Easily-accessible fullerenol as a cathode buffer layer for inverted organic photovoltaic devices

Abstract

An easily-accessible hydroxyl group-containing fullerene derivative, fullerenol, has been used as a cathode buffer layer (CBL) for inverted polymer solar cells. The performance of the inverted solar cells could be tuned by simply altering the annealing temperature of the fullerenol layer, which was due to the unique thermal-cleavage property of fullerenol. P3HT-PC₆₁BM based cells with fullerenol as buffer layer showed power conversion efficiency (PCE) approaching 3.80%, as control, the bare ITO and ZnO modified ITO devices showed PCEs of 1.92% and 3.42%, respectively.

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1. Introduction

Organic photovoltaic devices (OPVs) have attracted much attention due to their great advantages of flexibility, lightweight, and low-cost fabrication.^1–7 In the past decades, OPVs based on conjugated organic materials and fullerene derivatives have shown a remarkable breakthrough; the highest reported power conversion efficiency (PCE) has exceeded 9% for single junction solar cells⁸ and 10% for tandem solar cells.⁹ Despite these achievements, the commercial application of the OPVs is still limited by some factors including lower PCEs compared to those of silicon-based cells, tedious synthesis procedures for most of the organic molecules and unknown long-term stability of the devices.¹⁰ So there remains a significant challenge for OPVs toward commercialization.

OPVs have two main types of device structures, which are the normal structure (ITO substrate/hole selective buffer layer/active layer/electron selective buffer layer/metal electrode)¹¹ and inverted structure (ITO substrate/electron selective buffer layer/active layer/hole selective buffer layer/metal electrode).¹² It can be seen that both types of devices mentioned above are composed of ITO substrate, active layer, buffer layer, and metal electrode. The development of efficient donors and acceptors for the active layer of OPVs is undoubtedly one of the most original and important driving forces to improve the device performance,^13–17 meanwhile, it is noticeable that the interfacial material is also critical for efficient and stable OPVs,^18,19 because the interfaces between layers can not only provide favorable energy level alignment of the device,^20,21 but also optimize the physical properties of both anode and cathode such as the work functions and compatibility with the active layers.²² For example, a thin layer of inorganic semiconductors,^23–25 water/alcohol soluble conjugated polymers with highly polar pendant groups,^26,27 polyviologen derivatives,²⁸ poly(vinylpyrrolidone),²⁹ alcohol soluble fullerene derivatives,^30–34 amphiphilic surfactant oleamide³⁵ and solution-processed tetra-n-alkyl ammonium bromides³⁶ are used as buffer layer between the active layer and metal electrode. As for the ITO electrode, poly(3,4-ethylenedioxylenethiophene):poly(styrenesulfonic acid) (PE-DOT:PSS),^37–42 self-propagating molecule-based assemblies,⁴³ phosphate group-containing fullerene derivative,⁴⁴ and organic molecules coated on the surface of inorganic semiconductors^45–49 are introduced on the ITO surface for improving the interface properties between ITO and the active layer. Among the above interfacial materials, the conventionally used inorganic semiconductors suffer from the drawbacks of critical deposition conditions, which is not compatible with the large-scale solution processing techniques.^24,50 Additionally, most of inorganic layers are sensitive to the UV-irradiation and the surface adsorption of oxygen,^44,51 which will further limit the widespread application of these materials as the interfacial layer. To overcome the above drawbacks of inorganic interfacial layer, organic semiconductors, which have the virtue of structural tunability and can be spin-coated from the solution, are developed as one of the possible candidates for the buffer layer of the photovoltaic devices.^19,20,22 Especially, fullerene-based interfacial materials draw much attention of researches due to their following unique characteristics. Firstly, it is structurally similar to the conventionally used fullerene acceptors such as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), which could smoothly bridge electrons transporting;^31,32 secondly, the physical properties such as the solubility, thermo-stability can be tailored by simply chemical modification of fullerene core. All these features make fullerene-based materials one of the effective candidates as the organic interfacial layer for the photovoltaic devices.²⁰

