Correlation between Distribution of Polymer Orientation and Cell

Sep 17, 2018 - ... both orientation types co-exist in their polymer/fullerene blend films. ... The difference in the distribution of the backbone orie...
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Correlation between Distribution of Polymer Orientation and Cell Structure in Organic Photovoltaics Masahiko Saito,† Tomoyuki Koganezawa,‡ and Itaru Osaka*,† †

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan ‡ Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo 679-5198, Japan

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S Supporting Information *

ABSTRACT: The backbone orientation of semiconducting polymers is one of the important structural factors that determines the charge transport and thus the performance of optoelectronic devices. Here, we study two sets of thiophene− thiazolothiazole polymers, which primarily form edge-on and face-on orientations, termed “edge-on-polymers” and “faceon-polymers”, respectively; both orientation types co-exist in their polymer/fullerene blend films. Interestingly, we find that the dependence of the photoactive layer thickness on the fill factor in the photovoltaic cells, with the inverted and conventional structures, is quite distinct in the edge-onpolymer; however, this is not evident in the face-on-polymers. An in-depth study by grazing incidence X-ray diffraction analysis reveals that the face-on/edge-on ratio is unevenly distributed through the film thickness in the edge-on-polymers, while it is evenly distributed in the face-on-polymers. The difference in the distribution of the backbone orientation correlates with the difference in the thickness dependence on the fill factor. We thus propose that the distribution of the backbone orientation is an important factor to understand the performance of polymer-based photovoltaic cells and that the cell structure should be carefully selected by considering the distribution for maximizing the performance. KEYWORDS: organic photovoltaics, semiconducting polymers, π-conjugated polymers, orientation, thiazolothiazole

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INTRODUCTION Organic photovoltaics (OPVs) with bulk-heterojunction (BHJ) photoactive layers that use semiconducting polymers have been attracting much attention, as they can be fabricated by solution processes that are low-cost and low-energyconsumption methods; further, they are flexible, lightweight, and semitransparent.1−3 One of the important issues in OPVs is to improve the power conversion efficiency (PCE). Numerous studies toward improving the PCE, mainly by developing new semiconducting polymers, have been conducted in the last decade, which have resulted in OPVs surpassing a milestone value of 10%.4−10 The electronic properties of semiconducting polymers, such as the band gap and frontier orbital (highest occupied molecular orbital and lowest unoccupied molecular orbital) energy levels, largely affect the light absorption and charge separation, which determine the short-circuit current density (JSC) and open-circuit voltage (VOC). The charge carrier transport is also an important parameter that affects JSC and the fill factor (FF). The charge carrier transport of semiconducting polymers is highly dependent on their ordering structure, that is, crystallinity and/or backbone orientation.11−16 As the photoactive layer is vertically stacked between the electrode in an OPV cell, high charge carrier transport in the out-ofplane direction with respect to the substrate (electrode) plane © XXXX American Chemical Society

is required. Hence, it is highly desirable that the backbones lie flat on and π-stack vertically to the substrate plane, known as “face-on” orientation, rather than stand on and π-stack laterally along the substrate, known as “edge-on” orientation.17 The backbone orientation of semiconducting polymers is altered by several factors such as side chain (regularity, attachment density, length, and topology),11,14−20 molecular weight,13,21,22 thermal annealing,23 and blending with fullerene derivatives.13,24 Recently, we reported that the orientation of a thiophene−thiazolothiazole copolymer system (PTzBT: Figure 1) in a polymer neat film can be “controlled” by carefully

Figure 1. Chemical structure of a thiophene−thiazolothiazole polymer system (PTzBTs). Received: June 22, 2018 Accepted: September 4, 2018

A

DOI: 10.1021/acsami.8b10460 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. Chemical structures of PTzBTs with different side chains (a,b) and 2D GIXD patterns of the PTzBT neat films (c,d) and PTzBT/PCBM blend films (e,f). 14HD and EHOD provide the π−π stacking diffraction along the qxy axis (c), which is assignable to the edge-on orientation in the neat film but provide it mainly along the qz axis (e), which is assignable to the face-on orientation, in the blend film. 12OD and BOHD provide the π−π stacking diffraction along the qz axis in both the neat (d) and blend films (f).

