Roll-coating fabrication of ITO-free flexible solar cells based on a non-fullerene small molecule acceptor

Abstract

We report organic solar cells (OSCs) with non-fullerene small molecule acceptors (SMAs) prepared in large area via a roll coating process. We employ all solution-processed indium tin oxide (ITO)-free flexible substrates for inverted solar cells with a new SMA of F(DPP)₂B₂. By utilizing poly(3-hexylthiophene) as donor blended with F(DPP)₂B₂ as acceptor, ITO-free large-area flexible SMA based OSCs were produced under ambient conditions with the use of slot-die coating and flexographic printing methods on a lab-scale compact roll-coater that is readily transferrable to roll-to-roll processing. The effect of different processing solvents on the device performance was investigated, and the best performance with a power conversion efficiency of 0.65%, an open circuit voltage of 0.85 V, a short-circuit current density of 2.19 mA cm⁻², and a fill factor of 35% was obtained.

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Introduction

Organic photovoltaics (OPV) show promise as a next-generation green energy technology because of their ease of fabrication, flexibility, and capability for large-scale and low-cost production.^1–4 With the rapid progress during the last few years,^5–9 power conversion efficiencies (PCEs) exceeding 10% have been achieved for single junction organic solar cells (OSCs),^10–12 and up to nearly 12% (11.5% (ref. 13) and 11.83% (ref. 14)) for triple-junction OSCs. In order for OSCs to succeed as a commercial photovoltaic technology, significant efforts have been made when progressing from lab-scale to industry-scale production.¹⁵ However, the commonly used deposition methods, such as spin-coating in inert atmosphere and vacuum thermal evaporation deposition, are not compatible with mass production. Recently, roll-to-roll (R2R) based production methods have been demonstrated as an promising way for industrial mass production of OSCs and attracted increasing attention due to their merits of high speed, high throughput, ambient conditions, indium tin oxide (ITO)-free electrode, large area and flexibility.^16–20 To date, the best reported PCEs for fully printed, ITO- and vacuum-free single junction polymer and small molecule flexible solar cells is 3.5% (ref. 21) and 1.01% (ref. 22) (over an area of 1 cm²), respectively, which were much lower than those based on small device area in laboratory. Therefore, more efforts have to be made towards large area fabrication combined with high throughput R2R processing methods.

Conventional bulk-heterojunction (BHJ) OSCs are based on polymers or small molecules as electron donors and fullerene derivatives as electron acceptors.^23–25 Despite of the essential role of fullerenes in achieving the best-performing BHJ OSCs, fullerene derivatives have been shown with several drawbacks such as potential lack of morphological stability, less tunable electronic structure, weak absorption in the visible region and high production cost.^26–28 For this reason, non-fullerene small molecule acceptors (SMAs) have been reported as alternatives which will not only retain the favorable properties of fullerene derivatives, but that will also overcome their insufficiencies.^29–34 They also offer possibilities to fabricate devices with materials obtained with simple and cost effective synthetic processes. Driven by these promises, there have been intensive research efforts in non-fullerene SMAs based BHJ OSCs, with high PCEs up to 5.0–6.3% achieved by a few research groups in laboratory.^35–39 Nevertheless, there were few reports in large area non-fullerene SMAs based BHJ OSCs with R2R processing regardless of their growing position in electron acceptors for OPV. Chen et al. reported the first roll coated BHJ OSCs based on a cross-linkable SMA with an efficiency of 0.067%,⁴⁰ which was relatively low and large efforts are still needed to improve the performance.

We recently designed and synthesized a new diketopyrrolopyrrole (DPP) derivative based acceptor compound,⁴¹ namely F(DPP)₂B₂, in which a fluorene ring is functioned as the core, two DPP units as the arms, and two benzene rings as the end groups (as shown in Fig. 1a). F(DPP)₂B₂ exhibits a relatively high electron mobility of 2.8 × 10⁻⁴ cm² V⁻¹ s⁻¹ and shows intense absorptions in the range of 550–700 nm. The highest occupied molecular orbital (HOMO) energy level and the lowest unoccupied molecular orbital (LUMO) energy level of F(DPP)₂B₂ are −5.21 and −3.39 eV, respectively, which matching those of poly(3-hexylthiophene) (P3HT). Thus, when it was blended with P3HT acting as electron acceptor, the resulting small area (0.054 cm²) device exhibits a PCE up to 3.17%, which is among the highest values ever reported for P3HT and non-fullerene acceptor based OSCs.⁴¹

Fig. 1 (a) A schematic illustration of the device structure and chemical structures of P3HT and F(DPP)₂P₂. (b) Pictures showing slot-die coating of the active layer/PEDOT:PSS layers (top), flexographic printing of the silver grids electrode (middle) and completed roll coated ITO-free flexible solar cells (bottom).

