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Enhanced efficiency of organic solar cells by mixed orthogonal solvents
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Enhanced efficiency of organic solar cells by mixed orthogonal solvents
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Yubin Xiao a, Shuang Zhou a, Yaorong Su a, Han Wang a, Lei Ye a, Sai-Wing Tsang b,⇑, Fangyan Xie c, Jianbin Xu a,⇑ a Department of Electronic Engineering and Materials Science and Technology Research Centre, The Chinese University of Hong Kong, Shatin, Hong Kong Special Administrative Region b Department of Physics and Materials Science, The City University of Hong Kong, Kowloon Tong, Hong Kong Special Administrative Region c Instrumental Analysis and Research Center, Sun Yat-sen University, Guangzhou 510275, PR China
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i n f o
Article history: Received 6 December 2013 Received in revised form 19 March 2014 Accepted 13 May 2014 Available online xxxx
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Keywords: Organic solar cells BHJ PCBM-rich layer Orthogonal solvents
a b s t r a c t In this work, the effects of mixed solvents on donor–acceptor vertical phase separation and light absorption was investigated. By using mixed orthogonal solutions of 1,2 o-dichlorobenzene (o-DCB) and dichloromethane (DCM), a PCBM([6,6]-phenyl-C61-butyric acid methyl ester)-rich top layer was induced in typical poly(3-hexylthiophene-2,5diyl)(P3HT):PCBM bulk heterojunction structure. By carefully adjusting the o-DCB:DCM volume ratio, the contact between active layer and the Al cathode was significantly improved due to the precipitation of PCBM on the top surface, which resulting in an electron transport preferable interface between the active layer and cathode. Meanwhile, light absorption was also effectively improved due to the increased crystallinity of polymers under mixed solvents. Overall, the short circuit current was greatly increased, and the efficiency was improved from 3.07% in the control sample to 3.97% by adding 30% DCM. The detailed mechanism of the formation of PCBM-rich layer and enhanced light absorption with o-DCB:DCM solution was expatiated. Our findings suggest a facile spin coating method to fabricate efficient BHJ solar cells, which can pave the way for the large scale application of organic photovoltaic devices (OPVs) in the future. Ó 2014 Elsevier B.V. All rights reserved.
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1. Introduction
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Organic photovoltaic devices (OPVs) which are composed of conjugated polymers and fullerene blends have provided an attractive approach for renewable energy applications due to its low cost, light weight, mechanical flexibility and manufacturing advantages [1–3]. In 1992, Sariciftci et al. found photon induced electron can be
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⇑ Corresponding authors. Tel.: +852 53243645 (S.-W. Tsang). E-mail addresses: [email protected] (Y. Xiao), [email protected]. hk (S. Zhou), [email protected] (Y. Su), [email protected] (H. Wang), [email protected] (L. Ye), [email protected] (S.-W. Tsang), [email protected] (F. Xie), [email protected] (J. Xu).
effectively transferred from conducting polymer to buckminsterfullerene, C60 and foreseed the possibility of C60 derivatives as acceptors in solar cells [4]. Later on, the bulk heterojunction (BHJ) that dominating today’s organic solar cell structures was proposed to form the active layer with inter penetrating networks of donor and acceptor phases and therefore, facilitate more efficient exciton dissociation [5]. Among plenty of donor/acceptor materials, poly(3-hexylthiophene-2,5-diyl)(P3HT) and ([6,6]-phenylC61-butyric acid methyl ester)(PCBM) is still the prototypical system that has been intensively investigated because of the remarkable power conversion efficiency (3–5%) [6–8]. However, some drawbacks of this BHJ structure still exist. For example, many studies had shown that a vertical
http://dx.doi.org/10.1016/j.orgel.2014.05.011 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
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phase gradient was formed after the device fabrication process in such BHJ structure, where a PCBM-rich layer was dominated near the anode and a P3HT-rich layer was introduced on the top surface next to cathode due to the different surface energy of these two ingredients and their interactions with the substrates [9]. Such a vertical distributed structure was conflicted with the ideal solar cell construction, in which the active layer should be sandwiched between a donor-rich layer near the high work function anode and an acceptor-rich layer close to the low work function cathode to promote the charge extraction efficiency. To further increase the cell efficiency, this critical issue should be overcome. Recently, several approaches were proposed to solve this problem, such as employing an inverted device structure [9,10], pre-coating a P3HT buffer layer [11] or thermally depositing a PCBM layer on the top of the P3HT/PCBM active layer. Effective of those approaches though, they require one more processing step which is neither economically efficient nor simple in device fabrication. Alternatively, mixed solvents on the microstructures of the active layer have been investigated by many research groups and demonstrated a simpler approach. Kim et al. found that by mixing dichlorobenzene with different solvents, such as chloroform and chlorobenzene, the crystallinity of the active layer and its surface morphology can be controlled [12]. Peet et al. also observed a longer carrier lifetime in P3HT/PCBM with ordered structure in morphology by adding alkanethiol to toluene, which resulted in an enhanced device performance [13]. Recently, Wang et al. reported that the ideal device structure can be achieved in fabrication of P3HT and PCBM concentration gradient bilayers with optimized o-DCB and DCM composition, which giving a highest power conversion efficiency (PCE) of 2.85% [14]. Herein, we proposed a facile approach to achieve spontaneous formation of a PCBM-rich layer in the interface of P3HT/PCBM and the cathode. By controlling the amount of dichloromethane (DCM) in 1,2 o-dichlorobenzene (o-DCB), the most commonly used solvent, a PCBM-rich layer can be created at the top surface of the BHJ film which minimizes the active layer–electrode interface contact problems or the free electron transport lost to cathode. Also the Jsc was largely increased from 8.2 to 12.0 mA/cm2 due to the enhanced light absorption by the improved molecular chain ordering of P3HT, as a result, the cell efficiency is significantly enhanced from 3.07% to 3.97%. The mechanism involved will be discussed in details.
