Tellurophene-Based N-type Copolymers for Photovoltaic Applications

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Tellurophene-based N-type Copolymers for Photovoltaic Applications Lei Lv, Xiaofen Wang, Xinlong Wang, Lei Yang, Tao Dong, Zhou Yang, and Hui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11041 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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ACS Applied Materials & Interfaces

Tellurophene-based

N-type

Copolymers

for

Photovoltaic Applications Lei Lv, †, §Xiaofen Wang, †, ‡, § Xinlong Wang, † Lei Yang, † Tao Dong, † Zhou Yang, ‡ Hui Huang*, † †College of Materials Science and Opto-Electronic Technology&CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, P. R. China ‡Beijing Key Laboratory of Function Materials for Molecular&Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, P. R. China. KEYWORDS: tellurophene, n-type copolymers, molecular engineering, all polymer solar cells, bromine detection

ABSTRACT: Novel tellurophene-based n-type copolymers are synthesized and characterized with thermal analysis, electrochemistry, optical spectroscopy, and DFT calculations. The copolymers demonstrate reversible interactions with bromine. Through tuning the building blocks and alkyl chains, together with device engineering, the maximum power conversion efficiency (PCE) of all polymer solar cells improves from 2.8% to 4.3%, which is supported by photoluminescence, AFM, TEM, SCLC, and exciton dynamic studies. These results suggest that tellurophene-based n-type copolymers are promising electron acceptors for organic solar cells and potential sensor materials for bromine detection.

ACS Paragon Plus Environment

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1. INTRODUCTION Organic photovoltaics is a promising technology for harvesting solar energy for its merits of flexibility, low weight and cost effective fabrication for large areas.1-2 Nowadays, bulk heterojunction (BHJ) structure is the most widely used configuration to construct organic solar cells, which include an active layer blended with p- and n-type conjugated polymers.3 Through molecular engineering, a large amount of p-type conjugated materials have been designed and synthesized for high performance organic solar cells.4-6 To date, the record efficiency for polymer/fullerene system has reached over 11%.7 In comparison to the variety of p-type donors, fullerene derivatives are primarily used as n-type acceptors for their fast charge split capability and excellent electron mobilities.4, 8-10 However, the fullerenes have several drawbacks, for instance, high-cost production and low absorption coefficient in the visible region.11 Recently, non-fullerene n-type molecular/polymeric materials have attracted attentions because of their adjustable energy levels, enhanced absorption in visible region and potentially low productive costs.

12-13

Various small molecules have been designed and synthesized based on

electron-deficient building blocks,14 like perylene diimide (PDI),15-18 naphthalene diimide (NDI),19-20 diketopyrrolopyrrole

(DPP),21-22

phthalimide,23

fluoranthene-fused

imide,24

quinacridone,25

decacyclene triimide26 and others.23, 27-29 As a result, the PCE of organic solar cells based on n-type small molecules reached over 7% in many examples.30-32 Polymer/polymer based solar cells could flexibly control the solution viscosity, a significant parameter for solution-processing production of large area OPV module.33 Great efforts have been made to develop new n-type conjugated polymers for high efficient all polymer solar cells.34-37 However, the PCE of all polymer solar cells has lagged with relatively few examples of efficiency over 5%.36, 38-40 Among them, the perylene diimide and naphthalene diimide are the dominant electron with-drawing building blocks for n-type conjugated polymers due to their high planarity and fused structures, possibly leading to high electron mobilities.


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Tellurophene is an attractive building block for constructing organic optoelectronic materials due to the metalloid characteristics of tellurium41-43 and its large spin-orbit coupling, possibly leading to long-lived triplet excited states for tellurophene-based materials.44 Several tellurophene-based ptype conjugated polymers were employed for sensors, thin film transistors, and organic solar cells etc.45-49 However, to the best of our knowledge, tellurophene was never employed to construct n-type organic semiconductors. For this contribution, we present a series of tellurophene-based n-type polymeric semiconductors. The relationship between the structures and the optoelectronic properties was studied via the DFT calculations and physicochemical characterizations. The metalloid characteristics of tellurium was studied through the reversible interactions of PTe-NDI(OD) with bromine. PTB7-Th50-51 was selected as a donor due to its intense optical absorption, depicted in Figure 1. Through molecular engineering from PDI to NDI together with tuning the alkyl chains, a high efficiency over 4% is achieved for all polymer solar cells. AFM, TEM, SCLC, and photoluminescence were employed to understand the morphology, charge transport and exciton quenching of the blend films, which are well corresponding to the performance of the all polymer solar cells.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of the Polymers. Three tellurophene-based copolymers were synthesized by Stille coupling of 2,5-bis(trimethylstannyl)tellurophene52 and 2BrPDI(OD)53/2Br-NDI(OD) and (HD)54-55 (Scheme 1 and Figure 1). These polymers pssess excellent solubility in organic solvents including chloroform (CF), chlorobenzene (CB) and dichlorobenzene (DCB). The copolymers’ structures were confirmed by 1H NMR (Figure S1-S3). The thermal and


