Photonic Flash Sintering of Ink-Jet-Printed Back Electrodes for Organic

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Photonic flash sintering of inkjet printed back electrodes for organic photovoltaic applications Giuseppina Polino, Santhosh Shanmugam, Guy Bex, Robert Abbel, Francesca Brunetti, Aldo Di Carlo, Ronn Andriessen, and Yulia Galagan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11394 • Publication Date (Web): 24 Dec 2015 Downloaded from http://pubs.acs.org on January 3, 2016

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ACS Applied Materials & Interfaces

Photonic flash sintering of inkjet printed back electrodes for organic photovoltaic applications Giuseppina Polino

a,b

, Santhosh Shanmugam c, Guy J.P. Bex a, Robert Abbel a, Francesca

Brunetti b, Aldo Di Carlo b, Ronn Andriessen c, Yulia Galagan c,*

(a) Holst Centre, High Tech Campus 31, 5656AE Eindhoven, The Netherlands (b) CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of Rome Tor Vergata, via del Politecnico 1, 00133 Rome, Italy (c) Holst Centre – Solliance, High Tech Campus 21, 5656AE Eindhoven, The Netherlands

KEYWORDS: polymer solar cells, inverted structure, photonic flash sintering, interfaces, inkjet printing.

ABSTRACT: A study of the photonic flash sintering of a silver nanoparticle ink printed as the back electrode for organic solar cells is presented. A number of sintering settings with different intensities and pulse durations have been tested on both full area and grid-based silver electrodes, using the complete emission spectrum of the flash lamps from UV-A to NIR. However, none of these settings was able to produce functional devices with performances comparable to reference

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cells prepared using thermally sintered ink. Different degradation mechanisms were detected in the devices with a flash sintered back electrode. The P3HT:PCBM photoactive layer appears to be highly heat sensitive and turned out to be severely damaged by the high temperatures generated in the silver layer during the sintering. In addition, UV induced photochemical degradation of the functional materials was identified as another possible source of performance deterioration in the devices with grid-based electrodes. Reducing the light intensity does not provide a proper solution, because in this case the Ag electrode is not sintered sufficiently. For both types of devices, with full area and grids electrodes, these problems could be solved by excluding the short wavelength contribution from the flash light spectrum using a filter. Optimized sintering parameters allowed manufacturing OPV devices with equal performance to the reference devices. Photonic flash sintering of the top electrode in organic solar cells was demonstrated for the first time. It reveals the great potential of this sintering method for the future Roll-to-Roll manufacturing of organic solar cells from solution.

INTRODUCTION Polymer based organic photovoltaics (OPV) play a significant role as an alternative source of energy in the actual power generation request. However, the future industrialization requires a pathway towards large-scale and economical manufacturing. High throughput production can be obtained by Roll-to-Roll (R2R) processes, and in this context, solution-processing techniques represent a low-cost fabrication approach, suitable also for flexible substrates

1,2,3

. Printed

electronics technologies, already widely used for electronic circuits fabrication, have been successfully applied in the deposition of organic layers in optoelectronic devices 4,5,6. Impressive progress has been made in realizing up-scalable solution processing methods for the electrodes.


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The most common techniques used for the deposition of electrodes in OPV devices are represented by spray coating 7, screen printing 8, flexography

9

and inkjet printing

10

. The

advantages of printing are an accurate and rapid deposition of a wide range of inks in virtually any desired pattern. The drawback of all these solution processing techniques for the preparation of the back electrodes is that they are conventionally combined with a prolonged heat treatment (thermal sintering) in order to reach a proper conductivity of the electrode materials

11,12

. This

slow process results in long overall processing times, which are not compatible with R2R processing and in the case of back electrodes, which are printed on top of the device stack, can in addition lead to degradation of the organic layers 13. Alternative sintering methods, that are able to accelerate the entire process without damaging the underlying layers and plastic substrates, are therefore in high demand in order to enable fast and efficient R2R processing, regardless of the specific printing process used. Photonic flash sintering (PFS) has been successfully applied to Ag current collecting grids deposited on plastic substrates by inkjet, flexo and screen printing 1,2,14. Whereas common plastic foils are typically transparent, silver nanoparticle inks are deeply colored and strongly absorb visible light. Consequently, by choosing a lamp with an appropriate emission spectrum, energy can be coupled selectively into the printed ink structures without directly affecting the substrate. This method of sintering significantly reduces the processing time, compared to conventional thermal sintering

15

, and in addition, offers much more parameters for process optimalization:

Spectral composition, light intensity, duration, number and shape of the pulses as well as flashing frequency can all be varied, whereas oven sintering offers only temperature and time as variables. As drying and sintering of conductive inks constitutes a complex process involving solvent evaporation, decomposition of organic ink ingredients and the formation of conduction


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paths between the particles, the exact flashing conditions have a significant influence on the final result. PFS has until now only been employed in the processing of conductive inks on chemically rather robust substrates like bare plastic foils, where the main problem is deformation due to local melting. It has never been applied so far to the sintering of back electrodes, where sensitive functional organic materials are located underneath the layer of silver. In this context, the main challenge is to apply PFS to the back electrodes deposited on top of an OPV stack containing ZnO, a photoactive layer and PEDOT:PSS. In this paper, we investigate the effect of PFS of the back electrode on the organic layers also in terms of solar cell performance. In particular, the parameters are selected in such a way that destructive effects of the flash light on the photoactive layer in an inverted cell configuration are minimized and a possible solution to resolve the critical issues is proposed. The current work demonstrates the feasibility of PFS for the production of back electrodes on top of a stack of sensitive functional materials without sacrificing their functionality, speeding up the sintering process by more than a factor of 1000, and thereby revealing its R2R compatibility.