Herein, we select fullerenol as the cathode buffer layer material for the inverted organic solar cells. Fullerenol is one class of the most attractive reported fullerenic derivatives, which has drawn extensive attention for their promising application in many fields such as fuel cells,⁵² visible-light sensitizers⁵³ and biomedical materials.⁵⁴ However, to the best of our knowledge, fullerenol has not been used as buffer layer in the photovoltaic devices. In fact, while remaining the good features of fullerene derivatives such as the unique three-dimension structure⁵⁵ and excellent electron mobility,⁵⁶ fullerenol has some other advantages such as the good solubility in water,⁵⁷ good film forming ability and one-step simple synthesis procedure from C60.⁵⁴ These features make fullerenol meet the requirement of the buffer layer for the organic solar cells. In our experiment, commercial available poly-3-hexylthiophene (P3HT) and PC₆₁BM were used as the donor and the acceptor in the active layer. An inverted photovoltaic device was fabricated with the structure of ITO/fullerenols/P3HT:PC₆₁BM/MoO₃/Ag and showed a PCE of 2.97% after drying the interfacial layer at room temperature. Next, the fullerenols interfacial layer on ITO substrate was annealed at four temperatures (90 °C, 160 °C, 200 °C and 250 °C) and obtained different PCEs ranging from 3.09% to 3.80%. For comparison, the reference devices without a buffer layer and with ZnO as buffer layer were made under the same condition, showing PCE of 1.92% and 3.42%, respectively. The effect of the thermal annealing on the morphology and the surface wettability of the fullerenols buffer layer was explored and analyzed by atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM) and contact angle measurement.

2. Experimental

2.1 Materials

All reagents were bought from commercial sources and were used without further purification. The solvents were purified with standard methods. The composition of the as-prepared fullerenol products were estimated from thermo-gravimetric analysis (TGA) and element analysis.

2.2 Measurements

The Fourier-transform infrared (FT-IR) spectroscopy was recorded as KBr pellets using a NicoLET 6700 Spectrometer. TGA was performed by a STA 409 under the protection of inert atmosphere at a heating rate of 10 °C min⁻¹. Elemental analysis was carried out using a Vario EL cube from Elementar analyser (Hanau, Germany) and inductively coupled plasma measurements were carried out on TERRACON GmbH (Juterbog, Germany). The energy levels of the fullerene derivatives were investigated by cyclic voltammograms (CV). The CV was performed in a solution of tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) (0.1 M) in acetonitrile. A three-electrode cell consisting of an ITO glass working electrode, a Pt counter electrode and an Ag/AgCl reference electrode was used. The scan rate was 100 mV s⁻¹. The films were cast on an ITO glass by dipping the corresponding solutions onto the surface. The potential of ferrocene/ferrocenium (Fc/Fc⁺) was measured to be 0.43 V compared to the Ag/AgCl electrode under the same conditions. It is assumed that the redox potential of Fc/Fc+ has an absolute energy level of −4.8 eV to vacuum. The ultraviolet-visible (UV-Vis) spectra were measured on a Varian Cary 50 spectrophotometer. AFM images were obtained on an Agilent 5400 scanning probe microscope using AC mode. FE-SEM images were taken on a JSM 6700F NT instrument. The water contact angles measurements were carried out on a JY-82 contact angle system (Chengdeng, China). The crystallization of the active layer was analyzed using XRD (Mac Science, Cu Ka wavelength of 0.154056 nm). The films of fullerenols and active layer for the above measurements were prepared under the same experimental conditions as that of optimized photovoltaic devices fabrication.

2.3 Fabrication of photovoltaic devices

The inverted photovoltaic devices with the structure of ITO/fullerenols/active layer/MoO₃/Ag were prepared under conditions as follows. All cells were fabricated on ITO glass substrates with a nominal sheet resistance of 15 Ω sq⁻¹. The substrates were cleaned in an ultrasonic bath with detergent, ultra-pure water, acetone, and isopropyl alcohol sequentially for 20 min, and then dried in a laboratory oven at 80 °C for one night. The ITO surfaces were coated by the aqueous solution of fullerenols (4 mg mL⁻¹) with spin-coated speed at 2500 rpm for 60 s, and then, dried at 25 °C or treated in an oven for 10 minutes at different temperature, respectively. The thickness of the fullerenol film was 5 nm. The P3HT and PC₆₁BM were dissolved in chlorobenzene (18 mg mL⁻¹) in weight ratio of 1 : 1. After stirring for one night, the blend solution was then spin-coated on fullerenols-coated ITO substrate to form the active layer at 600 rpm. Followed by annealing at 150 °C for 10 min, a thin layer of MoO₃ (3 nm) and Ag (80 nm) were thermal evaporated under 2.0 × 10⁻⁴ Pa to complete the inverted structure. Finally, the cells with an active area of 0.1 cm² were measured. The fabrication process of the device was conducted in the nitrogen glove box. The film thickness was measured with Veeco Dektak 150 surface profiler. The current density–voltage (J–V) characteristics were recorded with Keithley 2420 source measurement unit under simulated 100 mW cm⁻² (AM1.5G) irradiation from a Newport solar simulator. Two reference devices without a buffer layer and with ZnO as buffer layer were made under the same condition. The annealing temperature for ZnO was 150 °C.

