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Cite this: DOI: 10.1039/c4cp03484h
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High-efficiency inverted polymer solar cells controlled by the thickness of polyethylenimine ethoxylated (PEIE) interfacial layers Ping Li,ab Gang Wang,ab Lun Cai,ab Baofu Ding,c Dachen Zhou,ab Yi Hu,ab Yujun Zhang,ab Jin Xiang,ab Keming Wan,ab Lijia Chen,ab Kamal Alamehc and Qunliang Song*ab In this work, we investigate the effect of the thickness of the polyethylenimine ethoxylated (PEIE) interface layer on the performance of two types of polymer solar cells based on inverted poly(3-hexylthiophene) (P3HT):phenyl C61-butryric acid methyl ester (PCBM) and thieno[3,4-b]thiophene/ benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Maximum power conversion efficiencies of 4.18% and 7.40% were achieved at a 5.02 nm thick PEIE interface layer, for the above-mentioned solar cell types, respectively. The optimized PEIE layer provides a strong enough dipole for the best charge collection while maintaining charge tunneling ability. Optical transmittance and atomic force microscopy measurements indicate that all PEIE films have the same high
Received 5th August 2014, Accepted 16th September 2014 DOI: 10.1039/c4cp03484h
transmittance and smooth surface morphology, ruling out the influence of the PEIE layer on these two parameters. The measured external quantum efficiencies for the devices with thick PEIE layers are quite similar to those of the optimized devices, indicating the poor charge collection ability of thick PEIE layers. The relatively low performance of devices with a PEIE layer of thickness less than 5 nm is the
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result of a weak dipole and partial coverage of the PEIE layer on ITO.
Introduction Polymer solar cells (PSCs) have attracted considerable attention due to their high power conversion efficiency (PCE) (exceeding 10%) as well as attractive features of light weight, flexibility in large area applications and low-cost fabrication through roll-to-roll processing.1–5 Both conventional and inverted PSC structures have been widely studied.6–10 Thin poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) hole transporting layers have conventionally been deposited onto transparent conductive indium tin oxide (ITO) anodes to improve the efficiency of conventional PSCs.6–8 The main drawback of such PSC devices is that the hygroscopic and acidic PEDOT:PSS etch the ITO, thus decreasing the stability of the device. In contrast, a cathode buffer layer (CBL) modified ITO cathode has been used in conjunction with an air stable high work function (WF) anode to realize inverted PSC devices of improved stability a
Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, P. R. China. E-mail: [email protected], [email protected] b Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, Chongqing 400715, P. R. China c Electron Science Research Institute, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA, 6027 Australia
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and manufacturing compatibility.9,10 Because of the high WF of ITO, a good CBL is indispensable to lower the ITO’s WF, thus achieving high PCE inverted PSCs. Spin-coated thin inorganic metal oxide11–15 films have been used as CBLs. However, inorganic metal oxides typically have poor solubility with organic solvents. In contrast, organic molecules have excellent solubility and compatibility with organic solvents and can lower the ITO’s WF.16,17 For example, a basic poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA)16 solution treatment has been demonstrated to improve the device performance by reducing the WF of ITO from 4.38 to 3.94 eV, enhancing charge transport and decreasing the leakage current. Another organic material, namely polyethylenimine ethoxylated (PEIE), has been widely used to enhance electron injection in organic light emitting diodes.18 PEIE has been used to reduce the WF of ITO and other oxide electrodes,19–21 hence improving the performance and stability of inverted devices. PEIE has also been applied to modify the recombination layer in high performance tandem organic solar cells.22,23 However, since PEIE is an insulator with a band gap of 6.2 eV,19 it might not be a good charge extraction layer from the energy point of view. Hence, it is very necessary to systematically investigate the effect of the PEIE layer’s thickness on the performance of PSCs. Such an investigation has not been reported.
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In this paper, the effect of the thickness of the PEIE layer on the performance of inverted poly(3-hexylthiophene) (P3HT): phenyl C61-butryric acid methyl ester (PCBM) and thieno[3,4-b] thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM)-based PSCs is systematically studied. Experimental results show that an optimum PEIE layer thickness of 5.02 nm leads to PCEs of 4.18% and 7.40%, respectively, for the two investigated PSC devices. Optical transmittance, atomic force microscopy (AFM), and thickness determination rule out the change in light absorption in the devices and morphology modification of the active layers and interfaces. External quantum efficiency (EQE) measurements identify the poor charge transfer as the reason for the poor performance of PSC devices employing thick PEIE layers.
Experimental
was modified by PEIE layers from spin-coating PEIE solution with different concentrations19 at 4000 rpm for 60 s, then baked in air at 100 1C for 10 min. The active layers deposited on the PEIE layers of the nine fabricated samples were prepared by spincoating either a P3HT : PCBM (1 : 1 w/w) in 1,2-dichlorobenzene (DCB) solution with a concentration of 34 mg ml1 without any additives or PTB7 : PC71BM (1 : 1.5 w/w) in a mixed solvent of chlorobenzene : 1,8-diiodooctane (0.97 : 0.03) with a concentration of 25 mg ml1 at a speed of 870 rpm for 60 s. The nine samples were then transferred into a vacuum chamber, in which 6 nm thick MoO3 and 100 nm thick Ag electrodes were thermally evaporated at rates of 0.02 and 0.1 nm s1, respectively, through a shadow mask at a base pressure of B107 mbar. Finally all P3HT:PCBM-based devices were thermally annealed24 at 135 1C for 10 min inside the nitrogen filled glove box and stored in the same glove box before measurements. The device area, defined by the overlap between ITO and Ag electrodes, was 9 mm2.
