Despite significant growth in photovoltaics (PV) over the last few years, only approximately 1.07 billion kWhr of
electricity is estimated to have been generated from PV in the US during 2008, or 0.27% of total electrical generation.
PV market penetration is set for a paradigm shift, as fluctuating hydrocarbon prices and an acknowledgement of the
environmental impacts associated with their use, combined with breakthrough new PV technologies, such as thin-film
and BIPV, are driving the cost of energy generated with PV to parity or cost advantage versus more traditional forms of
In addition to reaching cost parity with grid supplied power, a key to the long-term success of PV as a viable energy
alternative is the reliability of systems in the field. New technologies may or may not have the same failure modes as
previous technologies. Reliability testing and product lifetime issues continue to be one of the key bottlenecks in the
rapid commercialization of PV technologies today. In this paper, we highlight the critical need for moving away from
relying on traditional qualification and safety tests as a measure of reliability and focus instead on designing for
reliability and its integration into the product development process. A drive towards quantitative predictive accelerated
testing is emphasized and an industrial collaboration model addressing reliability challenges is proposed.
Electromodulation (EM) spectroscopy has been used to probe the electric field distribution in polymer light-emitting diodes. Below the turn-on bias, the EM spectrum is dominated by electroabsorption of the emissive layer. The electroabsorption signal vanishes at the turn-on bias. Under operation, the EM spectrum is due to excited state absorption from injected charge and bleaching of the ground state absorption of the emissive layer. We conclude that the internal electric field is effectively screened by accumulation of electrons at the anode.
An appropriate choice of the cathode material and the process of cathode deposition is a key issue in the development of polymer light emitting devices. In this paper, we report on the impact of low work function metals on the luminescence efficiency of thin films of polyfluorene type polymers. Photoluminescence as well as electroluminescence experiments are presented, and in both cases, a strong correlation between the metal layer thickness and the luminescence efficiency is demonstrated. By means of time-of-flight secondary ion mass spectroscopy (TOF-SIMS), the distribution of the metal contamination within the polymer layers is determined. The results strongly suggest that impurity quenching of excitons by metal atoms inside the polymer layer takes place and strongly affects luminescence and device efficiency.
This paper describes the development of light emitting diode technology based on fluorene-containing polymers, prepared through the coupling of 9,9-disubstituted 2,7-bis-1,3,2- dioxaborolanyl-fluorene with a variety of aromatic dibromides. In these polymers the polyphenylene-like backbone provides the mechanical and chemical robustness and the C-9 of fluorene provides a site for physical property modifications without introducing significant torsional strain which would adversely affect conjugation. Polymer optical and electronic properties are tailored through selective incorporation of different aromatic unit into the AB alternating structure. LED devices emitting in blue, green, red and other colors are thus obtained.
We report our progress to date in the materials and processing aspects of developing organic light emitting diode technology based on high molecular weight polymers. A portfolio of fluorene-related polymers are prepared through the coupling of 9,9-disubstituted 2,7-bis-1,3,2- dioxaborolanyl-fluorene with a variety of aromatic dibromides. In the case of fluorene homopolymers, the polyphenylene main chain provides the mechanical, electrical and electronic properties and the C-9 maintains coplanarity of the biphenylene unit and a site for property modification without altering effective conjugation. In the case of alternating polymers, the optical and electronic properties of the polymers are tailored through selective incorporation of different aromatic unit into the system. These polymers are soluble in common organic solvents and are readily processed into uniformed films of high quality by spin casting. Unlike PPV and related materials, LED devices with fluorene polymers in a conventional configuration appear to have electrons as the majority carrier and their performance is markedly improved when modified with an appropriate polymeric hole transporting layer. Bright green light with a luminance of 10000 Cd/m2 is achieved at a very low drive voltage of < 6 V attributable in part to the high hole mobility of fluorene-based polymers.