Construction of photosensitised semiconductor cathodes
thesisposted on 05.09.2012, 14:57 by Mohd Mat-Teridi
Recent studies suggest that the performance of dye-sensitised solar cells (DSC) has appeared to have reached a limit, therefore solar cells based on semiconductor materials, such as extremely thin absorber (ETA) solar cells and tandem solar cells are currently the subject of intense research in the framework of low-cost photovoltaic devices as sources of harvesting sunlight to generate electricity. Generally, semiconductor solar cells have been divided into two different types, namely anodic and cathodic type solar cells. Extensive research and development work has been focused on anodic semiconductor sensitised solar cells to date. In contrast, the cathodic semiconductor sensitised solar cells have received no attention which is very surprising. Developing the cathodic semiconductor sensitised solar cell concept is very important in the development of tandem solar cells as well as other new solar cell configurations. The main reason for the lack of research in this area was due to the rarity of p-type semiconductor materials, which made it difficult to find suitable materials to match the energy band edges for cathodic semiconductor sensitised solar cells (CSSC) as well as solid-state cathodic semiconductor solar cells (SS-CSSC). The primary aim of this thesis was to construct cathodic semiconductor sensitised solar cells as well as their solid-state analogues (SS-CSSC). The work conducted within this doctoral study presents state-of-art materials and thin film processing/preparation methods, their characterisation and developing CSSCs and SS-CSSCs employing such films in cascade configurations. No reports have been published in the literature on SS-CSSC to date. The first stage of this thesis is focused on optimising the morphology and the texture (porosity) of the CuI and NiO semiconductor photocathode, by the introduction of new deposition methods namely, pulsed-electrodeposition (PED) and Aerosol-Assisted Deposition (AAD) and Aerosol-Assisted Chemical Vapour Deposition (AACVD). The electrodes prepared by employing the methods mentioned above and controlling the deposition parameters systematically, we have achieved significant improvement in the film morphology and the texture of the deposited films. The resulting electrodes showed excellent improvement in the photoelectrochemical performance which made it suitable for application in construction of both CSSC and SS-CSSC. The photoelectrochemical performance of the electrodes can be seen clearly through the photocurrent density data. For the case of bare CuI, the PEC performance of electrode prepared by the AAD and PED compared against that of continuous-electrodeposition (ED) electrodes. The photocurrent density achieved for the electrodes prepared by AAD and PED was reported around 175 and 75 µAcm-2 respectively which are way higher than the ED case. At the second stage of this study, the work focused on fabrication and characterisation of the CSSCs. Cathodic sensitised PEC solar cells (CuI/Cu2S/(Eu2+/Eu3+) and NiO/Cu2S/(I3-/I-)) were fabricated by deposition of p-Cu2S on the texture controlled CuI and NiO photocathodes. The morphological properties of the photocathode, in particular layer thickness, particle size and film porosity, play an important role in the PEC performance of CSSCs. Optimisation of these parameters led to increased adsorption of the Cu2S light harvester on the photocathode s surface. As a result, the charge injection from Cu2S to the wide band gap photocathode material (CuI and NiO) was significantly improved. Due to this, the CSSC performance showed significant improvement as semiconductor sensitised cathodic solar cells (CSSC). The IPCE and photocurrent density of the CSSC achieved in this study was around (19 and 7 %) and (1 and 0.5 mAcm-2) for the CuI/Cu2S and NiO/Cu2S electrodes respectively. Finally, the SS-CSSC has been fabricated by employing n-Fe2O3 electron transport layer. The construction of SS-CSSC for the first time using the n-Fe2O3 electron transport layer (CuI/Cu2S/Fe2O3 and NiO/Cu2S/Fe2O3) allowed us to study the materials, optical and photoelectrochemical properties of this device. Under AM 1.5 illumination, the SS-CSSC shows a photocurrent density of 6 and 9 µAcm-2 for CuI/Cu2S/Fe2O3 and NiO/Cu2S/Fe2O3 solar cells, respectively. The results of this work indicated low performance for both SS-CSSC compared to CSSC results, due to the lack of adsorption between the absorber and Fe2O3 electrode. However, this study proved the concept of SS-CSSC based on semiconductor material, which is valuable for the future work of cathodic semiconductor sensitised solar cells as well as solid-state tandem solar cells.