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Two fully non‐fused acceptors are precisely designed and easily prepared. The side chain encapsulation can induce a planar molecular backbone conformation, endowing the acceptor with broad light absorption. Thermal annealing promotes molecular rearrangement to form J‐aggregates with even broader absorption and higher absorption coefficient. A PCE over 10 % is one of the highest PCE for fully non‐fused ring acceptors.

Abstract

Fused‐ring electron acceptors have made significant progress in recent years, while the development of fully non‐fused ring acceptors has been unsatisfactory. Here, two fully non‐fused ring acceptors, o‐4TBC‐2F and m‐4TBC‐2F, were designed and synthesized. By regulating the location of the hexyloxy chains, o‐4TBC‐2F formed planar backbones, while m‐4TBC‐2F displayed a twisted backbone. Additionally, the o‐4TBC‐2F film showed a markedly red‐shifted absorption after thermal annealing, which indicated the formation of J‐aggregates. For fabrication of organic solar cells (OSCs), PBDB‐T was used as a donor and blended with the two acceptors. The o‐4TBC‐2F‐based blend films displayed higher charge mobilities, lower energy loss and a higher power conversion efficiency (PCE). The optimized devices based on o‐4TBC‐2F gave a PCE of 10.26 %, which was much higher than those based on m‐4TBC‐2F at 2.63 %, and it is one of the highest reported PCE values for fully non‐fused ring electron acceptors.

A terpolymer donor PM6‐Tz20 was developed by incorporating the third 5,5′‐dithienyl‐2,2′‐bithiazole (DTBTz) unit into the PM6 backbone. The introduction of DTBTz can tailor the molecular ordering, orientation, and aggregation properties, and then optimize the morphology and electrical properties of devices, ultimately improving fill factor (0.77) and thus device efficiency (17.6 %).

Abstract

Regulating molecular structure to optimize the active layer morphology is of considerable significance for improving the power conversion efficiencies (PCEs) in organic solar cells (OSCs). Herein, we demonstrated a simple ternary copolymerization approach to develop a terpolymer donor PM6‐Tz20 by incorporating the 5,5′‐dithienyl‐2,2′‐bithiazole (DTBTz, 20 mol%) unit into the backbone of PM6 (PM6‐Tz00). This method can effectively tailor the molecular orientation and aggregation of the polymer, and then optimize the active layer morphology and the corresponding physical processes of devices, ultimately boosting FF and then PCE. Hence, the PM6‐Tz20: Y6‐based OSCs achieved a PCE of up to 17.1% with a significantly enhanced FF of 0.77. Using Ag (220 nm) instead of Al (100 nm) as cathode, the champion PCE was further improved to 17.6%. This work provides a simple and effective molecular design strategy to optimize the active layer morphology of OSCs for improving photovoltaic performance.

A dissymmetric fused‐ring acceptor BTIC‐2Cl‐γCF₃ with chlorine and trifluoromethyl end groups give a power conversion efficiency (PCE) of over 17 % which is the highest among polymer solar cells processed by halogen‐free solvents. Dissymmetric chlorination and trifluoromethylation is a practical approach towards a low band‐gap acceptor for eco‐compatible processed photovoltaic applications.

Abstract

To elevate the performance of polymer solar cells (PSC) processed by non‐halogenated solvents, a dissymmetric fused‐ring acceptor BTIC‐2Cl‐γCF₃ with chlorine and trifluoromethyl end groups has been designed and synthesized. X‐ray crystallographic data suggests that BTIC‐2Cl‐γCF₃ has a 3D network packing structure as a result of H‐ and J‐aggregations between adjacent molecules, which will strengthen its charge transport as an acceptor material. When PBDB‐TF was used as a donor, the toluene‐processed binary device realized a high power conversion efficiency (PCE) of 16.31 %, which improved to 17.12 % when PC71ThBM was added as the third component. Its efficiency of over 17 % is currently the highest among polymer solar cells processed by non‐halogenated solvents. Compared to its symmetric counterparts BTIC‐4Cl and BTIC‐CF₃‐γ, the dissymmetric BTIC‐2Cl‐γCF₃ integrates their merits, and has optimized the molecular aggregations with excellent storage and photo‐stability, and also extending the maximum absorption peak in film to 852 nm. The devices exhibit good transparency indicating a potential utilization in semi‐transparent building integrated photovoltaics (ST‐BIPV).

A series of PM6 polymers with different weight‐average molecular weights and polydispersity index are synthesized, and the effects of PM6 polymerization degree on the efficiency and degradation behaviors of the Y6‐based photovoltaic system are systematically studied.