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2. Experiment
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Firstly, patterned ITO glass was successively cleaned with detergent, deionized water, acetone and isopropanol, respectively, followed by oxygen plasma treatment for 1.5 min. Secondly, the PEDOT:PSS was spun-cast onto ITO glass with a thickness of ca. 40 nm, and then annealed at 145 °C for 10 min in air. P3HT and PCBM were dissolved in a mixed solution composed of 1,2 o-dichlorobenzene (o-DCB) and dichloromethane (DCM) with DCM volume concentration of 0%, 10%, 20%, 30%, and 40% and were spun-cast on the top of PEDOT:PSS layer at a slow speed
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of 500 rpm for 5 s and then higher speed varied from 850 rpm to 1550 rpm for 50 s in order to control the active layer thickness of 150 nm at different DCM concentrations. Following that, 20 min soft annealing and a further annealing at 120 °C for 10 min were conducted in nitrogen atmosphere. The mixed solution had a concentration of 20 mg/mL in P3HT with P3HT:PCBM weight ratio of 1:1. A thin LiF interfacial layer (ca. 1 nm) was deposited by thermal evaporation under a high vacuum (base pressure lower than 2 10 4 Pa). Finally, a 100 nm thick film of Al was thermally evaporated onto the device. The evaporator was BOC Edwards Auto 306 and the active layer area of the device was defined by a shadow mask of 2 mm 6 mm. P3HT:PCBM in pure o-DCB and a 30% DCM mixed solution were prepared to examine the contact angles of deionized water as well as X-ray photoelectron spectroscopy (XPS). Electrical measurements were performed by a semiconductor characterization system (Kei-thley 236) at room temperature in air under the spectral output from solar simulator (Newport) using an AM 1.5G filter with a light power of 100 mW/cm2. The light intensity was precisely calibrated by a calibrated solar cell. For all devices, no external package or encapsulation was applied after device fabrication. The morphologies of the P3HT:PCBM blend thin films were characterized by atomic force microscopy (AFM) in tapping mode. The thickness of the active layer and evaporated layers were recorded with a thickness monitor (Sigma SQM-160), and also verified by AFM. The surface composition was investigated by XPS.
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3. Results and discussion
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3.1. Study of the surface microstructure
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Basically, DCM is orthogonal to o-DCB, in other words, these two solvents have selective solubility to different materials. In this case, P3HT can easily dissolves in o-DCB but has a poor solubility in DCM, whereas PCBM could dissolves in o-DCB and shows a saturated concentration of 10 mg/mL in DCM. It is known that DCM and o-DCB have different boiling points (39.8 °C for DCM and 180.4 °C for o-DCB, respectively), and they have significant difference in dissolving capacities to P3HT and PCBM. When mixing these two solvents together, one could imagine the following scenario by considering their different volatilities and dissolving capacities: after P3HT/PCBM in the mixed solvent was spun-cast onto PEDOT:PSS, during the soft annealing process, because of the very lower solubility of P3HT in DCM than in o-DCB, it will enrich in o-DCB, while PCBM exists in both DCM and o-DCB. Furthermore, due to the low boiling point of DCM, it will evaporate much faster compared with o-DCB, during its upward movements, the dissolved PCBM will move together with DCM and finally reach the top (air) surface to form a PCBM-rich layer. To confirm this assumption, contact angle measurement was conducted on a control sample of P3HT:PCBM film spincoated with pure o-DCB solvent along with the sample prepared with P3HT:PCBM in 30% DCM mixed solvent. It has been known that P3HT possesses a surface energy of 26.9 mN/m [15], which is lower than that of PCBM
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(38.2 mN/m) [16], making it preferably accumulates on the top air surface in order to reduce the overall blend film energy. During the spin-coating and the later on soft annealing process, a thermodynamically favorable state was formed by means of vertical phase separation. Other researchers have also perceived the similar surface enrichment phenomena [16,17]. Therefore, contact angle measurement was performed with DI water drops on samples of P3HT:PCBM dissolving in pure o-DCB and 30% DCM added, as well as pristine P3HT and PCBM. It is clearly observed that the control sample has similar contact angles (98°) with pristine P3HT (99°) as shown in Fig. 1s. The contact angle then decreased from 98° to 90° upon adding 30% DCM in the solvent. The slight change in contact angles suggests that the surface changes from hydrophobic to relatively hydrophilic, indicating that the top surface is dominated by a higher surface energy material (PCBM, 89°) instead of P3HT which has a lower surface energy [18–20]. Yang et al. also performed same contact angle examination on the P3HT:PCMB blend films treated with high pressure carbon dioxide (CO2) and observed similar contact angle variation tendency [21]. To further explore the underlying mechanism of the contact angle variation, atomic force microscopy (AFM) was used to examine the top air surface morphologies and the phase images of P3HT/PCBM active layer with different volume ratios of o-DCB:DCM, as shown in Fig. 1. The corresponding root mean square (RMS) roughness values are summarized in Table 1 and the amplitude images are shown in Fig. 2s. For the AFM phase images in real case as shown in Fig. 1(a), with pure o-DCB solvent, the film surface is dominated by fibre-like patterns which can be assigned to typical P3HT microcrystals. The corresponding RMS value obtained from the morphology image is 3.93 nm. The above observation suggests that the free air
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surface of P3HT/PCBM is enriched by P3HT, which is proven to be adverse to the electron extraction through the cathode. Wang et al. have also proposed a double junction model and assumed a Schottky junction existing between P3HT and Al electrode due to the undesirable vertical phase separation, thus limiting the open circuit voltage [22]. Therefore, eliminating such negative influence of P3HT adjacent to cathode is extremely important in improving the device performance. Fig. 1(b) shows the phase image of active layer upon adding 10% DCM to the solvent. Comparing with pure o-DCB, the fibre-like patterns observed in later case becomes vague with shorter length. The estimated RMS value is 2.43 nm, which is slightly smaller than that of pure o-DCB. With further increasing the volume ratio of DCM to 20% and 30%, a monotonous decrease in RMS values is observed (2.25 nm and 1.61 nm, respectively). At the same time, the fibre-like patterns are gradually vanished. Based on the above observations, it is proposed that with increasing the volume of DCM, on one hand, more PCBM is dissolved by DCM and then diffuses upward to the top surface. On the other hand, during the evaporating process of the blend solvents under annealing, o-DCB has less chance to move into the free air due to the large vapor pressure difference resulted from its high boiling point, thus the upward movement of P3HT was selectively suppressed, and then resulting in a PCBM-rich free air surface instead, which is in good agreement with the results obtained in AFM characterizations. To further optimizing the volume ratio of the blend solvent, more DCM is added into o-DCB to probe the evolution of AFM images. Unfortunately, by increasing the DCM to 40% (Fig. 1(e)), an opposite transition is observed. The RMS value increases markedly to 4.28 nm indicating a much rougher surface is formed. With the increasing volume ratio of DCM, the volume of o-DCB is reduced,
Fig. 1. Tapping mode atomic force microscopy phase images of the P3HT/PCBM composite cast from various DCM concentration.
