Effect of solvent and thermal annealing on D18/Y6 polymer solar cells

Organic solar cells (OSCs) as emerging generation solar cells are required to face climate and energy challenges. In this regard, OSCs based on the D18:Y6 active layer with a ratio of 1:1.6 with thermal and solvent annealing as a post-treatment were fabricated. The effect of different thermal annealing with chloroform on the active layer and the cell performance was studied. Optical, morphological and thermal analysis are executed to investigate the effect of thermal with solvent annealing on the D18:Y6 active layer. Photoluminescence (PL), transmission electron microscope (TEM) and atomic force microscope (AFM) reveal that D18:Y6 film treated at 55 °C with chloroform for 5.0 min had the lowest PL intensity, interpenetrating grain networking structures and more smoother surface leads to optimize photo-induced charge transfer and exciton dissociation in the active layer. D18: Y6 blend film annealed at 80 °C with chloroform for 5.0 min exhibits higher roughness of 17.81 nm than 11.60 nm for D18:Y6 blend film treated at 55 °C. As a result, the optimal performance of the fabricated conventional OSCs based on active layer treated at 55 °C with chloroform had short-current density (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency of 60 mA/cm2, 0.70V, 39.8% and 16.5%, respectively. This study indicates additional thermal annealing with chloroform as a post-treatment enhances the device performance of OSCs. Studying the effect of solvent vapor annealing with thermal annealing of D18:Y6 layer as post-treatment on the performance of organic solar cells.


Introduction
Over the last decades, the world is subjected to climate change and global warming due to utilizing fossil fuels.There is an effort to replace this fuel with renewable and green energy sources such as solar energy.Solar cells are one of the main keys to reduce the harmful environmental impacts caused by fossil fuels [1][2][3].Solar cells harness light energy and convert it directly into electrical energy.Silicon solar cells are known to be the first generation of solar cells while the second one is based on thin-film solar cells.Third-generation solar cells include emerging technologies such as organic, perovskite, dye-sensitized, and quantum dots-sensitized photocells.Compared to other solar cells, organic solar cells (OSCs) are flexible, lightweight and have the potential for large-scale production with low cost [4][5][6].
OSCs are mainly based on bulk-heterojunction (BHJ) architecture by blend of donor and acceptor materials and fullerene derivatives have been commonly used as acceptors in the past decades.Poly(3-hexylthiophene) (P3HT) and its derivatives are blended with phenyl-C61-butyric acid methyl ester (PCBM) and delivered an efficiency of more than 10% [7,8].The serious disadvantage of this type of solar cells is the limited absorption bands in the whole solar spectrum.To overcome this challenge of new donors and non-fullerene acceptors (NFAs) materials have been developed [7,8].
The main objective of this work is to investigate the effect of SVA combined with thermal annealing (TA) as a post-treatment for the D18:Y6 active layers on OSCs performance.In addition, the effect of thermal with solvent annealing of D18:Y6 active layer on optical, structural, morphological, and surface energy properties is studied.The device performance is significantly enhanced by chloroform as solvent vapor treatment with thermal annealing at 55 °C with PCE values of 16.5% using an optimized ratio of D18:Y6.Furthermore, this work has studied the effect of different thermal annealing combined with chloroform on the active layer to better understand and analyze the performance of the D18:Y6 at high temperatures.From this work, D18:Y6 solar cell can operate at high temperatures sufficiently.

Fabrication of D18/Y6 solar cells
The etching process was carried out for ITO by painting Zn powder in ethanol onto the uncovered part of ITO and then it was immersed horizontally in dilute hydrochloric acid with 1:1 of HCl: H 2 O at 25 °C for 10 min.After etching, ITO substrates were washed with deionized water, acetone, and dried nitrogen flow.ITO substrates were sonicated in Hellmanex (2% v/v solution in deionized water), two times in deionized water, acetone and isopropanol for 15 min for each step.Finally, ITO substrates were blow-dried with N 2 and followed by thermal treatment for 24 h at 150 °C [34,35].OSCs in architecture of glass/ITO/PEDOT: PSS/D18:Y6/PDINN/ Ag as shown in Fig. 1a were fabricated.PEDOT: PSS as hole transport layer (HTL) was spin-coated on ITO substrates at 3500 rpm for 50 s in the air and annealed at 150 °C for 10 min, then they were transferred into a nitrogenfilled glovebox (Innovative Technology (PL-HE-2 GB)).Mixture with 11 mg/mL consisted of 4.2 mg D18 and 6.7 mg Y6 was dissolved in 1.0 mL chloroform.D18:Y6 solution was stirred overnight at 25 °C.This active material was spin-coated at 3800 rpm for 30 s onto the etched ITO/ PEDOT: PSS substrates.The effect of solvent and thermal annealing onto the active layer was carried out by placing some ITO/PEDOT: PSS/D18:Y6 layers in petri dish containing 100 µL chloroform for 5.0 min and the other layers were also exposed to 55, 80 and 100 °C, respectively.To deposit the final layer, 2.0 mg PDINN was dissolved in 1 mL of methanol and was spin-coated on the active layer with 4000 rpm for 30 s. Finally, Ag electrode was thermally evaporated (Edwards coating system E306A) through a shadow mask at applied current of 25 A and pressure of 3*10 -5 bar [36,37].

