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All-polymer solar cells (all-PSCs) have been intensively investigated due to their excellent thermal and mechanical stabilities. However, the efficiency of all-PSCs is still far behind that of organic solar cells (OSCs), which are based on small molecule acceptors. Improving the efficiency of all-PSCs is of great urgency. In this work, the solvent additive named p-anisaldehyde (AA) was introduced to improve the performance of all-PSCs based on PBDB-T:N2200. It is demonstrated that AA helps to form a better network interpenetrating track and more uniform phase separation. With the assistance of AA, which has both an oleophilic methoxy group and a hydrophilic aldehyde group, the interfacial contact between the donor and the acceptor (D/A) is improved, increasing the contact area at D/A and promoting efficient exciton dissociation. More importantly, AA acts as a “bridge” between the oleophilic active layer and the hydrophilic PEDOT:PSS layer, improving the interfacial compatibility between the active layer and the PEDOT:PSS layer, reducing the interfacial resistance, and facilitating the carrier transport. Finally, the all-PSCs based on PBDB-T:N2200 exhibits a superior power conversion efficiency (PCE) of 7.24% which is a record efficiency for the current all-PSCs with the same architecture. In this work, a promising and effective strategy for achieving high efficiency all-PSCs is provided.

The cathode interfacial layer plays a key role in enhancing the efficiency and stability of organic solar cells. Constructing efficient two-layer interfaces is one of the important strategies to improve device performance. In this work, we deposited a thin layer of lithium fluoride (LiF) beneath the electron transport layer poly[(9,9bis(3′-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9dioctylfluorene)]dibromide (PFN-Br) and (N,N-dimethyl-ammonium N-oxide)propyl perylene diimide (PDINO), respectively, to prepare conventional organic solar cells featuring a double electron transport layer. The V oc and FF of the optimized devices with a double electron transport layer were improved to achieve a power conversion efficiency (PCE) of 16.4% for LiF/PFN-Br-based devices and 16.0% for LiF/PDINObased devices. The LiF-containing double electron layers reduce the work function of the active layer, makes electron injection more efficient, and promotes charge extraction and collection. As an optical spacer layer, the thin layer of LiF also changes the internal light intensity distribution, enabling the active layer to absorb more photons and generate more excitons. In addition to PCE enhancement, the LiF-containing double layers still reduce photodegradation of the active layer and improve the stability of the device.

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Using optical pump-terahertz (THz) time-domain spectroscopy measurements along with ab initio density functional theory (DFT) calculations, we studied the effect of 3,9-bis(2-methylene-(3-(1, 1-dicyanomethylene)-5-methylindanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno [1,2-b:5,6-b′]-dithiophene (IT-M) molecules on the ultrafast dynamics and photoconductivity of the active layers of ternary organic solar cells (OSCs), which are composed by the conjugated polymer donor and fullerene derivative acceptor. The THz photoconductivity measurements show that the introduction of IT-M molecule increases the photoconductivity of the ternary OSC and the maximum value of photoconductivity is obtained in the sample with IT-M weight ratio of 15%. To reveal the mechanism of IT-M doping on the photoconductivity of OSC material, ab initio DFT calculations were performed to identify the band structures of the ternary OSC heterostructures. Resonant charge transfers induced by the band alignment of IT-M with the conjugated polymer donor and fullerene derivative acceptor, which lead to an increase of photoconductivity with a reduction of electron–hole recombination rate, were obtained from DFT calculations. In addition to the resonant charge transfer process, defect scattering induced by the doping of IT-M molecules leads to a reduction of photoconductivity. By competing these two effects, maximum photoconductivity of ternary OSC materials is obtained with IT-M weight ratio of 15%. Moreover, the charge carrier densities and scattering times of ternary OSC materials were derived from the measured THz photoconductivity via the classical Drude–Smith model. With these material properties, we identified that the maximum value of photoconductivity in OSC material with IT-M weight ratio of 15% is induced by its long scattering time instead of high carrier density.