Nanoscale Local Contacts Enable Inverted Inorganic Perovskite Solar Cells with 20.8 % Efficiency

Inorganic perovskite solar cells (IPSCs) have gained significant attention due to their excellent thermal stability and suitable band gap (~1.7 eV) for tandem solar cell applications. However, the defect-induced non-radiative recombination losses, low charge extraction efficiency, energy level mismatches, and so on render the fabrication of high-efficiency inverted IPSCs remains challenging. Here, the use of 3-amino-5-bromopyridine-2-formamide (ABF) in methanol was dynamically spin-coated on the surface of CsPbI_2.85Br_0.15 film, which facilitates the limited etching of defect-rich subsurface layer, resulting in the formation of vertical PbI₂ nanosheet structures. This enabled localized contacts between the perovskite film and the electron transport layer, suppress the recombination of electron-hole and beneficial to electron extraction. Additionally, the C=O and C=N groups in ABF effectively passivated the undercoordinated Pb²⁺ at grain boundaries and on the surface of CsPbI_2.85Br_0.15 film. Eventually, we achieved a champion efficiency of 20.80 % (certified efficiency of 20.02 %) for inverted IPSCs with enhanced stability, which is the highest value ever reported to date. Furthermore, we successfully prepared p-i-n type monolithic inorganic perovskite/silicon tandem solar cells (IPSTSCs) with an efficiency of 26.26 %. This strategy provided both fast extraction and efficient passivation at the electron-selective interface.     

14.31 % Power Conversion Efficiency of Sn-Based Perovskite Solar Cells via Efficient Reduction of Sn4+

The photoelectric properties of nontoxic Sn-based perovskite make it a promising alternative to toxic Pb-based perovskite. It has superior photovoltaic performance in comparison to other Pb-free counterparts. The facile oxidation of Sn²⁺ to Sn⁴⁺ presents a notable obstacle in the advancement of perovskite solar cells that utilize Sn, as it adversely affects their stability and performance. The study revealed the presence of a Sn⁴⁺ concentration on both the upper and lower surfaces of the perovskite layer. This discovery led to the adoption of a bi-interface optimization approach. A thin layer of Sn metal was inserted at the two surfaces of the perovskite layer. The implementation of this intervention yielded a significant decrease in the levels of Sn⁴⁺ and trap densities. The power conversion efficiency of the device was achieved at 14.31 % through the optimization of carrier transportation. The device exhibited operational and long-term stability.     

Ultra Robust and Highly Efficient Flexible Organic Solar Cells with Over 18 % Efficiency Realized by Incorporating a Linker Dimerized Acceptor

The wearable application of flexible organic solar cells (f-OSCs) necessitates high power conversion efficiency (PCE) and mechanical robustness. However, photoactive films based on efficient non-fullerene small molecule acceptors (NF-SMAs) are typically brittle, leading to poor mechanical stability in devices. In this study, we achieved a remarkable PCE of 18.06 % in f-OSCs while maintaining ultrahigh mechanical robustness (with a crack-onset strain (COS) of higher than 11 %) by incorporating a linker dimerized acceptor (DOY-TVT). Compared to binary blends, ternary systems exhibit reduced non-radiative recombination, suppressed crystallization and diffusion of NF-SMAs, and improved load distribution across the chain networks, enabling the dissipation of the load energy. Thus, the ternary f-OSCs developed in this study achieved, high PCE and stability, surpassing binary OSCs. Moreover, the developed f-OSCs retained 97 % of the initial PCE even after 3000 bending cycles, indicating excellent mechanical stability (9.1 % higher than binary systems). Furthermore, the rigid device with inverted structure based on the optimal active layer exhibited a substantial increase in efficiency retention, with 89.6 % after 865 h at 85 °C and 93 % after more than 1300 h of shelf storage at 25 °C. These findings highlight the potential of the linker oligomer acceptor for realizing high-performing f-OSCs with ultrahigh mechanical robustness.     

