Fluorination-enabled optimal morphology leads to over 11% efficiency for inverted small-molecule organic solar cells

Abstract

Solution-processable small molecules for organic solar cells have attracted intense attention for their advantages of definite molecular structures compared with their polymer counterparts. However, the device efficiencies based on small molecules are still lower than those of polymers, especially for inverted devices, the highest efficiency of which is <9%. Here we report three novel solution-processable small molecules, which contain π-bridges with gradient-decreased electron density and end acceptors substituted with various fluorine atoms (0F, 1F and 2F, respectively). Fluorination leads to an optimal active layer morphology, including an enhanced domain purity, the formation of hierarchical domain size and a directional vertical phase gradation. The optimal morphology balances charge separation and transfer, and facilitates charge collection. As a consequence, fluorinated molecules exhibit excellent inverted device performance, and an average power conversion efficiency of 11.08% is achieved for a two-fluorine atom substituted molecule.

ARTICLE

Received 15 Jun 2016 | Accepted 31 Oct 2016 | Published 19 Dec 2016

Dan Deng1,*, Yajie Zhang1,*, Jianqi Zhang1, Zaiyu Wang2, Lingyun Zhu1, Jin Fang1,3, Benzheng Xia1, Zhen Wang1, Kun Lu1, Wei Ma2 & Zhixiang Wei1,3

Solution-processable small molecules for organic solar cells have attracted intense attention for their advantages of denite molecular structures compared with their polymer counterparts. However, the device efciencies based on small molecules are still lower than those of polymers, especially for inverted devices, the highest efciency of which is o9%.

Here we report three novel solution-processable small molecules, which contain p-bridges with gradient-decreased electron density and end acceptors substituted with various uorine atoms (0F, 1F and 2F, respectively). Fluorination leads to an optimal active layer morphology, including an enhanced domain purity, the formation of hierarchical domain size and a directional vertical phase gradation. The optimal morphology balances charge separation and transfer, and facilitates charge collection. As a consequence, uorinated molecules exhibit excellent inverted device performance, and an average power conversion efciency of 11.08% is achieved for a two-uorine atom substituted molecule.

DOI: 10.1038/ncomms13740 OPEN

Fluorination-enabled optimal morphology leads to over 11% efciency for inverted small-molecule organic solar cells

1 CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China. 2 State Key Laboratory for Mechanical Behavior of Materials, Xian Jiaotong University, Xian 710049, China. 3 University of Chinese Academy of Sciences, Beijing 100049, China. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to K.L. (email: mailto:[email protected]

Web End [email protected] ) or to W.M. (email: mailto:[email protected]

Web End [email protected] ) or to Z.W. (email: mailto:[email protected]

Web End [email protected] ).

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Organic solar cells (OSCs) have attracted intense attentions due to their potential for solution processing cheap and exible devices. Comparing with conventional device

architectures, inverted devices exhibit improved environmental stability and more preferable for industrial applications1. Polymer solar cells with power conversion efciency (PCE) higher than 10% are mainly obtained through inverted devices24, and the highest efciency has reached 11.7% (ref. 5). Solution-processable small molecules for OSCs have attracted intense attentions for their advantages of high purity and denite molecular structures compared with polymers611. To date, the PCE based on small molecules has reached 10% by conventional devices8,12. However, attempts to develop inverted devices for small molecules are not as successful as that of polymers1316, and the highest PCE reported is 8.84% (ref. 17), which is much lagging behind their polymer couterparts.

