Three-dimensional observation of a light-soaked photo-reactant layer

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Three-dimensional observation of a light-soaked photo-reactant layer in BTR:PCBM solar cells treated with/without solvent vapor annealing Yu Jin Kim, Won Suk Shin, Chang Eun Song, and Chan Eon Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02871 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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ACS Applied Materials & Interfaces

Three-dimensional observation of a light-soaked photo-reactant layer in BTR:PCBM solar cells treated with/without solvent vapor annealing

Yu Jin Kim1,a, Won Suk Shin2, Chang Eun Song2* and Chan Eon Park1*

1

POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Poha

ng University of Science and Technology (POSTECH), Pohang, 790-784, Republic of Korea. 2

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT),

141 Gajeongro, Yuseong, Daejeon, 34114, Republic of Korea.

a

Current address: Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass

Ave. Lemont, IL 60439, USA.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract A key challenge to the commercialization of solution-processed solar cells is a proper understanding of morphological variations during long-term periods, particularly under lightsoaking conditions. Many research groups have competitively reported solvent vapor annealing (SVA)-treated small molecule devices with efficiency rates exceeding 11%; however, their light-soaking effects were rarely studied. Here, we investigate the morphological changes of light-soaked devices with/without SVA treatments depending on the illumination time via three-dimensional observations. From the results, we found that the trends of morphological variations differ in the surface and bulk parts of the active film and that the difference is closely related to the device performance capabilities. Therefore, our research will enhance the underlying knowledge of the light-soaking effect on active morphologies over the long term.

Keywords Small molecule solar cell, Solvent vapor annealing, Light-soaking, long-term stability, Crystalline structure


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Introduction Given the rapidly increasing power conversion efficiency rates (PCEs) to more than 11%, organic solar cells (OSCs) have become a promising class of solar-power-generation devices to meet the growing demand for clean and renewable resources1-3. Recently, tremendous levels of attention regarding the lifetimes of plastic photovoltaics have made this issue central to their commercialization, with both light and heat shown to be stress factors that can induce device degradation4-5. Clearly, OSCs are exposed to continuous illumination and are monitored over time; thus, extrinsic degradation is known to affect the deformation of photoactive layers significantly, ultimately affecting their lifetimes as well.4-7 Therefore, to develop OSCs with good long-term stability of the external factors, a thorough understanding of this type of degradation is essential. Recent attention has focused on solvent vapor annealing (SVA) treatments of active layers, particularly for small-molecule solar cells (SMSCs) to obtain a dramatic effect with regard to device performance levels. Indeed, the highest PCEs were achieved in SVA-treated SMSCs, and these have since been upgraded.8-12 However, most researchers have focused on the efficiency rates of these devices considering their short-term performances. They rarely studied variations in SVA-treated films caused by light-soaking or the links to device results from a long-term perspective. Thus, the origin of light-driven morphological variations remains unclear. Studies of the kinetics of the morphological changes are therefore needed. In this paper, we investigate morphological variations by light degradation depending on the illumination time in SMSCs. To understand this in detail, we evaluated various active layers from a three-dimensional perspective. To do this, we fabricated SMSCs using BTR and fullerene [6,6]-phenyl C70-butyric acid methyl ester (PC71BM) as the semiconducting materials (Figure 1a). We chose the BTR compound because it has a distinct effect of a SVA treatment (in other words, the device performance varies dramatically from the as-cast to the



SVA treatment conditions)13; hence, it offers a clear point for comparison. Furthermore, we investigated light-soaking degradation in both conventional and inverted device architectures (Figure 1b). To the best of our knowledge, very few studies have analyzed and investigated 3D light-soaked SM films. Notably, these findings provide a promising way to develop SVAprocessed SMSCs with high efficiency rates and long device lifetimes towards practical applications.

Results and discussions Photovoltaic performances according to the light-soaking time (as-cast versus SVA) We initially consider the impact of light-soaked active layers with/without SVA on the performance of benchmark BTR:PC71BM photovoltaic cells. We used AM 1.5G irradiation with light illumination times of 0, 2, 5, 10, 30, or 60 min. The photocurrent-voltage (J-V) curves and the device performance parameters of the resulting solar cells are shown and summarized in Figure 2 and Table 1-2, respectively. Before considering device performances depending on the light-soaking time, we found that SVA-treated devices with tetrahydrofuran (THF) showed considerably higher performance results (efficiency rates which were two to three times higher) than pristine cells in both types of devices, i.e., conventional BTR:PC71BM devices with PCE = 3.97 % and with an open-circuit voltage (Voc) of 0.99 V, a short-circuit current (Jsc) of 10.08 mA cm-2, and a fill factor (FF) of 40 % without a SVA treatment, and PCE = 8.85 % with a Voc of 0.94 V, a Jsc of 13.39 mA cm-2, and a FF of 71 % for SVA-treated cells. Inverted devices were studied as well, with PCE = 1.57% and with a Voc of 0.87 V, a Jsc of 5.70 mA cm-2, and a FF of 32 % for pristine cells, with PCE = 6.76 % and with a Voc of 0.83 V, a Jsc of 13.52 mA cm-2, and a FF of 61% for SVA-processed cells. These results indicate that the BTR material is also a very sensitive compound for SVA treatments with other commercial small molecules, such as DR3TBDTT14 and DR3TSBDT15.


