Photoinduced electron transfer at the interface of semiconductor and organic donor-acceptor layer

This work investigates the electron transfer process (ET) at the interface between organic donor-acceptor dyads and semiconductor layers. The main goal was to identify possible combinations of organic dyads and semiconductors to be used in molecular heterojunction organic solar cells. The chosen donor acceptor pairs were porphyrin-fullerene (PC60) and perylene diimide-fullerene (PDIC60) whereas the studied semiconductors were zinc oxide (ZnO) and titanium dioxide (TiO2), with alumina (Al2O3) being the insulator and reference layer. Free-based porphyrin (CPTPP) was initially used to study the formation of monolayers. Spectroscopic studies were carried out to characterize the organic compounds both in solution and as monolayers on different surfaces. The organic monolayer absorption was studied by means of Langmuir-Blodgett (LB) deposition on flat glass substrates and on glass substrates covered by a thin spin-coated ZnO layer. Subsequent self-assembled monolayers (SAM) were prepared by dipping method on the same ZnO layer and their quality was compared to that of the LB ones. Photo-voltage measurements were carried out, by means of time-resolved Maxwell displacement charge (TRMDC) method, to study the vectorial electron transfer process in the organic layer and between the organic and semiconductor layers. Three different sets of samples were appositely prepared for these measures. An Al2O3 layer was deposited via atomic-layer-deposition (ALD) on ITO supports and served as insulating and reference layer with no electron transfer activity. Additional layers of either ZnO or TiO2 were deposited via ALD on top of the Al2O3 layer to yield two other sets of samples. SAM monolayers of the organic dyads were then deposited on each set and then covered with insulating LB ODA layers. The preparation of PDIC60 containing samples was satisfactory only on ZnO substrate whereas the one of PC60 on all substrates. Spectroscopic measurements were carried out to characterize the samples and confirm the presence of each molecular layer. TRMDC measures revealed the formation of a charge separated state in the active layer and the following charge recombination process in each sample. In particular the effect of the additional semiconductor layers was characterized. ZnO serves as efficient secondary electron acceptor as concluded from ten folds increase of the photo-voltage response of the PC60 SAM on ZnO as compared to that on Al2O3. For SAMs on TiO2 the response was somewhat lower in intensity as compared to Al2O3 samples. For both semiconductors long-lived charge separated states were observed, further confirming the oxide role as secondary electron acceptors. Despite of the observed sample degradation, these structures were considered as promising for organic photovoltaic applications