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Table 1 Photovoltaic parameters and surface roughness of devices based on P3HT:PCBM active layer cast from 1,2-dichlorobenzene and dichloromethane with different o-DCB: DCM volume ratio.
0% DCM 10% DCM 20% DCM 30% DCM 40% DCM
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Voc (V)
Jsc (mA/cm2)
FF (%)
Eff. (%)
RMS (nm)
J (EQE) (mA/cm2)
Rs (X cm2)
Rsh (X cm2)
0.61 0.61 0.62 0.61 0.61
8.2 9.0 10.2 12.0 6.7
61.6 58.0 55.2 54.3 48.0
3.07 3.17 3.49 3.97 1.97
3.93 2.43 2.25 1.61 4.28
7.7 8.6 10.1 11.5 6.3
58.4 53.0 45.6 34.7 67.1
823.9 911.1 676.5 407.4 468.2
the solvation of P3HT in the blend solvent will reach saturation at a certain point since the concentration of P3HT is kept constant. As a result, the excess P3HT will then precipitate out and aggregate together forming large domains or particles, which can be responsible for the increased RMS value as observed in the case of 40% DCM. To determine the surface composition of active layers, X-ray photoelectron spectroscopy (XPS) was used to characterize the active layer surfaces of different samples. The polymer blends for XPS analysis were spun-coat on 300 nm SiO2/Si substrates. Fig. 2 shows the XPS spectra of S 2p peaks on pristine P3HT and P3HT/PCBM that dissolved in blend solvent with 0% DCM and 30% DCM. The atomic ratios for each elements were summarized in Table 2. (The intensities have normalized by C 1s peak intensity of each sample.) Gaussian–Lorentzian model is used to analyze the XPS data so as to obtain element ratio and exact positions of peaks. The S 2p spectrum was splitted into two peaks (S 2p3/2 and S 2p1/2) due to spin–orbit splitting, as indicated in the Fig. 2. The first S 2p3/2 peak has a binding energy of 163.65 eV while the second S 2p1/2 peak has a binding energy of 164.85 eV. The area for each peak can be estimated by fitting the XPS spectra by a XPS peak 41 software. For pristine P3HT, S 2p has an area of 6401 (a.u.). For XPS spectra obtained from 0% and 30% DCM, the peak areas of S 2p are 4019 (a.u.) and 2371 (a.u.), respectively. It can also notice that the sulphur ratio on surface decrease from 3.80% to 2.04% after 30% DCM added as shown in Table 2. The decreasing S 2p peak area and intensity indicate that the occupancy of P3HT on film surface is decreased after adding 30% DCM in o-DCB, which
Fig. 2. XPS spectra of S 2p peaks on pristine P3HT and P3HT/PCBM prepared from 0% to 30% DCM. The intensities have been normalized by their respective C 1s peak intensities.
Table 2 Percentage of the surface atoms and their atomic ratios from XPS measurements.
Pristine P3HT P3HT/PCBM (0% DCM) P3HT/PCBM (30% DCM) Pristine PCBM
C1s
O1s
Si2p
S2p
75.56 79.16 83.78 83.14
12.01 11.01 8.38 11.51
7.40 6.03 5.80 5.35
5.03 3.80 2.04 –
again proves that the surface composition has changed to PCBM-rich.
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3.2. Current–Voltage characterization
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Fig. 3(a) displays the device structure of the P3HT/ PCBM-based solar cells. Noting that a PCBM-rich layer will be spontaneously formed above the P3HT/PCBM photoactive layer as discussed in previous section. Fig. 3(b) shows the current density–voltage (J–V) curves of the devices under illumination of AM 1.5 (Intensity = 100 mW/cm2) at various DCM concentrations. The performance of the solar cells, i.e., short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) are summarized in Table 1. By inspection, it can be clearly noticed that with additional DCM, the PCE increases monotonously and then reaches the maximum value of 3.97% with 30% DCM, which is about 1.3 times than the control sample with PCE of 3.07%, For the sample with adding 30% DCM, the Voc = 0.61 V, Jsc = 12.0 mA/cm2, and FF = 54.3%, whereas for the control sample, the Voc = 0.61 V, Jsc = 8.2 mA/cm2 and a FF of 61.6%. Interestingly, the Jsc exhibits a drastic increase after adding 30% DCM, which is responsible for the enhanced PCE. As having been discussed previously, in conventional solar cell fabrication process, most of the top surface was composed of a P3HT-rich layer, which obstructing the pathway for electrons due to its intrinsic electron blocking property. However, after adding certain amount of low boiling point DCM to o-DCB, a PCBM-rich layer will emerge on the top and form a better contact at the interface between Li/Al cathode and active layer, which providing a preferred conduction channel for electron transport. However, with more DCM concentration at 40%, the cell performance suddenly become worse. As discussed previously, with increasing DCM, due to the very poor solubility of P3HT in DCM, P3HT will reach its saturation concentration and aggregate to the film surface because of the decreasing volume ratio of o-DCB, therefore, the contact between active layer and cathode will become worse again. The following discussion
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Fig. 3. (a) Device structure and (b) J–V characteristic of solar cells at various DCM concentrations. The inset shows the FF and Jsc variations with DCM concentration.
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in Section 3.3 further shows that the 40% DCM cell has relatively lower light absorption and poorer polymer crystallinity which should also responsible for the decreasing PCE.