Figure 3. Thickness dependence of the photovoltaic parameters for the PTzBT-based inverted cells (a−c) and conventional cells (d−f). (a,d) JSC, (b,e) FF, (c,f) PCE.

designing the combination of alkyl side chains, R1 and R2, in terms of length and topology (linear and branched).18 Interestingly, we observed that, in OPV cells with a conventional structure, the FF decreased gently with the increase of the photoactive layer thickness when PTzBTs with the face-on orientation were used, whereas the FF decreased significantly when PTzBTs with the edge-on orientation were used. Although this likely originated due to the difference in the backbone orientation, it should be noted that all these

polymers formed the face-on orientation when blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) regardless of the primary orientation in the neat film. This prompted us to further investigate the correlation between the backbone orientation and OPV performance in this polymer system. Herein, we show that, in sharp contrast to the conventional cells, the thickness dependence of FF between PTzBTs, which primarily form the edge-on and face-on orientations, is similar in the inverted cells, where both orientation systems showed a B

DOI: 10.1021/acsami.8b10460 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

with the increase in the film thickness for all the polymers. However, the change in FF was quite distinct between the edge-on- and face-on-polymers. The FF decreased more significantly for the cells that used the edge-on-polymers than for those that used the face-on-polymers: the FF for the edge-on-polymers decreased from ca. 0.75 at around 60 nm to below 0.6 at >300 nm, whereas the FF for the face-onpolymers stayed above 0.65. As a result, PCEs increased for the face-on-polymers and decreased for the edge-on-polymers as a function of the photoactive layer thickness, completely reproducing the results in our previous report. To investigate the reason for the contrasting behavior in the thickness dependence of FF between the inverted and conventional cells, we conducted a qualitative study on the difference in charge recombination by measuring the light intensity dependence of J−V curves.9,25 Figure 4 depicts the

gentle decrease in FF with increasing thickness. We also found by means of grazing incidence X-ray diffraction (GIXD) measurements and analyses that the distribution of the edgeon/face-on ratio through the film thickness is quite distinct between the two orientation systems. The difference in the distribution correlates with the difference in the thickness dependence of FF. Thus, we propose that the distribution of the backbone orientation through the film thickness is a key factor to determine the performance of polymer-based BHJ cells and that the cell structure must be carefully chosen to better suit the distribution and thereby maximize the performance of the cells.

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RESULTS AND DISCUSSION Photovoltaic Performance. The PTzBTs with different side chain combinations employed in this work are displayed in Figure 2a,b. PTzBT-14HD (R1 = tetradecyl, R2 = 2hexyldecyl) and -EHOD (R1 = 2-ethylhexyl, R2 = 2octyldodecyl) (Figure 2a), which will be called hereafter 14HD and EHOD, primarily form the edge-on orientation as they provide a diffraction corresponding to the π−π stacking along the qxy axis in the two-dimensional GIXD (2D GIXD) pattern of the neat film (Figure 2c). They are hence referred to as “edge-on-polymers”. PTzBT-12OD (R1 = dodecyl, R2 = 2octyldodecyl) and -BOHD (R1 = 2-butyloctyl, R2 = 2hexyldecyl) (Figure 2b), which will be called hereafter 12OD and BOHD, mainly form a face-on orientation as they provide a diffraction corresponding to the π−π stacking mainly along the qz axis in the 2D GIXD patterns (Figure 2d). They are hence referred to as “face-on-polymers”. However, again, all these polymers provide 2D GIXD patterns assignable to the face-on orientation when blended with PCBM (Figure 2e,f). We first fabricated inverted cells using polymers with five different photoactive layer thicknesses (50−350 nm), with the stacking structure of ITO/ZnO/PTzBT:PCBM (1:2 w/w)/ MoOx/Ag. The polymer-to-PCBM weight ratio was 1:2 for all the polymers. The current density−voltage (J−V) curves were measured by illuminating the cells with a simulated solar light (air mass 1.5G, 100 mW/cm2). All the J−V curves and external quantum efficiency spectra of the cells and the photovoltaic parameters are summarized in the Supporting Information (Figure S1a−h and Table S1). Figure 3a−c depicts the thickness dependence of JSC, FF, and PCE of the inverted cells. In all cases, JSC increased from ca. 8 to 12 mA/cm2 as the photoactive layers thickened, reflecting the increased volume of the photoactive layer. The FF values for both the edge-onpolymers and face-on-polymers were almost the same (∼0.75) when the photoactive layers were very thin (ca. 70 nm), which similarly decreased gently to around 0.7 when the layers were thickened to ca. 300 nm. As a result, PCEs increased as a function of the photoactive layer thickness for both the edgeon- and face-on-polymers. It is noted that this trend is in sharp contrast to the conventional cells as we reported previously, in which the FF decreased more significantly in the edge-on-polymers than in the face-on-polymers. In addition, the hole mobility, evaluated by the space-charge limited current model, also supports this: the face-on-polymers tended to show higher mobilities than the edge-on-polymers.18 To reconfirm this, we also fabricated conventional cells with the stacking structure of ITO/ PEDOT:PSS/PTzBT:PCBM (1:2 w/w)/Ca/Al. The plots of JSC, FF, and PCE as a function of the thickness are shown in Figure 3d−f. As is the case in the inverted cell, JSC increased

