Improved power conversion efficiency by insertion of RGO–TiO2 composite layer as optical spacer in polymer bulk heterojunction solar cells

We report that the power conversion efficiency (PCE) can be enhanced in polymer bulk heterojunction solar cells by inserting an interfacial electron transporting layer consisting of pristine TiO₂ or reduced graphene oxide–TiO₂ (RGO–TiO₂) between the active layer and cathode Al electrode. The enhancement in the PCE has been analyzed through the optical absorption, current–voltage characteristics under illumination and estimation of photo-induced charge carrier generation rate. It was found that either TiO₂ or RGO–TiO₂ interfacial layers improve the light harvesting, as well as the charge extraction efficiency, acting as a blocking layer for holes, and also reducing charge recombination. The combined enhancement in light harvesting property and charge collection efficiency improves the PCE of the organic solar cell up to 4.18% and 5.33% for TiO₂ and RGO–TiO₂ interfacial layer, respectively, as compared to a value of 3.26% for the polymer solar cell without interfacial layer.     

Highly Conductive Cable‐Like Bicomponent Titania Photoanode Approaching Limitation of Electron and Hole Collection

TiO₂‐based materials are cheap and stable choices for photoelectrochemical devices. However, the activity is still limited by the inefficient charge extraction. Here a highly conductive cable‐like bicomponent titania photoanode, consisting of reduced anatase‐coated TiO₂‐B nanowires, is proposed to simultaneously establish effective electron and hole transport channels separately, which meets the requirements of electronic dynamics for efficient water splitting. A synergistic effect of charge separation from the built‐in electric field is demonstrated with this 1D TiO₂‐B/anatase heterojunction, in which a high electron collection efficiency of up to 97.1% at 0.6 V versus reversible hydrogen electrode is achieved. The efficient electron collection approaching the limitation is also attributed to the large electron conducting region in the photoanode. Moreover, the O‐deficient amorphous layer is found to be more catalytic toward the oxygen evolution reaction through quantifying rate constants for charge recombination and charge transfer. It can reduce onset potential and suppress charge‐carrier recombination simultaneously, prompting surface hole collection efficiency up to 95% at 0.6 V versus reversible hydrogen electrode.     

Influences of Injection Barrier and Mobility on Recombination Rate and Zone in OLEDs

The luminous efficiency of organic light-emitting devices depends on the recombination probability of electrons injected at the cathode and holes at the anode. A theoretical model to calculate the distribution of current densities and the recombination rate in organic single layer devices is presented taking into account the charge injection process at each electrode, charge transport and recombination in organic layer. The calculated results indicate that efficient single-layer devices are possible by adjusting the barrier heights at two electrodes and the carrier mobilities. Lowering the barrier heights can improve the electroluminescent（EL） efficiency pronouncedly in many cases, and efficient devices are still possible using an ohmic contact to inject the low mobility carrier, and a contact limited contact to inject the high mobility carrier. All in all, high EL efficiency needs to consider sufficient recombination, enough injected carriers and well transport.     

Separation of local bulk and surface recombination in crystalline silicon from luminescence reabsorption

Spontaneous photoemission of crystalline silicon provides information on excess charge carrier density and thereby on electronic properties such as charge carrier recombination lifetime and series resistance. This paper is dedicated to separating bulk recombination from surface recombination in silicon solar cells and wafers by exploiting reabsorption of spontaneously emitted photons. The approach is based on a comparison between luminescence images acquired with different optical short pass filters and a comprehensive mathematical model. An algorithm to separate both front and back surface recombination velocities and minority carrier diffusion length from photoluminescence (PL) images on silicon wafers is introduced. This algorithm can likewise be used to simultaneously determine back surface recombination velocity and minority carrier diffusion length in the base of a standard crystalline silicon solar cell from electroluminescence (EL) images. The proposed method is successfully tested experimentally. Copyright © 2009 John Wiley & Sons, Ltd.     