In this work, we report the fully roll coating fabrication of ITO-free flexible solar cells based on the non-fullerene small molecule acceptor F(DPP)₂B₂. Our devices were fabricated on a mini roll-coater which is constructed to mimic the coating performed on full scale R2R processing equipment.^42,43 As shown in Fig. 1a, an inverted device geometry was adopted with P3HT:F(DPP)₂B₂ blend as the active layer and poly(ethyleneterephthalate) (PET) foil with silver grids as the transparent substrate. All the fabrication procedures were completed at atmosphere with the silver grids (bottom and top layers) printed by flexographic printing whereas the remaining layers all fabricated by slot-die coating (Fig. 1b). Through carefully optimizing of devices, we achieved a high PCE of 0.65%, combined with an open circuit voltage of 0.85 V, a short-circuit current density of 2.19 mA cm⁻² and a fill factor of 35% for our devices, which, so far, was the best performance for roll coated ITO-free flexible SMA based BHJ OSCs.

Experimental

Materials

P3HT with a M_n of 40 000 Da was obtained from Plextronics. F(DPP)₂B₂ was synthesized in our laboratory according to the literature procedure.⁴¹ All the solvents were commercially available and used as received.

Coating was performed on a ITO-free semitransparent PET with silver grid substrate known as Flextrode, which comprises a silver grid, semitransparent conductor and electron transport layer (ETL), and provides similar performances to analogous OSCs prepared on ITO covered substrates as described in previous literature.⁴⁴ The PET substrate Melinex ST506 was obtained from Dupont-Teijin. Front silver was PFI-722 from PCHEM. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) PH1000 from Heraeus diluted with isopropyl alcohol in the ratio 10 : 3 was employed as semitransparent conductor. The front PEDOT:PSS electrode had a sheet resistivity of 60–90 Ω per square and PET substrate with Ag grid had sheet resistivity of 10 Ω per square. ETL was coated using a stabilized ZnO nanoparticle solution in acetone (49 mg mL⁻¹). Three different PEDOT:PSS formulations (AI 4083 and F010 diluted with isopropanol in different ratios) were coated at the back of devices. In the case of PEDOT:PSS 4083 it was mixed in a ratio of 1 : 2 with isopropanol. The other two PEDOT:PSS F010 was diluted with IPA in a ratio of 1 : 4 or 1 : 1 respectively. The printable silver back electrode used was PV410 from Dupont. The sheet resistivity of the back PEDOT:PSS electrode was 60 Ω per square on its own and the sheet resistivity of the top silver electrode was 0.1 Ω per square. The cells were encapsulated between two 15 × 15 mm² glass slides with a UV curable epoxy resin (DELO LP655) adhesive and cured for 2 min under the solar simulator (1000 W m⁻², AM 1.5G).

Roll-coated device fabrication

We prepared active material inks (concentration, 25 mg mL⁻¹) comprising a blend of P3HT:F(DPP)₂B₂ (w/w, 1 : 1.5) in different solvents (chloroform, chlorobenzene, dichlorobenzene), which then were transferred from an external container to the slot-die head via a pump. The coating of the active layer was conducted at a speed of 0.65 m min⁻¹. The roll was heated to 70 °C to allow a quick and uniform drying of the films.²⁰ The flow through the head was set to 0.072 mL min⁻¹ resulting in a wet thickness of 11 μm and an estimated dry thickness of 250 nm. The top electrode consisted of 4 layers; a thin layer of PEDOT:PSS F010 (dilution ratio 1 : 4) was coated as a compatibility layer with a flow of 0.06 mL min⁻¹ for a wet thickness of 8 μm. On top of the wetting layer, a PEDOT:PSS 4083 as a hole transport layer was employed, coated with a flow of 0.15 mL min⁻¹ for a wet thickness of 23 μm. Then a highly conducting layer was coated by using PEDOT:PSS F010 (dilution ratio 1 : 1) with a flow of 0.25 mL min⁻¹ for a wet thickness of 38 μm, following a drying process at 70 °C on the roll for about 20 min. The top silver grids were applied by flexographic printing of a silver paste PV410 (Dupont) with a web speed of 1.2 m min⁻¹ and roll temperature of 80 °C. The completed solar cells were then removed from the roll and annealed at 120 °C in a hot air oven for 5 min. Finally, hundreds of individual cells with an active area of 1 cm² were divided from the substrates for encapsulation and characterization. The completed roll coated ITO-free flexible solar cells were shown in Fig. 1b.

