Vertical Stratification Engineering for Organic Bulk-Heterojunction

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Vertical Stratification Engineering for Organic Bulk-Heterojunction Devices Liqiang Huang, Gang Wang, Weihua Zhou, Boyi Fu, Xiaofang Cheng, Lifu Zhang, Zhibo Yuan, Sixing Xiong, Lin Zhang, Yuanpeng Xie, Andong Zhang, Youdi Zhang, Wei Ma, Weiwei Li, Yinhua Zhou, Elsa Reichmanis, and Yiwang Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00439 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Vertical Stratification Engineering for Organic Bulk-Heterojunction Devices Liqiang Huang a, #, Gang Wang b, *, #, Weihua Zhou a, #, Boyi Fu b, Xiaofang Cheng a, Lifu Zhang a, Zhibo Yuan b, Sixing Xiong c, Lin Zhang d, Yuanpeng Xie a, Andong Zhang e, Youdi Zhang a, Wei Ma d, Weiwei Li e, Yinhua Zhou c, Elsa Reichmanis b, * and Yiwang Chen a, * L. Huang, Prof. W. Zhou, X. Cheng, L. Zhang, Y. Xie, Dr. Y. Zhang, Prof. Y. Chen a

College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031,

China Dr. G. Wang, Dr. B. Fu, Z. Yuan, Prof. E. Reichmanis b

School of Chemical and Biomolecular Engineering, School of Chemistry and

Biochemistry, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, 30332, USA S. Xiong, Prof. Y. Zhou c

Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic

Information, Huazhong University of Science and Technology, Wuhan 430074, China L. Zhang, Prof. W. Ma d

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

University, Xi’an 710049, China A. Zhang, Prof. W. Li e

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of

Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China #

Author contributions: L. H., G. W., and W. Z. contributed equally to this work.

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Keywords: stratification engineering, surface energy, organic bulk-heterojunction, organic photovoltaics, interface engineering, self-assembled small molecule, external quantum efficiency Abstract: High efficiency organic solar cells (OSCs) can be produced through optimization of component molecular design, coupled with interfacial engineering and control of active layer morphology. However, vertical stratification of the bulk-heterojunction (BHJ), a spontaneous activity which occurs during the drying process, remains an intricate problem yet to be solved. Routes toward regulating the vertical separation profile and evaluating the effects on the final device should be explored to further enhance the performance of OSCs. Herein, we establish a connection between the material surface energy, absorption and vertical stratification which can then be linked to photovoltaic conversion characteristics. Through assessing the performance of temporary, artificial vertically stratified layers created by the sequential casting of the individual components to form a multilayered structure, optimal vertical stratification can be achieved. Adjusting the surface energy offset between the substrate results in donor and acceptor stabilization of that stratified layer. Further, a trade-off between the photocurrent generated in the visible region and the amount of donor or acceptor in close proximity to the electrode was observed. Modification of the substrate surface energy was achieved using self-assembled small molecules (SASM), which in turn, directly impacted the polymer donor to acceptor ratio at the interface. Using three 2 ACS Paragon Plus Environment

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different donor polymers in conjunction with two alternative acceptors in an inverted organic solar cell architecture, the concentration of polymer donor molecules at the ITO (Indium tin oxide)/BHJ interface could be increased relative to the acceptor. Appropriate selection of SASM facilitated a synchronized enhancement in external quantum efficiency and power conversion efficiencies over 10.5%.


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Organic solar cells (OSCs) fabricated using an electron donor (D) and an electron acceptor (A) endowed with well-designed molecular structures, suitable energy levels and strong, broad absorption in the visible region, have produced photovoltaic devices with efficiency exceeding 13%.1 Compared with its planar alternative, the bulk-heterojunction (BHJ) structure has fulfilled the promise of achieving high-performance OSCs. Appropriate domain size and sufficient interfacial contact between the donor and acceptor components comprising the BHJ adequately promote exciton dissociation into free charges.2 However, elaborate control of the micromorphology, including coordination between component crystallinity, phase separated domain size, molecular orientation, and vertical component distribution in a BHJ is a sophisticated task.3,4 D/A ratio, solvent and additives in solution, thermal and solvent annealing conditions in film fabrication and so on, affect the morphology of blend films significantly and ultimately determine device performance.5 Among the multitude of fundamental considerations, vertical stratification, which originates from spontaneous phase segregation of the donor and acceptor in the vertical dimension during the film drying process, is a key factor that closely correlates with OSC performance.6-17 Quite simply, delivery of a hole to the anode along consecutive donor domains benefits from interfaces that are donor-rich at the anode,

and

vice

versa.

For

instance,

it

was

proposed

that

increasing

poly(3-hexylthiophene) (P3HT) surface coverage at the anode would yield a good conventional OSC; however, device performances were not strictly determined by the measured level of vertical stratification.18 In another example, continuous fullerene 4 ACS Paragon Plus Environment

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channels throughout the active layer achieved by a two-step off-center spinning method were also shown to enhance device performance,19 and fullerene and non-fullerene acceptors typically performed better with an inverted vs. conventional device architecture.20,21 The active layer region of interest was viewed as three sub-layers having different enrichment levels of donor or acceptor.22 Three active material sub-layers, which can be manipulated by anode and cathode interfacial characteristics and BHJ solution blend ratio, were identified in the region between the underlying substrate and upper metal electrode. For both conventional and inverted structures, PC71BM ([6,6]-phenyl C71 butyric acid methyl ester) clusters tend to aggregate on ITO (Indium tin oxide)/ZnO (Zinc Oxide) or

ITO/PEDOT:PSS

(poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate))

surfaces due to similarities in their surface energies; while donor materials are inclined to aggregate at BHJ/air interface as polymer donor materials have relatively higher surface energy and lower solubility in chlorobenzene and chloroform. In practice, the evolution of BHJ morphology is a complicated process, determined by both thermodynamic and kinetic factors.23 While various additives and interfacial materials have been demonstrated to enhance OSC performance, the question of how to regulate the interfacial properties at the electrodes to induce optimal vertical stratification and thus, photoelectric performance remains unanswered. Meanwhile, for

the

promising

non-fullerene

acceptor

system

based

on

ITIC24

(3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexy lphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene), the exploration 5 ACS Paragon Plus Environment

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and optimization on the unrevealed vertical phase segregation are meaningful. Therefore, a universal guideline to achieve a desirable vertically stratified layer leading to an optimized BHJ is an urgent need to further improve the performance of state-of-the-art OSCs. In this study, we used self-assembled small-molecules (SASMs) to modulate the interface surface energy between the electrodes and BHJ to induce different levels of vertically stratified donor and acceptor materials in a BHJ. The impact of surface energy and the relationship between vertical stratification and material surface energy were investigated using a diketopyrrolopyrrole (DPP) -based polymer, HD-PDPPTPT and a benzodithiophene (BDT)/thieno[3,4-b]thiophene (TT) -based polymer, PTB7-Th (Figure 1a) in fullerene-based OSCs. We proposed that the level of PC71BM enrichment on ITO surface can be decreased with a decrease in surface energy of the interlayer. Correlated with the wider charge transportation path and thus larger effective conductivity, the inverted devices produce higher photocurrent than corresponding conventional devices. Further, the ITIC acceptor was found to have similar

behavior

when

used

in

conjunction

with

a

BDT/BDD

(benzodithiophene-4,8-dione) -based polymer, PBDB-T (Figure 1a). Charge transfer in the inverted device was facilitated because ITIC favors the ZnO surface. Additionally, through a sequential casting process and SASM modification, an optimal vertically stratified profile was induced to form in a PBDB-T:ITIC BHJ which was reported to exhibit an improved photovoltaic conversion efficiency (PCE) of 10.8%. Through manipulation of electrode surface energy, we have demonstrated a 6 ACS Paragon Plus Environment

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relationship between electrode surface energy, device active layer vertical stratification, and external quantum efficiency (EQE). A trade-off between the photocurrent in the visible region and the quantity of donor or acceptor in close proximity to the transparent electrode is proposed. Whereas an anode surface enriched with donor material facilitates the conversion of light absorbed by the acceptor, conversion of light absorbed by the donor is simultaneously suppressed. Using the fundamental insights into the ‘surface energy - device active layer vertical stratification - external quantum efficiency’ relationships developed here, we designed and demonstrated a strategy that induces desired vertical stratification leading to higher short-circuit current density (Jsc) with greater photocurrent generated in the red or blue region. The elevated EQE in the red region (ITIC absorption region) can contribute the Jsc more obviously than that in the blue region (PC71BM absorption region). With an improved whole spectral response to sunlight achieved through vertical stratification engineering, the performance of state-of-the-art OSCs can be further enhanced. RESULTS AND DISCUSSION Surface energy induced vertical stratification Among the efforts to optimize OSCs through interface engineering, most research has aimed at matching materials work functions and carrier mobilities, whereas the effect of surface energy on OSC performance has not received significant attention. Historically, it was believed that the substrate surface energy directly determined the wettability of the D/A mixed solution. The relationship between substrate surface 7 ACS Paragon Plus Environment

