Chen Weijie

A recast strategy is proposed to optimize the spatial distribution of components in organic bulk‐heterojunction (BHJ) films in an all‐inorganic perovskite/BHJ integrated solar cells, leading to extended photoresponse, enhanced ambipolar charge transport, and suppressed charge carrier recombination. A record power conversion efficiency of 11.08% and robust thermal stability are obtained.

Abstract

All‐inorganic CsPbIBr₂ perovskite solar cells (pero‐SCs) exhibit excellent overall stability, but their power conversion efficiencies (PCEs) are greatly limited by their wide bandgaps. Integrated solar cells (ISCs) are considered to be an emergent technology that could extend their photoresponse by directly stacking two distinct photoactive layers with complementary bandgaps. However, rising photocurrents always sacrifice other photovoltaic parameters, thereby leading to an unsatisfactory PCE. Here, a recast strategy is proposed to optimize the spatial distribution components of low‐bandgap organic bulk‐heterojunction (BHJ) film, and is combined with an all‐inorganic perovskite to construct perovskite/BHJ ISCs. With this strategy, the integrated perovskite/BHJ film with a top‐enriched donor‐material spatial distribution is shown to effectively improve ambipolar charge transport behavior and suppress charge carrier recombination. For the first time, the ISC is not only significantly extended and enhanced the photoresponse achieving a 20% increase in current density, but also exhibits a high open‐circuit voltage and fill factor at the same time. As a result, a record PCE of 11.08% based on CsPbIBr₂ pero‐SCs is realized; it simultaneously shows excellent long‐term stability against heat and ultraviolet light.

Three homologous small molecule donors with hydrogen, fluorine, and chlorine substitution afford organic solar cells with efficiencies over 10% in combination with a common acceptor. The chlorinated derivative exhibits a more crystalline nanomorphology with relatively pure domains and provides more than 12% efficiency.

Abstract

Compared to conjugated polymers, small‐molecule organic semiconductors present negligible batch‐to‐batch variations, but presently provide comparatively low power conversion efficiencies (PCEs) in small‐molecular organic solar cells (SM‐OSCs), mainly due to suboptimal nanomorphology. Achieving precise control of the nanomorphology remains challenging. Here, two new small‐molecular donors H13 and H14, created by fluorine and chlorine substitution of the original donor molecule H11, are presented that exhibit a similar or higher degree of crystallinity/aggregation and improved open‐circuit voltage with IDIC‐4F as acceptor. Due to kinetic and thermodynamic reasons, H13‐based blend films possess relatively unfavorable molecular packing and morphology. In contrast, annealed H14‐based blends exhibit favorable characteristics, i.e., the highest degree of aggregation with the smallest paracrystalline π–π distortions and a nanomorphology with relatively pure domains, all of which enable generating and collecting charges more efficiently. As a result, blends with H13 give a similar PCE (10.3%) as those made with H11 (10.4%), while annealed H14‐based SM‐OSCs have a significantly higher PCE (12.1%). Presently this represents the highest efficiency for SM‐OSCs using IDIC‐4F as acceptor. The results demonstrate that precise control of phase separation can be achieved by fine‐tuning the molecular structure and film formation conditions, improving PCE and providing guidance for morphology design.

Publication date: November 2020

Source: Nano Energy, Volume 77

Author(s): Ahra Yi, Sangmin Chae, Sejeong Won, Hyun-June Jung, In Hwa Cho, Jae-Hyun Kim, Hyo Jung Kim

A promising route to widespread deployment of photovoltaics is to harness inexpensive, highly-efficient tandems. We perform holistic life cycle assessments on the energy payback time, carbon footprint, and environmental impact scores for perovskite-silicon and perovskite-perovskite tandems benchmarked against state-of-the-art commercial silicon cells. The scalability of processing steps and materials in the manufacture and operation of tandems is considered. The resulting energy payback time and greenhouse gas emission factor of the all-perovskite tandem configuration are 0.35 years and 10.7 g CO₂-eq/kWh, respectively, compared to 1.52 years and 24.6 g CO₂-eq/kWh for the silicon benchmark. Prolonging the lifetime provides a strong technological lever for reducing the carbon footprint such that the perovskite-silicon tandem can outcompete the current benchmark on energy and environmental performance. Perovskite-perovskite tandems with flexible and lightweight form factors further improve the energy and environmental performance by around 6% and thus enhance the potential for large-scale, sustainable deployment.

