The Effect of Solution Processing on the Power ... - Science Direct

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ScienceDirect Energy Procedia 50 (2014) 237 – 245

The International Conference on Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES14

The effect of solution processing on the power conversion efficiency of P3HT-based organic solar cells Burak Y. Kadema, Mohammed K.Al-hashimib, A. K. Hassana* a

Material and Engineering Research Institute, Sheffield Hallam University, Sheffield, UK. b Mathematics department, Missan University, Iraq.

Abstract Chloroform, Chlorobenzene, 1, 2-Dichlorobenze and co-solvent Chlorobenzene with Chloroform (1:1) were used to investigate the influence of the solvent on the morphology of P3HT:PCBM (with the ratio 1:1) as well as the photovoltaic properties of organic solar cells based on this polymer blend. Using co-solvents leads to higher absorption intensity as compared to other studied solvents. XRD measurements reveal improved crystallinity in P3HT films prepared by chloroform and chlorobenzene while large aggregates appeared in films prepared using dichlorobenzene. Films produced using co-solvents show well-ordered structure and improved crystallinity; these results were further confirmed by SEM images. The J(V) measurements show that a maximum PCE of 1.47% is obtained in films produced using co-solvents. This was associated with significant improvement in current density (11.77 mA/cm2) and a fill factor of 0.4 which was determined from the series and shunt resistances. Open circuit voltage has decreased in film prepared by co-solvents (0.31 Volt), and improved significantly (0.54 Volt) in film prepared in chloroform. © Ltd. This is an openbyaccess article under the CC BY-NC-ND license © 2014 2014Elsevier The Authors. Published Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD). Selection and peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) Keywords: P3HT:PCBM blend; Organic-based solar cell; Co-solvents process; Power conversion efficiency.

1.

Introduction

Organic photovoltaic cells (OPVs) are receiving significant attention during the last two decades. Low cost, mechanical flexibility and ease of device fabrication are among the most attractive aspects of the new technology [1-5]. The device performance could be affected by improving materials synthesis and fabrication processes. One of the key factors that affect the efficiency of the active layer is the blend organization within the OPV layer, which could improve charge carriers separation and transportation [6].

1876-6102 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) doi:10.1016/j.egypro.2014.06.029

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The major principle of OPVs is based on the formation of bulk hetrojunction (BHJ) where a donor can blend with an acceptor [7]. Poly (3-hexylthiophene) (P3HT), which represents the donor and the C60-derivative [6,6]-phenylC61- butyric acid methyl ester (PCBM), acting as the acceptor are widely used as bulk hetrojunction (BHJ) for allorganic solar cells. Because of their conjugated structures, the P3HT:PCBM blends can absorb incident light within the visible spectrum up to the ultraviolet and produces excitons and transfer charge carries through the blend rapidly [8]. PCBM which is commonly used as electron acceptor in polymer solar cells has the advantages of its side chain which enhances its solubility for solution processed OPVs. Moreover, the higher lowest unoccupied molecular orbital (LUMO) related to C60 gives higher open-circuit voltage (Voc) and improves the electron transfer in the blend. On the other hand, P3HT which is used as an electron donor has the advantages of its high holes mobility and stability. In addition, poly (3, 4- ethylenedioxythiophene):poly styrene sulfonate (PEDOT: PSS) which is known as p-type semiconducting polymer is generally used as a buffer layer in OPV devices because of its higher conductivity, high transmission to visible light, good film forming and excellent stability [8, 9]. A considerable number of studies is available in the literature investigating the effects of solvents processing on P3HT:PCBM blends morphology and its performance as active layers in OPV. The nature of solvent, the melting point and its dipole momentum can dramatically change the active layer crystallinity and morphology [1, 6]. In organic thin films, chloroform was used to dissolved P3HT, while chlorobenzene has a good solubility for PCBM. It has invariably been shown that the solvents are the most important concern for the organic semiconducting active materials processing. Moreover, the morphologies are determined by the solubility and phase segregation process of the organic molecules forming the blends [9-11]. Yue Sun and co-workers [12] have employed co-solvents to control the morphology of P3HT:PCBM by controlling the film formation process with different evaporation rate. They have shown that PV active layers with 1.5 times larger power conversion efficiency are produced with mixed solvents as compared to layers produced with one solvent. A comparative study of P3HT:PCBM- based on three different fullerene derivatives, have shown that the use of chlorobenzene (CB) and Di chlorobenzene (DCB) as well as their combination has revealed similar improvement in the PV properties [1]. It was found that the co-solvent is the key factor to obtain highly improved morphology and to provide proper spreading and wetting [1]. Chlorobenzene and toluene co-solvents used for the P3HT:PCBM blend films have influenced the organization of the blend, depending on the nature of the solvent [6]. The optical, morphological and electrical properties of P3HT:PCBM blend films have been shown to be significantly affected by the purity and the type of mixed solvents [11]. In this study, the influence of solvent and co-solvent on the morphologies, crystal structures, optical and electrical properties of BHJ based on P3HT:PCBM blends have been investigated using XRD, SEM, UV-visible spectroscopy and DC electrical measurements under illumination. The solar cells were fabricated using chlorobenzene, chloroform, dichlorobenzene and a mixture of chloroform and chlorobenzene in order to determine the solvents effects on these devices performance. 2.

