Performance Of Organic Photovoltaic Devices Research Articles

High efficiency homojunction tandem organic solar cells with all solution processable interconnect layer

In the field of organic photovoltaics, the power conversion efficiency of single junction solar cells continues to improve. However, tandem organic solar cells are poised to push the efficiency limits even further and offer a promising avenue for improving the performance of organic photovoltaic devices. This study reports the development of an all-solution processed interconnecting layer (ICL) based on ZnO NPs:PEI/PEI/PEDOT:PSS/2PACz for tandem solar cells. The PM6:BTP-eC9 active layer material was adjusted for its donor-to-acceptor (D/A) ratio and film thickness as the front and back sub-cells. The ICL exhibits favorable mechanical, electrical and optical properties. Through multidimensional modulation, the front and rear sub-cells have been optimized to obtain highly efficient homojunction tandem solar cells. The tandem solar cell has a structure of indium tin oxide (ITO)/PEDOT:PSS/2PACz/active layer/ICL/active layer/PNDIT-F3N/Ag. This optimization resulted in a power conversion efficiency (PCE) of 19.9 %, which is the highest reported efficiency for homojunction tandem organic solar cells to date. Our research demonstrates that the PCE of homojunction tandem cells can be significantly improved by careful design of the interconnecting layers and optimization of the donor-to-acceptor (D/A) ratio. This strategy may provide guidance for further improvements in homojunction tandem cells.

Enhancing the photovoltaic properties of phenothiazine-based chromophores via structural integration of selenophene analogues

A series of oligomer-type donor photovoltaic materials (MP2–MP9) were designed via the incorporation of selenophene unit (n = 1–8) in MP1 as π-spacer. In order to accomplish the electronic, photophysical, and photovoltaic (PV) behavior of MPR and MP1–MP9, quantum chemical calculations were performed through the density functional theory (DFT) approach. The M06/6-311G (d,p) level was selected for current investigation through a benchmark study between experimental and DFT values of λmax at various functionals of MPR compound. Descending in energy gap (3.058–2.476 eV) with wider absorption wavelength (652.998–513.392 nm) in dichloromethane along with greater charge transmission were probed with the addition of selenophene unit as compared to MPR. The studied chromophores were perceived to have Voc and FF ranging from 1.320–1.728 V and 0.903–0.923 %, respectively. These findings provide valuable insights into the design of oligomer-type donor materials with optimized electronic and photophysical properties, which could enhance the performance and efficiency of organic photovoltaic devices.

Free carrier generation efficiency in organic photovoltaic films determined using photo-MIS-CELIV

Solution processed organic photovoltaic (OPV) devices are promising for low-embedded energy and large-scale renewable energy production. The efficiency of charge carrier generation is a critical factor influencing the performance of photovoltaic devices. However, quantifying charge carrier generation can be challenging, with the results from experimental methods not always being easily correlated with solar cell performance. In this paper, we describe how photoinduced metal-insulating-semiconductor charge-extraction-by-linearly-increasing-voltage (photo-MIS-CELIV) can be used to determine the free charge carrier generation efficiency (FCGE) in OPV films. One of the benefits of this approach is that the FCGE can be measured alongside the charge mobility to provide a holistic picture of the fate of charges, from generation to extraction. We demonstrate this method through quantifying the FCGE of bulk heterojunctions of PCE10:ITIC-4F, D18:Y6 and PPDT2FBT:PC71BM, obtaining values of 47.4 ± 1.6 %, 75.0 ± 2.5 % and 70.6 ± 4.6 %, respectively. The measured FCGEs for these blends were consistent with the device-based external quantum efficiencies (EQEs) at the excitation wavelength used. The use of photo-MIS-CELIV for quantifying the FCGE increases its utility beyond simple charge mobility measurements and provides an extra method to enable optimisation of OPV device performance.

Progress on the Development of Boron Subphthalocyanines (BsubPcs) and Subnaphthalocyanines (BsubNcs) Including Hybrids and Their Sustainability