To obtain highly efcient OSCs, decisive parameters, namely, open circuit voltage (Voc), ll factor (FF) and short-circuit current (Jsc) should be enhanced. Each parameter could be expressed as following general formula:

L Lmax Lloss L Voc; FF or Jsc

1

A proved successful strategy in molecular design is to increase Lmax. The approaches involve increasing ionization potential of the donor (IPD), narrowing bandgap and enhancing mobility to achieve high Voc, Jsc and FF, respectively1820. However, IPD enhancement would lead to bandgap increase19, and (Voc Jsc)max is limited by ShockleyQueisser model. To further

maximize PCE, an alternative strategy is to minimize Lloss. The main loss of Voc is related to the disorder arrangement of donors/ acceptors and their poor contact with electrode2123; the main losses of FF and Jsc are ascribed to the recombination induced by undesirable distribution of intermix and crystalline phases, interfacial traps and poor domain purity24,25. Consequently, all losses are related to the bulk-heterojunction (BHJ) morphology, the optimization of which could simultaneously minimize Lloss

for all three parameters.

The adjustment of the donoracceptor (D:A) interaction is an essential approach to optimize the BHJ morphologies. A difference in surface free energies between donors and acceptors would result in a repulsive interaction (lower miscibility), which acts as the internal driving force to form phase separation26,27. Hence, a proper disparity of surface free energies is signicant to achieving optimized lateral morphology. Furthermore, a lower surface free energy of donors in comparison with acceptors would induce surface enrichments and vertical phase separation28, both of which could effectively decrease losses of Voc, Jsc and FF

through suppression of recombination by modifying interface contact, forming charge-blocking regions and facilitating charge collection24,28.

In this paper, three medium bandgap molecules are designed and synthesized with thiophene-substituted benzodithiophene (TBDT) as a core, 2-(thiophen-2-yl)thieno [3,2-b]thiophene as p-bridges and end-capped with 1H-indene-1,3(2H)-dione, 4-uoro-1H-indene-1,3(2H)-dione or 4,7-diuoro-1H-indene-1,3(2H)-dione; these molecules are abbreviated as BTID-0F, BTID-1F and BTID-2F (Fig. 1a). With incremental introduction of uorine to end-capped units, the PCE for inverted devices increases from 8.30% for BTID-0F to 10.4% for BTID-1F and to11.3% for BTID-2F. A hierarchical morphology with higher domain purity, enhanced surface enrichment and more directional vertical phase distribution is induced by uorination, thereby Voc, Jsc and FF are increased simultaneously.

ResultsDesign and characterization of small molecules. To benet the charge carrier transport, a two-dimensional donor unit, TBDT was selected as the core29. A new p-bridge, namely, 2-(thiophen-2-yl)thieno[3,2-b]thiophene, was designed with two advantages: rst, it introduces stronger aromaticity units of thieno [3,2-b]thiophene to increase IPD, aiming to increase attainable

Voc. Second, it presents an inner gradient-decreased electron density distribution (Supplementary Fig. 1), beneted for backbone hole transfer. To lower the miscibility and increase dielectric constant, 1H-indene-1,3(2H)-dione was selected as acceptor because of its stronger polarity and aromatic difference in comparison with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)26. To decrease molecular surface free energies and miscibility with PC71BM, we introduced uorine atoms30 to the end-capped acceptor units and obtained two new acceptor units, namely, 4-uoro-1H-indene-1,3(2H)-dione and 4,7-diuoro-1H-indene-1,3(2H)-dione. Various studies have discussed uorination effects on photovoltaic performance but most focused on modifying molecular internal part to stabilize molecular conformation31,32. In addition, the positions of alkyl chains were designed to prevent backbone torsion to highest extent. The different alkyl chains attached to TBDT units for BTID-0F, BTID-1F and BTID-2F were used to ensure their sufcient solubility. The synthesis routes of the three molecules are shown in Supplementary Fig. 2. The dielectric constants of BTID-0F, BTID-1F and BTID-2F are calculated as ca. 4.0 (Supplementary Table 1, Supplementary Fig. 3a), which was a high value among the conjugated small molecules or polymers33.

