Chen Weijie

Publication date: Available online 23 April 2020

Source: Joule

Author(s): Nicole Moody, Samuel Sesena, Dane W. deQuilettes, Benjia Dak Dou, Richard Swartwout, Joseph T. Buchman, Anna Johnson, Udochukwu Eze, Roberto Brenes, Matthew Johnston, Christy L. Haynes, Vladimir Bulović, Moungi G. Bawendi

The amorphous ICBA is incorporated into the host binary PM6:BTP‐4Cl system to fabricate ternary organic solar cells. Both photovoltaic efficiency and ultraviolet durability are enhanced in optimal ternary devices due to the increased photon harvesting, boosted hole transfer from BTP‐4Cl to PM6, highly efficient Förster resonance energy transfer between ICBA and BTP‐4Cl, more balanced charge transport, and more stable film morphology.

A ternary bulk‐heterojunction (BHJ) strategy that synergistically combines the merits of fullerene and nonfullerene acceptors has been regarded as a promising approach to enhance the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Herein, the fullerene derivative ICBA as the morphology regulator is incorporated into a nonfullerene‐based PBDB‐T‐2F:BTP‐4Cl (PM6:BTP‐4Cl) system to fabricate the high‐performance ternary OSCs. The amorphous ICBA prefers to homogeneously distribute in the BTP‐4Cl phase to form the well‐mixed acceptor domains due to their better miscibility, which distinctly reduces the exciton decay loss driven by the unfavorable phase separation and enhances BHJ morphology stability of ternary blends. The appropriate addition of ICBA induces the efficient long‐range Förster resonance energy transfer to BTP‐4Cl and facilitates the ultrafast hole transfer process from BTP‐4Cl to PM6, thereby contributing to charge carrier generation in the actual devices. Ultimately, the optimal ternary OSCs not only yield an average PCE higher than 16.5% but also show the superior ultraviolet photostability relative to binary control devices due to the increased harvesting of ultraviolet photons, boosted charge transfer, more balanced charge transport, and more stable nanostructural morphology. The results provide new insights to enable the simultaneously improved device performance and ultraviolet durability in state‐of‐the‐art ternary OSCs.

PM6‐Ir1.5, with an iridium complex‐incorporated polymer backbone, yields highly efficient power conversion efficiencies of over 16.5% for halogenated and non‐halogenated non‐fullerene polymer solar cells, with suitable nanoscale morphology, improved physical dynamics, and eco‐compatible processability.

The field of polymer solar cells (PSCs) has seen rapid development after the reports of high‐performance photovoltaic materials. Herein, iridium (Ir) complexes (1.5 mol%) are introduced to the polymer backbone of PM6, and a new π‐conjugated polymer PM6‐Ir1.5 is developed. Further analysis indicates that this approach can rationally modify the molecular packing order of PM6 to achieve suitable nanoscale morphology, efficient charge transport properties, and reduced carrier recombination losses, and significantly improve the photovoltaic performance of the resulting PSCs based on Y6‐C2 as non‐fullerene acceptor with good solubility in common solvents. Optimized devices obtain a power conversion efficiency (PCE) of 17.09%, whereas the corresponding devices fabricated using the green solvent achieve a remarkable PCE of 16.52%, which is the highest value reported so far in the literature for non‐halogenated PSCs.

A secondary bond‐constructed isotropic electron transfer 3D‐network is fabricated based on biomass‐derived demethylated kraft lignin (D_MeKL). Secondary bonds successfully modify the contact of the perylene diiminde/active layer and conjugate‐blocked linkages in D_MeKL, to overcome anisotropy‐aroused electron transfer barriers at the cathode interface. The enhancement of cross/vertical‐sectional electron transfer performance and well‐matched energy levels yields the highest power conversion efficiency reported among biomaterial‐based organic solar cells.