EXPERIMENTAL SECTION Device preparation. Glass substrates of 30 mm x 30 mm covered with an ITO layer (sheet resistance 10 Ω/sq), patterned by photolithography, were obtained from Naranjo. Zinc oxide (ZnO) nanoparticles solution was purchased from Genes’ink and used as received. Spin coating of the ZnO solution was done at 1000 rpm (acceleration 1000 rpm/s) for 60 s and annealed at 120°C for 10 min in ambient atmosphere. The resulting layer thickness was about 50 nm. Poly(3hexylthiophene) (P3HT, Plextronics Plexcore OS 2100) and [6,6] phenyl-C61-butyric acid methyl ester (PCBM, 99%, Solenne BV) were used as received. P3HT and PCBM in weight ratio


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of 1:1 were dissolved with a concentration of 26 mg/mL each in ortho-dichlorobenzene (o-DCB) purchased from Sigma Aldrich. The solution was stirred for 16 h at 90 °C and filtered before usage. The solution was spin coated in nitrogen atmosphere at 1000 rpm for 60 s (acceleration 1000 rpm/s, with open lid); the resulting layer thickness was approximately 240 nm. Annealing of the photoactive layer was performed at 120 °C for 10 min in nitrogen. Poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Agfa, S315) was purchased from Agfa and used as received. The spin coating was performed in ambient atmosphere at 1000 rpm (acceleration 1000 rpm/s) for 60 s and annealed at 120 °C for 10 min in nitrogen. The final thickness of PEDOT:PSS layer was 160 nm. The silver back electrode (100 nm) was thermally evaporated in a vacuum chamber through shadow masks at a base pressure of 1 x 10–6 mbar. For the inkjet-printed Ag electrodes (400nm) a commercially available nano-particles ink (Sun Chemicals, Slough UK) consisting of 20 wt% Ag (U5603) was used. The silver ink was printed using a drop-on-demand piezoelectric DMP2800 printer (Dimatix-Fujifilm Inc., USA), equipped with a 10 pL cartridge. Printing was performed using a drop space of 20 µm, a voltage of 20 V, a print head temperature of 30 °C, a frequency of 10 kHz and a customized waveform. After printing, all samples were fast dried on hot plate for 3 min at 90 °C, resulting in an initial resistivity in the order of 60 - 80 · 10-8 Ωm. The reference samples were oven sintered at 130°C for different times using a Memmert hot air oven, while for PFS, samples were flash sintered using a PulseForge 1300 equipment from Novacentrix. The energy fluence of a pulse sequence onto the sample plate can be quantified using an integrated bolometer and used to determine the power density. The spatial uniformity of the light intensity distribution of the lamp has been studied by locally measuring the maximum temperature achieved during a light flash over an area of 130 mm by 210 mm (step size 3.8 and


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15.0 mm, respectively), using a setup as described in ref.

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. These measurements were carried

out for different distances between the substrate plate and the light source (range 6 – 24 mm, step size 1 mm), revealing no larger variations than ± 5 % within the area where the samples are placed. Device Characterization. The thickness of the organic layers was measured with a Veeco Dektak Profilometer. UV-vis absorption spectra were measured using an Uv-vis NIR Spectrophotometer (Agilent). Atomic force microscopy (AFM) images were acquired using Park NX10 system. Scanning electron microscopy (SEM) images (see the Supporting Information, Figure S1) were acquired using a Nova 200 Nanolab Small Dual Beam. Current-density voltage (J–V) characteristics were measured in nitrogen with a Keithley 2400 source meter using simulated AM 1.5 global solar irradiation (100 mW/cm2) with a halogen lamp. Short-circuit currents and PCEs representing AM1.5G illumination conditions were estimated by integrating the EQE with the AM1.5G solar spectrum (the EQE data are available in the Supporting Information in Figure S2). The average performance of at least five identical devices was reported. Two different cell areas were chosen in order to evaluate the solar cell performance respectively on small (0.089 cm2) and large area (0.805 cm2) devices. The active area is defined by the overlap between ITO and the area of the Ag electrode. Measurements were done with a mask to obtain a sharply defined illuminated area. The grid electrode consists respectively of three lines (small area) or four lines (large area) with a pitch size of 2 mm and a deposition width of at least 100 µm. The final width of the lines depended on the deposition method. Temperature modeling. The temperature evolution within an OPV stack with a top electrode consisting of a full area coverage of silver ink was calculated using an adapted version of the


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Simpulse Software package which is supplied by Novacentrix together with their PulseForge PFS tool. The basis for the calculations was the device architecture as shown in Figure 1 (right schematic with full area top electrode). Based on the emission spectrum of the flash lamp, its overlap with the absorption spectrum of PI (for settings with filter), the reflectivity of the dried silver ink and the known energy output of the flash settings (from the bolometer measurements), the absorbed energy was calculated and released at the surface of the top silver layer as heat. Mass density, heat capacity and thermal conductivity of the various materials were obtained from literature (Table S1), and based on this input, the heat flow and the corresponding temperature distribution in time throughout the materials stack was calculated using the one-dimensional heat transport equations. Since the software can only handle up to five different materials, the organic films (PEDOT:PSS and P3HT:PCBM) were lumped together as one layer with averaged materials properties. This simplification can be justified since both have quite similar values compared to the other stack components and it is therefore not expected to have a major impact on the results. The calculated peak temperatures on top of a layer during the first and last flash of a particular sintering setting were used as a measure for the heat exposure a material would suffer during PFS.

