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.     

Unraveling Charge Separation and Transport Mechanisms in Aqueous‐Processed Polymer/CdTe Nanocrystal Hybrid Solar Cells

Recently great progress has been achieved in highly effective hybrid solar cells fabricated using aqueous materials. The state‐of‐the‐art energy conversion efficiency has been close to 5% with high photocurrent. However, charge separation and transport mechanism in the aqueous‐processed hybrid solar cells are rarely reported and are usually assumed to be similar to oil‐phase processed systems; that is, self‐assembly polymers are mainly responsible for charge separation and carrier transport. To date, this assumption has prohibited further improvement of the conversion efficiency in aqueous‐processed hybrid systems by adopting any appropriate technique routes. Here, ultrafast carrier dynamics in these hybrid solar cells consisting of poly(p‐phenylenevinylene) (PPV)‐based aqueous polymers and water‐solution CdTe nanocrystals (NCs) are investigated in detail. Self‐charge separation in grown CdTe NC partly capped CdS shell layers after anneal treatment is unambiguously identified. Different from their oil‐soluble counterparts, these core/shell nanocrystals do not have the restrictions of quantum confinement and surface ligands, form effective charge transport networks, and play a dominant role in the charge separation and carrier transport processes. These findings provide a greater understanding on the fundamental photophysics in aqueous‐processed hybrid systems.     

Influence of Charge Transport and Defects on the Performance of Planar and Mesostructured Perovskite Solar Cells

Photoinduced charge selective carrier extraction by linearly increasing voltage technique allows straightforward assessment of charge transport properties within planar and mesostructured perovskite solar cells with respect to light intensity and signal delay time. Charge sensitive device architecture is realized through implementation of insulating layer between the anode or cathode to prevent extraction of unwanted type of carriers. Resulting behavior of comparatively efficient mesoporous and planar solar cells exhibits well balanced charge transport with slight dependence of charge mobility on applied laser pulse fluence, for given pulse delay times. Very similar charge carrier mobilities are present within mesoporous devices, whereas holes trail approximately half an order of magnitude behind electrons in planar structured specimens. Moreover, dispersive transport is identified in the electron selective devices with titanium oxide electron transporter, suggesting considerable presence of trapping states at the perovskite interface, whereas no such behavior characterizes planar samples. Variation in delay time between laser pulse and extraction ramp only affects initial charge concentration present within the device, while transient outlay remains unchanged, indicating absence of film charging effect.     

Improving Perovskite Solar Cells: Insights From a Validated Device Model

To improve the efficiency of existing perovskite solar cells (PSCs), a detailed understanding of the underlying device physics during their operation is essential. Here, a device model has been developed and validated that describes the operation of PSCs and quantitatively explains the role of contacts, the electron and hole transport layers, charge generation, drift and diffusion of charge carriers and recombination. The simulation to the experimental data of vacuum‐deposited CH₃NH₃PbI₃ solar cells over multiple thicknesses has been fit and the device behavior under different operating conditions has been studied to delineate the influence of the external bias, charge‐carrier mobilities, energetic barriers for charge injection/extraction and, different recombination channels on the solar cell performance. By doing so, a unique set of material parameters and physical processes that describe these solar cells is identified. Trap‐assisted recombination at material interfaces is the dominant recombination channel limiting device performance and passivation of traps increases the power conversion efficiency (PCE) of these devices by 40%. Finally, guidelines to increase their performance have been issued and it is shown that a PCE beyond 25% is within reach.     

