**5. Conclusion**

190 Advanced Photonic Sciences

To fabricate those structures, the ITO substrate is first patterned using standard photolithography and wet-etching processes before spin-coating of a very thin 30 nm-thick layer of PEDOT:PSS. As we mentioned before, this thin layer of PEDOT:PSS plays the dual role of facilitating the hole-injection while alleviating the detrimental effects of the surface roughness of the ITO film on the structural properties of the hole-transporting polymer. A toluene-based TFB solution is then spin-coated to provide a 120 nm-thick TFB film atop the PEDOT:PSS. Since the PEDOT:PSS is immune to toluene, there is no solvent-compatibility issues. Then, a solution of PbS quantum dots suspended in hexane is spin-coated atop the TFB, prior to performing the ligand-exchange process using the dithiol molecule diluted in acetonitrile. While the PEDOT:PSS would be affected by both the hexane and acetonitrile solutions, the TFB provides an efficient protection barrier for both solvents, thus preserving

Fig. 16. High-Performance near-infrared LEDs using all solution-based processing. (a) Cross-sectional SEM micrograph showing the optimized structure of the device. (b) Device current-voltage characteristics for hybrid LED structure (•) and a polymer-only control device (◦). The inset shows the actual device atop the entry port of an integrating sphere while the near-IR electroluminescence is collected through the transparent substrate. (c) Emission spectra of a typical 1050 nm LED. The inset image shows the actual near-IR emission of a 1 mm2 device measured using a near-IR camera coupled to a 2X objective.

As shown in Figure 16(b), the polymer-only control device reaches a clear single-carrier (hole) trap-limited regime around 1 Volt, before reaching a space-charge limited operation regime around 2 Volts. This is consistent with the large energy barrier at the TFB-aluminum interface. For the LED device with the BDT-treated nanocrytalline film structure, measurements indicate a much higher current density at low voltages originating from the efficient electron-injection at the metal-PbS interface. Here, the lower slope in the traplimited region simply suggests different transport and trapping mechanisms in the nanocrystalline film compared with the TFB. These highly-efficient LED structures can operate anywhere between 1000 and 1600 nm depending on the nanocrystals used while providing external quantum efficiencies as high as 0.7% and ouput powers close to 80 µW. While the conventional ethanedithiol (EDT)-based ligand-exchange treatment is known to work well for photovoltaic structures (Luther et al., 2008), it yields only relatively poor LED structures compared to the phenomenal results achieved using benzendithiol (BDT)-based treatment. This disparity can be readily explained now based on the conductivity and mobility results presented in Figure 14. Indeed, the EDT treatement provides higher conductivities due to higher p-type doping and comparable electron- and hole- mobilities. As such, both the hole current and nonradiative Auger recombination will be orders of

the structural integrity of the whole structure.

While π-conjugated polymer-based light-emitting diodes are perfectly suited for the visible, their potential for near-infrared operation remains limited. However, the hybrid integration of semiconductor nanocrystals and conjugated polymer material systems can provide an easy pathway for (1) improving the conjugated polymer-based devices optoelectronic properties and/or (2) providing added functionality to the conjugated polymer-based device structures. Because the oleate capping groups keeping colloidal lead-chalcogenide nanocrystals stable also inhibit carrier transport, previously-proposed hybrid near-infrared LED structures usually rely on nanocrystals embedded within a polymer host matrix, or use a self-assembled monolayer of colloidal quantum dots located at the junction of an organic heterostructure directly between hole- and electron-transporting organics. We have demonstrated why both these hybrid polymer-nanocrystal blends and the hybrid bilayered heterostructures greatly suffer from poor injection efficiencies and from significant carrier losses into the organic layers, while limited electron-transporting materials especially limit the overall performances of those polymer-based optoelectronic devices.

Here, we report an all solution-based method producing efficient hybrid polymernanocrystal multilayered heterostructures for light-emission in the near-infrared (1050-1600 nm). After optimization device structure, we obtain low-cost near-infrared light-emitting diodes with external quantum efficiency (EQE) as high as 0.7% and up to 80 µW output from devices entirely processed in ambient air and with no encapsulation. This approach relies on a carefully-controlled layer-by-layer benzenedithiol (BDT) ligand-exchange to achieve direct charge injection and better transport. In comparison with this BDT treatment, the conventional ethanedithiol (EDT)-based treatment provides poor LED structures. As we show, the high performances of our devices can be explained by the different doping levels and electron & hole mobilities resulting from the BDT versus EDT treatments.

In the future, this easy, robust, low-temperature and substrate-independent approach has the potential to become extremely useful for flexible and/or reconfigurable integrated opto-

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#### **6. Acknowledgment**

I am most thankful to my entire team and all my collaborators of the last 5 years for their contribution direct or indirect to this research. Most especially, I would like to thank Xin Ma and Fan Xu who have done tremendous work to advance this emerging field of research. Finally, most of this research work was kindly supported through the AFOSR (FA9550-10-1- 0363), the DARPA–COMPASS and the DARPA-Young Faculty Award programs, to whom I am most thankful. The majority fraction of this work was conducted while the author worked at the University of Delaware.

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**8** 

*Japan* 

Koichi Okamoto

**Plasmonics for Green Technologies:** 

*Institute for Materials Chemistry and Engineering, Kyushu University* 

**Toward High-Efficiency LEDs and Solar Cells** 

Nowadays, energy issues became very important problem for us. We spend a lot of energy for illumination at night, so developing high-efficiency light sources is very important to save our energy. Recently, solid-state light-emitting devices have been developed and expected as new-generation light sources because of their advantages such as small, light-weight, long lifetime, easy operation, and saving energy. Since 1993, InGaN quantum wells (QW)-based light-emitting diodes (LEDs) have been continuously improved and commercialized as light sources in the ultraviolet (UV) and visible spectral regions. In 1996, white light LEDs, in which a blue LED is combined with yellow phosphors, have been developed and offer a replacement for conventional incandescent and fluorescent light bulbs. However, these devices have not fulfilled their original promise as solid-state replacements for light bulbs as their lightemission efficiencies have been limited. The most important requirement for competitive LEDs for solid-state lighting is improvement of their quantum efficiencies of light

Making energy is also very important so much as saving energy. Renewable energies have attracted a great deal of attention as a new energy source instead of fossil resource which is going to be exhausted. The solar energy is one of the most important renewable energy resources and the photocurrent conversion efficiencies of several kind of solar cells have been rapidly developed. Especially, the crystalline solar cells with silicon or compound semiconductors were well developed and their efficiencies were almost reached to the theoretical limits. The drastic cost reduction is much important for such crystalline solar cells to use for much wider areas. For example, making ultra-thin device structures is required to save the materials. On the other hand, amorphous or organic solar cells are very cheap and easy to treat them but the efficiencies are still very low. The improvements of the efficiencies and device lifetime are most important for such

A lot of effort and time have been used to improve the efficiencies of LEDs and solar cells, but still it has been very difficult to achieve dramatic improvements. Here I introduce the unique approach to increase these efficiencies based on "Plasmonics". These studies should

bring the new application field of plasmonics for green technologies.

**1. Introduction** 

emissions.

solar cells.

600-fold increase in maximum power output via control of the ligand barrier, *Applied Physics Letters*, Vol.87 (2005), pp. 233101-3

