How Molecules with Dipole Moments Enhance the Selectivity of Electrodes in Organic Solar Cells – A Combined Experimental and Theoretical Approach

Simple organic molecules with permanent dipole moments – amino acids and heterocycles – have been successfully employed in bulk‐heterojunction organic solar cells as interlayer between photoactive material and electron contact. A large increase of open‐circuit voltage and fill factor can be observed for four different polymers as donor material in the photoactive layer. A combination of current–voltage curves, scanning Kelvin‐probe atomic force microscopy, ultraviolet photoelectron spectroscopy, and electroluminescence measurements as well as numerical simulations are carried out to clarify in detail the underlying mechanisms. All results fully confirm the hypothesis that the main effect is an accumulation of electrons and a depletion of holes in the photoactive layer in the vicinity of the electron contact induced by a decrease of its effective work function. Further, density functional theory calculations and literature reports of the energy levels of the dipole molecules strongly suggest that the charge carriers tunnel through the thin dipole layer which does however not limit the current. This represents a versatile, simple, and cheap method to realize highly selective contacts which may also be beneficial for other types of solar cells and devices where contact selectivity is crucial.     

Identifying the Impact of Surface Recombination at Electrodes in Organic Solar Cells by Means of Electroluminescence and Modeling

This work reports on combining current‐voltage characteristics, electroluminescence (EL) measurements, and modeling to identify the selectivity of the electrodes in bulk‐heterojunction organic solar cells. Devices with the same photoactive layer but different contact materials are compared and the impact of surface recombination at the contacts on their performance is determined. The open‐circuit voltage, V _OC, depends strongly on the selectivity of the electrodes and it is observed that the EL signal of cells with lower V _OC is dramatically reduced. This is ascribed to an enhanced rate of surface recombination, which is a non‐radiative recombination pathway and does therefore not contribute to the EL yield. In addition, these cells have a lower current in forward direction despite the fact that the surface recombination occurs in addition to the recombination in the bulk. A theoretical model was set up and in the corresponding numerical simulations all three findings (lower V _OC, strongly reduced EL signal and lower forward current) could be clearly reproduced by varying just one single parameter which determines the selectivity of the electrode.     

Carrier‐Selectivity‐Dependent Charge Recombination Dynamics in Organic Photovoltaic Cells with a Ferroelectric Blend Interlayer

Interfacial energetics determines the performance of organic photovoltaic (OPV) cells based on a thin film of organic semiconductor blends. Here, an approach to modulating the “carrier selectivity” at the charge collecting interfaces and the consequent variations in the nongeminate charge carrier recombination dynamics in OPV devices are demonstrated. A ferroelectric blend interfacial layer composed of a solution‐processable ferroelectric poly­mer and a wide bandgap semiconductor is introduced as a tunable electron selective layer in inverted OPV devices with non‐Ohmic contact electrodes. The direct rendering of dipole alignment within the ferroelectric blend layer is found to increase the carrier selectivity of the charge collecting interfaces up to two orders of magnitude. Transient photovoltaic analyses reveal that the increase of carrier selectivity significantly reduces the diffusion and recombination among minority carriers in the vicinity of the electrodes, giving rise to the 85% increased charge carrier lifetime. Furthermore, the carrier‐selective charge extraction leads to the constitution of the internal potential within the devices, even with energetically identical cathodes and anodes. With these carrier‐selectivity‐controlled interlayers, the devices based on various photoactive materials commonly display significant increments in the device performances, especially with the high fill factor of up to 0.76 under optimized conditions.     

