Incorporating an Inert Polymer into the Interlayer

DOI: 10.1002/chem.201703382

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& Photovoltaics

Incorporating an Inert Polymer into the Interlayer Passivates Surface Defects in Methylammonium Lead Halide Perovskite Solar Cells Shiqing Bi,[a] Xuning Zhang,[a] Liang Qin,[a] Rong Wang,[b] Jiyu Zhou,[b] Xuanye Leng,[a] Xiaohui Qiu,[a] Yuan Zhang,*[b] Huiqiong Zhou,*[a] and Zhiyong Tang[a] Abstract: The hysteresis effect and instability are important concerns in hybrid perovskite photovoltaic devices that hold great promise in energy conversion applications. In this study, we show that the power conversion efficiency (PCE), hysteresis, and device lifetime can be simultaneously improved for methylammoniumlead halide (CH3NH3PbI3-xClx) solar cells after incorporating poly(methyl methacrylate) (PMMA) into the PC61BM electron extraction layer (EEL). By choosing appropriate molecular weights of PMMA, we obtain a 30 % enhancement of PCE along with effectively lowered hysteresis and device degradation, adopting inverted planar device structure. Through the combinatorial study using Kelvin probe force microscopy, diode mobility meas-

Introduction Organic–inorganic hybrid lead halide solar cells have emerged as highly competitive sources of energy conversion applications, with demonstrated state-of-the art power conversion efficiencies (PCE) in excess of 20 %.[1–3] Alongside the effort to boost PCEs, obtaining improved stability and reducing hysteresis have become central topics in the perovskite communities. The stability is intimately influenced by the perovskite film quality, which can be improved either by bulk or interface engineering; for example, by applying molecular or solvent additives to increase the moisture tolerance of the perovskite layer,[4–6] or by modifying interface properties to enhance the [a] Dr. S. Bi, X. Zhang, L. Qin, X. Leng, X. Qiu, Prof. H. Zhou, Prof. Z. Tang CAS Key Laboratory of Nanosystem and Hierachical Fabrication CAS Center for Excellence in Nanoscience National Center for Nanoscience and Technology No. 11, ZhongGuanCun BeiYiTiao Beijing 100190 (P. R. China) E-mail: [email protected] [b] R. Wang, J. Zhou, Y. Zhang School of Chemistry Beihang University No. 37, Xueyuan Road Beijing 100191 (P. R. China) E-mail: [email protected] Supporting information for this article can be found under: https://doi.org/10.1002/chem.201703382. Chem. Eur. J. 2017, 23, 14650 – 14657

urements, and irradiation-dependent solar cell characterization, we attribute the enhanced device parameters (fill factor and open circuit voltage) to the surface passivation of CH3NH3PbI3-xClx, leading to mitigating charge trapping at the cathode interface and resultant Shockley-Read-Hall charge recombination. Beneficially, modified by inert PMMA, CH3NH3PbI3-xClx solar cells display a pronounced retardation in performance degradation, resulting from improved film quality in the PC61BM layer incorporating PMMA which increases the protection for underneath perovskite films. This work enables a versatile and effective interface approach to deal with essential concerns for solution-processed perovskite solar cells by air-stable and widely accessible materials.

protection of perovskites from ambient conditions using charge extraction layers.[7–11] In the aspect of interfacial modification, electron extraction layers (EEL) in inverted perovskite solar cells are generally based on fullerene ([6,6]-phenyl-C61-butyric acid methyl ester, or PCBM) derivatives, which have been proven to be able to suppress ion motion and passivate Pb@I antisite defects in perovskite films.[12, 13] Generally, Pb@I defects can originate from non-stoichiometry during the thermal annealing for perovskite films with a poor thermal stability.[14–19] Although PCBM has been widely adopted, it may not be able to completely passivate the Pb2 + cations because of the Lewis acidic nature. Very recently, a more efficient trap passivation approach was proposed by using the EEL based on organic non-fullerene molecules, in which the Lewis basic groups can form Lewis adducts with Pb2 + .[20] So far, many of the emerging non-fullerene alternatives tend to have an inferior carrier mobility compared to PCBM. Thus, to minimize the electron loss via EEL, a very low film thickness (a few nanometers) may be required when used as EELs. This not only curtails the protection for perovskite films by EEL, but also causes the complexity in device fabrication through printing techniques. To this end, the development of effective EELs based on PCBM derivatives with improved trap passivation capability will be highly desirable to attain high performance perovskite solar cells with long-term stability.

