Improved charge extraction in inverted perovskite solar cells with dual-site-binding ligands

A solar cell certified at twenty-six point fifteen percent efficiency is a record for inverted perovskite devices. What got it there wasn't a new material or a redesigned architecture, but rather the orientation of a single molecule lying flat against a surface instead of standing up. That shift, measured in ångströms, is the focus of this paper. Hao Chen and colleagues at the University of Toronto and collaborating institutions set out to solve a tension that has defined perovskite solar cell research for years. There are two main architectural designs of these cells. The conventional "nip" design stacks layers in a particular order and has long produced the highest efficiencies. The inverted "pin" design reverses that order, and it turns out to be significantly more stable under real operating conditions. The problem is that pin cells have always trailed nip cells in efficiency, and the losses happen at a specific location: the interfaces between the perovskite absorber and the charge transport layers on either side of it. At those interfaces, undercoordinated lead ions, which are lead atoms with unfilled bonding sites, act as traps. Charges generated by sunlight fall into these traps instead of flowing to the electrodes, and that recombination negatively impacts both voltage and the fill factor. Fill factor is essentially how square the current-voltage curve appears; a perfect cell has a fill factor of one, while real cells fall short whenever charge extraction is inefficient. Fix the traps, and you recover those losses. That's the premise of surface passivation, and it's been the standard approach for years. However, passivation carries a catch that Chen and colleagues identified as the root of the problem. The typical passivating ligands bind to a single lead ion, neutralizing it chemically. They work, but the issue is that these single-site ligands pack densely and orient themselves perpendicular to the perovskite surface, like a picket fence of molecules standing between the perovskite and the electron transport layer. That fence is electrically resistive. The very molecules that suppress recombination also impede the extraction of the charges they just freed. Fix one problem, and you introduce another. This is the conflict that prior work hadn't resolved. The team's insight was to reframe passivation as an orientation problem. If a ligand lies flat against the surface rather than standing upright, it removes the resistive barrier. If it can reach two neighboring lead sites simultaneously, it passivates more defects per molecule while doing so. Chen and colleagues used density functional theory, which involves quantum mechanical calculations that predict how molecules prefer to sit on a surface, to search for this kind of dual-site, planar ligand. They screened three benzenesulfonate candidates. The critical variable was molecular length relative to the spacing between neighboring lead surface sites in the perovskite lattice, which is about six point three ångströms. Plain benzenesulfonate was too short and preferred to stand up. A para-methyl version, measuring six point fourteen ångströms, was close but still favored the perpendicular configuration. The para-chloro version, sodium four-chlorobenzenesulfonate or four Cl-BZS, came in at six point thirty-seven ångströms, just slightly longer than the lead-to-lead spacing. The density functional theory calculations showed that the parallel, flat-lying configuration was energetically preferred. The chlorine substituent provides an additional surface interaction that tips the balance. Geometry and chemistry aligned perfectly. The electrostatic potential maps made this clear: regions of charge accumulation in the four Cl-BZS treated interface matched the positions of two neighboring lead ions, confirming dual-site coordination. When the team calculated how strongly the electron transport layer material, the fullerene C60, adsorbs onto perovskite surfaces treated with each ligand, the numbers were telling. The C60 adsorption energy on four Cl-BZS treated perovskite was minus zero point eighty-five electron volts, compared with minus zero point forty-six for plain BZS and minus zero point thirty-nine for the methyl version. This indicates a much stronger interaction, meaning better electronic coupling across the interface where charge needs to flow. These predictions held true under measurement. X-ray photoelectron spectroscopy confirmed that lead four F peaks shifted to lower binding energy in treated films, consistent with passivation. Time-of-flight secondary ion mass spectrometry showed the ligands concentrated at the surface rather than entering the bulk lattice, exactly where you want them. The passivation was effective and remained stable. The photoluminescence data quantified what that meant for the charges themselves. Photoluminescence quantum yield, which is the fraction of absorbed photons re-emitted as light instead of lost to traps, rose from twenty percent in control films to forty-one percent with the four Cl-BZS treatment. This twofold increase corresponds to a projected twenty millivolt gain in quasi-Fermi level splitting, a direct measure of how much voltage the cell can produce. In full device stacks, quasi-Fermi level splitting rose from one point ten to one point seventeen volts. Time-resolved photoluminescence showed carrier lifetimes increasing from zero point six microseconds in control films to three point zero microseconds with four Cl-BZS, meaning charges survived five times longer before recombining and far more reached the electrodes. That's the mechanism. Now, let's look at the device numbers. Chen and colleagues sent cells for independent certification, which is important because certified quasi-steady-state efficiency is harder to alter than a peak scan. Quasi-steady-state measurement holds the cell at maximum power and waits for output to stabilize, reflecting what the device truly delivers under sustained operation. The small-area device, which measures zero point zero five square centimeters, achieved twenty-six point fifteen percent efficiency, with an open-circuit voltage of one point seventeen volts, a short-circuit current density of twenty-six point one milliamps per square centimeter, and a fill factor of eighty-five point two percent. The larger device, measuring one point zero four square centimeters, certified at twenty-four point seventy-four percent, with a fill factor of eighty point one percent. Both numbers represent the highest certified efficiencies reported for inverted perovskite cells at the time of publication, placing pin performance squarely in the range that had previously belonged only to nip devices. The stability result is equally important because efficiency without durability doesn't matter for deployment. Encapsulated, four Cl-BZS treated devices were held at maximum power point under continuous one sun illumination with a heatsink temperature of sixty-five degrees Celsius, which are genuinely harsh conditions. After one thousand two hundred hours, the devices retained ninety-five percent of their initial efficiency. That's fifty days of nonstop operation under full sunlight and elevated heat. Their initial efficiency under those aging conditions was twenty-three point two percent, and it barely changed during the test period. The team also tested whether the same ligand strategy translates to the bandgap compositions needed for all-perovskite tandem cells. In these designs, two perovskite layers with different bandgaps are stacked to capture more of the solar spectrum. A one point seventy-eight electron volt wide-bandgap mixed-halide composition and a one point twenty-five electron volt narrow-bandgap mixed lead-tin composition both showed efficiency increases with four Cl-BZS treatment. This approach generalizes across chemistries, not just the single composition that set the record. The throughline of this work is a design principle rather than just a one-off recipe. Passivating ligands have been central to perovskite solar cell improvement for years, but the field had largely optimized what they bind to instead of how they sit. Chen and colleagues showed that orientation is as consequential as chemistry. A molecule lying flat at the perovskite surface can simultaneously reduce defects and improve charge extraction, while the same molecule standing upright would solve one problem while creating another. The dual-site binding isn't an incidental feature; it's precisely what allows the ligand to adopt the planar geometry without sacrificing passivation coverage. The efficiency gap between inverted and conventional perovskite cells had persisted for years, and the explanation was always "interface losses," a broad category that obscured the specific mechanism. This work narrows it down to a single structural variable: the orientation of the passivant. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.