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3.3. Light absorption and influences of crystallinity
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To further clarify the cause of the increment of PCE after adding certain amount of DCM, the light absorption of the P3HT/PCBM films obtained from pure o-DCB and various concentrations of DCM were examined and the results were shown in Fig. 4. Comparing with the film spun-cast from pure o-DCB solvent, the absorption of the films obtained from blend solvent is much stronger, as can be seen from Fig. 4. Meanwhile, the three vibronic absorption peaks at ca. 530 nm, 560 nm and 610 nm become much more pronounced, indicating a higher level of molecular chain ordering. Because of the improved polymer crystallinity, a slightly red-shifting of the absorption peak can be observed after DCM added. On the other hand, in the case of 40% DCM, in order to maintain the thickness of P3HT/PCBM layer, the spin speed increases to 1550 rpm to make a similar 150 nm-thick film which resulting in fast evaporation of both DCM and o-DCB to evaporate in a very
short time, giving an almost dried P3HT/PCBM film with poor orientation of P3HT supermolecules during spin-coating process. The poor ordering of P3HT in the active layer is responsible for the weak absorption along the full wavelength [23]. To confirm the phenomenon observed in light absorption, we further did X-ray diffraction (XRD) to examine the crystallinity of P3HT. Fig. 5 shows the XRD spectra of P3HT:PCBM active layers prepared from various DCM concentrations. The peak intensity at 2h 5.2 gradually increased as DCM added till 30%, indicating a highly self-organized P3HT. The regular chain structure of P3HT can be tuned by controlling the film solidify time or the film growth rate [24]. Hence, during spun-cast of active layer and the later on soft annealing, the competing vaporization between o-DCB and DCM lead to a slow growth rate of P3HT conjugated polymer that assist the formation of self-organized P3HT which gives a higher peak intensity. However, as DCM added to 40%, due to the faster spun-cast speed, the blend solvents evaporate very fast and P3HT/PCBM solidify in shorter time which results in a very low peak intensity, indicating the amorphous state of P3HT.
Fig. 4. UV–Vis absorption spectra at various amount of DCM concentration. The films were spun cast at spin range from 850 rpm to 1550 rpm for 60 s (film thickness 150 nm). A vertical line was added in the figure to assist the eye.
Fig. 5. X-ray diffraction spectra (XRD) of P3HT:PCBM active layers prepared from various DCM concentrations. The peaks at 2h 5.2 come from the P3HT (1 0 0).
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Furthermore, external quantum efficiency (EQE) was an effective approach to study the transfer efficiency of photon to electron induced by light absorption. Therefore, EQE was measured to confirm the contribution of the increased light absorption. As shown in Fig. 6, the area of the EQE curve enlarged gradually as DCM added till 30%. Also the calculated current density from EQE at each DCM fraction are comparable with the current density obtained from J–V measurement, which proves the validity of the J–V characteristics and the current value at each concentrations are summarized in Table 1.
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3.4. Shunt and series resistance investigation
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Though the increased short circuit current, the fill factor decreases obviously as DCM added. Fundamentally, the fill factor mainly depends on shunt resistance (Rsh) and series resistance (Rs), other parameters, on the other hand, such as mobility, lifetime of the photoactive materials, thickness of active layer and morphology of the cathode-polymer interface should also be paid close attention to [25]. It is reported that the low surface roughness is benefit for the cathode-polymer contact [1,12]. Since the roughness of the active layer decreases with increasing DCM till 30% as confirmed by AFM analysis, so a small contact resistance between active layer and electrodes could be expected. Therefore, we speculate that the possible reason for the decreased fill factor is due to the variation of shunt resistance of the solar cells. To confirm this speculation, shunt resistance (Rsh) and series resistance (Rs) were extracted out from the J–V characteristics (Fig. 3(b)) and were summarized in Table 1. It shows that without DCM fraction, the series resistance is 58.4 X cm2 and after blend with DCM, the series resistance shows a decreasing tendency to 34.7 X cm2 at 30% DCM. The decreasing series resistance is owing to the optimized film-electrodes interface as mentioned before. While for shunt resistance, it presents a reverse trend. At 0% DCM, the Rsh is 823.9 X cm2. after adding 10% DCM, the Rsh stays at the similar level but then decreases quickly to 676.5 X cm2 and 407.4 X cm2 at 20% and 30% DCM respectively. It is well know that shunt resistance denotes the currents losses in the solar cells, such as the current leakage from the edge of the cell, traps or pin-
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Fig. 6. The measured external quantum efficiency (EQE) at various DCM concentrations from 0% to 40%.
holes in the active layer or current losses due to electron– hole recombination. In this specific case, as proved by the UV–Vis absorption spectra, the light absorption of the cell increases significantly after adding DCM. Hence, we speculate that with the increasing light absorption, a larger number of photoinduced charges will generate and results in the increasing Jsc. However, as photoinduced charges increases, the possibility of charge recombination could also reasonably increase because of the limited charge collection efficiency. Hence, more leakage current will be produced and leads to a decreasing shunt resistance. However, more powerful approach should be employed to verify this deduction. Therefore, based on the above analysis, we conclude that the increment of Jsc is mainly due to the increased light absorption as well as the decreased series resistance, whereas, the 10% around decreasing fill factor can be explained by the gradually decreased shunt resistance. To further verify the above speculations, more investigations are still needed to be done and the related results will be reported elsewhere.
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4. Conclusions
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In summary, we have proposed a simple method to increase the PCE of P3HT/PCBM blend solar cells by adding certain amount of DCM to o-DCB. Contact angle, AFM and XPS measurements confirm that the interface between photoactive layer and Li/Al electrode became PCBM-rich in the DCM, o-DCB mixed solvent sample, which can effectively block the hole transport from P3HT to Al. The J–V characteristics of the devices show that current density increases significantly as compared with the control sample and thus resulting in a 29% increment in PCE. Furthermore, the light absorption measurement reveals that the enhanced absorption is due to the improved supermolecular ordering of P3HT according to XRD analysis, which also responsible for the increased current density. By investigate the variation of shunt and series resistance, we observe that the decreased FF is owing to the decreased shunt resistance. These findings suggest that the blend solvent processing method can be an effective approach to modify the morphology of the polymer-cathode interface, which is of great importance to the development of organic solar cell in practical application.
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Acknowledgements
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The work is in part supported by Research Grants Council of Hong Kong, particularly, via Grant Nos. AoE/P-03/08, CUHK4179/10E, N_CUHK405/12, T23-407/13-N, AoE/P-02/ 12. J.B. Xu would like to thank the National Science Foundation of China for the support, particularly, via Grant Nos. 61229401 and 60990314.
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Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.05.011.