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Full Paper Incorporating stable and non-conductive polymers into charge-extracting layers has been demonstrated to be viable to improve the device characteristics in perovskite solar cells. For example, PMMA was mixed into a carbon nanotube-doped P3HT hole extraction layer for CH3NH3PbI3-xClx solar cells with retarded thermal degradation.[21] Polystyrene was incorporated into PC61BM EELs to achieve enhanced PCEs, a result of forming smooth and uniform EELs that prevent interfacial charge recombination.[22] Moreover, polyethylene glycol was applied to perovskite films as additives to generate self-healing effects.[23] Notwithstanding these achievements, mechanisms of how these inert polymers impact the device physics in operational solar cells still remain poorly understood. Obtaining insights into these aspects could benefit the advances in interface engineering and solar cell performance. In this article, we investigate the effect of incorporation of PMMA into PC61BM EELs on the photovoltaic behavior in CH3NH3PbI3-xClx solar cells adopting an inverted structure. We show that the hysteresis in CH3NH3PbI3-xClx solar cells is effectively suppressed, associated with enlargements of fill factor (FF) and PCE. By Kelvin probe force microscopy (KPFM), we observe the rising of the contact potential difference for PC61BM incorporated with PMMA, implying that the electronic structure at the cathode interface is changed. The surface trap passivation with PMMA is evidenced by the reduction in Shockley–Read–Hall (SRH) recombination based on illumination-dependent open-circuit voltage (Voc) measurements. With the surface modification, the electron transport in CH3NH3PbI3-xClx single-carrier devices can be slightly increased, which correlates to the reduced surface charge-trapping, and explains the improved FF and Voc. Additionally of benefit, the lifetime of PMMA-modified CH3NH3PbI3-xClx solar cells is dramatically improved, which remains 75 % of the initial performance after 30 days of storage. The enhanced stability can be ascribed to the improved film-forming quality in the PC61BM layer after incorporating inert PMMA.

Results and Discussion In this study, we chose PMMAs with three different molecular weights (PMMA1 = 1.2 V 105 g mol@1, PMMA2 = 3.6 V 105 g mol@1, and PMMA3 = 9.96 V 105 g mol@1). Figure 1 b illustrates the utilized inverted planar structure for CH3NH3PbI3-xClx solar cells, in which the PEDOT:PSS layer serves as the hole extraction layer, and PC61BM incorporated with PMMA is the EEL. We acknowledge that inverted structure generally produces a lower PCE compared to a mesoporous TiO2-based device, whereas the former offers a straightforward test bed to investigate the effect of PMMA on the cathode interface properties in perovskite devices. As a background study, we compared thin-film absorbance of CH3NH3PbI3@xClx/PC61BM incorporated with various PMMA (see Figure 1 c). Clearly, there are no substantial changes in the presence of PMMA, indicating that the optical properties are nearly unaffected,[24] at least in the bulk of CH3NH3PbI3-xClx films. Then we examined the PMMA concentration dependence for device performance. Figure 1 d–f shows current density versus voltage (J–V) characteristics of CH3NH3PbI3@xClx solar cells with various PMMAs under standard 1 sun irradiation (100 mW cm@2) scanned in the reverse direction (defined for scan bias from positive to negative and abbreviated as R). Detailed device parameters are summarized in Table S1–S3 in the Supporting Information. The photovoltaic behavior depends on both the PMMA concentration and MW. At a fixed MW, the Jsc monotonously decreases incrementally with the varying incorporated amount of PMMA. On the contrary, the Voc and FF display a non-monotonous change at higher PMMA concentrations. In all cases, we found that the FF and Voc are maximized with addition of 5 wt % PMMA (to PC61BM). When we further increase the PMMA concentration (beyond 5 wt %), all device parameters decrease, which is possibly caused by an increased resistivity in the EEL with PMMA. This is based on the consideration that PMMA is nearly an insulating polymer with a wide band gap,[21, 36] thus a decreased

