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molecular bulk heterojunction (BHJ), or be arranged in alternating thin layers to create a planar (or linear) heterojunction. The most effi cient OSCs also contain electron/hole blocking and transport layers to facilitate Ohmic extraction at the relevant electrodes. Laboratory-scale power conversion effi ciencies (PCEs) in OSCs now exceed 10% in both solution processed and evaporated junctions, with predictions of >13% within the next 2 years. [3][4][5] However, these PCEs have yet to be translated to the module-scale, with best efforts being serial or parallel connected narrow-strip "minimodules" from IMEC at ≈5-6% (16 cm 2 ), Heliatek 7.7% (140 cm 2 ) and Toshiba 9% (25 cm 2 ). [6][7][8] The narrow-strip architecture is a consequence of the relatively high sheet resistances ( R sh = 10-15 Ω/square) of currently available transparent conducting electrodes: predominantly indium tin oxide (ITO) or fl uorine doped tin oxide (SnO:F). Jin et al. recently showed that the TCE sheet resistance is a dominant scaling parameter controlling cell fi ll factor (FF) for carrier collection path lengths greater than ≈1.0-1.5 cm ( R sh ≈ 10 Ω/square). [ 9 ] Devices with collection path lengths >1 cm in any dimension suffer dramatic loss in FF and short circuit current density which have fi rst order effects upon cell effi ciency. This is clearly a major issue limiting the creation of high effi ciency OSC modules with large active areas (so called monolithic architectures). Connected narrow-strip geometries impose manufacturing complexity and substantial additional cost, plus lead to loss of active area (versus substrate usage) due to the need for extensive interconnection.In this regard, attempts to improve TCE performance have focused on three key strategies: i) The use of very thin, semitransparent metal layers sandwiched between transparent extraction and refractive index matching layers. [10][11][12][13][14][15] The idea behind these insulator/metal/insulator (IMI) stack electrodes originates from low emissivity coatings for windows. [ 16,17 ] By changing the thickness of the individual layers the electrical and optical properties of the stack can be adjusted, allowing for tuning of the sheet resistance and the optical transmission of the electrode. Unfortunately, it is not yet possible to achieve a high conductivity (sheet resistance <5 Ω/square) and a broadband optical transmission from 400-1000 nm for effi cient The high power conversion effi ciencies (PCEs) of laboratory-scale polymerbased organic solar cells are yet to translate to large area modules because of a number of factors including the relatively large sheet resistance of available transparent conducting electrodes (TCEs), and the high defect densities associated with thin organic semiconductor junctions. The TCE problem limits device architectures to narrow connected strips (<1 cm) causing serious fabrication diffi culties and extra costs. Thin junctions are required because of poor charge transport (imbalanced mobilities) in the constituent organic semiconduct...

Efficient and stable perovskite solar cells with a simple active layer are desirable for manufacturing. Three-dimensional perovskite solar cells are most efficient but need to have improved environmental stability. Inclusion of larger ammonium salts has led to a trade-off between improved stability and efficiency, which is attributed to the perovskite films containing a two-dimensional component. Here, we show that addition of 0.3 mole percent of a fluorinated lead salt into the three-dimensional methylammonium lead iodide perovskite enables low temperature fabrication of simple inverted solar cells with a maximum power conversion efficiency of 21.1%. The perovskite layer has no detectable two-dimensional component at salt concentrations of up to 5 mole percent. The high concentration of fluorinated material found at the film-air interface provides greater hydrophobicity, increased size and orientation of the surface perovskite crystals, and unencapsulated devices with increased stability to high humidity.