Film and device characterization

The current density–voltage (J–V) curves were measured with a Keithley 2400 sourcemeter at room temperature in air. The photovoltaic response was measured under a KHS 575 solar simulator with an AM 1.5G 100 mW cm⁻² intensity, and the light intensity was calibrated with a standard photovoltaic reference cell. The external quantum efficiency (EQE) spectra were measured using a Solar Cell Spectral Response Measurement System QEX10. The atomic force microscopy (AFM) images were taken on a Veeco Multimode atomic force microscopy in the tapping mode. The light beam induced current (LBIC) experiments were carried out using a custom made setup⁴⁵ with 410 nm laser diode (5 mW output power, 100 μm spot size ≈ 65 W cm⁻², ThorLabs) mounted on a computer controlled XY-stage and focused to a spot size of <100 μm. The short circuit current from the device under study was measured using a computer controlled source measure unit (SMU, a Keithley 2400 instrument). A custom written computer program was used to scan the solar cell devices in a raster pattern in 200 μm steps in the X and the Y directions, logging the coordinates and measured current. The results were then converted to yellow/blue colored bitmaps in 255 different hues with another custom written program. Bright yellow represents the highest absolute current extracted while blue represents the lowest current. Current profiles along selected directions were taken from these maps to visualize the relative differences in different regions.

Results and discussion

Active material inks comprising a blend of P3HT:F(DPP)₂B₂ in different solvents, such as chloroform (CF), chlorobenzene (CB), dichlorobenzene (DCB) were prepared. The preliminary optimizing of devices were conducted by varying different donor/acceptor (D/A) weight ratios and active layer thicknesses. The optimum performance for the CB based devices was obtained with 1.0 : 1.5 D/A weight ratios and 250 nm film thickness. The effects of solvents on the film quality and device performance were further investigated and discussed in details. Their J–V curves and EQE curves with different solvents of CF, CB, and DCB are shown in Fig. 2, and the corresponding performance parameters are summarized in Table 1. Devices with the three different solvents demonstrated a distinct difference in performance. By coating from CB solution with a boiling point of 131 °C, which is often reported for R2R processed solar cells, the most even and uniform films were achieved (Fig. 3a) with >90% yield of working devices. The highest PCE was 0.65%, with a short-circuit current (J_sc) of 2.19 mA cm⁻², open-circuit voltage (V_oc) of 0.85 V and fill factor (FF) of 0.35. Compared to CB based devices, the uniformity at the edge of the coated stripe was slightly worse for CF based films which lead to a lower device yield of 70%, due to the low boiling point of CF (61 °C) and therefore fast drying of the solution in the outer parts of the slot-die head before being transferred to the substrate. The performance of CF based devices shown a maximum PCE of 0.60% with a decreased V_oc of 0.78 V while almost the same J_sc and FF as that of CB based devices. However, we could not obtain higher performance despite of the higher boiling point of DCB (approximately 180 °C) which allows for a longer drying time and thereby a longer time for the crystallization of the bulk heterojunction. Some holes and non-uniform agglomeration were developed in the coated film during drying and half of the produced devices can not work (Fig. 3a). These pinholes could also lead to large leakage current and decrease the V_oc.⁴⁶ As we can see from the working devices, a much lower PCE of 0.23% was achieved due to the much lower V_oc and FF. It was also found that different solvents resulted in a remarkable difference of EQE values covering the range of 300–700 nm (Fig. 2b), displaying a similar value of 12% for CF and CB based devices, a lower value of 10% for DCB based devices. These results are consistent with the varying trend of the J_sc as illustrated previously.

Fig. 2 (a) J–V and (b) EQE curves of devices with the structure Ag/PEDOT:PSS/ZnO/P3HT:F(DPP)₂B₂ (w/w, 1 : 1.5)/PEDOT:PSS/Ag prepared from different solvents.

Table 1 Summary of the J–V data of devices based on different solvents

Solvent	Voc (V)	Jsc (mA cm^−2)	FF	PCE (%)		Yield (%)
				Max	Average
a Roll-coated device.b Spin-coated device with P3HT:F(DPP)2B2 as active layer.
CF^a	0.78	2.20	0.35	0.60	0.53 ± 0.06	70
CB^a	0.85	2.19	0.35	0.65	0.61 ± 0.03	>90
DCB^a	0.44	1.94	0.27	0.23	0.17 ± 0.04	50
CF^b	1.18	5.35	0.50	3.17	—	—

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Fig. 3 (a) Film uniformity of the as coated P3HT:F(DPP)₂B₂ film before the coating of PEDOT:PSS (from top to bottom: CF based film, CB based film and DCB based film). (b) PCE values and deviation values for the cells produced. The inset is the LBIC image of a solar cell fabricated from CB solvent.

