At present Dye sensitized solar cells (DSSC) acts as a promising alternative to the conventional silicon solar cells due to their superior benefits of low cost, easy fabrication and processing and suitable efficiency. One of the research issues related to the DSSC performance enhancement is the innovation and development of nanomaterials as photosensiztiers and modification of the nasnosturcured $TiO_2$ electrode for the enhancement of the cell performance.
Recently inorganic semiconductor nanocrystals known as as quantum dots have been received a wider attention as photosensitizers due to their superior and interesting properties like tunable bandgaps and impact ionization induced by quantum size effects. Additionally quantum dots have high extinction coefficients as compared to dyes. Contrary to all such advantages the efficiency and the fill factors of Quantum dot sensitized solar cells (QDSC) are still very low (Fill Factor even less than 0.50) as compared to DSSC.
Focusing the issue of well assembly of QDs into mesoporous $TiO_2$, an attempt was made by mimicking the dye adsorption upon the $TiO_2$ surface in DSSC. In case of DSSC, the carboxylic acid groups in N3 dyes plays an important role in adsorption of the dyes upon $TiO_2$ surface via electronic coupling. Such an approach was followed to enhance the adsorption of the CdS QDs into mesoporous $TiO_2$ electrode via surface modification of the CdS QDs with organic compound 2-mercaptoethanol (ME). 2-mercaptoethanol as revealed form formula $HOCH_2CH_2SH$ consists of thiol group (SH) at one end and hydroxyl group (OH) at the other end. The thiol groups of ME get linked to the CdS nanocrystal surface while the hydroxyl group at the other end binds to the $TiO_2$ surface. But the cell performance but unfortunately found to be very low. The main reason was low photocurrent may be due to the spatial distance between CdS QDs and $TiO_2$ electrode and partial coverage of CdS QDs by ME. Additional surface treatment of the m-CdS-QDs sensitized $TiO_2$ electrode was performed by ionic solution providing sulfur ions. Surface treatment was performed by simply dipping the m-CdS QDs sensitized $TiO_2$ electrode into $Na_2S$ ionic solution. After the surface treatment an increase in the photocurrent was observed which improves the cell efficiency comparatively indicating that the mercaptoethanol modified QDs still have unpassivated sites upon their surface. It can be considered that ME acting as effective capping and linking agent, but partially passivates the uncoordinated surface cadmium ions. Some surface sites, not covered by ME were passivated by sulfur ions and enhance the electron density by converting the surface cadmium ions to CdS optically active site.
In the second investigation, an effort was done to captivate the advantages of such approach employing in CdS-QDSC. Following such approach, the electrode/sensitizer that is $TiO_2$/ CdS quantum dots interface was modified by inserting a thin layer of ZnO with the expectations of reduction in the electron recombination processes and enhancement in QDSC performance. ZnO was chosen as a suitable material energy barrier for its well known conducting properties and due its suitable alignment of conduction band edge (0.1~0.2eV) with the $TiO_2$ conduction band, allowing favorable flow of electrons and acting as potential barrier for the electrons to backflow from $TiO_2$ conduction band.
The ZnO thin film was inserted between the interface via chemical bath deposition upon mesoporous $TiO_2$ electrode with simple chemical process known as SILAR (successive ionic layer adsorption and reaction). The presence of ZnO upon $TiO_2$ was confirmed by different characterization tools including UV-Visible spectroscopy, x-ray diffraction and energy dispersive x-ray analysis. After the confirmation of the presence of ZnO layer upon mesoporous $TiO_2$ electrode, the $TiO_2$/ZnO electrode was sensitized with CdS quantum dots with already optimized number of deposition cycles for CdS(8 CBD) carried by SILAR method. It was observed that the formation of ZnO layer upon $TiO_2$ enhances the QDSC (Quantum dot sensitized solar cell) performance when compared to the QDSC employing electrode without ZnO energy barrier. The increase in performance was mainly contributed by an increase in open circuit voltage ($V_{oc}$) with a little increase in photocurrent. This increase in $V_{oc}$ is attributed to the increase in electron density due to reduced electron recombination.
현재 염료감응 태양전지는 낮은 가격, 쉬운 제조공정, 높은 효율로 인해 실리콘 태양전지를 대체가능할 것으로 기대하고 있다. 염료감응 태양전지의 효율 향상을 위한 연구 이슈 중 하나로 광감응재로서의 나노물질 개발 및 $TiO_2$ 전극의 개질에 관한 것이다.
최근 양자점이라고 불리우는 무기 반도체 나노결정체는 밴드갭 조절이 가능 하고 양자 크기 효과에 의한 이온화 등의 장점으로 인해 광감응재로 주목을 받고 있다. 게다가 양자점은 염료와 비교시에 상대적으로 많은 양의 빛을 흡수할 수 있다. 하지만 이러한 장점과는 달리, 양자점 태양전지의 효율은 여전히 낮은 수준이다.
첫 번째 주제는 $TiO_2$ 표면에 링커를 이용하여 양자점을 잘 조립하는 것이다. 그 후에 $Na_2S$ 용액을 이용, 표면처리 효과를 확인하였다.
두 번째 주제는 $TiO_2$/CdS 계면에 ZnO를 이용해 얇은 막을 코팅하여 코어쉘 구조를 형성하여 양자점 태양전지로 응용하는 것이다
