Crack-Free Nanoporous Titania Films and Nanostructured Polymer Solar Cells Made by Spray Pyrolysis Revealed by National Taiwan University Chemists

National Taiwan University (Taipei, TW) chemists have devised a spray pyrolysis process to produce nanostructured solar cells employing nanoporous titania films.  Photovoltaic cells were fabricated by electrochemical polymerization, followed by the evaporation deposition of an 80 nm-thick Au electrode in a vacuum.  

Polymer solar cells attract great research interests because they have various advantages compared to the traditional silicon-based solar cells. For example, the low manufacturing energy and cost, light weight, flexibility and potential large-area fabrication, etc. However, the efficiency of polymer solar cell is still low. Thus, developing a high-efficiency polymer solar cell becomes an important research target. 

A polymer solar cell with a multi-layered nanostructure that is used to generate, transport, and collect electric charges was developed by Professor of Polymer Science and Engineering Lee-Yih Wang, Professor of Polymer Science and Engineering Wen-Yen Chiu, Chemical Engineer Yi-Jun Lin, and Professor Wei-Fang Su disclose  The multi-layered nanostructure comprises a cathode, a hole-blocking layer, a photo-active layer, and an anode. 

The hole-blocking layer is made of the material selected from the group consisting of the following: inorganic semiconducting material, metal oxide material and mixture of inorganic and metal oxide materials. The photo-active layer comprises a porous body and a conjugated polymer filler. The porous body is used as an electron acceptor while the conjugate polymer filler is as an electron donor. The conjugated polymer filler is formed in the pores of the porous body by in-situ polymerization.  U.S. Patent Application 20090183769 details NTU's method for preparing multi-layered nanostructured solar cells.

One main object of the method is to use in-situ polymerization technique to polymerize monomers and directly fill the pores of porous materials with thus-formed polymers. The conjugated polymer is an electron donor while the porous material is an electron acceptor. The conjugated polymer and the porous material act as a photo-active layer in a solar cell. Monomers are so small that they can easily penetrate the pores of the porous material. Then, the conjugated polymer material can be formed in the pores via the in-situ polymerization, thus to increase the interfacial area between the electron acceptor and the electron donor.

The manufacturing method can use a wide choice of conjugate polymers. Since the conjugate polymers are prepared by the in-situ polymerization according to the invention, the solvent-dissolvable monomers are more than the solvent-dissolvable conjugate polymers. Thus, fabrication technique provides convenient processing procedures, besides a wide variety of usable conjugate polymers can be used. The technique has economic advantages for industrial applications.

The resulting solar cell has a multi-layered structure that is use to generate, transport, and collect electric charges. The multi-layered nanostructure comprises a cathode, a hole-blocking layer, a photo-active layer, and an anode. The hole-blocking layer is made of inorganic semiconducting material, metal oxide material, or mixture of inorganic and metal oxide materials. The photo-active layer comprises a porous body and a conjugated polymer filler. The porous body is used as an electron acceptor while the conjugate polymer filler is as an electron donor. The conjugated polymer filler is formed in the pores of the porous body by in-situ polymerization. In addition, the invention discloses a method for preparing a solar cell.
FIG. 2 shows scanning electron microscope (SEM) images of TiO2 hole-blocking layers formed by spraying different number of layers according to example 5 of the present invention, where (a) FTO surface; (b) one layer; (c) three layers; (d) five layers; and (e) thirty layers. 

FIG. 2 show the SEM images of bare FTO glass and compact TiO2 films on FTO glass. The bare FTO surface exhibits characteristic morphology of tin oxide crystals (FIG. 2a), and differs markedly from the smooth surface of ITO substrates (not shown). One spraying cycle of TiO2 made the FTO surface smoother (FIG. 2b), but the edges of the FTO particles are still visible. TiO2 was grown and sintered on top of the FTO particles, adopting the structure of the surface relief. FIG. 2c depicts the surface after three cycles of TiO2 spray deposition.

Most of the small FTO particles were covered by TiO2, and the sharp edges of the larger FTO particles were rounded off by the deposition, such that the TiO2 surface morphology is somewhat "smoothed", but the shapes and contrast between the underneath FTO particles and the TiO2 pattern on the surface can still just be recognized. As the spray deposition was increased to five or ten cycles (FIGS. 2d-2e), the sharp edges have almost completely disappeared because of the repeated efficient formation of compact layers. Only very small particles with gentle edge-curves are seen; the surface is smooth with very low roughness, and hardly any trace of the surface morphology of the starting FTO is preserved.

However, the TiO2 film became rough and cracked with an irregular and heterogeneous distribution of titania particles after 20 cycles of spray pyrolysis deposition, as shown in FIG. 2f. Obviously, the spraying deposition technique allows the thickness of the film to be easily controlled by varying the number of spraying cycles in the preparation of compact TiO2 films, the film thickness of each layer was estimated to be around 2.5 nm. An uniform, strongly adhering and crack-free film can be produced in five to ten deposition cycles.

FIG. 3 shows the I-V characteristic curves of the solar cell comprised of the TiO2 hole-blocking layers formed by spraying pyrolysis deposition method with different number of spraying cycles. 

The effective cell area was adjusted to approximately 0.12 cm². I-V characteristics of the cell were measured with a Keithley SMU 2400 unit under AM 1.5G irradiation with an intensity of 100 mW/cm².