Zhizhe Wang

Performance Comparison of Conventional and Inverted Organic Bulk Heterojunction Solar Cells From Optical and Electrical Aspects

The conventional and inverted organic solar cells (OSC and IOSC) based on the bulk heterojunction structure are investigated from both optical and electrical aspects. When the optical aspect is considered only, with the increase of the active layer thickness, the number of photons absorbed in the active layer and the external quantum efficiency tend to increase with the obvious interference behavior for both OSC and IOSC. However, compared to OSC, IOSC shows a better performance except for the thicknesses around which the interference maxima of OSC are obtained. When the electrical aspect is also considered, an effective area in the active layer will be induced by the charge drift length (L), and only the photons absorbed in this effective area have contribution to the photocurrent. By considering optical and electrical aspects together, OSC and IOSC show different behaviors. Compared to IOSC, OSC performs better for relatively thick active layers. Simultaneously, the optical modulation effect is also investigated by introducing an optical spacer layer. It is found that the optical spacer layer can notably enhance the performance of OSC with thin and thick active layers, while it could only degrade the performance of IOSC with relatively thick active layers.

Efficient inverted polymer solar cells using low-temperature zinc oxide interlayer processed from aqueous solution

In this work, an aqueous solution method that entails processing at low temperatures is utilized to deposit a ZnO interlayer in poly(3hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester-based inverted polymer solar cells (PSCs). The effect of ZnO annealing temperature from 50 to 150°C on PSC performance is systemically studied and it is found that the transition point is approximately 80°C. When the ZnO annealing temperature is higher than 80°C, PSCs show similar current density–voltage (J–V) characteristics and achieve a power conversion efficiency higher than 3.5%. Transmittance spectrum, PL spectrum, and surface morphology studies show that an annealing temperature above 80°C is sufficient for ZnO to achieve a relatively good quality, and that a higher temperature only slightly improves ZnO quality, which is confirmed from statistical results. Furthermore, flexible PSCs based on PET substrates show a comparable power conversion efficiency and good flexibility.

Efficient indium-tin-oxide free inverted organic solar cells based on aluminum-doped zinc oxide cathode and low-temperature aqueous solution processed zinc oxide electron extraction layer

Indium-tin-oxide (ITO) free inverted organic solar cells (IOSCs) based on aluminum-doped zinc oxide (AZO) cathode, low-temperature aqueous solution processed zinc oxide (ZnO) electron extraction layer, and poly(3-hexylthiophene-2, 5-diyl):[6, 6]-phenyl C61 butyric acid methyl ester blend were realized in this work. The resulted IOSC with ZnO annealed at 150 °C shows the superior power conversion efficiency (PCE) of 3.01%, if decreasing the ZnO annealing temperature to 100 °C, the obtained IOSC also shows a PCE of 2.76%, and no light soaking issue is observed. It is found that this ZnO film not only acts as an effective buffer layer but also slightly improves the optical transmittance of AZO substrates. Further, despite the relatively inferior air-stability, these un-encapsulated AZO/ZnO IOSCs show comparable PCEs to the referenced ITO/ZnO IOSCs, which demonstrates that the AZO cathode is a potential alternative to ITO in IOSCs. Meanwhile, this simple ZnO process is compatible with large area deposition an...

Effects of interlayer growth condition on the transport properties of heterostructures with InGaN channel grown on sapphire by metal organic chemical vapor deposition

The effects of AlN interlayer growth condition on the properties of InAlN/InGaN heterostructures are investigated in detail. Since the properties of InGaN channel are different from the traditional GaN channel, two-step AlN interlayer is proposed, which is proven to be more suitable for the InGaN channel heterostructures than the interlayers grown at constant temperature. Test results show that two-step AlN interlayer can not only significantly improve the interface morphology between the InGaN channel and barrier layers but also make an effective protection of the high-quality InGaN channel. The electron mobility of the InAlN/InGaN heterostructure with two-step AlN interlayer achieves 890 cm2/V s with a high two-dimensional-electron-gas density of 1.78 × 1013 cm−2. The gratifying results indicate that the InGaN channel heterostructure with two-step interlayer is a promising candidate for microwave power devices.

Effects of growth temperature on the properties of InGaN channel heterostructures grown by pulsed metal organic chemical vapor deposition

Pulsed metal organic chemical vapor deposition (P-MOCVD) is introduced into the growth of high quality InGaN channel heterostructures. The effects of InGaN channel growth temperature on the structural and transport properties of the heterostructures are investigated in detail. High resolution x-ray diffraction (HRXRD) and Photoluminescence (PL) spectra indicate that the quality of InGaN channel strongly depends on the growth temperature. Meanwhile, the atomic force microscopy (AFM) results show that the interface morphology between the InGaN channel and the barrier layer also relies on the growth temperature. Since the variation of material properties of InGaN channel has a significant influence on the electrical properties of InAlN/InGaN heterostructures, the optimal transport properties can be achieved by adjusting the growth temperature. A very high two dimension electron gas(2DEG) density of 1.92 × 1013 cm−2 and Hall electron mobility of 1025 cm2/(V⋅s) at room temperature are obtained at the optimal growth temperature around 740 °C. The excellent transport properties in our work indicate that the heterostructure with InGaN channel is a promising candidate for the microwave power devices, and the results in this paper will be instructive for further study of the InGaN channel heterostructures.

