Organic Small Molecular Materials for Perovskite Solar Cells and Bulk Heterojunction Solar Cells

Licheng Sun^a, Ming Cheng^a

^aKTH The Royal Institute of Technology, Roslagstullsbacken 21, Stockholm, Sweden

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Ming Cheng, 312
Publication date: 5th February 2015

The development of cost-effective, high-efficiency solar cells that can meet the ever increasing demand of sustainable energy is one of the great challenges in the 21st century. Over the past two years, the emerging family of metal-halide-based perovskites with the general formula (RNH₃)MX₃ (R = alkyl, M = Pb, X = I, Br or Cl) havebeen demonstrated to constitute highly promising photovoltaic materials to fulfill this target.^1-4To date, extraordinary power conversion efficiencies (PCEs) of over 19% havebeen reported for cells based on a lead-containingperovskite as light-absorbing material and 2, 2, 7, 7-tetrakis(N,N-di-p-methoxyphenyl-amine)-9, 9-Spirobifluorene (Spiro-OMeTAD) as the HTM. ⁵However, the high cost of Spiro-OMeTAD impedes the growth and advancement of highly efficient and cost-effective PSCs. Moreover, Spiro-OMeTAD suffers from low hole mobility and conductivity in its pristine form; therefore, p-type dopants are necessary in order to promote high device performance. However, some of the p-type dopants used have been observed to accelerate the cell degradation due to their deliquescent properties. ⁶In addition, the p-type doping strategy increases the overall cost and also requires a strict optimization of the doping conditions, such as the solvent and dopants used, as well as the doping concentrations.Therefore, it is imperative to develop low-cost, dopant-free HTMs for efficient PSCs. We have designed and synthesizedof aseries of Acceptor-Donor-Acceptor (A-D-A) structured small-molecule materials(M1-M4) based on a rigid benzo[1,2b:4,5b’]-dithiophene (BDT) central building block and phenoxazine (POZ) linkersand they were used both as dopant-freehole-transport materials(HTMs) in (CH₃NH₃)PbI₃-perovskite-based solar cells and as photoactive donor materials(PDMs) in bulk heterojunction (BHJ) organic solar cells (OSCs). Excellent PCEof 13.2% under 100 mW·cm^-2irradiation was achievedusing M1 as dopant-freeHTM in PSCs, as compared to 12.4% obtained for devices containing the p-type doped Spiro-OMeTADas HTM.When applied in BHJ OSCs, a PCE of 6.91% was achieved using an optimized device structure of ITO/ZnO/polyethylenimine, 80% ethoxylated (PEIE) /M1:PC₇₀BM (1:1.2)/MoO₃/Ag. To the best of our knowledge, this is the first case where a small-molecule organic material has been shown to act both as an HTM in PSCs and as a PDM in BHJ OSCs and showing excellent conversion efficiencies in both types of solar cell systems.
Device structure and J-V performance of M1 based perovskite solar cells
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