Donor–p–donor type hole transporting materials: marked p-bridge effects on optoelectronic properties, solid-state structure, and perovskite solar cell efficiency† S. Paek, a I. Zimmermann, a P. Gao, * a P. Gratia, a K. Rakstys, a G. Grancini, a Mohammad Khaja Nazeeruddin, * a Malik Abdul Rub, b Samia A. Kosa, b Khalid A. Alamry b and Abdullah M. Asiri b Donor–p-bridge–donor type oligomers (D–p–D) have been studied intensively as active materials for organic optoelectronic devices. In this study, we introduce three new D–p–D type organic semiconductors incorporating thiophene or thienothiophene with two electron-rich TPA units, which can be easily synthesized from commercially available materials. A thorough comparison of their optoelectronic and structural properties was conducted, revealing the strong influence of the extent of longitudinal p-bridge conjugation on both the solid structure of the organic semiconductive materials and their photovoltaic performance when applied as hole transporting materials (HTM) in perovskite solar cells. Single-crystal measurements and time-resolved photoluminescence (TRPL) studies indicate that these coplanar donor–p–donor type HTMs could be promising alternatives to state-of-the-art spiro-OMeTAD, due to the multiple intermolecular short contacts as charge transporting channels and efficient charge extraction properties from the perovskite layer. The optimized devices with PEH-9 exhibited an impressive PCE of 16.9% under standard global AM 1.5 illumination with minimized hysteretic behaviour, which is comparable to that of devices using spiro-OMeTAD under similar conditions. Ambient stability after 400 h revealed that 93% of the energy conversion efficiency was retained for PEH-9, indicating that the devices had good long-term stability. Introduction Organometal halide perovskite solar cells (PSCs) exhibiting high power conversion efficiencies (PCEs) may provide inex- pensive, renewable sources of solar electricity via low-cost materials and fabrication techniques. 1–3 PCEs of PSCs have been quickly increased from 3.8 to 22.1% as certied by the National Renewable Energy Laboratory (NREL) 4 due to their intrinsic advantages such as broad absorption in the visible region, 5 high absorption coefficients, 6 high charge carrier mobility 7 and long diffusion length. 8 In such devices, the photoactive layer nor- mally consists of a pure/blended polycrystalline layer of perov- skite semiconductor [APbX 3 ,A ¼ MAI, FAI, Cs; X ¼ Cl, Br, I] that is imbedded between a layer of electron transporting material (ETM) and a hole transporting material (HTM). 2 An attractive approach to push PSCs to industry and market, besides devel- oping unconventional device structures 9 and more complicated perovskite compositions, 10 is to explore new contact/interfacial materials, particularly HTMs. 11 HTMs play an important role in determining the photovoltaic performance and long-term stability of the perovskite solar cells. Among the many HTMs developed, 2,2 0 ,7,7 0 -tetrakis(N,N-di-p-methoxyphenylamine)- 9,9 0 -spirobiuorene (spiro-OMeTAD) is by far the most studied and used molecular p-type HTM with a recently reported PCE of 20.8%. 12 However, spiro-OMeTAD is very expensive owing to the need for sublimation for purication. In this regard, the development of cost-effective and efficient HTMs remains a problem. Recently, impressive photovoltaic performance has been achieved using molecular HTMs, such as thiophene deriva- tives, 13,14 3,4-ethylenedioxythiophene derivatives, 15–17 spiro- OMeTAD derivatives, 18,19 truxene-based derivatives, 20 carbazole derivatives, 21 etc. 22 Their characterization provides fundamental information on how molecular modications affect PCE by altering arylamine-substitution, p-system size, steric geometry, and carrier mobility. However, the interesting question of how a Group for Molecular Engineering of Functional Materials, Ecole Polytechnique Federale de Lausanne Valais Wallis, Rue de l'Indutrie 17, 1950 Sion, Valais, Switzerland. E-mail: mdkhaja.nazeeruddin@ep.ch; peng.gao@ep.ch b Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz, University, Jeddah, Saudi Arabia † Electronic supplementary information (ESI) available. CCDC 1446682–1446684. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc01478j Cite this: Chem. Sci., 2016, 7, 6068 Received 4th April 2016 Accepted 24th May 2016 DOI: 10.1039/c6sc01478j www.rsc.org/chemicalscience 6068 | Chem. Sci., 2016, 7, 6068–6075 This journal is © The Royal Society of Chemistry 2016 Chemical Science EDGE ARTICLE Open Access Article. Published on 24 May 2016. Downloaded on 5/16/2022 10:09:23 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue