Inverted organic light-emitting diodes (IOLEDs) can effectively improve device stability because they concentrate the air-stable anode and high work function (WF) metal oxide hole-injection layer (HIL) at the top of the devices. In this work, we report a facile solution-processed ultra-thin alumina film used as an electron-injection layer in IOLEDs and present significantly improved device performance. We achieved a high current efficiency of 5.12 cd A−1 at 10 mA cm−2 and the best current efficiency approaching 5.5 cd A−1 at 40 mA cm−2 without doping of an emission layer (EML) for a single Alq3-based green fluorescent IOLED, and a high current efficiency for a green phosphorescent IOLED as well. Furthermore, the extrapolated 50% decay lifetime (t50) shows that our Alq3-based green fluorescent IOLED is about 5 times more stable than the conventional OLED.

A class of polymers (POT-DH, POT-HCN and POT-DCN) were synthesized and they contain the same donor (BDT) and acceptor units but different numbers of cyano (CN)-groups, i.e. 0, 1 and 2. We investigated for the first time the effects of different CN-group numbers on the optoelectronic, molecule packing, film morphology and photovoltaic properties of three conjugated polymers. With increased CN-group number, broader absorption, smaller optical bandgap and lower HOMO level can be obtained. The planarity of polymers also decreases with increasing CN-groups, leading to different inter-molecular packing and morphology. The incorporation of two CN-groups results in poor morphology, inefficient charge transfer and very low device performance. Due to the optimized POT-HCN polymer possessing the most balanced properties, the best PCE of 4.21% was demonstrated by POT-HCN with one CN-group. Thus we believe that, by controlling the number of introduced CN-groups, we can generally fine-tune the planarity and LUMO/HOMO levels of this class of polymers to achieve desired optoelectronic properties and morphology for high photovoltaic performance. This also provides a feasible way for optimizing other photovoltaic semiconducting polymers by adjusting the number of electron-withdrawing units.

•It is a facile method to prepare metal oxides for interfacial layer in PSC.•Good device performance can be achieved with the prepared metal-oxide interfacial layers.•Applicability was demonstrated with both polymer P3HT and PBDT-T8-TPD.We report a facile approach to prepare metal oxides for the interfacial layer in polymer solar cells (PSCs), in which the precursor solutions were obtained by dissolving commercial metal oxide/hydroxide in ammonia water. This approach can be adopted as a general method to prepare various solution-processable metal oxides for interfacial layers in PSCs, such as MoOx, VOx, WOx and ZnOx. The photovoltaic performance of PSCs buffered by these metal-oxide layers was studied and the applicability of these interfacial layers was demonstrated both with P3HT and a low band-gap polymer PBDT-T8-TPD.Graphical abstract

•Four copolymers composed of donors and acceptors were designed and synthesized.•Influence of side chain and π-bridge on polymer's properties was studied.•Broader absorption and narrower band-gap were achieved for the synthesized polymers.•Deep HOMOs between −5.48 eV and −5.37 eV were confirmed for all the polymers.•Prospectively high Vocs of the devices based on the four polymers were achieved.Four new homologous copolymers (POP, POT, PTP and PTT), composed of benzo[1,2-b:4,5-b′]dithiophene (BDT) donors and fumaronitrile (BCNV) acceptors, were designed and synthesized. The effects of different side-chains of BDT units and different π-bridges of BCNV acceptors on polymers’ thermal, optical, electrochemical, chain geometric, hole-transporting properties and photovoltaic performance were systematically investigated. POT and PTT exhibit broader absorption and narrower bandgaps than POP and PTP due to the replacement of phenyl π-bridges with thienyl groups. All polymers show relatively deep highest occupied molecular orbitals (HOMOs) between −5.48 eV and −5.37 eV. Prospectively high open circuit voltages (Vocs) of the devices based on the four polymers were accomplished. The highest Voc 0.975 V was obtained with PTP. However, the obtained PCEs are still relatively low. We consider this is related to the polymers’ poor backbone planarity, relatively low LUMO levels or low polaron photogeneration efficiency resulted from BCNV.

Small molecules TPDA, TPDB, and TPDH, comprising the same backbone TPD (N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine) and different carboxyl side chains were designed and synthesized as hole-transporting materials in polymer solar cells. These small molecules demonstrated improved solubility in polar solvents due to the introduction of the weakly acidic carboxyl groups. The lengths of the side chains can also influence the film forming ability. Compared to conventional PEDOT:PSS, these small molecules showed higher transmittance in the visible range as revealed by UV-vis measurements. Desirable energy-level alignment for efficient hole-transporting and electron-blocking ability was indicated by their absorption and UPS spectra. Without applying any post-treatment to these buffer layers, comparable and improved device performances were achieved compared with PEDOT:PSS buffered control devices. A high power conversion efficiency of 6.51% was realized by a TPDB buffered device employing a low band gap polymer PBDTTPD as the active material, which showed ∼15% efficiency enhancement over the control devices. Equally important, better device stability was demonstrated by using TPDB as the new hole-transporting layer.

Inverted organic light-emitting diodes (IOLEDs) can effectively improve device stability because they concentrate the air-stable anode and high work function (WF) metal oxide hole-injection layer (HIL) at the top of the devices. In this work, we report a facile solution-processed ultra-thin alumina film used as an electron-injection layer in IOLEDs and present significantly improved device performance. We achieved a high current efficiency of 5.12 cd A−1 at 10 mA cm−2 and the best current efficiency approaching 5.5 cd A−1 at 40 mA cm−2 without doping of an emission layer (EML) for a single Alq3-based green fluorescent IOLED, and a high current efficiency for a green phosphorescent IOLED as well. Furthermore, the extrapolated 50% decay lifetime (t50) shows that our Alq3-based green fluorescent IOLED is about 5 times more stable than the conventional OLED.

Small molecules TPDA, TPDB, and TPDH, comprising the same backbone TPD (N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine) and different carboxyl side chains were designed and synthesized as hole-transporting materials in polymer solar cells. These small molecules demonstrated improved solubility in polar solvents due to the introduction of the weakly acidic carboxyl groups. The lengths of the side chains can also influence the film forming ability. Compared to conventional PEDOT:PSS, these small molecules showed higher transmittance in the visible range as revealed by UV-vis measurements. Desirable energy-level alignment for efficient hole-transporting and electron-blocking ability was indicated by their absorption and UPS spectra. Without applying any post-treatment to these buffer layers, comparable and improved device performances were achieved compared with PEDOT:PSS buffered control devices. A high power conversion efficiency of 6.51% was realized by a TPDB buffered device employing a low band gap polymer PBDTTPD as the active material, which showed ∼15% efficiency enhancement over the control devices. Equally important, better device stability was demonstrated by using TPDB as the new hole-transporting layer.