Junfeng Tong

Improving the performance of perovskite solar cells with glycerol-doped PEDOT:PSS buffer layer*

In this paper, we investigate the effects of glycerol doping on transmittance, conductivity and surface morphology of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)) (PEDOT:PSS) and its influence on the performance of perovskite solar cells. . The conductivity of PEDOT:PSS is improved obviously by doping glycerol. The maximum of the conductivity is 0.89 S/cm when the doping concentration reaches 6 wt%, which increases about 127 times compared with undoped. The perovskite solar cells are fabricated with a configuration of indium tin oxide (ITO)/PEDOT:PSS/CH3NH3PbI3/PC61BM/Al, where PEDOT:PSS and PC61BM are used as hole and electron transport layers, respectively. The results show an improvement of hole charge transport as well as an increase of short-circuit current density and a reduction of series resistance, owing to the higher conductivity of the doped PEDOT:PSS. Consequently, it improves the whole performance of perovskite solar cell. The power conversion efficiency (PCE) of the device is improved from 8.57% to 11.03% under AM 1.5 G (100 mW/cm2 illumination) after the buffer layer has been modified.

Dithieno[2,3-d:2′,3′-d′]naphtho[1,2-b:3,4-b′]dithiophene – a novel electron-rich building block for low band gap conjugated polymers

A feasible synthesis of 10,11-di(3,7-dimethyloctyloxy)dithieno[2,3-d:2′,3′-d′]naphtho[1,2-b:3,4-b′]dithiophene (NDT) was presented, and a novel low band gap (LBG) NDT-based polymer was prepared and characterized. The preliminary results indicate that NDT can be used as a novel electron-rich building block in the development of LBG conjugated polymers.

Large branched alkylthienyl bridged naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole-containing low bandgap copolymers: Synthesis and photovoltaic application

ABSTRACT Two donor-acceptor (D-A) type low bandgap (LBG) alternating conjugated copolymers containing larger conjugation planarity and stronger electro-withdrawing ability naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NT) unit, namely, poly[4,8-bis(5-(n-octylthio)thien-2-yl)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4,9-bis(4-(2-decyltetradecyl)thien-2-yl)naphtho- [1,2-c:5,6-c′]bis[1,2,5]thiadiazole-5,5′-diyl] (PBDT-TS-DTNT-DT) and poly[4,8-bis(triiso-propylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4,9-bis(4-(2-decyltetradecyl)-thien-2-yl)naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole-5,5′-diyl] (PBDT-TIPS-DTNT-DT), were prepared by the palladium-catalyzed Stille polycondensation and characterized by gel permeation chromatography (GPC), UV-Vis absorption, thermal gravimetric analysis (TGA), cyclic voltammetry (CV) etc. PBDT-TS-DTNT-DT and PBDT-TIPS-DTNT-DT show the broader absorption and deeper highest occupied molecular orbital (HOMO) energy level approximately −5.45 and −5.62 eV, respectively. Bulk-heterojuction solar cells based on the resulted polymers and [6,6] phenyl-C61 butyric acid methyl ester (PC61BM) blends, with the device configuration of ITO/PFN/polymer:PC61BM/MoO3/Ag were prepared and investigated. The results showed the power conversion efficiency (PCE) of 2.67% for PBDT-TS-DTNT-DT/PC61BM (w:w, 1:2) and 0.64% for PBDT-TIPS-DTNT-DT/PC61BM (w:w, 1:1), with relatively high open-circuit voltage (VOC) of 0.86 and 1.05 V, small short-circuit current (JSC) of 5.41 and 0.97 mA cm−2 and moderate fill factor (FF) of 57.8% and 62.4%, under an AM1.5 simulator (100 mWcm−2), respectively.

Synthesis of π-Extended Dithienobenzodithiophene-Containing Medium Bandgap Copolymers and Their Photovoltaic Application

