Imbalanced Mobilities Research Articles

Defect passivation and carrier management via a multifunctional additive for efficient and stable wide-bandgap perovskite solar cells with high fill factor

Wide-bandgap (WBG) Methylammonium (MA)-free based perovskite solar cells (PSCs) have attracted widespread research interests due to their broad applicability in multijunction tandem. Nevertheless, the efficiency of WBG PSCs is restricted by unfulfilled potential in fill factor and large deficits in open-circuit voltage. The imbalanced carrier mobility not only plays a critical role in bulk non-radiative recombination loss but also hinders efficient charge collection for mixed I/Br WBG perovskites. Herein, we introduced additive named 2,8-Diiododibenzothiophene to overcome above issues. Inverted devices with enhanced photostability and a record fill factor of 85.7 % are demonstrated. Moreover, combining the semitransparent WBG PSCs with 1.25 eV-bandgap perovskite bottom cells, we realized 4-terminal tandem devices and a champion PCE of 28 % is achieved. The multifunctional additive strategy demonstrated here, will open new avenues for improving WBG PSCs’ performance.

Eliminating the Imbalanced Mobility Bottlenecks via Reshaping Internal Potential Distribution in Organic Photovoltaics.

The imbalanced carrier mobility remains a bottleneck for performance breakthrough in even those organic solar cells (OSCs) with recorded power conversion efficiencies (PCEs). Herein, a counter electrode doping strategy is proposed to reshape the internal potential distribution, which targets to extract the low mobility carriers at far end. Device simulations reveal that the key of this strategy is to partially dope the active layer with a certain depth, therefore it strengthens the electric field for low mobility carriers near counter electrode region while avoids zeroing the electric field near collection electrode region. Taking advantage of these, PCE enhancements are obtained from 15.4% to 16.2% and from 16.9% to 18.0%, respectively, via cathode p-doping and anode n-doping. Extending its application from opaque to semitransparent devices, the PCE of dilute cell rises from 10.5% to 12.1%, with a high light utilization efficiency (LUE) of 3.5%. The findings provide practical solutions to the core device physical problem in OSCs.

Modeling the impacts of energetic disorder and charge transport on the exciton formation zone in organic light-emitting diodes

The width of exciton formation zone plays a vital role in determining the long-term stability of organic light-emitting diodes (OLEDs). Here, based on the general model of carrier device lifetimes, the dependences of the width and location of exciton formation zone on energetic disorder and charge transport have been simulated in OLEDs with emissive layer featuring hole and electron transport on host and guest molecules, respectively. The width of exciton formation zone increases with emissive layer’s energetic disorder increasing. The emissive layer with properly imbalanced carrier mobilities enables larger width of exciton formation zone than that with balanced carrier mobilities, due to the different hole and electron trap densities. Moreover, increasing hole transport layer’s hole mobility or decreasing electron transport layer’s electron mobility helps increase the width of exciton formation zone. There is no width of exciton formation zone obtained, when hole (electron) transport layer’s hole (electron) mobility is smaller than a certain value. The location of exciton formation relies on not only emissive layer’s carrier mobilities but also hole (electron) transport layer’s hole (electron) mobility. The general model provides the comprehensive picture to elucidate the roles of energetic disorder and charge transport in enhancing the stability of OLEDs, beneficial to improve the OLEDs designs towards the high stability at high luminance.

Facility or Transport Inequality? Decomposing Healthcare Accessibility Inequality in Shenzhen, China.

Accessibility to healthcare services is crucial for residents’ wellbeing. Numerous studies have revealed significant spatial inequality in healthcare accessibility across various contexts. However, it still remains unclear whether the inequality is caused by the unbalanced spatial distribution of healthcare facilities or by unequal transport access to them. This study decomposes inequality in healthcare accessibility into facility- and transport-driven inequality by comparing scenarios of healthcare accessibility, which consider various combinations of multidimensional components of accessibility using different distance measures. Using a case study in Shenzhen, this study reveals that both facility distribution and transport access substantially contribute to spatial inequality in healthcare accessibility. Facility distribution accounts for 61.3% and 50.8% of the overall accessibility inequality for driving and transit modes, respectively. The remaining inequality is induced by imbalanced mobility provided by transport networks. Furthermore, the impact of transport component on healthcare accessibility is unevenly distributed. This study highlights that both facility- and transport-related countermeasures should be considered to improve the accessibility and equality of healthcare services. It provides transferable methods for quantitatively decomposing facility- and transport-driven inequality in accessibility to healthcare or other facilities.

