The Critical Role of Vertical Phase Separation in Small Molecule

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Organic Electronic Devices

The Critical Role of Vertical Phase Separation in Small Molecule Organic Solar Cells Jin Fang, Dan Deng, Zaiyu Wang, Muhammad Abdullah Adil, Tong Xiao, Yuheng Wang, Guanghao Lu, Yajie Zhang, Jianqi Zhang, Wei Ma, and Zhixiang Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00886 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

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ACS Applied Materials & Interfaces

The Critical Role of Vertical Phase Separation in Small Molecule Organic Solar Cells Jin Fang1, Dan Deng1, Zaiyu Wang2, Muhammad Abdullah Adil1, Tong Xiao3, Yuheng Wang3, Guanghao Lu3, Yajie Zhang1, Jianqi Zhang1, *, Wei Ma2, *, Zhixiang Wei1,4 * 1

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence

in Nanoscience, National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing 100190, China, 2

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an

710049, China 3

Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China

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University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Keywords: small molecule organic solar cells, open-circuit voltage loss, vertical phase separation, crystallite orientation, charge transport

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Abstract: Inverted device structure is a more stable configuration than regular device structure for solution-processed organic solar cells. However, most of the solution-processed small molecule organic solar cells (SM-OSCs) reported in literatures were using regular device structure, and regular device normally exhibits a higher efficiency than that of inverted device. Herein, a representative small molecule DR3TBDTT was selected to figure out the reason for photovoltaic performance differences between regular and inverted devices. The mechanisms for a reduced open-circuit voltage (Voc) and fill factor (FF) in the inverted device were studied. The reduced Voc and FF is due to the vertical phase separation with excess PC71BM near air/blend surface, which leads to a reduction in build-in voltage and unbalance charge transport in the inverted device. Another reason for the reduced FF is the unfavorable DR3TBDTT crystallite orientation distribution along the film thickness, which is preferential edge-on crystallites in the top layer of blend film and the increased population of face-on crystallites in the bottom layer of blend film. This study illustrates that the morphology plays a key role on photovoltaic performance difference between regular and inverted structured devices, and provides useful guidelines for further optimizing the morphology of solution-processed SM-OSCs.


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1. Introduction Organic solar cells (OSCs) have attracted great attention as a promising candidate for future thin-film photovoltaic technology.1 Solution-processed small molecule organic solar cells (SMOSCs), composed of small molecule as electron donor and fullerene derivatives as electron acceptor have developed quickly over the past few years.2-10 Solution-processed SM-OSCs could be produced by regular or inverted device structures. In a regular device structure, a poly (3,4ethylenedioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS) coated upon indium tin oxide (ITO) is used as anode and a low work function metals such as aluminum are used as cathode. In an inverted device structure, air-stable high work function metals and n-type metal oxides coated upon ITO substrate are used as the anode and cathode, respectively.11 Up to know, most of high efficient SM-OSCs were produced with the regular device structures. For instance, Chen et al. synthesized a series of acceptor−donor−acceptor simple oligomer-like small molecules based on oligothiophenes, and an efficiency higher than 10% was obtained based on regular structure devices.7 Peng et al. reported low bandgap small molecules with two different electron-accepting groups, and the efficiency could reach as high as 11.53%.10 Generally, the inverted device structures are preferred over the regular device structures, which ultimately lead to better stability.11-14 In polymer solar cells, vertical phase separation structures also benefit photovoltaic performance due to surface-enrichment of polymer donors.15-16 For small molecules, we found that fluorination leaded to optimal active layer morphology and an average power conversion efficiency of 11.08% was achieved for a two-fluorine atom substituted molecule by using inverted structures.9,

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However, without properly substituted atoms or

special film processing methods, we found the small molecules usually show lower photovoltaic performance, peculiarly reduced Voc in inverted devices than in regular ones.18-26 In fact, only a


