Characterization of the mesoscopic structure in the photoactive layer of organic solar cells: A focused review

Materials Letters 90 (2013) 97–102

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Characterization of the mesoscopic structure in the photoactive layer of organic solar cells: A focused review Kiarash Vakhshouri a, Sameer Vajjala Kesava a, Derek R. Kozub a, Enrique D. Gomez a,b,n a b

Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, United States Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, United States

a r t i c l e i n f o

abstract

Article history: Received 19 January 2012 Accepted 20 August 2012 Available online 11 September 2012

Organic photovoltaics (OPVs) belong to a class of devices where the nanometer scale morphology of the active layer has a large impact on device performance. However, characterization of the morphology of organic semiconductor mixtures that make up the active layer of OPVs remains a challenge. Here, the characterization methods that can be used to quantitatively and qualitatively measure the mesoscopic structure of the active layer in organic solar cells are described. Specifically, we focus on the use of X-ray and neutron scattering, scanning probe microscopy, and electron and X-ray microscopy for morphological characterization of organic semiconductor mixtures at mesoscopic length scales. & 2012 Elsevier B.V. All rights reserved.

Keywords: Organic photovoltaics Organic semiconductors Morphology Electron microscopy X-ray scattering Interfaces

1. Introduction Blends of solution-processable organic molecules and conjugated polymers are of interest due to their potential use in low-cost, lightweight, scalable and flexible optoelectronic and photovoltaic devices. In order to build high performance organic photovoltaic (OPV) devices several requirements must be fulfilled: efficient light absorption, maximum separation of electron–hole pairs (excitons), transport of charge carriers to the electrodes, and efficient charge collection at the anode and cathode [1]. In OPVs, exciton dissociation is achieved by creating donor–acceptor interfaces where charge separation is energetically favorable. A promising family of OPV devices is composed of thin-film solution-processed solar cells which rely on self-assembly to create the morphology necessary to meet the aforementioned requirements [1]. These devices are often called bulk heterojunction solar cells, due to the large amount of interfaces created throughout the photoactive layer by mixing two semiconductors. However, the limited exciton diffusion length of organic semiconductors near 5–10 nm [2] places a limit to the domain size of donor–acceptor mixtures. Thus, the photoactive layer must not only possess bicontinuous morphologies capable of extracting electrons and holes but also nanometer-sized domains to promote efficient device performance.

n Corresponding author at: Department of Chemical Engineering, Pennsylvania State University, 106 Fenske Lab, University Park, PA 16802, United States. Tel.: þ 1 814 689 9394. E-mail address: [email protected] (E.D. Gomez).

0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.146

Many recent efforts have strived to understand the effect of nanoscale phase separation of the photoactive layer on the performance of OPVs. The goal of these studies is to move the field beyond empirical trial-and-error optimization by developing a basic understanding of the material parameters which govern structure, structural evolution and device performance. An important component of these efforts is structural characterization of the photoactive layer, which has received significant attention. Although both scattering and microscopy techniques have been utilized to gain insights on the morphology of donor–acceptor mixtures, characterization remains a challenge. The semicrystalline nature of the electron donor and acceptor and the apparent lack of strong mesoscopic order confound imaging results. As a result, ambiguity over analysis of scattering data exists; there is little consensus in the literature on appropriate models for scattering data from organic semiconductor mixtures. Thus, the challenge of characterizing the active layer morphology has hampered studies focused on the role of processing on the active layer structure and, in-turn, device performance. Here, we discuss efforts to characterize the mesoscopic structure (5–500 nm) of the photoactive layer of organic photovoltaics. Although molecular-level order may be critical for charge transport and charge separation, the mesoscopic level structure will strongly influence device performance by dictating domain sizes, the amount of interfacial area and the presence of continuous pathways. Consequently, herein we include efforts to characterize the mesoscopic order in thin films of organic semiconductor mixtures through X-ray and neutron small-angle scattering techniques and scanning probe, X-ray and electron

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microscopy. Characterization of molecular-level structure of organic semiconductors and the effect of molecular-level structure on OPV performance is beyond the scope of this perspective and the reader is referred to excellent recent reviews [3–6]. Furthermore, we focus on the mesoscopic structure along the plane of the film and exclude discussion of various techniques such as neutron reflectivity, variable-angle spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) which have been used to measure composition variations along the thickness of the photoactive layer of OPVs [7–9].