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In particular, in the SVA-treated devices with conventional structures, the device results (8.85 % PCE) showed values similar to those in a study by Jones et al.13, which implies that our data are quite reasonable. Furthermore, the veracity of the photocurrent values from the above results was proved from the external quantum efficiency (EQE) spectra, shown in Figure S1. Interestingly, when the device was soaked with light, the performance outcomes gradually decreased depending on the exposure time in both devices. Most importantly, a greater reduction was observed in the SVA-treated devices. When the light-soaking time was 60 min, the pristine cell with a conventional geometry showed a PCE of 2.45 %, whereas that of a SVA-treated cell was 2.68 % (see Table 1). Likewise, in inverted samples, devices without a SVA treatment showed a PCE of 1.31 %, whereas those which underwent a SVA treatment revealed a PCE of 3.16 % (Table 2). Therefore, the variation in light-soaked cells with SVA is close to 60 %, whereas that in the devices without SVA appears to be around 25 %. To confirm the data visually and systematically, we investigated the variation of each parameter as a function of the light-soaking time (Figure 3) (we indicate the results relative to normalized values). First, we can clearly confirm that all resulting parameters gradually declined as the illumination time was increased, regardless of the device type or SVA treatment status. This important point was also mentioned above and suggests that the parameters are considerably influenced by light damage. In other words, the BTR molecule with PC71BM is very sensitive to light exposure, resulting in strong driving force to move due to the light energy. Briefly, to support this statement, we investigated the variation of device performances under dark condition (Table S1). In both as-cast and SVA-treated devices, their performance parameters were mostly kept, which means that the materials our chose are very active to the light energy. Regarding the Voc factor, the variation was not great for both device types with/without SVA. Surprisingly, however, the Jsc and FF factors both showed



sharp drops, and the PCE values also showed rapid variations. This indicates that the different efficiency rates are mainly associated with the photocurrent and fill factor parameters, meaning that the morphological properties of active films, including the molecular packing structure and domain size, most likely change. More interestingly, we hypothesize that larger morphological changes may be revealed in SVA-treated films because these samples showed greater illumination time-dependent variations. Thus, we discuss these morphological aspects below with lateral and vertical morphology analyses. Additionally, for a clearer understanding of the light-soaking effect on the device performance, we examined encapsulated cells treated with UV-curable epoxy and cover slide glasses. Because they were mostly protected by external factors such as oxygen and water,16 we can mainly consider the illumination effect on the devices. Figure S2 and Table S2 show the resulting data for conventional cells, while that of the inverted devices are presented in Figure S3 and Table S3. Similarly, most performance trends followed the unencapsulated device results, i.e., greater variation of the parameters according to the time-dependent lightsoaking in the SVA-treated samples, and Jsc and FF factors which show greater decreases than the Voc values in both the conventional and inverted devices with/without SVA. Therefore, also considering non-external factors, the influence of light is very strong in the BTR:PC71BM devices, particularly in the solvent-annealed cells. To confirm whether these properties only arose in the BTR:PC71BM devices, we also investigated encapsulated DR3TSBDT:PC71BM devices (Tables S4-S5 and chemical structure of DR3TSBDT is shown in Figure S4).15 These outcomes also showed higher performance capabilities in SVA-treated films, but upon exposure to light, their performance results rapidly decreased. Larger variations were observed in both conventional and inverted cells upon a SVA treatment. From the data, for small molecules showing dramatic effects after the SVA treatment, we can carefully argue that they significantly show a strong response to light exposure. Therefore,


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small molecules having long-term stability against light response may be necessary in the future.

