An efficient selenophene-containing conjugated copolymer for organic solar cells

Abstract

A D–A polymer, PSeTPTI, was developed by copolymerizing a pentacyclic acceptor unit TPTI with a selenophene unit. PSeTPTI possesses a narrow optical bandgap, a low-lying HOMO energy level, and a high hole mobility of 0.26 cm² V⁻¹ s⁻¹. PSeTPTI/PC₇₁BM solar cells demonstrate a PCE of 6.04%, which is the highest efficiency for the conjugated copolymers using selenophene as the donor unit.

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Polymer solar cells (PSCs) with great commercialization potential show unique advantages, such as solution processing, lightweight, and flexibility.¹ To date, power conversion efficiencies (PCEs) of single and tandem PSCs have reached 9.2%² and 10.6%,³ respectively. D–A conjugated copolymers have been widely used to obtain efficient solar cells. An outstanding D–A copolymer should possess: (1) low bandgap for harvesting more photons; (2) low-lying HOMO level for high open-circuit voltage (V_oc); (3) high hole mobility for high photocurrent and fill factor (FF); (4) good solubility for facile solution processing and favourable nano-morphology. Polycyclic aromatic donor and acceptor building blocks function very well in PSCs since their extended conjugation can effectively reduce bandgap, enhance light absorbance, and improve charge carrier mobility.⁴ Recently, we developed a highly efficient D–A polymer PThTPTI using a pentacyclic aromatic lactam unit, thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione (TPTI), as the acceptor unit. PThTPTI/PC₇₁BM solar cells afforded a PCE of 7.8%.⁵ PThTPTI is among the best donor materials. However, PThTPTI exhibited relatively narrow absorption spectra with a moderate optical bandgap of 1.86 eV, which limited the absorption in long-wavelength region. One approach to broaden absorption spectra and reduce bandgap is to replace thiophene unit with selenophene unit. Compared with thiophene, the reduced aromaticity of selenophene increases the ground state quinoid resonance of the polymer, and selenophene-containing polymers possess enhanced planarity, extended conjugation length, and reduced bandgaps.⁶ Besides, selenium is more polarizable than sulfur, Se⋯Se interactions favor interchain charge hopping and enhance hole mobility.⁷ Selenophene was used in D–A copolymers as a donor unit,⁸ a π-bridge,⁹ or part of the donor unit or acceptor unit.¹⁰ In this work, we synthesized PSeTPTI based on TPTI acceptor unit and selenophene donor unit (Scheme 1). PSeTPTI exhibited good thermal stability, broad absorption, low optical bandgap, and high hole mobility of 0.26 cm² V⁻¹ s⁻¹. PSeTPTI/PC₇₁BM solar cells afforded a PCE of 6.04%. To the best of our knowledge, this is the highest efficiency for D–A copolymers using selenophene as the donor unit (Table 1).

Scheme 1 Synthesis of PSeTPTI.

Table 1 Performance data for PSeTPTI:PC₇₁BM (1 : 1, w/w) solar cells under AM 1.5G illumination (100 mW cm⁻²)

DIO [vol%]	Voc [V]	Jsc [mA cm^−2]	FF [%]	PCE [%]
3	0.85	11.10	64	6.04
Without	0.89	1.95	64	1.12

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The synthesis of compound TPTI-Br is described in the literature.⁵ Target polymer PSeTPTI was synthesized through Stille reaction of TPTI-Br and 2,5-bis(trimethylstannyl)selenophene in toluene at 115 °C. The crude product was purified by Soxhlet extraction using methanol, hexane, and chloroform in sequence. The chloroform fraction was added into methanol to obtain PSeTPTI as dark blue solid in 97% yield. The chemical structure of PSeTPTI was confirmed by ¹H NMR and elemental analysis. PSeTPTI is highly soluble in common organic solvents, such as toluene, chloroform, chlorobenzene, and o-dichlorobenzene. From gel permeation chromatography (GPC) against polystyrene standards in THF eluent, the number-average molecular weight (M_n) and the polydispersity index (PDI) of the polymer are 34.9 kDa and 3.51, respectively. PSeTPTI shows good thermal stability with a decomposition temperature T_d of 417 °C (5% wt loss) (Fig. S1†).

