Enhancing Efficiency of Perovskite Solar Cells via Surface Passivation

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Enhancing efficiency of perovskite solar cells via surface passivation with graphene oxide interlayer Hao Li, Leiming Tao, Feihong Huang, Qiang Sun, Xiaojuan Zhao, Junbo Han, Yan Shen, and Mingkui Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10773 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 15, 2017

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ACS Applied Materials & Interfaces

Enhancing Efficiency of Perovskite Solar Cells via Surface Passivation with Graphene Oxide Interlayer Hao Li†, Leiming Tao†, Feihong Huang†, Qiang Sun†, Xiaojuan Zhao†, Junbo Han‡, Yan Shen†, Mingkui Wang*,† †Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, Hubei, China ‡Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, Hubei, China

Keywords: energy lever, interface, passivation, perovskite, recombination, solar cell

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Abstract: Perovskite solar cells have been demonstrated as promising low-cost and highly-efficient next-generation solar cells. Enhancing VOC by minimization the interfacial recombination kinetics can further improve device performance. In this work, we for the first time reported on surface passivation of perovskite layers with chemical modified graphene oxides, which acts as efficient interlayer to reduce interfacial recombination and enhance hole extraction as well. Our modeling points out that the passivation effect mainly comes from the interaction between functional group (4-Fluorophenyl) and under-coordinated Pb ions. The resulting perovksite solar cells achieved highly efficient power conversion efficiency of 18.75% with enhanced high open circuit VOC of 1.11 V. Ultrafast spectroscopy, photovoltage/photocurrent transient decay and electronic impedance spectroscopy characterizations reveal the effective passivation effect and the energy loss mechanism. This work sheds light on the importance of interfacial engineering on the surface of perovskite layers and provides possible ways to improve device efficiency.

Introduction Since the first demonstration in solar cells,1 organic-inorganic hybrid halide perovskite compound has become one of the most promising candidates for efficient solar energy harvesting application. The power conversion efficiency (PCE) of perovskite solar cell (PSCs) has exhibited a meteoric rise from 3.8% to a certified 22.1% recently.2 For efficient PSCs with internal quantum efficiency close to 100% over a wide absorption spectrum, under standard testing conditions the reported photocurrent (~22-23 mA cm-2) has almost reached the theoretical limits for perovskite compounds when accounting the reflective and parasitic absorption losses.3 For example, the theoretical photocurrent has calculated to be 23.8 mA cm-2 for CH3NH3PbI3 perovskite compound with bandgap of 1.56 eV. However, a large proportion of the output voltage of these solar cells remains very much below ~ 1.1 V, which is lower than the theoretical VOC limit of 1.32 V.4 Therefore, how to further push forward the device voltage and thus the device performance has become one of hot topics for PSCs community. 2


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The energy loss in organic-inorganic hybrid halide PSCs is principally caused by non-radiative recombination of charge carriers due to trap states at grain boundaries (GBs) and surfaces as well as point defects such as interstitial defects or vacancies in the perovskite compound crystal lattice.5-9 Trap states can be unavoidably induced for a solution-processed inorganic-organic hybrid lead halide perovksite compound films.10-12 For example methylammonium (MA) iodide can be dissolute from the crystal, leaving unavoidable non-stoichiometry in composition. The decompose of MAPbI3 to PbI2 on the surface and at the GBs of polycrystalline films can be accelerated by thermal annealing process,13,14 where unsaturated or under-coordinated Pb ions have been most frequently found. Some theoretical studies have been focused on the defect chemistry in these hybrid compounds, in order to identify the causes for the energy loss accompanying with low VOC induced by trap states.15 The formation of Pb dimers and I trimers with strong covalent bonds at the intrinsic defects leads to deep charge-state transition levels, which could act as effective recombination centers.16 Therefore, in order to reduce interfacial charge carrier recombination, surface modification has become one of the most successful methods to tune the PSCs’ voltage without sacrificing short circuit current (JSC) and fill factor (FF). For instance, an additional FAPbBr3-xIx (FA, formamidinium) layer on top of (FAPbI3)0.85(MAPbBr3)0.15 active film as an electron blocking layer can effectively promote devices PCE to 21.3%.17 Huang et al further highlighted the importance of surface passivation for PSCs. For example, π-conjugated Lewis base combing various organic π-conjugated structures was proposed as interlayer between the perovskite and the cathode to passivate the traps on the perovskite films. It was evidenced that the device VOC increased from 1.03 V to 1.11 V.18 Liu et al introduced an ionic liquid methyltrioctylammonium trifluoromethanesulfonate (MATS) to passivate the traps and boundaries at the interface of the i-n junction, improves the PCE from 12.1% to 17.51%. Such improvement is highly speculated by the ion redistribution induced barrier reduction at the interface.19 Sargent et al report a contact-passivation strategy using chlorine-capped TiO2 colloidal nanocrystal film that mitigates 3