Electron‐Beam‐Related Studies of Halide Perovskites: Challenges and Opportunities

Electron beam microscopy and related characterization techniques play an important role in revealing the microstructural, morphological, physical, and chemical information of halide perovskites and their impact on associated optoelectronic devices. However, electron beam irradiation usually causes damage to these beam‐sensitive materials, negatively impacting their device performance, and complicating this interpretation. In this article, the electron microscopy and spectroscopy techniques are reviewed that are crucial for the understanding of the crystallization and microstructure of halide perovskites. In addition, special attention is paid to assessing and mitigating the electron beam‐induced damage caused by these techniques. Since the halide perovskites are fragile, a protocol involving delicate control of both electron beam dose and dose rate, coupled with careful data analysis, is key to enable the acquisition of reliable structural and compositional information such as atomic‐resolution images, chemical elemental mapping and electron diffraction patterns. Limiting the electron beam dose is critical parameter enabling the characterization of various halide perovskites. Novel methods to unveil the mechanisms of device operation, including charge carrier generation, diffusion, and extraction are presented in scanning electron microscopy studies combined with electron‐beam‐induced current and cathodoluminescence mapping. Future opportunities for electron‐beam‐related characterizations of halide perovskites are also discussed.     

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.     

Light Absorption and Recycling in Hybrid Metal Halide Perovskite Photovoltaic Devices

The production of highly efficient single‐ and multijunction metal halide perovskite (MHP) solar cells requires careful optimization of the optical and electrical properties of these devices. Here, precise control of CH₃NH₃PbI₃ perovskite layers is demonstrated in solar cell devices through the use of dual source coevaporation. Light absorption and device performance are tracked for incorporated MHP films ranging from ≈67 nm to ≈1.4 µm thickness and transfer‐matrix optical modeling is utilized to quantify optical losses that arise from interference effects. Based on these results, a device with 19.2% steady‐state power conversion efficiency is achieved through incorporation of a perovskite film with near‐optimum predicted thickness (≈709 nm). Significantly, a clear signature of photon reabsorption is observed in perovskite films that have the same thickness (≈709 nm) as in the optimized device. Despite the positive effect of photon recycling associated with photon reabsorption, devices with thicker (>750 nm) MHP layers exhibit poor performance owing to competing nonradiative charge recombination in a “dead‐volume” of MHP. Overall, these findings demonstrate the need for fine control over MHP thickness to achieve the highest efficiency cells, and accurate consideration of photon reabsorption, optical interference, and charge transport properties.     

Relation of Nanostructure and Recombination Dynamics in a Low‐Temperature Solution‐Processed CuInS2 Nanocrystalline Solar Cell

The understanding and control of nanostructures with regard to transport and recombination mechanisms is of key importance in the optimization of the power conversion efficiency (PCE) of solar cells based on inorganic nanocrystals. Here, the transport properties of solution‐processed solar cells are investigated using photo‐CELIV (photogenerated charge carrier extraction by linearly increasing voltage) and transient photovoltage techniques; the solar cells are prepared by an in‐situ formation of CuInS₂ nanocrystals (CIS NCs) at the low temperature of 270 °C. Structural and morphological analyses reveal the presence of a metastable CuIn₅S₈ phase and a disordered morphology in the CuInS₂ nanocrytalline films consisting of polycrystalline grains at the nanoscale range. Consistent with the disordered morphology of the CIS NC thin films, the CIS NC devices are characterized by a low carrier mobility. The carrier density dynamic indicates that the recombination kinetics in these devices follows the dispersive bimolecular recombination model and does not fully behave in a diffusion‐controlled manner, as expected by Langevin‐type recombination. The mobility–lifetime product of the charge carriers properly explains the performance of the thin (200 nm) CIS NC solar cell with a high fill‐factor of 64% and a PCE of over 3.5%.     

Correlation of Absorption Profile and Fill Factor in Organic Solar Cells: The Role of Mobility Imbalance

We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C₆₀ active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge‐carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C₆₀ that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non‐geminate recombination is the dominating loss mechanism in these devices. Numerical drift‐diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage.     