RESULTS AND DISCUSSION Thermal Sintering. Printed silver back electrodes require a post-treatment to enhance the drying rate of the ink and to obtain the proper conditions of sintering by improving the contacts between the nanoparticles. It is well known that the annealing conditions have a big influence on the performance of polymer solar cells

16,17

. In order to understand the effect of a prolonged

thermal sintering of the back electrode on the performance of the solar cells, three different


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batches

of

the

devices

with

the

following

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inverted

structure

ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag have been prepared. The full area back electrodes (cell area 0.089 cm2) were produced by both evaporation and inkjet printing. The first ones were thermally evaporated and considered as a reference. The samples with inkjet printed silver electrodes (layer thickness 400 nm) were sintered in a hot air oven at 130°C for (a) 10 min, (b) 20 min, (c) 30 min, respectively. The photovoltaic parameters of the these devices are shown in Table 1 and compared to the evaporated reference device. A small decrease in fill factor (FF) and short circuit current density (Jsc) is observed between 20 and 30 minutes of treatment, resulting in a decrease of power conversion efficiency, as shown in Figure S3. Moreover, increasing the sintering time decreases the reproducibility of the power conversion efficiency (PCE) for different samples prepared under the same conditions, resulting in a larger standard deviation. It is proposed that prolonged exposure to high temperatures in ambient atmosphere results in a steadily increasing degradation of the organic layers PEDOT:PSS and P3HT:PCBM 18

13

and unfavorable morphology changes in the photoactive layer

. The specific electric resistivity (ρ) of thermal sintered electrodes was also evaluated. The

resistivity of inkjet printed electrodes was calculated from the resistance R, using the following formula: ρ= R·A/ℓ where ℓ is the length and A is the cross sectional area of the electrode. It was found that with increasing sintering time the resistivity decreases. In particular, after a sintering time of 30 minutes it is possible to obtain ρ ~ 7x10-8 Ωm, 20 minutes of sintering provide ρ ~ 9x10-8 Ωm and a shorter sintering time of 10 minutes resulted in a value of ~ 11x10-8 Ωm. For comparison,


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the resistivity of the evaporated electrode was ρ ~ 2.5x10-8 Ωm. With regards to the electrode resistivity, longer sintering times are thus favorable, since they will result in lower resistive losses during current transport out of the device. This is particularly important when considering the further up-scaling of the active device area. On the other hand, it is obvious, from the above mentioned efficiency results, that increasing the sintering time has a negative influence on the device performance. Moreover, prolonged sintering time is not compatible with R2R processing. For this reason, the main purpose of this study is to identify an alternative sintering method for the printed electrodes which reduces the processing time and therefore allows up-scaling of the production to R2R manufacturing 19,20. Photonic Flash Sintering. Several alternative sintering methods have been reported that allow the selective heating of conductive inks, among which are laser sintering 21, microwave radiation 22

, electrical

23

and photonic flash sintering

15,24

. These techniques allow to reach high

conductivity values in a very short time 25. In particular, PFS is a very fast method (the typical pulse length is below 10 ms), which is performed using a Xenon flash lamp. These very short process times are attractive both from a manufacturing point of view (R2R compatible) and because of their potential to limit thermal damage to the other functional layers, like the photoactive layer and the transparent conducting materials. Therefore, we have investigated this technique in more detail for the sintering of solution deposited back electrodes on top of an OPV device stack. Although in this study, devices containing inkjet printed silver structures were used as a model system, a fast and efficient post-deposition treatment will always be necessary regardless of the specific printing technology used for the conductive inks. In order to guarantee the samples are illuminated uniformly, we have determined the spatial intensity distribution of the lamp. This has been done by measuring the maximum temperature achieved during a light


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flash at different spots on the exposed area and for different distances between lamp and substrate plate (cf. Exp. Section for details). For the closest possible distance (6 mm), the spatial variations were limited to ±5 % within a central area of 50 mm by 120 mm, and sharply increased outside this area. Given the sample size of 30 mm by 30 mm, a homogeneous illumination was therefore assured. For larger distances to the substrate plate, the spatial variations within the central area were even smaller, since the light intensity distribution smoothened out more and more with increasing distance. Various PFS settings were considered for sintering the printed Ag electrode in order to compare the obtained resistivities to that of a reference sample treated with thermal sintering (Table 2a). In all cases, thermally dried silver ink was used with an initial resistivity in the order of 60 – 80 · 10-8 Ωm. The exact PFS conditions are listed in Table 2b and can be summarized in three categories: low intensity (LI) settings with long pulses of low intensity, intermediate intensity (II) settings with long pulses of medium intensity and finally high intensity (HI) settings with short pulses of high intensity. The corresponding energy fluences for each PFS setting have been determined using an integrated bolometer and used to calculate the average power densities to which the samples are exposed for the various pulse sequences. Within the group of LI settings, both the number and duration of the pulses were varied, and the flashing conditions were chosen such as to provide approximately identical power intensities. Specifically, Set1 is characterized by 2 pulses with a length of 1700 µs, while Set 2 and Set 3 have 10 pulses of 5000 µs and 7500 µs, respectively. Given the pulse frequency of 1 Hz, these flash sequences correspond to a total processing time of a maximum of 10 s, whereas for thermal sintering, the shortest time applied was 10 minutes. The total energy fluences were 0.362 J/cm2, 1.357 J/cm2 and 1.741 J/cm2, with power densities of 212.9 W/cm2, 271.4 W/cm2 and 232.1 W/cm2 for the LI