Materials P3HT with regioregularity better than 98%, PCBM and 1,2-dichlorobenzene (DCB, anhydrous, 99%) were purchased from SigmaAldrich. Polyethylenimine, 80% ethoxylated (Mw = 70 000 g mol1), dissolved in H2O with a concentration of 35–40 wt% and 2-methoxy ethanol was also purchased from Sigma-Aldrich. PTB7 and PC71BM were purchased from One-material Chemscitech Inc. (St-Laurent, Quebec, Canada) and used as received. The structure of the PSC devices and chemical structures of materials used in the PSC devices are given in Fig. 1. Fabrication of inverted PSCs The structures of the fabricated inverted PSC devices were ITO/ PEIE/P3HT:PCBM and PTB7:PC71BM/MoO3/Ag, in which the thicknesses of the PEIE layers were 0 (i.e., without PEIE modification), 0.78, 1.41, 3.74, 5.02, 8.68, 14.22, 21.13, and 29.09 nm, respectively. These thicknesses were measured using an ellipsometer (J. A. Woollam M-2000UI). The sheet resistance of the ITO substrates was about 20 O per square. After sequential ultrasonic cleaning using detergents, acetone, and deionized water, the ITO
Fig. 1
Characterization of inverted PSCs and thin films UV-vis absorption measurements were carried out using a Shimadzu UV-21011C spectrometer. For measuring the spectral absorption of the active layers, ITO was used as a reference, whereas the optical transmittance of PEIE-coated ITO glass substrates was measured by using air as a reference. The thicknesses of the active layers were measured using a step profiler (DEKTEK 6M). Atomic Force Microscopy (AFM) was carried out in air for surface morphology measurements using a Dimension ICON AFM operating in tapping mode. The contact angle of the PEIE layer was measured using a contact angle meter (POWEREACH). Current–voltage (I–V) measurements were conducted using a Keithley 2400 with a Newport solar simulator (94043A) of 100 mW cm2 (AM 1.5 G) simulated sunlight. The devices were kept in the dark inside the glove box, except during the periods of short exposure to the simulated sunlight or monochromatic light. The EQE of the devices was calculated from the photocurrent measured using a lock-in amplifier (SR-830). A 150 W xenon lamp followed by a monochromator
(a) Device structures of the PSC devices and (b) chemical structures of materials used in the PSC devices.
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was used to supply the monochromatic light, and the light intensity at each wavelength was determined using a calibrated Si detector.
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Results and discussion All I–V characteristics of the fabricated P3HT:PCBM-based inverted polymer solar cells are shown in Fig. 2a and their performance parameters are summarized in Table 1. The device without a PEIE layer (denoted as 0 nm) exhibited a poor performance, similar to the previous ones.25–28 The shortcurrent density (Jsc) increases from 5.65 0.64 mA cm2 to 10.58 0.62 mA cm2 when the thickness of the PEIE layer increases from 0 nm to 8.68 nm, and then gradually decreases to 3.41 0.56 mA cm2 by further increasing the PEIE layer thickness to 21.13 nm. It was found that Jsc remarkably decreased down to 0.84 0.35 mA cm2 when the thickness of the PEIE layer was increased to 29.09 nm. The dark current rectification ratios are consistent with the Jsc variation, as shown in Fig. 2b. Interestingly, the fill factor (FF) exhibited a similar volcano shape with the change in the PEIE layer thickness, with a peak value of 63 3% at an optimum PEIE layer thickness of 5.02 nm. The open circuit voltage (Voc) was
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not sensitive to the thickness of the PEIE layer. The highest PCE value achieved was 4.18% for a 5.02 nm thick PEIE layer. This value is higher than the reported B3% value obtained using a similar PEIE modifier.19,23 The worst PCE was 0.11% for the device employing a 29.09 nm thick PEIE layer, at which Jsc and FF had their lowest values. The PTB7:PC71BM-based polymer solar cells exhibited similar results as shown in Fig. 2c and d and summarized in Table 2. The highest PCE achieved was 7.4%, also for a PEIE layer thickness of 5.02 nm. The results shown in Fig. 2 demonstrate that by simply changing the thickness of the PEIE layer, the performance of an inverted PSC device can be altered drastically. The energy diagram of a P3HT:PCBM-based cell is shown in Fig. 3. Similar discussion applies to PTB7:PC71BM-based solar cells. The dipole introduced by covering a thin layer of PEIE reduces the work function of ITO, thus facilitating charge transfer and improving the photocurrent. Electrons can transfer by tunneling to ITO through the very thin insulating PEIE layer. For a relatively thick PEIE layer, electron transfer from the active layer to ITO is blocked. In this investigation, the optimized PEIE layer thickness was found to be 5.02 nm, at which the tradeoff was achieved between the dipole-induced charge collection and charge blocking of the insulating PEIE layer. The photocurrent measurements with and without light bias typically determine
Fig. 2 I–V characteristics of the inverted P3HT:PCBM and PTB7:PC71BM polymer solar cells for various PEIE layer thicknesses under 100 mW cm2 AM 1.5 G illumination [(a) and (c)] and in the dark [(b) and (d)].
