Abstract:The time-resolved photoluminescence (PL) of C60 solid
film was measured at various detection wavelengths,
excitation laser fluences, and temperatures. Two emission bands
were identified which possess different
decay profiles, and these profiles exhibited completely opposite
temperature dependence. The bands were
attributed to free exciton states and self-trapped exciton states,
which decay through diffusive recombination
and activated intersystem crossing, respectively. For the latter
case, a distinct rise com… Show more
“…From the fit, we calculate the lifetime of the singlet excited state to be 1.12 ns. This value is in agreement with previous reports of singlet lifetime of C 60 and C 60 derivatives, both in film and solution. − The fluorescence decays of films stored in dry air are fit by a double-exponential function. In these films, the PL decay contains an additional component, a dominant decay with a lifetime of 267 ± 3 ps (Table S1), resulting in a weighted exciton lifetime of 388 ± 12 ps.…”
Fullerene-based molecules are the archetypical electron-accepting materials for organic photovoltaic devices. A detailed knowledge of the degradation mechanisms that occur in C60 layers will aid in the development of more stable organic solar cells. Here, the impact of storage in air on the optical and electrical properties of C60 is studied in thin films and in devices. Atmospheric exposure induces oxygen-trap states that are 0.19 eV below the LUMO of the fullerene C60. Moreover, oxygen causes a 4-fold decrease of the exciton lifetime in C60 layers, resulting in a 40% drop of short-circuit current from optimized planar heterojunction solar cells. The presence of oxygen-trap states increases the saturation current of the device, resulting in a 20% loss of open-circuit voltage. Design guidelines are outlined to improve air stability for fullerene-containing devices.
“…From the fit, we calculate the lifetime of the singlet excited state to be 1.12 ns. This value is in agreement with previous reports of singlet lifetime of C 60 and C 60 derivatives, both in film and solution. − The fluorescence decays of films stored in dry air are fit by a double-exponential function. In these films, the PL decay contains an additional component, a dominant decay with a lifetime of 267 ± 3 ps (Table S1), resulting in a weighted exciton lifetime of 388 ± 12 ps.…”
Fullerene-based molecules are the archetypical electron-accepting materials for organic photovoltaic devices. A detailed knowledge of the degradation mechanisms that occur in C60 layers will aid in the development of more stable organic solar cells. Here, the impact of storage in air on the optical and electrical properties of C60 is studied in thin films and in devices. Atmospheric exposure induces oxygen-trap states that are 0.19 eV below the LUMO of the fullerene C60. Moreover, oxygen causes a 4-fold decrease of the exciton lifetime in C60 layers, resulting in a 40% drop of short-circuit current from optimized planar heterojunction solar cells. The presence of oxygen-trap states increases the saturation current of the device, resulting in a 20% loss of open-circuit voltage. Design guidelines are outlined to improve air stability for fullerene-containing devices.
“…So far, the ISC processes for pristine fullerenes from C 60 to C 84 have been extensively studied experimentally with comprehensive elaborations of the ISC rates and ISC quantum yields over the past several years. ,,,− The measured ISC quantum yields for C 60 , C 70 , C 76 , C 78 , and C 84 are 0.93, 0.90, 0.05, 0.12, and 0.01, respectively, ,,, decreasing with the increase of carbon number of fullerenes. For higher fullerenes C n ( n > 84), experimental studies are lacking for ISC processes because of the difficult isolation from abundant isomers, such as C 92 , C 96 , C 100 , etc.…”
Section: Introductionmentioning
confidence: 99%
“…Based on Marcus formula, the ISC rate is heavily dependent on the three factors, namely, driving force, reorganization energy, and spin–orbit coupling (SOC). The driving force of singlet to triplet states is defined by the difference between the adiabatic energies of singlet and triplet states, and many experiments and theoretical calculations have been used to determine these values for C 60 to C 120 . ,,,− ,, However, to the best of our knowledge, for the other two factors, reorganization energy and spin–orbit coupling have not been systematically quantified for these large fullerene systems. The reorganization energy reflects the rigidity of system arising from energy relaxation from the equilibrium configurations of primary state to the final state, and spin–orbit coupling impacts the interaction between the initial singlet state and final triplet state.…”
Although the triplet states of fullerenes have prosperous applications, it remains unclear how the structural parameters of singlet and triplet states control the intersystem crossing (ISC) rates. Here, electronic structure calculations (reorganization energy, driving force, and spin-orbit coupling) and a rate theory (Marcus formula) are employed to quantitatively predict the ISC rates of isolated fullerenes C (n = 60-110). The results demonstrate that the driving force is not the only factor to predict the ISC rates. For instance, although C, C, and C have the favorable driving force, the ISC rates are close to zero because of small spin obit couplings, whereas small ISC rates of C and C result from quite small reorganization energies. Meanwhile, in addition to well-known C and C, C possesses good ISC property with obviously large ISC rate. C also has a higher triplet-state energy than singlet-state oxygen energy; it may thus have a good photoactive property.
“…Upon increasing the delay time, the intensity of the PL1 band decreases rapidly, while the PL2 band becomes a major component after 0.3 ns time delay. Although the number of PL bands is different from that of C 60 solid, the time evolution of spectral shape shown in these figures suggests that these PL bands have different origins, as in the case of C 60 . 1 (a) Time-integrated PL spectrum of C 70 solid collected over 1 μs after a picosecond dye laser excitation at 590 nm at 10 K. (b−f).…”
The exciton dynamics of C70 solid has been investigated by the time-resolved photoluminescence (PL) and
transient absorption (TA) technique and compared with the case of C60 solid. Three distinct PL bands were
observed at 685, 715, and 740 nm. The high-energy band at 685 nm displayed an almost purely single-exponential decay profile, while the other two exhibited a slow-decaying tail as well, especially at low
temperatures. Laser fluence dependence of PL decay profiles shows that the decay in C70 solid, in contrast to
the case of C60, is predominantly governed by fast (20 ps) nonradiative exciton-trap annihilation, with the
intersystem crossing playing a minor role in lower energy bands at 715 and 740 nm. On the other hand, laser
fluence dependence of transient absorption signals shows that the decay in C70 is much faster than PL decay,
which indicates the fast component arises from the exciton−exciton annihilation process under extremely
high laser fluence condition.
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