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energy and vertical stratification in the P3HT and PC61BM ([6,6]-phenyl C61 butyric acid methyl ester) blend system has been well considered.7,15 For a conventionally functionalized bottom electrode, its surface energy is always much larger than the donor and acceptor. If the surface energy of the donor is lower than that of the acceptor, the surface energy discrepancy will result in enrichment of the acceptor on the substrate, and vice versa. Because P3HT possesses lower surface energy than PC61BM, deposition of the blend is likely to afford PC61BM-rich domains on the substrate. Thus, in this work, the surface energy of the materials has been examined first. As shown in Table S1, ITO glass with simple air plasma treatment exhibited a surface energy of ~68.0 mN m-1 as determined by the Owens method.25 Coating the ITO glass substrate with a 30 nm PEDOT:PSS film led to an increase in surface energy to ~72.8 mN m-1. In the scope of interlayer energetics and surface energy, there are few stable, high work function SASM modifiers with a low enough surface energy. In general, SASMs functionalized with relatively long hydrophobic groups have a low surface energy, but in turn, may adversely affect charge transfer.26 As an alternative, we developed a series of Cl-substituted small molecules to serve as anode modifiers.27,28 Herein, a hydro-stable SASM, 4-chlorobenzenesulfonyl chloride (CBSC), with a chlorine atom/molecule mixed coverage of ~7×10-10 mol/cm2 was applied to the substrate (Table S2). The modified ITO exhibited a stable work function of 5.3 eV and a surface energy of 66.9 mN m-1. The surface energies of the polymer donor materials HD-PDPPTPT29 and PTB7-Th30 were 27.0 and 26.9 mN m-1, respectively. 8 ACS Paragon Plus Environment

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These values were somewhat smaller than that of PC71BM, 27.9 mN m-1, suggesting that PC71BM will be enriched at the substrate interface for both CBSC and PEDOT:PSS modified ITO. To study the impact of surface energy, vertical stratification of the BHJs was revealed through time-of-flight secondary ion mass spectrometry (TOF-SIMS). A ~90 nm thick HD-PDPPTPT:PC71BM film was fabricated on neat Si wafers with 2.2 nm thick native oxide layer. At the same time, the film surface was protected with a ~23 nm thick

poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-

dioctylfluorene)] (PFN)31 film and maintained the conventional OSC condition at the same time (Figure 1b,c). Sulfur was used to track the polymer donors, and nitrogen was used to define the active layer and PFN interface. For a classical P3HT:PC61BM blend system, the profiles can be normalized according to the reference.32 In our system, it is difficult to accurately depict the vertical composition profiles because of the different sputter speeds of materials. In addition, diffusion, interface roughness, and the contribution of S signal in PEDOT:PSS and CBSC close to the anode cannot be ignored. Therefore, the profiles of the interface might be skewed to some extent. As shown in Figure 1d and 1e, an escalating S signal was obtained at the BHJ/PFN interface because of the rough BHJ surface. The higher S signal in the left sublayer indicated a donor-rich BHJ/PFN interface. However, the D/A ratio in the blend was 1:2, which indicated the proportion of PC71BM in the film was larger than HD-PDPPTPT, the enrichment level was compared to the S signal in the middle sublayer. Based on the intensity of the S signal, the BHJ vertical stratification can be 9 ACS Paragon Plus Environment

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divided into three parts. From the BHJ in contact with the bottom substrate (ITO/HTL), to the bulk material, and finally to the BHJ material in contact with the upper metal electrode (PFN/Al), the three sub-layers are controlled by the BHJ/HTL interface, D/A blend ratio in solution, and BHJ/PFN interface, respectively. Because the surface energy of the HTL is much larger than those of the active materials, and the surface energy of PC71BM is larger than HD-PDPPTPT, fullerene clusters are enriched in the right sub-layer as the solvent evaporates. When such a distinction between the HTL and active materials decreases, the degree of enrichment is weakened (Figure 1d,e). The enrichment of the fullerene acceptor reaches a minimum within the region controlled by the D/A blend ratio. The amount of D/A remains constant in the D/A blend ratio controlled region until it reaches the cathode controlled region. It had been pointed out that better delivery of charge was found when donor materials were enriched on the anode surface.18 Thus, narrowing the surface energy offset between the interfaces and the active material is expected to be beneficial in obtaining a more evenly distributed interface composition. Subsequently,

we

investigated

vertical

stratification

using

alternative

polymer/fullerene BHJs in both conventional and inverted conditions. The circumstances of BHJ films on PEDOT:PSS or CBSC, and on ZnO or ZnO/MPPA ((4-methoxyphenyl)phosphonic acid) substrates were shown in Figure S3 and S4, respectively. For PTB7-Th system with obvious diversity in molecular structure, the intensity of F signal at BHJ/CBSC/SiO2 interface improved significantly (Figure S3a,b), as a result of the interference effect of H3O+ fragment ion from the CBSC 10 ACS Paragon Plus Environment

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grafted SiO2. The F signal and S/C signal variation for each sample were almost the same. The vertical stratification profiles of HD-PDPPTPT and PTB7-Th are different, although they possess similar surface energy. One possible cause is the differences in chemical structures and dissolved solution, another possible cause is the influence of additive-induced vertical segregation, and the measurement was not carried out immediately as fabricated, this condition will be further discussed in the following sections. Similar phenomena were observed when the BHJ films were coated on the surface of ZnO or ZnO/MPPA with a structure of Si/ZnO/(MPPA)/BHJ/MoO3. At the same time, the skewed S signal which originated from PEDOT:PSS and CBSC can be eliminated, but the influence of O-O fragment ion still remained. After MPPA was grafted onto the ZnO surface, the surface energy decreased from 49.0 mN m-1 to 45.1 mN m-1. The constantly enhanced O signal and Zn signal indicated the proximity to the BHJ/ZnO interface. From the vertical profile spectra (Figure S4), the polymers inclined to aggregate on ZnO/MPPA surface to different extents. The S signal was obviously reduced for sensitive HD-PDPPTPT:PC71BM film on ZnO, but the signal showed indiscernible change for the same film on the ZnO/MPPA substrate. For the HD-PDPPTPT system on an ITO electrode with either PEDOT:PSS or ZnO interlayer, the polymer donor material enriched at the BHJ/air interface, and fullerene acceptor material enriched at the ITO/BHJ interface. However, for PTB7-Th on ZnO or ZnO/MPPA, the S and F signal discrepancy appears negligible from the spectral results. 11 ACS Paragon Plus Environment

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To further explore the relationship between the vertical stratification profile and surface energies of active materials and interlayers. The enthalpy of D/substrate or A/substrate interface formation, ∆G, was used to qualitatively weigh the acceptor-rich degree, according to Good's equation15:

(1) where the subscripts 1 and 2 refer to the photoactive material (D or A) and the substrate, respectively. Symbols d and p refer to the dispersion component and polar component of surface energy. The results were summarized in Table S3, a more negative value means that a more stable interface has formed. The relative degree to which the interface was acceptor-rich (∆) was approximated from the enthalpies of D/substrate and A/substrate. When the value of ∆ is greater than 1, the buried sub-layer is an acceptor-enriched layer. Different BHJ materials combinations examined here showed ∆ values greater than 1 on the four modified ITO substrates (Table S4). The values for both BHJs on PEDOT:PSS (ZnO) were slightly improved with respect to the values calculated for ITO/CBSC (ZnO/MPPA) substrates. Notably, ∆ values of the HD-PDPPTPT and PTB7-Th were different so that their tendency to undergo vertical stratification cannot be identical. OSC performance and Spectral response OSCs were fabricated with the architecture of ITO/hole transporting layer (HTL)/polymer:PC71BM/PFN/Al

and

ITO/electron

transporting

layer

(ETL)/polymer:PC71BM/MoO3/Ag. Current density versus voltage (J−V) curves for


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the best device for each polymer were presented in Figure S5, and corresponding device

parameters

were

summarized

in

Table

1.