Lateral surface morphology analysis - AFM Previously, we surmised that the dramatic variations of the photocurrent and fill factors caused by light-soaking are due to morphological changes of the active materials. In order to validate this hypothesis, we performed a lateral surface morphology analysis of light-soaked BTR:PC71BM films with a controlled illumination time. Figure 4 shows atomic force microscopy (AFM) topographic images of the as-cast (upper panel: a-e) and SVA-treated (lower panel: f-j) films. First, all layers showed sufficiently homogenous surface morphologies. However, the SVA-treated films exhibited more aggregated surfaces with larger domain sizes regardless of the illumination time. This higher phase-separated morphology was proved by the RMS roughness (1.56 – 3.97 nm range). With regard to the degree of morphological variation as a function of the light-soaking time, there were no dramatic changes visually in both the as-cast and SVA-treated films. However, the degree of roughness variation was found to be quite high in the SVA-treated films; the variation ranges from 0.34 nm to 0.39 nm for the pristine films, whereas that in the SVA-processed films is from 1.56 nm to 3.97 nm. Thus, we can roughly match the rapidly reduced performance outcomes in the SVA-treated devices to the phase separation caused by the non-ideally packed molecular domains.

Vertical bulk morphology analysis 1- ToF-SIMS In the above sections, we explored the lateral morphology of the BTR:PC71BM active layers; however, they only showed information about the surfaces. Thus, to understand the bulk morphology, particularly in relation to the 3D morphological characteristics, we investigated



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the active layers via time-of-flight secondary ion mass spectroscopy (ToF-SIMS) measurements. For these measurements, a 25 keV Bi3+ primary ion source with a 0.28 pA current was utilized.17 The negative elemental isotopes

12

C,

16

S, and

14

Si were analyzed

because S- and Si- are only in the BTR and substrates, respectively, and they can be regarded as indicators of the vertical composition in the active layer and substrate. Figure 5 shows the ToF-SIMS results along with the sputter time of the active films with/without SVA treatments. Regarding the Si- signals, they rapidly increased beyond a sputter time of 4000 s in all activecoated substrates, indicating that the active films have fairly similar thicknesses regardless of the SVA treatment and/or the light exposure time. In other words, the thicknesses of the BTR:PC71BM films were rarely varied by the external treatments; this was partially proved by examining the AFM sections. Furthermore, the S- intensity was significantly stabilized without any rapid drop in the sputter time range of 0 s to 4000 s. These results suggest that BTR molecules were uniformly distributed overall in the blended films.18-19 Through the Csignal features, it was also proved that the PC71BM compounds with BTR were evenly formed. To confirm the BTR distribution in the 3D structures visually, we conducted a 3D rendering analysis of the S- ions. The conclusions pertaining to the 3D morphology are summarized graphically in Figure S5 (pink and dark brown indicate the existence and absence of BTR molecules, respectively). Regardless of the SVA and/or light-soaking process, BTR existed evenly in the 3D vertical distribution, which feasibly supports the aforementioned depth profile results.

Vertical bulk morphology analysis 2- 2D-GIWAXS In order to explore in more detail the 3D morphology that governs the SVA and lightsoaking processes in our systems, we utilized the two-dimensional grazing incidence wideangle X-ray scattering (2D-GIWAXS) technique. To understand the molecular crystalline


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structures in 3D, we investigated two morphology sections of the surface and bulk. To do this, we used two different incident angles ( = 0.078 and 0.132°), where the X-ray angle with equal to 0.078° ( < critical incidence angle, = 0.105°) penetrates the near-surface region of the layer and the X-ray angle with equal to 0.132° ( > ) is diffracted on the bulk section20-21 (two incident angles including critical point were determined by Yoneda peak curve and penetration depth profile22-24 – Figure S6) (these are correspondingly shown via schematic illustrations in Figures. 6 and 7). We initially analyzed pristine BTR:PC71BM films. These results are shown in Figure 6. Regarding the surface 2D-GWIAXS image (Figure 6a), the diffraction revealed two strong halo peaks at qxy,z ≈ 0.32 Å−1 and qxy,z ≈ 1.36 Å−1, indicating that the BTR and PC71BM molecules are randomly packed in the film.21 Similar results were also noted for the bulk morphology (Figure 6f). Interestingly, when the active layer was soaked with light, the molecular structure, particularly the BTR, was very slightly rearranged in both parts; in the surface region (Figures 6b-e), the BTR molecule showed more ordered structures with the edge-on orientation ((200) and (300) vertical diffraction peaks were more distinctly observed).25-26 In the bulk parts (Figures 6g-j), the first-scattering random diffraction (qxy,z ≈ 0.32 Å−1) was brighter with higher intensity levels, indicating that the rearrangement of the BTR molecules led to more random structures.25,27 The UV-vis absorption curves can support the result (Figure S7). All data were confirmed in one-dimensional extraction profiles (Figure S8) in detail. From the data, we can thereby suggest that the BTR:PC71BM films without a SVA treatment have structures that are considerably randomly oriented. Moreover, despite the fact that they were soaked with light, there is no dramatic variation in the surface or bulk crystalline morphologies, which can also be confirmed in the PL results (Figure S9). Surprisingly, however, the crystalline structure of SVA-treated BTR:PC71BM shows a considerably well-ordered morphology along the x, y, and z axes. Clear (h00) vertically



