In-Operando Study of the Effects of Solvent ... - ACS Publications

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In-Operando Study of the Effects of Solvent Additives on the Stability of Organic Solar Cells Based on PTB7-Th:PC71BM Dan Yang,† Franziska C. Löhrer,† Volker Körstgens,† Armin Schreiber,† Sigrid Bernstorff,‡ Jillian M. Buriak,§ and Peter Müller-Buschbaum*,†,∥

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Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, James-Franck-Strasse 1, 85748 Garching, Germany ‡ Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14 km 163.5, AREA Science Park, Basovizza, 34149 Trieste, Italy § Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada ∥ Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstrasse 1, 85748 Garching, Germany S Supporting Information *

ABSTRACT: Degradation of organic solar cells is among the fundamental problems that hinder their successful breakthrough. We probe in-operando the degradation behavior of organic solar cells based on the high-efficiency low-bandgap benzodithiophene copolymer PTB7-Th and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The influence of the solvent additives 1,8-diiodooctane and o-chlorobenzaldehyde on the degradation mechanism is studied with in situ grazing incidence small-angle X-ray scattering during operation. Changes in the IV characteristics are correlated for the first time in situ with changes in the morphology showing that the use of a solvent additive causes a change in device degradation.

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aromatic additive is 1,8-diiodooctane (DIO), which has been applied in various devices using low-bandgap polymers to obtain high efficiencies.27−29 Because of its high bp of 332.5 °C and selective solubility for fullerenes, a highly phase-separated morphology can be developed to balance charge carrier mobilities and reduce charge recombination in bulk heterojunction (BHJ) solar cells.11,30 Recently, o-chlorobenzaldehyde (CBA) with a bp of 212 °C has been used as an aromatic additive, which is beneficial for 200−300 nm thick active layers to form nanoscale morphologies.31 Aside from optimizing the initial PCE of solar cells, their long-term stability is considered as a key for a successful real-world use of organic solar cells (OSCs).23 Therefore, the effect of different solvent additives on the stability of an optimized active layer morphology of OSCs should be identified. In this study, we have fabricated three kinds of devices based on the high-efficiency low-bandgap benzodithiophene copolymer. We studied blends of poly[4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-

rganic photovoltaics show great potential as a costeffective alternative to conventional solar cells, due to their scalability and solution-based manufacturability.1,2 The power conversion efficiencies (PCEs) of lab-scale devices have already exceeded 14%.3−6 Because further increases in efficiency are always desirable in terms of economic viability, much effort has been devoted to optimizing the device morphology, which plays the most important role in attaining high efficiencies.7−9 An ideal nanostructure of interpenetrating p- and n-type domains within a scale of 10− 20 nm is critically important for efficient charge carrier separation and transport.10−12 To achieve a favorable morphology, various approaches,13,14 such as thermal annealing,15−17 solvent annealing,18,19 and the use of solvent additives,20 have been studied.21−23 Among those, solvent additives have been identified as the most effective way to control the morphology with respect to largescale fabrication.11,24−26 In previous studies, the additives were classified as either non-aromatic or aromatic.11 Both types of solvent additives need to fulfill two features. (1) Their boiling point (bp) must be much higher than that of the host solvent, and (2) one component of the blend must be more soluble in the additive than the other. A famous example of a non© XXXX American Chemical Society

Received: November 28, 2018 Accepted: January 10, 2019

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DOI: 10.1021/acsenergylett.8b02311 ACS Energy Lett. 2019, 4, 464−470

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Cite This: ACS Energy Lett. 2019, 4, 464−470

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Figure 1. Horizontal line cuts (black dots) and modeling results (red lines) measured for devices (a) without a solvent additive, (b) with DIO, and (c) with CBA. In-operando measurement after device operation for 0, 3, 10, 20, 40, 60, 90, and 120 min (from bottom to top, respectively). The curves are shifted for the sake of clarity along the y axis. The area shielded by the beamstop is shaded in gray.