Publication date: 16 September 2020

Source: Joule, Volume 4, Issue 9

Author(s): Yihua Chen, Shunquan Tan, Nengxu Li, Bolong Huang, Xiuxiu Niu, Liang Li, Mingzi Sun, Yu Zhang, Xiao Zhang, Cheng Zhu, Ning Yang, Huachao Zai, Yiliang Wu, Sai Ma, Yang Bai, Qi Chen, Fei Xiao, Kangwen Sun, Huanping Zhou

A high‐performance all‐polymer solar cell (PCE of 12.06%) is achieved based on a novel polymer acceptor with a voltage loss of 0.52 eV, which is one of the smallest values reported for all‐polymer solar cells to date.

Abstract

Although the field of all‐polymer solar cells (all‐PSCs) has seen rapid progress in device efficiencies during the past few years, there are limited choices of polymer acceptors that exhibit strong absorption in the near‐IR region and achieve high open‐circuit voltage (V _OC) at the same time. In this paper, an all‐PSC device is demonstrated with a 12.06% efficiency based on a new polymer acceptor (named PT‐IDTTIC) that exhibits strong absorption (maximum absorption coefficient: 2.41 × 10⁵ cm⁻¹) and a narrow optical bandgap (1.49 eV). Compared to previously reported polymer acceptors such as those based on the indacenodithiophene (IDT) core, the indacenodithienothiophene (IDTT) core has further extended fused ring, providing the polymer with extended absorption into the near‐IR region and also increases the electron mobility of the polymer. By blending PT‐IDTTIC with the donor polymer, PM6, a high‐efficiency all‐PSC is achieved with a small voltage loss of 0.52 V, without sacrificing J _SC and FF, which demonstrates the great potential of high‐performance all‐PSCs.

Publication date: 16 September 2020

Source: Joule, Volume 4, Issue 9

Author(s): Yehao Deng, Zhenyi Ni, Axel F. Palmstrom, Jingjing Zhao, Shuang Xu, Charles H. Van Brackle, Xun Xiao, Kai Zhu, Jinsong Huang

A novel strategy of tuning perovskite crystal orientation toward ≈45° inclination with respect to the substrate is proposed with incorporating 2,3‐diaminopropionic acid monohydrochloride (2,3‐DAPAC) into FASnI₃, which facilitates charge transport in the perovskite film from bottom to top. The solar cells with 2,3‐DAPAC acquire a champion power conversion efficiency of 7.23% and improved stability.

Despite a higher power conversion efficiency (PCE) than other lead‐free perovskite solar cells (PSCs) due to intrinsically excellent optoelectronic properties and suitable bandgaps of tin (Sn) perovskites, Sn‐based PSCs still suffer from issues of stability and efficiency for practical applications. Herein, a novel strategy of tuning perovskite crystal orientation toward ≈45° with respect to the substrate by doping 2,3‐diaminopropionic acid monohydrochloride (2,3‐DAPAC) into formamidinium tin iodide (FASnI₃) is proposed, which facilitates charge transport in the perovskite film and consequent device performances. In addition, the incorporation of 2,3‐DAPAC into FASnI₃ enables dense and smooth high‐quality perovskite films with less Sn vacancies. Applications of the 2,3‐DAPAC‐treated FASnI₃ films into PSCs acquire a champion PCE of 7.23%, showing 37.2% enhancement compared with 5.27% of the control device. Moreover, the storage stabilities of both perovskite films and PSCs are significantly prolonged with improved film quality.

