JIALINGBO

Imidazolium-based zwitterionic ionic liquid 1-(1-ethyl-3-imidazolium)propane-3-sulfonate (EIMS) is studied as additive to enhance the performance of the FAPbI₃ perovskite solar cells. EIMS changes the morphology of PbI₂ to improve the quality of perovskite film, as well as accelerate the extraction of electron, which leads to an efficiency of 22.1% with a negligible hysteresis behavior in the solar cells.

Imidazolium-based ionic liquids have been established as promising candidates for additives in perovskite solar cells. Herein, 1-(1-ethyl-3-imidazolium)propane-3-sulfonate (EIMS) zwitterionic ionic liquid is studied as additive to enhance the performance of the FAPbI₃ perovskite solar cells. The addition of EIMS leads to the generation of porous PbI₂ film, which promotes the reaction of PbI₂ with FAI solution and thus improves the quality of FAPbI₃ perovskite film during the two-step sequential spin coating. It is revealed that EIMS accelerates the extraction of electron, inhibits the formation of pinhole, and reduces the defect density, which leads to an increased efficiency of 22.1%. The hysteresis index is also reduced from 0.142 to 0.009, which indicates the addition of EIMS can realize a negligible hysteresis behavior in the solar cells. Moreover, devices prepared with EIMS retain above 96% of the original power conversion efficiency (PCE) value after being stored in dry air for 45 days. All these results demonstrate that the imidazolium-based zwitterionic ionic liquid additive strategy is a promising way for high-performance perovskite solar cells.

In this study, a double-side passivation strategy is proposed to reduce the defects on both the top and buried surfaces of perovskite layer. To passivate both positive and negative defects, a multifunctional passivating molecule with both electron-rich and electron-poor domains is developed. As a result, these synergistic virtuous effects are used here to demonstrate flexible perovskite solar cells (f-PSCs) with unprecedented, simultaneous improvements in power conversion efficiency (PCE) (a high PCE of 24.40% and 22.04% for rigid and flexible PSCs, respectively), operational stability (>1000 h) and bending durability (>10 000 cycles), where refer to retention of 90% and 80%, respectively, of the initial PCE.

Abstract

Although much progress is made toward enhancing the efficiency of perovskite solar cells (PSCs), their operational reliability, particularly their mechanical stability, which is a crucial factor for flexible PSCs (f-PCSs), has not attracted sufficient attention. The defects in the perovskite layer, especially on the top and the buried surface of the perovskite layer, can induce perovskite fracture, highly limiting the performance of f-PSCs. Herein, a novel multifunctional organic salt, metformin hydrochloride, which can passivate cationic and anionic defects, is incorporated on both the top and buried surfaces of perovskite layer to suppress defects. As a result, a power conversion efficiency (PCE) of 24.40% for rigid PSCs and a PCE of 22.04% for f-PSCs are achieved. Simultaneously, the device can retain 90% and 80% of the initial efficiency after 1000 h of light illumination and 10 000 bending cycles, respectively, showing excellent operational stability. This study may provide a global way to design a passivation strategy and fabricate flexible perovskite solar cells with high efficiency and stability.

Perovskite-organic tandem solar cells (PO-TSCs) hold great promise as an excellent technology for converting sustainable solar energy into electricity, which combines the advantages of perovskites and organic semiconductors. In this perspective, we set the focus on the critical challenges of each sub-cell and the development roadmap of PO-TSCs. Moreover, we propose potential strategies that can help further enhance their efficiency and stability.

A cross-linkable biological molecule α-lipoic acid is first employed to construct self-healable polymer network in buried interface and perovskite layer via dynamic covalent disulfide bonds and intermolecular hydrogen bonds. The passivation and toughening effects endow perovskite films with excellent mechanical stability and self-healing capacity. The resultant bending-degraded flexible devices can recover to 95% of its original efficiency under mild condition.