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few of SM-OSCs based on the inverted devices have been reported in literatures, which shown better performance comparing with regular devices.9, 11, 17, 27-30 Therefore, it is highly needed to illustrate the reason why most of SM-OSCs are suitable for regular device structures instead of inverted device structures. Herein, we selected a representative small molecule DR3TBDTT (Figure 1a) as the electron donor31-32 to construct SM-OSCs with regular and inverted devices, and the reason for the opencircuit voltage (Voc) and fill factor (FF) difference between the two kind devices is investigated by various device and morphology measurements. The low performance of the inverted device is most probably due to an unfavorable vertical phase separation, including donor/acceptor composition and crystallite orientation. The unique characteristics of such vertical phase separation in the inverted structure reduce build-in voltage, impede charge transport and increase charge recombination, resulting in lower Voc and FF.

2. Results and discussion 2.1 Solar cell performance and charge recombination The chemical structures of DR3TBDTT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) are presented in Figure 1a. And the schematic illustration of regular device ITO/ molybdenum trioxide (MoO3)/active layer/Zinc oxide (ZnO)/Ag) and inverted device (ITO/ZnO/active layer/MoO3/Ag) are shown in Figure S1. To eliminate the influence of the electrode property, solution-processed ZnO nanoparticles and thermal evaporated MoO3 were used as the cathode and anode buffer layer both for the regular and inverted devices. Ultraviolet photoelectron spectroscopy (UPS) was carried out to obtain work function of the MoO3 and ZnO on ITO substrates (Figure S2).33 The work function of the MoO3 and ZnO buffer layers were 5.41 eV and 4.00 eV, respectively. The Fermi level of DR3TBDTT on ITO/MoO3 substrate and


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PC71BM on ITO/ZnO substrate was 4.49 eV and 4.06 eV, respectively. The highest occupied molecular orbital (HOMO) level for DR3TBDTT was 5.02 eV and lowest unoccupied molecular orbital (LUMO) level for PC71BM was 3.91 eV measured by cyclic voltammetry (CV) method.31, 34

According to the integer charge-transfer (ICT) model33, a Fermi level pinning happened

between MoO3 and DR3TBDTT, as well as between ZnO and PC71BM. The typical current density versus voltage characteristics (J-V) of the regular and inverted devices of DR3TBDTT:PC71BM are shown in Figure 1b, and the device parameters are summarized in Table 1. Regular device displays a power conversion efficiency (PCE) of 7.55%, with a Voc of 0.927 V, a short-circuit current density (Jsc) of 11.84 mA/cm2, and a FF of 68.8%. In the inverted device, the Jsc is improved from 11.84 to 12.84 mA/cm2 because of the better transmittance of ZnO than MoO3 when used as the transparence electrodes (Figure S3a), and the optical spacer effect for the inverted devices.35 The measured Jsc is consistent with the EQE curve as shown in Figure S3b. However, significant decrement in the PCE (4.66%) was observed in the inverted device, mainly due to the simultaneous reduction of the Voc to 0.760 V and the FF to 47.7%. The Voc difference (∆Voc) between regular and inverted devices is 0.167 V. The Voc value is related to the value of the dark saturation current density (Jsat) and the diode ideality factor (n). By assuming infinite shut resistance and zero series resistance, the equivalent circuit model for the ideal solar cell can be simplified as the following equation at one sun illumination: Voc=(nKT/q)ln(Jph(Voc)/Jsat), where Jph(Voc) is the photogenerated current density at Voc, which is equal to the dark current density at Voc. q is the elementary charge, k is the Boltzmann’s constant and T is temperature.36-37. To obtain the corresponding parameters (Jph(Voc), Jsat and n), the dark J-V curves were measured as shown in Figure 1c. The Jsat and n were evaluated by fitting the exponential regime of the dark J-V curves using the equivalent


