Highly efficient inverted polymer solar cells using fullerene derivative modified TiO₂ nanorods as the buffer layer

Abstract

We reported a significant improvement in the efficiency of polymer solar cells by introducing C₆₀ pyrrolidine tris-acid (CPTA) to modify the interface between inorganic TiO₂ nanorods and the organic active layer. The devices with CPTA-modified TiO₂ as the cathode buffer layer exhibited a power conversion efficiency (PCE) as high as 8.74%, accounting for a 12.8% enhancement compared with the bare TiO₂ based devices (7.75%) in the polymer thieno[3,4-b] thiophene/benzodithiophene:[6,6]-phenyl C₇₁-butyric acid methyl ester (PTB7:PC₇₁BM) system. We found that the CPTA layer improves the surface properties of the bare TiO₂ film so that charge transfer between the active layer and the TiO₂ layer is enhanced.

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Polymer solar cells (PSCs) based on bulk-heterojunction (BHJ) structures have attracted much attention, due to their advantages of high mechanical flexibility, light weight, large area fabrication by low cost solution processing, etc.^1–4 In a conventional BHJ PSC, the active layer, which contains a p-type donor and an n-type acceptor to form an interpenetrating nanoscale network, is sandwiched between a low-work-function metal cathode and a hole-conducting poly(3,4-ethylenedioxylenethiophene):poly(styrene sulfonic acid) (PEDOT:PSS) layer on top of a transparent indium tin oxide (ITO) substrate.^2,5 However, rapid oxidation of the low-work-function metal cathode (Ca, Ba etc.) and corrosion of ITO by the acidic PEDOT:PSS are potentially detrimental to the device stability.^6,7 To solve these problems and improve device lifetime, inverted PSCs have attracted increasing attention. In inverted PSCs, the polarity of charge collection is reversed compared to that in the conventional structure. The anode is substituted with high-work-function transition metal oxides (e.g. V₂O₅ and MoO₃) which are covered by a stable metal electrode, such as Au or Ag.^8,9 The cathode buffer layers, such as TiO₂,^8,10–13 Cs₂CO₃,¹⁴ PFN,^4,15 CdS,¹⁶ ZnO,^17,18 CPE,¹⁹ are introduced decrease the work function of ITO. As a consequence, ITO serves as the cathode to collect electrons. In such an inverted configuration, the electron extraction layer on top of the ITO becomes a key issue for the high performance of PSCs.

Titanium dioxide (TiO₂) has been extensively studied as an n-type semiconductor owing to high electron mobility, good transparency and nontoxic.^8,11,12,20 Over the past decades, TiO₂ has been widely applied in dye-sensitized solar cells (DSSCs),²¹ photocatalysis,²² and hybrid organic solar cells.^23,24 It has been demonstrated that TiO₂ can effectively help to collect electrons and block holes.^13,25,26 To further enhance the device performance, the interfacial modification of TiO₂ which enhances the compatibility and adhesion between the inorganic metal oxide and organic active layer has drawn considerable attention.^16,26,27 For example, a C₆₀ derivative functionalized with a carboxylic acid group, which can form a self-assembled monolayer (SAM) on the metal oxide surface, has been used successfully to simultaneously improve the morphology and charge selectivity of the inverted devices.^28,29 It is believed that interfacial modification by inserting an additional C₆₀ derivative interlayer between the organic active layer and the inorganic metal oxide could help to enhance device performance.^27,29–31

In this work, we introduced a polar fullerene derivative with carboxyl groups, C₆₀ pyrrolidine tris-acid (CPTA), between the organic active layer and anatase titania nanorods (NRs) (Fig. 1a) to improve the device performance. This is expected to improve the charge transport and charge collection in PSCs as well as reduce charge recombination probability. CPTA is soluble in methanol because of the polar property of carboxyl groups. It was reported that CPTA used as a cathode buffer layer has an excellent electron-collecting and hole-blocking ability.³² Low bandgap polymer thieno[3,4-b]-thiophene/benzodithiophene (PTB7) was used as the donor and phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) was used as the acceptor. A high performance device was fabricated with the inverted configuration ITO/TiO₂/CPTA/PTB7:PC₇₁BM/MoO₃/Al (Fig. 1a). For comparison, the reference device (without CPTA layer) was also made under the same conditions. As a result, the inverted device based on an ITO/TiO₂/CPTA/PTB7:PC₇₁BM/MoO₃/Al configuration exhibited an enhanced PCE of 8.74% in comparison to the reference device (PCE = 7.75%) without CPTA interlayer.