Flexible ITO-Free Organic Solar Cells Based on

Flexible organic solar cells (OSCs) using our proposed MoO 3 (2 nm)/Ag (9 nm) anode are fabricated on poly(ethylene terephthalate) (PET) substrates with poly(3hexylthiophene) (P3HT) and [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) films as the active layer. Power conversion efficiency (PCE) of 2.50% is achieved for such flexible indium tin oxide (ITO)-free OSCs under 1 sun AM 1.5G simulated illumination, which is comparable with that of the same devices fabricated on glass substrates (PCE of 2.71%) or that of ITO-based reference OSCs on glass (PCE of 2.85%). Meanwhile, such flexible ITO-free OSCs show good mechanical flexibility. A 10% degradation in PCE is observed after 500 inner bending cycles with a bending radius of 1.5 cm, whereas a 5% decrease in PCE is observed after 500 outer bending cycles. Furthermore, the flexibility of the structure PET/MoO 3 /Ag/MoO 3 has been investigated. Its transparency almost remains constant, and its sheet resistance varies negligibly after 1000 cycles of inner or outer bending tests. It shows the huge potential of our flexible electrodes, and it may be instructive for further research on flexible electrodes.

\hbox{MoO}_{3}/\hbox{Ag}

We present a numerical analysis on an ultra-short channel AlGaN/GaN HEMT-like planar Gunn diode based on the velocity-field dependence of two-dimensional electron gas (2-DEG) channel accounting for the ballistic electron acceleration and the inter-valley transfer. In particular, we propose a Schottky-ohmic composite contact instead of traditional ohmic contact for the Gunn diode in order to significantly suppress the impact ionization at the anode side and shorten the “dead zone” at the cathode side, which is beneficial to the formation and propagation of dipole domain in the ultra-short 2-DEG channel and the promotion of conversion efficiency. The influence of the surface donor-like traps on the electron domain in the 2-DEG channel is also included in the simulation.

Anodes

ITO-free semitransparent organic solar cells (OSCs) based on MoO3/Ag anodes with poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester films as the active layer are investigated in this work. To obtain the optimal transparent (MoO3)/Ag anode, ITO-free reference OSCs are firstly fabricated. The power conversion efficiency (PCE) of 2.71% is obtained for OSCs based on the optimal MoO3 (2 nm)/Ag (9 nm) anode, comparable to that of ITO-based reference OSCs (PCE of 2.85%). Then based on MoO3 (2 nm)/Ag (9 nm) anode, ITO-free semitransparent OSCs with different thickness combination of Ca and Ag as the cathodes are investigated. It is observed from our results that OSCs with Ca (15 nm)/Ag (15 nm) cathode have the optimal transparency. Meanwhile, the PCE of 1.79% and 0.67% is obtained for illumination from the anode and cathode side, respectively, comparable to that of similar ITO-based semitransparent OSCs (PCE of 1.59% and 0.75% for illumination from the anode and cathode side, resp.) (Sol. Energy Mater. Sol. Cells, 95, pp. 877–880, 2011). The transparency and PCE of ITO-free semitransparent OSCs can be further improved by introducing a light couple layer. The developed method is compatible with various substrates, which is instructive for further research of ITO-free semitransparent OSCs.

Ultra-short channel GaN high electron mobility transistor-like Gunn diode with composite contact

Inspired by the natural phenomena that rough sea surfaces often mask the ship target when the wind comes big, or another case in point, vehicles can usually hide themselves in the mountains with dense vegetation. Here in this paper, we demonstrate a practical strategy of hiding by using gradient-index meta-surfaces based on the quasi-conformal transformation optics. Different from the prevalent choice of the invisible cloak to virtually extinguish an object that is wrapped inside, our approach aims to suppress the detectability of the target through creating diffuse reflections by proposing the camouflage meta-surface underneath. Our design, possessing a flat and slim profile with solely a few randomly distributed dielectrics on a metallic sheet, can readily offer the perfect cover to mask the hiding object adjacently above in a broad frequency range.

ITO-Free Semitransparent Organic Solar Cells Based on Silver Thin Film Electrodes

Low in cost, light in weight and flexible in mechanics, the solution-processed organic solar cells have aroused worldwide interest and have been the promising alternative to the tradi‐ tional silicon-based solar cells [1-4]. However, they are still not available for the commercial‐ ization due to their low power conversion efficiency (PCE). Therefore, many research works have focused on the employing of new materials and device structures to improve the de‐ vice performance. The milestone is the introduction and application of the bulk heterojunc‐ tion structure consisting of an interpenetrating network of electron donor and acceptor materials [5]. By using this structure, the conventional organic solar cell (OSC) with poly(3hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) blend shows a su‐ perior performance. Recently, the inverted organic solar cell (IOSC, in which the polarities of the two electrodes are exchanged) has also been introduced as the possible candidate for OSC to remedy the low air stability of OSC [6]. Both OSC and IOSC are now attracting the research interest. However, most of the previous works are mainly done for OSC or IOSC separately, and almost no researches are reported about the systemic comparison between OSC and IOSC for their different performances besides the air stability. Since the reported PCE of IOSC is relatively lower than that of OSC in many researches, one may doubt that which structure is better, the conventional one or the inverted one? As a result, one section of this chapter aims to investigate the performance differences of OSC and IOSC.