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settings of Set 1, Set 2 and Set 3, respectively. None of these three settings were found to have any detectable positive influence on the conductivity of the dried silver ink (Table 2b). To investigate the effect of higher energy densities on the sintering process, the II setting (Set 4) was introduced. In contrast to the LI setting (Set 2), it has a higher voltage: 250V (Set 4) vs 200V (Set 2), which results in a fluence of 2.445 J/cm2 and a power density of 489 W/cm2. The total process time was again 10 seconds. However, although the II setting resulted in a slight improvement of the resistivity compared to the LI settings, neither was able to produce results comparable to thermal sintering, as shown in Table 2b. Only in the case of the HI settings, the obtained resistivity values were close to those achieved by thermal sintering of the inkjet printed silver electrode. The HI setting (Set 5) was characterized by two pulses with a voltage of 450V and a length of 500 µs. The flashing frequency was changed from 1 Hz, as in all previous cases, to 10 Hz, resulting in a shortening of the process duration to only 0.2 seconds. The fluence and power density generated under these sintering conditions were 1.684 J/cm2 and 3368 W/cm2, respectively (see Table 2). Furthermore, the influence of the photonic sintering conditions on the OPV device performance was evaluated. Two types of devices (with full area back electrode and grid-based electrode) were investigated (see Figure 1). OPV Devices With Grid-Based Electrode. To understand the effect of photonic sintering on OPV devices with grid-based back electrodes, several batches of the devices with LI, II and HI settings were produced. A very poor performance and high resistive losses were obtained in the devices with the use of LI sintering settings. Subsequently, the same experiments were carried out using the II and HI settings (Figure 2). Although the devices with II settings have Voc comparable to that of the devices with thermally sintered electrodes (Table 3a), II sintered


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devices show a strong decrease in the short circuit current (Jsc). Increasing the intensity of the sintering (HI settings) led to a pronounced degradation of the device performance (Figure 2 and Table 3a). It is obvious that photonic flash sintering under the condition applied so far had a negative influence on the device performance. In order to understand the origin of these negative effects, an optical and morphological analysis of the organic layers was performed. In particular, the effects of the II and HI settings were examined. In Figure 3a the absorbance of the photoactive layer (PAL) deposited on glass is shown after the three types of treatment. A red shift of the UVVis spectrum is observed when the IE settings were applied, as well as a pronounced drop in the vibronic peaks at 550-600nm after the HI photonic treatment. The UV-Vis spectra of the stack containing Glass/ZnO/PAL/PEDOT:PSS after the same treatments are shown in Figure 3b. In addition to a red shift of the absorbance curve in both cases, photo-bleaching 26 and photolysis 27 phenomena are evident in the case of the HI settings. This is confirmed by the disappearance of the typical P3HT:PCBM shoulders at 552nm. Furthermore,

the

morphology

of

both

the

photoactive

layer

and

the

stack

(glass/ZnO/PAL/PEDOT:PSS) was investigated by atomic force microscopy (AFM) after both thermal and photonic sintering. From the AFM images, it is evident that a change in the morphology of the active layer had occurred due to flash sintering. Moreover, in the case of the HI settings, the surface of the photoactive layer became more porous, probably due to decomposition of the material under light exposure in the presence of oxygen (Figure 4a). Even in the case where the PEDOT:PSS was deposited on top of the photoactive layer, a pronounced degradation of the film quality was observed (Figure 4b). This degradation is induced by the PEDOT:PSS layer, which is known to decompose under UV light with loss of sulfonic acid


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moieties

28,29

. The AFM images of the layers with other sintering settings are available in the

Supp. Inf. (Figure S4). Another important degradation mechanism induced by UV light is due to the ZnO layer when the inverted structure is used and depends on the generation and degeneration of shunts in the ZnO blocking layer as reported in the work of Manor et al 30. It has been shown that photonic flash sintering of the grid-based back electrodes has a strong negative influence on the organic layers, leading to their decomposition. As a result, the performance of the devices strongly deteriorated. We speculate that the usage of a full area Ag back electrode will protect the organic layers from the harmful influence of the light and might have positive effects on the performance of the thus produced OPV devices. Therefore, during the next stage of this study, OPV devices with full area Ag back electrodes were fabricated and subjected to photonic flash sintering. In a later stage of the study, adapted PFS conditions were also identified which allowed the sintering of silver grids on top of the OPV stack, while still resulting in fully functional solar cells with undamaged organic layers (vide infra). OPV Devices With Full Area Electrode. For full area back electrodes, the LI settings were also not able to provide devices with satisfactory performances. We speculate that incomplete sintering of the back electrodes (cf. resistivity values in Table 2) leads to the presence of residual solvents in the electrode layer, which has a negative influence on the layers underneath

14,31

.

