Chapter 7 Conclusion and Future Scope This chapter presents a summary of the work with concluding remarks for the research performed and reported in this thesis and then lays out the future scope pertaining to this work. Onkar S. Game 182 Savitribai Phule Pune University
7.1 Summary of the thesis Unprecedented growth in the demand for energy along with the awareness about environment and health hazards of fossil fuels have made scientists extensively investigate the alternative clean and renewable energy sources. Among various possible renewable sources solar energy has tremendous potential to fulfil the energy demands of future generations in an environment friendly and sustainable manner. Importantly the solar energy can also power creation of other clean forms of energy such as hydrogen which is predicted to be the fuel of future because of zero greenhouse gas emissions and high energy efficiency upon combustion. Current state of the art silicon-based photovoltaic (PV) technology is relatively expensive and therefore developing potentially cheaper PV technologies using solution processable earth abundant material systems is the need of the present time. With the advent of nanotechnology it has become increasingly possible to understand, control and manipulate the material properties in an unprecedented manner as a result of which several new concepts and designs of solar cells, hydrogen generation and optoelectronic devices have emerged. Among various material systems used in the new generation of energy or optoelectronic systems metal oxides nanomaterials have acquired the center stage because of their intriguing optical, chemical and mechanical properties. In the research work presented in this thesis, we have used metal oxide nanostructures with engineered optoelectronic properties for applications in the field of energy conversion viz. Dye-sensitized solar cells (DSSC) and photo-electrochemical (PEC) water splitting. We have also shown the applicability of these synthesized metal oxide nanostructures in the field of self-powered photosensing which is synergistically similar to the field of photovoltaics. The summary of the work done is as follows: 1. The multi-component architecture of DSSC imposes an inherent complexity in device optimization as the functionalities of these components are interdependent. For this reason, only a handful of research groups around the world could demonstrate efficiencies in the range of 9-10% for DSSCs. Therefore considerable time and efforts in this thesis work were dedicated to the optimization of several Onkar S. Game 183 Savitribai Phule Pune University
DSSC fabrication parameters such as TiO 2 paste making, electrode thickness, double layer architecture, electrolyte composition, electrode assembly and the sealing procedure. We successfully developed a protocol for the fabrication of sealed and stable DSSCs with an active area in the range of 0.15 cm 2-1 cm 2. Importantly, we demonstrated excellent power conversion efficiency of 10% on unmasked cell and 9% on the masked cell of active area 0.15cm 2. The issue of efficient charge collection encountered during the transition from small area (0.15 cm 2 ) to large area (5 cm 2 ) DSSC was resolved by deposition of silver collection electrodes on both the working and the counter electrode. The dramatic improvement in the fill factor was observed for large area DSSCs with the silver grids deposited on both electrodes (FF~60%) over the DSSCs without silver grid on either electrode (FF~29%). The sealed 5 cm 2 active area DSSCs showed 90-95% stability over a period two months. Further, these sealed DSSCs were used for low power applications such as a calculator, a pedometer and electrochemical water splitting for H 2 generation. We also assembled these solar cells on a farmer s hat to demonstrate the charging of a cell phone battery under the sunlight. 