Xichang Bao

High-Performance Photovoltaic Polymers Employing Symmetry-Breaking Building Blocks.

Two 1D-2D asymmetric benzodithiophenes (BDTs) as donor building blocks are designed and synthesized, combining the advantages of both 1D and 2D symmetric BDTs. The photovoltaic properties of the asymmetric BDT-based polymers are improved greatly in comparison with corresponding symmetric BDT-based polymers. This work provides a new approach to design prospective organic optoelectronic materials employing the symmetry-breaking strategy.

Simple planar perovskite solar cells with a dopant-free benzodithiophene conjugated polymer as hole transporting material

Dopant-free poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b] thiophene)-2,6-diyl] (PBDTTT-C) polymer is used as hole transporting material (HTM) in electron transporting material (ETM) free planar perovskite solar cells (PSCs). The devices with a PBDTTT-C HTM show higher power conversion efficiency (PCE = 9.95%) than the devices with a P3HT HTM (PCE = 6.17%) with enhanced short circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF) in a simple device configuration (ITO/CH3NH3PbI3/PBDTTT-C/MoO3/Ag), due to the suitable energy level, better carrier mobility and lower interfacial charge recombination.

Compact Layer Free Perovskite Solar Cells with a High-Mobility Hole-Transporting Layer.

A high-mobility diketopyrrolopyrrole-based copolymer (P) was employed in compact layer free CH3NH3PbI3 perovskite solar cells as a hole-transporting layer (HTL). By using the P-HTL, the 6.62% device efficiency with conventional poly-3-hexylthiophene was increased to 10.80% in the simple device configuration (ITO/CH3NH3PbI3/HTL/MoO3/Ag). With improved short circuit current density, open circuit voltage, and fill factor, the higher power conversion efficiency of P-HTL device is ascribed to the higher carrier mobility, more suitable energy level, and lower interfacial charge recombination. Advantages of applying P-HTL to perovskite solar cells, such as low cost, low-temperature processing, and excellent performance with simple cell structure, exhibit a possibility for commercial applications.

Significantly improved photovoltaic performance of the triangular-spiral TPA(DPP–PN)3 by appending planar phenanthrene units into the molecular terminals

A novel triangular-spiral conjugation molecule of TPA(DPP–PN)3 using triphenylamine (TPA) as the donor core, diketopyrrolopyrrole (DPP) as the acceptor arm and phenanthrene (PN) as the planar arene terminal, as well as its counterpart of TPA–3DPP without the PN terminal, were prepared. Their UV-vis absorption, electrochemistry and thermal stability, as well as hole mobility were investigated. Significantly red-shifted UV-vis absorption profiles were observed for TPA(DPP–PN)3 instead of TPA–3DPP in solution and solid state. A hole mobility of 1.67 × 10−4 cm2 V−1 s−1 was obtained for the TPA(DPP–PN)3/PC71BM blended film, which is 2.1 times higher than that of the TPA–3DPP/PC71BM blended film. Furthermore, the TPA(DPP–PN)3/PC71BM-based organic solar cells presented better photovoltaic property with a maximum power conversion efficiency of 3.67%, which is 1.9 times higher than that of TPA–3DPP/PC71BM-based devices. The results confirm that appending planar PN terminals to TPA–3DPP with a triangular-spiral shape is an efficient approach to improve the photovoltaic performance of its resulting molecules.

Hyperconjugated side chained benzodithiophene and 4,7-di-2-thienyl-2,1,3- benzothiadiazole based polymer for solar cells

A novel donor–acceptor (D–A) copolymer (P3TBDTDTBT), including hyperconjugated side chained benzodithiophene as a donor and 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT) as an acceptor, was designed and synthesized. Due to the introduction of the hyperconjugated side chain, the resultant polymer exhibited good thermal stability with a high decomposition temperature of 437 °C, a low band-gap of 1.67 eV with an absorption onset of 742 nm in the solid film, and a deep highest occupied molecular orbital (HOMO) energy level of −5.26 eV. Finally, the polymer solar cell (PSC) device based on this polymer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) showed the best power conversion efficiency (PCE) of 3.57% with an open-circuit voltage (Voc) of 0.78 V, a short-circuit current density (Jsc) of 8.83 mA cm−2 and a fill factor (FF) of 53%.

High efficiency solution-processed two-dimensional small molecule organic solar cells obtained via low-temperature thermal annealing

A new two-dimensional (2D) organic small molecule, DCA3T(T-BDT), was designed and synthesized for solution-processed organic solar cells (OSCs). DCA3T(T-BDT) exhibited a deep HOMO energy level (−5.37 eV) and good thermal stability. The morphologies of the DCA3T(T-BDT):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) blends were investigated by atomic force microscopy and the crystallinity was explored by X-ray diffraction (XRD) and 2D grazing incidence wide-angle X-ray scattering (GIWAXS), respectively. The morphologies of the blends were strongly influenced by the blend ratio of DCA3T(T-BDT):PC61BM and annealing temperature. The effect of thermal annealing on the photovoltaic performance of DCA3T(T-BDT)-based small molecule organic solar cells (SMOSCs) was studied in detail. When DCA3T(T-BDT) was used as a donor with PC61BM as an acceptor, high efficiency SMOSCs with a power conversion efficiency of 7.93%, a high Voc of 0.95 V, Jsc of 11.86 mA cm−2 and FF of 0.70 were obtained by a thermal annealing process at only 60 °C, which offers obvious advantages for large scale production compared with solvent additive or interfacial modification treatment.

