Mechanistic Studies of Effect of Dispersity on the Photovoltaic

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Mechanistic studies of effect of dispersity on the photovoltaic performance of PTB7 polymer solar cells Luyao Lu, Tianyue Zheng, Tao Xu, Donglin Zhao, and Luping Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5042953 • Publication Date (Web): 19 Dec 2014 Downloaded from http://pubs.acs.org on December 29, 2014

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Chemistry of Materials

Mechanistic studies of effect of dispersity on the photovoltaic performance of PTB7 polymer solar cells Luyao Lu,‡ Tianyue Zheng,‡ Tao Xu, Donglin Zhao, Luping Yu* Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637, United States. ABSTRACT: Bulk heterojunction (BHJ) polymer solar cells (PSCs) are a popular research subject currently pursued by many groups around the world. The state-of-the-art PSCs are composed of a polymer donor and a fullerene acceptor as the active layer and their overall photovoltaic performance is dependent on many factors, such as the electrical and optical properties of donor polymers, device architectures and interfacial layers used. Among them, the nature of donor polymer is without doubt one of the determining factors in performance of PSCs. In this work, we report for the first time the study of the influence of polymer dispersity (Ð) on the performance of PSCs composed of PTB7 and PC71BM as the active layer materials. It was found that polymers exhibiting large Ð contained structural defects that played the role of energy transfer and charge trapping/recombination centers. The power conversion efficiency of PTB7 devices decreased from 7.59% to 2.55% with increased Ð. The results highlighted the importance of controlling Ð of donor polymers for PSCs.

Introduction An extensive research effort was devoted to understand structure-property relationship in order to develop novel polymer materials to ensure effective harvesting of solar energy and high charge carrier mobilities.1-6 Now it was understood that donor polymers should exhibit a broad absorption in the solar spectrum and energy level match with fullerene acceptor;7-8 side chains need to be tuned to simultaneously optimize processibility and morphology;9 high molecular weight (Mw) is desirable to achieve high efficiency.10-12 However, the influence of dispersity (Ð) changes on polymer solar cells (PSCs) performance has not been widely investigated and only a few studies report that polymers with low Ð values showed better crystallinity or solar cell performance.13-15 Most of the solar cell polymers were synthesized via the Stille polycondensation method, which generally resulted in polymers exhibiting large Ð.16-17 In this work, for the first time, we found that Ð of PTB7 could dramatically influence its photovoltaic performance. Detailed chemical studies in model reaction and physical analysis for the variations in device performances revealed the underlining chemical and physical mechanism and led us to conclude that high Ð was a result of homo coupling during the polymerization process which introduced structural defects that could act as energy transfer and trapping centers in the solar cell devices that hinder the charge generation process and facilitate charge recombination in PTB7 bulk heterojunction (BHJ) organic photovoltaic (OPV) cells.18 The results also provide further evidence for the notion that internal polarization in polymer repeat unit is important for high solar efficiency.19 Experimental section Polymeriazion: ditin monomer (193.1 mg, 0.25 mmol) and dibromo monomer (118.0 mg, 0.25 mmol) are weighed into a flask together with Pd(PPh3)4 catalyst (12.5 mg, 0.01 mmol). After three cycles of vacuum and refilling with argon, the reaction system was protected by argon atmosphere and tol:DMF (4 mL:1 mL) was added. The reaction was carried out at 120 oC in dark. After cooling down to room temperature, the reaction mixture was