Investigating the Influence of Interfacial Contact Properties on Open Circuit Voltages in Organic Photovoltaic Performance: Work Function Versus Selectivity

The role of work function and thermodynamic selectivity of hole collecting contacts on the origin of open circuit voltage (V_OC) in bulk heterojunction organic photovoltaics is examined for poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and [6,6]‐phenyl‐C₇₁ butyric acid methyl ester (PC₇₁BM) solar cells. In the absence of a charge selective, electron blocking contact, systematic variation of the work function of the contact directly dictates the V_OC, as defined by the energetic separation between the relative Fermi levels for holes and electrons, with little change in the observed dark saturation current, J₀. Improving the charge selectivity of the contact through an increased barrier to electron injection from the fullerene in the blend into the hole contact results in a decreased reverse saturation current (decreased J₀ and increased shunt resistance, R_SH) and improved V_OC. Based on these observations, we provide a set of contact design criteria for tuning the V_OC in bulk heterojunction organic photovoltaics.     

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

The 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.     

Spatial Distribution Recast for Organic Bulk Heterojunctions for High‐Performance All‐Inorganic Perovskite/Organic Integrated Solar Cells

All‐inorganic CsPbIBr₂ perovskite solar cells (pero‐SCs) exhibit excellent overall stability, but their power conversion efficiencies (PCEs) are greatly limited by their wide bandgaps. Integrated solar cells (ISCs) are considered to be an emergent technology that could extend their photoresponse by directly stacking two distinct photoactive layers with complementary bandgaps. However, rising photocurrents always sacrifice other photovoltaic parameters, thereby leading to an unsatisfactory PCE. Here, a recast strategy is proposed to optimize the spatial distribution components of low‐bandgap organic bulk‐heterojunction (BHJ) film, and is combined with an all‐inorganic perovskite to construct perovskite/BHJ ISCs. With this strategy, the integrated perovskite/BHJ film with a top‐enriched donor‐material spatial distribution is shown to effectively improve ambipolar charge transport behavior and suppress charge carrier recombination. For the first time, the ISC is not only significantly extended and enhanced the photoresponse achieving a 20% increase in current density, but also exhibits a high open‐circuit voltage and fill factor at the same time. As a result, a record PCE of 11.08% based on CsPbIBr₂ pero‐SCs is realized; it simultaneously shows excellent long‐term stability against heat and ultraviolet light.     

Disorder‐Induced Open‐Circuit Voltage Losses in Organic Solar Cells During Photoinduced Burn‐In

The photoinduced open‐circuit voltage (V_oc) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn‐in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the V_oc losses observed during burn‐in are caused by a redistribution of charge carriers in a broader DOS. The temperature and light intensity dependence of the V_oc losses can be described with an analytical model that contains the amount of disorder broadening in a Gaussian DOS as the only fit parameter. Finally, the V_oc loss in solar cells made from amorphous and crystalline polymers is compared and an increased stability observed in crystalline polymer solar cells is investigated. It is found that solar cells made from crystalline materials have a considerably higher charge carrier density than those with amorphous materials. The effects of a DOS broadening upon aging are suppressed in solar cells with crystalline materials due to their higher carrier density, making crystalline materials more stable against V_oc losses during burn‐in.     

Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells

Charge transport in organic photovoltaic (OPV) devices is often characterized by space‐charge limited currents (SCLC). However, this technique only probes the transport of charges residing at quasi‐equilibrium energies in the disorder‐broadened density of states (DOS). In contrast, in an operating OPV device the photogenerated carriers are typically created at higher energies in the DOS, followed by slow thermalization. Here, by ultrafast time‐resolved experiments and simulations it is shown that in disordered polymer/fullerene and polymer/polymer OPVs, the mobility of photogenerated carriers significantly exceeds that of injected carriers probed by SCLC. Time‐resolved charge transport in a polymer/polymer OPV device is measured with exceptionally high (picosecond) time resolution. The essential physics that SCLC fails to capture is that of photo­generated carrier thermalization, which boosts carrier mobility. It is predicted that only for materials with a sufficiently low energetic disorder, thermalization effects on carrier transport can be neglected. For a typical device thickness of 100 nm, the limiting energetic disorder is σ ≈71 (56) meV for maximum‐power point (short‐circuit) conditions, depending on the error one is willing to accept. As in typical OPV materials the disorder is usually larger, the results question the validity of the SCLC method to describe operating OPVs.     

