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Title: Modulating the optical & electrical properties of mesoscopic solar cells
Authors: Yantara, Natalia
Keywords: DRNTU::Engineering::Materials::Energy materials
Issue Date: 2014
Abstract: Mesoscopic solar cells are a promising form of photovoltaic devices that offer lower production cost as compared to traditional silicon solar cells. The use of low quality abundant materials and low temperature solution deposition allow for low production cost in these solar cells. Efficient charge collection from the region that absorbs photons (optical depth) is essential for enhancing the solar cell performances. As solution processed materials generally suffer from poor charge collection lengths, a shorter optical depth is beneficial for solar cell performances. Optical depth could be reduced by increasing the optical path through nanostructuring. In this work, the impact of nanostructuring was studied in depth for two systems; Cu2O - where the optical depth and charge collection length were mismatched and CH3NH3PbI3 where the optical depth and charge collection lengths were matched. Mesoscopic devices were fabricated by employing nanostructured ZnO (i.e. nanorods and inverse opal morphology) as the scaffold to host the absorber film. The charge collection length of both Cu2O and CH3NH3PbI3 based solar cell were first examined in the planar configuration. Short charge collection length (~26 nm) as compared to its optical depth was observed in Cu2O based solar cell, reconfirming the need of nanostructuring in the system. On the other hand, charge collection length of CH3NH3PbI3 based devices was comparable with the absorber layer thickness, explaining its efficient photocarrier extraction. The roles of nanostructuring on the photocurrent were scrutinized via numerical optical simulations and consequently supported by experimental data. Nanostructuring enhanced the photocurrent by at least 50% and 30% for Cu2O and CH3NH3PbI3 based devices respectively. Optical simulation via finite difference time domain (FDTD) method highlighted the optical path increment due to nanostructuring in Cu2O based device. The extended electron selective contact towards the absorber film would further assist the charge extraction inside solar cells. Contrastingly, in CH3NH3PbI3 based devices, nanostructuring acted as scaffolds to retain more absorber film, leading to thicker films which boosted the photocurrent. Despite the photocurrent enhancement, nanostructuring reduces the photovoltage of the devices. Electrochemical impedance spectroscopy (EIS) studies revealed the high recombination rates in devices with increased interfacial area. In Cu2O based devices, the ZnO/Cu2O interface was the major interfacial recombination contributor. TiO2 interfacial layer was then introduced to resolve the interfacial recombination due to favorable band energy alignment. In the case of CH3NH3PbI3 based devices, the ZnO/hole transport layer interface could potentially decrease the photovoltage of the devices. Based on the studies presented in this thesis, the understanding of the impact of nanostructuring could be extended to other solar cell systems with unbalanced optical depth and charge collection length.
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