How the charge-neutrality level of interface states controls energy level alignment in cathode contacts of organic bulk-heterojunction solar cells

Blending Donors with Different Molecular Weights: An Efficient Strategy to Resolve the Conflict between Coherence Length and Intermixed Phase in Polymer/Nonfullerene Solar Cells

Long coherence lengths (CLs) of crystals and proper intermixed phase amount guarantee charge transport and exciton dissociate efficiently, which is crucial for organic solar cells (OSCs) to achieve high device performance. However, extending CLs usually reduces the intermixed phase, leading to an insufficient interface for exciton dissociation. Herein, a strategy using a binary polymer with different molecular weights as donor is employed, that is, poly(3-hexylthiophene-2,5-diyl) (P3HT) with high (P3HT-H) and low (P3HT-L) molecular weight are blended as donor, and (5Z,5'Z)-5,5'-(((4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR) is used as acceptor. In kinetics, the entanglements of P3HT-H are relieved due to the higher molecular diffusivity of P3HT-L. In thermodynamics, the miscibility of P3HT-L/O-IDTBR, P3HT-H/O-IDTBR, and P3HT-L/P3HT-H blends increases in turn. Hence, P3HT forms a more ordered structure with longer CLs after adding P3HT-L, which also drives O-IDTBR dispersed in P3HT crystalline regions diffuse to the O-IDTBR crystalline regions to further self-organize. Consequently, the CLs of both P3HT and O-IDTBR are extended, while keeping the intermixed phase amount proper. The optimized microstructure boosts device performance from 7.03% to 7.80%, which is one of the highest values reported for P3HT/O-IDTBR blends. This is a novel way to solve the conflict mentioned above, which may provide guidance to finely regulating the morphology of the active layer.

Impedance Spectroscopy of Metal Halide Perovskite Solar Cells from the Perspective of Equivalent Circuits

Impedance spectroscopy (IS) provides a detailed understanding of the dynamic phenomena underlying the operation of photovoltaic and optoelectronic devices. Here we provide a broad summary of the application of IS to metal halide perovskite materials, solar cells, electrooptic and memory devices. IS has been widely used to characterize perovskite solar cells, but the variability of samples and the presence of coupled ionic-electronic effects form a complex problem that has not been fully solved yet. We summarize the understanding that has been obtained so far, the basic methods and models, as well as the challenging points still present in this research field. Our approach emphasizes the importance of the equivalent circuit for monitoring the parameters that describe the response and providing a physical interpretation. We discuss the possibilities of models from the general perspective of solar cell behavior, and we describe the specific aspects and properties of the metal halide perovskites. We analyze the impact of the ionic effects and the memory effects, and we describe the combination of light-modulated techniques such as intensity modulated photocurrent spectroscopy (IMPS) for obtaining more detailed information in complex cases. The transformation of the frequency to time domain is discussed for the consistent interpretation of time transient techniques and the prediction of features of current-voltage hysteresis. We discuss in detail the stability issues and the occurrence of transformations of the sample coupled to the measurements.

Quantitative amplitude-modulation scanning Kelvin probe microscopy via the second eigenmode excitation

Amplitude modulation scanning Kelvin probe microscopy (AM-SKPM) is widely used to measure the contact potential difference (CPD) between probe and samples in ambient or dry inert atmosphere. However, AM-SKPM is generally considered quantitatively inaccurate due to crosstalk between the cantilever and the sample. Here we demonstrate that the accuracy of AM-SKPM-based CPD measurements is drastically improved by exciting the SKPM probe at its second eigenmode. In the second eigenmode of oscillation, there exists a stationary node at the cantilever towards its free end, across which the displacement bears opposite signs; therefore driving the SKPM probe at its second eigenmode helps to partially cancel the virtual work done by the cantilever and reduce the crosstalk effect. The improvement in accuracy is experimentally confirmed with interdigitating electrodes calibration samples as well as practical samples such as the cross-section of wafer-bonded GaAs/GaN heterojunction.