Experimental

2.1. Materials and Sample Preparation P3HT and PCBM were purchased from Sigma Aldrich. Different solvents were used to process the solution; these are chlorobenzene (CB), chloroform (CF), 1, 2-dichlorobenzene and co-solvent (CB:CF) with a ratio of (1:1). All solvents were purchased from Sigma Aldrich and used without further purification. P3HT and PCBM materials as blends with the ratio 1:1 were dissolved in each solvent as well as in co-solvents of CB:CF in order to study the effects of solvents on film absorption properties and their PV performance. The solutions were stirred overnight at 45 ஈC, and then filtered using PTFE filter with pore size of 0.45 —m. On the other hand, a solution of PEDOT:PSS was prepared by mixing with Methoxyehanol (ME) with the ratio of 1:0.04. The solution was stirred overnight without heating then filtered, and then the samples were kept in a desiccator overnight in the dark. The ITO coated glass slides with a sheet resistant of 8-12 ȍ/sq were etched on one end and cleaned in chloroform, acetone and DI water for 10 min each, respectively. The PEDOT:PSS solution was spin coated onto ITO-coated slides at 2000 rpm for 20 sec followed by post deposition annealing in a furnace set up at 120 ஈC for 10 min.

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The polymer blends solution was spin coated onto PEDOT:PSS-coated ITO slides at 1000 rpm for 50 sec. The films were then annealed on a hot plate at 130 ஈC for 10 min. Moreover, a blended P3HT:PCBM solutions with the ratio of 1:3 were prepared by mixing the two solutions (P3HT and PCBM separately dissolved in a mixture of CF and CB) for further investigation. A thin Al layer was finally deposited on the active layer for all samples with the thickness of 70 nm under vacuum of 10-5 mbar using thermal vacuum evaporation. The final devices were annealed in a furnace at 100 C for 10 min. The devices names and their structures are as listed in Table 1(D4 is bilayer device and D5 is blend device); Table 2 shows some physical properties for the solvents under study. Table 1. The OPV devices names and their structures. Solvent

Structure

Device name

CF CB

ITO/PEDOT:PSS /P3HT:PCBM/Al ITO/PEDOT:PSS /P3HT:PCBM/Al

D1 D2

DCB CF:CB

ITO/PEDOT:PSS /P3HT:PCBM/Al ITO/PEDOT:PSS /P3HT/PCBM/Al

D3 D4

CF:CB

ITO/PEDOT:PSS /P3HT:PCBM/Al

D5

Table 2. Physical properties of the various solvents used for the P3HT:PCBM active layer. Solvents

Boiling point ஈC

Molecular formula

Molecular weight (g/mol)

Vapor density (air =1)

CF

61.2

CHCl3

119.38

4.1

CB

132

C6-H5-CL

112.56

9.3

DCB

181

C6±H5±CL2

147

5.1

2.2. Characterization UV-visible spectrophotometer (Varian 50-scan UV-Vis) in the range of 300-800 nm was used to record the absorption spectra of P3HT:PCBM blends which were spin coated onto cleaned glass substrates. The active layer thickness was determined using M2000 (J.A. Woollam Co., Inc.) spectroscopic ellipsometer operating in the wavelength range 370-1000 nm. Furthermore, the blended structures were investigated by multipurpose X`Pert Philips X-ray defractometer (MPD) (Cu, Ȝ=0.154 nm). The active layer morphology was determined by FEI-Nova SEM. The DC electrical properties were studied using 4200 Keithley semiconductor characterization system and the photo current was generated under AM 1.5 solar simulator source of 100 mWcm-2. The fill factor (FF) and the overall light toelectrical energy conversion efficiency (Ș) of the solar cell were calculated according to the following equations [13]: ɄሺΨሻ ൌ