Abstract

Halide perovskites are qualified to meet the flexibility demands of optoelectronic field because of their merits of flexibility, lightness, and low cost. However, the intrinsic defects and deformation-induced ductile fracture in both perovskite and buried interface significantly restrict the photoelectric performance and longevity of flexible perovskite solar cells (PVSCs). Here, a dual-dynamic cross-linking network is schemed to boost the photovoltaic efficiency and mechanical stability of flexible PVSCs by incorporating natural polymerizable small molecule α-lipoic acid (LA). The LA therein can be autonomously ring-opening polymerized through dynamic disulfide bonds and hydrogen bonds, concurrently forming coordination bonds to interact with perovskite component. Importantly, the polymerization product can serve as efficacious passivating and toughening agents to simultaneously optimize interfacial contact, enhance perovskite crystallinity and sustain robust mechanical bendability. Subsequently, the rigid (or flexible) p-i-n device realizes a champion efficiency of 22.43% (or 19.03%) with prominent operational stability. Moreover, the dual-dynamic cross-linking network endows PVSCs with bendability and self-healing capacity, allowing the optimized devices to retain >80% efficiency after 3000 bending cycles, and subsequently restore to ≈95% of its initial efficiency under mild heat-treatment. This toughening and self-healing strategy provides a facile and efficient path to prolong operational lifetime of flexible device.

A green anti-solvent deposition method that allows precise control of crystallization and phase transition in wide-bandgap perovskite is reported. It can regulate the lead iodide (PbI₂) adduct to directly form α-perovskite, and effectively reduces nonradiative recombination in the bulk. Finally, a remarkable open-circuit voltage (V _OC) of 1.255 V and an optimal efficiency of 20.06% is achieved.

Abstract

Wide-bandgap perovskite solar cells (PSCs) have attracted a lot of attention due to their application in tandem solar cells. However, the open-circuit voltage (V _OC) of wide-bandgap PSCs is dramatically limited by high defect density existing at the interface and bulk of the perovskite film. Here, an anti-solvent optimized adduct to control perovskite crystallization strategy that reduces nonradiative recombination and minimizes V _OC deficit is proposed. Specifically, an organic solvent with similar dipole moment, isopropanol (IPA) is added into ethyl acetate (EA) anti-solvent, which is beneficial to form PbI₂ adducts with better crystalline orientation and direct formation of α-phase perovskite. As a result, EA-IPA (7-1) based 1.67 eV PSCs deliver a power conversion efficiency of 20.06% and a V _OC of 1.255 V, which is one of the remarkable values for wide-bandgap around 1.67 eV. The findings provide an effective strategy for controlling crystallization to reduce defect density in PSCs.

With the introduction of a low-viscosity solvent (acetonitrile), the ribbing effect in slot-die coated films is reduced and the morphology of films improved. A certified perovskite solar cell with steady state power conversion efficiency of 22.35% is achieved by slot die coating. Finally, the encapsulated devices are subjected to outdoor conditions for a one-year stability test.

Abstract

The next technological step in the exploration of metal-halide perovskite solar cells is the demonstration of larger-area device prototypes under outdoor operating conditions. The authors here demonstrate that when slot-die coating the halide perovskite layers on large areas, ribbing effects may occur but can be prevented by adjusting the precursor ink's rheological properties. For formamidinium lead triiodide (FAPbI₃) precursor inks based on 2-methoxyethanol, the ink viscosity is adjusted by adding acetonitrile (ACN) as a co-solvent leading to smooth FAPbI₃ thin-films with high quality and layer homogeneity. For an optimized content of 46 vol% of the ACN co-solvent, a certified steady-state performance of 22.3% is achieved in p-i-n FAPbI₃-perovskite solar cells. Scaling devices to larger areas by making laser series-interconnected mini-modules of 12.7 cm², a power conversion efficiency of 17.1% is demonstrated. A full year of outdoor stability testing with continuous maximum power point tracking on encapsulated devices is performed and it is demonstrated that these devices maintain close to 100% of their initial performance during winter and spring followed by a significant performance decline during warmer summer months. This work highlights the importance of the real-condition evaluation of larger area device prototypes to validate the technological potential of halide perovskite photovoltaics.

Herein, an asymmetric modification strategy is developed by incorporating a thiazole derivative, 1,3-thiazole-2,4-diammonium (TDA), into a perovskite/SnO₂ interface for high performance FACsPbI₃ perovskite solar cells (PSCs). N3 cures the free OH defects on the SnO₂ surface, while N1 passivates the Pb²⁺/I⁻ defects from the perovskite-buried interface. Consequently, the TDA-modified device achieves nearly 25% efficiency with a high V _oc of 1.20 V.