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interfacial recombination and improves interface binding, the fabricated planar PSCs shows certified efficiencies of 20.1%. The stronger binding at the TiO2-Cl/ perovskite interface and the suppressed interfacial recombination account for superior stability in planar PSCs based on TiO2-Cl.20 Due to the high optical transparency, good thermal conductivity, and high electrical conductivity, reduced graphene oxide (rGO) has been widely employed in biosensors, memories, transistors, and solar cells.21,22 Several studies have already demonstrated the utilization of rGO as an electron blocking material could bring great improvements to the PSCs device performance.23 rGO, which can be easily cast into ultrathin two dimensional crystalline films from a colloidal solution, has been recently demonstrated as an interfacial material in optoelectronics and as a structure for π-stacking of conjugated organic molecules.24, 25 Furthermore, rGO can be simply modulated via the rational functionalization of graphene sheets, which provides feature tunability of the interaction between rGO and the contacting material. Herein we propose moderated rGO linking-up with functionalized group onto perovskite film as an efficient interfacial layer, with the purpose of passivating the traps (e.g., under-coordinated Pb ions and Pb clusters) on the surface and at the GBs of MAPbI3-xCl3. Electrospray (e-spray) technique was firstly used to homogeneously deposit rGO-4FPH onto MAPbI3-xCl3 film surface to form a uniform and compact modified layer. As have been reported previously, such surface passivation materials are generally small molecules containing an opposite charge at the end group, to balance the undercoordinated excess charges in the perovskite. In this work, for the first time flaky-like material was used as an interfacial material in combination the passivation effect of small molecules and hole extraction ability of rGO. In addition, rGO-4FPH reported here has less effect on the formation of perovskite and the following HTM layer. Consequently, the hole extraction property can be significantly enhanced by introduction of functionalized rGO, which further contributes to VOC enhancement. Finally, efficient PSCs can be achieved with an overall PCE of 18.75% and VOC of 1.11 V. This work also points to a 4


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new route to design multifunctional interlayer materials based on new functionalized reduced graphene oxides by combing various organic groups, which could be further applied in optoelectronic devices, such as PSCs, light-emitting diodes and lasers. Experimental Section Synthesis of functional graphene (rGO-4FPH) The GO was synthesized via previously reported approaches.26 In brief, the prepared GO was dispersed in deionized water, then 4-Fluorophenyl-hydrazine hydrochloride (4FPH) was added, and the solution was stirred at 60 oC for 24 h. The schematic procedure for preparation process was shown in Figure 1. The prepared rGO-4FPH was composed of few-layer transparent graphene sheets with homogeneous distribution of C, O, and F on the surface of the graphene sheets (see Figure S1a-e). After the completion of the reduction process, sodium chloride was added into the solution to precipitate rGO4FPH. After dry in vacuum, the rGO-4FPH was dispersed in chlorobenzene at a certain concentration. And the functionalized group in rGO-4FPH was calculated to be 0.136 mg mL-1 in a 0.6 mg mL-1 rGO4FPH dispersion.

Figure 1. Schematic procedure for preparation of rGO-4FPH starting from graphene oxide.

