Effect of solvent additive and ethanol treatment on the performance of PIDTDTQx:PC71BM polymer solar cells

Solar Energy Materials & Solar Cells 132 (2015) 528–534

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Effect of solvent additive and ethanol treatment on the performance of PIDTDTQx:PC71BM polymer solar cells Xixiang Zhu a, Fujun Zhang a,n, Qiaoshi An a, Hui Huang b, Qianqian Sun a, Lingliang Li a, Feng Teng a,n, Weihua Tang c a

Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, PR China School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, PR China c Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, PR China b

art ic l e i nf o

a b s t r a c t

Article history: Received 24 July 2014 Received in revised form 29 September 2014 Accepted 4 October 2014

Solvent additive 1,8-diiodooctane (DIO) is not almighty for improving the performance of polymer solar cells (PSCs). In this paper, the effect of solvent additive DIO and ethanol treatment on the performance of polymer solar cells (PSCs) with poly{[4,9-dihydro-4,4,9,9-tetra(4-hexylbenzyl)-s-indaceno[1,2-b:5,6-b0 ]dithiophene-2,7-diyl]-alt-[2,3-bis(3-(octyloxy)phenyl)-2,3-dihydro-quinoxaline-2,20 -diyl] (PIDTDTQx) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the active layer was investigated. The power conversion efficiency (PCE) was decreased from 4.57% to 1.96% by adding 4 vol% DIO solvent additive for the active layer processed with 1,2-dichlorobenzene (DCB) as solvent. The negative effect of DIO on PCE from 1.94% to 1.18% of PSCs processed with chlorobenzene (CB) as solvent was further demonstrated. The PCE values of PSCs with DIO additive can be effectively increased from 1.96% to 3.71% for DCB as solvent and from 1.18% to 1.89% for CB as solvent by ethanol treatment on the active layer. The crystalline and morphology of active layers play the key role in determining the performance of PSCs. & 2014 Elsevier B.V. All rights reserved.

Keywords: Polymer solar cells Morphology Solvent additive Ethanol treatment

1. Introduction Polymer solar cells (PSCs) have attracted extensive attention for their potential to be clean and sustainable energy sources with several advantages, including low processing cost, light weight and flexible fabrication [1–4]. The manufacture and installation of a solar park based on PSCs following a concept has been realized by Krebs et al., a series of iconic research results have been achieved based on large scale PSCs [5–8], which strongly promote the basic research on performance improvement of PSCs. Since the discovery of the photo-induced electron transfer from conjugated polymer to fullerene derivatives, the bulk-heterojunction structure has been extensively applied for better exciton dissociation due to bicontinuous interpenetrating network structure of electron donor and acceptor [9]. The molecular arrangement of electron donor/ acceptor materials plays an important role in exciton dissociation and charge carriers transport, which strongly impacts on the performance of PSCs [10,11]. Many strategies have been successfully developed and employed to optimize the morphology of active layers, such as solution treatment, solvent additives, and thermal

n

Corresponding authors. Tel./fax: þ86 10 51684908. E-mail addresses: [email protected] (F. Zhang), [email protected] (F. Teng).

http://dx.doi.org/10.1016/j.solmat.2014.10.006 0927-0248/& 2014 Elsevier B.V. All rights reserved.

annealing treatments [12–16]. Among them, adopting solvent additives has been demonstrated to be an effective and simple method to adjust morphology of active layers. The common used solvent additives, such as 1-chloronaphthalene (CN), 1,8-diiodooctane (DIO) and diphenylether (DPE), have selective dissolution of fullerene derivatives and relative high boiling points [17–19]. The high boiling point (BP) solvent additives have played an important role in controlling electron donor or acceptor domain sizes, which would effect of the balance between exciton dissociation and charge carrier transport. In fact, DIO as a high BP solvent additive has been proved to be the most successful additive for performance improvement of PSCs [20–22]. Liang et al. reported that the PCE of PSCs with PTB7: [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as active layer was increased to 7.40% from 3.92% upon mixing of 3 vol% DIO to the host solvent chlorobenzene (CB) [23]. Klein et al. also reported that the PCE of PSCs with PCDTPBt:PC71BM as active layer was increased from 2.5% to 4.6% by adding 3 vol% solvent additive DIO to the host solvent 1,2-dichlorobenzene (DCB) [24]. Recently, Gedefaw et al. reported that the PCE of PSCs with PBDTFQ-T:PC61BM as active layer was increased from 2.18% to 5.30% by adding 3 vol% solvent additive DIO, the PCE improvement should be attributed to the finer adjustment on nanostructure of active layer for the better balance between charge transport properties and light absorption [25]. Up to now, there are few researches about the negative effect