As shown in Fig. 1, PSeTPTI exhibits two absorption peaks at 548 and 602 nm in chloroform solution. The film absorption peak at 638 nm originates from interchain π–π stacking. The absorption onset for PSeTPTI film is 701 nm, corresponding to an optical bandgap of 1.77 eV, which is narrower than that of its analogue PThTPTI (1.86 eV).^6,9b The electrochemical property of PSeTPTI film was studied by cyclic voltammetry (CV) and all potentials were calibrated against Fc/Fc⁺ redox couple. As shown in Fig. S2†, the oxidation onset potential (E^on_ox) and the reduction onset potential (E^on_red) of PSeTPTI are 0.61 and −1.92 V, respectively. According to the empirical formulae: HOMO = −(E^on_ox + 4.8) eV, LUMO = −(E^on_red + 4.8) eV,¹¹ the HOMO and LUMO energy levels of the polymer are calculated to be −5.41 and −2.88 eV, respectively. Compared with PThTPTI, PSeTPTI possesses similar HOMO energy level and lower LUMO energy level.^10b V_oc is proportional to the difference between the HOMO energy level of donor materials and the LUMO energy level of acceptor materials.¹² Deep HOMO energy level should render PSeTPTI high V_oc and good stability against oxidization.^9b The electrochemical bandgap of PSeTPTI is 2.53 eV, 0.76 eV higher than the optical bandgap, which results from the interface barrier between polymer films and electrode surface.¹³

Solar cells with a structure of ITO/PEDOT:PSS/PSeTPTI:PC₇₁BM/Ca/Al were fabricated to evaluate the photovoltaic performance of PSeTPTI. After optimization, solar cells with a D–A ratio of 1 : 1 (w/w), an active layer thickness of 140 nm, and 3 vol% 1,8-diiodooctane (DIO) gave the best result (see ESI†). The best device afforded a PCE of 6.04%, with a V_oc of 0.85 V, a J_sc of 11.10 mA cm⁻², and a FF of 64%. DIO addition greatly increases photocurrent by reducing domain size and increasing D–A interfaces in the active layer.¹⁴ External quantum efficiency (EQE) spectra indicate that the device with 3 vol% DIO exhibits much higher EQE values than the device without DIO. The integrated currents from EQE curves for the devices without and with DIO are 1.87 and 10.61 mA cm⁻², respectively, which consist with the J_sc values from J–V measurements (Fig. 2).

Fig. 1 Absorption spectra for PSeTPTI in chloroform and as thin films.

Fig. 2 (a) J–V curves for PSeTPTI/PC₇₁BM solar cells; (b) EQE spectra.

Bottom-gate and bottom-contact organic field-effect transistors (OFETs) were fabricated to explore the charge-transporting property of PSeTPTI film. As shown in Fig. 3, the devices show p-type behavior with typical transfer and output curves. PSeTPTI-based OFET exhibited a high hole mobility of 0.26 cm² V⁻¹ s⁻¹ with a large on/off current ratio of 1.89 × 10⁸ and a low threshold voltage of −6.51 V. In the annealing temperature range of 20–160 °C, the polymer showed similar mobility (Fig. S3†). The high mobility of PSeTPTI accounts for its good J_sc and FF in solar cells. X-ray diffraction (XRD) pattern of PSeTPTI films shows two diffraction peaks at 2θ = 4.46° and 25.12°, respectively (Fig. S4†). The primary peak at 4.46° corresponds to an interlayer d-spacing of 19.80 Å, and the peak at 25.12° corresponds to a d-spacing of 3.54 Å for π–π stacking. The lamellar structures existing in PSeTPTI film lead to its high mobility.

Fig. 3 (a) Transfer and (b) output curves for PSeTPTI-based OFETs. Device dimension: channel length (L) = 50 μm; channel width (W) = 1400 μm.

In summary, a D–A conjugated polymer, PSeTPTI, based on a pentacyclic acceptor unit, TPTI, and selenophene donor unit, was developed. The optical bandgap of PThTPTI was decreased successfully by replacing thiophene unit with selenophene unit. PSeTPTI possesses good solubility, excellent thermal stability, narrow optical bandgap, low-lying HOMO energy level, and high mobility. PSeTPTI/PC₇₁BM solar cells demonstrate a PCE of 6.04%, which is the highest efficiency for the D–A copolymers using selenophene as the donor unit.

Acknowledgements

This work was supported by the “100 Talents Program” of Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details including synthesis, measurements, and instruments. See DOI: 10.1039/c3ra47098a

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