Origin of high fill factor in polymer solar cells from semiconducting polymer with moderate charge carrier mobility

The fill factor of polymer bulk heterojunction solar cells (PSCs), which is mainly governed by the processes of charge carrier generation, recombination, transport and extraction, and the competition between them in the device, is one of the most important parameters that determine the power conversion efficiency of the device. We show that the fill factor of PSCs based on thieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C₇₁-butyric acid methylester (PC₇₁BM) blend that only have moderate carrier mobilities for hole and electron transport, can be enhanced to 76% by reducing the thickness of the photoactive layer. A drift–diffusion simulation study showed that reduced charge recombination loss is mainly responsible for the improvement of FF, as a result of manipulating spatial distribution of charge carrier in the photoactive layer. Furthermore, the reduction of the active layer thickness also leads to enhanced built-in electric field across the active layer, therefore can facilitate efficient charge carrier transport and extraction. Finally, the dependence of FF on charge carrier mobility and transport balance is also investigated theoretically, revealing that an ultrahigh FF of 80–82% is feasible if the charge mobility is high enough (∼10⁻³–10⁻¹ cm²/V s).     

Modeling of charge transfer by surface acoustic waves in a monolithic metal/ZnO/SiO2/Si system

This paper discusses modeling of minority-carrier charge transfer by surface acoustic waves (SAW's). An idealized, structure independent model which includes the mechanism of carrier diffusion is described and definitions of charge-transfer efficiency and charge capacity analogous to those of a conventional charge-coupled device are introduced and developed. The model is used to predict the fundamental upper limit performance of the device in the absence of surface-state trapping and bulk recombination generation. The parameters of the model are evaluated for the monolithic metal/ZnO/SiO2/Si system, and the model is used to predict charge distributions and charge capacity for surface wave frequencies in the range of 40 MHz to 2 GHz. At high frequencies, the predicted device performance is found to be limited by carrier diffusion and the SiO2thickness.     

Analytical modeling of organic solar cells including monomolecular recombination and carrier generation calculated by optical transfer matrix method

An analytical model of organic solar cell has been developed including the effect of monomolecular recombination and charge carrier generation rate, simultaneously. The charge carrier generation rate, depending on position and wavelength, obtained from optical transfer matrix method, has been incorporated into electrical transport equation of carriers; this has led to combining optical and electrical phenomena into this model. Charge carrier generation rate profile has been investigated and included to develop the model. The proposed model addresses the propagation of light and the effects of optical phenomena like reflection and interference inside the device. Compared to previous models, this model is an improved version because of considering recombination mechanism and position and wavelength dependent generation rate simultaneously. This analytical model is useful for finding the performance of the organic solar cell device such as current-voltage relation, power-voltage relation, efficiency, etc. avoiding the complexities of numerical calculations. The proposed model has been validated by comparing the results obtained from the model with that of published experimental works. This model may help to analyse organic solar cells and optimize their parameters for improving the performance.     

Novel Metal–Organic Framework Cocrystal Strategy for Significantly Enhancing Photocatalytic Performance

Owing to their high carrier recombination speed and low spectral utilization, it is difficult to further improve the performance of photocatalysts. In this study, a novel metal–organic framework (MOF) self-assembled cocrystal material is developed. The guest molecule is inserted and self-assembled with the existing MOF ligand to form an organic cocrystal. The highly ordered molecular arrangement and tight intermolecular distances between donor and acceptor molecules promote a strong π–π charge transfer interaction, facilitating the migration and separation of photogenerated charge carriers. In addition, efficient redshifts in the absorption wavelength enhance the response to visible light. Further, the unique porous structure of MOFs is beneficial for increasing the interfacial area of photocatalytic reactions, and metal ions can become the center of photogenerated carrier capture, effectively inhibiting carrier recombination. Consequently, the MOF cocrystal demonstrates remarkable efficiency in the degradation of pollutants in water, achieving a noteworthy removal efficiency of 95.31% within 15 min. Moreover, the photocatalyic reaction kinetics constant of the MOF cocrystal is 46.5 times higher, indicating the success of this new strategy in developing highly efficient photocatalytic systems.     

