The power conversion efficiency of small-molecular-weight and polymer organic photovoltaic cells has increased steadily over the past decade. This progress is chiefly attributable to the introduction of the donor-acceptor heterojunction that functions as a dissociation site for the strongly bound photogenerated excitons. Further progress was realized in polymer devices through use of blends of the donor and acceptor materials: phase separation during spin-coating leads to a bulk heterojunction that removes the exciton diffusion bottleneck by creating an interpenetrating network of the donor and acceptor materials. The realization of bulk heterojunctions using mixtures of vacuum-deposited small-molecular-weight materials has, on the other hand, posed elusive: phase separation induced by elevating the substrate temperature inevitably leads to a significant roughening of the film surface and to short-circuited devices. Here, we demonstrate that the use of a metal cap to confine the organic materials during annealing prevents the formation of a rough surface morphology while allowing for the formation of an interpenetrating donor-acceptor network. This method results in a power conversion efficiency 50 per cent higher than the best values reported for comparable bilayer devices, suggesting that this strained annealing process could allow for the formation of low-cost and high-efficiency thin film organic solar cells based on vacuum-deposited small-molecular-weight organic materials.

We demonstrate double-heterostructure copper phthalocyanine/C60 organic photovoltaic cells with series resistances as low as 0.1 Ω cm2. A high fill factor of ∼0.6 is achieved, which is only slightly reduced at very intense illumination. As a result, the power conversion efficiency increases with the incident optical power density, reaching a maximum of (4.2±0.2)% under 4–12 suns simulated AM1.5G illumination. The cell performance is accurately described employing an analysis based on conventional semiconductor p–n junction diodes. The dependence of the series resistance on the device area suggests the dominance of the bulk resistance of the indium-tin-oxide anode as a limiting factor in practical cell efficiencies.

The foaming solutions were prepared by mixing a cationic surfactant (tetradecyltrimethylamonium bromide, TTAB) or an anionic surfactant (sodium dodecylsulfate, SDS), with water, titanium ethoxide, and HCl. Typically, titanium ethoxide was added to an aqueous solution of TTAB (35 wt.-%) or SDS (15 wt.-%) in order to reach a proportion of 10 wt.-%. Then, the pH of the solution was adjusted to pH = 1 by adding HCl (37 %). The mixture was subjected to strong stirring for 30 min to homogenize the solution and to evaporate ethanol produced by the hydrolysis of titanium alkoxide. A particulate sol could be obtained by aging for 20 h. Foam was obtained by bubbling nitrogen through a porous glass disk into perfluorohexane in a 2.5 cm-diameter, 60 cm-high Plexiglas column. Different porosity glass disks (100±160 lm, 40±100 lm, 16±40 lm, or 10±16 lm) could be used to introduce nitrogen into the foaming solution. The reaction took place inside the Plexiglas column. During the reaction, the foam was wetted from above with the foaming solution. Imposing a sol flux Q at the top of the foam allowed the imposition of a constant and homogeneous liquid fraction to the entire sample. Varying the sol flux Q at the top of the foam varied the liquid fraction, and thus tuned the morphology of the foam. Metastable foams were recovered at the top of the column with a spatula and stored in a beaker. Then, the foam was immediately treated with an aqueous ammonia solution (20 wt.-%) with a pipette in order to promote titanium dioxide condensation. The quantity of ammonia used during the process depended upon the foam-liquid fraction. Typically, we used 0.5 mL of ammonia solution for 100 mL of foam and a sol flux of 0.024 g s ±1 , so the ratio was 2 mL/ 100 mL for a sol flux of 0.160 g s ±1 . The final foams were then frozen overnight and lyophilized for 5 h. The resulting hybrid organic±inor-ganic monolith-type materials were then thermally treated at 500 C in order to obtain the anatase structure of TiO 2 , or at 900 C to obtain the rutile structure. The heating rate was 2 C min ±1 , with a first Plateau at 200 C for 2 h. The cooling process was uncontrolled and depended upon oven cooling. The final inorganic scaffolds were then analyzed.Transmission electron microscopy (TEM) experiments were performed with a Jeol 2000 FX microscope (acceleration voltage of 200 kV). The samples were prepared as follows: TiO 2 scaffolds in a powder state were deposited on a copper grid coated with a Formvar/ carbon membrane. Scanning electron microscopy (SEM) observations were performed with a Jeol JSM-840A SEM operating at 10 kV. The specimens were gold-coated or carbon-coated prior to examination. Mesoscale surface areas and pore characteristics were obtained with a Micromeritics ASAP 2010 instrument, employing the Brunauer±Em-mett±Teller (BET) method. Prior to performing the nitrogen adsorption±desorption measurements, the macrocellular-foam monoliths were reduced to a powder state. Small-angle X-ray experiments were carried on with an 18 kW rotating-anod...

