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EPFL scientists develop hole-transport material that lowers perovskite solar costs and still reaches 20.2% efficiency

To address this problem, a team of researchers at EPFL developed a molecularly engineered hole-transporting material, called FDT

18 Jan 2016 | Editor

EPFL scientists have recently published a research paper in Nature Energy that describes how they have developed a solar-panel material that can cut down on photovoltaic costs while achieving competitive power-conversion efficiency of 20.2%.

According to the researchers some of the most promising solar cells today use light-harvesting films made from perovskites - a group of materials that share a characteristic molecular structure. However, perovskite-based solar cells use expensive "hole-transporting" materials, whose function is to move the positive charges that are generated when light hits the perovskite film.

EPFL scientists have now engineered a considerably cheaper hole-transporting material that costs only a fifth of existing ones while keeping the efficiency of the solar cell above 20%.

As the quality of perovskite films increases, researchers are seeking other ways of improving the overall performance of solar cells. Inadvertently, this search targets the other key element of a solar panel, the hole-transporting layer, and specifically, the materials that make them up. There are currently only two hole-transporting materials available for perovskite-based solar cells. Both types are quite costly to synthesise, adding to the overall expense of the solar cell.

To address this problem, a team of researchers led by Mohammad Nazeeruddin at EPFL developed a molecularly engineered hole-transporting material, called FDT, that can bring costs down while keeping efficiency up to competitive levels. Tests showed that the efficiency of FDT rose to 20.2% - higher than the other two, more expensive alternatives. And because FDT can be easily modified, it acts as a blueprint for an entire generation of new low-cost hole-transporting materials.

EPFL - 3-D illustration of FDT molecules on a surface of perovskite crystals

Figure: EPFL - 3-D illustration of FDT molecules on a surface of perovskite crystals

Mohammad Nazeeruddin, said, "The best performing perovskite solar cells use hole transporting materials, which are difficult to make and purify, and are prohibitively expensive, costing over €300 per gram preventing market penetration." Nazeeruddin added, "By comparison, FDT is easy to synthesize and purify, and its cost is estimated to be a fifth of that for existing materials - while matching, and even surpassing their performance."

The study was led by EPFL's Group for Molecular Engineering of Functional Materials, in collaboration with the Istituto di Scienze e Tecnologie Molecolari del Consiglio Nazionale delle Ricerche (Italy), Panasonic Corporation (Japan), EPFL's Laboratory for Photomolecular Science and Laboratory of Photonics and Interfaces, and the Qatar Environment and Energy Research Institute.

It was funded by the European Union Seventh Framework Programme (MESO; ENERGY; NANOMATCELL), the Swiss National Science Foundation, and Nano-Tera.

A molecularly engineered hole-transporting material for efficient perovskite solar cells

Nature Energy 15017 | 18 January 2016 | DOI: 10.1038/NENERGY.2015.17

Michael Saliba | Simonetta Orlandi | Taisuke Matsui | Sadig Aghazada | Marco Cavazzini | Juan-Pablo Correa-Baena | Peng Gao | Rosario Scopelliti | Edoardo Mosconi | Klaus-Hermann Dahmen | Filippo De Angelis | Antonio Abate | Anders Hagfeldt | Gianluca Pozzi | Michael Graetzel | Mohammad Khaja Nazeeruddin

Received: 07 September 2015 | Accepted: 26 November 2015 | Published online: 18 January 2016

Abstract

Solution-processable perovskite solar cells have recently achieved certified power conversion efficiencies of over 20%, challenging the long-standing perception that high efficiencies must come at high costs. One major bottleneck for increasing the efficiency even further is the lack of suitable hole-transporting materials, which extract positive charges from the active light absorber and transmit them to the electrode. In this work, we present a molecularly engineered hole-transport material with a simple dissymmetric fluorene–dithiophene (FDT) core substituted by N,N-di-p-methoxyphenylamine donor groups, which can be easily modified, providing the blueprint for a family of potentially low-cost hole-transport materials. We use FDT on state-of-the-art devices and achieve power conversion efficiencies of 20.2% which compare favourably with control devices with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). Thus, this new hole transporter has the potential to replace spiro-OMeTAD.

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