Jin Young Kim

Fused aromatic self‐assembled monolayers (SAMs) have emerged as effective p‐type contacts for organic solar cells, but the role of alkyl linker length in such bulky systems remains unclear. Here, two phosphonic acid SAMs bearing an identical fused phenanthro[1,10,9,8‐cdefg]carbazole (PCz) terminal group, PCz‐C2PA and PCz‐C4PA, were synthesized and implemented as hole‐selective layers in PM6:PY‐DT bulk‐heterojunction devices with an ITO/SAM/PM6:PY‐DT/PDINN/Ag architecture. The two SAMs exhibit nearly identical frontier energy levels and similar work‐function shifts on ITO, yet lead to distinct device characteristics: the PCz‐C4PA‐based cells achieve a power conversion efficiency of 11.40% compared to 9.14% for PCz‐C2PA, with comparable short‐circuit current densities but a substantially higher open‐circuit voltage and fill factor. Analysis of voltage losses and light intensity dependence indicates that the longer C4 linker reduces both nonradiative and trap‐assisted recombination, while transient measurements and morphology studies point to faster charge extraction, longer carrier lifetimes, and a smoother active‐layer interface for PCz‐C4PA. These findings demonstrate that, even for SAMs with the same bulky fused‐aromatic terminal group, the alkyl linker length is a critical handle for tuning interfacial recombination and charge transport, offering a straightforward molecular design guideline for p‐type SAMs in high‐performance organic photovoltaics.

Self‐assembled monolayers (SAMs) constitute foundational interfacial elements in high‐efficiency inverted perovskite solar cells (PSCs); however, ensuring chemical and electronic integrity across SAM‐modified interfaces remains challenging. Although post‐deposition solvent washing has been widely applied to remove loosely bound or aggregated SAM species, its topography‐dependent consequences for SAM‐modified textured FTO, including retention and solvent‐dependent removal, have not been systematically examined. Here, we identify texture‐induced retention of π–π‐stacked SAM aggregates within nanoscale valleys of FTO, rendering them resistant to conventional alcohol‐based washing. To address this limitation, we establish a substrate‐informed solvent engineering strategy that balances removal of SAM aggregates with preservation of chemisorbed SAMs. Complementary crystallographic and theoretical analyses reveal that N, N ‐dimethylformamide (DMF) provides the most balanced solvent–SAM interaction window among examined solvents, enabling removal of valley‐retained residues while maintaining the chemisorbed SAM interface. The resulting interfacial refinement establishes a homogeneous electrostatic landscape and improved energy alignment at the perovskite/SAM interface, while serving as a uniform growth template for high‐quality, strain‐relieved perovskite films. Consequently, DMF‐washed devices achieve a power conversion efficiency of 26.13% (certified 25.82%) and demonstrate enhanced stability under illumination and thermal stress. These findings highlight substrate‐informed solvent washing as a reliable strategy for SAM‐based interfacial engineering on textured electrodes.

A mixed Al ₂ O ₃ /Meo-4PACz hole-selective layer enables 22.85% perovskite/organic tandem photovoltaics with a 2.13 V photovoltage. The PV-EC system achieves bias-free nitrate-to-ammonia conversion with 3.7% solar-to-ammonia efficiency.

A series of donor–acceptor π-conjugated polymers based on Difluorophenazine acceptor and benzodithiophene donor units with different substituents, hydrogen (H), chlorine (Cl), fluorine (F), and sulfur (S), was designed and synthesized to investigate how donor-side substituent modification influences optoelectronic properties. The substituent variation systematically modulated the optical band gaps (1.72–1.82 eV) and HOMO energy levels (−5.42 to −5.58 eV). When applied in bulk heterojunction solar cells with the Y6 acceptor, these polymers delivered power conversion efficiencies ranging from 3.85% to 7.79%. The fluorinated polymer P(BDTTF-TffPzT) exhibited the best performance, with J_sc = 19.65 mA cm⁻², V_oc = 0.825 V, and FF = 0.481. These findings establish that donor-side substituent engineering is an effective molecular design strategy for tuning optoelectronic properties and device performance in phenazine-based polymer systems.

Efficient exciton dissociation and the suppression of its recombination are key to improving the performance of organic solar cells (OSCs). While bulk heterojunctions (BHJ) promote charge separation through extensive donor−acceptor (D/A) intermixing, layer-by-layer (LbL) processing offers an alternative to control D/A morphology. Although LbL active layers are frequently described as pseudo-bilayer, pseudo-BHJ, or quasi-planar heterojunction (PHJ) structures, the recombination consequences of the effective D/A interfacial area have rarely been benchmarked quantitatively against both BHJ and a low-intermixing PHJ-like reference. Using a benchmark PM6/L8-BO system, we establish BHJ/LbL/PHJ model devices and deduce recombination characteristics of LbL-processed active layers against BHJ and low-intermixing PHJ-like references. Non-radiative recombination loss analysis shows that PHJ achieves substantially lower loss (0.187 eV) than BHJ (0.248 eV) and LbL (0.249 eV), consistent with a reduced D/A interfacial area. Device-architecture-dependent J−V measurements are consistent with substantial acceptor penetration into the donor in LbL stacks, suggesting BHJ-like interfacial contact. Transient-absorption spectroscopy discloses the pronounced formation of the lowest-energy triplet state in BHJ and LbL films, strongly suppressed in the low-intermixing PHJ-like reference. This trend is consistent with an early-stage T1-mediated recombination pathway in the BHJ and LbL films, likely involving non-geminate recombination at D/A interfaces. The results show that LbL devices behave distinctly from PHJ yet BHJ-like, contrary to the general assumption that the LbL architecture represents intermediate recombination behavior. Our findings suggest that simply minimizing the D/A interfacial area is not universally practical for LbL OSCs, as achieving PHJ-like voltage-loss characteristics would require an extremely reduced interface, penalizing exciton harvesting when the exciton diffusion length is limited.