Abstract

The degree of polymerization can cause significant changes in the blend microstructure and physical mechanism of the active layer of non‐fullerene polymer solar cells, resulting in a huge difference in device performance. However, the diversity of stability issues, including photobleaching stability, storage stability, photostability, thermal stability, and mechanical stability, and more, poses a challenge for the degree of polymerization to comprehensively address the trade‐off between device efficiency and stability and reasonably evaluate the application potential of polymer materials. Herein, a series of PM6 polymers with different weight‐average molecular weights (M _w) and polydispersity index (PDI) are synthesized. The effects of the degree of PM6 polymerization on the efficiency and degradation behaviors of the photovoltaic systems based on Y6 as acceptor are investigated systematically. The findings regarding stability issues, together with the trade‐offs in the efficiency‐stability gap, formulate a complete guideline for the material design and performance evaluation in a way that relies much less on trial‐and‐error efforts.

This work proposes an efficient method to produce tri‐iodide ions, which has been known as an efficient additive that improves the crystallinity, grain size, and morphology of perovskite films in a precursor solution using a photoassited process within short time, resulting in achieving the device performance up to 22%.

Abstract

One of the most effective methods to achieve high‐performance perovskite solar cells (PSCs) is to employ additives as crystallization agents or to passivate defects. Tri‐iodide ion has been known as an efficient additive to improve the crystallinity, grain size, and morphology of perovskite films. However, the generation and control of this tri‐iodide ion are challenging. Herein, an efficient method to produce tri‐iodide ion in a precursor solution using a photoassisted process for application in PSCs is developed. Results suggest that the tri‐iodide ion can be synthesized rapidly when formamidinium iodide (FAI) dissolved isopropyl alcohol (IPA) solution is exposed to LED light. Specifically, the photoassisted FAI–IPA solution facilitates the formation of fine perovskite films with high crystallinity, large grain size, and low trap density, thereby improving the device performance up to 22%. This study demonstrates that the photoassisted process in FAI dissolved IPA solution can be an alternative strategy to fabricate highly efficient PSCs with significantly reduced processing times.

The working on the interfacial engineering toward efficient and stable perovskite solar cells (PSCs) is demonstrated. The key role of 2D interface modification for efficient and stable perovskite solar cells is highlighted, especially for the energy band alignment of PSCs. This paper sheds light on the significance of 2D interface modification in PSCs and provides critical guidance for development of highly efficient and stability PSCs.

Abstract

Interfacial engineering is essential for facilitating carrier separation, charge extraction, and enhancing the stability in organic–inorganic perovskite solar cells (PSCs). Herein, a facile and effective method is demonstrated not only to tune the electronic performance of electron transporting layer (ETL) but also to passivate the defects at the interface between the ETL and perovskite. On the top of the tin(IV) oxide (SnO₂) ETL, butylammonium chloride (BACl) and lead(II) iodide (PbI₂) are introduced as interface to modify the ETL/perovskite interface. The PSCs with interface modified exhibit a power conversion efficiency (PCE) of 21.15%, compared to 18.33% for the device without interface modified. Such enhancement in efficiency is mainly attributed to a better energy band alignment, and the quality of perovskite films is improved through the interface modification, thus enhancing photogenerated charge extraction and leading to low charge carrier recombination at the interface of ETL/perovskite. Furthermore, the device with interface modified exhibits significant stability. This work provides an alternative strategy on the ETL/perovskite interface to obtain highly stable and efficient PSCs.

A low‐temperature fabrication strategy for TiO₂ nanopillars electron transporting layer (ETL) is developed by one‐step glancing angle deposition (GLAD) for efficient and flexible perovskite solar cells. The ETL consisting of a bottom layer of planar TiO₂ and an upper layer of TiO₂ nanopillars array, which are fabricated both by GLAD consecutively, can significantly improve the device performance and flexibility.

Abstract

Organometal halide perovskite solar cells (PSCs) are promisingly applied to flexible solar cells because of the high power conversion efficiency (PCE) and intrinsic softness of perovskite materials. In the most efficient PSCs, mesoporous TiO₂ generally functions as the electron transporting layer. However, the mesoporous TiO₂ is typically generated through high‐temperature thermal annealing that is not suitable for producing flexible PSCs. In this work, TiO₂ nanopillar arrays are directly deposited on flexible substrates using glancing angle deposition at low substrate temperature. The TiO₂ nanopillars strongly adhere to the flexible substrates, improving light harvesting in the perovskite layers, facilitating electron extraction and transportation, and enhancing the mechanical flexibility of the PSCs. The flexible PSCs hybridized with the TiO₂ nanopillars show a PCE as high as 13.3% and excellent photovoltaic stability after 500 cycles of bending at a small radius of curvature.

