Elsevier

Nano Energy

Volume 32, February 2017, Pages 551-557
Nano Energy

Communication
Carbazole-based enamine: Low-cost and efficient hole transporting material for perovskite solar cells

https://doi.org/10.1016/j.nanoen.2017.01.015Get rights and content

Highlights

  • Palladium catalyst-free synthesis of HTMs for perovskite solar cells is proposed.

  • One step synthesis procedure and simplified purification offer significant HTM synthesis cost reduction.

  • Effectiveness of V950 as an alternative to the spiro-OMeTAD for a wide range of perovskite materials is evaluated.

Abstract

A simple carbazole-based conjugated enamine V950 was synthesized, fully characterized and incorporated into a perovskite solar cell, which displayed high power conversion efficiency close to 18%. The investigated hole transporting material was synthesized via an extremely simple route (one step, no expensive catalysts, no column chromatography or sublimation purification) from commercially available and relatively inexpensive starting reagents, resulting in more than one order of magnitude lower cost of the final product compared to the commercial 2,2′,7,7′-tetrakis(N,N-di-p-methoxy-phenylamine)-9-9′-spirobifluorene (spiro-OMeTAD). This material promises to be a viable p-type organic charge conductor to be employed in the scale-up and manufacturing of perovskite solar modules.

Graphical abstract

New carbazole-based conjugated enamine small-molecule hole conductor, that yields an overall efficiency of 17.8% in perovskite-based solar cells, was prepared in one step without use of expensive catalysts, column chromatography or sublimation purification.

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Introduction

Although known since the late 19th century, organic-inorganic perovskites have recently received an extraordinary attention of the research community because of their unique physical properties, which make them promising candidates for application in photovoltaic and optoelectronic devices.[1], [2], [3], [4] Past few years have witnessed rapid development in the perovskite based solar cells (PSC). Record efficiencies of lead halide perovskite (APbX3, A=methylammonium, formamidinium; X=Br, I)-based thin film photovoltaic devices exceeding 20% have been reported and certified [5], [6], [7], [8], [9], [10], [11]. In these cells, the hole transporting material (HTM) is one of the key components, which transport photo-generated holes to contact. The HTM should exhibit high enough hole mobility, suitable highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) energy levels [12]. Currently they are a bottleneck for the realization of cost-effective and stable devices [13]. Despite significant efforts dedicated towards development of new HTMs, the field is still dominated by costly small spiro-type molecules 2,2′,7,7′-tetrakis(N,N-di-p-methoxy-phenylamine)-9-9′-spirobifluorene (spiro-OMeTAD), 2′,7′-bis(bis(4-methoxyphenyl)amino)spiro-[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene] (FDT), and even more expensive macromolecule poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) [6], [8], [14], [15]. The high cost of these HTMs arises from the expensive synthesis and purification procedures which limit their potential for applications in low-cost solar cells. For example, spiro-OMeTAD is synthesized in five reaction steps that require low temperature (−78 °C), sensitive (n-butyllithium or Grignard reagents) and aggressive (Br2) reagents [16]. Moreover, spiro-OMeTAD, and other HTMs require costly sublimation steps for purification to obtain high-performance devices.

Recently, easily attainable branched methoxydiphenyl-amine-substituted fluorene-[17] and bifluorenylidene-based [18] HTMs were designed with PCE up to 20%, which is comparable to spiro-OMeTAD. An interesting approach towards the synthesis of low-cost, spiro-type HTM– X59 was also introduced [19]. However, these molecules, as most of other novel HTMs, are synthesized via cross-coupling reactions that require transition metal catalysts, inert reaction conditions, and extensive product purification in order to remove catalyst residue which in turn increases cost of the final product.

Herein we report synthesis and characterization of a novel enamine-based small-molecule V950 (Scheme 1) employing 3-amino-9-ethylcarbazole and 2,2-bis(4-methoxyphenyl)acetaldehyde moieties. The V950 HTM when incorporated in PSCs, gives power conversion efficiency (PCE) comparable to that of the state-of-the-art materials, such as spiro-OMeTAD, in a like to like comparison. The V950 can be obtained by a single-step reaction from readily available commercial starting materials making it very appealing for commercial prospects of PSCs. To the best of our knowledge, this is the first report dealing with the enamine-based molecules applied as HTM in photovoltaics.

Section snippets

Results and discussion

Enamine condensation chemistry offers an extremely simple route towards extended π-conjugated molecules [20]. The reaction can be performed under ambient conditions and water is the only by-product, making product purification uncomplicated [21]. The general synthesis procedure for the preparation of enamine V950 is shown in Scheme 1.

Commercially available precursor 3-amino-9-ethylcarbazole reacts with 2,2-bis(4-methoxyphenyl)acetaldehyde in the presence of (+/˗)camphor sulfonic acid to

Conclusions

In conclusion, we have demonstrated a new small-molecule hole conductor V950 that yields high overall efficiency up to 17.8% in perovskite-based solar cells. This is comparable to the state-of-the-art material spiro-OMeTAD on a like-to-like comparison. The simplicity of synthesis and purification should enable rapid advancement of this new class of hole-conductors for perovskite solar cells and other optoelectronic applications. The enamine condensation chemistry offers a cost-effective and

Acknowledgements

The authors acknowledge funding from the European Union Seventh Framework Programme [FP7/2007-2013] under Grant agreement no 604032 of the MESO project. This work was in part funded by EPSRC UK (EP/M015254/1). We thank E. Kamarauskas for his help with ionization potential measurements.

