New fullerene design enables efficient passivation of surface traps in high performance p-i-n heterojunction perovskite solar cells

Author’s Accepted Manuscript New Fullerene Design Enables Efficient Passivation of Surface Traps in High Performance p-i-n Heterojunction Perovskite Solar Cells Yue Xing, Chen Sun, Hin-Lap Yip, Guillermo C. Bazan, Fei Huang, Yong Cao www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(16)30123-9 http://dx.doi.org/10.1016/j.nanoen.2016.04.057 NANOEN1265

To appear in: Nano Energy Received date: 16 January 2016 Revised date: 11 March 2016 Accepted date: 28 April 2016 Cite this article as: Yue Xing, Chen Sun, Hin-Lap Yip, Guillermo C. Bazan, Fei Huang and Yong Cao, New Fullerene Design Enables Efficient Passivation of Surface Traps in High Performance p-i-n Heterojunction Perovskite Solar Cells, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.04.057 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.

New Fullerene Design Enables Efficient Passivation of Surface Traps in High Performance p-i-n Heterojunction Perovskite Solar Cells Yue Xinga,1, Chen Suna,1, Hin-Lap Yipa,*, Guillermo C. Bazana,b,**, Fei Huanga,*, and Yong Caoa a

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China b

Department of Chemistry and Biochemistry, Center for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106, USA

Abstract Defect states within perovskite crystals are thought to induce undesired charge recombination and photocurrent hysteresis in perovskite solar cells. Although the processing of perovskite films with electron-rich molecules that can efficiently passivate the surface traps, the exact mechanism remains unclear. As the electron-rich units are key components for such a function, a rigorous analysis using controlled electron density in passivators can provide the means to understand these underlying mechanisms and thereby improve future improvements. In the study reported here, we combined electron-rich functional groups with fullerenes to design a new series of hydrophilic fullerene derivatives, in which each phenyl group of the diphenylmethanofullerene (DPM) moiety was decorated with an oligoether (OE) side group. These new materials were introduced as alternative electron transport layers (ETLs) to replace the commonly used PCBM in p-i-n planar-heterojunction perovskite solar cells. Our tests indicate that electron-rich OE chains can both passivate perovskite trap states and reduce the work function of the metal cathode. By adjusting the numbers of OE chains, relevant properties such as the energy levels, charge carrier mobilities, surface energy and dipole layer features could be tuned at the interfaces. Furthermore, devices with these fullerene ETLs showed significant improvements in power conversion efficiency (PCE) compared to devices with PCBM ETLs. A high PCE of 16% was achieved by applying the monoadduct fullerene derivative C70-DPM-OE as the ETL of the device. Keywords: Perovskite solar cells; Traps passivation; Electron transport layers; Planar-heterojunction; Hydrophillic fullerene derivatives;

* Corresponding author. ** Corresponding author at: Department of Chemistry and Biochemistry, Center for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106, USA 1 These authors contributed equally to this work. E-mail address: [email protected] (F. Huang), [email protected] (H-L Yip), [email protected] (G. C. Bazan).