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Table 1 Performance parameters of inverted P3HT:PCBM polymer solar cells with various PEIE layer thicknesses. Data in parentheses give the performance of the champion devices
PEIE (nm)
Jsc (mA cm2)
0 0.78 1.41 3.74 5.02 8.68 14.22 21.13 29.09
5.65 7.20 8.01 9.51 10.56 10.58 9.97 3.41 0.84
0.64 0.87 0.71 0.68 0.55 0.62 1.01 0.56 0.35
(6.29) (8.07) (8.72) (10.19) (11.11) (11.20) (10.99) (3.97) (1.18)
FF (%) 37 33 54 53 63 57 56 18 16
2 2 4 3 3 2 2 2 3
Voc (V) (39) (35) (58) (56) (66) (59) (58) (18) (17)
0.28 0.52 0.54 0.52 0.57 0.57 0.56 0.43 0.51
PCE (%) 0.01 0.01 0.01 0.01 0.00 0.00 0.01 0.02 0.02
(0.29) (0.53) (0.55) (0.53) (0.57) (0.57) (0.57) (0.44) (0.53)
0.59 1.24 2.34 2.62 3.79 3.44 3.13 0.26 0.07
0.15 0.25 0.41 0.40 0.39 0.32 0.50 0.06 0.04
(0.71) (1.49) (2.78) (3.02) (4.18) (3.76) (3.63) (0.32) (0.11)
Table 2 Performance parameters of inverted PTB7:PC71BM polymer solar cells with various PEIE layer thicknesses. Data in parentheses give the best performance inverted solar cells
PEIE (nm)
Jsc (mA cm2)
0 0.78 1.41 3.74 5.02 8.68 14.22 21.13 29.09
9.05 13.98 14.59 15.61 16.82 15.92 14.60 9.91 0.03
0.81 1.08 0.98 0.69 0.78 0.87 0.71 0.98 0.02
(10.16) (15.36) (15.62) (16.30) (17.51) (16.85) (15.47) (12.74) (0.04)
FF (%) 37 48 48 54 59 55 55 21 19
3 2 2 3 2 4 4 4 4
Voc (V) (40) (50) (50) (57) (61) (60) (58) (19) (23)
Fig. 3 The PSC device architecture employed in this study and its energy diagram.
whether charge blocking is taking place or not. Since all the PSC devices have similar structures but different PEIE layer thicknesses, the exciton generation and then dissociation in the active layers are also similar. When the bias light is not applied, i.e. for a chopper modulated weak monochromatic light illumination, electrons can move to and then accumulate at the interface between the PEIE layer and the active layer, though most electrons cannot transfer across the thick PEIE layer. The movement of photogenerated carriers including the conducting current and the displacement current,29,30 which are generated from chopper modulated monochromatic light, is typically measured using a lock-in amplifier. The photocurrent measured using a lock-in amplifier enables the apparent EQE to be subsequently calculated. Under a steady state white light bias, some of the charges generated from white light accumulate at the interface between the PEIE layer and the active layer if charge blocking happens.
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0.44 0.63 0.63 0.69 0.69 0.69 0.64 0.67 0.50
PCE (%) 0.01 0.01 0.01 0.00 0.00 0.00 0.02 0.02 0.02
(0.45) (0.64) (0.64) (0.69) (0.69) (0.69) (0.66) (0.69) (0.52)
1.47 4.23 4.41 5.81 6.85 6.04 5.13 1.39 0.003
0.35 (1.82) 0.56 (4.92) 0.58 (5.04) 0.60 (6.41) 0.55 (7.40) 0.82 (6.98) 0.76 (5.92) 0.09 (1.67) 0.001 (0.005)
These accumulated charges would quench excitons or slow down the transfer of charges produced by the monochromatic light. Therefore, a smaller photocurrent is generated by the monochromatic light, and hence a small spectral integrated photocurrent (Jcal) can be calculated when the EQE measurement is conducted under a light bias. Indeed, the measured EQEs without and with a bias light shown in Fig. 4 confirm the above-mentioned physical description. Except for the PSC device without a PEIE layer, and that with a very thick PEIE layer, all other PSC devices exhibited high EQE values without the application of a bias light. Interestingly, the EQEs are relatively large for the PSC devices with very thick PEIE layers, though their Jsc values are very small. Several interpretations can explain this observed behavior, including the displacement current, space charge limited current (SCLC) and bimolecular recombination,29–34 with the displacement current originating from the thick PEIE layer being the major contributor. To further prove the existence of a displacement current in the PSC devices, especially with a thick PEIE layer, we quantitatively compare the calculated current density, Jcal, with the measured short circuit current density, Jsc, for P3HT:PCBM cells with different PEIE layer thicknesses. Jcal is calculated from the EQE shown in Fig. 4a as follows: ð EQEðlÞ PAM1:5 ðlÞ q l Jcal ¼ dl; (1) hc where PAM1.5 is the standard AM 1.5G solar spectral irradiance, and q, l, h, and c are the elementary charge, wavelength, Planck’s constant, and velocity of light in free space, respectively. The comparison results are shown in Fig. 5a. It is obvious that the Jcal values match the measured Jsc values quite well for
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Fig. 4 EQEs of the inverted P3HT:PCBM (a) and PTB7:PC71BM (b) PSCs with various PEIE layer thicknesses measured without light bias.
Fig. 5 (a) Jcal and Jsc of P3HT:PCBM-based PSCs as a function of the PEIE layer thickness for 100 mW cm2 input bias light. The ratio of Jcal/Jsc is shown in the inset. (b) Jcal, calculated from the apparent EQEs, as a function of intensity of bias light for devices with 5.02 nm and 29.09 nm thick PEIE layers.
