Qinjun Sun

Improved performance of organic solar cells by incorporating silica-coated silver nanoparticles in the buffer layer

It is demonstrated that the use of silica-coated silver nanoparticles (AgNPs) in the buffer layer improves the performance of organic solar cells (OSCs). It is found that only large sized AgNPs are advantageous for increasing the electric field distribution in the active layer, and therefore, increasing light absorption, caused by the localized surface plasmonic resonance and far-field scattering. Furthermore, the scattering of silica-coated AgNPs is more important to the light harvesting because of the existence of the silica coating. It is also demonstrated that the silica coating is favorable for enhancing the exciton dissociation because of the reduction of the exciton quenching that occurred at the interface between the bare AgNPs and the active layer. Furthermore, silica-coated AgNPs also promote hole transport and extraction, which is presumably explained by the introduction of “dopant” levels within the band gap of the poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) and reduction of hole trapping of a bare silver surface. The combination of all these benefits results in a 25.4% improvement in photocurrent density and an increase of 19.2% in power conversion efficiency. This work indicates that using silica-coated AgNPs as light trapping elements is more efficient than using bare AgNPs in plasmonic organic solar cells. The systematic exploration of the optical and electrical effects of silica-coated AgNPs contributes to a more comprehensive understanding of the mechanism of performance improvement of the plasmonic OSCs.

Efficiency enhancement in organic solar cells by incorporating silica-coated gold nanorods at the buffer/active interface

The performance of organic solar cells (OSCs) can be greatly improved by incorporating silica-coated gold nanorods (Au@SiO2 NRs) at the interface between the hole transporting layer and the active layer due to the plasmonic effect. The silica shell impedes the aggregation effect of the Au NRs in ethanol solution as well as the server charge recombination on the surface of the Au NRs otherwise they would bring forward serious reduction in open circuit voltage when incorporating the Au NRs at the positions in contact with the active materials. As a result, while the high open circuit voltage being maintained, the optimized plasmonic OSCs possess an increased short circuit current, and correspondingly an elevated power conversion efficiency with the enhancement factor of ~11%. The origin of performance improvement in OSCs with the Au@SiO2 NRs was analyzed systematically using morphological, electrical, optical characterizations along with theoretical simulation. It is found that the broadband enhancement in absorption, which yields the broadband enhancement in exciton generation in the active layer, is the major factor contributing to the increase in the short circuit current density. Simulation results suggest that the excitation of the transverse and longitudinal surface plasmon resonances of individual NRs as well as their mutual coupling can generate strong electric field near the vicinity of the NRs, thereby an improved exciton generation profile in the active layer. The incorporation of Au@SiO2 NRs at the interface between the hole transporting layer and the active layer also improves hole extraction in the OSCs.

High efficiency planar Sn–Pb binary perovskite solar cells: controlled growth of large grains via a one-step solution fabrication process

One-step solution fabrication of high-performance Sn-including perovskite solar cells (PSCs) is very challenging due to the rapid crystallization of the Sn-based perovskite layer, leading to a poor film morphology and low surface coverage. In this work, a well-controlled one-step method, assisted by a multi-step solvent treatment, is developed for the growth of a high-quality CH3NH3Pb(1−x)SnxI3 (0 ≤ x ≤ 1) perovskite film on a planar PEDOT:PSS substrate. The CH3NH3Sn0.25Pb0.75I3 perovskite films consisting of densely packed and uniformly distributed large crystal grains were obtained using sec-butyl alcohol solvent engineering and N,N-dimethylformamide solvent annealing under an N2 atmosphere. The CH3NH3Sn0.25Pb0.75I3-based PSCs with a maximum power conversion efficiency (PCE) of 12.08% and an average PCE of 11.01% were obtained. The PSCs also exhibit excellent performance reproducibility, good air stability and weak hysteresis behavior. The enhancement in the performance of the PSCs is attributed to the well-crystallized CH3NH3Sn0.25Pb0.75I3 film, resulting in simultaneous improvement in charge–carrier transport properties and reduction in charge–carrier recombination, a very promising approach to obtain high performance Sn-including perovskite solar cells.

