One‐dimensional nanostructures of niobium‐doped anatase TiO2 (Nb:TiO2) up to 5 at.% Nb were synthesized by electrospinning a polymeric solution containing titanium and niobium precursors and subsequent annealing. Thus obtained fibers had diameter ∼150 nm. The undoped TiO2 fibers were constituted by larger single crystalline grains of size ∼50 nm, whereas the doped ones had decreased grain sizes (∼30 nm) under similar processing conditions. The Nb doping decreased the BET surface area of TiO2. A strain‐induced lattice contraction was observed in Nb:TiO2. The continuous nanofibers were shortened to nanowires (NW) of aspect ratio 10:1 by ultrasonically dispersing them in acetic acid, which were developed as films of thickness ∼8–13 μm onto conducting glass substrates. The TiO2 and Nb:TiO2 nanowire films were further sensitized by a dye; the amount of dye anchored was found to decrease with increase in the dopant concentration. The dye‐sensitized solar cells fabricated using the doped fibers, although with a nominally increased current density (JSC), have reduced efficiency due to lower fill factor and open circuit voltage (VOC). The electron diffusion coefficient (Dn) and mobility (μn) of the TiO2 and Nb:TiO2 NW in the presence of iodide/triiodide ions were an order of magnitude higher compared with the undoped samples.
Inverted bulk heterojunction organic solar cells having device structure ITO/ZnO/poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) /MoO3/Ag were fabricated with high photoelectric conversion efficiency and stability. Three types of devices were developed with varying electron transporting layer (ETL) ZnO architecture. The ETL in the first type was a sol-gel-derived particulate film of ZnO, which in the second and third type contained additional ZnO nanowires of varying concentrations. The length of the ZnO nanowires, which were developed by the electrospinning technique, extended up to the bulk of the photoactive layer in the device. The devices those employed a higher loading of ZnO nanowires showed 20% higher photoelectric conversion efficiency (PCE), which mainly resulted from an enhancement in its fill factor (FF). Charge transport characteristic of the device were studied by transient photovoltage decay and charge extraction by linearly increasing voltage techniques. Results show that higher PCE and FF in the devices employed ZnO nanowire plantations resulted from improved charge collection efficiency and reduced recombination rate.
A new series of dithieno[3,2-b:2′,3′-d]pyrrole-incorporated oligomers was synthesized and characterized. The crystal structure, crystal packing, optical properties, electrochemical properties, and time-of-flight mobilities were investigated in detail. The oligomers are highly fluorescent in both solution and the solid state. The solution-state quantum yield of these new compounds ranged from 52 to 75%. Band gaps of these oligomers were found to be in the range of 2.5-2.8 eV. The surface morphology of the film was also characterized by atomic force microscopy. The material was found to be hole-transporting with a mobility on the order of 10 -6 cm 2 /(V s).
Charge carrier life-time in an inverted solar cell is studied as a function of differing thickness of biopolymer-based electron selective layer (ESL) using transient photovoltage (TPV) spectroscopy. Longer carrier lifetimes were obtained for devices with a thicker ESL layer, indicating better device performance. However, thick ESL devices showed lower charge mobility and poorer solar cell performance from double injection current (DI) spectroscopy and current density-voltage (J-V) measurements respectively as compared to thin ESL devices. The origin of this discrepancy is found to be due to enhanced charge carrier trapping in the thick ESL layer as supported by capacitance-voltage (C-V) measurements.Polymer solar cells (PSCs) have attracted much research attention due to its potential as a flexible and low cost alternative to silicon-based solar cell technology. 1-3 Efficiencies of up to 9.2% have been achieved 4 and the next step would be to improve device life-time and stability. Much effort has been placed in developing new photoactive materials and device architectures to improve solar cell performance. 5 The normal device architecture is found to suffer from degradation issues due to poor ambient stability of aluminum (Al) top electrode and the acidic nature of the bottom poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS layer). 6-8 To circumvent this issue, an inverted architecture is implemented which avoids the use of both an acidic PEDOT:PSS layer and a low workfunction (WF) Al electrode. Therefore, it has been proved to be an alternate option to enhance the device stability/efficiency. 4,[9][10][11] To collect electrons efficiently, the WF of ITO has to be substantially reduced in order to match the lowest unoccupied molecular orbital (LUMO) energy level of most of the commonly used acceptor materials such as [6,6]-phenyl C 61 butyric acid methyl ester (PCBM). To modify the WF of ITO, various interfacial materials such as TiO 2 , ZnO, LiF, CS 2 CO 3 and inorganicorganic hybrids [12][13][14][15] have been introduced between the photoactive layer and the electrodes. While it is important to lower the ITO WF from the perspective of energy consideration, it is essential to ensure that the interfacial layer does not degrade overall device performance. One of the most common signatures of poor device performance is the S-shaped J-V characteristics (S-curve) which reduces the fill factor (FF) and power conversion efficiency (PCE) of the device. The S-curve has been suggested to occur due to several mechanisms such as charge trapping, 16,17,19,20 surface dipoles, 18 series resistance effects 21 and charge accumulation in/near the interfacial layer. [21][22][23][24] In this work, inverted polymer solar cells with a biopolymer: poly(dimethylaminoethyl methacrylate) (PDMAEMA) interfacial layer which acts as an electron selective layer (ESL) were fabricated. 21 To further understand the role of the interfacial layer and charge transport processes occurring in the device, we perform current density-...
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