Chen Weijie

Publication date: 16 March 2022

Source: Joule, Volume 6, Issue 3

Author(s): Lingling Zhan, Shuixing Li, Yaokai Li, Rui Sun, Jie Min, Zhaozhao Bi, Wei Ma, Zeng Chen, Guangqing Zhou, Haiming Zhu, Minmin Shi, Lijian Zuo, Hongzheng Chen

The pure-phase FAPbI₃ perovskite material has not been prepared and the related properties are not deeply understood yet. This review summarizes the development and challenges of FA-based perovskite from component engineering to pure-phase perovskite technology and proposes a strategy for the development of pure-phase FAPbI₃ perovskite. This helps researchers to deeply understand the properties of FA-based perovskites.

Formamidinium lead triiodide (FAPbI₃) with a narrow bandgap, broad light absorption spectra, and high thermal stability has emerged as one of the promising active materials for perovskite solar cells. To date, the certified power conversion efficiency of FAPbI₃-based solar cells has reached 25.7%, comparable with that of monocrystalline silicon solar cells (26.7%). However, FAPbI₃ tends to form an undesirable metastable nonperovskite phase (α-FAPbI₃), which is the most fatal issue for the commercialization development of FAPbI₃-based perovskite solar cells. Many efforts are committed to stabilizing the α-FAPbI₃ phase. In this review, the strategies involving composition engineering in A-site (including double-cation, triple-cation, quadruple-cation systems) and X-site ions (halides and pseudohalides) to stabilize FA-based perovskites are summarized. To realize higher efficiencies and avoid the increase in bandgap and phase segregation issue induced by the multicomponent elements, the corresponding strategies for preparing a pure α-FAPbI₃ perovskite with various functional materials are discussed. Moreover, the perovskite crystal redissolution strategy to prepare perovskite films with high purity, precise stoichiometric ratio, high crystallinity, ideal crystal orientation, and low defect density is described for highly efficient FAPbI₃-based perovskite solar cells. Finally, the perspective for future research directions toward highly reproducible and large-area FAPbI₃-based photovoltaics is raised.

Two oligothiophene-based donor-acceptor polymers, PTTz-3HD and PTTz-4HD, are developed for use as electron donors in organic solar cells (OSCs). A prominent power conversion efficiency of 16.7% is achieved by PTTz-3HD, which suggests the renaissance of oligothiophene-based polymers in OSCs and opens a promising avenue to access high-efficiency OSCs from low-cost polymers.

Abstract

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have increased rapidly owing to the development of non-fullerene acceptors (NFAs). However, the development of polymer donors lags behind significantly. Currently, the polymer donors are dominated by a handful of thiophene-substituted benzo[1,2-b:4,5-b']dithiophene (BDTT) polymers, which suffer from lengthy synthesis and high production cost. Compared with BDTT-based polymers, oligothiophene-based donor-acceptor polymers feature much easier synthesis, which were the prevailing polymer donors in fullerene-based OSCs, but almost disappeared in non-fullerene OSCs. Herein, two oligothiophene-based donor-acceptor polymers (PTTz-3HD and PTTz-4HD) are reported to re-evaluate this kind of polymer in non-fullerene OSCs. Benefiting from the exquisite alkyl chain design, the polymer PTTz-3HD exhibits more planar conformation, stronger aggregation, and higher crystallinity, which in turn contributes to the formation of an optimal active layer morphology when blended with NFA. As a result, a PCE of 16.1% and 16.7% is achieved by PTTz-3HD in binary and ternary OSCs, respectively. Of particular note, the product of short-circuit current density and fill factor of PTTz-3HD is fully comparable to those of BDTT-based polymers. These results suggest the renaissance of oligothiophene-based donor-acceptor polymers in OSCs and demonstrate a promising avenue to access high-efficiency OSCs from low-cost materials.

Poly(ionic-liquid)s (PILs) are employed to construct an “ionic polymer network” in perovskites for defect passivation and ion immobilization. The device that incorporates quaternary ammonium-based PIL perfectly retains dark current characteristics during a cooling-heating (−40–85 °C) process. The corresponding perovskite solar cells maintain 80% of their original efficiency under either 1500 h light-soaking or 300 h thermal stress (85 °C).