Abstract

Fabricating high‐efficient electron transporting interfacial layers (ETLs) with isotropic features is highly desired for all‐directional electron transfer/collection from an anisotropic active layer, achieving excellent power conversion efficiency (PCEs) on nonfullerene acceptor (NFA) organic solar cells (OSCs). The complicated synthesis and cost‐consumption in exploring versatile materials arouse great interest in the development of binary‐doping interlayers without phase separation and flexible manipulation. Herein, for the first time, a novel cathode interfacial layer based on biomass‐derived demethylated kraft lignin (D_MeKL) is proposed. Features of multiple phenolic‐hydroxyl (PhOH) and uniform‐distributed render D_MeKL to exhibit an excellent bonding capacity with amino terminal substituted perylene diiminde (PDIN), and successfully form a high‐efficient isotropic electron transfer 3D network. Synchronously, secondary bonds completely modify conjugate‐blocked linkages of D_MeKL, significantly enhance the electron transporting performance on cross‐section and vertical‐sections, and repair the contact of PDIN with active layer. The D_MeKL/PDIN‐based 3D‐network exhibits well‐matched work function (WF) (–4.34 eV) with cathode (–4.30 eV) and energy level of electron acceptor (–4.11 eV). D_MeKL/PDIN‐based NFAs‐OSC shows excellent short‐circuit current density (26.61 mA cm^–2) and PCE (16.02%) beyond the classic PDIN‐based NFA‐OSC (25.64 mA cm^–2, 15.41%), which is the highest PCEs among biomaterials interlayers. The results supply a novel method to achieve high‐efficient cathode interlayer for NFAs‐OSCs.

Different concentrations of iridium complexes are introduced into the conjugated backbone of polymer donor PM6 (PM6‐Ir0), this strategy can rationally modify the molecular aggregations, effectively control the blend morphology and physical mechanisms, and finally improve the photovoltaic performance. This work affords an effective approach for further breakthroughs in the reported champion power conversion efficiency of polymer solar cells.

Abstract

The commercially available PM6 as donor materials are used widely in highly efficient nonfullerene polymer solar cells (PSCs). In this work, different concentrations of iridium (Ir) complexes (0, 0.5, 1, 2.5, and 5 mol%) are incorporated carefully into the polymer conjugated backbone of PM6 (PM6‐Ir0), and a set of π‐conjugated polymer donors (named PM6‐Ir0.5, PM6‐Ir1, PM6‐Ir2.5, and PM6‐Ir5) are synthesized and characterized. It is demonstrated that the approach can rationally modify the molecular aggregations of polymer donors, effectively controlling the corresponding blend morphology and physical mechanisms, and finally improve the photovoltaic performance of the PM6‐Irx‐based PSCs. Among them, the best device based on PM6‐Ir1:Y6 (1:1.2, w/w) exhibits outstanding power conversion efficiencies (PCEs) of 17.24% tested at Wuhan University and 17.32% tested at Institute of Chemistry, Chinese Academy of Sciences as well as a certified PCE of 16.70%, which are much higher than that of the control device based on the PM6‐Ir0:Y6 blend (15.39%). This work affords an effective approach for further break through the reported champion PCE of the binary PSCs.

Herein, a novel precursor (HCOOCs and HPbX₃) for deposition of high‐quality CsPbI₂Br films, irrespective of humidity is presented. CsPbI₂Br cells prepared in an atmosphere with 30% and 91% relative humidity exhibit efficiencies of 16.1% and 15.1%, respectively, which are the highest among all inorganic CsPbX₃ (X: I, Br, or mixed halides) PSCs prepared in a medium or high humid atmosphere.

Abstract

High temperature stable inorganic CsPbX₃ (X: I, Br, or mixed halides) perovskites with their bandgap tailored by tuning the halide composition offer promising opportunities in the design of ideal top cells for high‐efficiency tandem solar cells. Unfortunately, the current high‐efficiency CsPbX₃ perovskite solar cells (PSCs) are prepared in vacuum, a moisture‐free glovebox or other low‐humidity conditions due to their poor moisture stability. Herein, a new precursor system (HCOOCs, HPbI₃, and HPbBr₃) is developed to replace the traditional precursors (CsI, PbI₂, and PbBr₂) commonly used for solar cells of this type. Both the experiments and calculations reveal that a new complex (HCOOH•Cs⁺) is generated in this precursor system. The new complex is not only stable against aging in humid air ambient at 91% relative humidity, but also effectively slows the perovskite crystallization, making it possible to eliminate the popular antisolvent used in the perovskite CsPbI₂Br film deposition. The CsPbI₂Br PSCs based on the new precursor system achieve a champion efficiency of 16.14%, the highest for inorganic PSCs prepared in ambient air conditions. Meanwhile, high air stability is demonstrated for an unencapsulated CsPbI₂Br PSC with 92% of the original efficiency remaining after more than 800 h aging in ambient air.