For

the

conventional

polymer:PC71BM OSCs, the Jsc values for devices fabricated on ITO/CBSC improved within 6% compared to those on ITO/PEDOT:PSS, while changes in open-circuit voltage (Voc) were imperceptible. The change in Jsc was relatively weak than the change in EQE spectra. For HD-PDPPTPT based inverted device, the presence of MPPA had little effect on Jsc; however, the presence of MPPA had obviously effect on Jsc

for

PTB7-Th

based

inverted

devices.

Notably,

ITO/ZnO/MPPA/PTB7-Th:PC71BM/MoO3/Ag OSCs generated a maximum PCE of 10.6%. On the other hand, the inverted OSCs based on HD-PDPPTPT:PC71BM BHJ exhibited much higher Jsc values vs. the conventional counterpart, demonstrating that aggregation of the donor at the interface with the anode and aggregation of acceptor at the cathode interface could prove favorable for achieving higher overall efficiency. As different vertical stratification gradients became established, the donor to acceptor ratio throughout the thickness of the film might change, which in turn might alter the holistic spectral response and thus the Jsc. For this reason, the absorbance profiles of the blend films, as well as the pure donor and acceptor films, were studied as shown in Figure 2a. Examination of Figure 2a showed that longer wavelength light was primarily absorbed by HD-PDPPTPT; while PTB7-Th contributed to absorption of 600-750 nm light in conjunction with a contribution in the 320 nm region as well. PC71BM compensated the limited absorbance profile of the polymers through absorption of light in the shorter wavelength region. In addition to the absorption 13 ACS Paragon Plus Environment

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profile, extinction coefficient (k) plays a significant role, whereby an increase in k corresponds to enhanced absorption, suggesting that an improved photo-generated current could be obtained. Figure 2b,c presented the values of k measured for the blend films fabricated on PEDOT:PSS and CBSC modified quartz glass. The BHJ films fabricated on ITO/CBSC exhibited slightly increased k values in the long wavelength regions, coupled with somewhat decreased extinction coefficients at short wavelengths. These values can then be correlated with the donor to acceptor ratios throughout the entire film. The total absorption efficiency (TAE) includes the parasitic absorption that occurs outside the active layer region and can be derived from the UV-vis diffuse reflectance spectra (DRS) measurement as TAE = 1 - R.33 As shown in Figure S6, the CBSC modified devices showed higher TAE, which in turn leads to a higher Jsc. Since molecular orientation might be expected to affect absorbance and charge transfer, the molecular stacking was explored through grazing-incidence wide-angle X-ray scattering (GIWAXS). Based upon an analysis of GIWAXS data (Figure 2d), overall polymer crystallinity improved for films fabricated on Si/CBSC. Both donors exhibited well-defined (100) diffraction patterns along the qz (out-of-plane) axis arising from an ordered lamellar structure; and a (010) peak along the qxy (in-plane) axis corresponding to the π-π stacking of the polymer backbones. The lamellar d-spacing distance for HD-PDPPTPT, and PTB7-Th were 17.95 Å, 20.27 Å, respectively; and the π-π stacking distances were 3.83 Å, and 3.36 Å, respectively (Table S5). PTB7-Th and HD-PDPPTPT adopted both edge-on and face-on 14 ACS Paragon Plus Environment

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orientations; though the edge-on orientation was more distinct for samples fabricated on Si/CBSC (Figure S8), which exhibited stronger intermolecular interactions with the polymers. The introduction of PC71BM grains may alter polymer packing and the incidence of tie chains, and thus determines the charge transport of the full film. The change in photocurrent can be viewed as the result of changes in D to A ratio, absorption efficient and molecular crystallinity. The vertical stratification associated with conventional and inverted device architectures was illustrated in Figure 3a,b. Device EQE was measured in order to further study the spectral response, as shown in Figure 3c-f. The integrated Jsc (dashed line) for each solar cell fabricated on ITO/CBSC was higher than that for devices fabricated on ITO/PEDOT:PSS; and similarly, OSCs fabricated on ITO/ZnO/MPPA vs. ITO/ZnO also exhibited a higher integrated value of Jsc. For HD-PDPPTPT system with different device architectures, EQE improved in the region between 350 nm and 450 nm, which is the predominant absorption region for PC71BM. When HD-PDPPTPT was used as the donor, EQE in the region between 700 nm and 850 nm (HD-PDPPTPT predominant absorption region) decreased dramatically. Alternatively, when a PTB7-Th BHJ was combined with the conventional device structure, the EQE curves for devices fabricated with and without CSBC, intersected often; while the same BHJ fabricated with an inverted device archtecture exhibited an obviously improved EQE across the entire absorbance region. The increased spectral response in the longer wavelength regions contributes to higher photocurrent according to the formula as follows: 15 ACS Paragon Plus Environment

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(2) where e is the elementary charge, h is the Plank's constant, c is the speed of light, λ is the wavelength, and ф is the photon flux. As shown in Figure S9b, when EQE increased from 70% to 90% in the region of 300-550 nm (relative growth 28.6%), the Jsc increased from 19.0 mA cm-2 to 21.0 mA cm-2 (relative growth 10.5%); and when EQE increased from 70% to 90% in the 550-800 nm region (relative growth 28.6%), Jsc increased from 19.0 mA cm-2 to 22.5 mA cm-2 (relative growth 18.4%). Because the TAE spectrum can be altered by using a different interlayer, as well as the resultant film absorption and vertical stratification, higher device performance can be obtained through balancing and enhancing the EQE response in whole wavelengths. As the EQE values increased in the blue region but slightly decreased in the red region, higher EQE in the red region contributed the Jsc more significantly. Thus, the Jsc for the HD-PDPPTPT based devices fabricated on ITO/CBSC finally showed slight improvement, but the change in EQE (>10%) was noteworthy and beyond the error range. A relationship between the variation in EQE and vertical stratification can be observed for the HD-PDPPTPT:PC71BM layers. Specifically, EQE increased in the predominant absorption region of the acceptor materials when donor materials were somewhat enriched on the substrate. Additives have also been frequently used to enhance OSC performance, and thus the impact of additives on the performance of devices was mainly attributed to the change in crystallinity and domain size.34,35 According to the GIWAXS results (Figure S10),


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the crystallinity of PTB7-Th:PC71BM blend films with 1,8-diiodooctane (DIO) additive was shown to be improved. Resonant soft X-ray scattering (R-SoXS) was performed to investigate the phase separation of PTB7-Th:PC71BM films. The distribution of scattering profiles can be fitted by lognormal functions and represents the distribution function of spatial frequency (s, s = q/2π). Therefore, the distribution mode (smode) corresponds to the characteristic mode length scale (domain spacing, ξ = 1/smode) and the mode domain size (ξ/2).36-38 The ξ values were decreased from 179 nm (q = 0.035 nm-1) to 43 nm (q = 0.146 nm-1) with DIO additive, as shown in Figure S10d, corresponding to the mode domain size decreased from 89.5 nm to 21.5 nm. The decrease in domain size of PTB7-Th:PC71BM blend film was in accordance with previous observation on PTB7:PC71BM blend film with DIO additive.39 By comparing the EQE curves in Figure S11, the EQE derived from the donor and acceptor absorption showed simultaneous improvement after incorporation of DIO. At the same time, it has been proposed that combining the use of DIO additive with methanol extraction can produce a BHJ film that is fullerene-enriched at the air and relatively polymer-enriched at the substrate interface.13 Thus, the incorporation of DIO will result in a transition of the vertical stratification that may simultaneously alter the EQE. Whether or not DIO was incorporated into the film, the EQE derived from PC71BM improved after modification of the substrate with SASM having lower surface energy. Due to the presence of the DIO additive, the film drying time became longer, thereby providing time for vertical stratification induced by surface energy to occur. The alcohol soluble PFN solution was used for reference. To further 17 ACS Paragon Plus Environment

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demonstrate the impact of differences in surface energy on the interlayer independently, another DPP system, DT-PDPP4T:PC71BM was studied (Figure S12).40 The results consistently showed the PC71BM contribution to EQE improved after SASM modification without the incorporation of DIO. Transfer Matrix Method (TMM) was used to model the optical distribution in the devices (Figure 3g).41 Active layers on differently treated substrates were studied using variable angle spectroscopic ellipsometry (VASE), as shown in Figure S13. The change in phase angle (ψ and ∆) stands for the difference in index of refraction (n + ik), where k was shown in Figure 2b,c. The distinction in ψ and ∆ occurred in a region coincident with the EQE curves and further supported the validity of the observed result. The accurate n value can be observed by the method reported previously.22,42 The simulated and integrated Jsc from 350 nm to 800 nm increased, which was in accordance with the experimental device Jsc. In order to prove that the change in EQE is connected with the vertical stratification and donor to acceptor ratio of active layer, different stratified HD-PDPPTPT:PC71BM with featured parameters (n, k) combining with same device architecture (ITO/CBSC/HD-PDPPTPT:PC71BM (n1, k1 or n2, k2)/PFN/Al) was simulated by TMM method (Figure S15). It can be found that the EQE or the electric field intensity was in connection with the characteristics of active layer, not only the thickness of different interlayer. By excluding the reflection and simulated parasitic absorption, relatively accurate internal quantum efficiency (IQE) was obtained for the HD-PDPPTPT system (Figure S16). The exciton generation rate in the device improved, especially in the position close to the 18 ACS Paragon Plus Environment