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circuit equation as JD=Jsat[exp(qV/nkT)-1], where JD is the dark current density.36, 38 The resulting parameters are summarized in Table 1, where it can be seen that in terms of Jsat there is 5 orders of magnitude difference between the two type devices, as the regular device gave a Jsat of 3.1×10-11 mA/cm2 compared to that of 3.8×10-6 mA/cm2 in inverted device. Physically, Jsat is the number of injected charges able to overcome the energy barrier in the reverse bias direction36-37, which indicating the inefficient charge blocking in the inverted device. The diode ideality factor n changes from 1.43 of regular device to 2.52 of inverted device. The ideality factor reflects the different charge transport and recombination behavior of the devices in the regular and inverted devices.38-39 The Jph(Voc) is 3.01 mA/cm2 and 0.47 mA/cm2 for the regular and inverted devices, respectively. Hence, the lower Voc in inverted device could be explained by the larger saturation current density, larger ideality factor and lower photocurrent density. In parallel, the ideality factor under illumination (S) was found to be 1.0 and 1.5 for the regular and inverted devices, respectively, which was determined from the slope (SkT/q) of Voc as a function of the logarithm of light intensity (Plight) (as shown in Figure 1d).39 S is equal to 1.0 implies that the bimolecular recombination is dominating mechanism in the regular device. While for the inverted device, S is 1.5 when the light intensity is between 20 mW/cm2 to 100 mW/cm2, indicate increased monomolecular recombination in the inverted device. An increase in the slope is observed when the light intensity below 20 mW/cm2, indicating an increase in the ratio of the monomolecular recombination under low charge density. Figure 1e represents the photocurrent (Jph) as a function of light intensity. Jph is defined as J under illumination subtracts JD. Veff is defined V0 subtract V, where V0 is the compensation voltage at which Jph= 0 and V is the applied voltage. The experimental data are fitted by a power law dependence Jph∝(Plight)σ, where σ is the exponential factor.40 At high effective voltage, e.g. at Jsc condition, as shown in


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Figure S4a, the exponential factors were 0.99 and 0.96 for the regular and inverted devices, respectively. The charge recombination at Jsc condition of the inverted devices is slightly higher than that of the regular device, however, no build-up of net space-charge form in both devices at Jsc condition40. At low effective voltage of 0.1 V, the exponential factor σ deceases to 0.74 for the inverted device, near the 0.75 exponent for the space-charge-limited photocurrent40, implying a build-up of space charge and the increased bimolecular recombination, which cause Jph and charge collect probability decreased quickly around the Voc condition (low effective voltage range, as shown in Figure S4b). In contrast, σ for the regular device is 0.91, indicating no space charge build-up and lower bimolecular recombination than that of the inverted device. And as shown in Figure S4c, the charge collect probability of the regular device is better than that of the inverted device.

2.2. Charge transport property and vertical composition distribution AFM and TEM were used to study the morphology of the active layer on MoO3 and ZnO substrates. As shown in Figure S5, the morphology of the active layer show similar feature. Therefore, the reasonable contribution to the inefficient charge blocking, increased charge recombination and the space-charge-limited photocurrent in the inverted device would be the interface difference between the active layer and the electrodes. To verify the proposed mechanism, the charge transport behavior of the active layers in the direction normal to the substrate was investigated. We measured the hole and electron mobilities in the regular and inverted devices by using hole- and electron-only devices41-44, with the structures ITO/MoO3/active layer/MoO3/Ag and ITO/ZnO/active layer/ZnO/Ag, respectively. As shown in inset of Figure 2a and b, in the hole-only device, when ITO is forward or reverse biased, holes are injected from the bottom electrode (ITO/MoO3) or the top electrode


