used for microelectronic devices, solar cells and nano-devices.

Highlights

1. We demonstrate for the first time the utilization of GQDs as an additive to PCBM ETL of planar forward PSCs. 2. The device with PCBM: GQDs maintains about 80% of its original highest value under continuous full spectrum sunlight for a period of over 300 h. 3. The introduction of GQDs in PCBM resulted in a high efficient (17.56%) PSC with negligible hysteresis. 4. All the device component layers are deposited at room temperature and treated below 100℃.