Chao
Yi
a,
Xiaowen
Hu
a,
Huckleberry C.
Liu
a,
Rundong
Hu
b,
Chin-Hao
Hsu
a,
Jie
Zheng
b and
Xiong
Gong
*a
aCollege of Polymer Science and Polymer Engineering, The University of Akron, Akron, OH 44325, USA. E-mail: xgong@uakron.edu
bDepartment of Chemical & Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA
First published on 30th October 2014
In this study, we investigate the device performance of bulk heterojunction (BHJ) polymer solar cells (PSCs) fabricated from pristine chlorobenzene (CB) solution, CB solutions with different concentrations of solvent processing additive, chloronaphthalene (CN) which has a high boiling point temperature, and pristine CN solution. An efficiency of 7.12% is observed from PSCs processed from pristine CN solution as compared with 4.01% of that from pristine CB solution. The correlation between the efficiency of PSCs with the concentrations of CN was systematically studied by absorption spectra, atomic force microscopy and cross-section transmission electron microscopy images, wide angle X-ray diffraction and grazing incidence small angle X-ray patterns of BHJ active layers and impedance spectroscopies of BHJ PSCs. It was found that the addition of CN into CB solution does not affect the crystallization or the molecular packing of the donor polymer in BHJ layers, but it changes the film morphology of the BHJ layers. The phase separation between the donor polymer and fullerene derivatives was reduced and BHJ layers were redistributed as the concentration of CN is increased in CB solutions. As a result, increased ratios of the donor polymer to fullerene derivatives, and high hole mobilities of the donor polymer in BHJ layers were obtained for the resultant films. Consequently, a high efficiency was observed from PSCs processed from CN solution rather than from CB solution. Our findings provide a method to approach highly efficient PSCs.
In this study, we report the efficient PSCs fabricated from solvent processing additive chloronaphthalene (CN, b. p. 259 °C) solution. The power conversion efficiencies (PCEs) of PSCs fabricated using poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were increased to 7.12% from 4.01% as the PTB7:PC71BM BHJ composites processed from CN solution as compared to that from chlorobenzene (CB), with a b. p. of 131 °C. It was found that the additional CN in CB solutions could affect the film formation process hence to modify the film morphology of PTB7:PC71BM BHJ active layers, which result in an enhanced hole mobility of PTB7 in PTB7:PC71BM BHJ active layers and enhanced light capture by PTB7 due to the increased composition ratios of PTB7 to PC71BM in the resultant layers, and reduced internal charge transport resistances in PSCs. To the best of our knowledge, this is the first report that a high b. p. solvent is used as the solvent for approaching highly efficient PSCs.
Device | V OC (V) | J SC (mA cm−2) | FF (%) | PCE (%) |
---|---|---|---|---|
a Device parameters with each composition shown in the table are mean values over 20 devices. The deviations in device performance are less than 5%. | ||||
Pristine CB | 0.75 | 11.20 ± 0.30 | 48.0 ± 1.0 | 4.01 ± 0.05 |
CB w/5% CN | 0.72 | 13.00 ± 0.11 | 50.3 ± 0.8 | 4.58 ± 0.11 |
CB w/10% CN | 0.72 | 13.65 ± 0.03 | 54.2 ± 0.1 | 5.18 ± 0.15 |
CB w/20% CN | 0.71 | 13.80 ± 0.20 | 55.1 ± 1.1 | 5.40 ± 0.21 |
CB w/40% CN | 0.70 | 14.32 ± 0.05 | 58.5 ± 0.2 | 5.86 ± 0.05 |
CB w/80% CN | 0.69 | 15.17 ± 0.11 | 59.1 ± 1.2 | 6.27 ± 0.09 |
Pristine CN | 0.70 | 15.92 ± 0.16 | 63.9 ± 0.1 | 7.12 ± 0.07 |