Abstract

Longevity is a key constraint for hybrid perovskite based photovoltaics. Here it is demonstrated that ion migration-induced degradation can be eliminated by incorporation of multifunctional poly(ionic-liquid)s (PILs) additives, resulting in ultrastable perovskite solar cells (PVSCs). The presence of PILs suffices to construct an “ionic polymer network,” providing the functionalities of defect passivation and ion immobilization by concurrently forming a physical barrier and chemical bonding. Compared with the defect passivation effect for the imidazolium-based PIL (PIL-Im) case, the quaternary ammonium-based PIL (PIL-Am) shows a higher interaction energy with the perovskite due to the stronger electronic coupling ascribed to the additional complexation, which endows the corresponding perovskite with higher migration energy for iodide ions. As a result, the power conversion efficiency (PCE) of anion-cation-mixed hybrid n-i-p PVSCs with PIL-Am is remarkably improved from 20.26% to 22.22%. Specifically, the PILs-modified device perfectly retains its dark current characteristics upon a cooling (−40 °C)–heating (85 °C) process. The unencapsulated PIL-Am stabilized PVSC maintains 80% of the initial PCE under AM 1.5G light soaking for nearly 1500 h. The corresponding device also displays pronounced stability under thermal stress or realistic operation conditions.

Y6 passivation layer can enhance SnO₂ ETL durability during bending processes. The SnO₂/Y6-based FPSCs achieve a PCEof 20.09% and retain over 80% of their initial efficiency after 1000 bending cycles at a curvature radius of 8 mm, while SnO₂-based devices only retain 60% of their initial PCE (18.60%) upon the same bending cycles.

Abstract

Flexible perovskite solar cells (FPSCs) represent a promising technology in the development of next-generation photovoltaic and optoelectronic devices. SnO₂ electron transport layers (ETL) typically undergo significant cracking during the bending process of FPSCs, which can significantly compromise their charge transport properties. Herein, the semi-planar non-fullerene acceptor molecule Y6 (BT-core-based fused-unit dithienothiophen [3,2-b]-pyrrolobenzothiadiazole derivative) is introduced as the buffer layer for SnO₂-based FPSCs. It is found that the Y6 buffer layer can enhance the ability of charge extraction and bending stability for SnO₂ ETL. Moreover, the internal stress of perovskite films is also reduced. As a result, SnO₂/Y6-based FPSCs achieved a power conversion efficiency (PCE) of 20.09% and retained over 80% of their initial efficiency after 1000 bending cycles at a curvature radius of 8 mm, while SnO₂-based devices only retain 60% of their initial PCE (18.60%) upon the same bending cycles. In addition, the interfacial charge extraction is also effectively improved in conjunction with reduced defect density upon incorporation of Y6 on the SnO₂ ETL, as revealed by femtosecond transient absorption (Fs-TA) measurements.

A highly efficient semitransparent organic photovoltaic system with excellent natural color perception is successfully constructed by employing a synergy of ternary strategy and optical engineering.

Abstract

Power conversion efficiency (PCE) and color rendering index (CRI) are two important parameters for realizing the potential application of semitransparent organic photovoltaics (ST-OPVs) into the field of building-integrated photovoltaics (BIPVs). Herein, to extend the PCE limit while showing desirable neutral-colored perception, formally in a trade-off relationship, of ST-OPVs, new ternary photoactive layers composed of polymer donor PM6-Ir1, non-fullerene acceptor BTP-eC9, and fullerene acceptor PC₇₁BM as the third component are employed, which are effective to improve opaque device efficiency while modulating CRI in relevant ternary blends. Furthermore, multifunctional ST-OPVs are realized via simultaneously optimizing the content of PC₇₁BM in acceptors and employing a simple photonic reflector, leading to not only excellent neutral color perception with a CRI of 96.5 but also a PCE of 14.09% with an average visible transmittance higher than 20%. Vivid backgrounds can be clearly seen through the 1-DBR-based ternary ST-OPVs. Overall, these results represent the best-performing multifunctional ST-OPVs, and this synergistic effect can pave the road for ST-OPVs as promising BIPVs of high device performance and good color neutrality.

A grain boundary stress release strategy is proposed for high-stability flexible perovskite indoor photovoltaics by the grain boundary penetration with borax 3D stretchable molecules. The full-dimensional grain boundary stress release enables the flexible perovskite photovoltaics deliver a champion power conversion efficiency (PCE) of 21.63% under AM 1.5G illumination and an indoor PCE of 31.85% under 1062 lux.

