Design and implementation of new generation solar cells

The UPC will design and fabricate new generation solar cells following the concept of optical absorber plus complementary charge-carrier selective contacts. The active layers developed by the partners in this project will be implemented in novel device architectures.

The expertise of the UPC team is the fabrication of high-efficiency solar cells with innovative technologies. In this sense, it has been one of the first groups demonstrating the viability to fabricate dopant-free heterojunction solar cells in Europe. In this project, the UPC will focus on charge-carrier selective contacts obtained by thermal evaporation and atomic-layer-deposition (ALD). Alternative light absorbers for thin-film devices will be also investigated. In collaboration with CIEMAT and UB, unconventional transparent electrodes and antireflection coatings will be implemented in photovoltaic devices. The UPC group will also collaborate with CL-UPM to develop original laser-assisted processes for more compatible and cost-effective fabrication routes.

The UPC coordinates the research of the consortium in the SCALED project. This proposal is built on a long and consolidated scientific cooperation.


Hole Transport Layer based on atomic layer deposited V2Ox films: paving the road to semi-transparent CZTSe solar cells

Rosa Almache; Benjamín Pusay; Kunal Tiwari, Eloi Ros, Gerard Masmitjà, Ignacio Becerril, Isidro Martín, Cristóbal Voz, Joaquim Puigdollers, Edgardo Saucedo, Pablo Ortega

This work explores the use of very thin transparent vanadium oxide films deposited by atomic layer deposition (ALD) technique as hole transport layer for CZTSe solar cells as alternative of opaque molybdenum based contacts. Contact resistivity between the CZTSe absorber and the ALD V2Ox contact was measured. In order to improve contact quality, a cleaning bath using hydrofluoric acid (HF) dip, was also analyzed and its influence on kesterite surface was studied. Elementary material characterization and composition analysis of the V2Ox layers was performed. Contact quality was assessed yielding contact resistivity values below 9 and 30 mΩcm2 for ALD and thermal evaporated V2Ox films respectively.

The proposed ALD V2Ox based hole transport layer was deposited onto a glass covered with a transparent conductive oxide forming part of the rear contact scheme of a vertical CZTSe solar cell with a conventional ITO/CdS stack as electron transport layer. The impact of subsequent thermal post-annealing treatments in the cell performance was also analyzed yielding efficiency up to 3.9% on a semi-transparent CZTSe solar cell without any additional optimization process. In this way, a CZTSe solar cell with both transparent electrodes has been demonstrated paving the way to obtain in the future high efficiency bifacial and/or semitransparent Building –integrated photovoltaic devices.

Atomic layer deposition of vanadium oxide films for crystalline silicon solar cells

Eloi Ros; Gerard Masmitjà; Rosa Almache; Benjamín Pusay; Kunal Tiwari; Edgardo Saucedo; Byung Chul Kim; C. Justin Raj, Isidro Martín, Joaquim Puigdollers, Cristóbal Voz, Pablo Ortega

Transition Metal Oxides (TMOs) are promising materials to develop selective contacts on high-efficiency crystalline silicon solar cells. Nevertheless, the standard deposition technique used for TMOs is thermal evaporation, which could add potential scalability problems into the industrial photovoltaic fabrication processes. As an alternative, Atomic Layer Deposition (ALD) is a thin film deposition technique already used for dielectric deposition in semiconductor device industry that has a straight forward up scalable design. This work reports on the results of vanadium oxide (V2O5) films deposited by ALD acting as hole-selective contact for n-type crystalline silicon (c-Si) solar cells frontal transparent contact without additional PECVD passivating layer. Reasonable specific contact resistance of 100 mΩcm2 was measured by transfer length method. In addition, measurements suggest the presence of an inversion layer at the c-Si/V2O5 interface with a sheet resistance of 15 kΩ/sq. The strong band bending induced at the c-Si surface is confirmed by capacitance-voltage measurements with a built-in voltage value of 683 mV. Besides low contact resistance, vanadium oxide films provide excellent surface passivation with effective lifetime values up to 800 μs. Finally, proof-of-concept both-side contacted solar cells exhibit efficiencies beyond 18%, shedding light on the possibilities of TMOs deposited by atomic layer deposition technique.

Expanding the Perspective of Polymeric Selective Contacts in Photovoltaic Devices Using Branched Polyethylenimine

Eloi Ros, Thomas Tom, David Rovira, Julià Lopez, Gerard Masmitjà, Benjamin Pusay, Estefania Almache, Isidro Martin, Maykel Jimenez, Edgardo Saucedo, Eva Tormos, Jose Miguel Asensi, Pablo Ortega, Joan Bertomeu, Joaquim Puigdollers, and Cristobal Voz

This work studies the use of polymeric layers of polyethylenimine (PEI) as an interface modification of electron-selective contacts. A clearly enhanced electrical transport with lower contact resistance and significant surface passivation (about 3 ms) can be achieved with PEI modification. As for other conjugated polyelectrolytes, protonated groups of the polymer with their respective counter anions from the solvent create an intense dipole. In this work, part of the amine groups in PEI are protonated by ethanol that behaves as a weak Brønsted acid during the process. A comprehensive characterization including high-resolution compositional analysis confirms the formation of a dipolar interlayer. The PEI modification is able to eliminate completely Fermi-level pinning at metal/semiconductor junctions and shifts the work function of the metallic electrode by more than 1 eV. Induced charge transport between the metal and the semiconductor allows the formation of an electron accumulation region. Consequently, electron-selective contacts are clearly improved with a significant reduction of the specific contact resistance (less than 100 mΩ·cm2). Proof-of-concept dopant-free solar cells on silicon were fabricated to demonstrate the beneficial effect of PEI dipolar interlayers. Full dopant-free solar cells with conversion efficiencies of about 14% could be fabricated on flat wafers. The PEI modification also improved the performance of classical high-efficiency heterojunction solar cells.