Researchers at the Indian Institute of Technology Bombay have fabricated a large-area inverted perovskite solar cell based on a mixed self-assembled monolayer (SAM) that improves perovskite film quality, enhances charge transport, reduces interfacial recombination losses, and increases device efficiency and stability.
SAMs have emerged as key components in high-efficiency perovskite solar cells, particularly in inverted architectures, where they serve as ultrathin hole-selective contacts that improve charge extraction and reduce interfacial recombination losses. Previous research has shown that SAMs can also enhance device stability and mechanical reliability, while optimizing energy-level alignment between the perovskite absorber and the underlying electrode. More recently, scientists have developed co-assembled SAM strategies to improve surface coverage, suppress defects, and promote better perovskite crystallization. Further advances in SAM molecular engineering have enabled efficiencies approaching 27%, highlighting their potential for developing efficient, stable, and scalable perovskite solar cells.
“We have developed a mixed-carbazole-based SAM strategy for wide-bandgap perovskite solar cells, targeting both surface wettability and suppression of non-radiative recombination at the buried interface. By combining different carbazole-based SAMs, we were able to tune the interfacial properties and promote improved perovskite film formation for the scalabilty of device,” corresponding author Dinesh Kabra told pv magazine. “The impact of the mixed-SAM interface on perovskite film formation and charge-carrier recombination was systematically investigated using morphological and optical characterization.”
The scientists explained that scaling perovskite solar cells from laboratory-scale devices of around 0.1 cm² to larger areas remains a major challenge, particularly for inverted (p-i-n) architectures with wide-bandgap (WBG) absorbers. Although significant efficiency improvements have been achieved in small-area devices, increasing their size typically results in substantial performance losses.
With this in mind, they developed a mixed SAM combining [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz) and [4-(7H-dibenzo[c,g]carbazol-7-yl)butyl]phosphonic acid (4PADCB). While Me-4PACz provides favorable energy-level alignment, low interfacial defect density, and efficient hole extraction, 4PADCB improves wettability and promotes more uniform perovskite crystallization and larger grain formation.
The team built the solar cell with a glass and indium tin oxide (ITO) substrate, a hole transport layer (HTL) based on the mixed SAM, a perovskite absorber, an aluminum oxide (AlOx) interlayer, a fullerene (C60) electron transport layer, a tin oxide (SnOx) buffer layer, and a silver (Ag) metal contact.
The researchers said the mixed-SAM approach combines the favorable electrical properties of Me-4PACz with the improved film-forming characteristics of 4PADCB, resulting in better device performance and uniformity.
In cells with an active area of 1.2 cm², the mixed-SAM configuration achieved a power conversion efficiency of 20.30% and a fill factor of 81.23%, compared with efficiencies of 18.17% and 19.30% for devices using Me-4PACz and 4PADCB alone, respectively. Surface and morphological analyses revealed improved wettability, more compact perovskite films, larger columnar grains, and fewer interfacial defects. Photoluminescence mapping and conductive atomic force microscopy also indicated more homogeneous optoelectronic properties and improved local charge transport.
When the active area was increased to 5.2 cm², the mixed-SAM cell achieved a maximum efficiency of 18.88%, compared with 17.20% and 17.96% for devices based on Me-4PACz and 4PADCB alone, respectively. The larger mixed-SAM cells also maintained an average efficiency of 17.81%, compared with 18.96% for their smaller counterparts, with limited device-to-device variation.
Further analysis showed that the efficiency losses associated with scaling were mainly attributable to increased series resistance at other interfaces and in the transparent conductive oxide electrode, rather than changes in intrinsic recombination characteristics.
Under damp-heat testing at 85 C and 65% relative humidity, the cells took more than 680 hours to fall to 75% of their initial efficiency. According to the researchers, the results demonstrate that mixed-SAM interface engineering can improve efficiency, stability, and reproducibility while limiting performance losses in large-area inverted perovskite solar cells.
“Importantly, the present study isolates interface- and area-dependent losses using a consistent spin-coating process; the solution-processable nature of the mixed-SAM strategy provides potential compatibility with scalable deposition techniques, such as blade coating, slot-die coating, and spray coating, which represent important pathways toward further scale-up and module-level fabrication,” the academics concluded.
The device was described in “Mixed Carbazole Self-Assembled Monolayers Enable Scalable Large-Area (>5 cm2) Efficient Wide Bandgap Perovskite Solar Cells,” published in ACS Applied Materials & Interfaces.
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