Boundary condition model for the simulation of organic solar cells

Organic solar cells (OSCs) are promising photovoltaic devices to convert solar energy into electrical energy. Their many advantages such as lightweight, flexibility and low manufacturing costs are intrinsic to the organic/polymeric technology. However, because the performance of OSCs is still not competitive with inorganic solar cells, there is urgent need to improve the device performance using better designs, technologies and models. In this work, we focus on developing an accurate physics-based model that relates the charge carrier density at the metal-organic boundaries to the current density in OSCs. This analysis is based on our previous studies on single-carrier and bipolar diodes. The model for the boundary condition of the charge carrier density at the interfaces of OSCs follows a power-law function with the current density, both in dark and under illumination. Simulated current-voltage characteristics are verified with experimental results. The numerical simulations of the current-voltage characteristics of OSCs consider well-established models for the main physical and optical processes that take place in the device: light absorption and generation of excitons, dissociation of excitons into free charge carriers, charge transport, recombination and injection-extraction of free carriers. Our analysis provides important insights on the influence of the metal-organic interfaces on the overall performance of OSCs. The model is also used to explain the anomalous S-shape current-voltage curves found in some experimental data.     

Effect of spatial distribution of generation rate on bulk heterojunction organic solar cell performance: A novel semi-analytical approach

We present a physics-based semi-analytical model of bulk heterojunction (BHJ) organic solar cell (OSC) for predicting the electrical characteristics of the device, taking into account the space dependency of generation rate profiles. The model enables us to derive the J-V characteristics of BHJ OSC without the need of a closed form expression of arbitrary carrier generation rate (which may not exist), hence avoiding the cumbersome numerical fitting method employed in literature previously. Using the proposed model, we perform an extensive analysis to study the effect of spatial distribution of generation rate profiles on the device performance. For this purpose, we use Gaussian shaped profiles that have a common average value thus retaining the total number of generated carriers same. We vary the position of the generation peak and its sharpness (width of the Gaussian peak) as well as number of peaks to analyze their effects on device efficiency. For the considered profiles, results show that the optimized profile has a peak carrier generation rate exactly halfway through the active layer and falls off sharply on either side. In the end, we propose methods of controlling the generation profiles by modifying the device structure and perform optical simulation to show the corresponding generation profiles. Thus, we show the usefulness of our derived model in finding the spatial distribution of a given number of carriers along the active layer that yields the best device performance.     

Plasmonic Electrically Functionalized TiO2 for High‐Performance Organic Solar Cells

Optical effects of the plasmonic structures and the materials effects of the metal nanomaterials have recently been individually studied for enhancing performance of organic solar cells (OSCs). Here, the effects of plasmonically induced carrier generation and enhanced carrier extraction of the carrier transport layer (i.e., plasmonic‐electrical effects) in OSCs are investigated. Enhanced charge extraction in TiO₂ as a highly efficient electron transport layer by the incorporation of metal nanoparticles (NPs) is proposed and demonstrated. Efficient device performance is demonstrated by using Au NPs incorporated TiO₂ at a plasmonic wavelength (560–600 nm), which is far longer than the originally necessary UV light. By optimizing the concentration ratio of the Au NPs in the NP‐TiO₂ composite, the performances of OSCs with various polymer active layers are enhanced and efficiency of 8.74% is reached. An integrated optical and electrical model, which takes into account plasmonic‐induced hot carrier tunneling probability and extraction barrier between TiO₂ and the active layer, is introduced. The enhanced charge extraction under plasmonic illumination is attributed to the strong charge injection of plasmonically excited electrons from NPs into TiO₂. The mechanism favors trap filling in TiO₂, which can lower the effective energy barrier and facilitate carrier transport in OSCs.     

Mapping charge carrier density in organic thin-film transistors by time-resolved photoluminescence lifetime studies

The device performance of organic transistors is strongly influenced by the charge carrier distribution. A range of factors effect this distribution, including injection barriers at the metal-semiconductor interface, the morphology of the organic film, and charge traps at the dielectric/organic interface or at grain boundaries. In our comprehensive experimental and analytical work we demonstrate a method to characterize the charge carrier density in organic thin-film transistors using time-resolved photoluminescence spectroscopy. We developed a numerical model that describes the electrical and optical responses consistently. We determined the densities of free and trapped holes at the interface between the organic layer and the SiO₂ gate dielectric by comparison to electrical measurements. Furthermore by applying fluorescence lifetime imaging microscopy we determine the local charge carrier distribution between source and drain electrodes of the transistor for different biasing conditions. We observe the expected hole density gradient from source to drain electrode.     

