Chen Weijie

The partial substitution of methylammonium (MA) and formamidinium (FA) cations by cesium (Cs) or rubidium (Rb) cations in multiple‐cation lead mixed halide perovskites, (FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃), reduces the trap‐assisted (k₁) and radiative (k₂) charge carrier recombination rate. Furthermore, Urbach energies are reduced, indicating improved perovskite film microstructure. Consequently, photovoltaic devices with Cs/Rb‐incorporated perovskites exhibit improved power conversion efficiency.

Incorporating cesium (Cs) or rubidium (Rb) cations into multiple‐cation lead mixed halide perovskites (FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃) increases their photovoltaic performance. Herein, the fundamental photophysics of perovskites are investigated by steady‐state and transient optical spectroscopy and the reasons for the performance increase are revealed. Cs/Rb‐cation incorporation slightly increases the bandgap, whereas exciton binding energies remain in the range of a few meV. Urbach energies are reduced, suggesting improved perovskite microstructure upon Cs/Rb incorporation. Carrier density‐induced broadening of the photo‐bleaching following the Burstein–Moss model is observed, and the effective carrier masses are determined to be a few tenths of the electron rest mass. From fits of the high‐energy tail of the perovskite's photo bleach to Boltzmann's distribution, subpicosecond hot‐carrier cooling is revealed, implying strong carrier–phonon coupling. Importantly, the charge carrier recombination dynamics indicate that Cs/Rb‐incorporation reduces both the first‐order (trap‐assisted) and the second‐order (radiative) recombination, which appears to be the main reason for the observed performance increase upon Cs/Rb‐cation incorporation. Overall, this work presents a detailed study of the photophysics of multiple‐cation mixed halide lead perovskites and develops a concise picture of the impact of cesium/rubidium incorporation on the photophysics and device performance.

By varying thermal annealing conditions, a thermally induced perovskite crystal control process of the wide‐bandgap perovskite films provides an opportunity to exploit both lead‐iodide passivation and perovskite orientation strategies with a fixed E _g of 1.73 eV. Based on this concept, the device efficiency is improved from 15.76% to 18.60% and the operational photostability is also enhanced without any encapsulation in ambient conditions.

Wide‐bandgap perovskite solar cells (WBG PSCs) have gained attention as promising tandem partners for silicon solar cells due to their complementary absorption, superb open‐circuit voltage, and an easy solution process. Recently, both their performance and stability have been improved by compositional engineering or defect passivation strategies, due to the modulation of perovskite crystal size and reduction of crystal defects. Herein, a report on the thermally induced phase control (TIPC) strategy is provided, which enables efficient and photostable WBG PSCs without compositional engineering by exploring a thermal annealing process window (100–175 °C and 3–60 min) of the WBG perovskite films. Within this window, a key annealing regime is found that produces preferred crystal orientations of lead iodide and the WBG perovskite, suppressing phase segregation and reducing charge recombination in the perovskites. The WBG PSCs (composition of FA_0.75MA_0.15Cs_0.1PbI₂Br and E _g of 1.73 eV) optimized by TIPC exhibit an excellent power conversion efficiency (PCE) of 18.60% and improved operational stability, maintaining >90% of the maximum PCE (during maximum power point tracking) without encapsulation after 12 h of operation (air mass 1.5 global irradiation in ambient air conditions) or after 500 h of operation (white light‐emitting diode irradiation (100 mW cm⁻²) in N₂ conditions).

A strategy is proposed to precisely control CsPbIBr₂ crystallization behaviors by incorporating sulfamic acid sodium salt (SAS), thus resulting in a high‐quality film. More importantly, SAS in perovskite possibly introduces an additional internal electric field effect that favors the electron transport and injection. Encouragingly, a higher efficiency of 10.57% is achieved with this strategy.

Abstract

Excellent power conversion efficiency (PCE) and stability are the primary forces that propel the all‐inorganic cesium‐based halide perovskite solar cells (PSCs) toward commercialization. However, the intrinsic high density of trap state and internal nonradiative recombination of CsPbIBr₂ perovskite film are the barriers that limit its development. In the present study, a facile additive strategy is introduced to fabricate highly efficient CsPbIBr₂ PSCs by incorporating sulfamic acid sodium salt (SAS) into the perovskite layer. The additive can control the crystallization behaviors and optimize morphology, as well as effectively passivate defects in the bulk perovskite film, thereby resulting in a high‐quality perovskite. In addition, SAS in perovskite has possibly introduced an additional internal electric field effect that favors electron transport and injection due to inhomogeneous ion distribution. A champion PCE of 10.57% (steady‐output efficiency is 9.99%) is achieved under 1 Sun illumination, which surpasses that of the contrast sample by 16.84%. The modified perovskite film also exhibits improved moisture stability. The unencapsulated device maintains over 80% initial PCE after aging for 198 h in air. The results provide a suitable additive for inorganic perovskite and introduce a new conjecture to explain the function of additives in PSCs more rationally.

Purely vibrationally excited lead–iodide perovskites are prepared using off‐resonance, infrared optical excitation far below the bandgap. The transient optical response manifested as bandgap oscillations below and above the static bandgap is attributed to the A _g optical phonon mode at 25 cm⁻¹. This mode, arising from antiphase octahedral rotations, is observed in both 3D perovskite CH₃NH₃PbI₃ and layered 2D perovskite [CH₃(CH₂)₃NH₃]₂PbI₄.

Abstract

Hybrid organic–inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy‐relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time‐resolved optical spectroscopy is performed, with off‐resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off‐resonant pumping scheme permits a simplified observation of changes in the bandgap owing to the A _g vibrational mode, which is qualitatively different from vibrational modes of other symmetries and without increased complexity of photogenerated electronic charges. The work demonstrates a strong correlation between the lead–iodide octahedral framework and electronic transitions, and provides further insights into the manipulation of coherent optical phonons and related properties in hybrid perovskites on ultrafast timescales.

The {100} facets of ≈3 nm PbS quantum dots (QDs) are minimized by tuning the balance between the growth kinetics and thermodynamics in the synthesis. Compared to PbS QDs from thermodynamics‐dominated growth, the PbS QDs with less {100} facets show fewer trap states in the QD solids, leading to a better photovoltaic device performance with a power conversion efficiency of 11.5%.

Abstract

Trap states in colloidal quantum dot (QD) solids significantly affect the performance of QD solar cells, because they limit the open‐circuit voltage and short circuit current. The {100} facets of PbS QDs are important origins of trap states due to their weak or missing passivation. However, previous investigations focused on synthesis, ligand exchange, or passivation approaches and ignored the control of {100} facets for a given dot size. Herein, trap states are suppressed from the source via facet control of PbS QDs. The {100} facets of ≈3 nm PbS QDs are minimized by tuning the balance between the growth kinetics and thermodynamics in the synthesis. The PbS QDs synthesized at a relatively low temperature with a high oversaturation follow a kinetics‐dominated growth, producing nearly octahedral nanoparticles terminated mostly by {111} facets. In contrast, the PbS QDs synthesized at a relatively high temperature follow a thermodynamics‐dominated growth. Thus, a spherical shape is preferred, producing truncated octahedral nanoparticles with more {100} facets. Compared to PbS QDs from thermodynamics‐dominated growth, the PbS QDs with less {100} facets show fewer trap states in the QD solids, leading to a better photovoltaic device performance with a power conversion efficiency of 11.5%.

Publication date: 20 May 2020

Source: Joule, Volume 4, Issue 5

Author(s): Jia Li, Hao Wang, Xin Yu Chin, Herlina Arianita Dewi, Kurt Vergeer, Teck Wee Goh, Jia Wei Melvin Lim, Jia Haur Lew, Kian Ping Loh, Cesare Soci, Tze Chien Sum, Henk J. Bolink, Nripan Mathews, Subodh Mhaisalkar, Annalisa Bruno

A strategy is demonstrated for efficacious regulation of perovskite crystallinity using glycolic acid (GA) against nonvolatile thioglycolic acid (TGA) following dimethyl sulfoxide sublimation, resulting in enhanced device performance. A champion power conversion efficiency as high as 21.32% is achieved for the GA‐based device, which is almost 13% or 20% higher than those of the control device or TGA‐based device.

Abstract

A strategy for efficaciously regulating perovskite crystallinity is proposed by using a volatile solid glycolic acid (HOCH₂COOH, GA) in an FA_0.85MA_0.15PbI₃ (FA: HC(NH₂)₂; MA: CH₃NH₃) perovskite precursor solution that is different from the common additive approach. Accompanied with the first dimethyl sulfoxide sublimation process, the subsequent sublimation of GA before 150 °C in the FA_0.85MA_0.15PbI₃ perovskite film can artfully regulate the perovskite crystallinity without any residual after annealing. The improved film formation upon GA modification induced by the strong interaction between GA and Pb²⁺ delivers a champion power conversion efficiency (PCE) as high as 21.32%. In order to investigate the role of volatility in perovskite solar cells (PSCs), nonvolatile thioglycolic acid (HSCH₂COOH, TGA) with a similar structure to GA is utilized as an additive reference. Large perovskite grains are obtained by TGA modification but with obvious pinholes, which directly leads to an increased defect density accompanied by a decline in PCE. Encouragingly, the champion PCE achieved for GA‐based PSC device (21.32%) is almost 13% or 20% higher than those of the control device or TGA‐based device. In addition, GA‐modified PSCs exhibit the best stability in light‐, thermal‐, and humidity‐based tests due to the improved film formation.

Tin fluoride and methylammonium iodide are employed as precursors for the fabrication of methylammonium tin iodide (MASnI₃) film via an ion exchange/insertion reactions approach, and a highly uniform, pinhole‐free perovskite film is obtained with a high concentration of SnF₂ and a low content of Sn⁴⁺. The corresponding solar cell exhibits the highest power conversion efficiency of 7.78% with high reproducibility and stability.

Abstract

The low toxicity, narrow bandgaps, and high charge‐carrier mobilities make tin perovskites the most promising light absorbers for low‐cost perovskite solar cells (PSCs). However, the development of the Sn‐based PSCs is seriously hampered by the critical issues of poor stability and low power conversion efficiency (PCE) due to the facile oxidation of Sn²⁺ to Sn⁴⁺ and poor film formability of the perovskite films. Herein, a synthetic strategy is developed for the fabrication of methylammonium tin iodide (MASnI₃) film via ion exchange/insertion reactions between solid‐state SnF₂ and gaseous methylammonium iodide. In this way, the nucleation and crystallization of MASnI₃ can be well controlled, and a highly uniform pinhole‐free MASnI₃ perovskite film is obtained. More importantly, the detrimental oxidation can be effectively suppressed in the resulting MASnI₃ film due to the presence of a large amount of remaining SnF₂. This high‐quality perovskite film enables the realization of a PCE of 7.78%, which is among the highest values reported for the MASnI₃‐based solar cells. Moreover, the MASnI₃ solar cells exhibit high reproducibility and good stability. This method provides new opportunities for the fabrication of low‐cost and lead‐free tin‐based halide perovskite solar cells.

This work mainly focuses on materials composition and working mechanism of the hydroiodic acid (HI) hydrolysis‐derived intermediate compound DMAI/DMAPbI _x . Importantly, the main component of the CsPbI₃ film prepared by such precursor is proved to be still inorganic. Finally, the optimized CsPbI₃ film–based device shows significantly enhanced stability in ambient environment with a high power conversion efficiency of 17.32%.

Abstract

Introducing hydroiodic acid (HI) as a hydrolysis‐derived precursor of the intermediate compounds has become an increasingly important issue for fabricating high quality and stable CsPbI₃ perovskite solar cells (PSCs). However, the materials composition of the intermediate compounds and their effects on the device performance remain unclear. Here, a series of high‐quality intermediate compounds are prepared and it is shown that they consist of DMAI/DMAPbI _x . Further characterization of the products show that the main component of this system is still CsPbI₃. Most of the dimethylammonium (DMA⁺) organic component is lost during annealing. Only an ultrasmall amount of DMA⁺ is doped into the CsPbI₃ and its structure is stabilized. Meanwhile, excessive DMA⁺ forms Lewis acid–base adducts and interactions with Pb²⁺ on the CsPbI₃ surface. This process passivates the CsPbI₃ film and decreases the recombination rate. Finally, CsPbI₃ film is fabricated with high crystalline, uniform morphology, and excellent stability. Its corresponding PSC exhibits stable property and improved power conversion efficiency (PCE) up to 17.3%.

Nanoporous Au film is successfully introduced into perovskite solar cells to replace the typical thermal deposition of metal electrode with a high efficiency of 19.0% on rigid substrate and sustains an excellent bending durability of 98.5% even after 1000 cycles testing on a flexible device, while its facile and recycled utilization significantly reduces the device fabrication cost, noble metal consuming, and environmental pollution.

Abstract

Perovskite solar cells (PSCs) using metal electrodes have been regarded as promising candidates for next‐generation photovoltaic devices because of their high efficiency, low fabrication temperature, and low cost potential. However, the complicated and rigorous thermal deposition process of metal contact electrodes remains a challenging issue for reducing the energy pay‐back period in commercial PSCs, as the ubiquitous one‐time use of a contact electrode wastes limited resources and pollutes the environment. Here, a nanoporous Au film electrode fabricated by a simple dry transfer process is introduced to replace the thermally evaporated Au electrode in PSCs. A high power conversion efficiency (PCE) of 19.0% is demonstrated in PSCs with the nanoporous Au film electrode. Moreover, the electrode is recycled more than 12 times to realize a further reduced fabrication cost of PSCs and noble metal materials consumption and to prevent environmental pollution. When the nanoporous Au electrode is applied to flexible PSCs, a PCE of 17.3% and superior bending durability of ≈98.5% after 1000 cycles of harsh bending tests are achieved. The nanoscale pores and the capability of the porous structure to impede crack generation and propagation enable the nanoporous Au electrode to be recycled and result in excellent bending durability.

The highly crystalline uniaxial‐orientated FA‐MA mixed cation perovskite films, fabricated by amino‐functionalized carbon nanotube (CNT‐NH₂) and methylammonium chloride additive, exhibit improved charge‐carrier dynamics to facilitate charge extraction and transport. The champion device based on the uniaxial‐orientated MA_0.85FA_0.15PbI₃ films with optimal amino‐functionalized CNT‐NH₂ content exhibits efficiency of 21.05%.

Abstract

High‐quality perovskite thin film with few defects/traps is the key to assemble high‐performance perovskite solar cells (PSCs). Because of the high defects/traps density in polycrystalline perovskite films and challenging fabrication of single crystal devices, the polycrystalline perovskite films consisting of the oriented quasi‐single crystals with reduced defect density are the ideal compromise. In this work, the fabrication of highly crystalline uniaxial‐orientated perovskite thin films is demonstrated, with enhanced charge‐carrier transport through amino‐functionalized carbon nanotube (CNT‐NH₂) and methylammonium chloride (MACl) additive. X‐ray scattering, including synchrotron‐based grazing incidence wide‐angle X‐ray scattering, reveal that the MA_0.85FA_0.15PbI₃ thin films grown with the assistance of both CNT‐NH₂ and MACl additives exhibit strong (110) orientation. Because of the highly crystallinity, uniaxial‐orientation, and large crystal grain size, the FA‐MA mixed cation perovskite films exhibit improved charge‐carrier dynamics to facilitate charge extraction and transport. The champion PSC device based on these perovskite films achieves power conversation efficiency of 21.05%. This work provides a general approach for preparing high crystalline uniaxial‐orientated perovskite films with enhanced charge‐carrier transport for various optoelectronic applications.

A tungsten oxide (WO _x ) layer with niobium oxide surface treatment is introduced as a sputter buffer for semitransparent perovskite solar cells. Compared to devices with an untreated WO _x buffer, using the surface‐treated buffer significantly recovers the fill factor, which is possibly explained via electronic‐trap shifting toward the band edge. Incorporation of this approach is demonstrated for four‐terminal perovskite‐silicon tandems.

Abstract

For semitransparent devices with n‐i‐p structures, a metal oxide buffer material is commonly used to protect the organic hole transporting layer from damage due to sputtering of the transparent conducting oxide. Here, a surface treatment approach is addressed for tungsten oxide‐based transparent electrodes through slight modification of the tungsten oxide surface with niobium oxide. Incorporation of this transparent electrode technique to the protective buffer layer significantly recovers the fill factor from 70.4% to 80.3%, approaching fill factor values of conventional opaque devices, which results in power conversion efficiencies over 18% for the semitransparent perovskite solar cells. Application of this approach to a four‐terminal tandem configuration with a silicon bottom cell is demonstrated.

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.

A tungsten oxide (WO _x ) layer with niobium oxide surface treatment is introduced as a sputter buffer for semitransparent perovskite solar cells. Compared to devices with an untreated WO _x buffer, using the surface‐treated buffer significantly recovers the fill factor, which is possibly explained via electronic‐trap shifting toward the band edge. Incorporation of this approach is demonstrated for four‐terminal perovskite‐silicon tandems.

Abstract

For semitransparent devices with n‐i‐p structures, a metal oxide buffer material is commonly used to protect the organic hole transporting layer from damage due to sputtering of the transparent conducting oxide. Here, a surface treatment approach is addressed for tungsten oxide‐based transparent electrodes through slight modification of the tungsten oxide surface with niobium oxide. Incorporation of this transparent electrode technique to the protective buffer layer significantly recovers the fill factor from 70.4% to 80.3%, approaching fill factor values of conventional opaque devices, which results in power conversion efficiencies over 18% for the semitransparent perovskite solar cells. Application of this approach to a four‐terminal tandem configuration with a silicon bottom cell is demonstrated.

Stable perovskite thin films and solar cells are obtained by judicious incorporation of multifunctional tetrapropylammonium (TPA) cations in methylammonium iodide (MAPbI₃). Upon addition of TPA, a heterostructure is formed, which leads to the passivation of defects along with improved morphology. This study highlights a new strategy to enhance the stability of perovskite solar cells while maintaining high performance.

Abstract

Improving the performances of photovoltaic (PV) devices by suppressing nonradiative energy losses through surface defect passivation and enhancing the stability to the level of standard PV represents one critical challenge for perovskite solar cells. Here, reported are the advantages of introducing a tetrapropylammonium (TPA⁺) cation that combines two key functionalities, namely surface passivation of CH₃NH₃PbI₃ nanocrystals through strong ionic interaction with the surface and bulk passivation via formation of a type I heterostructure that acts as a recombination barrier. As a result, nonencapsulated perovskite devices with only 2 mol% of TPA⁺ achieve power conversion efficiencies over 18.5% with higher V _OC under air mass 1.5G conditions. The devices fabricated retain more than 85% of their initial performances for over 1500 h under ambient conditions (55% RH ± 5%). Furthermore, devices with TPA⁺ also exhibit excellent operational stability by retaining over 85% of the initial performance after 250 h at maximum power point under 1 sun illumination. The effect of incorporation of TPA⁺ on the structural and optoelectronic properties is studied by X‐ray diffraction, ultraviolet–visible absorption spectroscopy, ultraviolet photon–electron spectroscopy, time‐resolved photoluminescence, and scanning electron microscopy imaging. Atomic‐level passivation upon addition of TPA⁺ is elucidated employing 2D solid‐state NMR spectroscopy.

A power conversion efficiency of 16.88% (certified as 16.4%) is achieved based on PM6:Y6 by morphology optimization, which is the most efficient for organic solar cells. Through the study of single structure and film morphology, a well‐ordered 2D crystal is found, which helps to enhance ultrafast hole and electron transfer, thus improving performance.

Abstract

Single‐layered organic solar cells (OSCs) using nonfullerene acceptors have reached 16% efficiency. Such a breakthrough has inspired new sparks for the development of the next generation of OSC materials. In addition to the optimization of electronic structure, it is important to investigate the essential solid‐state structure that guides the high efficiency of bulk heterojunction blends, which provides insight in understanding how to pair an efficient donor–acceptor mixture and refine film morphology. In this study, a thorough analysis is executed to reveal morphology details, and the results demonstrate that Y6 can form a unique 2D packing with a polymer‐like conjugated backbone oriented normal to the substrate, controlled by the processing solvent and thermal annealing conditions. Such morphology provides improved carrier transport and ultrafast hole and electron transfer, leading to improved device performance, and the best optimized device shows a power conversion efficiency of 16.88% (16.4% certified). This work reveals the importance of film morphology and the mechanism by which it affects device performance. A full set of analytical methods and processing conditions are executed to achieve high efficiency solar cells from materials design to device optimization, which will be useful in future OSC technology development.

A method is introduced to experimentally measure the efficiency potential of any neat perovskite film on glass with/without attached transport layers using intensity‐dependent photoluminescence measurements. This approach allows decoupling efficiency losses due to insufficient charge transport, bulk, interface, and surface recombination. These findings also shine light on the ideality factor in perovskite solar cells and thereby fill factor limitations.

Abstract

Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1‐sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non‐radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open‐circuit voltage and the internal quasi‐Fermi level splitting (QFLS), the transport resistance‐free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity‐dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non‐radiative fill factor and open‐circuit voltage loss. It is found that potassium‐passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit.