Electrochemiluminescent Transistors: A New Strategy toward Light-Emitting Switching Devices.

Light-emitting transistors (LETs) have attracted a significant amount of interest as multifunctional building blocks for next-generation electronics and optoelectronic devices. However, it is challenging to obtain LETs with a high carrier mobility and uniform light-emission because the semiconductor channel should provide both the electrical charge transport and optical light-emission, and typical emissive semiconductors have low, imbalanced carrier mobilities. In this work, a novel device platform that adapts the electrochemiluminescence (ECL) principle in LETs, referred to as an ECL transistor (ECLT) is proposed. ECL is a light-emission phenomenon from electrochemically excited luminophores generated by redox reactions. A solid-state ECL electrolyte consisting of a network-forming polymer, ionic liquid, luminophore, and co-reactant is employed as the light-emitting gate insulator of the ECLT. Based on this construction, high-performance LETs that make use of various conventional non-emissive semiconductors (e.g., poly(3-hexylthiophene), zinc oxide, and reduced graphene oxide) are successfully demonstrated. All the devices exhibit a high mobility (0.9-10 cm2 V-1 s-1 ) and a uniform light-emission. This innovative approach demonstrates a novel LET platform and provides a promising pathway to achieve significant breakthroughs to develop electronic circuits and optoelectronic applications.

Effect of Imbalanced Charge Transport on the Interplay of Surface and Bulk Recombination in Organic Solar Cells

Surface recombination has a major impact on the open-circuit voltage ($V_\text{oc}$) of organic photovoltaics. Here, we study how this loss mechanism is influenced by imbalanced charge transport in the photoactive layer. As a model system, we use organic solar cells with a two orders of magnitude higher electron than hole mobility. We find that small variations in the work function of the anode have a strong effect on the light intensity dependence of $V_\text{oc}$. Transient measurements and drift-diffusion simulations reveal that this is due to a change in the surface recombination rather than the bulk recombination. We use our numerical model to generalize these findings and determine under which circumstances the effect of contacts is stronger or weaker compared to the idealized case of balanced charge transport. Finally, we derive analytical expressions for $V_\text{oc}$ in the case that a pile-up of space charge is present due to highly imbalanced mobilities.

Modeling the Taxi Drivers’ Customer-Searching Behaviors outside Downtown Areas

A popular phenomenon in the street-hailing taxi system is the imbalanced mobility services between city central and outside downtown areas, which leads to unmet demand outside downtown areas and competitions in city central areas. Understanding taxi drivers’ customer-searching behaviors is crucial to addressing the phenomenon and redistributing the taxi supply. However, the current literature ignores or simply models the taxi drivers’ behaviors, in particular, lacks the in-depth discussions on individuals’ heterogeneity. This study introduces the latent class model to identify the internal and external factors influencing the taxi drivers’ destination choice after the last drop-offs. Beyond the influencing factors, the modeling structure captures the heterogeneity in vacant taxicab drivers through introducing latent classes. The proposed model outperforms other discrete choice models, for instance, multinomial logit, nested logit, and mixed logit, based on the two study cases developed from the New York City yellow taxicab system. The empirical results first statistically indicate the existence of latent classes, which further empirically prove the heterogeneity in the choices by vacant taxicab drivers while searching customers. Moreover, we obtain a set of internal and external factors influencing the customer searching behaviors. For example, the taxicab drivers are sensitive to the demand at the search destination areas and the distance from the last drop-off location to the search destination areas and behave identically in particular under the conditions of high demand and short search distance. On the other hand, the external variables have different impacts on customer searching behaviors across the different groups of drivers in the both study cases, including peak hours, weekday, holiday, earned fare from last occupied trip, raining hours, and flight arrivals at airports. In final, the proposed modeling structure and findings are useful as a sub-model of taxi system modeling while developing strategies, as well as as a regional planning tool for taxi supply estimations.