Only a slightly better performance was found for the II setting (Table 3b and Figure 5), which is in accordance with the very minor improvements in silver resistivity achieved under these PFS conditions (Table 2b). Surprisingly, however, applying the HI settings, also gave rise to poorly performing devices. Since under these conditions, sufficient levels of conductivity can be achieved in the printed back electrodes, complete sintering and solvent removal can be implied. Moreover, the full area Ag layer is assumed to have protected the organic layers underneath from


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the aggressive effects of the light. Therefore, the disappointing results hint at another origin of the device failure. Our following studies were focused on understanding how the heat created during the flash sintering can affect the device performance. In previous studies 14,32, it has been demonstrated that within few flashes, temperatures above 300 °C can be achieved by exposure of conductive inks to short light pulses. Theoretical calculations based on significantly higher intensities and shorter pulse durations have even claimed much higher values

33

. Using such a

theoretical model, we have also simulated the temperatures which are reached in our systems under the flashing conditions we have applied (Figure 6 and Table S1). All LI settings were found to give only very minor temperature increases, always staying below 100 oC. Together with the short exposure times, this explains why no significant effect on the silver resistivity was found and the corresponding devices worked so poorly. By contrast, the II setting gave clearly higher peak temperatures (around 140 oC), which apparently were already sufficient to induce some sintering in the silver layers, although still in this case, the effect was not strong enough to achieve a satisfying device performance. The HI settings, by contrast, gave peak temperatures above 300 oC, which was clearly high enough to sinter the silver well. The heat generated in the Ag layer will be easily transferred into the thin organic layers underneath (both PEDOT:PSS and photoactive layer), which are known to degrade thermally at elevated temperatures 34. Moreover, the solar cells with the full area electrodes are subjected to prolonged heating 26 due to the large amount of silver that absorbs in the same range as P3HT:PCBM 27. This will result in a lowering of the open circuit voltage (Voc) since it is well known that an increase in temperature reduces the band gap of a semiconductor during operation 35. Furthermore, the decreased fill factor (FF) and low shunt resistance (Figure 5b) suggest a P3HT:PCBM degradation. The higher Voc observed in the case of the grid based cells confirms that the effect described above is lower for


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this type of devices, because the amount of heat transferred from the grid-based electrodes into the organic layers is smaller than in the case of the full area electrodes. Optimization of Flash Sintering Conditions. Given the above mentioned problems, a possible solution, in order to get working devices, is to perform the PFS in an inert atmosphere. The absence of oxygen should reduce the degradation of the organic layers. However, such experiments did not show any positive effect, suggesting that photochemical reactions without oxygen being involved are the main contributor to the device deterioration. For example, it is well known that UV light can decompose PEDOT:PSS even in the absence of oxygen 36. The Xenon lamp used in this study is known to emit a broad spectrum ranging from 350 to 900 nm. A rather obvious solution, especially for the grid-based electrode structures, is to selectively filter out the damaging short wavelength components before they hit the silver and to attenuate the total intensity so that the peak temperatures in the functional materials will be reduced (see Supp. Inf., Figure S4). Specifically, a polyimide foil (KAPTON) was used as the UV filter, since it strongly absorbs up to 500 nm and thus will cut off any wavelength below, as shown in Figure 7. Although the Ag ink shows a high reflectance in the visible (VIS) and near infrared (NIR), still a significant fraction of the incident light is absorbed by the nanoparticles. As the UV filter blocks the wavelengths with high energies, it is important to know whether the residual light has sufficient energy to perform complete sintering of the back electrode. In Table 2c the resistivity values are displayed, which have been obtained using the HI settings and a polyimide UV-Vis filter. Set 6 (HI settings with a filter) is characterized by the same flashing parameters, and thus also by the same short total process time of 0.2 seconds, as Set 5 (HI settings without filter). However, due to filtering out a significant part of the spectrum, both fluence and power density were much lower for Set 6 than for Set 5. Specifically, a fluence of


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0.891 J/cm2 and power density of 1782 W/cm2 were measured for Set 6 compared to 1.684 J/cm2 and 3368 W/cm2 obtained for Set 5. It is obvious that the resistivity values obtained with Set 6 were higher than the values obtained without a filter. To compensate for this, the energy output of the lamp was increased even more by introducing high intensity and long exposure (HI-LE) settings with high voltage and a wide pulse length. The pulse length of 500 µs used for Set 6 was increased to 750 µs in the HI-LE setting (Set 7). The resulting fluence of 1.139 J/cm2 was higher than that for Set 6, but the power density was lower, namely 1518.7 W/cm2, as shown in Table 2, whereas the total process time for this flash sequence remained unchanged. These HI-LE settings in combination with a KAPTON filter provide a resistivity of the printed Ag of 13x10-8 Ω·m, which is in the range of our goal and compatible with the values obtained by thermal sintering. In order to evaluate the influence of the sintering conditions on the device performance, another batch of devices was realized, the performances of which are reported in Table 4. A comparison between thermal sintering and flash sintering (settings are HI and HI-LE with UV filter) was carried out and evaluated on small (0.089cm2) and large (0.805cm2) area solar cells with full area and grid-based back electrodes. In Table 4a the results for the full area electrodes are shown. Comparable performances of the devices with both small and large active areas are obtained for thermal and PFS devices using the HI settings with a filter (Figure 8a and Figure 8b). This implies that the full area silver electrode has been completely sintered and the underlying organic layers have not been damaged. By contrast, in the case of the HI-LE settings a decrease of the devices performance is observed. In the case of the small area devices a strong decrease of Jsc confirms the degradation of the photoactive layer due to overheating. In the case of the large area devices, even an abrupt