A chemically orthogonal hole transport layer for lead sulfide colloidal quantum dot (CQD) solar cells is introduced. By substituting the 1,2‐ethanedithiol‐treated CQDs with malonic‐acid‐treated CQDs, the surface chemistry of the active layer is preserved. This increases the charge diffusion length by 1.4×, enabling near‐unity charge extraction efficiency at the back electrode, achieving 13.0% efficiency.

Abstract

Colloidal quantum dots (CQDs) are of interest in light of their solution‐processing and bandgap tuning. Advances in the performance of CQD optoelectronic devices require fine control over the properties of each layer in the device materials stack. This is particularly challenging in the present best CQD solar cells, since these employ a p‐type hole‐transport layer (HTL) implemented using 1,2‐ethanedithiol (EDT) ligand exchange on top of the CQD active layer. It is established that the high reactivity of EDT causes a severe chemical modification to the active layer that deteriorates charge extraction. By combining elemental mapping with the spatial charge collection efficiency in CQD solar cells, the key materials interface dominating the subpar performance of prior CQD PV devices is demonstrated. This motivates to develop a chemically orthogonal HTL that consists of malonic‐acid‐crosslinked CQDs. The new crosslinking strategy preserves the surface chemistry of the active layer beneath, and at the same time provides the needed efficient charge extraction. The new HTL enables a 1.4× increase in charge carrier diffusion length in the active layer; and as a result leads to an improvement in power conversion efficiency to 13.0% compared to EDT standard cells (12.2%).

By finely optimizing the alkyl chains, the nonfullerene acceptor named BTP‐eC9 is synthesized and a maximum power conversion efficiency of 17.8% in organic photovoltaic cells is recorded. This work demonstrates that the optimization of alkyl chains to get suitable solubility and enhanced intermolecular packing has a great potential in further improving its photovoltaic performance.

Abstract

Optimizing the molecular structures of organic photovoltaic (OPV) materials is one of the most effective methods to boost power conversion efficiencies (PCEs). For an excellent molecular system with a certain conjugated skeleton, fine tuning the alky chains is of considerable significance to fully explore its photovoltaic potential. In this work, the optimization of alkyl chains is performed on a chlorinated nonfullerene acceptor (NFA) named BTP‐4Cl‐BO (a Y6 derivative) and very impressive photovoltaic parameters in OPV cells are obtained. To get more ordered intermolecular packing, the n‐undecyl is shortened at the edge of BTP‐eC11 to n‐nonyl and n‐heptyl. As a result, the NFAs of BTP‐eC9 and BTP‐eC7 are synthesized. The BTP‐eC7 shows relatively poor solubility and thus limits its application in device fabrication. Fortunately, the BTP‐eC9 possesses good solubility and, at the same time, enhanced electron transport property than BTP‐eC11. Significantly, due to the simultaneously enhanced short‐circuit current density and fill factor, the BTP‐eC9‐based single‐junction OPV cells record a maximum PCE of 17.8% and get a certified value of 17.3%. These results demonstrate that minimizing the alkyl chains to get suitable solubility and enhanced intermolecular packing has a great potential in further improving its photovoltaic performance.

Phenanthrene‐fused‐quinoxaline (PFQ) is demonstrated as an efficient auxiliary acceptor to realize long‐wavelength response, enhancing the photocurrent as well as power conversion efficiency (PCE) of copper‐electrolyte‐based dye‐sensitized solar cells. The resulting dye, termed HY64, achieves an outstanding PCE of 12.5 %.

Abstract

Dye‐sensitized solar cells (DSSCs) based on Cu^II/I bipyridyl or phenanthroline complexes as redox shuttles have achieved very high open‐circuit voltages (V _OC, more than 1 V). However, their short‐circuit photocurrent density (J _SC) has remained modest. Increasing the J _SC is expected to extend the spectral response of sensitizers to the red or NIR region while maintaining efficient electron injection in the mesoscopic TiO₂ film and fast regeneration by the Cu^I complex. Herein, we report two new D‐A‐π‐A‐featured sensitizers termed HY63 and HY64, which employ benzothiadiazole (BT) or phenanthrene‐fused‐quinoxaline (PFQ), respectively, as the auxiliary electron‐withdrawing acceptor moiety. Despite their very similar energy levels and absorption onsets, HY64‐based DSSCs outperform their HY63 counterparts, achieving a power conversion efficiency (PCE) of 12.5 %. PFQ is superior to BT in reducing charge recombination resulting in the near‐quantitative collection of photogenerated charge carriers.