Passivation is like a pair of magic hands, which can heal defective perovskite cubes to a perfect light absorption layer. Herein, the origin of various defects as well as their detrimental effects on perovskite solar cells (PSCs) performance and the targeted passivation strategies for specific defects are summarized. Finally, the future development trend on passivation is provided.

With a certificated record efficiency of 25.2%, organometal halide perovskite (OHP) solar cells have experienced unprecedentedly rapid development in the past decade due to their extraordinary photoelectronic properties. However, because of the rapid processing conditions and complex precursor compositions, there are a large number of defects in polycrystalline OHP films, including point defects and 2D defects along grain boundary and on the surface. Unfortunately, these defects serve as the nonradiative recombination centers and exert negative effects on the degradation and performance of OHP layers, heavily limiting their further application for efficient photovoltaic devices. Herein, the formation origin of various defects as well as their detrimental effects on the efficiency and stability of perovskite solar cells (PSCs) are discussed, and recent passivation strategies for specific defects to minimize defect state density in the perovskite films are summarized. Finally, a brief outlook on the development trend of future passivation engineering is provided for deeper understanding of efficient and stable PSCs.

A new fluorinated organic ammonium halide salt, 4‐trifluoromethyl phenethylammonium iodide (CFPEAI), is utilized to passivate the surface of CsPbI₂Br perovskite for solar cells with enhanced efficiency as well as improved stability.

Surface modification is demonstrated as an efficient strategy to enhance the efficiency and stability of perovskite solar cells (PVSCs). Fluorinated organic ammonium salts featuring a strong hydrophobic nature are seldom used as passivation agents for the surface modification of CsPbI₂Br perovskites. Herein, a fluorinated organic ammonium halide salt, 4‐trifluoromethyl phenethylammonium iodide (CFPEAI), is incorporated into the surface of CsPbI₂Br perovskite for the first time. After the CFPEAI modification, the defects of CsPbI₂Br perovskite are significantly passivated with reduced trap densities. The best‐performance PVSC with CFPEAI modification shows an excellent power conversion efficiency (PCE) of 16.07% with a high fill factor (FF) of 84.65%, a short‐circuit current density (J _SC) of 15.45 mA cm⁻², and an open‐circuit voltage (V _OC) of 1.23 V. In contrast, the control PVSCs without the surface modification exhibit a lower PCE of 14.50% with a FF of 80.56%, a J _SC of 15.05 mA cm⁻², and a V _OC of 1.20 V. With CFPEAI passivation, the CsPbI₂Br perovskite film exhibits enhanced hydrophobicity, thereby leading to improved stability for the corresponding PVSC in comparison with the control PVSC without any surface modification.

A zwitterionic conjugated polyelectrolyte presents high hole mobility, compatible covalence level, and the ability for passivating surface defects of the perovskite film. The formation of a weak double‐layer capacitance, which is not strong enough to induce the migration of MA⁺ ions, contributes to low carrier transport resistance and interfacial charge accumulation, leading to high efficiency and stability.