X. Zhu et al. / Solar Energy Materials & Solar Cells 132 (2015) 528–534

of DIO additive on performance of PSCs. Kyaw et al. reported that the PCE of small-molecule solar cells with p-DTS(FBTTh2)2):PC71BM as active layer was decreased to 3.32% from 8.03% by an excess amount of DIO (from 0.4 vol% to 0.8 vol%), resulting in the overaggregation of the donor phase [26]. Do et al. reported that the PCE of PSCs based on 3T-BO20:PC61BM reduced from 1.92% to 1.76% by addition of 1 vol% DIO additive [27]. Therefore, the underlying reason of DIO additive function on performance of PSCs should be further needed to be clarified in the different polymers system. Another effective strategy for PCE improvement of PSCs is using low boiling point polar solvent treatment on pristine active layer, which can wash out used solvent and solvent additive from active layer before deposition of metal electrodes. Many research results exhibited that polar solvent, such as ethanol and methanol, have positive effect on the device performance for optimization of the phase separation in the active layer, the increase of built-in potential due to the passivation of surface traps [28,29]. Guo et al. successfully synthesized a novel band gap polymer poly{[4,9-dihydro-4,4,9,9tetra(4-hexylbenzyl)-s-indaceno[1,2-b:5,6-b0 ]-dithiophene-2,7-diyl] -alt-[2,3-bis(3-(octyloxy)phenyl)-2,3-dihydro-quinoxaline-2,20 -diyl] (PIDTDTQx) and obtained a high PCE of 7.11% with optimized PIDTDTQx:PC71BM doping weight ratio as 1:4 [30]. In this paper, we investigated the effect of solvent additive DIO on performance of PSCs based on the blend of PIDTDTQx/PC71BM with DCB or CB as solvent, respectively. It is apparent that PCE values of PSCs with DIO decreased from 4.57% to 1.96% for DCB as solvent and decreased to 1.18% from 1.94% for CB as solvent. The PCE values of PSCs with DIO additive were a large extent restored from 1.96% to 3.71% and from 1.18% to 1.89% by ethanol treatment on the active layers.

2. Experimental details The indium tin oxide (ITO) glass substrates with sheet resistance 15 Ω/sq were cleaned continuously in the ultrasonic baths containing acetone, detergent, de-ionized water, and ethanol. Then, the cleaned ITO substrates were blow-dried by high pure nitrogen gas and then treated by UV ozone for 10 min. PEDOT:PSS (Clevios P VP.Al 4083,

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purchased from H.C. Starck co. Ltd.) films were fabricated onto the cleaned ITO substrate by the spin coating method at 5000 rpm for 40 s and then annealed at 120 1C for 10 min in room conditions. The polymer material PIDTDTQx (Product no: OT51501, purchased from Organtec Materials Inc.), and PC71BM (Product no: LT-S923, purchased from Luminescence Technology Corp) with a weight ratio of 1:4 were dissolved in DCB or CB at a concentration of 50 mg/ml, respectively. The 1,8-diiodooctance (DIO) (Product no: A10867, purchased from Alfa Aesar Chemical Co., Ltd.) additive was added into the solutions before the spin-coating process. The blend solutions were stirred with a magnetic stirrer at 70 1C for 12 h. Then the active layers were fabricated by the spin-coating method at 2500 rpm for 40 s in a high-purity nitrogen-filled glove box. And then 10 min for drying the active layer, ethanol solvent was spin-coated on top of the active layers for further optimizing the morphology of active layer. The LiF/Al (0.9 nm/100 nm) combined cathode was deposited by thermal evaporation under 10  4 Pa conditions. The thickness was monitored by a quartz crystal microbalance. The vertical overlap of the ITO anode and Al cathode is defined as the active area about 3.8 mm2. The current density voltage (J–V) characteristics of the devices were measured using a Keithley 4200 source measurement unit. The absorption spectra of the films were obtained using a Shimadzu UV-3101 PC spectrometer. The external quantum efficiency (EQE) spectra of the ternary PSCs were measured by a Zolix Solar Cell Scan 100. The X-ray diffraction (XRD) spectra of the PIDTDTQx:PC71BM films drop-cast onto PEDOT:PSS/ITO substrate were obtained by using a Bruker D8 Advance X-Ray diffractometer (XRD). The morphology and phase images of blend films were investigated with atomic force microscopy (AFM) using a multimode Nanoscope IIIa operated in tapping mode. The chemical structures and energy levels of the used materials as well as the schematic diagram of PSCs are shown in Fig. 1.