diluted with CHCl3, and passed through celite. The solution was concentrated and precipitated in MeOH. The solid was collected and soxhlated with acetone, hexane, CHCl3. The CHCl3 portion was concentrated and precipitated in hexanes, collected and dried in vacuum. For PTB7 (Ð 2.1), the reaction mixture was refluxed for 24 hrs with a yield of 77.5% from CHCl3 extraction after soxhlation; for PTB7 (Ð 3.5) and PTB7 (Ð 4.3), the reaction mixture was refluxed for 12 hrs with yields of 55.6% from hexane extraction (Ð 4.3) and 41.8% from CHCl3 extraction (Ð 3.5) after soxhlation. 1HNMR spectroscopy (recorded at 500MHz on Bruker Ultrashield 500 Plus spectrometer) (Figure S1 to S3) and elemental analysis results of three batches of PTB7 are listed below: PTB7 (Ð 2.1): 1HNMR (CDCl2CDCl2): δ 0.39-1.26 (45H, br), 3.60-3.77 (6H, br), 6.64-7.01 (2H, br). Elemental analysis: calcd: C 65.04, H 7.06, F 2.51, S 16.94; found: C 64.62, H 7.06, F 2.43, S 16.78. PTB7 (Ð 3.5): 1HNMR (CDCl2CDCl2): δ 0.39-1.26 (45H, br), 3.59-3.69 (6H, br), 6.65-7.02 (2H, br). Elemental analysis: calcd: C 65.04, H 7.06, F 2.51, S 16.94; found: C 65.26, H 7.18, F 2.42, S 16.74. PTB7 (Ð 4.3): 1HNMR (CDCl2CDCl2): δ 0.38-1.26 (45H, br), 3.58-3.69 (6H, br), 6.64-7.03 (2H, br). Elemental analysis: calcd: C 65.04, H 7.06, F 2.51, S 16.94; found: C 64.89, H 7.18, F 2.52, S 16.82. Model reaction conditions are the same as polymerization, but with relative starting materials. Monotin monomer (223 mg, 0.291 mmol), dibromo monomer (68.6 mg, 0.145 mmol), Pd(PPh3)4 (7.5 mg, 0.0077 mmol), tol/DMF=2.4/0.6 mL. The reaction mixture was separated through column chromatography with hexanes/CH2Cl2 = 3/1 as eluenet, yielding A (145.4 mg, 66.7%), B (10.6 mg, 4.04%), C (15.4 mg, 8.91%). 1HNMR spectroscopy and MALDI (Mass spectra were obtained on a Bruker Daltonics UltrafleXtreme MALDI-TOF system, a Reflective Positive (RP) method using dithranol as matrix) results of A, B and C are listed below: A: 1HNMR (CDCl3): δ 0.94-0.97 (18H, m), 1.04-1.06 (12H, m), 1.17-1.19 (36H, d, J=7.5Hz), 1.36-1.50 (26H, m), 1.60-1.80 (13H, m), 1.81-1.90 (4H, m), 4.21-4.36 (10H, m), 7.62-7.63 (3H, t), 7.89 (1H, s). MALDI, calcd, 1516.5; found M+, 1516.8.

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B: 1HNMR (CDCl3): δ 0.94-0.97 (26H, m), 1.04-1.06 (12H, m), 1.17-1.19 (36H, d, J=7.5Hz), 1.36-1.50 (40H, m), 1.60-1.80 (16H, m), 1.81-1.90 (4H, m), 4.23-4.33 (12H, m), 7.63-7.93 (4H, m). MALDI, calcd, 1828.9; found M+, 1828.5. C: 1HNMR (CDCl3): δ 0.94-0.97 (12H, m), 1.04-1.06 (12H, m), 1.17-1.19 (36H, d, J=7.5Hz), 1.36-1.50 (22H, m), 1.60-1.80 (12H, m), 1.81-1.90 (4H, m), 4.21-4.22(4H,d, J=5.5Hz), 4.24-4.25 (4H, d, J=5.5Hz), 7.62 (2H, s), 7.64 (2H, s). MALDI, calcd, 1204.1; found [M-H]+, 1203.2. Solar cell fabrication: The ITO-coated glass substrate (20±2 ohms/sq.) was cleaned stepwise in water, acetone and isopropyl alcohol with sonication for 15 minutes each. After that, glasses were exposed to ultraviolet ozone irradiation for 60 mins. Then PEDOT:PSS water solution was spin coated on top of ITO and annealed at 80 oC for 30 mins. Polymer and fullerene mixture was dissolved in chlorobenzene and 1, 8-diodooctane (v/v, 97:3) solution at 80 oC for 12 h. After that, the solution was spin coated on top of PEDOT:PSS inside glove box. Metal cathode (20 nm Ca and 80 nm Al) was thermal evaporated with a chamber pressure of 1.0 × 10-6 torr in the glove box.