Figure 2. Temporal evolution of diameters of small (black) and large (red) polymer structures present in the devices (a) without a solvent additive, (b) with DIO, and (c) with CBA.

positioned below the sample to decrease the temperature of the device as well as the level of photodegradation. Vacuum conditions are applied to further reduce the level of degradation from oxygen and moisture (details are introduced in the Supporting Information). It should be noted that with special encapsulation, in general, the lifetimes of organic solar cells can be improved; however, to match the accessible time scale of a synchrotron radiation experiment, no encapsulation was used. The kinetic changes of the active layer are probed in real time and on a nanometer scale during the degradation due to device operation.30,38,40−44 To avoid scattering signals from the metal contacts of the probed solar cells, the devices are aligned in such a way that the X-rays impinge on the active layer close to the electrodes. Before illumination of the solar cells, an initial two-dimensional (2D) GISAXS data set is measured (denoted as 0 min). During the illumination, current−voltage curves are periodically recorded every 26 s via a source meter for 120 min. In parallel, 2D GISAXS data are taken with an exposure time of 5 s after illumination for 3, 10, 20, 40, 60, 90,

(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate2-6-diyl] (PTB7-Th) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in chlorobenzene (CB) (a) without any solvent additives, (b) with 3 vol % DIO, and (c) with 5 vol % CBA. PTB7-Th can yield record PCE values of ≥10%.30,32,33 In our work, devices without an additive have PCEs of 3.8%, which is a value very close to published results.12,34 The PCEs of devices with a solvent additive are around 8% (see the Supporting Information), which also match the reported results of PTB7-Th:PC71BM solar cells.35−37 Here, we present the first comparative in-operando GISAXS study performed on a series of PTB7-Th-based devices. Our previous work demonstrated that in-operando measurements simultaneously reveal the nanostructures in the active layer and enable its correlation with the photovoltaic characteristics of the respective device.38−41 All in-operando measurements are performed in a homemade measurement chamber,35,37 which is mounted in the Austrian SAXS beamline of the ELETTRA synchrotron source in Trieste, Italy. A pattern mask with eight illumination holes for the individual solar cell pixels is 465


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Figure 3. Time evolution of the photovoltaic characteristics of devices (a) without a solvent additive, (b) with DIO, and (c) with CBA. PCE (black), Voc (red), Jsc (blue), and FF (purple) are normalized to their initial values for the sake of clarity.

of the Jsc, which then dominates the entire device degradation (see Figure S10). Such a decrease in Jsc can be attributed to the increasing polymer domain sizes, which reduces the chances of charge carrier separation and thereby reaching Jsc. A similar behavior was reported by Schaffer et al. for organic solar cells based on P3HT:PC61BM.38 Panels b and c of Figure 2 reveal the temporal evolution of characteristic structure sizes in devices with solvent additives DIO and CBA, respectively. Irrespective of the type of additive, the polymer domain sizes decrease during device operation. It was proposed by Schaffer et al. that such a decrease is caused by the evaporation of the solvent additive.41 As a result, the connection of neighboring polymer domains in the interpenetrating network is lost.41 Such disconnected domains, which are like isolated islands in the active layer, will act as traps for free charge carriers. Therefore, smaller domain sizes result in higher chances of charge carrier recombination, thereby reducing the FF. However, a decrease in domain size can also cause a more efficient exciton splitting, which results in more free charge carriers being transported and collected. Consequently, a trade-off between Jsc and FF would occur in shrinking domains.41 We find this trend also in the present J− V characteristics. Panels b and c of Figure 3 show the timeresolved photovoltaic parameters of the devices with solvent additives DIO and CBA. The FF decays the strongest in the case of the CBA solvent additive, similar to what was reported earlier for OSCs based on PCPDTBT:PC71BM with 1,8octanedithiol (ODT, bp of 270 °C).41 Meanwhile, a slight increase in Jsc can be observed from 80 to 120 min (Figure 3c), which is due to there being more collected free charge carriers from the smaller domains. In contrast, for DIO the FF does not play a key role in the degradation process for the full period under observation. For the first 2400 s, the FF decay dominates the performance of this solar cell. At longer time scales, the decrease in FF is less pronounced, while the Jsc exhibits a strong decrease, which dominates degradation. To determine the content of the residual additive solvent, we calculate the ratio of the intensity around the critical angle of an additive solvent to that of PTB7-Th.37 The temporal evolution of the scattering intensity ratio (Iadd_sol/Ipol) during the in-operando study is summarized in Figure S7. We can observe that both scattering intensity ratios decrease with time, which indicates a loss of residual DIO or CBA during device operation. Moreover, the stronger decay in the intensity ratio