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel 5 6
Enhanced efficiency of organic solar cells by mixed orthogonal solvents
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Yubin Xiao a, Shuang Zhou a, Yaorong Su a, Han Wang a, Lei Ye a, Sai-Wing Tsang b,⇑, Fangyan Xie c, Jianbin Xu a,⇑ a Department of Electronic Engineering and Materials Science and Technology Research Centre, The Chinese University of Hong Kong, Shatin, Hong Kong Special Administrative Region b Department of Physics and Materials Science, The City University of Hong Kong, Kowloon Tong, Hong Kong Special Administrative Region c Instrumental Analysis and Research Center, Sun Yat-sen University, Guangzhou 510275, PR China
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a r t i c l e
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i n f o
Article history: Received 6 December 2013 Received in revised form 19 March 2014 Accepted 13 May 2014 Available online xxxx
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Keywords: Organic solar cells BHJ PCBM-rich layer Orthogonal solvents
a b s t r a c t In this work, the effects of mixed solvents on donor–acceptor vertical phase separation and light absorption was investigated. By using mixed orthogonal solutions of 1,2 o-dichlorobenzene (o-DCB) and dichloromethane (DCM), a PCBM([6,6]-phenyl-C61-butyric acid methyl ester)-rich top layer was induced in typical poly(3-hexylthiophene-2,5diyl)(P3HT):PCBM bulk heterojunction structure. By carefully adjusting the o-DCB:DCM volume ratio, the contact between active layer and the Al cathode was significantly improved due to the precipitation of PCBM on the top surface, which resulting in an electron transport preferable interface between the active layer and cathode. Meanwhile, light absorption was also effectively improved due to the increased crystallinity of polymers under mixed solvents. Overall, the short circuit current was greatly increased, and the efficiency was improved from 3.07% in the control sample to 3.97% by adding 30% DCM. The detailed mechanism of the formation of PCBM-rich layer and enhanced light absorption with o-DCB:DCM solution was expatiated. Our findings suggest a facile spin coating method to fabricate efficient BHJ solar cells, which can pave the way for the large scale application of organic photovoltaic devices (OPVs) in the future. Ó 2014 Elsevier B.V. All rights reserved.
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1. Introduction
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Organic photovoltaic devices (OPVs) which are composed of conjugated polymers and fullerene blends have provided an attractive approach for renewable energy applications due to its low cost, light weight, mechanical flexibility and manufacturing advantages [1–3]. In 1992, Sariciftci et al. found photon induced electron can be
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⇑ Corresponding authors. Tel.: +852 53243645 (S.-W. Tsang). E-mail addresses: [email protected] (Y. Xiao), [email protected]. hk (S. Zhou), [email protected] (Y. Su), [email protected] (H. Wang), [email protected] (L. Ye), [email protected] (S.-W. Tsang), [email protected] (F. Xie), [email protected] (J. Xu).
effectively transferred from conducting polymer to buckminsterfullerene, C60 and foreseed the possibility of C60 derivatives as acceptors in solar cells [4]. Later on, the bulk heterojunction (BHJ) that dominating today’s organic solar cell structures was proposed to form the active layer with inter penetrating networks of donor and acceptor phases and therefore, facilitate more efficient exciton dissociation [5]. Among plenty of donor/acceptor materials, poly(3-hexylthiophene-2,5-diyl)(P3HT) and ([6,6]-phenylC61-butyric acid methyl ester)(PCBM) is still the prototypical system that has been intensively investigated because of the remarkable power conversion efficiency (3–5%) [6–8]. However, some drawbacks of this BHJ structure still exist. For example, many studies had shown that a vertical
http://dx.doi.org/10.1016/j.orgel.2014.05.011 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
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phase gradient was formed after the device fabrication process in such BHJ structure, where a PCBM-rich layer was dominated near the anode and a P3HT-rich layer was introduced on the top surface next to cathode due to the different surface energy of these two ingredients and their interactions with the substrates [9]. Such a vertical distributed structure was conflicted with the ideal solar cell construction, in which the active layer should be sandwiched between a donor-rich layer near the high work function anode and an acceptor-rich layer close to the low work function cathode to promote the charge extraction efficiency. To further increase the cell efficiency, this critical issue should be overcome. Recently, several approaches were proposed to solve this problem, such as employing an inverted device structure [9,10], pre-coating a P3HT buffer layer [11] or thermally depositing a PCBM layer on the top of the P3HT/PCBM active layer. Effective of those approaches though, they require one more processing step which is neither economically efficient nor simple in device fabrication. Alternatively, mixed solvents on the microstructures of the active layer have been investigated by many research groups and demonstrated a simpler approach. Kim et al. found that by mixing dichlorobenzene with different solvents, such as chloroform and chlorobenzene, the crystallinity of the active layer and its surface morphology can be controlled [12]. Peet et al. also observed a longer carrier lifetime in P3HT/PCBM with ordered structure in morphology by adding alkanethiol to toluene, which resulted in an enhanced device performance [13]. Recently, Wang et al. reported that the ideal device structure can be achieved in fabrication of P3HT and PCBM concentration gradient bilayers with optimized o-DCB and DCM composition, which giving a highest power conversion efficiency (PCE) of 2.85% [14]. Herein, we proposed a facile approach to achieve spontaneous formation of a PCBM-rich layer in the interface of P3HT/PCBM and the cathode. By controlling the amount of dichloromethane (DCM) in 1,2 o-dichlorobenzene (o-DCB), the most commonly used solvent, a PCBM-rich layer can be created at the top surface of the BHJ film which minimizes the active layer–electrode interface contact problems or the free electron transport lost to cathode. Also the Jsc was largely increased from 8.2 to 12.0 mA/cm2 due to the enhanced light absorption by the improved molecular chain ordering of P3HT, as a result, the cell efficiency is significantly enhanced from 3.07% to 3.97%. The mechanism involved will be discussed in details.