Bimolecular recombination and fill factor in crystalline polymer solar cells

Fill factor (FF) is one of the important photovoltaic parameters, which is limited mainly by charge recombination in solar cells. It is still a challenging issue to realize a high FF with a thick active layer, which is required for the development of efficient bulk-heterojunction polymer solar cells. In this study, we discussed how recombination losses limit charge collection efficiency in polymer solar cells with low FF by transient optoelectronic techniques. As a result, we found that geminate recombination is almost voltage-independent, that bimolecular recombination is suppressed significantly under open-circuit condition, and that hole and electron mobilities are low and imbalanced. We therefore concluded that the low collection efficiency is caused by the bimolecular recombination of accumulated charges due to low and imbalanced mobilities.

Imbalanced Student Mobility in India: A Serious Concern

A major challenge that India is facing is the existing imbalance between inbound and outbound student mobility. The inflow–outflow ratio of about 1:10 is a matter of concern. Evidence shows that it is not only the human capital that is flowing out of India, but also a substantial amount of revenue. In connection with this, concerns have been raised, which need immediate attention from Indian policy makers.

Imbalanced Student Mobility in India: A Serious Concern

A major challenge that India is facing is the existing imbalance between inbound and outbound student mobility. The inflow–outflow ratio of about 1:10 is a matter of concern. Evidence shows that it is not only the human capital that is flowing out of India, but also a substantial amount of revenue. In connection with this, concerns have been raised, which need immediate attention from Indian policy makers.

On the Impact of Contact Selectivity and Charge Transport on the Open‐Circuit Voltage of Organic Solar Cells

The selectivity of electrodes of solar cells is a critical factor that can limit the overall efficiency. If the selectivity of an electrode is not sufficient both electrons and holes recombine at its surface. In materials with poor transport properties such as in organic solar cells, these surface recombination currents are accompanied by large gradients of the quasi‐Fermi energies as the driving force. Experimental results from current–voltage characteristics, advanced photo‐ and electroluminescence as well as charge extraction of three different photoactive materials are shown and compared to drift‐diffusion simulations. It can be concluded that in cases of electrodes with reduced selectivity the decrease of the open‐circuit voltage can be divided into two distinct contributions, the reduction of the overall steady‐state charge carrier density and the gradients of the quasi‐Fermi energies. The results clearly show that for photoactive layers with poor transport properties, the gradient of the quasi‐Fermi energy in the vicinity of the contact is the main contribution to the loss in open‐circuit voltage. For imbalanced mobilities, this gives rise to the phenomenon that it is more challenging to realize a selective contact for the less mobile charge carrier, i.e., the hole contact in most organic solar cells.

A route to high gain photodetectors through suppressed recombination in disordered films

Secondary photocurrents offer an alternative mechanism to photomultiplier tubes and avalanche diodes for making high gain photodetectors that are able to operate even at extremely low light conditions. While in the past secondary currents were studied mainly in ordered crystalline semiconductors, disordered systems offer some key advantages such as a potentially lower leakage current and typically longer photocarrier lifetimes due to trapping. In this work, we use numerical simulations to identify the critical device and material parameters required to achieve high photocurrent and gain in steady state. We find that imbalanced mobilities and suppressed, non-Langevin-type charge carrier recombination will produce the highest gain. A low light intensity, strong electric field, and a large single carrier space charge limited current are also beneficial for reaching high gains. These results would be useful for practical photodetector fabrication when aiming to maximize the gain.