This review focuses on the usefulness of spatially resolved analysis of halide perovskite solar cells. Methods sensitive to open circuit voltage, short circuit current, fill factor, and cell efficiency are discussed, and the specific value of the spatial information is demonstrated in quantitative loss analyses.

Abstract

This review explores the current state of the art in spatially resolved characterization of mixed‐halide perovskite solar cells. As the size of perovskite cells and modules continues to grow, quantification of the spatial distribution of key cell parameters will become increasingly valuable in predicting ultimate cell‐level performance and tracking process homogeneity. Here, both high resolution microscopic approaches using scanning techniques and camera‐based methods for full‐area cell and/or module analysis are discussed. The value of this local data in predicting performance losses at the cell level is particularly emphasized. Measurable physical parameters sensitive to losses of voltage, current, fill factor, and efficiency are discussed together with selected experimental results. It is demonstrated that a combination of spatially resolved cell parameter mapping/imaging can be used to quantitatively discriminate various loss contributions at high resolution. The impact and control of inhomogeneities become particularly important when upscaling from small devices to large formats compatible with industrial mass production.

High‐efficiency and stable dopant‐free poly(3‐hexylthiophene) (P3HT)‐based CsPbI₂Br solar cells are achieved by introducing an optimized preannealing process to engineer the nucleation and crystallization of CsPbI₂Br films. Further incorporation of an ultrathin wide‐bandgap diphenylamine derivative layer (poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co ‐(4,4′‐(N ‐(4‐sec‐butylphenyl)diphenylamine)]) to regulate the band alignment of CsPbI₂Br and P3HT delivers a record‐high efficiency of 15.50% for dopant‐free P3HT‐based CsPbI₂Br solar cells.

Abstract

CsPbI₂Br is emerging as a promising all‐inorganic material for perovskite solar cells (PSCs) due to its more stable lattice structure and moisture resistance compared to CsPbI₃, although its device performance is still much behind this counterpart. Herein, a preannealing process is developed and systematically investigated to achieve high‐quality CsPbI₂Br films by regulating the nucleation and crystallization of perovskite. The preannealing temperature and time are specifically optimized for a dopant‐free poly(3‐hexylthiophene) (P3HT)‐based device to target dopant‐induced drastic performance degradation for spiro‐OMeTAD‐based devices. The resulting P3HT‐based device exhibits comparable power conversion efficiency (PCE) to spiro‐OMeTAD‐based devices but much enhanced ambient stability with over 95% PCE after 1300 h. A diphenylamine derivative is introduced as a buffer layer to improve the energy‐level mismatch between CsPbI₂Br and P3HT. A record‐high PCE of 15.50% for dopant‐free P3HT‐based CsPbI₂Br PSCs is achieved by alleviating the open‐circuit voltage loss with the buffer layer. These results demonstrate that the preannealing processing together with a suitable buffer layer are applicable strategies for developing dopant‐free P3HT PSCs with high efficiency and stability.

In situ optical spectroscopy during two‐step deposition of narrow bandgap Pb–Sn hybrid perovskite films reveals the film formation kinetics. Homogeneous crystallization and passivation of iodide vacancies on the perovskite surface gives solar cells with a power conversion efficiency of 16.1% at a bandgap of 1.23 eV.

Abstract

Developing efficient narrow bandgap Pb–Sn hybrid perovskite solar cells with high Sn‐content is crucial for perovskite‐based tandem devices. Film properties such as crystallinity, morphology, surface roughness, and homogeneity dictate photovoltaic performance. However, compared to Pb‐based analogs, controlling the formation of Sn‐containing perovskite films is much more challenging. A deeper understanding of the growth mechanisms in Pb–Sn hybrid perovskites is needed to improve power conversion efficiencies. Here, in situ optical spectroscopy is performed during sequential deposition of Pb–Sn hybrid perovskite films and combined with ex situ characterization techniques to reveal the temporal evolution of crystallization in Pb–Sn hybrid perovskite films. Using a two‐step deposition method, homogeneous crystallization of mixed Pb–Sn perovskites can be achieved. Solar cells based on the narrow bandgap (1.23 eV) FA_0.66MA_0.34Pb_0.5Sn_0.5I₃ perovskite absorber exhibit the highest efficiency among mixed Pb–Sn perovskites and feature a relatively low dark carrier density compared to Sn‐rich devices. By passivating defect sites on the perovskite surface, the device achieves a power conversion efficiency of 16.1%, which is the highest efficiency reported for sequential solution‐processed narrow bandgap perovskite solar cells with 50% Sn‐content.