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Chemical structures of PTB7 and PC71BM are shown in Figure 1. Three batches of PTB7 used in this work have Ð values at 2.1, 3.5 and 4.3 with similar Mw values at 100, 118 and 103 kg/mol, respectively, measured with Gel Permeation Chromatography by using polystyrene as standard. The structures of polymers were confirmed by 1HNMR spectra (Figure S1-S3) and elemental analysis. RO

CO2R O

O

S OR

F

OR

S

S S

S

O

OR

S

S

S n

S n

S

OR

RO2C

R=2-ethylhexyl

R=2-butyloctyl PBB3

PTB7

PC71BM

Figure 1. Chemical structures of PTB7, PBB3 and PC71BM.

Results and discussion

Figure 2. (a), Current-voltage characteristics of PTB7 solar cells with different Ð values. (b), EQE curves of PTB7 solar cells with different Ð values and PBB3 solar cell. (c), Photocurrent density (Jph) versus effective voltage (Veff) characteristics for PTB7 solar cells with different Ð values. (d), UV-vis absorption spectra of PTB7 with different Ð values. a Table 1. Summary of Jsc, Voc, FF and PCE values from devices with different Ð values.

Ð

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%) [avg (best)]

2.1

15.0±0.08

0.72±0.01

67.4±0.56

7.20±0.10 (7.59)

3.5

12.4±0.48

0.66±0.01

55.6±0.11

4.57±0.27 (4.75)

4.3

9.58±0.22

0.59±0.01

44.0±0.64

2.48±0.06 (2.55)

a

Each value represents the average result from ten solar cells. These polymers were used as the donor materials in solar cells with the device structure of ITO/PEDOT:PSS/PTB7:PC71BM/Ca/Al. The area of the devices is 3.14 mm2. The corresponding current density versus voltage (J-

V) characteristics of the solar cells under AM 1.5 G illumination at 100 mW/cm2 are shown in Figure 2a. Dramatic differences in the solar cell parameters for these devices can be observed as summarized in Table 1 (both average and best results). The PTB7


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with 2.1 Ð gave a PCE of 7.59% with an open circuit voltage (Voc) at 0.71 V, a short circuit current density (Jsc) at 15.5 mA/cm2 and a fill factor (FF) at 68.8%. Under the same condition, the PTB7 device with a Ð at 3.5 showed a PCE of 4.75% with a Voc at 0.66 V, a Jsc at 12.8 mA/cm2 and a FF at 56.2%. All three device parameters decreased significantly. Further increasing the Ð to 4.3 led to the worst solar cell performance with a Voc at 0.59 V, a Jsc at 9.87 mA/cm2 and a FF at 44.0%, resulting in a poor PCE of 2.55%. External quantum efficiency (EQE) of the three solar cells was measured to unravel the deceased Jsc for high Ð devices. Figure 2b presents EQE curves for PTB7 with different Ð values. PTB7 with Ð at 2.1 showed the highest EQE values from 300 nm to 700 nm while for PTB7 with higher Ð values, EQEs were extended to longer wavelength and EQE values decreased dramatically from 300 nm to 700 nm. The integrated Jsc values for the three devices were 9.58 mA/cm2 (Ð 4.3), 12.69 mA/cm2 (Ð 3.5) and 15.11 mA/cm2 (Ð 2.1), respectively. The difference between calculated Jsc and measured Jsc is within 3%, indicating that the J-V measurements are reliable. Next, we investigated changes in saturation current density (Jsat) and charge dissociation probabilities P(E,T) of devices with different Ð values to attain more insight into the influence of Ð on light absorption and exciton dissociation process. Saturation current density (Jsat) will only be limited by total amount of absorbed incident photons if all the photogenerated excitons are dissociated into free charge carriers and collected by electrodes at a high Veff (i.e. Veff = 2 V).20,21 Here, photocurrent density (Jph) is defined as Jph = JL - JD, where JL and JD are the current densities under illumination and dark, respectively. Effective voltage (Veff) is defined as Veff = V0 - Va, where V0 and Va are the voltage where Jph equals zero and applied bias voltage. Figure 2c illustrates Jph versus Veff characteristics for the three devices. The Jsat values for the three devices were 172.0 A m-2 (Ð 2.1), 149.7 A m-2 (Ð 3.5) and 129.0 A m-2 (Ð 4.3), respectively. The results suggested that decreasing Ð values could strengthen light absorption in solar cell devices. We further calculated P(E, T) by normalizing Jph with Jsat. P(E, T) values for the three devices under Jsc condition were 90.1% (Ð 2.1), 85.5% (Ð 3.5) and 76.5% (Ð 4.3), respectively. It is clear that high Ð not only led to less efficient exciton generation but also impeded exciton dissociation inside the devices and this matches well with largely decreased EQE from 300 nm to 700 nm in high Ð devices. To understand these dramatic changes in Jsc for PTB7 with different Ð values, we examined the basic properties of polymers. Figure 2d showed UV-vis absorption spectra of PTB7 thin films with different Ð values. It is clear that absorption onset was redshifted when Ð value increased by stealing the absorption strength from maximum peak at 674 nm. This is in accordance with decreased EQE from 300 nm to 700 nm as well as extended EQE from 750 nm to 900 nm in Figure 2b for high Ð devices.

