penghuang

All-polymer solar cells (all-PSCs) utilizing p-type polymers as electron-donors and n -typepolymers as electron-acceptors have attracted a great deal of attention, and their efficiencies have been improved considerably. Here, five polymer donors with different molecular orientations are synthesized by random copolymerization of 5-fluoro-2,1,3-benzothiadiazole with different relative amounts of 2,2′-bithiophene (2T) and dithieno[3,2-b;2′,3′-d]thiophene (DTT). Solar cells are prepared by blending the polymer donors with a naphthalene diimide-based polymer acceptor (PNDI) or a [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) acceptor and their morphologies and crystallinity as well as optoelectronic, charge-transport and photovoltaic properties are studied. Interestingly, charge generation in the solar cells is found to show higher dependence on the crystal orientation of the donor polymer for the PNDI-based all-PSCs than for the conventional PC₇₁BM-based PSCs. As the population of face-on-oriented crystallites of the donor increased in PNDI-based PSC, the short-circuit current density (J_SC) and external quantum efficiency of the devices are found to significantly improve. Consequently, device efficiency was enhanced of all-PSC from 3.11% to 6.01%. The study reveals that producing the same crystal orientation between the polymer donor and acceptor (face-on/face-on) is important in all-PSCs because they provide efficient charge transfer at the donor/acceptor interface.

Five polymer donors showing different molecular orientations are synthesized by carrying out random copolymerization, and their photovoltaic properties are investigated by fabricating all-polymer solar cells using a PNDI polymer acceptor. As compared with PC₇₁BM-based devices, charge generation in the PNDI-based devices is found to be highly dependent on the orientation of the polymer donor.

The Back Cover picture shows a pioneering graphene-based structure for mesoscopic perovskite solar cells that combines the extraordinary conduction properties of graphene with the exceptional light-harvesting behavior of perovskites. The graphene flakes, produced by liquid-phase exfoliation of pristine graphite, are intercalated into the mesoporous TiO₂ photoelectrode, while graphene oxide is used as interlayer between the perovskite active material (brown) and the spiro-OMeTAD hole-transporting layer (green). The proposed architecture yields a power conversion efficiency of up to 18.2 %. Moreover, graphene/perovskite solar cells show higher stability with respect to conventional perovskite solar cells, demonstrating the central role of graphene-modified interfaces to inhibit aging mechanisms in the device. More details can be found in the Full Paper by Agresti et al. on page 2609 in Issue 18, 2016 (DOI: 10.1002/cssc.201600942).

The Cover picture shows the outdoor testing performance of perovskite solar cells in a hot and humid environment (Hong Kong summer). Different encapsulation techniques are examined for perovskite solar cells. The cells encapsulated with a SiO₂ thin film and cover glass packaging incorporating desiccant and UV-curable epoxy exhibit good performance in accelerated lab testing under illumination and at 85 °C, as well as in outdoor testing in Hong Kong. More details can be found in the Full Paper by Dong et al. on page 2597 in Issue 18, 2016 (DOI: 10.1002/cssc.201600868).

Although perovskite solar cells (PSCs) surpassing 20 % in certified power conversion efficiency (PCE) have been demonstrated with organic hole-transporting layers (HTLs), thermal degradation remains one of the key issues for practical applications. We fabricated PSCs using low temperature solution-processed CuSCN as the inorganic hole-transport layer (HTL), which possesses a highly stable crystalline structure and is robust even at high temperatures. The best-performing cell delivers a PCE of 18.0 %, with 15.9 % measured at the stabilized power output. Here we report the thermal stability of PSCs based on CuSCN in comparison with commonly used 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD). The PSC fabricated with organic spiro-OMeTAD degrades to 25 % of initial PCE after annealing for 2 h at 125 °C in air under 40 % average relative humidity. However, CuSCN-based PSCs maintain approximately 60 % of the initial value, exhibiting superior thermal stability under identical conditions. This work demonstrates that high efficiency and improved thermal stability are simultaneously achieved when CuSCN is used as an HTL in PSCs.

Stability from within: The performance and thermal stability of CuSCN- and spiro-OMeTAD-based perovskite solar cells are investigated. Comparable power conversion efficiency values (PCE) are obtained for both devices; however, thermal stability tests at 125 °C in air showed improved stability of the CuSCN analogues. The intrinsic thermal stability of CuSCN and its ability to act as a protective barrier for the perovskite layer could contribute to improved thermal stability of the devices.