Preparation of e-spray rGO-4FPH film 5



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0.3 mg mL-1 aqueous rGO-4FPH solution (chlorobenzene) was deposited directly onto perovskite/TiO2/ITO using an e-spray technique. First, the rGO-4FPH solution was loaded into a plastic syringe and the feed rate was controlled by the syringe pump (KD Scientific Model 220) at a constant flow rate of 10-30 µL min-1. An electric field of 1-2 kV cm-1 was applied between the FTO glass and the micro tip using a high voltage power supply (BERTAN SERIES 205B) at a distance of 10 cm. In order to form a uniform thickness over a large area, the nozzle and substrate were placed on a motion control system using a microprocessor. Other experimental details including device fabrication, characterizations are shown in the supporting information. Results and Discussion In this work, rGO was used to passivate the trap states at perovskite films surface, due to its high optical transparency, good thermal conductivity, and high electrical conductivity. 4FPH was further selected to manipulate the graphene surface properties (hydrophilic and functionalized) during the reduction process. The resulted 4FPH-rGO could possess an intergraded favorite with high solubility and conductivity.27 The conversion from GO to rGO by 4FPH was confirmed with Fourier transform infrared spectroscopy (FTIR) characterization (Figure 2a). The presence of oxygen-containing groups in the GO samples, such as C-O (stretching vibrations at 1057 cm-1), C-OH (stretching vibrations at 1230 cm-1) and C=O (carboxylic acid and carbonyl moieties at 1730 cm-1) was reduced to a certain extent in the rGO-4FPH sample.28,29 The successful synthesis of rGO-4FPH can be demonstrated by new peaks (C-F stretching vibration) in the range 1000 to 1400 cm-1. This result can be further confirmed with Raman spectra characterization as shown in Figure S2. Furthermore, optical absorption was conducted to verify the functionalized rGO. The rGO-4FPH presents the same absorption peak at 277 nm with 4FPH sample in Figure 2b. The less absorption intensity of rGO-4FPH than the mixture of GO and 4FPH in the range of 300 nm to 500 nm indicates intramolecular energy transfer via the C=N binding 6


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(Figure 1), which forms during the reduction process. The above-mentioned feature demonstrates the successful linking up of fluorophenylhydrazine group into rGO. Surface wetting is an important factor for the solution-coating process. Figures 2c and 2d compare the optical images of contact-angle for perovskite films with and without rGO-4FPH interlayer. The contact angle decreases from 77o to 72o after depositing a rGO-4FPH layer. This indicates polyaromatic rings and fluorophenylhydrazine groups brought by rGO-4FPH benefit the contact between MAPbI3Cl3-x and spiro-OMeTAD.30 In order to further identify the influence of interlayer on the film formation of SpiroOMeTAD, the cross-section SEM images of devices with and without interlayer are given in Figure S3a and 3d, which show that the thickness of Spiro-OMeTAD is about 110-120 nm. This is consistent with the thickness (112-115 nm) obtained by profile-meter (Veeco Dektak 150). The surface morphology of the Spiro-OMeTAD were confirmed to be almost the same by the top view SEM and AFM images of films with and without the interlayer (Figure S 3b, 3c, 3e, 3f). These results suggest that the interlayer has little influence on the formation and surface of Spiro-OMeTAD.

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Figure 2. (a) FTIR transmittance spectra for 4FPH, GO and rGO-4FPH, respectively, (b) UV–vis absorption spectra of the GO, rGO-4FPH, (GO+4FPH) mixture and 4FPH, respectively. The contact angle of water on the varied substances: (c) perovskite sample, (d) perovskite/rGO-4FPH sample, respectively. Figure 3a illustrates the schematic of a planar ITO/TiO2/MAPbI3Cl3-x/spiro-OMeTAD/Au device structure in this study and the possible passivation effect brought by the rGO-4FPH which will be discussed below. The rGO-4FPH was deposited on top of MAPbI3Cl3-x layer as an interface modification layer through e-spray method. PSCs devices without rGO-4FPH interlayer were also fabricated for comparison purpose. Figure 3b illustrates the preparation process of rGO-4FPH interlayer. The e-spray is a gentle and efficient deposition method that produces uniform and lower material consumption. The SEM, AFM and optical microscope images of the perovskite surface with and without the interfacial treatment were given in Figure S4 a-f. It can be observed that rGO-4FPH exhibit 8


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thin and uniform distribution on the surface without any big cluster. Figure 3c shows a cross-section image of rGO-4FPH modified PSCs device, from which a well fabricated and uniform structure of planar PSCs can be informed.