Meanwhile, PTB7 with different Ð values showed similar HOMO energy levels around 5.10 eV and LUMO energy levels around 3.21 eV. Moreover, the red shifted absorption with increased Ð values would suggest that during polymerization, there might be some side reactions producing species with red absorption. The redshifted absorption region is reminiscent of our polymer PBB3 (Figure 1), which contains monomer units of thienothiophene (TT) dimer and exhibits an absorption edge on 900 nm.19 The EQE curves of the solar cell fabricated from PBB3 and PC71BM and the extended EQE range for PTB7 with Ð values at 3.5 and 4.3 matches well between 750 nm to 900 nm (Figure 2b). This again provides evidence for the existence of structural defects like PBB3 in our high Ð samples. This hypothesis is further supported by photoluminescence (PL) study of PTB7 solutions. If structural defects like PBB3 existed in high Ð PTB7 samples, one would expect decreased emission intensity for high Ð PTB7 solution due to photoinduced energy transfer from normal PTB7 units to TT units, leading to red shift in emission. Figure 3a showed PL spectra of PTB7 solutions excited at 680 nm, which was the maximum absorption for PTB7. It is clear that as the Ð increased, the light emission shifted towards longer wavelength and the emission strength was reduced. The existence of such structural defects offer sound explanation for the decreased hole mobility since PBB3 only showed a poor hole mobility at 1.08×10-4 cm2 V-1 s1 19 . Hole mobility was measured using space-charge-limited current (SCLC) method. The mobility slightly decreased from 5.87×10-4 cm2 V-1 s-1 (Ð 2.1) to 4.39×10-4 cm2 V-1 s-1 (Ð 3.5) and 3.44×10-4 cm2 V-1 s-1 (Ð 4.3) for PTB7 hole only devices (Figure 3b). It was pointed out in our early studies that the repeating units of PBB3 polymer exhibited a cancelled ground state dipole moment due to the TT dimer structure and a small dipolar change between ground and excited state, namely a small polarization, all of which caused the diminished charge separation rate and enhanced charge recombination in high Ð devices.19 The question is how these defects can be formed? One possible explanation is the side reaction involving homo coupling in the Pd catalyzed carbon-carbon formation reaction.22-24 It was reported recently that the intrachain homo coupling defects could lead to dramatically decreased Jsc and Voc for PDPPTPT PSCs.25 It is likely that during polymerization, dimer species of fluorinated thienothiophene (FTT) and benzodithiophene (BDT) could be formed via aryl-for-aryl exchange between PdRXL2 complexes and Aryl stananne monomer, followed by normal Stille coupling reaction (Scheme 1). Considering the fact that polymer PBB3,19 with thienothiophene dimer in the polymer repeating units, showed a red shifted absorption and inferior device performance compared to PTB7, we believe that dimer species of FTT or BDT in the polymer chain are the structural defects that could act as energy transfer and charge trapping centers for our PTB7 devices.