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(MoO3/Ag), respectively. In the electron-only device, when ITO is forward or reverse biased, electrons are injected from the top electrode (ZnO/Ag) or the bottom electrode (ITO/ZnO), respectively. Figure 2a and b show the current density-voltage curve of the hole- and electrononly devices under dark condition, respectively. Hole-only device demonstrates a lower measured current under reverse biased voltage. The asymmetric curve indicates a hole injection barrier near the top electrode (MoO3/Ag) is presented, which is usually related to the vertical compositional gradient in blend film.42 While electron-only device shows a relatively symmetric behavior, suggesting Ohmic contact forms between ZnO and the blend film. The process of charge extraction is the opposite of the charge injection process. Here we considered the hole and electron measured current under the forward and reverse biased voltages to be similar to the hole and electron transport in the regular and inverted devices, respectively. From this point of view, as shown in Figure S6, a more balanced charge transport is achieved in the regular device. For the inverted device, a strong unbalanced charge transport is observed, resulting in the holes accumulation in the device and ultimately leads to the formation of space-charge-limited charges at low effective voltage40. To investigate the compositional gradient of the blend films in the regular and inverted devices, depth-profiled X-ray photoelectron spectroscopy (XPS) was used.42, 45 As sulfur element is only present in the small molecule, S/C ratio is used to identify the distribution of DR3TBDTT. As shown in Figure 2c, the curves of S/C ratio for the regular and inverted devices are almost the same, with thin layer (about 5 nm) DR3TBDTT enrichment at the blend/air interface (Zone 1), followed by PC71BM enrichment (more than 20 nm) near the blend/air interface (Zone 2), and then the donor and acceptor mixed well (Zone 3). The DR3TBDTT enrichment at top interface is benefit for hole injection from the MoO3/Ag electrode. However,


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PC71BM aggregation in zone 2 form a hole injection barrier that lowers hole injected current under reverse biased voltage in the hole-only device (Figure 2a). At low effective voltage, hole cannot overcome the barrier, which causes the lower Voc and FF, and with the effective voltage increasing, hole can flow tough zone 2 under high Coulomb force (as shown in Figure S4b and c). In contrast, in the regular device, where the photo-generated hole transfer to the bottom MoO3 layer, while the enrichment of the donor on the top surface don’t influence electron extraction46. Water contact angle characterizations were used as semiquantitative method to calculate the surface component of the active layer by Cassie−Baxter equation.47 The average contact angles of the pure DR3TBDTT, pure PC71BM film and the blend film are 97.9°, 105.3° and 104.6° and 100.0°, respectively. The calculated surface content ratio of PC71BM was about 10%, which is consistent with the XPS data. After spin-coating isopropanol (the solvent of ZnO nanopartiles) on the blend film, the surface content ratio of PC71BM increased to about 70%, which is more benefit for the electron extraction in the regular device. This vertical composition distribution would facilitate hole transport to the bottom layer, while block hole transport to the top layer in the regular device. Mott-Schottky (MS) analysis was performed to study the influence of the hole injection barrier (enriched PC71BM layer) on the build-in voltage.48-49 The values of Vbi were calculated by the relation A2/C2=2/qεε0N(Vbi-V), where A is the active area, C is the capacitance, q is the elementary charge, εε0 is the dielectric permittivity of the active layer, N is the doping density and V is applied voltage.50 Figure 2d shows the MS plots of hole-only, regular and inverted devices in the dark at 10 kHz frequency. The values of Vbi are 0.118 V, 0.841 V and 0.736 V for hole-only, regular and inverted devices, respectively. The Vbi difference between regular and inverted devices is 0.105 V, which is close to 0.118 V, indicating the hole injection barrier