Capture of charge carriers and output power of a quantum well laser

The effect of noninstantaneous carrier capture by a nanoscale active region on the power characteristics of a semiconductor laser is studied. A laser structure based on a single quantum well is considered. It is shown that delayed carrier capture by the quantum well results in a decrease in the internal differential quantum efficiency and sublinearity of the light-current characteristic of the laser. The main parameter of the developed theoretical model is the velocity of carrier capture from the bulk (waveguide) region to the two-dimensional region (quantum well). The effect of the capture velocity on the dependence of the following laser characteristics on the pump current density is studied: the output optical power, internal quantum efficiency of stimulated emission, current of stimulated recombination in the quantum well, current of spontaneous recombination in the optical confinement layer, and carrier concentration in the optical confinement layer. A decrease in the carrier capture velocity results in a larger sublinearity of the light-current characteristic, which results from an increase in the injection current fraction expended to parasitic spontaneous recombination in the optical confinement layer and, hence, a decrease in the injection current fraction expended to stimulated recombination in the quantum well. A comparison of calculated and experimental light-current characteristics for a structure considered as an example shows that good agreement between them (up to a very high injection current density of 45 kA/cm²) is attained at a capture velocity of 2 × 10⁶ cm/s. The results of this study can be used to optimize quantum well lasers for generating high optical powers.     

Influence of Phase Segregation on Recombination Dynamics in Organic Bulk‐Heterojunction Solar Cells

The recombination dynamics of charge carriers in organic bulk‐heterojunction (BHJ) solar cells made of the blend system poly(2,5‐bis(3‐dodecylthiophen‐2‐yl)thieno[2,3‐b]thiophene) (pBTCT‐C₁₂):[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) with a donor–acceptor ratio of 1:1 and 1:4 are studied here. The techniques of charge‐carrier extraction by linearly increasing voltage (photo‐CELIV) and, as local probe, time‐resolved microwave conductivity are used. A difference of one order of magnitude is observed between the two blends in the initially extracted charge‐carrier concentration in the photo‐CELIV experiment, which can be assigned to an enhanced geminate recombination that arises through a fine interpenetrating network with isolated phase regions in the 1:1 pBTCT‐C₁₂:PC₆₁BM BHJ solar cells. In contrast, extensive phase segregation in 1:4 blend devices leads to an efficient polaron generation that results in an increased short‐circuit current density of the solar cells. For both studied ratios a bimolecular recombination of polarons is found using the complementary experiments. The charge‐carrier decay order of above two for temperatures below 300 K can be explained on the basis of a release of trapped charges. This mechanism leads to delayed bimolecular recombination processes. The experimental findings can be generalized to all polymer:fullerene blend systems allowing for phase segregation.     

Charge carrier mobility,bimolecular recombination and trapping in polycarbazole copolymer:fullerene (PCDTBT:PCBM) bulk heterojunction solar cells

Organic photovoltaic devices based on the donor:acceptor blend of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) have received considerable attention in recent years due to their high power conversion efficiencies and the ability to achieve close to 100% internal quantum efficiency. However, the highest efficiencies were all attained using active layers of less than 100 nm, which is not ideal for either maximised potential performance or commercial viability. Furthermore, more recent reports have documented significant charge carrier trapping in these devices. In this paper two charge extraction techniques (photo-CELIV and time-of-flight) have been used to investigate the mobility and recombination behaviour in a series of PCDTBT:PCBM devices. The results not only confirm significant charge carrier trapping in this system, but also reveal close to Langevin-type bimolecular recombination. The Langevin recombination causes a short charge carrier lifetime that results in a short drift length. The combination of these two characteristics (trapping and fast bimolecular recombination) has a detrimental effect on the charge extraction efficiency when active layers greater than ∼100 nm are used. This accounts for the pronounced decrease in fill factor with increasing active layer thickness that is typically observed in PCDTBT:PCBM devices.     

受体浓度影响体异质结有机太阳能电池复合损耗研究

制备基于P3HT:PCBM的复合体异质结太阳能电池 ,研究受体浓度对载流子复合特性的影响。测试结果表明, 受体浓度影响器件的电荷收集和复合损耗, 从而直接影响体异质结有机太阳能电池的光电性能; 光生电流随偏 置电压的增加而下降,光生电流下降趋势反映了载流子的复合损耗特性。理论分析进一步 表明,给体中电子与受体中 电子的密度比与受体浓度有直接关系,受体浓度改变双分子复合常数的大小,从而影响载流 子的复合特性。     

Measurement of radiative and nonradiative recombination rates in InGaAsP and AlGaAs light sources