Reduced efficiency roll-off in phosphorescent OLEDs with a stack emitting layer facilitating triplet exciton diffusion

A high efficiency and low efficiency roll-off phosphorescent organic light-emitting diode (PHOLED) is demonstrated based on a stacked emissive layer by alternating [CBP : 4 wt% Ir(ppy)3 (5 nm)] and [CBP : 8 wt% Ir(ppy)3 (5 nm)] ultrathin films. The results show that when the number of the stack cell is equal to three, the peak current efficiency of ca. 48.8 cd A−1 and the peak external quantum efficiency (EQE) of ca. 19.6% are obtained and kept until 1000 cd m−2. As the luminance further increases from 1000 to 100 000 cd m−2, there only exists a less than 25.1% drop in current efficiency and 25.5% drop in EQE. The improved properties for the proposed PHOLED are attributed to greatly weakened triplet exciton quenching due to the very effective triplet exciton diffusion by the abovementioned stacked emissive structure.

Fabrication of benzothiadiazole–benzodithiophene-based random copolymers for efficient thick-film polymer solar cells via a solvent vapor annealing approach

We adopted a ternary copolymerization strategy to construct a series of D–A1–D–A2 random polymers (P2FBT-25, P2FBT-50 and P2FBT-75), utilizing difluorinated benzothiadiazole (2FBT) and alkoxy substituted benzothiadiazole (2ORBT) as complementary acceptor units and thienyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) as the donor unit. By changing the molar ratio of the two acceptor units, the optical, electrochemical, and charge transport properties can be tuned. The optimal ratio of 75 : 25 between 2FBT and 2ORBT in the random terpolymers enhanced the efficiency of the P2FBT-75:PC71BM blend via a CHCl3 solution vapor annealing approach, exhibiting an efficiency of 5.72% due to improved light absorption and balanced charge transport. Moreover, an efficiency of over 5.60% can be obtained with an active layer thickness of 100–170 nm, and the PCE of over 4.52% was observed even though the thickness was around 270 nm. Noticeably, the three random copolymers that differ considerably in composition, all afford high PCEs (5.00% for P2FBT-25, 4.40% for P2FBT-50, and 5.72% for P2FBT-75), suggesting that high performance materials can be developed within a reasonable composition range via random copolymerization.

Efficiency Enhancement of Perovskite Solar Cells via Electrospun CuO Nanowires as Buffer Layers

CuO nanowires (NWs) with the diameters ranging from 130 to 275 nm have been successfully prepared by electrospinning technique, followed by a calcination process. Inverted planar heterojunction perovskite solar cells (PSCs) with the structure of indium tin oxide/CuO NWs/poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrenesulphonate) (PSS)/CH3NH3PbI3/phenyl C61-butyric acid methyl ester/Bphen/Ag were designed, achieving a best power conversion efficiency (PCE) of 16.87%, which is 21% improvement compared to that of the control PSCs without CuO NWs. By the characterizations of an optical microscope, X-ray diffraction, and scanning electron microscopy, it was found that CuO NWs have uniform morphology and orderly arrangement. Electrochemical impedance spectrometry and external quantum efficiency were used to reveal the effect of CuO NWs on the performance of PSCs. Compared to ZnO NWs with the same diameters and quantitative analysis based on a simple model, we conclude that the improvement of PCE by about 13% can be ascribed to the increase of the PEDOT:PSS/CH3NH3PbI3 interface area and the remaining increase of 8% can be attributed to the higher hole mobility of the CuO NWs/PEDOT:PSS composite film. The results indicate that the efficiency of PSCs will have a significant enhancement when the optimal CuO NWs are introduced into the charge transport layer.