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Figure 3. (a) A schematic image of the device cross-section, and the possible passivation effect brought by rGO-4FPH, (b) schematic representation of an e-spray setup for depositing rGO-4FPH, (c) the device cross-section SEM image. The photovoltaic performance of PSCs with and without rGO-4FPH interlayer was investigated under simulated one-sun illumination (AM 1.5G). Table S1 and S2 summarize the photovoltaic parameters for these devices. The devices’ PCE varies significantly with the rGO-4FPH thickness. We found the optimized thickness for rGO-4FPH was about 9 nm. A further increase in the thickness resulted in a deterioration in the performance due to an increase in electrical resistance (Table S2). Figure 4a shows 9



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J-V characteristics of those PSCs. The device A with rGO-4FPH modified achieved a PCE of 18.75% with VOC of 1.11 V, JSC of 21.5 mA cm-2, and FF of 78.57 when measured under reverse scanning. The device B without rGO-4FPH showed a PCE of 15.58% with a VOC of 1.03 V, a JSC of 20.1 mA cm-2, and a FF of 75.03 at the same condition. The enhancement of efficiency for device A mainly comes from an improvement of VOC and FF, which might be caused by passivation of traps states by reduction interfacial recombination rate between the perovskite layer and HTM interface. In addition, an increased JSC contributes to device performance, which could be due to higher charge extraction efficiency brought by the rGO-4FPH modification layer. This will be discussed and confirmed by following tests. Figure 4b presents external quantum efficiency (EQE) of devices A and B. The EQE curves of all devices show a wide photo-response from 350 to 800 nm, which are consistent with the absorption spectrum of MAPbI3Cl3-x (Figure S5). The device A has a higher EQE response than device B in the whole range. The integrated JSC obtained from the EQE spectra are 21.48 and 20.7 mA cm-2 for the rGO-4FPH modified and control devices, respectively. Both are in good agreement with the measured values of Jsc. The histogram of the VOC and PCE distribution over 30 devices in Figure 4c and d shows superior VOC and PCE performance for the rGO-4FPH modified devices (average VOC=1.054 V and PCE=14.57 %) compared to the devices without rGO-4FPH modified (average VOC=1.015 V and PCE=13.23 %). Also, the distribution of JSC and FF have been given out in Figure S6 for better understanding of performance improvement. Furthermore, the long-term stability tests were conducted to identify the influence of the interlayer on the PSCs devices. Figure S7 shows evolution of normalized PCE for Device A and Device B measured under illumination (one sun intensity). Device A and B stored in glove box with nitrogen atmosphere at room temperature without encapsulation shows almost the same stability, also devices A and B show the similar stability while stored in the air (with humidity at about 30%). The humidity effect should be take into account as the PCE is test in air with the humidity greater than 50%. But device A shows slightly higher stability while stored under constant 110


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sun illumination in a N2-filled glove-box. This might be attributed to the passivated trap states which

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Figure 4. (a) J-V curves of the Device A and Device B using a metallic mask with an aperture area of 0.108 cm2 under AM 1.5G one sun (100 mW cm-2) illumination with a delay time of 0.005 s in reverse scan direction, (b) IPCE spectra, and integrated current date of corresponding devices. Histogram for PSCs based on rGO-4FPH modified devices and without rGO-4FPH modified devices: (c) VOC and (d) PCE. Figure 5a compares the dark current for devices A and B in logarithmic scale. Device A shows less leakage current compared with device B. As the leakage current shows the same trend to the shunt resistance (Rsh), and a larger Rsh indicates a smaller charge carrier recombination in the active layer and 11



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high charge selectivity collection.33,

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We suggest that the rGO-4FPH on perovskite surface can

effectively restrain the leakage current under reverse bias, which in turn enhances charge selectivity collection as reflected by increasing JSC. Here the dark J-V curves of PSCs were modeled using the Shockley diode equation for a single junction device ( JD = J 0[exp(