oriented peaks up to (300) and π-π stacking and (010) diffractions with face-on and edge-on orientations were noted. These reflections are revealed in both the surface and bulk regions (Figures 7a and 7f), demonstrating why the SVA-processed BTR:PC71BM showed significantly higher device performances, particularly with regard to the photocurrent and fill factor values compared to those of the as-cast system.13,28 Most of all, once the films were light-soaked, especially for more than 10 min, the molecular packing structures varied greatly. For the surface section, the (h00) diffractions were prominent, with larger spots and higher scattering intensity levels, and the (010) peak along the in-plane direction was strongly diffracted, indicating that the BTR molecules were more organized with ordered structures in the perpendicular and horizontal directions.25 From these data, we expected that the lightsoaked devices have higher performance capabilities given their well-stacked molecules29; however, the results showed the opposite. Thus, the reason for the lower performances can be found in the bulk crystalline morphology. Clearly, in the bulk section, the crystalline patterns showed trend which were opposite to those of the surface; all scattering peaks except the PC71BM halo diffraction exhibited less ordered packing structures with lower reflected intensity levels. In particularly, the (010) peak was rarely found. Although the BTR molecules were more oriented in the surface region, their crystallinity was collapsed in the bulk region. Regarding the (010) face-on stacking peak, it plays an important role in the perpendicular movement for charge carriers because the cells were vertically stacked. Thus, inefficient charge transport may arise in our systems.30 In addition, the data can be supported by TEM images of pure BTR and PC71BM films; the BTR molecules had smaller aggregated domains under longer illumination time, whereas the PC71BM compounds kept their random structure (Figure S10). In the UV-vis absorption and PL data, we can also find that these bulk morphologies are considerably reflected on the molecular absorption feature and carrier quenching results (Figure S11). For this phenomenon, we can guess that the longer


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illumination time provides larger heat exposure to the device, which leads to larger molecule movement. The BTR compound has larger driving force than PC71BM because BTR has a glass transition (Tg) of 38 °C (at mixed amorphous phase, 2/3 of the clearing temperature of 193 °C) and that of PC71BM is 163 °C.31 The energy by the heat and light allows a larger moving behavior of the BTR molecule, thereby different morphologies were occurred. Therefore, we can match the lower performances of the light-soaked devices with the SVA treatments to the less-oriented BTR packing structures in the bulk section. Through a 3D morphological analysis, we can distinctly find and understand the relationship between the performances and molecular structures in the light-soaked photovoltaic cells via 3D observations, which marks likely the first time this has been accomplished in the solar cell field. In addition, to understand how the BTR crystalline structure in the bulk region differed quantitatively depending on the light-soaking time, we calculated the mean crystallite size via Scherrer’s formula with the full width at half maximum (FWHM) of a (200) diffraction peak (qz ≈ 0.67 Å−1). The equation is ( ) = /FWHM, where P is the dimensionless shape function, 0.9, and λ is the X-ray beam wavelength32. The FWHM values were obtained from a Gaussian fitting (Figures 8a-b). The estimated BTR crystallite size gradually decreased: 67 Å for 0 min, 53 Å for 5 min, 41 Å for 10 min, 39 Å for 30 min, and 36 Å for 60 min. The transmission electron microscopy (TEM) results support above data (Figure S12). They also explain the lower device results as a function of the illumination time, as the smaller crystallite size is relatively unfavorable for charge carrier transport.33-36 Furthermore, when we investigated the crystallite orientation, the BTR crystal showed a fairly different orientation formation according to the light-soaking time. Clearly, the BTR crystallites showed vertically stacked structures (Figure 8c), but as the light-soaking time was increased, the molecules were relatively tilted-stacked in the substrate direction.29 The degree



of the tilt orientation can be observed in Figure 8d in detail; for the non-light-soaked film, the azimuthal distribution was broader and that of the light-soaked films, which showed a narrower shape. This means that the BTR crystallite was relatively tilted toward the horizontal direction in the light-soaked films exposed for 0 minutes, whereas for those exposed for 5 minutes and beyond, the molecules were vertically stacked without any tilted formation.37-38 As mentioned above, the stronger face-on stacking features allow more efficient charge carrier movement39-40; thus, the lower device efficiency results gradually appear in the following sequence: 0 min > 5 min > 10 min > 30 min > 60 min. Therefore, we can also find a trend in the performance results with the BTR crystallite orientation in the bulk morphology.