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influences the built-in voltage of the inverted device.49 Interestingly, when measured with 100 mW/cm2 illumination, the hole-only device showed photovoltaic property with a Voc of 0.158 V (details in Supporting Information, Figure S7). The data is also close to the Voc difference (∆Voc = 0.167 V) between the regular and inverted devices, suggesting the decrease of Voc in the inverted device is due to the the built-in voltage difference origin from the hole injection barrier. Surface energies is one the factors influence the vertical composition distribution.16, 51 The contact angles of different liquids with the known dispersive and polar component were measured on DR3TBDTT, PC71BM films52, as shown in Table S1. By using the Owens/Wendt theory52, the surface energy of DR3TBDTT and PC71BM was calculated to be 26.9 mJ/m2 and 37.7 mJ/m2, respectively. Hence, in order to minimize free energy at the blend/air interface, DR3TBDTT enrichment takes place. Although it is believed that a donor-rich top layer is usually followed with a thin PCBM-rich sublayer due to the limited PCBM diffusion in the rapid filmdrying process16, 53, the reason for such a thick PC71BM aggregation layer underneath the top surface cannot be fully proved at this stage. We supposed during the film dry processing, at beginning, DR3TBDTT and PC71BM mix well; and then DR3TBDTT and PC71BM aggregate as the solvent evaporated; at the final stage, the excess PC71BM will aggregate in the top layer of the blend due to the better solubility of PC71BM and the worse miscibility between the ordered DR3TBDTT and PC71BM.54 Therefore, We designed an experiment to reduce the influence of PC71BM aggregation on the Voc by lowering the ratio of PC71BM in the film.55 Table S2 summarized the photovoltaic performance of the inverted devices with different D/A ratio. From the table it can be seen that by increasing D/A ratio to 10:1, an increased Voc of 0.932V was observed, which is as high as the regular device. However, the FFs of the inverted devices with different D/A ratio are still lower than that of the regular device.


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2.3. Crystallite Orientation Distribution To further gain insight to the low FF, GIWAXS was used to study the molecular packing variation in the vertical direction of active layer by varying the incident angles.56 When the incident angle is under the critical angle, surface structure information was revealed from the GIWAXS. With the increased incident angle, the thickness of the blend structure information can be probed. 2D-GIWAXS of blend films at the surface of MoO3 and ZnO under different incident angles are shown in Figure S8 and S9. The critical angle of MoO3 and ZnO is about 0.13° and 0.12°, respectively. At the critical angle, the blend film on MoO3 exhibited ring-like scattering (as shown in Figure 3a) arising for π-π stacking around 1.75 Å-1, suggesting almost no preferential orientation in the blend films. While the blend film on ZnO also showed ring-like scattering but more pronounced intensity in the in-plane direction (as shown in Figure 3b), indicating a preferential “edge-on” orientation. This could be attributed to the difference in wettability of the solution on the substrate surfaces.57 However, as shown in Figure S8 and S9, under the critical angle, the scattering intensity of π-π stacking in the out-of-plane direction is much lower than that in the in-plane direction, suggesting edge-on is the preferential orientation of the small molecules in the upper part of the active layer. To get a deeper insight into the orientation, in-plane and out-of-plane π-π stacking GIWAXS profiles of the blend films were plotted in Figure 3c and d. We defined the integrated area of the π-π stacking diffraction along out-of-plane (Az) and in-plane (Axy) direction as the fraction of the face-on and edge-on crystallites. It is interesting to note that the ratio of Az to Axy (Az /Axy) for the active layer on MoO3 substrate was 0%, 1.5%, 6.2%, 16.0% and 103.2% at 0.09°, 0.10°, 0.11°, 0.12° and 0.13° incident angle, respectively, while the ratio for the active layer on ZnO substrate was 2.4%, 2.6%, 9.5% and 36.1% at 0.09°, 0.10°, 0.11° and 0.12°, respectively. The


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more favorable face-on orientation of the small molecule crystallites on the MoO3 substrate indicates the surface energies of the substrate surface influence the molecular alignment in the blend films.58 The population of the face-on crystallite increase from the top surface to the bottom of blend films. It is believed that the charge mobility of the edge-on crystallite is smaller than that of the face-on crystallite.57, 59 The edge-on crystallite is another factor which contributes to low hole mobilities and unbalanced charge transport in the inverted device. To confirm the vertical crystallite orientation in the blend films, the lamellar stacking around 0.3 Å-1 were studied, which is perpendicular to the π-π stacking in the blend film.60 As shown in Figure S10, the ratio of the face-on to edge-on crystallites increased with the incident angles, suggesting the increased population of face-on crystallites along the film thickness. Based on the above discussion, the vertical phase separation of the blend film with vertical composition and crystallites orientation distribution is schematically shown in Figure 4. Where, the small molecule DR3TBDTT enrich on the air/blend surface with edge-on orientation (Zone 1) and PC71BM enrich underneath the thin layer of DR3TBDTT (Zone 2). After isopropanol treatment, the PC71BM ratio in the surface will increase. This distribution in the upper part of the blend film is detrimental for the hole transport in the inverted device and is an efficient hole blocking layer in the regular device. Then, DR3TBDTT:PC71BM form well mixed bulk heterojunction (Zone 3), and the proportion of the face-on orientation DR3TBDTT increase along the thickness of the active layer (Zone 3), which facilitates the hole transport to the bottom electrode. Overall, the vertical phase separation with such composition and crystallite orientations is suitable for the regular device rather than the inverted device in SM-OSCs.