q (V − JDRS ) ) − 1] , where J0 is the nKBT

reverse saturation current density, JD is the dark current density, V is the applied bias, q is the electron charge, KB is the Boltzmann constant, T is the temperature, and n is the ideal factor of the real diode).35 Figure 5b shows the plots of -dV/dJ as a function of (JSC-J)-1 and the corresponding fitting results. With the modification of rGO-4FPH, the series resistance RS decreased from 4.39 to 3.06 Ω cm2. A lower RS is necessary to eliminate charge loss at high bias voltages and to achieve high fill factor. Furthermore, the reverse saturation current density J0 was evaluated to be 8.2×10-9 mA cm-2 for device A and 1.0×10-8 mA cm-2 for device B by using fitting the plots of ln(JSC-J) against (V+RSJ) (Figure S8). This result suggests that rGO-4FPH as an interfacial modification layer processes high hole selectivity.33 The obtained diode parameters such as RS, and J0, for the studied devices are summarized in Table S1. Likewise, the interfacial charge-recombination/transport in the PSCs was further investigated with transient photovoltage/photocurrent decay measurements. Figure S9 shows transient photovoltage decay curves for both devices serving as an example. Figure 5c compares the interfacial charge recombination lifetime (right coordinate) of devices A and B. Since both devices were fabricated under the same condition, the difference in lifetimes could be contributed to the rGO-4FPH interlayer on the interfacial recombination between electron from CH3NH3PbI3Cl3-x and hole from spiro-OMeTAD. A long interfacial recombination lifetime guarantees effective charge collection efficiency, thus the device output photovoltage.36 The interfacial charge recombination lifetime for device A is about one time longer than device B, well explaining the augmented output voltage by 1.11 V for the former at room temperature. Therefore, this result indicates that the augment in VOC for rGO-4FPH modified devices 12


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can be ascribed to longer interfacial charge recombination lifetime at the HTM/CH3NH3PbI3Cl3-x interface. Similarly, the charge-transport time can be obtained from the transient photocurrent decay measurements. As shown in Figure 5c (left coordinate), the hole charge transport lifetime decreases by inserting rGO-4FPH interlayer between CH3NH3PbI3Cl3-x and HTM layers. The hole diffusion coefficient was calculated to be ~3.5 × 10-4 cm2 s-1. Therefore, the hole diffusion length can be further obtained under different incident irradiation intensities as shown in Figure 5d. Long charge diffusion length indicates high charge collection, principally delivering a higher output photocurrent.37 After introduction of rGO-4FPH interlayer, hole diffusion length increases from 400 nm to 700 nm, which is benefited for charge extraction. This can be contributed to high hole conducting rGO-4FPH and fine contact for each layer. Electronic impedance spectroscopy measurement further confirms the reduction

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Figure 5. (a) J-V curves of the device with and without rGO-4FPH modified under dark condition, (b) plots of -dV/dJ vs (JSC-J)-1 and the linear fitting curves in which date comes from J-V curves of the device with or without rGO-4FPH modified under dark condition. (c) transport time (τt) (left coordinate) and recombination time (τr) (right coordinate) of both device A (blue) and devices B (red) comes from fitting curves of transient photovoltage/photocurrent decay measurements with a single-exponential equation. (d) the hole diffusion length calculated from transient photovoltage/photocurrent decay measurements. The PL peaks were found at 777 and 773 nm for the samples glass/perovskite and glass/perovskite/rGO-4FPH as shown in Figure 6a. The blue-shifted emission PL peak for glass/perovskite/rGO-4FPH can be correlated with band bending close to the surface of the film induced by the existence of surface trap states on the pervoskite surface or grain boundary.7 Filling the trap states enables the recovery of the bandgap and blue shifts the PL peak. This result reveals that rGO-4FPH could passivate the trap states close to the surface of the perovskite film. Time-resolved transient photoluminescence decays (TRPL) were used to elucidate the dynamics of charge transfer process at the HTM/MAPbI3Cl3-x interface as shown Figure 6b. The PL lifetimes (τPL) were fitted by a doubleexponential and the corresponding detailed parameters are summarized in Table S3. The fast decay lifetime τ1 is attributed to the quenching of the photogenerated free carriers transporting from the perovskite layer to the HTL, and the slow decay lifetime τ2 originates from the radiative recombination of free carriers before charge collection.23, 38 In the case of the spiro-OMeTAD/perovskite/glass sample, the faster decay lifetime of τ1=1.05 ns with a weight fraction of 64.8% and the slow lifetime τ2=6.99 ns with a weight fraction of 35.2% indicate that the radiative recombination is dominant in the depopulation

of

photo-generated

carries.

