P3HT

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Morphology and Optoelectronic Properties Tuning of ZnO/ P3HT/P3HT-b-PEO Hybrid Films via Spray Deposition Method Kun Wang, Lorenz Bießmann, Matthias Schwartzkopf, Stephan V. Roth, and Peter Muller-Buschbaum ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05459 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Applied Materials & Interfaces

Morphology and Optoelectronic Properties Tuning of ZnO/P3HT/P3HT-b-PEO Hybrid Films via Spray Deposition Method b

Kun Wanga, Lorenz Bießmanna, Matthias Schwartzkopf , Stephan V. Rothb,c, Peter MüllerBuschbauma,* a

Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München,

James-Franck-Strasse 1, 85748 Garching, Germany b

c

Deutsches Elektronen-Synchrontron (DESY), Notkestrasse 85, 22603 Hamburg, Germany

KTH Royal Institute of Technology, Department of Fibre and Polymer Technology,

Teknikringen 56-58, SE-100 44 Stockholm, Sweden

ABSTRACT The self-assembly of amphiphilic diblock copolymers yields the possibility to use them as a template for tailoring the film morphologies of sol-gel chemistry derived inorganic electron transport materials, such as mesoporous ZnO and TiO2. However, additional steps including etching and backfilling are required for the common bulk heterojunction fabrication process when using insulating diblock copolymers. Here, we use the conducting diblock copolymer poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) in which P3HT acts as charge carrier transport material and light absorber, while PEO serves as template for ZnO synthesis. The initial solution is subsequently spray coated to obtain the hybrid film. Scanning

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electron microscopy (SEM) and grazing-incidence small-angle X-ray scattering (GISAXS) measurements reveal a significant change in the morphology of the hybrid films during deposition. Optoelectronic properties illustrate the improved charge separation and charge transfer process. Both, the amount of the diblock copolymer and the annealing temperature play an important role to tune the morphology and the optoelectronic properties. Hybrid films being sprayed from a solution with a ratio of ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:1 and subsequent annealing at 80 °C show the most promising morphology combined with an optimal photoluminescence (PL) quenching. Thus, the presented simple, reagents- and energy-saving fabrication method provides a promising approach for a large scale preparation of bulk heterojunction P3HT:ZnO films on flexible substrates.

KEYWORDS: P3HT:ZnO, spray coating, morphology, optoelectronic properties, GISAXS

1. INTRODUCTION Block copolymers, with two or more chemically dissimilar polymer blocks which are covalently bonded to a single molecule, have attracted considerable attention over the past decades due to many foreseeable applications.1-3 Because of the competing forces arising from the enthalpy contribution of mixing (or de-mixing) and the entropy penalty associated with elongating the polymer chains, the obtainable structures have very well-defined spacing with a typical size on the nanometer length scale via microphase separation.4, 5 Moreover, the nanoscale hybridization of different units permits the combination of distinct properties and the interesting nanoscale


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assembly phenomena which cannot be accessed through simple blending of non-bonded blocks. With this, block copolymers hold high potential for functional nanotechnology applications.6-8 The most important classes among the synthetic systems for tailoring self-assembled nanostructures are di- and tri-block copolymers. Via self-assembly process, it is possible to obtain ordered nanostructures which are tunable towards manifold morphologies, e.g. spheres, cylinders, lamellae, and bicontinuous structures in volume as well as in thin film geometry. 9-11 These self-assembled patterns have also been considered as templates for further synthesis of inorganic nanostructures.12-15 Generally, this can be realized by growing inorganic nanostructures, such as metal oxides, in a nanoporous template pre-formed via the self-assembly of the block copolymer. For example, porous PS templates were fabricated by removing the other block in the diblock copolymer of polystyrene-block-poly(methyl methacrylate) (PS-bPMMA) or polystyrene-block-polyisoprene (PS-b-PI), followed by preparing zinc oxide (ZnO) in the porous template.16,

