Fabrication and investigation of heterojunction solar
\ncells based on near infrared quantum dots and one
\ndimensional nanostructures.

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\nSolar cell technology, which harvests the solar energy and converts it to direct current electricity,
\nis a viable alternative to provide clean and renewable energy from an abundant source to meet the
\nenergy demand of the growing world population in the future. For a solar cell technology to be
\napplicable practically it has to compete with other sources of energy in terms of cost. Furthermore
\nthe power conversion efficiency (PCE) of a solar cell is important. Since the discovery of the
\nphotovoltaic effect in 1839 by the French scientist Alexandre- Edmond Becquerel [1], various
\nmaterials and device structures have been investigated resulting in development of devices of high
\nPCE and even commercialized for practical applications. Among figures of merit, for example,
\nsingle junction devices have achieved a record PCE of 28.3 % [2] and multijunction devices (4
\njunctions) have achieved a PCE of 46 % [3]. The most widely commercialized single crystalline
\nsilicon solar cells have achieved a record PCE of 25.6 %. However, these solar cells are produced
\nat very high processing cost and it is quite necessary to develop solar cells, which can be produced
\nat lower cost and with reasonable PCE so that they can compete with other sources of energy.
\nThe solar cells investigated by a plethora of researchers so far are classified as different generations
\nbased on their cost, PCE, device characteristics, and the nature of the materials involved. While
\nthe first generation solar cells, such as single crystalline silicon solar cells, achieved high PCE,
\ntheir price is high. The second generation solar cells have low cost, but their efficiency is low. The
\nconcept of the third generation solar cells is later on proposed towards making low cost and high
\nPCE solar cells. Quantum dots (QDs) based solar cells are considered as promising third generation
\nsolar cells candidates. Among QDs synthesized by different techniques, colloidal QDs, which are
\nsynthesized in solution, are attractive for applications in solar cells because of their easy, low cost
\nsynthesis and their low temperature solution processability into solar cell devices. Moreover, they
\npossess several attributes, which endow them high potential to achieve high PCE. QDs are strong
\nabsorbers of light due to their high extinction coefficient and thus thin layer of QDs can be used
\nin designing solar cells. The absorption of photons by QDs can be tuned by varying their size
\nthanks to the quantum confinement effect. Especially near infrared (NIR) QDs are very attractive
\nas they can absorb NIR part of the solar spectrum, which is wasted by other solar cell materials,
\nbut yet comprise about 50 % of the solar spectrum, in addition to higher energy photons. QDs also
\noffer a possibility of utilizing multiple exciton generation (MEG) and extraction of hot carriers in
\ndesigning very high PCE solar cell devices and intermediate band solar cells, all of which have
\npotential to surpass Shockley-Queisser limit for single junction solar cells. Further considering
\ndevice stability, recently core-shell QDs attracted attention in solar cells due to the presence of a
\nrobust, inorganic shell on the surface.
\nWhile designing solar cell devices from QDs, several factors need to be taken into consideration.
\nIn devices involving continuous film of QDs, the QDs should be closely packed for efficient
\nmobility of charge carriers through the film and the charge carrier recombination sites should be
\nminimized. To alleviate this problem, combining the QDs with one dimensional (1D)
\nnanostructure to form continuous charge transport pathways is one solution in improving the
\nperformance of QD based solar cell devices. Another means of overcoming limited charge carrier
\ndiffusion in thick QD film is integrating plasmonic nanoparticles with thinner QD film to increase
\nabsorption of light, while maintaining efficient charge carrier extraction.
\nIn the first part of this work, solar cells fabricated by combining colloidal PbS and PbS/CdS coreshell
\nQDs with rutile TiO2 nanorod arrays (2 and 4 μm long) have been investigated. The general
\nstructure of the solar cell devices investigated is fluorine doped tin oxide
\n(FTO)/TiO2/QDs/interfacial layer/Au. Two types of QDs (PbS and PbS/CdS core-shell) and two
\ntypes of interfacial layer (Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)
\n(PEDOT:PSS) and MoO3) between the QD film and the back electrode have been investigated. It
\nhas been found out that both the processing atmosphere and the interfacial layer influence the
\nperformance of the solar cell devices. A maximum PCE of 2.14 % has been achieved under air
\nmass (AM) 1.5 illumination with devices involving PbS/CdS core-shell QDs, 2 μm long TiO2
\nnanorod arrays and MoO3 interfacial layer. Moreover these devices were processed in ambient
\natmosphere and have shown better performance (by about 40 %) than devices involving PbS QDs
\nprocessed under inert atmosphere in glove box.
\nIt has been encouraging to demonstrate the use of PbS/CdS core-shell QDs in solar cells with easy
\nprocessability and better performance compared to the PbS QDs. However, there still remained
\nroom for improvement of the performance by further optimization. Accordingly, in the second
\npart, the solar cell devices involving PbS/CdS core-shell QDs have been further optimized and a
\nPCE as high as 4.43 % has been achieved. This PCE has been achieved by utilizing uniform
\nsputter-deposited TiO2 seed layer on FTO glass prior to the growth of TiO2 nanorod arrays,
\noptimizing the length of the TiO2 nanorod arrays and post deposition mild thermal annealing of
\nthe PbS/CdS core-shell QD film under inert atmosphere. The performance of the solar cell devices
\nhas been found to depend on the length of TiO2 nanorod arrays and reach maximum at optimum
\nTiO2 nanorod array length of about 450 nm.
\nIn the third part, plasmon-enhanced bulk heterojunction (BH) solar cells involving Au nanostars
\nincorporated into NIR PbS/CdS core-shell QD film, which was spin coated onto TiO2 nanorod
\narrays film on FTO glass substrate are studied. The effects of the density of the Au nanostars and
\ntheir location in between the TiO2 nanorod arrays and the back electrode on the performance of
\nthe device were investigated. After optimizing the density and the location of the Au nanostars a
\nPCE of 4.16 % has been achieved. This is about 16 % increase compared to the device without Au
\nnanostars with a PCE of 3.59 %. The improvement in the PCE as a result of Au nanostars
\nincorporation is mainly due to increase in the short circuit current (Jsc) (about 26 % in this case).
\nThis indicates that the presence of Au nanostars enhances the charge carriers generation by
\nimproving the absorption of photons. This was further confirmed by the enhancement of
\nphotoresponse, as evidenced by external quantum efficiency (EQE) spectra of the devices.

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