Expanding The Compatibility of P3HT Hole Transport Layers for Stable NIP Perovskite Solar Cells

In order for perovskite solar cells to be commercially competitive, attaining robust environmental stability is of the upmost importance. In a standard nip device stack the material selection for the surface hole transport layer (HTL) is critical, as it will dictate the degree that air and moisture will penetrate into the sensitive perovskite layer. Present high performing devices rely upon the established small molecule: Spiro-OMeTAD due to its well aligned energy levels, high hole mobility and broad compatibility with differing perovskite chemistries and interfacial passivation layers.<br/>Conductive organic polymers offer several advantages over small molecule materials like Spiro-OMeTAD, such as fast intramolecular transport, greater environmental stability, and reduced cost. Poly(3-hexylthiophene-2,5-diyl) (P3HT) is one such polymer, and has been successfully implemented as an HTL in numerous reported perovskite solar cells, attaining efficiencies up to 24.6% [1]. However, typically these high performances rely upon interfacial perovskite treatments to induce favourable P3HT orientation and packing, and thus introduce restrictions on choice of perovskite chemistry or passivation method, that are not present for Spiro-OMeTAD. While reports that rely on oxidative dopants such as F4TCNQ, to increase the conductivity of P3HT HTL, exhibit lower efficiencies [2]. Therefore, presently P3HT based devices lag behind their Spiro-OMeTAD counterparts, in terms of performance, and versatility.<br/>Recently the novel “ion-modulated radical doping strategy” for Spiro-OMeTAD based HTLs has outlined a doping strategy that relies upon a stable radical (Spiro²⁺TFSI) to increase conductivity, in conjunction with an ionic modulator (TMBP-TFSI) to improve hopping transport between molecules [3].<br/>Drawing inspiration from the IM-Radical doping strategy, this work aims to develop an analogous co-doping approach, for P3HT (or other low-cost polymers). Oxidising dopants such as charge transfer materials, Lewis acids, photo-initiator salts, and electrophilic salts have been screened in order to maximise HTL conductivity, and thus short circuit current, while various ionic modulators have been employed to align material work-fucntions and maximise the open circuit voltage. Through this combined doping approach we aim to develop a low cost P3HT based HTL that can match the performance of established Spiro-OMeTAD based devices and offer imroved environmental stability, without imposing material restrictions upon the underlying device stack.<br/><br/><b>References</b><br/>[1] M. J. Jeong, K. M. Yeom, S. J. Kim, E. H. Jung, and J. H. Noh, “Spontaneous interface engineering for dopant-free poly(3-hexylthiophene) perovskite solar cells with efficiency over 24%,” <i>Energy Environ. Sci.</i>, vol. 14, no. 4, pp. 2419–2428, 2021, doi: 10.1039/d0ee03312j.<br/>[2] H. C. V. Tran, W. Jiang, M. Lyu, and H. Chae, “Tetrahydrofuran as Solvent for P3HT/F4-TCNQ Hole-Transporting Layer to Increase the Efficiency and Stability of FAPbI3-Based Perovskite Solar Cell,” <i>J. Phys. Chem. C</i>, vol. 124, no. 26, pp. 14099–14104, 2020, doi: 10.1021/acs.jpcc.0c03890.<br/>[3] T. Zhang <i>et al.</i>, “Ion-modulated radical doping of spiro-OMeTAD for more efficient and stable perovskite solar cells,” <i>Science (80).</i>, vol. 377, no. 6605, pp. 495–501, 2022, doi: 10.1126/science.abo2757.