PEIE layer thicknesses below 14.22 nm. For larger PEIE layer thicknesses the ratio of the Jcal/Jsc value gradually increases with the PEIE layer thickness. Especially, the Jcal value of the PSC device employing a 29.09 nm thick PEIE layer is approximately eight times the measured Jsc value, as shown in the inset of Fig. 5a. As shown in Fig. 5b, the best device with a 5.02 nm thick PEIE layer shows a stable EQE response under different intensities of light bias, thus the almost-unchanged Jcal value can be calculated. However, the EQE response and then the Jcal value of the PSC device with a 29.09 nm thick PEIE layer drop continuously with an increase in the intensity of the bias light. The Jcal value under 100 mW cm2 bias light is equal to the measured Jsc value of 1.18 mA cm2 for a P3HT:PCBM cell with a 29.09 nm thick PEIE layer. The same phenomenon was observed for other devices with a thick PEIE layer and also for PTB7: PC71BM-based PSCs. These results confirm that the displacement current comes from the thick insulating PEIE layer. To confirm that the device performance is solely controlled by the thickness of the PEIE layer, other possible reasons for performance variation must be ruled out. Since the PEIE layer is a transparent wide band gap insulator, it should have little influence on the light transmittance. As shown in Fig. 6a, ITO substrates coated with PEIE layers of different thicknesses show
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similar optical transmittance curves. This result rules out the influence of the PEIE layer on the optical transmittance of the ITO substrates. For all PEIE coated ITO substrates, the low transmittance at wavelengths below 350 nm can be ascribed to the strong absorption of glass in this UV range. Fig. 6b shows the optical transmittance of all PEIE coated ITO substrates at 517, 555 and 605 nm respectively. These specific wavelengths are the representative absorption of the active layer (P3HT:PCBM). Almost constant optical transmittance values of around 90% were observed for all PEIE-coated ITO substrates. Another possible reason for the different Jsc values for various PEIE layer thicknesses is that the thickness of the active layer might have been influenced by the PEIE layer thickness, leading to different light absorbance values for the PSC devices. The measured water contact angle of each PEIE-modified ITO and also the thickness of the active layer (P3HT:PCBM or PTB7:PC71BM) on it are displayed in Fig. 7. As shown in Fig. 7a, the contact angle of the ITO without a PEIE layer is about 60 degree. After PEIE coating, the contact angle of the ITO drops continuously until it saturates at about 10 degree when the PEIE layer thickness reaches 21.13 nm. The result of contact angle versus the PEIE layer thickness is quite similar to the trend of the work function of ITO.19 However, the thickness of the active layer on top of each PEIE-modified ITO is not affected,
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Fig. 6 (a) Optical transmittance of ITO coated with PEIE layers of different thicknesses. (b) Optical transmittance of the PEIE-coated ITO substrates at the three representative absorbance wavelengths of P3HT:PCBM layers.
Fig. 7 (a) The water contact angle and (b) the active layer thickness as a function of the PEIE layer thickness.
Fig. 8 AFM images of ITO (a) without and with (b) 1.41 nm, (c) 5.02 nm, (d) 8.68 nm, (e) 29.09 nm thick PEIE layers. The surface RMS are 1.23, 0.85, 0.65, 0.52, and 0.75 nm, respectively. The scan size is 1.2 mm 1.2 mm.
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as evident from Fig. 7b, where an almost-constant P3HT:PCBM layer thickness of around 210 nm was measured for all PEIE layer thicknesses. Similarly, a fairly-constant active layer thickness of 110 nm was measured for all PTB7:PC71BM-based PSC devices. These results confirm that though the contact angle and also the strength of dipole are affected by the thickness of the PEIE layer, the active layer thickness of the PSCs is constant. The strength of dipole and thus the charge collection ability is enhanced with an increase in the thickness of the PEIE layer. However, the charge tunnel ability would decrease with an increase in the thickness of the PEIE layer. The tradeoff between charge collection and charge tunneling is achieved at about 5 nm, as found in this study. Therefore, the reason for the poor performance of PSC devices with thinner PEIE layers (o5.02 nm) is the result of the weak dipole provided by thinner PEIE layers or the possibility of not fully covering the ITO with PEIE layers. Finally, the contact interface between the active layer and the PEIE layer was investigated. Morphologies of the PEIE layers were imaged using an AFM system as shown in Fig. 8(a)–(e) root mean square (RMS) roughness of the ITO substrate (B1.23 nm) was found to be reduced after coating it with the PEIE layer. Specifically, RMS values of 0.85, 0.65, 0.52, and 0.75 nm were found for PEIE layers of thicknesses 1.41, 5.02, 8.68, and 29.09 nm, respectively. These results indicate that the PEIE layer can smoothen the ITO surface. However, all the surface RMS roughness values for ITOs modified by different thicknesses of PEIE layers are almost identical. In summary, the above experimental results indicate that the impact of the PEIE layer on the optical transmittance, the active layer thickness and the contact interface between the active layer and the PEIE layer is negligible. The strong dipole originating from the optimized PEIE layer facilitates charge collection and maintains charge tunneling through the PEIE layer at the same time. Though thicker PEIE layers can supply a stronger dipole for charge collection, they decrease the tunneling ability of the PSC device, thus degrading its performance.
Conclusions Systematic characterization has been conducted to understand the effect of the thickness of PEIE-modified ITO on the performance of inverted P3HT:PCBM and PTB7:PC71BM solar cells. Experimental results have shown that an optimum PEIE layer thickness of 5.02 nm results in a PCE peak of 4.18% and 7.4%, respectively, for the two PSC types. The optimized PEIE layer is the tradeoff between charge collection and charge tunneling. Experimental results have also confirmed that there is a negligible influence of the PEIE layer thickness on the optical transmittance, thickness of the active layer, and morphology of the PEIE-modified ITO. Finally, it has been confirmed that the displacement current typically observed in PSC devices with thick PEIE layers is the result of the charge accumulation at the PEIE-active layer interface.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 11274256), the Natural Science
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Foundation Project of CQ CSTC (2011BB6012), and the Fundamental Research Funds for the Central Universities (XDJK2014A006). This work was also partially sponsored by SRF for ROCS, SEM and Doctoral Fund of Ministry of Education of China (20120182110008). In addition, this research work was supported by Edith Cowan University, the Department of Industry, Innovation, Science, Research and Tertiary Education, Australia. The authors would like to thank Prof. Zhisong Lu for AFM measurements and Wenpo Li for the PEIE layer thickness measurements.