1. Introduction Organic-inorganic hybrid perovskites CH3NH3PbX3(X = Cl, Br, I) have emerged as a useful materials component in photovoltaic and optoelectronic applications due to their excellent characteristics, including high absorption coefficient, direct bandgap, long carrier lifetime, high and balanced hole and electron mobilities and tunable bandgaps [1-8]. These properties have enabled the quick rise in the power conversion efficiencies (PCEs) of perovskite solar cells (PVKSCs), which now exceed 20% [9]. Such rapid development and breakthroughs in enhancing the efficiency of the solar cells have been attributed in large part to the development of new interfacial materials [10-14] and to the careful optimization of device architecture and fabrication processes [15-23]. However, crystalline perovskites tend to contain a high density of defect states, and they exhibit a high degree of disorder. There remains considerable scope for further enhancement, as significant impurities and defects are introduced during the fabrication process, and debate continues on the mechanism by which traps impact solar cell function. Recent reports of trap states at the surfaces and grain boundaries of perovskite crystals suggest that these traps may act as non-radiative recombination centers, which lead to solar cell photocurrent hysteresis [24]. Planar semiconductor devices commonly require high-quality materials to achieve a higher performance. Therefore, understanding the defect chemistry of perovskite crystals and taking appropriate action to passivate electronic defects at the boundaries has been of central importance to the advancement of PVKSC device performance [25]. Snaith et al. proposed that excess iodine ions on the surfaces of perovskite crystals could be passivated using iodopentafluorobenzene [26]. Afterwards, the same team used organic Lewis basic thiophene and pyridine heterocycles to passivate the crystal surface halide vacancy, and observed significantly inhibited nonradiative decay within the treated perovskite films [27]. As an alternative approach, depositing [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) fullerene layers on perovskites can also effectively passivate charge trap states and eliminate photocurrent hysteresis. Based on these observations, it was proposed that the origin of the photocurrent hysteresis in planar heterojunction (PHJ) PVKSCs was due to the presence of a high-density charge [28]. Yet another effective way to evade this challenge is to improve the perovskite thin film morphology by introducing additives in the precursor solution, such as 1,8-diiodooctane (DIO) [29, 30], hydroiodic acid (HI) [31] or hypophosphorous acid (HPA) [32]. These additives appear to provide specific interactions with perovskites that reduce crystal defects and enhance film morphology [33, 34]. Recent work from our laboratories has also demonstrated that surface traps can be passivated using electron-rich amine functional groups in electron transport layers (ETLs). These functional groups provide passivation by donating lone-pair electrons to the halide vacancy and forming a chemical bond with the perovskite surface, thereby providing better contact at the interfaces of ETL/cathode and ETL/perovskite. Such improved contact can lead to significant improvements of performance in the PHJ architecture [35]. In addition to controlling crystal growth in the perovskite film, interfacial engineering is a relevant approach for improving the performance of PVKSCs [32, 36-39]. As defect states exist at the grain boundaries, the interfaces between the charge-selective layers and the perovskite active layer can thus be regarded as strategic locations for minimizing interfacial resistance losses and for the formation of ohmic contacts. Hole transport layer (HTL) materials have recently attracted attention as a means to improve the performance of PVKSCs. However, few new ETL materials have been reported. Metal oxides are n-type semiconductors, and are widely used as the ETLs in n-i-p type PVKSCs. The ETLs used to enable high performance in p-i-n type PHJ PVSCs have been mainly limited to fullerene derivatives such as PC61BM or PC71BM. These derivatives are useful due to their electron mobility, their suitable lowest unoccupied molecular orbital (LUMO) energy level, and their capacity to be processed at room temperature with an orthogonal solvent [40]. As mentioned above, fullerenes can effectively passivate the charge trap states of perovskite surfaces, but this effect is relatively weak. Treatment with polycrystalline perovskite retains the defects that hinder carrier generation and transport. A universal approach for improvement thus remains to be achieved [41].

Scheme 1. The configuration of PHJ PVKSC device and the chemical structures of new fullerene derivatives used in this study. Energy levels of C60-DPM-OE, C60-(DPM-OE)2, C60-DPM-OC10H21, C70-DPM-OE, C70-(DPM-OE)2, PC61BM and PC71BM calculated from the CV curves.

Taking all the issues discussed above into consideration, we designed and synthesized a new series of fullerene derivatives, namely C60-DPM-OE, C60-(DPM-OE)2, C70-DPM-OE and C70-(DPM-OE)2. In our Scheme 1, various phenyl groups of the diphenylmethanofullerene (DPM) moiety were decorated with oligoether (OE) chains, and used as ETLs in PHJ PVKSCs. By applying a controlled quantity of OE chains, a quantitative analysis of effects from increasing the electron density in the ETLs was expected to aid in the understanding of the underlying mechanisms of trap passivation. For comparison, another new compound was prepared, specifically C60-DPM-OC10H21, in which the OE chain was replaced with an alkyl chain of similar length. We incorporated the fullerene derivatives into p-i-n planar PVKSCs, and studied how these electron-rich ETLs impact planar p-i-n PVKSCs relative to ETLs fabricated with PC61BM and PC71BM. Moreover, we studied how the number of ether linkages modifies energy levels, mobilities, surface energy and dipole strength at the perovskite/ETL and ETL/metal cathode interfaces. The PCEs of the p-i-n PHJ PVKSCs based on these new fullerene derivatives were significantly higher than those of devices with PCBM ETLs. Devices based on monoadduct fullerene ETLs proved superior, with PCE up to 16%. For comparison, control devices based on PC71BM ETL showed a PCE of 13%. 2. Experimental section 2.1. Materials All of the chemical reagents, unless otherwise specified, were purchased from Alfa Aesar or Sigma-Aldrich, and were used as received. All of the solvents used were further purified prior to use. All reactions were carried out under a nitrogen or argon atmosphere. Unless otherwise noted, all materials were used as received, without further purification. Methylammonium iodide (CH3NH3I) was synthesized according to the recommended procedure [16]. To prepare the perovskite precursor solution, CH3NH3I, PbI2 and PbCl2 (Sigma-Aldrich) were mixed in anhydrous N, N-dimethylformamide (DMF) (Sigma-Aldrich) at a molar ratio of 4:1:1, respectively. The solutions were stirred at 60˚C overnight, and were then filtered using 0.45 μm PTFE