Figure 1. a) Molecular structure of PMMA. b) Diagram of the CH3NH3PbI3-xClx solar cells. c) Absorbance of CH3NH3PbI3-xClx/PC61BM incorporated with PMMA in various molecular weights (MW). d–f) Current density versus voltage (J–V) characteristics of CH3NH3PbI3-xClx solar cells incorporated with various amounts of PMMA at different MWs under standard 1.5 AM G 1 sun irradiation (100 mW cm@2). Chem. Eur. J. 2017, 23, 14650 – 14657

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Figure 2. Comparison of the hysteresis effect in J–V characteristics of CH3NH3PbI3-xClx solar cells under 1 sun irradiation in the forward (F) and reverse (R) scan directions without PMMA a), and with PMMA in different molecular weights b–d).

electrical conductance in PC61BM should be expected after incorporating PMMA. To confirm this, we carried out OFET measurements (see results in Figure S1) and the electron mobility in PC61BM FETs was determined to be 2.9 V 10@2 cm2 Vs@1 for pristine PC61BM, and 1.1 V 10@2 cm2 Vs@1 for PCBM with PMMA 1 (5 wt %). Upon enlarging the MW of PMMA, the electron mobility further decreases to 6.7 V 10@4 cm2 Vs@1 (with 5 wt % PMMA3). This result indicates an increased resistivity in PC61BM with likely discontinuous transporting networks in the OFET conduction channel. For simplicity in our analysis, hereinafter all PMMA relevant measurements were performed based on the optimal concentration (5 wt %). To investigate the MW-dependence on solar cell characteristics, we compare J–V characteristics of CH3NH3PbI3-xClx solar cells without and with PMMA in different MWs scanned in reverse and forward (defined for scan bias from negative to positive and abbreviated as F) scan directions (Figure 2). As seen from the solar cell parameters summarized in Table 1, low MW PMMA1 (1.2 V 105 g mol@1) can provide the optimal performance with FF and Voc enhanced from 0.6 to 0.74 and from 0.971 V to 1.03 V, respectively. This leads to a maximal improvement for PCE to 14.3 % (in reverse scan). Clearly, incorporation of PMMA in all cases results in an increase in FF and Voc regardless of MW; however, this comes with the compromise of a slight reduction in Jsc. Figure S2 shows Incident photon to current efficiency (IPCE) spectra of these devices, in which there are no substantial changes in the presence of PMMA, although we observe a slight decrease in regimes between 600– 750 nm (with PMMA2 and PMMA3) and 400 nm–500 nm (with PMMA3). This trend agrees with the slightly reduced Jsc and can be interpreted by the enlargement of electrical resistance in PC61BM with PMMA, revealed by OFET measurements. BeneChem. Eur. J. 2017, 23, 14650 – 14657

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Table 1. Device parameters of CH3NH3PbI3-xClx solar cells measured in the forward (F) and reverse (R) scan direction with various PMMA.

Sample PCBM-F PCBM-R PMMA1-F PMMA1-R PMMA2-F PMMA3-R PMMA3-F PMMA3-R

Voc [V]

Jsc [mA cm@2]

FF [%]

PCE [%]

0.97 0.98 1.03 1.03 1.01. 1.00 1.01. 1.00

18.64 18.84 18.41 18.51 17.91 18.48 17.54 17.40

59.8 64.4 74.0 75.0 68.5 70.3 69.5 70.9

10.8 11.9 14.0 14.3 12.4 13.0 12.3 12.3

Hysteresis index 0.108 0.019 0.051 0.012

ficially, the overall PCE of CH3NH3PbI3-xClx devices with different PMMA remains improved in comparison to the control solar cell. As shown in Figure 2 a, a noticeable hysteresis in J–V characteristics is observed for the device without PMMA. Such inconsistency mainly occurs at the bias near Voc, resulting in an under- (in F-scan) or overestimated (in R scan) FF and PCE. This effect has been proposed to be ascribed to combined results of ion motion and charge trapping, or ferroelectricity, which can give rise to the accumulation of charges or specific ions near the contact area to re-screen the electrical field and changes the internal built-in potential (Vbi).[25–28] Importantly, the electrical hysteresis is suppressed in the presence of PMMA, leading to nearly overlapped photocurrent irrespective of the scan direction. This suggests that ionic motion and surface trap trapping may be mitigated by PMMA near the surface of perovskites. The hysteresis index (HI) for these devices was calculated according to the equation,[29]

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Figure 3. a–d) Surface topographic images of pure PC61BM, PC61BM + PMMA1, PC61BM + PMMA2, and PC60BM + PMMA3 films captured by AFM in tapping mode. e–h) Mapping of contact potential difference (CPD) of PC61BM(PMMA) films in a–d) casted on ITO substrates captured by KPFM. (scale bar = 400 nm).