Absorption spectra of solution and lms are shown in Fig. 1b. The absorption coefcient in solution of the three molecules denoted the competition between the content of alkyl chains and uorine atoms. Although, four uorine atoms were introduced into BTID-2F, the alkyl chain (no contribution to absorption) content of which was the highest. Thus, BTID-2F exhibited the lowest absorption coefcient in solution, whereas BTID-1F showed the highest. Furthermore, introduction of uorine not only enhanced absorption coefcient gradually but also slightly redshifted the absorption spectrum. From solution to lms, all the molecules clearly manifested ca. 70 nm redshifts, illustrating good aggregation in lms. With the increase of uorine atoms, the intensity ratios of the shoulder peak (attributed to pp stacking)

to the absorption peak from internal charge transfer increased. The different tendencies of lm absorption coefcient and the solution absorption coefcient further revealed that uorination led to dense and ordered pp stacking.

Ultraviolet photoelectron spectroscopy (UPS) was carried out to obtain IPD in pristine lms and blends with PC71BM on

PEDOT:PSS/ITO substrates (Fig. 1c)34, and the IP and EA (electron afnity) of PC71BM were obtained from literatures35.

The IPD of the three pristine lms were 4.91, 4.98 and 5.05 eV, respectively, indicating uorination of the end acceptors increase IPD comparing with those of uorine atoms attached to the internal part of polymers36,37. The detailed IPD and EAD of

the three molecules are shown Fig. 1d, and the EAD was calculated from: EAD Eoptg IPD (Eoptg is optical energy

bandgap). However, after blending with PC71BM, the IPD were changed to 4.98, 4.92 and 4.95 eV for BTID-0F, BTID-1F and BTID-2F, and their corresponding changed values were 0.09,

0.05 and 0.09 eV. The change from pristine to blended lms

was mainly due to the shifts in work function (Fig. 1c) indicating dipole polarity differs after PC71BM addition. Changes in dipole direction after uorination were probably due to various surface enrichment and vertical distributions, which will be discussed in the later section. Moreover, the molecules based on the new designed p-bridge exhibited low highest occupied molecular

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Figure 1 | Molecular structures and properties. (a) Small molecular structures; (b) solution coefcient of samples in chloroform (left Y axis) and lm (right Y axis); (c) UPS results of the pristine lms and blended lms with PC71BM; and (d) molecular energy levels measured from UPS, the lowest unoccupied molecular orbital (LUMO) levels calculated from optical bandgap and UPS.

orbital (HOMO) levels (measured by cyclic voltammetry, Supplementary Fig. 3b,c), which is the basis for obtaining high Voc in BHJ OSCs5.

To check the miscibility and molecular surface free energies, the contact angles of chloroform solution of three materials were measured on ZnO/ITO and PEDOT/ITO substrates (Supplementary Fig. 3d,e). The increase of contact angles for the uorinated molecules indicated uorination lowered the surface free energies of small molecules. On the other hand, the increase of deviation in contact angles between uorinated molecules and PC71BM indicated uorination also reduced the miscibility of small molecules with PC71BM. These results implied that surface free energies and miscibility were successfully adjusted through molecular design.

Fabrication and performance of inverted solar cells. To investigate the photovoltaic properties of the three small molecules, devices with a structure of ITO/ZnO/active layer/MoOx/Ag were fabricated (Fig. 2a). The highest PCE for BTID-0F, BTID-1F and BTID-2F were 8.30, 10.4 and 11.3%, and other detailed parameters are shown in Fig. 2b,c and Table 1, and the optimization process of D:A ratios is shown in Supplementary Table 2. The device based on BTID-2F was certied at an accredited laboratory, certifying a PCE of 11.0% (Supplementary Fig. 4ae). Notably, the active layers of all devices were obtained without any additives and post-treatment, which were facilitated for future industrial manufacturing38.

