The current–voltage characteristics of methanofullerene [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM)‐based devices are investigated as a function of temperature. The occurrence of space–charge limited current enables a direct determination of the electron mobility. At room temperature, an electron mobility of μe = 2 × 10–7 m2 V–1 s–1 has been obtained. This electron mobility is more than three orders of magnitude larger than the hole mobility of donor‐type conjugated polymer poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene) (OC1C10‐PPV). As a result, the dark current in PCBM/OC1C10‐PPV based devices is completely dominated by electrons. The observed field and temperature‐dependence of the electron mobility of PCBM can be described with a Gaussian disorder model. This provides information about the energetic disorder and average transport‐site separation in PCBM.

In this study, the influence of the TiCl(4) post-treatment on nanocrystalline TiO(2) films as electrodes in dye-sensitized solar cells is investigated and compared to nontreated films. As a result of this post-treatment cell efficiencies are improved, due to higher photocurrents. On a microscopic scale TiO(2) particle growth on the order of 1 nm is observed. Despite a corresponding decrease of BET surface area, more dye is adsorbed onto the oxide surface. Although it seems trivial to match this finding with the improved photocurrent, this performance improvement cannot be attributed to higher dye adsorption only. This follows from comparison between incident photon to current conversion efficiency (IPCE) and light absorption characteristics. Since the charge transport properties of the TiO(2) films are already more than sufficient without treatment, the increase in short circuit current density J(SC) cannot be related to improvements in charge transport either. Transient photocurrent measurements indicate a shift in the conduction band edge of the TiO(2) upon TiCl(4) treatment. It is concluded that the main contribution to enhanced current originates from this shift in conduction band edge, resulting in improved charge injection into the TiO(2).

The transient photoconductivity of blends of a highly soluble C60 derivative ͑PCBM͒ and a dialkoxyphenylene-vinylene polymer ͑MDMO-PPV͒ has been studied using the electrodeless flash-photolysis timeresolved microwave conductivity technique ͑FP-TRMC͒. Films approximately 100 nm thick on a quartz substrate were prepared by spin-coating PCBM/PPV solutions with PCBM weight fractions (W PCBM ) from 0.2 to 0.95. For all blends, the wavelength dependence of the photoconductivity in the range 420-700 nm closely resembled the photon attenuation spectrum, indicating that photoexcitation of both components contributes to mobile charge carrier formation. The product of the quantum yield for charge separation and the sum of the charge carrier mobilities ⌺ was determined from the maximum ͑end-of-pulse͒ value of the transient photoconductivity. On excitation at 500 nm, ⌺ remained almost constant in going from W PCBM ϭ0.2 to 0.6 with an average value of 0.6ϫ10 Ϫ3 cm 2 /V s. Above W PCBM ϭ0.6, ⌺ increased dramatically, reaching a maximum value of 83ϫ10Ϫ3 cm 2 /V s for W PCBM ϭ0.85. This effect is attributed to the occurrence of phase separation above W PCBM ϭ0.6 and to the resulting formation of highly mobile electrons within PCBM-rich aggregates. The much lower value of ⌺ observed below W PCBM ϭ0.6 is assigned mainly to mobile holes within the polymer component of the blend. Possible explanations for the decrease in ⌺ with increasing light intensity, found for all blend compositions, are discussed.

Dye-sensitized TiO 2 solar cells have attracted considerable interest since it was shown that conversion efficiencies of up to 10 % can be reached. [1,2] In recent years successful attempts have been made to replace the liquid electrolyte solution in these cells with elastomeric polymers or gelators filled with an ionic redox couple, [3±7] and by p-type organic or inorganic semiconductors, [1±3,8] resulting in conversion efficiencies of up to 3.2 %.[4] A particularly attractive approach is to replace the dye and liquid electrolyte with a single conjugated polymer which combines the functions of light absorption and hole transport.[5±9] However, one of the problems associated with conjugated polymers and nanocrystalline TiO 2 (nc-TiO 2 ) is the inefficient filling of the pores of the TiO 2 matrix, especially because small (~20 nm) pores are required to ensure that the majority of photoexcited charge carriers in the polymer reach the interface with TiO 2 where charge separation occurs. In practice, hybrid polymer/nc-TiO 2 photovoltaic devices are now limited by charge transport (incomplete filling) and charge generation (large pores).Here, we present a new and simple procedure for preparing bulk-heterojunction hybrid organic±inorganic solar cells in which a continuous interpenetrating network of TiO 2 is created inside a thin conjugated polymer film. The characteristic distance of the polymer:TiO 2 bulk-heterojunction is in the nanometer range and results in an efficient formation of photoinduced charges that can be collected when the film is incorporated in a photovoltaic device.To prepare the bulk-heterojunction, a dry tetrahydrofuran (THF) solution containing poly[2-methoxy-5-(3¢,7¢-dimethyloctyloxy)-p-phenylene vinylene] (MDMO-PPV; Scheme 1) and titanium(IV) isopropoxide (Ti(OC 3 H 7 ) 4 ), a precursor for TiO 2 , is spin-coated on a substrate to obtain a mixed film with a thickness of about 50±70 nm. Subsequent conversion in the dark of the Ti(OC 3 H 7 ) 4 precursor via hydrolysis in air results in the formation of a TiO 2 phase in the polymer film. As a consequence of the presence of TiO 2 , the bulk-heterojunction film becomes resistant to scratching and can no longer be wiped off the substrate. Furthermore, the polymer in the bulk-heterojunction no longer dissolves in organic solvents such as toluene.X-ray photoelectron spectroscopy (XPS) has been used to assess the formation of TiO 2 under these conditions by examining the C/Ti and Ti/O elemental ratios, which are expected to change from 12:1 and 1:4 in Ti(OC 3 H 7 ) 4 to 0:1 and 1:2 in TiO 2 . Because the elemental composition is strongly influenced by the polymer in the film, the conversion of Ti(OC 3 H 7 ) 4 into TiO 2 under ambient conditions (in the absence of polymer) was also studied and compared to a film of TiO 2 prepared via the same procedure followed by conversion at high temperature (400 C) in air for 30 min. Both samples were then placed under vacuum overnight. The XPS spectra of the low-and high-temperature converted pure Ti(OC 3 H 7 ) 4 f...