Fig. 2a presents EQE spectra of PSCs based on the PTB7:PC71BM BHJ composite processed from pristine CB, CB solutions with different concentrations of CN and pristine CN. The JSC values estimated from the EQE spectra of all PSCs and the JSC values observed from J–V characteristics are presented in Fig. 2b. It is clear that the estimated JSC values are consistent with the measured JSC values. Interestingly, the PSCs based on the PTB7:PC71BM BHJ composite processed from CB solutions with different concentrations of CN possess different profiles of EQE spectra. As the concentrations of CN are increased, the EQE intensities at wavelengths ranging from 550 nm to 720 nm are increased, but those at wavelengths ranging from 400 nm to 550 nm are decreased. In order to understand these changes, the absorption spectra of PTB7:PC71BM BHJ films cast from pristine CB solution, CB solutions mixed with different concentrations of CN and pristine CN solution were measured under the same conditions. The normalized absorption spectra of these films are presented in Fig. 3. Both absorption spectra of the pristine PTB7 film and pristine PC71BM are also presented in Fig. 3 for comparison studies. The EQE spectra of PSCs are consistent with the absorption spectra of BHJ active layers. It is clear that the enhanced EQE intensities at wavelengths ranging from 550 nm to 700 nm are contributed from PTB7 (ref. 15) and decreased EQE intensities at wavelengths ranging from 400 nm to 550 nm are assigned to PC71BM.16 The differences in both EQE profiles of PSCs and absorption spectra of BHJ thin films indicate that the film morphologies of BHJ active layers processed from pristine CB solution, CB solutions mixed with different concentrations of CN, and pristine CN are different; the component ratios of PTB7 to PC71BM in the resultant layers are increased as the concentrations of CN are increased in CB solutions.
Fig. 2 (a) EQE spectra of PSCs processed from different solutions and (b) JSC from both J–V curves and EQE correlated with concentrations of CN in CB solutions. |
Fig. 3 Absorption spectra of active layers deposited from precursors with different concentrations of CN, PTB7 thin films deposited from pristine CB and pristine CN, and PC71BM thin film. |
Both WAXRD and GISAX are used to characterize the molecular structures of PTB7 in PTB7:PC71BM BHJ thin films. The WAXRD patterns of PTB7:PC71BM BHJ layers are shown in Fig. 4a. All peaks are located in the same position, with no significant difference in the peak intensities, indicating that there is no obvious change in the molecular packing of PTB7.17–19 The GISAX patterns of PTB7:PC71BM films as shown in Fig. 4b are also the same; PTB7 does not show any face-on or edge-on packing in any films, suggesting that PTB7 is hard to crystallize.20 Thus WAXRD and GISAX studies indicate that the molecular structures of PTB7 in all PTB7:PC71BM films are the same and no effect of CN on the molecular packing of PTB7 is observed.
In order to further understand the underlying changes in both absorption spectra and EQE profiles, cross-section TEM was carried out to investigate the film morphology of the PTB7:PC71BM BHJ composite. Fig. 5 presents the cross-section TEM images of PTB7:PC71BM BHJ films processed from pristine CB solution, CB solutions with different concentrations of CN, and pristine CN. In the cross-section TEM images of PTB7:PC71BM/PEDOT:PSS/ITO/glass, where the PTB7:PC71BM BHJ composite is processed from pristine CB solution, the light layer is the PTB7:PC71BM active layer and the dark layer underneath the PTB7:PC71BM layer is the PEDOT:PSS layer.3,21 The thickness of the PEDOT:PSS layer underneath the PTB7:PC71BM layer is ∼30 nm, which is consistent with the measured thickness of the PEDOT:PSS layer (see the Experimental section). It was found that the thickness of the dark layer is increased as the PTB7:PC71BM BHJ composite is processed from the CB solutions with increased concentrations of CN. For instance, the thickness of the dark layer is 48 nm as the PTB7:PC71BM BHJ composite is processed from the CB solution with 40% of CN. The thickness of the dark layer is increased to 86 nm as the PTB7:PC71BM BHJ composite is processed from pristine CN solution. All these results demonstrate that the PTB7:PC71BM BHJ layer is re-distributed to the PCBM-rich (n-type layer) layer, PTB7:PC71BM BHJ layer (intrinsic