and 120 min. No X-ray radiation-induced damage has been observed during these in-operando measurements, which has been verified by reference measurements (see the Supporting Information). The 2D GISAXS data of devices with and without a solvent additive are displayed in Figure S4. Because the morphological degradation stems mainly from the photoactive layer, horizontal line cuts are performed at the strongest scattering signal (Yoneda region)45 of PTB7-Th at around 0.16° (see the Supporting Information). The respective horizontal line cuts for each device are plotted in Figure 1. The horizontal line cuts from the 2D GISAXS data look very similar on the first view, which indicates small changes in the film morphology during aging (see Figure S5). To extract the changes, a rigorous data analysis via model fitting is needed. Here, the data are modeled on the basis of the effective interface approximate and local monodisperse approximation (details are introduced in the Supporting Information).46,47 The average structure sizes of polymer domains in the active layer are extracted from the fitting of the horizontal line cuts. Figure 2 shows the size evolution of different structures present in the active layer of the studied device. To fit the horizontal line cuts, we need two characteristic structures for films prepared with and without a solvent additive (Figure S6). Thus, the morphology of active layers is modified by the use of a solvent additive as described in the literature.36,48 For the device fabricated without a solvent additive (Figure 2a), the polymer domains grow with an increase in operation time, whereas the devices with a solvent additive show the inverse trend of domain shrinkage. In general, it is known that excitons have an optimal diffusion length in a particular organic material.49,50 When the polymer domain sizes increase, the chances for excitons to reach an interface decrease, which will have a pronounced influence on the solar cell performance.38,51 Figure 3a shows the temporal evolution of the photovoltaic parameters of the solar cell without a solvent additive. In the beginning, the open-circuit voltage (Voc) shows the most pronounced decrease but stabilizes quickly and does not fall below 90% of its original value. In contrast, the decay of the fill factor (FF) and short-circuit current density (Jsc) become the main factors of the degradation after operation for only ∼20 min. In particular, the decay rate of the FF is quickly surpassed by that 466


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investigate the influence of the particular type of solvent additive on device degradation, we separately compare the decay rates of the PCE, Voc, Jsc, and FF of the devices (Figure S10). From Figure S10b−d, it becomes obvious that the degradation of the Voc, Jsc, and FF of the device with DIO additive is more significant than for the other devices. From the IV curves, we know that the device with the DIO additive has the lowest Voc (0.71 V) compared with those of solar cells fabricated without an additive (0.73 V) or with CBA (0.75 V) (Figure S1). The lower Voc can be explained with defect states at the interface between the polymer and PC71BM being dissolved in the residual DIO and thus additive-dependent.62 In Figure S10d, we can see that FF undergoes a large change during the first 2400 s, in which DIO is evaporating. However, the temporal evolution of Jsc is more complex when the device is made with DIO. Because the changes in vertical and horizontal directions inside the active layer all will impact the actual value of Jsc, no easy correlation is found. In general, we conclude that the presence and type of residual solvent additive have a considerable influence on the stability of solar cells. In conclusion, we investigate and compare the degradation of PTB7-Th:PC71BM-based solar cells fabricated without and with different types of solvent additives by in-operando measurements. The solar cell performance is probed simultaneously with structure characterization via GISAXS measurements. Figure 4 schematically summarizes the morphological changes found in the active layers in the solar cells without a solvent additive, with DIO, and with CBA

