Charge transport and interfacial transfer in CdSe tetrapods. In addition to solar-to-electricity conversion, solar energy can also be converted as chemical fuels. In this approach, the photon generated electron hole pairs in the light absorbers are separated by delivering the electrons to reduce water or CO2 to hydrogen or carbon-rich fuels and the holes to oxidize the water into oxygen. Compared with conventional dye molecules, the artificial atoms and molecules possess superior advantages in light harvesting and charge delivery steps in this photon conversion process due to the large absorption coefficient over broad spectral range and high charge separation yield. Indeed, the artificial atoms and molecules have been successfully coupled with various catalysts for solar-to-fuel conversions, like water splitting6, 7, 24, 25, 28, 48, 49, 114-123 and CO2 reduction.124, 125 The previously published report also pointed out that the efficiency of the light harvesting and charge separation can be further improved in the branched artificial molecules due to the giant absorption coefficient of one dimensional branch and high degree of asymmetry in electron and hole wavefunctions.24, 126 Although the interfacial charge separation and recombination processes between various artificial molecules and electron acceptors or donors have been thoroughly examined and also correlated with the quantum yield of photon conversion,21-24, 127, 128 due to complexity arising from the band alignment, trapping, highly overlapped transitions, and electron-hole interaction the carrier transport mechanism in the heterostructured nanocrystals still remains open questions.129-133 Compared with the intensively studied dot-in-rod structures, tetrapod structures can have the similar band alignment but with ~10 time larger absorption coefficient resulting from the four perfectly arranged branches that can harvest photons without depending on the direction and polarization.134 In this study, we chose the CdSe tetrapod as a model system to study the carrier transport in the heterostructured light harvesters. As consequence of the different crystal structures and quantum confinements in the branches and core, a quai-type II band alignment was formed in the CdSe tetrapods. The transient absorption measurement showed that the lifetime of the electron transport from the branch to the core was ~1.5 ps. By comparing the electron transport rate in unexcited and excited tetrapods, we found that the electron transport from branch to the core was not affected by...