Fast screening of precursor composition in mixed halide MAPbI_{3‐ x} Br _x perovskite films is accomplished by a multi‐channel pumping ultrasonic spray‐coating technique combined with vacuum‐assisted thermal annealing towards realizing large‐scale high‐throughput commercial production of mixed halide‐based perovskite solar cells.

Abstract

The upscaling of perovskite solar cells to achieve high‐throughput and highly efficient photovoltaic devices is still challenging. In this work, spray deposition of MAPbI_{3‐ x} Br _x mixed halide perovskite is achieved using a ternary channel pumping system to deposit methylammonium iodide, PbI₂, and PbBr₂ precursor solutions. The compositions of six kinds of perovskite films (x = 0–0.5) are manipulated by adjusting the flow rate of the syringe pump rather than through traditional methods of preparing and changing the chemical management of the halogen precursors. Large clusters in highly oriented crystalline perovskite films are fabricated as MAPbI_2.91Br_0.09 film (x = 0.09) with a champion power conversion efficiency (PCE) of 15.60%. When vacuum‐assisted thermal annealing is used, the optimized PCE increased further up to 17.07%. To confirm the homogeneity of the perovskite film over the large area (7 × 10 cm²), eight divided subcells with dimensions of 2.5 × 2.5 cm² showed the highest and average PCE of 17.11% and 16.25 ± 0.77%, respectively. These findings highlight the relevance of high‐throughput screening of perovskite compositions and the versatility of scalable solution process, in which active layers are deposited by spray‐coating.

A buffer layer free semitransparent perovskite solar cell with sputtering deposited indium‐zinc oxide as the rear transparent electrode achieves a conversion efficiency of 16.23% along with high average transmittance of 39.46% from 300 to 1200 nm. That enables a high‐performance four‐terminal perovskite/c‐Si tandem solar cell with an integrated conversion efficiency of 24.60%.

Abstract

Rear transparent electrode (RTE) dominates the quality of semitransparent solar cells, which determines the overall performance of tandem devices as the top cells. Here, sputtering deposited indium‐zinc oxide (IZO) is demonstrated as effective RTE for buffer layer free semitransparent perovskite solar cell (PSC). It is revealed that the vacuum‐evaporated electron‐transport layer (C₆₀/BCP, v‐ETL) has better tolerance of the bombardment in the deposition process of IZO and hence superior device performance compared to the solution‐processed ETL (PCBM/BCP, s‐ETL). Systematical characterizations evidence that the ETL surface potential shifts at the ETL/IZO interface rather than the surface morphology variation accounts for the device performance difference. By further optimizing the thickness of BCP and perovskite layers, the semitransparent PSC exhibits the best conversion efficiency of 16.23% as well as a high transmittance of 39.46% in the wavelength span from 300 to 1200 nm. By employing the optimized semitransparent PSC as the top cell in a four‐terminal perovskite/c‐silicon tandem solar cell, a combined conversion efficiency of 24.60% is achieved. This work highlights the use of sputtered IZO in the combination of IZO/C60 effectively for high‐performance semitransparent devices.

Publication date: March 2021

Source: Nano Energy, Volume 81

Author(s): Chengxi Zhang, Yan-Na Lu, Wu-Qiang Wu, Lianzhou Wang

Publication date: March 2021

Source: Nano Energy, Volume 81

Author(s): Yutong Ren, Ming Ren, Xinrui Xie, Jianan Wang, Yaohang Cai, Yi Yuan, Jing Zhang, Peng Wang

Publication date: 20 January 2021

Source: Joule, Volume 5, Issue 1

Author(s): So Me Yoon, Hanul Min, Jong Beom Kim, Gwisu Kim, Kyoung Su Lee, Sang Il Seok

The role of methylammonium chloride (MACl) in sequentially deposited bromine (Br)‐free formamidinium lead iodide (FAPbI₃)‐based perovskite is systematically demonstrated to regulate the PbI₂/FAI reaction, tune the phase transition at room temperature, and adjust the PbI₂ residual through an intermediate‐related perovskite decomposition during thermal annealing. The resulting optimized solar cells achieve a remarkable efficiency of 23.1% with considerably improved photostability.

