Thesis Laura Caliò/ Download the thesis here
Universidad Pablo de Olavide
Abstract /Thesis download here
Third generation photovoltaics (PV) have demonstrated to have all the requirements to replace
conventional inorganic silicon-based solar cells due to their appealing characteristics, such a easy processability, low cost and reduced material usage. In particular, organic photovoltaics have been intensely investigated in the latest decades since it can lead to clean, affordable, and flexible devices. Nevertheless, their limited power conversion
efficiency needs to be addressed, in order to pave the way for a large-scale commercialization. Lately, the major breakthroug among PV technology was indeed achieved with the introduction of hybrid organic-inorganic perovskite solar cells, due to the astonishing properties of perovskite, such as broad light absorption, easy fabrication, versatile deposition process and high solar-to-electricity power conversion efficiencies (PCE). In just a few years, certified values of >22% of PCE were obtained, representing an astonishing rise from their first introduction in 2009, and an unprecedent escalation among the PV technology. These results leaded perovskite under the limelight and entitled them as strong bet for future PV technology. Although considerable progress has been achieved in this field during last eight years, some issues still remain to be addressed, like high cost materials and device stability, which represents a big limitation for the large-scale production.
The use of hole transport material (HTM) is indispensable in order to fabricate efficient deviceE Spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-di-
methoxyphenylamine)-9,9′-spirobifluoren represents the state of the art HTM, working as electron blocking and p-type material . However, its high price due to the multistep synthetic route and complex purification, hinder progress toward commercialization of
perovskite PV technology. Furthermore, it needs p-type additives in order to efficiently extract and conduct the holes through the devices, due to it relatively poor hole mobility and conductivity. The use of such dopants and additives in the hole transporting layer reduces series resistance due to an enhancement in the hole injection.
Though, device long term stability remains a matter of concern, since the usage of highly hygroscopic dopants, such as lithium salts, can dissolve the water sensitive perovskite, which is very moisture sensitive.
In the present work, three main objectives were addressed in order to overcome the cited drawbacks. Firstly, novel small organic molecules have been designed and synthesized, and they have been effectively implemented in perovskite based solar devices as innovative and cost-effective HTMs. The rationally designed molecules present a thiophene-based core and a triphenylamine- or carbazole-derivative side arms. The opto-electrical properties of the novel molecules were measured and tuned as such to yield better photovoltaic parameters for efficient device fabrication. Moreover, two novel phthalocyanine-based HTMs were implemented and tested in PSCs. Advantages such as easy
synthesis, low cost and encouraging results in term of device performance provide a potential application as the replacement of the expensive Spiro OMeTAD.
Secondly, some of the newly synthesized thiophene-based molecules were implemented as exciton blocking layer in DBP/C70-based planar heterojunction organic solar cells (OPVs). The enhancement in the photocurrent demonstrated the efficient integration of the novel molecules and will allow to design and integrate other similar small
molecules as exciton blocking layers in OPVs in the future.
Finally, pyridine- and imidazolium-based ionic liquids were introduced as unique hydrophobic p-type dopants, with the aim of boosting the conductivity and hole mobility of Spiro-OMeTAD, and thus avoiding the use of hygroscopic and corrosive additive materials, with the aim of
enhancing the long-term stability of the PSCs. The carried studies prove the effectiveness of the use of the presented pyridinium-based hydrophobic ionic liquid as novel and unique dopant for PSCs, used in a very minute quantity. The impressive results pave the way to the use of a range of organic semiconductors as hydrophobic dopant, which can
work as effective dopant for HTMs and can effectively enhance the lifetime of PSCs devices.
Open access article:
Cu(II) and Zn(II) based Phthalocyanine as Hole selective layers for Perovskite Solar Cells, L. Calio, J. Follana-Berná, S. Kazim, M. Madsen, H.-G. Rubahn, Á. Sastre-Santos and S. Ahmad, Sustainable Energy Fuels, 2017
DOI: 10.1039/C7SE00367F (2017)