Determining Material-Specific Morphology of Bulk-Heterojunction

Article pubs.acs.org/JPCC

Determining Material-Specific Morphology of Bulk-Heterojunction Organic Solar Cells Using AFM Phase Imaging Vladislav Jovanov,† Nivedita Yumnam,† Arne Müller,† Mathias Gruber,†,‡ and Veit Wagner*,† †

Department of Physics and Earth Sciences, Jacobs University Bremen, 28759 Bremen, Germany PolyIC GmbH & Co. KG, 90763 Fürth, Germany

‡

S Supporting Information *

ABSTRACT: A technique for determining the materialspecific morphology of a polymer−fullerene blend is presented. This technique is applied to solution processed bulk-heterojunction organic solar cells with different weight ratios of polymer−fullerene blend using the PTB7:PCBM material system. Optical and electrical characterizations show that the light absorption increases for larger polymer (PTB7) content, while the fill factor of the fabricated solar cells is improved for larger fullerene (PCBM) content. The materialspecific morphologies of polymer−fullerene bulk-heterojunctions are measured by employing AFM phase imaging. The measured AFM phase images reveal that fullerene material forms flake-like clusters which are embedded in the polymer network. The size of the flakes is increasing with larger content of fullerene material. By correlating the optical and electrical properties with the measured bulk-heterojunction morphologies, the relation between bulk-heterojunction structure and solar cell performances is discussed.

1. INTRODUCTION To harvest solar energy, different types of solar cells have been developed.1−5 The largest share of the energy production market belongs to solar cells based on inorganic materials due to their high efficiency and well-established fabrication processes.1−4 These factors also make it difficult for newly emerging photovoltaic technologies to compete with inorganic solar cells in these mainstream applications. The situation is rather different for novel and nonstandardized photovoltaic applications, where emerging technologies can outperform inorganic solar cells. Promising candidates for these novel applications are organic materials due to their unique features. Semi-transparency or even full transparency of organic materials in the visible wavelength range6 allows for the realization of photovoltaic windows.7,8 Furthermore, organic materials can be solution processed, which is suitable for costefficient printing techniques and roll-to-roll fabrication methods.9 This enables production of light-weight solar cell modules printed on flexible plastic substrates that can be used in outdoor applications or mounted on bendable surfaces.10,11 Finally, solution processed organic solar cells have reached power conversion efficiency (PCE) values close to 10%,12,13 which has been seen as a critical value for commercialization. Use of organic materials in photovoltaic applications imposes specific requirements. Photogenerated excitons exhibit a high binding energy.14,15 This is caused by a low dielectric constant of these materials, which results in a pronounced Coulomb interaction. Therefore, special measures have to be taken to © XXXX American Chemical Society

enable efficient exciton separation. The active layer of solution processed organic solar cells is composed of an electron donor (polymer) and electron acceptor (fullerene) material. The proper energy difference between the donor and acceptor band edges allows for the dissociation of photogenerated excitons. The exciton dissociation can only occur at the interface between the two materials, and a large interface area is required to enable an efficient dissociation process. To achieve the large interface area, donor and acceptor materials are intermixed to form a bulk-heterojunction (BHJ). As the photogenerated excitons reach the donor−acceptor interface by diffusion, domain sizes of a material where light absorption occurs should be at most twice the exciton diffusion length.14 After the exciton dissociation, free electrons are transported by the fullerene network to an electron transport layer (ETL)16 and further to the cathode contact. On the other hand, free holes are transported by the polymer network to a hole transport layer (HTL)17 and later to the anode contact. As stated above, the BHJ plays a crucial role in the exciton dissociation and extraction of the free charge carriers. To fulfill these tasks, a bicontinuous organization of donor and acceptor phase (schematically shown in Figure 1) is needed.14 However, the phase organization depends on the used solvent, annealing steps, and ratio between donor and acceptor material in a Received: February 27, 2017 Revised: April 3, 2017

A

DOI: 10.1021/acs.jpcc.7b01924 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

proper instrument settings for feedback loop as well as for strength of tip−surface interaction. Nagarjuna et al. have used AFM phase imaging to investigate solar cells based on PTB7 and different derivatives of fullerene.38 However, the phase images they present are strongly affected by the sample topographies.38 Also, important requirement for proper AFM phase image interpretation is to obtain an unambiguous identification of polymer and fullerene material. In previous studies, identification of materials by AFM phase imaging has not been reported.22−25,34−38 Alongside with AFM, transmission electron microscopy (TEM) measurements have also been used to investigate BHJ morphology. TEM measurements have been able to identify immense fullerene aggregates due to larger electron densities.18,24,39 However, for optimized BHJ morphologies, TEM failed to resolve structure of smaller clusters18,24,39 most likely due to the signal integration over the film thickness. In our study, we have achieved an unambiguous identification of polymer and fullerene material by employing AFM phase imaging on a proper reference sample with pure PTB7 and PCBM domains located next to each other. By determining the phase angles that correspond to PTB7 and PCBM, we are able to identify their respective domains in different BHJ blends. Unlike previous studies where the AFM phase imaging measurements have been conducted only at the film surface, we have measured material-specific BHJ morphologies within the films after removing the surface layer by scratching. The measured material-specific morphologies of the BHJ are correlated with optical and electrical properties of fabricated solar cells in order to determine the influence of the BHJ structure on the solar cell electric performances.

