Efficient Inverted Perovskite Solar Cells by Employing N-Type (D-A1 -D-A2 ) Polymers as Electron Transporting Layer

Sublimed C60 for efficient and repeatable perovskite-based solar cells

Thermally evaporated C60 is a near-ubiquitous electron transport layer in state-of-the-art p-i-n perovskite-based solar cells. As perovskite photovoltaic technologies are moving toward industrialization, batch-to-batch reproducibility of device performances becomes crucial. Here, we show that commercial as-received (99.75% pure) C60 source materials may coalesce during repeated thermal evaporation processes, jeopardizing such reproducibility. We find that the coalescence is due to oxygen present in the initial source powder and leads to the formation of deep states within the perovskite bandgap, resulting in a systematic decrease in solar cell performance. However, further purification (through sublimation) of the C60 to 99.95% before evaporation is found to hinder coalescence, with the associated solar cell performances being fully reproducible after repeated processing. We verify the universality of this behavior on perovskite/silicon tandem solar cells by demonstrating their open-circuit voltages and fill factors to remain at 1950 mV and 81% respectively, over eight repeated processes using the same sublimed C60 source material. Notably, one of these cells achieved a certified power conversion efficiency of 30.9%. These findings provide insights crucial for the advancement of perovskite photovoltaic technologies towards scaled production with high process yield.

Benzothiadiazole versus Thiazolobenzotriazole: A Structural Study of Electron Acceptors in Solution-Processable Organic Semiconductors

Despite the rapid progress of organic electronics, developing high-performance n-type organic semiconductors is still challenging. Donor-acceptor (D-A) type conjugated structures have been an effective molecular design strategy to achieve chemically-stable semiconductors and the appropriate choice of the acceptor units determines the electronic properties and device performances. We have now synthesized two types of A1 -D-A2 -D-A1 type conjugated molecules, namely, NDI-BTT-NDI and NDI-TBZT-NDI, with different central acceptor units. In order to investigate the effects of the central acceptor units on the charge-transporting properties, organic field-effect transistors (OFETs) were fabricated. NDI-TBZT-NDI had shallower HOMO and deeper LUMO levels than NDI-BTT-NDI. Hence, the facilitated charge injection resulted in ambipolar transistor performances with the optimized hole and electron mobilities of 0.00134 and 0.151 cm2  V-1  s-1 , respectively. In contrast, NDI-BTT-NDI displayed only an n-channel OFET performance with the electron mobility of 0.0288 cm2  V-1  s-1 . In addition, the device based on NDI-TBZT-NDI showed a superior air stability to that based on NDI-BTT-NDI. The difference in these OFET performances was reasonably explained by the contact resistance and film morphology. Overall, this study demonstrated that the TBZ acceptor is a promising building block to create n-type organic semiconductors.

Polymers in High-Efficiency Solar Cells: The Latest Reports

Third-generation solar cells, including dye-sensitized solar cells, bulk-heterojunction solar cells, and perovskite solar cells, are being intensively researched to obtain high efficiencies in converting solar energy into electricity. However, it is also important to note their stability over time and the devices' thermal or operating temperature range. Today's widely used polymeric materials are also used at various stages of the preparation of the complete device-it is worth mentioning that in dye-sensitized solar cells, suitable polymers can be used as flexible substrates counter-electrodes, gel electrolytes, and even dyes. In the case of bulk-heterojunction solar cells, they are used primarily as donor materials; however, there are reports in the literature of their use as acceptors. In perovskite devices, they are used as additives to improve the morphology of the perovskite, mainly as hole transport materials and also as additives to electron transport layers. Polymers, thanks to their numerous advantages, such as the possibility of practically any modification of their chemical structure and thus their physical and chemical properties, are increasingly used in devices that convert solar radiation into electrical energy, which is presented in this paper.

Employing Equivalent Circuit Models to Study the Performance of Selenium-Based Solar Cells with Polymers as Hole Transport Layers

Selenium(Se)-based solar cells (SSCs), known as one of the oldest solar cells, have regained intense attention due to the advantages of Se including direct bandgap, good stability, and single absorber. Among all kinds of device structures, conventional n-i-p SSCs with top organic hole transport layers (HTLs) show great potential since organic HTLs could be well-designed to smoothly extract holes from the Se single absorber and protect the Se surface. However, till now, the performance of Se solar cells with organic HTLs is not as good as expected. To address this issue, herein, the SSCs are first presented with organic polymers as the HTLs with the improved efficiency up to 4.3%, which is the highest one in organic HTLs-based SSCs. Additionally, comparing with perovskite solar cells, it is found that the recombination process is the key factor that influences the performance of SSCs. It is believed that the further optimization of the Se active layer and the design of new and suitable organic HTLs for SSCs should be the main focus to suppress the undesired recombination processes of Se films. Such realization would boost the efficiency of the as-fabricated SSCs.