The high efciencies of the three materials were ascribed to simultaneous increment in Voc, Jsc and FF. The high Voc was in

good agreement with the IPD of their blends obtained by UPS. As calculated from the formula: eVoc IPD EAA DV

(refs 39,40), the Voc losses (DV) based on all three small molecules were ca. 0.2 eV ( IPD and EAA values shown in

Fig. 1d), which were much less than empirical value 0.3 0.5 eV

reported39,40. The low Voc loss were attributed to their high dielectric constant (Supplementary Fig. 3a, Supplementary Table 1)33,40 and ideal morphologies, which will be discussed in following sections. The Jsc of BTID-2F and BTID-1F cells was higher than that of BTID-0F cells partly because of the larger photocurrent generated in the red region (Fig. 2c). On the other hand, the enhancements of FF and partial Jsc for BTID-2F and

BTID-1F compared with BTID-0F, illustrated lower loss of FF and Jsc with uorination. This phenomenon could be supported by the relation of photocurrent density (Jph) versus effective voltage (Veff) (Fig. 2d), where Jph JL JD (JL and JD are the

current density under illumination and in the dark) and Veff V0 Va, (V0 is the voltage at Jph 0 and Va is the

measured voltage under different current density). The ratios of Jph/Jph,sat are used to judge the overall efciency of exciton dissociation and charge collection2,41. Under short-circuit condition, the ratios were 0.93, 0.95 and 0.96 for BTID-0F, BTID-1F and BTID-2F, suggesting the effective exciton dissociation of the three molecules, especially for BTID-1F and BTID-2F. Under maximal power output circumstances (Veff 0.2 V), the ratios of Jph/J

ph,sat were 0.71, 0.83 and 0.86, respectively, indicating a considerably higher charge collection and lower bimolecular recombination after uorine substitution. The superior JphVeff characteristics clearly demonstrated that

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Figure 2 | Device structures and photovoltaic properties for inverted solar cells. (a) Structures of inverted device; (b) optimized JV curves for inverted devices; (c) EQE corresponding to devices in b,d photocurrent density versus effective voltage (JphVeff) characteristics for devices under constant incident light intensity (AM 1.5 G, 100 mWcm 2).

Table 1 | Optimized photovoltaic performance of inverted device based on the three molecules (device structure: ITO/ ZnO/active layer/MoOx/Ag).

Donors Voc (V) Jsc (mA cm 2) FF (%) PCE (%)

Best Average BTID-0F 0.93 14.0 64.0 8.30 8.21

BTID-1F 0.94 15.3 72.0 10.4 10.37 BTID-2F 0.95 15.7 76.0 11.3 11.08

uorination could reduce bimolecular recombination, thereby improving Jsc and FF simultaneously.

Hierarchical morphology. The small loss in Jsc, FF and Voc for

devices based on the three molecules and their variation in loss should be ascribed to the optimized but different lateral/vertical phase distributions. To study morphologies in the lateral direction, we characterized small molecules/PC71BM blending lms on

ZnO/Si substrates and pristine lms on Si substrates by grazing incidence wide-angle X-ray scattering (GIWAXS) (Fig. 3af), and their corresponding one-dimension curves are shown in Fig. 3g,h. Whether in pristine or blended lms, all three molecules exhibited preferable edge-on molecular packing orientation with a small ratio of face-on packing orientation, because evident multiple higher-order (h00) reections in the out-of-plane direction and an evident (010) reection of pp stacking in the in-plane direction were observed for all samples. From the calculated face-on to edge-on ratios (Supplementary Table 3), it could be easily found: in pristine lms, molecules adopted a more favourable edge-on packing mode; while in blends, the ratios of face-on to edge-on orientation were similar for the three molecules.

The differences in d-spacing in the (100) direction were ascribed to the varying lengths of the alkyl (corresponds to the short axis periodicity). The pp stacking distance of BTID-0F, BTID-1F and BTID-2F were 3.63, 3.57 and 3.55 , illustrating a more condensed stacking in the pp direction after uorination; a result was consistent with the absorption in lms. The coherence lengths calculated from the pp stacking (010) peaks were 51.0,67.3 and 70.4 for BTID-0F, BTID-1F and BTID-2F, suggesting uorination of end-capped units increases order range. In comparison with pristine lm (Fig. 3ac), additions of PC71BM

decreased the coherence length by 13.2, 10.4 and 2% for BTID-0F, BTID-1F and BTID-2F, indicating that uorination decreases the inuence of PC71BM on molecular aggregation, especially for BTID-2F; the phenomenon could be ascribed to their decreased miscibility with PC71BM. In addition, PC71BM in all blends exhibited strong aggregation with coherence lengths ca. 20 (Supplementary Table 3). The good aggregations for PC71BM and small molecules reduced interfacial energy disorder, which was benecial for further lowering loss of Voc (ref. 22).