Abstract

So far, the combination of methylammonium bromide/methylammonium chloride (MABr/MACl) or methylammonium iodide (MAI)/MACl is the most frequently used additives to stabilize formamidinium lead iodide (FAPbI₃) fabricated by the sequential deposition method. However, the enlarged bandgap due to the addition of bromide and the ambiguous functions of these additives in lead iodide (PbI₂) transformation are still worth considering. Herein, the roles of MACl in sequentially deposited Br‐free FA‐based perovskites are systematically investigated. It is found that MACl can finely regulate the PbI₂/FAI reaction, tune the phase transition at room temperature, and adjust intermediate‐related perovskite crystallization and decomposition during thermal annealing. Compared to FAPbI₃, the perovskite with MACl exhibits larger grain, longer carrier lifetime, and reduced trap density. The resultant solar cell therefore achieves a champion power conversion efficiency (PCE) of 23.1% under reverse scan with a stabilized power output of 23.0%. In addition, it shows much improved photostability under 100 mW cm⁻² white illumination (xenon lamp) in nitrogen atmosphere without encapsulation.

In article number 2004367, Linfeng Lu, Dongdong Li, and co‐workers present MoO _X /c‐Si heterojunction solar cells with improved efficiency and stability by introducing an innovative stacked structure (c‐Si/SiO _X /MoO _X /V₂O _X /ITO/Ag). The SiO _X tunneling passivation layer suppresses the redox reaction at the MoO _X /c‐Si interface, while the ultra‐thin V₂O _X layer improves the stability of the heterojunction structure in air exposure and its resistance to sputtering damage.

Stable oxide thin films on both sides of the MoO _X film are employed in MoO _X ‐heterocontacted solar cells on p‐type CZ silicon wafers. The SiO _X tunneling layer formed by UV/O₃ treatment and the ultrathin V₂O _X capping layer maintains the work function and hole selectivity of MoO _X at a higher level. The p‐Si/SiO _X /MoO _X /V₂O _X /ITO/Ag solar cell demonstrates a stable efficiency of 20.0%.

Abstract

Crystalline silicon heterojunction solar cells based on hole‐selective MoO _X contacts provide obvious merits in terms of the decent passivation and carrier selectivity but face the challenge of long‐term stability. With the aim to improve the performance and stability of solar cells with full area MoO _X /metal contacts, a SiO _X tunneling layer on silicon surface is intentionally formed by UV/O₃ treatment and an indium tin oxide (ITO) film is sputtered as a high‐work‐function electrode. Before ITO sputtering, an ultrathin V₂O _X capping layer is introduced to efficiently prevent MoO _X film from air exposure and the damage by sputtering bombardment. The insertion of SiO _X , V₂O _X , and ITO keeps the work function of MoO _X at a high level, which improves the hole selectivity as well as the stability of the contact. The p‐Si/SiO _X /MoO _X /V₂O _X /ITO/Ag solar cell demonstrates an efficiency of 20.0% with improved stability, which is the highest value for MoO _X heterocontacts class on p‐type silicon to date.

PbSe quantum dot (QD) infrared (IR) solar cells are promising devices for improved photovoltaic performance by harvesting the low‐energy IR photons unabsorbed by common solar cells. Here, a strategy to protect PbSe QDs is developed via combination of epitaxially coating a thin PbS shell and in situ halide passivation, breaking the V _OC–J _SC trade‐off in the traditional QD solar cells.

Abstract

Lead chalcogenide quantum dot (QD) infrared (IR) solar cells are promising devices for breaking through the theoretical efficiency limit of single‐junction solar cells by harvesting the low‐energy IR photons that cannot be utilized by common devices. However, the device performance of QD IR photovoltaic is limited by the restrictive relation between open‐circuit voltages (V _OC) and short circuit current densities (J _SC), caused by the contradiction between surface passivation and electronic coupling of QD solids. Here, a strategy is developed to decouple this restriction via epitaxially coating a thin PbS shell over the PbSe QDs (PbSe/PbS QDs) combined with in situ halide passivation. The strong electronic coupling from the PbSe core gives rise to significant carrier delocalization, which guarantees effective carrier transport. Benefited from the protection of PbS shell and in situ halide passivation, excellent trap‐state control of QDs is eventually achieved after the ligand exchange. By a fine control of the PbS shell thickness, outstanding IR J _SC of 6.38 mA cm⁻² and IR V _OC of 0.347 V are simultaneously achieved under the 1100 nm‐filtered solar illumination, providing a new route to unfreeze the trade‐off between V _OC and J _SC limited by the photoactive layer with a given bandgap.