Achieving rapid extraction and equivalent transport of charge carriers is an effective way to improve the performance of perovskite solar cells (PSCs). Herein, a thiophene‐based zwitterionic conjugated polyelectrolyte (poly(5‐amino‐5‐carboxy‐3‐oxapentyl)‐2,5‐thiophene [POWT]) is introduced into PSCs as a hole‐transporting and interfacial material. The polyelectrolyte has a high hole mobility of 5.74 × 10⁻³ cm² V⁻¹ s⁻¹ (similar to that of poly(triarylamine) [PTAA]) and compatible covalence level relative to the perovskite. Terminated with a zwitterionic pair of a‐amino acid, POWT layer builds up a weak double‐layer capacitance at the interface, which is not strong enough to induce the migration of MA⁺ ions in the perovskite layer. Deep electrical study on the PSC with the structure of indium tin oxide (ITO)/POWT/FA_0.2MA_0.8PbI_2.9Br_0.1/C₆₀/bathocuproine (BCP)/Ag discloses that the device has low carrier transfer resistance, low leakage current density, and minor interfacial charge accumulation. The open‐circuit voltage and the short‐circuit current density are much improved, and the power conversion efficiency (PCE) is up to 17.5%. With a‐amino acid zwitterions, POWT passivates the surface charge defects and grain boundaries of the perovskite film. The PSC presents negligible hysteresis and high stability. After 56 days, the unencapsulated PSC still remains at 85% of the original efficiency.

Nature, Published online: 29 July 2020; doi:10.1038/s41586-020-2526-z

A homochiral molecular ferroelectric was incorporated into a perovskite film to enlarge the built‐in electric field of the perovskite solar cell (PSC), thereby facilitating charge separation and transportation. The molecular ferroelectric component of the PSC passivates the defects in the perovskite active layers to induce an approximately eightfold enhancement in photoluminescence intensity and a reduction in electron trap‐state density.

Abstract

The nonradiative recombination of electrons and holes has been identified as the main cause of energy loss in hybrid organic–inorganic perovskite solar cells (PSCs). Sufficient built‐in field and defect passivation can facilitate effective separation of electron–hole pairs to address the crucial issues. For the first time, we introduce a homochiral molecular ferroelectric into a PSC to enlarge the built‐in electric field of the perovskite film, thereby facilitating effective charge separation and transportation. As a consequence of similarities in ionic structure, the molecular ferroelectric component of the PSC passivates the defects in the active perovskite layers, thereby inducing an approximately eightfold enhancement in photoluminescence intensity and reducing electron trap‐state density. The photovoltaic molecular ferroelectric PSCs achieve a power conversion efficiency as high as 21.78 %.

A series of copolymers via a random copolymerization approach are designed and synthesized. The well‐defined fibril interpenetrating morphology with appropriate phase separation in PT2‐based blends can efficiently suppress the unfavorable aggregation, resulting in excellent morphological stability and high efficiency. The work demonstrates the importance of optimization of fibril network morphology in realizing high‐efficiency and ambient‐stable polymer solar cells.

Abstract

Morphological stability is crucially important for the long‐term stability of polymer solar cells (PSCs). Many high‐efficiency PSCs suffer from metastable morphology, resulting in severe device degradation. Here, a series of copolymers is developed by manipulating the content of chlorinated benzodithiophene‐4,8‐dione (T1‐Cl) via a random copolymerization approach. It is found that all the copolymers can self‐assemble into a fibril nanostructure in films. By altering the T1‐Cl content, the polymer crystallinity and fibril width can be effectively controlled. When blended with several nonfullerene acceptors, such as TTPTT‐4F, O‐INIC3, EH‐INIC3, and Y6, the optimized fibril interpenetrating morphology can not only favor charge transport, but also inhibit the unfavorable molecular diffusion and aggregation in active layers, leading to excellent morphological stability. The work demonstrates the importance of optimization of fibril network morphology in realizing high‐efficiency and ambient‐stable PSCs, and also provides new insights into the effect of chemical structure on the fibril network morphology and photovoltaic performance of PSCs.

An industry compatible slot‐die coating process combined with near‐infrared irradiation heating enables rapid manufacture of large‐area and uniform perovskite solar cells in air. The highest power conversion efficiency for a device, which is fabricated using the slot‐die coated four layer, is nearly 11%.