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ITO transparent electrode. Thus, it can be concluded that the improved Jsc originated from the optimized vertical stratification of active layers. CBSC and PEDOT:PSS modification slightly influenced the optical distribution in the device, but the consequent vertical stratification in the active layers played a crucial role in the optical distribution. Time-resolved photoluminescence (TRPL) was carried out to investigate the exciton dissociation and charge transfer in polymer:PC71BM blends (Figure S17).43,44 Only PC71BM emission under 405 nm excitation and the quenching of PC71BM emission by the polymer on different substrates was studied. A shorter decay time was observed for the HD-PDPPTPT system, while the PTB7-Th BHJ on a CBSC modified substrate exhibited a longer decay time. No direct relationship has been found between the fluorescence lifetime and the improved EQE, as the unpredictable effect of different HTLs may result in discrepancies. To broaden the application scope of the observed trends using a fullerene acceptor, PBDB-T:ITIC was introduced to fabricate an artificial vertically stratified BHJ.45 Here, a sequential spin-coating method was adopted to explore the optimized vertical gradation and exclude possibility that SASM treatment might in of itself influence the results. Considering the lower enthalpy of the ITIC/ZnO interface in comparison to the PBDB-T/ZnO interface (Table S3), ITIC is expected to preferentially gather on the ZnO surface, which guarantees a favorable electron transfer channel. The insertion of a pure PCBM layer at the anode can significantly suppress charge generation in solar cells, however suppression of charge generation is relatively weak when a pure P3HT layer was inserted at the cathode.46,47 Here, the impact of inserting a pure 19 ACS Paragon Plus Environment

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PBDB-T or ITIC film was studied (Table S6 and S7); notably, the inhibiting effect associated with both PBDB-T and ITIC were weaker than what was found for PCBM. The differences between ITIC and PCBM might be due to their respective differences in energy levels (ITIC, LUMO, -3.8 eV, HOMO, -5.5 eV; PC71BM, LUMO, -4.0 eV, HOMO, -6.0 eV) and hole mobility. Then, four different D/A ratios (9:1, 5:1, 1:5, and 1:9) with the same total concentration (3 mg/mL) were used to fabricate the buried sub-layer under a normal PBDB-T:ITIC (1:1, 20 mg/mL) active layer (Figure 4a). The thickness of the first as prepared buried layer was about 25 nm, and the conventional PBDB-T:ITIC film had a thickness of 110 nm. The sequentially prepared films were 124 ± 5 nm thick. The sequential preparation process of the upper layer will inevitably dissolve the buried active layer, and at the same time, provide for better connectivity. According to the reported results, the sequential casting method created a pseudo-bilayer structure with a mixed or rough region at the interface (about 15 nm) between the two BHJ layers.48 The diffusion of donor and acceptor materials is ongoing during the solvent drying process. To avoid the diffusion effect from further expanding, thermal annealing was not utilized.49,50 The Jsc was improved for the device with a thick sequentially prepared film. For the fresh devices after thermal evaporation of the top metal electrode, the device Voc decreased due to inappropriate donor or acceptor enrichment on the substrate. The circumstance of acceptor enrichment on the substrate was better than donor enrichment for the inverted device (Table 1). Interestingly, the performance of the device from the sequential process dramatically changed after 20 hours. Although accompanied by attenuation of device 20 ACS Paragon Plus Environment

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performance, the performance of the device with a buried layer from the D/A blend ratio of 9:1 improved because of material diffusion. The sequential casting method is an effective way to probe the advantageous vertical stratification for a BHJ system. In view of the results, SASM modifier was used to fabricate thermodynamically stable vertical stratification to fix the optimized gradient. When the surface energy of ZnO was decreased through MPPA modification to achieve a suitable work function, the proportion of ITIC that enriched the material in direct contact with ZnO decreased and higher device efficiency (10.8%) was obtained. The degree to which PBDB-T:ITIC on ZnO/MPPA was acceptor-rich was 2.16, which was smaller than that observed for ZnO, namely, 2.30. Therefore, there might be an overly enriched ITIC interface on untreated ZnO. EQE curves of fresh sequentially prepared devices were measured for comparison (Figure 4b). When the proportion of acceptor increased, EQE in the 350 nm to 540 nm region, mainly relying on the donor absorption improved, while EQE in the 540 nm to 800 nm region, mainly associated with acceptor absorption, deteriorated. With its lower surface energy, MPPA modification reduced aggregation of ITIC on the ZnO surface and resulted in a higher EQE. The improved EQE can be deconvoluted into an enhanced EQE component from the ITIC absorption region and a lower EQE from the PDBD-T absorption region. Meanwhile, comparing the EQE data, it can be concluded that the higher overall EQE for the MPPA modified device is not only related to vertical stratification, but also dependent on the interface properties, such as the passivated ZnO surface defects. Higher Jsc with greater photocurrent generated in the red region was achieved 21 ACS Paragon Plus Environment

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through the reduction of the amount of strongly absorbing. Although the change in EQE for ITIC system was relatively smaller than for the PC71BM analog, the improved EQE in the red region effectively increased the Jsc of ITIC-based device. Through adjusting the vertical segregation profiles, the PCE varied from 9.4% to 10.8% (relative change 14.9%). The TRPL intensity of films was also studied with selective excitation of PBDB-T and ITIC, respectively (Figure 4c,d).51,52 More efficient quenching of PBDB-T emission by ITIC was observed as ITIC became enriched on the substrate. At the same time, more efficient quenching of ITIC emission by PBDB-T was observed as PBDB-T was enriched on the substrate. Hence, an optimized D/A ratio of pre-coated active layer exists to balance the enrichment of D and A on the substrate. In addition, the decay time of films fabricated on MPPA was slightly longer than for films on untreated ZnO. Thus, the high Jsc observed for MPPA modified devices can be attributed to the higher exciton generation rate from the optimized vertical stratification. The vertical stratification of PBDB-T:ITIC was verified through dynamic X-ray photoelectron spectroscopy (DXPS). Nitrogen (N) is a feature atom of ITIC, and PBDB-T possesses more sulfur (S) compared to ITIC in the same volume, thus, the S/N ratio can be used to determine the variation in D/A ratio. A higher S/N ratio suggests a PBDB-T-rich sub-layer (Figure 5). The prominent P2p signal after 2000 s etching provided evidence of MPPA on the ZnO surface, and diffusion of MPPA into the BHJ was insignificant. The S2p and N1s signals were enhanced at the ZnO(MPPA)/BHJ interface to a different extent. By comparing the S/N ratios, it can 22 ACS Paragon Plus Environment

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be concluded that the air/BHJ interface was enriched with PBDB-T. With the assistance of MPPA, a less ITIC-rich interface was obtained. The result can be used to support the observed variations in EQE. On the basis of previous reports, the depth profiles of P3HT:PCBM films showed that P3HT diffused toward the surface during annealing, verifying that the measured vertical stratification was thermodynamically driven.53 DXPS results indicated that the vertical stratification obtained via the sequential process that deposited double layers from two solutions with different concentrations was not stable. The vertical stratification will eventually reach a thermodynamic equilibrium state, and thus an ITIC-rich interface. In addition, devices with longer storage time of active layer before electrode deposition were fabricated, as shown in Figure S20. The efficiency increased in the first four hours. With longer periods of time, there was no obvious change in solar cell efficiency. The above results indicated a good compatibility between PBDB-T and ITIC. Hence, while it is possible to optimize BHJ vertical stratification by sequentially spin-coating double active layers, it is also necessary to stabilize that stratification, perhaps by using suitable surface modifiers. DISCUSSION The vertical stratification of BHJ films was previously assumed to be invariant when combined with different interfacial layers. Further, the improved device performance observed upon introduction of interfacial layers was principally attributed to interlayer energy levels and carrier mobility. However, the effect of vertical stratification in the active layer should not be ignored, and results presented here suggest that it could be 23 ACS Paragon Plus Environment

