Carrier-selective heterojunctions are important for low-cost silicon-based photovoltaic applications. A low temperature (<100 °C) chemical vapor deposition technique is used here to deposit ultrathin (n-type) titanium dioxide (TiO₂) layers onto hydrogen-passivated surfaces of crystalline-silicon (c-Si). Energy level alignment and chemical composition at these abrupt, interfacial layer-free TiO₂/Si heterojunctions are investigated via ultraviolet, X-ray, and inverse photoemission spectroscopy, for c-Si doping ranging from p⁺⁺(10¹⁹) to n⁺⁺(10¹⁹). The interface Fermi level position and device-relevant TiO₂/Si band offsets are found to shift monotonically as a function of the Si doping, revealing the absence of Fermi level pinning at the c-Si interface and pointing to simple Fermi level equilibration as the driving mechanism behind the interface energy level alignment. Electrical transport measurements performed on TiO₂/Si-based diodes confirm the energy level alignment yielded by spectroscopic measurements and the hole-blocking properties of the TiO₂/Si heterojunction, exclude hole conduction in the TiO₂ as a transport mechanism, and show carrier recombination at the TiO₂/p-Si heterojunction.

Poly(3-hexylthiophene) (P3HT) is p-doped by the new soluble dopant molybdenum tris[1-(methoxycarbonyl)-2-(trifluoromethyl)-ethane-1,2-dithiolene] and investigated via photoemission spectroscopy and transport measurements. Soft-contact transfer lamination of thin layers of the doped P3HT on undoped polymer layers is used to create spatially-confined doped regions, which serve as hole-injection contacts on P3HT diodes. This strategy is then used to create efficient hole-collecting contacts on solution-processed inverted polymer solar cells.

Device performance and photoinduced charge transfer are studied in donor/acceptor blends of the oxidation-resistant conjugated polymer poly[(4,8-bis(2-hexyldecyl)oxy)benzo[1,2-b:4,5-b′]dithiophene)-2,6-diyl-alt-(2,5-bis(3-dodecylthiophen-2-yl)benzo[1,2-d;4,5-d′]bisthiazole)] (PBTHDDT) with the following fullerene acceptors: [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM); [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM); and the indene-C₆₀ bis-adduct IC₆₀BA). Power conversion efficiency improves from 1.52% in IC₆₀BA-based solar cells to 3.75% in PC₇₁BM-based devices. Photoinduced absorption (PIA) of the PBTHDDT:fullerene blends suggests that exciting the donor polymer leads to long-lived positive polarons on the polymer and negative polarons on the fullerene in all three polymer fullerene blends. Selective excitation of the fullerene in PC₇₁BM or PC₆₁BM blends also generates long-lived polarons. In contrast, no discernible PIA features are observed when selectively exciting the fullerene in a PBTHDDT/IC₆₀BA blend. A relatively small driving force of ca. 70 meV appears to sustain charge separation via photoinduced hole transfer from photoexcited PC₆₁BM to the polymer. The decreased driving force for photoinduced hole transfer in the IC₆₀BA blend effectively turns off hole transfer from IC₆₀BA excitons to the host polymer, even while electron transfer from the polymer to the IC₆₀BA remains active. Suppressed hole transfer from fullerene excitons is a potentially important consideration for materials design and device engineering of organic solar cells.