layer) and PTB7-rich layer (p-type layer).9,11 The PTB7-rich layer is on top of the PEDOT:PSS layer.9,11 The re-distribution of the PTB7:PC71BM BHJ layer into the n–i–p structure would benefit the charge carrier to be transported into the respective electrodes.11,22–24 In order to verify this hypothesis, the hole-only devices were fabricated and characterized to investigate the hole mobility of PTB7 in the PTB7:PC71BM BHJ layer. The hole mobilities of PTB7 from PTB7:PC71BM BHJ layers processed from pristine CB solution, CB solutions with different concentrations of CN, and pristine CN solution are summarized in Table 2. The hole mobility of PTB7 from the PTB7:PC71BM BHJ layer processed from pristine CB solution is 3.94 × 10−4 cm2 V−1 s−1, while the hole mobilities of PTB7 from PTB7:PC71BM BHJ layers are increased as PTB7:PC71BM BHJ layers are processed from CB solutions with increased concentrations of CN. The hole mobility of PTB7 from the PTB7:PC71BM BHJ layer processed from pristine CN solution is 10.83 × 10−4 cm2 V−1 s−1, which is more than two times higher than that processed from pristine CB solution. These results confirm our hypothesis from TEM images of PTB7:PC71BM BHJ layers that hole transport could be facilitated in PTB7:PC71BM BHJ layers processed from CB solutions with increased concentrations of CN. The thicknesses of the active layers cast from pristine CB, the CB solutions with different concentrations of CN and pristine CN are from 175 ± 10 nm.
Fig. 5 Cross-sectional TEM images of BHJ/PEDOT:PSS layers where BHJ layers processed from CB solutions with different concentrations of CN. |
BHJ layers processed from different solutions | μ h of PTB7 (cm2 V−1 s−1) |
---|---|
Pristine CB | 3.94 × 10−4 |
CB w/5% CN | 4.67 × 10−4 |
CB w/10% CN | 5.37 × 10−4 |
CB w/20% CN | 5.80 × 10−4 |
CB w/40% CN | 6.41 × 10−4 |
CB w/80% CN | 7.46 × 10−4 |
Pristine CN | 10.83 × 10−4 |
The film morphology of PTB7:PC71BM BHJ layers processed from different solutions was investigated by AFM. Fig. 6 presents the AFM height images of PTB7:PC71BM BHJ layers. It was found both the sizes of the polymer domain and fullerene aggregation are reduced along with the increased concentrations of CN in CB solutions. The reduced polymer domain and fullerene aggregation would result in a moderate phase separation, which facilitates charge carriers to be separated and transported within the BHJ layer.10,25,26
Fig. 6 AFM height images of BHJ layers processed from CB solutions with different concentrations of CN. |
IS is carried out to investigate the detailed electrical properties of devices, which cannot be observed by direct current measurements. The internal resistance of the PSCs is composed of the sheet resistance (RSH) of the electrodes, and the charge-transport resistance (RCT) at the BHJ/electrode interfaces and inside the BHJ active layer. Since all the electrodes, ITO anode and Al cathode are the same for all PSCs, the sheet resistance of all PSCs should be the same. Thus the changes in the internal resistance originate from inside the BHJ active layers which are processed from CB solutions with different concentrations of CN. Fig. 7 displays the Nyquist plot of all PSCs. The internal resistance gradually reduced from 1450 Ω, 879 Ω, 830 Ω, 800 Ω, 760 Ω to 550 Ω, for the PSCs processed from pristine CB solution, CB solutions with 10%, 20%, 40%, 80% CN, and pristine CN solution, respectively. The decreased internal resistance further indicates that the film morphology of PTB7:PC71BM BHJ layers was affected by the incorporation of CN with CB solutions, which is in good agreement with the observation from cross-section TEM images. Furthermore, the decreased internal resistance would facilitate the charge transport in PSCs and the PSCs with high JSC and PEC are anticipated.27,28
Fig. 7 Nyquist plot of PSCs where BHJ layers processed from CB solutions with different concentrations of CN. |
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