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finely tuned by coordinating the surface energy of each component in the device. The proposed hypothesis that photovoltaic conversion performance can be improved through surface energy induced vertical stratification is presented in Figure 6. Through altering the surface energy offset between the interface and active materials, a donor-rich or acceptor-rich buried sublayer can be achieved at the BHJ/substrate interface. Acceptor materials have higher surface energies than donor materials, and traditional interlayers have relatively high surface energies (Table S1).54-56 Consequently, an acceptor-rich BHJ/ITO interface is always in presence. Meanwhile, a trade-off exists between the photocurrent in the visible region and the amount of donor or acceptor close to the ITO electrode. Enrichment of the acceptor on the ITO surface can facilitate donor absorption and conversion into the current. By properly reducing the amount of donor or acceptor at the BHJ/ITO interface, a higher short-circuit current density with greater photocurrent generation can be obtained. Because the enhanced EQE in the long wavelength region can remarkably improve the final Jsc value, for common acceptor materials, such as PCBM (strong ultraviolet absorption) and ITIC (strong absorption in the red), increasing the surface energy offset between the interlayer and PCBM or decreasing the surface energy offset between the interlayer and ITIC (improving the photocurrent in the red region) will be conducive to obtaining higher EQE in red region and higher Jsc. The performance of OPVs has been obstructed by the unsatisfactory EQE values. Through equalizing the EQE in the blue and red region and realizing higher EQE in whole solar spectra from vertical stratification engineering, it has the potential to realize higher Jsc and thus 24 ACS Paragon Plus Environment

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higher PCE. CONCLUSION In summary, the vertical stratification profile of the BHJ has been carefully studied through interfacial modification and sequential casting of multilayered films. Our results offer a universal guideline on constituting superior vertical stratification with higher Jsc. For both conventional and inverted devices, fullerene and non-fullerene acceptor materials aggregate on ITO surface, which increases EQE contributed from the polymer donor and decreases EQE contributed from the acceptor. By decreasing the surface energy of ITO through SASM modification, the amount of polymer donor increases at the ITO surface. The EQE contributed from the acceptor improved. Thus, the desired vertical stratification profile can be established by employing interfacial materials with objective surface energy, and the Jsc can be modulated by the vertical stratification directly. Under appropriate selection of SASM, a simultaneous increase in external quantum efficiency and power conversion efficiencies over 10.5% were achieved in fullerene and non-fullerene acceptor based OSCs. Our work provides a perspective for designing interfacial materials and donor/acceptor materials with desired surface energies and device optimization to further improve the efficiency of state-of-the-art OSCs. METHODS Device fabrication and characterization. Patterned ITO transparent electrode (ITO-P001-1, Zhuhai Kaivo Optoelectronic Technology Co. Ltd., 8-10 Ω per square) was cleaned by sequential sonication in acetone, soap and deionized water, deionized water, and isopropanol. For conventional devices, after 2 min air plasma (AP) (Plasma Cleaner PPC862) treatment, CBSC (TCI Co. Ltd.) methanol solution (0.5 mg/mL) or PEDOT:PSS (CLEVIOS P VP AI 4083) was subsequently prepared on ITO (4000 25 ACS Paragon Plus Environment

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rpm, 1 min). HD-PDPPTPT:PC71BM (1:2, 15 mg/mL for all, 6% o-DCB in CHCl3, 90 o C) solution was prepared and spin-coated (2100 rpm, 1 min) on the HTL. DT-PDPP4T:PC71BM (1:2, 9 mg/ml for all, 7.5% o-DCB in CHCl3, 90 oC) solution was prepared and spin-coated (1000 rpm, 1 min) on the HTL. PTB7-Th:PC71BM (1:2, 18 mg/mL for all, 3% DIO in CB, 70 oC) solution was prepared and spin-coated (1000 rpm, 2 min) on the HTL. A 5 nm PFN interlayer was obtained by spin-casting on the top of active layers at 2500 rpm for 30 s from a methanol and glacial acetic acid (99.7:0.3 vol%) mixed solution with a concentration of 1 mg/mL. The device was completed after the deposition of 100 nm of Al as the top electrode. The effective area of the fullerene-based devices is 0.37 cm2. For inverted devices, a 30 nm thick ZnO film was prepared on ITO by spin-coating at 4,000 rpm from a ZnO precursor solution. PBDB-T and ITIC (Solarmer Materials Inc.) with different blend ratios were dissolved in CB with 0.5% of DIO. A 110 nm PBDB-T:ITIC layer was spin-coated from the solution (10:10, 20 mg/mL for all) at 2500 rpm. The additional buried active layer under the conventional PBDB-T:ITIC active layer was spin-coated from the dilute solution (9:1, 5:1, 1:5, 1:9, 3 mg/mL for all) at 1000 rpm. Inverted devices were completed after deposition of 5.5 nm MoO3 and 70 nm Ag (base pressure < 3 × 10−6 Torr). The effective area of the ITIC-based devices is 0.41 cm2. Current density-voltage (J−V) characteristics of the devices were measured using a Keithley 2400 Source Meter and Abet Solar Simulator Sun 2000. The light source was calibrated by using silicon reference cells with an AM 1.5 Global solar simulator with an intensity of 100 mW/cm2. The external quantum efficiency (EQE) was detected under monochromatic illumination (Oriel Cornerstone 260 1/4m monochromator equipped with an Oriel 70613NS QTH lamp), and the calibration of the incident light was performed using a monocrystalline silicon diode. Surface energy characterization. Contact angle measurements of films were performed at a Krüss DSA100s Drop Shape Analyzer. Water (secondary reverse osmosis method, 72.8 mN m-1, 25 oC) and hexadecane were used as probe liquids. TOF-SIMS characterization. TOF-SIMS experiments were carried out on a Physical Electronics PHI TRIFT III spectrometer. Samples for TOF-SIMS were prepared using the same methods for the active layer for the organic photovoltaics. XPS profiling. X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) was used for binding energy and element distribution analysis. And all XPS spectra were calibrated by setting the peak corresponding to aliphatic carbon to 285 eV. Samples for XPS were prepared using the same methods for the active layer for the organic photovoltaics. For the etching parameters, high etching power was applied and each etching cycle was 200 s. GIWAXS characterization. GIWAXS measurements were performed at beamline 7.3.357 at the Advanced Light Source. Samples were prepared on Si substrates using identical blend solutions as those used in devices. The 10 keV X-ray beam was 26 ACS Paragon Plus Environment

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incident at a grazing angle of 0.12°-0.16°, selected to maximize the scattering intensity from the samples. The scattered x-rays were detected using a Dectris Pilatus 2M photon counting detector. R-SoXS characterization. R-SoXS transmission measurements were performed at beamline 11.0.1.258 at the Advanced Light Source. Samples for R-SoXS measurements were prepared on a PSS modified Si substrate under the same conditions as those used for device fabrication, and then transferred by floating in water to a 1.5 mm × 1.5 mm, 100 nm thick Si3N4 membrane supported by a 5 mm × 5 mm, 200 µm thick Si frame (Norcada Inc.). 2-D scattering patterns were collected on an in-vacuum CCD camera (Princeton Instrument PI-MTE). The sample detector distance was calibrated from diffraction peaks of a triblock copolymer poly(isoprene-b-styrene-b-2-vinyl pyridine), which has a known spacing of 391 Å. The beam size at the sample is approximately 100 µm by 200 µm. AFM analysis. The morphologies of active layers were investigated by Bruker Multimode8 high resolution scanning probe microscope. The thickness of the active layer was roughly estimated by AFM scanning. XPS/UPS analysis. The X-ray photoelectron spectroscopy studies were performed on a Thermo-VG Scientific ESCALAB 250 photoelectron spectrometer using a monochromated Al Ka (1,486.6 eV) X-ray source. All recorded peaks were corrected for electrostatic effects by setting the C−C component of the C1s peak to 284.8 eV. The energy level of the film was investigated by ultraviolet photoelectron spectroscopy using Thermo-VG Scientific ESCALAB 250 with a He I (21.22 eV) discharge lamp. A bias of −8.0 V was applied to the samples for separation of the sample and the secondary edge of the analyzer. Optical characterizations. The UV-Vis absorption and reflection spectra of the films were taken with a Lambda 750 UV-Vis spectrometer. The thickness of interlayer and refraction of films were measured by RISE-Zenith spectroscopic ellipsometer system (Syscos Instruments) with a light source of L2174-01 xenon lamp (Hamamatsu Photonics). TRPL measurements were performed on Edinburgh FLS980 with a picosecond pulse laser (20 MHz-20 KHz). TRPL of PBDB-T:ITIC films were performed with 510 nm excitation and detected 690 nm emission, or with 655 nm excitation and detected 800 nm emission. TRPL of polymer:PC71BM films were performed with 405 nm excitation and detected 710 nm emission. Cyclic voltammetry. The adsorption spectra were obtained using a photoelectrochemical workstation Zahner CIMPS. Adsorption study of SASM was performed with a standard three-electrode configuration in acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate; saturated Ag/AgCl was used as a reference electrode, platinum as a counter electrode, and ITO (SASM-modified ITO) (area = 0.8 cm ×1.5 cm) as the working electrode. The sweep rate in both sets of 27 ACS Paragon Plus Environment

