- Nano Express
- Open Access
5-nm LiF as an Efficient Cathode Buffer Layer in Polymer Solar Cells Through Simply Introducing a C60 Interlayer
© The Author(s). 2017
- Received: 4 July 2017
- Accepted: 30 August 2017
- Published: 21 September 2017
Lithium fluoride (LiF) is an efficient and widely used cathode buffer layer (CBL) in bulk heterojunction polymer solar cells (PSCs). The LiF thickness is normally limited to 1 nm due to its insulting property. Such small thickness is difficult to precise control during thermal deposition, and more importantly, 1-nm-thick LiF cannot provide sufficient protection for the underlying active layer. Herein, we demonstrated the application of a very thick LiF as CBL without sacrificing the device efficiency by simply inserting a C60 layer between the active layer and LiF layer. The devices with the C60/LiF (5 nm) double CBLs exhibit a peak power conversion efficiency (PCE) of 3.65%, which is twofold higher than that (1.79%) of LiF (5 nm)-only device. The superior performance of the C60/LiF (5 nm)-based devices is mainly attributed to the good electrical conductivity of the C60/LiF (5 nm) bilayer, arising from the intermixing occurred at the C60/LiF interface. Besides, the formation of a P3HT/C60 subcell and the optical spacer effect of C60 also contribute to the increase in short-circuit current density (J sc) of the device. With further increase of LiF thickness to 8 nm, a PCE of 1.10% is attained for the C60/LiF-based device, while the negligible photovoltaic performance is observed for the LiF-only device. All in all, our results show that C60/LiF bilayer is a promising alternative to LiF single layer due to its high tolerance to the LiF thickness variations.
- Polymer solar cells
- Thick LiF buffer layer
- C60/LiF bilayer
- Mixed morphology
Solution-processed bulk heterojunction polymer solar cells (PSCs) have received increasing attention in recent decades because of their potential advantages such as low cost, light weight, and possibility to fabricate large-scale, flexible, and semitransparent devices [1–5]. By far, the relatively low power conversion efficiency (PCE) compared to silicon-based solar cells is still a major limitation that hinders their practical application. To achieve commercialization of this promising technology, extensive research efforts have focused on increasing the efficiency of PSCs. Until now, PCEs in the range of 11–13% have been demonstrated, primarily owing to the development of novel conjugated polymer donor and non-fullerene acceptor materials [6–12]. Besides, the introduction of anode/cathode buffer layer between the active layer and the electrode provides another efficient means to improve the device performance [13–21].
PSCs can be divided into conventional and inverted structures according to whether the indium-tin-oxide (ITO) electrode serves as the anode or the cathode. For the conventional PSCs with ITO as anode, a low work function metal such as Ca is commonly used as cathode buffer layer (CBL) to reduce the work function of the cathode (e.g., Al, Ag). However, Ca is easily oxidized when exposed to air, resulting in the poor stability of the devices. Another widely used CBL in PSCs is lithium fluoride (LiF), which has been demonstrated to enhance the device performance through the formation of an interfacial dipole at the cathode interface . Nevertheless, the thickness of LiF is limited to less than 2 nm (generally ~ 1 nm) due to its insulating property [23, 24]. Such a small thickness is very difficult to be controlled via thermal deposition. Furthermore, 1-nm-thick LiF cannot provide sufficient protection for the underlying active layer during the evaporation of hot metal atoms [17, 25].
To address these problems, we have previously reported five stacks of C60/LiF CBL, which substantially improved the device efficiency and stability of PSCs due to its good electrical conductivity even though a very thick LiF was used . However, the five-stacked C60/LiF film was prepared by alternating deposition of C60 and LiF layers. This preparation process is very complicated and time-consumed, and significantly increases the cost of device fabrication. In this work, we adopted a C60/LiF bilayer as CBL to achieve the same effect as five-stacked C60/LiF CBL. After depositing a C60 layer prior to the LiF evaporation, a thick LiF is allowed to be used without sacrificing the device efficiency. The PSCs with C60/LiF double CBLs maintained a ~ 3% PCE over a wide range of LiF thickness (1~6 nm), and showed a PCE of 1.10% even at a very thick LiF, 8 nm. In contrast, the PSCs with LiF single CBL exhibited a rapid decrease of PCE with increasing LiF thickness and had negligible photovoltaic performance at LiF thickness of 8 nm. Besides, the peak efficiency (3.77%) of C60/LiF-based devices is ~ 23% higher than that (3.06%) of LiF-only device. Taken all together, these results indicate that C60/LiF bilayer is a more promising candidate as a CBL compared to single LiF layer.