Charge carrier behavior To support the above statements regarding the relationship between less ordered BTR crystalline structure, smaller domain size, and relatively less titled molecules and lower device performances, we investigated charge hole carrier transport properties of BTR:PC71BM films under various light-soaking time using space-charge limited current density (SCLC) measurement. Hole mobilities were determined in a hole-only device, [ITO/PEDOT:PSS/BTR:PC71BM/Au (100 nm)], and estimated from the Mott-Gurney equation.41 When we first investigated the mobility of our devices with as-cast and SVAtreated conditions (Figure 9a), we found that they showed quite similar values (as-cast cell: 1.8 ×10-4 cm2 V-1 s-1 and SVA-treated cell: 9.2 ×10-4 cm2 V-1 s-1) and trend with the literature reported by Jones’s group13, indicating that our data are quite reasonable. Under different illumination time conditions, the mobility gradually decreased as the time increased; 9.2 × 10-4 cm2 V-1s-1 for 0 min, 8.7 × 10-4 cm2 V-1s-1 for 5 min, 8.0 × 10-4 cm2 V-1s-1 for 10 min, 5.1 × 10-4 cm2 V-1s-1 for 30 min, and 3.9 × 10-4 cm2 V-1s-1 for 60 min (Figure 9b). It


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suggests that more random stacked BTR molecules in the bulk morphology indeed lead inefficient charge carrier movement, resulting in lower device performances.42-43 Therefore, we can argue that the mobility data sufficiently support the above results. In addition, to investigate charge recombination probabilities as well as exciton dissociation and charge collection efficiencies in the SVA-treated devices under different illumination time, photocurrent (Jph, defined as JL (illumination current) - JD (dark current)) versus effective voltage (Veff, defined as V0 (the voltage at which Jph = 0) – V (the applied voltage)) analysis was conducted.44 From Figure 9c, we can find that the Jph linearly response to the higher voltage at a low Veff region. At a high Veff section, the Jph value was saturated for the devices with 0 and 5 min exposure time, however it was gradually increased in the devices with over 10 min exposure condition. This indicates that the recombination rate was minimized by the high internal electric field in the 0 or 5 min illumination condition, however it was gradually increased beyond 5 min illumination time. Because the Veff is quite associated with the charge carrier extraction and transport, we can demonstrate the recombination probability for the curve features.45 When we investigated Jph/J at 2V, the value appears to 0.94 for 0 min, 0.92 for 5 min, 0.88 for 10 min, 0.87 for 30 min, and 0.84 for 60 min under the short-circuit conditions. It means that the device with longer light exposure time has less charge collection efficiency with smaller exciton dissociation rate.44-45

Conclusion In conclusion, we conducted a detailed study of burn-in behavior via light-soaking in BTR:PC71BM photovoltaic devices with/without SVA treatments, particularly from a threedimensionally morphological perspective. We discovered that light-soaked devices regardless of whether a SVA treatment was done showed gradually decreased performance results depending on the illumination time, with the degree of degradation greater in the SVA-treated



devices. Among the performance parameters, the photocurrent and fill factor values showed more dramatic variation; thus, we hypothesized that these results were mainly related to morphological changes. Indeed, when we investigated the BTR:PC71BM morphology, particularly from a vertical viewpoint, the morphological aspects, including the molecular packing structure, degree of ordering, and crystallite orientation, showed greater changes in the SVA-treated devices as a function of the light-soaking time. Interestingly, as the light exposure time increases, the BTR molecules became more ordered in the surface region, whereas in the bulk section, the corresponding stacking structure was relatively collapsed with a smaller crystalline size without any tilted features toward a related face-on orientation. This indicated why the light-soaked devices treated with SVA showed lower performance outcomes with rapid degradation, a significant point in the SVA-treated small-molecule solar cell research from a long-term perspective.

Experimental section Materials BTR (product # YY10059) and DR3TSBDT (product # NK1503) and PC71BM (99%) were obtained from 1-Material, Inc. and Nano-C Co., Ltd., respectively, and were used as received. Polyethylenimine ethoxylated (PEIE) (Mw = 70,000 g mol-1) were received as 50 wt% aqueous solutions from Sigma-Aldrich. Other halogenated/non-halogenated solvents for cleaning, as the active solution, and for solvent vapor annealing (acetone, iso-propanol, chloroform (CF) and tetrahydrofuran (THF), 99% anhydrous) were purchased from SigmaAldrich Inc. and were used without further purification.