Highly stable and efficient cathode-buffer-layer-free inverted perovskite solar cells

A simpler and less expensive fabrication process is one of the essential demands for the commercialization of perovskite solar cells (PeSCs). Especially, inverted PeSCs (I-PeSCs) require a cathode buffer layer (CBL) for fabricating highly efficient and stable PeSCs. However, this increases the number of fabrication step. Here, we demonstrate highly stable and efficient cathode-buffer-layer-free I-PeSCs via additive engineering on an ETL, which is based on phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) with a small amount of poly(methyl methacrylate) (PMMA). This modified ETL shows not only a simplified fabrication process but also effective extraction of charge from the perovskite to a high work function copper electrode (Cu) by formation of an interfacial dipole at the interfaces between the ETL and the Cu. Additionally, it exhibits good passivation of the trap density existing along the grain boundaries and surface of the perovskite layer, reducing the non-radiative recombination and consistent with the increases in open-circuit voltage (V_oc). As a result, I-PeSCs with a blend PC₆₁BM : PMMA ETL demonstrate an enhancement in the power conversion efficiency (PCE) from 13.55% (without PMMA) to 18.38%. Furthermore, they exhibit both burn-in-free behaviour in photostability measurements by maximum power-point tracking (MPPT) method and long-term air-stability (30 days for T₉₀) in ambient air. Lastly, we obtained PCE of 15.03% and 16.83% for large-area (1 cm²) I-PeSCs with PC₆₁BM and PC₆₁BM : PMMA, respectively. This method provides an alternative route to reduce the fabrication time and budget for commercialization of I-PeSCs without sacrificing device performance.

Work-Function-Tunable Electron Transport Layer of Molecule-Capped Metal Oxide for a High-Efficiency and Stable p-i-n Perovskite Solar Cell

The composite electron transporting layer (ETL) of metal oxide with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) prevents perovskite from metal electrode erosion and increases p-i-n perovskite solar cell (PVSC) stability. Although the oxide exhibits protective function, an additional work function modifier is still needed for good device performance. Usually, complicated multistep synthesis is employed to have a highly crystalline film that increases manufacturing cost and inhibits scalability. We report a facile synthesis of a novel organic-molecule-capped metal oxide nanoparticle film for the composite ETL. The nanoparticle film not only has a dual function of electron transport and protection but also exhibits work function tunability. Solvothermal-prepared SnO₂ nanoparticles are capped with tetrabutylammonium hydroxide (TBAOH) through ligand exchange. The resulting TBAOH-SnO₂ nanoparticles disperse well in ethanol and form a uniform film on PCBM. The power conversion efficiency of the device dramatically increases from 14.91 to 18.77% using this layer because of reduced charge accumulation and aligned band structure. The PVSC thermal stability is significantly enhanced by adopting this layer, which prevents migration of I^- and Ag. The ligand exchange method extends to other metal oxides, such as TiO₂, ITO, and CeO₂, demonstrating its broad applicability. These results provide a cornerstone for large-scale manufacture of high-performance and stable PVSCs.

Boosting perovskite nanomorphology and charge transport properties via a functional D-π-A organic layer at the absorber/hole transporter interface

The photovoltaic efficiency and stability challenges encountered in perovskite solar cells (PSCs) were addressed by an innovative interface engineering approach involving the utilization of the organic chromophore (E)-3-(5-(4-(bis(2',4'-dibutoxy-[1,1'-biphenyl]-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid (D35) as an interlayer between the perovskite absorber and the hole transporter (HTM) of mesoporous PSCs. The organic D-π-A interlayer primarily improves the perovskite's crystallinity and creates a smoother perovskite/HTM interface, while reducing the grain boundary defects and inducing an energy level alignment with the adjacent layers. Champion power conversion efficiencies (PCE) as high as 18.5% were obtained, clearly outperforming the reference devices. Interestingly, the D35-based solar cells present superior stability since they preserved 83% of their initial efficiency after 37 days of storage under dark and open circuit (OC) conditions. The obtained results consolidate the multifunctional role of organic D-π-A molecules as perovskite interface modifiers towards performance enhancement and scale-up fabrication of robust PSCs.

Improved stability and efficiency of polymer-based selenium solar cells through the usage of tin(iv) oxide in the electron transport layers and the analysis of aging dynamics

Well-performing SSCs with SnO₂ as the ETL and P3HT as the HTL, showing a long-term stability (more than 1500 h) were fabricated. Moreover, the aging process of the SSCs was analyzed in detail to explore the factors that affect the device behaviors.