Figure 3. (a), PL spectra of PTB7 solution with different Ð vaues. (b), Hole mobility of PTB7 with different Ð values.



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Ar1-Ar2

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Ar1-Br

LnPd(0)

Ar2 Ln Pd(II) Br

2

Ar1 Ar -SnMe 3 Ln Pd(II) Br

Ar1 Ln Pd(II) Ar2

Ar2-SnMe3

Ar1-Br

1

Ar -SnMe 3

Ar1-Ar1

Pd cata.

Ar2-SnMe 3

BrSnMe3

Ar2-Ar2 + LnPd(0)

Scheme 1. Possible side reaction mechanism for dimer species.

HEO O

OEH S

Polymerization

Sn

OEH

F

S

+

Sn

S

OEH

S

Br

S

PTB7

O

OEH Model reaction

Si

Sn

+

OEH HEO

F

S

O

OEH

S

F OEH

S S

S Br

OEH

O OEH

S S

Br

S

Si

S

OEH

OEH

F S

OEH

HEO

S

Si

S S

S

OEH S

Si F

Si

HEO A, 66.7%

OEH S

S

S

OEH

Si S

n

OEH

EH = 2-ethylhexyl

S

F

S S

S Br

OEH

O

Si S

S OEH

OEH

OEH C, 8.91%

O

B , 4.04% (3 isomers)

Scheme 2. Reaction formula for normal polymerization and model reaction. To offer firm evidence for this hypothesis, we carried out a model reaction to mimic the polymerization conditions. A monotin compound of BDT (TIPS-BDT-tin) with one end blocked by triisopropylsilyl (TIPS) group was used to react with FTT dibromo monomers at the same reaction conditions as polymerization (Scheme 2). Indeed, in addition to the expected normal Stille product TIPS-BDT-FTT-BDT-TIPS (A), small amounts of two side products, namely TIPS-BDT-FTT-FTT-BDT-TIPS (B) (three regioisomers) and TIPS-BDT-BDT-TIPS (C) were detected, which came from the homo coupling of reactants. The structures of the three products were identified by mass (Figure S4) and 1 HNMR spectra (Figure S5 to S7). The results of model reaction strongly support the hypothesis of existence of structural defects in PTB7 as proposed above. We would like to note that though model reaction shows an appreciable content of homo coupling, the final polymers would not have such a high content of defects since homo coupling possibly involves a transmetallation step, which may require higher activation energy for polymers. Besides, small molecules moves freer and faster than polymer molecules in the reaction system, which gives model reaction higher content for homo coupling.

The effect of structural defects on photovoltaic properties is manifested in almost every aspect of associated physical properties. For example, it is reflected in deterioration of bimolecular recombination of charge carriers, as determined by measuring the dependence of log(Jsc) as a function of log(Plight), where Plight is illumination intensities.23 Generally, a linear dependence of log(Jsc) on log(Plight) with a slope close to 1 suggests weak bimolecular recombination in the OPV devices while a slope far less than 1 indicates partial loss of charge carriers due to bimolecular recombination between free holes and electrons during charge transport process. Figure 4a shows log(Jsc) versus log(Plight) characteristics of the three devices in this study. The fitted slope (S) decreased from 0.96 to 0.94 and finally to 0.91 for devices with Ð values at 2.1, 3.5 and 4.3, respectively. The results clearly suggested that increasing the Ð of donor polymer would result in more severe bimolecular recombination inside solar cell devices. The increased bimolecular recombination is the result of decreased hole mobility in high Ð samples as discussed above. Evidences are obtained on the structural defects acted as trapping centers in the devices by measuring the dependence of log(Voc) on light intensity. It is known if bimolecular recombina-