Abstract

Perovskite photovoltaics are strong potential candidates to drive low-power off-grid electronics for indoor applications. Compared with rigid devices, flexible perovskite devices can provide a more suitable surface for indoor small electronic devices, enabling them have a broader indoor application prospect. However, the mechanical stability of flexible perovskite photovoltaics is an urgent issue solved. Herein, a kind of 3D crosslinking agent named borax is selected to carry out grain boundary penetration treatment on perovskite film to realize full-dimensional stress release. This strategy improves the mechanical and phase stabilities of perovskite films subjected to external forces or large temperature changes. The fabricated perovskite photovoltaics deliver a champion power conversion efficiency (PCE) of 21.63% under AM 1.5G illumination, which is the highest one to date. The merit of low trap states under weak light makes the devices present a superior indoor PCE of 31.85% under 1062 lux (LED, 2956 K), which is currently the best flexible perovskite indoor photovoltaic device. This work provides a full-dimensional grain boundary stress release strategy for highly stable flexible perovskite indoor photovoltaics.

Publication date: 1 June 2022

Source: Nano Energy, Volume 96

Author(s): Yipeng Han, Guang Zhang, Haibing Xie, Tengfei Kong, Yahong Li, Yang Zhang, Jing Song, Dongqin Bi

In this study, an effective strategy is developed to dissolve a defective layer with a stoichiometric ratio and expose a fresh surface with high crystallinity and lattice continuity. The champion cell delivers a power conversion efficiency and V _OC of 17.51% and 1.37 V, respectively, which renders it one of the best wide-bandgap perovskite cells.

Abstract

The existence of a defective area composed of nanocrystals and amorphous phases on a perovskite film inevitably causes nonradiative charge recombination and structural degradation in perovskite photovoltaics. In this study, a stoichiometric etching strategy for the top surface of a defective cesium lead halide perovskite is developed by using ionic liquids. The dissolution of the original defective area substantially exposes the underlying perovskite, which is a high-quality surface with retained stoichiometry and lattice continuity. The ionic liquid molecules are adsorbed on the perovskite surface via Coulombic interactions and passivate the undercoordinated surface lead centers. Such a structural modulation considerably reduces the trap density of the perovskite devices and enables a record power conversion efficiency of 17.51% and an open-circuit voltage of 1.37 V of the CsPbI₂Br cell with a perovskite bandgap of 1.88 eV. This work provides a novel technical route to improve the efficiency and environmental resilience of perovskite-based optoelectronic devices.

Two donor–acceptor (DA) covalent organic frameworks (COFs) show high crystallinity, good porosity, and excellent stability, which are incorporated into the FAPbI₃ layer of perovskite solar cells. The highest power-conversion efficiency observed for perovskite solar cells constructed with DA-COFs is 23.19% with excellent humidity stability, which provides a pathway for using DA-COFs to fabricate perovskite solar cells with high efficiency and stability.

Abstract

Covalent organic frameworks (COFs) as a new class of crystalline, porous materials have attracted extensive attention in the fields of photocatalytic and photovoltaic applications. Generally, donor–acceptor (DA) structures play an important role in the charge separation efficiency of solar cells. In this study, two DA-COFs with high crystallinity, good porosity, and excellent stability are incorporated into the FAPbI₃ layer of perovskite solar cells. This addition of DA-COFs reduces the defect concentration and shallows the defect state. The donor–acceptor system in COFs also possesses strong charge-transfer pathway, which strongly prevents charge recombination to afford more efficient charge separation efficiency. The highest power-conversion efficiency of perovskite solar cells constructed with DA-COFs is 23.19% with excellent humidity stability of the solar cells. Therefore, this work provides a pathway for using DA-COFs to fabricate perovskite solar cells with higher efficiency and stability.

The low molecular weight (M _w) issue associated with the polymer solar cells has mostly been resolved by utilizing the tetraphenylethylene (TPE) as a ternary component in the PM6:Y6 binary system with varying donor molecular weights. Consequently, the inclusion of TPE enables the low M _w ternary systems to produce power conversion efficiencies comparable to that of standardized high M _w binary devices.

Despite producing the best device performances, several major issues such as the batch-to-batch variation and low molecular weight (M _w) optimization greatly hinder the progress for the organic solar cells. Herein, tetraphenylethylene (TPE) as a third component has been added in a series of PM6:Y6 blends with different donor molecular weights (42–129 kDa). Ultimately, ternary devices fabricated by the PM6 with about half the M _w (66 and 74 kDa) of the standard donor (129 kDa) produce an excellent power conversion efficiency (PCE) of 15.09% and 15.15% owing to enhanced morphology and charge mobility, as compared to a standard binary PCE of 15.65%, respectively, indicating the possibility of attaining high-performance attributes from low M _w polymers. Furthermore, the TPE inclusion in the 129 kDa binary blend leads to a remarkable PCE of 16.48%. This strategy, therefore, illustrates a relatively simple approach to overcome the batch-to-batch variation problem associated with polymers for organic solar cells.