Mapping Spatially Resolved Charge Collection Probability within P3HT:PCBM Bulk Heterojunction Photovoltaics

Material properties in polymer and fullerene bulk heterojunctions (BHJs) such as donor to acceptor volume fraction, morphology, and molecular orientation critically influence light absorption, exciton dissociation, charge transport, and recombination, all of which are crucial device properties in organic photovoltaics (OPV). Spatial variation of BHJ properties normal to the substrate, caused by phase segregation, can thereby create corresponding spatial variations in the OPVs optoelectronic properties. Here, normally incident and wave‐guided optical modes are used to selectively excite localized regions within an inverted poly(3‐hexythiophene‐2,5‐diyl) and phenyl‐C61‐butyric acid methyl ester BHJ OPV and corresponding internal quantum efficiencies are measured to study the spatial‐dependent charge carrier collection probability within the BHJ. An electron‐limited charge collection profile is observed for a thick (920 nm) BHJ due to fullerene‐poor regions as a result of phase segregation. As the thickness of the BHJ is reduced (100 nm), charge transport is seen to be unaffected by the phase segregation. This has the potential to be a versatile non‐destructive characterization technique for measuring the spatially varying charge collection probability in thin film photovoltaics and will help enable optimum device design and characterization.     

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

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

Impact of Fullerene on the Photophysics of Ternary Small Molecule Organic Solar Cells

Ternary organic solar cells (OSCs) are among the best‐performing organic photovoltaic devices to date, largely due to the recent development of nonfullerene acceptors. However, fullerene molecules still play an important role in ternary OSC systems, since, for reasons not well understood, they often improve the device performance, despite their lack of absorption. Here, the photophysics of a prototypical ternary small‐molecule OSC blend composed of the donor DR3, the nonfullerene acceptor ICC6, and the fullerene derivative PC₇₁BM is studied by ultrafast spectroscopy. Surprisingly, it is found that after excitation of PC₇₁BM, ultrafast singlet energy transfer to ICC6 competes efficiently with charge transfer. Subsequently, singlets on ICC6 undergo hole transfer to DR3, resulting in free charge generation. Interestingly, PC₇₁BM improves indirectly the electron mobility of the ternary blend, while electrons reside predominantly in ICC6 domains as indicated by fast spectroscopy. The improved mobility facilitates charge carrier extraction, in turn leading to higher device efficiencies of the ternary compared to binary solar cells. Using the (photo)physical parameters obtained from (transient) spectroscopy and charge transport measurements, the device's current–voltage characteristics are simulated and it is demonstrated that the parameters accurately reproduce the experimentally measured device performance.     

Charge Creation and Recombination in Multi‐Length Scale Polymer:Fullerene BHJ Solar Cell Morphologies

While the extremes in organic photovoltaic bulk heterojunction morphology (finely mixed or large pure domains) are easily understood and known to be unfavorable, efficient devices often exhibit a complex multi‐length scale, multi‐phase morphology. The impact of such multiple length scales and their respective purities and volume fractions on device performance remains unclear. Here, the average spatial composition variations, i.e., volume‐average purities, are quantified at multiple size scales to elucidate their effect on charge creation and recombination in a complex, multi‐length scale polymer:fullerene system (PBDTTPD:PC₇₁BM). The apparent domain size as observed in TEM is not a causative parameter. Instead, a linear relationship is found between average purity at length scales <50 nm and device fill‐factor. Our findings show that a high volume fraction of pure phases at the smallest length scales is required in multi‐length scale systems to aid charge creation and diminish recombination in polymer:fullerene solar cells.     

The lipid raft hypothesis revisited - New insights on raft composition and function from super-resolution fluorescence microscopy

Recently developed super‐resolution microscopy techniques are changing our understanding of lipid rafts and membrane organisation in general. The lipid raft hypothesis postulates that cholesterol can drive the formation of ordered domains within the plasma membrane of cells, which may serve as platforms for cell signalling and membrane trafficking. There is now a wealth of evidence for these domains. However, their study has hitherto been hampered by the resolution limit of optical microscopy, making the definition of their properties problematic and contentious. New microscopy techniques circumvent the resolution limit and, for the first time, allow the fluorescence imaging of structures on length scales below 200 nm. This review describes such techniques, particularly as applied to the study of membrane organisation, synthesising newly emerging facets of lipid raft biology into a state‐of‐the art model. Editor's suggested further reading in BioEssays: Super‐resolution imaging prompts re‐thinking of cell biology mechanisms Abstract and Quantitative analysis of photoactivated localization microscopy (PALM) datasets using pair‐correlation analysis Abstract     