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After removing the PS templates from the films, pure ZnO

nanostructures such as nanorods, nanowalls, gyroid or worm-like structures were obtained.16, 17 Apart from this, another more efficient approach is to use the sol-gel method coupled with an amphiphilic block copolymer as a template. It picks up the idea of modifying the sol-gel technique by the use of phosphate surfactants to nanostructure metal oxides.18 The hydrophilic and hydrophobic polymer blocks of the amphiphilic block copolymers have different functionality in the task of tailoring the metal oxide nanostructures. With the introduction of a good-poor solvent pair during the sol-gel preparation, micelles are induced in the solution with metal oxides preferential to growing in the hydrophilic blocks. After the self-assembly process of the block copolymer, tailored metal oxide nanostructure can be obtained. For example, many studies used the tri-block copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene


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oxide) being commercially easily available (such as Pluronic P123) as the structure-directing agent.19-21 Different PEO-based block copolymers were used to fine-tune the pore sizes of mesoporous metal oxide films.22 Among these approaches, Cheng et al. prepared multiple titanium dioxide (TiO2) structures via combining polystyrene-block-polyethyleneoxide (PS-bPEO) with the sol-gel method and investigated the phase diagram of the template-assisted TiO2 films with various morphologies.10 Polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) was also widely used as a template to control the structure of the inorganic materials, especially of metal oxides.23,

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These metal oxide films templated by the block copolymer provide numerous

applications, of which their use in hybrid solar cells appears very promising.25 Regarding hybrid solar cells, generated excitons can only separate into negative and positive charge carriers at the interfaces of p-type and n-type semiconductors. Therefore, it is crucial to increase the specific surface area of the metal oxides, such as ZnO, which serves as an n-type semiconductor with e.g. a p-type conjugated polymer. Moreover, a network structure is beneficial since it provides continuous pathways for the charge carrier transport to the corresponding electrodes. Such network-like nanostructures can be realized with amphiphilic block copolymers as a template for the sol-gel synthesis of metal oxide nanostructures, for example ZnO nanoparticles linked into a network.26 For the commonly used amphiphilic block copolymer PS-b-PEO,7, 10, 15 ZnO nanoparticles are derived from a zinc precursor inside the PEO blocks. A microphase separation of PS-b-PEO, which is induced by a good-poor solvent pair, enables to control the morphology of the interconnected network ZnO nanostructures with tunable surface area. The PEO block plays an important role in structure directing while the other block, PS in this case, has only the task to enable microphase separation to form well-defined nanostructures. In order to transform the ZnO/polymer nanocomposite film into a nanostructured


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ZnO film, the block copolymer needs to be removed due to its insulating behavior. Wellestablished removal procedures are calcination or other high temperature treatments, which have the drawback of a high energy input. Alternatively, the block copolymer removal might involve toxic solvents which could also hinder large-scale application. To establish the active layer of a hybrid solar cell, the nanostructured metal oxide film, e.g. the ZnO network, needs to be backfilled with a conducting polymer as for example poly (3-hexyl thiophene) (P3HT). This backfilling is challenging due to potentially unfavorable interaction of the polymer solution with the metal oxide nanostructure as well as steric problems of the conjugated polymer induced by the metal oxide nanostructure. As a consequence, the backfilling turned out to be a very efficiency limiting step for hybrid solar cells since the polymer infiltration into the metal oxide nanostructures may be incomplete and thus the polymer:metal oxide interface is not fully established.27-29 Among the different possibilities to solve this backfilling problem, an interesting approach is to use a different type of block copolymer, which already introduces additional functionality. When directly combining the p-type conducting polymers with the inorganic ntype semiconductors in a simple blend instead of using a block copolymer with functional blocks, non-continuous large domains will form due to the immiscibility of both components. Such large and non-connected domains are not useful for hybrid solar cells, because they will hinder the exciton separation and charge carrier transport.30, 31 In contrast, when an amphiphilic diblock copolymer with a conducting block is used, microphase separation will force nanoscale structures.32 In the present work, we use the diblock copolymer poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) in which P3HT acts as charge carrier transport material and light absorber, while PEO serves as template for ZnO synthesis. Thus, no further polymer removal and