Heterojunction Perovskite Solar Cells: Opto-Electro-Thermal Physics, Modeling, and Experiment

Organic-inorganic heterojunction perovskite solar cell (PSC) is promising for low-cost and high-performance photovoltaics. To further promote the performance of PSCs, understanding and controlling the underneath photoconversion mechanisms are highly necessary. Here, we present a comprehensive opto-electro-thermal (OET) study on the heterojunction PSCs by quantitatively addressing the coupled optical, carrier transport, and thermodynamic behaviors within the device. With achieving a good agreement with the experiment, we theoretically explore the thermodynamic mechanisms involving the energy conversions and focus especially on the origins of the various energy losses in PSCs. We summarize six categories of microscopic heat conversion processes in the heterojunction PSC, where the Joule and Peltier heats can be defined as the intrinsic losses in PSCs. Moreover, we also discuss the possible manipulation methods to decrease the energy losses, for example, by tailoring the doping concentration and energy-level alignment. An exemplified OET optimization is also presented, which predicts that the PCE of the fabricated PSC can be enhanced from 21.37% to 23.84%.

Reconfiguration of interfacial energy band structure for high-performance inverted structure perovskite solar cells

Charged defects at the surface of the organic–inorganic perovskite active layer are detrimental to solar cells due to exacerbated charge carrier recombination. Here we show that charged surface defects can be benign after passivation and further exploited for reconfiguration of interfacial energy band structure. Based on the electrostatic interaction between oppositely charged ions, Lewis-acid-featured fullerene skeleton after iodide ionization (PCBB-3N-3I) not only efficiently passivates positively charged surface defects but also assembles on top of the perovskite active layer with preferred orientation. Consequently, PCBB-3N-3I with a strong molecular electric dipole forms a dipole interlayer to reconfigure interfacial energy band structure, leading to enhanced built-in potential and charge collection. As a result, inverted structure planar heterojunction perovskite solar cells exhibit the promising power conversion efficiency of 21.1% and robust ambient stability. This work opens up a new window to boost perovskite solar cells via rational exploitation of charged defects beyond passivation.

Charged surface defects are expected to undermine the charge extraction in organic-inorganic perovskite solar cells. Here Zhang et al. design ionic fullerene derivatives to not only passivate the charged defects, but also optimize the interfacial energy due to aligned orientation of the fullerenes.

Investigation of the buried planar interfaces in multi-layered inverted organic solar cells using x-ray reflectivity and impedance spectroscopy

The hole and electron extracting interlayers in the organic solar cells (OSCs) play an important role in high performing devices. The present work focuses on an investigation of Zinc oxide/bulk heterojunction (ZnO/BHJ) and BHJ/MoO _x (Molybdenum oxide) buried planar interfaces in inverted OSC devices using the optical contrast in various layers along with the electrical measurements. The x-ray reflectivity (XRR) analysis demonstrates the formation of additional intermixing layers at the interfaces of ZnO/BHJ and BHJ/MoO _x . Our results indicate infusion of PC71BM into ZnO layer up to ~4 nm which smoothen the ZnO/BHJ interface. In contrast, thermally evaporated MoO _x molecules diffuse into PTB7-Th dominant upper layers of BHJ active layer resulting in an intermixed layer at the interface of MoO _x /BHJ. The high recombination resistance (~5 kΩ cm²) and electron lifetime (~70 μs), obtained from the impedance spectroscopy (IS), support such vertical segregation of PTB7-Th and PC71BM in the active layer. The OSC devices, processed in ambient condition, exhibit high power conversion efficiency of 6.4%. We consider our results have great significance to understand the structure of buried planar interfaces at interlayers and their correlation with the electrical parameters representing various interfacial mechanisms of OSCs.

Treating the Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Surface with Hydroquinone Enhances the Performance of Polymer Solar Cells

The introduction of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a standard hole transport layer greatly increased the efficiency of early organic solar cells. However, because PEDOT:PSS has a metallic property, it can still form a barrier by means of metal-semiconductor contact at its interface with the photoactive layer. In this study, we modified the PEDOT:PSS surface with hydroquinone (HQ) to remove that barrier. HQ treatment of the PEDOT:PSS surface lowered the hole transport barrier at the interface between the PEDOT:PSS and the active layer. In addition, because of the secondary doping effect of HQ, the sheet resistance of the PEDOT:PSS surface decreased by almost 2 orders of magnitude. As a result, the device fabricated with the HQ-modified PEDOT:PSS showed a 28% increase in efficiency compared to the device without HQ treatment. Modifying the PEDOT:PSS surface with HQ solution is an easy way to effectively boost the performance of polymer solar cells.