Reduced Recombination in High Efficiency Molecular Nematic Liquid Crystalline: Fullerene Solar Cells

Bimolecular recombination in bulk heterojunction organic solar cells is the process by which nongeminate photogenerated free carriers encounter each other, and combine to form a charge transfer (CT) state which subsequently relaxes to the ground state. It is governed by the diffusion of the slower and faster carriers toward the electron donor–acceptor interface. In an increasing number of systems, the recombination rate constant is measured to be lower than that predicted by Langevin's model for relative Brownian motion and the capture of opposite charges. This study investigates the dynamics of charge generation, transport, and recombination in a nematic liquid crystalline donor:fullerene acceptor system that gives solar cells with initial power conversion efficiencies of >9.5%. Unusually, and advantageously from a manufacturing perspective, these efficiencies are maintained in junctions thicker than 300 nm. Despite finding imbalanced and moderate carrier mobilities in this blend, strongly suppressed bimolecular recombination is observed, which is ≈150 times less than predicted by Langevin theory, or indeed, more recent and advanced models that take into account the domain size and the spatial separation of electrons and holes. The suppressed bimolecular recombination arises from the fact that ground‐state decay of the CT state is significantly slower than dissociation.

Charge Transport without Recombination in Organic Solar Cells and Photodiodes

Decoupling charge generation and extraction is critical to understanding loss mechanisms in polymer:fullerene organic solar cells and photodiodes but has thus far proven to be a challenging task. Using steady-state and time-resolved light intensity dependent photocurrent (iPC) measurements in combination with transient photovoltage, we estimate the total charge inside a typical device during steady-state photoconduction, which is defined by the trapped, doping-induced, and mobile charge populations. Our results show that nongeminate recombination of any order can be avoided as long as this charge is much less than that capable of being stored on the electrodes—a criterion that is typically met in the linear iPC regime in donor:fullerene systems even with low, imbalanced mobilities. Knowing the conditions under which nongeminate recombination is essentially absent is an important device and materials design consideration. Our work also demonstrates that the technique of iPC is not only useful to assess the charge extraction efficiency but can also be used to estimate the efficiency of free carrier generation in fully operational devices.

Efficient, Large Area, and Thick Junction Polymer Solar Cells with Balanced Mobilities and Low Defect Densities

The high power conversion efficiencies (PCEs) of laboratory‐scale polymer‐based organic solar cells are yet to translate to large area modules because of a number of factors including the relatively large sheet resistance of available transparent conducting electrodes (TCEs), and the high defect densities associated with thin organic semiconductor junctions. The TCE problem limits device architectures to narrow connected strips (<1 cm) causing serious fabrication difficulties and extra costs. Thin junctions are required because of poor charge transport (imbalanced mobilities) in the constituent organic semiconductors. These issues are addressed using a combination of approaches to create thick junctions conformally coated on low sheet resistance metal grid TCEs. An essential feature of these thick junctions is balanced carrier mobilities, which affords high fill factors and efficient carrier extraction. Conformal coating is achieved by promoting enhanced intermolecular interactions in the coating solution using a high molecular weight polymeric semiconductor and appropriate solvent system. This combination of balanced mobilities, conformal coating and metallic grid TCEs is a simple and generic approach to the fabrication of defect‐free large area organic solar cells (OSCs). The approach is demonstrated with 25 cm2 monolithic devices possessing aperture‐corrected power conversion efficiencies of 5% and fill factors exceeding 0.5.

Balanced Carrier Mobilities: Not a Necessary Condition for High‐Efficiency Thin Organic Solar Cells as Determined by MIS‐CELIV

A novel technique based upon injection‐charge extraction by linearly increasing voltage (i‐CELIV) in a metal‐insulator‐semiconductor (MIS) diode structure is described for studying charge transport in organic semiconductors. The technique (MIS‐CELIV) allows selective measurement of both electron and hole mobilities of organic solar cells with active layers thicknesses representative of operational devices. The method is used to study the model high efficiency bulk heterojunction combination poly[N‐9′′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) and [6,6]‐phenyl C70‐butyric acid methyl ester (PC70BM) at various blend ratios. The absence of bipolar transport in PCDTBT‐and‐PC70BM‐only diodes is shown and strongly imbalanced carrier mobility is found in the most efficient “optimized” blend ratios. The mobility measurements are correlated with overall device performance and it is found that balanced and high charge carrier mobility are not necessarily required for high efficiencies in thin film organic solar cells.