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backfilling steps will be necessary to establish the active layer of hybrid solar cells. In a simple one-step approach the active layer is directly realized in combination with sol-gel synthesis from a zinc precursor. We introduce different ratios of P3HT-b-PEO to fabricate ZnO/P3HT/P3HT-bPEO hybrid films with the aim of improving the morphology and the optoelectronic properties of the ZnO/P3HT bulk heterojunction films. Different from the earlier reported research,30, 31, 33 ZnO nanoparticles are directly synthesized in the PEO block during the sol-gel process and form the interface with P3HT to complete the heterojunction. As ZnO, P3HT and P3HT-b-PEO reveal different polar properties, two miscible solvents 1,2-dichlorobenzene and dimethyl sulfoxide are used to dissolve all of them. Afterwards the blend is spray-coated on the substrate to fabricate thin film. The simple and low-cost spray coating technique is used, as it allows a film deposition directly from solution and it is potentially up-scalable.9, 34, 35 A subsequent annealing step is also explored to investigate the resulting morphology changes. Apart from the surface morphology probed by scanning electron microscopy (SEM), the inner film morphology is essential for potential applications in hybrid solar cells which is explored by grazing-incidence small-angle Xray scattering (GISAXS). Moreover, photoluminescence (PL) measurements demonstrate the improvement of charge carrier separation yield and a more efficient charge transfer.

2. EXPERIMENTAL SECTION 2.1

Materials.

The

amphiphilic

diblock

copolymer,

poly(3-hexylthiophene)-block-

poly(ethylene oxide) (P3HT-b-PEO), was purchased from Polymer Source Inc., Canada, with the number average molecular weights, Mn = 3 kg mol-1 for the P3HT block and 90 kg mol-1 for the PEO block, respectively. Ethanolamine (MEA, 98%), zinc acetate (99.99% trace metals basis), 1,2-dichlorobenzene (DCB, anhydrous, 99%) and dimethyl sulfoxide (DMSO, anhydrous,


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≥99.9%) were purchased from Sigma-Aldrich. All the materials were used as received. Silicon (100) and glass substrates are pre-cleaned in an acid bath. 2.2 Sample Preparation. The deposition method we used to prepare the ZnO/P3HT/P3HT-bPEO hybrid films was spray coating. Figure 1 provides a schematic representation of the hybrid thin film preparation routine and its post-treatment with thermal annealing. 10 mg of P3HT and an appropriate amount of the diblock copolymer P3HT-b-PEO were dissolved in a 4 mL solution mixture of DCB and DMSO (volume ratio of 7:1). The solution was stirred for 30 min at 80 °C to dissolve both, the P3HT and the PEO components completely in the solution. At the same time, 50 mg zinc acetate was dissolved in a different vial in the same mixed solvent and stirred for 30 min at 80 °C. The weight ratio of ZnO and P3HT was kept constant at 2:1. Afterwards, 19.76 µL of MEA was added to the polymer solution and stirred for further 30 min at 80 °C. In the next step, both solutions were mixed together using a syringe pump, PHD 2000 infuse/withdraw, Harvard Apparatus, by using a constant infuse rate of 1 mL min-1 and then stirred for 1 h at 80 °C, subsequently. Therefore, the sol–gel was obtained with P3HT as charge carrier transport material and PEO chains as a template on which the precursor of ZnO was preferentially incorporated. Spray deposition was carried out subsequently onto pre-cleaned silicon (100) and glass substrates, which were kept at 80 °C. More detailed information about the film deposition is given in the supporting information. The obtained composites were then annealed at different temperatures below 160 °C for half an hour to realize the final hybrid films. For the hybrid films annealed at 80 °C, the film thickness is plotted as a function of the P3HT-bPEO ratio with respect to P3HT as shown in Figure S1. The thicknesses for the films are around 180 nm.


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Figure 1. Schematic representation of the steps involved in the fabrication of the ZnO/P3HT/P3HT-b-PEO hybrid films via spray coating and subsequent thermal annealing (as example at 80 °C).