HI ¼

JRS ð0:8Voc Þ @ JFS ð0:8Voc Þ JRS ð0:8Voc Þ

ð1Þ

in which JRS(0.8Voc) and JFS(0.8Voc) denote the current in reverse scan and forward bias = 0.8 Voc. The HI for the control cell is 0.108, and it decreases substantially to 0.019, 0.054, and 0.012 with PMMA1, PMMA2, and PMMA3, respectively. The reduced hysteresis leads to a greater consistency of the determined PCE in the two scan directions (see Table 1). These results demonstrate the ability of PMMA to mitigate the undesirable hysteresis in perovskite solar cells through the suppression of ionic motion or surface charge trapping. To further understand the underlying mechanisms for the mitigated hysteresis and enhanced device parameters (FF and Voc), we examined surface topography and contact potential difference of PC61BM (PMMA) by KPFM. As shown in Figure 3 a– d, PC61BM films surface with or without PMMA are all characteristic of a featureless surface morphology, which is consistent to previous results.[30] The surface roughness of PC61BM (PMMA) films slightly reduce from 1.15 nm (without PMMA) to 0.95 nm under the optimal conditions (PMMA1). In the surface topography images, we cannot observe any phase separation in the presence of PMMA, which is likely due to the relatively low concentration. The reduced roughness of PC61BM (PMMA) films may be attributed to a lower degree of aggregation in PC61BM with intermolecular interactions with PMMA.[31] To unravel the surface electronic properties, local contact potential difference (CPD) was measured based on the same set of films. Figure 3 e–h show the KPFM measurement on the corresponding PC61BM films. All images exhibit the homogeneous feature, implying low spacial variations on CPD. This also hints that the PMMA may be well mixed within the PC61BM matrix. From Table S4, in which the CPD values for different perovskite films are provided, the CPD of pristine PC61BM is on average about 70.3 mV with respect to the work function (WF) of the AFM tip. Markedly, the CPD with PMMA1 is lifted to 162 mV and further increases to 227 mV with PMMA2, whereas it slightly reduces Chem. Eur. J. 2017, 23, 14650 – 14657

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to 213 mV with PMMA3. The increasing surface potential can be correlated to the shift of Fermi level toward to the vacuum level, indicative of a changed interface electronic structure in the presence of PMMA. The shifted CPD (towards vacuum level) with PMMA implies that the Vbi in solar cells may be benefited. In this scenario, the charge recombination and space-charge effect may be mitigated, which prompts the increase in FF and Voc. Charge recombination has been identified as a major loss channel in solar cells.[32–34] In the presence of traps, the recombination can be dominated by the SRH (trap-assisted) process, which tends to elevate the carrier losses in devices.[35, 36] To understand how PMMA modified surface properties affect this important photophysical process, we explore irradiation-dependent Jsc and Voc. Figure 4 b shows the Jsc of CH3NH3PbI3-xClx solar cells measured under various irradiation intensities (Plight). In principle, the photocurrent (Jphoto) in solar cells should exhibit a power law dependence that typically varies between 0.8 & 1.[37, 38] The power parameter a in the Jsc versus Plight plot provides a gauge on the significance of bimolecular or monomolecular types for charge recombination. Note that there are still active debates on whether a = 1 should account for the bimolecular or monomolecular process. Within the framework of assuming a bimolecular recombination with approaching unity, the slopes ( & 1) in Figure 4 a indicate that the recombination in the devices may occur primarily through the mobile electrons and holes generated in the CH3NH3PbI3-xClx layer, which is consistent with previous observations.[39, 40] Irradiation-dependent Voc characteristics provide a diagnostic tool to interrogate the influences of traps on recombination. Generally, in solar cells with a dominant bimolecular charge recombination through mobile (non-trapped) carriers without SRH recombination, the slope of Voc vs. Plight plot should equal the thermal voltage kBT/q. If the SRH recombination plays a role, commonly associated with noticeable amounts of impurities/defects in the bulk film or at the interface,[28, 41] the slope should exceed kBT/q. On this basis, we examined the slope of Voc vs. Plight