3. Conclusions


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In summary, we studied the photovoltaic performance difference with regular and inverted devices, using small molecule DR3TBDTT as donor and PC71BM as acceptor. Our study demonstrates vertical phase separation formed in the active layer; DR3TBDTT enrichment at the top of the photoactive layer because of low surface energy of DR3TBDTT. Excessive PC71BM enrichment takes place near the top of the blend film. Such vertical phase separation decreased the built-in potential and increased the Voc loss in the inverted device. Importantly, we found that small molecule crystallites with increased face-on crystallites are present at the bottom part of the active layer, and the edge-on orientation is enriched at the upper part of the active layer. The vertical phase separation with such crystallite orientations also causes the low FF in the inverted device. These results are useful for synthesizing a new small molecule for the inverted device with proper surface energy and using some proper ways to enrich the donor on the top of the active layer while control the molecular orientation.

4. Experimental Section 4.1 Materials and device fabrication DR3TBDTT and PC71BM were purchased from 1-Material Inc. and American Dye Source, Inc. (99.5% purity), respectively. ZnO nanoparticles was synthesized according to the reference, dispersed in isopropanol.61 ITO substrates were cleaned by using deionized water, ethanol, acetone, and isopropanol thrice in an ultrasonic bath, step wise for 10 min each respectively. The substrates were then dried using compressed N2 gas followed by ultraviolet/ozone treatment for 5 min. A 10 nm thick MoO3 anode buffer layer was thermally evaporated at a rate of 0.1 Å s−1 under a vacuum of about 1×10-4 Pa. ZnO nanoparticle solution was spin-coated at 3000 rpm to obtain a 30 nm thick cathode buffer layer. The active layers were spin-coated at 3000 rpm in a N2 filled glove box


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from an 18 mg/ml solution of DR3TBDTT:PC71BM (1:0.8 weight ratio) in chloroform, leading to an active layer thickness of approximately 100 nm. The as-cast films were first thermally annealed at 110°C for 10 min, followed by chloroform solvent vapor annealing, where the substrates were placed in a glass petri dish containing chloroform, for 1 min. Ultimately, the silver electrode was deposited by thermal evaporation under a vacuum of about 1×10-4 Pa. The active area of the SM-OSCs device was 0.04 cm2. 4.2 Measurements and Characterization J-V characteristics measured by a Keithley 2400 source meter under AM 1.5G (100 mW/cm2) spectrum from a solar simulator (Enli Technology Ltd.) in the glove box. Solar simulator illumination intensity was calibrated using a standard Si photovoltaic cell with a KG-5 filter window. External quantum efficiencies (EQEs) were characterized by a solar cell spectral response measurement system (Enli Technology Ltd.) in the glove box. AFM measurements were performed using a Dimension 3100 scanning probe microscope (Veeco) in tapping mode. TEM specimens were prepared by floating an active layer film onto the surface of 1M HCl solution, then transferred to copper grids and washed with deionized water thrice. Measurements were performed using a Tecnai G2 U-TWIN (FEI) transmission electron microscope. In-depth XPS (Thermo Scientific™ ESCALAB 250Xi) profiling was performed on active layers (ITO/MoO3/active layer for regular device structure, ITO/ZnO/active layer for inverted device structure). The samples were sputtered with an Art gun at 2,000 eV and were etched from the air/blend interface with a Kα X-ray photoelectron spectrometer. Integrated areas from the C 1s, S 2p, Zn 2p and Mo 3d peaks were used for data analysis.