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electrochemical properties of these polymers were systematically studied by thermal gravimetrical analysis (TGA), differential scanning calorimetry (DSC) and cyclic voltammetry (CV). The corresponding data are summarized in Table 1. The molecular weights (Mn) of polymers PTePDI(OD), PTe-NDI(OD), PTe-NDI(HD) by gel permeation chromatograph (GPC) are 13.7 kDa (PDI = 1.85), 67.6 kDa (PDI = 2.87), 67.2 kDa (PDI = 2.49), respectively. The TGA studies showed that these polymers are thermally stable with onset decomposition temperature (Td) over 400 oC under nitrogen flow (Figure S4). The thermal characeteristics of the polymers were also studied by DSC and provided evidence for amorphous characteristics without detectable endotherms and exotherms. Scheme 1. Synthetic Route of Polymers


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Figure 1. Chemical structures of polymer donor and polymer acceptors. 2.2. Optical and Electrochemical Properties. The solution and film optical absorption spectra of the polymers are shown in Figure 2 and the data are summarized in Table 1. Density functional theory (DFT) calculations at the level of B3LYP/ LANL2DZ were also carried out to probe the electronic structures of the polymers. In solution, the PTe-PDI(OD) has a broad absorption ranging from 300 to 700 nm with a maximum absorption peak at 566 nm, bathochromic shifted comparing to thiophene (556 nm) polymeric analogue.56 This is caused by the stronger electron donating characteristics of tellurophene than thiophene. Not surprisingly, the PTe-NDI(OD) and PTe-NDI(HD) showed similar absorption spectra with two major absorption bands at 300-400 nm and 500-700 nm, similar to their reported thiophene-57-59 and selenophene-based35,

60

polymeric

analogues, which can be assigned to π-π* transition of NDI and an intramolecular charge transfer (ICT) band. The thin film absorption spectra are generally similar to the solution spectra. The absorption peak of PTe-PDI(OD) thin film is at 554 nm, an obvious hypsochromic shift by 12 nm comparing to the peak absorption of solution, indicating the solid state of the polymer may adopt H aggregation.61 Furthermore, the absorption peaks of the thin films of PTe-NDI(OD) and PTe-


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NDI(HD) are also slightly blue-shifted comparing to those of the solution. The optical bandgap (Eg) was calculated from the onset of the film absorption spectra. The Eg (1.68 eV) of PTe-PDI(OD) is larger than those of PTe-NDI(OD) (1.51 eV) and PTe-NDI(HD) (1.53 eV), which can be ascribed to the more twisted structure of PTe-PDI(OD) based on DFT calculations as shown in Figure 3b. The DFT calculations for the repeating unit of the parent polymer showed that HOMO/LUMO energy levels of PTe-PDI and PTe-NDI are -6.14/-3.82 and -6.37/-3.79 eV, respectively. As a result, the calculated HOMO-LUMO gap (2.32 eV) of PTe-PDI is smaller than that of PTe-NDI (2.58 eV), contradictory to the experimental bandgaps of thin films. This discrepancy may be accounted for the stronger intermolecular interactions in PTe-NDI in the solid states due to its more planar structure, in which the dihedral angle between tellurophene and naphthalene diimide is 21o, smaller than that between tellurophene and perylene diimide (54o).

Figure 2. UV-vis absorption spectra of polymers PTe-NDI(OD), PTe-NDI(HD), and PTe-PDI(OD) in DCB solution (a); as-cast pristine films (b); and blend films on glass substrates (c). As shown in Figure 2c the absorption spectra of PTB7-Th:PTe-NDI based blend films are the overlap of the spectra of each individual polymer, indicating no obvious charge transfer underground states. Furthermore, the absorption spectra of PTB7-Th:PTe-PDI(OD) blend film has a more intense absorption between 400 and 600 nm in comparison to PTB7-Th:PTe-NDI blend films. This is ascribed to the broad absorption of PTe-PDI(OD) pristine film in this range.


