Feilong Cai

Eliminated hysteresis and stabilized power output over 20% in planar heterojunction perovskite solar cells by compositional and surface modifications to the low-temperature-processed TiO2 layer

Fabrication of efficient, hysteresis-free perovskite solar cells with stabilized power output in a planar heterojunction configuration via a low-temperature solution process remains a big challenge. Herein, we report an effective approach to prepare high performance n–i–p type planar heterojunction perovskite solar cells with eliminated hysteresis and stabilized power output over 20% via compositional and surface modifications to a low-temperature-processed TiO2 electron-transport layer (ETL). The fullerene derivative layer between the ETL and perovskite layers significantly improves electron extraction and suppresses charge recombination by reducing trap density at the ETL interface. This work manifests the presence of diffused TiO2/fullerene and fullerene/perovskite interfaces as an efficient interfacial engineering strategy to prepare high-performing perovskite solar cells.

Light‐Soaking‐Free Inverted Polymer Solar Cells with an Efficiency of 10.5% by Compositional and Surface Modifications to a Low‐Temperature‐Processed TiO2 Electron‐Transport Layer

Compositional modification and surface treatments of a TiO2 film prepared by a low-temperature route are carried out by a new promising method. Inverted polymer solar cells incorporating the post-treated TiO2 :TOPD electron-transport layer achieve the highest efficiency of 10.5%, and more importantly, eliminate the light-soaking problem that is commonly observed in metal-oxide-based inverted polymer solar cells.

Sodium bromide additive improved film morphology and performance in perovskite light-emitting diodes

Organometal halide perovskite is a promising material to fabricate light-emitting diodes (LEDs) via solution processing due to its exceptional optoelectronic properties. However, incomplete precursor conversion and various defect states in the perovskite light-emitting layer lead to low luminance and external quantum efficiency of perovskite LEDs. We show here the addition of an optimum amount of sodium bromide in the methylammonium lead bromide (MAPbBr3) precursor during a one-step perovskite solution casting process can effectively improve the film coverage, enhance the crystallinity, and passivate ionic defects on the surface of MAPbBr3 crystal grains, resulting in LEDs with a reduced turn-on voltage from 2.8 to 2.3 V and an enhanced maximum luminance from 1059 to 6942 Cd/m2 when comparing with the pristine perovskite-based device.

Restrained light-soaking and reduced hysteresis in perovskite solar cells employing a helical perylene diimide interfacial layer

An n-type helical molecule perylene diimide (PDI2) has been explored as an efficient interfacial layer between TiO2 and perovskite for the preparation of perovskite solar cells. The extended π-conjugation of PDI2 ensures a high electron conductivity for efficient charge transport, and the oxygen atoms of the carbonyl groups can chelate with uncoordinated Pb2+ to passivate the surface defects of perovskite crystals. It thereby suppresses interfacial recombination, enhances efficiency, and reduces hysteresis and the light-soaking instability. The power conversion efficiency (PCE) of our perovskite solar cells showed negligible dependence on the thickness of the PDI2 interlayer, and the champion device achieved a high PCE of 19.84% and the hysteresis value (ΔPCE) was reduced to 2.34% compared to 6.46% in the perovskite device without the presence of the PDI2 interlayer.

The impacts of PbI2 purity on the morphology and device performance of one-step spray-coated planar heterojunction perovskite solar cells

We have fabricated planar heterojunction perovskite solar cells by one-step ultrasonic spray-coating of the perovskite precursor solution in air. Uniform perovskite films with high surface coverage can be prepared after optimization of the precursor solution and spray-coating parameters. We found that the purity of PbI2, although varying only from 98 to 99.9%, can significantly affect the crystallinity, grain size and boundaries of MAPbI3 films that were fabricated via one-step spray-coating, and ultimately determined the power conversion efficiency (PCE) of perovskite devices. PbI2 with a purity of 98% resulted in a low conversion of precursors to perovskites, whilst a high purity of 99.9% led to perovskites with a high crystallinity, large grain size and narrow grain boundaries. Our p–i–n type, planar heterojunction solar cell ITO/PEDOT:PSS/MAPbI3/PCBM/Ag made from MAI and 99.9% purity PbI2 achieved a maximum PCE of 12.6% without hysteresis, whereas the 98% purity PbI2-based device showed a low PCE of only 4.9% with the presence of hysteresis. However, the impacts of PbI2 purity on the device efficiency can be minimized by changing the deposition method from one-step spray-coating to a two-step spin casting approach.

Halogen-substituted Fullerene Derivatives for Interface Engineering of Perovskite Solar Cells

The interface between perovskites and charge transport layers is crucial for the power conversion efficiency and stability of perovskite solar cells (PSCs). We report for the first time the exploration of three novel fullerene derivatives, N-ethyl-2-arylvinyl-5-methyl fulleropyrrolidine (NAMF) with H, Cl or Br substitutions on the pyrrolidine side-chain (namely NAMF-H, NAMF-Cl and NAMF-Br), via a low cost and one-step reaction from C60, as the interlayer to enhance the efficiency and eliminate hysteresis in planar heterojunction PSCs. The crystallinity of MAPbI3 perovskite cast on these interlayer materials was found to be similar, but the grain size increased and the grain boundaries reduced. All three fullerene derivatives have well-aligned LUMO levels that facilitate electron transport from the perovskite to TiO2, and reduce trap density and charge recombination in PSCs, leading to enhanced device efficiency and reduced hysteresis. Notably, the NAMF-Cl interlayer was found to chlorinate the TiO2 electron transport layer, leading to an enhanced PCE of 19.2% in the reverse scan and 17.7% in the forward scan, which were greater than those of PCBM-based PSC devices. Our work adds a new direction to the design of new fullerene derivatives with favorable physical and electrical properties as interlayer materials to improve the performance of PSCs.

Eliminating Light-Soaking Instability in Planar Heterojunction Perovskite Solar Cells by Interfacial Modifications

Stability remains as a challenge of perovskite solar cells although encouraging progress has been made toward their maximum achievable power conversion efficiency in recent years. Light-soaking issue, where the device performance improves upon continuous light illumination and reduces upon storage in dark, is widely observed and marked as the early-stage instability during device operation. In this work, we have employed conjugated polymer PCDTBT as the dopant-free hole-transport layer to fabricate devices without hysteresis but with reversible light-soaking instability. The introduction of n-type molecules, either organic molecule PDI2 or fullerene derivative PC61BM, as the interfacial layer between TiO2 and perovskite layers can effectively reduce or eliminate this instability owing to the efficient charge transport and defect passivation at the electron-transport layer interface, accompanied with an efficiency of 15.7 and 17.7%, respectively. We conclude that the light-soaking instability of these perovskite solar cells is mainly originated from the charge accumulation at the TiO2/perovskite interface and can be eliminated once the interfacial charge can be suppressed by interfacial modifications to improve charge transport at the interface.