3. Results and discussion Solvent additive DIO has been widely applied in improving performance of PSCs by adjusting the donor/acceptor phase separation,

Fig. 1. (a) Chemical structures of PIDTDTQx and PC71BM. (b) Schematic configuration of the PSCs. (c) Energy levels of the used materials.

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X. Zhu et al. / Solar Energy Materials & Solar Cells 132 (2015) 528–534

Current Density (mA/cm2)

2 0 -2 -4

DCB/DIO (v/v) 100:0 100:2 100:4

-6 -8 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V) Fig. 2. J–V characteristics curves of the PSCs under illumination of 100 mW/cm2 AM 1.5G simulated solar light.

Table 1 Key photovoltaic parameters of PSCs with DCB as solvent and different DIO additive doping ratios. DIO (vol%)

Voc (V)

Jsc (mA/cm2)

FF (%)

Best PCE (%)

Ave PCE (%)

0 2 4

0.86 0.85 0.86

8.28 3.67 3.73

64 56 61

4.57 1.76 1.96

4.48 1.63 1.94

The averaged PCE values are calculated based on 30 cells.

especially to the system with narrow band gap polymer as electron donor. A series of PSCs with PIDTDTQx:PC71BM as the active layer and DCB as solvent were fabricated to investigate the effect of DIO on the cell0 s performance. The J–V characteristics curves of PSCs were measured under illumination of 100 mW/cm2 AM 1.5G simulated solar light and are shown in Fig. 2. According to the J–V characteristic curves, the key parameters of PSCs are summarized in Table 1. The open circuit voltage (Voc) was kept constant, and fill factor (FF) was slightly decreased by adding DIO additive. It is apparent that the short circuit current dentistry (Jsc) of PSCs was distinctly decreased to 3.67 mA/cm2 from 8.28 mA/cm2, resulting in the decrease of PCEs from 4.57% for the PSCs without DIO additive to 1.76% for the PSCs with 2 vol% DIO additive. And the PCE of PSCs was decreased to 1.96% from 4.57% by adding 4 vol% DIO additive. Based on more than 30 samples, it could be adequately confirmed that the effect of DIO additive on performance of PSCs with PIDTDTQx:PC71BM as the active layer is negative, the principal influence factor is on the decrease of Jsc. Meanwhile, the FFs of PSCs with DIO additive are larger than 55%, exhibiting a relative effective charge carrier transport and collection by their individual electrode. In order to clarify the reason of DIO additive on the decrease of PSCs performance, the absorption spectra of blend films with or without DIO additive were measured under the same conditions and are shown in Fig. S1. It is apparent that the absorption spectra of blend films with or without DIO additive are almost entirely coincidence. The underlying reason of the effect of DIO additive on performance of PSCs may be attributed to the excess donor/acceptor aggregation resulting in less exciton to be dissociated into free charge carriers. In order to further check our understanding on this phenomenon, ethanol treatment on active layers was carried out to adjust donor/acceptor phase separation degree. In order to give a solid experimental result, all PSCs were fabricated in the same batch keeping all processing parameters constant for each experiment. The J–V characteristic curves of PSCs were measured with and without ethanol treatment under illumination of 100 mW/cm2 AM 1.5G simulated solar light and in dark conditions, as shown in Fig. 3. According to the J–V characteristic curves, the key parameters of PSCs are summarized