As shown in Fig. 3b, not only the average PCE is the highest for devices coated using CB solvent but also that the deviation in the PCEs of the devices is the smallest when the CB solvent is used because more uniform films are achieved. Ten working devices fabricated from CF solution show the PCEs from 0.45% to 0.6%, with a range of 0.15%. For the devices coated from DCB solution, their PCEs ranged from 0.12% to 0.23%, varying by 0.11%. However, the PCEs of the devices prepared by CB solution differed by the smallest amount of 0.09%, ranging from 0.56% to 0.65%. The large deviation of PCE values for the devices prepared using CF or DCB solutions can be attributed to the too quickly or too slowly drying of the films, which resulted in uncontrollable wave or drying defects, respectively. To further investigate the solar cell behavior, we carried out the mapping of devices coated from CB solution by employing a light beam induced current (LBIC) technique⁴⁵ and the optical image was shown in the inset of Fig. 3b, which indicates a good current response over the whole region due to the low concentration of defects in the devices.

The active layer morphology of the OSC devices prepared from CF, CB and DCB was examined by atomic force microscopy (AFM) in tapping mode to further investigate the solvent effect on the device performance. Fig. 4 shows the AFM height and phase images of the blend films of P3HT:F(DPP)₂B₂ from different solvents. The blend films based on P3HT:F(DPP)₂B₂ prepared from different solvents exhibited typical cluster structures with many crystalline domains, with root-mean-square (RMS) roughness of 9.6, 11.0, 6.1 nm for CF, CB, and DCB solvents, respectively. The domain sizes are all in the range of 100–200 nm shown in the phase images, indicating efficient exciton dissociation and charge carrier transport in these three films. However, compared to the film coated from DCB solution, the film coated from CB or CF solution shows more distinctive morphology with clearer nanophase separation, which accounts for the higher values of J_sc and FF, resulting in an enhancement in the overall PCE.²⁵

Fig. 4 AFM height images ((a)–(c)) and phase images ((d)–(f)) of P3HT:F(DPP)₂B₂ blend films coated from different solvents.

Furthermore, a comparison was carried out between our roll-coated device in this work and the spin-coated device as described in the previous report.⁴¹ Their J–V and EQE curves are shown in Fig. 5a and b, while their performance parameters are summarized in Table 1. We found the PCE of our roll-coated device was nearly five times lower than that of spin-coated device due to the drop in overall performance including V_oc, J_sc and FF. It should be noted that the difference of device performance can be affected by many factors related to the different processing conditions such as substrates, solvents, blend ratios, operating humidity and temperatures.^19,22,47 However, an obviously important reason for the remarkable disparity of device performance can be attributed to the different morphologies of roll-coated and spin-coated films, which were revealed by AFM measurement and shown in Fig. 5c–f. Compared with our roll-coated film, the spin-coated film exhibits a much smaller domain size (40–60 nm) and displays more interconnected donor–acceptor phase separation, giving the percolated network for more efficient exciton separation, charge transportation and thus better photovoltaic properties. This comparison indicates that the film morphologies is still a limitation factor for efficiency of roll-coated devices and more efforts need to be devoted to the morphology control with the roll-coated films.

Fig. 5 Comparison of J–V curves (a), EQE curves (b), AFM height images ((c) and (d)) and phase images ((e) and (f)) between roll-coated device and spin-coated device with P3HT:F(DPP)₂B₂ as active layer.

Conclusions

In conclusion, we successfully reported the first example of R2R fabrication of ITO-free flexible solar cells based on non-fullerene small-molecule acceptor. The devices were prepared by slot-die roll coating and flexographic printing techniques on a mini roll coater without the use of spin-coating or vacuum evaporation methods. Effects of different processing solvents were investigated and a PCE up to 0.65% with a yield of >90% was achieved for the cells fabricated from a CB solution of P3HT:F(DPP)₂B₂ blend. Although the PCEs of these ITO-free flexible solar cells are relatively low compared to those of small area solar cells fabricated on ITO glass substrates, this work opens a venue for the potential application of small-molecule, nonfullerene acceptor in R2R flexible solar cells and will be helpful for the further advance of non-fullerene organic solar cells.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants 51261130582 and 91233114), the Danish National Research Foundation and the Major State Basic Research Development Program (2014CB643503), and the program for Innovative Research Team in University of Ministry of Education of China (IRT13R54). This work was carried out in the Danish Chinese Centre of Organic-Based Photovoltaics with Morphological Control.

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