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High-efficiency inverted polymer solar cells controlled by the thickness of polyethylenimine ethoxylated (PEIE) interfacial layers Ping Li,ab Gang Wang,ab Lun Cai,ab Baofu Ding,c Dachen Zhou,ab Yi Hu,ab Yujun Zhang,ab Jin Xiang,ab Keming Wan,ab Lijia Chen,ab Kamal Alamehc and Qunliang Song*ab In this work, we investigate the effect of the thickness of the polyethylenimine ethoxylated (PEIE) interface layer on the performance of two types of polymer solar cells based on inverted poly(3-hexylthiophene) (P3HT):phenyl C61-butryric acid methyl ester (PCBM) and thieno[3,4-b]thiophene/ benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Maximum power conversion efficiencies of 4.18% and 7.40% were achieved at a 5.02 nm thick PEIE interface layer, for the above-mentioned solar cell types, respectively. The optimized PEIE layer provides a strong enough dipole for the best charge collection while maintaining charge tunneling ability. Optical transmittance and atomic force microscopy measurements indicate that all PEIE films have the same high
Received 5th August 2014, Accepted 16th September 2014 DOI: 10.1039/c4cp03484h
transmittance and smooth surface morphology, ruling out the influence of the PEIE layer on these two parameters. The measured external quantum efficiencies for the devices with thick PEIE layers are quite similar to those of the optimized devices, indicating the poor charge collection ability of thick PEIE layers. The relatively low performance of devices with a PEIE layer of thickness less than 5 nm is the
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result of a weak dipole and partial coverage of the PEIE layer on ITO.
Introduction Polymer solar cells (PSCs) have attracted considerable attention due to their high power conversion efficiency (PCE) (exceeding 10%) as well as attractive features of light weight, flexibility in large area applications and low-cost fabrication through roll-to-roll processing.1–5 Both conventional and inverted PSC structures have been widely studied.6–10 Thin poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) hole transporting layers have conventionally been deposited onto transparent conductive indium tin oxide (ITO) anodes to improve the efficiency of conventional PSCs.6–8 The main drawback of such PSC devices is that the hygroscopic and acidic PEDOT:PSS etch the ITO, thus decreasing the stability of the device. In contrast, a cathode buffer layer (CBL) modified ITO cathode has been used in conjunction with an air stable high work function (WF) anode to realize inverted PSC devices of improved stability a
Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, P. R. China. E-mail: [email protected], [email protected] b Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, Chongqing 400715, P. R. China c Electron Science Research Institute, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA, 6027 Australia
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and manufacturing compatibility.9,10 Because of the high WF of ITO, a good CBL is indispensable to lower the ITO’s WF, thus achieving high PCE inverted PSCs. Spin-coated thin inorganic metal oxide11–15 films have been used as CBLs. However, inorganic metal oxides typically have poor solubility with organic solvents. In contrast, organic molecules have excellent solubility and compatibility with organic solvents and can lower the ITO’s WF.16,17 For example, a basic poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA)16 solution treatment has been demonstrated to improve the device performance by reducing the WF of ITO from 4.38 to 3.94 eV, enhancing charge transport and decreasing the leakage current. Another organic material, namely polyethylenimine ethoxylated (PEIE), has been widely used to enhance electron injection in organic light emitting diodes.18 PEIE has been used to reduce the WF of ITO and other oxide electrodes,19–21 hence improving the performance and stability of inverted devices. PEIE has also been applied to modify the recombination layer in high performance tandem organic solar cells.22,23 However, since PEIE is an insulator with a band gap of 6.2 eV,19 it might not be a good charge extraction layer from the energy point of view. Hence, it is very necessary to systematically investigate the effect of the PEIE layer’s thickness on the performance of PSCs. Such an investigation has not been reported.
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In this paper, the effect of the thickness of the PEIE layer on the performance of inverted poly(3-hexylthiophene) (P3HT): phenyl C61-butryric acid methyl ester (PCBM) and thieno[3,4-b] thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM)-based PSCs is systematically studied. Experimental results show that an optimum PEIE layer thickness of 5.02 nm leads to PCEs of 4.18% and 7.40%, respectively, for the two investigated PSC devices. Optical transmittance, atomic force microscopy (AFM), and thickness determination rule out the change in light absorption in the devices and morphology modification of the active layers and interfaces. External quantum efficiency (EQE) measurements identify the poor charge transfer as the reason for the poor performance of PSC devices employing thick PEIE layers.
Experimental
was modified by PEIE layers from spin-coating PEIE solution with different concentrations19 at 4000 rpm for 60 s, then baked in air at 100 1C for 10 min. The active layers deposited on the PEIE layers of the nine fabricated samples were prepared by spincoating either a P3HT : PCBM (1 : 1 w/w) in 1,2-dichlorobenzene (DCB) solution with a concentration of 34 mg ml1 without any additives or PTB7 : PC71BM (1 : 1.5 w/w) in a mixed solvent of chlorobenzene : 1,8-diiodooctane (0.97 : 0.03) with a concentration of 25 mg ml1 at a speed of 870 rpm for 60 s. The nine samples were then transferred into a vacuum chamber, in which 6 nm thick MoO3 and 100 nm thick Ag electrodes were thermally evaporated at rates of 0.02 and 0.1 nm s1, respectively, through a shadow mask at a base pressure of B107 mbar. Finally all P3HT:PCBM-based devices were thermally annealed24 at 135 1C for 10 min inside the nitrogen filled glove box and stored in the same glove box before measurements. The device area, defined by the overlap between ITO and Ag electrodes, was 9 mm2.
