Revealing the Active Layer Morphology within Complete Solar Cell

Article pubs.acs.org/JPCC

Revealing the Active Layer Morphology within Complete Solar Cell Devices via Spectroscopic Ellipsometry Sebastian Engmann, Vida Turkovic, Harald Hoppe, and Gerhard Gobsch* Institut für Physik, Technische Universität Ilmenau, Weimarer Strasse 32, D-98693 Ilmenau, Germany S Supporting Information *

ABSTRACT: We used spectroscopic ellipsometry to simultaneously reveal the distribution of the fullerene component, as well as the formation of higher ordered polymer domains, within the active layer of organic solar cells. In contrast to other measurement techniques, both informations can be obtained nondestructively on postcathode annealed encapsulated solar cell devices. We observe a preferential attraction of [6,6]-phenyl-C61-butyric acid methyl ester toward the aluminum cathode, which is consistent with simple energetic considerations based on the estimated free surface energy of the cathode. Because of the increased nucleation rate in regions with high fullerene concentration and at the interfaces, we observed a reduction of the average domain size of ordered poly(3-hexylthiophene) regions in these areas, which manifests itself in a reduction of the volume fraction of highly ordered polymer domains.

electron microscopy methods (TEM), such as energy filtered TEM,18 three-dimensional (3D)-TEM tomography,19 as well as spectroscopic methods, such as spectroscopic ellipsometry (SE),20,21 photoelectron spectroscopy and profiling,22 and near edge X-ray absorption fine structure spectroscopy (NEXAFS).23 Our study further demonstrates the applicability of spectroscopic ellipsometry in revealing morphological informations on polymer/fullerene active layers, via through-the-glass ellipsometry. Ellipsometric measurements on full devices through an optical thick substrate have been reported so far on CdTe Solar cells by Chen et al.,24 as well as on OLED devices by Ramsdale et al.,25 while they are still missing for OPV devices based on polymer/fullerene active layers. Recently, we have shown that SE is capable to reveal quantitative information about the degree of ordered polymer domains within thin polymer/fullerene films.26,27 This study further gives further proof for the applicability of the reported model to encapsulated organic solar cell devices. We extracted crucial information determining the solar cell performance, the information on the distribution of higher ordered polymer domains and the fullerene distribution over the film thickness. It is to be stressed here that spectroscopic ellipsometry enabled us to obtain these data simultaneously and in a nondestructive manner.

1. INTRODUCTION Bulk heterojunction (BHJ) solar cells, based on π-conjugated polymer (donor) and fullerene derivative (acceptor) blends, are a hot topic in physics nowadays, as their application promises flexible, low-weight, and low-cost devices.1−3 In this kind of device architecture, understanding and manipulating the morphology within the BHJ is one of the most important issues to be addressed on the technological route toward better device performance. Organic solar cell (OSC) device characteristics are strongly depending on the spatial distribution of both donor and acceptor phase over the active area, especially their domain sizes, as well as the spatial order of the polymer backbone. Not only the light absorption is strongly correlated to the order within the blend but also the charge carrier generation, the electron and hole transport within the device and charge carrier extraction at the electrodes.4−12 Over the years, empirical data was collected on how the processing conditions affect the morphology and how the blend morphology can be altered after film formation. Typical morphology optimization routes include the variation of the relative concentration of each of the blend constituents and the casting solvent, which influence the drying time of the film, and the annealing of the films after casting, either by exposure to solvent vapor or to elevated temperatures, which allows morphological reorganization of the dried films. Interestingly, different processing schemes might lead to the same device performance, for example, slow drying of the active layer might lead to a morphology close to those of devices that were fast dried and then annealed.13−17 A detailed understanding on the variables affecting the morphology requires a deeper insight into the active layer morphology. Thus, over the last years an important goal of the OPV community was to develop and adapt bulk morphology sensitive measurement techniques to thin organic films. To be mentioned here are transmission © 2013 American Chemical Society

2. EXPERIMENTAL SECTION In this study, the active layer of organic solar cells consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was analyzed regarding the selforganization of the polymer within the blend via spectroscopic ellipsometry. P3HT, with a regioregularity RR of ∼92%, Received: October 24, 2013 Published: October 25, 2013 25205

dx.doi.org/10.1021/jp410537y | J. Phys. Chem. C 2013, 117, 25205−25210

The Journal of Physical Chemistry C

Article

factors were varied. Further fit parameters were the overall film thickness of the active layer and the thickness nonuniformity. The dielectric functions (DFs) of PCBM and higher and lower ordered P3HT domains were published previously.26