filters before being used for device fabrication. The concentration of the perovskite precursor solution was 40 wt%. Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) (Clevios PH), [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were purchased from Heraeus and Nano-C, respectively. 2.2. Instruments and Measurements 1

H and 13C-NMR spectra were acquired using a Bruker AVANCE 500 MHz spectrometer in a deuterated chloroform solution at 298˚K. Chemical shifts were reported as δ values (ppm) relative to an internal standard of tetramethylsilane (TMS). MALDI-TOF-MS was measured using a Bruker BIFLEXIII. Cyclic voltammetry (CV) experiments were conducted on a CHI600D electrochemical workstation with a platinum working electrode and a Pt wire counter electrode at a scanning rate of 50 mV s-1 against an Ag/Ag+ (0.1 M of AgNO3 in acetonitrile) reference electrode, with a nitrogen-saturated anhydrous solution of 0.1 mol L-1 tetrabutylammonium hexafluorophosphate in acetonitrile. The UV-vis absorption spectra were recorded on an HP 8453 spectrophotometer. 2.3. Synthesis of Fullerene Derivatives Scheme 2. Synthetic routes of C60-DPM-OE, C60-(DPM-OE)2, C60-DPM-OC10H21, C70-DPM-OE, C70-(DPM-OE)2a.

Reagents and conditions: (a) p-toluenesulfonyl chloride, triethylamine, THF, 0 ℃, then room temperature, overnight; (b) 4,4'-Dihydroxybenzophenone, K2CO3, DMF, 70 ℃, 24 h; (c) p-toluene-sulfonyl hydrazide, MeOH, reflux, 10 h; (d) MeONa, pyridine, 30 min; C60/C70, 75 ℃, 16 h; and then reflux 24 h. a