Computational Study of Ternary Devices: Stable, Low-Cost, and Efficient Planar Perovskite Solar Cells

Although perovskite solar cells with power conversion efficiencies (PCEs) more than 22% have been realized with expensive organic charge-transporting materials, their stability and high cost remain to be addressed. In this work, the perovskite configuration of MAPbX (MA = CH₃NH₃, X = I₃, Br₃, or I₂Br) integrated with stable and low-cost Cu:NiO _x hole-transporting material, ZnO electron-transporting material, and Al counter electrode was modeled as a planar PSC and studied theoretically. A solar cell simulation program (wxAMPS), which served as an update of the popular solar cell simulation tool (AMPS: Analysis of Microelectronic and Photonic Structures), was used. The study yielded a detailed understanding of the role of each component in the solar cell and its effect on the photovoltaic parameters as a whole. The bandgap of active materials and operating temperature of the modeled solar cell were shown to influence the solar cell performance in a significant way. Further, the simulation results reveal a strong dependence of photovoltaic parameters on the thickness and defect density of the light-absorbing layers. Under moderate simulation conditions, the MAPbBr₃ and MAPbI₂Br cells recorded the highest PCEs of 20.58 and 19.08%, respectively, while MAPbI₃ cell gave a value of 16.14%.

Crucial Role of the Electron Transport Layer and UV Light on the Open-Circuit Voltage Loss in Inverted Organic Solar Cells

Understanding the degradation mechanisms in organic photovoltaics is crucial in order to develop stable organic semiconductors and robust device architectures. The rapid loss of efficiency, referred to as burn-in, is a major issue to be addressed. This study reports on the influence of the electron transport layer (ETLs) and UV light on the drop of open-circuit voltage (V_oc) for P3HT:PC₆₀BM-based devices. The results show that V_oc loss is induced by the UV and, more importantly, that the ETL can amplify it, with TiO_x yielding a stronger drop than ZnO. Using impedance spectroscopy (IS) and X-ray photoelectron spectroscopy (XPS), different degradation mechanisms were identified according to whether the ETL is TiO_x or ZnO. For TiO_x-based devices, the formation of an interface dipole was identified, resulting in a loss of the flat-band potential (V_fb) and, thus, of the V_oc. For ZnO-based devices, chemical modifications of the metal oxide and active layer at the interface were detected, resulting in a doping of the active layer which impacts the V_oc. This study highlights the role of the architecture and, more specifically, of the ETL in the severity of burn-in and degradation pathways.

Insights into the Influence of Work Functions of Cathodes on Efficiencies of Perovskite Solar Cells

Though various efforts on modification of electrodes are still undertaken to improve the efficiency of perovskite solar cells, attributing to the large scope of these methods, it is of significance to unveil the working principle systematically. Herein, inverted perovskite solar cells based on indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/CH₃ NH₃ PbI₃ /phenyl-C61-butyric acid methyl ester (PC₆₁ BM)/buffer metal/Al are constructed. Through the choice of different buffer metals to tune work function of the cathode, the contact nature of the active layer with the cathode could be manipulated well. In comparison with the device using Au/Al as the electrode that shows an unfavorable band bending for conducting the excited electrons to the cathode, the one with Ca/Al presents a dramatically improved efficiency over 17.1%, ascribed to the favorable band bending at the interface of the cathode with the active layer. Details for tuning the band bending and the corresponding charge transfer mechanism are given in a systematic manner. Thus, a general guideline for constructing perovskite photovoltaic devices efficiently is provided.

