Bilayer organic solar cells

Yu Zhao

The most efficient organic solar cells produced to date are bulk heterojunction (BHJ) photovoltaic devices based on blends of semiconducting polymers such as poly(3-hexylthiophene-2,5-diyl) (P3HT) with fullerene derivatives such as [6,6]-penyl-C61-butyric-acid-methyl-ester (PCBM). The need for blending the two components is based on the idea that the exciton diffusion length in polymers like P3HT is only∼10 nm, so that the polymer and fullerene components must be mixed on this length scale to efficiently split the excitons into charge carriers. Related researches show that the BHJ geometry is not necessary for high efficiency, and that all-solution-processed P3HT/PCBM bilayer solar cells can be nearly as efficient as BHJ solar cells fabricated from the same materials.

Three kind of organic solar cells geometry were introduced briefly . Then two important articles about bilayer organic solar cells were discussed.

In the first article ,the method of fabricating bilayer organic solar cells was introduced firstly and we can see that the thickness of each layer plays an important role in device photovoltaic performance. In particular, the most efficient devices are made with P3HT layers that are about four times thicker than the PCBM layers.The bilayer organic solar cells showed power conversion efficiencies in excess of 3.5%, which is slightly higher than BHJ device fabricated by the same materials.

In the second article, For device optimization,the author varied the following four parameters: (1) P3HT layer thickness (by adjusting rotation speed). (2) ICBA layer thickness(by adjusting solution concentration. ICBA thickness used here is its thickness before annealing, when serious inter-diffusion has not occurred.).(3) Annealing temperature. (4) Annealing time. They found the optimal values for the four parameters are 55 nm (spin coated at 1000 rpm), 25 nm (spin-coated at 5mg/ml), 150 C and 20 min, respectively. The optimized device exhibits a PCE of 5.12%, with an open circuit voltage (Voc) of 0.886 V,a short-circuit current density (Jsc) of 8.21 mA/cm2, and a fill factor (FF) of 70.4%.

At the end , I put forward the idea of fabricating bilayer hybrid solar cells with CuInGaSe2(CIGS) layer embedded between ITO and P3HT or PCBM or P3HT&PCBM blends. The advantages of CIGS films deposited by pulsed laser deposition were stated, including high absorption coefficient (about 105cm-1),high electron and hole mobility(100 cm2V-1S-1) , easily adjusted energy level and conduction type.

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Spectroscopic characterization and temperature determination for highly reactive nitrogen/aluminum plasma generated by ECR microwave discharge of N2 gas and pulsed laser ablation of Al target

Peipei Liang

The reactive nitrogen/aluminum plasma generated by electron cyclotron resonance (ECR) microwave discharge of N2 gas and pulsed laser ablation (PLA) of an Al target was characterized spectroscopically by time integrated and resolved optical emission spectroscopy (OES). The plasma emits strong emissions from a variety of excited species including ambient nitrogen and ablated aluminum and exhibits unique characteristics in optical emission and temperature evolution compared with the plasma generated solely by ECR discharge or pulsed laser ablation. OES measurements show that due to the interactions between the nitrogen plasma generated by ECR discharge and the aluminum plume induced by PLA, the ambient N2 is first excited by ECR discharge and the excitation of nitrogen is further enhanced by the rapid expansion of the aluminum plume, while the excitation of the ablated aluminum is prolonged during the expansion of the aluminum plume in the ECR nitrogen plasma.    

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Enhancement of perpendicular magnetic anisotropy in

Co/Nimultilayers by in-situ annealing the Ta/Cu under-layers

Di Wu

In order to maintain strong thermal stability and lower the current shunting for the nanoscale devices, it is necessary to acquire sufficient strong PMA with a thin under layer. It is known that the PMA in Co/Ni MLs can be enhanced by post-annealing at a moderate temperature below 300�C, commonly believed as a result of the increased degree of Co/Ni (111) texture. However, by this way the PMA enhancement is not much since the post-annealing temperature could not be raised too high, excessive heating at elevated temperatures would give rise to atomic intermixing across the interfaces and thus the loss of PMA.

In this work, we have investigated the PMA in Ta/Cu/[Co/Ni]4/Ta MLs systematically. In order to greatly enhance the PMA without increasing the under-layer thickness or introducing interlayer diffusion, the Ta/Cu under-layers were in-situ pre-heated at various temperatures for 30 min before the deposition of the Co/Ni stack. By combining the in-situ annealing of Cu with the subsequent post-annealing treatment taken on the whole ML films, we obtain not only a significant increase in the perpendicular magnetic coercivity, but a deep insight into the role of interface roughness, layer interdiffusion, and (111) texture which are responsible for such increase as well.

We have shown that the perpendicular magnetic properties of [Co/Ni] MLs can be significantly enhanced by in-situ annealing the Ta/Cu under-layers (TCu) and post-annealing the whole ML stack (TMLs). Both the perpendicular coercivity Hc� and anisotropy energy Ku exhibit an unusual non-monotonous dependence on the TCu, showing minimum values both at TCu=100�C, and maxima at 400�C for Ku while at 550�C for Hc�. Such variation tendency can be interpreted as the combined effect that the ordering degree of fcc (111) texture promotes the PMA but the Cu surface roughness prevents it. In-situ annealing at proper temperatures can promote the growth of (111) texture and form a relatively smooth Cu surface which are beneficial to the PMA of Co/Ni MLs. Interestingly, as TCu is over 400�C, the Ku starts to drop while the Hc� continues to rise until TCu=550�C, which can be ascribed to the lattice defects of the rough Cu surface that provides a negative contribution to the PMA but a positive contribution to the Hc�. However, if TCu is as high as 600�C, severe heating gives rise to pronounced reduction in both Ku and Hc� due to the decreased degree of (111) texture and increased Cu surface roughness. Our results demonstrate that by combining the in-situ annealing at TCu =550�C and post-annealing at TMLs =250�C, the Hc� value can be significantly enhanced up to 620 Oe, more than 4 times higher than the samples without suffering any heating treatment. Our results would make these films more attractive for practical spintronic applications that require strong PMA, high spin polarization, but thin underlayers.

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