2-Alkyl-5-thienyl-Substituted Benzo[1,2-b:4,5-b′]dithiophene-Based Donor Molecules for Solution-Processed Organic Solar Cells

TL;DR: Two novel, symmetrical, and linear A-D-A-type π-conjugated donor molecules each containing a planar electron-rich 2-octylthiene-5-yl-substituted benzodithiophene (TBDT) unit as the core are synthesized and end-capped with electron-deficient cyanoacetate (CNR) or dicyanovinyl (CN) units.

Abstract: In this study, we have strategically designed and convergently synthesized two novel, symmetrical, and linear A–D–A-type π-conjugated donor molecules (TBDTCNR, TBDTCN), each containing a planar electron-rich 2-octylthiene-5-yl-substituted benzodithiophene (TBDT) unit as the core, flanked by octylthiophene units and end-capped with electron-deficient cyanoacetate (CNR) or dicyanovinyl (CN) units. We thoroughly characterized both of these materials and investigated the effects of the end groups (CNR, CN) on their optical, electrochemical, morphological, and photovoltaic properties. We then fabricated solution-processed bulk heterojunction organic solar cells incorporating TBDTCNR and TBDTCN. Among our tested devices, the one containing TBDTCNR and [6,6]-phenyl-C61-butyric acid methyl ester in a 1:0.40 ratio (w/w) exhibited the highest power conversion efficiency (5.42%) with a short-circuit current density (Jsc) of 9.08 mA cm–2, an open circuit voltage (Voc) of 0.90 V, and an impressive fill factor (FF) of ...

Summary

1. INTRODUCTION

• Solution-processed polymer solar cells (PSCs) are emerging as promising alternatives to silicon-based solar cells; they provide enormous benefits as a result of their low cost, amenability, high-throughput roll-to-roll processing, and ready structural modifications.
• 6,8,9 Nevertheless, research into SMOSCs requires much further attention if the authors are to solve their shortcomings.

2. EXPERIMENTAL SECTION

• 1. Fabrication and Characterization of Organic Solar Cells (OSCs).
• After routine solvent cleaning, the ITO substrates were treated with UV ozone for 15 min and then spin-coated with the PEDOT:PSS layer (∼30 nm) at 4000 rpm.
• The photovoltaic performance was characterized within a N2-filled glove box under an AM 1.5G filter (100 mW cm−2) using a Newport Oriel Solar Simulator (Thermal Oriel 1000W), as calibrated using a Si photodiode.
• The configurations of the hole- and electron-only devices were ITO/ PEDOT:PSS/small molecule:PC61BM/V2O5/Al and ITO/Cs2CO3/ small molecule:PC61BM/Ca/Al, respectively.

3. RESULTS AND DISCUSSION

• Scheme 1 presents the synthetic route the authors followed to prepare TBDTCNR and TBDTCN.
• TBDTCN possessed a relatively low HOMO energy level relative to that of TBDTCNR, due to its high onset oxidation potential resulting from the presence of its more highly electron deficient CN units.
• The corresponding devices incorporating TBDTCNR:PC61BM at weight ratios of 1:0.25, 1:0.50, and 1:0.75 provided lower EQEs obtained at lower values of Jsc, because of imbalances in the hole and electron mobilities and unfavorable morphologies , as the authors discuss below.
• Transmission electron microscopy (TEM) images were recorded both pristine films of TBDTCNR and the blend film TBDTCNR/PC61BM (wt. ratio 1:0.4) and presented in Supporting Information Figure S10.

4. CONCLUSIONS

• The authors have synthesized TBDTCNR and TBDTCN two πconjugated small molecules having A−D−A architectures based on TBDT as the central donating core and modified with different end groups and correlated their photophysical and photovoltaic properties.
• These TBDT-based species possessed deep HOMO energy levels and provided devices exhibiting good values of Voc. TBDTCNR showed not only good packing in the solid state but also superior charge transport properties and favorable nanoscale morphology relative to TBDTCN and thus pronounced to the higher PCE values.
• Further engineering of the molecular structure and optimization of the morphology, and variation of the acceptor moieties, are currently underway in their laboratory in the quest to boost device efficiencies.
• ■ ASSOCIATED CONTENT *S Supporting Information 1HNMR spectra, 13CNMR spectra, TGA and DSC curves, AFM height images, synthesis details, characterization, DFT calculations AFM, TEM images of the blends and transfer curves.
• This materials are available free of charge via the Internet at http://pubs.acs.org.

Figures

Figure 6. EQE curves of BHJ solar cell devices incorporating TBDTCNR:PC61BM (1:0.4, w/w) and TBDTCN:PC61BM (1:0.6, w/ w) blends (optimized weight ratios).

Figure 7. Tapping-mode AFM phase images of blend films cast from CHCl3 solutions for (a, b) TBDTCNR:PC61BM at weight ratios of (a) 1:0 and (b) 1:0.4 and (c, d) TBDTCN:PC61BM at weight ratios of (c) 1:0 and (d) 1:0.6.

Figure 1. Chemical structures of TBDTCNR and TBDTCN.

Figure 2. Normalized UV−vis absorption spectra of TBDTCNR and TBDTCN as dilute solutions in CHCl3 and as thin films on glass surfaces at room temperature.

Figure 4. XRD patterns of TBDTCNR and TBDTCN films spincoated from CHCl3. Inset: Expanded view of the pattern for TBDTCNR and TBDTCN in the range 6−14 Å.

Figure 3. Cyclic voltammograms of TBDTCNR and TBDTCN as thin films (scan rate = 100 mV s−1).

Table 2. Photovoltaic Properties of TBDTCNR and TBDTCN

Figure 5. (a) J−V curves of BHJ solar cell devices incorporating blend ratios of TBDTCNR:PC61BM (w/w) and (b) TBDTCN:PC61BM (w/ w).

Frequently asked questions

Q1. What are the contributions mentioned in the paper "2‐alkyl-5-thienyl-substituted benzo[1,2‐b:4,5‐b′]dithiophene-based donor molecules for solution-processed organic solar cells" ?

In this study, the authors have strategically designed and convergently synthesized two novel, symmetrical, and linear A− D−A-type π-conjugated donor molecules ( TBDTCNR, TBDTCN ), each containing a planar electron-rich 2-octylthiene-5-ylsubstituted benzodithiophene ( TBDT ) unit as the core, flanked by octylthiophene units and end-capped with electron-deficient cyanoacetate ( CNR ) or dicyanovinyl ( CN ) units. The authors thoroughly characterized both of these materials and investigated the effects of the end groups ( CNR, CN ) on their optical, electrochemical, morphological, and photovoltaic properties. The FFs of these solutionprocessed small-molecule organic solar cells ( SMOSCs ) are outstanding when compared with those recently reported for benzodithiophene ( BDT ) -based SMOSCs, because of the high crystallinity and excellent stacking properties of the TBDT-based compounds.