Figure 1. Schematic cross section of an organic solar cell in inverted configuration with bicontinuous organization of polymer−fullerene blend and material system used in this study.

blend.18−25 In this study, we have investigated the organization of donor and acceptor material (material-specific morphology) in solution processed BHJ solar cells for blends with different weight compositions. Determined material-specific morphologies are then used to further elucidate measured performances of fabricated solar cells. As electron donor material, we have chosen poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7). PTB7 falls into the class of low bandgap polymers, which exhibit increased absorption in the near-infrared part of the spectrum.26,27 Solution processed solar cells based on PTB7 are able to achieve excellent performances with PCE of more than 7% for solar cells with a low work function metal as cathode contact (regular configuration).18,28 More recently, solar cells based on PTB7 and [6,6]-phenyl-C71-butyric acid methyl ester (PC[70]BM) realized in an inverted configuration have reached PCE of more than 9%.12,13 In this investigation, we have selected materials and solar cell design such that we achieve compatibility with large scale production and prolonged lifetime of devices. The solar cell design and used materials are schematically presented in Figure 1. The fabricated solar cells have been realized in inverted configuration, where the high work function metal is used as the anode contact. Compared to regular configuration, organic solar cells in inverted configuration exhibit longer lifetime and better stability.14,29 As ETL and HTL materials, we have employed zinc oxide (ZnO) and poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) (PEDOT:PSS), respectively, since these materials are cost efficient and solution processable.14,29 As an electron acceptor material, [6,6]-phenylC61-butyric acid methyl ester (PCBM) is used in this study. Additionally, we have realized the complete preparation and fabrication process of our solar cells, excluding the indium tin oxide (ITO) and metal contact, in a normal air environment (see Experimental Section). In order to investigate the material-specific morphology of the BHJ, we have used atomic force microscopy (AFM) phase imaging.30−33 This approach is often employed to investigate the surface of organic BHJ solar cells.22−25,34−38 Yet, despite numerous studies, a clear model of the material-specific BHJ morphology is still not presented. To obtain meaningful results with AFM phase imaging, special care is needed to select

2. EXPERIMENTAL SECTION Device Fabrication. The complete fabrication of our BHJ solar cells and preparation of all solutions have been conducted in ambient air according to the following procedure. In the first step, glass substrates coated with ITO, which is used as the cathode contact of our solar cells, are cleaned by rinsing with acetone and 2-propanol followed by 10 min in an ultraviolet (UV)-ozone cleaner to remove any organic residue. After the cleaning step, a solution processed ETL is spin-coated onto the ITO and annealed for 15 min at 110 °C. The ETL is based on a sol−gel-derived ZnO, and the solution is prepared by dissolving the precursor zinc acetate dihydrate into 2-methoxyethanol and ethanolamine.16 In the next step, the active layer solution is doctor bladed and annealed for 3 min at 75 °C. The preparation of the active layer solutions is realized by creating PTB7:PCBM blends with different weight ratios (1:1, 1:2, 1:3) in 1,2-dichlorobenzene (DCB) solvent. A solution processed HTL is deposited by spin-coating and annealed for 10 min at 75 °C. The HTL solution is a mixture of PEDOT:PSS (Heraeus, Clevios HTL solar) and Zonyl FSN-100 (SigmaAldrich). In the last step, the anode contact is prepared by sputtering a 50 nm thick layer of silver through a shadow mask. The resulting thickness of individual solar cell layers is determined by a surface profilometer (Dektak-3) after each deposition step. The thicknesses of the ZnO, active, and PEDOT:PSS layer of the fabricated solar cells are 20, 200, and 50 nm, respectively. Single Film Preparation. Fabrication of the PTB7:PCBM films has also been realized in ambient air. In the first step, glass substrates are rinsed with acetone and 2-propanol. The B


Article

The Journal of Physical Chemistry C remaining organic residue is then removed by exposing the glass substrates to UV light in the UV-ozone cleaner for 10 min. Afterward, the PTB7:PCBM films are deposited by doctor blading. For this purpose, the same solutions (PTB7:PCBM blends with weight ratio 1:1, 1:2, and 1:3) prepared for solar cell fabrication are used. The thickness of the deposited films of 200 nm is measured by a surface profilometer. To prepare the reference sample for the AFM measurements, the PTB7 material is dissolved in DCB and PCBM is dissolved in cyclohexanone. In the first step, the PTB7 film is doctor bladed and annealed to remove the solvent. Afterward, a droplet of PCBM solution is deposited in one corner directly onto the PTB7 film. Since cyclohexanone is not able to dissolve PTB7, mixing of the two materials has been prevented. In the last step, the reference sample is annealed to remove remaining solvent from the PCBM droplet. Absorptance Measurements. To measure the absorptance of the films prepared from different PTB7:PCBM blends, an UV−VIS spectrometer (PerkinElmer Lambda 12) has been used. In the first step, the transmittance (T) of the single deposited films is measured. Afterward, the reflectance (R) of the single films is measured. Finally, the absorptance (A) is calculated as:

Figure 2. Absorptance of 200 nm thick PTB7:PCBM films with weight ratios of 1:1, 1:2, and 1:3.