Abstract

Surmounting complicated defects at the electron transport layer (ETL) and perovskite interface plays a non-trivial role in improving efficiency and stability of perovskite solar cells (PSCs). Herein, an asymmetric interface modification strategy (AIMS) is developed to passivate the defects from both a SnO₂ ETL and the perovskite buried surface via incorporating 1,3-thiazole-2,4-diammonium (TDA) into the SnO₂/perovskite interface. Detailed experimental and calculated results demonstrate that N3 (the nitrogen atom bonding to the imine) in the TDA preferentially cures the free hydroxyl (OH), oxygen vacancy (V _O), and the Sn-related defects on the SnO₂ surface, while N1 (the nitrogen atom bonding to the vinyl) is more inclined to passivate the Pb²⁺ and I⁻ related defects at the perovskite buried surface. As a result, the TDA-modified FACsPbI₃ PSC yields a champion power conversion efficiency (PCE) of 24.96% with a gratifying open-circuit voltage (V _oc) of 1.20 V. In addition, the optimized PSCs exhibit charming air-operational stability with the unencapsulated device sustaining 97.04% of its initial PCE after storage in air conditions for 1400 h. The encapsulated device maintains 90.21% of its initial PCE after maximum power point tracking for 500 h.

A comprehensive passivation strategy based on 4-(chloromethyl) benzonitrile is proposed for defect treatments of both hole and electron transport layers. This additive can improve wettability of poly(triarylamine) and reduce agglomeration of [6,6]-phenyl-C61-butyric acid methylester particles. It also demonstrates improvement of interfacial contact between the charge transport layers and perovskites for fabrication of efficient and stable inverted perovskite solar cells.

Abstract

Poor carrier transport capacity and numerous surface defects of charge transporting layers (CTLs), coupled with misalignment of energy levels between perovskites and CTLs, impact photoelectric conversion efficiency (PCE) of inverted perovskite solar cells (PSCs) profoundly. Herein, a collaborative passivation strategy is proposed based on 4-(chloromethyl) benzonitrile (CBN) as a solution additive for fabrication of both [6,6]-phenyl-C61-butyric acid methylester (PCBM) and poly(triarylamine) (PTAA) CTLs. This additive can improve wettability of PTAA and reduce the agglomeration of PCBM particles, which enhance the PCE and device stability of the PSCs. As a result, a PCE exceeding 20% with a remarkable short circuit current of 23.9 mA cm⁻², and an improved fill factor of 81% is obtained for the CBN- modified inverted PSCs. Devices maintain 80% and 70% of the initial PCE after storage under 30% and 85% humidity ambient conditions for 1000 h without encapsulation, as well as negligible light state PCE loss. This strategy demonstrates feasibility of the additive engineering to improve interfacial contact between the CTLs and perovskites for fabrication of efficient and stable inverted PSCs.

Antisolvent-assisted in situ cation exchange of perovskite quantum dots (PQDs) is demonstrated. The vacancy-mediated cation diffusion can efficiently passivate the surface defects of PQDs without affecting the size distribution and surface ligand chemistry of PQDs. Meanwhile, using the antisolvent-assisted cation exchange method, the PQD solid with a flattened energy landscape, improved stacking orientation, and favorable charge carrier transport is realized.

Abstract

Cesium-formamidinium lead iodide perovskite quantum dots (FA _x Cs_{1− x} PbI₃ PQDs) show high potential for next-generation photovoltaics due to their outstanding optoelectronic properties. However, achieving composition-tunable hybrid PQDs with desirable charge transport remains a significant challenge. Herein, by leveraging an antisolvent-assisted in situ cation exchange of PQDs, homogeneous FA _x Cs_{1− x} PbI₃ PQDs with controllable stoichiometries and surface ligand chemistry are realized. Meanwhile, the crystallographic stability of PQDs is substantially improved by substituting the cations of the PQDs mediated by surface vacancies. Consequently, PQD solar cell delivers an efficiency of 17.29%, the highest value among the homostructured PQD solar cells. The high photovoltaic performance is attributed to the broadened light harvesting spectra, flattened energy landscape, and rationalized energy levels of highly oriented PQD solids, leading to efficient charge carrier extraction. This work provides a feasible approach for the stoichiometry regulation of PQDs to finely tailor the optoelectronic properties and tolerance factors of PQDs toward high-performing photovoltaics.