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Surface energy of the small molecule and PC71BM films was performed with an OCA 20 contact angle system (Dataphysics Instruments GmbH, Germany). Mott-Schottky(MS) analysis was performed in the dark condition using a VMP3 Photentiostat/Galvanostat (EG&G, Princeton Applied Research). The amplitude was 25 mV to maintain response linearity, and the frequency applied during measurement was 10 kHz. Grazing incidence wide-angle X-ray scattering (GIWAXS) characterizations were performed at beamline 7.3.3 at the Advanced Light Source (ALS). Samples were prepared on MoO3/Si and ZnO/Si substrates. The X-ray beam was incident at a grazing angle of 0.09°-0.18°. A Dectris Pilatus 2M photo counting detector was using to detect the scattering X-rays.56


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zone 3

zone 2 0.06 0.04 0.02 0

300

600

900

1200

1500

16000 Regular device Inverted device Hole-only device

-1 2 2

Blend film on ZnO Blend film on MoOx

zone 1

12000

2

0.12

[A/C] ([cm µF ] )

0.14

S/C ratio

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


8000 4000 0 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Etching time (s)

Figure 2. Double-logarithmic current density versus voltage characteristics of a) hole-only device and b) electron-only device. The insets schematically show hole and electron inject from different electrodes in the forward and reverse bias directions. c) XPS depth profiles of S/C intensity ratios as a function of etching time for the DR3TBDTT:PC71BM regular and inverted devices (etching begins at the air/film interface). d) Mott-Schottky plots of hole-only device, regular and inverted devices in dark condition measured at constant frequency of 10 kHz.


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Figure 3. GIWAXS 2D patterns of DR3TBDTT:PC71BM blend films on a) MoO3/Si and b) ZnO/Si substrates at the critical angles. c) and d) is π-π stacking in in-plane and out-of-plane directions correspond to a) and b) under different incident angles.


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Figure 4. Schematic illustration of vertical composition and crystallite orientation distribution of DR3TBDTT/PC71BM blend film. Table 1. Photovoltaic performance of regular and inverted devices under illumination of AM 1.5G, 100 mW/cm2, and parameters from the dark J-V of the regular and inverted devices. Voc

FF

Jsc

PCE

Jph(Voc)

Jsat n

(V)

(mA/cm2) (%)

(%)

(mA/cm2)

(mA/cm2)

0.927

11.84

68.8

7.55

3.1×10-11

1.43

3.01

Inverted 0.760

12.84

47.7

4.66

3.8×10-6

2.52

0.47

Regular

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:. Additional characterization data (Device structures, UPS, UV-vis spectrum, EQE, Jph -Veff, AFM, TEM, J-V characteristic of hole-only device and different D/A ratios, surface energy, GIWAXS) (PDF) AUTHOR INFORMATION Corresponding Author *Email: [email protected]. *Email: [email protected]. *Email: [email protected]. ORCID Jin Fang: 0000-0002-8268-1879


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Jianqi Zhang: 0000-0002-3549-1482 Wei Ma: 0000-0002-7239-2010 Zhixiang Wei: 0000-0001-6188-3634 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by the Ministry of Science and Technology of China (No 2016YFA0200700), the National Natural Science Foundation of China (Grant Nos 21534003, 91427302, 51773047, 21604017 and 21504066) and the ‘Strategic Priority Research Program’ of the Chinese Academy of Sciences (Grant No XDA09040200). Two-dimensional GIWAXS experiments were acquired at beamlines 7.3.3 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Con-tract No DE-AC02-05CH11231. REFERENCES (1)

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The Critical Role of Vertical Phase Separation in Small Molecule

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