The procedure for the synthesis of the new fullerene derivatives is shown in Scheme 2, while the complete experimental details can be found in the Supporting Information. Compound 3 was the key intermediate to provide 1,3-dipoles for subsequent [3+2] cycloaddition reactions to C60/C70, and this compound was synthesized via a three-step procedure. The introduction of OE or alkyl chains onto C60/C70 was performed using [3+2] cycloaddition from Compound 3 to C60/C70, to yield the target compounds. 2.4. Device fabrication and characterization Indium tin oxide (ITO, 15 ohm/sq) glass substrates were cleaned sequentially under sonication for 20 min with acetone, detergent, deionized water and isopropyl alcohol. Then the substrates were dried at 80˚C in a baking oven overnight, followed by a plasma treatment for 4 min. PEDOT:PSS (filtered through a 0.45 μm PES filter) was spun onto ITO substrates, and was dried at 140˚C for 20 min in air. The CH3NH3PbI3-xClx precursor solution was spun onto the PEDOT:PSS layer to form a thin-film perovskite layer with a thickness of ~400 nm, and was then further annealed at 100˚C for 1 h. The PC61BM/PC71BM (25 mg/ml) or fullerene derivatives (25 mg/ml) were dissolved in chlorobenzene and then were spun-cast onto the perovskite layer at 1200 rpm for 30 s. Finally, a silver electrode (90 nm) was deposited by thermal evaporation through a shadow mask under a base pressure of 1 × 10-6 mbar, which defined an active cell area of 0.05 cm2. To improve the accuracy of measurement, the J–V curves for all devices were measured by masking the active area with a metal mask (with an area of 0.04 cm2). The device photocurrent was measured under an AM 1.5G solar simulator (Japan, SAN-EI, XES-40S1). The current density-voltage (J-V) characteristics for the devices were recorded with a Keithley 2400 source meter, and no poling or light soaking process was applied before the J-V measurement. The illumination intensity of the light source was calibrated before the tests by using a standard silicon solar cell with a KG5 filter, which was calibrated using a National Renewable Energy Laboratory (NREL) calibrated silicon photodiode, giving a value of 100 mW cm-2. All of the J-V curves reported in this study were measured under a reversed voltage bias. The hysteresis property was tested by sweeping the J-V measurement from both forward and reversed directions with a scan rate of 2 V s-1 (Supporting Information, Figure S7). Photoluminescence (PL) spectra were recorded on an FLS920 spectrofluorimeter (Edinburgh Instruments). A 150W, Ozone-free xenon arc lamp was used for the PL measurement. Spot sizes and excitation power were consistent and slit with resolution of 1 nm was used. Atom force microscopy (AFM) measurements were carried out using a Digital Instrumental DI Multimode Nanoscope IIIa in tapping mode. Measurements with a Kelvin Probe (Besocke Delta Phi, with a probe diameter of around 3 mm) were taken on three spots. Average values and standard deviations were generated from the measurements of these spots. A highly ordered pyrolytic graphite (HOPG) sample with a work function of 4.6 eV was used as the reference sample. 3. Results and discussion In principle, free electrons generated in the perovskite absorber in p-i-n PHJ PVKSCs can be extracted by the cathode under bias via the LUMO levels of fullerenes that serve as electron transport interlayers. Thus, the energy levels of the fullerene derivatives are important parameters that influence photovoltaic performance. In this study, the energy levels of the fullerene derivatives were determined by CV (summarized in Scheme 1), and the CV curves are shown in Figure 1. The valence band (VB) and conduction band (CB) values of the CH3NH3PbI3–xClx perovskite semiconductor were obtained from the literature [42]. LUMOs of these fullerene derivatives are below the CB of CH3NH3PbI3–xClx (-3.8 eV), thus making them suitable for electron extraction. In addition, the highest occupied molecular orbital (HOMO) energy levels are significantly deeper than the VB of the perovskite films (-5.3 eV), and should therefore be effective for blocking holes. However, a comparison of the single and double functionalizations showed that the monoadduct C60-DPM-OE has a higher LUMO energy level (-3.88 eV) than that of the bisadduct C60-(DPM-OE)2 (-3.99 eV), and the same trend can be observed between C70-DPM-OE and C70-(DPM-OE)2.

Double functionalization of fullerenes is often found to be an effective strategy for raising LUMO energy levels, because the second functionalization on the core structure of the mono-substituted C60/C70 can further reduce π-conjugation and electron delocalization in the fullerene cage [43-47]. Thus, bisadduct C60/C70 derivatives usually have higher LUMO energy levels than their corresponding monoadduct C60/C70 analogues. Interestingly, we obtained an inverse result with our two bisadduct fullerene derivatives. Both of these derivatives had lower LUMO energy levels than their corresponding monoadduct derivatives. Indeed, this result was not unexpected. According to previous work [48] the electron-rich OE side chains located in close vicinity of the fullerene can provide a stabilization effect to the electrons in the fullerene, and therefore can increase the electron affinity of the molecules. The increased number of OE side chains in the fullerene bisadduct further enhances the stabilization effect and leads to even lower LUMO energy. To confirm the proposed effect of the OE chains on the LUMO energy level, we synthesized C60-DPM-OC10H21 as a reference molecule, in which the OE chains were replaced by alkyl chains with the same chain length. It was found that the LUMO level of C60-DPM-OC10H21 (-3.81 eV) was 0.07 eV higher than that of C60-DPM-OE (-3.88 eV), which was in good agreement with our hypothesis and results reported in the literature [48].

Figure 1. Cyclic voltammetry curves of (a) C60-DPM-OE, C60-(DPM-OE)2, C60-DPM-OC10H21, PC61BM films and (b) C70-DPM-OE, C70-(DPM-OE)2, PC71BM films.

To determine the effect that varying the numbers of OE chains has on the charge carrier properties, we measured the electron mobilities of different ETLs using the space-charge-limited current (SCLC) model with electron-only devices (ITO/Al/ETL/Al), as shown in Figure 2 and Table 1 [49]. The PC71BM-based and PC61BM-based electron-only devices show reasonably high electron mobilities in the range of 10-3 cm2 V-1 s-1 (1.0 × 10-3, 1.3 × 10-3 cm2 V-1 s-1, respectively). With long peripheral OE or alkyl chains, the mobility decreases as the number of side chains increases. Among the monoadduct fullerene derivatives, the OE chains fullerene exhibit slightly better mobility than the alkyl chains-containing fullerene. Perhaps the OE chains yield better contact with the electrode, but the disparity is relatively small, as both results were within the same order of magnitude. The electron mobilities of the bisadduct fullerene derivatives C60-(DPM-OE)2 and C70-(DPM-OE)2 were determined to be 1.8 × 10-5 cm2 V-1 s-1 and 1.7 × 10-5 cm2 V-1 s-1, respectively. In these derivatives, the additional presence of “insulating” side groups seem to impede electron transport, as the larger number side chains limit charge transport.