Co-La-Based Hole-Transporting Layers for Binary Organic Solar Cells with 18.82 % Efficiency

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a widely used hole transporting layer (HTL) in organic solar cells (OSCs), but its acidity severely reduces the stability of devices. Until now, very few HTLs were developed to replace PEDOT:PSS toward stable and high-performance OSCs. Herein, a new cobalt-lanthanum (Co-La) inorganic system was reported as HTL to show a high conversion efficiency (PCE) of 18.82 %, which is among the top PCEs in binary OSCs. Since electron-rich outer shell of La atom can interact with Co atom to form charge transfer complex, the work function and conductivity of the Co-La system could be simultaneously enhanced compared to Co or La-based HTLs. This Co-La system could also be applied into other OSCs to show high performance. All these results demonstrate that binary Co-La systems as HTL can efficiently tackle the issue in hole transporting and show powerful application in OSCs to replace PEDOT:PSS.     

Suppressing Interfacial Charge Recombination in Electron-Transport-Layer-Free Perovskite Solar Cells to Give an Efficiency Exceeding 21 %

The performances of electron-transport-layer (ETL)-free perovskite solar cells (PSCs) are still inferior to ETL-containing devices. This is mainly due to severe interfacial charge recombination occurring at the transparent conducting oxide (TCO)/perovskite interface, where the photo-injected electrons in the TCO can travel back to recombine with holes in the perovskite layer. Herein, we demonstrate for the first time that a non-annealed, insulating, amorphous metal oxyhydroxide, atomic-scale thin interlayer (ca. 3 nm) between the TCO and perovskite facilitates electron tunneling and suppresses the interfacial charge recombination. This largely reduced the interfacial charge recombination loss and achieved a record efficiency of 21.1 % for n-i-p structured ETL-free PSCs, outperforming their ETL-containing metal oxide counterparts (18.7 %), as well as narrowing the efficiency gap with high-efficiency PSCs employing highly crystalline TiO₂ ETLs.     

Boosting the Efficiency of Dye-Sensitized Solar Cells by a Multifunctional Composite Photoanode to 14.13 %

We report a new strategy to fabricate a multifunctional composite photoanode containing TiO₂ hollow spheres (TiO₂-HSs), Au nanoparticles (AuNPs) and novel NaYF₄ : Yb,Er@NaLuF₄ : Eu@SiO₂ upconversion nanoparticles (UCNPs). The AuNPs are grown on the photoanode film including TiO₂-HSs and UCNPs by a simple in situ plasmonic treatment. As a result, an impressive power conversion efficiency of 14.13 % is obtained, which is a record for N719 dye-based dye-sensitized solar cells, demonstrating great potential for the solar cells toward commercialization. This obvious enhancement is ascribed to a collaborative mechanism of the TiO₂-HSs exhibiting excellent light-scattering ability, of the UCNPs converting near-infrared photons into visible photons and of the AuNPs presenting outstanding surface plasmon resonance effect. Notably, a steady-state experiment further reveals that the champion cell exhibits 95.33 % retainment in efficiency even after 180 h of measurements, showing good device stability.     

‘Radicalize’ the Performance of Perovskite Solar Cells with Radical Compounds

Perovskite solar cells(PSCs) have attracted tremendous attention due to their outstanding performance within a short development. Radical molecules with unpaired single electrons have been widely used in energy-related fields, such as organic light-emitting diodes(OLEDs), organic field-effect transistors(OFETs), organic and dye-sensitized solar cells, batteries, thermoelectric conversion devices, etc. However, as far as we know, there has never been a systemic collection and analysis of the application of radical molecules in PSCs. Herein, we summarized the role of the radical molecule on perovskite(passivate trap defects, enhance oxygen stability and make perovskite band-bending) and charge transport layer(improve conductivity and mobility, enhance oxygen stability, modulate work function and decrease by-product generating). Meanwhile, future directions of making full use of radical molecules in improving the performances of PSCs were envisioned.     

