The performance of perovskite solar cells with inverted polarity (
p-i-n
) is still limited by recombination at their electron extraction interface, which also lowers the power conversion efficiency (PCE) of
p-i-n
perovskite-silicon tandem solar cells. A ~1 nm thick MgF
x
interlayer at the perovskite/C
60
interface through thermal evaporation favorably adjusts the surface energy of the perovskite layer, facilitating efficient electron extraction, and displaces C
60
from the perovskite surface to mitigate nonradiative recombination. These effects enable a champion
V
oc
of 1.92 volts, an improved fill factor of 80.7%, and an independently certified stabilized PCE of 29.3% for a ~1 cm
2
monolithic perovskite-silicon tandem solar cell. The tandem retained ~95% of its initial performance following damp-heat testing (85 Celsius at 85% relative humidity) for > 1000 hours.
Organic n‐type materials (e.g., fullerene derivatives, naphthalene diimides (NDIs), perylene diimides (PDIs), azaacene‐based molecules, and n‐type conjugated polymers) are demonstrated as promising electron transport layers (ETLs) in inverted perovskite solar cells (p–i–n PSCs), because these materials have several advantages such as easy synthesis and purification, tunable frontier molecular orbitals, decent electron mobility, low cost, good solubility in different organic solvents, and reasonable chemical/thermal stability. Considering these positive factors, approaches toward achieving effective p–i–n PSCs with these organic materials as ETLs are highlighted in this Review. Moreover, organic structures, electron transport properties, working function of electrodes caused by ETLs, and key relevant parameters (PCE and stability) of p–i–n PSCs are presented. Hopefully, this Review will provide fundamental guidance for future development of new organic n‐type materials as ETLs for more efficient p–i–n PSCs.
It is highly desirable to employ n‐type polymers as electron transporting layers (ETLs) in inverted perovskite solar cells (PSCs) due to their good electron mobility, high hydrophobicity, and simplicity of film forming. In this research, the capability of three n‐type donor–acceptor1–donor–acceptor2 (D–A1–D–A2) conjugated polymers (pBTT, pBTTz, and pSNT) is first explored as ETLs because these polymers possess electron mobilities as high as 0.92, 0.46, and 4.87 cm2 (Vs)−1 in n‐channel organic transistors, respectively. The main structural difference among pBTT, pBTTz, and pSNT is the position of sp2‐nitrogen atoms (sp2‐N) in the polymer main chains. Therefore, the effect of different substitution positions on the PSC performances is comprehensively studied. The as‐fabricated p–i–n PSCs with pBTT, pBTTz, and pSNT as ETLs show the maximum photoconversion efficiencies of 12.8%, 14.4%, and 12.0%, respectively. To be highlighted, pBTTz‐based device can maintain 80% of its stability after ten days due to its good hydrophobicity, which is further confirmed by a contact angle technique. More importantly, the pBTTz‐based device shows a neglected hysteresis. This study reveals that the n‐type polymers can be promising candidates as ETLs to approach solution‐processed highly‐efficient inverted PSCs.
The unexpected synthesis and characterization of imidazole-fused azaacenes are presented. Their optical and electrochemical properties have been investigated and compared with these of previously reported imidazole-fused azaacenes. Application of these two imidazole-fused azaacenes in memory devices showed distinctly different resistive behaviors.
Because of its less toxicity and direct band gap, selenium (Se) has been considered as a promising single-element absorber in photovoltaics. In this work, a vacuum-evaporated selenium film has been applied in the inverted p-i-n device structure. After the gradual annealing process, the crystalline Se film could work well as an active layer on the top of a tellurium/ poly(3,4-ethylenedioxythiophene) polystyrene sulfonate /ITO substrate, and phenyl-C61-butyric acid methyl ester was used as the electron transport layer to fulfill the configuration of solar cells. In our research, we found that the deposition rate had great influence on the orientation and grain size of crystalline Se in films as well as on the surface roughness of the annealed Se films. By controlling the thermal evaporation rate of Se, the grain orientation could be uniformly located along the (100) plane with larger grain sizes, which efficiently improved the open current voltage and power conversion efficiency of devices. The device based on the Se layer with the deposition rate of 0.3 nm/s possessed the highest efficiency (3.9%). Moreover, the issues that may impede the application of Se in the inverted solar cell structure and their possible solutions have been discussed.
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