A series of tetrathiophene‐based fully non‐fused ring acceptors (4T‐1, 4T‐2, 4T‐3, and 4T‐4), which can be paired with the star donor polymer PBDB‐T to fabricate highly efficient organic solar cells are developed. Tailoring the size of lateral chains can tune the solubility and packing mode of acceptor molecules in neat and blend films. It is found that the incorporation of 2‐ethylhexyl chains can effectively change the compatibility with the donor polymer PBDB‐T, and an encouraging power conversion efficiency of 10.15% is accomplished by 4T‐3‐based organic solar cells. It also presents good compatibility with the other polymer donor and an even higher power conversion efficiency (PCE) of 12.04% is achieved based on D18:4T‐3 blend, which is the champion PCE for the fully non‐fused acceptors. Importantly, these inexpensive tetrathiophene fully non‐fused ring acceptors provide cost‐effective photovoltaic performance. The results demonstrate a high photovoltaic performance from synthetically inexpensive materials could be achieved by the rational design of non‐fused ring acceptor molecules.

Engineering the interface between the perovskite absorber and the charge-transporting layers has become an important method for improving the charge extraction and open-circuit voltage ( V) of hybrid perovskite solar cells. Conjugated polymers are particularly suited to form the hole-transporting layer, but their hydrophobicity renders it difficult to solution-process the perovskite absorber on top. Herein, oxygen plasma treatment is introduced as a simple means to change the surface energy and work function of hydrophobic polymer interlayers for use as p-contacts in perovskite solar cells. We find that upon oxygen plasma treatment, the hydrophobic surfaces of different prototypical p-type polymers became sufficiently hydrophilic to enable subsequent perovskite junction processing. In addition, the oxygen plasma treatment also increased the ionization potential of the polymer such that it became closer to the valance band energy of the perovskite. It was also found that the oxygen plasma treatment could increase the electrical conductivity of the p-type polymers, facilitating more efficient charge extraction. On the basis of this concept, inverted MAPbI perovskite devices with different oxygen plasma-treated polymers such as P3HT, P3OT, polyTPD, or PTAA were fabricated with power conversion efficiencies of up to 19%.

The development of organic solar cells (OSCs) with thick active layers is of crucial importance for the roll-to-roll printing of large-area solar panels. Unfortunately, increasing the active layer thickness usually results in a significant reduction in efficiency. Herein, we fabricated efficient thick-film OSCs with an active layer consisting of one polymer donor and two non-fullerene acceptors. The two acceptors were found to possess enlarged exciton diffusion length in the mixed phase, which is beneficial to exciton generation and dissociation. Additionally, layer by layer approach was employed to optimize the vertical phase separation. Benefiting from the synergetic effects of enlarged exciton diffusion length and graded vertical phase separation, an efficiency of 17.31% (certified value of 16.9%) is obtained for the 300 nm-thick OSC, with a short-circuit current density of 28.36 mA cm−2, and a high fill factor of 73.0%. Moreover, the device with an active layer thickness of 500 nm also shows an efficiency of 15.21%. This work provides valuable insights into the fabrication of OSCs with thick active layers.

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The power conversion efficiencies (PCEs) of small molecule acceptor (SMA)‐based organic solar cells have already exceeded 18%. However, the development of polymer acceptors still lags far behind their SMA counterparts mainly due to the lack of efficient polymer acceptors. Herein, a series of polymer acceptors named PY‐X (with X being the branched alkyl chain) are designed and synthesized by employing the same central core with the SMA L8‐BO but with different branched alkyl chains on the pyrrole motif. It is found that the molecular packing of SMA‐HD featuring 2‐hexyldecyl side chain used in the synthesis of PY‐HD is similar to L8‐BO, in which the branched alkyl chains lead to condensed and high‐order molecular assembly in SMA‐HD molecules. When combined with PM6, PY‐HD‐based all polymer solar cell (all‐PSC) exhibits a high PCE of 16.41%, representing the highest efficiency for the binary all‐PSCs. Moreover, the side‐chain modification on the pyrrole site position further improves the performance of the all‐PSCs, and the PY‐DT‐based device delivers a new record high efficiency of 16.76% (certified as 16.3%). The work provides new insights for understanding the structure–property relationship of polymer acceptors and paves a feasible avenue to develop efficient conjugated polymer acceptors.