11), we can derive a boundary for the minimum energy difference between the HOMO of CuPc and the LUMO of C 60. Finally, from the open-circuit voltage of the CuPc:C 60 heterojunction solar cell, which corresponds to ~0.5 eV (ref. This corresponds to the charge-separated state configuration on the C 60 side of the heterojunction, where C 60 is now negatively charged. Accordingly, the LUMO orbital of a fully occupied C 60 is located at an energy between 2.0 and 2.3 eV above the HOMO. Gas-phase spectra of negatively charged C 60 − indicate an energy difference of 2.0 eV between the X 1A g ground state and the B 1S 1 lowest singlet excited state of neutral C 60 (ref. Combining photoemission and inverse photoemission spectroscopy, the onset of the HOMO–LUMO gap in solid C 60 was determined to 2.3 eV (ref. This allows to determine the position of the HOMO of C 60 on the right side of Fig. Photoemission measurements established an offset of 1.45 eV between the HOMO of CuPc and C 60 (ref. Singlet and triplet excitons in CuPc are located ~2 and 1.2 eV, respectively, above the ground state 3, 4, 5, 6, 7. The energy level diagram of the CuPc:C 60 heterojunction, based on a combination of various spectroscopic data, is shown in Fig. Let us review the knowledge about the energy landscape of the relevant electronic states involved. Photoemission and inverse photoemission spectroscopy demonstrated that this process is energetically enabled by the electronic level alignment of the compounds forming the heterojunction, including CuPc and C 60 (ref. Adding a small amount of C 60, the fluorescence radiation from the recombination of the excitonic state of the chromophore is quenched 1, 2, indicating a vastly improved efficiency of charge generation. For many organic systems, C 60 is an excellent acceptor, capturing the electron and thus separating the charges. Light harvesting in CuPc:C 60 is initiated through creation of an excitonic state at the chromophore (CuPc), while the desired final state consists of a separated electron–hole pair with a vacancy in the chromophore and a free electron in C 60. Evidently, a better theoretical understanding and novel experimental approaches are needed to validate or dismiss fundamental assumptions, regarding the nature and fate of photoexcited states in organic heterojunctions. Even more concerning, partly contradicting interpretations and models have been presented regarding the question, which initial excitations contribute to charge generation, and which do not. Copper-phthalocyanine (CuPc):C 60 is a canonical model system for this class of devices, but despite a significant body of research, fundamental mechanisms for charge separation remain obscure. Improving the efficiency of the underlying light-harvesting and charge generation processes, however, requires detailed knowledge of all the steps from the initial light-induced excitation of the chromophore to the final state, where charges are separated in the donor and acceptor phases. Organic donor–acceptor systems are particularly intriguing candidates for light-harvesting applications, as their properties can be readily modified using well-established chemical synthesis techniques. Photoinduced charge generation plays a central role in a broad range of physical, chemical, and biological processes that underlie natural and engineered photocatalytic and photovoltaic systems.
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