PTB7. By increasing the content of PCBM, the PTB7 absorption peak is decreasing, and the PCBM absorption peak is slightly increasing. From Figure 2 it can be observed that the highest overall absorption under the illumination of air mass 1.5 (AM 1.5) sun spectrum41,42 occurs for PTB7:PCBM blend with 1:1 weight ratio, while the 1:3 blend results in the smallest overall absorption. These results show that the PTB7 is mainly responsible for the sunlight absorption. In the next step of our investigation, BHJ solar cells are fabricated using the same PTB7:PCBM blends that have been employed for the single film characterization. For each blend (1:1, 1:2, and 1:3), a set of six solar cell devices is produced. The fabricated solar cells are then characterized by measuring their current voltage (I−V) characteristics under AM 1.5 illumination.43 The results are shown in Figure 3.

A=1−R−T

AFM Measurements. To determine the material-specific bulk morphology of PTB7:PCBM films, a NanoSurf Mobile S AFM system with AppNano ACLA AFM tips has been used. During the measurements, the AFM tip has been excited at the resonant frequency to achieve a free oscillating amplitude that corresponds to 800 mV output of the integrated detector. During the scan the bulk of the deposited films after scratching, the set point has been set to 60% of the free oscillating amplitude. To ensure the accuracy of measurements, an internal PID controller has been adjusted to keep the oscillating amplitude constant, which is confirmed by the measured data. The measurements have been realized in a closed environment with controlled humidity of 12−17%. The scanned area has been set to 1 × 1 μm2 and 0.5 × 0.5 μm2 with resolution of 256 × 256 points and scan frequency of 51.2 Hz per point. For the reference sample, the scanned areas have been 0.79 × 0.79 μm2 and 2.1 × 2.1 μm2. The measured phase difference between excitation force and AFM tip oscillation has been processed by the instrument controller, so that the phase difference is set to zero at the resonant frequency for the free oscillation conditions.

3. RESULTS Optical and Electrical Measurements. The optical absorption of an active layer of solution processed organic solar cells depends on the ratio of donor and acceptor material in the blend.40 Therefore, in the first step of our investigation, single films with different content of PTB7 (polymer donor) and PCBM (fullerene acceptor) are deposited on glass substrates from DCB solvent, and their absorptance is measured. During the preparation of the films, the weight ratio of PTB7 and PCBM in the blend is varied from 1:1 to 1:2 and 1:3, while the film thickness of 200 nm is kept constant. The results are presented in Figure 2. Figure 2 displays the absorptance of single PTB7:PCBM films for different blend ratios. The absorptance exhibits two distinct peaks. The absorption peak at 330 nm corresponds to PCBM, while the broader peak at 650 nm corresponds to

Figure 3. Average current voltage characteristics of fabricated solar cells for different PTB7:PCBM blends.

Figure 3 shows the mean of measured I−V curves of the fabricated solar cells for different PTB7:PCBM blends. It can be seen that the highest short circuit current density is achieved for PTB7:PCBM blend of 1:2, while the lowest is obtained for the 1:1 blend. By increasing reverse bias voltage to −2 V, more current is extracted from solar cell with 1:1 blend, and the short circuit current density becomes equal to that of solar cell with 1:2 blend. The I−V curves of all fabricated solar cells can be found in the Supporting Information (Figures S1−S3). Table 1 shows mean values and standard deviations of the extracted electrical parameters of the fabricated solar cells for different PTB7:PCBM blends. The series and shunt resistance C


Article


Table 1. Mean Value and Standard Deviation of Electrical Parameters of Fabricated Solar Cells for Different Weight Ratios of PTB7 and PCBM PTB7:PCBM

Voc (mV)

Jsc (mA/cm2)

Rsh (Ω cm2)

Rs (Ω cm2)

FF (%)

PCE (%)