On

the

contrary,

for

the

spiro-OMeTAD/rGO-

4FPH/perovskite/glass sample, a shorter τ1 indicates faster photogenerated carriers transporting and a longer τ2 suggested the relativity slow recombination rate. (τ1=0.72 ns with a weight fraction of 78.7%, 14


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τ2=7.15 ns with a weight fraction of 21.3%). In short, the photo-generated carriers can be effectively extracted and transferred from the perovskite film to the HTM layer with the help of interlayer. The trap densities of state (t-DOS) of the devices were characterized to confirm the passivation effects brought by the rGO-4FPH. Here we investigated the variation of the t-DOS distribution by extracting the capacitance (C) by means of impedance spectroscopy at the open-circuit voltage (VOC) under different illumination. It has been established that the capacitance in this type of measurement is proportional to the t-DOS, as C reflects the capability of a photovoltaic device to accept or release additional charge carriers as a result of shifts in the quasi-Fermi level. The distribution of the t-DOS is then fitted by Gaussian equation39, 40 ( gn ( E − EL ) =

Nn ( EL − E ) 2 exp[− ] , where E is the Fermi lever, 2σ n2 2πσ n

EL is the energy center of the t-DOS, Nn is the total density per unit volume and σn is the disorder parameter that represents the broadness of the t-DOS). Figure 6c presents the result of the t-DOS distributions for both devices. Device A shows a narrow t-DOS distribution with a σn of 81.4 meV compared with device B which shows a distribution of 150.8 meV. The narrow t-DOS distribution and reduce of deep traps (located at low VOC) demonstrate that rGO-4FPH indeed passives traps states at the perovskite surface as schematically illustrated in Figure S11. This could effectively assurance the holes transporting to the Au electrode. Therefore, an augmented VOC are expected for the devices with MAPbI3Cl3-x/rGO-4FPH. Nanosecond transient absorption spectroscopy (ns-TAS) characterization was further performed to understand the effect of rGO-4FPH interlayer on charge transfer at the HTM/MAPbI3Cl3-x interface. As shown in Figure 6d and 6e the photobleaching (PB) negative and photoabsorption (PA) positive peaks can be easily observed at around 760 and 500-600 nm, respectively.41,42 The PB negative peak appearing at 760 nm is related to the band gap or exciton transition of MAPbI3-xClx film, whereas the PA positive peak at about 500-600 nm can be attributed to the absorption of transient species.39 After depositing the rGO-4FPH on top of perovskite film, the ns-TAS spectra exhibit several distinct features 15



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from those without modification. An obvious decrease of the peak intensity was observed by comparing the negative PB peak at about 760 nm between glass/MAPbI3-xClx/rGO-4FPH/spiro-OMeTAD and glass/MAPbI3-xClx/spiro-OMeTAD samples. This phenomenon indicates the existence of rGO-4FPH affects exciton transition or charge extraction process within the MAPbI3-xClx film. Figure 6f shows the kinetic decay traces of photo-bleaching features. The time constant τTAS (obtained with a single exponential function fitting) is related to the excited-stated decay and free carrier recombination dynamics in the perovskite layers. The τTAS of glass/MAPbI3-xClx/rGO-4FPH/spiro-OMeTAD sample (~ τTAS1 = 258 ns) is higher than that of glass/MAPbI3-xClx/spiro-OMeTAD (~ τTAS1 = 207 ns), indicating that the photo-excited carriers in the case of rGO-4FPH have higher opportunity to be collected.43-45 This allows us to analysis the interfacial charge recombination velocity (defined as k=1/τTAS) which decreases from 4.83×106 s-1 to 3.87×106 s-1 after introduction of rGO-4FPH interlayer.

16


1.0

0.5

773±1 1

0.0 700

750

777±1 1

800

1 perovskite+spiro perovskite+rGO-4FPH+spiro

0.1

0.01

1E-3

850

Wavelength (nm)

0

20

40

Delay time (ns)

d)

2 Device A Device B

perovskite/spiro-OMeTAD

0.1

∆ O.D.

Measured DOS (*1016cm-3eV-1)

c)

b) Glass/Perovskite Glass/Perovskite/rGO-4FPH

Normalized PL intensity

a)

1

0.0 30ns 130ns 230ns 330ns 430ns

-0.1

0.4

0.6

0.8

1.0

Voc (V)

e)

500

f)

600

0.0 30ns 130ns 230ns 330ns 430ns

-0.1 500

600

700

800

800

1 Spiro/perovskite Spiro/rGO-4FPH/perovskite

Normalized ∆ O.D.

