Replacing the metal oxide layer with a polymer surface modifier for high-performance inverted polymer solar cells

Abstract

Replacing ZnO with PEIE, both as a surface modifier for the low work function electrode and as an electron selective layer, enhances the performance and air stability of inverted polymer solar cells by improving electron transport, wettability between the active layer and the cathode, and maximizing light absorption within the active layer without light interference.

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Polymer solar cells (PSCs) have attracted much attention because of their many advantages including their low cost, light weight, and solution-based fabrication on large-area and flexible substrates.^1–4 Significant efforts in designing semiconducting polymers,^5–8 developing device architectures,^9–11 and controlling the morphology of the active layer via post-treatments,^3,12,13 have improved the power conversion efficiency (PCE) of conventional PSCs (cPSCs) by up to 9%. For the fabrication of cPSCs, poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) and low work function (WF) metals, such as aluminum (Al) and calcium (Ca), are commonly used for the hole transporting layer and the metal cathode, respectively. However, the acidic nature of PEDOT:PSS is detrimental to the active layer because it can etch the indium tin oxide (ITO) substrate and increases the interfacial resistance through indium diffusion into the active layer.¹⁴ In addition, low WF metals are vulnerable to oxidation in air, leading to the degradation of device performance.^15,16

To overcome the poor device stability of cPSCs, an inverted device structure has been developed by employing metal oxides as the electron selective layer (ESL) and high WF metals (e.g. gold) as the anode.^17–20 In spite of the many advantages of metal oxides, such as their excellent electron mobility and transparency in the visible light region, a metal oxide layer can reduce light absorption within the active layer by light absorption in the ultraviolet (UV) wavelength region and light interference.^21,22 Furthermore, the inherent incompatibility between the organic active layer and the inorganic metal oxide leads to high contact resistance and charge recombination loss which limit the performance of inverted PSCs (iPSCs).^9,23,24 The introduction of various organic surface modifiers, such as conjugated polyelectrolytes,^9,25,26 self-assembled monolayers,^27–29 and ionic molecules,^24,30 on the metal oxide layer can alleviate interfacial resistance at the interface of the active layer/metal oxide and thus improve the device efficiency. However, this layer-by-layer process makes device fabrication long and complex. Recently, polymer surface modifiers, such as ethoxylated polyethylenimine (PEIE) and poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), have been introduced to produce low WF electrodes and showed good electron selectivity for organic optoelectronic devices without an n-type metal oxide layer.^10,15,31

Here, we demonstrate highly efficient iPSCs by replacing zinc oxide (ZnO) with PEIE as both a surface modifier for the low WF electrode and as the ESL. The interfacial dipole formed by the PEIE layer leads to an electron transport cascade from the active layer to the ITO cathode without a ZnO layer. The iPSCs with PEIE exhibited a higher short-circuit current density (J_SC) of 14.4 mA cm⁻², fill factor (FF) of 0.68, and power conversion efficiency (PCE) of 7.02%, compared to those of ZnO (J_SC = 13.4 mA cm⁻², FF = 0.65, and PCE = 6.26%). This enhanced device performance is attributed to the PEIE layer, which plays a versatile role to maximize the light absorption within the active layer, to facilitate electron transport by lowering the energy barrier between the active layer and the ITO cathode, and to improve the wettability between them.

The PEIE layer is known for producing low WF electrodes in organic optoelectronics.³¹ Fig. 1 shows energy band diagrams of components of bilayer and bulk heterojunction (BHJ) devices, where the PEIE layer effectively reduced the WF of ZnO and ITO in bilayer (Fig. 1a) and BHJ devices (Fig. 1b). The surface of ZnO or ITO was modified with a PEIE solution (0.4 wt% in 2-methoxyethanol) via a spin-coating method. The PEIE layer makes interfacial dipoles at the ZnO/polymer and ITO/active layer interfaces, leading to a reduced WF of ZnO (from 4.5 eV to 3.8 eV) and ITO (from 5.1 eV to 4.1 eV).¹⁵ Since the open-circuit voltage (V_OC) of bilayer devices is affected by the difference between the HOMO level of the donor polymer and the conduction band of ZnO,^24,32 introducing PEIE at the donor–acceptor interface can maximize V_OC. Furthermore, PEIE can facilitate electron transport by lowering the energy barrier between the active layer and the ITO cathode in BHJ devices.

Fig. 1 Energy band diagrams of components of (a) bilayer and (b) BHJ devices with and without a PEIE layer.

