Conventional polymer solar cells with power conversion efficiencies increased to >9% by a combination of methanol treatment and an anionic conjugated polyelectrolyte interface layer

Abstract

The power conversion efficiency of PTB7:PC₇₁BM polymer solar cells was improved to 9.1% by treatment with methanol followed by a water- and alcohol-soluble conjugated polyelectrolyte cathode interface layer. This improvement in efficiency is a result of a combination of an enriched PC₇₁BM ratio on the top surface in the active layer and the presence of the preferred dipole at the cathode interface.

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Bulk heterojunction polymer solar cells (PSCs) have attracted much attention as a result of their light weight, flexible properties and low cost of manufacturing based on roll-to-roll processing.^1–3 The power conversion efficiency (PCE) of PSCs has improved rapidly over the past few years with the development of new conjugated polymers,^4–7 the application of new device structures^8–11 and interface engineering.^12–14 Interface engineering is critical in PSCs and the performance of these devices can be dramatically improved by incorporating a functional interfacial layer between the active layers and the electrodes.¹⁵ Various interfacial materials, such as metal oxides,^16–19 self-assembled monolayers²⁰ and hydrophilic polymers^21–24 have been successfully applied to enhance the PCE of fabricated PSCs. As an alternative, the use of water- or alcohol-soluble polyelectrolytes has been explored because these materials avoid intermixing between the active layer and the cathode interface layer. Different kinds of water- or alcohol-soluble hydrophilic polymers,²⁵ such as conjugated polyelectrolytes (CPEs) (e.g., PFN, PFN-Br, P3TMAHT and PFCn6:K^+15,22–24) and non-conjugated polymers (e.g., PEO, PEI and PEIE^26–28) have been used as cathode interface layers (CILs) in PSCs and have been used to achieve significantly enhanced PCEs. By incorporating PFN or a derivative of PFN with a metallic backbone as a CIL, PCEs >9.0% have been obtained in inverted PTB7:PC₇₁BM PSCs.^29,30

It has been reported that the performance of PSCs may be enhanced by treating the active layers with polar solvents before the deposition of the metal electrodes.^31–35 The positive effects of the polar solvents include: the optimization of the phase separation in the active layer and a possible influence on the interface between the active layer and the PEDOT:PSS layer underneath;³¹ an increase in the built-in voltage across the device because of the passivation of surface traps; and a corresponding increase in the surface charge density.³² Polar solvents, including methanol and ethanol, are widely used to prepare CPE solutions. Low concentrations are used to minimize the thickness and to prevent possible complications from ion motion and the concomitant redistribution of the internal electric field in these devices. However, it is not clear whether the improvement in the performance of the PSCs after CPE deposition is a result of the combination of the effects of methanol treatment and the presence of the thin CPE layer. These interesting phenomena and attractive effects need to be investigated further.

In this study, a series PSCs based on poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b]dithiophene-2,6-diyl-alt-ethylhexyl-3-uorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) (Fig. 1) was fabricated to investigate the possible influence of the polar solvent and the CPEs. Methanol was chosen as the solvent because it is widely used as a solvent for CPEs.^15,23 These experiments involved the deposition of methanol on top of the active layer followed by a sequence of steps similar to those used for CPE devices. An anionic CPE, poly(9,9-bis(4-(sulfonatobutyl)-2,7-fluorene)-alt-2,7-(9,9-bis(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-fluorene)) (PF_EOSO₃Na) was used as the CIL. The PCE of PTB7:PC₇₁BM PSCs increases in a sequence according to the modifications of the interface: methanol < Ca < PF_EOSO₃Na < methanol/PF_EOSO₃Na. A combination of methanol treatment followed by a PF_EOSO₃Na layer gave the highest PCE of 9.1% by a combination of the effect of methanol treatment and the presence of the thin PF_EOSO₃Na layer.

Fig. 1 Schematic diagram of the structure of PTB7:PC₇₁BM PSCs and the chemical structures of PF_EOSO₃Na, PTB7 and PC₇₁BM.

Fig. 1 shows the device configuration of the PSCs ITO/PEDOT:PSS (40 nm)/PTB7:PC₇₁BM with or without a CIL of Al (100 nm). The optimized concentration for the PF_EOSO₃Na solution was 0.25 mg mL⁻¹. Fig. 2a shows the current density–voltage (J–V) curves for the optimized PTB7:PC₇₁BM PSCs with methanol treatment and a PF_EOSO₃Na CIL, together with some control devices. Table 1 summarizes the key parameters of the PSCs, including the short-circuit current density (J_sc), open-circuit voltage (V_oc), fill factor (FF) and PCE. The PCE increases in sequence according to the interface modifications: methanol < Ca < PF_EOSO₃Na < methanol/PF_EOSO₃Na. All the interfacial treatments improve the PCE of PTB7:PC₇₁BM PSCs compared with the control devices without interface modification. A combination of methanol treatment followed by PF_EOSO₃Na CIL gives the highest PCE of 9.06%, with a J_sc of 17.1 mA cm⁻², a V_oc of 0.74 V and a FF of 71.6%. The J_sc calculated by integrating the external quantum efficiency spectrum (Fig. 2b) is 16.1 mA cm⁻², which is very close to the measured J_sc.