Figure 2. J-V curves of electron-only devices with (a) C60 derivatives and (b) C70 derivatives. Table 1. Electron mobilities of different ETLs. ETL

Electron Mobility(cm2 V-1 s-1)

C60-DPM-OE

5.0x10-4

C60-(DPM-OE)2

1.8x10-5

PC61BM

1.3x10-3

C60-DPM-OC10H21

1.1x10-4

C70-DPM-OE

3.3x10-4

C70-(DPM-OE)2

1.7x10-5

PC71BM

1.0x10-3

The surface properties of the ETL fullerene derivatives are determined by their interfacial compatibility with perovskites. As the surface energy of a material is related to its polarity, crystallinity and surface interactions, a series of contact angle measurements were performed on individual fullerene films to investigate the influence by the different chemical structures [50]. Figure 3 shows an example, illustrating the water contact angles of C60 derivatives. Films of C60-DPM-OC10H21 and PC61BM exhibit contact angles greater than 90°, and the contact angles of the fullerene derivatives functionalized with OE chains fall by around 20°. These measurements revealed an increased polarity and a hydrophilic tendency caused by the OE chains. Moreover, the bisadduct was more polar and hydrophilic than the monoadduct fullerene. The ethylene glycol contact angles of C60 derivatives and the contact angles of two different solvents on C70 derivatives films exhibited a similar trend to that seen in the water contact angles of the C60 derivatives shown in Figure 3 (also see Figure S6 in the Supporting Information). The surface tensions (γtot) of these derivatives were calculated from contact angle data, and are summarized in Table 2. The γtot increases as the numbers of OE chains increases, and thus the γtot of the bisadducts are closer to those of CH3NH3PbI3-XClX [51]. These findings indicate that bisadducts have a polarity similar to that of the perovskite surfaces, which should allow them to permeate the perovskite bulk more deeply and to passivate the boundary traps of perovskite more efficiently. This effect is further discussed in the section on photovoltaic devices.

Figure 3. Water contact angle of the C60 derivatives films coated on ITO substrate. Table 2. Contact Angle Data of different ETLs and Perovksite Films. ETL

θ1(°) Water

θ2(°) Ethylene glycol

γtot(mN/m)

γd(mN/m)

γp(mN/m)

C60-DPM-OE

76.7

58.3

28.1

13.3

14.8

C60-(DPM-OE)2

65.2

52.1

36.7

8.2

28.4

PC61BM

90.7

72.0

20.2

13.0

7.2

C60-DPM-OC10H21

92.1

76.3

18.4

11.1

7.4

C70-DPM-OE

78.1

56.9

28.5

17.0

11.5

C70-(DPM-OE)2

63.4

50.9

34.7

29.8

4.9

PC71BM

91.1

71.6

20.7

13.5

7.2

Varying the numbers of OE chains incorporated into the new fullerene derivatives has a strong effect on the photovoltaic performance of the PVKSCs, which was evaluated by using the device structure of ITO/PEDOT:PSS (~40 nm)/CH3NH3PbI3-XClX(~400 nm)/ETL(~80 nm)/Ag (Scheme 1). Complete details for the device fabrication can be found in the Supporting Information. The resulting solar cell performances, measured under 100 mW cm-2 and air mass 1.5 global (AM 1.5 G) illumination, are summarized in Table 3. The current density-voltage (J-V) curves are shown in Figure 4. The PCEs of the p-i-n PHJ PVKSCs having C60-DPM-OE or C60-(DPM-OE)2 ETLs reached 15.5% and 13.8%, respectively. These PCEs were higher than the PCE of devices with PC61BM (12.4%). Notably, the Voc increased from 0.91 V to 0.96 V and from 0.91 V to 0.93 V, respectively, and the FF greatly improved from 66% to more than 75%. Similar to C60 derivatives, devices with C70-DPM-OE and C70-(DPM-OE)2 ETLs exhibited higher Voc and FF than those with PC71BM ETLs. When applying C70-DPM-OE as the ETL of the device, a maximum PCE of 16.0% has been achieved, which is over 23% higher than that of devices containing PC71BM ETLs. Obviously, the introduction of the OE chains to ETLs is capable of improving all solar cell parameters, particularly in the case of monoadduct fullerenes.