The wettability test
The contact angles were measured by using goniometer (Rame´-hart CA instrument, model 190-F2) at ambient air condition by placing a drop of water and ethylene glycol (EG) on D18 and Y6 layers.A 4.2 mg of D18 and 6.7 mg of Y6 are dissolved in 1.0 mL of chloroform, separately.D18 and Y6 solutions are stirred overnight at 25 °C.The neat solutions of D18 and Y6 were spin coated at 3800 rpm for 30 s on glass substrates, separately.The effect of solvent and thermal annealing onto D18 and Y6 layers were carried out.Subsequently, the contact angle for each film was measured by using drops of water and ethylene glycol (EG).From contact angle data, the surface tension (γ) of D18 and Y6 layers was determined using the Wu's method [38]: where θ is the contact angle, and γ water and γ EG are the surface tensions of the water and ethylene glycol; and (1) γd water , γ p water , γ d EG and γ p EG are the dispersion and polarity components of γ Water and γ EG .
The Flory-Huggins parameters (χ) were measured to calculate the interfacial energy χ X−Y between D18 and Y6 [38]: where γ x and γ y are the surface energies of the donor and acceptor, respectively.

Characterization techniques
The optical properties of the active layer D18:Y6 were obtained by UV-visible spectroscopy (spectrophotometer, Thermo Scientific 600, USA).Photoluminescence spectra were recorded for D18 and D18:Y6 layers deposited on glass substrates at an excitation wavelength of 600 nm using spectrophotometer Perkin Elmer L55, USA.The surface topography of D18:Y6 layers was characterized using Nano scope IV scanning probe (Shimadzu SPM-(9700HT), USA) to obtain atomic force microscope (AFM) images.The morphology and the homogeneity of these active layers were investigated using transmission electron microscope (TEM ''JEOL JSM-6360 LA'', Japan).Thermogravimetric analyses (TGA) and Differential Scanning Calorimetry (DSC) were carried out by SDT Q600 instrument under N 2 flow with 10°Cmin −1 .Current density versus voltage (J-V) curves of the fabricated OSCs were measured using Metrohm Autolab B.V. Potentiostat (PGS STAT 204) with a scan rate of 0.1 V/s in the potential range from -0.5 to 1.2 V.The Xenon lamp was calibrated by I-V 400W photovoltaic panel analyzer with a power intensity of 100 mW.cm −2 .

Optical properties
Figure 1b illustrates absorption spectra of neat D18 and Y6 films spin-coated onto glass substrates with chloroform as a post-treatment.The neat film of D18 absorbs the spectra from 400 to 650 nm with two maximum absorption peaks at 550 and 580 nm due to π-π* transition.D18 film shows complementary absorption to Y6 film which absorb the spectra from 600 to 950 nm with a maximum absorption peak at 830 nm.This facilitates the efficient generation of charge carriers in the whole solar spectrum [39].The effect of thermal annealing at different temperatures with SVA on the absorption of D18 films shows a similar behavior to D18 with only SVA as a post-treatment as indicated in Fig. 1c.Y6 films annealed at 55, 80,100 °C with SVA exhibit a slightly bathochromic absorption peak at 840 compared with the pristine Y6 with only SVA.D18:Y6 blend films treated at different thermal annealing with SVA cover the spectra from 400 to 950 nm [39,40].However, Y6 annealed film at 55 °C with SVA has an absorption peak with a slightly blue-shifted relative to the other films, which is valuable for harvesting short and medium wavelength photons as shown in Fig. 1d [41,42].
To study the exciton behavior, photoluminescence (PL) spectra of D18 neat films at different thermal temperatures with chloroform and D18:Y6 (1:1.6)films treated with solvent annealing at different temperatures are proceeded with 600 nm light excitation as displayed in Fig. 2 [43].D18 neat film treated at 55 °C with chloroform has the strongest emission peak at 730 nm due to radiative decay of the excitons from the excited to ground states.On the other hand, this peak is quenched for D18:Y6 film treated with only chloroform due to charge transfer from D18 donor to Y6 acceptor levels.Moreover, PL intensity for D18:Y6 film treated at 55 °C with chloroform slightly declined as presented in Fig. 2b.PL spectra of D18 neat film at 55 °C with chloroform and D18:Y6 films treated at different temperatures with chloroform are presented in Fig. 2c.D18:Y6 films at 55 and 80 °C with chloroform have the lowest PL intensity attributed to the effective photo-induced charge transfer and exciton dissociation in the active layer [44].However, the enhancement in PL intensity of D18:Y6 film thermal annealed at 100 °C with chloroform is due to the reduction of D/A interfaces and low exciton dissociation as shown in Fig. 2c [45].