Fabrication of PSCs
ITO-coated glass substrates (Delta Technologies, LTD) were cleaned in acetone and isopropyl alcohol (IPA) under sonication for 5 min each and then treated by O2 plasma for 60 s to generate the hydrophilic surface. The filtered poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution (H. C. Starck, Clevios PH 500) was spin-coated onto the cleaned glass/ITO substrates at a speed of 2000 rpm for 50 s, followed by baking at 110 °C for 20 min under nitrogen atmosphere. Subsequently, the samples were transferred to a N2-purged glovebox (< 0.1 ppm O2 and H2O) for spin-coating of photoactive layer.
P3HT (Rieke Metals Inc., 4002-EE, 91–94% regioregularity) and PCBM (American Dye Source, purity > 99.5%) were dissolved in chlorobenzene with a weight ratio of 1:1. The mixed solution was filtered using a 0.45 μm filter and then spin-coated on top of the PEDOT:PSS layer at 1000 rpm for 50 s, followed by thermal annealing at 130 °C for 20 min, which produced a ~ 160-nm-thick active layer measured using a Dektek surface profiler. The C60, LiF, and Al (75 nm) electrode were sequentially deposited by thermal evaporation at a base pressure of 1 × 10− 6 mbar. The deposition rate and film thickness were monitored with a quartz crystal sensor. A circular-shaped shadow mask of 1 mm diameter was put on the sample to define the active area before the Al deposition.
where μ is the charge carrier mobility, d is the thickness of the active layer, A is the voltage rise speed, t max is the time when the extraction current reaches the maximum value, ∆j is the current extraction peak height, and j(0) is the displacement current of the capacitance.
Photovoltaic parameters for the P3HT:PCBM-based PSCs with and without different thicknesses of C60 inserted between the active layer and 5-nm-thick LiF layer
J sc (mA/cm2)
V oc (V)
R s (Ω cm2)
R sh (Ω cm2)
C60 (3 nm)/LiF
C60 (5 nm)/LiF
C60 (8 nm)/LiF
C60 (12 nm)/LiF
C60 (15 nm)/LiF
C60 (25 nm)/LiF
C60 (35 nm)/LiF
Photovoltaic parameters for the P3HT:PCBM-based PSCs using LiF single and C60 (25 nm)/LiF double CBLs with different thicknesses of LiF
J sc (mA/cm2)
V oc (V)
R s (Ω cm2)
R sh (Ω cm2)
LiF (0.5 nm)
LiF (1 nm)
LiF (6 nm)
LiF (8 nm)
C60/LiF (0.5 nm)
C60/LiF (1 nm)
C60/LiF (6 nm)
C60/LiF (8 nm)
As shown in Table 2, the improvement in PCE for the C60 (25 nm)/LiF-based PSCs mainly arises from the increase in FF and J sc due to the reduced R s. To better understand the R s reduction, we investigate the charge transport properties of the LiF single layer and C60/LiF bilayer using the photo-CELIV technique [30, 31]. Additional file 1: Figure S1 shows the photo-CELIV current transients, recorded at varying voltage rise speeds, for the PSCs with the LiF single and C60/LiF double CBLs. In photo-CELIV, the time of extraction current maximum (t max) is used for estimating the charge carrier mobility according to Eq. 1 . The calculated mobilities of the LiF (6 nm)-only device are 3.71, 3.40, and 3.59 × 10− 5 cm2 V− 1 s− 1 for the voltage slopes of 10, 20, and 30 kV/s, respectively, implying that the mobility is independent on the voltage rise speed. In contrast, the estimated mobilities of the C60 (25 nm)/LiF (6 nm)-based device are 3.81, 3.56, and 3.09 × 10− 4 cm2 V− 1 s− 1 for the voltage slopes of 10, 20, and 30 kV/s, respectively, which are one order of magnitude higher than those of the LiF (6 nm)-only device. The increased mobility after introducing a C60 layer can be attributed to the improved electrical conductivity arising from the intermixing occurred at the C60/LiF interface. In addition, it is noted that the photo-CELIV peak for the LiF (6 nm)-only device is broader than that for the C60 (25 nm)/LiF (6 nm)-based device, which indicates a more dispersive charge transport resulting from the larger imbalance between the electron and hole mobilities [32, 33]. This imbalance is attributed to the extremely low electron mobility for the LiF (6 nm)-only device considering that the extraction of electrons is blocked by the thick LiF layer. The accumulated electrons at the P3HT:PCBM/LiF interface screen the applied electric field and thereby decrease the rate of charge extraction in the device. In contrast, the narrow peak for the C60 (25 nm)/LiF (6 nm)-based device implies the balanced electron and hole mobilities as well as the improved electron extraction owing to the good conductivity of the C60 (25 nm)/LiF (6 nm) bilayer.
