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Perovskite solar cells (PSCs) have drawn unprecedented attention due to their skyrocketed power conversion, however the reported fill factors (FFs) still lag behind commercialized solar cells, and there lacks comprehensive...

Low-temperature solution-processed TiO 2 nanocrystals (LT-TiO 2 ) have been extensively applied as electron transport layer (ETL) of perovskite solar cells (PSCs). However, the low electron mobility, high density of electronic trap states, and considerable photocatalytic activity of TiO 2 result in undesirable charge recombination at the ETL/perovskite interface and notorious instability of PSCs under ultraviolet (UV) light. Herein, LT-TiO 2 nanocrystals are in situ fluorinated via a simple nonhydrolytic method, affording formation of Ti─F bonds, and consequently increase electron mobility, decrease density of electronic trap states, and inhibit photocatalytic activity. Upon applying fluorinated TiO 2 nanocrystals (F-TiO 2 ) as ETL, regular-structure planar heterojunction PSC (PHJ-PSC) achieves a champion power conversion efficiency (PCE) of 22.68%, which is among the highest PCEs for PHJ-PSCs based on LT-TiO 2 ETLs. Flexible PHJ-PSC devices based on F-TiO 2 ETL exhibit the best PCE of 18.26%, which is the highest value for TiO 2 -based flexible devices. The bonded F atoms on the surface of TiO 2 promote the formation of Pb─F bonds and hydrogen bonds between F − and FA/MA organic cations, reinforcing interface binding of perovskite layer with TiO 2 ETL. This contributes to effective passivation of the surface trap states of perovskite film, resulting in enhancements of device efficiency and stability especially under UV light.

Ionic defects at the surfaces of organolead halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells (PSCs). Herein, sodium p-toluenesulfonate (STS) is applied during the surface modification of perovskite layer for the first time, leading to the efficient surface passivation of the perovskite film and consequently significant enhancements in both efficiency and stability of mixed-cation PSC devices. Upon incorporating STS atop the perovskite layer, the power conversion efficiency of the Cs 0.05 MA 0.12 FA 0.83 PbI 2.55 Br 0.45 (abbreviated as CsMAFA) mesoporous-structure mixed-cation PSC devices improves from 18.70% to 20.05% with reduced hysteresis. The sulfonate (-SO 3 À ) anion of STS coordinates with the Pb 2þ of CsMAFA perovskite, and the Na þ cation of STS electrostatically interacts with the anions (I À /Br À ) of CsMAFA perovskite, resulting in the surface passivation of the CsMAFA perovskite film with reduced electron and hole trap state densities. In addition, STS modification induces an upshift of the valence band of perovskite, facilitating hole extraction from the perovskite layer to the hole transport layer with suppressed interfacial charge recombination. Moreover, such a trap state passivation of perovskite film leads to improvement of the ambient stability of PSC devices.

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2D perovskites possess superior humidity stability but inferior power conversion efficiency (PCE) compared with 3D perovskites due to their typically insulating spacers. Size of the spacer cation is determinative for the formation of 2D perovskite, and fullerene is believed not to be capable of templating 2D perovskite structure because of its larger size than the width of the lead-halide octahedron despite its well-known strong electronaccepting ability. Herein, a novel amino-functionalized fullerene derivative (abbreviated as C 60 -BPAM) is developed and an 'improbable' spacer for 2D/3D hybrid perovskite solar cells (PSCs), achieving enhanced electron transport is applied. Unlike most of the reported alkylammonium spacers that are based on insulating organic tails, the incorporation of a highly conductive fullerene tail within C 60 -BPAM 2+ leads to increased electron density in 2D/3D perovskite and induces an additional built-in electric field, facilitating electron transport in PSCs. Besides, the 2D/3D hybrid structure helps to passivate both of the shallow-and deep-level defects within perovskite. As a result, the PCE of 2D/3D PSCs improves from 19.36% (3D MAPbI 3 PSCs) to 20.21%. Moreover, the 2D/3D PSCs show significant improvement in the humidity stability compared to the 3D counterparts.

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Fullerene derivatives, especially [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), have been widely applied as electron transport layers of inverted planar heterojunction perovskite solar cells (PSCs). However, the solution-processed PCBM capping layer suffers from limited surface wetting which hinders the improvement in efficiency and scalability of PSCs. Herein, we develop a facile hybrid solvent strategy that enables very fast wetting of the PCBM capping layer atop of the perovskite surface, leading to an improved interfacial contact and electron transport. The significantly enhanced wettability of the PCBM solution fulfilled through blending isopropyl alcohol into the commonly used chlorobenzene (CB) is attributed to the reduced surface tension while retaining viscosity. As a result, the electron mobility and electric conductivity of the PCBM capping layer increase by around two times, and the PSC devices exhibit the highest power conversion efficiency (PCE) of 19.92%, which is improved by ∼18% relative to that of the control device (16.78%). Importantly, this strategy is also applicable for other alcohols (ethanol and methanol) and CB blends. Moreover, the fast wetting approach enables us to deposit the PCBM capping layer using a facile drop-casting method, affording comparable PCEs to those obtained by the conventional spincoating method, which is not achievable by using the conventional single solvent. This fast wetting PCBM capping layer also contributes to efficiency improvement of large-area (1 cm 2 ) devices. These advances hold great potential for other scalable deposition methods such as blade-coating and slot-die coating.