Solution preparation The donor-acceptor mixed active solutions were placed and stirred on a hot plate with a


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temperature at 30 °C and a stirring condition of 400 rpm. BTR:PC71BM and DR3TSBDT:PC71BM blends were created at a 1.0:1.0 weight ratio (concentration = 48 mg ml-1, 24 mg:24 mg) and 1.0:0.8 ratio (concentration = 20 mg ml-1, 11.1 mg:8.89 mg), respectively, in 1 ml of chloroform. The blended solutions were stirred for 1 h in a nitrogen full-filled glove box prior to spin-coating.

Solar cell fabrication and measurement Indium tin oxide (ITO)-coated glass substrates were cleaned in warm detergent, deionized water, acetone and iso-propanol in an ultrasonic bath successively, followed by UV-ozone treatment for 20 min. For the conventional architecture devices, an aqueous solution of PEDOT:PSS (Clevios P VP AI 4083) was spin-cast at 4000 rpm for 60 s onto the substrates and baked at 120 °C for 20 min on the hot plate. For the inverted structure devices, a ZnO nanoparticle solution (the particles were dispersed in iso-propanol) was dropped onto the substrates and spun at 4000 rpm for 40 s. Subsequently, the ZnO-coated glass substrates were baked at 100 °C for 10 min on the hot plate. In addition, the PEIE layer was deposited onto the ZnO films from the PEIE solution at 4000 rpm for 60 s. Both substrates were coated with two different inter-layers and were then transferred to a nitrogen glovebox. The homogeneously mixed active solutions, BTR:PC71BM and DR3TSBDT:PC71BM, were spincast onto both the conventional and inverted architecture substrates at 3000 rpm for 30 s and 1000 rpm for 40 s, respectively. The thicknesses of the deposited active layers were ca. 278 nm for BTR:PC71BM and ca. 230 nm for DR3TSBDT:PC71BM. As a comparison group, SVA treatments were performed and BTR:PC71BM active-coated substrates were placed into a petri dish filled with 5 mL of THF solvent, and DR3TSBDT:PC71BM substrates were put into a petri dish with 5 mL of CF solvent. After sealing the petri dishes, the two active-coated substrates were indirectly exposed to THF and CF solvent vapor, respectively, at various



times. Finally, conventional devices used ca. 100 nm of aluminium as the cathode, and inverted devices used ca. 5 nm of molybdenum oxide and ca. 100 nm of silver as the anode. These metal electrodes were thermally deposited at pressures of less than 2 × 10-6 Torr. For the encapsulated devices, the solar cells were encapsulated by UV-curable epoxy (Epotexk OG112-6 by Epoxy Tech. Inc.) with cover glasses under a nitrogen atmosphere. The J-V characteristics were recorded using a Keithley 2400 source meter under 1 sun illumination of AM 1.5G at 100 mW cm-2 in air from a Newport solar simulator with an Oriel white light source calibrated by a silicon reference cell. The light-soaking treatment was done using the same lamp source, where the devices were continuously illuminated, and the data were periodically acquired. During the testing process, each cell was carefully masked by a calibrated mask to prevent excess photocurrent from being generated from the parasitic device regions outside the overlapped electrode area. The EQE spectra were acquired using a photo-modulation spectroscopic setup with a 100 W tungsten lamp light source and a monochromator (Oriel Instruments, Stanford, CT, USA). A Thorlabs SMR05 silicon photodiode with a known spectral response was used to calibrate the EQE values.

Hole-only device fabrication Fabrication of hole-only devices for the mobility measurement was followed by above solar cell device procedure except for the top cathode, Au. The Au layer was thermally deposited onto the active layer-coated ITO substrates at a base pressure of 1.8 × 10−6 Torr with ca. 100 nm.

Active film characterization Film thicknesses were determined using an ellipsometer (J. A. Woollam. Co., Inc.). The surface morphologies of the active films were characterized by AFM (tapping mode, Digital


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Instruments Multimode IIIa). The vertical active morphology (depth profiling) was analyzed by TOF-SIMS using a SIMS V instrument (ION-TOF GmbH, Germany) at the National Nano Fab Center of the Korea Advanced Institute of Science and Technology. 2D-GIWAXS measurements were conducted in a grazing-incidence geometry at the 3C beamline (SAXS I) (X-ray beam wavelength = 0.1213 nm) of the Pohang Accelerator Laboratory (Pohang, Korea). The bulk morphologies of the active films were investigated by TEM (JEOL-2100F, 200 kV accelerating voltage). UV-vis absorption spectra were obtained from Carry-50 spectrophotometer. Photoluminescence measurement was conducted on a calibrated fluorescence spectrophotometer (FP-6500, JASCO).

