Substantial broadband photoconductive gain has been realized for organic, thin-film photodetectors with a poly(3-hexylthiophene):phenyl-C61-butyric-acid-methyl-ester (P3HT:PCBM) active layer at low bias voltages. External quantum efficiencies upwards of 1500% were achieved when a semicontinuous gold layer was introduced at the anode interface. Significant gain was also observed in the sub-band gap, near infrared region where the external quantum efficiency approached 100% despite the lack of a sensitizer. The gain response was highly dependent on the thickness of the active layer of the photodetector with the best results achieved with the thinnest devices. The gain is the result of the injection of secondary electrons due to hole charge trapping at the semicontinuous gold layer.
The need for extreme-duration light-weight power sources for space applications motivates the study and development of polymer-based betavoltaics. The betavoltaic device, based on the semiconductive polymer-fullerene blend of poly(3-hexylthiophene): indene-C 60 bisadduct (P3HT:ICBA), is demonstrated here for the first time. Both direct and indirect energy conversion methods were explored. For the indirect conversion method, a phosphor intermediate layer of ceriumdoped yttrium aluminum garnet (Ce:YAG) was used on top of the polymer device. A high open circuit voltage of 0.56 V has been achieved in the betavoltaic device fabricated on a polyethylene terephthalate (PET) substrate with indirect energy conversion at 30-keV electron kinetic energy. The maximum output electrical power of 62 nW was achieved at 30-keV input electron beam (e-beam) energy. The highest betavoltaic power conversion efficiency of 0.78% was achieved at an e-beam energy of 10 keV.Using the thin PET substrate instead of a glass substrate for the polymer device and phosphor screen fabrication, the betavoltaic device performance has been significantly improved due to a reduction in physical distance between photon-generating Ce:YAG phosphor screen and photon-absorbing P3HT:ICBA layer. The use of the PET substrates helped by significantly decreasing the directional and external interaction losses. IndexTerms-Beta rays, betavoltaic, degradation, electron beam (e-beam), optical polymers, phosphors, poly(3-hexylthiophene): indene-C 60 bisadduct (P3HT:ICBA), scintillator.
The ongoing advanced space exploration requires the novel energy sources that can generate power for extreme duration without need of refill. The long duration betavoltaic devices are presented using conjugated polymer with scintillators. The Monte Carlo simulations are used to study the interaction of electron beam with two different scintillators, Cerium doped Yttrium Aluminum Garnet (Ce:YAG) and Thallium doped Cesium Iodide (CsI:Tl). The catholuminescence profiles from simulation showed that CsI:Tl is more-efficient to generate photons when hit by electron beam compared to Ce:YAG. The semiconductive conjugated polymer device stack of ITO/PEDOT:PSS/P3HT:ICBA/Al are then fabricated and tested with Ce:YAG and CsI:Tl scintillators under different electron beam energies. The electrical current is successfully extracted from these betavoltaic devices when illuminated with electron beams. As expected, the betavoltaic devices with CsI:Tl scintillator performed better compared with Ce:YAG. The maximum power conversion efficiency (PCE) of 0.24% is obtained at 10 kV electron beam with CsI:Tl, while PCE in device with Ce:YAG is 0.16%. The short circuit current in devices with CsI:Tl is about 57%, greater than in devices with Ce:YAG. The experimental result showed that output electrical power increased with increase in incident electron beam energy.
P3HT:PCBM based photovoltaic devices with different active layer thicknesses (ALTs) were examined in photodetector and solar cell operation modes. The photodetector photocurrent spectra and solar cell current density-voltage characteristics were measured. All experimental results were reproduced by the unique drift-diffusion model which excludes the optical interference and allows the parameters of photogeneration, transport, and recombination to be ALT dependent. The active layer optical characterization indicated a thickness dependence of optical parameters too. A conclusion was drawn that the P3HT:PCBM film thickness and morphology are strongly correlated which leads to a non-monotonic change of film parameters with its thickness.
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