16

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decrease of all photovoltaic parameters is observed. These losses in the efficiency can be ascribed to a physical degradation of the ZnO layer 30. Presumably, the device performance deterioration here is much more pronounced than in the case of the small area devices, since lateral heat transport is more efficient for the small cells. This size effect can be expected to limit the maximum peak temperatures more efficiently in the cells with small than in those with large area, leading to stronger damage in the larger devices. Thus, using a UV filter has a positive effect on the sintering of the back electrode, effectively reducing the energy and thus preventing overheating of the organic layers. However, whereas such protection from overheating is very efficient in the case of the HI settings, it is not sufficient for the HI-LE settings. In this case, the overheating of the organic layers still takes place, resulting in low device performances. This interpretation is also supported by heat flow simulations as already described above for settings 1 to 5: Set 6 resulted in peak temperatures around 175 oC, whereas Set 7 gave values above 200 oC (Figure 6). Although other parameters such as the duration of the heat exposure are probably also relevant, apparently, there is only a rather narrow range of peak temperatures the OPV material can tolerate without losing its functionality, while at the same time allowing sufficient sintering of the silver. In the case of the devices with grid electrodes (Table 4b), an identical trend is found. The HI settings are able to provide sufficient sintering of the back electrode without a significant influence on the organic layers, when the short wavelength components are filtered out. Comparable performances as with thermal sintering are obtained (Figure 8c and 8d). By contrast, the use of the HI-LE settings results in a rather strong performance drop due to photodegradation of the organic layers, in particular the P3HT:PCBM. This occurs because the intensity of the lamp is very high and not all wavelengths affecting the photoactive layers are


17


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Page 18 of 35

fully blocked. There is still a significant overlap in the non-filtered area between the silver reflectance and the P3HT:PCBM absorbance at 517 nm (related to the π- π absorption in P3HT 37, 38

). Therefore, we can conclude that on the one hand, the energy used for flash sintering of the

back electrode should be as high as possible to provide proper sintering of the Ag, but at the same time should not affect the organic layers, either by photo-bleaching (in the case of an electrode grid) or by overheating (in the case of a full area electrode). The flash sintering settings should be chosen such as to provide a trade-off satisfying both these boundary conditions. In summary, we have reported a complete comparison of the efficiency trends between two types of back electrodes in OPV cells prepared in three different ways: evaporated and printed, followed by either thermal sintering or PFS. In the last case, we have analyzed the effects of a number of settings with different intensities on the device performance. An effective use of flash sintering for producing printed back electrodes is only possible with a UV filter, which blocks part of the light. Observing the efficiency trend we can conclude that the HI settings with UV filter result in devices showing power conversion efficiencies (PCE) comparable with conventional sintering for both small and large area cells. The general trend of the device performance in relation to the sintering conditions is reported in Figure 9 for both full area and grid-based electrodes with large and small areas.

CONCLUSIONS The principal aim of this study was to understand the possible effects of PFS of the back electrodes in OPV cells with inverted structure on the device performance. We have demonstrated the feasibility of photonic flash sintering of a solution processed back electrode deposited on top of an organic stack without damaging of the latter. This was possible due to the


18

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use of a filter that can block the UV components in the emission spectrum of the light source used, which would otherwise cause photo-degradation of the polymeric solar cells. These negative effects are expected to be especially severe when the device production is carried out at ambient conditions and under exposure to moisture and oxygen. By choosing the correct settings of intensity, pulse length, flashing frequency and number of flashes, and by adjusting the spectral composition of the flash light, we were able to prepare cells with highly conductive silver electrodes, while at the same time limiting thermal and photochemical damage to the functional layers. The devices thus prepared rivalled the performances of conventionally (i. e. thermally) sintered cells, indicating a similar (and obviously minor) degree of materials degradation in both processes, despite the very different thermal histories. In addition, the processing time for the PFS step was orders of magnitude shorter, thus representing an important step towards R2R upscalable solar cell manufacturing. Indeed, further development will be aimed at realizing flexible polymeric solar cells with all layers deposited through large area printing techniques and both solution processed electrodes sintered using the PFS method. This is expected to contribute to a reduction of the production cost.