3. Results and discussion

3.1 Synthesis and characterization of fullerenol

Fullerenol was prepared via a simple one-step procedure.⁵⁴ As shown in Scheme 1, the toluene solution of [60]fullerene was oxidized by hydrogen peroxide at the aid of sodium hydroxide and tetrabutylammonium hydroxide. The fullerenol product was obtained as dark solid in a high yield of 95%. As shown in Fig. S1,† the FT-IR spectrum of the fullerenol product showed four characteristic peaks at 3400, 1610, 1390, and 1080 cm⁻¹, which were assigned for νO–H, νC=C, δ_SC–O–H, and νC–O absorption,⁵⁸ respectively. The as-prepared fullerenol product had excellent solubility in water (12.4 mg mL⁻¹) due to the formation of hydroxyl groups on [60]fullerene. The thermal stability of the fullerenol product was investigated by TGA. As shown in Fig. S2,† two ranges of decomposition were found for the fullerenol product, which was partitioned on the basis of differential thermo-gravimetric (DTG) curve. Firstly, a clear desorption with a total weight loss of 9.57% at temperature below 160 °C was observed, which could be ascribed to the elimination of the physically absorbed water on the surface of fullerenol.⁵⁹ It should be noted that one minimum peak at 90 °C was found, which meant the existence of the inflection point temperature for the loss of physically adsorbed water within this range. The second weight loss range (>160 °C) might be attributed to dehydration of introduced hydroxyl groups and the decomposition of the fullerene nucleus.^59,60 The changing trend of the TGA curve for our fullerenol product was quite similar with the reported one by Kokubo et al.⁶⁰ The difference of detailed decomposition temperature for the each range was probably induced by the specific reaction conditions, which would result in different number of hydroxyl groups and H₂O physically adsorbed to the fullerenol molecule. The average number of hydroxyl groups and the amount of physically adsorbed water per fullerenol molecule were estimated according to the element analysis and TGA spectrometer. The characteristic results of our fullernol product were summarized in Table S1.† The LUMO levels of the fullerenol and PC₆₁BM were investigated through cyclic voltammograms (CV) under the same experiment conditions. As shown in Fig. S3,† the onset potentials of the first reduction process were evaluated to be −0.53 and −0.66 V, respectively. The reference electrode was calibrated by the ferrocene/ferrocenium (Fc/Fc⁺) (4.8 eV below vacuum level) to obtain accurate energy levels. The LUMO level of the fullerenol film was calculated to be −3.84 V, which was lower than that of PC₆₁BM (−3.71 V). This phenomenon could be understood from the fact that the polar hydroxyl substitute groups might strengthen the intermolecular interaction of fullerenol, and thus had some interaction on the electrochemical properties of the fullerenol film.^44,45 The above results indicated that the thin film of fullerenol had compatible energy level, and could be applied to inverted type devices with a structure of ITO/fullerenol/active layer/hole transporting layer/Ag as an interfacial layer.

Scheme 1 The synthesis procedure for fullerenol.

3.2 Optical property

The transmission spectra of the fullerenol-coated ITO glass were shown in Fig. 1. The spectrum of the bare ITO glass was included as a reference. Because of surface Fresnel reflection,²¹ the transmission of the ITO-coated glass was around 80% between 400 nm and 800 nm. No obvious transmission changes in this region were found in the case of fullerenol-coated ITO glass, which was due to the very thin thickness (5 nm) of the fullerenol films and the weak light harvesting ability of fullerene derivatives. This result indicated that fullerenol film had excellent light transmission ability.

Fig. 1 Transmission spectra of bare ITO glass and fullerenol films on ITO annealing at different temperatures.