Ultra-bright alternating current organic electroluminescence

We report on an alternating current (AC) field induced organic electroluminescence (EL) device with internal charge carrier generation and recombination luminance of over 5000 cd m^?2 under AC drive without charge carrier injection from external electrodes. The ultra-bright AC-EL is attributed to an optical optimization performed on the devices via numerical optical simulations based on an optical thin film model as well as an increase in the number of charge carriers achieved via the concept of molecular doping within the device. The luminance levels achieved are highest reported so far in literature for AC organic light emitting devices.     

Analysis of the dynamic short-circuit resistance in organic bulk-heterojunction solar cells: relation to the charge carrier collection efficiency

This work studies the charge carrier collection efficiency in organic bulk-heterojunction solar cells based on polymer:fullerene blends. An equivalent circuit with a specific recombination term is proposed to describe the behavior of this type of devices. It is experimentally shown that this recombination term determines the slope of the current–voltage characteristic at the short-circuit condition. The variation of this dynamic resistance with the light intensity can be interpreted considering a dominant first-order recombination process. Finally, an analytical model under a constant electric field approximation is presented that can be used to calculate the charge carrier collection efficiency of the device. This model can be also used to estimate an effective mobility–lifetime product, which is characteristic of the quality of the active layer.     

Improvement of photovoltaic performance of inverted hybrid solar cells by adding single-wall carbon nanotubes in poly(3-hexylthiophene)

Solution prepared hybrid solar cells show promising low cost technology for electricity generation from sun light, although their power conversion efficiency has to be improved. One of the approaches is to increase the absorbance or charge carrier mobility of organic semiconductors. In this work, pristine single walled carbon nanotubes (SWCNT) were added into poly(3-hexylthiophene) (P3HT) solution to form P3HT:SWCNT composite films with different weight percent (wt%) of SWCNT. It is observed that optical absorbance spectra as well as the morphology of the composite films were modified by the addition of SWCNTs. This phenomenon could be explained by the π-π interaction between the conjugated polymer and carbon nanotubes. Most importantly, the electrical conductivities of the composite films increased with the SWCNT wt%. When these films were used as hole conductor layers in inverted planar hybrid solar cell, with CdS thin films as electron acceptor layers, the fill factor (FF) and open-circuit voltage (Voc) of the corresponding cells were decreased with the increase of the wt% of SWCNT. However, the short-circuit current density (Jsc) and the power conversion efficiency (PCE) showed a maximum value at about 0.4 wt% of SWCNT in P3HT. The transient photovoltage measurements (TPV) revealed that the presence of SWNCT promoted the charge recombination process at P3HT/CdS interface, and as a result, reduced the Voc. The photovoltaic performance of the hybrid solar cells could be optimized by choosing an adequate weight percentage of SWCNT in P3HT to balance the charge carrier transport and charge recombination processes at the donor-acceptor interface.     

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.     

Design rules for semi-transparent organic tandem solar cells for window integration

For window integration of semi-transparent solar cells in living and working areas, color neutral transparency perception and good color rendering are of pivotal importance. In order to tune the optical device properties, we simulate a parallel tandem configuration with two different absorber materials. Within a regime of convenient transparency perception, the transparency can be adjusted between 20% and 40% by choosing the right absorber layer thickness combination. From the optical field in the tandem devices we calculate the charge carrier generation profile and subsequently correlate the optical properties with the electrical device properties as derived from drift-diffusion modelling – altogether allowing for a comprehensive assessment of the transparency, the transparency perception and the device performance and their interdependencies.     

Organic Field‐Effect Devices as Tool to Characterize the Bipolar Transport in Polymer‐Fullerene Blends: The Case of P3HT‐PCBM

In organic bulk heterojunction solar cells (oBHJ) the blend morphology in combination with the charge transport properties of the individual components controls the extracted photocurrent. The organic field‐effect transistor (OFET) has been proved as a powerful instrument to evaluate the unipolar carrier transport properties in a wide range of cases. In our work we extend the OFET concept to the evaluation of the bipolar transport properties in polymer‐fullerenes blends and propose a method to improve the accuracy of the evaluation. The method is based on capacitance–voltage (C–V) measurements on MOS structures prepared on the same blends and delivers complementary information on the bulk heterojunction to the one obtained with FETs. The relevance for photovoltaic applications is investigated through the correlation between the current–voltage behavior of solar cells and the bipolar mobility for composites with varying polymer molecular weight and processed from different solvents. In particular the transport features of solar cells produced from o‐Xylene (oX), a non chlorinated solvent more suitable to production requirements, have been compared to the one of devices cast from Chlorobenzene (CB) solution. For the P3HT‐PCBM blend a consistent correlation between the mobility and the electrical fill factor and power performance was found. A significant asymmetry in the bipolar carrier mobility, together with low electron mobility dependent on the M_w value, affects the performances of thick o‐Xylene cast devices. In the case of devices processed from Chlorobenzene the slower carrier has higher mobility and the small electrical losses detected are eventually more related to the formation of space‐charge and eventually to surface recombination. This results in an efficient charge collection that is almost thickness independent. We report a dependence of the slow‐carrier type (electrons or holes) and their mobility on the specific combination of molecular weight and solvent. The mobility data and the solar cell performance coherently fit to the prediction of a device model only based on the drift of carriers under the built‐in electric field originated in the donor‐acceptor oBHJ.     