After introducing a C60 layer between the P3HT:PCBM and LiF layers, the optical field distribution within the solar cell is most likely altered, which will cause the variation in J sc [26, 37]. To investigate this effect, we first simulated the electric field intensity inside the P3HT:PCBM active layer for the devices with and without the C60 interlayer. As shown in Additional file 1: Figure S2a, the integrated field intensity for the devices incorporating a C60 layer is weaker in the short-wavelength region and stronger in the long-wavelength region as compared to the device without C60 interlayer. This trend becomes more remarkable, and simultaneously, a red shift is observed with increasing the C60 thickness. Additional file 1: Figure S2b shows the absorption spectra of the pristine C60 film, and the P3HT:PCBM films with and without different CBLs deposited on top. Comparing the absorption spectra of P3HT:PCBM/C60 (25 nm) films with and without 8-nm-thick LiF, the two curves overlap almost completely, indicating that LiF does not absorb visible light. On the other hand, the P3HT:PCBM/C60 films have higher absorption in the wavelength ranges of 400~510 nm and 580~680 nm when compared to the pristine P3HT:PCBM film. This absorption enhancement becomes more pronounced with increasing C60 thickness. Intuitively, the absorption enhancement in the 400~510 nm wavelength range arises from the C60 absorption (400~550 nm). Additional file 1: Figure S2c shows the incident photon-to-current conversion efficiency (IPCE) spectra of the PSCs with LiF (5 nm) single and C60 (25 nm)/LiF (5 nm) double CBLs. Compared to the LiF-only device, the device with C60/LiF double CBLs has a lower IPCE at the short wavelengths due to the parasitic absorption in the C60 film, and shows a higher IPCE at long wavelengths, owing to the optical spacer effect as well as the contribution of P3HT/C60 subcell.
From Table 2, it is noticed that the C60 (25 nm)/LiF (8 nm)-based device exhibits a low PCE of 1.10% although this efficiency is still much higher than that (0.06%) of the LiF (8 nm)-only device. The low PCE is the result of the small J sc and FF, which is caused by the large R s. As discussed above, the C60 (35 nm)/LiF (5 nm) film has good electrical conductivity due to the formation of the mixed morphology between C60 and LiF layers (see Fig. 2). To find the reason for the high resistance of the C60 (25 nm)/LiF (8 nm) film, AFM measurements were performed on P3HT:PCBM films without and with the C60 (25 nm), LiF (8 nm), and C60 (25 nm)/LiF (8 nm) layers deposited on top. As shown in Additional file 1: Figure S3, large spherical aggregates are formed in the C60 (25 nm) film while relatively small aggregates are formed in the LiF (8 nm) film, which is similar to the observation in Fig. 2. When depositing 8-nm-thick LiF on top of the C60 (25 nm) layer, the morphology (small aggregates) is very similar to that of the pristine LiF film, indicating that the underlying C60 aggregates are completely covered by 8-nm-thick LiF. Therefore, we speculate that a thick LiF accumulates at the top of the C60 (25 nm)/LiF (8 nm) bilayer film, which hinders the electron extraction and therefore leads to the high R s of the device.
In summary, we have demonstrated that a thick LiF can be used as CBL in P3HT:PCBM-based PSCs by simply introducing a C60 layer between the active layer and the LiF layer. The devices with the C60/LiF (5 nm) double CBLs exhibit a peak efficiency of 3.65%, while the LiF (5 nm)-only device shows a two times lower PCE of 1.79%. The improved device performance mainly results from the high FF due to the good electrical conductivity of the C60/LiF bilayer. In addition, the J sc is also improved after introducing a C60 interlayer, which can be attributed to the contribution of P3HT/C60 subcell as well as the optical spacer effect of C60. Further increasing the LiF thickness to 8 nm leads to the rapid decrease of PCE to 1.10 and 0.06% for the C60/LiF-based device and LiF-only device, respectively. The decline in PCE of the device with C60/LiF (8 nm) double CBLs is caused by the impeded electron transport, owing to the accumulated LiF at the top of the C60 (25 nm)/LiF (8 nm) bilayer. All in all, these results indicate that the C60/LiF bilayer is a more promising CBL as compared to LiF single layer for fabricating highly efficient and large-scale PSCs.
This work was supported by the National Natural Science Foundation of China (61604101) and the Scientific Research Foundation of UESTC for Young Teacher (ZYGX2016KYQD134).
XDL carried out the experiments, analyzed the data, and wrote the manuscript. LJG and YHZ provided helpful suggestions. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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