Nature Energy, Published online: 23 February 2023; doi:10.1038/s41560-023-01204-z

The design of low-dimensional interface materials for perovskite solar cells is limited in the choice of the metal cation. By processing metal and ammonium halides together, Ye et al. expand the metal cation library for these interface materials.

3D polydentate complexing agents are explored to synchronously passivate defects of perovskite absorber directly in multiple spatial directions. The strong electron-donating groups (H₂PO₄) in the phytic acid can passivate non-coordinated Pb²⁺ at the ground boundaries/surface and modulate perovskite crystallization. Especially, phytic acid dipotassium (PAD) containing additional (K→PO) push–pull structure passivates the amphoteric defects. Therefore, the PAD-passivated perovskite solar cells deliver a champion photoelectric conversion efficiency of 23.18%.

Abstract

The grain boundaries (GBs)/surface defects within perovskite film directly impede the further improvement of photoelectric conversion efficiency (PCE) and stability of planar perovskite solar cells (PSCs). Herein, 3D phytic acid (PA) and phytic acid dipotassium (PAD) with polydentate are explored to synchronously passivate the defects of perovskite absorber directly in multiple spatial directions. The strong electron-donating groups (H₂PO₄) in the PA molecule afford six anchor sites to bind firmly with uncoordinated Pb²⁺ at the GBs/surface and modulate perovskite crystallization, thus enhancing the quality of perovskite film. Particularly, PAD containing an additional (K→PO) push–pull structure promotes the dominant coordination of phosphate group (PO) with Pb²⁺ and passivates halide anion defects due to the complexation of potassium ions (K⁺) with iodide ions (I^-). Consequently, the PAD-complexed PSCs deliver a champion PCE of 23.18%, which is remarkably higher than that of the control device (19.94%). Meanwhile, PAD-complexed PSCs exhibit superior moisture and thermal stability, remaining 79% of their initial PCE after 1000 h under continuous illumination, while the control device remain only 48% of their PCE after 1000 h. This work provides important insights into designing multifunctional 3D passivators for the purpose of simultaneously enhancing the efficiency and stability of devices.

Quinoxaline-based X-shaped molecules (1-4) are designed and synthesized as p-type self-assembled monolayer (SAM) for tin perovskite solar cells (TPSC). SAM; TQxD (4) exhibits excellent device performance of 8.3%, and shows great enduring stability for the performance retaining ≈90% of their original values for shelf storage over 1600 h. This is the current best result for SAM-based TPSC ever reported.

Abstract

Four X-shaped quinoxaline-based organic dyes, PQx (1), TQx, (2), PQxD (3), and TQxD (4) (D = dye sensitizers) are developed and served as p-type self-assemble monolayer (SAM) for tin perovskite solar cells (TPSC). Thermal, optical, and electrochemical properties of these SAMs are thoroughly investigated and characterized. Tin perovskite layers are successfully deposited on these four SAM surfaces according to a two-step approach and the devices exhibit power conversion efficiency in the order of TQxD (8.3%) > TQx (8.0%) > PQxD (7.1%) > PQx (6.1%). With thiophene π-extended conjugation unit in SAM structure, TQxD (4) exhibits the highest hole extraction rates, greatest hole mobilities, and slowest charge recombination to achieve great device performance of 8.3%, which is the current best result for SAM-based TPSC ever reported. Furthermore, all devices except PQx shows great enduring stability for the performance retaining ≈90% of their original values for shelf storage over 1600 h.

An ionic-liquid polymer additive (poly[Se-MI][BF₄]) is developed to stabilize perovskite precursor inks used to fabricate perovskite solar cells (pero-SCs). A chemical homogeneity of the inks contributes to improving the quality of perovskite films by stabilizing the colloids and compositions for over two months. Moreover, polymers anchored at the grain boundaries can further suppress the migration of I⁻ ions. Thus, the pero-SCs exhibit overall improved stability and efficiency.