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experiments was 50 mV/s. ASSOCIATED CONTENT The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author Tel.: +86 791 83968703; Fax: +86 791 83969561. E-mail: [email protected] (Y. Chen) Tel: +1-404-894-0316; Fax: +1-404-894-286. E-mail: [email protected] (E. Reichmanis) E-mail: [email protected] (G. Wang) Acknowledgements L. H., G. W., and W. Z. contributed equally to this work. This work was financially supported by the National Science Fund for Distinguished Young Scholars (51425304), National Natural Science Foundation of China (51673091, 51563016 and 21764009), and Jiangxi Province Innovation Fund for Graduate Student (YC2017-B007). Z. Y. and E. R. thank the National Science Foundation for support under DMR-1507205. E. R. is also grateful for support from the Georgia Institute of Technology and the Brook Byers Institute for Sustainable Systems. X-ray data are acquired at beamlines 7.3.3 (WAXS) and 11.0.1.2. (R-SoXS) 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 Contract No. DE-AC02-05CH11231. Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff)) Supporting Information Available Additional data and discussion. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J., Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148-7151. (2) Nakano, K.; Tajima, K., Organic Planar Heterojunctions: From Models for Interfaces in Bulk Heterojunctions to High-Performance Solar Cells. Adv. Mater. 2017, 29, 1603269. (3) Richter, L. J.; Delongchamp, D. M.; Amassian, A., Morphology Development in Solution-Processed Functional Organic Blend Films: An In Situ Viewpoint. Chem. 28 ACS Paragon Plus Environment

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Polymer-Fullerene Bulk Heterojunction. Adv. Energy Mater. 2014, 4, 1301879. (16) Chou, K. W.; Yan, B.; Li, R.; Li, E. Q.; Zhao, K.; Anjoum, D. H.; Alvarez, S.; Gassaway, R.; Biocca, A.; Thoroddsen, S. T.; Hexemer, A.; Amassian, A., Spin-Cast Bulk Heterojunction Solar Cells: A Dynamical Investigation. Adv. Mater. 2013, 25, 1923-1929. (17) Jin, Y.; Chen, Z.; Dong, S.; Zheng, N.; Ying, L.; Jiang, X.-F.; Liu, F.; Huang, F.; Cao, Y., A Novel Naphtho[1,2-c:5,6-c′]Bis([1,2,5]Thiadiazole)-Based Narrow-Bandgap π-Conjugated Polymer with Power Conversion Efficiency Over 10%. Adv. Mater. 2016, 28, 9811-9818; (18) Jasieniak, J. J.; Treat, N. D.; McNeill, C. R.; de Villers, B. J. T.; Della Gaspera, E.; Chabinyc, M. L., Interfacial Characteristics of Efficient Bulk Heterojunction Solar Cells Fabricated on MoOx Anode Interlayers. Adv. Mater. 2016, 28, 3944-3951. (19) Huang, J.; Carpenter, J. H.; Li, C.-Z.; Yu, J.-S.; Ade, H.; Jen, A. K.-Y, Highly Efficient Organic Solar Cells with Improved Vertical Donor-Acceptor Compositional Gradient via an Inverted Off-Center Spinning Method. Adv. Mater. 2016, 28, 967-974. (20) Xia, D.; Wu, Y.; Wang, Q.; Zhang, A.; Li, C.; Lin, Y.; Colberts, F. J. M.; van Franeker, J. J.; Janssen, R. A. J.; Zhan, X.; Hu, W.; Tang, Z.; Ma, W.; Li, W., Effect of Alkyl Side Chains of Conjugated Polymer Donors on the Device Performance of Non-Fullerene Solar Cells. Macromolecules 2016, 49, 6445-6454. (21) Guo, X.; Zhang, M.; Ma, W.; Ye, L.; Zhang, S.; Liu, S.; Ade, H.; Huang, F.; Hou, J., Enhanced Photovoltaic Performance by Modulating Surface Composition in Bulk Heterojunction Polymer Solar Cells Based on PBDTTT-C-T/PC71BM. Adv. Mater. 2014, 26, 4043-4049. (22) Germack, D. S.; Chan, C. K.; Kline, R. J.; Fischer, D. A.; Gundlach, D. J.; Toney, M. F.; Richter, L. J.; DeLongchamp, D. M., Interfacial Segregation in Polymer/Fullerene Blend Films for Photovoltaic Devices. Macromolecules 2010, 43, 3828-3836. (23) Yan, Y.; Liu, X.; Wang, T., Conjugated-Polymer Blends for Organic Photovoltaics: Rational Control of Vertical Stratification for High Performance. Adv. Mater. 2017, 29, 1601674. (24) Lin, Y.; Wang, J.; Zhang, Z.-G.; Bai, H.; Li, Y.; Zhu, D., Zhan, X., An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv. Mater. 2015, 27, 1170-1174. (25) Ma, K.-X.; Ho, C.-H.; Zhu, F.; Chung, T.-S., Investigation of Surface Energy for Organic Light Emitting Polymers and Indium Tin Oxide. Thin Solid Films 2000, 371, 140-147. (26) Paniagua, S. A.; Giordano, A. J.; Smith, O. N. L.; Barlow, S.; Li, H.; Armstrong, N. R.; Pemberton, J. E.; Brédas, J.-L.; Ginger, D.; Marder, S. R., Phosphonic Acids for Interfacial Engineering of Transparent Conductive Oxides. Chem. Rev. 2016, 116, 7117-7158. (27) Huang, L.; Chen, L.; Huang, P.; Wu, F.; Tan, L.; Xiao, S.; Zhong, W.; Sun, L.; 30 ACS Paragon Plus Environment

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Chen, Y., Triple Dipole Effect from Self-Assembled Small-Molecules for High Performance Organic Photovoltaics. Adv. Mater. 2016, 28, 4852-4860. (28) Cheng, X.; Huang, L.; Zhang, L.; Ai, Q.; Chen, L.; Chen, Y., Multi-Chlorine-Substituted Self-Assembled Molecules As Anode Interlayers: Tuning Surface Properties and Humidity Stability for Organic Photovoltaics. ACS Appl. Mater. Interfaces 2017, 9, 9204-9212. (29) Li, W.; Hendriks, K. H.; Furlan, A.; Roelofs, W. S. C.; Meskers, S. C. J.; Wienk, M. M.; Janssen, R. A. J., Effect of the Fibrillar Microstructure on the Efficiency of High Molecular Weight Diketopyrrolopyrrole-Based Polymer Solar Cells. Adv. Mater. 2014, 26, 1565-1570. (30) Liao, S.-H.; Jhuo, H.-J.; Cheng, Y.-S.; Chen, S.-A., Fullerene Derivative-Doped Zinc Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low-Bandgap Polymer (PTB7-Th) for High Performance. Adv. Mater. 2013, 25, 4766-4771. (31) He, Z.; Xiao, B.; Liu, F.; Wu, H.; Yang, Y.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y., Single-Junction Polymer Solar Cells with High Efficiency and Photovoltage. Nature Photon. 2015, 9, 174-179. (32) Bulle-Lieuwma, C. W. T.; van Gennip, W. J. H.; van Duren, J. K. J.; Jonkheijm, P.; Janssen, R. A. J.; Niemantsverdriet, J. W., Characterization of Polymer Solar Cells by TOF-SIMS Depth Profiling. Appl. Surf. Sci. 2003, 203-204, 547-550. (33) Sergeant, N. P.; Hadipour, A.; Niesen, B.; Cheyns, D.; Heremans, P.; Peumans, P.; Rand, B. P., Design of Transparent Anodes for Resonant Cavity Enhanced Light Harvesting in Organic Solar Cells. Adv. Mater. 2012, 24, 728-732. (34) Kwon, S.; Kang, H.; Lee, J.-H.; Lee, J.; Hong, S.; Kim, H.; Lee, K., Effect of Processing Additives on Organic Photovoltaics: Recent Progress and Future Prospects. Adv. Energy Mater. 2016, 7, 1601496. (35) Liao, H.-C.; Ho, C.-C.; Chang, C.-Y.; Jao, M.-H.; Darling, S. B.; Su, W.-F., Additives for Morphology Control in High-Efficiency Organic Solar Cells. Mater. Today 2013, 16, 326-336. (36) Xu, X.; Bi, Z.; Ma, W.; Wang, Z.; Choy, W. C. H.; Wu, W.; Zhang, G.; Li, Y.; Peng, Q., Highly Efficient Ternary-Blend Polymer Solar Cells Enabled by a Nonfullerene Acceptor and Two Polymer Donors with a Broad Composition Tolerance. Adv. Mater. 2017, 29, 1704271. (37) Liu, F.; Zhao, W.; Tumbleston, J. R.; Wang, C.; Gu, Y.; Wang, D.; Briseno, A. L.; Ade, H.; Russell, T. P. Understanding the Morphology of PTB7:PCBM Blends in Organic Photovoltaics. Adv. Energy Mater. 2014, 4, 1301377 (38) Yin, J.; Zhou, W.; Zhang, L.; Xie, Y.; Yu, Z.; Shao, J.; Ma, W.; Zeng, J.; Chen, Y., Improved Glass Transition Temperature towards Thermal Stability via Thiols Solvent Additive versus DIO in Polymer Solar Cells. Macromol. Rapid Commun. 2017, 38, 1700428. (39) Collins, B. A.; Li, Z.; Tumbleston, J. R.; Gann, E., McNeill, C. R.; Ade, H., Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7:PC71BM Solar Cells. Adv. Energy Mater. 2013, 3, 65-74. 31 ACS Paragon Plus Environment