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2. Experiment
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Firstly, patterned ITO glass was successively cleaned with detergent, deionized water, acetone and isopropanol, respectively, followed by oxygen plasma treatment for 1.5 min. Secondly, the PEDOT:PSS was spun-cast onto ITO glass with a thickness of ca. 40 nm, and then annealed at 145 °C for 10 min in air. P3HT and PCBM were dissolved in a mixed solution composed of 1,2 o-dichlorobenzene (o-DCB) and dichloromethane (DCM) with DCM volume concentration of 0%, 10%, 20%, 30%, and 40% and were spun-cast on the top of PEDOT:PSS layer at a slow speed
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of 500 rpm for 5 s and then higher speed varied from 850 rpm to 1550 rpm for 50 s in order to control the active layer thickness of 150 nm at different DCM concentrations. Following that, 20 min soft annealing and a further annealing at 120 °C for 10 min were conducted in nitrogen atmosphere. The mixed solution had a concentration of 20 mg/mL in P3HT with P3HT:PCBM weight ratio of 1:1. A thin LiF interfacial layer (ca. 1 nm) was deposited by thermal evaporation under a high vacuum (base pressure lower than 2 10 4 Pa). Finally, a 100 nm thick film of Al was thermally evaporated onto the device. The evaporator was BOC Edwards Auto 306 and the active layer area of the device was defined by a shadow mask of 2 mm 6 mm. P3HT:PCBM in pure o-DCB and a 30% DCM mixed solution were prepared to examine the contact angles of deionized water as well as X-ray photoelectron spectroscopy (XPS). Electrical measurements were performed by a semiconductor characterization system (Kei-thley 236) at room temperature in air under the spectral output from solar simulator (Newport) using an AM 1.5G filter with a light power of 100 mW/cm2. The light intensity was precisely calibrated by a calibrated solar cell. For all devices, no external package or encapsulation was applied after device fabrication. The morphologies of the P3HT:PCBM blend thin films were characterized by atomic force microscopy (AFM) in tapping mode. The thickness of the active layer and evaporated layers were recorded with a thickness monitor (Sigma SQM-160), and also verified by AFM. The surface composition was investigated by XPS.
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3. Results and discussion
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3.1. Study of the surface microstructure
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Basically, DCM is orthogonal to o-DCB, in other words, these two solvents have selective solubility to different materials. In this case, P3HT can easily dissolves in o-DCB but has a poor solubility in DCM, whereas PCBM could dissolves in o-DCB and shows a saturated concentration of 10 mg/mL in DCM. It is known that DCM and o-DCB have different boiling points (39.8 °C for DCM and 180.4 °C for o-DCB, respectively), and they have significant difference in dissolving capacities to P3HT and PCBM. When mixing these two solvents together, one could imagine the following scenario by considering their different volatilities and dissolving capacities: after P3HT/PCBM in the mixed solvent was spun-cast onto PEDOT:PSS, during the soft annealing process, because of the very lower solubility of P3HT in DCM than in o-DCB, it will enrich in o-DCB, while PCBM exists in both DCM and o-DCB. Furthermore, due to the low boiling point of DCM, it will evaporate much faster compared with o-DCB, during its upward movements, the dissolved PCBM will move together with DCM and finally reach the top (air) surface to form a PCBM-rich layer. To confirm this assumption, contact angle measurement was conducted on a control sample of P3HT:PCBM film spincoated with pure o-DCB solvent along with the sample prepared with P3HT:PCBM in 30% DCM mixed solvent. It has been known that P3HT possesses a surface energy of 26.9 mN/m [15], which is lower than that of PCBM
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(38.2 mN/m) [16], making it preferably accumulates on the top air surface in order to reduce the overall blend film energy. During the spin-coating and the later on soft annealing process, a thermodynamically favorable state was formed by means of vertical phase separation. Other researchers have also perceived the similar surface enrichment phenomena [16,17]. Therefore, contact angle measurement was performed with DI water drops on samples of P3HT:PCBM dissolving in pure o-DCB and 30% DCM added, as well as pristine P3HT and PCBM. It is clearly observed that the control sample has similar contact angles (98°) with pristine P3HT (99°) as shown in Fig. 1s. The contact angle then decreased from 98° to 90° upon adding 30% DCM in the solvent. The slight change in contact angles suggests that the surface changes from hydrophobic to relatively hydrophilic, indicating that the top surface is dominated by a higher surface energy material (PCBM, 89°) instead of P3HT which has a lower surface energy [18–20]. Yang et al. also performed same contact angle examination on the P3HT:PCMB blend films treated with high pressure carbon dioxide (CO2) and observed similar contact angle variation tendency [21]. To further explore the underlying mechanism of the contact angle variation, atomic force microscopy (AFM) was used to examine the top air surface morphologies and the phase images of P3HT/PCBM active layer with different volume ratios of o-DCB:DCM, as shown in Fig. 1. The corresponding root mean square (RMS) roughness values are summarized in Table 1 and the amplitude images are shown in Fig. 2s. For the AFM phase images in real case as shown in Fig. 1(a), with pure o-DCB solvent, the film surface is dominated by fibre-like patterns which can be assigned to typical P3HT microcrystals. The corresponding RMS value obtained from the morphology image is 3.93 nm. The above observation suggests that the free air
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surface of P3HT/PCBM is enriched by P3HT, which is proven to be adverse to the electron extraction through the cathode. Wang et al. have also proposed a double junction model and assumed a Schottky junction existing between P3HT and Al electrode due to the undesirable vertical phase separation, thus limiting the open circuit voltage [22]. Therefore, eliminating such negative influence of P3HT adjacent to cathode is extremely important in improving the device performance. Fig. 1(b) shows the phase image of active layer upon adding 10% DCM to the solvent. Comparing with pure o-DCB, the fibre-like patterns observed in later case becomes vague with shorter length. The estimated RMS value is 2.43 nm, which is slightly smaller than that of pure o-DCB. With further increasing the volume ratio of DCM to 20% and 30%, a monotonous decrease in RMS values is observed (2.25 nm and 1.61 nm, respectively). At the same time, the fibre-like patterns are gradually vanished. Based on the above observations, it is proposed that with increasing the volume of DCM, on one hand, more PCBM is dissolved by DCM and then diffuses upward to the top surface. On the other hand, during the evaporating process of the blend solvents under annealing, o-DCB has less chance to move into the free air due to the large vapor pressure difference resulted from its high boiling point, thus the upward movement of P3HT was selectively suppressed, and then resulting in a PCBM-rich free air surface instead, which is in good agreement with the results obtained in AFM characterizations. To further optimizing the volume ratio of the blend solvent, more DCM is added into o-DCB to probe the evolution of AFM images. Unfortunately, by increasing the DCM to 40% (Fig. 1(e)), an opposite transition is observed. The RMS value increases markedly to 4.28 nm indicating a much rougher surface is formed. With the increasing volume ratio of DCM, the volume of o-DCB is reduced,
Fig. 1. Tapping mode atomic force microscopy phase images of the P3HT/PCBM composite cast from various DCM concentration.