molecular weight Mw of 66 kDa, and polydispersity index PDI of 2.3, was purchased from Rieke Metals Inc. PCBM of purity 99% was delivered by Solenne BV. Approximately 90 nm thick films (spin-frequency 1000 rpm) of mass ratio P3HT/PCBM 3/2 were spin-coated inside the glovebox (H2O < 1 ppm; O2 < 1 ppm) on PEDOT:PSS (approximately 40 nm) coated ITO substrates (Sheet resistance 10−15Ω/□, film thickness approximately 150 nm), using stock solutions of P3HT and PCBM (each 2 wt % in CB). After spin-coating, a nontransparent thick aluminum cathode was evaporated at a base pressure lower than 5 × 10−6 mbar. Note that no additional drying steps of the active layer were performed prior to evaporation of the aluminum electrode. Postcathode annealing at 150 °C for 5 min was performed inside the glovebox prior to encapsulation of the devices. Solar cells with this architecture typically yield power conversion efficiencies around 3% under AM1.5 conditions. The prepared devices were then investigated via spectroscopic ellipsometry through the glass substrate in the energy range between 1.2−4.5 eV and incident angles ranging from 25 to 65°. The data was analyzed using a commercial piece of software, namely J. A.Woollam Co. WVASE32, which is pretty convenient as incoherent reflections arising from reflections of the top and bottom of the thick glass substrate are already implemented. The software implementation is based on the model by Kildemo et al.28 and Yang et al.29 Furthermore, the corrected incident angle depending on the photon energy of the incident light beam on the active layer is automatically calculated using Snell’s law. The optical model used for the simulation of the experimental data is shown in Figure 1. Active layer modeling

3. RESULTS AND DISCUSSION Prior to the analysis of the solar cell devices, the dielectric functions and film thicknesses of the ITO-glass substrate and PEDOT:PSS hole extraction layer were determined. First, the characterization of the commercially available ITO-glass (150 nm ITO on 1.1 mm float glass) is discussed. The dielectric functions of glass and ITO were determined by simultaneous evaluation of the following samples: first, glass in standard reflection ellipsometry, where the ITO was removed via abrasive blasting leading to a roughened back surface, which enabled data acquisition without the need for multiple reflections correction; second, Al/glass/ITO in backside corrected ellipsometry; third, glass/ITO/Al in backside corrected ellipsometry; finally, transmission measurements of the glass/ITO substrate in the photon energy range between 0.6..5 eV were performed. The latter one is especially important to determine the DF and transmission cutoff energy of the glass substrate, as well as the ITO conductivity. Here, we follow the suggestions of Johs et al.,30 who recommend measurements from both sides of the layer stack, complemented with transmission measurements. Prior to the above-described measurements, the dielectric function of aluminum was determined on multiple devices, which have been additionally oxidized on the surface by an ozone plasma. The multiple sample analysis allowed for the determination of the DF of the aluminum electrode, eliminating uncertainties due to the fast oxidation of the metal surface.25 It was shown by Synowicki et al. in the late 90s that thin sputtered ITO films on glass can be described best using slightly different optical properties over the ITO film thickness, which is due to different stoichiometry and sputter conditions during film deposition.31 D′Elia et al. have shown that a Drude oscillator model can be used to investigate the conductivity via spectroscopic ellipsometry for various film deposition conditions.32 Thus we chose to divide the ITO layer into three independent sublayers, each modeled using the GenOsc parametrized optical constants layer material of the WVASE32 software. This allows for the easy combination of a Drude oscillator model in order to model the low photon energy properties and Gaussian peaks to model the high energy transitions hν > 3.5 eV. Equation 1 shows the complex

Figure 1. Schematic representation of the investigated solar cell layer stack. The film thickness of each sublayer was determined by the fit of the ellipsometric data according to the optical model described in the manuscript.

was done using a graded EMA layer approach, where the film was divided into 4 nodes, at 0, 20, 80, and 100% of the film thickness. At each node, the fullerene volume fraction, the volume fraction of ordered polymer domains, and the screening

Figure 2. Complex dielectric function of ITO (left) and PEDOT:PSS (right), as extracted from the SE results. The ITO layer on top of the glass substrate was divided into three sublayers (enumeration starting from the layer closest to the substrate). Layer 1 corresponds to a low conductivity region of the ITO layer near the glass substrate, Layers 2 and 3 are higher conductive film regions. 25206



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Figure 3. In-depth volume fractions of the fullerene phase and the fraction of higher ordered polymer domains with respect to the total P3HT amount across the active layer within a postcathode annealed solar cell device (left). Experimentally obtained ellipsometric data ψ and the fit according to the model described within the manuscript (right). The film thickness of the active layer P3HT/PCBM 3:2 was approximately 84 nm. Annealing of the device was performed in inert atmosphere at 150 °C for 5 min after the deposition of the aluminum electrode (postcathode annealing).