Probing Charge Recombination Dynamics in Organic Photovoltaic Devices under Open‐Circuit Conditions

Large perturbation transient photovoltage and impedance spectroscopy measurements are used to gain insights into recombination in organic photovoltaic devices. The combination of these two simple optoelectronic techniques enables characterization of recombination order as well as mobile and trapped charge evolution over a large range of carrier densities. The data show that trapped charge is approximately equal to total charge at low carrier densities in the high efficiency devices measured. Between low and high charge carrier density, the order of recombination is observed to vary from monomolecular to bimolecular to higher order. The new techniques and methods presented can be applied to any type of photovoltage device to gain insight into device operation and limitations.     

Charge‐Carrier Mobility Requirements for Bulk Heterojunction Solar Cells with High Fill Factor and External Quantum Efficiency >90%

To increase the efficiency of bulk heterojunction (BHJ) solar cells beyond 15%, 300 nm thick devices with 0.8 fill factor (FF) and external quantum efficiency (EQE) >90% are likely needed. This work demonstrates that numerical device simulators are a powerful tool for investigating charge‐carrier transport in BHJ devices and are useful for rapidly determining what semiconductor pro­perties are needed to reach these performance milestones. The electron and hole mobility in a BHJ must be ≈10^?2 cm² V^?1 s^?1 in order to attain a 0.8 FF in a 300 nm thick device with the recombination rate constant of poly(3‐hexyl­thiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM). Thus, the hole mobility of donor polymers needs to increase from ≈10^?4 to ≈10^?2 cm² V^?1 s^?1 in order to significantly improve device performance. Furthermore, the charge‐carrier mobility required for high FF is directly proportional to the BHJ recombination rate constant, which demonstrates that decreasing the recombination rate constant could dramatically improve the efficiency of optically thick devices. These findings suggest that researchers should prioritize improving charge‐carrier mobility when synthesizing new materials for BHJ solar cells and highlight that they should aim to understand what factors affect the recombination rate constant in these devices.     

Charge Formation in Pentacene Layers During Solar‐Cell Fabrication: Direct Observation by Electron Spin Resonance

Microscopic characterization of charge carriers in solar cells is useful for high‐performance cell fabrication because the formation and accumulation of charges in cells greatly affect the device performance. Electron spin resonance (ESR) is suitable for such characterization because it can directly observe charge carriers with spins in these cells. In this work, the ESR method is applied to organic thin‐film solar cells to investigate charge formation in such devices. Heterojunction cells of indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS)/pentacene/C₆₀/bathocuproine (BCP)/Al are investigated. Clear ESR signals are observed by inserting a typical PEDOT:PSS hole buffer layer. From analysis of the dependence of the ESR characteristics on the external magnetic field direction, the bias voltage, and the duration of solar‐simulated irradiation, the charges (mobile holes) in pentacene layers are successfully identified and it can be deduced that these holes are formed at the PEDOT:PSS/pentacene interface during device fabrication. This ESR analysis provides useful knowledge for understanding device operation and improving device performance at the microscopic level.     

Quantifying Charge‐Carrier Mobilities and Recombination Rates in Metal Halide Perovskites from Time‐Resolved Microwave Photoconductivity Measurements

The unprecedented rise in the power conversion efficiency of solar cells based on metal halide perovskites (MHPs) has led to enormous research effort to understand their photophysical properties. The progress made in understanding the mobility and recombination of photogenerated charge carriers from nanosecond to microsecond time scales, monitored using electrodeless transient photoconductivity techniques, is reviewed. In addition, a kinetic model to obtain rate constants from transient data recorded using a wide range of laser intensities is presented. For various MHPs the temperature dependence of the mobilities and recombination rates are evaluated. Furthermore, it is shown how these rate constants can be used to predict the upper limit for the open‐circuit voltage V_oc of the corresponding device. Finally, the photophysical properties of MHPs that are not yet fully understood are explored, and recommendations for future research directions are made.