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Hybrid cells based on ZnO/P3HT heterojunctions have the advantage of better device stability, but suffer poor photovoltaic performance compared to all-organic cells which use PCBM as the electron acceptor. The photovoltaic effect in these hybrid systems is accomplished via photoinduced charge separation at the interface between the absorbing polymer (P3HT) and the electron acceptor (ZnO). Efforts to improve device performance in these hybrid systems have centered on reducing the required diffusion length for P3HT excitons by creating bulk heterojunctions from either ZnO nanoparticles and P3HT or using ZnO precursors which convert in situ to form ZnO networks inside a polymer matrix. In this study, we use transient photoinduced absorption to access the lifetimes of P3HT polarons and excitons in bulk heterojunctions constructed using P3HT and ZnO nanoparticles or ZnO precursors and compare to those in planar ZnO/P3HT devices. Steady-state photoinduced absorption spectra of ZnO/P3HT show characteristic of sub-bandgap transitions associated with the formation of long-lived (msec lifetimes) radical cations (polarons) in P3HT. Similar short-lived polarons (psec lifetimes) are observed by picosecond transient photoinduced absorption in addition to infrared absorption due to excitons. Here we examine the lifetimes of both the excitons and polarons in ZnO:P3HT bulk heterojunctions using both picosecond and millisecond techniques in an effort to understand the effect of the structure and morphology of the electron acceptor on charge separation. We will also compare the relative photoexitation lifetimes, hence charge separation efficiency, for the planar and bulk heterojunction hybrid system to an all-organic P3HT:PCBM system.

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TYPE Conference YEAR 2010

OSTI

Final LDRD report : enhanced spontaneous emission rate in visible III-nitride LEDs using 3D photonic crystal cavities

Fischer, Arthur J.; Subramania, Ganapathi S.; Lee, Yun-Ju L.; Koleske, Daniel K.; Li, Qiming L.; Wang, George T.; Luk, Ting S.; Fullmer, Kristine W.

The fundamental spontaneous emission rate for a photon source can be modified by placing the emitter inside a periodic dielectric structure allowing the emission to be dramatically enhanced or suppressed depending on the intended application. We have investigated the relatively unexplored realm of interaction between semiconductor emitters and three dimensional photonic crystals in the visible spectrum. Although this interaction has been investigated at longer wavelengths, very little work has been done in the visible spectrum. During the course of this LDRD, we have fabricated TiO{sub 2} logpile photonic crystal structures with the shortest wavelength band gap ever demonstrated. A variety of different emitters with emission between 365 nm and 700 nm were incorporated into photonic crystal structures. Time-integrated and time-resolved photoluminescence measurements were performed to measure changes to the spontaneous emission rate. Both enhanced and suppressed emission were demonstrated and attributed to changes to the photonic density of states.

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TYPE SAND Report YEAR 2009

OSTI DOI