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Figure 4. Irradiation-dependent a) short-circuit current Jsc and b) open-circuit voltage of CH3NH3PbI3-xClx solar cells with various PMMA. c) Dark J–V characteristics of single carrier devices in configuration of ITO/ZnO/perovskite/PC61BM(PMMA)/Al used to determine the electron mobility in perovskite layer. d) Nyquist plot of the impedance measured on the same solar cells under 1 sun irradiation (bias at Voc). Lines are the results of equivalent circuit modelling.

characteristics (Figure 4 b) for perovskite solar cells with different PMMA. We attained a slope of 1.4 kBT/q for the cell without PMMA, indicating the presence of SRH recombination. This may be caused by the grain boundaries in CH3NH3PbI3-xClx bulk film or Pb2 + defects due to non-stoichiometry near the surface. Of note, the slope for devices incorporating PMMA1 reduces to 1.3 kBT/q, suggestive of a slight mitigation on the SRH recombination, primarily due to the passivation of surface traps. Of interest, with increasing the PMMA MW, the slopes in Figure 4 b recover to the original value for the control device. This suggests that the SRH recombination may be resurrected. At this stage, this phenomenon is not entirely understood, and further investigation is needed to fully explain. It should be mentioned that none of these devices show a slope approaching the unit of thermal voltage, suggesting that certain levels of traps, perhaps in the bulk films of perovskites, can still cause the occurrence of SRH recombination. As the incorporated PMMA mainly leads to the interfacial modification, it may be of necessity to resort to complementary strategies to further reduce the total amount of defects/traps to boost the ultimate device performance. With the demonstrated reduction in surface charge-trapping with PMMA, it will be of interest to examine how the electron transport in CH3NH3PbI3-xClx is influenced. For this purpose, we examined the electron mobility in CH3NH3PbI3-xClx using singlecarrier devices, which were attained by sandwiching the perovskite layer and PC61BM (PMMA) between ITO/ZnO (bottom) and Al (top) contacts. By controlling the polarity of the bias, electrons can be injected through the PC61BM/Al interface to the LUMO of CH3NH3PbI3-xClx while maintaining the blocking for counterpart holes. Figure 4 c compares obtained J–V charChem. Eur. J. 2017, 23, 14650 – 14657

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acteristics of single carrier devices at room temperature. A quadratic voltage dependence within a large bias range is observed, indicating the transport characteristic of space–charge limited current (SCLC). This behavior contrasts to the observed trap-filling behavior in CH3NH3PbI3,[42, 43] which may be due to the smaller device thickness of CH3NH3PbI3-xClx ( & 200 nm), leading to higher carrier densities in the SCLC device that could mask the effect of traps.[14] Consistent to the improved surface trap passivation, we observe an enhanced electron current for the device containing PMMA1. When incorporating PMMA with larger MW, the current enhancement becomes less obvious. We further determined the electron mobility (me) with the well-established Mott–Gurney law as, 9 ðV @ Vbi Þ2 J ¼ e0 er me 8 L3

ð2Þ

in which J is the current density, e0er is the permittivity of perovskite films ( & 25), and L is the active layer film thickness. As summarized in Table S5, the me is increased from 2.9 V 10@3 cm2 Vs@1 to 3.45 V 10@3 cm2 Vs@1, and to 3.35 V 10@3 cm2 Vs@1 with PMMA2 and PMMA3, respectively, and is further improved to 4.1 V 10@3 cm2 Vs@1 when using PMMA1. Since the bulk conductance in perovskite photoactive layers should be hardly affected, the enhanced electron transport can be primarily ascribed to the interface modification with the suppressed surface trap trapping. Although the lateral conductivity in PC61BM EEL is reduced with PMMA, the transport in the perovskite layer along the out-of-plane direction is improved, which is more relevant to solar cell operation.

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Figure 5. a) Comparison of the decay of power conversion efficiencies (PCE) of CH3NH3PbI3-xClx solar cells normalized to initial performance as a function of storage time. b–e) SEM images of perovskite layers with b) PC61BM, c) PC61BM(PMMA1), d) PC61BM(PMMA2), and e) PC61BM(PMMA3), scale bar = 1 mm.