1:1 1:2 1:3

755 ± 11 744 ± 8 732 ± 16

5.8 ± 0.3 7.3 ± 0.5 6.2 ± 0.4

186 ± 12 229 ± 16 384 ± 35

117 ± 11 55 ± 2 40 ± 5

28.4 ± 0.5 37.6 ± 0.5 42.2 ± 0.4

1.2 ± 0.1 2.0 ± 0.1 1.9 ± 0.2

are extracted from I−V curves by using the slope method.44,45 The increase of the open circuit voltage with decreasing shunt resistance indicates that the extracted values of shunt resistance are strongly affected by the series resistance values. Based on this observation, we have concluded that the fill factor of all fabricated solar cells is dictated only by the series resistance. The lowest short circuit current density of 5.8 mA/cm2 is achieved for the 1:1 blend. This blend also exhibits the lowest fill factor and PCE of 28.4% and 1.2%, respectively. The lowest fill factor is a consequence of the highest series resistance. By increasing the PCBM content, the series resistance of solar cells drops and the fill factor increases. Additionally, a drop of the open circuit voltage occurs, which we have attributed to the change of the effective band gap. The highest short circuit current density of 7.3 mA/cm2 and PCE of 2% is obtained for the 1:2 blend. Although the 1:3 blend exhibits the short circuit current density of 6.2 mA/cm2 (almost the same as the 1:1 blend), the PCE of these cells is almost equal to the 1:2 blend. This is attributed to the better electrical properties with the lowest series resistance and highest fill factor of 42.2%. The absolute performance of the fabricated solar cells does not affect the validity or generality of the presented results. These solar cells do not achieve highest PCEs reported in the literature,12,13 which is caused by the following factors. First, the complete fabrication process of the solar cells in this study is realized in ambient air, while high efficiency solar cells are usually produced in an inert atmosphere.12,13 Second, materials used to fabricate our solar cells are suited for large area production.14,25 On the other hand, to achieve PCE of more than 7% high performance materials are required.12,13 Finally, as a solvent we have used DCB (see Experimental Section), while high efficiency solar cells are usually deposited from a mixture of DCB and 1,8-diiodooctane (DIO).12,13 If we compare our solar cells to solar cells that are also fabricated under ambient conditions using similar materials and fabrication process,46,47 achieved PCEs are in the same range. The presented results show that the lowest light absorption occurs for the 1:3 blend. However, the lowest short circuit current density is achieved for the 1:1 blend, despite the fact that this blend exhibits the largest light absorption. By applying larger reverse bias voltage, the situation changes and the highest current is achieved for the 1:1 blend as expected. In order to explain these results, the morphology of BHJ has to be taken into account. AFM Measurements. To determine the BHJ morphology of PTB7:PCBM blends, AFM phase imaging is used in this study. A short outline of the relevant AFM theory is given here to emphasize that phase imaging is suitable for such an investigation. In AFM tapping mode measurements, the phase shift (ϕ) between the cantilever tip oscillations and the excitation force depends on the interaction with materials that are present at the sample surface.30−33,48 The expression for the phase shift (ϕ) can be derived from the equation of motion and is given by:32

sin(ϕ) =

E ⎞ Aω ⎛ ⎜1 + ts ⎟ A 0ω0 ⎝ Emed ⎠

(1)

where A0 is the free oscillation amplitude (cantilever tip is not in contact with sample) at the resonant frequency (ω0), A is the oscillation amplitude during tip interaction with the surface at a driving frequency (ω), Ets is the energy dissipated in the tip− surface interaction, and Emed is the energy dissipated in the tip interaction with a surrounding medium (air) during one oscillation period. The dissipated energies are given by the following expressions:32

Ets =

∫0

Emed =

T

Ftsz ̇ dt

kA2ωπ Qω0

(2)

(3)

where T is the period of the cantilever oscillation (T = 2π/ω), Fts is the tip−surface interaction force, ż is the velocity of the cantilever tip movement, k is the cantilever spring constant, and Q is the cantilever quality factor. Equation 1 is valid for in-air measurements using cantilevers with high Q-factors.32 Equations 1 and 2 show that the phase shift (ϕ) is sensitive to the tip−surface inelastic interaction. At the same time, the tip− surface interaction depends on the properties of the material that is being probed by the cantilever tip. Consequently, the presence of different materials at the surface of the sample is indicated by a contrast in the AFM phase image. However, in order to achieve sufficient contrast in the phase image, the instrument settings such as free amplitude, set point, and feedback loop have to be chosen with care.48 The instrument settings for our AFM measurements can be found in the Experimental Section. To make an unambiguous identification of PTB7 and PCBM domains by AFM phase imaging, a reference sample is used. For this purpose, a pure PTB7 film is doctor bladed on a glass substrate. Afterward, a droplet of PCBM is deposited on top of the PTB7 film. Finally, the transition region between PTB7 and PCBM is measured by AFM. The results are shown in Figure 4. Figure 4a displays the surface morphology of the transition region between the PTB7 film and PCBM droplet. Based on the fabrication procedure of the reference sample, the PTB7 film is identified as the regions with a lower height (bottom-left parts of Figure 4a), while the regions with a larger height are identified as the PCBM droplet (top-right parts of Figure 4a). The corresponding phase image is shown in Figure 4b. By comparing Figures 4a and 4b, we have determined that for our AFM measurement system the phase angle of the PTB7 is lower than that of the PCBM. The phase image shows a clear contrast between the PTB7 and PCBM domains. From Figure 4b it can be observed that the PTB7 film is also present in the top-left part of Figure 4b and that a small PCBM region is located in the bottom-center part of Figure 4b. By correlating the surface topography and phase image, we have concluded that at the edge of the droplet the PCBM does not form a D