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tion is the main loss mechanism in the device, log(Voc) would show a linear dependence on log(Plight) with a slope of kT/q, where k is Boltzmann’s constant, T is temperature and q is elementary charge. In case of trap assisted recombination is dominating, log(Voc) would show a stronger dependence on log(Plight) with a slope of 2 kT/q.26-28 As shown in Figure 4b, device with 4.3 Ð exhibited strongest Voc dependence on light intensity with a slope at 1.78 kT/q while device with 3.5 Ð also possessed a larger slope at 1.31 kT/q compared to 1.22 kT/q for 2.1 Ð device. The results

indicated that species from the homo coupling acted as trapping centers in high Ð devices while in low Ð devices, bimolecular recombination is still dominating due to a relative lack of trapping centers or defects. We carried out transmission electron microscopy (TEM) measurement to investigate the effect of Ð on phase separations in our thin films. Figure 5 presents TEM images of three PTB7:PC71BM devices with different Ð values. Unlike the Mn effect, where larger Mn gave more severe phase separations for PTB7,12 it was found that all three films showed almost identical nanomorphology with finely dispersed phase separated domains. Thus the difference in solar cell performance should not result from morphology changes in these devices.

Figure 4. (a), Dependence of Jsc on light intensity for PTB7 solar cells with different Ð values. (b), Dependence of Voc on light intensity for PTB7 solar cells with different Ð values.

Figure 5. TEM images of PTB7 with different Ð values. (a), Ð 2.1. (b), Ð 3.5. (c), Ð 4.3. It has been previously reported that Mn effect had a significant influence on PTB7 polymer solar cell performance.12 To confirm that changes of device performance in our work is truly resulted from Ð effect other than Mn effect, we prepared two PTB7 solar cells with similar Ð values (2.49 and 2.50) but different Mn (6.5 k and 5.7 k). The corresponding J-V curves of these two devices are shown in Figure 6. It is clear from Figure 6 that Jsc increased from 13.1 mA/cm2 to 14.4 mA/cm2 with increased Mn while both Voc and FF remained almost the same. The changes in PCE are within 15% due to Mn effect. This is also in good agreement with Mn effect on other polymer systems where increased Mn only improved Jsc values in solar cells.10,11 Thus, we can safely conclude that the simultaneously decreased Jsc, Voc and FF in devices with high Ð values are not caused by Mn effect.

Figure 6. Current-voltage characteristics for PTB7 solar cells with different Mn and similar Ð.



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Conclusions In conclusion, we find that Ð played a significant role in determining the performance of PSCs fabricated from different batches of donor polymer PTB7. When Ð increases from 2.1 to 3.5 and 4.3, PCE decreases from 7.59% to 4.75% and finally to 2.55%. We demonstrated that the decreased PCE is due to the existence of side reactions which introduced structural defects into PTB7. We further showed that increased Ð could result in red shifted absorption, weaker exciton generation and dissociation, stronger bimolecular recombination and trap assistant recombination as well as lower charge carrier mobilities in our devices. Considering the fact that homo coupling defects have also been observed in PDPPTPT polymer system with diketopyrrolopyrrole (DPP) unit very recently25, it is reasonable to assume that many other high performance push-pull donor copolymers based on DPP and BDT units synthesized from Stille polycondensation reactions reported in the literatures may also suffer from above problems. We show here that at least for PTB7 polymer, these defects will manifest themselves in the high Ð values and extended absorption spectra. Further studies are absolutely needed to fully understand whether or not this could be a universal method to indicate the existence of homo coupling defects in other polymer systems and how to avoid these homo coupling defects effectively. Overall, these results highlighted the importance of achieving low Ð during the synthesis of donor polymers to avoid possible structural defects and achieve high performance of PSCs.

ASSOCIATED CONTENT Supporting Information. This material contains Figures for 1HNMR and Mass spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected].

Author Contributions ‡These authors contributed equally.

Notes

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The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is supported by U. S. National Science Foundation grant (NSF CHE-1229089, DMR-1263006, Air Force Office of Scientific Research and NSF MRSEC program at the University of Chicago, DOE via the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DE-SC0001059.

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7

Mechanistic Studies of Effect of Dispersity on the Photovoltaic

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