Fig. 2 (a) Current density–voltage (J–V) characteristics of the PSCs with various treatments under AM 1.5G irradiation (100 mW cm⁻²). (b) External quantum efficiency spectra of the solar cells with methanol pre-treatment and a PF_EOSO₃Na cathode interface layer.

Table 1 Performance of PSCs with various interfacial treatments

Cathode	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)	Rs (Ω cm^2)	Rsh (kΩ cm^2)
None	0.52	13.8	55.3	3.95	8.39	0.43
Methanol	0.66	14.3	59.8	5.65	8.28	0.63
Ca	0.72	14.4	67.6	7.02	6.22	0.78
PFEOSO3Na	0.74	16.5	69.5	8.50	5.30	1.13
Methanol/PFEOSO3Na	0.74	17.1	71.6	9.06	2.45	5.46

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The PCE of the PSCs with methanol treatment on the active layer is 5.65%, much higher than the 3.95% obtained from devices without any modification of the interface. To study the effect of the modification by methanol treatment on the PTB7:PC₇₁BM films, we examined the surface morphology using atomic force microscopy (AFM), the surface potential by scanning Kelvin probe microscopy and the surface composition by X-ray photoelectron spectroscopy (XPS). The surface morphology of the PTB7:PC₇₁BM films before and after methanol treatment is shown in Fig. 3a and b. No obvious change in the morphology and roughness of the film was observed. In addition, no obvious change in film thickness was observed by profilometry after treatment with methanol. However, the average surface potential of the PTB7:PC₇₁BM films after methanol treatment was −266.43 mV, i.e., higher than the −316.59 mV obtained for the pristine PTB7:PC₇₁BM film. Methanol treatment lifts the vacuum level on the metal side, which reduces the electron injection barrier at the organic/metal interface. The XPS survey of the pure PTB7 film, pristine PTB7:PC₇₁BM film and PTB7:PC₇₁BM film with methanol pre-treatment is given in Fig. S1.† Sulfur is used as the characteristic element for PTB7 because there is no sulfur in PC₇₁BM. The C/S atomic ratios were used to calculate the weight ratios of the PC₇₁BM on the top surfaces of the PTB7:PC₇₁BM film. The C/S ratios of the pure PTB7 film and the PTB7:PC₇₁BM films with or without methanol pre-treatment are 10.22, 14.43 and 15.81, respectively. The C/S ratios of 10.22 for the pure PTB7 film obtained from the XPS measurements are close to the C/S ratio of 10.25 from the molecular formula of PTB7, implying that the measurement is reliable. After introducing PC₇₁BM, the C/S ratios of the PTB7:PC₇₁BM films with or without methanol pre-treatment increased, although the C/S ratios of the PTB7:PC₇₁BM film with methanol pre-treatment increased more obviously. This means that PC₇₁BM is richer at the surface of the PTB7:PC₇₁BM film after methanol pre-treatment, which is beneficial for electron extraction in a conventional device. It has been reported that minor changes are observed in the surface morphology and composition after treatment with polar solvents, including methanol, acetonitrile and ethanol.³¹

Fig. 3 AFM height images (5 μm × 5 μm) of PTB7:PC₇₁BM films with (a) no treatment and (b) with methanol treatment. Surface potential images (1 μm × 1 μm) of PTB7:PC₇₁BM films with (c) no treatment and (d) methanol pre-treatment.

In comparison with PTB7:PC₇₁BM PSCs using a Ca cathode, a simultaneous enhancement of the values of V_oc (0.74 V vs. 0.72 V), J_sc (16.5 mA cm⁻² vs. 14.4 mA cm⁻²), FF (69.5% vs. 67.6%) and the PCE (8.50% vs. 7.02%) was achieved for the PSCs with PF_EOSO₃Na CILs. In contrast with the morphology of the PF_EOSO₃Na CIL on P3HT:PC₆₁BM PSCs,³⁶ many holes with a radii of about 200–300 nm and a height of about 3–5 nm were observed in the PF_EOSO₃Na CIL on the PTB7:PC₇₁BM films (Fig. 4a). The surface potential of the PF_EOSO₃Na CIL (9.3 mV) is about 300 mV which is more positive than that of the active layer (Fig. 4b) and this provides a strong interfacial dipole layer between the cathode and the active layer and increases the V_bi. Thus, a favorable electrical field was developed as a result of the interfacial dipole and this influences charge transport and extraction.

