Minghong Tong

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.

Strain and Hückel Aromaticity: Driving Forces for a Promising New Generation of Electron Acceptors in Organic Electronics

The tangible possibility of fabricating flexible, lightweight organic photovoltaic devices (OPVs) by using roll-to-roll coaters, similar to those used in the production of print magazines and newspapers, renders this technology a valid alternative to expensive crystalline silicon photovoltaic cells. The most widely used active layer for these OPVs, the so-called bulk heterojunction (BHJ), 5] is based on photoinduced charge transfer from an electron-donating material, such as a light-absorbing and hole-conducting polymer, to an electron-accepting component, typically fullerene[60] and its derivative 1-(3-methoxycarbonyl) propyl-1phenyl-[6,6]-C61 ([C60]PCBM). [6, 7] Several research groups have reported a wide range of new polymeric donor structures that absorb light over a broad wavelength range, and have a narrow energy gap and increased charge transport and collection at the electrode. However, there have been fewer reports on new structures of acceptor components that do not contain fullerene derivatives. C60 and C70 PCBMs are currently considered the most successful acceptor architectures, despite only slight improvements when modifying these functionalized fullerenes. For example, the insertion of electron-donating groups on the phenyl ring of the [C60]PCBM to tune the lowest unoccupied molecular orbital (LUMO) energy levels improved the open-circuit voltage (Voc), while maintaining a relatively unchanged efficiency. Furthermore, [C70]PCBM, which absorbs a wider range of wavelengths than [C60]PCBM, [16] was employed with low-band-gap polymers such as poly[2,6-(4,4-bis-(2-ethylhexyl-4H-cyclopenta[2,1b;3,4b’]-dithiophene)-alt-4,7-(2,1,3-benzothiadizole)] (PCPDTBT), to broaden the photocurrent spectral range. Although encouraging photocurrent and photovoltage values were obtained, a low overall efficiency, which arises from loss mechanisms, was observed. Despite the wide use of these fullerene derivatives, the synthesis of new acceptors with energy levels significantly different from those of current C60 derivatives, and wide versatility in terms of derivatization and functionalization is urgently required. Herein, we report the inherent potential of a new generation of acceptor compounds based on the 9,9’-bifluorenylidene (99’BF) backbone. 99’BF could be considered a tetrabenzofulvalene with an atom numbering that reflects fluorene linked by a double bond between the 9 and 9’ carbon atoms. In the ground state, 99’BF is forced to be coplanar because of the presence of the double bond, but the repulsive interaction between the H1– H1’ and H8–H8’ protons twists the structure of the dimer. The addition of one electron across the C9–C9’ bond is highly favorable for two main reasons: steric (“twist”) strain relief and gain in aromaticity to a 14-p-electron system (Scheme 1).

Bulk Heterojunction Solar Cells with Large Open-Circuit Voltage: Electron Transfer with Small Donor-Acceptor Energy Offset

e 99BF 99BF(.) Power conversion effi ciencies (PCEs) (in response to solar AM1.5 radiation) as high as 6–8% have been reported for bulk heterojunction (BHJ) polymer solar cells. [ 1 , 2 ] In order to attain PCEs over 10%, BHJ materials capable of generating larger open circuit voltage (V oc ) are required. [ 3 , 4 ] One approach to increase V oc is to develop low-bandgap semiconducting polymers with deeper highest occupied molecular orbital (HOMO) energies. An alternative approach is to develop new electron acceptors with higher lowest unoccupied molecular orbital (LUMO) energies. The pathway to low-bandgap semiconducting polymers with deeper HOMOs is well understood, and BHJ solar cells fabricated by semiconducting polymers with deeper HOMOs have successfully exhibited larger V oc . [ 5 ] However, the discovery of new (non-fullerene) electron acceptors with higher LUMOs remains undeveloped. [ 6 ]

A Porphyrin–Fullerene Dyad with a Supramolecular “Double‐Cable” Structure as a Novel Electron Acceptor for Bulk Heterojunction Polymer Solar Cells

Bulk heterojunction (BHJ) polymer solar cells (PSCs) offer a promising, low-cost, large-area, fl exible, light-weight, clean, and quiet alternative energy source for both indoor and outdoor applications. [ 1–4 ] Power conversion effi ciencies (PCEs) in response to solar AM1.5 radiation as high as 6–8% have been reported for BHJ PSCs. [ 5 , 6 ] In order to achieve PCEs over 10%, BHJ materials capable of generating higher short circuit current ( J sc ) and larger open circuit voltage ( V oc ) are required. [ 7 , 8 ]

Semiconducting Polymer Photodetectors with Electron and Hole Blocking Layers: High Detectivity in the Near-Infrared

Sensing from the ultraviolet-visible to the infrared is critical for a variety of industrial and scientific applications. Photodetectors with broad spectral response, from 300 nm to 1,100 nm, were fabricated using a narrow-band gap semiconducting polymer blended with a fullerene derivative. By using both an electron-blocking layer and a hole-blocking layer, the polymer photodetectors, operating at room temperature, exhibited calculated detectivities greater than 1013 cm Hz1/2/W over entire spectral range with linear dynamic range approximately 130 dB. The performance is comparable to or even better than Si photodetectors.

Extension of the spectral responsivity of the photocurrent in solution-processed small molecule composite via a charge transfer excitation

Incorporating [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in solution-processed composites comprising the two small molecular semiconductors 9,10-Diphenylanthracene (DPA) and 5,6,11,12-tetraphenylnaphthacene (Rubrene) extends significantly the onset wavelength of the steady-state photoconductivity from 610 nm in the pristine DPA:Rubrene composite to 900 nm in the DPA:Rubrene:PCBM composite. The experimental data indicate carrier generation in the near IR spectral region arising from inter-molecular charge transfer (IMCT) excitation that potentially could be useful for extending solar radiation light harvesting. Pump/probe photoinduced absorption (PIA) measurements indicate instantaneous carrier generation at sub-band-gap photon energies, confirming the viability of IMCT excitations as the underlying carrier generation mechanism at the near IR spectral region.

Ultrasensitive solution processed polymer photodetectors

We demonstrate a polymer photodetector with spectral response from 300nm to 1450nm by using a narrow-band-gap semiconducting polymer blended with a fullerene derivative. Operating in room temperature, the polymer photodetectors exhibit detectivity greater than 1013Jones (1Jones =1cm Hz1/2/W) from the UV well into the near-infrared out to 1150nm and greater than 1012Jones from 1150nm to 1450nm. The linear dynamic range is over 100dB. To our knowledge, there is no inorganic material system (not even Si-Ge alloys) capable of such high performance photodetectivty over such a wide spectral range.