Photoanode And Counter Electrode Studies Of Quantum Dot Sensitized Solar Cells

Quantum dot sensitized solar cells(QDSSCs)have attracted increasing attention because of QDs' low production cost,size tunable band gap,and high optical extinction coefficient.Although the theoretical efficiency of QDSSCs is as high as 66%in view of hot photogenerated carriers and multiple exciton generation originating from impact ionization effect,the highest power conversion efficiency of QDSSCs in present is 11.66%,the general conversion efficiency of QDSSCs is around 4-6%,with even lower efficiency of around 2%for single species sensitizer of CdS and 1%for single species sensitizer of CdSe.In addition to the redox electrolyte,the key reasons for the low efficiencies of QDSSCs are attributed to the inefficient charge injection and electron transport in photoanode and the poor electrocatalytic activity of the counter electrodes(CE).This thesis aims at the above problems to study the following aspects:(1)Synthesis of Mn doped zinc blende CdSe quantum dots for an enhanced performance on QDSSCs QDs.Pure CdSe and Mn doped CdSe quantum dots(QDs)with a zinc blende structure were synthesized via a phosphine-free method in octadecene(ODE)and oleic acid.QDSSCs based on the above-synthesized QDs as sensitizers were fabricated.The photovoltaic performance of the CdSe and Mn-doped CdSe QDSSCs were further comparative investigated.An improvement in efficiency to 1.84%was achieved as compared with 0.94%for the QDSSCs based on the pure CdSe QDs.The improvement was ascribed to the existence of long lived high-energy doping levels on the large-sized Mn doped CdSe QDs,which provides a significant driving force for faster charge separation and electron transfer.(2)Engineered band structure for an enhanced performance on QDSSCs.Solvothermal synthesized Mn:ZnO(MZnO)nanowire are spin coated on fluorine doped tin oxide(FTO)glass with P25 paste to serve as photoanode after calcinations.Mn doped CdS(MCdS)was deposited on the MZnO film by the successive ionic layer adsorption and reaction(SILAR)method.A photon-to-current efficiency of 2.93%is received for the MCdS QDSSCs using MZnO nanowire as photoanode.The enhanced performance may be due to the long lived excitation energy state of Mn2+ is located inside the conduction band in the wide bandgap ZnO and under the conduction band of CdS,which increases the energetic overlap of donor and acceptor states,reducing the "loss-in-potential",inhibiting charge recombination,and accelerating electron injection.(3)Performance enhancement in titania based QDSSCs through incorporation of disc shaped ZnO particles into photoanode.Zinc oxide along c-axis possesses higher carrier mobility than in other directions.Uniform single-crystalline ZnO round discs have been synthesized hydrothermally.The disc is dominated by circular column-based thin disc of about 1-1.5?m in diameter and 200-300nm in thickness.The growth is favored along directions perpendicular to the[0001]zone axis with the typical growth along the[0001]direction suppressed.Disc shaped ZnO particles with different content of 2.5%,5%,and 7.5%are embedded into traditional titanium dioxide photoanodes and QDSSCs are assembled using these electrodes.With the aid of ZnO discs cells display enhanced performance that peaks at 5%disc loadings with a power conversion efficiency of 5.36%.The performance enhancement is as a result of improved conductivity of ZnO discs in the photoanodes.(4)Lead selinide thin film deposited by pulsed voltage deposition as counter electrode for high performance QDSSCs.Lead selenide(PbSe)thin films were deposited on fluorine doped tin oxide(FTO)glass by a facile one-step pulse voltage electrodeposition method,and used as counter electrode(CE)in CdS/CdSe QDSSCs.A power conversion efficiency of 4.67%is received for the CdS/CdSe co-sensitized solar cells,which is much better than that of 2.39%received using Pt CEs.The enhanced performance is attributed to the extended absorption in the near infrared region,superior electrocatalytic activity and p-type conductivity with a reflection of the incident light at the back electrode in addition.