2.3 Film Characterization. X-ray diffraction (XRD) measurements were carried out on a Bruker D8 ADVANCE X-ray diffractometer at a wavelength of λ = 1.54 Å generated from CuKα radiation. Bruker DektakXT stylus profiler was used to detect the film thickness. A Zeiss Gemini Ultra Plus field emission scanning electron microscope (SEM) was used at an electron acceleration voltage of 3 kV. GISAXS measurements were performed at the P03 MINAXS beamline36 of the PETRA III storage ring at DESY (Hamburg, Germany) with a wavelength of λ = 0.954 Å and a sample-detector distance of 3.10 m. The incident angle was αi = 0.4° and the detector used was a Pilatus 1 M detector (Dectris Ltd.; pixel size of 172 µm × 172 µm) to record


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the scattered signal. Absorption spectra were measured with a UV-Vis spectrometer (Cary 60, Agilent Technologies) in a wavelength range from 300 to 800 nm. Photoluminescence (PL) spectra of the hybrid films were measured by a Fluorolog-3 FL3-22 (Horiba Jobin Yvon GmbH) spectrometer.

3. RESULTS AND DISCUSSION Spray coating was used in the present work to deposit ZnO/P3HT/P3HT-b-PEO hybrid films on the silicon (100) and glass substrates directly from the initial solution. The solvent evaporation from the droplets during the whole spray process is considered as an important factor for the determination of the spray parameters. A more detailed description and the used spray parameters are given in the supporting information. In the bulk heterojunction approach of the active layer of a hybrid solar cell, the conjugated polymer (P3HT) block serves as light absorber to generate excitons. These quasiparticles eventually split at the P3HT/ZnO interface, and subsequently the positive charge carriers are transported through the P3HT and the negative charge carriers through the sol-gel derived ZnO nanoparticle network to their respective electrodes. In organic solar cells for example P3HT and the fullerene derivate PCBM, it is suitable to mix both components directly together. However, this approach does not work for hybrid solar cells containing e.g. P3HT and ZnO, due to their chemical incompatibility. When simply blending P3HT and ZnO nanoparticles, large domains of P3HT and ZnO are formed instead of the required small interconnected structures. By introducing the block copolymer P3HT-b-PEO, the interfacial tension and the phase separation are reduced, due to the introduction of the covalently bonded block copolymer and interaction of the oxygen atoms of the PEO chains and the ZnO precursor.30 Therefore, the connection and the interface between


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P3HT and ZnO is improved, which is assumed to enhance the separation of the initially generated excitons (in the P3HT phase). The crystalline structures of P3HT and ZnO in the hybrid films are investigated by XRD measurement as shown in Figure S2. The prominent peak in Figure S2a is attributed to the (100) plan of P3HT, corresponding to the chain-chain interlayer distance. The three characteristic peaks in Figure S2b indicate the wurtzite phase of ZnO prepared from sol-gel method, which is considered with good charge transport properties.37 To explore the morphology of the hybrid films, SEM and GISAXS are mainly used to investigate the characteristic length scales of the sprayed films.

3.1 Film Morphology Influenced by the Amount of the Diblock Copolymer. SEM images of ZnO/P3HT/P3HT-b-PEO hybrid films with different ratios of the diblock copolymer are shown in Figure 2. They give a detailed view of the surface morphology on the local scale. Because of the high electron density of ZnO, the SEM images show large contrast between polymers and the ZnO phase. All SEM images illustrate the interconnected ZnO phase build-up from ZnO nanoparticles and in turn the interconnected P3HT phase as well. These network-like structures are expected to facilitate the charge carrier transport through the n-type ZnO and p-type polymer phase to their corresponding electrodes. Compared to the simple ZnO/P3HT blend film (Figure 2a), the hybrid films with added diblock copolymer (Figure 2b-d) exhibit a more homogeneous distribution of the ZnO at the surface. Instead of a random distribution of the ZnO nanoparticles they are enriched in the PEO domains of the diblock copolymer. Due to the microphase separation and the interfacial compatibilizer effect of the diblock copolymer, the ZnO nanoparticles in the ZnO/P3HT/P3HT-b-PEO hybrid films become


