Transparent Conductive Oxide-free hybrid and organic solar cells

In organic Bulk Hetero Junction (BHJ) and hybrid/perovskite solar cells, the most employed material used as transparent electrode for the charges collection is transparent conductive oxide (TCO) such as indium doped tin oxide (ITO) or fluorine doped tin oxide (FTO). Beside the high transparency and conductivity (80% on glass substrates and 15 Ω/□, respectively) of ITO and FTO, there are many critical issues: i) limited indium sources, ii) high cost due to the deposition techniques (sputtering, evaporation, pulsed laser deposition and electroplating etc.), iii) high temperature processing and iv) high mechanical brittleness. For these reasons, it is necessary to investigate new materials. The discovery of graphene, in 2004, that led Novoselov and Geim to win the Nobel Prize has opened up new areas of scientific research. In particular, its surprising physical, optical, mechanical and electrical properties have made the graphene one of the most promising material in the modern electronic applications and in particular in the 3rd generation solar cells technology that can be produced cheaply and very fast from solution with printing processes both on plastic and rigid substrates. This work is mainly focused on the use of graphene as a replacement of the conventional transparent conductive oxides. In particular, most of the problems (wettability, annealing temperature etc.) for fabricate solar cells on graphene electrodes were solved. A simple way to decrease the sheet resistance of graphene electrode, by the addition of a metal grid, is presented as well. With the aim to realize high efficiency solar cells, both BHJ with low band gap polymers as active layer and perovskite-based solar cells have been investigated. Firstly, the effects of two different materials (Ni and MoO3), used as p-dopant on bare graphene, were studied and the thickness was optimized in order to reduce the graphene sheet resistance and increase the solar cells performances. Moreover, was investigated the feasibility to realize graphene-based solar cells starting to optimize the deposition of the organic active layer material (blend of P3HT: PC [60] BM or PTB7: PC [70] BM) in terms of annealing temperature and thickness. iv Furthermore, in order to increase the solar cells efficiency, organic-inorganic perovskite ( CH3NH3PbI3-xClx ) material was studied as active layer. As first step, the growth of perovskite active layer was optimized in terms of annealing temperature, photoluminescence and morphology both for direct and inverted architectures. Then, using a planar direct structure, efforts were made to solve the issues related to the realization of perovskite solar cells on graphene electrode. While, in the direct structure, Titania ordered photonics nanostructures were introduced as electron transporting layer (ETL) to increase the light absorbed by the perovskite active layer and the photo-generated current in the solar cells. With the view to replace the conventional transparent conductive electrode, conductive polymers were also investigated. The most promising organic material is PEDOT: PSS, which is a semitransparent and conductive polymer. However, the pristine PEDOT: PSS film, deposited from aqueous solution, has a lower conductivity than the conventional transparent conductive oxide. For this reason, many strategies have been employed to improve the conductivity of this material to obtain a low cost, low temperature and TCO-free perovskite planar heterojunction solar cells on flexible substrate. In particular, it is demonstrated that the highly conductive polymeric material shows potential as a practical replacement for expensive and brittle ITO/PET. Moreover, in the bending test, the ITO-free perovskite solar cells with PEDOT anodes on flexible substrate manifested superior mechanical robustness compared with ITO-based cells, showing the high flexibility of perovskite layer.