Complete Peripheral Fluorination of the Small-Molecule Acceptor in Organic Solar Cells Yields Efficiency over 19 %

Due to the intrinsically flexible molecular skeletons and loose aggregations, organic semiconductors, like small molecular acceptors (SMAs) in organic solar cells (OSCs), greatly suffer from larger structural/packing disorders and weaker intermolecular interactions comparing to their inorganic counterparts, further leading to hindered exciton diffusion/dissociation and charge carrier migration in resulting OSCs. To overcome this challenge, complete peripheral fluorination was performed on basis of a two-dimensional (2D) conjugation extended molecular platform of CH-series SMAs, rendering an acceptor of CH8F with eight fluorine atoms surrounding the molecular backbone. Benefitting from the broad 2D backbone, more importantly, strengthened fluorine-induced secondary interactions, CH8F and its D18 blends afford much enhanced and more ordered molecular packings accompanying with enlarged dielectric constants, reduced exciton binding energies and more obvious fibrillary networks comparing to CH6F controls. Consequently, D18:CH8F-based OSCs reached an excellent efficiency of 18.80 %, much better than that of 17.91 % for CH6F-based ones. More excitingly, by employing D18-Cl that possesses a highly similar structure to D18 as a third component, the highest efficiency of 19.28 % for CH-series SMAs-based OSCs has been achieved so far. Our work demonstrates the dramatical structural multiformity of CH-series SMAs, meanwhile, their high potential for constructing record-breaking OSCs through peripheral fine-tuning.     

On the Conformation of Dimeric Acceptors and Their Polymer Solar Cells with Efficiency over 18 %

The determination of molecular conformations of oligomeric acceptors (OAs) and their impact on molecular packing are crucial for understanding the photovoltaic performance of their resulting polymer solar cells (PSCs) but have not been well studied yet. Herein, we synthesized two dimeric acceptor materials, DIBP3F-Se and DIBP3F-S, which bridged two segments of Y6-derivatives by selenophene and thiophene, respectively. Theoretical simulation and experimental 1D and 2D NMR spectroscopic studies prove that both dimers exhibit O-shaped conformations other than S- or U-shaped counter-ones. Notably, this O-shaped conformation is likely governed by a distinctive “conformational lock” mechanism, arising from the intensified intramolecular π–π interactions among their two terminal groups within the dimers. PSCs based on DIBP3F-Se deliver a maximum efficiency of 18.09 %, outperforming DIBP3F-S-based cells (16.11 %) and ranking among the highest efficiencies for OA-based PSCs. This work demonstrates a facile method to obtain OA conformations and highlights the potential of dimeric acceptors for high-performance PSCs.     

Formamidinium Lead Iodide-Based Inverted Perovskite Solar Cells with Efficiency over 25 % Enabled by An Amphiphilic Molecular Hole-Transporter

Formamidinium lead iodide (FAPbI₃) represents an optimal absorber material in perovskite solar cells (PSCs), while the application of FAPbI₃ in inverted-structured PSCs has yet to be successful, mainly owing to its inferior film-forming on hydrophobic or defective hole-transporting substrates. Herein, we report a substantial improvement of FAPbI₃-based inverted PSCs, which is realized by a multifunctional amphiphilic molecular hole-transporter, (2-(4-(10H-phenothiazin-10-yl)phenyl)-1-cyanovinyl)phosphonic acid (PTZ−CPA). The phenothiazine (PTZ) based PTZ−CPA, carrying a cyanovinyl phosphonic acid (CPA) group, forms a superwetting hole-selective underlayer that enables facile deposition of high-quality FAPbI₃ thin films. Compared to a previously established carbazole-based hole-selective material (2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl)phosphonic acid (MeO−2PACz), the crystallinity of FAPbI₃ is enhanced and the electronic defects are passivated by the PTZ−CPA more effectively, resulting in remarkable increases in photoluminescence quantum yield (four-fold) and Shockley-Read-Hall lifetime (eight-fold). Moreover, the PTZ−CPA shows a larger molecular dipole moment and improved energy level alignment with FAPbI₃, benefiting the interfacial hole-collection. Consequently, FAPbI₃-based inverted PSCs achieve an unprecedented efficiency of 25.35 % under simulated air mass 1.5 (AM1.5) sunlight. The PTZ−CPA based device shows commendable long-term stability, maintaining over 90 % of its initial efficiency after continuous operation at 40 °C for 2000 hours.     