Temperature-dependent Schottky barrier in high-performance organic solar cells

Organic solar cells (OSCs) have attracted great attention in the past 30 years, and the power conversion efficiency (PCE) now reaches around 10%, largely owning to the rapid material developments. Meanwhile with the progress in the device performance, more and more interests are turning to understanding the fundamental physics inside the OSCs. In the conventional bulk-heterojunction architecture, only recently it is realized that the blend/cathode Schottky junction serves as the fundamental diode for the photovoltaic function. However, few researches have focused on such junctions, and their physical properties are far from being well-understood. In this paper based on PThBDTP:PC₇₁BM blend, we fabricated OSCs with PCE exceeding 10%, and investigated temperature-dependent behaviors of the junction diodes by various characterization including current-voltage, capacitance-voltage and impedance measurements between 70 to 290 K. We found the Schottky barrier height exhibits large inhomogeneity, which can be described by two sets of Gaussian distributions.

Recent Advances to Understand Morphology Stability of Organic Photovoltaics

Organic photovoltaic devices are on the verge of commercialization with power conversion efficiencies exceeding 10 % in laboratory cells and above 8.5 % in modules. However, one of the main limitations hindering their mass scale production is the debatable inferior stability of organic photovoltaic devices in comparison to other technologies. Adequate donor/acceptor morphology of the active layer is required to provide carrier separation and transport to the electrodes. Unfortunately, the beneficial morphology for device performance is usually a kinetically frozen state which has not reached thermodynamic equilibrium. During the last 5 years, special efforts have been dedicated to isolate the effects related to morphology changes taking place within the active layer and compare to those affecting the interfaces with the external electrodes. The current review discusses some of the factors affecting the donor/acceptor morphology evolution as one of the major intrinsic degradation pathways. Special attention is paid to factors in the nano- and microscale domain. For example, phase segregation of the polymer and fullerene domains due to Ostwald ripening is a major factor in the microscale domain and is affected by the presence of additives, glass transition temperature of the polymers or use of crosslinkers in the active layer. Alternatively, the role of vertical segregation profile toward the external electrodes is key for device operation, being a clear case of nanoscale morphology evolution. For example, donor and acceptor molecules actually present at the external interfaces will determine the leakage current of the device, energy-level alignment, and interfacial recombination processes. Different techniques have been developed over the last few years to understand its relationship with the device efficiency. Of special interest are those techniques which enable in situ analysis being non-destructive as they can be used to study accelerated degradation experiments and some will be discussed here.

Characterization of Capacitance, Transport and Recombination Parameters in Hybrid Perovskite and Organic Solar Cells

The application of small perturbation frequency techniques to solar cells provides a great deal of information in terms of capacitive and resistive processes that are related to the photophysical mechanisms that lie at the basis of the photovoltaic operation. These methods can be exhaustively exploited to determine bulk and contact effects in the solar cells, and henceforth improve and optimize materials and interfaces. For photovoltaic devices, the main effects of interest in impedance spectroscopy are the capacitive charge storage and the resistive processes of transport and recombination. The combination of these parameters provides important information about properties such as conductivity, diffusion length and carrier lifetime. In this chapter, we provide an extensive review of the present status of knowledge about these aspects of solar cell operation for organic solar cells and hybrid organic–inorganic perovskite solar cells. We describe an exhaustive characterization of capacitive processes, including dielectric relaxation processes, and examine the interpretation of transport and recombination based on a variety of experimental techniques.

Interplay between Interfacial Structures and Device Performance in Organic Solar Cells: A Case Study with the Low Work Function Metal, Calcium

A better understanding of how interfacial structure affects charge carrier recombination would benefit the development of highly efficient organic photovoltaic (OPV) devices. In this paper, transient photovoltage (TPV) and charge extraction (CE) measurements are used in combination with synchrotron radiation photoemission spectroscopy (SRPES) to gain insight into the correlation between interfacial properties and device performance. OPV devices based on PCDTBT/PC71BM with a Ca interlayer were studied as a reference system to investigate the interfacial effects on device performance. Devices with a Ca interlayer exhibit a lower recombination than devices with only an Al cathode at a given charge carrier density (n). In addition, the interfacial band structures indicate that the strong dipole moment produced by the Ca interlayer can facilitate the extraction of electrons and drive holes away from the cathode/polymer interface, resulting in beneficial reduction in interfacial recombination losses. These results help explain the higher efficiencies of devices made with Ca interlayers compared to that without the Ca interlayer.