Article


1:2, and 1:3 are measured. For this investigation, the single films on the glass substrates previously prepared for absorptance measurements are used. In order to access the bulk, a scratch is made on these films with special care so that only a surface layer is removed. Finally, AFM measurements are performed within the created scratch regions. The results are shown in Figure 5. To demonstrate the reproducibility of AFM measurements, we have also measured a smaller area within the same scratch, and these results are shown in Figures S5−S7. The bulk topographies of PTB7:PCBM films for different weight ratios are shown in Figures 5a−c. The height range varies from 10 nm (1:1 mixture) to 23 nm (1:2 and 1:3 blend). Since the thickness of each film is around 200 nm, the results shown in Figure 5 represent 5−12% of the film bulk, giving a meaningful insight into its structure and morphology. The corresponding phase images are shown in Figures 5d−f. Some variations in the measured range of the phase angles are observed from measurement to measurement (Figures 4b and 5d−f). These variations are caused by the working temperature of the electronics inside the AFM controller, and phase angles that correspond to PCBM and PTB7 are equally affected. Except for this offset, the shown phase images are quantitatively comparable. Figures 5d−f show that the PCBM again forms flakes similar to the reference sample (Figure 4b). These flakes are embedded in polymer network, which means that PTB7 is located in the space between neighboring flakes. This is confirmed by the fact that the regions with lower phase angle are located not only around PCBM flakes but also on top of them (Figures 5d−f). Furthermore, the distance between flakes (Figures 5d−f) is larger compared to the reference sample (Figure 4b), which also confirms that the polymer material is placed between PCBM flakes. By increasing the content of PCBM, we have estimated that the size of flakes increases from ∼100 nm (1:1 blend in Figure 5d) to ∼200 nm (1:2 blend in Figure 5e) and ∼275 nm (1:3 blend in Figure 5f). Three-dimensional (3D) material-specific morphology of a PTB7:PCBM bulk heterojunction is schematically shown in Figures 5g−i. The top surface of these figures represents an overlay of the measured topographies and phase images. These overlays nicely show that each flake corresponds to PCBM (phase angle is higher) and that PTB7 is located around and on top of the flakes (phase angle is lower). Furthermore, stacking of PCBM flakes in vertical direction can also be observed. Based on these overlays, a schematic of 3D material-specific morphology is created. For the 1:1 blend, PCBM flakes are smallest and PTB7 regions are wrapped around them in every direction. By increasing the PCBM content, the flake size increases and the space between the flakes that is occupied by PTB7 becomes smaller. Proposed 3D material-specific morphology is also able to explain how the surface layers of the film are removed by scratching. The tearing of the film occurs at the vertical boundary between PCBM flakes as the point of the lowest mechanical stability.

Figure 4. (a) Topography and (b) phase image of the reference sample with pure PTB7 (bottom) and PCBM domains (top). (c) Topography and phase image line profiles from bottom to top along direction “A”.

uniform layer but flake-like clusters. The flake boundaries are observed in Figure 4b as darker lines between PCBM regions, indicating different mechanical properties compared to the flake surface. Figure 4c displays an overlay of the surface topography and phase angle from bottom to top part along direction “A” that is depicted in Figures 4a and 4b. The measured phase angle shows that the difference between the PTB7 and PCBM is around 2°, but for the each material the phase angle has scattering of almost 0.3° possibly influenced by measurements noise. Furthermore, the boundary between two PCBM flakes (Figure 4c at position 300−330 nm) is represented by the phase angle that is 0.5° lower than for pure PCBM. The heightto-radius ratio of the PCBM flakes is estimated to be around 0.1. The flake-like formation of the PCBM and accuracy of AFM phase imaging are confirmed by measuring a larger area of the transition region between PTB7 and PCBM. These results can be found in Figure S4. In the next part of the investigation, the material-specific BHJ morphologies of PTB7:PCBM films with weight ratios of 1:1,

4. DISCUSSION In this investigation, we have shown that the highest light absorption occurs for the PTB7:PCBM blend with the weight ratio of 1:1. At the same time, the solar cells fabricated by using this blend exhibit the lowest short circuit current density and highest series resistance. The measured phase images show that PCBM flakes embedded in polymer (PTB7) network are smallest for the 1:1 blend. The overlap between PCBM flakes E


Article


Figure 5. (a−c) Topography and (d−f) phase images within the scratch region of PTB7:PCBM thin films for blends with weight ratio of 1:1 (a, d), 1:2 (b, e), and 1:3 (c, f). (g−i) Overlay of topography and phase images combined with schematic representation of the bulk morphology for 1:1 (g), 1:2 (h), and 1:3 (i) blend.

in the vertical direction is then also small, resulting in a morphology that hinders the transport of the free electrons. Only by applying a larger reverse bias voltage, we can overcome the poor extraction properties, and the extracted current for the 1:1 blend becomes significantly larger (Figure 3). By increasing the PCBM content, the absorption properties of the PTB7:PCBM blend reduce since less PTB7 is present in the film. At the same time, the size of PCBM flakes is increased. Larger PCBM flakes are able to achieve better overlap in the vertical direction, creating a morphology that is favorable for the electron transport. Consequently, the extraction of the free electrons is improved resulting in an increased short circuit current density and lower series resistance, which is observed for the 1:2 blend. However, if the content of PTB7 becomes too small, the light absorption in the film is too much reduced. The short circuit current density starts to drop despite excellent extraction properties of the PCBM flakes, which is observed in the case of the 1:3 blend. By comparing the material-specific BHJ morphologies of the 1:1, 1:2, and 1:3 blends, it can be observed that the dimensions of PTB7 network are sufficiently small so that the photogenerated excitons can reach the interface with PCBM flakes. This material arrangement enables efficient dissociation of photogenerated excitons, and therefore, it is favored for solar