Materials P3HT with regioregularity better than 98%, PCBM and 1,2-dichlorobenzene (DCB, anhydrous, 99%) were purchased from SigmaAldrich. Polyethylenimine, 80% ethoxylated (Mw = 70 000 g mol1), dissolved in H2O with a concentration of 35–40 wt% and 2-methoxy ethanol was also purchased from Sigma-Aldrich. PTB7 and PC71BM were purchased from One-material Chemscitech Inc. (St-Laurent, Quebec, Canada) and used as received. The structure of the PSC devices and chemical structures of materials used in the PSC devices are given in Fig. 1. Fabrication of inverted PSCs The structures of the fabricated inverted PSC devices were ITO/ PEIE/P3HT:PCBM and PTB7:PC71BM/MoO3/Ag, in which the thicknesses of the PEIE layers were 0 (i.e., without PEIE modification), 0.78, 1.41, 3.74, 5.02, 8.68, 14.22, 21.13, and 29.09 nm, respectively. These thicknesses were measured using an ellipsometer (J. A. Woollam M-2000UI). The sheet resistance of the ITO substrates was about 20 O per square. After sequential ultrasonic cleaning using detergents, acetone, and deionized water, the ITO
Fig. 1
Characterization of inverted PSCs and thin films UV-vis absorption measurements were carried out using a Shimadzu UV-21011C spectrometer. For measuring the spectral absorption of the active layers, ITO was used as a reference, whereas the optical transmittance of PEIE-coated ITO glass substrates was measured by using air as a reference. The thicknesses of the active layers were measured using a step profiler (DEKTEK 6M). Atomic Force Microscopy (AFM) was carried out in air for surface morphology measurements using a Dimension ICON AFM operating in tapping mode. The contact angle of the PEIE layer was measured using a contact angle meter (POWEREACH). Current–voltage (I–V) measurements were conducted using a Keithley 2400 with a Newport solar simulator (94043A) of 100 mW cm2 (AM 1.5 G) simulated sunlight. The devices were kept in the dark inside the glove box, except during the periods of short exposure to the simulated sunlight or monochromatic light. The EQE of the devices was calculated from the photocurrent measured using a lock-in amplifier (SR-830). A 150 W xenon lamp followed by a monochromator
(a) Device structures of the PSC devices and (b) chemical structures of materials used in the PSC devices.
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was used to supply the monochromatic light, and the light intensity at each wavelength was determined using a calibrated Si detector.
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Results and discussion All I–V characteristics of the fabricated P3HT:PCBM-based inverted polymer solar cells are shown in Fig. 2a and their performance parameters are summarized in Table 1. The device without a PEIE layer (denoted as 0 nm) exhibited a poor performance, similar to the previous ones.25–28 The shortcurrent density (Jsc) increases from 5.65 0.64 mA cm2 to 10.58 0.62 mA cm2 when the thickness of the PEIE layer increases from 0 nm to 8.68 nm, and then gradually decreases to 3.41 0.56 mA cm2 by further increasing the PEIE layer thickness to 21.13 nm. It was found that Jsc remarkably decreased down to 0.84 0.35 mA cm2 when the thickness of the PEIE layer was increased to 29.09 nm. The dark current rectification ratios are consistent with the Jsc variation, as shown in Fig. 2b. Interestingly, the fill factor (FF) exhibited a similar volcano shape with the change in the PEIE layer thickness, with a peak value of 63 3% at an optimum PEIE layer thickness of 5.02 nm. The open circuit voltage (Voc) was
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not sensitive to the thickness of the PEIE layer. The highest PCE value achieved was 4.18% for a 5.02 nm thick PEIE layer. This value is higher than the reported B3% value obtained using a similar PEIE modifier.19,23 The worst PCE was 0.11% for the device employing a 29.09 nm thick PEIE layer, at which Jsc and FF had their lowest values. The PTB7:PC71BM-based polymer solar cells exhibited similar results as shown in Fig. 2c and d and summarized in Table 2. The highest PCE achieved was 7.4%, also for a PEIE layer thickness of 5.02 nm. The results shown in Fig. 2 demonstrate that by simply changing the thickness of the PEIE layer, the performance of an inverted PSC device can be altered drastically. The energy diagram of a P3HT:PCBM-based cell is shown in Fig. 3. Similar discussion applies to PTB7:PC71BM-based solar cells. The dipole introduced by covering a thin layer of PEIE reduces the work function of ITO, thus facilitating charge transfer and improving the photocurrent. Electrons can transfer by tunneling to ITO through the very thin insulating PEIE layer. For a relatively thick PEIE layer, electron transfer from the active layer to ITO is blocked. In this investigation, the optimized PEIE layer thickness was found to be 5.02 nm, at which the tradeoff was achieved between the dipole-induced charge collection and charge blocking of the insulating PEIE layer. The photocurrent measurements with and without light bias typically determine
Fig. 2 I–V characteristics of the inverted P3HT:PCBM and PTB7:PC71BM polymer solar cells for various PEIE layer thicknesses under 100 mW cm2 AM 1.5 G illumination [(a) and (c)] and in the dark [(b) and (d)].