Interfacial Degradation of Planar Lead Halide Perovskite Solar Cells

The stability of perovskite solar cells is one of the major challenges for this technology to reach commercialization, with water believed to be the major degradation source. In this work, a range of devices containing different cathode metal contacts in the configuration ITO/PEDOT:PSS/MAPbI3/PCBM/Metal are fully electrically characterized before and after degradation caused by steady illumination during 4 h that induces a dramatic reduction in power conversion efficiency from values of 12 to 1.8%. We show that a decrease in performance and generation of the S-shape is associated with chemical degradation of the metal contact. Alternatively, use of Cr2O3/Cr as the contact enhances the stability, but modification of the energetic profile during steady illumination takes place, significantly reducing the performance. Several techniques including capacitance-voltage, X-ray diffraction, and optical absorption results suggest that the properties of the bulk perovskite layer are little affected in the device degradation process. Capacitance-voltage and impedance spectroscopy results show that the electrical properties of the cathode contact are being modified by generation of a dipole at the cathode that causes a large shift of the flat-band potential that modifies the interfacial energy barrier and impedes efficient extraction of electrons. Ionic movement in the perovskite layer changes the energy profile close to the contacts, modifying the energy level stabilization at the cathode. These results provide insights into the degradation mechanisms of perovskite solar cells and highlight the importance to further study the use of protecting layers to avoid the chemical reactivity of the perovskite with the external contacts.

Effects of electronic coupling and electrostatic potential on charge transport in carbon-based molecular electronic junctions

Molecular junctions consisting of 2-20 nm thick layers of organic oligomers oriented between a conducting carbon substrate and a carbon/gold top contact have proven to be reproducible and reliable, and will soon enter commercial production in audio processing circuits. The covalent, conjugated bond between one or both sp(2)-hybridized carbon contacts and an aromatic molecular layer is distinct from the more common metal/molecule or silicon/molecule structures in many reported molecular junctions. Theoretical observations based on density functional theory are presented here, which model carbon-based molecular junctions as single molecules and oligomers between fragments of graphene. Electronic coupling between the molecules and the contacts is demonstrated by the formation of hybrid orbitals in the model structure, which have significant electron density on both the graphene and the molecule. The energies of such hybrid orbitals correlate with tunneling barriers determined experimentally, and electronic coupling between the two graphene fragments in the model correlates with experimentally observed attenuation of transport with molecular layer thickness. Electronic coupling is affected significantly by the dihedral angle between the planes of the graphene and the molecular π-systems, but is absent only when the two planes are orthogonal. Coupling also results in partial charge transfer between the graphene contacts and the molecular layer, which results in a shift in electrostatic potential which affects the observed tunneling barrier. Although the degree of partial charge transfer is difficult to calculate accurately, it does provide a basis for the "vacuum level shift" observed in many experiments, including transport and ultraviolet photoelectron spectroscopy of molecular layers on conductors.

Interface modification strategy based on a hybrid cathode buffer layer for promoting the performance of polymer solar cells

An interface modification strategy based on a hybrid cathode buffer layer is proposed and demonstrated for promoting charge generation and extraction.

Low band gap diketopyrrolopyrrole-based small molecule bulk heterojunction solar cells: influence of terminal side chain on morphology and photovoltaic performance

Two low band gap small molecules based on DPP with different terminal side chains were synthesized. They show similar physical properties but different photovoltaic property.

Lanthanum Hexaboride As Novel Interlayer for Improving the Thermal Stability of P3HT:PCBM Organic Solar Cells

For efficient organic photovoltaic (OPV) solar cells, a low work function electrode is necessary to enhance the built-in voltage of the active layer, thereby improving the overall efficiency. Calcium is often used for this purpose in the laboratory; however, its development on a larger scale is impaired by its high reactivity with oxygen and water and the resulting low stability of solar cells under operation. The influence of a novel interlayer, lanthanum hexaboride (LaB6), on the electronic properties of OPV is studied in this work. Similarly to calcium, when LaB6 is used as an interlayer, it enhances the built-in voltage in the device, leading to a higher fill factor (FF) and optimal open circuit voltage (V(oc)). As a result, optimized LaB6-based devices present significantly improved power conversion efficiencies. More importantly, while calcium/aluminum (Ca/Al) and aluminum (Al) cathodes lose their capacity to enhance the internal electrical field during thermal aging, the LaB6/aluminum (LaB6/Al) electrode remains stable. This remarkable effect results in a highly stable V(oc) and flat-band potential during aging.

Quantitative operando visualization of the energy band depth profile in solar cells

The energy band alignment in solar cell devices is critically important because it largely governs elementary photovoltaic processes, such as the generation, separation, transport, recombination and collection of charge carriers. Despite the expenditure of considerable effort, the measurement of energy band depth profiles across multiple layers has been extremely challenging, especially for operando devices. Here we present direct visualization of the surface potential depth profile over the cross-sections of operando organic photovoltaic devices using scanning Kelvin probe microscopy. The convolution effect due to finite tip size and cantilever beam crosstalk has previously prohibited quantitative interpretation of scanning Kelvin probe microscopy-measured surface potential depth profiles. We develop a bias voltage-compensation method to address this critical problem and obtain quantitatively accurate measurements of the open-circuit voltage, built-in potential and electrode potential difference.

The energy band alignment of solar cell materials is highly relevant to the device performance, but its measurement is challenging. Here, the authors report direct visualization of energy band alignment in operating organic photovoltaic devices using scanning Kelvin probe microscopy imaging.

Exploring the open-circuit voltage of organic solar cells under low temperature

Open-circuit voltage (V_OC) in organic solar cells (OSCs) is currently still not well-understood. A generally acceptable view is that V_OC is mainly determined by the energy level offset between donor and acceptor materials. Recently in ternary blend OSCs, V_OC is found to be dependent on the blend composition. But contrary to expectation, this dependence is not a simple linear relationship, which adds complications to understanding on V_OC. Here, in order to figure out the origin of V_OC, we performed a series of experiments on both binary and ternary blend OSCs in a wide temperature range from 15 K to 300 K. It is observed that the devices behave like Schottky barrier (SB) diode. By fitting the experimental results with SB diode model, the detailed device parameters of ternary blend OSCs are extracted and it is found that V_OC is determined by the energetics of organic molecules and metal at the cathode interface, and the inhomogeneity of the SB also play a great role in the origin of V_OC at low temperatures. This work not only paves the way to deep understanding on the origin of V_OC, but also opens a door to further exploring the general working principle of OSCs.

Role of vertical segregation in semitransparent organic photovoltaics

In this work, the efficiency of semitransparent organic photovoltaic (OPV) devices for low intensity applications is investigated as a function of the processing conditions. It is observed that a thermal treatment of the organic layer induces fullerene migration toward the active layer/air interface. This physical process gives rise to different vertical segregation profiles of donor and acceptor molecules. Once the back contact is deposited, the amount of fullerene covering the surface will determine the contact selectivity and leakage current of the device. Control of this leakage current may not be essential for devices fabricated for high illumination condition applications. However, devices to be used under low illumination conditions may be highly influenced by the presence of this parasitic dark current which flows in the opposite direction to photogenerated current. At the proximity of the contacts, the vertical segregation profile is inferred from optical and electrical measurements. In particular, external quantum efficiency (EQE) measurements carried out from a relatively opaque back contact provide local information on the materials spatially close to the light source. Alternatively, capacitance-voltage measurements enable calculation of the percentage of fullerene molecules covering the cathode contact. Overall, a versatile method that can be used in regular and inverted configuration is presented that explains the different behavior observed for devices to be used under low illumination conditions.

Uncovering the role of cathode buffer layer in organic solar cells

Organic solar cells (OSCs) as the third generation photovoltaic devices have drawn intense research, for their ability to be easily deposited by low-cost solution coating technologies. However the cathode in conventional OSCs, Ca, can be only deposited by thermal evaporation and is highly unstable in ambient. Therefore various solution processible cathode buffer layers (CBLs) are synthesized as substitute of Ca and show excellent effect in optimizing performance of OSCs. Yet, there is still no universal consensus on the mechanism that how CBL works, which is evidently a critical scientific issue that should be addressed. In this article detailed studies are targeted on the interfacial physics at the interface between active layer and cathode (with and without treatment of a polar CBL) by using ultraviolet photoelectron spectroscopy, capacitance-voltage measurement, and impedance spectroscopy. The experimental data demonstrate that CBL mainly takes effect in three ways: suppressing surface states at the surface of active layer, protecting the active layer from being damaged by thermally evaporated cathode, and changing the energy level alignment by forming dipole moments with active layer and/or cathode. Our findings here provide a comprehensive picture of interfacial physics in devices with and without CBL.

Classification of solar cells according to mechanisms of charge separation and charge collection

This paper elaborates a general description of solar cells based on a single absorber material, according to the mechanisms of charge separation and charge collection.

Formation of cathode buffer layer by surface segregation of fluoroalkyl-modified ZnO for polymer solar cells

A cathode buffer layer, formed by surface segregation of fluoroalkyl modified ZnO, was present in polymer solar cells based on P3HT:PCBM.

Iron pyrite thin film counter electrodes for dye-sensitized solar cells: high efficiency for iodine and cobalt redox electrolyte cells

Iron pyrite has been the material of interest in the solar community due to its optical properties and abundance. However, the progress is marred due to the lack of control on the surface and intrinsic chemistry of pyrite. In this report, we show iron pyrite as an efficient counter electrode (CE) material alternative to the conventional Pt and poly(3,4-ethylenedioxythiophene (PEDOT) CEs in dye-sensitized solar cells (DSSCs). Pyrite film CEs prepared by spray pyrolysis are utilized in I3(-)/I(-) and Co(III)/Co(II) electrolyte-mediated DSSCs. From cyclic voltammetry and impedance spectroscopy studies, the catalytic activity is found to be comparable with that of Pt and PEDOT in I3(-)/I(-) and Co(III)/Co(II) electrolyte, respectively. With the I3(-)/I(-) electrolyte, photoconversion efficiency is found to be 8.0% for the pyrite CE and 7.5% for Pt, whereas with Co(III)/Co(II) redox DSSCs, efficiency is found to be the same for both pyrite and PEDOT (6.3%). The excellent performance of the pyrite CE in both the systems makes it a distinctive choice among the various CE materials studied.

Surface-charge accumulation effects on open-circuit voltage in organic solar cells based on photoinduced impedance analysis

The accumulation of dissociated charge carriers plays an important role in reducing the loss occurring in organic solar cells.

Package-free flexible organic solar cells with graphene top electrodes

Package-free flexible organic solar cells are fabricated with multilayer graphene as top transparent electrodes, which show the highest power conversion efficiency of about 3.2% and excellent flexibility and bending stability. The devices also show good air stability, indicating that multilayer graphene is a promising environmental barrier that can protect the organic solar cells from air contamination.

Interplay between fullerene surface coverage and contact selectivity of cathode interfaces in organic solar cells

Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces has an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques such as variable-angle spectroscopic ellipsometry and capacitance-voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in three additional donor/acceptor systems: PCPDTBT, P3HT, PCDTBT, and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene-rich interface at the cathode is a prerequisite to enhance contact selectivity and consequently power conversion efficiency.

Elucidating Operating Modes of Bulk-Heterojunction Solar Cells from Impedance Spectroscopy Analysis

We discuss the progress and challenges in the application of impedance spectroscopy analysis to determine key processes and parameters in organic bulk-heterojunction solar cells. When carrier transport or outer interface extraction do not severely influence the solar cell performance, a simple method to quantify the open-circuit voltage loss caused by the kinetics of charge carrier recombination is provided, based on the determination of chemical capacitance and recombination resistance. This easily allows distinguishing between energetic and kinetic effects on photovoltage, and establishes a benchmark for the performance comparison of a set of different cells. A brief discussion of impedance analysis in the much less studied case of collection-limited solar cells is introduced.