Figure 4. Dependence of FF on light intensity for the inverted cells with thin (ca. 70 nm) (a) and thick (ca. 300 nm) (b) photoactive layers and for the conventional cells with thin (ca. 70 nm) (c) and thick (ca. 300 nm) (d) photoactive layers.

plots of FF as a function of light intensity for the inverted and conventional cells for thin (ca. 70 nm) and thick (ca. 300 nm) photoactive layers. In the inverted cells, light intensity dependence was mostly the same for all the polymers. For the thin-layer cells (Figure 4a), the FF was almost insensitive to the light intensity, whereas, for the thick-layer cells (Figure 4b), the FF slightly decreased with the increase in the light intensity. Note that JSC increased linearly as the light intensity increased, wherein the number of free carriers increased (Figure S3). This light intensity dependence in FF suggests that the bimolecular recombination is negligible for the thinlayer cells, whereas the recombination increased when the layer was thickened, which is most likely due to the elongated distance for the charges to be transported.26 The result also indicates that bimolecular recombination occurs similarly regardless of the orientation. In the conventional cells, the FF was also mostly insensitive to the light intensity in all cases for the thin-layer cells (Figure 4c). However, in the thick-layer cells, the trend between the C

DOI: 10.1021/acsami.8b10460 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces face-on- and edge-on-polymers was quite distinct (Figure 4d). In the face-on-polymers, the FF slightly decreased as the light intensity increased, as is the case in the inverted cells. In contrast, the decrease in FF with increasing light intensity was much more significant in the edge-on-polymers. This suggests that bimolecular recombination was considerably increased in the edge-on-polymers compared to the face-on-polymers. The difference in the bimolecular recombination may be one of the main reasons for the difference in the thickness dependence of FF. Investigation of Backbone Orientation. As discussed above, interestingly, the trend in the thickness dependence of photovoltaic performances was distinct between the inverted and conventional cells. We therefore conducted GIXD studies for the PTzBT/PCBM films fabricated on the ZnO surface (ZnO coated ITO glass) and on the PEDOT:PSS surface (PEDOT:PSS-coated ITO glass) with different thicknesses. The films were fabricated exactly according to the procedure for the photovoltaic cells and had film thicknesses similar to the photovoltaic cells. 2D GIXD patterns indicated that in the films with different thicknesses, PTzBT mainly formed the face-on orientation with small fractions of edge-on orientation (Figures S6 and S7). In addition, the lamellar diffraction appearing at the small-angle region showed that there were clear differences in the ratio of the face-on crystallite to the edge-on crystallite by polymer, substrate surface, and thickness. Thus, in order to quantitatively discuss the backbone orientation, we performed a pole figure analysis for the lamellar diffraction.7,27,28 Figure 5a−d depicts the line cut profiles of the lamellar diffraction along the azimuth angle (χ) for 14HD and 12OD on the ZnO and PEDOT:PSS surfaces. The face-on/edge-on ratio can be quantified by dividing the peak area for the face-on crystallite (Axy: χ = 0−45° and 135− 180°) by that for the edge-on crystallite (Az: χ = 55−125°) (see Supporting Information). Note that Axy/Az does not necessarily indicate the real face-on/edge-on ratio because the scattering from the direct beam overlaps with the first-order lamellar diffraction especially along the qz axis, which overestimates the Az value. Figure 5e,f shows the plots of Axy/Az as a function of the thickness for the blend PTzBT/ PCBM films fabricated on ZnO and PEDOT:PSS, respectively. In all the polymers, the Axy/Az values were slightly higher for the ZnO surface than for the PEDOT:PSS surface, indicating that the polymers have a higher tendency to form face-on orientation on the ZnO surface than the PEDOT:PSS surface. In both surfaces, the face-on-polymers (12OD and BOHD) showed higher Axy/Az values than the edge-on-polymers (14HD and EHOD), reflecting the primary orientation, in the thinner films. Interestingly, whereas the Axy/Az value did not change by the film thickness for the face-on-polymers, it gently increased as the film thickness increased for the edgeon-polymers. Notably, 14HD (edge-on-polymer) was found to show even higher Axy/Az values, thus possessing a larger population of the face-on crystallite, than BOHD (face-onpolymer) in the thicker films (>250 nm). However, the difference in Axy/Az by the thickness does not directly correlate with the difference in the photovoltaic performances by the thickness: although the edge-on-polymers had a larger population of the face-on crystallite at larger thicknesses, which should be favorable for the OPV performance, they afforded lower FF. The increase in the face-on crystallite with increasing film thickness in the edge-on-polymers may rather suggest that the