The mediated interface properties in CH3NH3PbI3-xClx solar cells are further confirmed by impedance spectroscopy. Figure 4 d shows Nyquist plots of the impedance measured under 1 sun irradiation. An equivalent circuit modeling (ECM) with single component of resistance R and capacitance C in parallel components can provide satisfactory fittings. Based on ECM, the series resistance (Rs) was determined to decrease to 87.81 W, 90.05 W, and 97.76 W after incorporating PMMA1, 2, and 3, respectively, from the original value of 135.8 W in the control sample without PMMA (Table S6, SI). The reduction in Rs explicitly manifests the modified interfacial properties with PMMA. Also, we found that the Rs is inversely proportional to MW, which seems to correlate with trend in Jsc. Furthermore, we note the reduction in radius of the semi-arc for devices with PMMA in Figure 4 d, which is indicative of a decreased resistivity, related to bulk transport in perovskite solar cells. This result can to some degree correlate with the slightly enhanced carrier mobility with the PCBM(PMMA) EEL determined by single carrier devices. At last, we explore the incorporation of PMMA for improving the stability in perovskite solar cells. To minimize the influence of electrodes degradation to our analysis, all devices were tested under nitrogen environment without encapsulation. Figure 5 a shows the normalized PCE of CH3NH3PbI3-xClx solar cells (with respect to fresh performance) as a function of storage time. After about 1 month of storage, the cell without PMMA exhibits considerable degradation (90 %). The cell with PMMA1 degrades to approximately 50 % after the same storage time. Interestingly, enlargement of the PMMA MW leads to decelerating degradation process. 75 % of the fresh PCE is maintained by using high MW (PMMA3). To clarify the reason behind this, we explored the film morphology of CH3NH3PbI3-xClx capped with PC61BM(PMMA) by scanning electron microscopy (see Figure 5 b–e). The presence of pinholes can be clearly identified in CH3NH3PbI3-xClx without PMMA, and such a feature correlates to some degree with the reduced FF in the corresponding solar cells. In contrast, the size and density of pinholes are both lowered after incorporating PMMA. This modification is also consistent to the reduced leakage current in CH3NH3PbI3xClx solar cells (see Figure S3). The decreased pinholes along Chem. Eur. J. 2017, 23, 14650 – 14657

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with the reduced leakage currents may increase the shunt resistance and ultimately improve the diode quality and resulting FF. Furthermore, we examined the surface wettability (see results in Figure S4) and found that the hydrophobicity was not enhanced by PMMA, revealed by the reduced contact angles. Combining these results, we attribute the improved stability mainly to the improvement of the film quality in PCBM (PMMA), which enables the formation of a dense and compact layer with fewer pinholes, to strengthen the robustness and improve the protection of the perovskite layer. Based on the demonstrated improvements on solar cell parameters and device lifetimes by PMMA, it is advantageous that these two metrics can be simultaneously tuned and balanced by choosing appropriate MW for PMMA.

Conclusions To summarize, we have demonstrated a facile and effective interface strategy based on inert PMMA that allows for the simultaneous improvement of the PCE, suppression of the hysteresis, and deceleration of the degradation in performance for solution-processed CH3NH3PbI3-xClx solar cells, for which a 30 % enhancement of PCEs has been achieved based on a low MW PMMA. We attribute the improved performance and reduced hysteresis to the passivation of surface traps related to non-coordinated Pb2 + by the functional groups in PMMA, leading to mitigating the SRH recombination such that the FF and ultimate PCE are enhanced. The changes in electronic structure at the cathode interface with PMMA are evidenced by the shifts in CPD by Kelvin probe force microscopy. The considerably elongated device lifetime is found to originate from the improved protection for CH3NH3PbI3-xClx films provided by the dense and stable PC61BM(PMMA) EEL with a reduced density of pinholes. Through choosing the appropriate MW for PMMA, the enhancements of the device parameters and stability can be simultaneously tuned and balanced. This work demonstrates a promising opportunity to tackle important concerns of perovskite solar cells by using low-cost and accessible polymeric materials.