Nongeminate Recombination in Planar and Bulk Heterojunction Organic Solar Cells

Nongeminate recombination in organic solar cells based on copper phthalocyanine (CuPc) and C₆₀ is investigated. Two device architectures, the planar heterojunction (PHJ) and the bulk heterojunction (BHJ), are directly compared in view of differences in charge carrier decay dynamics. A combination of transient photovoltage (TPV) experiments, yielding the small perturbation charge carrier lifetime, and charge extraction measurements, providing the charge carrier density is applied. In organic solar cells, charge photogeneration and recombination primarily occur at the donor–acceptor heterointerface. Whereas the BHJ can often be approximated by an effective medium due to rather small scale phase separation, the PHJ has a well defined two‐dimensional heterointerface. In order to study nongeminate recombination dynamics in PHJ devices the charge accumulation at this interface is most relavent. As only the spatially averaged carrier concentration can be determined from extraction techniques, the charge carrier density at the interface n_int is derived from the open circuit voltage. Comparing the experimental results with macroscopic device simulation, the differences of recombination and charge carrier densities in CuPc:C₆₀ PHJ and BHJ devices are discussed with respect to the device performance. The open circuit voltage of BHJ is larger than for PHJ at low light intensities, but at 0.3 sun the situation is reversed: here, the PHJ can finally take advantage of its generally longer charge carrier lifetimes, as the active recombination region is smaller.     

Quantifying Efficiency Loss of Perovskite Solar Cells by a Modified Detailed Balance Model

A modified detailed balance model is built to understand and quantify efficiency loss of perovskite solar cells. The modified model captures the light‐absorption‐dependent short‐circuit current, contact and transport‐layer‐modified carrier transport, as well as recombination and photon‐recycling‐influenced open‐circuit voltage. The theoretical and experimental results show that for experimentally optimized perovskite solar cells with the power conversion efficiency of 19%, optical loss of 25%, nonradiative recombination loss of 35%, and ohmic loss of 35% are the three dominant loss factors for approaching the 31% efficiency limit of perovskite solar cells. It is also found that the optical loss climbs up to 40% for a thin‐active‐layer design. Moreover, a misconfigured transport layer introduces above 15% of energy loss. Finally, the perovskite‐interface‐induced surface recombination, ohmic loss, and current leakage should be further reduced to upgrade device efficiency and eliminate hysteresis effect. This work contributes to fundamental understanding of device physics of perovskite solar cells. The developed model offers a systematic design and analysis tool to photovoltaic science and technology.     

Tantalum Nitride Electron‐Selective Contact for Crystalline Silicon Solar Cells

Minimizing carrier recombination at contact regions by using carrier‐selective contact materials, instead of heavily doping the silicon, has attracted considerable attention for high‐efficiency, low‐cost crystalline silicon (c‐Si) solar cells. A novel electron‐selective, passivating contact for c‐Si solar cells is presented. Tantalum nitride (TaN _x ) thin films deposited by atomic layer deposition are demonstrated to provide excellent electron‐transporting and hole‐blocking properties to the silicon surface, due to their small conduction band offset and large valence band offset. Thin TaN_x interlayers provide moderate passivation of the silicon surfaces while simultaneously allowing a low contact resistivity to n‐type silicon. A power conversion efficiency (PCE) of over 20% is demonstrated with c‐Si solar cells featuring a simple full‐area electron‐selective TaN_x contact, which significantly improves the fill factor and the open circuit voltage (V_oc) and hence provides the higher PCE. The work opens up the possibility of using metal nitrides, instead of metal oxides, as carrier‐selective contacts or electron transport layers for photovoltaic devices.     