2. Scanning probe microscopy Depending on the type of probe used, scanning probe microscopy (SPM) techniques provide the means for measuring the surface morphology, surface potential, conductivity, chemical composition, capacitance, thermal properties, and optical properties of polymer films with nanometer resolution. Surface or Atomic Force Microscopy (AFM) is one of the most common scanning probe methods in which a sharp tip is scanned on the surface, providing a measure of sample-tip interactions on nanometer length scales. In some cases, these interactions can then be correlated to the topology, elastic modulus and chemistry of the sample. AFM has been widely successful in providing useful information about the crystal morphology in organic semiconductors, where the contrast is mostly due to differences in topology and in the elastic modulus between amorphous and crystalline phases. Thus, initial efforts to characterize the structure of organic semiconductor mixtures relied on AFM. Films based on poly(pphenylene vinylene)/phenyl-C61-butyric acid methyl ester (PCBM) and polymer donor/polymer acceptors revealed a coarse phase separation through AFM, on the order of hundreds of nanometers [10]. Given the large features found in these blends, AFM captured much of the structure despite being limited to probing the surface, where the contrast is once again most likely from topology and material stiffness differences. AFM, however, is not suitable for all systems. Mixtures based on regioregular poly(3-hexylthiophene) (P3HT)/PCBM, for example, have finer morphologies and consequently do not exhibit the same structure within the film as the surface of the film; this is in part due to P3HT selectively wetting the top surface of P3HT/PCBM films [11–13]. One SPM technique which is capable of examining the properties of films beyond the surface is conducting atomic force microscopy (c-AFM). c-AFM measures the local conductivity by passing a current through the tip and the sample. For example, Rice et al. studied the local electrical properties of P3HT nanowire/PCBM blends using c-AFM [14]. Fig. 1 shows the topography and correlated dark currents at a positive and negative tip bias. Interestingly, the images are markedly different depending on the tip bias; the authors suggest that positive tip bias is a result of hole conduction while a negative tip bias probes electron conduction [14,15]. Regardless of the physics behind the image contrast, the heterogeneity in the electrical properties of P3HTXPCBM films is markedly clear. The image in Fig. 1C shows the presence of ‘‘hot spots’’ or regions of significantly higher conduction. We note that image interpretation of maps of electron conduction (under a negative tip bias, Fig. 1C) is not trivial due to the preferential wetting of P3HT at the air interface [16]. Thus, the small regions of high conduction visible in Fig. 1C could be due to regions of exposed PCBM; NEXAFS studies have determined that about 3% of PCBM exists at the air interface [12]. An interesting extension to c-AFM can be achieved by illuminating the sample with a diffraction-limited laser beam from the bottom while simultaneously measuring the electrical properties

Fig. 1. Topography (A) and conductive AFM images (B image is current with þ 3 V applied to the tip; C image is current with  3 V applied to the tip) of P3HT nanowires mixed with PCBM. Reprinted with permission from Ref. [14]. Copyright 2011 American Chemical Society.

of the film. With this technique, called photoconducting AFM (pc-AFM), the photocurrent distribution in organic photovoltaic blends can be mapped with  20 nm resolution [15,17]. Furthermore, by modulating the wavelength of the light it is possible to fill shallow traps without exciting electrons from the highest occupied molecular orbital and obtain a local measurement of trap energy and density through pc-AFM. We note two concerns to keep in mind in both c-AFM and pc-AFM. First, high fields applied to organic molecules can oxidize the material in air, and as such, c-AFM and pc-AFM experiments should be carried in vacuum or inert atmospheres. Second, electrical currents will follow the path of least resistance, which is not always straight down through the film. As such, the map of electrical conductance is not necessarily correlated with the in-plane morphology of the film in a trivial manner.

