Triplet exciton and polaron dynamics in phosphorescent dye blended polymer photovoltaic devices

The triplet exciton and polaron dynamics in phosphorescent dye (PtOEP) blended polymer (MEH-PPV) photovoltaic devices are investigated by quasisteady-state photo-induced absorption (PIA) spectroscopy. According to the low-temperature PIA and photoluminescence (PL) results, the increase in strength of the triplet-triplet (T₁-T_n) absorption of MEH-PPV in the blend system originates from the triplet-triplet energy transfer from PtOEP to MEH-PPV. The PtOEP blended MEH-PPV/C₆₀ bilayer photovoltaic device shows a roughly 30%-40% enhancement in photocurrent and power-conversion efficiency compared to the device without PtOEP. However, in contrast to the bilayer device results, the bulk heterojunction photovoltaic devices do not show a noticeable change in photocurrent and power-conversion efficiency in the presence of PtOEP. The PIA intensity, originating from the polaron state, is only slightly higher (within the experimental error), indicating that carrier generation in the bulk heterojunction is not enhanced in the presence of PtOEP. The rate and probability of the exciton dissociation between PtOEP and PCBM is much faster and higher than that of the triplet-triplet energy transfer between PtOEP and MEH-PPV.