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The electrochemical properties of the new polymers thin films were studied with cyclic voltammetry (CV), using ferrocene/ferrocenium (Fc/Fc+) redox couple as an internal standard (Figure S6a and Table 1). Based on the onset of reduction potential of the polymers from the CV, the LUMO energy levels were estimated LUMO=-(Eredonset+4.44) eV. With this method, the LUMO energy levels of PTe-PDI(OD), PTe-NDI(OD), and PTe-NDI(HD) are determined to be -4.03, -3.98, and -3.90 eV, respectively. By subtracting the Eg from the LUMO, the HOMO energy levels of PTePDI(OD), PTe-NDI(OD), and PTe-NDI(HD) are -5.71, -5.49, and -5.43 eV, respectively. The HOMO and LUMO energy levels of PTB7-Th were also measured to be -5.42 and -3.81 eV, respectively (Figure S6b). The LUMO-LUMO offset between PTB7-Th and the n-type polymers are from 0.09 to 0.22 eV, which is challenging to split excitons in polymer/polymer system. Table 1. Physicochemical Characteristics of Polymers Polymer

Mn

PDI

(kDa)

Td

λmaxsolution

λmaxfilm

Egopt

HOMO

LUMO

(oC)

(nm)

(nm)

(eV)a

(eV)b

(eV)

PTePDI(OD)

13.8

2.49

438

566

554

1.68

-5.69

-4.01

PTeNDI(OD)

67.6

2.89

417

375, 702

379, 701

1.51

-5.49

-3.98

PTeNDI(HD)

67.2

1.85

415

372, 699

376, 694

1.53

-5.43

-3.90

a

Determined from the onset of UV−vis absorption spectra; bCalculated from LUMO = HOMO + Egopt.


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Figure 3. Experimentally determined energy levels (a), and calculated energies of the frontier orbitals (b) of polymers and PTe-NDI adduct with Br2. 2.3. Reversible Interactions with Br2. Tellurophene-based p-type conjugated polymers were reported to reversibly react with bromine to form coordination complexes,41 which can be used for bromine detection.52 To explore the photophysical properties of these tellurophene-basd conjugated


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polymers, the CHCl3 solution (ca.0.01 mg ml-1 in CHCl3) and thin films of PTe-NDI (OD) was selected to be treated with Br2 and monitored with UV-vis spectroscopy as shown in Figure 4. Solutions of polymer PTe-NDI(OD) was titrated with bromine in CHCl3. Titration of PTeNDI(OD) with Br2 (0-0.4 equiv, with respect to Te atom, based on the molecular weight)41 results in an blue-shift in the absorption spectra with decrease of the absorption peak at 705 nm and increase of the absorption peak at 605 nm, even though no obvious color changed in the solution (Figure 4b,d), which indicates the bromine interacts with tellurium centers of PTe-NDI(OD). This observation is different from the reported results of p-type copolymers that addition of Br2 usually leads to a red shift in optical absorption.41, 62 To investigate the reversibility of the interaction between polymer and Br2 (Figure 4a, c and e), the pristine films of PTe-NDI(OD) were exposed to Br2 atmosphere for 5s, resulting in an instantaneous change of colors from blue to purple. This is consistent with the change of absorption spectra in which an obvious hypsochromic shift of absorption peaks from 708 to 616 nm. Furthermore, the purple film turned back to blue after annealed at 150 oC for 5 min., in accordant with the recovered absorption spectra, indicating reversible interactions between Br2 and PTeNDI(OD). DFT calculations (B3LYP/LANL2DZ) were employed to understand the change in optical and electronic properties resulted from the tellurium-bromine interactions as shown in Figure 3b. The basic repeating units were to investigate the trends of the change of MO energy levels. The calculated HOMO/LUMO energy levels of PTe-NDI(OD)-Br2 adduct are -6.86 and -4.39 eV, respectively. As a result, the bandgap is 2.47 eV, slightly narrower than that of parent polymer (2.58 eV). This is contradictory to the experimental results that the optical absorption of PTe-NDI(OD)-Br2 adduct blue shifted in comparison to PTe-NDI(OD). In order to understand the discrepancy, the


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Figure 4. (a) Demonstration of reversible interaction of Br2 with tellurium atom of PTe-NDI(OD). Absorption spectra (b) and picture (d) of a solution of PTe-NDI(OD) treated with various concentrations of Br2 (0-0.4 equiv). Absorption spectra (c) and picture (e) of a film of PTe-NDI(OD) exposed to Br2. calculated geometry of repeating units was investigated. The dihedral angle between tellurophene and NDI planes in the PTe-NDI(OD)-Br2 adduct is 26o, higher than that of the parent polymer (21o). Furthermore, the bromines adopt a pseudo-axial geometry relative to the tellurophene ring. Both


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characteristics disrupt the planarity and limit the conjugation length along the polymer chain. As a result, it is not surprising that adduction of Br2 increased the bandgap of the polymer.