in the case of DIO doping indicates a larger loss of residual DIO than of CBA. Optical microscopy (OM) images, which extend the morphology information to more macroscopic surface structures, confirm these observations (Figure S8). The few black dots in the OM images appear on the surface of a device with DIO only after the in-operando experiment. It has been widely reported that DIO selectively dissolves PCBM aggregates in the BHJ solution.20,52,53 Figure S8c shows that the dots are originally blurry in the film, but after operation for 2 h in the vacuum chamber, the dots become clear and obvious. This phenomenon is observed only in the devices with DIO, as DIO has the highest boiling point among the applied solvents and it was verified that some residual DIO existed in the film after device fabrication.21,54,55 Moreover, the top surface should contain PC71BM domains due to the kinetics of film formation and processing.56 Therefore, we conclude that the few black dots are ascribed to domains of PC71BM. DIO evaporation causes PC71BM to phase separate and form domains. Without an additive and with CBA, the surfaces do not undergo changes on length scales resolved in the OM images. To further understand the more complex behavior of the devices with DIO (FF decay is dominant only at the beginning of device operation), we analyze the vertical film structure. Vertical line cuts of 2D GISAXS data at qy = 0 are shown in Figure S9. The amplitude of the resonant diffuse scattering along the qz direction increases significantly with time (Figure S9b) for the device with DIO. The increase in these interference fringes with an increase in time indicates the formation of a strongly correlated interface during device operation (details are introduced in the Supporting Information).46,57 The thickness of this correlated layer (d) can be determined by the distance between adjacent minima in the intensity via the equation d = 2π/Δqz,58,59 as the resonant diffuse scattering fulfills the one-dimensional Bragg condition.60,61 In this approach, the significantly smaller qx component is neglected.57 Thus, we conclude that a distinguished top layer is formed on the basis of the GISAXS observation, which has a thickness of 35 nm and might be hardly detectable by other characterization methods. Assuming that DIO is more likely evaporated from the top part of the active layer, we can identify a 35 nm thick layer forming at the film surface by DIO evaporation via the in-operando measurement. After 60 min, the intensity of the resonant diffuse scattering remains almost constant, meaning that this formed layer does not change further during operation of the device for the probed times. Upon analysis of the amplitude of the resonant diffuse scattering for the devices with CBA, a different behavior is observed. Immediately after preparation, the intensity oscillations are present, meaning that the initial film has a layered structure in the vertical direction. Likely, this is induced by CBA evaporation during film preparation. During device operation, slight changes in diffuse scattering are visible in terms of decreases in the amplitudes of the intensity oscillations at larger qz values. This indicates that the interlayer is disappearing, which can be explained by the evaporation of a small amount of residual CBA from the active layer. During device operation, the active layer is becoming more homogeneous. Hereby, we conclude that the residual CBA in the active layer can be almost fully evaporated, which causes the changes in the lateral film structure discussed above accompanied by a continuous degradation of FF. To further

Figure 4. Schematic illustration of changes in the morphology of PTB7-Th:PC71BM solar cells (a) without a solvent additive, (b) with DIO, and (c) with CBA from the initial state (left) to an operated state (right). The different components are indicated. The residual solvent is shown by the color bar. 467