19


Figure 1. Schematic illustration of the OPV device architecture

9 Themal Sintering 130°C 7x 10min Set 4 (II)

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 20 of 35

5

Set 5 (HI)

3 1

-1

-0.8

-0.6

-0.4

-0.2 -1 0

0.2

0.4

0.6

0.8

1

-3 -5 -7 -9 -11 -13

Cell Voltage [V] Figure 2. J-V Characteristics of grid based OPV devices obtained by applying thermal sintering, II and HI settings (active area 0.089cm2)


20

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a) 1.4

Glass/PAL

Absorbance [a.u]

1.2

130°C x 10min Set 4 (II)

1

Set 5 (HI) 0.8

Thetmal

0.6

Set 4 0.4

Set 5

0.2 0 350

450

550

650

750

Wavelength [nm] 1.4

b)

Glass/ZnO/PAL/PEDOT 1.2

Absorbance [a.u]

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


130°C x 10min Set 4 (II)

1

Set 5 (HI)

0.8 0.6

Thermal Set 4

0.4

Set 5 0.2 0 350

450

550

650

750

Wavelength [nm] Figure

3.

UV-Vis

comparison

thermal

annealing

vs

PFS

(a)

glass/PAL

(b)

glass/ZnO/PAL/PEDOT


21


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Page 22 of 35

Figure 4. Atomic force microscopy phase images of the polymer surfaces. Comparison between thermal and different setting of PFS on (a) glass/PAL (b) glass/ZnO/PAL/PEDOT


22

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9 Thermal Sintering 130°C7x10min Set 4 (II)

5

Set 5 (HI)

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


3 1 -1

-0.8

-0.6

-0.4

-0.2 -1 0

0.2

0.4

0.6

0.8

1

-3 -5 -7 -9 -11 -13

Cell Voltage [V] Figure 5. J-V Characteristics of full electrode based OPV devices obtained by applying thermal sintering, II and HI settings (active area 0.089cm2)


23


Figure 6. Calculated peak temperatures achieved during flash sintering in the organic layers for all PFS settings used in this study (full area electrodes). The error bars represent the differences between the first and the last pulse

1.6

80

Ag reflectance

70

1.2

60

1

50

Xe Lamp Emission

0.8

40

0.6

30

0.4

20

Kapton Filter absorbance

0.2

Reflectance

P3HT:PCBM absorbance

1.4

Absorbance

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 24 of 35

10

0

0 300

400

500

600

700

800

Wavelength (nm)

Figure 7. UV-Vis spectra: Absorption of the P3HT:PCBM, normalized emission spectrum of the flash lamp and reflectance of the silver electrode


24

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Figure 8. J-V Characteristics comparison between thermal and PFS with PI filter: Full area electrode (a) - small area 0.089 cm2, (b) - large area 0.805cm2; Grids electrode (c) - small area 0.089cm2 (d) - large area 0.805 cm2


25


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Page 26 of 35

Figure 9. Efficiency trend (obtained considering Jsc integrated from EQE) for the devices with different sintering conditions of the top electrodes: (a) full area; (b) grids electrode

Table 1. Main photovoltaic parameters obtained with thermal sintering of the back electrodes for different times (active area 0.089 cm2)

Sintering condition

Voc (mV)

Jsc (mA/cm2)

JscEQE (mA/cm2)

FF (%)

ƞ MPP (%)

ƞ EQE (%)

Evaporated

543.4 ± 0.07

10.7 ± 0.02

10.6 ± 0.02

62.3 ± 0.1

3.6 ± 0.09

3.4 ± 0.09

130°C x 10 min

557.4 ± 0.03

9.2 ± 0.01

8.7 ± 0.01

61.4 ± 0.1

3.2 ± 0.01

3.0 ± 0.01

130°C x 20 min

560.8 ± 0.07

8.7 ± 0.02

8.7 ± 0.05

58.1 ± 0.05

2.8 ± 0.10

2.8 ± 0.10

130°C x 30 min

560.4 ± 0.10

8.1 ± 0.01

8.2 ± 0.01

56.2 ± 0.1

2.5 ± 0.12

2.5 ± 0.12


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Table 2. Sintering settings applied to inkjet printed silver ink (Suntronic U5603) and corresponding specific electric resistivities Setting

Resistivity [Ω·m]

(a)

Thermal sintering

(b)

PFS without filter

10.8 x10-8

10 min at 130°C Voltage [V]

Pulse Length [µs]

Flashing Frequency [Hz]

Number of pulses

Power [J/cm2]

Power density [W/cm2]

Processing time [s]


Set 1

180

1700

1

2

0.362

212.9

2

65 x10-8

Set 2

200

5000

1

10

1.357

271.4

10

76 x10-8

Set 3 Intermediate Intensity (II) Set 4

200

7500

1

10

1.741

232.1

10

72 x10-8

250

5000

1

10

2.445

489.0

10

44 x10-8

450

500

10

2

1.684

3368.0

0.2

16.2 x10-8

Voltage

Pulse Length

Flashing frequency

Number of pulses

Power [J/cm2]

Power density [W/cm2]

Processing time [s]


450

500

10

2

0.891

1782.0

0.2

18 x10-8

450

750

10

2

1.139

1518.7

0.2

13 x10-8

Low Intensity (LI)

High Intensity (HI) Set 5 (c)

PFS with PI filter

High Intensity (HI) Set 6 High Intensity – Long exposure (HI-LE) Set 7

Table 3. Performance of organic solar cells (active area 0.089 cm2), containing (a) grid-based and (b) full area back electrodes

(a)

(b)

Voc (mV)

Jsc (mA/cm2)

JscEQE (mA/cm2)

FF (%)

ƞ MPP (%)

ƞ EQE (%)