High-Pressure Fabrication of Binary Organic Solar Cells with High Molecular Weight D18 Yields Record 19.65 % Efficiency

In this work, inspired by the principles of a pressure cooker, we utilized a high-pressure method to address the processing challenges associated with high molecular weight polymers. Through this approach, we successfully dissolved high molecular weight D18 in chloroform at 100 °C within a pressure-tight vial. The increased steam pressure raised the boiling point and dissolving capacity of chloroform, enabling the creation of a hybrid film with superior properties, including more ordered molecular arrangement, increased crystallinity, extended exciton diffusion length, and improved phase morphology. Organic solar cells (OSCs) based on D18 : L8-BO prepared using this high-pressure method achieved an outstanding power conversion efficiency of 19.65 %, setting a new record for binary devices to date. Furthermore, this high-pressure method was successfully applied to fabricate OSCs based on other common systems, leading to significant enhancements in device performance. In summary, this research introduces a universal method for processing high molecular weight D18 materials, ultimately resulting in the highest performance reported for binary organic solar cells.     

Manipulation of the Buried Interface for Robust Formamidinium-based Sn−Pb Perovskite Solar Cells with NiOx Hole-Transport Layers

Low band gap tin-lead perovskite solar cells (Sn−Pb PSCs) are expected to achieve higher efficiencies than Pb-PSCs and regarded as key components of tandem PSCs. However, the realization of high efficiency is challenged by the instability of Sn²⁺ and the imperfections at the charge transfer interfaces. Here, we demonstrate an efficient ideal band gap formamidinium (FA)-based Sn−Pb (FAPb_0.5Sn_0.5I₃) PSC, by manipulating the buried NiO_x/perovskite interface with 4-hydroxyphenethyl ammonium halide (OH-PEAX, X=Cl⁻, Br⁻, or I⁻) interlayer, which exhibits fascinating functions of reducing the surface defects of the NiO_x hole transport layer (HTL), enhancing the perovskite film quality, and improving both the energy level matching and physical contact at the interface. The effects of different halide anions have been elaborated and a 20.53 % efficiency is obtained with OH-PEABr, which is the highest one for FA-based Sn−Pb PSCs using NiO_x HTLs. Moreover, the device stability is also boosted.     

Double Asymmetric Core Optimizes Crystal Packing to Enable Selenophene-based Acceptor with Over 18 % Efficiency in Binary Organic Solar Cells

Side-chain tailoring is a promising method to optimize the performance of organic solar cells (OSCs). However, asymmetric alkyl chain-based small molecular acceptors (SMAs) are still difficult to afford. Herein, we adopted a novel asymmetric n-nonyl/undecyl substitution strategy and synthesized two A-D₁A′D₂-A double asymmetric isomeric SMAs with asymmetric selenophene-based central core for OSCs. Crystallographic analysis indicates that AYT9Se11-Cl forms a more compact and order intermolecular packing compared to AYT11Se9-Cl , which contributed to higher electron mobility in neat AYT9Se11-Cl film. Moreover, the PM6 : AYT9Se11-Cl blend film shows a better morphology with appropriate phase separation and distinct face-on orientation than PM6 : AYT11Se9-Cl . The OSCs with PM6 : AYT9Se11-Cl obtain a superior PCE of 18.12 % compared to PM6 : AYT11Se9-Cl (17.52 %), which is the best efficiency for the selenium-incorporated SMAs in binary BHJ OSCs. Our findings elucidate that the promising double asymmetric strategy with isomeric alkyl chains precisely modulates the crystal packing and enhances the photovoltaic efficiency of selenophene-incorporated SMAs.     

13.34 % Efficiency Non-Fullerene All-Small-Molecule Organic Solar Cells Enabled by Modulating the Crystallinity of Donors via a Fluorination Strategy

Non-fullerene all-small-molecule organic solar cells (NFSM-OSCs) have shown potential as OSCs, owing to their high purity, easy synthesis and good reproducibility. However, challenges in the modulation of phase separation morphology have limited their development. Herein, two novel small molecular donors, BTEC-1F and BTEC-2F, derived from the small molecule DCAO3TBDTT, are synthesized. Using Y6 as the acceptor, devices based on non-fluorinated DCAO3TBDTT showed an open circuit voltage (V_oc) of 0.804 V and a power conversion efficiency (PCE) of 10.64 %. Mono-fluorinated BTEC-1F showed an increased V_oc of 0.870 V and a PCE of 11.33 %. The fill factor (FF) of di-fluorinated BTEC-2F-based NFSM-OSC was improved to 72.35 % resulting in a PCE of 13.34 %, which is higher than that of BTEC-1F (61.35 %) and DCAO3TBDTT (60.95 %). To our knowledge, this is the highest PCE for NFSM-OSCs. BTEC-2F had a more compact molecular stacking and a lower crystallinity which enhanced phase separation and carrier transport.     