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much smaller and show an increased monodispersity of the domains compared to those in the ZnO/P3HT film. With increasing the ratio of the diblock copolymer from 0.2 to 1, the size of ZnO nanoparticles and the average distance decrease. ZnO nanoparticles form a more homogeneous network, which is an indication for a successful templating of the ZnO nanostructures via the diblock copolymer. The smaller sized ZnO nanoparticle network provides larger interfaces with P3HT, which is beneficial for the charge carrier separation. In addition, this is expected to enhance the electron injection process.38 To investigate the morphology on a larger scale, SEM images with different magnifications for the hybrid films with the ratio of 2:1:1 are shown in Figure S3. It still shows very homogeneous morphology at lower magnification (Figure S3a). Despite that coffee rings are observed on the micrometer length scale in Figure S3b, which is mainly due to the complicated flow behaviors during the spray deposition process,39-41 the homogeneous interconnected network structures on the nanometer length scale benefit for the improvement of the interfaces between ZnO and P3HT.

Figure 2. SEM images of the ZnO/P3HT/P3HT-b-PEO hybrid films deposited by spray coating and annealing at 80 °C. Ratios of ωZnO:ωP3HT:ωP3HT-b-PEO are (a) 2:1:0, (b) 2:1:0.2, (c) 2:1:0.5, and (d) 2:1:1. ZnO appears bright and polymer phase dark.


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For the bulk heterojunction layers, excitons can only separate into free charge carriers at the interfaces between P3HT (light absorber) and ZnO (electron acceptor). Moreover, to decrease the probability of exciton recombination, the charge carriers are required to be transported through the p-type polymer and n-type semiconductor. Thus, their domain sizes are also of great significance for hybrid solar cells. Although, the SEM measurements clearly indicate a structural evolution of the film surface, these structures might not extend into the inner film volume.42-44 In addition, a quantitative analysis of the length scales is important in order to develop an accurate routine to tune the structures for solar cell application.45, 46 Accordingly, GISAXS measurements are performed on the samples with different diblock copolymer ratios. As seen from the 2D GISAXS data in Figure 3, the specular peak, which appears at the exit angle equal to the incident angle of the X-ray beam, is covered by a beamstop to prevent oversaturation or damage of the detector. Furthermore, this highlights the scattered signal in the Yoneda peak region.47-50 Therefore, the maximum scattering intensity for all of the samples is originated in the Yoneda peak, which depends on the critical angle of the material.51 When the incident angle of the X-ray beam is larger than the critical angle of the material, the beam is able to penetrate the whole film, and therefore the structures buried inside the film can be detected.52, 53 Apart from the scattering signal in vertical direction (along qz), a prominent lateral scattering (along qy) is also observed for all samples. The lateral scattering of the samples with different diblock copolymer ratios appears significantly different, which is an indication for differences in the characteristic structures inside the films. The scattering signal becomes much wider when increasing the ratio of the diblock copolymer, which indicates smaller domain sizes existing in these films.


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Figure 3. 2D GISAXS data of the ZnO/P3HT/P3HT-b-PEO hybrid films deposited by spray coating and annealing at 80 °C. Ratios of ωZnO:ωP3HT:ωP3HT-b-PEO = (a) 2:1:0, (b) 2:1:0.2, (c) 2:1:0.5, and (d) 2:1:1 are shown. The specular peak is covered by a beamstop. All the images have the same intensity scale as shown in the scale bar.

Vertical cuts are performed at qy=0 nm-1. The cuts are plotted from bottom to top with increasing ratio of the diblock copolymer in Figure 4a. The position of the exit angle αf = 0° indicates the sample horizon, which is along the sample plane. The grey box indicates the gap of the detector below the sample horizon. The drop in intensity at around 0.4° in all curves is caused by the shielding of the specular peak with a beamstop as described above. Between the sample horizon and the shielded specular beam region, the intensity is dominated by the Yoneda peak. The peaks located at αf = 0.133°~0.138° shaded blue in all vertical cuts come from the SiOx/Si substrate, which is consistent with the theoretical value of its critical angle.15 The theoretical critical angle of P3HT is about 0.098°,53 which is shaded red in Figure 4a. A peak contributed from the critical angle of P3HT is hardly visible, mainly due to the dominating scattering of the substrate.