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Table 1 Photovoltaic parameters and surface roughness of devices based on P3HT:PCBM active layer cast from 1,2-dichlorobenzene and dichloromethane with different o-DCB: DCM volume ratio.
0% DCM 10% DCM 20% DCM 30% DCM 40% DCM
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Voc (V)
Jsc (mA/cm2)
FF (%)
Eff. (%)
RMS (nm)
J (EQE) (mA/cm2)
Rs (X cm2)
Rsh (X cm2)
0.61 0.61 0.62 0.61 0.61
8.2 9.0 10.2 12.0 6.7
61.6 58.0 55.2 54.3 48.0
3.07 3.17 3.49 3.97 1.97
3.93 2.43 2.25 1.61 4.28
7.7 8.6 10.1 11.5 6.3
58.4 53.0 45.6 34.7 67.1
823.9 911.1 676.5 407.4 468.2
the solvation of P3HT in the blend solvent will reach saturation at a certain point since the concentration of P3HT is kept constant. As a result, the excess P3HT will then precipitate out and aggregate together forming large domains or particles, which can be responsible for the increased RMS value as observed in the case of 40% DCM. To determine the surface composition of active layers, X-ray photoelectron spectroscopy (XPS) was used to characterize the active layer surfaces of different samples. The polymer blends for XPS analysis were spun-coat on 300 nm SiO2/Si substrates. Fig. 2 shows the XPS spectra of S 2p peaks on pristine P3HT and P3HT/PCBM that dissolved in blend solvent with 0% DCM and 30% DCM. The atomic ratios for each elements were summarized in Table 2. (The intensities have normalized by C 1s peak intensity of each sample.) Gaussian–Lorentzian model is used to analyze the XPS data so as to obtain element ratio and exact positions of peaks. The S 2p spectrum was splitted into two peaks (S 2p3/2 and S 2p1/2) due to spin–orbit splitting, as indicated in the Fig. 2. The first S 2p3/2 peak has a binding energy of 163.65 eV while the second S 2p1/2 peak has a binding energy of 164.85 eV. The area for each peak can be estimated by fitting the XPS spectra by a XPS peak 41 software. For pristine P3HT, S 2p has an area of 6401 (a.u.). For XPS spectra obtained from 0% and 30% DCM, the peak areas of S 2p are 4019 (a.u.) and 2371 (a.u.), respectively. It can also notice that the sulphur ratio on surface decrease from 3.80% to 2.04% after 30% DCM added as shown in Table 2. The decreasing S 2p peak area and intensity indicate that the occupancy of P3HT on film surface is decreased after adding 30% DCM in o-DCB, which
Fig. 2. XPS spectra of S 2p peaks on pristine P3HT and P3HT/PCBM prepared from 0% to 30% DCM. The intensities have been normalized by their respective C 1s peak intensities.
Table 2 Percentage of the surface atoms and their atomic ratios from XPS measurements.
Pristine P3HT P3HT/PCBM (0% DCM) P3HT/PCBM (30% DCM) Pristine PCBM
C1s
O1s
Si2p
S2p
75.56 79.16 83.78 83.14
12.01 11.01 8.38 11.51
7.40 6.03 5.80 5.35
5.03 3.80 2.04 –
again proves that the surface composition has changed to PCBM-rich.
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3.2. Current–Voltage characterization
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Fig. 3(a) displays the device structure of the P3HT/ PCBM-based solar cells. Noting that a PCBM-rich layer will be spontaneously formed above the P3HT/PCBM photoactive layer as discussed in previous section. Fig. 3(b) shows the current density–voltage (J–V) curves of the devices under illumination of AM 1.5 (Intensity = 100 mW/cm2) at various DCM concentrations. The performance of the solar cells, i.e., short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) are summarized in Table 1. By inspection, it can be clearly noticed that with additional DCM, the PCE increases monotonously and then reaches the maximum value of 3.97% with 30% DCM, which is about 1.3 times than the control sample with PCE of 3.07%, For the sample with adding 30% DCM, the Voc = 0.61 V, Jsc = 12.0 mA/cm2, and FF = 54.3%, whereas for the control sample, the Voc = 0.61 V, Jsc = 8.2 mA/cm2 and a FF of 61.6%. Interestingly, the Jsc exhibits a drastic increase after adding 30% DCM, which is responsible for the enhanced PCE. As having been discussed previously, in conventional solar cell fabrication process, most of the top surface was composed of a P3HT-rich layer, which obstructing the pathway for electrons due to its intrinsic electron blocking property. However, after adding certain amount of low boiling point DCM to o-DCB, a PCBM-rich layer will emerge on the top and form a better contact at the interface between Li/Al cathode and active layer, which providing a preferred conduction channel for electron transport. However, with more DCM concentration at 40%, the cell performance suddenly become worse. As discussed previously, with increasing DCM, due to the very poor solubility of P3HT in DCM, P3HT will reach its saturation concentration and aggregate to the film surface because of the decreasing volume ratio of o-DCB, therefore, the contact between active layer and cathode will become worse again. The following discussion
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Fig. 3. (a) Device structure and (b) J–V characteristic of solar cells at various DCM concentrations. The inset shows the FF and Jsc variations with DCM concentration.
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in Section 3.3 further shows that the 40% DCM cell has relatively lower light absorption and poorer polymer crystallinity which should also responsible for the decreasing PCE.
