Sudakar Chandran

We present a detailed study on the photovoltaic performance of dye and quantum dot (QD) sensitized solar cells fabricated using nanoporous TiO2 photoanodes prepared by a template approach. Nanoporous TiO2 particles (TNPO) are prepared by sol-gel method using dextran (TNPO-D) and glucose (TNPO-G) as templates. The decomposition of templating agents during calcination introduces nanopores (∼10 nm) in TiO2 nanoparticles as evidenced from the HRTEM studies. Oxygen vacancies are also introduced in TiO2 nanoparticles due to prevailing reducing ambient during decomposition. Oxygen vacancy (VO) related defects are evidenced from EPR spectra from a strong absorption signal observed at g-value of 1.994 related to Ti3+-VO-Ti3+ defect sites. The defect concentration estimated from EPR spectra is found to be ∼5 to 15 times larger for nanoporous TiO2 particles compared to Degussa P25-TiO2. TNPO samples also show enhanced defect level photoluminescence emission. The photovoltaic efficiency (η) and the IPCE of DSSC and QDSSC are strongly influenced by the defect concentration and seen to decrease with increasing VO in TiO2 nanoparticles. Twofold lower η is found in both DSSC (∼6.38% to 3.66%) and QDSSC (∼1.69% to 0.46%) devices made using nanoporous TiO2 compared to P25-TiO2 photoanode solar cells. P25-TiO2 also shows significant decrease in η when oxygen vacancies are introduced by hydrogen annealing process, which reveal detrimental effect of VO on the photovoltaic properties. EIS studies suggest increased interfacial resistance in solar cells made from nanoporous TiO2 photoanodes.

Perovskite Solar Cells (PSCs) gained attention by solar photovoltaic community owing to excellent optoelectronic properties, solution processability, low cost and competing efficiencies with established technologies. Methyl ammonium lead iodide (MAPbI3) is the most commonly used perovskite absorber layer in PSCs owing to its optimum bandgap, ease of phase stabilization and high performance. MAPbI3 based PSCs are known to best perform when the perovskite films are prepared by 1-step recipe followed by conventionally heat treating at 100 °C. Here we report a different precursor where such heat treatment is unwanted and is in fact detrimental in MAPbI3 absorber layer-based PSCs. MAPbI3 films are developed by liquefying their single crystal counterparts in methylamine gas. The obtained precursor from the single crystals is suitable for both lab scale as well as large scale depositions. Here we study its crystallization mechanism in lab-scale spin coating process. The precursor crystallized instantaneously during spin-coating itself without any post annealing step. Our results indicate that annealing of the films is detrimental to its morphological, structural and optical properties thus rendering it unsuitable for photovoltaic applications. The films without annealing have resulted in an efficiency greater than 13% while the annealed films have resulted in less than 2%.

CuInS2 (CIS) based solar cell devices are fabricated by sensitizing TiO2 photoanodes with CIS quantum dots (CIS-QDs). Morphologically different TiO2, viz. Degussa P25 nanoparticles, smooth and fibrous microspheres (SμS and FμS respectively) are used to fabricate photoanodes. CIS-QDs are synthesized using dodecanethiol (DDT), CuI and In(OAc)3 precursors by solvothermal method. DDT surfactant present on the CIS QDs surface is replaced with 3-mercaptopropionic acid in a single phase one step procedure to enable efficient loading of QDs onto photoanode and as linker molecule for charge carrier extraction. The CIS QDs sensitized on SμS and FμS microsphere photoanode layers exhibit a photoconversion efficiency (η) of 3.2% and 1.6%, respectively, in comparison to η ≈ 2.1% for nanoparticulate TiO2 (Degussa P25). Further increase in efficiency is obtained (3.8% for SμS and 2.5% for FμS) when composite photoanode films made of porous microspheres filled with nanoparticulate P25 are used. A maximum efficiency of 3.8% (with JSC ≈ 6.2 mA, VOC ≈ 926 mV and FF ≈ 66 for cell area ≈ 0.25 cm2 and thickness ≈ 20 µm) is realized when 4.6 nm CIS QDs sensitized on composite photoanode (consisting of 80 wt. % SμS and 20 wt. % P25) is used. High VOC observed is unprecedented and is possible due to combined effect of SμS+P25 composite photoanode properties such as fewer defects, good connectivity between particles, effective light scattering, minimum recombination, and effective electron transport and size optimized CuInS2 QDs. Electrochemical impedance spectroscopy studies reveal a low interfacial resistance and longer electron life time in SμS+P25 composite photoanodes.

Anatase TiO2 microspheres with smooth and fibrous morphology (SμS and FμS) resembling laddu and dandelion are synthesized by solvothermal and hydrothermal methods, respectively. A detailed analysis of these two microstructures using XRD, UV–vis spectroscopy, electron microscopy and surface area measurement techniques are presented. Photoanodes fabricated using these microspheres and their composites with nanoparticles (Degussa P25) are tested for photovoltaic (PV) performance using a standard Grätzel-type dye-sensitized solar cell (DSSC) configuration. The DSSC made up of SμS and FμS microspheres exhibit an efficiency (η) of 8.4% and 6.4% respectively, in comparison to η≈7.1% for nanoparticulate P25. Further enhancement in η is realized in the composite photoanode films made of porous SμS/FμS microspheres filled with nanoparticulate P25 TiO2. High power conversion efficiency of 8.9% (for cell area of 0.5 cm2 and thickness of ~25 µm) was achieved in composite photoanode film consisting of 80 wt% SμS and 20 wt% P25. Electrochemical impedance spectroscopy studies reveal a low interfacial resistance in composite photoanodes, which is desired for efficient electron generation and transport. Composite microspheres filled with P25 nanoparticles in their voids show enhanced efficiency than the mesoporous TiO2 microspheres. Thus, SμS-nanoparticle composite TiO2 film possesses essential attributes necessary for an efficient photoanode viz. large surface area for dye adsorption, good connectivity between nanocrystallites for the efficient electron transport, and higher scattering properties for better light harvesting efficiency.