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Owing to the excitonic nature of photoexcitations in organic semiconductors, the working mechanism of organic solar cells relies on the donor-acceptor (D/A) concept enabling photoinduced charge transfer at the interface between two organic materials with suitable energy-level alignment. However, the introduction of such a heterojunction is accompanied by additional energy losses compared to an inorganic homojunction cell due to the presence of a charge-transfer (CT) state at the D/A interface. By careful examination of planar heterojunctions of the molecular semiconductors diindenoperylene (DIP) and C${}_{60}$ we demonstrate that three different analysis techniques of the temperature dependence of solar-cell characteristics yield reliable values for the effective photovoltaic energy gap at the D/A interface. The retrieved energies are shown to be consistent with direct spectroscopic measurements and the D/A energy-level offset determined by photoemission spectroscopy. Furthermore, we verify the widespread assumption that the activation energy of the dark saturation current $\ensuremath{\Delta}E$ and the CT energy ${E}_{\mathrm{CT}}$ may be regarded as identical. The temperature-dependent analysis of open-circuit voltage ${V}_{\mathrm{OC}}$ and dark saturation current is then applied to a variety of molecular planar heterojunctions. The congruency of $\ensuremath{\Delta}E$ and ${E}_{\mathrm{CT}}$ is again found for all material systems with the exception of copper phthalocyanine/C${}_{60}$. The general rule of thumb for organic semiconductor heterojunctions, that ${V}_{\mathrm{OC}}$ at room temperature is roughly half a volt below the CT energy, is traced back to comparable intermolecular electronic coupling in all investigated systems.

We optically probe the spectrum of ground and excited state transitions of an individual, electrically tunable self-assembled quantum dot molecule. Photocurrent absorption measurements show that the spatially direct neutral exciton transitions in the upper and lower dots are energetically separated by only ~2 meV. Excited state transitions ~8-16 meV to higher energy exhibit pronounced anticrossings as the electric field is tuned due to the formation of hybridized electron states. We show that the observed excited state transitions occur between these hybridized electronic states and different hole states in the upper dot. By simultaneously pumping two different excited states with two laser fields we demonstrate a strong (88% on-off contrast) laser induced switching of the optical response. The results represent an electrically tunable, discrete coupled quantum system with a conditional optical response.

We demonstrate the ability to control the spontaneous emission dynamics of self-assembled quantum dots via the local density of optical modes in two-dimensional s2Dd photonic crystals. We show that an incomplete 2D photonic band gap is sufficient to significantly lengthen the spontaneous emission lifetimes.2 3 d over a wide bandwidth sDl o40 nmd. For dots that are both spectrally and spatially coupled to strongly localized fVmodes, 1.5sl / nd 3 g, high Q, 2700 optical modes, we have directly measured a strong Purcell-enhanced shortening of the emission lifetime o5.63, limited only by our temporal resolution. Analysis of the spectral dependence of the recombination dynamics shows a maximum lifetime shortening of 19± 4. From the directly measured enhancement and suppression we show that the single-mode coupling efficiency for quantum dots in such structures is at least b = 92% and is estimated to be as large as ,97%.