penghuang

End groups in small-molecule photovoltaic materials are important owing to their strong influence on molecular stability, solubility, energy levels, and aggregation behaviors. In this work, a series of donor–acceptor pentads (D₂–A–D₁–A–D₂) were designed and synthesized, aiming to investigate the effect of the end groups on the materials properties and photovoltaic device performance. These molecules share identical central A–D₁–A triads (with benzodithiophene as D₁ and 6-carbonyl-thieno[3,4-b]thiophene as A), but with various D₂ end groups composed of alkyl-substituted thiophene (T), thieno[3,2-b]thiophene (TT), and 2,2′-bithiophene (BT). The results indicate a relationship between conjugated segment/alkyl chain length of the end groups and the photovoltaic performance, which contributes to the evolving molecular design principles for high efficiency organic solar cells.

Finding the balance: Three novel donor–acceptor conjugated small molecule pentads (D₂–A–D₁–A–D₂) are synthesized and applied in bulk-heterojunction solar cells along with PC₇₁BM. By varying the conjugation length and alkyl substitution of the D₂ end groups, the performance of the devices is optimized. The subtle demands of designing small molecules for OPV are highlighted, in which both the nature of the π system and the solubilizing side chains are critical.

[70]Fullerene is presented as an efficient alternative electron-selective contact (ESC) for regular-architecture perovskite solar cells (PSCs). A smart and simple, well-described solution processing protocol for the preparation of [70]- and [60]fullerene-based solar cells, namely the fullerene saturation approach (FSA), allowed us to obtain similar power conversion efficiencies for both fullerene materials (i.e., 10.4 and 11.4 % for [70]- and [60]fullerene-based devices, respectively). Importantly, despite the low electron mobility and significant visible-light absorption of [70]fullerene, the presented protocol allows the employment of [70]fullerene as an efficient ESC. The [70]fullerene film thickness and its solubility in the perovskite processing solutions are crucial parameters, which can be controlled by the use of this simple solution processing protocol. The damage to the [70]fullerene film through dissolution during the perovskite deposition is avoided through the saturation of the perovskite processing solution with [70]fullerene. Additionally, this fullerene-saturation strategy improves the performance of the perovskite film significantly and enhances the power conversion efficiency of solar cells based on different ESCs (i.e., [60]fullerene, [70]fullerene, and TiO₂). Therefore, this universal solution processing protocol widens the opportunities for the further development of PSCs.

Glass half full-erene: [70]Fullerene and [60]fullerene are incorporated as the electron-selective contact in normal-architecture perovskite solar cells to study their suitability. Importantly, despite the low electron mobility and significant visible-light absorption of [70]fullerene, the presented protocol allows the use of [70]fullerene as an efficient electron-selective contact.

The efficiency of dye-sensitized solar cells (DSSCs) is generally limited by the mismatch between the absorption spectrum of the photosensitizer and the solar irradiation spectrum. This work describes the use of a mixture that containing proper proportions of SiO₂ coated Au nanospheres (AuNSs@SiO₂) and Au nanorods (AuNRs@SiO₂) (the mixture was denoted as AuNCs@SiO₂) to enhance the sunlight utility in DSSCs. The incorporation of AuNCs@SiO₂ into the TiO₂ photoanode induced broadband light-harvesting at both low- and long- wavelengths and thus enhanced the photocurrent compared to that of plasmonic solar cells based on either AuNSs@SiO₂ or AuNRs@SiO₂. Upon the doping of AuNCs@SiO₂, the overall power conversion efficiency (PCE) increased from 7.39 to 9.12 % for DSSCs based on organic liquid electrolytes.

Gold medal! A mixture that contains appropriate proportions of SiO₂-coated Au nanospheres (AuNSs@SiO₂) and Au nanorods (AuNRs@SiO₂) is applied to enhance the sunlight utility of dye-sensitized solar cells.

The Back Cover image illustrates oxygen permeation through a metal top electrode in organic photovoltaics. Oxygen molecules can easily penetrate into the metal electrode via nanoscale pinholes formed among metal grains, resulting in oxidation-induced degradation of devices. This work reports on a simple and effective strategy for reducing the oxygen permeation. Controlling the morphology of metal electrodes enables the electrode by itself to serve as an efficient oxygen barrier. This self-passivating strategy can greatly improve the environmental stability of devices without complicated additional fabrication steps. More details can be found in the Full Paper by Lee et al. on page 445 in Issue 5, 2016 (DOI: 10.1002/cssc.201501536).

Surface plasmon resonance using noble metal nanoparticles is regarded as an attractive and viable strategy to improve the optical absorption and/or photocurrent in dye-sensitized solar cells (DSSCs). However, no significant improvement in device performance has been observed. The bottleneck is the stability of the noble-metal nanoparticles caused by chemical corrosion. Here, we propose a simple method to synthesize high-performance DSSCs based on polyvinylpyrrolidone-coated Au–TiO₂ microspheres that utilize the merits of TiO₂ microspheres and promote the coupling of surface plasmons with visible light. When 0.4 wt % Au nanoparticles were embedded into the TiO₂ microspheres, the device achieved a power conversion efficiency (PCE) as high as 10.49 %, a 7.9 % increase compared with pure TiO₂ microsphere-based devices. Simulation results theoretically confirmed that the improvement of the PCE is caused by the enhancement of the absorption cross-section of dye molecules and photocurrent.

Vibrant microspheres: Au–TiO₂ microspheres with controllabe sizes and shell thicknesses are synthsized to investigate the effects of localized surface plasmon resonance on the performance of dye-sensitized solar cells (DSSCs). The Au–TiO₂ microspheres are designed to capitalize on the merits of microspheres and increase the absorption cross-section of the dye molecules of DSSCs, which allows us to decrease the thickness of photoanodes.

Device scale-up and long-term stability constitute two major hurdles that the emerging perovskite solar technology will have to overcome before commercialization. Here, a comparative study was performed between ZnO and TiO₂ electron-selective layers, two materials that allow the low-temperature processing of perovskite solar cells on polymer substrates. Although the use of TiO₂ is well established on glass substrates, ZnO was chosen because it can be readily printed at low temperature and offers the potential for the large-scale roll-to-roll manufacturing of flexible photovoltaics at a low cost. However, a rapid degradation of CH₃NH₃PbI₃ was observed if it was deposited on ZnO, therefore, the influence of the perovskite film preparation conditions on its morphology and degradation kinetics was investigated. This study showed that CH₃NH₃PbI₃ could withstand a higher temperature on TiO₂ than ZnO and that TiO₂-based perovskite devices were more stable than their ZnO analogues.

So solar: A comparison between CH₃NH₃PbI₃/ZnO- and CH₃NH₃PbI₃/TiO₂-based perovskite solar cells on polymer substrates demonstrates a higher thermal device stability if constructed on TiO₂. Exacerbated degradation of CH₃NH₃PbI₃ leads us to question the viability of ZnO as an effective electron-selective layer in low-temperature processed perovskite solar cells.