Structure property relationship for carbazole and benzothiadiazole based conjugated polymers

doi:10.1016/j.solmat.2015.11.005

Solar Energy Materials and Solar Cells

Volume 145, Part 3, February 2016, Pages 412-417

https://doi.org/10.1016/j.solmat.2015.11.005 Get rights and content

Highlights

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Structure–property effected by D and A blocks.
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Intramolecular charge transfer peak bands shift remarkably.
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Energy level substantially changed.
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Morphology controlled by molecular structure.
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Donor-accept blocks lengthen towards two directions.

Abstract

A series of conjugated polymers with carbazole monomer/dimer/trimer as the donor unit or benzothiadiazole monomer/trimer as the acceptor unit were synthesized and used as donor materials for polymer solar cells (PSCs). The influence of polymer structure on the optical properties and performance of PSCs was investigated in detail. P1 showed a power conversion efficiency (PCE) of 5.8%. The incorporation of longer donor or acceptor unit leads to a blue shift of absorption spectrum and the polymers became more amorphous due to the large torsion angels in the polymer backbone. After increasing the length of donor or acceptor, the PCE decreased dramatically to 0.43% for P2, 0.23% for P3, and 1.23% for P4. Our results have offered a useful insight to structure–property relationship for high performance polymer solar cells.

Graphical abstract

Introduction

Bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells have aroused a great interest for their potential applications and impressive device efficiency achieved in the past decade [1], [2], [3]. The progress of polymer solar cells (PSCs) is extremely encouraging and exciting. However, the achievement of PSCs still cannot meet the requirement of market for their exorbitant cost, low power conversion efficiency (PCE) and short device lifetime [4], [5], [6]. To obtain high PCE, the efficient absorption of solar energy for the active layer is very crucial [7]. The optical absorption profile of polymers should be matched well with the solar spectrum. Many experimental methods and new molecular design strategies have been applied to achieve strong light-absorbing ability [8], [9], [10], [11]. Hopefully, broad absorptions can be obtained by using donor and acceptor alternating polymer backbone design strategy via the internal charge transfer (ICT) from the donor to the acceptor unit. Decreasing the band gap can make the polymers absorb more efficiently and lowering the HOMO level can maximize the open circuit voltage (V_oc) of devices [12]. Narrow band gap polymers can be constructed by alternating an electron-rich donor (D) and an electron-deficient acceptor (A) segment in the polymer main chain [13], [14], [15]. The ICT effect can produce a pivotal light absorption in the region of visible light [16]. From the perspective of material structure–property, the influence of length of donor or acceptor segment on the optical properties of polymers and the photovoltaic performance of devices has not been intensively explored. In this contribution, we synthesized a series of conjugated polymers with carbazole monomer/dimer/trimer as the donor unit and benzothiadiazole monomer/trimer as the acceptor unit (Scheme 1). The influence of polymer structure on the optical properties of polymers and the performance of PSCs was studied in detail.

Section snippets

Synthesis

The syntheses of P1–4 are illustrated in Scheme 2. The structures of P1–4 were characterized by ¹H and ¹³C NMR spectroscopy (Figs. S1–7. Supporting information). Molecular weights of and molecular weight distributions of P1–4 were measured by GPC at room temperature using chloroform as an eluent and calibrated with polystyrene standards. The data are summarized in Table S1 (Supporting information). The relatively low molecular weight for polymer P3 and P4 was probably due to their poor

Conclusions

In summary, we have synthesized a series of conjugated polymers based on carbazole and benzothiadiazole. Increasing the length of donor or acceptor unit, the absorption spectra of the resulted polymers became narrower. Low molecular weight polymers were obtained by Suzuki polycondensation by using carbazole dimer/trimer as the donor unit or benzothiadiazole trimer as the acceptor unit. The carbazole dimer/trimer or benzothiadiazole trimer probably has a larger torsion angel, resulting larger

Materials

All chemicals were bought from Aldrich or Acros and used without further purification. Polymers HXS-1-3 published before [20], [21]. The solvents were dried according to standard procedures. All reactions were performed under an atmosphere of nitrogen and monitored by thin-layer chromatography with silica gel (GF254). Column chromatography was carried out on silica gel (200–300 mesh).

Characterization

¹H and ¹³C NMR was recorded on a Bruker AV400 spectrometer in CDCl₃. Electronic absorption spectra were obtained

Acknowledgment

This work was financial supported by Young Scientists Foundation of Henan Normal University (01026400061); Henan Province Basic and Frontier Technology Research Projects (1323004100247); National Natural Science Foundation of China (21402042).

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View full text

Structure property relationship for carbazole and benzothiadiazole based conjugated polymers

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis

Conclusions

Materials

Characterization

Acknowledgment

Nat. Commun.

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

A series of simple oligomer-like small molecules based on oligothiophenes for solution-processed solar cells with high efficiency

J. Am. Chem. Soc.

A high efficiency solution processed polymer inverted triple-junction solar cell exhibiting a power conversion efficiency of 11.83%

Energy Environ. Sci.

Photoconductive cathode interlayer for highly efficient inverted polymer solar cells

J. Am. Chem. Soc.