2.4. All-polymer BHJ Solar Cells. All-polymer BHJ solar cells with a structure of ITO/ZnO/PTB7-Th:polymer/MoO3/Ag were fabricated to investigate the photovoltaic performances of PTe-PDI(OD), PTe-NDI(OD), and PTe-NDI(HD). The typical J-V curves are shown in Figure 5a and the photovoltaic characteristics are summarized in Table 2. The results showed that the PTB7Th:PTe-PDI(OD) based solar cells afforded an average efficiency of 2.61% with a Voc of 0.68 V, Jsc of 9.08 mA/cm2, and FF of 41.3%. When PTe-NDI(OD) was used as a polymeric acceptor, the average efficiency of solar cells increased to 3.18% with an increased Voc of 0.71 V, Jsc of 9.08 mA/cm2, and an enhanced FF of 47.4%. The increased open circuit voltage is rooted from the shallow LUMO of PTe-NDI(OD). Replacement of the 2-octyldodecane alkyl chains with a slightly shorter alkyl chains 2-hexyldecyl, the efficiency of PTe-NDI(HD) based solar cells further increased to 3.36% with a Voc = 0.71 V, Jsc = 10.02 mA/cm2, and FF of 47.2%. To further optimize the performance, the ratio of PTB7-Th:PTe-NDI(HD) was tuned with increasing the amount of the polymeric acceptor. When the weight ratio of PTB7-Th:PTe-NDI(HD) changed to 1:1.2, the Voc slightly increased to 0.72 V, the Jsc was enhanced to be 11.02 mA/cm2, and the FF was improved to 51.0%. As a result, an average efficiency of 4.09% was achieved with a maximum efficiency of 4.29%. When the mass ratio of donor and acceptor changed to 1:1.4, the efficiency decreased to 3.84% with an obvious decreased of fill factor (47.9%), even though the Voc and Jsc are intrinsically same. The external quantum efficiency (EQE) of organic solar cells is shown in Figure 5b. The EQE plots cover the range from 300 to 800 nm, consistent with the UV-vis absorption spectra of blend films. The calculated Jsc based on EQE results are accordant with the Jsc values of the


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corresponding organic solar cells. For example, the integral current density of solar cells based on PTB7-Th: PTe-NDI(HD)=1:1.4 (w/w) is 10.33 mA/cm2, highest among the different systems, accordant with the value of J-V measurement. To illustrate the mechanism of the photovoltaic performances, the morphology, bulk charge transport properties, and photoluminescence characteristics of blend films were discussed later.

Figure 5. The typical J-V curves (a), and EQE plots of organic solar cells with different active layers (b).

2.5. Photoluminescence of BHJ Solar Cells. Photoluminescence (PL) spectroscopy is performed to probe the exciton diffusion and dissociation of the blend films (Figure S7). PL emission band of PTB7-Th was found in the range of 640-840 nm with a maximum peak at 763 nm. The PL spectra of the blend films of PTB7-Th:PTe-PDI(OD) (1:1, w/w), PTB7-Th:PTe-NDI(OD) (1:1,w/w) and PTB7-Th:PTe-NDI(HD) (1:1, w/w) are 85.5%, 90.9%, 93.2%, respectively. This indicates that excitons could be most efficiently split inside the PTB7-Th:PTe-NDI(HD) blend film, consistent with the highest Jsc (10.02 mA/cm2) for corresponding solar cells. Furthermore, the quenching efficiencies for blend films of PTB7-Th:PTe-NDI(HD) (1:1.2, w/w) and PTB7-Th:PTe-NDI(HD)


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(1:1.4, w/w) are 93.5% and 93.4%, respectively, slightly higher than that of PTB7-Th:PTe-NDI(HD) (1:1, w/w). This supports the photovoltaic characteristics that the Jsc of PTB7-Th:PTe-NDI(HD) (1:1.2, w/w) and PTB7-Th:PTe-NDI(HD) (1:1.4, w/w) based solar cells are higher than that of PTB7Th:PTe-NDI(HD) (1:1, w/w) based solar cells. Table 2. Parameters Summary of Photovoltaic Devices with PTe-PDI(OD), PTe-NDI(OD), and PTeNDI(HD) as Acceptors and PTB7-Th as a Donor PTB7-Th: acceptora

VOC

JSC

FF

PCE (best)

ૄh

ૄe

(w/w)