cell applications. At the same time, the polymer network is able to realize effective transport of free holes. However, to extract free electrons, the PCBM flakes have to achieve significant vertical overlap. We can conclude that the performances of the investigated solar cells with reduced content of PCBM (1:1 blend) are limited by extraction of free electrons and not by extraction of free holes or dissociation of photogenerated excitons. The exciton dissociation can become limiting factor only for lower PCBM concentrations. Investigation has also shown that by increasing the PCBM content electron extraction becomes better. This microscopic morphology model could be used in interpretation of macroscopic experiments such as charge extraction by linearly increasing voltage (CELIV) measurements on diodes or metal−insulator−semiconductor structures (MISCELIV measurements) or general transient current measurements.49−52 Finally, for large PCBM contents (1:3 blend), the solar cells performances become limited by the light absorption and not by charge extraction. To improve electron extraction in bulk heterojunction solar cells, different strategies are available. In this study, better electron transport is achieved by increasing the PCBM content. The limitation of this approach is that there is an optimal amount of PCBM that balances restrictions given by poor electron extraction and by low absorption of light. Another F



■

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the German Federal Ministry of Education and Research (BMBF) in the framework of the POPUP project (Grant 03EK3501D).

option to improve the acceptor network conductivity is to use different acceptor materials with better suited microstructure. As an alternative, the electron transport could be improved if the orientation of PCBM flakes is different. Based on the presented results, these PCBM flakes are oriented horizontally in the plane of the film. However, if they could be oriented normal to the plane of the film, only one flake would be needed to transport electrons from ETL to HTL. This could be possibly achieved by using different solvents, annealing temperatures, or additives. The presented material-specific AFM analysis is an ideal tool for further investigations related to optimization of BHJ structure.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01924. Current−voltage characteristics of all fabricated solar cells; AFM images of larger and smaller areas of reference sample and single films with different weight ratios of PTB7:PCBM blends (PDF)

■

REFERENCES

(1) Zhao, J.; Wang, A.; Green, M. A.; Ferrazza, F. 19.8% Efficient “Honeycomb” Textured Multicrystalline and 24.4% Monocrystalline Silicon Solar Cells. Appl. Phys. Lett. 1998, 73, 1991−1993. (2) Shah, A.; Torres, P.; Tscharner, R.; Wyrsch, N.; Keppner, H. Photovoltaic Technology: The Case for Thin-Film Solar Cells. Science 1999, 285, 692−698. (3) Konagai, M. Present Status and Future Prospects of Silicon ThinFilm Solar Cells. Jpn. J. Appl. Phys. 2011, 50, 030001−12. (4) Jackson, P.; Hariskos, D.; Lotter, E.; Paetel, S.; Wuerz, R.; Menner, R.; Wischmann, W.; Powalla, M. New World Record Efficiency for Cu(In,Ga)Se2 Thin-Film Solar Cells Beyond 20%. Prog. Photovoltaics 2011, 19, 894−897. (5) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, 15−26. (6) Lunt, R. R.; Bulovic, V. Transparent, Near-infrared Organic Photovoltaic Solar Cells for Window and Energy-Scavenging Applications. Appl. Phys. Lett. 2011, 98, 113305−1−3. (7) Colsmann, A.; Puetz, A.; Bauer, A.; Hanisch, J.; Ahlswede, E.; Lemmer, U. Efficient Semi-Transparent Organic Solar Cells with Good Transparency Color Perception and Rendering Properties. Adv. Energy Mater. 2011, 1, 599−603. (8) Chen, C.-C.; Dou, L.; Zhu, R.; Chung, C.-H.; Song, T.-B.; Zheng, Y. B.; Hawks, S.; Li, G.; Weiss, P. S.; Yang, Y. Visibly Transparent Polymer Solar Cells Produced by Solution Processing. ACS Nano 2012, 6, 7185−7190. (9) Krebs, F. C. Fabrication and Processing of Polymer Solar Cells: A Review of Printing and Coating Techniques. Sol. Energy Mater. Sol. Cells 2009, 93, 394−412. (10) Manceau, M.; Angmo, D.; Jørgensen, M.; Krebs, F. C. ITO-free Flexible Polymer Solar Cells: From Small Model Devices to Roll-toRoll Processed Large Modules Org. Org. Electron. 2011, 12, 566−574. (11) Søndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen, T. T.; Krebs, F. C. Roll-to-Roll Fabrication of Polymer Solar Cells. Mater. Today 2012, 15, 36−49. (12) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-Conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nat. Photonics 2012, 6, 593−597. (13) Das, S.; Keum, J. K.; Browning, J. F.; Gu, G.; Yang, B.; Dyck, O.; Do, C.; Chen, W.; Chen, J.; Ivanov, I. N.; et al. Correlating High Power Conversion Efficiency of PTB7:PC[71]BM Inverted Organic Solar cells with Nanoscale Structures. Nanoscale 2015, 7, 15576− 15583. (14) Grossiord, N.; Kroon, J. M.; Andriessen, R.; Blom, P. W. Degradation Mechanisms in Organic Photovoltaic Devices. Org. Electron. 2012, 13, 432−456. (15) Deibel, C.; Dyakonov, V. Polymer-Fullerene Bulk Heterojunction Solar Cells. Rep. Prog. Phys. 2010, 73, 096401−39. (16) Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low-TemperatureAnnealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679−1683. (17) Bom, S.; Ortel, M.; Wagner, V. Cyclic Potential Growth Mechanism for Electropolymerized Polythiophenes as Anode Buffer Layers in P3HT-PCBM Solar Cells. Phys. Status Solidi A 2014, 211, 1634−1639. (18) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright Future: Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, E135−E138. (19) Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.; Hinsch, A.; Meissner, D.; Sariciftci, N. S. Nanoscale Morphology of