in Table 2. The PCE of PSCs without DIO additive was slightly increased from 4.57% to 4.72% by ethanol treatment on the active layers, along with the increase of Jsc from 8.28 mA/cm2 to 8.46 mA/ cm2 and FF from 64% to 65%. According to the J–V characteristic curves of PSCs without DIO additive, the series resistance (Rs) was slightly decreased from 1.52 Ω cm2 to 1.26 Ω cm2 and the shunt resistance (Rsh) was increased from 99.67 Ω cm2 to 115.18 Ω cm2 by ethanol treatment. For the PSCs with DIO additive, the PCE values of PSCs were markedly decreased from 4.57% of PSCs (without DIO additive) to 1.96% by adding 4% DIO additive, the PCE was recovered from 1.96% to 3.71% by ethanol treatment, corresponding to about 89% PCE improvement. The Rs was slightly decreased from 2.11 Ω cm2 to 1.67 Ω cm2 and the Rsh was increased from 62.16 Ω cm2 to 96.36 Ω cm2 by ethanol treatment. It means that ethanol treatment can improve the contact between active layer and cathode, which is beneficial to charge carrier collection. The ethanol treatment can prevent PC71BM from depleting in the top surface by immediately washing out the residual DCB solvent and DIO additive from active layer due to the low boiling point 78.3 1C of ethanol. Ye et al. also reported that morphology stability and PCE reproducibility can be efficiently improved by using solvent treatment on the active layer [31]. It is known that DIO is a good solvent for fullerene derivations but poor solvent for donor components leading to polymer aggregation into more crystalline phases [32]. The different solubility of PIDTDTQx and PC71BM in DIO should induce more apparent phase separation, resulting in larger pure donor or acceptor domain size. The exciton dissociation would be limited when the domain size is larger than exciton diffusion length about 10–20 nm [33]. The boiling point of DIO and DCB is 169 1C and 180.4 1C, respectively. The PC71BM molecule would precipitate from the wet blend films due to the lower boiling point of DIO, and then the PIDTDTQx molecules may diffuse to the top surface of blend films impelled by DCB solvent evaporation from close to dry blend films. The molecular diffusion tread of PIDTDTQx in the blend films with DIO additive can be further demonstrated from the dark J–V characteristic curves (as shown in Fig. 2b), the PSCs with DIO additive show the smallest dark current density at low and reverse biased as compared with other PSCs. The dark current density of PSCs with DIO additive was recovered by ethanol treatment because polymer PIDTDTQx on the top surface of active layer can be washed out, resulting in the better contact between PC71BM and cathode. The similar phenomenon of PCE improvement by using ethanol treatment was also observed from PSCs without DIO additive. For further confirmation on the effect of DIO additive and ethanol treatment on performance of PSCs, the external quantum efficiency (EQE) spectra of PSCs were measured and are shown in Fig. 4. It is apparent that the EQE values of PSCs without DIO additive are larger than those of PSCs with DIO additive in the spectral range from 380 nm to 600 nm. For the PSCs with and without DIO additive, the EQE values were increased by ethanol treatment on active layer in the whole spectral range, resulting in the increase of Jsc and PCE. In addition, the J–V measurements are rather accurate as further examined by integrating the EQE data with the AM1.5G solar spectrum. The calculated Jsc values of PSCs with and without DIO additive as well as ethanol treatment are summarized in Table 2, the errors between measured values and calculated values are less than 5%. In order to verify the above findings and presumption, the morphology and crystalline structure of active layers with and without DIO additive as well as ethanol treatment were investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD). The XRD patterns for PIDTDTQx:PC71BM blend films are shown in Fig. 5. The parameters of four diffraction peaks (2θ E 301, 351, 501, 601) were calculated according to XRD spectra and are summarized in Table S1. The diffraction peak intensity of blend

2

Current Density (mA/cm2)

Current Density (mA/cm2)

X. Zhu et al. / Solar Energy Materials & Solar Cells 132 (2015) 528–534

DCB DCB+Ethanol

0

DCB/DIO DCB/DIO+Ethanol

-2 -4 -6 -8 -10 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

531

DCB DCB+Ethanol DCB/DIO DCB/DIO+Ethanol

1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 -1.0

Voltage (V)

-0.5

0.0

0.5

1.0

Voltage (V)

Fig. 3. (a) J–V characteristics of the PSCs processed with DCB as solvent under illumination of 100 mW/cm2 AM 1.5G simulated solar light, (b) the dark J–V characteristics curves.

Table 2 Key photovoltaic parameters of PSCs processed with DCB as solvent. Voc (V)

Jsc (mA/ cm2)

Cal. Jsc (mA/ cm2)

FF Rs Rsh Best (%) (Ω cm2) (Ω cm2) PCE (%)

Ave PCE (%)

DCB DCB þ ethanol DCB\DIO DCB\DIOþ ethanol

0.86 0.86 0.86 0.84

8.28 8.46 3.73 6.92

7.96 8.10 3.61 6.60

64 65 61 65

4.48 4.62 1.94 3.60

1.52 1.26 2.11 1.67

99.67 115.18 62.16 91.36

4.57 4.72 1.96 3.71

Intensity (a.u)

Conditions

DCB DCB+Ethanol DCB/DIO DCB/DIO+Ethanol

The averaged PCE values are calculated based on 30 cells.