3.3 Surface wettability of the fullerenol films

As shown in Fig. 2, contact angle measurement was used to analyze the surface wettability alteration of bare ITO electrode, and ITO with ZnO or fullerenol as buffer layer. The contact angle of bare ITO glass was found to be 68.88°. After coated with ZnO on the surface of ITO, the contact angle decreased to 64.55°. When the ITO surface was coated by fullerenols, the contact angle dramatically decreased to 4.56°, which was ascribed to the remaining large amount of hydrophilic hydroxyl groups on the fullerenols at room temperature. The fullerenol buffer layer was annealed with increasing temperature from 25 °C to 250 °C, one can observe that the contact angle was slightly increased, which clearly reflected the thermo-cleavage process on the surface. The fullerenol buffer layer can be tuned to suitable contact situation between the electrode and active layer by altering annealing temperature, which would be beneficial to the performance of the photovoltaic devices.⁶¹

Fig. 2 Contact angles of ITO glass (A), ZnO-coated ITO glass (B) and fullerenol-coated ITO glass annealed at 25 °C (C), 90 °C (D), 160 °C (E), 200 °C (F) and 250 °C (G). The insets show the photographic images of dropped water on the surface.

3.4 Morphology

The ability of controlling the morphology of the buffer layer is very important for optimizing device performance, since the series resistance across the device is strongly dependent on the morphology of the interlayer in devices.^12,30 As shown in Fig. 3g, the surface root mean square (RMS) roughness for ZnO coated ITO glass is 1.8 nm, which is 5.3 nm smaller than that of the bare ITO glass (Fig. 3a). The morphology of ITO and ZnO coated on ITO can be observed more visually through the FE-SEM images. An irregular fish-scale-shaped surface morphology was found in the case of bare ITO glass (Fig. S4b†). Differently, the ZnO buffer layer showed a smooth and pinhole-free morphology (Fig. S4h†), which would efficiently prevent the leakage current at the interface between the active layer and ITO electrode, and thus could enhance the efficiencies of devices.⁴⁵

Fig. 3 AFM topographic images (2 μm × 2 μm) of ITO glass (a), fullerenol-coated ITO glass annealed at 25 °C (b), 90 °C (c), 160 °C (d), 200 °C (e) and 250 °C (f), and ZnO coated ITO glass (g).

The surface morphology of fullerenol film coated on ITO glass was investigated at different annealing temperature. All the films were prepared under the same conditions as those used for the photovoltaic devices. Similar to ZnO buffer layer, the inclusion of fullerenol interlayer on the surface of ITO glass significantly decreased the roughness of the ITO electrode (Fig. 3b–f). Interestingly, the RMS data of the fullerenol films were highly related to the annealing temperature. When the film was annealed at 25 °C, the RMS value was 3.4 nm (Fig. 3b). With the temperature increasing to 160 °C, the RMS decreased to 2.3 nm (Fig. 3d). This result could be understood from the thermal stability of the fullerenol product (Fig. S2†). With increasing of the annealing temperature from 25 °C to 160 °C, the secondary water physically adsorbed to the fullerenol molecule would be lost,⁵⁹ which may change the aggregation morphology of the fullerenol film (Fig. S4d and S4f†). When the annealing temperature increased to 200 °C even higher, the dehydration of introduced hydroxyl groups occurred and resulted in a cross-linked structure,⁶⁰ which would further decrease the roughness of the surface to less than 2 nm (Fig. 3e and f).

To further investigate and compare the change of the donor and acceptor distribution when casting the P3HT/PC₆₁BM active layer on different surfaces, we performed the AFM images and XRD spectra of the active layer on the bare ITO electrode, ITO electrode coated with ZnO and fullerenol buffer layer, respectively. As shown in Fig. S5,† a similar surface morphology and RMS roughness ranging from 1.4 nm to 1.8 nm were found while spin-coating the mix solution of P3HT and PC₆₁BM on different substrate. However, the crystallinity of the active layer was different (Fig. S6†). The diffraction intensity of the active layer P3HT/PC₆₁BM blend films on ITO electrode with fullerenol as buffer layer was much higher than those of on bare ITO and ZnO coated ITO substrate, which might cause higher J_sc values of fulllerenol devices.³⁵

