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Colloidal quantum dot bulk heterojunction solids for efficient infrared solar cells are presented. A mixture of n‐type CQDs and p‐type CQDs enables a photoelectric conversion efficiency of >80% in the spectral regime beyond silicon's bandgap and lead to a 17% PCE relative improvement compared to that of best previous reported CQD solids.

Abstract

Colloidal quantum dots (CQDs) are of interest for optoelectronic applications owing to their tunable properties and ease of processing. Large‐diameter CQDs offer optical response in the infrared (IR), beyond the bandgap of c‐Si and perovskites. The absorption coefficient of IR CQDs (≈10⁴ cm⁻¹) entails the need for micrometer‐thick films to maximize the absorption of IR light. This exceeds the thickness compatible with the efficient extraction of photogenerated carriers, a fact that limits device performance. Here, CQD bulk heterojunction solids are demonstrated that, with extended carrier transport length, enable efficient IR light harvesting. An in‐solution doping strategy for large‐diameter CQDs is devised that addresses the complex interplay between (100) facets and doping agents, enabling to control CQD doping, energetic configuration, and size homogeneity. The hetero‐offset between n‐type CQDs and p‐type CQDs is manipulated to drive the transfer of electrons and holes into distinct carrier extraction pathways. This enables to form active layers exceeding thicknesses of 700 nm without compromising open‐circuit voltage and fill factor. As a result, >90% charge extraction efficiency across the ultraviolet to IR range (350–1400 nm) is documented.

A systematic study of a series of polymer:non‐fullerene acceptor blends is conducted to unify the cumulative effects of voltages losses, charge generation efficiencies, non‐geminate recombination and extraction dynamics, and nuanced morphological differences to the device performance. Deconvolution of the major loss processes in these blends and their connections to the nuanced bulk‐heterojunction morphology and energetics are established.

Abstract

Even though significant breakthroughs with over 18% power conversion efficiencies (PCEs) in polymer:non‐fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses, ii) charge generation efficiencies, iii) non‐geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. An extension to advanced morphological techniques, such as solid‐state NMR (for atomic level insights on the local ordering and donor:acceptor ππ interactions) and resonant soft X‐ray scattering (for donor and acceptor interfacial area and domain spacings), provide detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.

Publication date: October 2020

Source: Nano Energy, Volume 76

Author(s): Jia Zhang, Bin Hu

The nematic liquid‐crystalline small‐molecule donor benzodithiophene terthiophene rhodamine (BTR) works as a successful third component in PM6:Y6‐based organic solar cells. The doping of BTR significantly enhances the crystallinity with slightly reducing donor/acceptor phase separation of the photoactive layer. This uncommon morphology evolution facilitates charge separation, transport, and collection and ultimately boosts the efficiency from 15.7% to 16.6%.

Abstract

Achieving an ideal morphology is an imperative avenue for enhancing key parameters toward high‐performing organic solar cells (OSCs). Among a myriad of morphological‐control methods, the strategy of incorporating a third component with structural similarity and crystallinity difference to construct ternary OSCs has emerged as an effective approach to regulate morphology. A nematic liquid‐crystalline benzodithiophene terthiophene rhodamine (BTR) molecule, which possesses the same alkylthio‐thienyl‐substituted benzo moiety but obviously stronger crystallinity compared to classical medium‐bandgap polymeric donor PM6, is employed as a third component to construct ternary OSCs based on a PM6:BTR:Y6 system. The doping of BTR (5 wt%) is found to be enough to improve the OSC morphology—significantly enhancing the crystallinity of the photoactive layer while slightly reducing the donor/acceptor phase separation scale simultaneously. Rarely is such a morphology evolution reported. It positively affects the electronic properties of the device—prolongs the carrier lifetime, shortens the photocurrent decay time, facilitates exciton dissociation, charge transport, and collection, and ultimately boosts the power conversion efficiency from 15.7% to 16.6%. This result demonstrates that the successful synergy of liquid‐crystalline small‐molecule and polymeric donors delicately adjusts the active‐layer morphology and refines device performance, which brings vibrancy to the OSC research field.

An efficient and stable tin perovskite solar cell with a graded heterostructure which is composed of narrow‐bandgap and wide‐bandgap tin perovskites is reported. Such heterostructure facilitates charge extraction and suppresses the oxidation process of Sn²⁺ to Sn⁴⁺. Consequently, the device achieves a maximum power conversion efficiency of 11% with better operational stability.

Lead‐free tin perovskite solar cells (TPSCs) have attracted widespread attention in recent years due to their low toxicity, suitable bandgap, and high carrier mobility. However, the photovoltage and efficiency of TPSCs are still much lower than those of the lead counterparts because of the high trap density and unfavorable band structure in tin perovskite films. To overcome these issues, efficient and stable TPSCs with a graded heterostructure of light‐absorbing layer are reported, in which the narrow‐bandgap tin perovskite dominates at the bulk, whereas the wide‐bandgap tin perovskite is distributed with a gradient from bulk to surface. This heterostructure can selectively extract the photogenerated charge carriers at the perovskite/electron transport layer interface, reduce the density of trap states, and impede the oxidation process of Sn²⁺ to Sn⁴⁺ in air. As a consequence, this graded heterostructure of tin perovskite layer contributes to an increase of 120 mV in the open‐circuit voltage and a maximum power conversion efficiency of 11% for TPSCs with longer operational stability.

Publication date: September 2020

Source: Nano Energy, Volume 75

Author(s): Wu-Qiang Wu, Jun-Xing Zhong, Jin-Feng Liao, Chengxi Zhang, Yecheng Zhou, Wenhuai Feng, Liming Ding, Lianzhou Wang, Dai-Bin Kuang

In this article, a perovskite/Si dual‐absorber tandem cell for stand‐alone solar water splitting is reported. An unprecedented, over 17% solar‐to‐hydrogen conversion efficiency, is achieved when a Si photocathode is paired in tandem with a high bandgap (≈1.75 eV) semitransparent perovskite solar cell.

Abstract

Realizing solar‐to‐hydrogen (STH) efficiencies close to 20% using low‐cost semiconductors remains a major step toward accomplishing the practical viability of photoelectrochemical (PEC) hydrogen generation technologies. Dual‐absorber tandem cells combining inexpensive semiconductors are a promising strategy to achieve high STH efficiencies at a reasonable cost. Here, a perovskite photovoltaic biased silicon (Si) photoelectrode is demonstrated for highly efficient stand‐alone solar water splitting. A p⁺nn⁺ ‐Si/Ti/Pt photocathode is shown to present a remarkable photon‐to‐current efficiency of 14.1% under biased condition and stability over three days under continuous illumination. Upon pairing with a semitransparent mixed perovskite solar cell of an appropriate bandgap with state‐of‐the‐art performance, an unprecedented 17.6% STH efficiency is achieved for self‐driven solar water splitting. Modeling and analysis of the dual‐absorber PEC system reveal that further work into replacing the noble‐metal catalyst materials with earth‐abundant elements and improvement of perovskite fill factor will pave the way for the realization of a low‐cost high‐efficiency PEC system.

In article number https://doi.org/10.1002/aenm.2020006872000687, Qinye Bao and co‐workers systematically investigate the energetics and energy loss in 2D Ruddlesden‐Popper perovskite (RPP) solar cells. The crucial scenario found at the 2D RPP/electron transport layer interface is that the potential gradient across ligands promotes separation of the photogenerated carrier, with electrons transferring from the perovskite crystal to the electron transport layer.

CsPbI₃, with its excellent chemical stability, possesses a suitable bandgap for single‐junction and tandem solar cells, yet the poor phase stability hinders its application. In article https://doi.org/10.1002/adma.2020001862000186, Wan‐Jian Yin, Wallace C. H. Choy, and co‐workers reveal the nature of the photoactive CsPbI₃ phase transition from the perspective of PbI₆ octahedral rotation and develop a facile method to simultaneously stabilize the photoactive phase and reduce the defect density of the CsPbI₃.

Publication date: 15 July 2020

Source: Joule, Volume 4, Issue 7

Author(s): Bin Sun, Andrew Johnston, Chao Xu, Mingyang Wei, Ziru Huang, Zhang Jiang, Hua Zhou, Yajun Gao, Yitong Dong, Olivier Ouellette, Xiaopeng Zheng, Jiakai Liu, Min-Jae Choi, Yuan Gao, Se-Woong Baek, Frédéric Laquai, Osman M. Bakr, Dayan Ban, Oleksandr Voznyy, F. Pelayo García de Arquer

Halogenation of D–A (donor–acceptor)‐type materials is an effective method to improve the performance of polymer solar cells (PSCs). In this work, recent developments of PSCs by the chlorination strategy are summarized, including the intrinsic property of chlorine atoms, the progress of chlorinated polymers and small molecules, and the synergetic effect of chlorination with other methods to elevate solar conversions.

Abstract

This work summarizes recent developments in polymer solar cells (PSCs) prepared by a chlorination strategy. The intrinsic property of chlorine atoms, the progress of chlorinated polymers and small molecules, and the synergistic effect of chlorination with other methods to elevate solar conversions are discussed. Halogenation of donor–acceptor (D–A) materials is an effective method to improve the performance of PSCs, which mainly affects the push–pull of electrons between donor and acceptor units due to their strong electron‐withdrawing capabilities. Although chlorine is less electronegative than fluorine, it can form very strong noncovalent interactions, such as Cl···S and Cl···π interactions, because its empty 3d orbits can help to accept the electron pairs or π electrons. This synergistic effect of electronegativity together with the empty 3d orbits of chlorine atoms leads to increased intramolecular and intermolecular interactions and a much stronger capability to down‐shift the molecular energy levels. This work is intended to support a better understanding of the chlorination strategy to modify the material properties, and thus improve the performance of solar devices. Eventually, it will provide the research community with a clearer pathway to choose proper substitution methods according to different situations for high and stable solar energy conversion.

The role of energy offset between the optical bandgap and charge transfer (CT) state energies in nonradiative voltage loss ΔV _nr in organic solar cells is discussed. It is found that the ΔV _nr reduces considerably down to 0.185 V, when local excited and CT states are remarkably close in energy.

The voltage loss incurred by nonradiative charge recombination should be reduced to further improve the power conversion efficiency of organic solar cells (OSCs). This work discusses the nonradiative voltage loss in OSCs with systematically controlled energy offset between optical bandgap and charge transfer (CT) states. It is demonstrated that the nonradiative voltage loss is a function of the energy offset; it drops sharply with decreasing energy offset. By measuring the quantum yields of electroluminescence from OSCs and decay kinetics of CT states, it is found that the radiative decay rate of CT states becomes larger when the energy offset is negligible compared with those in conventional OSCs with sufficient energy offset. This behavior is rationalized by hybridization between CT and local excited states, resulting in a considerable enhancement of the oscillator strength of CT states. Based on a trend observed in this study, the precise mechanism by which the energy offset affects the nonradiative voltage loss is discussed.

Three novel donor–π‐bridge–acceptor (D–π–A)‐type small organic molecules are designed and synthesized as dopant‐free hole transport materials for perovskite solar cells. Combination of triazatruxene donor, terthiophene π‐bridge, and dicyanovinylene N‐ethyl rhodanine electron‐accepting unit as CI‐B3 creates well‐ordered edge‐on aggregated π–π stacking. Solar cell performance and long‐term stability are significantly improved.

Three donor–π‐bridge–acceptor (D–π–A)‐type organic small molecules coded CI‐B1, CI‐B2, and CI‐B3 are designed, synthesized, and used as dopant‐free hole transporting materials (HTMs) for perovskite solar cells (PSCs). The strong electron‐donating triazatruxene central core (D), terthiophene conjugated arms (π), and three different strong electron‐accepting units (A) provide high intramolecular charge transfer nature and eliminate the need of dopants during the fabrication of PSCs. HTMs are investigated to understand the effect of terminal functional groups on the PSC performance. Interestingly, due to the change of end‐capping, three different organizations of self‐assembly with π–π stacking are observed in the solid thin films. Dopant‐free CI‐B1, CI‐B2, CI‐B3, and spiro‐OMeTAD with dopants are used with triple cation perovskite composition Cs_0.1(MA_0.15FA_0.85)_0.9Pb(I_0.85Br_0.15)₃ (MA: CH₃NH₃ ⁺, FA: NHCHNH₃ ⁺) in n‐i‐p architecture. The cells prepared with CI‐B3 not only exhibits a comparable power conversion efficiency (PCE) of 17.54% to the state‐of‐art of spiro‐OMeTAD with dopants (18.02%), but also demonstrates improved long‐term stability, maintaining 88% of its original PCE after 1000 h of illumination. The superior photovoltaic performance, synthetic simplicity, dopant‐free nature, high durability, and edge‐on molecular orientation of CI‐B3 show its great promise as a HTM candidate for efficient and stable PSCs.

A synthetic polyhalide ligand (2‐picolyl)amine triiodide as a molecular glue is used to passivate halide vacancies at grain boundaries directionally and throughout grain bulk of perovskites. The inverted perovskite solar cells after passivation are allowed to be more efficient, and are profoundly stabilized in both ambient air and light‐soaking circumstances.

The fundamental instability of hybrid perovskite solar cells originates from the considerable halide vacancies. Furthermore, the local roles of halide vacancies between grain boundaries and grain bulk generally conflict, thus inhibiting complete passivation. To overcome this obstacle, a rational polyhalide ligand, di‐(2‐picolyl)amine triiodide, is designed as a molecular “glue” to achieve comprehensive passivation. Unlike a monohalide ligand, this ligand has multiple iodide ions and a quasiplanar tridentate chelation capability, contributing to directional passivation along the grain boundaries and overall passivation throughout the grain bulk. Using this molecular glue passivation, the best inverted solar cell yields an efficiency of 20.02%. Moreover, the relative stability of this cell in ambient air (≈40% humidity, 800 h aging) and under light‐soaking conditions (500 h aging) is profoundly enhanced by 33.33% and 22.26%, respectively. Herein, important insights into the design of passivating molecules to achieve low‐defect perovskites toward the development of multifunctional devices are provided.

A successful double bulk heterojunction is fabricated with a blade coating method, which sheds light on innovative approaches to rationally realize balanced crystallization kinetics for donors and acceptors and further improve the photovoltaic efficiency of organic solar cells.

Abstract

Sequential deposition has great potential to achieve high performance in organic solar cells due to the resulting well‐controlled vertical phase separation. In this work, double bulk heterojunction organic solar cells are fabricated by sequential‐blade cast in ambient conditions. Probed by the in situ grazing incidence X‐ray diffraction and in situ UV–vis absorption measurements, the seq‐blade system exhibits a different tendency from each of the binary films during the film formation process. Due to the extensive aggregation of FOIC, the binary PBDB‐T:FOIC film displays a strong and large phase separation, resulting in low current density (J _sc) and unsatisfactory power conversion efficiency. In the seq‐blade cast system, the bottom layer PBDB‐T:IT‐M produces many crystal nuclei for the top layer PBDB‐T:FOIC, so the PBDB‐T molecules are able to crystallize easily and quickly. Balanced crystallization kinetics between polymer and small molecule and an ideal percolation network in the film are observed. In addition, the balanced crystallization kinetics are favorable toward realizing lower recombination loss through charge transport processes.

In article number https://doi.org/10.1002/adfm.2020012942001294, Maria A. Loi, Giuseppe Portale, and co‐workers unveil the mechanism of crystallization of Ruddlesden‐Popper Sn‐based perovskites using in situ X‐ray measurements. The addition of a small fraction of the 2D component to the 3D perovskite causes suppression of the bulk crystallization and promotes oriented surface crystallization. Knowledge of the mechanism of crystal formation will help scientists to further optimize the efficiency of lead‐free perovskite solar cells.

n‐Type polymer semiconductors with a broad absorption and ultranarrow bandgap down to 1.28 eV are synthesized. When applied as electron acceptor materials, a power conversion efficiency of over 10% with a photoresponse reaching 950 nm is realized for all‐polymer solar cells.

Abstract

Compared to organic solar cells based on narrow‐bandgap nonfullerene small‐molecule acceptors, the performance of all‐polymer solar cells (all‐PSCs) lags much behind due to the lack of high‐performance n‐type polymers, which should have low‐aligned frontier molecular orbital levels and narrow bandgap with broad and intense absorption extended to the near‐infrared region. Herein, two novel polymer acceptors, DCNBT‐TPC and DCNBT‐TPIC, are synthesized with ultranarrow bandgaps (ultra‐NBG) of 1.38 and 1.28 eV, respectively. When applied in transistors, both polymers show efficient charge transport with a highest electron mobility of 1.72 cm² V⁻¹ s⁻¹ obtained for DCNBT‐TPC. Blended with a polymer donor, PBDTTT‐E‐T, the resultant all‐PSCs based on DCNBT‐TPC and DCNBT‐TPIC achieve remarkable power conversion efficiencies (PCEs) of 9.26% and 10.22% with short‐circuit currents up to 19.44 and 22.52 mA cm⁻², respectively. This is the first example that a PCE of over 10% can be achieved using ultra‐NBG polymer acceptors with a photoresponse reaching 950 nm in all‐PSCs. These results demonstrate that ultra‐NBG polymer acceptors, in line with nonfullerene small‐molecule acceptors, are also available as a highly promising class of electron acceptors for maximizing device performance in all‐PSCs.