3. X-ray and electron microscopy Scanning transmission X-ray microscopy (STXM) is a relatively novel technique capable of generating contrast in organic films due to mass, elemental or chemical differences with a resolution of 20–50 nm [10,18]. STXM utilizes a variable-energy focused X-ray beam which is scanned across a sample to measure local variations in the X-ray absorption. Thus, by selecting the

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appropriate energies, chemical bonding differences can be exploited. As shown in Fig. 2, McNeill et al. have imaged blends of poly(9,9-dioctylfluorene-co-bis-N,N0 -(4-butylphenyl)-bis-N,N0 phenyl-1,4-phenylene-diamine) (PFB) and poly(9,90 -dioctylfluoreneco-benzothiadiazole) (F8BT) using STXM [10]. As-cast films show structures with domains exhibiting an average size of  85 nm and little difference in composition (within 10 wt%). After thermal annealing of the films, the STXM images in Fig. 2d and e exhibit larger domains and higher contrast. This demonstrates that after thermal annealing the PFB/F8BT blends phase separate further, leading to coarsening of domains and an enhancement in domain purity. Although STXM is clearly a powerful technique for imaging soft materials, it may not be well-suited for highly efficient OPVs. Ideal domain sizes for the photoactive layer of OPVs are likely close to 10 nm [19], and as such, the morphology of high-performance organic semiconductor mixtures are currently beyond the resolution limit of STXM. Transmission electron microscopy (TEM) is another versatile technique for imaging the structure of soft materials. Soft matter electron microscopy, however, differs in subtle but important ways from the microscopy of hard materials. For example, the extension of the Rose equation by Glaeser describes how the resolution is limited by the maximum acceptable radiation dose and contrast in soft matter microscopy [20]: S=N dC Z pffiffiffiffiffiffi f jt

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apparent disagreement. Electron tomography reconstructions require 50–100 images and are highly radiation-intensive; therefore, damage may have degraded the fibers near the top surface. Also, assuming that the presence of fibers denotes the composition ignores amorphous P3HT. Consequently, the disagreement between electron tomography and NEXAFS/XPS results exemplifies one of the challenges of TEM experiments on the active layer of OPVs: the origin of contrast is not entirely clear, especially in poorly-ordered systems. An extension to conventional TEM is achieved by taking advantage of inelastic losses due to electron–sample interactions in the electron microscope. This technique, termed energy-filtered transmission electron microscopy (EFTEM), can map the local elemental composition by noting the presence of specific excitations at different energies. Plasmon excitations (near 20 eV) are broad, often overlap and vary little between different compounds; nevertheless, a principal component analysis of the plasmon peaks can be used to image the fibrillar structure of P3HT/PCBM mixtures [23–25]. In addition, Kozub et al. obtained high contrast images of P3HT fibers in P3HT/PCBM mixtures by mapping differences in core-loss excitations (100–300 eV) given the large differences in sulfur and carbon densities (Fig. 3) [26]. As expected, thermal annealing promotes the formation of P3HT fibers which