Benzotriazole-Based 3D Four-Arm Small Molecules Enable 19.1 % Efficiency for PM6 : Y6-Based Ternary Organic Solar Cells

A third component featuring a planar backbone structure similar to the binary host molecule has been the preferred ingredient for improving the photovoltaic performance of ternary organic solar cells (OSCs). In this work, we explored a new avenue that introduces 3D-structured molecules as guest acceptors. Spirobifluorene (SF) is chosen as the core to combine with three different terminal-modified (rhodanine, thiazolidinedione, and dicyano-substituted rhodanine) benzotriazole (BTA) units, affording three four-arm molecules, SF-BTA1, SF-BTA2, and SF-BTA3, respectively. After adding these three materials to the classical system PM6 : Y6, the resulting ternary devices obtained ultra-high power-conversion efficiencies (PCEs) of 19.1 %, 18.7 %, and 18.8 %, respectively, compared with the binary OSCs (PCE=17.4 %). SF-BTA1-3 can work as energy donors to increase charge generation via energy transfer. In addition, the charge transfer between PM6 and SF-BTA1-3 also acts to enhance charge generation. Introducing SF-BTA1-3 could form acceptor alloys to modify the molecular energy level and inhibit the self-aggregation of Y6, thereby reducing energy loss and balancing charge transport. Our success in 3D multi-arm materials as the third component shows good universality and brings a new perspective. The further functional development of multi-arm materials could make OSCs more stable and efficient.     

Parallel Planar Heterojunction Strategy Enables Sb2S3 Solar Cells with Efficiency Exceeding 8 %

Solution-processed solar cells based on inorganic heterojunctions provide a potential approach to the efficient, stable and low-cost solar cells required for the terrestrial generation of photovoltaic energy. Antimony trisulfide (Sb₂S₃) is a promising photovoltaic absorber. Here, an easily solution-processed parallel planar heterojunction (PPHJ) strategy and related principle are developed to prepare efficient multiple planar heterojunction (PHJ) solar cells, and the PPHJ strategy boosts the efficiency of solution-processed Sb₂S₃ solar cells up to 8.32 % that is the highest amongst Sb₂S₃ devices. The Sb₂S₃-based PPHJ device consists of two kinds of conventional planar heterojunction (PHJ) subcells in a parallel connection: Sb₂S₃-based PHJ subcells dominating the absorption and charge generation and CH₃NH₃PbI₃-based PHJ subcells governing the electron transport towards collection electrode, but it belongs to an Sb₂S₃ device in nature. The resulting PPHJ device combines together the distinctive structural features of Sb₂S₃ absorbing layer as a main absorber and the duplexity of well-crystallized/oriented CH₃NH₃PbI₃ layer in charge transportation as an additional absorber, while the presence of perovskite does not affect device stability. The PPHJ strategy maintains the facile preparation by the conventional sequential depositions of multiple layers, but eliminates the normal complexity in both tandem and parallel tandem PHJ systems.     

Impact of A–D–A-Structured Dithienosilole- and Phenoxazine-Based Small Molecular Material for Bulk Heterojunction and Dopant-Free Perovskite Solar Cells

Herein, the synthesis of the novel acceptor–donor–acceptor (A–D–A)-structured small molecule Si-PO-2CN based on dithienosilole (DTS) as building block flanked by electron-rich phenoxazine (POZ) units, which are terminated with dicyanovinylene, is presented. Si-PO-2CN showed unique electrochemical and photophysical properties and has been successfully employed in perovskite solar cells (PSCs) as well as in bulk heterojunction organic solar cells (OSCs). The PSCs fabricated with dopant-free Si-PO-2CN as hole-transport material (HTM) exhibited a power conversion efficiency (PCE) of 14.1 % (active area=1.02 cm²). Additionally, a PCE of 5.6 % has been achieved for OSCs, which employed Si-PO-2CN as p-type donor material when blended with a [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) acceptor. The versatile application of Si-PO-2CN provides a pathway for further implementation of DTS-based building blocks in solar cells for designing new molecules.     