Figure 4. Current density–voltage (J-V) curves for the best-performing solar cells with (a) C60 derivatives and (b) C70 derivatives ETLs. Table 3. Photovoltaic parameters of PHJ perovskite solar cells with different ETLs.

a)

ETL

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

Rs (Ω cm2)

Rp (Ω cm2)

C60-DPM-OE

0.96a) (0.95±0.02)b)

21.4 (20.8±0.8)

0.76 (0.74±0.02)

15.5 (14.0±1.5)

5.7

1129.3

C60-(DPM-OE)2

0.93 (0.92±0.01)

20.7 (20.0±1.0)

0.71 (0.70±0.02)

13.8 (12.2±1.6)

8.7

1033.4

PC61BM

0.91 (0.90±0.01)

20.6 (19.8±0.9)

0.66 (0.65±0.03)

12.4 (11.1±1.3)

9.1

853.9

C60-DPM-OC10H21

0.90 (0.88±0.02)

19.9 (19.0±0.9)

0.60 (0.58±0.04)

10.8 (9.3±1.5)

10.7

614.1

C70-DPM-OE

0.97 (0.96±0.02)

21.9 (21.0±0.9)

0.75 (0.73±0.02)

16.0 (14.6±1.4)

5.6

1218.1

C70-(DPM-OE)2

0.94 (0.92±0.01)

21.0 (20.0±1.0)

0.71 (0.70±0.02)

14.0 (12.4±1.6)

7.8

1074.2

PC71BM

0.92 (0.90±0.02)

21.0 (20.2±0.8)

0.67 (0.65±0.04)

13.0 (11.8±1.2)

8.7

828.9

Best device; b) Average values calculated over 10 devices.

Monoadduct C60/C70 derivatives have slightly higher LUMO energy levels; a feature that should make them effective for increasing the Voc of solar cell devices. Indeed, as shown in Table 3, we find that the Voc of devices containing monoadducts are higher than those obtained using the corresponding bisadducts. A higher LUMO level of the ETL yields better energy-level alignment with the CB of the perovskite, and it can therefore reduce photovoltage losses. Our tests also show that the Jsc and the FF of monoadduct C60/C70 derivatives are higher than those of bisadduct cells. We attributed this result to the higher electron mobilities of monoadduct fullerenes. This characteristic of monoadduct fullerenes also result in a decreased series resistance (Rs). However, the Jsc and the FF of PC61BM and C60-DPM-OC10H21 were both lower than those of monoadduct C60-DPM-OE, despite their high electron mobilities. Furthermore, when compared with PC61BM/PC71BM, the two bisadduct fullerene derivatives show lower electron mobilities (on the order of 10-5 cm2 V-1 s-1), but the Jsc of the bisadducts are on the same order, or even little higher, and the FF of the bisadducts are larger. These results are attributed to the effect of the OE chains, which contain electron lone pairs in the oxygen atoms. They may donate electrons to the halide vacancy and form a coordinative or dative-covalent bond with the Pb atom on the perovskite surface, then passivate the surface traps of perovskite to improve the electron extraction properties at the perovskite/ETL interface. The OE chains also proved able to facilitate ohmic contact formation at the cathode interface (as is further discussed subsequently) leading to the increase of Jsc and FF. The external quantum efficiency (EQE) spectra for the solar cells also show that the monoadduct-based devices have higher overall photon-to-electron conversion efficiency (Supporting Information, Figure S5), in good agreement with the observed trend of Jsc values. These results suggest that the incorporation of OE containing fullerene interlayers between the metal electrodes and perovskite layers passivates interfacial trap states, minimizes charge recombination losses at the perovskite/ETL interface, and improves charge transfer from the perovskite layers to the metal electrodes, thus resulting in superior PCEs.

