Large area flexible electronics rely on organic or hybrid materials prone to degradation limiting the device lifetime. For many years, photo-oxidation has been thought to be one of the major degradation pathways. However, intense illumination may lead to a burn-in or a rapid decrease in performance for devices completely isolated from corrosive elements as oxygen or moisture. The experimental studies we present in here indicate that a plausible triggering for the burn-in is a spin flip after a UV photon absorption leading to the accumulation of electrostatic potential energy that initiates a rapid destruction of the nanomorpholgy in the fullerene phase of a polymer cell. To circumvent this and achieve highly stable and efficient devices, we induce a robust nano-crystalline ordering in the PCBM phase prior to UV illumination. In that event, PTB7-Th:PC 71 BM cells are shown to exhibit T 80 lifetimes larger than 1.6 years under a continuous UV-filtered 1-sun illumination, equivalent to 7 years for sunlight harvesting at optimal orientation and 10 years for vertical applications.

The length of the transit region of a Gunn diode determines the natural frequency at which it operates in fundamental modethe shorter the device, the higher the frequency of operation. The long-held view on Gunn diode design is that for a functioning device the minimum length of the transit region is about 1.5μm, limiting the devices to fundamental mode operation at frequencies of roughly 60 GHz. Study of these devices by more advanced Monte Carlo techniques that simulate the ballistic transport and electron-phonon interactions that govern device behaviour, offers a new lower bound of 0.5μm, which is already being approached by the experimental evidence that has shown planar and vertical devices exhibiting Gunn operation at 600nm and 700nm, respectively. The paper presents results of the first ever THz submicron planar Gunn diode fabricated in In0.53Ga0.47As on an InP substrate, operating at a fundamental frequency above 300 GHz. Experimentally measured rf power of 28 µW was obtained from a 600 nm long ×120 µm wide device. At this new length, operation in fundamental mode at much higher frequencies becomes possible -the Monte Carlo model used predicts power output at frequencies over 300 GHz.

We report a thin and robust interconnecting layer (ICL) for polymer tandem solar cells. This ICL shows low absorption, good electrical contacts, large work function contrast and robustness. Its use yields tandem cells with a very high fill factor of 76%, making this ICL a promising component of future highly efficient multijunction organic solar cells.Bulk heterojunction polymer solar cells represent a promising technology which could be an important player in the future photovoltaic market due to their light weight, exibility, semitransparency, low cost, and rapid energy payback time. 1-6 The power conversion efficiency (PCE) of these polymer solar cells (PSCs) has been improved markedly in recent years 7-10 and has now exceeded the milestone value of 10%, 11 which makes their industrialization to eventually commercialize them very promising. Moreover, by using the tandem concept, 12 in which two or more subcells with complementary absorption spectra are stacked on top of each other and connected in series or parallel conguration, the PCE has been recently boosted to near 11%. The FF characterizes how "rectangular" the J-V curve is and it represents how "difficult" or "easy" the photogenerated carriers can be extracted out of a photovoltaic device. This parameter is crucial to determine the performance of an organic solar cell together with the short circuit current density (J sc ) and the open-circuit voltage (V oc ). There are several factors that can inuence the FF in a complex way, thus making the FF one of the least understood parameters among the ones determining the PCE. 19 Commonly, in the fabrication of PSCs, several actions have shown to be benecial for improving the FF and hence the efficiency like, for instance, improving the morphology 20,21 or choosing appropriate buffer layers that can help diminish the contact resistance (reduce R s ) or the current leakage (increase R sh ). 22 The buffer layers normally used as transporting layers in single junction solar cells also play another important role in multijunction solar cells as they are commonly part of the interconnecting layer (ICL). In tandem cells, the ICL serves as the charge recombination zone between the front and rear subcells; the ICL is vital in realizing high performance in tandem cells. 23 Further requirements of the ICL are transparency and robustness to protect the underlying layers against damage from further solution processing of the rear subcell.In this study, we report a novel and very thin ICL for multijunction congurations which provides FFs exceeding the ones of the single cells forming part of it and reaching values up to 76%, never reported before in organic tandem solar cells to the best of our knowledge. Herein, we employed a MoO 3 /Ag/PFN ICL layer, less than 20 nm thick, connecting two identical, inverted PTB7:PC71BM subcells in series. By using identical polymers in both subcells, one can overcome the decrease in J sc , FF, and ultimately PCE usually observed in single-junction cells when the thickness of the ac...

No abstract

The degradation of AlGaN/GaN high electron mobility transistors after off-state stress is studied by means of electroluminescence (EL) analysis, gate leakage current (Ig) monitoring, and atomic force microscopy (AFM) mapping of the semiconductor surface. It is found that the degradation of Ig upon stress is due to the combined effect of the individual defects underlying each of the EL spots, which contribute a few μA each to the total Ig. After removal of contacts and passivation, a direct one-to-one correspondence between EL spots and pits on the semiconductor surface is found. Reverse bias, conducting-tip AFM imaging showed that these surface pits do indeed act as leakage paths. Thus, the direct relationship between EL hot spots, surface pits, and gate current leakage is demonstrated. Discussion on the morphology of the surface pits and their possible origin is also provided.