Dopant-free hole-transporting polymers for efficient, stable, and hysteresis-less perovskite solar cells

Four novel polymers have been investigated as promising solution processable, dopant-free polymeric hole-transporting materials (HTMs) for efficient and stable perovskite solar cells. The poly(1-(4-hexylphenyl)-2,5-bis(5-methylthiophen-2-yl)-1H-pyrrole) p(hPhDTP), poly(1-(4-methoxyphenyl)-2,5-bis(5-methylthiophen-2-yl)-1H-pyrrole)p(mPhDTP), poly(3-hexyl-5,5′-dimethyl-2,3′-bithiophene) p(hBT) and poly(5,5′-dimethyl-2,3′-bithiophene) p(BT) HTMs have successfully been synthesized from relatively cheap raw materials with a facile synthetic route. The fabricated polymers are potentially cost-effective and exhibit favourable HOMO level with respect to the valence band of the perovskite active layer. The incorporation of the new HTMs in the perovskite solar cell architecture exhibited excellent overall power conversion efficiency (PCE) of 16.20, 14.45, 11.10 and 9.63% for P(hPhDTP), P(mPhDTP), P(hBT), and P(BT), respectively with minimal hysteresis without the use of any dopants or additives, which is higher than the 8.33% obtained using P3HT-based devices. The high efficiency can be attributed to the positive HOMO energy level and the high hole (h⁺) mobility. In addition, the long-term stability of the polymers as HTM devices were studied, where they maintained their initial efficiency for over 1200 h, whereas devices made using P3HT as HTM drops down to almost half of its value after 600 h.