A small signal method is used to measure the carrier lifetime as a function of injected carrier density, and the results are used to determine the radiative and nonradiative recombination rates for AlGaAs LED's and 1.3 μm InGaAsP lasers. For AlGaAs LED's the radiative recombination constant decreases with injected carrier density and the rate equation contains a small nonradiative Cn³term. The low internal efficiency of 1.3 μm InGaAsP lasers is found to be primarily caused by two factors: a radiative coefficientB(n)which strongly decreases with the injected carrier density, and CHHS Auger recombination having a recombination coefficient of1-2 times 10^{-29}cm⁶/s. A recombination term representing carrier leakage is observed in some devices, but it is not the principal cause of low internal efficiency.     

Performance and stability improvement in organic photovoltaics using non-toxic PEIE interfacial layers

The improvement in current density–voltage characteristics and external quantum efficiency of organic photovoltaics with non-toxic polyethylenimine ethoxylated (PEIE) as a modified layer between the electron transport layer and the active layer were demonstrated. The mechanisms of carrier transport, photon generation current, and the carrier recombination influenced by the PEIE layer were systematically studied. An extra PEIE layer was demonstrated having a shorter relaxation time via pump-probe spectroscopy, which provides better charge transfer ability. From capacitance-voltage measurement, the effective capacitance of the device increase with an addition PEIE layer, corresponding to the increase of accumulation carriers generated at the interfaces. Furthermore, electrochemical impedance spectroscopy elucidated that the PEIE layer can reduce the carrier recombination probability in the devices. As a result, the lifetime of the devices with a PEIE layer is improved as compared to the devices without PEIE layer.     

Intensity dependent but temperature independent charge carrier generation in organic photodiodes and solar cells

Measurements of the transient photoresponse of organic photodiodes and solar cells show a strong saturation effect in the quantum efficiency at laser fluences above approximately 3.3 μJ/cm². By a comparison of the measured intensity, temperature and field dependence of the transient pulse responses with extended drift–diffusion simulations, the loss of charge carriers can be traced back to a quadratic loss channel in the charge carrier generation process. In contrast to the predictions of the commonly used Onsager–Braun charge carrier generation model, we demonstrate that the dissociation of bound electron–hole-pairs is temperature independent but slightly field dependent.     

Efficient Perovskite Hybrid Solar Cells via Ionomer Interfacial Engineering

The surface of the solution‐processed methylammonium lead tri‐iodide (CH₃NH₃PbI₃) perovskite layer in perovskite hybrid solar cells (pero‐HSCs) tends to become rough during operation, which inevitably leads to deterioration of the contact between the perovskite layer and the charge‐extraction layers. Moreover, the low electrical conductivity of the electron extraction layer (EEL) gives rises to low electron collection efficiency and severe charge carrier recombination, resulting in energy loss during the charge‐extraction and ‐transport processes, lowering the efficiency of pero‐HSCs. To circumvent these problems, we utilize a solution‐processed ultrathin layer of a ionomer, 4‐lithium styrenesulfonic acid/styrene copolymer (LiSPS), to re‐engineer the interface of CH₃NH₃PbI₃ in planar heterojunction (PHJ) pero‐HSCs. As a result, PHJ pero‐HSCs are achieved with an increased photocurrent density of 20.90 mA cm^?2, an enlarged fill factor of 77.80%, a corresponding enhanced power conversion efficiency of 13.83%, high reproducibility, and low photocurrent hysteresis. Further investigation into the optical and electrical properties and the thin‐film morphologies of CH₃NH₃PbI₃ with and without LiSPS, and the photophysics of the pero‐HSCs with and without LiSPS are shown. These demonstrate that the high performance of the pero‐HSCs incorporated with LiSPS can be attributed to the reduction in both the charge carrier recombination and leakage current, as well as more efficient charge carrier collection, filling of the perforations in CH₃NH₃PbI_3, and a higher electrical conductivity of the LiSPS thin layer. These results demonstrate that our method provides a simple way to boost the efficiency of pero‐HSCs.     