୎ౣ౗౮ ୚ౣ౗౮ ୔౟౤

And the Fill Factor (FF) is given by:  ୎ ୚ ൌ ౣ౗౮ ౣ౗౮ ୎౩ౙ୚౥ౙ

šͳͲͲΨ ൌ 

୚౥ౙ ୎౩ౙ୊୊ ୔౟౤

šͳͲͲΨ

(1)

(2)

Where JSC is the short-circuit current density (mAcm-2), VOC is the open-circuit voltage (V), P in is the incident light power and J max (mAcm-2) and Vmax (V) are the current density and voltage at the point of maximum power output in the J-V curves, respectively.

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3.

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Results and discussion

3.1. Optical properties Figure 1 shows the absorption spectra of the blended P3HT:PCBM active layers. It is well known that P3HT:PCBM blends exhibit spectral absorption in the range between 400-650 nm [14]. The measured spectra fall in the range starting at ~ 650 nm down to ~ 300 nm for all blends with different used solvents and are characterized with different absorption intensities. For the individual solvents, the highest absorption intensity was observed for blend films dissolved in chloroform and is around 483 nm. The center of the absorption band for chlorobenzene- blended film is at 505 nm, and for the dichlorobenzene-blended film is at 517 nm. The absorption intensity in the co- solvents-blended film exhibited the highest absorption intensity around 518 nm, a property which is thought to enhance photon harvesting leading to more exciton generation. The observed absorption peaks can be attributed to the ʌ-ʌ*) transitions of the blend P3HT:PCBM [8, 15,16,17]. An absorption shoulder in the range 600-610 nm appeared to be more pronounced in the co-solvent-blended film which can be ascribed to the inter-chain interaction within the P3HT aggregates. In addition, the absorption spectra in the range 300-400 can be related to the PCBM in the blend [18]. The absorption intensity for the co-solvent (CF:CB) is higher than other individual solvent, bearing in mind that the films¶ thicknesses are all in the region of 190±10 nm which were determined by spectroscopic ellipsometer. Furthermore, the concentration used for all solutions was fixed as 20 mg/ml. Although the absorption spectra of the P3HT:PCBM films could be attributed to the nature of the solvents, however the spectrum could be explained directly by two major processes taking place inside the blends. Firstly the arrangement of the polymer chains is influenced by blending the PCBM with P3HT [19]. Furthermore, the polymer disorder could strongly lead to a reduction in the absorption intensity. Secondly, the interaction and charge transfer between P3HT and PCBM affect the absorption spectra; this could lead to a reduction in the photo-generated charge carrier density and t will produce lower efficiency [17]. 3.2. Structure analysis In order to study films¶ structures of the P3HT:PCBM blended by different solvents, X-ray diffraction (XRD) measurements were carried out for spun blended layers on cleaned glass substrates. Figure (2) displays XRD diffraction patterns showing a single diffraction peak at 5.38±0.09ࡈ which is related to the P3HT crystallites at the- axis orientation and side chain perpendicular to the substrates [20]. To determine the structure parameters, the XRD peaks at 2ș were fitted with one Gaussian to obtain the peaks¶ width and the lattice constants as illustrated in Table 3. Moreover, the grain sizes of the nanocrystalline structure for the studied blends were calculated using Scherer equation [20]: ൌ

଴Ǥଽସ஛

(3)

ஒୡ୭ୱሺ஘ሻ

Table 3. Summary of the peak positions and grain sizes of the P3HT: PCBM blends as prepared with different solvents. Solvents

Pos. (ƒș)

d-spacing (nm)

FWHM (°ș)

Highest Peak (cts)

D (nm)

CB

5.3336

1.656937

0.5845

608.6

14.23

CF

5.3381

1.655551

0.4546

558.69

18.36

DCB

5.2859

1.671894

0.8443

157.93

9.9

CB:CF

5.4793

1.612915

0.4546

241.17

18.3

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1.8

In CF In DCB In CB In CF:CB

1.6

Absorption

1.4 1.2

1 0.8 0.6 0.4 0.2 0 320

360

400

440

480 520 560 Wavelength (nm)

600

640

680

720

Figure 1. UV-Vis absorption spectra of the blended P3HT:PCBM prepared by different solvents;

Intensity (Counts)

CB, CF, DCB and co-solvents (CB:CF) with a ratio of (1:1) 900 600 P3HT:PCBM in CB