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(40) Li, W.; Hendriks, K. H.; Furlan, A.; Roelofs, W. S. C.; Wienk, M. M.; Janssen, R. A. J., Universal Correlation between Fibril Width and Quantum Efficiency in Diketopyrrolopyrrole-Based Polymer Solar Cells J. Am. Chem. Soc. 2013, 135, 18942-18948. (41) Burkhard, G. F.; Hoke, E. T.; McGehee, M. D., Accounting for Interference, Scattering, and Electrode Absorption to Make Accurate Internal Quantum Efficiency Measurements in Organic and Other Thin Solar Cells. Adv. Mater. 2010, 22, 3293-3297. (42) Karagiannidis, P. G.; Georgiou, D.; Pitsalidis, C.; Laskarakis, A.; Logothetidis, S., Evolution of Vertical Phase Separation in P3HT:PCBM Thin Films Induced by Thermal Annealing. Mater. Chem. Phys. 2011, 129, 1207-1213. (43) Hedley, G. J.; Ward, A. J.; Alekseev, A.; Howells, C. T.; Martins, E. R.; Serrano, L. A.; Cooke, G.; Ruseckas, A.; Samuel, I. D. W., Determining the Optimum Morphology in High-Performance Polymer-Fullerene Organic Photovoltaic Cells. Nat. Commun. 2013, 4, 2867; (44) Choi, H.; Mai, C.-K.; Kim, H.-B.; Jeong, J.; Song, S.; Bazan, G. C.; Kim, J. Y.; Heeger, A. J., Conjugated Polyelectrolyte Hole Transport Layer for Inverted-Type Perovskite Solar Cells. Nat. Commun. 2015, 6, 7348. (45) Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganäs, O.; Gao, F.; Hou, J., Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734-4739. (46) Wang, H.; Shah, M.; Ganesan, V.; Chabinyc, M. L.; Loo, Y.-L., Tail State-Assisted Charge Injection and Recombination at the Electron-Collecting Interface of P3HT:PCBM Bulk-Heterojunction Polymer Solar Cells. Adv. Energy Mater. 2012, 2, 1447-1455. (47) Wang, H.; Gomez, E. D.; Kim, J.; Guan, Z.; Jaye C.; Fischer, D. A.; Kahn, A., Loo, Y.-L., Device Characteristics of Bulk-Heterojunction Polymer Solar Cells are Independent of Interfacial Segregation of Active Layers. Chem. Mater. 2011, 23, 2020-2023. (48) Ghasemi, M.; Ye, L.; Zhang, Q.; Yan, L.; Kim, J.-H.; Awartani, O.; You, W.; Gadisa, A.; Ade, H., Panchromatic Sequentially Cast Ternary Polymer Solar Cells. Adv. Mater. 2017, 29, 1604603. (49) Chen, D.; Liu, F.; Wang, C.; Nakahara, A.; Russell, T. P., Bulk Heterojunction Photovoltaic Active Layers via Bilayer Interdiffusion. Nano Lett. 2011, 11, 2071-2078. (50) Yim, J. H.; Joe, S.-y.; Nguyen, D. C.; Ryu, S. Y.; Ha, N. Y.; Ahn, Y. H.; Park, J.-y.; Lee, S., True Nature of Active Layers in Organic Solar Cells Fabricated by Sequential Casting of Donor and Acceptor Layers. Phys. Status Solidi RRL 2017, 11, 1600415. (51) Park, G. E.; Choi, S.; Park, S. Y.; Lee, D. H.; Cho, M. J.; Choi, D. H., Eco-Friendly Solvent-Processed Fullerene-Free Polymer Solar Cells with over 9.7% Efficiency and Long-Term Performance Stability. Adv. Energy Mater. 2017, 7, 1700566. (52) Zhao, W.; Li, S.; Zhang, S.; Liu, X.; Hou, J., Ternary Polymer Solar Cells based 32 ACS Paragon Plus Environment

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on Two Acceptors and One Donor for Achieving 12.2% Efficiency. Adv. Mater. 2017, 29, 1604059. (53) Clark, M. D.; Jespersen, M. L.; Patel, R. J.; Leever, B. J., Predicting Vertical Phase Segregation in Polymer-Fullerene Bulk Heterojunction Solar Cells by Free Energy Analysis. ACS Appl. Mater. Interfaces 2013, 5, 4799-4807. (54) Fu, P.; Huang, L.; Yu, W.; Yang, D.; Liu, G.; Zhou, L.; Zhang, J.; Li, C., Efficiency Improved for Inverted Polymer Solar Cells with Electrostatically Self-assembled BenMeIm-Cl Ionic Liquid Layer as Cathode Interface Layer. Nano Energy 2015, 13, 275-282. (55) Lee, E. J.; Heo, S. W.; Han, Y. H.; Moon, D. K., An Organic-Inorganic Hybrid Interlayer for Improved Electron Extraction in Inverted Polymer Solar Cells. J. Mater. Chem. C 2016, 4, 2463-2469. (56) Lin, Y.; Cai, C.; Zhang, Y.; Zheng, W.; Yang, J.; Wang, E.; Hou, L., Study of ITO-Free Roll-to-Roll Compatible Polymer Solar Cells Using the One-Step Doctor Blading Technique. J. Mater. Chem. A 2017, 5, 4093-4102. (57) Hexemer, A.; Bras, W.; Glossinger, J.; Schaible, E.; Gann, E.; Kirian, R.; MacDowell, A.; Church, M.; Rude, B.; Padmore, H., A SAXS/WAXS/GISAXS Beamline with Multilayer Monochromator. J. Phys. Conf. Ser. 2010, 247, 012007. (58) Gann, E.; Young, A. T.; Collins, B. A.; Yan, H.; Nasiatka, J.; Padmore, H. A.; Ade, H.; Hexemer, A.; Wang, C., Soft X-Ray Scattering Facility at the Advanced Light Source with Real-Time Data Processing and Analysis. Rev. Sci. Instrum. 2012, 83, 045110.


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Figure 1. Chemical structure and vertical stratification. (a) Schematic illustration of the molecular structure and surface energy relation. Schematic illustration of vertical stratification of HD-PDPPTPT:PC71BM on (b) CBSC and (c) PEDOT:PSS modified Si wafer. TOF-SIMS spectra of HD-PDPPTPT:PC71BM on (d) CBSC and (e) PEDOT:PSS modified Si wafer.

Figure 2. Film absorption and crystallinity. (a) Absorption of the donor materials (D), the acceptor material (A), and D:A blend films on quartz glass. (b,c) Extinction coefficient (k) of PTB7-Th:PC71BM and HD-PDPPTPT:PC71BM on substrates with different treatments. (d) Out-of-plane and in-plane cuts of two-dimensional GIWAXS patterns of active layers on PEDOT:PSS and CBSC modified Si substrate.

Figure 3. Vertically stratified BHJ structure, EQE spectra and light intensity distribution simulation of devices. Illustration of the aggregation of donor (blue) and acceptor (orange) materials at anode and cathode interface in (a) conventional and (b) inverted devices, the hole transporting layer 1 (HTL1) is PEDOT:PSS or CBSC and the electron transporting layer 1 (ETL1) is ZnO or ZnO/MPPA. And their external quantum efficiency (EQE) spectra based on (c,e) HD-PDPPTPT:PC71BM, and (d,f) PTB7-Th:PC71BM BHJ films. (g) The light intensity distribution of the active layer region for conventional devices with PEDOT:PSS and CBSC modified ITO.

Figure 4. Results of the non-fullerene device with sequential casting process and 34 ACS Paragon Plus Environment

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MPPA modification. (a) Current density versus voltage (J−V) characteristics of inverted devices with the structure of ITO/ZnO/(MPPA)/(PBDB-T:ITIC(a:b))/ PBDB-T:ITIC(10 mg/mL:10 mg/mL)/MoO3/Ag. (b) Corresponding EQE spectra of devices. Time-resolved photoluminescence (TRPL) of BHJ with (c) 510 nm excitation and detected 690 nm emission, or (d) 655 nm excitation and detected 800 nm emission.

Figure 5. Vertical stratification of the non-fullerene system. Dynamic X-ray photoelectron spectroscopy (DXPS) spectra of PBDB-T:ITIC on (a) MPPA modified ZnO film and (b) pristine ZnO film, from left to right are the S2p scan, N1s scan, and S/N ratio spectra, respectively. (c) A P2p scan of PBDB-T:ITIC on ZnO/MPPA film. (d) S/N ratio of sequentially prepared BHJ films after stored for 15 days.

Figure 6. Proposed models through which surface energy can induce optimized vertical stratification to improve final photoelectric conversion performance.


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Table 1. Performance characteristics for the reported device architectures based on different BHJs.

Active Layer

HD-PDPPTPT:PC71BM (1:2, o-DCB:CHCl3 6.0%)

HTL

ETL

PEDOT:PSS

PFN

Voc (V)

FF (%)

14.22 ± 0.41 (13.43)b 0.807 ± 0.002 60.3 ± 1.87 b

PCEavec (PCEbest)d PCE (%) (%)d 6.86 ± 0.17 (7.1)

PFN

14.31 ± 0.21 (14.05) 0.811 ± 0.005 61.2 ± 1.58

6.98 ± 0.23 (7.3)

MoO3

ZnO

17.01 ± 0.72 (16.44)b 0.787 ± 0.001 56.1 ± 2.14

7.50 ± 0.08 (7.6)

PEDOT:PSS CBSC MoO3 MoO3 MoO3 MoO3 PBDB-T:ITIC (1:1, DIO:CB 0.5%)

Jsc (mA/cm2)

CBSC

MoO3

PTB7-Th:PC71BM (1:2, DIO:CB 3%)

Buried layer

MoO3

ZnO/MPPA

8.89 ± 0.07 (9.0)

b

9.13 ± 0.10 (9.2)

b

9.42 ± 0.10 (9.5)

16.84 ± 0.11 (16.18) 0.785 ± 0.003 66.6 ± 1.41

PFN

17.55 ± 0.68 (16.87) 0.786 ± 0.002 66.0 ± 2.84

ZnO

17.61 ± 0.51 (17.05) 0.785 ± 0.002 68.1 ± 0.99 b

ZnO/MPPA

ZnO

7.51 ± 0.06 (7.6)

b

17.39 ± 0.59 (16.61) 0.787 ± 0.001 55.2 ± 1.64

PFN

ZnO

b

19.50 ± 0.45 (18.78) 0.796 ± 0.002 67.5 ± 2.23 10.47 ± 0.08 (10.6) 9:1

a

5:1

a

17.70 ± 0.44 (17.25)b 0.867 ± 0.002 60.7 ± 1.82

9.31 ± 0.05 (9.4)

9.7

17.81 ± 0.42 (17.26) 0.880 ± 0.001 62.9 ± 0.20 9.89 ± 0.19 (10.0)

9.9

b b

ZnO

16.98 ± 0.18 (16.40) 0.885 ± 0.001 66.8 ± 0.76 10.05 ± 0.22 (10.2) 9.9 a

18.07 ± 0.37 (17.09)b 0.877 ± 0.001 64.3 ± 0.40 10.18 ± 0.10 (10.3) 9.7

MoO3

ZnO

1:5

MoO3

ZnO

1:9a 17.99 ± 0.38 (17.13)b 0.876 ± 0.007 64.5 ± 0.47 10.14 ± 0.22 (10.3) 9.9

MoO3

ZnO/MPPA

18.03 ± 0.59 (17.14)b 0.885 ± 0.002 66.1 ± 2.58 10.56 ± 0.18 (10.8) 10.5

a

The ratio of buried PBDB-T:ITIC (a:b, 3 mg/ml for all) BHJ layer; b Intergrated Jsc values from EQE measurement, the admissible error is in 6%; c the average PCE is obtained from over 20 devices; d Best performance of device; e PCE value of devices kept in a nitrogen atmosphere after 20 hours.


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Routes toward regulating the interfacial surface energy, vertical segregation profile, and spectra response of bulk-heterojunction are explored to further enhance the performance of OSCs. A path towards higher short-circuit current density with greater photocurrent generated in the red or blue region is proposed by adjusting the amount of component owning strong absorption. stratification engineering, surface energy, organic bulk-heterojunction, organic photovoltaics, interface engineering, self-assembled small molecule, quantum efficiency L. Huang, G. Wang*, W. Zhou, B. Fu, X. Cheng, L. Zhang, Z. Yuan, S. Xiong, L. Zhang, Y. Xie, A. Zhang, Y. Zhang, W. Ma, W. Li, Y. Zhou, E. Reichmanis* & Y. Chen* Vertical Stratification Engineering for Organic Bulk-heterojunction Devices


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Figure 1. Chemical structure and vertical stratification. (a) Schematic illustration of the molecular structure and surface energy relation. Schematic illustration of vertical stratification of HD-PDPPTPT:PC71BM on (b) CBSC and (c) PEDOT:PSS modified Si wafer. TOF-SIMS spectra of HD-PDPPTPT:PC71BM on (d) CBSC and (e) PEDOT:PSS modified Si wafer. 140x67mm (300 x 300 DPI)


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Figure 2. Film absorption and crystallinity. (a) Absorption of the donor materials (D), the acceptor material (A), and D:A blend films on quartz glass. (b,c) Extinction coefficient (k) of PTB7-Th:PC71BM and HDPDPPTPT:PC71BM on substrates with different treatments. (d) Out-of-plane and in-plane cuts of twodimensional GIWAXS patterns of active layers on PEDOT:PSS and CBSC modified Si substrate. 140x155mm (300 x 300 DPI)


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Figure 3. Vertically stratified BHJ structure, EQE spectra and light intensity distribution simulation of devices. Illustration of the aggregation of donor (blue) and acceptor (orange) materials at anode and cathode interface in (a) conventional and (b) inverted devices, the hole transporting layer 1 (HTL1) is PEDOT:PSS or CBSC and the electron transporting layer 1 (ETL1) is ZnO or ZnO/MPPA. And their external quantum efficiency (EQE) spectra based on (c,e) HD-PDPPTPT:PC71BM, and (d,f) PTB7-Th:PC71BM BHJ films. (g) The light intensity distribution of the active layer region for conventional devices with PEDOT:PSS and CBSC modified ITO. 140x109mm (300 x 300 DPI)


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Figure 4. Results of the non-fullerene device with sequential casting process and MPPA modification. (a) Current density versus voltage (J−V) characteristics of inverted devices with the structure of ITO/ZnO/(MPPA)/(PBDB-T:ITIC(a:b))/ PBDB-T:ITIC(10 mg/mL:10 mg/mL)/MoO3/Ag. (b) Corresponding EQE spectra of devices. Time-resolved photoluminescence (TRPL) of BHJ with (c) 510 nm excitation and detected 690 nm emission, or (d) 655 nm excitation and detected 800 nm emission. 140x102mm (300 x 300 DPI)


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Figure 5. Vertical stratification of the non-fullerene system. Dynamic X-ray photoelectron spectroscopy (DXPS) spectra of PBDB-T:ITIC on (a) MPPA modified ZnO film and (b) pristine ZnO film, from left to right are the S2p scan, N1s scan, and S/N ratio spectra, respectively. (c) A P2p scan of PBDB-T:ITIC on ZnO/MPPA film. (d) S/N ratio of sequentially prepared BHJ films after stored for 15 days. 140x70mm (300 x 300 DPI)


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Figure 6. Proposed models through which surface energy can induce optimized vertical stratification to improve final photoelectric conversion performance. 140x53mm (300 x 300 DPI)


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Toc Figure 54x50mm (300 x 300 DPI)


Vertical Stratification Engineering for Organic Bulk-Heterojunction

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