A Hybrid Planar–Mixed Molecular Heterojunction Photovoltaic Cell

Abstract

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...

Solar cells utilizing small molecular weight organic semiconductors

“…This can be accomplished by using a hybrid planar-mixed-molecular heterojunction (PM-HJ), consisting of a mixed DA layer sandwiched between homogeneous donor and acceptor layers. 23,110 An optimized hybrid PM-HJ structure has a thinner mixed layer than a bulk HJ cell, and the thicknesses of the homogeneous layers are approximately equal to L D in each respective layer. In this way, the device has the high η ED of a mixed HJ, and the low resistance to charge transport of a planar HJ.…”

“…19,20 Of these, the solid-state DSSC, polymer, and small molecular weight based devices have all achieved efficiencies of between 4 and 5% for a single cell. 13,[21][22][23] Keeping pace with this rapid increase in device efficiency is the number of refereed journal publications related to organic PV materials and devices, 24 which has increased exponentially for approximately the past three decades, as shown in Figure 1. Here, we focus our attention on the physics and operation of thin film PV cells based upon HJs of small molecular weight organic semiconductors.…”

Structural templating as a route to improved photovoltaic performance in copper phthalocyanine/fullerene (C60) heterojunctions

“…1-3 Fullerene ͑C 60 ͒ has become the preferred electron acceptor and it has been shown that cell structures containing mixed donor-acceptor layers, fabricated by codeposition of CuPc and C 60 , show significant improvement in device performance over simple bilayer heterojunctions. [4][5][6] Although CuPc/ C 60 OPV devices have reached power conversion efficiencies ͑ p ͒ of ϳ5%, 5 their performance remains limited by the low exciton diffusion length of CuPc and poor charge transport in the mixed CuPc/ C 60 layers.…”

Enhancing spectral contrast in organic red-light photodetectors based on a light-absorbing and exciton-blocking layered system

“…As shown in the figure, employing a mixed photoactive layer enhanced IPCE for both red and blue light. The enhancement on the conversion by mixing phthalocyanine and fullerene has been reported in the field of organic PV cells, 26 and the mechanism of the IPCE enhancement observed in this study is probably the same as in the case of organic PV cells. Here we define spectral contrast ratio ͑SCR͒ as an IPCE ratio at 620 and 450 nm.…”