Thermal analysis of organic solar cells using an enhanced opto-thermal model

In this paper, an opto-thermal model is presented in order to specify the dominant thermal phenomena in organic solar cells (OSCs), as rather low efficiency photovoltaic devices. This model is capable of predicting the amount of optical heat generation (Q_{th_opt}), also the transient and steady state thermal behavior of an organic photovoltaic cell combining both the optical and thermal models. In a typical organic solar cell, Q_{th_opt} plays a significant role in heating up the device while the electric heat generation (Q_{th_elec}) does not effectively have such a role. Developing an optical model for a solar cell, Q_{th_opt} can be determined in every position of the device; also, the contribution of each layer in heat generation is precisely specified. The device thermal behavior is predicted by feeding the thermal model with Q_{th_opt}. This is done for an organic solar cell with a typical architecture and it is shown that thermal convection and radiation are two determinative thermal phenomena while conduction plays a minor role; furthermore, the electrodes, Aluminum (Al) cathode and Indium Tin Oxide (ITO) anode, are two strong light absorbers which contribute to more than 80% of optical heat generation. Assuming Stefan–Boltzman radiation loss, the temperature rise for a typical single junction OSC is estimated under different conditions. The device temperature rise might be even larger for other architectures consisting of several layers depending on their thicknesses and absorption coefficients. This temperature increase enhances the OSCs’ efficiency while degrading the lifetime. The model can be applied to thermal analysis of other types of photovoltaic cells and optoelectronic devices with minor modification.     

Using Deep Machine Learning to Understand the Physical Performance Bottlenecks in Novel Thin‐Film Solar Cells

There is currently a worldwide effort to develop materials for solar energy harvesting which are efficient and cost effective, and do not emit significant levels of CO₂ during manufacture. When a researcher fabricates a novel device from a novel material system, it often takes many weeks of experimental effort and data analysis to understand why any given device/material combination produces an efficient or poorly optimized cell. It therefore takes the community tens of years to transform a promising material system to a fully optimized cell ready for production (perovskites are a contemporary example). Herein, developed is a new and rapid approach to understanding device/material performance, which uses a combination of machine learning, device modeling, and experiment. Providing a set of electrical device parameters (charge carrier mobilities, recombination rates, trap densities, etc.) in a matter of seconds thus offers a fast way to directly link fabrication conditions to device/material performance, pointing a way to further and more rapid optimization of light harvesting devices. The method is demonstrated by using it to understand annealing temperature and surfactant choice and in terms of charge carrier dynamics in organic solar cells made from the P3HT:PCBM, PBTZT‐stat‐BDTT‐8:PCBM, and PTB7:PCBM material systems.     

浴铜灵有机分子材料对钙钛矿薄膜表面的钝化作用

采用2,9-二甲基-4,7-二苯基-1,10-邻二氮杂菲(浴铜灵,缩写:BCP)有机小分子作为钙钛矿薄膜与电子传输层之间的界面修饰层,从而使得反型结构的钙钛矿太阳电池性能得到显著改善。通过扫描电子显微镜研究发现:BCP分子可在钙钛矿薄膜样品表面的晶界间充分填充,推测其抑制了界面缺陷态的产生。进一步研究器件内部界面电荷的累积,并结合交流阻抗谱的分析,证实经BCP钝化的钙钛矿太阳电池中界面电荷的累积减少,光生载流子的复合被抑制,电池的光电转换效率由原来的15.7%提升到了17.4%。     

Novel Approach for Alternating Current (AC)‐Driven Organic Light‐Emitting Devices

A novel approach for alternating current (AC)‐driven organic light‐emitting devices is reported, which uses the concept of molecular doping in organic semiconductors. Doped organic charge‐transport layers are used to generate charge carriers within the device, hence eliminating the need for injecting charge carriers from external electrodes. Bright luminance of up to 1000 cd m^?2 is observed when the device is driven with an AC bias. The luminance observed is attributed to charge‐carrier generation and recombination, leading to the formation of excitons within the device, without injection of charge carriers through external electrodes. A mechanism for internal charge‐carrier generation and the device operation is proposed.     

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。     

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.     