300 0

Intensity (Counts)

Intensity (Counts)

4.5

6.5

7.5

2ɽ

8.5

9.5

800 600 400

P3HT:PCBM in CF

200 0 4.5

5.5

6.5

2ɽ

7.5

8.5

250 200 150 100 50 0

9.5

P3HT:PCBM in DCB

4.5

Intensity (Counts)

5.5

5.5

6.5

2ɽ

7.5

8.5

9.5

400 300 200

P3HT:PCBM in (CF:CB)

100 0 4.5

5.5

6.5

2ɽ 7.5

8.5

9.5

Figure 2. XRD of the blended P3HT:PCBM prepared by different solvents; CB, CF, DCB and co-solvents (CB:CF) with a ratio of (1:1)

P3HT has a good self-organization which gives various characteristics; it is considered one of the main conjugated polymers known as regioregular polymer [21]. This regular chain structure enables the P3HT molecules selforganization via inter-chain stacking into two dimensions leading to high crystallinity [21]. Despite the change in the

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peak intensities which reflects the order of organization of the structures, however these peaks show that the structure crystallinity in the blend is dominated by amorphous environment. According to Fig. 2 the films prepared using CB and CF solvents exhibited a well regular arrangement and the P3HT in the blend kept its ordered structure and was not affected by addition of PCBM. On the other hand, a noticeable decrease in the peak intensity was observed in films prepared in DCB solvent. This decrease could be related to the decrease in polymer crystallinity [6]. Moreover, this reduction could be attributed to the aggregation of the PCBM which leads to increased disorder in the blend and decreases the P3HT crystallinity [6]. Using co-solvents has shifted the XRD peak slightly and showed lower intensity than the CF or CB solvent blended films. This could be attributed to a lower ordering in the P3HT, which may be caused by PCBM, however this could be further attributed to the different boiling points of the two solvents. On the other hand, using Sherrer equation, the crystallite sizes of the blends have changed when using co-solvents, this means that the ordering of the alkyl chain increasing within main thiophene chain [15]. Over and above, it can be inferred that the choice of a good solvent to dissolve blends of P3HT and PCBM can lead to obtaining uniform film structures with increased crystallinity. This is expected to enhance both charge carrier transportation by creating percolation pathways and thus influence the solar cell performance properties. Contrary to that, a bad choice of solvent could lead to dissolving the P3HT and the PCBM in different rates or order which could lead to lower crystallinity and reduced charge transport properties; these inferences are in agreements with published results [6, 15, 22]. Further investigation was carried out to examine the morphology of the blend films using SEM. Figure (3) shows SEM images for P3HT:PCBM blended in different solvents. For CF and CB blended films, it can be noted that there is no clear phase separation or aggregations. In these films the lighter regions are PCBM rich domains, due to the different boiling points between the solvents which lead to different drying process. The PCBM molecules could be carried by the solvent in a vertical direction leading to rough film surface and larger aggregates.

a

b

c

d

Figure 3. SEM images for the blended P3HT:PCBM in different solvent (a) CB (b) CF (c) DCB and (d) CF:CB

Burak Y. Kadem et al. / Energy Procedia 50 (2014) 237 – 245

Table 4. Solar cell Performance of the P3HT: PCBM blends as prepared with various solvents. Sample

Voc (Volt)

Jsc (mA/cm2)

