Weichao Chen

Preparation and photoluminescence of highly ordered TiO2 nanowire arrays

Highly ordered TiO2 nanowire (TN) arrays were prepared in anodic alumina membranes (AAMs) by a sol-gel method. The TNs are single crystalline anatase phase with uniform diameters around 60 nm. At room temperature, photoluminescence (PL) measurements of the TN arrays show a visible broadband with three peaks, which are located at about 425, 465, and 525 nm that are attributed to self-trapped excitons, F, and F+ centers, respectively. A model is also presented to explain the PL intensity drop-down of the TN arrays embedded in AAMs: the blue PL band of AAMs arises from the F+ centers on the pore walls, and the TNs first form in the center area of the pores and then extend to the pore walls.

Fluorine substitution enhanced photovoltaic performance of a D–A1–D–A2 copolymer

A new alternating donor-acceptor (D-A1-D-A2) copolymer containing two electron-deficient moieties, isoindigo and quinoxaline, was synthesized. The photovoltaic performance of this polymer could be improved by incorporating fluorine atoms into the quinoxaline units, resulting in an efficiency of 6.32%. This result highlights the attractive promise of D-A1-D-A2 copolymers for high-performance bulk heterojunction solar cells.

Getting high-efficiency photoluminescence from Si nanocrystals in SiO2 matrix

Silicon nanocrystals in SiO2 matrix are fabricated by plasma enhanced chemical vapor deposition followed by thermal annealing. The structure and photoluminescence (PL) of the resulting films is investigated as a function of deposition temperature. Drastic improvement of PL efficiency up to 12% is achieved when the deposition temperature is reduced from 250 °C to room temperature. Low-temperature deposition is found to result in a high quality final structure of the films in which the silicon nanocrystals are nearly strain-free, and the Si/SiO2 interface sharp. The demonstration of the superior structural and optical properties of the films represents an important step towards the development of silicon-based light emitters.

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.

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.

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.

Novel donor–acceptor polymers containing o-fluoro-p-alkoxyphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene units for polymer solar cells with power conversion efficiency exceeding 9%

In this work, a new electron-rich building block, o-fluoro-p-alkoxyphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) unit, has been used to construct donor (D)–acceptor (A) conjugated copolymers with electron-deficient units 5,6-difluoro-4,7-di(4-(2-ethylhexyl)-2-thienyl)-2,1,3-benzothiadiazole (C8DTBTff) and 5,6-difluoro-4,7-di(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (C6DTBTff), named P-o-FBDTP-C8DTBTff (P2) and P-o-FBDTP-C6DTBTff (P3), respectively. The experimental results indicate that the incorporation of fluorine into the ortho-position of the alkoxyphenyl substituted BDT unit can enable its resultant polymer to efficiently tune the energy levels and improve the mobility of the derived bulk heterojunction layer, which results in a much higher power conversion efficiency (PCE) of P2 (8.10%). Moreover, replacing the 2-ethylhexyl chains on the DTBTff unit with hexyl chains can improve the planarity of the conjugated backbone of the polymer, which makes the P3/PC71BM blends exhibit higher carrier mobility than P2/PC71BM. Finally, a PCE of 9.02% for the device of P3 is obtained without any additive treatment, which is the highest value achieved for the widely reported D–A polymers with fluorine substituted BDT as the electron-donor unit in single junction polymer solar cells.

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.

Simple O2 Plasma-Processed V2O5 as an Anode Buffer Layer for High-Performance Polymer Solar Cells

UNLABELLED A simple O2 plasma processing method for preparation of a vanadium oxide (V2O5) anode buffer layer on indium tin oxide (ITO)-coated glass for polymer solar cells (PSCs) is reported. The V2O5 layer with high transmittance and good electrical and interfacial properties was prepared by spin coating a vanadium(V) triisopropoxide oxide alcohol solution on ITO and then O2 plasma treatment for 10 min [V2O5 (O2 plasma)]. PSCs based on P3HT:PC61BM and PBDTTT-C:PC71BM using V2O5 (O2 plasma) as an anode buffer layer show high power conversion efficiencies (PCEs) of 4.47 and 7.54%, respectively, under the illumination of AM 1.5G (100 mW/cm(2)). Compared to that of the control device with PBDTTT-C:PC71BM as the active layer and PEDOT PSS (PCE of 6.52%) and thermally annealed V2O5 (PCE of 6.27%) as the anode buffer layer, the PCE was improved by 15.6 and 20.2%, respectively, after the introduction of a V2O5 (O2 plasma) anode buffer layer. The improved PCE is ascribed to the greatly improved fill factor and enhanced short-circuit current density of the devices, which benefited from the change in the work function of V2O5, a surface with many dangling bonds for better interfacial contact, and the excellent charge transport property of the V2O5 (O2 plasma) layer. The results indicate that an O2 plasma-processed V2O5 film is an efficient and economical anode buffer layer for high-performance PSCs. It also provides an attractive choice for low-cost fabrication of organic electronics.

Efficient polymer solar cells based on a new benzo[1,2-b:4,5-b ']dithiophene derivative with fluorinated alkoxyphenyl side chain

A novel fluorine-containing benzo[1,2-b:4,5-b′]dithiophene (BDT) derivative (BDTPF) was designed to construct a donor–acceptor (D–A)-structured polymer (PBDTPF-DTBT) with the electron-withdrawing unit 4,7-di(4-(2-ethylhexyl)-2-thienyl)-2,1,3-benzothiadiazole (DTBT). The resulting polymer exhibits a broad absorption spectrum, relatively low lying HOMO energy level (−5.39 eV) and a good film-forming ability. The field-effect mobility of PBDTPF-DTBT is 0.034 cm2 V−1 s−1. Bulk heterojunction organic solar cells (OSCs) based on PBDTPF-DTBT and PC71BM were prepared and showed a good photovoltaic performance with power conversion efficiency (PCE) of 7.02%. This work demonstrates that a BDT unit with fluorinated alkoxyphenyl side chains is a promising candidate as an electron-rich building block for high performance solution-processed OSCs.