2. Because of the contradictory reports in the literature regarding the role of TiO 2 oxygen vacancies in DSSC, we carried out a systematic study of the influence of O-vacancies on the performance of DSSCs. Initially, Pulsed Laser Deposition based non-porous thin film platform was developed to simplify the complex/interdependent DSSC architecture and to study the TiO 2 -dye-electrolyte interface without being interfered by the FTO-electrolyte interface. The results obtained on the thin film platform were further verified using the mesoporous TiO 2 based DSSCs made with air annealed (A-TiO 2 ) and vacuum annealed (Vc-TiO 2 ) mesoporous TiO 2 films. The Vc-TiO 2 based DSSC showed lower V oc and J sc as compared to A-TiO 2 based DSSC, thus following the same trend as the results on thin films. The origin of decreased V oc and J sc was probed on Vc-TiO 2 and A-TiO 2 based DSSCs using home-built transient optoelectronic setup with the help and advice of Prof. Brian O'Reagan. The transient measurements revealed that the presence of surface O-vacancies on TiO 2 leads to downshift in the conduction band edge along with more than one order of magnitude lower recombination lifetime than A-TiO 2, which explains the observed trends in V oc and J sc in the case of oxygen Onkar S. Game 184 Savitribai Phule Pune University
deficient TiO 2 based DSSC. Therefore, we could definitively conclude from a detailed optoelectronic characterization that surface oxygen vacancies in TiO 2 are not beneficial to the performance of DSSC. 3. In an attempt to bring the absorption of ZnO into the visible region, we developed a simple solution based process to synthesize Nitrogen Doped Zinc oxide (N:ZnO) nanorods with a high aspect ratio of 50-70. We derived the idea for this synthesis based on a theoretical study which predicted extended and enhanced visible absorption by the incorporation of nitrogen along with Zn interstitial. We showed that the N:ZnO nanorods synthesized by a suitably modified technique to induce Zn interstitials along with Nitrogen incorporation exhibited enhanced broadband visible light absorption extending from UV to deep visible region up to 650 nm. By using several characterizations we established the substitutional doping of nitrogen accompanied by other complex defects such as zinc interstitials and oxygen vacancy in N:ZnO and this resulted in an effectively reduced band gap of 2.1 ev, in very good agreement with recent theoretical predictions. Photo-electrochemical water splitting experiment demonstrated a much superior performance of N:ZnO nanorods over undoped ZnO. The Incident Photon to Current conversion Efficiency (IPCE) of N-ZnO brought out its capability to convert photons with wavelengths up to 650 nm into usable current, which agreed well with the absorption spectrum of N:ZnO. 4. We also considered the photosensing application of metal oxide nanostructures which are synergistically similar in working principles as photovoltaics. We developed a facile solution-based fabrication of a self-powered organic-inorganic hybrid photodetector using nanorods of environmentally-friendly and earth-abundant metal oxide semiconductor n-type ZnO and an organic p-type semiconductor Spiro-MeOTAD. Initially, we fabricated a visible blind UV photodiode using undoped ZnO nanorod arrays impregnated with Spiro-MeOTAD. Such UV photodetector showed a UV to Visible rejection ratio of 10 2 with a high photocurrent density of 50μA/cm 2 under a low intensity (3mW/cm 2 ) UV illumination. A rapid photoresponse with rise time of 200 μs and decay time 950 μs was observed in the case of undoped ZnO-Spiro-MeOTAD UV photodiode. Importantly, we further showed that a slight modification in reaction conditions Onkar S. Game 185 Savitribai Phule Pune University
(doping nitrogen in ZnO nanorods) could convert the UV operative ZnO-Spiro- MeOTAD junction to be highly responsive even in the visible region (400 nm-650 nm). The N:ZnO-Spiro-MeOTAD photodiode showed a rapid and strong photoresponse under blue (460 nm), Green (530 nm) and Red (635 nm) LED illuminations, thereby making it capable of sensing white light. Interestingly, this self-powered visible photodetector showed a high photocurrent density of 120 na/cm 2 under low intensity (28μW/cm 2 ) white light. The photosensing parameters of N:ZnO-Spiro-MeOTAD photodiode were much superior to previously reported organic-inorganic hybrid photo-detectors. High visible light sensitivity of N:ZnO- Spiro-MeOTAD junction makes it an attractive alternative over toxic element (Cd, Pb) based current visible light photodetectors. 