Fig. 1 (a) The device structure of the inverted PSC. (b) The molecular structures of CPTA and PC₇₁BM.

Fig. 2 shows the energy levels diagram of the materials involved in the inverted device. The details of anatase TiO₂ synthesis are described in the ESI† and the dimensions of TiO₂ NRs were ∼35–45 nm in length and 3–5 nm in diameter as shown in TEM image (Fig. S1†). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of TiO₂ are located at 4.1 eV and 7.3 eV, respectively.³³ The higher LUMO energy level of CPTA has been reported to be 3.94 eV, which was located between the LUMO of PC₇₁BM (3.69 eV) and the conduction band of TiO₂ (4.1 eV).³² From the energy levels diagram (Fig. 2), it is clear that CPTA functions as an intermediate in the energy gradient. Therefore, electrons in the PC₇₁BM domain can be efficiently extracted by CPTA and transported to the TiO₂ through an energetically favorable pathway. In addition, the HOMO of CPTA is low-lying relative to the HOMO of PC₇₁BM, which would efficiently block the holes and reduce the recombination at the interfaces.

Fig. 2 Energy levels diagram of the components involved in the inverted device.

The current density versus voltage (J–V) characteristics of inverted PSCs obtained under 100 mW cm⁻² AM 1.5G illumination are shown in Fig. 3a. Table 1 summarized the specific parameters of devices A (ITO/TiO₂/PTB7:PC₇₁BM/MoO₃/Al) and B (ITO/TiO₂/CPTA/PTB7:PC₇₁BM/MoO₃/Al). The device A without CPTA layer showed an open circuit voltage (V_oc) of 0.758 V, a short current density (J_sc) of 14.69 mA cm⁻², a fill factor (FF) of 69.6% and a PCE of 7.75%. When CPTA was inserted between TiO₂ and active layer, the J_sc and FF were enhanced significantly. The device B with an optimized concentration 0.5 mg ml⁻¹ (see Table S1 in the ESI†) exhibited an impressive improved PCE of 8.74%, which was enhanced 12.8% compared with the device A (7.75%). The device B showed a higher J_sc and FF than device A, indicating that CPTA could efficiently extract and transport the electron to the TiO₂ through an energetically favorable pathway, which can be proved by the obvious decrease of series resistance (R_s) from 5.24 Ω cm² (device A) to 3.88 Ω cm² and the increased shunt resistance (R_sh) of 1021 Ω cm² (device A) from 1567 Ω cm² (Table 1). It indicated better interface property in the device B, which will lead to more efficient electron transport, collection and less charge carrier recombination at the TiO₂ interface. The external quantum efficiency (EQE) (see Fig. 3b) of devices B was much higher at almost whole wavelength range compared with that of devices A. It further confirmed that using organic CPTA to modify inorganic TiO₂ interface was an efficient method to improve the device performance for the inverted PSCs.

Fig. 3 (a) J–V curves of the inverted PSCs with device architectures of ITO/TiO₂/PTB7:PC₇₁BM/MoO₃/Al and ITO/TiO₂/CPTA/PTB7:PC₇₁BM/MoO₃/Al under 100 mW cm⁻² AM 1.5G illumination and in the dark. (b) External quantum efficiency (EQE) of the device with or without the CPTA layer.