Two medium bandgap alternating conjugated copolymers, namely, poly{5,10-di(2-hexyldecyloxy)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-2,7-diyl-alt-thiophene-2,5-di-yl} (P1) and poly{5,10-di(2-hexyldecyloxy)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-2,7-di-yl-alt-thieno[3,2-b]thiophene-2,5-diyl} (P2), were prepared by the palladium-catalyzed Stille polycondensation and characterized by gel permeation chromatography (GPC), UV-Vis absorption and photoluminescence (PL) spectra, thermal gravimetric analysis (TGA), cyclic voltammetry (CV) etc. The resultant copolymers show moderate solubility in common organic solvents and enough stabilities for photovoltaic application. And both copolymers absorb the solar light from 300–600 nm, with the optical band gaps () calculated from the onset of absorption in the solid film of ca. 2.1 eV. The highest occupied molecular orbital (HOMO) levels of two copolymers determined by CV were at about −5.35 eV. Photovoltaic propertites of the polymers were investigated by using the polymers as donor and [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) as acceptor with a weight ratio of polymer:PC71BM of 1:2. The power conversion efficiencies (PCEs) of polymer solar cells based on PDTBTT-TT reached 2.50%, with an open-circuit voltage (Voc) of 0.70 V, a short-circuit current density (Jsc) of 6.89 mA cm−2, and a fill factor (FF) of 52%, under the illumination of AM1.5, 100 mW cm−2. These results indicate that dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) is a promising building block for the high-performance organic electronic materials.

Effect of alkylthiophene spacers and fluorine on the optoelectronic properties of 5,10-bis(dialkylthien-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-alt-benzothiadiazole derivative copolymers

Alternating conjugated copolymers based on 5,10-bis(dialkylthien-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) and 2,1,3-benzothiadiazole (BT) or 5,6-difluoro-2,1,3-benzothiadiazole (FBT) with alkylthiopene spacers were synthesized, and the effect of insertion of alkylthiophene spacers and fluorine atoms on the characteristics of the copolymers, such as the energy levels, intrachain π–π interaction, dielectric constants, photovoltaic properties, etc., were systematically investigated. It has been found that: (i) the introduction of alkylthiophene spacers not only led to an increase in the intrachain interaction of the copolymers, but also resulted in an increase in the highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels, and (ii) the inclusion of fluorine atoms resulted in a decrease in both HOMO and LUMO energy levels with enhancement of the planarity and hole mobility. However, the inclusion of fluorine atoms had little effect on the LUMO levels relative to the decrease in the HOMO levels, and almost did not affect the dielectric constant of the copolymers. Use of the materials in polymeric photovoltaic cells led to high performance photovoltaic cells (PVCs) with power conversion efficiencies of 6.04–7.12%. The results demonstrated that the optoelectronic and aggregation properties of the 5,10-bis(alkylthien-2-yl)dithieno-[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-alt-benzothiadiazole derivative copolymers can be effectively regulated by the introduction of alkylthiophene spacers and/or fluorine atoms into the backbone.

A two-dimension medium band gap conjugated polymer based on 5,10-bis(alkylthien-2-yl)dithieno[3,2-d:3′,2′-d′]benzo[1,2-b:4,5-b′]dithiophene: Synthesis and photovoltaic application

ABSTRACT A two-dimension medium band gap copolymer poly{5,10-bis(4,5-didecylthien-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-2,7-diyl-alt-2,5-di(3-octylthien-2-yl) thiophen-5,5′-diyl}, named as PDTBDT-T-3T, was prepared by the palladium-catalyzed Stille cross coupling reaction and characterized. The resulting polymer exhibits good solubility in common organic solvents, excellent thermal stability, and extensive light absorption from 300 nm to 650 nm with an optical band gap of 1.92 eV, the highest occupied molecular orbital (HOMO) level of −5.03 eV and the hole mobility up to 1.92 × 10−4 cm2·V−1·s−1. The power conversion efficiencies (PCEs) of 2.02%–3.19% have been achieved in the traditional PVCs for the copolymer. It should be noted that the PCEs of 4.2% for the inverted PVCs from the copolymer with PFN (poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl- fluorene)]) as cathode modifying interlayer, were similar with the PCEs of 4.39% for the inverted PVCs from P3HT:PC71BM at the same condition. These results indicated that the copolymer could be used as potential candidate for P3HT.