The significant energy loss in organic solar cells mainly results from the charge transfer loss and the nonradiative recombination loss. In view of this, herein, the recent advances in energy loss reduction according to different strategies are systematically summarized. On this basis, some fundamental questions in this topic are proposed to improve future investigations.

Compared with conventional inorganic solar cells (ISCs), energy loss (E _loss) in organic solar cells (OSCs) is usually much higher, limiting their maximum achievable power conversion efficiency (PCE). In view of this, a hot topic in OSC research is how to make E _loss as low as possible. To date, in some typical organic donor/acceptor (D/A) blends, although E _loss has been reduced to the values comparable with those in ISCs, the PCEs of the corresponding devices still fails to meet expectations. One crucial issue is that the physics behind the photovoltaic process in these D/A blends and the corresponding energy loss remain unclear. Herein, combining with an analysis of the photovoltaic process in OSCs, the mechanisms of different energy loss pathways are first discussed. On this basis, the recent advances focusing on E _loss are systematically summarized according to different strategies: 1) optimizing the energy offset of the D/A blend; 2) optimizing the morphology of the D/A blend; 3) ternary modulation; and 4) spin modulation. Finally, the summary and prospects are presented, where some fundamental questions to be cleared up in the photovoltaic process are proposed, such that more targeted photovoltaic design can be carried out in the future investigations of OSCs.

The hygroscopic characteristics of dopants in 2,2′,7,7′‐tetrakis(N ,N ‐di‐p ‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐MeOTAD) hole‐transporting layers (HTLs) result in the degradation of both HTL morphology and device performance. A detailed study on the effects of initial morphology is presented. Accumulated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is the key factor causing poor stability. Performing thermal annealing on HTL can improve the air stability greatly.

Abstract

Doped 2,2′,7,7′‐tetrakis(N ,N ‐di‐p ‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐MeOTAD), which acts as a hole‐transporting layer (HTL), endows perovskite solar cells (PSCs) with excellent performance. However, the intrinsically hygroscopic nature of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dopants also aggravates the moisture instability of PSCs. In this work, the origins of the moisture instability of spiro‐MeOTAD HTLs are explored and strategies to enhance moisture resistance are proposed. After 780 h of aging in air, 52% of the initial power conversion efficiency (PCE) can be sustained by prolonging the mixing time of the precursor solution of spiro‐MeOTAD to reduce accumulated LiTFSI. In contrast, only 7% of the initial PCE remains if the precursor solution is mixed briefly. By thermally annealing an HTL to evaporate residual tBP in spiro‐MeOTAD, pinholes are completely eliminated and 65% of the initial PCE remains after the same aging time. In this study, the significance of the initial morphology of spiro‐MeOTAD HTLs on device stability is analyzed and strategies based on physical morphology for controlling PSC moisture instability induced by HTL dopants are developed.

The study presents the curious case of ionic defect migration in halide perovskites. Using photoluminescence in samples of different grain sizes coupled with molecular dynamic simulation, this study highlights the light‐induced ionic defect movement in relation to the material microstructure. In particular, it is shown that ionic defect migration is blocked by grain boundaries in methylammonium lead iodide perovskite.

Abstract

Halide perovskites are emerging as revolutionary materials for optoelectronics. Their ionic nature and the presence of mobile ionic defects within the crystal structure have a dramatic influence on the operation of thin‐film devices such as solar cells, light‐emitting diodes, and transistors. Thin films are often polycrystalline and it is still under debate how grain boundaries affect the migration of ions and corresponding ionic defects. Laser excitation during photoluminescence (PL) microscopy experiments leads to formation and subsequent migration of ionic defects, which affects the dynamics of charge carrier recombination. From the microscopic observation of lateral PL distribution, the change in the distribution of ionic defects over time can be inferred. Resolving the PL dynamics in time and space of single crystals and thin films with different grain sizes thus, provides crucial information about the influence of grain boundaries on the ionic defect movement. In conjunction with experimental observations, atomistic simulations show that defects are trapped at the grain boundaries, thus inhibiting their diffusion. Hence, with this study, a comprehensive picture highlighting a fundamental property of the material is provided while also setting a theoretical framework in which the interaction between grain boundaries and ionic defect migration can be understood.