DCB DCB+Ethanol DCB/DIO DCB/DIO+Ethanol

60

EQE (%)

50 40 30 20 10 0 300

400

500

600

700

800

Wavelength (nm) Fig. 4. EQE spectra of PSCs with and without DIO additive as well as ethanol treatment.

films without DIO additive at 2θ E301 was slightly increased by using ethanol treatment. It indicates that PIDTDTQx molecular arrangement may be more ordered for the blend films processed with ethanol treatment. The diffraction peak intensity of blend films at 2θ E301 significantly increased when added 4 vol% DIO. This indicates the crystallinity of the film has been increased because polymer PIDTDTQx would diffuse onto the top surface of films, while the solvent additive DIO can drive the PIDTDTQx to self-stack to form the larger domain size. However, the diffraction peak intensity of the blend films with DIO at 2θ E301 was remarkably decreased because part of PIDTDTQx on the top surface of active layer can be washed by ethanol treatment. The morphology images of blend films with and without DIO additive as well as ethanol treatment are shown in Fig. 6. The rootmean-square roughness (RMS) of blend films without DIO additive is decreased from 0.604 nm to 0.554 nm by ethanol treatment due

20

40

60

80

θ (Degree) 2θ Fig. 5. XRD patterns of the PIDTDTQx:PC71BM blend films with and without DIO additive as well as ethanol treatment.

to the rearrangement of PIDTDTQx on the surface. The RMS (0.339 nm) of blend films with DIO PIDTDTQx is smaller than that (0.604 nm) of blend films without DIO additive because polymer PIDTDTQx would diffuse onto the top surface of films when DCB solvent evaporate from the blend films. The RMS of blend films with DIO additive was increased to 0.604 nm by ethanol treatment because part of polymer PIDTDTQx was washed out and PC71BM particles appear on the surface of blend films. The presumption can be supported from the improvement performance of PSCs due to the better contact between PC71BM and cathode by ethanol treatment, with the decrease of Rs and increase of Rsh. The AFM phase images of PIDTDTQx:PC71BM blend films with and without DIO additive as well as ethanol treatment are shown in Fig. S2. A series of PSCs with CB as solvent were fabricated to further confirm the effect of DIO additive and ethanol treatment on performance of PSCs based on PIDTDTQx:PC71BM as the active layer. The solvent CB has a relative low boiling point 131.7 1C compared with 180.4 1C of DCB, which is also lower than that of DIO (169 1C). The morphology of blend films should be influenced by the evaporation processes of solvent and solvent additive. The AFM morphology images of blend films with and without DIO additive as well as ethanol treatment are shown in Fig. 7, which are markedly different from the relative morphology images of blend films processed with DCB as solvent (shown in Fig. 6). The relative phase images of blend films processed with CB as solvent are shown in Fig. S2. The RMS of blend films without DIO additive was about 15.934 nm and was decreased to 15.691 nm by using ethanol treatment. The relative larger RMS of blend films

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X. Zhu et al. / Solar Energy Materials & Solar Cells 132 (2015) 528–534

Fig. 6. AFM morphology images of PIDTDTQx:PC71BM blend films with and without DIO additive as well as ethanol treatment, (a) DCB (RMS: 0.604 nm), (b) DCB with ethanol treatment (RMS: 0.554 nm), (c) DCB:DIO (RMS: 0.339 nm), (d) DCB:DIO with ethanol treatment (RMS: 0.604 nm).

Fig. 7. AFM height images of PIDTDTQx:PC71BM blend films with and without DIO additive as well as ethanol treatment, (a) CB (RMS: 15.934 nm), (b) CB with ethanol treatment (RMS:15.691 nm), (c) CB:DIO (RMS: 23.351), (d) CB:DIO with ethanol treatment (RMS: 7.526 nm).

processed with CB as solvent should be attributed to the PIDTDTQx aggregation induced by CB fast evaporation from active layer. The RMS of blend films with DIO additive was 23.351 nm due to

PIDTDTQx over aggregation in wet blend films. The RMS of blend films with DIO additive was markedly decreased to 7.526 nm by ethanol treatment because the larger PIDTDTQx particles can be

X. Zhu et al. / Solar Energy Materials & Solar Cells 132 (2015) 528–534

1E-5

1

Current Density (mA/cm2)

Current Density (mA/cm2)