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ACS Energy Letters during operation. Without a solvent additive (Figure 4a), the polymer domains become larger. With solvent additives DIO (Figure 4b) and CBA (Figure 4c), the polymer domains become smaller and more isolated after the residual solvent additives are evaporated during the in-operando process. Because DIO has a higher boiling point, more of it is left in the film compared to CBA. The final film still contains some DIO. When polymer domains in the solar cells without a solvent additive become larger during operation, device degradation is dominated by the decay of Jsc. Thus, the degradation of PTB7Th:PC71BM-based solar cells is analogous to that of P3HT:PC61BM solar cells studied previously. For the P3HT:PC61BM device, no solvent additive was used and P3HT domains became coarser, which caused a Jsc-driven degradation.38 In contrast, PTB7-Th domains in the solar cells with solvent additives become smaller because the residual solvent additives are evaporated from the active layer during the operation process. Initially, connected domains lose their connection when they shrink and charge carriers become trapped in such structures that are not well-connected. Consequently, the level of charge recombination increases and causes degradation driven by a decrease in FF. This kind of device degradation was reported before in the case of PCPDTBT:PC71BM solar cells fabricated with the solvent additive ODT.41 When directly comparing the two different additives (DIO vs CBA), we see differences. Only a part of residual DIO in the active layer can be evaporated during the in-operando experiment from PTB7Th:PC71BM. Moreover, for PTB7-Th:PC71BM films, a vertical film structure is induced by fabrication (CBA) or established during operation (DIO). The presence and temporal change of vertical structures further complicate the morphological degradation process. In summary, in-operando characterizations give us a better understanding of the complex degradation processes in organic solar cells when using low-bandgap polymers and solvent additives. Two major types of morphological degradation processes, namely, domain growth and domain shrinkage, are confirmed for the high-efficiency system PTB7-Th:PC71BM depending on the presence of solvent additives. Such general knowledge will be important for improving the stability of organic solar cells and thereby contribute to a successful realworld use of this class of next-generation solar cells.

MoO3 (∼10 nm) and a layer of Al were deposited successively via thermal evaporation under vacuum conditions of ∼2 × 10−5 mbar for 1 h. The active area of each pixel of the devices was ∼12 mm2.

EXPERIMENTAL SECTION For device fabrication, indium-doped tin oxide (ITO)-coated glass substrates were sequentially cleaned via ultrasonication in Alconox, ethanol, acetone, and isopropyl alcohol, each for 10 min. Afterward, the substrates were dried with N2 and subjected to an O2-plasma cleaning step (Plasma-SystemNano, Diener Electronic GmbH). The ZnO precursor solution, which was prepared by following the process described in an earlier report,34 was spin-cast at 3000 rpm on top of cleaned ITO substrates and subsequently annealed at 150 °C for 30 min in air, resulting in a transparent ZnO thin film with a thickness of ∼30 nm. For the active layer solution, PTB7-Th and PC71BM (1:1.5 ratio, 20 mg/mL) were dissolved in chlorobenzene. The solution was stirred and heated at 70 °C overnight. Later, 3.0 vol % 1,8-diiodooctane (DIO) or 5 vol % o-chlorobenzaldehyde (CBA) was added to the solutions, and they were stirred for an additional 2 h. Then, the active layer (∼100 nm) was cast by spin-coating the solution on the top of the ZnO layer. Finally, a thin layer of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b02311.

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Performance of solar cells, measurement setup, radiation damage test, GISAXS measurements and analysis, optical analysis, vertical line cuts of 2D GISAXS data, and temporal evolution of degradation rates (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dan Yang: 0000-0002-3927-4047 Franziska C. Löhrer: 0000-0002-9530-0115 Sigrid Bernstorff: 0000-0001-6451-5159 Jillian M. Buriak: 0000-0002-9567-4328 Peter Müller-Buschbaum: 0000-0002-9566-6088 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funding from the International Research Training Group 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS), TUM.solar in the context of the Bavarian Collaborative Research Project Solar Technologies Go Hybrid (SolTech), the Excellence Cluster Nanosystems Initiative Munich (NIM), the Center for NanoScience (CeNS), and the Future Energy Systems (https://futureenergysystems.ca; grant number T12P04) of the University of Alberta. D.Y. acknowledges China Scholarship Council (CSC) funding. The authors thank Bernhard Kalis for building the homemade in-operando chamber.

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REFERENCES

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