Investigation of Driving Forces for Charge Extraction in Organic Solar Cells: Transient Photocurrent Measurements on Solar Cells Showing S‐Shaped Current–Voltage Characteristics

AbstractThe role of drift and diffusion as driving forces for charge carrier extraction in flat heterojunction organic solar cells is examined at the example of devices showing intentional S‐shaped current–voltage (J‐V) characteristics. Since these kinks are related to energy barriers causing a redistribution of the electric field and charge carrier density gradients, they are suitable for studying the limits of charge extraction. The dynamics of this redistribution process are experimentally monitored via transient photocurrents, where the current response on square pulses of light is measured in the μs to ms regime. In combination with drift‐diffusion simulation data, we demonstrate a pile‐up of charge carriers at extraction barriers and a high contribution of diffusion to photocurrent in the case of injection barriers. Both types of barrier lead to S‐kinks in the J‐V curve and can be distinguished from each other and from other reasons for S‐kinks (e.g. imbalanced mobilities) by applying the presented approach. Furthermore, it is also helpful to investigate the driving forces for charge extraction in devices without S‐shaped J‐V curve close to open circuit to evaluate whether their electrodes are optimized.

Imbalanced mobilities causing S-shaped IV curves in planar heterojunction organic solar cells

We show that S-kinks in the current voltage characteristics, which decrease the fill factor significantly, can be caused by a strong imbalance of charge carrier mobilities (hole mobility in donor and electron mobility in acceptor) in planar/flat heterojunction organic solar cells. Electrical simulations according to a drift-diffusion model predict the occurrence of an S-kink for a mobility mismatch factor larger than 100. By combining a low-mobility donor material, (1,2,3,4,9,10,11,12-octaphenyl-diindeno[1,2,3-cd:1′,2′,3′-lm]perylene), with the acceptors C60 and N,N′-dimethylperylene-3,4:9,10-dicarboximide, which show different electron mobilities, we experimentally verify the predictions. Our results demonstrate that not only interface effects but also the photoactive material itself can cause S-kinks.

Imbalanced charge mobility in oxygen treated polythiophene/fullerene based bulk heterojunction solar cells

The effect of oxygen induced traps on charge mobility in bulk heterojunction solar cells using poly(3-hexylthiophene) (P3HT):l-(3-methoxycarbonyl)-propyl-l-phenyl-(6, 6) methanofullerene (PCBM) blend have been studied using photoinduced charge extraction by linearly increasing voltage (PhotoCELIV) technique. The solar cells exposed to oxygen exhibit dual PhotoCELIV peaks, whereas the solar cell without oxygen treatment show single PhotoCELIV peak with the charge mobility of the order of 10−4 cm2/V s. It is demonstrated that the oxygen treatment imbalance the charge mobility in the P3HT/PCBM photoactive layer, which affects the power conversion efficiency and lifetime of the solar cell. The single PhotoCELIV peak for the device without oxygen treatment indicates that the charge mobility is balanced, that causes the overlapping of electron and hole transients.

Loss of photocurrent efficiency in low mobility semiconductors: Analytic approach to space charge effects

We derive an analytic expression for the photocurrent efficiency as a function of the optical excitation power within the framework of space charge limit of Mott and Gurney [Electronic Processes in Ionic Crystals (Oxford University Press, London, 1940)]. This complements the approach based on charge recombination and we show that the two give similar expressions. Namely, in low mobility and intrinsic semiconductors (as conjugated polymers) based photocell, where recombination follows Langevin’s expression, the onsets of space charge and of charge recombination coincide. The analysis shows that the onset of space charge or recombination depends only on the slow carrier mobility value and do not require imbalanced mobility values.