Thermal

550.9 ± 0.06

7.8 ± 0.03

7.6 ± 0.03

59.5 ± 0.08

2.6 ± 0.07

2.5 ± 0.07

Set 3 (LI)

537.5 ± 0.1

1.3 ± 0.2

1.3 ± 0.2

27.2 ± 0.01

0.2 ± 0.05

0.2 ± 0.05

Set 4 (II)

553.0 ± 0.09

3.9 ± 0.1

3.9 ± 0.1

51.1 ± 0.1

1.1 ± 0.03

1.1 ± 0.03

Set 5 (HI)

124.0 ± 0.01

1.3 ± 0.2

1.3 ± 0.2

23.8 ± 0.01

0.1 ± 0.01

0.1 ± 0.01

Full area

Voc (mV)

Jsc (mA/cm2)

JscEQE (mA/cm2)

FF (%)

ƞ MPP (%)

ƞ EQE (%)

Thermal

557.4 ± 0.03

9.2 ± 0.01

8.7 ± 0.01

61.4 ± 0.1

3.2 ± 0.01

3.0 ± 0.01

Set 3 (LI)

281.4 ± 0.07

2.0 ± 0.3

2.0 ± 0.3

26.5 ± 0.01

0.2 ± 0.09

0.2 ± 0.09

Set 4 (II)

313.4 ± 0.04

8.6 ± 0.1

8.6 ± 0.1

25.5 ± 0.04

0.7 ± 0.1

0.7 ± 0.1

Grids


27


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

Set 5 (HI)

264.2 ± 0.09

3.3 ± 0.1

2.1 ± 0.1

26.3 ± 0.01

Page 28 of 35

0.2 ± 0.05

0.2 ± 0.05

Table 4. Performance comparison of the realized devices between thermal sintering and PFS with PI filter: (a) grids electrode (b) full area electrode

(a)

(b)

Grids

area (cm2)

Voc (mV)

Jsc (mA/cm2)

JscEQE (mA/cm2)

FF (%)

ƞ MPP (%)

ƞ EQE (%)

Thermal

0.089

550.9 ± 0.06

7.8 ± 0.03

7.6 ± 0.03

59.5 ± 0.08

2.6 ± 0.07

2.5 ± 0.07

Set 6 (HI)

0.089

535.8 ± 0.05

5.8 ± 0.1

5.5 ± 0.1

53.3 ± 0.03

1.7 ± 0.03

1.6 ± 0.03

Set 7 (HI-LE)

0.089

184.7 ± 0.07

7.0 ± 0.1

7.0 ± 0.1

24.7 ± 0.05

0.3 ± 0.05

0.3 ± 0.05

Thermal

0.805

528.0 ± 0.01

7.0 ± 0.06

7.0 ± 0.06

54.0 ± 0.02

2.0 ± 0.05

2.0 ± 0.05

Set 6 (HI)

0.805

550.7 ± 0.01

6.7 ± 0.03

6.4 ± 0.03

57.3 ± 0.04

2.1 ± 0.1

2.0 ± 0.1

Set 7 (HI-LE)

0.805

515.9 ± 0.02

6.5 ± 0.08

5.9 ± 0.08

43.8 ± 0.04

1.5 ± 0.02

1.3 ± 0.02

Full area

area (cm2)

Voc (mV)

Jsc (mA/cm2)

JscEQE (mA/cm2)

FF (%)

ƞ MPP (%)

ƞ EQE (%)

Thermal

0.089

557.4 ± 0.08

9.2 ± 0.01

8.7 ± 0.02

62.0 ± 0.1

3.2 ± 0.02

3.0 ± 0.02

Set 6 (HI)

0.089

553.4 ± 0.02

8.7 ± 0.05

8.7 ± 0.05

61.8 ± 0.06

3.0 ± 0.09

3.0 ± 0.09

Set 7 (HI-LE)

0.089

577.5 ± 0.02

8.0 ± 0.01

8.2 ± 0.01

56.1 ± 0.05

2.6 ± 0.01

2.7 ± 0.01

Thermal

0.805

555.2 ± 0.03

8.9 ± 0.02

8.5 ± 0.02

58.6 ± 0.01

2.9 ± 0.01

2.8 ± 0.01

Set 6 (HI)

0.805

545.3 ± 0.03

8.9 ± 0.04

8.9 ± 0.04

57.4 ± 0.07

2.8 ± 0.08

2.8 ± 0.08

Set 7 (HI-LE)

0.805

430.5±0.04

6.8 ± 0.03

7.7 ± 0.03

26.8 ± 0.04

0.8 ± 0.05

0.9 ± 0.05

ASSOCIATED CONTENT Supporting Information Scanning electron Microscopy (SEM) images of inkjet printed silver electrodes after thermal and photonic flash sintering. EQE for all analyzed devices. Average trend of power conversion efficiencies with time during thermal sintering of the printed full area silver back electrodes. AFM images of surfaces treated with various PFS settings. Materials properties used as input for the temperature simulations.


28

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AUTHOR INFORMATION Corresponding Author * Dr. Yulia Galagan, E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS G. Polino, F. Brunetti, and A. Di Carlo acknowledge for the support the Regione Lazio through Polo Solare Organico

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Photonic Flash Sintering of Ink-Jet-Printed Back Electrodes for Organic

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