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Full Paper Experimental Section Materials: Various PMMAs (molecular structure is shown in Figure 1 a) in different molecular weights (MW) were purchased from Sigma Aldrich. PbCl2 (99.999 %, Sigma) and CH3NH3I (purity scale: 99.8 %, Dyesol) were used as received. PEDOT:PSS (AL 4083 Clevios) and PC61BM were purchased from H. C. Starck GmbH and Lumtec (Taiwan), respectively. Device fabrication: For solar cells, the precursor solutions were prepared by solubilizing the mixture of PbCl2 and CH3NH3I (1:3 in molar ratio) in dimethylformamide with a total concentration of 27 wt %. PC61BM(PMMA) solutions were prepared by dissolving 20 mg of PC61BM in 1 mL of chlorobenzene by mixing with desired amounts of PMMA additives. To start, ITO substrates were cleaned by soap-scrubbing and sonicating sequentially in ultra-pure water, acetone, and isopropanol and finally drying at 80 8C for 20 min before use. PEDOT:PSS layers were spin-coated at 2000 rpm, leading to an average thickness of 40 nm determined by a profilometer (KLA-Tencor). The CH3NH3PbI3-xClx thin films and PC61BM (PMMA) EELs were both solution-deposited in a nitrogen-purged glove box. CH3NH3PbI3-xClx precursor solutions were spin-coated on PEDOT:PSS-coated ITO substrates at 2000 rpm. The thickness of CH3NH3PbI3-xClx layer was determined to be & 270 nm using the profilometry. The cast CH3NH3PbI3-xClx films were then thermally annealed on a hotplate at 90 8C for 90 min. Following the deposition of perovskite layer, the PC61BM (PMMA) solutions were cast on top of the annealed CH3NH3PbI3-xClx films at selected spin-rates to attain different film thicknesses. Finally, 80 nm of Au was thermally evaporated on top of PC61BM (PMMA) through shadow masks, resulting in an effective device area of 0.04 cm2. For single carrier devices, active layers of CH3NH3PbI3-xClx and PC61BM (PMMA) EELs were prepared identically to solar cells. The CH3NH3PbI3-xClx was solution-cast on the ITO glass substrates precoated with 30 nm of ZnO thin film. Organic field-effect transistors (OFET) based on active layers of PC61BM (PMMA) were fabricated as follows: Si/SiO2 substrates (oxide layer thickness = 300 nm) with pre-patterned Au source-drain electrodes defined by photolithography were cleaned following the same procedures for preparing the ITO substrates. Prior to spin-coating, the Si/SiO2 substrates were passivated with OTS-8 through self-assembly. The PC61BM (PMMA) films were deposited on top of OTS-treated substrates by spin-coating (2000 rpm), leading to a film thickness of & 50 nm. Device testing and materials characterization: Current-density versus voltage (J–V) characteristics of CH3NH3PbI3-xClx solar cells and single carrier devices were recorded by a Keithley 2400 Sourcemeter under nitrogen atmosphere. CH3NH3PbI3-xClx solar cells were tested under irradiation by using a solar simulator calibrated by a standard Si cell, and single carrier devices were tested in dark. The irradiation intensity was adjusted by an optical wheel filter and determined according to the current reading of the calibration cell. IPCE spectra of CH3NH3PbI3-xClx solar cells were measured by a Newport external quantum measurement system (Model 66920). PC61BM (PMMA) OFETs were characterized in a Lakeshore vacuum probe station by a Keithley semiconductor parameter analyzer (model 4200 SCS). Impedance spectroscopy of CH3NH3PbI3-xClx solar cells was measured by a potentiostat under nitrogen environment and the data were analyzed by using the ZView 2 program. For topography and local contact potential difference (CPD) measurements, PC61BM (PMMA) thin films were directly cast on the conductive ITO substrates. The KPFM measurement of PC61BM (PMMA) surfaces was performed by an atomic force microscopic (AFM) system (model Bruker Dimension Icon) under a dual-pass lift mode. In the first pass, the topography is obtained in tapping mode and Chem. Eur. J. 2017, 23, 14650 – 14657

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then the probe is lifted 30 nm above the surface to measure the CPD between the tip and sample. A gold sample was used to calibrate the work function of Pt/Ir coated silicon tip before and after each experiment.

Acknowledgements This work was supported by the Chinese Academy of Sciences (100 Top Young Scientists Program and QYZDB-SSW-SLH033) and the National Key Research and Development Program of China (2017YFA0206600). Y.Z. thanks for the financial support by the National Natural Science Foundation of China (Grant No. 21674006).

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Manuscript received: July 21, 2017 Accepted manuscript online: August 17, 2017 Version of record online: September 18, 2017

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