PEDOT:PSS layer. The determined film thicknesses including the etched interface region were then used for the evaluation of the full solar cell device, which were completed by an approximately 90 nm thick spin-coated P3HT/PCBM layer and evaporated opaque aluminum electrode. Prior to characterization, the device was encapsulated using a UV-curable epoxy resin and a glass cover. Full devices were analyzed with respect to the fullerene distribution across the film thickness of the active layer, which became quite a standard for thin films since Campoy-Quiles et al. reported on vertical phase segregation.20,35−37 Additionally, we analyzed the degree of polymer order within the film. This analysis is based on our recent reports in which we derived the dielectric functions of higher and lower ordered polymer domains and modeled various films either containing different PCBM volume fractions or annealed at various temperature.26,27 Order in the sense that we refer to here, and as accessible via SE, is not interchangeable with the polymer crystallinity, or relative volume fraction of polymer crystals, but corresponds more likely to polymer domains that show a large average conjugation length. Thus, these regions are included in crystalline domains but not necessarily restricted to them, as small torsions and kinks are allowed, as long as the electro-optic properties (conjugation) are conserved. It is worth noticing that there is a much closer relation between the amount of “optical order” and the electronic properties, like the charge carrier mobility, than “crystalline order” and electronic device properties.38 The determined distribution of the fullerene component, as well as the volume fraction of higher ordered polymer domains across the active layer thickness is shown in Figure 3. Because of the preferential wetting of PCBM at the PEDOT:PSS and aluminum interfaces, a PCBM enrichment layer is present at both interfaces, while previous investigations on thin films, annealed in the absence of the cathode interface, yielded P3HT-rich surface layers.21,22,35,37 It should be mentioned here that spectroscopic ellipsometry is an indirect method and that the uncertainty in the obtained results grows drastically with the number of fit parameters. In a multiparametric space, it is evident that there might be a large number of local minima of the means squared error of generated ellipsometric data and experimental data (MSE0). In order to verify that the shown results correspond to the global minimum within the parameter space, we performed the so-

dielectric function εDrude = ε1,Drude + iε2,Drude in dependence on the photon energy E for the Drude oscillator model εDrude =

−ℏ2 ε0ρ(τE2 + iℏE)

(1)

where ε0 is the vacuum dielectric permittivity,ρ the resistivity and τ is the average time between collision of the electrons. The complex DF εgauss = ε1, gauss + iε2,gauss within the Gaussian peak model is given by the imaginary part ε2,gauss according to ⎧ ⎡ ⎛ E − E ⎞2 ⎤ ⎡ ⎛ E + E ⎞ 2 ⎤⎫ 0⎟ 0⎟ ⎜ ⎢ ⎥ ⎢−⎜ ⎥⎬ ε2,gauss = A ⎨ − + exp exp ⎪ ⎪ ⎝ ⎠ ⎝ ⎠ ⎦⎭ Br Br ⎦ ⎣ ⎩ ⎣ ⎪

⎪

(2)

and the Kramers-Kronig consistent calculation of the real part ε1,Gauss. In eq 2, the parameter A represents a dimensionless amplitude, E0 is the position of the peak center, and Br is the peak width. The best overall agreement between the experimental data and the simulated model was achieved for one thin (∼10 nm) layer with low conductivity near the glass substrate, and two (∼70 and 50 nm) highly conductive ITO layers with slightly different dielectric functions. A list of all obtained parameter values can be found in the Supporting Information Table S 1. The total ITO film thickness was 146 nm including 9 nm surface roughness simulated by an EMA consisting of the upper ITO layer and 90% of the layer above it. The conductivity of the ITO was determined from the Drude oscillator model to be around 13Ω/sq, which is in good agreement with the data provided by the manufacturer (10− 15Ω/□). Figure 2 shows the determined dielectric functions for each of the three sublayers, as well as the DF of the holeextraction PEDOT:PSS layer. The latter one, known to show optical uniaxial behavior,33,34 was investigated on silicon substrates and the DF was modeled by multiple Gaussian peaks (a detailed list of the peak parameter can be found in the Supporting Information Table S 2). Using the shown DFs of ITO and PEDOT:PSS, an incomplete layer stack (glass/ITO/PEDOT:PSS) was investigated in order to elucidate the effects of ITO etching due to the acidic nature of aqueous solutions of PEDOT:PSS. The optical model in this case consisted of the initial ITO/glass substrate where the thickness of the last of the three sublayers was varied, an etched region with unique DF exhibiting properties of both the PEDOT:PSS and ITO layer and a thin 25207



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Table 1. Surface Free Energy γ for Typical Materials Used in Organic Solar Cell Devices or As Substrates for Thin Film Characterizationa material 42

P3HT PCBM43 SiO243 ITO42 PEDOT42 Al44 Au44

γLW [ mJ/m2]

γ+ [ mJ/m2]

γ− [ mJ/m2]

γ [ mJ/m2]

ΔGP3HT‑X [ mJ/m2]