0.1

700

Wavelength (nm)

perovskite/rGO-4FPH/spiro-OMeTAD

∆ O.D.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normalized Intensity (a.u.)

Page 17 of 30

τTAS1=258ns

τTAS2=207ns

0

Wavelength (nm)

300

600

Delay time (ns)

Figure 6. Photoelectronic properties of spiro-OMeTAD/perovskite (MAPbI3Cl3-x) films with or without rGO-4FPH modified. (a) Steady-state PL spectra of the films excited by a 500 nm light source from the air side. (b) Time-resolved PL decay transients measured at 770 nm for Glass/MAPbI3-xClx/rGO4FPH/spiro-OMeTAD (red) and Glass/MAPbI3-xClx/spiro-OMeTAD (black) samples after excitation at 500 nm. (c) Measured DOS of devices with or without rGO-4FPH modification extracted from their impedance spectra. The disorder parameter σn is obtained by fitting the DOS curves with a Gaussian 17



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distribution. The solid lines are the double-exponential fits of the PL decay transients. Transient absorption spectra of (d) perovskite(MAPbI3Cl3-x)/spiro-OMeTAD films and (e) perovskite (MAPbI3Cl3-x)/rGO-4FPH/spiro-OMeTAD films, excitation at 500 nm, black (30 ns), red (130 ns), blue (230 ns), pink (330 ns), green (430 ns). (f) Normalized kinetic traces for photobleaching probed at 760 nm for perovskite layer covered by spiro-OMeTAD (black) or spiro-OMeTAD/rGO-4FPH (red).

In order to study the impact of the functional interlayer, we further carried out density functional theory (DFT) calculations on the rGO-4FPH/perovskite interface. Figure 7a and 7d present the geometry for the configuration of the optimized structures for perovskite and perovskite-(rGO-4FPH) respectively. Figure 7b and 7e show the calculated electron density (ED) in the region of perovskite(rGO-4FPH). After modification of rGO-4FPH, the electrons of Pb atoms are dragged upwards to the F atom, suggesting a strong coupling exists between them. This is an indication of efficient passivation.46 Furthermore, the bonding property was investigated with electron localization function (ELF). In the Figure 7c and 7f of ELF profile, the values are in the range of (0, 1) with 1 (red) meaning complete localization and 0.5 (green) corresponding to a delocalized (metallic) electron gas. High ELF values are found on the top of under-coordinated Pb ions, indicating their function of trap sites. No electrons are left at the Pb vacancy position in the perovskite-(rGO-4FPH) configuration, suggesting Pb vacancy defects positively charged with respect to the original anion site.47 This points out that the passivation effect mainly comes from the interaction between functionalized end group (4-Fluorophenyl) and undercoordinated Pb ions, which is further confirmed by reduction of the deep trap states as presented in tDOS distribution in Fig. 6c.

18


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a)

b)

c)

d)

e)

f)

Figure 7. (a) The atomic configuration (the light grey atom is C, the white atom is H, and the dark grey atom is Pb, the dark red atom is I, the blue atom is N). (b) The distribution of valence electron density, (c) electron localization function (ELF) of valence electrons at the perovskite interface. (d) The atomic configuration (the light grey atom is C, the white atom is H, and the dark grey atom is Pb, the dark red atom is I, the blue atom is N). (e) The distribution of valence electron density, (f) ELF of valence electrons at the perovskite interface/rGO-4FPH.

Conclusion In summary, we introduced functionalized graphene oxides (rGO-4FPH) into PSCs as interlayer between the perovskite and the HTL. The functionalized 4FPH group can passivate the traps on the perovskite films and benefit for charge extraction from perovskite layers. The MAPbI3Cl3-x/rGO-4FPH based solar cells yielded a VOC of 1.11 V and a final PCE up to 18.75%. Our findings point out the VOC 19