Maryte Daskeviciene received her Ph.D. (1994) degree in the department of Organic chemistry from Kaunas University of Technology. She is currently Senior Researcher in the same department. Her research interests involve molecular engineering of functional materials. She is co-author of 36 journal papers and over 20 patents.

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      Additionally, as shown in Fig. 3b, during the first heating scan, only melting was detected at 232.7 °C, while no melting but a glass transition was observed solely during the second heating cycle. This demonstrated that K-DHP-1, similarly as spiro-OMeTAD, can exist in both amorphous and crystalline states [64]. In contrast, the melting transition of K-DHP-2 took place during both the first and second heating scans, and a cold crystallization was also observed (Fig. S11b) [65], revealing that K-DHP-2 has a stronger tendency to crystallize.

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    Maryte Daskeviciene received her Ph.D. (1994) degree in the department of Organic chemistry from Kaunas University of Technology. She is currently Senior Researcher in the same department. Her research interests involve molecular engineering of functional materials. She is co-author of 36 journal papers and over 20 patents.

    Sanghyun Paek received his M.S. (2011) and Ph.D. (2013) degree in department of materials chemistry from Korea University. He is currently post-doctor position in the EFPL. His main research interests include the design, synthesis and characterization of new hole-transporting materials based on perovskite solar cells for high efficiency and good stability.

    Zhiping Wang is a postdoctoral researcher at the University of Oxford. After receiving his Ph.D. in 2012 from the University of Toyama, Japan, he worked at the National Institute of Advanced Industrial Science and Technology (AIST), focusing on organic optoelectronics. From 2015, he joined Prof. Henry Snaith’s group and his research is now focused on perovskite solar cells. His current research interests include development of new perovskite compounds, solar cell device engineering, especially for long term stability.

    Tadas Malinauskas received his Ph.D degree from Kaunas University of Technology in 2007. He is currently associate professor at Kaunas University of Technology, Faculty of Chemical Technology, with research interests in development and investigation of new materials for optoelectronic applications.

    Gabriele Jokubauskaite was educated at the Kaunas University of Technology where she received a B.Sc. (2015). She is a M.Sc. student at the Kaunas University of Technology from 2015. Her main research interests are design and synthesis of hole transporting materials for the perovskite solar cells.

    Kasparas Rakstys is a Ph.D. student at EPFL from 2013. He received a B.Sc. (2011) and M.Sc. (2013) degrees in applied organic chemistry from Kaunas University of Technology. His main research interests are design, synthesis and characterization of novel organic sensitizers for dye-sensitized solar cells and highly efficient hole transporting materials for perovskite solar cells.

    Kyung Taek Cho was educated at the Hanyang University where he received a B.S. (2012) and at Korea Advanced Institute of Science and Technology (KAIST) where he received M.S. (2014). He is a Ph.D. student at EPFL (2014-present). His main research interests include the design of perovskite solar cells for high efficiency and new perovskite materials sustaining in high humidity.

    Artiom Magomedov was educated at the Kaunas University of Technology where he received a B.Sc (2013) and M.Sc. (2015). He is a Ph.D. student at the Kaunas University of Technology from 2015. His main research interests include the design and synthesis of novel organic small molecules for the application in the various electronic devices (Dye-sensetized solar cells, perovskite solar cells etc.).

    Vygintas Jankauskas is an associate professor and scientific researcher in Faculty of Physics at Vilnius University. He works at the Vilnius University since 1982. His research focuses now on electronics and carrier transport phenomena in disordered and organic semiconductors.

    Shahzada Ahmad is a Principal Scientist at Abengoa Research. He finished his Ph.D. (2006) and later moved to the Max Planck Institute for Polymer Research, as Alexander von Humboldt Fellow to work with Prof. H.-J. Butt (surface and interface studies of electrodeposition in ionic liquids). He was a regular visitor to Prof. Michael Grtzel’s group at cole polytechnique federale de Lausanne, where he developed nanoporous films for metal-free electro-catalysis. His research targets are in energy conversion, conservation, and storage materials.

    Prof. Henry J. Snaith FRS is a professor of physics at Oxford University and is CSO and Founder of Oxford PV Ltd. His research is focused on developing new materials for photovoltaics and understanding and controlling the optoelectronic processes occurring within the devices and at heterojunctions. He was awarded the institute of Physics Patterson Medal in 2012, named as one of "natures ten" people who mattered in 2013, received the Materials Research Society Outstanding Young Investigator award in 2014, elected as a member of the Royal Society in 2015 , assessed to be the 2nd "most influential scientific mind" in 2016 and will be awarded the Royal Society Kavli Medal and Lecture in 2017.

    Vytautas Getautis is a Professor at the Kaunas University of Technology, head of the group for synthesis and investigation of functional materials. His research focuses on design and synthesis of organic photoconductors, especially in the areas of xerography, photovoltaic, and organic light emitting diodes. He has published more than 100 journal papers and filed over 50 patents in regard to these technologies.

    Mohammad Khaja Nazeeruddin is a Professor, and directs Group for Molecular Engineering of Functional Materials at EPFL. He published over 500 peer-reviewed scientific publications, 12 review/invited book chapters and inventor or co-inventor of over 50 patents with an h index of 112. His total number of citations is over 55,000. He is an expert in Dye-sensitized Solar Cells, Perovskite solar cells and Organic Light Emitting Diodes. He has been appointed as a world-class university professor at South Korea, Eminent Professor at Brunei and Distinguished Professor at King Abdul Aziz University.

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    These authors contributed equally to this work.

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