MXene‐Contacted Silicon Solar Cells with 11.5% Efficiency

MXene, a new class of 2D materials, has gained significant attention owing to its attractive electrical conductivity, tunable work function, and metallic nature for wide range of applications. Herein, delaminated few layered Ti₃C₂T_x MXene contacted Si solar cells with a maximum power conversion efficiency (PCE) of ≈11.5% under AM1.5G illumination are demonstrated. The formation of an Ohmic junction of the metallic MXene to n⁺‐Si surface efficiently extracts the photogenerated electrons from n⁺np⁺‐Si, decreases the contact resistance, and suppresses the charge carrier recombination, giving rise to excellent open‐circuit voltage and short‐circuit current density. The rapid thermal annealing process further improves the electrical contact between Ti₃C₂T_x MXene and n⁺‐Si surface by reducing sheet resistance, increasing electrical conductivity, and decreasing cell series resistance, thus leading to a remarkable improvement in fill factor and overall PCE. The work demonstrated here can be extended to other MXene compositions as potential electrodes for developing highly performing solar cells.     

Phthalimide Polymer Donor Guests Enable over 17% Efficient Organic Solar Cells via Parallel‐Like Ternary and Quaternary Strategies

Ternary strategies show over 16% efficiencies with increased current/voltage owing to complementary absorption/aligned energy level contributions. However, poor understanding of how the guest components tune the active layer structures still makes rational selection of material systems challenging. In this study, two phthalimide based ultrawide bandgap polymer donor guests are synthesized. Parallel energies between the highest occupied molecular orbitals of host and guest polymers are achieved via incorporating selnophene on the guest polymer. Solid‐state ¹⁹F magic angle spinning nuclear magnetic spectroscopy, graze‐incidence wide‐angle X‐ray diffraction, elemental transmission electron microscopy mapping, and transient absorption spectroscopy are combined to characterize the active layer structures. Formation of the individual guest phases selectively improves the structural order of donor and acceptor phase. The increased electron mobility in combination with the presence of the additional paths made by the guest not only minimizes the influence on charge generation and transport of the host system but also contributes to increasing the overall current generation. Therefore, phthalimide based polymers can be potential candidates that enable the simultaneous increase of open‐circuit voltage and short‐circuit current‐density via fine‐tuning energy levels and the formation of additional paths for enhancing current generation in parallel‐like multicomponent organic solar cells.     

Photo‐Carrier Recombination in Polymer Solar Cells Based on P3HT and Silole‐Based Copolymer

Photo‐current loss in donor‐acceptor (DA) polymer‐fullerene bulk heterojunction solar cells was studied via carrier transport and recombination measurements. Focusing on the DA polymer poly((4,4‐dioctyldithieno (3,2‐b:2',3'‐d) silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl) (DTS‐BTD) we found that the carrier transport is well‐balanced and attribute the loss mechanism in DTS‐BTD solar cells to carrier recombination. Using carrier extraction with linear increasing voltage (photo‐CELIV) and transient photo‐voltage (TPV), we show that carrier recombination plays an important role in photo‐current extraction at open circuit conditions due to increase in photo‐excited carrier concentration. Delay time dependent photo‐CELIV and temperature dependent transport studies suggest that the recombination rate is related to the degree of energetic disorder in the polymer: fullerene blends.     

Achieving High Open‐Circuit Voltages up to 1.57 V in Hole‐Transport‐Material‐Free MAPbBr3 Solar Cells with Carbon Electrodes

An open‐circuit voltage (V_oc) of 1.57 V under simulated AM1.5 sunlight in planar MAPbBr₃ solar cells with carbon (graphite) electrodes is obtained. The hole‐transport‐material‐free MAPbBr₃ solar cells with the normal architecture (FTO/TiO₂/MAPbBr₃/carbon) show little hysteresis during current–voltage sweep under simulated AM1.5 sunlight. A solar‐to‐electricity power conversion efficiency of 8.70% is achieved with the champion device. Accordingly, it is proposed that the carbon electrodes are effective to extract photogenerated holes in MAPbBr₃ solar cells, and the industry‐applicable carbon electrodes will not limit the performance of bromide‐based perovskite solar cells. Based on the analysis of the band alignment, it is found that the voltage (energy) loss across the interface between MAPbBr₃ and carbon is very small compared to the offset between the valence band maximum of MAPbBr₃ and the work function of graphite. This finding implies either Fermi level pinning or highly doped region inside MAPbBr₃ layer exists. The band‐edge electroluminescence spectra of MAPbBr₃ from the solar cells further support no back‐transfer pathways of electrons across the MAPbBr₃/TiO₂ interface.     