Influences of Structural Modification of Naphthalenediimides with Benzothiazole on Organic Field-Effect Transistor and Non-Fullerene Perovskite Solar Cell Characteristics

Developing air-stable high-performance small organic molecule-based n-type and ambipolar organic field-effect transistors (OFETs) is very important and highly desirable. In this investigation, we designed and synthesized two naphthalenediimide (NDI) derivatives (NDI-BTH1 and NDI-BTH2) and found that introduction of 2-(benzo[d]thiazol-2-yl) acetonitrile groups at the NDI core position gave the lowest unoccupied molecular orbital (LUMO; -4.326 eV) and displayed strong electron affinities, suggesting that NDI-BTH1 might be a promising electron-transporting material (i.e., n-type semiconductor), whereas NDI-BTH2 bearing bis(benzo[d]thiazol-2-yl)methane at the NDI core with a LUMO of -4.243 eV was demonstrated to be an ambipolar material. OFETs based on NDI-BTH1 and NDI-BTH2 have been fabricated, and the electron mobilities of NDI-BTH1 and NDI-BTH2 are 14.00 × 10^-5 and 8.64 × 10^-4 cm²/V·s, respectively, and the hole mobility of NDI-BTH2 is 1.68 × 10^-4 cm²/V·s. Moreover, a difference in NDI-core substituent moieties significantly alters the UV-vis absorption and cyclic voltammetry properties. Thus, we further successfully employed NDI-BTH1 and NDI-BTH2 as electron transport layer (ETL) materials in inverted perovskite solar cells (PSCs). The PSC performance exhibits that NDI-BTH2 as the ETL material gave higher power conversion efficiency as compared to NDI-BTH1, that is, NDI-BTH2 produces 15.4%, while NDI-BTH1 gives 13.7%. The PSC performance is comparable with the results obtained from OFETs. We presume that improvement in solar cell efficiency of NDI-BTH2-based PSCs is due to the well-matched LUMO of NDI-BTH2 toward the conduction band of the perovskite layer, which in turn increase electron extraction and transportation.

Lead-Free Double Perovskite Cs2 SnX6 : Facile Solution Synthesis and Excellent Stability

Long-term instability and possible lead contamination are the two main issues limiting the widespread application of organic-inorganic lead halide perovskites. Here a facile and efficient solution-phase method is demonstrated to synthesize lead-free Cs₂ SnX₆ (X = Br, I) with a well-defined crystal structure, long-term stability, and high yield. Based on the systematic experimental data and first-principle simulation results, Cs₂ SnX₆ displays excellent stability against moisture, light, and high temperature, which can be ascribed to the unique vacancy-ordered defect-variant structure, stable chemical compositions with Sn⁴⁺ , as well as the lower formation enthalpy for Cs₂ SnX₆ . Additionally, photodetectors based on Cs₂ SnI₆ are also fabricated, which show excellent performance and stability. This study provides very useful insights into the development of lead-free double perovskites with high stability.

Significant Difference in Semiconducting Properties of Isomeric All-Acceptor Polymers Synthesized via Direct Arylation Polycondensation

The direct arylation polycondensation (DArP) appeared as an efficient method for producing semiconducting polymers but often requires acceptor monomers with orienting or activating groups for the reactive carbon-hydrogen (C-H) bonds, which limits the choice of acceptor units. In this study, we describe a DArP for producing high-molecular-weight all-acceptor polymers composed of the acceptor monomers without any orienting or activating groups via a modified method using Pd/Cu co-catalysts. We thus obtained two isomeric all-acceptor polymers, P1 and P2, which have the same backbone and side-chains but different positions of the nitrogen atoms in the thiazole units. This subtle change significantly influences their optoelectronic, molecular packing, and charge-transport properties. P2 with a greater backbone torsion has favorable edge-on orientations and a high electron mobility μ_e of 2.55 cm²  V^-1  s^-1 . Moreover, P2-based transistors show an excellent shelf-storage stability in air even after the storage for 1 month.

Recent Progress in Organic Electron Transport Materials in Inverted Perovskite Solar Cells

Organic n-type materials (e.g., fullerene derivatives, naphthalene diimides (NDIs), perylene diimides (PDIs), azaacene-based molecules, and n-type conjugated polymers) are demonstrated as promising electron transport layers (ETLs) in inverted perovskite solar cells (p-i-n PSCs), because these materials have several advantages such as easy synthesis and purification, tunable frontier molecular orbitals, decent electron mobility, low cost, good solubility in different organic solvents, and reasonable chemical/thermal stability. Considering these positive factors, approaches toward achieving effective p-i-n PSCs with these organic materials as ETLs are highlighted in this Review. Moreover, organic structures, electron transport properties, working function of electrodes caused by ETLs, and key relevant parameters (PCE and stability) of p-i-n PSCs are presented. Hopefully, this Review will provide fundamental guidance for future development of new organic n-type materials as ETLs for more efficient p-i-n PSCs.