Organic Non-Fullerene Acceptors as Efficient Electron Transporting Materials in Inverted Perovskite Solar Cells
Ahmed Said a, Qichun Zhang a, Dada Shaikh b, Sidhanath Bhosale b, Yang Wang c, Tsuyoshi Michinobu c
a School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798
b Polymers and Functional Materials Division and Academy of Scientific and Innovative Research (AcSIR), CSIR-Indian Institute of Chemical Technology, Hyderabad-500007, Telangana, India
c Department of Materials Science and Engineering Tokyo Institute of Technology
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Ahmed Said, 156
Publication date: 11th February 2019

During the development of perovskite solar cells (PSCs), numerous breakthroughs have been done by scientists. One of this breakthrough is the birth of inverted perovskite solar cells (p-i-n) PSCs. (1) p-i-n PSCs initiated new route in PSCs fabrication on flexible substrate and at low temperature. (2, 3) These properties are missed from regular perovskite solar cells (n-i-p) PSCs, which hampered n-i-p PSCs toward marketing. Charge transporting layers are crucial for extraction of electrons and holes, which are generated inside the perovskite layer and  transporting charges to cathode and anode. Electron transporting layer (ETL) has crucial effect on the efficiency of PSC as reported by Lam et al (4) compared with hole transporting layer (HTL), meanwhile HTL-free PSC showed decent efficiency . (5) PCBM as a derivatives of fullerene has deep experience as an ETL in perovskite solar cells. Although PCBM-based PSCs achieved high efficiency (21%), (6) it is highly desirable to find new candidates instead of PCBM as ETLs in PSCs, due to its drawbacks, which include poor film forming, high synthesis cost and poor  stability. (7-9) A lot of researches have been devoted to find new ETLs. Non-fullerene acceptors such as organic small molecules and n-type polymers are good alternatives of PCBM, due to their ease of synthesis, high solubility in different organic solvents and desired stability.  In this abstract we present our researches, which focus on fabrication of p-i-n PSCs based non-fullerene ETLs. We started our work by using two of Naphthalene diimide (NDI)-based ETLs, which termed DS1 and DS2 to replace PCBM in p-i-n PSCs. DS2-based PSCs achieved efficiency (11.4%) higher than DS1-based PSCs (9.1%). We attributed this result to the excess number of sulfur atoms in DS2. Since, sulfur atoms passivate the surface of perovskite layer by coordinating with under-coordinated Pb-atoms, which are originated during fabrication of perovskite layer and considered as electron trap centers. The  passivation by sulfur atoms  declines the charge recombination and enhances the efficiency. PCBM-based PSCs achieved efficiency (13.5%) at the same condition . (10) N-type conjugated polymers have all advantages of non-fullerene acceptors, in addition its high hydrophobicity and better film forming. We utilized three n-type conjugated (D-A1-D-A2) polymers, which termed pBTT, pBTTz and pSNT as ETLs in p-i-n PSCs. The main difference between each polymers is the position of embedded sp2 nitrogen in acceptor unit. The sp2 nitrogen has effect on the electron mobility and Molecular Frontier orbitals of each polymer. Although, pBTTz showed lowest electron mobility (0.47 cm2/V.s), pBTTz-based PSCs showed highest efficiency (14.4 %) compared with PCE of pBTT-based PSCs and efficiency of pSNT-based PSCs. The decisive factor, which controlled efficiencies was LUMO and HOMO of pBTTz. LUMO of pBTTz well matched with conduction band of perovskite layer, therefore the electrons transported easily from perovskite layer to cathode through pBTTz ETL. Furthermore, the deep HOMO of pBTTz -6.01 eV blocked the movement of holes from perovskite layer to cathode, so that charge recombination declined and current leakage decreased. Such factors delivered high efficiency. Although the LUMO of pSNT (-3.88 eV) is matched well with conduction band of perovskite layer, pSNT-based PSCs showed lower efficiency (12 %). The shallow HOMO of pSNT (-5.45 eV) compared with valence band of perovskite layer has neglected holes blocking properties, which enhanced charge recombination and leakage current, so efficiency decreased. Interestingly, after ten days pBTTz-based device retained 80% of its original efficiency, which is higher than PCBM-based device with a stability retention of 73%. (11) Such results strongly recommend the appropriateness of organic small molecules and n-type conjugated polymer as potential alternatives of PCBM in p-i-n PSCs.

We acknowledge our group members, our collaborators and financial support from AcRF Tier 1 (RG 111/17, RG 2/17,
RG 114/16, RG 8/16) and Tier 2 (MOE 2017-T2-1-021), Singapore.

© Fundació Scito
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