1. Introduction

As a promising alternative for low-cost renewable energy technology in wearable and portable equipment, flexible polymer solar cells (FPSCs) with the inverted construction are attracting extensive interest due to promising power conversion efficiency (PCE), mechanical flexibility, as well as the compatibility with high-volume large area fabrication and roll-to-roll processing [1–4]. Recent advances in polymer material development have led to the conversion efficiencies exceeding 10% in the inverted bulk-heterojunction solar cell architecture [5, 6]. The conventional inverted bulk-heterojunction solar cell consists of a blended active layer with polymer donor and fullerene acceptor, a transparent anode, and a low work function (WF) metal cathode (e.g., Al, Ca) [7–9]. Indium tin oxide (ITO) is the most commonly used transparent anode in polymer photoelectric devices [10]. However, due to the increasing costs of indium, fragile property and difficulty to fabricate on flexible substrate, ITO anode is not an excellent choice for FPSCs [11]. Ultrathin metal films are one of the potential replacement for ITO electrodes in conventional FPSCs, but their application in inverted flexible devices as the transparent top electrode still needs more experimental evidence. In addition, electron selective layers at the cathode interface play an important role in improving the PCEs by reducing the WF of bottom electrodes and increasing the electron transport and extraction [12]. Fabricating parts of common cathode interlayers (metal oxide, CsCO₃) needs a high temperature process [13–15] that is not compatible with flexible substrate and metallic bottom electrode, which can seriously lead to the degradation of FPSCs during the high temperature process. So up to now, there are still stumbling blocks in terms of conversion efficiencies and mechanical stability of inverted FPSCs to realize commercial application.

In this paper, we used multilayer transparent top electrodes with the structures of MoO₃/Au/Ag/NPB (N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine) (MAN structure) as the replacement of ITO to improve the performance of inverted FPSCs. In the MAN electrode, 5 nm MoO₃ and 1 nm Au acted as the wetting layer and seed layer respectively to prepare the ultrathin Ag film, and NPB as the anti-reflection film could improve the total transparency in visible light wavelength region of the multilayer transparent top electrodes. Moreover, we used vacuum evaporated organic small molecule TPBi (1, 3, 5-Tris (N-phenylbenzimidazol-2-yl) benzene) to replace high temperature solution-processed metal oxides as the cathode interface layer to avoid destruction of metallic bottom electrode and flexible substrate caused by high temperature. Combining with the excellent performance of transparent top MAN electrode and cathode interface layer, the inverted FPSC obtained a power conversion efficiency of 6.02%, which is comparable to that of the ITO-based equivalent with the similar architecture [16], and it had a good mechanical stability and flexibility under 3000 bending cycles.

2. Experimental

The MAN transparent electrodes composed of a MoO₃ layer (5 nm), a Au film (1 nm), a ultrathin Ag film (7 nm), and a NPB film (0, 20, 40, 50 nm), which were deposited on flexible substrates in sequence by thermal deposition under the vacuum of 5×10⁻⁴ Pa. The deposition rates were set to be 1 Å/s for all layers, and the thicknesses were monitored by quartz crystals. The transmission spectra and the sheet resistance of the MAN transparent electrodes were measured by a UV-Vis spectrophotometer (UV-2550, SHIMADZU Co.Inc.) and a four-probe meter, respectively.

Poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiad -iazole)] (PCDTBT) and [6,6]-phenyl C70-butyric acid methyl ester (PC₇₁BM) were the photoactive material of the bulk-heterojunction in inverted FPSCs. The inverted FPSCs with the architecture of Ag/ TPBi/ PCDTBT: PC₇₁BM/ MAN top electrodes were fabricated by the following processes. Firstly, the Si substrates were pretreated with Octadecyltrichlorosilane (OTS) to realize a hydrophobic surface with lowered surface energy, and the ultrathin photopolymer film (NOA63, Norland Inc.) was spin-coated onto the Si substrate at 6000 rpm for 60 s, and then exposed to an ultraviolet light source for 5 min with a power of 125 W. The thickness of NOA63 film was about 50 μm. After that Ag films (80 nm) as the bottom electrode and TPBi as the cathode interface layer with various thicknesses (20, 30, 40, 50 nm) were thermal deposited onto the NOA63 substrate at a rate of 1 Å/s. A blend solution of PCDTBT and PC₇₁BM (1:4 w/w, total polymer concentration of 25 mg/mL in 1,2-dichlorobenzene) was spin-coated on top of the TPBi layer at 3000 rpm for 30 s as the photoactive blend layer. The PCDTBT: PC₇₁BM layer with a thickness of about 80 nm was annealed at 70 °C for 1 h in the glove box. Then, the MAN transparent electrode was deposited on the photoactive blend layer. Finally, The NOA63 film coated PSC device was peeled-off from the Si wafer to complete fabrication of inverted FPSCs. The inverted FPSCs were characterized under illumination of a solar simulator (A.M. 1.5 Global spectrum with a light intensity of 100 mW cm⁻²), and the J-V characteristics were recorded using a Keithley 2400 source meter.