FF %

PCE %

4.38

22.83

0.44

D1

0.44

D2

0.54

2.97

22.73

0.364

D3

0.29

10.88

30.43

0.96

D4

0.28

10.79

31.38

0.948

D5

0.31

11.77

40.34

1.472

The CF-blended film exhibited smoother texture than the CB-blended films, which indicates that the boiling point of the solvents play an important role in the morphology of the formed films. On the other hand, DCB-blended film was characterized with a noticeably different features and this may be ascribed to the phase separation of the two materials constituting the film. A few large aggregates were formed in this film, as a result of the slow evaporation rate of DCB leading to a competition between the amorphous and crystalline region growth in the DCBblended film [23, 24]. This result is in agreement with the XRD and UV-Vis measurements. 3.3. Photovoltaic device performance The open circuit voltage (Voc), short circuit current density (J SC), fill factor (FF) and power conversion efficiency (PCE) of the devices under study were calculated using the J(V) measurements under solar simulated AM1.5 illumination with an output power of 100 mWcm-2. These parameters are summarized in Table (4). Figure (4) shows that J (V) curves for P3HT-based devices fabricated using different solvents with almost same thickness. To compare the devices processed with CF and CB solvents with DCB-processed device, it can be noticed that there is a significant enhancement in the PCE (up to 2 times larger), while it is 3 times larger when co-solvents processing was used. The VOC however has decreased despite the improvement in PCE. It is well known that VOC is determined by the difference between the HOMO of the donor and the LUMO of the acceptor [25]. Therefore, the lower VOC can be associated with the loss in charge carriers at the electrodes. In addition, the interface between the P3HT and PCBM molecules could play an important role in modifying the VOC [26]. In the bulk hetrojunction layer, the holes moved through the conjugated polymer matrix while the electrons transfer by hopping between the PCBM molecules [27, 28]. Light absorbed by the P3HT:PCBM results in excitons generation which are then separated into electron-hole pairs. The P3HT:PCBM hetrojunction controlled the separation and transfer of the charge carriers to the electrodes and prevent the recombination between them leading to improved photocurrent. This result appeared more pronounced in the co-solvents processed device, which exhibited dramatic enhancement in the current density from 2.97 mA/cm2 in D2 to 11.77 mA/c m2 in D5. It is clear that the diffusion of the PCBM molecules out of the P3HT matrix improves the P3HT crystallinity and chain ordering (this was noticed in the CF and CB-processed films). The blended P3HT:PCBM processed in co- solvents probably show different PCBM diffusion in the P3HT matrix, which seems to have led to improved distribution of both P3HT and PCBM in the structure resulting in an increase in charge carriers mobility and reducing hopping distance between the molecules [27]. The fill factor has improved in the co-solventsprocessed device, as did the PCE, indicating enhanced device performance in the case of co-solvents-processed device. This improvement could be assigned to the improved current density, despite the low VOC, an improvement, which may be the result of interface modification between either of the two electrodes and the blend film with improved morphology. Furthermore, the maximum power generated by a solar cell is dependent on the fill factor. A lower FF than the ideal value could be attributed to voltage drop due to high series resistance of the solar cell. Moreover, the voltage drop could be due to leakage current through the shunt resistance of the device, as well as leakage current caused by the current through local defects in the junction or due to the shunts at the edges of the solar cell [29]. Furthermore, recombination in a non-ideal solar cell results in a decrease in the FF which was probably the case in D1 and D2 devices. Further investigation was carried out to fabricate P3HT/PCBM bilayers, P3HT and PCBM were spun on the PEDOT coated ITO layer after layer under the same conditions. This film (D4) show good PCE compared to D1, D2 and D3. Besides, an improvement in the current density (10.79 mA/cm2) and FF (31.34 %) were observed but associated with a drop in VOC (0.28 Volt).

4.

Conclusion

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To sum up, it was found that the introduction of different solvents would affect the device performance. It has been inferred that improved polymer blend morphology was the key factor behind improved device performance through enhancement of photon harvesting. The PV device processed in a mixed solvent show the best performance among other solvents used, with PCE reaching 1.472 % with JSC =11.77 mA/c m2 and FF= 40.34% while VOC =0.31 V. These results could be an important milestone to evaluate the performance of solution processed PV hetrojunction structures produced using different solvents as well as different donor-acceptor organic blends. The principle aim would always be to produce fully organic PV devices with further improved performance. 0.5 0

0

0 -0.1 -0.5

0.1

0.3

-0.1

0.5

0.1

-0.1

0.5

0.3

-1

D1

-2 -2.5

-2

D2 -3

-3

-4

-3.5

-5

Voltage, V

0.1

0.2

0.3

-4 -6

D3

-8

Voltage, V

-12

1 0

0.1

0.2

D4

-8 -10 -12

-1 -0.05

0.05

0.15

0.25

0.35

-3

-4

-6

0.3

J, (mA/cm2)

J, (mA/cm2)

-2

0

-10 Voltage, V

0 -0.1

J, (mA/cm2)

J, (mA/cm2)

J, (mA/cm2)

-1 -1.5

-2

-5 -7

D5

-9 -11

Voltage, V

-13

Voltage, V

Figure 4. The J-V curves for the samples prepared with different solvents and different structures under illumination of simulated AM 1.5 sunlight at 100 mW/cm2

Acknowledgements I would like to acknowledge my supervisor Dr. Aseel K. Hassan and my colleagues in MERI at Sheffield Hallam University. This work is fully sponsored by Iraqi government / Ministry of Higher Education and Scientific Research / Babylon University, College of Science, Physics department. References 1. 2. 3. 4. 5.

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