The morphologies were further investigated by atomic force spectroscopy (AFM) and transmission electron microscopy (TEM). The domain in AFM phase images (Fig. 4ac) increased in size after uorinations in accordance with their enhanced crystalline. As seen in TEM images (Fig. 4df), nanostructures were observed for BTID-0F/PC71BM blends with diameters ca.

25 nm. After uorination, the diameters of nanostructure increased evidently for BTID-1F/PC71BM blends because of increased order range. Interestingly, with a further increase in uorine atoms, an evident network of whiskers with diameter ca. 15 nm was observed, interpenetrating in the larger domains.

Resonant soft X-ray scattering (RSoXS) was employed to investigate the above mentioned ne microstructure (Fig. 4g). With increment of uorine atoms, the dominated domain size increased from 24 to 43, and nally to 53 nm, as in good

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Figure 3 | Microstructures of pristine and blend lms. (af) GIWAXS images in pristine lms on Si substrate and GIWAXS images in blends lms on ZnO/Si substrates; (g) corresponding out-of-plane curves; and (h) corresponding in-plane curves.

agreement with the increased crystallinity. However, by carefully analysing the RSoXS proles, we could see that the scattering distribution could be tted by two log-normal functions, with the other peak in the longer q-values: ca. 0.25 nm1 for BTID-2F (corresponding to domain size of 12.8 nm, calculated from plot tting in Supplementary Fig. 5) and a less evident interference appeared at ca. 0.22 nm1 for BTID-1F (corresponding to domain size of 14.4 nm). As for BTID-0F, no obvious difference was found between the two tting domain sizes (24 and 20 nm). In other words, BTID-2F/PC71BM and BTID-1F/PC71BM blends demonstrated formation of a hierarchical morphology with secondary domain sizes, and the secondary domain size was closed to the exciton diffusion length of ca. 10 nm (Fig. 4h). Moreover, the relative domain purity for BTID-0F, BTID-1F and BTID-2F was calculated as 0.70, 0.93 and 1, in good agreement with increased crystallinity and decreased miscibility by increased uorine introduction (Fig. 4g,h). The hierarchical morphology is reported to well balance domain size and purity to facilitate charge separation and transfer; the smaller donor phase accounts

for charge separation, whereas the larger donor phase is responsible for charge transport42,43. Therefore, the hierarchical morphology consisting 1020 nm structure and enhanced domain purity could increase FF and Jsc simultaneously.

Surface enrichment and vertical phase distribution. X-ray photoelectron spectroscopy (XPS) was carried out to study surface enrichment and vertical phase distribution. In the blends of small molecules: PC71BM, sulfur and uorine atoms could be used as characteristic elements of the small molecules due to the absence of the two elements of PC71BM. A parameter named surface enrichment degree was introduced to characterize surface enrichment, which was equal to divide the S:C (or F:C) ratio measured through XPS by the ideal S:C (or F:C) ratio calculated from D:A ratio (Fig. 5a). The histogram in Fig. 5a illustrated that all three molecules were prone to accumulating on the surface. With uorination, surface enrichment was enhanced from 1.69 for BTID-0F to 1.79 for BTID-1F and 1.81 for BTID-2F, as

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a b c

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Figure 4 | Lateral morphologies and microstructures of blend lms on ZnO/ITO substrates. (ac) AFM phase images; (df) TEM images; (g) RSoXs proles; and (h) relative domain purity and domain sizes.

calculated from S:C ratios. The surface enrichment degree calculated from F:C ratios were markedly higher (2.1 for BTID-1F and 2.6 for BTID-2F) than those of S:C ratios, indicating that the uorine-substituted groups are more prone to be enriched at the active layer surface.