To investigate the effect of PEIE on device performance, we fabricated bilayer devices with a configuration of ITO/ZnO/(PEIE)/polymer/MoO₃/Au and BHJ devices with a configuration of ITO/ESL/active layer/MoO₃/Au using different ESLs (ZnO, PEIE, and ZnO/PEIE). For the bilayer devices, we used poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]thieno-[3,4-b]thiophenediyl]] (PTB7) and poly(3-hexylthiophene) (P3HT) as electron donors and ZnO as an electron acceptor. For BHJ devices, we used a blended solution of PTB7:[6,6]-phenyl-C₇₀ butyric acid methyl ester (PC₇₀BM) and P3HT:PC₆₀BM as the active layer. Fig. 2a and b show the current density versus voltage (J–V) characteristics of bilayer and BHJ devices, respectively. The detailed characteristics of the corresponding devices are summarized in Table 1. The PTB7/ZnO bilayer device exhibited a short-circuit current density (J_SC) of 0.79 mA cm⁻², V_OC of 0.46 V, fill factor (FF) of 0.51, and power conversion efficiency (PCE) of 0.19%. The introduction of a PEIE layer between PTB7 and ZnO led to a 54% enhancement (0.46 V → 0.71 V) in V_OC. Similarly, the P3HT/ZnO device with a PEIE layer also showed remarkable V_OC enhancement from 0.38 V to 0.80 V (Fig. S1a†). Interfacial dipoles formed by the PEIE layer effectively reduced the work function of ZnO by shifting the band edge of the ZnO closer to the vacuum level of the polymer, thus leading to an enhanced V_OC. However, the resulting PCEs of bilayer devices with PEIE were not higher than those of the devices without PEIE due to significant decreases in J_SC and FF, which can be attributed to the interruption of charge transfer from the polymer to the ZnO or charge trapping by interfacial dipoles within the PEIE layer.^24,33 The PTB7:PC₇₀BM BHJ device with ZnO exhibited a J_SC of 13.40 mA cm⁻², V_OC of 0.71 V, FF of 0.65, and PCE of 6.26%. Replacing ZnO with a PEIE layer significantly improved the device performance. The PTB7:PC₇₀BM BHJ device with PEIE showed a J_SC of 14.40 mA cm⁻², V_OC of 0.71 V, FF of 0.68, and PCE of 7.02%. In contrast, a combined layer of ZnO and PEIE (ZnO/PEIE) resulted in a decreased J_SC (12.60 mA cm⁻²) and maintained similar V_OC and FF. Since the V_OC value is determined by the energy difference between the HOMO of the electron donor and the LUMO of the electron acceptor in BHJ devices, there were no changes in the V_OC values of devices with different ESLs. The same tendencies were observed in P3HT:PC₆₀BM BHJ devices (Fig. S1b and Table S1†). The P3HT:PC₆₀BM BHJ device with PEIE showed a higher J_SC of 11.40 mA cm⁻² and PCE of 4.04% than those of the devices with ZnO (J_SC: 9.03 mA cm⁻², PCE: 3.21%) or ZnO/PEIE (J_SC: 8.76 mA cm⁻², PCE: 3.12%), whereas V_OC and FF remained almost constant. The differences in device performances caused by different ESLs mainly originated from an increase or decrease in J_SC, which are consistent with the EQE measurements (Fig. 2c and S1c†). Compared to the devices with ZnO and ZnO/PEIE, devices with a PEIE layer exhibited broad and high EQEs ranging from 300 nm to 750 nm in both PTB7:PC₇₀BM and P3HT:PC₆₀BM BHJ devices. In addition, the PEIE layer plays an important role in facilitating electron transport and blocking hole transport from the active layer to the ITO cathode as an ESL instead of ZnO, which is supported by the J–V characteristics in darkness (inset of Fig. 2b and S1d†). The devices with a PEIE layer showed a low leakage current at reverse voltage and a high rectification ratio (∼10⁵), compared to the devices with only a ZnO layer (<10⁵). The highest rectification ratio (>10⁵) belonging to the devices with a combined layer of ZnO and PEIE is attributed to the well-aligned dipoles of PEIE on top of the ZnO layer. These results can support a high FF for the devices with a PEIE layer.

Fig. 2 Comparison of the J–V characteristics in (a) bilayer and (b) BHJ devices with different ESLs, and (c) the corresponding EQE curves of BHJ devices. The inset of (b) shows the J–V characteristics in darkness of BHJ devices with different ESLs.

Table 1 Characteristics of PTB7/ZnO-based bilayer and PTB7:PC₇₀BM-based BHJ devices with different ESLs

Device structure	Device configuration	JSC [mA cm^−2]	VOC (V)	FF	PCE [%]	JSC (calc.) [mA cm^−2]
Bilayer	ITO/ZnO/PTB7/MoO3/Au	0.79	0.46	0.51	0.19	—
	ITO/ZnO/PEIE/PTB7/MoO3/Au	0.27	0.71	0.29	0.06	—
BHJ	ITO/ZnO/PTB7:PC70BM/MoO3/Au	13.40	0.71	0.65	6.26	13.25
	ITO/PEIE/PTB7:PC70BM/MoO3/Au	14.40	0.71	0.68	7.02	14.59
	ITO/ZnO/PEIE/PTB7:PC70BM/MoO3/Au	12.60	0.71	0.66	5.88	12.68

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To investigate the effect of the PEIE layer on J_SC enhancement, we performed reflectance measurements on BHJ devices with the same architecture (Fig. 3a). We also prepared a device without an ESL as a reference. A blended solution of PTB7:PC₇₀BM was used as the active layer. The reflectance spectra of the devices with ZnO, PEIE, and ZnO/PEIE layers are shown in Fig. 3b. The reflectance spectrum of the device with PEIE was almost identical to that of reference device, whereas the reflectance spectra of the devices with ZnO and ZnO/PEIE were entirely changed by the ZnO layer. To clarify the change in light absorption by different ESLs, we calculated absorption changes (Δα) from the reflectance measurements (Fig. 3c).^2,34,35 Compared to the reference device without an ESL, there was a negligible change in light absorption for the device with PEIE. In contrast, significant decreases in light absorption in the ranges of 350–450 nm (UV region) and 500–650 nm (visible region) were observed in the devices with ZnO and ZnO/PEIE. This implies that the enhanced J_SC caused by replacing ZnO with a PEIE layer is attributed to an increase in light absorption.

