The narrow indirect bandgap of Si hinders the full absorption and utilization of the solar light. Energy band engineering to the Si based photovoltaic devices is essential to address this problem. Here we report the fabrication of ZnSe nanoribbon (ZnSeNR)/Si nanowire (SiNW) p–n heterojunction arrays by directly growing or simply drop-casting the p-type ZnSeNRs on highly aligned n-type SiNW arrays. Phosphorus (P) was used as dopant to achieve robust p-type doping in the ZnSe nanostructures with a hole concentration as high as 6.5 × 1018 cm−3. Thanks to the matched bandgap and the light trapping arising from the one-dimensional (1D) array structure, the heterojunction arrays exhibited improved light absorption, particularly in the blue/UV wavelength range. Moreover, by taking advantage of the ohmic contact between graphene and p-ZnSeNR, heterojunction array solar cells with graphene transparent electrode were fabricated, giving rise to a power conversion efficiency of ∼0.78% under AM 1.5G illumination. It was found that the surface passivation to the SiNW array played an important role in determining the device performance; an efficiency up to 2.27% was obtained after surface modification with methyl groups. Our results demonstrate the heterojunction arrays, together with the graphene transparent electrodes, could be promising candidates for high-performance and low-cost photovoltaic applications.
The preparation of high specific-capacity pseudocapacitive transition-metal-oxide/hydroxide (TMH) materials into transparent micro-structure electrodes is a great challenge and has become the bottleneck restricting the performance of transparent flexible supercapacitors . Beyond that, extending the operating voltage window is an urgent need for improving energy density . Here, Co(OH) 2 nanosheets and amorphous FeOOH nanowires were prepared into transparent films through a simple and scalable bio-inspired method at the air-solution interface. Both nanostructures are enwrapped in graphene shells (Co@Gr-TFandFe@Gr-TF) for use as transparent asymmetric pseudocapacitive electrodes. The unique encapsulation structures (1) provide pseudocapacitive electrode materials, especially an amorphous FeOOH structure cathode, (2) build up rapid three-dimensional electron and ion transport pathways together with the underlying ITO layer, (3) alleviate the exfoliation and dissolution of the active materials into electrolyte, and (4) extend the potential window of the FeOOH cathode from −0.8 ∼ 0 V to −1.2 ∼ 0 V with minimal cycling stability degradation. The asymmetric transparent and flexible supercapacitor based on Co@Gr-TF//Fe@Gr-TF operated at 1.8 V exhibits good transparency (50.5% at 550 nm), high specific capacitance (25.5 mF cm −2 , one order higher than the maximum value reported for FFT-GP), and high energy density (1.04 mWh cm −3 , comparable to that of nontransparent devices) as well as a long cycle life (10 000 cycles) with high-capacity retention (83.5%).
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