Morphological investigation of D18:Y6 films
To further understand the thermal annealing effect with chloroform on blend films of D18:Y6, the morphological and microstructure properties are studied.Figure 3 displays TEM images of D18:Y6 blend films at different thermal annealing temperatures with chloroform and high magnification images are inserted as inset images.From Fig. 3a, the blend film treated with chloroform has one phase and a homogenous layer with a granular-like structure indicating a good mixing between D18 and Y6.Meanwhile, the blend film annealed at 55 °C with chloroform exhibits different grains networking with each other to from different pathways with average grain size of 129.85 nm and this good for the charge separation and transportation of the generated excitons (Fig. 3b) [46,47].The annealed blend film treated with chloroform at 80 °C has cracks at the grain boundaries with.The dimensions of elliptical grains with major and minor axes are about 221.17 and 139.7 nm, respectively (Fig. 3c).Insufficient material crystallization and segregation lead to larger grains size [48].For the blend film annealed at 100 °C with SVA, distinct isolated grains with continuous cracks are observed in the active layer and this is insufficient for the charge separation and strong charge recombination as illustrated in Fig. 3d.As displayed in Fig. 3c, d, the small dark agglomerated particles appear due to the annealing effect on Y6 [13,49,50].
To investigate how thermal annealing with SVA affects topology and roughness of D18:Y6 films, AFM is used.Figure 4 shows 2D and 3D AFM images of 5 µm × 5 µm region of D18:Y6 blend films treated at 55 and 80 °C.Root mean square (RMS) is a statistical parameter to characterize surface roughness [51].D18: Y6 blend film treated at 80 °C with chloroform for 5.0 min exhibits higher roughness of 17.81 nm than 11.60 nm for D18:Y6 blend film treated at 55 °C (Fig. 4a, b) [51,52].The blend film treated at 55 °C with chloroform has a stone-like structure and exhibits a smoother and more uniform surface than the blend film annealed at 80 °C, which is advantageous for charge transfer and extraction from the active layer to the electrode of OSCs.Increasing thermal annealing to 80 °C with chloroform results in the formation of surfaces with high RMS values and reduces charge dissociation and PCE.Thermal annealing with chloroform can act as a morphological adjuster to optimize the surface morphology of D18:Y6 layers [53].

Wettability properties
From the previous optical and morphological results, the thermal annealing at 55 °C with chloroform for D18: Y6 has improved the performance of the device compared to performance of the other devices.To evaluate the compatibility and wettability of the used materials, the contact angles (CA) measurement of neat D18 and Y6 films are carried out at different annealing temperatures with chloroform based on liquid drops of water and EG as shown in Fig. 5 [54].The water and EG contact angles for neat D18 samples are almost the same value (101°) as shown in Fig. 5, referring to a small influence of the temperature on the variation of the hydrophobic and hydrophilic properties of the neat D18 layer.On the other hand, the values of water and EG contact angles on neat Y6 samples treated at 55 0 C with chloroform increase to 107.0 °C and 64.3 °C, respectively.The increase in hydrophobicity of Y6 improves the compatibility between Y6 and D18.However, there is a slight decrease in the contact angle values when the temperature rises to 80 and 100 °C.The surface tension of the pure D18 annealing at 55 °C with chloroform (31.0 mN m −1 ) is closer to Y6 (25.0 mN m −1 ) as illustrated in Table 1.This closer of surface tensions of the two components indicate that D18:Y6 blend at 55 °C with chloroform has better miscibility, charge transport and extraction [55].The χ values are predestined to be 0.6 mN m −1 for D18:Y6 (SVA), 0.22 mN m −1 for D18:Y6 (SVA + 55 °C), 0.57 mN m −1 for D18:Y6 (SVA + 80 °C) and 0.39 mN m −1 for D18:Y6 (SVA + 100 °C) as listed in Table 1.The smallest χ value for D18:Y6 (SVA + 55 °C) film indicates strong donor-acceptor interaction and good compatibility when compared to other films [44,56,57].