5. CONCLUSIONS In summary, we have investigated the material-specific BHJ morphology of the solution processed solar cells. As a material system we have chosen the polymer (donor) PTB7 and the fullerene (acceptor) PCBM, which are suitable candidates for large scale applications. In our investigation, we have varied the PTB7:PCBM blend content from 1:1 to 1:2 and 1:3 weight ratio. We have measured the material-specific BHJ morphology by using AFM phase imaging. This method is based on the phase difference between cantilever tip oscillations and excitation (driving) force. This phase shift depends on the dissipated energy during interaction of the cantilever tip with the material. We have obtained material identification by measuring a reference sample composed of pure PTB7 film with a PCBM droplet on top. The method is then employed on PTB7:PCBM films after removing the surface layer. The determined material-specific morphologies are correlated with optical properties of single films and electrical properties of fabricated solar cells. We have shown that PCBM self-organizes into nanoscale flakes that are embedded into PTB7 polymer network and oriented in the plane of the film. This arrangement enables efficient exciton dissociation. For low contents of PCBM (1:1 blend), the limiting factor on the performances of the solar cells is attributed to the extraction of free electrons. The free electrons are transported by PCBM flakes in vertical direction. By increasing the size of PCBM flakes, the vertical overlap can be improved and electron extraction becomes more efficient as it is shown for 1:2 and 1:3 blend. If the content of PTB7 becomes too small (1:3 blend), the limiting factor on the solar cell performances becomes low absorption of light.

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (V.W.). ORCID

Vladislav Jovanov: 0000-0002-2220-1394 Notes

The authors declare no competing financial interest. G


Article

The Journal of Physical Chemistry C Conjugated Polymer/Fullerene Based Bulk-Heterojunction Solar Cells. Adv. Funct. Mater. 2004, 14, 1005−1011. (20) Hoppe, H.; Sariciftci, N. S. Morphology of Polymer/Fullerene Bulk Heterojunction Solar Cells. J. Mater. Chem. 2006, 16, 45−61. (21) Moulé, A.; Meerholz, K. Controlling Morphology in PolymerFullerene Mixtures. Adv. Mater. 2008, 20, 240−245. (22) Chan, S.-H.; Hsiao, Y.-S.; Hung, L.-I.; Hwang, G.-W.; Chen, H.L.; Ting, C.; Chen, C.-P. Morphology Evolution of Spin-Coated Films of Poly(thiophene-phenylene-thiophene) and [6,6]-Phenyl-C71-butyric Acid Methyl Ester by Solvent Effect. Macromolecules 2010, 43, 3399−3405. (23) Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. Processing Additives for Improved Efficiency from Bulk Heterojunction Solar Cells. J. Am. Chem. Soc. 2008, 130, 3619−3623. (24) Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency approaching 100%. Nat. Photonics 2009, 3, 297−302. (25) Karagiannidis, P.; Georgiou, D.; Pitsalidis, C.; Laskarakis, A.; Logothetidis, S. Evolution of Vertical Phase Separation in P3HT:PCBM Thin Films Induced by Thermal Annealing. Mater. Chem. Phys. 2011, 129, 1207−1213. (26) Bundgaard, E.; Krebs, F. C. Low Band Gap Polymers for Organic Photovoltaics. Sol. Energy Mater. Sol. Cells 2007, 91, 954−985. (27) Zhu, Y.; Champion, R. D.; Jenekhe, S. A. Conjugated DonorAcceptor Copolymer Semiconductors with Large Intramolecular Charge Transfer: Synthesis, Optical Properties, Electrochemistry, and Field Effect Carrier Mobility of Thienopyrazine-based Copolymers. Macromolecules 2006, 39, 8712−8719. (28) Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.-T.; Son, H.-J.; Li, G.; Yu, L. Development of New Semiconducting Polymers for High Performance Solar Cells. J. Am. Chem. Soc. 2009, 131, 56−57. (29) Jørgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.; Krebs, F. C. Stability of Polymer Solar Cells. Adv. Mater. 2012, 24, 580−612. (30) García, R.; Tamayo, J.; Paulo, Á . S. Phase Contrast and Surface Energy Hysteresis in Tapping Mode Scanning Force Microscopy. Surf. Interface Anal. 1999, 27, 312−316. (31) Paulo, Á . S.; García, R. Tip-Surface Forces, Amplitude, and Energy Dissipation in Amplitude-Modulation (Tapping Mode) Force Microscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 64, 193411. (32) Paulo, Á . S.; García, R. Unifying Theory of Tapping-Mode Atomic-Force Microscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 041406. (33) García, R.; Magerle, R.; Perez, R. Nanoscale Compositional Mapping with Gentle Forces. Nat. Mater. 2007, 6, 405−411. (34) van Duren, J. K.; Yang, X.; Loos, J.; Bulle-Lieuwma, C. W.; Sieval, A. B.; Hummelen, J. C.; Janssen, R. A. Relating the Morphology of Poly (p-phenylene vinylene)/methanofullerene Blends to Solar-Cell Performance. Adv. Funct. Mater. 2004, 14, 425−434. (35) Tremolet de Villers, B.; Tassone, C. J.; Tolbert, S. H.; Schwartz, B. J. Improving the Reproducibility of P3HT: PCBM Solar Cells by Controlling the PCBM/cathode Interface. J. Phys. Chem. C 2009, 113, 18978−18982. (36) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Polymer Solar Cells with Enhanced Open-Circuit Voltage and Efficiency. Nat. Photonics 2009, 3, 649−653. (37) Gao, H.; Zhang, X.; Meng, J.; Yin, Z.; Zhang, L.; Wu, J.; Liu, X. Quantitative Characterization of Phase Separation in the Photoactive Layer of Polymer Solar Cells by the Phase Image of Atomic Force Microscopy. Thin Solid Films 2015, 576, 81−87. (38) Nagarjuna, P.; Bagui, A.; Hou, J.; Singh, S. P. New Electron Acceptor Derived from Fluorene: Synthesis and Its Photovoltaic Properties. J. Phys. Chem. C 2016, 120, 13390−13397. (39) Martens, T.; D’Haen, J.; Munters, T.; Beelen, Z.; Goris, L.; Manca, J.; D’Olieslaeger, M.; Vanderzande, D.; De Schepper, L.; Andriessen, R. Disclosure of the Nanostructure of MDMO-