Associated content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:~. EQE spectra, encapsulated BTR:PC71BM device performance results, DR3TSBDT molecular structure, 3D structure, 1D profiles and TEM images of pure BTR, PC71BM and BTR:PC71BM films, and Yoneda peak curve.

Author Information Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements This research was supported by a New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean Government



through the Ministry of Knowledge Economy (20123010010140). This work was also supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015M1A2A2056214).

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Figure 1. (a) Small molecule BTR and fullerene derivative PC71BM were used as the donor and acceptor semiconducting materials, respectively, in the photovoltaic cells. (b) Both conventional (left) and inverted (right) device architectures were monitored by timedependent light-soaking.


Page 24 of 35

2

(a) w/o SVA -2 -4 0 min 2 min 5 min 10 min 30 min 60 min

-6 -8 -10 0.2

0.4

Current Density (mA/cm )

2

0

0.0

0.6

0.8

0

-6

0 min 2 min 5 min 10 min 30 min 60 min

-9 -12 -15

1.0

(b) w/ SVA

-3

0.0

0.2

0

2

(c) w/o SVA -2 -4 -6


-8 -10 0.0

0.2

0.4

0.6

0.4

0.6

0.8

1.0

1.2

Voltage (V)


Voltage (V)

Current Density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 25 of 35

0.8

1.0

0

(d) w/ SVA -3 -6


-9 -12 -15

0.0

0.2

Voltage (V)

0.4

0.6

0.8

1.0

1.2

Voltage (V)

Figure 2. Light-soaking effects on photovoltaic performance (J-V plots) for as-cast (a and c) and SVA-treated devices (b and d) under AM 1.5G sunlight. First row (a and b) is for conventional structure devices and second row (c and d) is for inverted structure devices.



Figure 3. Evolution of normalized Voc (a and e), Jsc (b and f), FF (c and g) and PCE (d and h) values of conventional device architectures (left column) and inverted device architectures (right column). All devices were degraded under AM 1.5G simulated illumination from 0 min to 60 min in atmosphere condition.


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Figure 4. AFM morphology images (5 µm × 5 µm) of BTR:PC71BM samples without (ae)/with (f-j) SVA treatments as a function of light-soaking time: (a and f) 0 min, (b and g) 5 min, (c and h) 10 min, (d and i) 30 min, and (e and j) 60 min.


ACS Applied Materials & Interfaces active layer

(f) 1.00x105

Intensity (arb. units)


active layer

Si

(a) 7.50x104 5.00x104 2.50x104 C−

S−

Si−

0

1000

2000

3000

7.50x104 5.00x104 2.50x104 C−

0

4000

1000

Sputter time (s)

(g)



1.00x105 7.50x104 5.00x104 2.50x104 S−

Si−

0

1000

2000

3000

4000

5.00x104 2.50x104 C−

S−

Si−

0

1000

2000

3000

(h) 1.00x10 Intensity (arb. units)

7.50x104 5.00x104 2.50x104 C−

S−

4000

Sputter time (s)

5


4000

7.50x104

5000

(c) 1.00x10

Si−

0.00

5

7.50x104 5.00x104 2.50x104 C−

S−

Si−

2000

3000

0.00 0

1000

2000

3000

4000

0

1000

Sputter time (s)

(i)

1.00x105 7.50x104 5.00x104 2.50x104 C−

S−

4000

Sputter time (s)



3000

1.00x105

Sputter time (s)

1.00x105 7.50x104 5.00x104 2.50x104 C−

Si−

S−

Si−

0.00

0.00 0

1000

2000

3000

4000

0

5000

1000

(j) Intensity (arb. units)

1.00x105 7.50x104 5.00x104 2.50x104 C−

S−

2000

3000

4000

Sputter time (s)

Sputter time (s)

(e)

2000

Si−

0.00

0.00

(d)

S−

Sputter time (s)

(b)

C−

Si

1.00x105

0.00

0.00


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

1.00x105 7.50x104 5.00x104 2.50x104 C−

Si−

S−

Si−

2000

3000

0.00

0.00 0

1000

2000

3000

4000

5000

0

1000

Sputter time (s)

4000

Sputter time (s)

Figure 5. Negative ion ToF-SIMS depth profiles (C-, S- and Si-) for BTR:PC71BM layers with photoinduced degradation according to different illumination time (0 min: a and f, 5 min: b and g, 10 min: c and h, 30 min: d and i and 60 min: e and j). BTR:PC71BM layers were treated with SVA (f-j) or not (a-e) and deposited on a silicon substrate.