Photoelectrochemical (PEC) water splitting offers a direct route to solar hydrogen, but organic semiconductor-based photocathodes have been limited by insufficient photovoltage and interfacial instability. Here, we demonstrate monolithic all-organic semiconductor tandem photocathodes that address these coupled photovoltage and interfacial-stability constraints. A MoOx/2PACz hybrid interconnecting layer (ICL) replaces conventional PEDOT:PSS-based interconnects, suppressing interfacial recombination and enabling stable sequential deposition of front and back subcells. The resulting tandem photocathodes reach an onset potential of 1.67 V vs. RHE and a 7.9% solar-to-hydrogen (STH) efficiency under unbiased two-electrode operation, which, to our knowledge, is the highest reported for organic semiconductor-based photocathodes operating without external bias, while retaining 86% of their photocurrent over 10 h under three-electrode operation. Coupled with an earth-abundant NiFeCo OER catalyst and an ultralow-Pt CoSMoS-Pt HER catalyst, the system operates in a single alkaline electrolyte without an ion-exchange membrane. Large-area modules up to 320 cm2 demonstrate outdoor bias-free hydrogen production. Techno-economic analysis yields a levelized hydrogen cost of $8.74 kg-1 under current conditions, with sensitivity analysis projecting $4.17 kg-1 under optimized STH efficiency and production scale.

Perovskite/organic tandem architectures provide a versatile platform for achieving high photovoltage and efficient solar-energy conversion, yet their performance and operational stability are critically constrained by electronic losses and chemical instability at buried hole-selective interfaces, particularly in wide-bandgap perovskite top cells. Here, we present interface engineering strategies that address these limitations by simultaneously controlling interfacial energetics, chemical robustness, and strain evolution in tandem systems spanning photovoltaics and photoelectrochemistry. A multi-functional hole-selective layer based on blended phosphonic acid self-assembled molecules is shown to homogenize surface potential, improve energy level alignment, and suppress lattice strain and light-induced phase segregation in wide bandgap perovskites, enabling perovskite/organic tandem solar cells with open-circuit voltage (Voc) exceeding 2.2 V, fill factors above 84%, and efficiencies approaching 25%. In addition, a chemically stabilized self-assembled monolayer formed via potassium carbonate mediated deprotonation of carbazole-based phosphonic acids yields solvent-resistant and strongly anchored hole-selective interfaces that minimize non-radiative recombination and preserve interfacial integrity during perovskite processing. This robust interfacial chemistry further enables the first transparent, metal-free perovskite/organic tandem photocathode capable of stable operation in aqueous electrolyte, delivering photovoltages above 2.1 V with external quantum efficiency validated photocurrent generation. Together, these results establish chemically engineered hole selective interfaces as a unifying and scalable design principle for high Voc tandem photovoltaics and integrated solar-to-chemical energy conversion systems.

Organic solar cells (OSCs) are emerging as a promising renewable energy technology due to their intrinsic advantages, including mechanical flexibility, lightweight, cost‐effective processing, and easy bandgap tuning. Among advanced device architectures, tandem OSCs have been extensively investigated to improve solar energy harvesting through the development of new organic semiconductors and optimized device designs. In this work, we studied solution‐processed hetero‐tandem OSCs featuring a ternary blend (PBDTTPD‐HT: PC ₇₁ BM: IDIC) as the bottom sub‐cell and a binary blend (PTB7‐Th: o ‐IO1) as the top sub‐cell. Through morphological control of the ternary active layer and optical optimization, a power conversion efficiency (PCE) of 15.53% was achieved for tandem OSCs employing a 5:5 weight ratio of acceptors in the ternary bottom sub‐cell. Furthermore, we demonstrated a tandem OSCs mini‐module consisting of four serially connected unit tandem OSCs, which showed a PCE of 9.16% with an output voltage of 6.11 V. Finally, as a proof‐of‐concept application, the tandem OSCs were integrated into a solar‐to‐ammonia (PV‐EC) system for electricity‐driven electrolysis, achieving a solar‐to‐ammonia (STA) conversion efficiency of 4.65%.

Perovskite solar cells (PSCs) are highly attractive for space applications due to their high power-to-weight ratio, yet their reliability under extreme thermal environments remains unclear. Here we investigate interfacial stability in devices employing polymeric PTAA, self-assembled monolayer MeO-2PACz, and a sequential PTAA/MeO-2PACz bilayer. Devices were evaluated under space-relevant conditions, including vacuum, AM0 illumination and thermal cycling spanning -40 °C to 90 °C. While MoO-2PACz enables high efficiency through interfacial engineering, it exhibits severe degradation under thermal tress due to thermomechanical mismatch with the perovskite layer, leading to interfacial defects and phase instability. In contrast, PTAA provides improved thermal stability but lower efficiency. The bilayer structure achieves both high efficiency and enhanced durability, retaining 73% of its initial performance after thermal cycling. This improvement arises from the polymeric layer mitigating thermomechanical stress and suppressing δ-phase transitions. These results suggest efficient interfacial thermomechanical engineering and strategy to thermal durability in PSCs for harsh space environments.