N-phenylindole-diketopyrrolopyrrole-containing narrow band-gap materials for dopant-free hole transporting layer of perovskite solar cell

Novel conjugated materials, DPIO and DPIE, having same molecular configuration of both an electron donating N-phenylindole and an electron accepting diketopyrrolopyrrole derivative, exhibited different aggregation behavior because of the applied side chains. When DPIO and DPIE were applied to as hole transporting materials in perovskite solar cell, DPIO showed better device performance than ones with DPIE, mostly due to the aggregation-assisted enhanced electrical property. DPIO effectively extracted hole from the perovskite layer, providing over 10% PCE of cell efficiency without any chemical doping. Incident-photon-to-electron conversion efficiency (IPCE) measurement confirmed that DPIO’s strong absorption in the longer wavelength region partly contributed to the light harvesting of the solar cell device. In addition, time-resolved photoluminescence (TRPL) and transient photovoltage (TPV) studies proved that the DPIO-based device, compared to the conventional Spiro-MeOTAD-based device, has better charge extraction ability and reduced charge recombination.     

Hot-carrier-induced degradation of LDD polysilicon TFTs

In order to improve the stability of polysilicon thin-film transistors (TFTs) several drain junction architectures have been proposed. In this paper, the hot-carrier (HC) related stability of the lightly doped drain (LDD) TFT architecture is analyzed by using an iterative algorithm that relates the HC induced damage to the carrier injection across the device interfaces with gate and substrate oxide. The resulting creation of interface states and trapped charge is taken into account by using a system of rate equations that implements mathematically the Lais two step model, in which the generation of interface states is attributed to the trapping of hot-holes by centres into the oxide followed by the recombination with hot electrons. The rate equations are solved self-consistently with the aid of a device simulation program. By successive iterations, the time evolution of the interface state density and positive trapped charge distribution has been reconstructed, and the electrical characteristics calculated with this model are in good agreement with experimental data. This algorithm represent an improvement of an already proposed degradation model, in which the interface states formation dynamics is accounted by using a phenomenological approach. The present model has been applied to reproduce the degradation pattern of LDD TFTs and it is found that generation of interface states proceed almost symmetrically on the front and back device interfaces, starting from the points in which the transverse electric field peaks, and moving toward the drain side of the device. The final interface states distribution determines a sort of "bottleneck" in the active layer carrier density, that can explain the sensitivity to HC induced damage of both transfer and output characteristics.     

Optical and electrical simulation of hybrid solar cell based on conjugated polymer and size-tunable CdSe quantum dots: Influence of the QDs size

The photovoltaic performance of hybrid solar cell based on poly(3-hexylthiophene) (P3HT) and size-tunable CdSe quantum dots is analyzed by combination of optical and electrical simulations. The employed optical and electrical models describe the dependency of solar cell characteristics on CdSe QDs diameter and active layer thickness. The device performance improvement is observed by increasing CdSe QDs diameter from 2.3 nm to 8.3 nm. The short circuit current density (Jsc) shows significant ascending trend by QDs diameter which is due to electron mobility (μ_n) and absorption range enhancement. However, the maximum achievable open circuit voltage (V_OC) decreases by QDs size growth, V_OC shows ascending trend with CdSe QDs diameter increase, because of the higher dissociation probability and lesser recombination rate for larger nanocrystals. As the electron mobility proportionally increases by CdSe QDs size, the performance dependency on the charge mobility is studied. Results show that by growing the size of CdSe QDs, charge extraction is dominant in the competition between recombination rate increase and charge extraction increase.     

Influence of Blend Morphology and Energetics on Charge Separation and Recombination Dynamics in Organic Solar Cells Incorporating a Nonfullerene Acceptor

Nonfullerene acceptors (NFAs) in blends with highly crystalline donor polymers have been shown to yield particularly high device voltage outputs, but typically more modest quantum yields for photocurrent generation as well as often lower fill factors (FF). In this study, we employ transient optical and optoelectronic analysis to elucidate the factors determining device photocurrent and FF in blends of the highly crystalline donor polymer PffBT4T‐2OD with the promising NFA FBR or the more widely studied fullerene acceptor PC₇₁BM. Geminate recombination losses, as measured by ultrafast transient absorption spectroscopy, are observed to be significantly higher for PffBT4T‐2OD:FBR blends. This is assigned to the smaller LUMO‐LUMO offset of the PffBT4T‐2OD:FBR blends relative to PffBT4T‐2OD:PC₇₁BM, resulting in the lower photocurrent generation efficiency obtained with FBR. Employing time delayed charge extraction measurements, these geminate recombination losses are observed to be field dependent, resulting in the lower FF observed with PffBT4T‐2OD:FBR devices. These data therefore provide a detailed understanding of the impact of acceptor design, and particularly acceptor energetics, on organic solar cell performance. Our study concludes with a discussion of the implications of these results for the design of NFAs in organic solar cells.