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In order to get detailed information about the lateral structures, horizontal line cuts are performed at the critical angle of P3HT (Figure 4b). Scattering features are observed in the horizontal line cuts which correspond to the size of the P3HT domains and the distances between them. It can be seen that dominant shoulders are present at high qy values, which is related to small P3HT domain sizes in the hybrid films. Moreover, with increasing the diblock copolymer ratio, the shoulder located at high qy values shifts towards larger qy direction, demonstrating the decreasing P3HT domain sizes.

Figure 4. (a) Vertical line cuts at qy = 0 nm-1 and (b) horizontal line cuts of the 2D GISAXS data (symbols) along with the fits (black curves) for the ZnO/P3HT/P3HT-b-PEO hybrid films deposited by spray coating and annealing at 80 °C with ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:0, 2:1:0.2, 2:1:0.5 and 2:1:1 from bottom to top. In (a) the grey shaded region is the detector gap and the red and blue shaded regions are the critical angle regions of P3HT and SiOx/Si, respectively. The intensity decrease at about 0.4° in (a) is due to the shielding of the specular beamstop.

Quantitative information is obtained from fitting the horizontal line cuts with a model assuming a one-dimensional paracrystal type of order of three types of cylindrically shaped objects in the framework of the distorted-wave Born approximation and local monodisperse approximation


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(LMA).54 The cylindrical objects represent the P3HT domains in the films. Thus, two kinds of characteristic length scales can be extracted from the fits, namely the diameter of the cylinders which represents the domain size of P3HT in the hybrid film and the distance between the cylinders, which gives the length scale of the ZnO/PEO domains. The obtained P3HT domain sizes and ZnO/PEO domain sizes are plotted as a function of the amount of the diblock copolymer in the films in Figures 5a and 5b. With increasing the amount of the diblock copolymer the P3HT domain sizes decrease on all three characteristic lengths scales, whereas the ZnO/PEO domain sizes change in a non-monotonous way. The small-sized ZnO/PEO domains almost remain constant in spite of the increasing template ratio. While for the middle- and largesized ZnO/PEO domains, their average sizes first decrease and then increase. When the template ratio is 0.2, the decreased ZnO/PEO domain size indicates the improvement of the interfaces with P3HT after introducing the diblock copolymer as a compatibilizer. However, upon increasing the template ratio further, the ZnO/PEO domains become slightly larger, which might be due to the increased amount of PEO block in the hybrid films. Anyhow, films with added diblock copolymer have smaller domains compared with those in ZnO/P3HT films, due to the interface compatibilization by the diblock copolymer. The microphase separation of the diblock copolymer causes the more complex trend. The P3HT domains in the film with a weight ratio of ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:1 exhibit the smallest sizes (large domain size: 82.4 nm, middle domain size: 24.0 nm, small domain size: 3.6 nm). As the exciton diffusion length for P3HT is about 10 nm,55, 56 this sample is expected to show the best exciton separation performance.


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Figure 5. (a) P3HT domain size and (b) ZnO/PEO domain size in the ZnO/P3HT/P3HT-b-PEO hybrid films as a function of P3HT-b-PEO weight ratio. Triangles (blue) indicate small-sized structures, circles (red) middle-sized structures and squares (black) large-sized structures.

3.2 Film Morphology Influenced by the Annealing Temperature. Apart from the influence of the amount of added diblock copolymer, the temperature also plays an important role in the structure formation of the hybrid film. The SEM images in Figure 6 show the surface morphology of the hybrid films with a composition of ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:1 processed at different annealing temperatures of 80 °C, 120 °C and 160 °C. With increasing annealing temperature, the network-like appearance of the nanostructure is preserved and characteristic length scales coarsen. It seems like parts of the ZnO particles aggregate with each other and form larger domains. Very obviously, this cannot be caused by an increasing compatibility of the two blocks in the diblock copolymer when the annealing temperature is raised because this would favor smaller domains.57-59 In contrast, it is a kinetic process which drives the system towards an equilibrium structure via thermal annealing and with different annealing temperatures. After fixed times of annealing different states in the underlying kinetics are reached.


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Figure 6. SEM images of ZnO/P3HT/P3HT-b-PEO hybrid films with ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:1 annealed at (a) 80 °C, (b) 120 °C, and (c) 160 °C for 0.5 hours.