3.5 Photovoltaic device performance

P3HT/PC₆₁BM is the most representative and extensively studied blend system in PSCs. Therefore, P3HT/PC₆₁BM was used to verify the feasibility of fullerenol as buffer layer in the PSCs. Fig. 4 shows the J–V characteristics of inverted polymer solar cells with and without buffer layer between the active layer and ITO electrode. Considering the thermo-stability of the fullerenols, the annealing temperatures for the fullerenol buffer layer were set at 25 °C, 90 °C, 160 °C, 200 °C and 250 °C, which were consistent with the inflection point temperatures of the weight loss of fullerenol in the DTG curve (Fig. S2†). As shown in Table 1, when annealing treatment at different temperature, the open circuit voltage (V_oc) of the devices remained stable at 0.60 V, however, the short current density (J_sc) and fill factor (FF) values were related to the annealing temperature, and finally the PCEs ranged from 2.97 to 3.80%. The optimized annealing temperature for the fullerenol buffer layer was 160 °C, which was just the inflection point of the elimination the physically adsorbed water on the surface of fullerenol. The loss of water could form smooth surface of the fullerenol buffer layer (Fig. 3) and better contact with the active layer, which would result in higher J_sc and FF values of the devices. When inserting a thin layer of sol–gel processed ZnO as interlayer, the device's PCE reached 3.42% with a significantly enhanced V_oc of 0.64 V, an improved FF of 64.36%. It could be seen that the fullerenol device performance improvement compared to ZnO device was the photocurrent, although simultaneous losses in FF and V_oc were also found. As shown in Fig. S6,† the higher crystallinity of the P3HT/PC₆₁BM blend film on fullerenol buffer layer could cause higher J_sc values of fulllerenol devices.³⁵ As for the lower FF and V_oc values of fullerenol devices, we presumed that one possible reason was that the morphology of fullerenol buffer layer was easier to be changed comparing with ZnO buffer layer, and thus caused that the fullerenol film was collapsed on the local scale,^28,44 especially while the active layer was spin-coated on it. Under the same experiment conditions, the photovoltaic device with bare ITO electrode showed only PCE of 1.92%, with a V_oc of 0.47 V, J_sc of 8.54 mA cm⁻², and a FF of 47.50%. It should be noted that the devices with fullerenol as buffer layers showed much higher PCE values than that of the bare ITO electrode device, and comparable performance with the device using ZnO as buffer layer. This result indicates that fullerenol is a promising buffer layer material applicable in photovoltaic devices.

Fig. 4 (a) Current density–voltage curves of inverted-type P3HT:PC₆₁BM solar cells with the fullerenol as cathode buffer layer. The buffer layer on ITO substrate was annealed at different temperatures. (b) Current density–voltage curves of inverted-type P3HT:PC₆₁BM solar cells without buffer layer, and with ZnO or fullerenol as buffer layer (the fullerenol buffer layer was annealed at 160 °C).

Table 1 Photovoltaic performance of the P3HT:PC₆₁BM-based devices with different buffer layer

Buffer layer	Annealing temperature^a (°C)	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)
a The buffer layer was annealed after spin-coated on the ITO substrate.
No interlayer	N/A	0.47	8.54	47.50	1.92
ZnO	150	0.64	8.30	64.36	3.42
Fullerenol	25	0.60	9.02	54.80	2.97
	90	0.60	9.35	55.18	3.09
	160	0.60	10.58	59.71	3.80
	200	0.60	9.42	58.34	3.32
	250	0.60	9.40	55.25	3.11

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4. Conclusions

In conclusion, a water soluble hydroxyl group-containing fullerenol was prepared through one-step simple synthesis procedure. The fullerenol shows excellent optical transmission and electron transporting properties. The surface morphology and wettability of the fullerenol thin film on ITO coated glass could be tuned by simply altering annealing temperature, which finally would change the photovoltaic performance of the inverted devices with fullerenol as cathode buffer layer. The inverted device ITO/fullerenol/P3HT:PC₆₁BM/MoO₃/Ag showed a PCE of 2.97% after drying the interfacial layer at room temperature. The PCE could be further improve to 3.80% after annealed at 160 °C, which indicates that fullerenol is a promising interfacial material for organic photovoltaic devices.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21204097, 51173199, 61107090), Chinese Academy of Sciences (KGCX2-YW-399+9-2), Shandong Provincial Natural Science Foundation (ZR2011BZ007), Qingdao Municipal Science and Technology Program (11-2-4-22-hz), and the Key Project Foundation of Heilongjiang Province University Material Research and Application Key Laboratory (2014).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra03045a

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