We have fabricated organic photovoltaic cell using tetrabenzoporhyrin (BP) as a donor material. Tetrabenzoporhyrin is
formed by thermal conversion of the soluble precursor that has four bicyclo rings. Upon heat treatment above 150°C, the
precursor molecule is converted to semiconductive tetrabenzoporphyrin, which is insoluble against conventional organic
solvents. Taking advantage of this insoluble character of BP, p-i-n bulk heterojunction photovoltaic cell is successfully
fabricated from solution, with BP/BP:fullerene/fullerene trilayer, where p-layer is crystal BP, i-layer consists of both BP
and fullerene, and fullerene acts as n-layer. We have reported the
p-i-n photovoltaic cell using [6,6]-phenyl-C61-butyric
acid n-butyl ester (PCBNB) as an acceptor. This solution-processed
p-i-n device has achieved power conversion
efficiency as high as 3.4% (Jsc=9.8mA/cm2; Voc=0.62V; FF=0.56). The performance of BP device with p-i-n junction is
further improved by introducing a new fullerene for the i- and
n-layers. Taking into consideration LUMO level,
solubility and thermal properties, a novel fullerene derivative,
is designed and synthesized. This new di-adduct fullerene , SIMEF, improves power conversion efficiency up to 4.1%
(Jsc=9.1mA/cm2; Voc=0.76V; FF=0.59). This improvement is largely due to an increased Voc. We have analyzed
BP:fullerene composite films to find that formation of meta-stable phase of BP is fully suppressed by SIMEF. Depth
profile of this p-i-n cell is measured by means of TOF-SIMS method to observe the expected vertical distribution of the
donor and acceptor that has been designed.
Organic photovoltaic cell is fabricated with tetrabenzoporhyrin (BP) as a donor material. Tetrabenzoporhyrin is formed
by thermal conversion of the soluble precursor that has four bicycle rings instead of benzo-rings. Upon heat treatment
above 150°C, the precursor molecule is converted to semiconductive benzoporphyrin, which is insoluble against
conventional organic solvents. Hetero junction OPV cell is made of benzoporhyrin/fullerene layers with power
conversion efficiency of 2.2%. Taking advantage of insoluble character of BP, p-i-n heterojunction OPV is successfully
fabricated from solution, with BP/BP:fullerene/fullerene layers. This solution-processed p-i-n device exhibited further
improvement of power conversion efficiency, 3.0%.
We have developed a new type of polymers having TPD unit combined with non-conjugated spacer group, poly(arylene ether sulfone)-containing and poly(arylene ether ketone)- containing tetraphenylbenzidine (PTPDES an PTPDEK) and also polymers with directly coupled triphenyl amine units (PPBA). When these polymers are mixed with strong acceptor such as TBPAH or DDQ, they indicated higher conductivity and facilitated hole injection from ITO to the hole transport layer. Spin-coating of such polymer from an organic solution on ITO was found to improve the surface roughness of ITO, resulting in reduced defects that cause electric short circuit between ITO and cathode. These buffer materials lowered the operation voltage and improved the thermal stability of the device. After storage of 1,000 hours at 85 degrees Celsius, the device with polymer buffer showed no degradation in luminance and small increase of operation voltage. In comparison with CuPc buffer, it is clear that the doped polymer is superior in terms of both efficiency and thermal stability.
Three interfaces, anode interface, hole blocking layer and cathode interface were considered mainly from the viewpoint of materials. Vinyl polymers containing triphenylamine as a side group were investigated as an ITO buffer layer. When these polymers were doped with strong acceptor, they lowered operation voltage of OLED and also improved the thermal stability. Employment of high Tg hole transport material was also found effective for the thermally stable EL characteristics. Hole blocking material with a wider optical gap improved color purity of blue-emitting device. Various inorganic compounds were examined as a cathode interface layer to demonstrate that MgF2 was effective to improve operation lifetime of OLED.