7.2 Scope for future work During the course of the research work presented here several interesting and outstanding developments in the field of energy have occurred on the international front. Due to major influx of solar cell/panels from China and Japan the silicon photovoltaics sector has seen a price drop of almost 67% as compared to the year 2010, which has certainly strained the current research and development in this field. Therefore future developments in the field of energy (photovoltaics or PEC water splitting) should be based on the earth abundant and inexpensive materials with low processing cost in order to be able to survive in the cut-throat competition with the silicon technology. Recent outstanding advances in the field of organic-inorganic hybrid perovskite materials for energy applications have certainly boosted the hopes to develop energy solutions at very low cost as compared to silicon technology. Also, the solution-processed engineered nanostructures certainly offer hope to achieve high energy output at low cost. In the present thesis as well we have attempted to engineer the nanostructures via solution process routes to achieve high performance parameters in solar cells, water splitting and photosensing applications. Based on the insights gained through these efforts, the following points appear important for further research in this field: 1. In DSSC, we successfully demonstrated a power conversion efficiency of 10% on the active area of 0.15 cm 2. However, transition from a small area to a larger Onkar S. Game 186 Savitribai Phule Pune University
area of 5 cm 2 posed several design and technical difficulties which resulted in the lower charge collection efficiencies. Therefore fabrication of DSSC modules of size 10 cm 10 cm or higher is certainly a challenging task where one may have to opt for various designs such a monolithic/w-shaped/parallel design depending on the tradeoffs between the efficiency, device fabrication and cost. 2. Recently organic-inorganic hybrid perovskite materials such as CH 3 NH 3 PbI 3, CH 3 NH 3 PbBr 3 etc. have been used as light harvesting agents instead of typical organo-metallic dyes in solid state sensitized solar cells which have resulted in the power conversion efficiencies above 15%. Currently, there is a limited understanding of the active or passive role of the metal oxide nano-porous network in the working of hybrid perovskite based solar cells. Pulsed laser deposition based metal oxide thin film approach presented in the chapter 4 combined with the transient optoelectronic characterization can certainly shine light on the role of metal oxide-perovskite interface in the perovskite solar cells. Also the nature of metal oxide-hybrid perovskite interface is identical to conventional p-n junction and free from complexities of semiconductor-electrolyte junction as in DSSC one may expect the positive effects of doping in metal oxide nanoparticles over the undoped metal oxide-perovskite junction. Therefore, influence of various dopants (trivalent or pentavalent) in TiO 2 matrix on the performance of hybrid perovskite solar cell can be important from scientific and technological point of view. Replacement of Pb by less toxic element such as Sn in the perovskite structure and also replacement of expensive hole transport material (Spiro- MeOTAD) with cheaper p-type metal oxide semiconductors (e.g. Cu 2 O or NiO) are matters of immediate scientific interest. 3. Photo-electrochemical (PEC) water splitting is certainly advantageous over electrochemical water splitting due to the requirement of lower applied bias in the former. However, unassisted water splitting using various tandem assemblies of nanomaterials based photoelectrodes and/or photovoltaic (PV) cell has been an area interest lately. One such example is the dual absorber system which utilizes an n-type semiconductor as anode and a p-type as cathode whose band gaps and alignments are such that the electrochemical potential developed by this assembly in electrolyte is Onkar S. Game 187 Savitribai Phule Pune University
self-sufficient to split water. Another example of tandem assembly for unassisted water splitting includes semiconductor-pv assembly. Therefore, it will be interesting to check the performance of engineered n-type metal oxide semiconductors developed in this thesis with p-type metal oxide systems such as Cu 2 O in tandem assembly. Novel metal oxide-hydroxide catalyst systems coupled to low-cost, efficient PV systems such as DSSC or perovskite solar cell is also a matter of tremendous interest lately. 4. In this thesis, we successfully developed an inorganic-organic hybrid selfpowered photosensor with quality factors superior to the previously reported hybrid systems. However, the commonly used organic p-type hole transport materials are expensive. Recently discovered hybrid perovskites possess intrinsic p-type nature which, therefore, makes them ideal candidates for self-powered p-n junction photodetectors employing n-type metal oxides. Onkar S. Game 188 Savitribai Phule Pune University