Wanzhu Cai

High-Detectivity Polymer Photodetectors with Spectral Response from 300 nm to 1450 nm

Polymer Photodetectors Optical sensing is used in a wide range of applications, such as low-light detection systems in cars and cameras. Most photodetectors have a limited spectral range and can only detect a narrow range of wavelengths. Gong et al. (p. 1665, published online 13 August) developed polymer photodetectors with extremely broad spectral response and exceptionally high sensitivity that can exceed the response of an inorganic semiconductor detector at liquid helium temperature. A key aspect in the device design is the inclusion of blocking layers to reduce significantly the dark current or noise in the devices. Well-designed polymer photodetectors show performance comparable with the best inorganic devices. Sensing from the ultraviolet-visible to the infrared is critical for a variety of industrial and scientific applications. Today, gallium nitride–, silicon-, and indium gallium arsenide–-based detectors are used for different sub-bands within the ultraviolet to near-infrared wavelength range. We demonstrate polymer photodetectors with broad spectral response (300 to 1450 nanometers) fabricated by using a small-band-gap semiconducting polymer blended with a fullerene derivative. Operating at room temperature, the polymer photodetectors exhibit detectivities greater than 1012 cm Hz1/2/W and a linear dynamic range over 100 decibels. The self-assembled nanomorphology and device architecture result in high photodetectivity over this wide spectral range and reduce the dark current (and noise) to values well below dark currents obtained in narrow-band photodetectors made with inorganic semiconductors.

Synthesis of donor–acceptor copolymers based on anthracene derivatives for polymer solar cells

A series of narrow band gap donor–acceptor type conjugated copolymers based on 2,6-linked anthracene derivatives are synthesized via Suzuki copolymerization. The resulting copolymers typically exhibit dual absorption characteristics in both solution and as thin films with optical band gaps in the range of 1.85–2.13 eV. By varying the substitutions from alkyloxy to aromatic thienyl and phenyl groups in the 9,10-positions of the anthracene unit, the constructed two-dimensional 2,6-linked anthracene structures lead to broader absorption, lower-lying highest occupied molecular orbitals, as well as improved charge carrier mobilities of the resulting copolymers relative to the alkyloxy side chain substituted counterparts. Additionally, it was found that the sizes of the substitutions in the benzo[c][1,2,5]thiadiazole acceptor also play an important role in the optoelectronic properties of these anthracene based conjugated copolymers. The best polymer solar cell device with a power conversion efficiency of 4.34% and a high open circuit voltage of 0.98 V was realized based on the resulting materials. Our results indicate that substantial optimization in the sizes and patterns of substitutions of both the 2,6-linked anthracene donor and benzo[c][1,2,5]thiadiazole acceptor may potentially lead to high performance narrow band gap copolymers for solar cell applications.

The influence of amino group on PCDTBT-based and P3HT-based polymer solar cells: Hole trapping processes

Polymer solar cells (PSCs) based on aliphatic-amino-functionalized materials presented low performance with negligibly small efficiency, the prime mechanism of which is found to be hole trapping induced by the amine end groups. We propose that such hole trapping behavior depends on the relative energetic position of the hole transport states and the trapping states. Herein, we comparatively study the photovoltaic properties of PSCs based on amino-functionalized fullerene derivative blended with poly [N-9′-heptadecanyl-2, 7-carbazole-alt-5, 5-(4′, 7′-di-2-thienyl-2′, 1′, 3′-benzothiadiazole)] (PCDTBT) or poly (3-hexylthiophene) (P3HT). The former polymer has a lower-positioning highest occupied molecular orbital (HOMO) level, whereas the latter has a comparable HOMO level relative to the ionization state of tertiary aliphatic amine in energy. Our investigation confirms our proposition, revealing an ultrafast trapping process in PCDTBT:amino-group-functionalized fullerene derivative film, which seriously crippled hole transport, consequently results in very poor device performance. In contrast, trapping process is almost negligible in P3HT systems.

Synthesis, Characterization and Photovoltaic Properties of Polycarbazole Derived Random Copolymers With Enhanced Light Absorption

Through the introduction of 2,5-bis(3-octylthiophen-2-yl)thiazolo[5,4-d]thiazole (DTTz) and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) units with different light-absorption capabilities into the polycarbazole main chains, five random copolymers PCzTz, PCzTz3BT1, PCzTz1BT1, PCzTz1BT3 and PCzBT were synthesized by Suzuki polycondensation. Their thermal, optical, electrochemical and photovoltaic properties were characterized detailedly. The band gaps and absorption performance can be tuned feasibly via adjusting the molar ratio of DTTz to DTBT. When the molar ratio of DTTz to DTBT was 1:3 in PCzTz1BT3, it displayed the lowest optical band gap of 1.78 eV within the enhanced and broadened light absorption comparing with other polymers. The photovoltaic devices were fabricated under the device structure of ITO/PEDOT:PSS/polymer:PC61BM/Ca/Al, where the PCzTz1BT3 based polymer solar cell gave the highest photovoltaic performance with the short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) of 6.27 mA cm−2, 0.85 V, 61% and 3.45%, respectively.