Figure 5. Line cut profiles of the lamellar diffraction along the azimuth angle (χ) in the 2D GIXD patterns for 14HD/PCBM (a,b) and 12OD/PCBM (c,d) blend films on the ITO/ZnO (a,c) and ITO/ZnO (b,d) substrates. Dependence of Axy/Az on film thickness, where Axy/Az corresponds to the ratio of the face-on to edge-on orientation on ZnO (e) and PEDOT:PSS (f) substrate.

face-on to edge-on ratio is not distributed evenly along the film thickness, as we discussed for naphthobisthiadiazole polymers.7 It is reasonable to assume that the thickness of the “interfacial” layer of the PTzBT/PCBM film around the bottom surface (ZnO or PEDOT:PSS) and the face-on to edge-on ratio in the interfacial layer are independent of the total film thickness. Hence, the increase in the face-on fraction mainly occurs in the bulk and arises from the increase in the bulk volume in thicker films. This indicates that the face-on fractions are more abundant in the bulk than at the interface of the film-bottom surface. On the other hand, the fact that the face-on/edge-on ratio did not change with the film thickness in the face-onpolymers suggests that the face-on to edge-on ratio is distributed evenly along the film thickness. Correlation between Thickness Dependence in FF and Backbone Orientation. The distribution of the backbone orientation, in particular, through the film thickness correlates with the difference in the thickness dependence of FF between the edge-on-polymers and the face-on-polymers. In the edge-on-polymers, the population of the face-on crystallite is relatively larger in the bulk, and the population of the edge-on crystallites is relatively larger at the film-bottom interlayer interface. This distribution could facilitate the vertical hole transport in the inverted cell, where the generated holes flow toward the top interlayer (MoOx) through the faceD

DOI: 10.1021/acsami.8b10460 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Schematic images of the blend films of the edge-on-polymer (a,c) and the face-on-polymer (b,d), showing the distribution of the orientation in the inverted (a,b) and conventional cells (c,d). Note that PCBM molecules/aggregates are not illustrated for the sake of simplicity.

results indicate that the distribution of the backbone orientation is an important factor for understanding the performance of polymer-based BHJ solar cells and that the cell structure should be carefully selected by considering the distribution of the orientation for maximizing the performance.

on rich region (Figure 6a). In contrast, this distribution of the orientation should be detrimental to the conventional cell, where the generated holes flow toward the bottom interlayer through the edge-on rich region (Figure 6c). Note that we did not observe the segregation of PCBM at the substrate interface,29 which would also affect the device performance, by the energy-dispersive X-ray spectroscopy (EDS) for the cross section of the cells (Figure S7). It is thus quite reasonable that, with the better matching between orientation distribution and the cell stack, the edge-on-polymers showed a gentler decrease in FF with increasing film thickness in the inverted cell than in the conventional cell. Whereas, for the face-onpolymers, the face-on to edge-on ratio is distributed evenly through the film thickness (Figure 6b,d). It is thus assumed that the generated holes flow similarly in both the upper and lower directions, and thereby, the thickness dependence in FF is insensitive to the cell structure.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b10460. Materials and instruments, polymer orientation and photovoltaic parameters of polymer-based BHJ cells in inverted and conventional device structures, J−V curves and EQE spectra of inverted and conventional cells, 2D GIXD patterns, and EDS profiles (PDF)

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CONCLUSIONS In summary, we fabricated BHJ cells with inverted and conventional structures using PTzBTs that primarily form edge-on orientation (edge-on-polymers) and face-on orientation (face-on-polymers). In inverted cells, both the edge-onpolymers and face-on-polymers showed similar dependence in the photovoltaic performance by the BHJ active layer thickness, where the FF gently decreased with increasing thickness. Meanwhile, in the conventional cells, the edge-onpolymers showed a more significant decrease in FF with the increase in thickness than the face-on-polymers. This most likely correlated with the difference in the distribution of the backbone orientation through the film thickness between the edge-on-polymers and face-on-polymers, which was revealed by an in-depth analysis of the GIXD patterns of the PTzBTs/ PCBM blend films. In the edge-on-polymers, in which large fractions of edge-on orientation turn into face-on orientation in the blend film, the edge-on crystallites are relatively abundant around the substrate surface and the face-on crystallites are relatively abundant in the bulk. Therefore, generated holes could be transported faster in the upper direction than in the lower direction. This distribution would better match the structure of the inverted cell compared to that of the conventional cell. In the face-on-polymers, the face-on and edge-on crystallites are evenly distributed through the film thickness. In such a case, holes can be equally transported in both upper and lower directions, and thus, the photovoltaic performance would be insensitive to the cell structure. These

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Itaru Osaka: 0000-0002-9879-2098 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from JST (grant no. JPMJAL1404). 2D GIXD experiments were performed at the BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal nos. 2015A1952 and 2015B1904).

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DOI: 10.1021/acsami.8b10460 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX