Interfacial engineering and down-conversion of ultraviolet light for efficient perovskite solar cells

The addition of a tiny amount of fluorescent organic material N, N'-di (naphtha-1-yl)-N, N'-diphenyl-benzidine (NPB) into polymeric hole transport layer (HTL) is shown to yield a significant enhancement in down-conversion of ultraviolet (UV) light into visible light. As a result, NPB incorporated HTL based devices exhibit enhancement in power conversion efficiency (PCE) of 18.2% compared to 15.1% for pristine devices. This work demonstrated that fluorescent down-conversion of UV light based on doped HTLs is a novel and effective approach for enhanced performance of perovskite solar cells (PSCs). In addition, the incorporated fluorescent material NPB provides a favorable energetic alignment at the interface for low voltage loss. These results are further supported by time-resolved photoluminescence (TRPL) measurements that are performed on perovskite layers deposited on top of NPB doped Poly (triaryl amine) (PTAA), which shows an improvement in lifetime of the charge carriers with increasing concentration of NPB. An increase in the lifetime of the charge carriers is observed on the perovskite layer that is deposited on top of 2.5 wt% NPB doped PTAA layer. The lifetime enhancement is due to additional charge carriers that are generated from the conversion of high energy photons to low energy photons. These additional charge carriers that are generated in HTL occupy the charge transport states after filling the trap states in the perovskite layer and hence the lifetime of the charge carriers increases. The TRPL lifetime decreases with further increase in the NPB concentration more than 2.5 wt% due to efficient charge carrier extraction.