(V)

(mA/cm2)

(%)

(%)

(cm2/V·s)

(cm2/V·s)

PTe-PDI(OD)

0.68±0.01

9.08±0.40

41.3±1.0

2.61±0.20 (2.81)

1.04×10-4

8.94×10-7

47.4±0.9

3.18±0.03 (3.21)

1.03×10-4

3.04×10-7

47.2±0.4

3.36±0.18 (3.54)

1.80×10-4

5.01×10-7

51.0±1.3

4.09±0.20 (4.29)

6.24×10-5

5.96×10-7

47.9±1.0

3.84±0.16 (4.00)

1.79×10-4

5.07×10-6

(8.11)b

(1: 1) PTe-NDI(OD)

0.71±0.01

(8.32)b

(1: 1) PTe-NDI(HD)

0.71±0.01

0.72±0.01

(1: 1.4)

11.02±0.05 (10.15)b

(1: 1.2) PTe-NDI(HD)

10.02±0.04 (9.61)b

(1: 1) PTe-NDI(HD)

9.08±0.40

0.71±0.01

11.23±0.18 (10.33)b

a

Average values and standard deviations are calculated from at least 10 devices. bIntegrated from EQE data.

2.6. Morphology of BHJ Solar Cells. The blend film morphologies were characterized by atomic force microscopy (AFM) in tapping-mode and TEM. Figure 6 shows the topographic and phase images of blend films. Both blend films of PTB7-Th:PTe-PDI(OD) (1:1, w/w) and PTB7Th:PTe-NDI(OD) (1:1, w/w) demonstrated huge phase separation (>200 nm) with large root mean


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square (RMS) roughness of 2.01 and 1.98 nm, respectively, which are detrimental for charge transport from donor to acceptor.63 In comparison, the blend film of PTB7-Th:PTe-NDI(HD) (1:1, w/w) exhibits a much smaller phase separation with a smoother roughness (RMS=0.99 nm), which facilitates the charge generation and transport. These observations support that the Jsc and FF of PTB7-Th:PTe-NDI(HD) based solar cells are higher than those of PTB7-Th:PTe-PDI(OD) and PTB7-Th:PTe-NDI(OD) based solar cells.

Figure 6. AFM topographic and phase images (5µm×5µm) of blend films of PTB7-Th:PTe-PDI(OD) (1: 1, w/w) (a, f), PTB7-Th:PTe-NDI(OD) (1: 1, w/w) (b, g), PTB7-Th:PTe-NDI(HD) (1: 1, w/w) (c, h), PTB7-Th:PTe-NDI(HD) (1: 1.2, w/w) (d, i), and PTB7-Th:PTe-NDI(HD) (1: 1.4, w/w) (e, j). TEM images of blend films of (k) PTB7-Th: PTe-PDI(OD) (1: 1, w/w), (l) PTB7-Th: PTe-NDI(OD) (1: 1, w/w) , (m) PTB7-Th: PTe-NDI(HD) (1: 1, w/w), (n) PTB7-Th: PTe-NDI(HD) (1: 1.2, w/w), and (o) PTB7-Th: PTe-NDI(HD) (1: 1.4, w/w).


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TEM was employed to further probe the relationship between the morphological features and the photovoltaic performances. Figure 6k showed that the blend film of PTB7-Th:PTe-PDI(OD) (1:1, w/w) showed huge domain size, consistent with the AFM images. Figure 6m showed the morphology image of PTB7-Th:PTe-NDI(HD) (1:1, w/w) that demonstrated large and non-continuous phases. Furthermore, the TEM image of PTB7-Th:PTe-NDI(HD) (1:1.4, w/w) exhibited similar features as PTB7-Th:PTe-NDI(HD) (1:1, w/w). In comparison, the TEM image of PTB7-Th:PTe-NDI(HD) (1:1.2, w/w) showed a much fine structure with continuous phases, beneficial to charge generation and transport (Figure 6n). These observations are consistent with the photovoltaic performances that PTB7-Th:PTe-NDI(HD) (1:1.2, w/w) based solar cells exhibit relatively high Jsc and FF.