Fig. 4 AFM height images (5 μm × 5 μm) of PTB7:PC₇₁BM films with (a) PF_EOSO₃Na CIL and (b) with methanol treatment and PF_EOSO₃Na CIL. Surface potential images (1 μm × 1 μm) of PTB7:PC₇₁BM films with (c) PF_EOSO₃Na CIL and (d) methanol treatment and PF_EOSO₃Na CIL.

A combination of methanol treatment followed by a PF_EOSO₃Na CIL further improves the PCE of PTB7:PC₇₁BM PSCs up to 9.06%. This major improvement is attributed to the values of the J_sc (17.1 mA cm⁻² vs. 16.5 mA cm⁻²) and FF (71.6% vs. 69.5%) compared with PSCs using PF_EOSO₃Na as the CIL. Although no obvious change in the morphology and thickness of the film (Fig. 4a and b) was observed, the surface potential of the PF_EOSO₃Na layer with methanol pre-treatment was 113.90 mV, i.e., about 100 mV higher than the 9.30 mV of the PF_EOSO₃Na layer. These results indicate that a combination of methanol treatment followed by a PF_EOSO₃Na layer can further improve the performance of PSCs by a combination of the effect of methanol treatment and the presence of the thin PF_EOSO₃Na layer, although the PF_EOSO₃Na solutions use methanol as the solvent. In addition, with a combination of methanol treatment and a PF_EOSO₃Na layer, the series resistance is reduced from 8.39 to 2.45 Ω cm², the shunt resistance is increased from 0.43 kΩ cm² to 5.46 kΩ cm² and the hole mobility is increased from 1.7 × 10⁻⁴ cm⁻² V⁻¹ s to 4.0 × 10⁻⁴ cm⁻² V⁻¹ s compared with PSCs without surface modification (Fig. S3†).

Conclusion

The PCE of PSCs based on PTB7:PC₇₁BM in a conventional configuration is increased to 9.1% by a combination of methanol treatment followed a PF_EOSO₃Na layer. The methanol treatment enriched the amount of PC₇₁BM at the surface of the PTB7:PC₇₁BM films and lifted the vacuum level on the metal side, which reduces the electron injection barrier at the organic/metal interface. The incorporation of an ultra-thin PF_EOSO₃Na interlayer contributed a strong interfacial dipole, which improved the surface potential of the active layer and benefited the collection of charge. A combination of methanol treatment followed by a PF_EOSO₃Na layer can further improve the performance of PSCs by a combination of the effect of methanol treatment and the presence of the thin PF_EOSO₃Na layer, although the PF_EOSO₃Na solutions use methanol as the solvent. This result provides a deeper insight into the mechanism of surface engineering by polar solvent processing and the introduction of polyelectrolytes. This strategy may offer a simple and efficient method to improve the PCEs of PSCs in laboratory studies or industrial fabrication.

Experimental section

Device fabrication

PTB7 was purchased from 1-Material Chemscitech Inc. (St-Laurent, Quebec, Canada) and was used as received. The device configuration was ITO/PEDOT:PSS/PTB7:PC₇₁BM/Ca/Al. The PTB7:PC₇₁BM solution was prepared at a ratio of 1 : 1.5 by weight in o-dichlorobenzene–1,8-diiodooctane (97 : 3 vol%). The PTB7:PC₇₁BM active layer with a thickness of 100 nm was prepared by spin-coating the o-dichlorobenzene solution at 1200 rpm for 90 s. The films were dried at 4 × 10⁻⁴ Pa for 30 min before subjecting them directly to thermal evaporation or methanol treatment. Methanol was spin-coated at 2500 rpm on top of the active layers. The PF_EOSO₃Na was dissolved in methanol and its thickness was adjusted by changing the concentration of the solution. The Ca (10 nm) and Al (100 nm) electrodes were deposited onto the active layers by thermal evaporation at 2 × 10⁻⁴ Pa with a metal mask. The metal electrodes had an area of 6.6 mm².

Characterization and measurement

The current density–voltage curves for the PTB7:PC₇₁BM PSCs were characterized under an illumination from a AM 1.5G lamp at 100 mW cm⁻² (Oriel Solar 3A Simulator) with a computer-controlled Keithley 2400 source measuring unit. The solar simulator illumination intensity was determined by a monocrystalline silicon reference cell (M-91150V with a KG5 filter certified by NREL). The external quantum efficiency was characterized using a QTest Station 2000 ADI system (Crowntech Inc., USA). Atomic force microscopy images and scanning Kelvin probe microscopy images were obtained using a Bruker Metrology Nanoscope III-D atomic force microscope. XPS measurements were carried out using a VG ESCALAB MKII system with a monochromatized Al Kα source at a pressure of 5 × 10⁻⁷ Pa.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation of China (no. 51073063, 21274048, 21374120) and the National Significant Program (2013CB922104). J. Zhang acknowledges the support of the 100 Talents Program of the Chinese Academy of Sciences.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08904a

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