To elucidate the reasons behind this performance enhancement and further study the trap filling effect, a complementary set of experiments was conducted. The experiments included steady-state photoluminescence (PL) measurement, time-resolved photoluminescence (TRPL) decay measurement and Kelvin probe measurement. These various measurements provided a more complete analysis of the underlying mechanism of trap passivation and its effects on device efficiency. TRPL profiles of perovskite without and with different ETLs are illustrated in Figures 5a and 5b. Samples were pumped using picosecond laser pulses with a wavelength at 406 nm at a repetition rate of 200 kHz, and were probed at 770 nm. PL lifetimes were calculated by fitting the exponential decay with double exponential functions. This analysis revealed the presence of two distinct processes of carrier generation and recombination. The charge population bearing a short lifetime (τ1) was attributable to two possible processes, namely the nonradiative charge recombination within the perovskite layer and the charge transfer from the perovskite to the ETL quenchers. The long decay component (τ2) was associated with radiative recombination within the perovskite layers, which included the trap-assisted recombinations [27, 52, 53]. Our results reveal that the PL lifetime τ1 is similar in all of the cases with lifetimes < 10 ns. However, the τ2 values of all the ETL-coated perovskite films were found to decrease significantly, from 380 ns with the pristine perovskite film to lower than 100 ns, as is shown in Table 4. The τ2 values for the samples having fullerenes with OE chains are all longer than those having fullerene derivatives without OE chains. The longest τ2 values of 91.5 ns and 99.4 ns were found in the C70 and C60 bisadducts, respectively. We attributed the increased τ2 values to the more effective passivation of surface trap states of the perovskite films when the OE-containing fullerenes were used. To verify this hypothesis, we conducted steady-state PL measurements and studied the PL peaks from the perovskite films. We did so because it is well known that trap filling enables the recovery of the bandgap and blue-shifts the PL peak of perovskite films [28, 35]. Figures 5c and 5d show the PL characteristics of the C60- and C70-coated perovskite films, respectively. These figures clearly show that fullerene derivatives containing OE chains passivate trap states close to the top surface of the perovskite film, as was confirmed by the blue-shift of the PL emission.

Figure 5. Time-resolved PL decay transients measured at 770 nm for (a) C 60 derivatives films and (b) C70 derivatives films after excitation at 406 nm. The solid lines are the double-exponential fits of the PL decay transients. Photoluminescence spectra of (c) perovskite/C 60 derivatives films and (d) perovskite/C70 derivatives films excited by a 406 nm light source from the air side.

Table 4. PL lifetimes of different ETLs-coated perovskite films. ETL

PL lifetimes (ns)

C60-DPM-OE

56.5

C60-(DPM-OE)2

99.4

PC61BM

51.1

C60-DPM-OC10H21

43.4

C70-DPM-OE

50.5

C70-(DPM-OE)2

91.5

PC71BM

45.8

An excellent ETL in the PVKSC may modify not only the ETL/perovskite interface, but also the ETL/cathode interface. It is known that electron-rich groups such as amine can form interfacial dipoles with metal surfaces and alter the work function (WF), and that the OE groups also have this capability. In our study, the WF of the pristine Ag film was -4.72 eV, as measured by a Kelvin probe force microscope. As shown in Table 5, the result of the Kelvin probe measurement indicated that the WF of Ag could be decreased when it was adjacent to these fullerene derivatives. Fullerene derivatives functionalized with OE chains were able to shift the WF of Ag to a greater extent than fullerene derivatives without OE chains. However, monoadduct fullerene derivatives reduced the WF of Ag more than the bisadducts did. This result may be due to the increase of dipole-dipole repulsion after the incorporation of an excess of OE chains, or it may be related to the number of OE side chains, as crowded side chains may make the molecules more bulky. In that situation, the number of fullerenes in touch with the Ag cathode would be smaller, thus reducing the interfacial dipole effect. In addition, the lower WF of Ag modified with monoadducts resulted in devices with a higher Voc than that of devices having Ag modified with bisadducts. Table 5. Scanning Kelvin probe microscopy (SKPM) of different ETLs treated Ag electrodes. ETL