2.7. Bulk Charge Transport in BHJ Solar Cells. The space charge limit current (SCLC) method was used to understand the bulk charge transport characteristics in blend films and FF of the solar cells.64 The hole mobilities were measured with the device structure of ITO/PEDOT:PSS/active layer/MoO3/Ag, while electron mobilities were characterized with the device structure of ITO/ZnO/active layer/Ca/Al. The J1/2 -V curves are shown in Figure S8 and calculated hole (µh) and electron (µe) mobilities of blend films are summarized on Table 2. For the blends composed with donor:acceptor=1:1 (w/w), the electron mobility is calculated to be 8.94×10-7,3.04×10-7, and 5.01×10-7 cm2/V•s for PTe-PDI(OD), PTe-NDI(OD), and PTe-NDI(HD) based devices, while the hole mobility is 1.04×10-4, 1.03×10-4, and 1.80×10-4 cm2/V•s, respectively. Although, the µh/µe of PTe-NDI(HD) is relatively high, it still possesses the highest Jsc.65 Furthermore, the electron mobility is calculated to be 5.96×10-7 and 5.07×10-6 for PTB7-Th:PTe-NDI(HD) (1:1.2, w/w) and PTB7-Th:PTe-NDI(HD) (1:1.4, w/w), while the hole mobility is 6.24×10-5 and 1.79×10-4 cm2/Vs,


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respectively. Note that the µh/µe of PTB7-Th:PTe-NDI(HD) (1:1.4, w/w) is most balanced, in accordant with the highest Jsc.

2.8. Exciton Dissociation and Bimolecular Recombination Kinetics. To conduct a deep understanding of light absorption and dissociation process, the saturation current density Jsat and charge dissociation probabilities P(E,T) of PTB7-Th:PTe-NDI(HD) (1:1, w/w), PTB7-Th:PTeNDI(HD) (1:1.2, w/w), and PTB7-Th:PTe-NDI(HD) (1:1.4, w/w) devices were investigated. Figure 7a shows photocurrent density (Jph) versus effective voltage (Veff) curves for the solar cells. Here, Jph is defined as Jph = JL- JD, where JL and JD are the photocurrent densities under illumination and in the dark, respectively. Veff is defined as Veff = V0-Va, where V0 is the voltage where Jph equals zero and Va is the applied bias voltage. At a high reverse voltage (i.e., Veff ≥ 2 V), Jph reaches saturation (Jsat), indicating that the photogenerated excitons are fully split into free charges and gathered by the electrodes. So, the excitons dissociation and charge collection can be evaluated using the P(E,T).66 The PTB7-Th:PTe-NDI(HD) (1:1.4, w/w) based solar cells demonstrated a largest P(E,T) value (90.2%), higher than those of PTB7-Th:PTe-NDI(HD) (1:1, w/w) (80.9%) and PTB7-Th:PTeNDI(HD) (1:1.2, w/w) (88.2%), indicating a highest photogenerated excitons dissociation and charge collection efficiency in PTB7-Th:PTe-NDI(HD) (1:1.4, w/w) based solar cells. This is accordant with the highest Jsc of the PTB7-Th:PTe-NDI(HD) (1:1.4, w/w) based solar cells.


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Figure 7. (a) Photocurrent density (Jph) vs. effective voltage (Veff) characteristics. (b) Short current density (Jsc) vs. light intensity for the PTB7-Th:PTe-NDI(HD) based solar cells. Additionally, we also investigated the relationship between Jsc and illumination strength for the three solar cells to probe the recombination dynamics. In general, there is a power-law between Jsc and light strength. In theory, a sublinear relationship between photocurrent and Plight would suggest partial loss of charge carriers during the charge transport procedure due to bimolecular recombination, while a linear relationship demonstrates weak bimolecular recombination.67 As shown in Figure 7b, the exponential factors of PTB7-Th:PTe-NDI(HD) (1:1, w/w), PTB7-Th:PTeNDI(HD) (1:1.2, w/w), and PTB7-Th:PTe-NDI(HD) (1:1.4, w/w) are 1.05, 1.09, and 1.1, respectively. All three systems demonstrated similarly linear scaling of photocurrent with Plight, indicating the bimolecular recombination is weak in those systems.

3. CONCLUSIONS A series of n-type conjugated copolymers were synthesized through copolymerization of tellurophene and PDI/NDI derivatives as acceptors for all polymer solar cells. The studies of physicochemical properties of polymers showed that PTe-NDI copolymers possess relatively planar


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Page 18 of 33

conformation in comparison to PTe-PDI copolymers, leading to better π-π stacking, relatively high LUMO energy levels, and better bulk electron transport capabilities. As a result, through tuning the building block and alkyl chains, the best efficiency of PTe-NDI(HD) based solar cells reached 4.3%, an 54% enhancement in comparison to that of PTe-PDI(OD) based solar cells. This improvement was ascribed to the improved mrophologies, and more efficient exciton separation, transportation and collection in the blend films, supported by the photoluminescence, AFM, TEM, SCLC, and bimolecular recombination kinetic studies. Finally, the reversible interactions between tellurophene polymers and bromine indicate their potential for bromine detection.