GISAXS measurements are performed to probe the inner film morphology after annealing at different temperatures. 2D GISAXS data of the sprayed films with a ratio of ωZnO:ωP3HT:ωP3HT-bPEO

= 2:1:1 are shown in Figure S4 for the different tested annealing temperatures. Again, a more

detailed quantitative information is obtained from horizontal line cuts along the Yoneda peak position of P3HT. The cuts are depicted in Figure 7a for hybrid films annealed at 80 °C, 120 °C and 160 °C from bottom to top. From the line cuts it is observed that the most prominent shoulders are located at a high qy value of about 0.6 nm-1. The position of the shoulders remains almost unchanged, indicating a constant P3HT domain size for the small-sized structures. In contrast, another weaker feature in the scattering curves, which are positioned at a low qy value of about 0.03 nm-1, move to the lower qy direction, which indicates larger P3HT domains coarsen in the films with increasing annealing temperature. For a more detailed analysis the same model consisting of three form and structure factors is used to fit the line cuts and to determine the characteristic domain sizes (Figure 7b and 7c). Large- and middle-sized P3HT domains increase with increasing the annealing temperature whereas the small-sized P3HT structure remains almost unchanged. Due to the exciton diffusion length of P3HT being small, the sample annealed


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at 80 °C is expected to have the best exciton separation as compared to the samples annealed at higher temperatures. In general, the ZnO/PEO domains show an increase in domain size as well when raising the annealing temperature. The observed increase in sizes of both P3HT domains and ZnO/PEO domains agrees with the changes in the surface morphology probed with SEM, which means that the structures inside the hybrid films follow similar kinetics as the surface structures.

Figure 7. (a) Horizontal line cuts of the 2D GISAXS data (symbols) shown with their corresponding fits (black curves) for the ZnO/P3HT/P3HT-b-PEO hybrid films deposited by spray coating with ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:1 after annealing at different temperatures: 80 °C, 120 °C, and 160 °C from bottom to top. (b) P3HT domain size and (c) ZnO/PEO domain size in the ZnO/P3HT/P3HT-b-PEO hybrid films as a function of increasing annealing temperature. Triangles (blue) indicate small-sized structures, circles (red) middle-sized structures and squares (black) large-sized structures.

3.3 Optoelectronic Properties.


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UV-Vis absorption measurements and photoluminescence (PL) measurements are carried out. These optoelectronic measurements are useful to get information about the aggregation of the P3HT polymer in the film, the exciton separation, and the charge transfer between the p-type polymer and the n-type semiconductor. The UV-Vis spectra of the hybrid films are shown in Figure 8a for the variation of the diblock copolymer content. The influence of different annealing temperatures on the absorption spectra of the samples with a ratio of ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:1 are shown in Figure 8b. For all films, an absorption at around 330 nm is observed, with a value similar to the reported work, which suggests a successful synthesis of ZnO nanoparticles.31, 60, 61

The increased intensity of the peak observed for the hybrid films with the ratio of

ωZnO:ωP3HT:ωP3HT-b-PEO=2:1:1 might be due to the decreasing size of the formed ZnO nanoparticles and the improved monodispersity of the ZnO nanoparticles.62, 63 The other three absorption peaks present at approximately 518, 550 and 600 nm originate from the P3HT polymer. The maximum absorption at 518 nm and the two distinct shoulders at 550 nm and 600 nm are assigned to 0-2, 0-1 and 0-0 transitions in P3HT.64, 65 The absorption shoulder related to 600 nm can be assigned to the intermolecular π-π stacking of P3HT. No shift in this peak position is observed, which indicates P3HT crystallizes in all of the diblock copolymers in a similar way as the P3HT homopolymers. As seen in the UV-Vis data in Figure 8 the absorption of ZnO nanoparticles is mainly located in the UV region, which will not hinder the light absorption of P3HT, as P3HT mainly absorb visible light. Therefore, the ZnO nanoparticles serves as additional UV filter to prevent degradation of the P3HT component by the harmful UV light. To record the PL spectra the hybrid films are excited at 460 nm, while the emission spectra are recorded in the range of 550–850 nm (Figure 8c). Compared to the film without the diblock