Subsequently, in-depth XPS measurements in the vertical direction of BTID-0F and BTID-2F blends on ZnO/ITO substrates were carried out to characterize the vertical phase distribution (Fig. 5b). We dened the active layer/ZnO interface by the appearance of a large amount of Zn element. The corrosion time of active layer was slightly different because of the material and lm thickness differences. BTID-0F exhibited enrichments both on top surface (air/active layer) and bottom interface (active layer/ZnO). As for BTID-2F, surface enrichment degree was increased evidently, whereas enrichment at the bottom and bulk was suppressed. As calculated from F:C ratios, the top surface was nearly 100% of BTID-2F (Supplementary Methods), indicating an electron blocking layer was formed at the active layer/MoOx interface. The more directional vertical phase distribution and formation of electron blocking layer facilitated the charge extraction/collection and recombination suppression, leading to a higher FF and Jsc in inverted devices than those of conventional devices for BTID-2F (Supplementary Fig. 6,

Supplementary Table 4). Moreover, the vertical distribution and surface enrichment reduced recombination and increased the build-in potential of inverted devices in comparison of its conventional devices, which thereby further reduced Voc loss.

The inuence of surface enrichment and vertical phase separation on the device performance was further certied in the Supplementary evidence of the role of surface enrichment and vertical phase separation (Supplementary Discussions, Supplementary Fig. 7, Supplementary Tables 3 and 5).

The surface enrichment and vertical phase distribution were further manifested by results of charge carrier mobility (Fig. 5c). All the carrier mobility were used average values measured by space-charge limited current method (Supplementary methods), obtained from thickness between 100 and 130 nm (Supplementary Fig. 8). The average hole mobility of pristine lm for BTID-0F, BTID-1F and BTID-2F were 8.7 10 4

cm2 V 1 s 1, 6.4 10 4 cm2 V 1 s 1 and 3 10 4 cm2 V 1

s 1, the decrease of hole mobility with uorination should be mainly resulted from their decreased face-on packing ratios (Supplementary Table. 3) and longer alkyl chain lengths. However, after blending of PC71BM, BTID-2F showed markedly higher hole mobility than BTID-0F, the average of which were1.4 10 3 cm2 V 1 s 1 and 4.7 10 4 cm2 V 1 s 1. On the

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Figure 5 | Vertical morphologies of blended lms. (a) Surface enrichments of BTID-0F/PC71BM, BTID-1F/PC71BM and BTID-2F/PC71BM on ZnO/ITO substrate; (b) in-depth XPS proles of BTID-0F/PC71BM and BTID-2F/PC71BM on ZnO/ITO substrates, the line were obtained from tting all the S:C value from various etching time; (c) carrier mobilities of blends and pristine lms, the average values were obtained from the mobility measured from the thickness between 100 and 130 nm, the error bars come from the mobility value from thickness differences and their measurements errors; and(d) schematic illustrations of lateral and vertical phase distribution of BTID-2F/PC71BM.

other hand, the electron mobility of BTID-2F/PC71BM blends

was considerably lower than that of BTID-0F/PC71BM blends. The hole and electron mobility of BTID-1F/PC71BM were in

between. The different trends for hole and electron mobility could be explained by the largest surface enrichment of BTID-2F, which facilitated hole transport while blocked electron transport. The increase in hole mobility of blends compared with pristine lms for uorine-substituted molecules could be explained by their increased ratios of face-on to edge-on after PC71BM

blending (Supplementary Table 3) and favourable backbone orientation for vertical charge transport10.

DiscussionBased on hierarchical morphology at lateral direction and vertical phase distribution, the morphology of active layer was schematically shown as Fig. 5d for BTID-2F/PC71BM. In lateral direction, BTID-2F formed a hierarchical morphology with high domain purity, composing domains with diameters of ca. 53.0 nm and an interpenetrating whisker network with diameters of ca.12.8 nm. A similar hierarchical morphology for BTID-1F/ PC71BM was observed but with a lower domain purity, while no obvious difference for two domain sizes in BTID-0F/PC71BM.

In vertical direction, BTID-2F formed hole-transporting layers on the top interface and more directional vertical phase distributions of the active layer than those of BTID-0F.

Therefore, uorination-enabled optimal hierarchical morphology and surface enrichment, which could increase Voc, Jsc and FF

simultaneously and thereby obtained a high PCE in inverted devices. To further verify the inuence of hierarchical morphology on device performance (Supplementary Discussions), the substrate temperature was increased from 28 C (normal) to 40 C (hot) during lm formation. Due to a faster solvent evaporation on hot substrate, the hierarchical morphology was not formed as proved by TEM images and RSoXS images (Supplementary Fig. 9). As a consequence, the device performance based on the hot substrate was decreased (Supplementary Table 6). Hence, the hierarchical morphology is one of the most important factors to obtain high performance devices.

For the molecular design, acceptordonoracceptor have been widely used for organic photovoltaic. To obtain a low HOMO level and a high hole mobility simultaneously, a novel p-bridge between donor and acceptor unit, 2-(thiophen-2-yl) thieno [3,2-b]thiophene was introduced, which presented an inner gradient-decreased electron density distribution, and facilitated the backbone charge transfer. Different from that of polymers, the end acceptor played an important role to tune molecular packing and miscibility with PC71BM. Therefore, uorinated end-capped acceptor were introduced in the molecular design, which lowered surface tension and their miscibility with PCBM, As a result, the lateral and vertical morphology of the active layer was optimized. BTID-2F formed a hierarchical morphology in the active layer,

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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms13740

which will inspire more investigations on the effects of p-bridges and end acceptors for high performance OSCs.

In summary, by combining traditional molecular design strategy with ne tuning surface tension and miscibility with PC71BM by uorination, we designed and synthesized three novel molecules, BTID-0F, BTID-1F and BTID-2F, with incremental uorine atoms. The three molecules exhibited excellent molecular properties, such as low HOMO levels, good crystallinity and high hole mobility, because of well-designed molecular structures, including new gradient-decreased electron density p-bridges and proper polarity of aromatic acceptors. Through device measurement and morphology characterization, we emphasized the importance of uorination to hierarchical morphology, surface enrichment and directional vertical phase distribution. The optimal morphology was benecial to charge transfer, charge collection and recombination suppression, which reduced the loss of Voc, Jsc and FF simultaneously. As a result, a record PCE of11.3% was obtained in inverted OSCs based on small molecules, with Voc of 0.95 V, Jsc of 15.7 mA cm 2 and FF of 76%.

Methods

Solar cell fabrication and measurements. Inverted devices were fabricated with a structure of glass/ITO/ZnO/donor:acceptor/MoOx/Ag. The ZnO precursor solution was prepared by dissolving 0.14 g of zinc acetate dihydrate (Zn(CH3COO)2 2H2O, 99.9%, Aldrich) and 0.5 g of ethanolamine

(NH2CH2CH2OH, 99.5%, Aldrich) in 5 ml of 2-methoxyethanol (CH3OCH2-CH2OH, 99.8%, J&K Scientic). Patterned ITO glass with a sheet resistance of15 O sq1 was purchased from CSG HOLDING Co., Ltd. The ITO-coated glass substrates were cleaned by ultrasonic treatment in detergent, DI water, acetone and isopropyl alcohol under ultrasonication for 20 min at each step. A thin layer of ZnO precursor was spin-coated at 5,000 r.p.m. onto the ITO surface. After being baked at 200 C for 30 min, the substrates were transferred into a nitrogen-lled glove box. The mixture of small molecules and PC71BM with total concentration ca. 18 mg ml 1 stirred at 60 C in chloroform for ca. 0.5 h until they are intensively dissolved. Subsequently, the active layer was spin-coated from chloroform solutions of blends. Finally, a layer of ca. 5 nm MoOx and then an Ag layer of ca. 160 nm was evaporated subsequently under high vacuum (o1 10 4 Pa).