ð1Þ

Here d is the size of a feature to be resolved, C is the image contrast, f is the fraction of electrons which contribute to the image, j is the current density through the object, t is the exposure time and S/N is the minimum acceptable signal-tonoise ratio. Eq. (1) exemplifies the challenge of soft matter electron microscopy—the low inherent contrast and low tolerance of organic molecules to radiation limits the minimum value of d that can be imaged. One approach to enhance the contrast in a TEM is to take advantage of phase interference in the objective lens by defocusing the microscope [21]. Unfortunately, the objective lens transfer function oscillates at length scales smaller than a critical length scale set by the instrumentation, thereby creating intensity fluctuations which are artifacts. Defocusing increases this length scale, and as such, the structure of poorly-ordered materials can be challenging to distinguish from microscope artifacts. Although contrast between P3HT and PCBM domains is low, there have been some reports of in-focus bright field TEM images of highly ordered systems taken at low magnification [22]. Despite the low contrast, the authors of these studies were able to reconstruct the three-dimensional structure from a series of images taken at different tilt angles. Analysis of the 3D morphologies suggested that since less P3HT fibers were found at the top interface, the top surface was PCBM-rich; this is in direct contradiction with multiple NEXAFS and XPS experiments [11–13]. We surmise that two difficulties may be responsible for the

Fig. 3. Sulfur elemental maps of 1:1 by mass P3HT/PCBM mixtures annealed at various temperatures generated through EFTEM. The scale bar is 200 nm. Reprinted with permission from Ref. [26]. Copyright 2011 American Chemical Society.

Fig. 2. STXM images of PFB/F8BT blends with different annealing conditions: (a) as-spun film and annealed at (b) 140 1C, (c) 160 1C, (d) 180 1C and (e) 200 1C. Reprinted with permission from Ref. [10]. Copyright IOP Publishing, Inc. 2008.

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drives the structure formation process in polythiophene/fullerene mixtures. Since core-losses are used to generate the elemental maps, the intensities of the images are directly proportional to the sulfur and P3HT concentrations within domains. Analysis of the image intensities reveals that the fibers are essentially pure P3HT, while the matrix is composed of PCBM-rich regions which are 10%–45% P3HT by volume. Consequently, Fig. 3 suggests partial miscibility, which was further confirmed by analysis of the Flory–Huggins interaction parameter for P3HTXPCBM mixtures [26,27]. Given that the differences in structure can be subtle at times, Fourier transforms of images are useful to quantify contrast and discriminate differences by length scales. For example, since regiorandom P3HT/PCBM mixtures are amorphous for some processing conditions, the lack of crystallinity enables studies of the miscibility between amorphous phases through the evolution of structure. Unfortunately, the differences in sulfur maps of amorphous polythiophene/fullerene mixtures at the early stages of phase separation, prior to crystallization, are subtle. Nevertheless, the radially integrated intensities of Fourier transforms of sulfur maps show an enhancement in intensity at periodicities above 40 nm for compositions which are fullerene rich [27]. The rise in intensity suggests that phase separation occurs at high fullerene content, consistent with estimates of the phase diagram between amorphous P3HT and PCBM obtained from Flory– Huggins theory and the interaction parameter [26]. As demonstrated by the aforementioned examples, EFTEM is a powerful technique which can yield quantitative information on the composition of the domains with nanometer resolution and qualitative information about the active layer morphology of OPVs. Nevertheless, it is important to keep in mind that 109–1011 images are required to sample a volume of 1 mm3 in an electron microscope. Furthermore, electron micrographs are two dimensional projections of three dimensional objects, sometimes making images challenging to interpret. For example, the size of spheres in a film containing a large concentration of objects with random packing is not trivial to determine from TEM images if the thickness of the sample is significantly larger than the diameter of the spheres [28,29]. The next section is focused on X-ray and neutron scattering, which complement direct imaging techniques by obtaining quantitative data on structure and morphology.