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Figure 6. 2D-GIWAXS images of pristine BTR:PC71BM films (blend films were light-soaked by AM 1.5 illumination to different ray of light time: 0 min (a and f), 5 min (b and g), 10 min (c and h), 30 min (d and i) and 60 min (e and j)). Data are the diffractions in which the incidence angle of X-ray was set to be 0.078° (below the critical angle of the film) and 0.132° (above the critical angle), to scatter the surface and bulk region of the film, respectively.



Page 30 of 35

Figure 7. 2D-GIWAXS patterns of BTR:PC71BM blend films with SVA treatments, which were light-soaked under AM 1.5G simulated sunlight (illumination time: 0 min (a and f), 5 min (b and g), 10 min (c and h), 30 min (d and i), and 60 min (e and j)). For surface section patterns, the scattering images were obtained at the incidence angle of 0.078° and for that of bulk region, the incidence angle (αi = 0.132°) of the X-ray beam is used.


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Figure 8. (a) 1D out-of-plane profiles extracted from 2D-GIWAXS scattering results of bulk parts for SVA-treated films and (b) their magnified curves with multiple Gaussian fitting lines. (c) Azimuthal angle distribution for (200) peak obtained from Figure 7f-j and (d) their corresponding magnified results range from 50 to 130 degree.



Figure 9. Charge hole carrier mobilities of BTR:PC71BM devices: (a) as-cast vs. SVA-treated cells and (b) under various light-soaking time. (c) Photocurrent trend of the SVA-treated devices as a function of effective voltage plots.


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Table 1. Photovoltaic parameters of conventional BTR:PC71BM solar cells without/with SVA treatments under AM 1.5G illumination light-soaking time.

Active layer

SVA

Time [min]

VOC [V]

0

JSC

[mA/cm ]

FF [%]

PCE [%]

0.99±0.01

10.08±0.75

40±0.32

3.97±0.25

2

0.97±0.02

9.75±1.03

38±0.41

3.63±0.36

5

0.97±0.02

9.57±0.92

38±0.25

3.52±0.32

10

0.97±0.01

9.47±0.83

37±0.18

3.42±0.12

30

0.96±0.01

9.34±0.79

36±0.22

3.22±0.18

60

0.96±0.02

8.06±0.77

32±0.16

2.45±0.20

0

0.94±0.01

13.39±1.10

71±0.27

8.85±0.12

2

0.93±0.01

12.58±0.55

68±0.35

7.92±0.16

5

0.92±0.02

11.67±0.64

66±0.21

7.08±0.20

10

0.91±0.02

11.28±0.70

61±0.19

6.23±0.18

30

0.90±0.01

7.56±1.03

60±0.20

4.08±0.27

60

0.90±0.01

5.58±0.96

53±0.47

2.68±0.31

2

w/o

BTR:PC71BM

w/



Page 34 of 35

Table 2. Photovoltaic performance results, under different sunlight time via Standard illumination, of inverted small molecule solar cells based on as-cast BTR:PC71BM and SVAtreated BTR:PC71BM.

Active layer

SVA

Time [min]

VOC [V]

0

JSC

[mA/cm ]

FF [%]

PCE [%]

0.87±0.01

5.70±0.85

32±0.34

1.57±0.27

2

0.84±0.02

5.64±0.69

31±0.28

1.49±0.20

5

0.82±0.02

5.65±0.72

31±0.31

1.44±0.22

10

0.80±0.01

5.66±1.03

31±0.40

1.37±0.33

30

0.80±0.02

5.58±0.77

30±0.37

1.33±0.24

60

0.81±0.01

5.40±0.82

30±0.36

1.31±0.23

0

0.83±0.01

13.52±0.43

61±0.15

6.76±0.13

2

0.82±0.01

13.33±0.51

59±0.20

6.39±0.14

5

0.82±0.02

13.39±0.47

57±0.19

6.30±0.14

10

0.80±0.01

12.69±0.65

54±0.23

5.52±0.17

30

0.78±0.02

11.07±0.58

50±0.28

4.27±0.20

60

0.74±0.03

10.10±0.72

42±0.31

3.16±0.18

2

w/o

BTR:PC71BM

w/


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Table of Content (TOC)


Three-dimensional observation of a light-soaked photo-reactant layer

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