4. EXPERIMENTAL SECTION 4.1. Materials. All chemicals were purchased from Aladdin, Energy Chemical and were used as received. The solvents were freshly distilled before their usage for chemical synthesis. All synthetic procedures were performed with nitrogen protection. The monomers were synthesized based on the literatures reported previously: 2,5-bis (trimethylstannyl) tellurophene,52 2Br-PDI(OD),53 2BrNDI(OD and HD).54-55 PTB7-Th (Mw>40 kDa, PDI=1.8-2.0) was purchased from Solarmer Materials Inc., and used without any purification. 4.2. Synthesis of Polymers. All polymers were synthesized by the Stille coupling. PTe-PDI(OD): In a 25ml Schlenk tube, 2Br-PDI(OD) (147.8 mg, 0.15 mmol), 2,5-bis (trimethylstannyl) tellurophene (75.8 mg, 0.15 mmol), Pd2(dba)3 (8 mg, 6 mol.%) and P(o-tol)3 (11 mg, 24 mol.%) were added. The tube was degassed and filled with nitrogen for three times. Afterwards, chlorobenzene (8 mL) was added and N2 was used to purge the mixture for 20 min. The mixture was refluxed for 72 h under N2. Afterwards, trimethyl(thiophen-2-yl) stannane (37 mg, 0.15 mmol) and 2-bromothiophene (49 mg, 0.3 mmol) were added subsequently with 4 h interval to end


18

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cap the reaction. After being cooled to room temperature, the mixture was added into CH3OH (300 mL) dropwise to precipitate the product. The crude product was collected by filtration, and was then dried under vacuum, and loaded into Soxhlet extraction with different solvents in a row, acetone, hexane, THF and CHCl3. At last, the CHCl3 part was reduced by evaporation and re-precipitated into CH3OH (300 mL), followed by filtration with a filter paper, which afforded a dark purple solid (108 mg, 64%). GPC: Mn = 13.8 kDa, Mw= 25.5 kDa, PDI= 1.85. Td (5% loss) = 438 °C. Elemental analysis calcd (%) C68H90N2O4Te: C, 72.34; H, 8.21; N, 2.48; Found: C, 71.92; H, 8.05; N, 2.42. The 1

H NMR is provided in Figure S1.

PTe-NDI(OD):

In

a

Schlenk

tube,

2Br-NDI(OD)

(78

mg,

0.079

mmol),

2,5-bis

(trimethylstannyl)tellurophene (40 mg, 0.079 mmol), Pd2(dba)3 (5 mg, 6 mol.%) and P(o-tol)3 (6 mg, 24 mol.%) were added under N2. Afterwards, chlorobenzene (8 mL) was added and N2 was used to purge the mixture for 20 min. The reaction mixture was refluxed for 72 h under N2. Afterwards, trimethyl(thiophen-2-yl) stannane (20 mg, 0.079 mmol) and 2-bromothiophene (26 mg, 0.158 mmol) were subsequently added with 4 h interval to end cap the reaction. After cooling to r.t., the solution was added into methanol. This precipitate was collected with a filter paper and treated with Soxhlet extraction with different solvents in a row of acetone, hexane, THF, chloroform and chlorobenzene. Finally, chlorobenzene part was re-precipitated with methanol and collected with a filter paper, which afford a blue solid (Chlorobenzene fraction: 35mg, 44%). GPC: Mn = 67.6 kDa, Mw = 194.2 kDa, PDI= 2.87. Td (5% loss) = 417 °C. Elemental analysis calcd (%) C58H86N2O4Te: C, 69.32; H, 8.83; N, 2.79. Found: C, 69.56; H, 8.67; N, 2.71.The 1H NMR is provided in Figure S2. PTe-NDI (HD): 2Br-NDI(HD) (96 mg, 0.11 mmol), 2,5-bis (trimethylstannyl) tellurophene (55.6 mg, 0.11 mmol), Pd2(dba)3 (6 mg, 6 mol.%) and P (o-tol) 3 (8 mg, 24 mol.%) were added into a Schlenk tube under N2. Afterwards, chlorobenzene (8 mL) was added. N2 was used to purge the