3. Results and discussion

Transparent top electrode with good electronic and optical characteristics is a key factor for improving efficiency of the inverted FPSCs. We have reported an ultrathin Ag electrode with structure of M_OO₃/Au/Ag on NOA63 flexible substrate in our previous work [17]. Owing to the combined effect of M_OO₃ wetting layer and Au seed layer, the percolation threshold of Ag films was reduced. The ultrathin transparent M_OO₃/Au/Ag electrode on NOA63 flexible substrate exhibited a high conductivity with the square resistance of 7 Ω/□, but its transmittance in visible wavelength region was only about 80% which could hardly compare to the ITO electrode. In order to further enhance transmittance of transparent top electrode, we deposited the NPB film on the top of M_OO₃/Au/Ag electrode as anti-reflection layer. Figure 1 shows the transmittance spectra of the transparent MAN electrodes with 0, 20, 30, 40, 50 nm NPB anti-reflection layers. Due to the refractive index modification, the improved transmittance of MAN electrode is observed by introducing the NPB layer. With increasing of thickness of anti-reflection film, a red shift of transmittance peak position can be found and the transmittance above 500 nm wavelength region gradually increases. After the thickness of NPB layer increased to 40 nm, the light transmittance blew 550 nm has an obvious decrease. So, we chose the thickness of NPB film to be 40 nm in the MAN transparent top electrode for inverted FPSCs.

 

Fig. 1 The transmittance spectra of the MAN electrode with 0, 20, 30, 40, 50 nm NPB coatings.

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The performance of inverted FPSC has a close relationship with the electronic energy band structures of materials [18–21]. The inverted device architectures and energy level diagram are shown in Fig. 2. The HOMO and LUMO levels of TPBi match well with the HOMO of the PC₇₁BM acceptor and LUMO of MoO₃. Compared with PC₇₁BM (6.0 eV), the higher HOMO level of the TPBi (6.5 eV) indicates that there is a very efficient hole blocking from active layer to cathode, but compared with MoO₃ (2.3 eV), the lower LUMO level of the thin TPBi layer (2.7 eV) does not block the electron extraction from the PC₇₁BM phase. From the energy levels of FPSCs, we can conclude that the electron transport and extraction will be improved by introducing the TPBi cathode interface layer, and a higher short-circuit current (J_SC) and fill factor (FF) could be expected. Moreover, the TPBi film can be deposited directly by vacuum evaporation, which is fully compatible with flexible substrates and metallic electrodes.

 

Fig. 2 (a) The device structure of the flexible inverted PCDTBT: PC₇₁BM solar cell. (b) Energy level diagram of the inverted FPSCs.

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The thickness effect of the cathode interface layer has been explored in detail. The inverted FPSCs with different thickness of TPBi interlayer have been fabricated, and an inverted FPSC without NPB anti-reflection layer was also prepared as comparison. Figure 3a shows the current density (J) vs. voltage (V) curves for the inverted FPSCs measured under AM 1.5 G solar irradiation at the intensity of 100 mW cm⁻². The detailed information about the open-circuit voltage (V_OC), J_SC, FF, and PCE are summarized in Table 1. In experiment, all devices with the thickness of TPBi less than 30 nm were breakdown because thickness of TPBi is so thin that organic solution of 1, 2-dichlorobenzene can contact with rough Ag electrode directly and lead to breakdown easily. From the Table 1, it can be seen that the J_SC and FF have a decline with increasing of thickness of TPBi film, which arising from the limitation of electron diffusion length. The J_SC of the device with NPB film is 8.3% higher than the device without NPB coating which is consistent with the increasing transmittance of top electrode by NPB anti-reflection layer. From the above analysis, we have demonstrated that the device with a 30-nm TPBi interlayer and a 40-nm NPB film is an optimal inverted FPSC construction. The PCE of the optimized devices is about 6.02%, which is comparable to reported performance of the ITO-based equivalent with the similar architecture [16]. The UV-visible absorption and incident photon-to-current efficiency (IPCE) spectra of the optimal device are shown in Fig. 3(b). The absorption spectrum covers entire visible spectral range and the maximum IPCE of 71% is achieved.

 

Fig. 3 (a) J – V characteristics of flexible inverted polymer solar cells with 30, 40, 50 nm TPBi interlayers and 40 nm NPB coating layer, and the device with 30 nm TPBi interlayer and without NPB coating layer was used as control device to verify the function of 40 nm NPB coating; (b) The UV-visible absorption and IPCE spectras of inverted PCDTBT: PC₇₁BM solar cells.

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Table 1. The photovoltaic parameters of the inverted FPSCs with different thicknesses of TPBi interlayers.

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Bending tests have been conducted to evaluate the mechanical robustness of the inverted FPSCs. The photograph of inverted FPSCs under bending demonstrates the great flexibility as shown in the inset of Fig. 4 (a). The inverted FPSCs exhibit excellent mechanical robustness, and the performance parameters consisting of V_OC, I_SC, FF, and PCE are maintained after repeated bending with a radius of curvature of about 2.5 mm (Fig. 4). The PCE of inverted FPSC has a slight decrease with increasing number of bending and after 3000 bending cycles, the PCE remains to be 4.5% which is 75% of its original efficiency (6.02%). The degradation PCE should be ascribed to the degradation of the V_OC which has multiple causes including the increase of temperature during test, air oxidation and damage of functional layers and so on.

 

Fig. 4 I_SC (a), V_OC (b), PCE (c) and FF (d) of the inverted FPSCs as a function of bending cycles. The inset shows the photograph of the inverted FPSCs after bending.

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4. Conclusions

In summary, we successfully fabricated inverted flexible polymer solar cells with transparent top MoO₃/Au/Ag/NPB electrode. The PCE of the inverted FPSC can reach to about 6.02% by optimizing the thicknesses of NPB anti-reflection layer and cathode modification of TPBi, and the inverted FPSC exhibits excellent flexibility and mechanical robustness. Moreover, the use of transparent top MoO₃/Au/Ag/NPB electrode and the cathode interface layer of vacuum evaporated TPBi provide a broad range of choice for flexible substrates and bottom electrode. The high efficiency and mechanical robustness indicate the potential application of the ITO-free inverted FPSCs in wearable electronics.