Work Function

Ag

-4.72 eV

Ag/C60-DPM-OE

-4.20 eV

Ag/C60-(DPM-OE)2

-4.37 eV

Ag/PC61BM

-4.45 eV

Ag/C60-DPM-OC10H21

-4.53 eV

Ag/C70-DPM-OE

-4.27 eV

Ag/C70-(DPM-OE)2

-4.42 eV

Ag/PC71BM

-4.47 eV

4. Conclusion In summary, we have developed a new series of fullerene derivatives that are functionalized with OE chains and which serve as efficient ETLs. These new ETLs show potential for replacing the PC61BM/PC71BM layers used in p-i-n planar-heterojunction PVKSCs. The interposition of the OE chains produced several desirable effects, including the enhancement of interfacial charge transport efficiency, the modification of the WF of cathodes and the passivation of interfacial trap states at the perovskite surface or gain boundaries. All of these effects combine to result in higher PCEs. Devices based on C60/C70 derivative ETLs with OE chains show significant improvement in performance compared to control devices based on PCBM ETLs or monoadduct C60 derivatives without OE chains. Among all of these ETL materials, the device with C70-DPM-OE as the ETL had the most efficient PCE of 16.0%, which was over 23% higher than that of the device with a PC71BM ETL. Furthermore, analysis using controlled electron density in ETLs was carried out to examine the underlying mechanism of passivation. We found that the passivation effect should be considered through a

comprehensive analysis related to several parameters, including energy levels, carrier mobilities, surface energy and dipole intensity. In summary, our work provides a new series of interfacial fullerene materials that can enhance the performance of PVKSCs through methods that are simple to apply. The results suggest a design strategy for the development of high-performance fullerene derivatives for use as ETLs in PVKSC applications. We expect that further refinement should be possible with respect to the functional groups in the solubilizing side chains and demension of the overall molecule in order to concurrently improve several device properties in PVKSCs. Acknowledgements The work was financially supported by the Ministry of Science and Technology (No. 2014CB643500), the Natural Science Foundation of China (No. 21520102006, 51521002, 51323003 and 51361165301) and Guangdong Natural Science Foundation (Grant No. S2012030006232). References [1] [2] [3] [4] [5] [6] [7]

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Yue Xing received her BS degree from Dalian University of Technology in 2011. Currently, she is a Ph.D. candidate in the Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology. Her research interest focuses on the design and synthesis of carbon and organic functional materials for organic solar cells and perovskite solar cells.

Chen Sun is a graduate student in Materials Science working in South China University of Technology under the supervision of Prof. Fei Huang since 2013. Her research interests are the device and physics of high efficiency perovskite and polymer solar cells.

Hin-Lap Yip received his BS and MS degree in Materials Science from Chinese University of Hong Kong in 2001 and 2003, respectively. He received his PhD degree in Materials Science and Engineering from University of Washington in 2008 under the supervision of Prof. Alex Jen. After postdoctoral work at University of Washington with Prof. Alex K.-Y. Jen, he began his academic career in 2013 as a professor of South China University of Technology. His research focuses on an integrated approach combining materials, interface, device, and process engineering to improve the organic solar cells and perovskite solar cells.

Guillermo C. Bazan obtained his BS degree from the University of Ottawa and Ph.D. from the Massachusetts Institute of Technology under the advisement of Professor Richard R. Schrock. After working at the California Institute of Technology as a postdoctoral fellow with advisor Professor John E. Bercaw, he started his academic career at the University of Rochester in 1992. In 1998, he became a Professor of the University of California, Santa Barbara. His research focuses on the molecular design and synthesis of innovative, functional materials for applications in organic electronics and bioelectrochemical devices.

Fei Huang received his BS degree in Chemistry from Peking University in 2000 and gained his PhD degree in Materials Science from the South China University of Technology in 2005 under the supervision of Prof.

Yong Cao. After postdoctoral work at University of Washington with Prof. Alex K.-Y. Jen, he began his academic career in 2009 as a professor of South China University of Technology. His main interests are in the fields of organic functional materials and devices for opto-electronics.

Yong Cao, physical chemist, Professor of South China University of Technology since 1999. BS from Department of Chemistry, Leningrad (now Saint Petersburg) University, PhD degree from Tokyo University, Japan. He was visiting senior researcher at University of California, Santa Barbara in 1988–1990 and senior scientist of UNIAX Corporation in 1990–1998. Current research interest: polymer optoelectronic materials and devices.

TOC Graphic:

Highlights  A new series of hydrophilic fullerene derivatives was introduced as electron transport layers in p-i-n planar-heterojunction perovskite solar cells.  Tests showed that the electron-rich OE chains of fullerene derivatives both passivate the trap states of perovskite and reduce the work function of the metal cathode.

 A rigorous analysis using controlled electron density in passivators was demonstrated, which enabled a better understanding of the underlying mechanism of trap passivation.  The highest PCE of 16.0% could be achieved with FF of 0.75, Voc of 0.97 V and Jsc of 21.9 mA cm−2.