Development of selective contacts, transparent electrodes and new absorbers

The research activity of UB is focused on the development of new selective contacts, dipolar layers and the development of metal oxides as alternative absorbers for dopant-free heterojunction solar cells.

The Solar Energy Group at Universitat de Barcelona (UB) investigates non-conventional materials for their use in dopant-free silicon-based heterojunction solar cells. In particular, the group explores the use of reactive sputtering for the development of hole transport layers based on transition-metal-oxides, the use of new approaches for the transparent contacts based on dielectric-metal-dielectric structures, and the deposition of dipolar layers by spin-coating for the modification of the work function at the electrodes. In addition, the development of new light absorbers based on metal oxides is being addressed. Jointly with UPC, the group applies these materials to the fabrication of heterojunction solar cells.

Finally, the UB group is coordinating the work package related to the characterization of the materials and the devices developed in the project.


Deposition and characterisation of sputtered molybdenum oxide thin films with hydrogen atmosphere

Nicolau López-Pintó; Thomas Tom; Joan Bertomeu; Jose Miguel Asensi; Eloi Ros; Pablo Ortega; Cristóbal Voz

Sputtered films of reduced molybdenum oxide (MoOx) with a molybdenum trioxide target in different pressures and atmospheres were deposited in varying temperatures. Compositional, optic, and electric characteristics of the samples were studied. X-ray photoelectron spectroscopy revealed reduced states when working in the hydrogen + argon atmosphere implying that stoichiometry could be controlled by adding some hydrogen in the sputtering chamber. The effect of slightly increasing the substrate temperature during deposition was also studied and lead to the presence of metastable Mo4+ states at 3 mTorr. Optical properties match the ones already in the literature, and transmittances of 90% were achieved. The results support sputtering as a viable method of depositing MoOx films apart from thermal evaporation for many applications.

Influence of Co-Sputtered Ag:Al Ultra-Thin Layers in Transparent V2O5/Ag: Al/AZO Hole-Selective Electrodes for Silicon Solar Cells.

Thomas Tom; Nicolau López-Pintó; Jose Miguel Asensi; Jordi Andreu; Joan Bertomeu; Eloi Ros; Joaquim Puigdollers; Cristóbal Voz


As optoelectronic devices continue to improve, control over film thickness has become crucial, especially in applications that require ultra-thin films. A variety of undesired effects may arise depending on the specific growth mechanism of each material, for instance a percolation threshold thickness is present in Volmer-Webber growth of materials such as silver. In this paper, we explore the introduction of aluminum in silver films as a mechanism to grow ultrathin metallic films of high transparency and low sheet resistance, suitable for many optoelectronic applications. Furthermore, we implemented such ultra-thin metallic films in Dielectric/Metal/Dielectric (DMD) structures based on Aluminum-doped Zinc Oxide (AZO) as the dielectric with an ultra-thin silver aluminum (Ag:Al) metallic interlayer. The multilayer structures were deposited by magnetron sputtering, which offers an industrial advantage and superior reliability over thermally evaporated DMDs. Finally, we tested the optimized DMD structures as a front contact for n-type silicon solar cells by introducing a hole-selective vanadium pentoxide (V2O5) dielectric layer.

Deoxyribonucleic Acid-Based Electron Selective Contact for Crystalline Silicon Solar Cells

Thomas Tom, Eloi Ros,David Rovira, Julian López-Vidrier, José Miguel Asensi, Pablo Ortega, Joaquim Puigdollers, Cristobal Voz, Joan Bertomeu


Development of carrier selective contacts for crystalline silicon solar cells has been recently of great interest toward the further expansion of silicon photovoltaics. The use of new electron and hole selective layers has opened an array of possibilities due to the low-cost processing and non-doping contacts. Here, a non-doped heterojunction silicon solar cell without the use of any intrinsic amorphous silicon is fabricated using Deoxyribonucleic acid (DNA) as the electron transport layer (ETL) and transition metal oxide V2O5 as the hole transport layer (HTL). The deposition and characterization of the DNA films on crystalline silicon have been studied, the films have shown a n-type behavior with a work function of 3.42 eV and a contact resistance of 28 mΩ cm2. This non-doped architecture has demonstrated a power conversion efficiency of 15.6%, which supposes an increase of more than 9% with respect to the cell not containing the biomolecule, thus paving the way for a future role of nucleic acids as ETLs.

Polymeric Interlayer in CdS-Free Electron-Selective Contact for Sb2Se3 Thin-Film Solar Cells

David Rovira,Eloi Ros,Thomas Tom, Maykel Jiménez,José Miguel Asensi ,Cristobal Voz, Julian López-Vidrier,Joaquim Puigdollers, Joan Bertomeu and Edgardo Saucedo


High open-circuit voltage in Sb2Se3 thin-film solar cells is a key challenge in the development of earth-abundant photovoltaic devices. CdS selective layers have been used as the standard electron contact in this technology. Long-term scalability issues due to cadmium toxicity and environmental impact are of great concern. In this study, we propose a ZnO-based buffer layer with a polymer-film-modified top interface to replace CdS in Sb2Se3 photovoltaic devices. The branched polyethylenimine layer at the ZnO and transparent electrode interface enhanced the performance of Sb2Se3 solar cells. An important increase in open-circuit voltage from 243 mV to 344 mV and a maximum efficiency of 2.4% was achieved. This study attempts to establish a relation between the use of conjugated polyelectrolyte thin films in chalcogenide photovoltaics and the resulting device improvements.