Table 1 Characteristics of device A and device B measured under 100 mW cm⁻² AM 1.5 G illumination

Device^a	Voc [V]	Jsc [mA cm^−2]	FF (%)	η^b (%) best (average)	Rs [Ω cm^2]	Rsh [Ω cm^2]
a Configurations: device A, ITO/TiO2/PTB7:PC71BM/MoO3/Al; device B, ITO/TiO2/CPTA/PTB7:PC71BM/MoO3/Al.b Average of eight devices. The device parameter distribution maps are presented in Fig. S2 in the ESI.
A	0.758	14.69	69.6	7.75 (7.56 ± 0.19)	5.24	1021
B	0.757	15.73	73.5	8.74 (8.45 ± 0.20)	3.88	1567

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Fig. 4a and b exhibit the atomic force microscopy (AFM) tapping mode height images (5 × 5 μm) of the surface of bare TiO₂ and TiO₂/CPTA on ITO substrate, respectively. In the case of bare ITO/TiO₂ sample, the image showed a root mean square (RMS) roughness of 4.85 nm. After a layer of CPTA was deposited on top of bare ITO/TiO₂, the RMS roughness decreased to 4.03 nm. The decreased surface roughness reveals that voids in the uneven TiO₂ NRs surface are filled and covered by the CPTA to form a smoother surface. The result suggests that the CPTA modification would vary the morphology of the bare TiO₂ film, which would further affect the morphology of the active layer. Water contact angle measurements were further performed for bare TiO₂ film and the TiO₂/CPTA film, as shown in Fig. 4c and d. The bare TiO₂ film exhibited a water contact angle of 84.1°, which could be due to the oleic acid and pyridine attaching on TiO₂ NRs surface.³⁴ The wetting contact angle of the CPTA-modified TiO₂ film was further increased to 99.2° from 84.1° (bare TiO₂ film). The increased hydrophobicity could be mainly attributed to the intrinsic hydrophobicity of fullerene unit in the CPTA molecule. Owing to the hydrophobic organic solvent (chlorobenzene) used in the deposition of the active layer, the enhanced hydrophobicity of buffer layer would help the flow and the spread of the active layer solution to deposit on the buffer layer.^10,26,30,35 Consequently, CPTA modification of the bare TiO₂ surface can help to improve the morphology of the active layer and provide a better percolation conduction pathway at the interface.²⁸ The improved interface properties could also reduce the recombination of charge carriers and contact resistance, improving charge transfer at the interface and leading to better device performance.^26,36

Fig. 4 (a) AFM images (5 × 5 μm) of bare TiO₂ and (b) TiO₂/CPTA. Contact angle images by dropping deionized water on the surface of bare TiO₂ (c) and TiO₂/CPTA (d).

In conclusion, the inverted PSCs were fabricated by using TiO₂ NRs and CPTA-modified TiO₂ NRs as the cathode buffer layer. We found that modification of the interface between the inorganic TiO₂ NRs and organic active layer with a fullerene derivative (CPTA) can improve the device performance. The devices with CPTA modifier on the TiO₂ NRs surfaces lead to a significant improvement compared with bare TiO₂. A high PCE of 8.74% was achieved, which was enhanced 12.8% compared with the bare TiO₂ based devices (7.75%). The CPTA layer improves surface property of the bare TiO₂ film so that the modified TiO₂ film has lower contact resistance, improved charge transfer and better adhesion with the active layer. We believe that this study will stimulate further research on understanding how to improve the interface, enhance charge transfer properties and achieve better device performance in inverted PSCs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51273208 and 51202264) and the Hundred Talent Program of the Chinese Academy of Sciences; Zhejiang Provincial Natural Science Foundation of China; China Postdoctoral Science Foundation funded project; the Ningbo Natural Science Foundation of China (2012A610114, 2013A610132).

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Footnote

† Electronic supplementary information (ESI) available: Specific devices parameters based on TiO₂ NRs modified with the different concentrations of CPTA in PTB7:PC₇₁BM blends, the device parameters distribution map using TiO₂ NRs and TiO₂ modified with CPTA as buffer layer in PTB7:PCB₇₁M system. See DOI: 10.1039/c4ra02254h

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