Thermal analysis
High temperatures can accelerate OSCs degradation.The actual working temperature of the solar cells in outdoor environments is from 30 °C to 80 °C and the used annealing temperatures ranging from 55 to 100 °C are selected to study the effect of thermal annealing on OSCs performance.The thermal analysis is carried out with a heating rate of 10 °C min −1 under nitrogen gas flow.The polymeric solar modules are fabricated and encapsulated under N 2 atmosphere to protect them from humidity and oxidation.TGA plots of D18 and Y6 exhibit good stability with a decomposition temperature above 200 °C under N 2 as shown in Fig. 6a.There is a slight weight loss of 2% between 20 and 200 °C implying the desorption of the absorbed water [58,59].DSC is also applied to study the properties of the molecular aggregation and crystallinity for D18 and Y6 as shown in Fig. 6b.There is no endothermic or exothermic peak recorded in D18 and Y6 in the selected temperature range indicating their good crystallinity and stability below 200 °C [60].

Photovoltaic properties
To study the effect of different thermal annealing with chloroform for 5.0 min on the performance of D18:Y6 based OSCs, the conventional structure ITO/PEDOT: PSS/D18:Y6/PDINN/Ag is fabricated.J-V curves and photovoltaic parameters including open circuit voltage (V OC ) and fill factor (FF) strongly depend on the shunt (R sh ) and series (R s ) resistances.R sh is reduced by the leakage current because of the pinholes and charge carriers' recombination.The morphology of the device's layers has been processed carefully to reduce the device's pinholes and recombination.A small R sh lowers the current that flows through the junction and thereby reduces V oc [61].The dark J-V curves of OSCs devices are displayed in Fig. 7a.The solar cells have  a smaller leakage current except for OSC based active layer treated at 100 °C with chloroform.This improvement of dark J-V characteristics is reflected in the improvement of Rsh and Voc.As shown in Fig. 7a, the leakage current of OSC based active layer treated at 80 °C with chloroform is the lowest and the corresponding values of R sh and Voc are 636.9Ω.cm 2 and 0.77 V, respectively.Moreover, the higher current in the space charge limited current dominated regime indicates an increase of charge transport and a reduction in R s [62].The OSCs devices based active layer treated at   different temperatures with chloroform are shown in Fig. 7b.The photovoltaic parameters including shortcircuit current density (J SC ), V OC , FF, PCE, R sh and R s of all devices are listed in Table 2.The first OSCbased active layer without annealing has achieved PCE of 9.6% with Jsc of 28 mA/cm 2 and V oc of 0.74 V.
The second OSC-based active layer treated with thermal annealing at 55 °C and chloroform has achieved 16.5% PCE, 60 mA/cm 2 Jsc and 0.70 V V oc .The third and fourth OSCs-based active layer treated with thermal treatment at 80 °C and 100 °C with chloroform have 13.5, 6.6% PCE, 43, 34 mA/cm 2 J sc and 0.77, 0.53 V V oc , respectively.The increase in the efficiency of the second OSC is contributed from the increase of material crystallinity and the grains networking with high charge separation for the generated excitons.For the third and fourth OSCs, the resulting loss of efficiency is might due to the increase of disordered molecular structure owing to cracks in the active layer when the thermal annealing temperature is increased over 55 °C [63,64].

Conclusion
The effect of SVA with thermal annealing of D18:Y6 layer as post-treatment on OSCs based on D18 as conjugated polymer and Y6 non-fullerene acceptor using PDINN as the ETL and PEDOT: PSS as HTL was investigated.The applied thermal annealing at 55 °C with SVA on D18:Y6 layer led to optimize phase separation and enhance surface morphology of the active layer.An efficiency of 16.5 % was obtained for device with D18:Y6 thermal annealed at 55 °C with SVA as post-treatment.

Figure 1
Figure 1 OSCs architecture and chemical structure of active layer (a), UV-visible absorbance spectra of D18 and Y6 thin films onto glass without thermal annealing (b), UV-visible absorbance spectra of D18 and Y6 with different thermal anneal-

Figure 2
Figure 2 PL spectra of D18 neat films treated at different temperatures with chloroform (a), D18 and D18:Y6 (1:1.6)treated with only chloroform (b) and D18 at 55 °C treated with chloroform and D18:Y6 films at different temperatures with chloroform (c).

Figure 6
Figure 6 TGA a and DSC b of pure D18 and Y6 powder.

Figure 7
Figure 7 Dark J-V curve for D18:Y6(1:1.6)solar cells based active layer treated at different annealing temperatures with chloroform (a) and J-V curves of D18:Y6(1:1.6) solar cells-based active layer treated at different annealing temperatures with chloroform (b).

Table 1
Surface tension values of neat donor and acceptors films and interfacial energy values between blend films