The photogeneration pathways in two non‐fullerene acceptor solar cells are investigated by charge extraction measurements. They reveal higher photogeneration yield and weaker electric field dependence for the nonrelaxed charge transfer states in PTB7‐Th:ITIC. The low molecular quadrupole moment of h‐ITIC, the dipolar analogue of ITIC, reduces delocalization of electron–hole pairs at the donor–acceptor interface lowering the yield.

Abstract

In organic solar cells, photogenerated singlet excitons form charge transfer (CT) complexes, which subsequently split into free charge carriers. Here, the contributions of excess energy and molecular quadrupole moments to the charge separation process are considered. The charge photogeneration in two separate bulk heterojunction systems consisting of the polymer donor PTB7‐Th and two non‐fullerene acceptors, ITIC and h‐ITIC, is investigated. CT state dissociation in these donor–acceptor systems is monitored by charge density decay dynamics obtained from transient absorption experiments. The electric field dependence of charge carrier generation is studied at different excitation energies by time delayed collection field (TDCF) and sensitive steady‐state photocurrent measurements. Upon excitation below the optical gap, free charge carrier generation becomes less field dependent with increasing photon energy, which challenges the view of charge photogeneration proceeding through energetically lowest CT states. The average distance between electron–hole pairs at the donor–acceptor interface is determined from empirical fits to the TDCF data. The delocalization of CT states is larger in PTB7‐Th:ITIC, the system with larger molecular quadrupole moment, indicating the sizeable effect of the electrostatic potential at the donor–acceptor interface on the dissociation of CT complexes.

A new DPP‐based alternating D–A copolymer (PD2FCT‐29DPP) is developed for use as a hole‐transport layer. PD2FCT‐29DPP addresses the different requirements for an HTL, offering favorable energetics, near‐infrared absorption, and efficient charge transfer. Therefore, a PD2FCT‐29DPP‐based device achieves a remarkable FF of 70% and the highest PCE of 14.0% among PbS CQD‐SCs.

Abstract

The need for optoelectronic and chemical compatibility between the layers in colloidal quantum dot (CQD) photovoltaic devices remains a bottleneck in further increasing performance. Conjugated polymers are promising candidates as new hole‐transport layer (HTL) materials in CQD solar cells (CQD‐SCs) owing to the highly tunable optoelectronic properties and compatible chemistries. A diketopyrrolopyrrole‐based polymer with benzothiadiazole derivatives (PD2FCT‐29DPP) as an HTL in these devices is reported. The energy level, molecular orientation, and hole mobility of this HTL are manipulated through molecular engineering. By levering the polymer's optical absorption spectrum complementary to that of the CQD active layer, EQE across the visible and near‐infrared regions is maximized. As a result, a PD2FCT‐29DPP‐based device exhibits a fill factor of 70% and approximately 35% efficiency enhancement compared to a PTB7‐based device.

Here, a straightforward polyTPD passivation is introduced to reduce the defect‐mediated recombination by elucidating the imperfections on the surface and grain boundaries of perovskite materials. Suppressed non‐radiative recombination and improved interfacial hole extraction result in perovskite solar cells with stabilized efficiency exceeding 21%. Moreover, ultra‐hydrophobic and thermally robust polyTPD passivated devices retain 94% of the initial efficiency after 800 h under operational conditions.

Abstract

The failure of perovskite solar cells (PSCs) to maintain their maximum efficiency over a prolonged time is due to the deterioration of the light harvesting material under environmental factors such as humidity, heat, and light. Systematically elucidating and eliminating such degradation pathways are critical to imminent commercial use of this technology. Here, a straightforward approach is introduced to reduce the level of defect‐states present at the perovskite and hole transporting layer interface by treating the various perovskite surfaces with poly(N,N′‐bis‐4‐butylphenyl‐N,N′‐bisphenyl)benzidine (polyTPD) molecules. This strategy significantly suppresses the defect‐mediated non‐radiative recombination in the ensuing devices and prevents the penetration of degrading agents into the inner layers by passivating the perovskite surface and grain boundaries. Suppressed non‐radiative recombination and improved interfacial hole extraction result in PSCs with stabilized efficiency exceeding 21% with negligible hysteresis (≈19.1% for control device). Moreover, ultra‐hydrophobic polyTPD passivant considerably alleviates moisture penetration, showing ≈91% retention of initial efficiencies after 300 h storage at high relative humidity of 80%. Similarly, passivated device retains 94% of its initial efficiency after 800 h under operational conditions (maximum power point tracking under continuous illumination at 60 °C). In addition to interfacial passivation function, hole‐selective role of dopant‐free polyTPD is also evaluated and discussed in this study.