Influence of Crystallinity and Energetics on Charge Separation in Polymer–Inorganic Nanocomposite Films for Solar Cells

Abstract

The dissociation of photogenerated excitons and the subsequent spatial separation of the charges are of crucial importance to the design of efficient donor-acceptor heterojunction solar cells. While huge progress has been made in understanding charge generation at all-organic junctions, the process in hybrid organic:inorganic systems has barely been addressed. Here, we explore the influence of energetic driving force and local crystallinity on the efficiency of charge pair generation at hybrid organic:inorganic semiconductor heterojunctions. We use x-ray diffraction, photoluminescence quenching, transient absorption spectroscopy, photovoltaic device and electroluminescence measurements to demonstrate that the dissociation of photogenerated polaron pairs at hybrid heterojunctions is assisted by the presence of crystalline electron acceptor domains. We propose that such domains encourage delocalization of the geminate pair state. The present findings suggest that the requirement for a large driving energy for charge separation is relaxed when a more crystalline electron acceptor is used.

Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells

“…The impact of fullerene nanocrystallinity on charge separation has been noted previously, where it was attributed to high local electron mobility within the traditional Onsager framework 11,30 as well as to shifts in electron affinity of the fullerene crystallites 31 . In contrast, our results here ARTICLE point to the role of delocalization, analogous to recent findings for organic-inorganic HJs 32 , and are consistent with a growing number of recent reports in which charge separation is driven by accessing band-like CT states that subsequently decay on a sub-50 ps timescale (an upper limit based on the resolution of our PL transient data) into separated, free charge carriers 3,4,29,33 . In summary, these results support a model of delocalized CT state dissociation and show that this process depends critically on nanoscale fullerene crystallinity.…”

Effect of Bath Temperature on Optical and Morphology Properties of CdS Thin Films Grown by Chemical Bath Deposition

“…According to standard JCPDS data card # 41-1049, these peaks correspond to hexagonal CdS [30]. The CdS films deposited at various deposition temperatures clearly shows that these films have (002) preferred orientation of crystallites.…”

Ultrafast electron and hole transfer in bulk heterojunctions of low-bandgap polymers

“…A fascinating example of the effect of molecular structure on charge separation is a pair of well-known polymers: PCPDTBT [33][34][35] and BTT-DPP [36][37][38] (Fig. 1).…”