2

533

CB CB+Ethanol

0 CB\DIO CB\DIO+Ethanol

-1 -2 -3 -4 -5 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

CB CB+Ethanol CB\DIO CB\DIO+Ethanol

1E-6 1E-7 1E-8 1E-9 1E-10 1E-11 -1.0

-0.5

0.0

0.5

1.0

Voltage (V)

Voltage (V)

Fig. 8. (a) J–V characteristics of the PSCs processed with CB as solvent under illumination of 100 mW/cm2 AM 1.5G simulated solar light, (b) the dark J–V characteristics curves

Table 3 Key photovoltaic parameters of PSCs processed with CB as solvent. Conditions

Voc (V)

Jsc (mA/ cm2)

Cal. Jsc (mA/ cm2)

FF Rs Rsh Best (%) (Ω cm2) (Ω cm2) PCE (%)

Ave PCE (%)

CB CB þethanol CB\DIO CB\DIO þethanol

0.86 0.86 0.86 0.82

3.79 3.88 2.88 4.56

3.93 4.06 3.00 4.78

60 60 48 51

1.90 1.96 1.15 1.84

1.13 1.03 1.46 1.07

47.02 50.53 29.45 45.33

1.94 1.99 1.18 1.89

The averaged PCE values are calculated based on 30 cells.

35

CB CB+Ethanol CB/DIO CB/DIO+Ethanol

EQE (%)

30 25 20 15 10

decrease of Rs. The key photovoltaic parameters of PSCs are summarized in Table 3 according to the J–V characteristics curves. The EQE spectra of PSCs processed with CB as solvent are shown in Fig. 9. It is apparent that EQE values of all PSCs processed with CB as solvent are lower than those of PSCs with DCB as solvent, which should be attributed to poor morphology of active layers. The PCE values of PSCs can be increased by ethanol treatment, the PCE improvement should be due to the increase of Jsc and FF. It is known that the EQE result is proportional to Jsc (higher EQEs suggests higher Jsc). The errors between calculated Jsc and measured Jsc are less than 5%, which further supports the observed phenomenon induced by adding DIO additive and ethanol treatment. Experimental results from two series of PSCs with CB or DCB as solvent solid demonstrate that DIO additive plays a negative effect on performance of PSCs with PIDTDTQx as electron donor. The dynamic aggregation process of PIDTDTQx and PC71BM is vital to form nanoscale bi-continuous interpenetration network during the solvent and solvent additive evaporation from wet blend films. The ethanol treatment on the active layer provides an effective and simple method to eliminate the residual solvent and optimize the contact between active layer and cathode for the better performance, stability and reproducibility of PSCs.

5 0 300

4. Conclusion 400

500

600

700

800

Wavelength (nm) Fig. 9. EQE spectra of PSCs processed with CB as solvent.

dissolved by ethanol, which could be supported from the tassel of particles, as shown in Fig. 7d. To further identify the morphology of active layer effect on performance of PSCs, the J–V characteristics curves of PSCs with CB as solvent were measured and are shown in Fig. 8. The similar performance variation trend of PSCs dependence on DIO additive and ethanol treatment was observed compared with the PSCs processed with DCB as solvent. The PCE values of PSCs were decrease from 1.94% to 1.18% by adding 4 vol% DIO additive, the decreased PCE should be attributed to excess phase separation (with larger RMS) resulting in the limited exciton dissociation. The PCE values of PSCs with and without DIO additive can be increased from 1.18% to 1.89% and from 1.94% to 1.99% by ethanol treatment, respectively. The PCE improvement induced by ethanol treatment should be due to the better contact between the active layer and cathode, resulting in the increase of Jsc and Rsh as well as the

Solvent additive DIO is not almighty for performance improvement of PSCs, which was demonstrated from PSCs with PIDTDTQx: PC71BM as the active layer processed with DCB or CB as solvent, respectively. For the PSCs with DCB as solvent, the PCE values was decreased from 4.57% to 1.96% by adding 4 vol% DIO additive, which can be largely recovered from 1.96% to 3.71% by using ethanol treatment on active layer. The similar phenomenon is also observed from the PSCs with CB as solvent. It is highlighted that tuning the morphology of active layers plays a vital role in determining the performance of PSCs. The morphology of active layers not only influences exciton dissociation dependence on the donor/acceptor domain size, but also the contact between active layer and electrode for charge carrier collection.

Acknowledgments This work was supported by Fundamental Research Funds for the Central Universities (No. 2014JBZ017); National Natural Science Foundation of China (No. 61377029); Beijing Natural Science Foundation

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