Fig. 3 (a) Device structures, (b) reflectance spectra, and (c) absorption changes (Δα) of PTB7:PC₇₀BM iPSCs with different ESLs.

To confirm that the J_SC improvement caused by the PEIE layer originates from increased light absorption, we compared the measured EQE enhancement and simulated EQE from optical modelling.^36,37 Fig. 4a shows the measured and simulated EQE difference (ΔEQE) between the devices with ZnO and PEIE layers. The EQE difference is calculated by subtracting the EQE of the device with ZnO from the EQE of the device with PEIE. For optical modelling, we used a ZnO layer with a thickness of 60 nm and an active layer with a thickness of 140 nm which are measured by a surface profiler. Since an ultrathin PEIE layer (below 10 nm) has a negligible effect on light absorption, diffraction, and scattering, we performed optical modeling using device structures with and without a ZnO layer. The simulated EQE difference from optical modelling is consistent with the measured EQE difference (Fig. 4a) as well as the Δα (Fig. 3c). Based on optical modeling, we also estimated the J_SC of the devices with and without ZnO as a function of the active layer thickness (Fig. S2†). The estimated J_SC values (ZnO: 13.8 mA cm⁻¹, PEIE: 14.5 mA cm⁻¹) are in good agreement with the measured values (ZnO: 13.4 mA cm⁻¹, PEIE: 14.4 mA cm⁻¹) at a real active layer thickness of 140 nm. The simulation results confirm that the enhanced J_SC originates from there being no substantial light absorption within the ultrathin PEIE layer.

Fig. 4 (a) Measured and simulated ΔEQE between the devices with ZnO and PEIE. (b) The dependence of PCE on air exposure time in PTB7:PC₇₀BM iPSCs with different ESLs.

We also used atomic force microscopy (AFM) and measured contact angles to investigate the changes in the surface morphology and hydrophobicity of ITO and ZnO caused by the PEIE layer. Bare ITO was prepared for comparison. In the AFM topography images (Fig. S3†), compared to bare ITO (root-mean-square (RMS) roughness of 0.80 nm), the PEIE film exhibited a smooth surface with RMS roughness of 0.30 nm, whereas ZnO and ZnO/PEIE films showed high RMS roughnesses of 1.40 nm and 1.30 nm, respectively. For contact angle measurements (Fig. S4†), we used a mixed solvent instead of water, which was the same solvent for device fabrication. The ITO and ZnO films showed contact angles of 5° and 17°, respectively. In contrast, there were no measurable contact angles after PEIE coating on top of both the ITO and ZnO layers. These results imply that the PEIE layer makes the surface of the metal oxide layer smooth and hydrophobic, leading to improved wettability between the active layer and the metal oxide.

The device stability of iPSCs with metal oxides is a considerable advantage over conventional PSCs. We tested the air stability of the devices with different ESLs over 50 h (Fig. 4b). Inevitably, the device with ZnO showed a slight decrease in PCE after air exposure times over 50 h. It is worthy of note that the air stability of devices with only a PEIE layer is comparable to that of the devices with ZnO and ZnO/PEIE. These results imply that the PEIE layer is an effective interfacial layer between the active layer and the electrode to simultaneously improve device efficiency and stability.

In conclusion, we have demonstrated high-performance iPSCs using a PEIE layer as both a surface modifier and an ESL. Although light absorption within the active layer can ideally be maximized by eliminating layers under the active layer in iPSCs, dissociated electrons within the active layer have difficulty in being transported to the ITO cathode in the absence of an ESL, leading to recombination loss and poor device performance. The PEIE layer maximizes light absorption within the active layer without light interference, while it facilitates electron transport by lowering the energy barrier between the active layer and the ITO electrode as an excellent ESL and also improves the wettability between them, giving rise to an increase in J_SC and FF. Furthermore, replacing ZnO with a PEIE layer not only enhances the device efficiency but also improves device stability in air. The solution-processability and low temperature manufacturing process of polymer surface modifiers offer a way to enhance the performance of organic optoelectronics via a roll-to-roll mass production technique.

Acknowledgements

This research was supported by the National Research Foundation of Korea Grant (NRF-2009-0093020, NRF-2013R1A2A2A01015342), the International Cooperation of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (2012T100100740).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Optical modeling, AFM images, and contact angle measurements as well as an Experimental section, additional J–V characteristics and tables of devices. See DOI: 10.1039/c3ra46180g

‡ These two authors contributed equally to this work.

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