Atomic layer deposition of thin films:from a chemistry perspective

Atomic layer deposition(ALD)has become an indispensable thin-film technology in the contemporary microelectronics industry.The unique self-limited layer-by-layer growth feature of ALD has outstood this technology to deposit highly uniform conformal pinhole-free thin films with angstrom-level thickness control,particularly on 3D topologies.Over the years,the ALD technology has enabled not only the successful downscaling of the microelectronic devices but also numerous novel 3D device structures.As ALD is essentially a variant of chemical vapor deposition,a comprehensive understanding of the involved chemistry is of crucial importance to further develop and utilize this technology.To this end,we,in this review,focus on the surface chemistry and precursor chemistry aspects of ALD.We first review the surface chemistry of the gas–solid ALD reactions and elaborately discuss the associated mechanisms for the film growth;then,we review the ALD precursor chemistry by comparatively discussing the precursors that have been commonly used in the ALD processes;and finally,we selectively present a few newly-emerged applications of ALD in microelectronics,followed by our perspective on the future of the ALD technology.

All-polymer solar cells with over 16%efficiency and enhanced stability enabled by compatible solvent and polymer additives

Considering the robust and stable nature of the active layers,advancing the power conversion efficiency(PCE)has long been the priority for all-polymer solar cells(all-PSCs).Despite the recent surge of PCE,the photovoltaic parameters of the stateof-the-art all-PSC still lag those of the polymer:small molecule-based devices.To compete with the counterparts,judicious modulation of the morphology and thus the device electrical properties are needed.It is difficult to improve all the parameters concurrently for the all-PSCs with advanced efficiency,and one increase is typically accompanied by the drop of the other(s).In this work,with the aids of the solvent additive(1-chloronaphthalene)and the n-type polymer additive(N2200),we can fine-tune the morphology of the active layer and demonstrate a 16.04%efficient all-PSC based on the PM6:PY-IT active layer.The grazing incidence wideangle X-ray scattering measurements show that the shape of the crystallites can be altered,and the reshaped crystallites lead to enhanced and more balanced charge transport,reduced recombination,and suppressed energy loss,which lead to concurrently improved and device efficiency and stability.

Fine-tuning HOMO energy levels between PM6 and PBDB-T polymer donors via ternary copolymerization

To achieve high-efficiency polymer solar cells(PSCs),it is not only important to develop high-performance small molecule acceptors(SMAs)but also to find a matching polymer donor to achieve optimal morphology and matching electronic properties.Currently,state-of-the-art SMAs mostly rely on a donor polymer named PM6.However,as the family of SMAs continues to expend,PM6 may not be the perfect polymer donor due to the requirement of energy level matching.In this work,we tune the energy level of PM6 via the strategy of ternary copolymerization.We achieve two donor polymers(named PL-1 and PL-2)with upshifted HOMO(the highest occupied molecular orbital)energy level(compared with PM6),and can thus match with the SMAs with upshifted HOMO energy levels compared with Y6.These two copolymers exhibit slightly higher order of molecular packing and similar charge transport properties,which demonstrate that the method of ternary copolymerization can fine tune the HOMO level of donor polymers,while the morphology and mobility of the blend film remain mostly unaffected.Among them,the best device based on PL-1:Y6 exhibits power conversion efficiencies(PCEs)of 16.37%with lower open circuit voltage(Voc)but higher short circuit current voltage(Jsc)and fill factor(FF)than that of the device based on PM6:Y6.This work provides an effective approach to find polymer matches for the SMAs with upshifted HOMO levels.