Abstract

The stability-related issues arising from the perovskite precursor inks, films, device structures and interdependence remain severely under-explored to date. Herein, we designed an ionic-liquid polymer (poly[Se-MI][BF₄]), containing functional moieties like carbonyl (C=O), selenium (Se⁺), and tetrafluoroborate (BF₄ ⁻) ions, to stabilize the whole device fabrication process. The C=O and Se⁺ can coordinate with lead and iodine (I⁻) ions to stabilize lead polyhalide colloids and the compositions of the perovskite precursor inks for over two months. The Se⁺ anchored on grain boundaries and the defects passivated by BF₄ ⁻ efficiently suppress the dissociation and migration of I⁻ in perovskite films. Benefiting from the synergistic effects of poly[Se-MI][BF₄], high efficiencies of 25.10 % and 20.85 % were exhibited by a 0.062-cm² device and 15.39-cm² module, respectively. The devices retained over 90 % of their initial efficiency under operation for 2200 h.

Mitigating surface problems of perovskite thin single crystals by one multifunctional molecule leads to improvement of power conversion efficiency as well as moisture and reverse-bias stability of single-crystal perovskite solar cells. The modified devices exhibit an impressive efficiency of 22.2%, which is the highest value for single-crystal MAPbI₃ perovskite solar cells.

Abstract

Metal halide perovskite single crystals are promising for diverse optoelectronic applications due to their outstanding properties. In comparison to the bulk, the crystal surface suffers from high defect density and is moisture sensitive; however, surface modification strategies of perovskite single crystals are relatively deficient. Herein, solar cells based on methylammonium lead triiodide (MAPbI₃) thin single crystals are selected as a prototype to improve single-crystal perovskite devices by surface modification. The surface trap passivation and protection against moisture of MAPbI₃ thin single crystals are achieved by one bifunctional molecule 3-mercaptopropyl(dimethoxy)methylsilane (MDMS). The sulfur atom of MDMS can coordinate with bare Pb²⁺ of MAPbI₃ single crystals to reduce surface defect density and nonradiative recombination. As a result, the modified devices show a remarkable efficiency of 22.2%, which is the highest value for single-crystal MAPbI₃ solar cells. Moreover, MDMS modification mitigates surface ion migration, leading to enhanced reverse-bias stability. Finally, the cross-link of silane molecules forms a protective layer on the crystal surface, which results in enhanced moisture stability of both materials and devices. This work provides an effective way for surface modification of perovskite single crystals, which is important for improving the performance of single-crystal perovskite solar cells, photodetectors, X-ray detectors, etc.

Nature Communications, Published online: 20 February 2023; doi:10.1038/s41467-023-36141-8

A mismatch between quasi-Fermi level splitting and open-circuit voltage is detrimental to wide bandgap perovskite pin solar cells. Here, through theoretical and experimental approaches, the authors optimize n- and p-type interfaces to achieve open-circuit voltage of 1.29 V and T80 of 3500 h at 85 °C.

Grain-boundary “ligaments” are developed to endow fragile perovskite grain boundaries with toughness, water resistance, and room temperature self-healing properties. Meanwhile, they can also passivate the defects and release the residual tensile strain in perovskite films. The resultant flexible perovskite solar cells achieve a record 23.84% power conversion efficiency and robust mechanical, operational, and ambient stabilities.

Abstract

Flexible perovskite solar cells (pero-SCs) are the best candidates to complement traditional silicon SCs in portable power applications. However, their mechanical, operational, and ambient stabilities are still unable to meet the practical demands because of the natural brittleness, residual tensile strain, and high defect density along the perovskite grain boundaries. To overcome these issues, a cross-linkable monomer TA-NI with dynamic covalent disulfide bonds, H-bonds, and ammonium is carefully developed. The cross-linking acts as “ligaments” attached on the perovskite grain boundaries. These “ligaments” consisting of elastomers and 1D perovskites can not only passivate the grain boundaries and enhance moisture resistance but also release the residual tensile strain and mechanical stress in 3D perovskite films. More importantly, the elastomer can repair bending-induced mechanical cracks in the perovskite film because of dynamic self-healing characteristics. The resultant flexible pero-SCs exhibit promising improvements in efficiency, and record values (23.84% and 21.66%) are obtained for 0.062 and 1.004 cm² devices; the flexible devices also show overall improved stabilities with T ₉₀ >20 000 bending cycles, operational stability with T ₉₀ >1248 h, and ambient stability (relative humidity = 30%) with T ₉₀ >3000 h. This strategy paves a new way for the industrial-scale development of high-performance flexible pero-SCs.