Benzo[1,2-b:4,5-b′]dithiophene-based conjugated polyelectrolyte for the cathode modification of inverted polymer solar cells

ABSTRACT A conjugated polyelectrolyte (CPE) named PBNBr, is prepared by post-quaternizing of poly{4,8-bis(octyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-9,9-bis(3′-((N,N-dimethylamino)propyl)fluorene-2,7-diyl} (PBN) with bromoethane. The chemical strucutes, electrooptical properties of the PBNBr is fully characterized. As compared with the PBN, the PBNBr exhibit much better methanol solution processibility, and more effectively tuning ability for the work function (Wf) of ITO (WF bare ITO, −4.8 eV, WF of ITO with PBN interlayer, −4.1 eV, WF of ITO with PBNBr interlayer −3.9 eV). The open circuit voltages (VOC) and power conversion efficiencies (PCEs) of polymer solar cells from the blend film of poly(3-hexylthiophene) (P3HT) and [6,6]-phenylC61-butyric acid methyl ester (PC61BM) with PBN and/or PBNBr modified ITO as cathode are respectively increased about 27% and 120% in contrast to those for the control devices with bare ITO as cathode. And PCEs of 4.21% and 4.53% are achieved in the PSCs with PBN and/or PBNBr modified ITO as cathode.

Efficiency boost significantly of ternary organic solar cells by doping low bandgap polymer

ABSTRACT Ternary blends composed of two donor absorbers and a complementary absorbing material provide an opportunity to enhance the short-circuit current and thus improved the power conversion efficiency (PCE) of polymer solar cells (PSCs). The used of PBT-T-DPP as the complementary electron donor in poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyricacid methyl ester (PC61BM) blend to construct the ternary PSCs. The PCE of the optimized ternary blend PSCs (with 5 wt% PBT-T-DPP) reached 3.65%, which was 15.51% higher than that of the PSCs with binary blends of P3HT:PC61BM. The ternary blend with 5 wt% PBT-T-DPP exhibited well-developed morphology while the blend with 15 wt% PBT-T-DPP showed phase separation with large-sized domains. Improved photovoltaic performance of ternary PSCs was mainly due to complementary absorption of two donors as well as facilitating charge separation and transport while suppressing charge recombination through a combination of cascade energy levels and optimized device morphology.

Enhanced efficiency of polymer solar cells through synergistic optimization of mobility and tuning donor alloys by adding high-mobility conjugated polymers

A diketopyrrolopyrrole-based small bandgap polymer (DPPT-TT) with high mobility is introduced as an additive to D–A1–D–A2 type thieno[3,4-b]thiophene-based random copolymer (P3):(6,6)-phenyl-C70-butyric acid methyl ester (PC71BM) polymer solar cells (PSCs). The average power conversion efficiencies (PCEs) were improved from 6.15% to 8.30% with the addition of 0.5% DPPT-TT. The photocurrent density versus effective voltage (Jph–Veff) curves, short-circuit current density (JSC) and open circuit voltage (VOC) as functions of incident light intensity, photoluminescence (PL) and time-resolved transient PL (TRTPL) spectra were investigated, and the results certified the effect of DPPT-TT as the third component material in terms of efficient exciton dissociation and weakened charge carrier recombination. The relationship between VOC and the weight ratio of DPPT-TT was explained with density functional theory (DFT) calculations and the electron density of states of unit mass (Ne), indicating the formation of a polymer alloy in ternary blend. With proper addition of DPPT-TT, the mobility of electrons and holes becomes more balanced and the efficiency of exciton utilization is improved due to the existence of Forster resonance energy transfer (FRET), which also contributes to the enhanced JSC and PCEs. Our work demonstrates that appropriate donor polymers forming a polymer alloy in blend is a rational strategy to improve photovoltaic performance.

Wide bandgap conjugated polymers based on bithiophene and benzotriazole for bulk heterojunction solar cells: Thiophene versus thieno[3,2-b]thiophene as π-conjugated spacers

ABSTRACT Two wide bandgap (WBG) conjugated polymers, P2T-DTTTAZ and P2T-DTTAZ, with donor-π-acceptor (D-π-A) structures was designed and synthesized, utilizing thieno[3,2-b]thiophene (TT) and/or thiophene (T) units as π-bridge in conjugated polymer backbone. And, the wider optical band gap (Eg) of approximately 1.98 eV for P2T-DTTTAZ and 2.09 eV for P2T-DTTAZ were observed. Obviously, replacing T unit with larger conjugated plane TT unit as π-bridges, P2T-DTTTAZ resulted in the red shifted absorption and the reduced band gap, compared with these of P2T-DTTAZ. The polymer solar cells (PSCs) with an inverted device structure based on P2T-DTTTAZ or P2T-DTTAZ as donor and [6,6] phenyl-C61 butyric acid methyl ester (PC61BM) as acceptor were assembled and the photovoltaic properties were also investigated. The power conversion efficiencies (PCEs) of 1.57% for P2T-DTTTAZ and 1.25% for P2T-DTTAZ were obtained.