PPV:PCBM Bulk Hetero-Junction Organic Solar Cells by a Combination of SPM and TEM. Synth. Met. 2003, 138, 243−247. (40) Ruderer, M. A.; Meier, R.; Porcar, L.; Cubitt, R.; MüllerBuschbaum, P. Phase Separation and Molecular Intermixing in Polymer-Fullerene Bulk Heterojunction Thin Films. J. Phys. Chem. Lett. 2012, 3, 683−688. (41) Riordan, C.; Hulstron, R. What is an Air Mass 1.5 Spectrum? [Solar Cell Performance Calculations]. Proc. 21st IEEE Photovolt. Spec. Conf. 1990, 2, 1085−1088. (42) Gueymard, C.; Myers, D.; Emery, K. Proposed Reference Irradiance Spectra for Solar Energy Systems Testing. Sol. Energy 2002, 73, 443−467. (43) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Accurate Measurement and Characterization of Organic Solar Cells. Adv. Funct. Mater. 2006, 16, 2016−2023. (44) Chegaar, M.; Ouennoughi, Z.; Guechi, F. Extracting DC Parameters of Solar Cells Under Illumination. Vacuum 2004, 75, 367− 372. (45) Ishibashi, K.-i.; Kimura, Y.; Niwano, M. An Extensively Valid and Stable Method for Derivation of All Parameters of a Solar Cell from a Single Current-Voltage Characteristic. J. Appl. Phys. 2008, 103, 094507. (46) Schneider, A.; Traut, N.; Hamburger, M. Analysis and Optimization of Relevant Parameters of Blade Coating and Gravure Printing Processes for the Fabrication of Highly Efficient Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2014, 126, 149−154. (47) Li, N.; Brabec, C. J. Air-Processed Polymer Tandem Solar Cells with Power Conversion Efficiency Exceeding 10%. Energy Environ. Sci. 2015, 8, 2902−2909. (48) Tamayo, J.; García, R. Relationship Between Phase Shift and Energy Dissipation in Tapping-Mode Scanning Force Microscopy. Appl. Phys. Lett. 1998, 73, 2926−2928. (49) Juška, G.; Arlauskas, K.; Viliunas, M.; Kočka, J. Extraction ̅ Current Transients: New Method of Study of Charge Transport in Microcrystalline Silicon. Phys. Rev. Lett. 2000, 84, 4946−4949. (50) Armin, A.; Juška, G.; Ullah, M.; Velusamy, M.; Burn, P. L.; Meredith, P.; Pivrikas, A. Balanced Carrier Mobilities: Not a Necessary Condition for High-Efficiency Thin Organic Solar Cells as Determined by MIS-CELIV. Adv. Energy Mater. 2014, 4, 1300954. (51) Hirwa, H.; Wagner, V. New Insights on Traps States in Organic Semiconductor applying Illumination-Free Transient Current Method. Org. Electron. 2015, 25, 112−120. (52) Yumnam, N.; Hirwa, H.; Wagner, V. Analytical and Numerical Analysis of Charge Carriers Extracted by Linearly Increasing Voltage in a Metal-Insulator-Semiconductor Structure Relevant to Bulk Heterojunction Organic Solar Cells. Submitted.

H


Determining Material-Specific Morphology of Bulk-Heterojunction

Recommend Documents