WO3 with surface oxygen vacancies as an anode buffer layer for high performance polymer solar cells

The exploration of inexpensive and efficient anode buffer layers is essential in large scale commercial applications of polymer solar cells (PSCs). Here, we report a simple way that can significantly enhance the power conversion efficiency (PCE) and extend the lifetime of PSCs. A solution-based tungsten oxide (WO3) layer with surface oxygen vacancies (VOs) is introduced as an efficient anode buffer layer between the active layer and indium tin oxide (ITO) glass. The PCEs of PSCs based on P3HT:PC61BM and PBDTTT–C:PC71BM active layers are improved by 24% (from 3.84% to 4.76%) and 27% (from 5.91% to 7.50%) with the introduction of the WO3 (VO) anode buffer layer, respectively, compared to that of the conventional PEDOT:PSS layer. The excellent performance is ascribed to the greatly improved fill factor and enhanced short circuit current density of the devices, which are benefited from the surface with lots of VOs for better interfacial contact and excellent charge transport properties of the WO3 (VO) layer. The impressive PCE, good stability, easy fabrication and compatibility with solution processed organic photovoltaic devices support this materials potential applications in PSCs for both wide bandgap and narrow bandgap polymers.

Highly Efficient Inverted Perovskite Solar Cells With Sulfonated Lignin Doped PEDOT as Hole Extract Layer

UNLABELLED Sulfonated-acetone-formaldehyde (SAF) was grafted with alkali lignin (AL) to prepare grafted sulfonated-acetone-formaldehyde lignin (GSL). Considering the rich phenolic hydroxyl groups in GSL, we detected a hole mobility of 2.27 × 10(-6) cm(2) V(-1) s(-1) with GSL as a hole transport material by space-charge-limited current model. Compared with nonconjugated poly(styrene sulfonic acid), GSL was applied as p-type semiconductive dopant for PEDOT to prepare water-dispersed PEDOT GSL. PEDOT GSL shows enhanced conductivity compared with that of PEDOT PSS. Simultaneously, the enhanced open-circuit voltage, short-circuit current density, and fill factor are achieved using PEDOT GSL as a hole extract layer (HEL) in sandwich-structure inverted perovskite solar cells. The power conversion efficiency is increased to 14.94% compared with 12.6% of PEDOT PSS-based devices. Our results show that amorphous GSL is a good candidate as dopant of PEDOT, and we provide a novel prospective for the design of HEL based on lignin, a renewable biomass and phenol derivatives.

Enhanced efficiency of polymer solar cells by incorporated Ag–SiO2 core–shell nanoparticles in the active layer

In this article, we creatively incorporated Ag–SiO2 core–shell nanoparticles (Ag–SiO2-NPs) into photo-/electro-active layers consisting of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) in polymer solar cells (PSCs). By this way, the photovoltaic performances of PSCs have largely been enhanced. The results demonstrate a 13.50% enhancement of short-circuit photocurrent density (Jsc) and a 15.11% enhancement of power conversion efficiency (PCE) as the weight percent of doped Ag–SiO2-NPs is 1.5 wt% in the active layer of corresponding PSCs. We attribute the enhancement to the localized surface plasmon resonance (LSPR) effect of Ag–SiO2-NPs, by which the incident light harvesting is enlarged. Whereas, the incorporated bare Ag nanoparticles (Ag-NPs) in the active layer of PSCs decreases the PCE, which is ascribed to the quenching of excitons at the surface of Ag-NPs and the poor dispersion of Ag-NPs in the active layer. Importantly, this work provides a new approach to enhance the performance of PSCs via the LSPR effect of Ag–SiO2-NPs other than via non-circular nanometals.

Facile preparation of TiOX film as an interface material for efficient inverted polymer solar cells

Titanium oxide (TiOX) is an effective electron transport layer in polymer solar cells (PSCs). Here, we report an efficient inverted polymer solar cell based on P3HT and fullerenes using a high density, single-step solution processed amorphous TiOX (α-TiOX) film as an electron transport layer. The α-TiOX film was prepared by spin coating tetrabutyl titanate (TBT) isopropanol solution onto ITO coated glass in a glovebox filled with N2 and then annealing at different temperatures in air. The films with high light transmittance are very smooth. The PSCs with the α-TiOX electron transport layer showed enhanced photovoltaic performance in comparison with the device using PEDOT:PSS as the anode buffer layer. The optimized power conversion efficiency (PCE) of the PSCs based on P3HT/PC61BM and P3HT/PC71BM with the α-TiOX electron transport layer reached 4.25% and 4.65%, respectively, under AM1.5G illumination (100 mW cm−2). In addition, the PSCs with the α-TiOX electron transport layer exhibited good stability. The results indicate that facile preparation of α-TiOX films using cheap TBT is promising for high-efficiency PSCs and large-scale fabrication of flexible electronics.