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Page 20 of 30

losses can be largely reduced via passivation the under-coordinated Pb ion traps. We also recognized the improvement in device performance may be underestimated due to a limited film fabrication process. In addition, the hole extraction ability of rGO indicates its suitability for the inverted planar structure perovskite solar cells as hole transfer material or interlayer material after proper functional modification. Further design for application may combine two or more different functional groups with rGO, through synergistic effect to implement fine tuning electronic property at the interface. This work therefore points out a new rout to design interlayer materials by combing the passivation effects of various functionalized organic group and the hole transport properties of functionalized reduced graphene oxides which supplies a useful strategy to reduce the energy loss, increase the VOC, finally enhance the efficiency of all-solid-state PSCs. Also, those kinds of multi-function materials could be further applied to other photovoltaic field as an interlayer or substitute the conventional single function materials to realize high performance devices as well.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Detailed material and device characterization. Additional figures showing EDS images; TEM image; Raman spectra; AFM images; UV–vis absorption spectra; ln(JSC-J) against V+RSJ plots; Transient 20


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photovoltage decay curves; EIS results of Nyquist plot, bode plot and fitting date; schematically illustrated the trap states; equivalent circuit for Electronic impedance spectroscopy data fitting; table S1S3; and additional references. Author Information Corresponding Author E-mail: [email protected] (M.W.) Author Contributions H. L. and M. W. designed the experiments. L. T., F. H., J. H. and M. W. gave suggestions for data analysis. H.L. and Q.S. performed the devices fabrication and characterization. H. L. and J.Z. carried out the SEM and TAS measurements. Q.S. conducted the PL measurement. All authors discussed the results and commented on the manuscript. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgements Financial support from the 973 Program of China (No. 2014CB643506), the Natural Science Foundation of China (No. 21673091), Major International (Regional) Joint Research Project NSFCSNSF (516611350), the Natural Science Foundation of Hubei Province (No. ZRZ2015000203), Technology Creative Project of Excellent Middle & Young Team of Hubei Province (No. T201511), and the Wuhan National High Magnetic Field Center (2015KF18). The authors thank the Analytical and Testing Centre of Huazhong University of Science & Technology for the measurements of the samples.

21



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Page 22 of 30

The authors thank Hongwei Han (Huazhong University of Science & Technology) for the Timeresolved transient photoluminescence decays (TRPL) measurements. Abbreviations ETL, electron transport layer; PCE, power conversion efficiency; MA, methylammonium; FA, formamidinium; FF, fill factor; HTL, hole transport layer; spiro-OMeTAD, 2,2’,7,7’-tetrakis(N,N-di-pmethoxyphenylamine)-9,9’- spiro bifluorene; 4FPH, 4-Fluorophenyl-hydrazine hydrochloride; e-spray, electrospray; ITO, indium tin oxide; rGO, reduced graphene oxide; FTIR, Fourier transform infrared spectroscopy; PV, photovoltaic; JSC, short-circuit current density; VOC, open-circuit voltages; PL, steady-state photoluminescence; J-V, photocurrent-voltage; EQE, external quantum efficiency; AC, alternating current; IS, impedance spectra; SEM, scanning electron microscope; ED, electron density; ELF, electron localization function. GBs, grain boundaries; TRPL, time-resolved transient photoluminescence decays; t-DOS, trap densities of state; ns-TAS, nanosecond transient absorption spectroscopy; PB, photobleaching; PA, photoabsorption; DFT, density functional theory; References (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as VisibleLight Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. Iodide Management in Formamidinium Lead Halide Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376-1379. (3) Ball, J. M.; Stranks, S. D.; Horantner, M. T.; Huttner, S.; Zhang, W.; Crossland, E. J. W.; Ramirez, I.; Riede, M.; Johnston, M. B.; Friend, R. H.; Snaith, H. J. Optical Properties and Limiting Photocurrent of Thin-Film Perovskite Solar Cells. Energy Environ. Sci. 2015, 8, 602-609. 22


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Inorganic Hybrid Perovskite CH3NH3Pb1−xCdxI3: An Experimental and Theoretical Study. Phys. Chem. Chem. Phys., 2015, 17, 23886-23896.

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SYNOPSIS TOC

20


15

Energy

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10

∆E Eab

eVoc

Trap states passivation Blend Circuit component

5

0 0.00

Device A Device B 0.25

VOC 0.50

0.75

1.00

Voltage (V) eVoc: Eab - ∆Etherm - ∆EEL- ∆Er; Eab: absorbed energy; ∆Etherm: thermalization of excess energy; ∆EEL: energetic loss by energy lever offset; ∆Er: recombiantion losses;

30


Enhancing Efficiency of Perovskite Solar Cells via Surface Passivation

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