Light Management Strategies and Nanostructuring Techniques to Improve Efficiency in Solar Cells

The demand for cheap and clean energy sources, not based on fossil fuels and having low impact on the environment, is becoming nowadays an urgent matter. In this regard photovoltaic (PV) power generation technology is one of the most promising. A large variety of photovoltaic devices, or most commonly “solar cells”, is currently on market but all of them share an identical research aspect: the need of increase their photovoltaic conversion efficiency in a cost effective way. In addition to improving the electronic properties of the materials used for solar devices construction, there is a very promising approach towards the efficiency enhancement based on the so called Light Management (LM) techniques. LM techniques are based on the introduction of particular photonic nanostructures which depending on the positioning along the cell architecture and depending on their morphological characteristics, can serve for the obtaining of different phenomena including: diffractive effects, modulation of the refractive index, coupling to waveguide modes through surface structuring, and modification of the photonic band structure of the device. Anyway, the goal of LM concept is the enhance of the probability of photons interaction with cell active layer for the generation of an increased quantity of charge carriers involved in the photovoltaic process. Continuous improvement in nanotechnology manufacturing field have led to a great attention for LM techniques applied to photovoltaics and the present work has given a contribute to this interesting field, focusing on a particular type of PV device, Dye Sensitized Solar Cells (DSC). A Bragg grating with defined morphological parameters (theoretically predicted by FEM calculation) has been realized on a high performance photoresist by means of Laser Interference Lithography (LIL) and then replicated on a mesoporous TiO2 layer. Replication process takes place by means of a low-cost Soft Lithographic (SL) process which exploits a PDMS mold for pattern transferring from one layer to the other. The nanostructures good quality replication, over a large area have been demonstrated by microscopic analysis. The nanostructured TiO2 layer was then soaked into a dye and the DSC cell assembled. PV properties of the build-up nanostructured cell and those of a traditional bare one, both realized following identical experimental procedures and differing only for the Bragg grating presence, were compared. Results confirmed an enhanced efficiency, in term of IPCE, of 31% for the nanostructured cell. Therefore, the most important achievement of this study has been the successful easy and low cost TiO2 nanostructuring. The second part of this work concerns on preliminary guidelines for the realization and ordering of different type of nanostructures. In particular a LIL method for 2D Bragg grating structure production has been proposed to be employed for photovoltaic antireflective coating. A transfer method that exploit PDMS mold to align gold nanoparticles (NPs) on a PEDOT:PSS layer of an organic solar cell was applied. The deposition and ordering of such Au NPs along specific patterns, permits to combine the photonic effect, whose effectiveness has been demonstrated in the first part of the work, with the plasmonic one. The presented result demonstrated the great potential of low-cost soft lithographic procedures in LM field.