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.     

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.     

Recombination Losses Above and Below the Transport Percolation Threshold in Bulk Heterojunction Organic Solar Cells

Achieving the highest power conversion efficiencies in bulk heterojunction organic solar cells requires a morphology that delivers electron and hole percolation pathways for optimized transport, plus sufficient donor:acceptor contact area for near unity charge transfer state formation. This is a significant structural challenge, particularly in semiconducting polymer:fullerene systems. This balancing act in the model high efficiency PTB7:PC70BM blend is studied by tuning the donor:acceptor ratio, with a view to understanding the recombination loss mechanisms above and below the fullerene transport percolation threshold. The internal quantum efficiency is found to be strongly correlated to the slower carrier mobility in agreement with other recent studies. Furthermore, second‐order recombination losses dominate the shape of the current density–voltage curve in efficient blend combinations, where the fullerene phase is percolated. However, below the charge transport percolation threshold, there is an electric‐field dependence of first‐order losses, which includes electric‐field‐dependent photogeneration. In the intermediate regime, the fill factor appears to be limited by both first‐ and second‐order losses. These findings provide additional basic understanding of the interplay between the bulk heterojunction morphology and the order of recombination in organic solar cells. They also shed light on the limitations of widely used transport models below the percolation threshold.     

Identifying the Nature of Charge Recombination in Organic Solar Cells from Charge‐Transfer State Electroluminescence

Charge‐transfer (CT) state electroluminescence is investigated in several polymer:fullerene bulk heterojunction solar cells. The ideality factor of the electroluminescence reveals that the CT emission in polymer:fullerene solar cells originates from free‐carrier bimolecular recombination at the donor‐acceptor interface, rather than a charge‐trap‐mediated process. The fingerprint of the presence of nonradiative trap‐assisted recombination, a voltage‐dependent CT electroluminescence quantum efficiency, is only observed for the P3HT:PCBM system, which is explained by a reduction of the competing bimolecular recombination rate. These results are in agreement with measurements of the illumination‐intensity dependence of the open‐circuit voltage.     

Solution Processable Inorganic–Organic Double‐Layered Hole Transport Layer for Highly Stable Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have reached their highest efficiency with 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD). However, this material can cause problems with respect to reproducibility and stability. Herein, a solution‐processable inorganic–organic double layer based on tungsten oxide (WO₃) and spiro‐OMeTAD is reported as a hole transport layer in PSCs. The device equipped with a WO₃/spiro‐OMeTAD layer achieves the highest efficiency (21.44%) in the tin (IV) oxide planar structure. The electronic properties of the double layer are thoroughly analyzed using photoluminescence, space‐charge–limited current, and electrochemical impedance spectroscopy. The WO₃/spiro‐OMeTAD layer exhibits better hole extraction ability and faster hole mobility. The WO₃ layer particularly improves the open‐circuit voltage (V_OC) by lowering the quasi‐Fermi energy level for holes and reducing charge recombination, resulting in high V_OC (1.17 V in the champion cell). In addition, the WO₃ layer as a scaffold layer promotes the formation of a uniform and pinhole‐free spiro‐OMeTAD overlayer in the WO₃/spiro‐OMeTAD layer. High stability under thermal and humid conditions stems from this property. The study presents a facile approach for improving the efficiency and stability of PSCs by stacking an organic layer on an inorganic layer.