The Role of Photon Energy in Free Charge Generation in Bulk Heterojunction Solar Cells

To determine the role of photon energy on charge generation in bulk heterojunction solar cells, the bias voltage dependence of photocurrent for excitation with photon energies below and above the optical band gap is investigated in two structurally related polymer solar cells. Charges generated in (poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothia­diazole)] (C‐PCPDTBT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) solar cells via excitation of the low‐energy charge transfer (CT) state, situated below the optical band gap, need more voltage to be extracted than charges generated with excitation above the optical band gap. This indicates a lower effective binding energy of the photogenerated electrons and holes when the excitation is above the optical band gap than when excitation is to the bottom of the CT state. In blends of PCBM with the silicon‐analogue, poly[(4,4‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (Si‐PCPDTBT), there is no effect of the photon energy on the electric field dependence of the dissociation efficiency of the CT state. C‐PCPDTBT and Si‐PCPDTBT have very similar electronic properties, but their blends with PCBM differ in the nanoscale phase separation. The morphology is coarser and more crystalline in Si‐PCPDTBT:PCBM blends. The results demonstrate that the nanomorphological properties of the bulk heterojunction are important for determining the effective binding energy in the generation of free charges at the heterojunction.     

Remarkably Enhanced Hydrogen Generation of Organolead Halide Perovskites via Piezocatalysis and Photocatalysis

To alleviate photoinduced charge recombination in semiconducting nanomaterials represents an important endeavor toward high‐efficiency photocatalysis. Here a judicious integration of piezoelectric and photocatalytic properties of organolead halide perovskite CH₃NH₃PbI₃ (MAPbI₃) to enable a piezophotocatalytic activity under simultaneous ultrasonication and visible light illumination for markedly enhanced photocatalytic hydrogen generation of MAPbI₃ is reported. The conduction band minimum of MAPbI₃ is higher than hydrogen generation potential (0.046 V vs normal hydrogen electrode), thereby rendering efficient hydrogen evolution. In addition, the noncentrosymmetric crystal structure of MAPbI₃ enables its piezoelectric properties. Thus, MAPbI₃ readily responds to external mechanical force, creating a built‐in electric field for collective piezophotocatalysis as a result of effective separation of photogenerated charge carriers. The experimental results show that MAPbI₃ powders exhibit superior piezophotocatalytic hydrogen generation rate (23.30 µmol h^?1) in hydroiodic acid (HI) solution upon concurrent light and mechanical stimulations, much higher than that of piezocatalytic (i.e., 2.21 µmol h^?1) and photocatalytic (i.e., 3.42 µmol h^?1) hydrogen evolution rate as well as their sum (i.e., 5.63 µmol h^?1). The piezophotocatalytic strategy provides a new way to control the recombination of photoinduced charge carriers by cooperatively capitalizing on piezocatalysis and photocatalysis of organolead halide perovskites to yield highly efficient piezophotocatalysis.     

In Situ Phase‐Induced Spatial Charge Separation in Core–Shell Oxynitride Nanocube Heterojunctions Realizing Robust Solar Water Splitting

Efficient spatial charge separation is critical for solar energy conversion over solid photocatalysts. The development of efficient visible‐light photocatalysts has been of immense interest, but with limited success. Here, multiband core–shell oxynitride nanocube heterojunctions composed of a tantalum nitride (Ta₃N₅) core and nitrogen‐doped sodium tantalate (NaTaON) shell have been constructed via an in situ phase‐induced etching chemical strategy. The photocatalytic water splitting performance of sub‐20‐nm Ta₃N₅@NaTaON junctions exhibits an extraordinarily high photocatalytic activity toward oxygen and hydrogen evolution. Most importantly, the combined experimental results and theoretical calculations reveal that the strong interfacial Ta? O? N bonding connection as a touchstone among Ta₃N₅@NaTaON junctions provides a continuous charge transport pathway rather than a random charge accumulation. The prolonged photoexcited charge carrier lifetime and suitable band matching between the Ta₃N₅ core and NaTaON shell facilitate the separation of photoinduced electron–hole pairs, accounting for the highly efficient photocatalytic performance. This work establishes the use of (oxy)nitride heterojunctions as viable photocatalysts for the conversion of solar energy into fuels.     

Advances in Piezo‐Phototronic Effect Enhanced Photocatalysis and Photoelectrocatalysis

Direct conversion of solar light into chemical energy by means of photocatalysis or photoelectrocatalysis is currently a point of focus for sustainable energy development and environmental remediation. However, its current efficiency is still far from satisfying, suffering especially from severe charge recombination. To solve this problem, the piezo‐phototronic effect has emerged as one of the most effective strategies for photo(electro)catalysis. Through the integration of piezoelectricity, photoexcitation, and semiconductor properties, the built‐in electric field by mechanical stimulation induced polarization can serve as a flexible autovalve to modulate the charge‐transfer pathway and facilitate carrier separation both in the bulk phase and at the surfaces of semiconductors. This review focuses on illustrating the trends and impacts of research based on piezo‐enhanced photocatalytic reactions. The fundamental mechanisms of piezo‐phototronics modulated band bending and charge migration are highlighted. Through comparing and classifying different categories of piezo‐photocatalysts (like the typical ZnO, MoS₂, and BaTiO₃), the recent advances in polarization‐promoted photo(electro)catalytic processes involving water splitting and pollutant degradation are overviewed. Meanwhile the optimization methods to promote their catalytic activities are described. Finally, the outlook for future development of polarization‐enhanced strategies is presented.     

Charge Pumping Strategy for Rotation and Sliding Type Triboelectric Nanogenerators

Triboelectric nanogenerator (TENG) is an emerging approach for harvesting energy from the living environment. But its performance is limited by the maximum density of surface charges created by contact electrification. Here, by rationally designing a synchronous rotation structure, a charge pumping strategy is realized for the first time in a rotary sliding TENGs, which is demonstrated to enhance the charge density by a factor of 9, setting up a record for rotary TENGs. The average power is boosted by more than 15 times compared with normal TENGs, achieving an ultrahigh average power density of 1.66 kW m^?3, under a low drive frequency of 2 Hz. Moreover, the charge pumping mechanism enables decoupling of bound charge generation and the severity of interfacial friction in the main TENG, allowing surface lubricants to be applied for suppressing abrasion and lowering heat generation. The adaptability of the strategy to rotation and sliding type TENGs in low‐frequency agitations provides a breakthrough to the bottleneck of power output for mechanical energy harvesting, and should have a great impact on high‐power TENG design and practical applications in various fields.     

Charge Accumulation and Hysteresis in Perovskite‐Based Solar Cells: An Electro‐Optical Analysis

Organic–inorganic hybrid perovskite solar cells based on CH₃NH₃PbI₃ have achieved great success with efficiencies exceeding 20%. However, there are increasing concerns over some reported efficiencies as the cells are susceptible to current–voltage (I–V) hysteresis effects. It is therefore essential that the origins and mechanisms of the I–V hysteresis can clearly be understood to minimize or eradicate these hysteresis effects completely for reliable quantification. Here, a detailed electro‐optical study is presented that indicates the hysteresis originates from lingering processes persisting from sub‐second to tens of seconds. Photocurrent transients, photoluminescence, electroluminescence, quasi‐steady state photoinduced absorption processes, and X‐ray diffraction in the perovskite solar cell configuration have been monitored. The slow processes originate from the structural response of the CH₃NH₃PbI₃ upon E‐field application and/or charge accumulation, possibly involving methylammonium ions rotation/displacement and lattice distortion. The charge accumulation can arise from inefficient charge transfer at the perovskite interfaces, where it plays a pivotal role in the hysteresis. These findings underpin the significance of efficient charge transfer in reducing the hysteresis effects. Further improvements of CH₃NH₃PbI₃‐based perovskite solar cells are possible through careful surface engineering of existing TiO₂ or through a judicious choice of alternative interfacial layers.     

Efficient Charge Extraction in Thick Bulk Heterojunction Solar Cells through Infiltrated Diffusion Doping

A new strategy for improving the charge extraction in thick bulk heterojunction (BHJ) polymer solar cells (PSCs) is reported. By the deposition of a solution‐processed vanadium oxide (s‐VO_x) onto BHJ active layers, conductive charge‐transport channels are formed inside the active layer via a charge‐transfer doping reaction between the lone‐pair electrons of the sulfur atoms in the polymer and the Lewis‐acidic vanadium atoms of the s‐VO_x. Because the charge‐transport channels significantly reduce charge recombination in the BHJ films, high internal quantum efficiencies (IQEs) of over 80% are achieved in the thick inverted PSCs (≈420 nm). This finding represents a new strategy for improving the efficiency and feasibility of printable photovoltaic devices.     

Local Intermolecular Order Controls Photoinduced Charge Separation at Donor/Acceptor Interfaces in Organic Semiconductors

How free charge is generated at organic donor–acceptor interfaces is an important question, as the binding energy of the lowest energy (localized) charge transfer states should be too high for the electron and hole to escape each other. Recently, it has been proposed that delocalization of the electronic states participating in charge transfer is crucial, and aggregated or otherwise locally ordered structures of the donor or the acceptor are the precondition for this electronic characteristic. The effect of intermolecular aggregation of both the polymer donor and fullerene acceptor on charge separation is studied. In the first case, the dilute electron acceptor triethylsilylhydroxy‐1,4,8,11,15,18,22,25‐octabutoxyphthalocyaninatosilicon(IV) (SiPc) is used to eliminate the influence of acceptor aggregation, and control polymer order through side‐chain regioregularity, comparing charge generation in 96% regioregular (RR‐) poly(3‐hexylthiophene) (P3HT) with its regiorandom (RRa‐) counterpart. In the second case, ordered phases in the polymer are eliminated by using RRa‐P3HT, and phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) is used as the acceptor, varying its concentration to control aggregation. Time‐resolved microwave conductivity, time‐resolved photoluminescence, and transient absorption spectroscopy measurements show that while ultrafast charge transfer occurs in all samples, long‐lived charge carriers are only produced in films with intermolecular aggregates of either RR‐P3HT or PC₆₁BM, and that polymer aggregates are just as effective in this regard as those of fullerenes.     

Band‐like Charge Photogeneration at a Crystalline Organic Donor/Acceptor Interface

Organic photovoltaic cells possess desirable practical characteristics, such as the potential for low‐cost fabrication on flexible substrates, but they lag behind their inorganic counterparts in performance due in part to fundamental energy loss mechanisms, such as overcoming the charge transfer (CT) state binding energy when photogenerated charge is transferred across the donor/acceptor interface. However, recent work has suggested that crystalline interfaces can reduce this binding energy due to enhanced CT state delocalization. Solar cells based on rubrene and C₆₀ are investigated as an archetypal system because it allows the degree of crystallinity to be moldulated from a highly disordered to highly ordered system. Using a postdeposition annealing method to transform as‐deposited amorphous rubrene thin films into ones that are highly crystalline, it is shown that the CT state of a highly crystalline rubrene/C₆₀ heterojunction undergoes extreme delocalization parallel to the interface leading to a band‐like state that exhibits a linear Stark effect. This state parallels the direct charge formation of inorganic solar cells and reduces energetic losses by 220 meV compared with 12 other archetypal heterojunctions reported in the literature.     

Doping‐Induced Screening of the Built‐in‐Field in Organic Solar Cells: Effect on Charge Transport and Recombination

We report on the effects of screening of the electric field by doping‐induced mobile charges on photocurrent collection in operational organic solar cells. Charge transport and recombination were studied using double injection (DI) and charge extraction by linearly increasing voltage (CELIV) transient techniques in bulk‐heterojunction solar cells made from acceptor‐donor blends of poly(3‐n‐hexylthiophene):phenyl‐C61‐butyric acid methyl ester (P3HT:PC60BM). It is shown that the screening of the built‐in field in operational solar cells can be controlled by an external voltage while the influence on charge transport and recombination is measured. An analytical theory to extract the bimolecular recombination coefficient as a function of electric field from the injection current is also reported. The results demonstrate that the suppressed (non‐Langevin) bimolecular recombination rate and charge collection are not strongly affected by native doping levels in this materials combination. Hence, it is not necessary to reduce the level of doping further to improve the device performance of P3HT‐based solar cells.     

Charge Transport Modulation of a Flexible Quantum Dot Solar Cell Using a Piezoelectric Effect

Colloidal quantum dots are promising materials for flexible solar cells, as they have a large absorption coefficient at visible and infrared wavelengths, a band gap that can be tuned across the solar spectrum, and compatibility with solution processing. However, the performance of flexible solar cells can be degraded by the loss of charge carriers due to recombination pathways that exist at a junction interface as well as the strained interface of the semiconducting layers. The modulation of the charge carrier transport by the piezoelectric effect is an effective way of resolving and improving the inherent material and structural defects. By inserting a porous piezoelectric poly(vinylidenefluoride‐trifluoroethylene) layer so as to generate a converging electric field, it is possible to modulate the junction properties and consequently enhance the charge carrier behavior at the junction. This study shows that due to a reduction in the recombination and an improvement in the carrier extraction, a 38% increase in the current density along with a concomitant increase of 37% in the power conversion efficiency of flexible quantum dots solar cells can be achieved by modulating the junction properties using the piezoelectric effect.     

Beyond Langevin Recombination: How Equilibrium Between Free Carriers and Charge Transfer States Determines the Open‐Circuit Voltage of Organic Solar Cells

Organic solar cells lag behind their inorganic counterparts in efficiency due largely to low open‐circuit voltages (V_oc). In this work, a comprehensive framework for understanding and improving the open‐circuit voltage of organic solar cells is developed based on equilibrium between charge transfer (CT) states and free carriers. It is first shown that the ubiquitous reduced Langevin recombination observed in organic solar cells implies equilibrium and then statistical mechanics is used to calculate the CT state population density at each voltage. This general result permits the quantitative assignment of V_oc losses to a combination of interfacial energetic disorder, non‐negligible CT state binding energies, large degrees of mixing, and sub‐ns recombination at the donor/acceptor interface. To quantify the impact of energetic disorder, a new temperature‐dependent CT state absorption measurement is developed. By analyzing how the apparent CT energy varies with temperature, the interfacial disorder can be directly extracted. 63–104 meV of disorder is found in five systems, contributing 75–210 mV of V_oc loss. This work provides an intuitive explanation for why qV_oc is almost always 500–700 meV below the energy of the CT state and shows how the voltage can be improved.     

Charge Transfer from Methylammonium Lead Iodide Perovskite to Organic Transport Materials: Efficiencies,Transfer Rates,and Interfacial Recombination

Perovskite‐based photovoltaics have been rapidly developed, with record power conversion efficiencies now exceeding 22%. In order to rationally design efficient and stable perovskite solar cells, it is important to understand not only charge trapping and recombination events, but also processes occurring at the perovskite/transport material (TM) interface, such as charge transfer and interfacial recombination. In this work, time‐resolved microwave conductivity measurements are performed to investigate these interfacial processes for methylammonium lead iodide and various state‐of‐the‐art organic TMs. A global kinetic model is developed, which accurately describes both the dynamics of excess charges in the perovskite layer and transfer to charge‐specific TMs. The authors conclude that for state‐of‐the‐art materials, such as Spiro‐OMeTAD and PCBM, the charge extraction efficiency is not significantly affected by intra‐band gap traps for trap densities under 10¹⁵ cm^–3. Finally, the transfer rates to C60, PCBM, EDOT‐OMeTPA, and Spiro‐OMeTAD are sufficient to outcompete second order recombination under excitation densities representative for illumination by AM1.5.     

From Recombination Dynamics to Device Performance: Quantifying the Efficiency of Exciton Dissociation,Charge Separation,and Extraction in Bulk Heterojunction Solar Cells with Fluorine‐Substituted Polymer Donors

An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, “the” technique to monitor all intermediate states over the entire relevant timescale, is combined with time‐delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two‐pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device‐relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide‐bandgap donor polymers: poly(4,8‐bis((2‐ethylhexyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene‐3,4‐difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8‐bis((2‐ethylhexyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene‐3,4‐thiophene) (PBDT[2H]T) combined with PC₇₁BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)‐state energy is shown to lead to significantly lower energy losses (resulting in higher V_OC) during charge generation compared to P3HT:PCBM.     

Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer

Although perovskite solar cells (PSCs) have emerged as a promising alternative to widely used fossil fuels, the involved high‐temperature preparation of metal oxides as a charge transport layer in most state‐of‐the‐art PSCs has been becoming a big stumbling block for future low‐temperature and large‐scale R2R manufacturing process. Such an issue strongly encourages scientists to find new type of materials to replace metal oxides. Except for expensive PC₆₁BM with unmanageable morphology and electrical properties, the past investigation on the development of low‐temperature‐processed and highly efficient electron transport layers (ETLs) has met some mixed success. In order to further enhance the performance of all‐solution‐processed PSCs, we propose a novel n‐type sulfur‐containing small molecule hexaazatrinaphtho[2,3‐c][1,2,5]thiadiazole (HATNT) with high electron mobility up to 1.73 × 10^?2 cm² V^?1 s^?1 as an ETL in planar heterojunction PSCs. A high power conversion efficiency of 18.1% is achieved, which is fully comparable with the efficiency from the control device fabricated with PC₆₁BM as ETL. This superior performance mainly attributes from more effective suppression of charge recombination at the perovskite/HATNT interface than that between the perovskite and PC₆₁ BM. Moreover, high electron mobility and strong interfacial interaction via S? I or S? Pb bonding should be also positive factors. Significantly, our results undoubtedly enable new guidelines in exploring n‐type organic small molecules for high‐performance PSCs.     

Quantitative Analysis and Visualized Evidence for High Charge Separation Efficiency in a Solid‐Liquid Bulk Heterojunction

In the past few decades, some novel low‐cost nanostructured devices have been explored for converting solar energy into electrical or chemical energy, such as organic photovoltaic cells, photoelectrochemical solar cells, and solar water splitting cells. Generally, higher light absorption and/or charge separation efficiency are considered as the main reasons for improved performance in a nanostructured device versus a planar structure. However, quantitative analysis and definite experimental evidence remain elusive. Here, using BiVO₄ as an example, comparable samples with porous and dense structures have been prepared by a simple method. The porous and dense films are assembled into a solid‐electrolyte bulk and planar heterojunction, respectively. Some quantitative results are obtained by decoupling photon absorption, interfacial charge transfer, and charge separation processes. These results suggest that higher charge separation efficiency is mainly responsible for enhanced performance in a solid‐electrolyte bulk heterojunction. Moreover, we also present visualized evidence to show higher charge separation efficiency comes from a shorter photo‐generated hole diffusion distance in a bulk heterojunction. These results can deepen understanding charge transfer in a bulk heterojunction and offer guidance to design a more efficient low‐cost device for solar conversion and storage.     

Charge Motion during the Photocycle of Bacteriorhodopsin

The function of bacteriorhodopsin in Halobacterium salinarum is to pump protons from the internal side of the plasma membrane to the external after light excitation, thereby building up electrochemical energy. This energy is transduced into biological energy forms. This review deals with one of the methods elaborated for recording the charge transfer inside the protein. In this method the current produced in oriented purple membrane containing bacteriorhodopsin is measured. It is shown that this method might be applied not only to correlate charge motion with the photocycle reactions but also for general problems like effect of water, electric field, and different ions and buffers for the functioning of proteins.     

Spatially Confined MnO2 Nanostructure Enabling Consecutive Reversible Charge Transfer from Mn(IV) to Mn(II) in a Mixed Pseudocapacitor‐Battery Electrode

Mn oxides are highly important electrode materials for aqueous electrochemical energy storage devices, including batteries and supercapacitors. Although MnO₂ is a promising pseudocapacitor material because of its outstanding rate and capacity performance, its electrochemical instability in aqueous electrolyte prevents its use at low electrochemical potential. Here, the possibility of stabilizing MnO₂ electrode using SiO₂‐confined nanostructure is demonstrated. Remarkably, an exceptionally good electrochemical stability under large negative polarization in aqueous (Li₂SO₄) electrolyte, usually unattainable for MnO₂‐based electrode, is achieved. Even more interestingly, this MnO₂–SiO₂ nanostructured composite exhibits unique mixed pseudocapacitance‐battery behaviors involving consecutive reversible charge transfer from Mn(IV) to Mn(II), which enable simultaneous high‐capacity and high‐rate characteristics, via different charge‐transfer kinetic mechanisms. This suggests a strategy to design and stabilize electrochemical materials that are comprised of intrinsically unstable but high‐performing component materials.     

Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States,Charge Transport,and Recombination

Adding cesium (Cs) and rubidium (Rb) cations to FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃ hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time‐of‐flight, time‐resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs‐incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open‐circuit voltage and fill factor of Cs‐containing devices. By comparison, Rb‐addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current–voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs‐Rb‐FA‐MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple‐cation perovskite solar cells, which are essential for the design of high‐performance devices.     

Leaf‐Mosaic‐Inspired Vine‐Like Graphitic Carbon Nitride Showing High Light Absorption and Efficient Photocatalytic Hydrogen Evolution

Green plants use solar energy efficiently in nature. Simulating the exquisite structure of a natural photosynthesis system may open a new approach for the construction of desirable photocatalysts with high light harvesting efficiency and performance. Herein, inspired by the excellent light utilization of “leaf mosaic” in plants, a novel vine‐like g‐C₃N₄ (V‐CN) is synthesized for the first time by copolymerizing urea with dicyandiamide‐formaldehyde (DF) resin. The as‐prepared V‐CN exhibits ultrahigh photocatalytic hydrogen production of 13.6 mmol g^?1 h^?1 under visible light and an apparent quantum yield of 12.7% at 420 nm, which is ≈38 times higher than that of traditional g‐C₃N₄, representing one of the highest‐activity g‐C₃N₄‐based photocatalysts. This super photocatalytic performance is derived from the unique leaf mosaic structure of V‐CN, which effectively improves its light utilization and affords a larger specific surface area. In addition, the introduction of DF resin further optimizes the energy band of V‐CN, extends its light absorption, and improves its crystallinity and interfacial charge transport, resulting in high performance. It is an easy and green strategy for the preparation of broad‐spectrum, high‐performance g‐C₃N₄, which presents significant advancement for the design of other nanophotocatalysts by simulating the fine structure of natural photosynthesis.