Abstract

Currently, high‐efficiency perovskite solar cells are mainly fabricated by the spin‐coating process, which limits the possibility of commercial mass‐production of perovskite solar cells. In this work, the slot‐die coating process is combined with near‐infrared irradiation heating to quickly manufacture perovskite solar cells in air. The composition of the perovskite precursor solution is tuned by adding n‐butanol, with its low boiling point and low surface tension, to increase the near‐infrared energy absorption, facilitate the evaporation of the solvent system and film formation, and accelerate the crystallization of perovskite. High‐quality uniform perovskite film can be prepared within 18 s. Moreover, the all slot‐die coating process is demonstrated to prepare over an area of 12 cm × 12 cm, four layers of uniform film overlay on top of each other for the devices except electrode in ambient air. A power conversion efficiency of ≈11% is achieved when this all slot‐die coated film is used to fabricate device. This facile process can greatly reduce the cost, time and bypass post‐annealing to fabricate high‐efficiency large‐area perovskite solar cells in ambient air.

Understanding the chemical structures of next‐generation small molecules will be a critical step for increasing the performance of organic photovoltaics (OPVs). This study reveals the importance of core structure on the device performance, and provides guidelines for the design of new materials for OPV technologies.

Understanding the chemical structures of next‐generation small molecules is a critical step for increasing the performance of organic photovoltaics (OPVs); an OPV's small molecule determines not only the extent of light absorption but also the morphology. Herein, four small molecules featuring different cores—indaceno dithiophene, dithienoindeno indaceno dithiophene (IDTT), substituted IDTT, and dithienothiophene‐pyrrolobenzothiadiazole—denoted as ID‐4Cl, IT‐4Cl, m‐ITIC‐OR‐4Cl, and Y7, respectively, are selected to form active layers with poly(quinoxaline) (PTQ10) and poly(benzodithiophene‐4,8‐dione) (PM6). The Y7 devices exhibit the best performance in both systems, with the power conversion efficiency (PCE) reaching 14.5%; in comparison, ID‐4Cl device gives a PCE of 10.0% for blending with PTQ10 and a relative efficiency enhancement of 45%. The same trend occurs for the cases of PM6 blend devices. This enhancement is attributed to i) the improved short‐circuit current density that is provided by the greater degree of conjugation in S, N‐heteroarenes ladder‐type fused‐ring cores of Y7, ii) an induced face‐on Y7 orientation and smaller domain sizes that result from the sp²‐hybridized nitrogen side chain, and iii) smaller energy loss. This study reveals the importance of the core structure on the device performance and provides guidelines for the design of new materials for OPV technologies.

Four new acceptors with different quinoid effects are prepared for solution‐processed organic solar cells. The excessive azole rings and weak steric repulsion are found to obviously decrease the stabilities, and the stabilities are remarkably reduced in solution state. Finally, by reducing the quinoid effects, the power conversion efficiencies of the devices are greatly boosted from 0.05% to 8.61%.

Although benzoazole‐fused rings with strong quinoid character have successfully been used to construct high‐performance small‐molecule non‐fullerene acceptors (NFAs), studies into how these units influence the stabilities of NFAs and their corresponding device performances are few to date. To address it, four new NFAs, SSTI, SNTI, NTI and NTTI, which adopt BBT, TBZ, and BTAZ as the cores, respectively, are designed and investigated. It is found that SSTI and SNTI based on BBT and TBZ cores with stronger quinoid resonance effects show features of more red‐shifted absorptions and deeper energy levels, but worse thermal and light stabilities than NTI and NTTI with a BTAZ core, especially in solutions and/or films blended with polymer donors. Through matrix‐assisted laser desorption ionization time of flight mass spectrometry analysis of the degradation products, it is disclosed that the CC double bond cleavage would be accelerated by stronger quinoid effects. Therefore, NTI and NTTI with relatively weaker quinoid characteristics show improved photovoltaic properties. Especially, NTTI based devices yield a good efficiency of 8.61% as the side chains on sp3‐hybrid C atoms can prevent the formation of large aggregates. These findings can provide invaluable knowledge for the molecular design of NFAs with both high‐efficiency and high‐stability