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3.3. Light absorption and influences of crystallinity
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To further clarify the cause of the increment of PCE after adding certain amount of DCM, the light absorption of the P3HT/PCBM films obtained from pure o-DCB and various concentrations of DCM were examined and the results were shown in Fig. 4. Comparing with the film spun-cast from pure o-DCB solvent, the absorption of the films obtained from blend solvent is much stronger, as can be seen from Fig. 4. Meanwhile, the three vibronic absorption peaks at ca. 530 nm, 560 nm and 610 nm become much more pronounced, indicating a higher level of molecular chain ordering. Because of the improved polymer crystallinity, a slightly red-shifting of the absorption peak can be observed after DCM added. On the other hand, in the case of 40% DCM, in order to maintain the thickness of P3HT/PCBM layer, the spin speed increases to 1550 rpm to make a similar 150 nm-thick film which resulting in fast evaporation of both DCM and o-DCB to evaporate in a very
short time, giving an almost dried P3HT/PCBM film with poor orientation of P3HT supermolecules during spin-coating process. The poor ordering of P3HT in the active layer is responsible for the weak absorption along the full wavelength [23]. To confirm the phenomenon observed in light absorption, we further did X-ray diffraction (XRD) to examine the crystallinity of P3HT. Fig. 5 shows the XRD spectra of P3HT:PCBM active layers prepared from various DCM concentrations. The peak intensity at 2h 5.2 gradually increased as DCM added till 30%, indicating a highly self-organized P3HT. The regular chain structure of P3HT can be tuned by controlling the film solidify time or the film growth rate [24]. Hence, during spun-cast of active layer and the later on soft annealing, the competing vaporization between o-DCB and DCM lead to a slow growth rate of P3HT conjugated polymer that assist the formation of self-organized P3HT which gives a higher peak intensity. However, as DCM added to 40%, due to the faster spun-cast speed, the blend solvents evaporate very fast and P3HT/PCBM solidify in shorter time which results in a very low peak intensity, indicating the amorphous state of P3HT.
Fig. 4. UV–Vis absorption spectra at various amount of DCM concentration. The films were spun cast at spin range from 850 rpm to 1550 rpm for 60 s (film thickness 150 nm). A vertical line was added in the figure to assist the eye.
Fig. 5. X-ray diffraction spectra (XRD) of P3HT:PCBM active layers prepared from various DCM concentrations. The peaks at 2h 5.2 come from the P3HT (1 0 0).
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Furthermore, external quantum efficiency (EQE) was an effective approach to study the transfer efficiency of photon to electron induced by light absorption. Therefore, EQE was measured to confirm the contribution of the increased light absorption. As shown in Fig. 6, the area of the EQE curve enlarged gradually as DCM added till 30%. Also the calculated current density from EQE at each DCM fraction are comparable with the current density obtained from J–V measurement, which proves the validity of the J–V characteristics and the current value at each concentrations are summarized in Table 1.
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3.4. Shunt and series resistance investigation
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Though the increased short circuit current, the fill factor decreases obviously as DCM added. Fundamentally, the fill factor mainly depends on shunt resistance (Rsh) and series resistance (Rs), other parameters, on the other hand, such as mobility, lifetime of the photoactive materials, thickness of active layer and morphology of the cathode-polymer interface should also be paid close attention to [25]. It is reported that the low surface roughness is benefit for the cathode-polymer contact [1,12]. Since the roughness of the active layer decreases with increasing DCM till 30% as confirmed by AFM analysis, so a small contact resistance between active layer and electrodes could be expected. Therefore, we speculate that the possible reason for the decreased fill factor is due to the variation of shunt resistance of the solar cells. To confirm this speculation, shunt resistance (Rsh) and series resistance (Rs) were extracted out from the J–V characteristics (Fig. 3(b)) and were summarized in Table 1. It shows that without DCM fraction, the series resistance is 58.4 X cm2 and after blend with DCM, the series resistance shows a decreasing tendency to 34.7 X cm2 at 30% DCM. The decreasing series resistance is owing to the optimized film-electrodes interface as mentioned before. While for shunt resistance, it presents a reverse trend. At 0% DCM, the Rsh is 823.9 X cm2. after adding 10% DCM, the Rsh stays at the similar level but then decreases quickly to 676.5 X cm2 and 407.4 X cm2 at 20% and 30% DCM respectively. It is well know that shunt resistance denotes the currents losses in the solar cells, such as the current leakage from the edge of the cell, traps or pin-
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Fig. 6. The measured external quantum efficiency (EQE) at various DCM concentrations from 0% to 40%.
holes in the active layer or current losses due to electron– hole recombination. In this specific case, as proved by the UV–Vis absorption spectra, the light absorption of the cell increases significantly after adding DCM. Hence, we speculate that with the increasing light absorption, a larger number of photoinduced charges will generate and results in the increasing Jsc. However, as photoinduced charges increases, the possibility of charge recombination could also reasonably increase because of the limited charge collection efficiency. Hence, more leakage current will be produced and leads to a decreasing shunt resistance. However, more powerful approach should be employed to verify this deduction. Therefore, based on the above analysis, we conclude that the increment of Jsc is mainly due to the increased light absorption as well as the decreased series resistance, whereas, the 10% around decreasing fill factor can be explained by the gradually decreased shunt resistance. To further verify the above speculations, more investigations are still needed to be done and the related results will be reported elsewhere.
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4. Conclusions
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In summary, we have proposed a simple method to increase the PCE of P3HT/PCBM blend solar cells by adding certain amount of DCM to o-DCB. Contact angle, AFM and XPS measurements confirm that the interface between photoactive layer and Li/Al electrode became PCBM-rich in the DCM, o-DCB mixed solvent sample, which can effectively block the hole transport from P3HT to Al. The J–V characteristics of the devices show that current density increases significantly as compared with the control sample and thus resulting in a 29% increment in PCE. Furthermore, the light absorption measurement reveals that the enhanced absorption is due to the improved supermolecular ordering of P3HT according to XRD analysis, which also responsible for the increased current density. By investigate the variation of shunt and series resistance, we observe that the decreased FF is owing to the decreased shunt resistance. These findings suggest that the blend solvent processing method can be an effective approach to modify the morphology of the polymer-cathode interface, which is of great importance to the development of organic solar cell in practical application.
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Acknowledgements
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The work is in part supported by Research Grants Council of Hong Kong, particularly, via Grant Nos. AoE/P-03/08, CUHK4179/10E, N_CUHK405/12, T23-407/13-N, AoE/P-02/ 12. J.B. Xu would like to thank the National Science Foundation of China for the support, particularly, via Grant Nos. 61229401 and 60990314.
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Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.05.011.
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Please cite this article in press as: Y. Xiao et al., Enhanced efficiency of organic solar cells by mixed orthogonal solvents, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.05.011
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References
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