Nature Energy, Published online: 20 April 2020; doi:10.1038/s41560-020-0602-0

Two compatible non‐fullerene acceptors with similar lowest unoccupied molecular orbital levels are finely selected to prepare efficient ternary organic solar cells (OSCs). The optimized ternary OSCs exhibit a power conversion efficiency of 15.74% and fill factor of 75.64%.

Abstract

Efficient organic solar cells (OSCs) are fabricated using polymer PM6 as donor, and IPTBO‐4Cl and MF1 as acceptors. The power conversion efficiency (PCE) of IPTBO‐4Cl based and MF1 based binary OSCs individually arrive to 14.94% and 12.07%, exhibiting markedly different short circuit current density (J _SC) of 23.18 mA cm⁻² versus 17.01 mA cm⁻², fill factor (FF) of 72.17% versus 78.18% and similar open circuit voltage (V _OC) of 0.893 V versus 0.908 V. The two acceptors, IPTBO‐4Cl and MF1, have similar lowest unoccupied molecular orbital levels, which is beneficial for efficient electron transport in the ternary active layer. The PCE of optimized ternary OSCs arrives to 15.74% by incorporating 30 wt% MF1 in acceptors, resulting from the simultaneously increased J _SC of 23.20 mA cm⁻², V _OC of 0.897 V, and FF of 75.64% in comparison with IPTBO‐4Cl based binary OSCs. The gradually increased FFs of ternary OSCs indicate the well‐optimized phase separation and molecular arrangement with MF1 as morphology regulator. This work may provide a new viewpoint for selecting an appropriate third component to achieve efficient ternary OSCs from materials and photovoltaic parameters of two binary OSCs.

Metal oxides (MO) with unique optoelectronic properties and outstanding stability are increasingly developed as effective electron transport layers (ETLs) for perovskite solar cells (PSCs). This Review focuses on the recent advances of MO ETLs from systematical synthesis to strategical optimization and provides feasible directions for future development of MO ETLs in higher‐performing PSCs.

Abstract

Organometallic mixed halide perovskite solar cells (PSCs) have emerged as a promising photovoltaic technology with increasingly improved device efficiency exceeding 24%. Charge transport layers, especially electron transport layers (ETLs), are verified to play a vital role in device performance and stability. Recently, metal oxides (MOs) have been widely studied as ETLs for high‐performance PSCs due to their excellent electronic properties, superb versatility, and great stability. This Review briefly discusses the development of PSCs' architecture and outlines the requirements for MO ETLs. Additionally, recent progress of MO ETLs from preparation to optimization for efficient PSCs is systematically summarized and highlighted to associate the versatility of MO ETLs with the performance of devices. Finally, a summary and prospectives for the future development of MO ETLs toward practical application of high‐performance PSCs are drawn.

Methylammonium thiocyanate additive‐assisted hot‐casting free deposition of a high‐quality 2D Dion–Jacobson perovskite film is reported. The optimized film exhibits high crystallinity, preferred orientation, and decreased defects. The corresponding device exhibits a maximum power conversion efficiency of 16.25%. The unsealed device retains 80% of its original efficiency after 35 days of storage in air with a humidity level of 45 ± 5%.

2D Dion–Jacobson (DJ) perovskite solar cells (PVSCs) with a high power conversion efficiency (PCE) are currently predominately fabricated via a hot‐casting process. The reason lies in the difficulty in preparing high‐quality perovskite films under mild conditions when the application of divalent ammonium removes the weak interaction from the spacer cation layer. Herein, the morphology of the 2D DJ perovskite film with a rigid piperidinium ring is tuned through a room‐temperature spin‐coating method, with the aid of a methylammonium thiocyanate (MASCN) additive. With the optimized amount of MASCN addition, the perovskite films deposited on the poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA)/poly[(9,9‐bis(30‐(N ,N‐dimethylamino)propyl)‐2,7‐uorene)‐alt‐2,7‐(9,9‐dioctylfuorene)] (PFN) substrate exhibit fine crystallinity, preferred orientation, decreased defects, and better energy‐level alignment with the hole transport layer. The device with the inverted planar structure presents a J _SC of 17.91 mA cm⁻², V _OC of 1.19 V, fill factor of 0.76, with a maximum PCE of 16.25%, which is the highest PCE for 2D DJ PVSCs free of hot casting. The unsealed device maintains around 80% of its initial efficiency after 35 days of exposure to air (Hr = 45 ± 5%). A potential route toward high‐performance 2D DJ PVSCs is provided.