Surface-confined monolayers (SCMs) of conducting polymers (CPs) have shown great promise as electrode materials in solar cell applications, owing to their outstanding operational stability, high reliability, and organized structures. Herein, the general features, anchoring mechanism, grafting approaches, and photovoltaic properties of CP SCMs are discussed. Then, recent advances and future perspectives of CP SCMs in different applications are summarized.

π-conjugated polymer (CPs)-functionalized semiconductor surfaces are important components of photovoltaic (PV) devices. In this context, the use of surface-confined monolayers (SCMs) of CPs has attracted great attention due to their operational stability, reliability, and excellent control over the nanostructure and morphology of CP films. The CPs with proper anchoring groups are covalently bonded to the semiconductor surface driven by the interactions between anchors and substrate, forming densely packed and orderly assembled polymer chains in thin films. The nanostructure of CP SCMs is highly dependent on the type and position of anchors, polymer structure, and chain length, as well as the film-forming techniques. Herein, general features, anchoring mechanism, grafting approaches, and PV properties of CP SCMs are described and analyzed. Then, the applications of CP monolayers as photosensitizers for dye sensitized solar cells and hole transport interlayers for bulk-heterojunction polymer solar cells and perovskite solar cells are summarized. With the structure tunability, future study directions for CP SCMs in terms of structure effect, assembly states, and efficiency enhancement are discussed.

The use of the dual-solvent-assisted sequential spin-coating method for precisely tuning active layer morphology to fabricate high efficient and stable organic solar cells is presented. Without any post-treatment, D18-Cl (THF + CB)/Y6-based devices achieved an efficiency of up to 17.73%, thus facilitating the future commercial production of organic solar cells.

The precise tuning of the active layer morphology to improve organic solar cells (OSCs) efficiency remains a key issue in the field of organic photovoltaics. Herein, a new solution to the above problem is provided by using the dual-solvent modulated polymer-assisted sequential spin-coating method. Herein, the sequential spin-coated OSCs based on the D18-Cl/Y6 system are prepared for the first time and an efficiency of 16.38% is obtained, similar to that of bulk heterojunction OSCs. On this basis, the performance is further improved by using a dual solvent to balance the dissolution and crystallization of D18-Cl, separately optimizing the morphology of the donor layer and allowing the subsequent spin-coated Y6 solution to penetrate uniformly into the D18-Cl framework. After the dual-solvent treatment, the D18-Cl (CF + CB)/Y6-based device obtains a power conversion efficiency (PCE) of 17.33% and the D18-Cl (THF + CB)/Y6-based device achieves an even better PCE of 17.73%. It is worth noting that no post-treatment is adopted here and after 2500 h of placement, the efficiency of the aforementioned devices is still 90% of the original. Thus, this work provides a simple method for tuning the film morphology to prepare efficient and stable devices, which is beneficial for future commercial production of OSCs.

A seed-assisted crystallization approach is demonstrated through addition of alkali salts for enabling homogeneous and highly crystalline large-area perovskite films via scalable slot-die coating technique. The slot-die coated methylammonium-free perovskite module with an active area of 57.5 cm² shows an efficiency of 16.22% and retains 82% of its initial efficiency after 4800 h under 30% RH without encapsulation.

Abstract

Typical fabrication methods for laboratory-scale (<1 cm²) perovskite solar cells (PSCs) are undeniably not scalable and the control of crystallization of large-area perovskite layer for commercial sized modules is also particularly challenging. Here, a seed-assisted crystallization approach is demonstrated through addition of alkali salts, CsPbBr₃ and KPb₂Br₅, to the perovskite precursor ink for enabling homogeneous and highly crystalline large-area Cs_0.15FA_0.85Pb(I_0.83Br_0.17)₃ (CsFA) perovskite films via scalable slot-die coating technique. X-ray photoelectron spectroscopy analysis reveals the segregation of potassium ions at SnO₂/perovskite interface which serve as nucleation sites for the crystallization of perovskite layer. The uniformly slot-die coated CsFA films (100 cm²) from the additives containing precursor inks possess larger grains with enhanced optoelectronic properties and the corresponding devices display higher reproducibility and consistency. A champion device efficiency of 18.94% under 1 sun illumination for slot-die coated PSCs in n-type/intrinsic/p-type structure is demonstrated with improved stability with 82% of its initial efficiency tested at 65 °C for 1150 h. The slot-die coated methylammonium-free perovskite module with an active area of 57.5 cm² shows an efficiency of 16.22% and retains 82% of its initial efficiency after 4800 h under 30% relative humidity without encapsulation.