Conventional devices were fabricated with a structure of glass/ITO/ PEDOT:PSS/Ca/Al. The ITO-coated glass substrates were cleaned by the same procedure with inverted devices. A thin layer of PEDOT:PSS was spin-coated at 4,000 r.p.m. onto the ITO surface. After being baked at 150 C for 15 min, the substrates were transferred into a nitrogen-lled glove box. The mixture of small molecules and PC71BM with total concentration ca. 18 mg ml 1 stirred at 60 C in chloroform for ca. 0.5 h until they intensively dissolved. Subsequently, the active layer was spin-coated from blend chloroform solutions of small molecules and PC71BM. Finally, a layer of B20 nm Ca and then 100 nm Al layer was evaporated under high vacuum (o1 10 4 Pa).

Device JV characteristics was measured under AM 1.5 G (100 mW cm 2) using a Newport Thermal Oriel 91159A solar simulator. Light intensity is calibrated with a Newport Oriel PN 91150V Si-based solar cell. JV characteristics were recorded using a Keithley 2400 source-measure unit. Typical cells have device areas of approximately 4 mm2. A mask with well-dened area was used to measure the JV characteristics as well. EQEs were performed in air with an Oriel Newport system (Model 66902) equipped with a standard Si diode. Monochromaticlight was generated from a Newport 300 W lamp source. We have usedmask for BTID-2F, the errors are in 5%.

Supplementary Methods including: characterization methods: (1) molecular structure characterization and calculation (nuclear magnetic resonance, mass spectrometry spectra, discrete Fourier transform); (2) molecular properties characterization (dielectric constant, UPS, ultravioletvis spectra, CV, UPS, contact angle); (3) TEM, AFM, XPS and in-depth XPS characterization; (4) GIWAXS characterization; (5) RSoXs characterization; and (6) Jph and mobility measurements. Calculation methods: (1) calculations of ionization potential of donor (IPD) from ultraviolet photoelectron spectroscopy (UPS); (2) calculation of the coherence length (Lc) of PC71BM and small molecules; (3) calculation of domain size and purity from RSoXs ; (4) calculation of surface enrichment degrees;(5) calculation of surface D:A ratio; and (6) calculations of mobility measured from space-charge limited current. Synthesis methods including: materials and synthesis.

Data availability. All relevant data are available from the authors.

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Acknowledgements

We acknowledge the nancial support by the Ministry of Science and Technology of China (No 2016YFA0200700), the National Natural Science Foundation of China (Grant Nos 21534003, 91427302, and 21504066) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No XDA09040200). X-ray data was acquired

at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Ofce of Science, Ofce of Basic Energy Sciences, of the U.S. Department of Energy under Contract No DE-AC02-05CH11231.

Author contributions

D.D. designed, synthesized and characterized materials and fabricated devices based on BTID-2F. Y.Z. performed device fabrication of BTID-0F and BTID-1F. B.X. helped device fabrication. J.Z. performed GIWAXS data analysis. Z.W. and W.M. performed GIWAXS/RSoXs measurements and data analysis. L.Z. and Z.W. performed theoretical simulation. J.F. performed dielectric constant measurements. D.D., K.L. and Z.W. prepared manuscript. All authors discussed and commented on the paper. K.L. and Z.W. supervise the project.

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How to cite this article: Deng, D. et al. Fluorination-enabled optimal morphology leads to over 11% efciency for inverted small-molecule organic solar cells. Nat. Commun. 7, 13740 doi: 10.1038/ncomms13740 (2016).

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