Liquid-state dithiocarbonate-based polymers with monodispersity render exceptionally high performance when used as additives for perovskite solar cells. The dithiocarbonate-based polymers are synthesized by connecting five-membered cyclic dithiocarbonates through the living cationic ring opening polymerization. The resulting devices exhibit efficiencies of 23.5% and 22.0% during the laboratory and certified measurement, respectively, which are the highest among the perovskite solar cells containing polymers.

Abstract

Dithiocarbonate-based non-hygroscopic polymers with a glass transition temperature (T _g) and polydispersity index (PDI) of ≈4 °C and 1, respectively, are synthesized through living cationic ring-opening polymerization. These liquid-state polymers are characterized by monodispersity based on the low T _g and PDI, rendering remarkable miscibility with the perovskite precursors without aggregation. Accordingly, these polymers are added to perovskite solar cells (PSCs) to enhance their power conversion efficiency (PCE). The PCE of reference PSCs increases from 19.70% to 23.52% after direct addition of the synthesized polymer. This efficiency improvement is attributed to the considerable increases in short-circuit current density (J _SC) and fill factor (FF), resulting from the augmented size and defect passivation of perovskite crystals induced by added polymers. In fact, the PCE and J _SC of the devices measured in the laboratory and the certification center are the highest among the reported polymer-added PSCs, thanks to the great miscibility of the new polymers leading to the large amount addition which enables more thorough passivation among the grain boundaries. The improvement in open-circuit voltage falls short as compared to that in J _SC and FF, ascribed to the relatively moderate interaction strength between perovskite materials and dithiocarbonate groups.

Publication date: May 2023

Source: Nano Energy, Volume 109

Author(s): Mengqi Jin, Chong Chen, Fumin Li, Zhitao Shen, Hu Shen, Dong Yang, Rong Liu, Huilin Li, Ying Liu, Chao Dong, Mingtai Wang

Lead-based perovskite photovoltaics have great potential. However, based on results of in vivo animal studies, lead exposure from devices is likely to exceed the maximum concentration tolerated by the human body. Therefore, some methods are introduced to reduce lead leakage. Additionally, comprehensive recycling strategies are necessary to solve the problem from the root and upgrade green energy in the future.

Abstract

Perovskite solar cells (PSCs) have developed rapidly in recent years due to their excellent photoelectric properties. Among them, lead-based perovskite photovoltaics have shown great potential for both outdoor and indoor applications, whose power conversion efficiency and stability are much higher than that of lead-free PSCs. However, based on results of in vivo animal studies, Kyoto Encyclopedia of Genes and Genomes annotations and pathway analysis of microbiota and metabolites influenced by lead, it has been proved that lead exposure from PSCs probably causes systematic toxicity to human body. For the purpose of reducing lead leakage, some methods mainly based on polymer resin protective layers and self-healing encapsulation have been introduced, which can increase lead capture rate up to 95% under harsh conditions. Eventually, the devices will still face damage and obsolescence, accompanied by lead leakage into the environment. Comprehensive recycling strategies are necessary to solve this problem from the root and also shorten the energy payback time for further transformation and upgrading of green energy. The vertical and in-depth collaborative strategy for lead leakage prevention and comprehensive recycling would provide an environmentally-friendly guarantee for the final large-scale market of perovskite photovoltaics.

Low-cost hole transport materials T3, featuring a substituted pyrrole core and triphenylamine peripheral arms, have been designed and synthesized for high performance perovskite solar cells. The capability of functionalization in the final synthetic step provides an efficient way to tune energy levels and other properties of T3 materials.

Abstract

Hole transport materials (HTMs) with high hole mobility, good band alignment and ease of fabrication are highly desirable for perovskite solar cells (PSCs). Here, we designed and synthesized novel organic HTMs, named T3, which can be synthesized in high yields with commercially available materials, featuring a substituted pyrrole core and triphenylamine peripheral arms. The capability of functionalization in the final synthetic step provides an efficient way to obtain a variety of T3-based HTMs with tunable energy levels and other properties. Among them, fluorine-substituted T3 (T3-F) exhibits the best band alignment and hole extraction properties, leading to PSCs with outstanding PCEs of 24.85 % and 24.03 % (certified 23.46 %) for aperture areas of 0.1 and 1 cm², respectively. The simple structure and tunable performance of T3 can inspire further optimization for efficient PSCs.

A solution-processed thick dielectric mask with nanoscale openings can maintain both open-circuit voltage and fill factor.