We provide a more compact fabrication process than the prior work using [6,6]-phenyl C61-butyric acid methyl ester (PCBM) buffer layer to enhance the performance of organic solar cells. The device with an extra PCBM layer exhibits an improved power conversion efficiency (PCE) of 20% as compared with the devices with conventional structures. Ultraviolet and x-ray photoemission spectroscopy indicate that thermal annealing treatments result in better mixture structures of PCBM layers with poly(3-hexylthiophene) at the blended surfaces. Therefore, the device with inserted PCBM provides more ideal hetero-structures and vertically graded PCBM distribution to optimize the charge extraction efficiency.
The effectiveness of carrier injection in electron transport layers has been investigated for high efficiency organic light
emitting devices. Via ultraviolet and x-ray photoemission spectroscopy (UPS and XPS), the carrier band structures,
interfacial interactions and electron-injection mechanisms are discussed. Acting as a good hole blocking layer with
higher mobility for electrons, 4,7-diphenyl-1, 10-phenanthroline (Bphen) was chosen to be the electron transport layer.
The performance of device used Rb2CO3 doped into Bphen is obviously better than the device even used LiF with
aluminum as cathode. According to the UPS spectra, the Fermi level of Bphen after doped with the ratio of 2% and 8%
rubidium carbonate (Rb2CO3) shifts toward the lowest unoccupied molecular orbital as a result of charge transfer from rubidium atom to Bphen, showing that electron-injection ability would be improved based on strong n-type doping
effect. Moreover, when aluminum is deposited as a thin layer on the surface of Bphen doped with Rb2CO3, the peak
around 5 eV, which is attributed to the delocalized Pi-electrons decreases as gap states appear around 2.8 eV at the top of
the highest occupied molecular orbital. There are changes in the binding energy of core levels of rubidium, nitrogen and
aluminum, which indicates a negative charge transfer to Bphen at the interface that could have the reduction of electroninjection
barrier height. Thus, the interfacial chemical reaction leads to the excellent electron injection ability could be demonstrated.
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