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mixture for 20 min. The mixture was heated up to reflux for 48 h under N2. Afterwards, trimethyl(thiophen-2-yl)stannane (27 mg, 0.11 mmol) and 2-bromothiophene (36 mg, 0.22 mmol) were subsequently added with 4 h interval to end cap the reaction. After cooling down to r.t., the mixture was dropped into methanol (200 mL) to form a dark blue solid and collected with a filter paper. The polymer was purified by Soxhlet extraction with acetone, n-hexane, THF, CHCl3 and chlorobenzene. Then, the chlorobenzene fraction was re-precipitated from methanol. After dried in air, a dark blue solid (18 mg, 18%) was obtained. GPC: Mn=67.2 kDa, Mw= 167.2 kDa, PDI= 2.49; Td (5% loss) = 415 °C. Elemental analysis calcd (%) C50H70N2O4Te: 67.27; H, 8.13; N, 3.14.Found: C, 66.77; H, 7.83; N, 3.06.The 1H NMR is provided in Figure S3.

4.3. Characterization and Measurement. electrochemical

cyclic

voltammetry

(CV),

1

H nuclear magnetic resonance (NMR),

UV-Vis

absorption

spectra,

gel

permeation

chromatography (GPC) analysis, thermogravimetric analysis (TGA) measurements, differential scanning calorimetry (DSC), AFM measurements, and transmission electron microscopy (TEM) characterization were carried with the reported methods.68

4.4. Fabrication of polymer solar cells. Ultrasonic baths with soap DI water, DI water, acetone and isopropanol was used to clean ITO glass substrate for 30 min. at each procedure. The dried substrates were then exposed in a UV-ozone chamber for 30 min. Afterwards, the ZnO precursor solution (0.5 M zinc acetate dehydrate in 0.5 M monoethanolamine and 2-methoxyethanol) was spin-coated on top of the ITO glass at 4500 rpm for 40 s. Furthermore, the substrates were annealed at 200 oC for 30 min. Then the annealed substrates were relocated to the glove box. Active layer solutions (e.g. PTBT-Th:PTe-PDI(OD), PTe-NDI(OD) and PTe-NDI(HD), (1:1;1:1;1:1/1.2/1.4,


20

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w/w)) were prepared in CB (donor concentration : 10mg/ml; 7mg/ml; 7mg/ml ) and spin-coated for 30 s. Afterwards, the wet films were dried for 2 h and relocated to a vacuum chamber inside the glove box. MoO3 (10 nm) and Ag (100 nm) were slowly deposited as an anode interlayer and the electrode under 2×10-5 Pa. The size of the solar cells was 0.04 cm2. The J-V curves of the solar cells was measured under AM 1.5 G (100 mW cm-2) using a Keithley 2400 Source Measure Unit. A Solar Cell Spectral Response Measurement System QER3011 (Enlitech, Taiwan) was used for external quantum efficiency (EQE) measurement. A Bruker Dektak XT profilometer was used to measure the thickness of the BHJ blends film.68

4.5. Space-Charge-Limited Current (SCLC) Measurement. Space-charge-limited current (SCLC) method was employed to measure the hole mobility and electron mobility of conjugated polymers.

Hole-only

or

electron-only

diodes

were

fabricated

with

a

structure

of

ITO/PEDOT:PSS/blend film/MoO3/Ag for holes and ITO/ZnO/blend film/Ca/Al for electrons, respectively. The mobilities were calculated with the following equation:

J = 9ε 0ε r µ (Vappl − Vbi ) 2 / 8L3

(1)

where ε0 means the permittivity of free space (8.85×10−12 F/m), εr means the material’s relative permittivity (assumed to be 3), µ means the hole mobility, Vappl means the applied voltage, Vbi means the built-in voltage (Vbi is 0.2 V for hole-only diodes; Vbi is 0 V for electron-only diodes),68 and L means the film’s thickness. Upon linearly fitting J1/2 with Vappl-Vbi, the mobilities were calculated based on the slope:


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µ h = slope 2 × 8 L3 / 9ε 0 ε r

Page 22 of 33

(2)

4.6 Molecular modelling:

Density functional theory (DFT) calculations by Gaussian 09

software package69 using B3LYP hybrid functional with basis set LANL2DZ was used to calculate the electronic structures of the small molecules.70 All the alkyl chains were substituted by methyl groups. Vertical electronic excitation energies were calculated upon the (DFT) method.70

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx /acsami.xxxxxxx. 1

HNMR of polymers; Thermogravimetric analysis (TGA) and DSC of polymers; Cyclic

voltammograms (CV) of polymers and PTB7-Th as thin films; Photoluminescence spectroscopy of blend films; Hole-only (a) and electron-only (b) devices based on PTB7-Th:polymers.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions §

These authors contributed equally to this work.


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Tellurophene-Based N-type Copolymers for Photovoltaic Applications

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