高反射间隔层在单基板染料敏化太阳能电池中的应用

Application of highly reflective spacer layer in monolithic dye-sensitized solar cells

提高太阳能电池对入射光的吸收是增强光电流的重要方法.本文将金红石二氧化钛@二氧化硅核壳复合纳米颗粒作为间隔层的基本组分,并在浆料中加入聚苯乙烯次微米球.烧结后,聚苯乙烯次微米球分解在间隔层薄膜中留下次微米孔,一方面作为散射中心有效地提高了间隔层对可见光的反射,另一方面作为电解质中氧化还原电对的快速扩散通道,从而提高了单基板染料敏化太阳能电池的短路电流.

Monolithic dye-sensitized solar cells （DSSCs） based on TiO2/spacer/carbon triple mesoporous films are low-cost, ease of fabrication with screen printing technology, possessing great potential to produce electricity from sunlight. The power conversion efficiency is still low compared to commercial photovoltaic technology. For DSSCs with platinum coated FTO or gold as counter electrode, the mirror like surface of counter electrode can reflect visible light back to be re-absorbed by dye-sensitized TiO2 film. However, for monolithic DSSCs, there is no reflection layer. To enhance absorbance of incident photons is an important way to improve photocurrent of monolithic DSSCs. Spacer film can be used as scattering layer to reflect visible light. However, spacer usually absorbs large amount of dyes due to large surface area and surface basic sites. Here, we report highly reflective spacer with no dyes anchoring onto surfaces of spacer. The scattering centers are sub-micrometer voids created by decomposing sub-micrometer polystyrene （PS） spheres. The building blocks are composed of rutile TiO2@SiO〉 Ruffle TiO2 was selected due to its large refractive index. SiO2 was coated on surface of rutile TiO2 to avoid dye adsorption. Moreover, the sub-micrometer voids provide fast transporting channels for redox species. To fabricate spacer paste that can be used with screen printing method to produce film, ruffle TiO2@SiO2 nanoparticles and PS spheres with diameter of 400 nm were first synthesized separately and then, they were mixed homogeneously by ball milling with addition of ethyl cellulose binder and organic dispersant medium （ct-terpineol）. Rutile TiO2 was obtained by reaction of TIC14 in HC1 aqueous solution at 80~C and sub-micrometer PS spheres were synthesized with emulsion polymerization of styrene monomer at 70~C. Rutile TiO2@SiO2 core@shell nanoparticles were obtained by hydrolysis of tetraethyl silicate on the surface of rutile TiO2. The UV-visible reflection spectra of sintered spacer film at 500~C showed that the spacer film with ratio （rutile TiO2@SiO2 to polystyrene） of 1：0.2 or 1：0.4 have maximum reflectivity in the visible range. However, the conventional ZrO2 spacer film with the same thickness has much weaker reflectivity. The dye absorption test of bilayer film with spacer lying top of TiO2 film reveals that the dye molecules can easily go through spacer film to absorb onto surface of mesoporous TiO2 film while the spacer film on the top keeps white color unchanged. Scanning electron images of spacer film also showed that sub-micrometer voids distributed uniformly in the film and the diameter of voids is similar to that of PS spheres. These sub-micrometer voids in spacer film can provide fast transporting channels for redox species, facilitating dye regeneration. By replacing the conventional ZrO2 spacer with the advanced spacer, photocurrent density of monolithic DSSCs was increased by 25 %. As a result, the power conversion efficiency has been improved to 4.61% from 3.95% with non-volatile iodine-free polymer gel electrolyte. These results endow spacer a new ability in the device and provide another guideline to enhance photocurrent of monolithic dye-sensitized solar cells. The design of spacer in this work may find other applications in the area of optics, catalyst, etc.