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Table 1 Performance parameters of inverted P3HT:PCBM polymer solar cells with various PEIE layer thicknesses. Data in parentheses give the performance of the champion devices
PEIE (nm)
Jsc (mA cm2)
0 0.78 1.41 3.74 5.02 8.68 14.22 21.13 29.09
5.65 7.20 8.01 9.51 10.56 10.58 9.97 3.41 0.84
0.64 0.87 0.71 0.68 0.55 0.62 1.01 0.56 0.35
(6.29) (8.07) (8.72) (10.19) (11.11) (11.20) (10.99) (3.97) (1.18)
FF (%) 37 33 54 53 63 57 56 18 16
2 2 4 3 3 2 2 2 3
Voc (V) (39) (35) (58) (56) (66) (59) (58) (18) (17)
0.28 0.52 0.54 0.52 0.57 0.57 0.56 0.43 0.51
PCE (%) 0.01 0.01 0.01 0.01 0.00 0.00 0.01 0.02 0.02
(0.29) (0.53) (0.55) (0.53) (0.57) (0.57) (0.57) (0.44) (0.53)
0.59 1.24 2.34 2.62 3.79 3.44 3.13 0.26 0.07
0.15 0.25 0.41 0.40 0.39 0.32 0.50 0.06 0.04
(0.71) (1.49) (2.78) (3.02) (4.18) (3.76) (3.63) (0.32) (0.11)
Table 2 Performance parameters of inverted PTB7:PC71BM polymer solar cells with various PEIE layer thicknesses. Data in parentheses give the best performance inverted solar cells
PEIE (nm)
Jsc (mA cm2)
0 0.78 1.41 3.74 5.02 8.68 14.22 21.13 29.09
9.05 13.98 14.59 15.61 16.82 15.92 14.60 9.91 0.03
0.81 1.08 0.98 0.69 0.78 0.87 0.71 0.98 0.02
(10.16) (15.36) (15.62) (16.30) (17.51) (16.85) (15.47) (12.74) (0.04)
FF (%) 37 48 48 54 59 55 55 21 19
3 2 2 3 2 4 4 4 4
Voc (V) (40) (50) (50) (57) (61) (60) (58) (19) (23)
Fig. 3 The PSC device architecture employed in this study and its energy diagram.
whether charge blocking is taking place or not. Since all the PSC devices have similar structures but different PEIE layer thicknesses, the exciton generation and then dissociation in the active layers are also similar. When the bias light is not applied, i.e. for a chopper modulated weak monochromatic light illumination, electrons can move to and then accumulate at the interface between the PEIE layer and the active layer, though most electrons cannot transfer across the thick PEIE layer. The movement of photogenerated carriers including the conducting current and the displacement current,29,30 which are generated from chopper modulated monochromatic light, is typically measured using a lock-in amplifier. The photocurrent measured using a lock-in amplifier enables the apparent EQE to be subsequently calculated. Under a steady state white light bias, some of the charges generated from white light accumulate at the interface between the PEIE layer and the active layer if charge blocking happens.
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0.44 0.63 0.63 0.69 0.69 0.69 0.64 0.67 0.50
PCE (%) 0.01 0.01 0.01 0.00 0.00 0.00 0.02 0.02 0.02
(0.45) (0.64) (0.64) (0.69) (0.69) (0.69) (0.66) (0.69) (0.52)
1.47 4.23 4.41 5.81 6.85 6.04 5.13 1.39 0.003
0.35 (1.82) 0.56 (4.92) 0.58 (5.04) 0.60 (6.41) 0.55 (7.40) 0.82 (6.98) 0.76 (5.92) 0.09 (1.67) 0.001 (0.005)
These accumulated charges would quench excitons or slow down the transfer of charges produced by the monochromatic light. Therefore, a smaller photocurrent is generated by the monochromatic light, and hence a small spectral integrated photocurrent (Jcal) can be calculated when the EQE measurement is conducted under a light bias. Indeed, the measured EQEs without and with a bias light shown in Fig. 4 confirm the above-mentioned physical description. Except for the PSC device without a PEIE layer, and that with a very thick PEIE layer, all other PSC devices exhibited high EQE values without the application of a bias light. Interestingly, the EQEs are relatively large for the PSC devices with very thick PEIE layers, though their Jsc values are very small. Several interpretations can explain this observed behavior, including the displacement current, space charge limited current (SCLC) and bimolecular recombination,29–34 with the displacement current originating from the thick PEIE layer being the major contributor. To further prove the existence of a displacement current in the PSC devices, especially with a thick PEIE layer, we quantitatively compare the calculated current density, Jcal, with the measured short circuit current density, Jsc, for P3HT:PCBM cells with different PEIE layer thicknesses. Jcal is calculated from the EQE shown in Fig. 4a as follows: ð EQEðlÞ PAM1:5 ðlÞ q l Jcal ¼ dl; (1) hc where PAM1.5 is the standard AM 1.5G solar spectral irradiance, and q, l, h, and c are the elementary charge, wavelength, Planck’s constant, and velocity of light in free space, respectively. The comparison results are shown in Fig. 5a. It is obvious that the Jcal values match the measured Jsc values quite well for
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Fig. 4 EQEs of the inverted P3HT:PCBM (a) and PTB7:PC71BM (b) PSCs with various PEIE layer thicknesses measured without light bias.
Fig. 5 (a) Jcal and Jsc of P3HT:PCBM-based PSCs as a function of the PEIE layer thickness for 100 mW cm2 input bias light. The ratio of Jcal/Jsc is shown in the inset. (b) Jcal, calculated from the apparent EQEs, as a function of intensity of bias light for devices with 5.02 nm and 29.09 nm thick PEIE layers.