We demonstrate high-efficiency organic photovoltaic cells by stacking two hybrid planar-mixed molecular heterojunction cells in series. Absorption of incident light is maximized by locating the subcell tuned to absorb long-wavelength light nearest to the transparent anode, and tuning the second subcell closest to the reflecting metal cathode to preferentially absorb short-wavelength solar energy. Using the donor, copper phthalocyanine, and the acceptor, C60, we achieve a maximum power conversion efficiency of ηP=(5.7±0.3)% under 1 sun simulated AM1.5G solar illumination. An open-circuit voltage of VOC⩽1.2V is obtained, doubling that of a single cell. Analytical models suggest that power conversion efficiencies exceeding 6.5% can be obtained by this architecture.

In this and the following paper ͑Parts I and II, respectively͒, we discuss the properties of mixed donor-acceptor organic thin films and their application to organic solar cells. In Part I, we present a study of the material properties of mixed donor-acceptor thin films. Through optical absorption, x-ray diffraction, microscopy, and charge transport measurements, we determine the relationships among film microstructure, mixing ratio, and charge conduction in mixtures of two organic molecular species. We find that mixed layers of the molecular pair of 1:1 ͑by weight͒ copper phthalocyanine in C 60 have electron and hole mobilities reduced by more than one order of magnitude compared to corresponding films of pure composition. In Part II, we demonstrate that the performance of organic hybrid planar-mixed heterojunction photovoltaic cells based on a mixed donor-acceptor molecular layer sandwiched between the donor and acceptor layers of homogeneous composition can have improved performance over conventional planar heterojunction cells containing no mixed composition layers.

An efficient organic solar cell with a vacuum codeposited donor–acceptor copper phthalocyanine (CuPc):C60 mixed layer is described. A device with a structure of indium tin oxide/330 Å CuPc:C60(1:1)/100 Å C60/75 Å 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline/Ag has a series resistance of only RS=0.25 Ω cm2, resulting in a current density of ∼1 A/cm2 at a forward bias of +1 V, and a rectification ratio of 106 at ±1 V. Under simulated solar illumination, the short circuit current density increases linearly with light intensity up to 2.4 suns. The maximum power conversion efficiency is ηP=(3.6±0.2)% at 0.3 suns (AM1.5G simulated solar spectrum) and ηP=(3.5±0.2)% at 1 sun. Although the fill factor decreases with increasing intensity, a power efficiency as high as ηP=(3.3±0.2)% is observed at 2.4 suns intensity.

We demonstrate efficient organic photovoltaic cells employing a photoactive region composed of a mixed donor-acceptor molecular layer, the properties of which were introduced in the preceding paper (Part I) [Rand et al., J. Appl. Phys. 98, 124902 (2005)]. The hybrid planar-mixed heterojunction (PM-HJ) device architecture consists of a film mixture of donor and acceptor molecules inserted between layers of pure donor and acceptor composition. Using the donor, copper phthalocyanine, and the acceptor, C60, we demonstrate a hybrid PM-HJ cell with a maximum power conversion efficiency of (5.0±0.3)% under 1–4suns simulated AM1.5 solar illumination. The current-voltage characteristics of the PM-HJ cell are described using a model based on the field-dependent charge collection length.

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