Francis R. Lin

Narrow‐bandgap Sn‐Pb perovskites have showcased great potential for constructing tandem perovskite solar cells (PSCs). However, Sn ^{2 +} in Sn‐Pb PSCs is readily oxidized to Sn ^{4 +} , and such B‐site Sn ^{4 +} will introduce excess positive charge and causes severe p‐type self‐doping, which pins the Fermi level and aggravates non‐radiative recombination. Additionally, the Sn‐based component crystallizes far more rapidly than that of Pb, resulting in unbalanced Sn‐Pb crystallization. To address these challenges, we develop a novel n‐type molecule NDBH as perovskite additive. This molecule offers three key advantages: prevents the oxidation of Sn ^{2 +} through chelation and blocks oxygen ingress along grain boundaries; balances the crystallization rates of Sn‐Pb perovskite by selectively chelating Sn ^{2 +} ; modifies the perovskite surface to be more n‐type. With this organic n‐type molecule, the inverted Sn‐Pb PSCs achieve power conversion efficiency (PCE) of 24.11%. The devices also exhibited remarkable long‐term stability, retaining over 90% of their initial efficiency after 600 h of maximum power point (MPP) tracking. This approach of designing multifunctional n‐type molecules offers a new perspective for enhancing the efficiency and stability of Sn‐Pb PSCs.

Organic solar cells (OSCs) have garnered extensive attention due to their advantage of low cost and lightweight properties. However, owing to the complex interplay between excitons and charge carriers, the intrinsic energy loss pathways and performance‐limiting factors in OSCs remain poorly understood. Herein, we established a comprehensive opto‐electro‐thermal (OET) model to quantify the exciton‐ and carrier‐related thermodynamic loss mechanisms in organic photovoltaics, and identified seven inherent energy loss channels in OSCs. To identify the key bottlenecks limiting device performance, we further systematically explored the influences of temperature and key electrical parameters on device performance. The results reveal that operating temperature and carrier mobility impose opposite impacts on exciton dissociation and carrier recombination behaviors, and balancing these two loss types enables optimized OSC performance. Guided by these insights, we experimentally fabricated BHJ and LBL‐type OSCs achieving a power conversion efficiency (PCE) of 19.7% and 19.9%, respectively. Furthermore, our results demonstrate that a PCE exceeding 22.2% can be expected for OSCs through the integration of rational material design and working temperature management. This study is critical for an in‐depth understanding of OET‐coupled physics in exciton‐ and carrier‐mixed devices and provides a theoretical guide for the design of high‐performance OSCs.

Self‐assembled monolayers (SAMs) have emerged as an effective interfacial strategy for improving charge extraction and interfacial energetics in organic solar cells (OSCs); however, limited operational stability, particularly under prolonged high‐temperature conditions, remains a critical challenge for practical deployment. Here, we systematically engineer SAM terminal groups to elucidate how interfacial molecular interactions between the SAM and the bulk‐heterojunction active layer govern device efficiency and thermal stability. Expanding the aromatic ring size of the SAM pendant groups enhances π–π and van der Waals interactions, leading to stronger molecular coupling and a more intact and robust interfacial structure at both the electrode/SAM and SAM/active‐layer interfaces. In particular, SAMs incorporating naphthalene pendant groups exhibit significantly strengthened intermolecular interactions, effectively suppressing thermally induced morphological degradation under elevated temperatures. As a result, PM6:BTP‐eC9‐based binary and ternary organic solar cells achieve power conversion efficiencies of 19.73% and 20.15%, respectively. Notably, devices employing this interfacial molecular locking strategy deliver a T ₉₀ operational lifetime of 150 h under maximum power point tracking at 85°C, representing an order‐of‐magnitude improvement compared to SAMs without pendant groups. These findings establish aromatic terminal group expansion as an effective molecular design strategy for simultaneously enhancing efficiency and thermal stability in organic solar cells.

Non-geminate recombination in organic photovoltaics (OPVs) forms low-energy spin-triplet excitons (T₁) that are known to result in irreversible, non-radiative relaxations^1,2,3,4,5. Here we experimentally show in an OPV system incorporating a non-fullerene acceptor with a narrowed singlet–triplet gap that T₁ excitons can be redissociated through the interfacial charge-transfer state to form free carriers. We corroborate this by identifying the increased population of free carriers following triplet sensitization of the acceptor in an OPV blend, and illustrate the way in which this mechanism alters the evolution of T₁ and free carrier populations. We reveal how the distribution of orbitals in the molecule and exciton delocalization in aggregates affect the singlet–triplet energetics of the acceptor in the condensed phase, rendering the traffic between T₁ and the spin-triplet charge-transfer state controllable. By introducing this acceptor as a ternary component into other host OPV systems, we manage to recover the triplet-mediated losses and improve OPV efficiencies by maximizing the number of extractable photocarriers. This study deepens our understanding of the fundamentals of OPVs, and shows how to develop future organic optoelectronics by demonstratating the recovery of low-energy T₁ excitons into usable charges for electricity or light generation instead of heat.