ΔGPCBM‑X [ mJ/m2]

25.00 39.97 40.28 35.20 27.00 36.91 42.10

0.40 0.00 2.78 0.00 2.10 0.19 0.42

3.60 6.52 47.11 9.20 48.00 16.00 18.93

27.40 39.97 63.18 35.20 47.08 40.40 47.83

−78.5 −56.0 −66.2 −67.5 −72.9

−88.0 −75.0 −73.1 −79.1 −85.4

The Lifshitz−van der Waals γLW, acid γ+, and basic γ− components of the surface free energy within the OCG model are given as found in the literature. The free energy of adhesion ΔG between each of the surfaces (X) and the polymer or fullerene was calculated according to the eq 4. a

called uniqueness fits, which are implemented in the WVASE software. These evaluations are based on the global variation of one parameter after the other and evaluation of the corresponding MSE0. The results of the uniqueness fits prove that the results here shown correspond to the global minimum (see Supporting Information). Interface wetting properties of different materials can be explained using the so-called Van Oss−Chaudhury−Good (OCG) theory,39−41 where the total surface energy of a material γ involves the sum of Lifshitz−van der Waals γLW and polar acid−base γAB interactions γ = γ LW + γ AB

heterogeneous growth and the exclusion of PCBM from crystalline regions is less significant than usually expected. And second, the distribution of ordered polymer domains prior to annealing, and their ripening upon annealing in combination with exclusion of PCBM from the crystalline domains, affects the distribution of the fullerene component significantly less than commonly expected. Each nucleus above the critical radius R* will grow to a larger grain at a temperature-dependent rate, until neighboring grains get into contact with each other and grain boundaries are formed. Thus, by an increasing fullerene concentration, the nucleation rate increases, leading to a reduction of the average domain size of ordered P3HT regions. Additionally, the nucleation rate at the electrode interfaces is further increased. Thus, the largest domain size of ordered P3HT domains could be found within the bulk of the film. As the average conjugation length is probed and analyzed by our ellipsometric model, we were able to directly observe this phenomenon.

(3)

In eq 3, the acid−base component is the geometric mean of two parameters γAB = 2(γ+γ−)1/2, where γ+ is a parameter describing the electron-acceptor (Lewis acid) and γ− is the electron-donor (Lewis base) properties. Water was chosen as the reference for γ+ and γ−, and their ratio was set to unity. If the surface free energy and its polar components are known, the free energy of adhesion Δ G between two surfaces in vacuum can be readily calculated ΔG12 = −2( γ1LWγ2LW +

γ1+γ2− +

γ1−γ2+ )

4. SUMMARY We have shown that information on the distribution of the fullerene component within organic solar cell devices can be successfully extracted from ellipsometric measurements on a real solar cell layer stack. Furthermore, the distribution of higher ordered polymer domains within the active layer of a solar cell could be simultaneously determined. In contrast to most of the other measurement techniques that can be applied to reveal the distribution of one of the two blend materials over the film thickness (e.g., XPS depth profiling), spectroscopic ellipsometry allows investigation without destroying the device. Therefore, the very same device can be investigated continuously, as required for lifetime measurements.

(4)

Table 1 shows the surface free energy and its components within the OCG model for typical organic photovoltaic materials. The free energy of adhesion was then calculated using eq 4. Note the preferential attraction of PCBM toward PEDOT as well as toward aluminum. Thus, postcathode annealing leads to a PCBM-rich aluminum interface as compared to precathode annealed devices. The distribution of higher and lower ordered P3HT domains in annealed thin films on silicon substrates was recently investigated by our group, and could be attributed to heterogeneous nucleation of the polymer at fullerene molecules.27 In blend systems with a large number of nucleation centers, the homogeneous nucleation rate Ṅ hom at excisting polymer crystals can be almost fully neglected, as compared to heterogeneous nucleation rate Ṅ het at PCBM molecules ⎛ ΔG hom(R *) − ΔG het(R *) ⎞ Ṅhet ∝ exp⎜ ⎟ Ṅhom kBT ⎠ ⎝

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ASSOCIATED CONTENT

S Supporting Information *

Additional information, tables, and figure. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Phone: +493677693701.

(5)

Notes

with ΔGhom(R*) > ΔGhet(R*) being the change in the Gibbs free energy for homogeneous, respectively, heterogeneous nucleation growth of nuclei with critical size R*. This leads to two conclusions. First, in regions of high PCBM concentration or near the interface the nucleation of ordered domains will happen more or less exclusively due to

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Ellipsometry measurements were performed using a J. A. Woollam VASE ellipsometer founded by the Thuringian 25208



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Ministry of Culture (EFRE B715-08015). We also thank the “Center for Micro- and Nanotechnologies” (ZMN) for providing the facilities needed in sample preparation.

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