4. X-ray and neutron scattering Reciprocal space methods for examining the structure rely on interference effects to generate patterns which can be correlated to structure after further analysis. The weak interactions of X-rays and neutrons (when compared to electrons) require samples on the order of 1 mm of thickness; consequently, the sampling volume for an X-ray or neutron scattering experiment is significantly larger than in TEM. For OPVs, the interest lies in the study of thin films; consequently, traditional transmission X-ray scattering experiments are not typically appropriate for the study of the active layer of organic solar cells. In this review, our discussion of X-ray scattering will focus on grazing-incidence small angle X-ray scattering (GISAXS) and resonant soft X-ray scattering (R-SoXS), two techniques capable of studying the morphology of thin films. In GISAXS, scattering is acquired in the reflection geometry with incident angles near the critical angle of the film to minimize the contribution of the substrate to the scattering intensities [30]. With this technique, a quantitative measure of the mesostructure in organic semiconductor mixtures can be obtained. For example, Chen et al. utilized GISAXS to study the morphological evolution of P3HTXPCBM blends as a function of annealing time. They found that the in-plane scattering from P3HT/PCBM films

increases quickly with annealing time when annealing takes place at 150 1C. However, the scattering intensities saturate quickly, suggesting that a steady state in the morphology is achieved even at elevated temperatures [31]. Further efforts by Kozub et al. determined that although little structure is apparent after film casting due to limited crystallization of P3HT [32], the annealing temperature determines whether coarsening occurs as a function of time [26]. Surprisingly, lower temperatures (below 150 1C) lead to coarsening while higher temperatures achieve a steady state within a few minutes. This phenomenon remains unexplained. Although features in scattering data suggest important length scales of the morphology, analysis of X-ray data require a model. Unfortunately, the transformation from scattering data in reciprocal space to real space is not one-to-one, since the phase information of the exit wave is lost when the data is collected as intensities. Thus, multiple models can, in principle, yield the same intensity profiles. The various models utilized for scattering data from organic semiconductor mixtures have been summarized in a recent review [33]. One concern is whether X-ray data is dominated by the form factor, i.e. shape of the domains, or the structure factor, i.e. correlations between domains. Using the high contrast elemental maps shown in Fig. 2 to select an appropriate model for the X-ray data, Kozub et al. used the Teubner–Strey scattering function, originally developed for oil/ water/microemulsions and later extended to polymer blends, to quantify the morphological evolution in terms of a domain spacing [26]. Teubner-Strey assumes point-scattering objects and thus only includes the structure factor. Here, we extend the work in Ref. [26] to study the effect of composition of P3HT/PCBM blends on domain spacing (spacing between P3HT fibers). Our results shown in Fig. 4a show that the peak positions of the scattering curves shift toward higher q with increasing P3HT fractions, which imply a decrease in separation distances between P3HT fibers. Fig. 4b shows that although the P3HT fiber diameter (estimated from EFTEM) remains unchanged for various P3HT contents, the domain spacing (calculated from Teubner–Strey fits to GISAXS data) decreases with increasing P3HT volume fraction. Thus, scattering from P3HT/PCBM films at conditions near optimum for OPVs is dominated from the structure factor of the morphology and domain–domain correlations. In addition to GISAXS, resonant soft X-ray scattering has been used recently for the morphological characterization of the photoactive layer in OPVs [10,34]. R-SoXS in a transmission geometry can provide direct information on the nanostructure of bulk heterojunction films at high spatial resolution. Soft X-rays have lower photon energies when compared to conventional hard Xrays and can match the core energy level of various atoms. Therefore, R-SoXS can strongly enhance scattering contrast and yield information about domain purity due to differences in the optical constants [34]. Swaraj et al. studied the morphology of a polymer/polymer system (PFB/F8BT) with R-SoXS, and showed that films cast from chloroform exhibit a hierarchy of length scales with impure domains. These domains become purer at the smallest length scales with annealing. Their study demonstrated that polymer/polymer blends contain a broad distribution of domain sizes with an average size significantly larger than the exciton diffusion length. This implies that domain sizes larger than the exciton diffusion length might be responsible for poor performance of all-polymer PFB/F8BT devices [34]. Small angle neutron scattering (SANS) has also been used to characterize the morphology of the photoactive layer in organic solar cells [35–37]. The scattering length densities of P3HT and fullerene derivatives are significantly different, providing large neutron contrast for conjugated polymer-fullerene blends without the need for deuteration. Similar to X-rays, the sample thickness requirements are near 1 mm; however, unlike X-rays,

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2

P3HT = 0.67

P3HT = 0.48

Intensity (a. u.)