Fermi Level Engineering of Passivation and Electron Transport Materials for p‐Type CuBi2O4 Employing a High‐Throughput Methodology

Metal oxide semiconductors are promising for solar photochemistry if the issues of excessive charge carrier recombination and material degradation can be resolved, which are both influenced by surface quality and interface chemistry. Coating the semiconductor with an overlayer to passivate surface states is a common remedial strategy but is less desirable than application of a functional coating that can improve carrier extraction and reduce recombination while mitigating corrosion. In this work, a data‐driven materials science approach utilizing high‐throughput methodologies, including inkjet printing and scanning droplet electrochemical cell measurements, is used to create and evaluate multi‐element coating libraries to discover new classes of candidate passivation and electron‐selective contact materials for p‐type CuBi₂O₄. The optimized overlayer (Cu_1.5TiO_z) improves the onset potential by 110 mV, the photocurrent by 2.8×, and the absorbed photon‐to‐current efficiency by 15.5% compared to non‐coated photoelectrodes. It is shown that these enhancements are related to reduced surface recombination through passivation of surface defect states as well as improved carrier extraction efficiency through Fermi level engineering. This work presents a generalizable, high‐throughput method to design and optimize passivation materials for a variety of semiconductors, providing a powerful platform for development of high‐performance photoelectrodes for incorporation into solar‐fuel generation systems.     

Peculiarities of radiative recombination in amorphous molecular semiconductors doped with polymethine dye

Processes of charge carrier photogeneration and recombination are investigated in films of poly-N-epoxypropylcarbazole doped with polymethine dye. Films with blocking contacts were illuminated with light from either the region of dye absorption or beyond this region. The kinetics of accumulation and relaxation of electron–hole pairs with lifetimes greater than tens or hundreds of seconds was studied. It is presumed that the reason for the growth of recombination luminescence intensity in an external electric field is connected with the increase in efficiency of radiative recombination stimulated by electrons captured from photogenerated excitons. © 1998 John Wiley & Sons, Ltd.     

Effect of surface recombination on the transient decay of excess carriers produced by short wavelength laser pulses

Effect of surface recombination on the transient decay of excess carriers injected very near to the surface region has been analysed for a semi-infinite semiconductor sample. The one dimensional continuity equation for the excess minority carriers has been solved analytically assuming that the surface recombination is important only on the front surface from which the excess carriers are injected. The decay of total excess carrier charge as a function of time is calculated for various values of the surface recombination velocities. It is found that values of S lower than 10³ cm/sec. have little effect on the decay of total excess charge and the decay is exponential. For values of S larger than 10⁴ cm/sec. the initial decay of excess charge is much faster and is dominated by the surface recombination. However, if one waits long enough for the excess carrier to diffuse into the semiconductor the decay becomes exponential when the excess charge decays to about 5% of its initial value. This exponential decay can be used to determine the excess carrier lifetime fairly accurately. A source of error in such experiments may arise from the heating of the silicon sample by the laser pulse. This effect, however is negligible for short duration pulses of low average power.     

Grain Boundary and Interface Passivation with Core–Shell Au@CdS Nanospheres for High‐Efficiency Perovskite Solar Cells

The plasmonic characteristic of core–shell nanomaterials can effectively improve exciton‐generation/dissociation and carrier‐transfer/collection. In this work, a new strategy based on core–shell Au@CdS nanospheres is introduced to passivate perovskite grain boundaries (GBs) and the perovskite/hole transport layer interface via an antisolvent process. These core–shell Au@CdS nanoparticles can trigger heterogeneous nucleation of the perovskite precursor for high‐quality perovskite films through the formation of the intermediate Au@CdS–PbI₂ adduct, which can lower the valence band maximum of the 2,2,7,7‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)9,9‐spirobifluorene (Spiro‐OMeTAD) for a more favorable energy alignment with the perovskite material. With the help of the localized surface plasmon resonance effect of Au@CdS, holes can easily overcome the barrier at the perovskite/Spiro‐OMeTAD interface (or GBs) through the bridge of the intermediate Au@CdS–PbI₂, avoiding the carrier accumulation, and suppress the carrier trap recombination at the Spiro‐OMeTAD/perovskite interface. Consequently, the Au@CdS‐based perovskite solar cell device achieves a high efficiency of over 21%, with excellent stability of ≈90% retention of initial power conversion efficiencies after 45 days storage in dry air.