Extending the π-Conjugated System in Spiro-Type Hole Transport Material Enhances the Efficiency and Stability of Perovskite Solar Modules

Hole transport materials (HTMs) are a key component of perovskite solar cells (PSCs). The small molecular 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl)-amine-9,9′-spirobifluorene (spiro-OMeTAD, termed “Spiro”) is the most successful HTM used in PSCs, but its versatility is imperfect. To improve its performance, we developed a novel spiro-type HTM (termed “DP”) by substituting four anisole units on Spiro with 4-methoxybiphenyl moieties. By extending the π-conjugation of Spiro in this way, the HOMO level of the HTM matches well with the perovskite valence band, enhancing hole mobility and increasing the glass transition temperature. DP-based PSC achieves high power conversion efficiencies (PCEs) of 25.24 % for small-area (0.06 cm²) devices and 21.86 % for modules (designated area of 27.56 cm²), along with the certified efficiency of 21.78 % on a designated area of 27.86 cm². The encapsulated DP-based devices maintain 95.1 % of the initial performance under ISOS-L-1 conditions after 2560 hours and 87 % at the ISOS-L-3 conditions over 600 hours.     

Electron Acceptor Molecule Doping Induced π–π Interaction to Promote Charge Transport Kinetics for Efficient and Stable 2D/3D Perovskite Solar Cells

Although the incorporation of 2D perovskite into 3D perovskite can greatly enhance intrinsic stability, power conversion efficiency (PCE) of 2D/3D perovskite is still inferior to its 3D counterpart due to poor carrier transport kinetics resulted from the quantum and dielectric confinement of 2D component. To overcome this issue, the electron acceptor molecule 1,2,4,5-tetracyanobenzene (TCNB) was introduced to trigger intermolecular π–π interaction in 2D perovskite along with the electronic doping of 2D/3D perovskite to improve charge transfer efficiency. By virtue of high electron affinity, TCNB can undergo electron transfer reaction and subsequently establish π–π interaction with 1-naphthalenemethylammonium (NMA) cations, greatly strengthening lattice rigidity and reducing exciton binding energy. Transmission electron microscopy results demonstrate that 2D phases are mainly distributed at grain boundaries, reducing defect density and weakening nonradiative recombination. Meanwhile, the p-type doping of perovskite by TCNB optimizes energy level alignment at perovskite/hole transport layer interface. Consequently, PCE of champion device is significantly boosted to 24.01 %. The unencapsulated device retains an initial efficiency close to 94 % after exposure to ambient environment for over 1000 h. This work paves a novel path for designing new mixed-dimensional perovskite solar cells with high PCE and superior stability.     

Electronic Configuration Tuning of Centrally Extended Non-Fullerene Acceptors Enabling Organic Solar Cells with Efficiency Approaching 19 %

In the molecular optimizations of non-fullerene acceptors (NFAs), extending the central core can tune the energy levels, reduce nonradiative energy loss, enhance the intramolecular (donor-acceptor and acceptor-acceptor) packing, facilitate the charge transport, and improve device performance. In this study, a new strategy was employed to synthesize acceptors featuring conjugation-extended electron-deficient cores. Among these, the acceptor CH-BBQ, embedded with benzobisthiadiazole, exhibited an optimal fibrillar network morphology, enhanced crystallinity, and improved charge generation/transport in blend films, leading to a power conversion efficiency of 18.94 % for CH-BBQ-based ternary organic solar cells (OSCs; 18.19 % for binary OSCs) owing to its delicate structure design and electronic configuration tuning. Both experimental and theoretical approaches were used to systematically investigate the influence of the central electron-deficient core on the properties of the acceptor and device performance. The electron-deficient core modulation paves a new pathway in the molecular engineering of NFAs, propelling relevant research forward.