PEIE layer thicknesses below 14.22 nm. For larger PEIE layer thicknesses the ratio of the Jcal/Jsc value gradually increases with the PEIE layer thickness. Especially, the Jcal value of the PSC device employing a 29.09 nm thick PEIE layer is approximately eight times the measured Jsc value, as shown in the inset of Fig. 5a. As shown in Fig. 5b, the best device with a 5.02 nm thick PEIE layer shows a stable EQE response under different intensities of light bias, thus the almost-unchanged Jcal value can be calculated. However, the EQE response and then the Jcal value of the PSC device with a 29.09 nm thick PEIE layer drop continuously with an increase in the intensity of the bias light. The Jcal value under 100 mW cm2 bias light is equal to the measured Jsc value of 1.18 mA cm2 for a P3HT:PCBM cell with a 29.09 nm thick PEIE layer. The same phenomenon was observed for other devices with a thick PEIE layer and also for PTB7: PC71BM-based PSCs. These results confirm that the displacement current comes from the thick insulating PEIE layer. To confirm that the device performance is solely controlled by the thickness of the PEIE layer, other possible reasons for performance variation must be ruled out. Since the PEIE layer is a transparent wide band gap insulator, it should have little influence on the light transmittance. As shown in Fig. 6a, ITO substrates coated with PEIE layers of different thicknesses show
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similar optical transmittance curves. This result rules out the influence of the PEIE layer on the optical transmittance of the ITO substrates. For all PEIE coated ITO substrates, the low transmittance at wavelengths below 350 nm can be ascribed to the strong absorption of glass in this UV range. Fig. 6b shows the optical transmittance of all PEIE coated ITO substrates at 517, 555 and 605 nm respectively. These specific wavelengths are the representative absorption of the active layer (P3HT:PCBM). Almost constant optical transmittance values of around 90% were observed for all PEIE-coated ITO substrates. Another possible reason for the different Jsc values for various PEIE layer thicknesses is that the thickness of the active layer might have been influenced by the PEIE layer thickness, leading to different light absorbance values for the PSC devices. The measured water contact angle of each PEIE-modified ITO and also the thickness of the active layer (P3HT:PCBM or PTB7:PC71BM) on it are displayed in Fig. 7. As shown in Fig. 7a, the contact angle of the ITO without a PEIE layer is about 60 degree. After PEIE coating, the contact angle of the ITO drops continuously until it saturates at about 10 degree when the PEIE layer thickness reaches 21.13 nm. The result of contact angle versus the PEIE layer thickness is quite similar to the trend of the work function of ITO.19 However, the thickness of the active layer on top of each PEIE-modified ITO is not affected,
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Fig. 6 (a) Optical transmittance of ITO coated with PEIE layers of different thicknesses. (b) Optical transmittance of the PEIE-coated ITO substrates at the three representative absorbance wavelengths of P3HT:PCBM layers.
Fig. 7 (a) The water contact angle and (b) the active layer thickness as a function of the PEIE layer thickness.
Fig. 8 AFM images of ITO (a) without and with (b) 1.41 nm, (c) 5.02 nm, (d) 8.68 nm, (e) 29.09 nm thick PEIE layers. The surface RMS are 1.23, 0.85, 0.65, 0.52, and 0.75 nm, respectively. The scan size is 1.2 mm 1.2 mm.
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as evident from Fig. 7b, where an almost-constant P3HT:PCBM layer thickness of around 210 nm was measured for all PEIE layer thicknesses. Similarly, a fairly-constant active layer thickness of 110 nm was measured for all PTB7:PC71BM-based PSC devices. These results confirm that though the contact angle and also the strength of dipole are affected by the thickness of the PEIE layer, the active layer thickness of the PSCs is constant. The strength of dipole and thus the charge collection ability is enhanced with an increase in the thickness of the PEIE layer. However, the charge tunnel ability would decrease with an increase in the thickness of the PEIE layer. The tradeoff between charge collection and charge tunneling is achieved at about 5 nm, as found in this study. Therefore, the reason for the poor performance of PSC devices with thinner PEIE layers (o5.02 nm) is the result of the weak dipole provided by thinner PEIE layers or the possibility of not fully covering the ITO with PEIE layers. Finally, the contact interface between the active layer and the PEIE layer was investigated. Morphologies of the PEIE layers were imaged using an AFM system as shown in Fig. 8(a)–(e) root mean square (RMS) roughness of the ITO substrate (B1.23 nm) was found to be reduced after coating it with the PEIE layer. Specifically, RMS values of 0.85, 0.65, 0.52, and 0.75 nm were found for PEIE layers of thicknesses 1.41, 5.02, 8.68, and 29.09 nm, respectively. These results indicate that the PEIE layer can smoothen the ITO surface. However, all the surface RMS roughness values for ITOs modified by different thicknesses of PEIE layers are almost identical. In summary, the above experimental results indicate that the impact of the PEIE layer on the optical transmittance, the active layer thickness and the contact interface between the active layer and the PEIE layer is negligible. The strong dipole originating from the optimized PEIE layer facilitates charge collection and maintains charge tunneling through the PEIE layer at the same time. Though thicker PEIE layers can supply a stronger dipole for charge collection, they decrease the tunneling ability of the PSC device, thus degrading its performance.
Conclusions Systematic characterization has been conducted to understand the effect of the thickness of PEIE-modified ITO on the performance of inverted P3HT:PCBM and PTB7:PC71BM solar cells. Experimental results have shown that an optimum PEIE layer thickness of 5.02 nm results in a PCE peak of 4.18% and 7.4%, respectively, for the two PSC types. The optimized PEIE layer is the tradeoff between charge collection and charge tunneling. Experimental results have also confirmed that there is a negligible influence of the PEIE layer thickness on the optical transmittance, thickness of the active layer, and morphology of the PEIE-modified ITO. Finally, it has been confirmed that the displacement current typically observed in PSC devices with thick PEIE layers is the result of the charge accumulation at the PEIE-active layer interface.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 11274256), the Natural Science
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Foundation Project of CQ CSTC (2011BB6012), and the Fundamental Research Funds for the Central Universities (XDJK2014A006). This work was also partially sponsored by SRF for ROCS, SEM and Doctoral Fund of Ministry of Education of China (20120182110008). In addition, this research work was supported by Edith Cowan University, the Department of Industry, Innovation, Science, Research and Tertiary Education, Australia. The authors would like to thank Prof. Zhisong Lu for AFM measurements and Wenpo Li for the PEIE layer thickness measurements.
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