The performance and operational stability of inverted organic solar cells (OSCs) are often limited by charge recombination and interfacial instability at the electron transport layer (ETL). To address this, we designed two fullerene‐based self‐assembled monolayers (SAMs)—C2‐PA and 4EG‐PA—as interfacial modifiers for zinc oxide (ZnO). Systematic comparisons reveal that the tetra(ethylene glycol) linker in 4EG‐PA induces a denser and more uniform SAM morphology than the alkyl chain in C2‐PA, which more effectively passivates the polar ZnO surface. This superior molecular packing translates into a champion power conversion efficiency of 19.46%. More critically, transient absorption spectroscopy (TAS) provides direct evidence that the 4EG‐PA‐modified interface facilitates the formation of a favorable charge‐transfer state, which not only promotes electron extraction but also enhances hole transfer efficiency from the acceptor to the donor, thereby suppressing non‐geminate recombination. Concurrently, the dense SAM acts as a robust buffer, improving the thermodynamic compatibility with the active layer and inhibiting its deleterious reaggregation. This dual mechanism—enhanced charge extraction and optimized interfacial morphology—underpins the exceptional operational stability, with devices retaining 84% of their initial performance after 2000 h. Our work elucidates the critical link between SAM molecular structure, interfacial properties, and device longevity, providing a strategic blueprint for future interfacial material design.

Non-radiative losses present a significant hurdle limiting the performance of organic solar cells (OSCs). Selenium substitution in non-fullerene acceptors (NFAs) is an effective approach for tuning intermolecular stacking and energy levels in state-of-the-art near-infrared absorbing OSCs. However, the inter-system crossing (ISC) induced by selenium may enhance the formation of triplet excitons in NFAs, aggravating non-radiative losses in OSCs. Herein, we have structurally engineered selenium-containing NFAs through introducing achiral N-alkyl substituents with varied branching sites at C2 or C3 position onto the pyrrole moiety of NFA core. We find that the C2-branched ones in our material system exhibit more intimate molecular packing, improved luminescence efficiency and reduced triplet exciton generation in both neat and blend films, reflected as improved external quantum efficiencies and open-circuit voltages in higher-performance devices. We utilize the best-performing NFA as a ternary component into the benchmark D18:L8-BO absorber to realize a high efficiency of 20.4% (certified as 19.88%). This work shows how precisely regulating the microstructure of NFAs by side-chain modification can reduce the non-radiative losses of OSCs.

Hole-selective self-assembled monolayers (SAMs)^1,2 have played a key role in driving the certified power conversion efficiency (PCE) of inverted perovskite solar cells^3,4,5 to 26.7% (ref. ⁶). However, their instability often compromises the operational performance of devices, strongly hindering their practical applications^7,8. Here we employ a cross-linkable co-SAM to enhance the conformational stability of hole-selective SAMs against external stresses, while suppressing the formation of defects and voids in SAM during self-assembly. The azide-containing SAM can be thermally activated to form a cross-linked and densely assembled co-SAM with a thermally stable conformation and preferred orientation. This effectively minimizes substrate surface exposure caused by wiggling of loose SAMs under thermal stress, preventing perovskite decomposition. This enables a certified PCE of 26.92% to be achieved for the best-performing cell, which also possesses excellent thermal stability with negligible decay under maximum-power-point tracking at 85 °C for 1,000 h. It also retains >98% of initial PCE after 700 repetitive thermal cycles between −40 °C and 85 °C, representing the state of the art of the field. This work offers an in-depth understanding of SAM degradation mechanisms to guide the design of a more robust buried interface for SAM-based devices adopting high-roughness substrates to realize highly efficient and durable perovskite solar cells.

Low-dimensional (LD)/three-dimensional (3D) heterostructure perovskite solar cells (PSCs) have achieved a power conversion efficiency (PCE) greater than 26%. However, the use of some ionic interfacial passivation materials in the construction of LD perovskites compromises device stability, as they can induce ion diffusion, particularly under high temperatures and light stress. In this study, we substitute the ammonium terminus (R-NH₃⁺) of conventional passivators with a carbamate terminus (R-NH-(CO)OR) and synthesized carbamate molecules featuring phenyl (PEA-Boc) and naphthalimide (ND-Boc) scaffolds. Through modulating the ionic terminus and enlarging the conjugated backbone of the passivation materials, the interlayer diffusion across PSCs was effectively inhibited. Moreover, the ND-Boc with electron-accepting moieties optimizes the band energy alignment, reduces defect density, and facilitates interfacial electron transfer of PSCs. As a result, the small-area target PSCs (0.04 cm²) and mini-modules (aperture area of 15.45 cm²) achieved a PCE of 26.04% and 21.83%, respectively. Notably, the encapsulated ND-Boc-based PSC maintained 96.7% of its initial PCE after being tracked at a maximum power point for 1500 h at 85 °C under argon (ISOS-L-2I protocol). Our strategy offers a simple and generally applicable passivation method for fabricating efficient and robust PSCs to facilitate their practical applications.

We engineer molecular packing in five derivatives of the nonfullerene acceptor Y6. Using transient absorption spectroscopy, we find evidence of the formation of a delocalized exciton in addition to the local exciton in neat films of the acceptors. Following selective photoexcitation of the acceptors in donor/acceptor blends with D18, we observe anion formation on the same timescale as in neat acceptor films, suggesting that D18 is a bystander to charge generation after photoexcitation of the acceptors. We quantify the recombination kinetics of the delocalized excitons with the monomolecular recombination constant (a) and find that both the hole transfer yield and the internal quantum efficiency in photovoltaic devices increase for acceptor films with lower a. In A1, relatively localized excitons with a limited charge transfer character have fast recombination kinetics (a = 3.2 × 10¹⁰ s^-1), leading to the lowest IQE (83.7%). In T1, more delocalized excitons with stronger charge transfer character have slower recombination kinetics (a = 5.3 × 10⁹ s^-1), leading to a higher IQE (97.2%). Grazing incidence wide-angle X-ray scattering of π-π stacking regions reveals that the tendency to pack face-on is a key driver of exciton delocalization across acceptors with similar molecular packing. We anticipate that this newly identified structural lever will help propel organic photovoltaics toward 20% efficiency.