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P3HT = 0.40

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0.5

0 0.1

0.01 qy (Å-1) 45 Domain spacing Fiber diameter

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20

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morphological and electrical properties in organic semiconductor mixtures utilized as the active layer of organic solar cells remains incomplete. As a consequence, the design of novel materials for OPVs is an unresolved challenge. Progress towards this aim will require proper characterization of the morphology to obtain basic knowledge of the factors which govern structural evolution. The combination of SPM, X-ray and electron microscopy, and scattering experiments will continue to be a critical part of the development of high performance OPVs. An example of how advances in mesostructure characterization can impact the development and optimization of OPVs can be found in Ref. [40]. Kozub et al. examined the photoactive layer of poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT)/phenyl-C71-butyric acid methyl ester (PC71BM) OPVs using GISAXS. Fitting the scattering data to the Teubner–Strey scattering model, similarly to P3HT/PCBM mixtures [26], enabled the quantification of the mesostructure as a function of processing conditions. As shown in Fig. 5, comparing the structural data to device data demonstrated a clear relationship between the length scale of the morphology and the device short circuit current. Furthermore, a simple model which assumes that only an outer shell of each domain can contribute to the photocurrent quantitatively describes the data, where the exciton diffusion length determines the thickness of this shell. Establishing such a relationship for polythiophene/fullerene mixtures enables optimization of the mesoscopic length scales by means of GISAXS as a diagnostic tool. Thus, the ability to characterize the photoactive layer mesostructure of OPVs leads towards directed or rational approaches for the optimization and implementation of novel high-performance materials. Much of this perspective has focused on characterization of P3HT/PCBM mixtures, as this is the most widely studied OPV material system. Nevertheless, the techniques discussed herein have been and continue to be applied to low band gap polymers which have demonstrated impressive power conversion efficiencies in devices. Noteworthy examples include R-SOXS measurements [41] and TEM imaging [42,43] of thienothiophene benzodithiophene alternating copolymers, where the addition of processing additives such as 1,8-diiodoctane lead to finer morphologies and devices with efficiencies greater than 7%. Whether the materials in the aforementioned studies and other low band gap polymers behave similarly to P3HT when mixed with fullerene remains unclear. As such, morphological studies will continue to be critical in the design and development of OPVs.

Fig. 4. (a) GISAXS intensity vs. scattering vector, q, for P3HT/PCBM mixtures of different mass ratios. (b) Domain spacing from GISAXS and fiber diameters obtained from EFTEM elemental maps for P3HT/PCBM blends of different mass ratios. The error bars denote the standard deviation over multiple measurements. Samples were annealed at 180 1C for 30 min.

8 Data 10 nm 7.5 nm 5 nm

7 JSC (mA/cm2)

silicon is relatively neutron transparent which allows scattering from multiple thin films supported by silicon wafers to be used for SANS experiments [35,37]. In addition, Yin and Dadmum have used drop cast bulk P3HT/PCBM blends (thickness  1mm). Their Porod analysis at low q suggests that the P3HT/ PCBM interface broadens with thermal annealing [36]. The implication is that P3HT/PCBM mixtures are at least partially miscible, which has been further supported by various reports [8,26,35,38,39].

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d (nm) 5. Conclusions and outlook Despite the remarkable increase in efficiency of organic solar cells, our understanding of the material parameters which govern

Fig. 5. Short-circuit current (JSC) of PBTTT/PC71BM devices vs. domain spacing, d, of the photoactive layer. The solid lines are from a morphological model described in Ref. [40] computed for three different exciton diffusion lengths (5, 7.5 and 10 nm). Error bars are the standard deviation from multiple measurements. Reproduced from Ref. [40] with permission from The Royal Society of Chemistry.

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