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copolymer, all PL spectra of ZnO/P3HT/P3HT-b-PEO hybrid films exhibit a better fluorescence quenching. This is mainly related to the smaller P3HT domain sizes and better interconnection of ZnO nanoparticles and P3HT due to the self-assembly and interfacial compatibilizer of the diblock copolymer. When increasing the ratio of the diblock copolymer, the PL intensity quenches stronger, indicating higher charge separation yield and a more efficient charge transfer process in the ZnO/P3HT/P3HT-b-PEO hybrid films. The influence of different annealing temperatures on the PL spectra (samples with ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:1) is seen in Figure 8d. With increasing annealing temperature the PL intensity increases, which indicates that the sample annealed at 80 °C possesses a better heterojunction than the other two samples annealed at higher temperatures.66 Apart from this, the PL signal of ZnO particles for the film with ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:1 annealed at 80 °C is also investigated with an excitation wavelength of 325 nm (Figure S5). An intensive peak at about 386 nm is caused by the radiative recombination of electrons which were excited into the conduction band with the holes left in the valence band.67 No clear peak is observed in the range of 500-700 nm, indicating the defects such as oxygen vacancies and zinc interstitials are very low.31, 68, 69


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Figure 8. (a,b) UV-Vis and (c,d) PL spectra of ZnO/P3HT/P3HT-b-PEO hybrid films (a,c) annealed at 80 °C with different compositions ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:0, 2:1:0.2, 2:1:0.5, and 2:1:1 and (b,d) for fixed composition ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:1 under different annealing temperatures: 80 °C, 120 °C, and 160 °C.

4. CONCLUSION In this work, hybrid films of ZnO/P3HT/P3HT-b-PEO are successfully fabricated via spray deposition, which is a promising approach for large-scale industrial applications. The surface and inner morphologies are investigated by SEM and GISAXS, respectively, to probe the influence of the diblock copolymer P3HT-b-PEO. With addition of the diblock copolymer, the interconnected network structure is sustained and both, the P3HT and ZnO domains become smaller. This is related to the preferential attachment of ZnO particles to the PEO chains and the self-assembly of the diblock copolymer which hinders the chemically incompatible two components from forming large integrated domains. The diblock copolymer acts as interfacial compatibilizer, and therefore improves the interfaces of ZnO particles and P3HT. When increasing the amount of the diblock copolymer, the P3HT domain sizes decrease while the ZnO/PEO domain sizes partly increase. These changes are beneficial for the exciton separation and the charge carrier transport along the ZnO and P3HT domains, which is supported by the PL measurements. However, aggregation of both, P3HT and ZnO particles is observed with increasing annealing temperature, which originates from kinetic changes of the film morphology. In summary, the most promising morphology for hybrid solar cell application and the optimal PL quenching is found at a ratio of ωZnO:ωP3HT:ωP3HT-b-PEO = 2:1:1 when annealing at 80 °C.


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With the approach to combine a diblock copolymer with the active layer material of a hybrid solar cell, a direct fabrication of the active layers is realized. Based on such fabrication it will be possible to successfully produce a nanostructured bulk heterojunction hybrid solar cell on an easy fabrication route. Moreover, the subsequent spray and annealing steps all at temperatures well below 160 °C provide the necessary prerequisites for a potential large scale preparation of hybrid solar cells on flexible substrates.

Supporting Information Additional experimental results including the details of spray deposition; film thickness of the hybrid films with different component ratios; GISAXS of the hybrid films annealed at variable temperatures as well as XRD, SEM images and PL of the hybrid films. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +49 (0)89 289 12473. Tel: +49 (0)89 289 12451. Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by funding from TUM.solar in the context of the Bavarian Collaborative Research Project Solar Technologies Go Hybrid (SolTech), the Excellence Cluster Nanosystems Initiative Munich (NIM), the Center for NanoScience (CeNS) and the International


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Research Training Group 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS). Kun Wang acknowledge the China Scholarship Council (CSC). The authors thank Yu Tong from LudwigMaximilians-Universität München for helping with the SEM, UV-Vis and PL measurements. Parts of this research were carried out at the synchrotron light source PETRA III at DESY. DESY is a member of the Helmholtz Association (HGF).

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