Organic photovoltaic cells and organic up-conversion devices

Organic electronic devices such as organic light emitting diodes, organic photovoltaic cells and organic photodetectors are attracting a great deal of attention because of their compatibility with flexible substrates, low manufacturing cost processes, and large area applications. In this work, we study several key factors affecting the power efficiency of organic photovoltaic cells and the use of novel organic up-conversion devices for infrared imaging applications. In the area of organic photovoltaics, the power efficiency has been limited to about 5%. The major limitations to power conversion efficiency are narrow spectral response, low open circuit voltages and small fill factors. Here, we present our results how to increase the open circuit voltage using materials with deep highest occupied molecular orbital (HOMO) energy, extend the photoresponse using infrared absorbing organic semiconductors, and increase the fill factor using molybdenum oxide (MoO3) interlayer. In the area of up-conversion devices, we will present our results on organic thin film photodetectors as well as on integrating organic photodetector with organic light emitting device (OLED) to demonstrate up-conversion for infrared imaging.Aluminum phthalocyanine chloride (AlPcCl) planar and bulk heterojunction cells were fabricated and the results were compared with heterojunction cells fabricated using copper phthalocyanine (CuPc). We demonstrated that in both AlPcCl and CuPc cells, the device performance was enhanced due to the bulk heterojunction effect. Comparing with the CuPc cells, the open-circuit voltages of AlPcCl cells is almost doubled compared with CuPc cells due to the deeper HOMO energy in AlPcCl.Tin phthalocyanine (SnPc) bulk heterojunction cells were fabricated. The absorption of CuPc is limited to wavelength below 800 nm. On the other hand, SnPc can extend the absorption wavelength to about 1000 nm. We demonstrated SnPc:C60 bulk heterojunction cells with photoresponse extended to about 900 nm. However, due to the suppression of SnPc dimer formation, the photoresponse of SnPc:C60 bulk heterojunction cells beyond 900 nm is significantly reduced.The effect of MoO3 interlayer on small molecule and polymer photovoltaic cells was studied. We found that it has a strongest effect on fill factors. In polymer cells, we found that the MoO3 interlayer increase the fill factor by 10--20%. On the other hand, in cells with small molecules such as CuPc, the enhancement in fill factor due to the MoO 3 interlayer can be as large as 30%. Our photoelectron spectroscopy results show there is a strong band bending at the organic/interlayer interface and the enhancement of fill factor is due to the strong built-in field at the interface leading to enhancement in carrier extraction.Organic photodetectors were demonstrated using both SnPc and CuPc. With bathocuproine (BCP) and MoO3 as the charge carrier blocking layers, the dark current is significantly reduced and external quantum efficiencies exceeding 90% were obtained.Novel infrared-to-visible up-conversion devices were demonstrated by fabricating an organic light emitting device in series with a photodetector. With SnPc as the infrared absorber and fac-tris(2-phenylpyridinato) iridium (III) (Irpy3) as an emitter, an infrared-to-green up-conversion device with a current efficiency exceeding 105 cd/A was demonstrated under 830 nm irradiation. The maximum photon-to-photon conversion efficiency is 2.7% at 15V. These results are consistent with the fact that the external quantum efficiency of the Irppy 3 based green emitting OLED is about 20% and the external quantum efficiency of the infrared photodetector is 10%. The maximum on/off ratio exceeds 1500 at an operating voltage of 12.7 V. The current efficiency of the OLED part of the device exceeds 100 cd/A. The high current efficiency in the OLED is due to photo-injected carriers resulting in enhanced charge balance.