Our group focuses on molecular design, synthetic methods and in-depth characterization of boron subphthalocyanines (BsubPcs) and subnaphthalocyanines (BsubNcs)[1] to apply them to organic electronics.[2] BsubPcs, BsubNcs and their hybrids called Bsub(Pc3-p-Ncp) are macrocycles with central boron atom and are p-conjugated macrocycles. Our focal point balances between the basic and applied chemistry, chemical engineering and their in-depth physical properties (electrochemistry included). Once materials are developed, we merge them into organic electronics for application into organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs)/organic solar cells (OSCs).For this presentation, all points above will be factored in and I will focus on our progress on the development of hybrids, and mixed alloys and none mixed alloys of BsubNcs,.Regarding hybrids, they are a mixture of BsubPcs and BsubNcs, we named as X-RnBsub(Pc3-p-Ncp) – ‘p’ being the numbers of the substitutions; they were formed via reaction of BCl3 and a statistical distribution of phthalonitrile and 2,3-dicyanonaphthalene; X the axial substituent, Rn the number of peripheral substitutions, Pcp the number of 6 p-conjugated bonds, Ncp the number of 10 p-conjugated bonds (Figure).[3] We found that X-RnBsub(Pc1-Nc2) have low photostability, therefore, we developed a ‘statistical’ synthetic methodology to target X-RnBsub(Pc2-Nc1)s, and therefore could be purified to enable physical characterization. Their absorption spectra are as wide if BsubPc+BsubNc was mixed together, therefore, has potential to harvest a larger scope of photons for OPVs or to achieved FRET transfer from a host to emit via OLED. We also confirmed their electrochemical oxidation and reduction stability and their photoluminescence.More recently, we developed a new methodology called ‘steric hinderance’ to have even a higher yield of the X-RnBsub(Pc3-p-Ncp) hybrids, and the achievement was also based on computational modelling. I will also outline this approach.In the past as we have shown that BsubNcs end up being a mixed alloyed composition with random bay-position halogenation that is formed during the reaction of BCl3 with 2,3-dicyanonaphthalene at temperature.[4] The random bay-position halogenation has been shown to be impactful in a positive way within OPV devices, negative within OLED devices and also has electrochemical variations. Ongoing, given it is random halogenation, and some of the positive outcomes, this justifies considering additional mixed alloyed compositions.We have recently been able to develop a separation method for the mixed alloyed BsubNc compositions and acquired data to show the impact of the percentage/number of bay-position halogens, chlorine and bromine included, on the electrochemical potentials and the photoluminescence.[5] We have also applied a computational model to look at the relative impact of the random bay-position halogenation on the electronics.[6] We have found that the frequency of halogenation has a larger impact on the predicted HOMO/LUMO energy levels than does the random halogen positioning around the bay-positions of the BsubNcs. As this computational data is therefore comparable to the acquired electrochemical data.I will also present a new synthetic methodology to avoid the random bay-position halogenation of the associated BsubNcs. We also have recently within these semesters developed alternative chemistry to more tune the mixed alloys of the BsubNcs. This will also be a part of the presentation.We have also recently developed an AI-ML method to further accelerate the BsubNc development. The focal point of this approach was to have bay-position substitutions to then not have any random bay-position halogenation of the BsubNcs. So, these current and upcoming outcomes will also be a part of the presentations.For this presentation, I will also be outlining the sustainability of these great macrocyclic compounds. Claessens, C. G.; et al, Subphthalocyanines, Subporphyrazines, and Subporphyrins: Singular Nonplanar Aromatic Systems. Chem. Rev. 2014, 114 (4), 2192-2277.Morse, G. E.; et al, Boron Subphthalocyanines as Organic Electronic Materials. ACS Appl. Mater. & Interfaces 2012, 4 (10), 5055-5068.Farac, N.F.; et al, Cs-Symmetric, Peripherally Fluorinated Boron Subphthalocyanine–Subnaphthalocyanine Hybrids: Shedding New Light on Their Fundamental Photophysical Properties and Their Functionality as Optoelectronic Materials. J. Phys. Chem. C 2023, 127 (1), 702–727.Dang, J. D.; et al, The mixed alloyed chemical composition of chloro-(chloro)n-boron subnaphthalocyanines dictates their physical properties and performance in organic photovoltaic devices. J Mater. Chem. A 2016, 4 (24), 9566-9577.Holst, D.P.; et al, Enhanced analytical and physical characterization of mixtures of random bay-position chlorinated boron subnaphthalocyanines enabled by an established partial separation method (a part 2). New J. Chem. 2021, 45, 21082-21091.Holst, D.P.; et al, Updated Calibrated Model for the Prediction of Molecular Frontier Orbital Energies and Its Application to Boron Subphthalocyanines. J. Chem. Inf. Model. 2022, 62 (4), 829–840. Figure 1

Synergistic passivation and energy level alignment by graphene quantum dots for high performance inverted CsPbI3 perovskite solar cells

Inorganic CsPbI3 perovskite solar cells (PSCs) have garnered considerable attention due to their high thermal stability and promising application in tandem devices. However, further advancement of the performance of CsPbI3 PSCs is restricted by severe nonradiative recombination, which is related to substantial defects and mismatched energy levels. Herein, the versatile graphene quantum dots (GQDs) are introduced to modify the CsPbI3 surface to improve interface contact and mitigate energy loss. GQD modification can not only effectively passivate surface defects via coordinating with the undercoordinated Pb2+ but also improve energy level alignment, contributing to efficient charge extraction and suppression of nonradiative recombination. Consequently, GQDs-based inverted CsPbI3 devices deliver a champion power conversion efficiency (PCE) of 18.98% with a high open-circuit voltage (VOC) of 1.141 V and are greatly superior to the control device obtaining a poor PCE of 13.29% with a VOC of 0.986 V. Moreover, GQDs can form a protective layer at the perovskite interface to resist external invasion, significantly boosting the device stability. Our findings establish the promising application of GQD modification as a compelling strategy for achieving high performance inorganic photovoltaic devices.

Printing and Coating Techniques for Scalable Organic Photovoltaic Fabrication.

Within recent years, there has been an increased interest towards organic photovoltaics (OPVs), especially with their significant device performance reaching beyond 19% since 2022. With these advances in the device performance of laboratory-scaled OPVs, there has also been more attention directed towards using printing and coating methods that are compatible with large-scale fabrication. Though large-area (>100 cm2) OPVs have reached an efficiency of 15%, this is still behind that of laboratory-scale OPVs. There also needs to be more focus on determining strategies for improving the lifetime of OPVs that are suitable for scalable manufacturing, as well as methods for reducing material and manufacturing costs. In this paper, we compare several printing and coating methods that are employed to fabricate OPVs, with the main focus towards the deposition of the active layer. This includes a comparison of performances at laboratory (<1 cm2), small (1-10 cm2), medium (10-100 cm2), and large (>100 cm2) active area fabrications, encompassing devices that use scalable printing and coating methods for only the active layer, as well as "fully printed/coated" devices. The article also compares the research focus of each of the printing and coating techniques and predicts the general direction that scalable and large-scale OPVs will head towards.

Enhancing the indoor performance of organic photovoltaic devices: interface engineering with an aminobenzoic-acid-based self-assembled monolayer

Significant advances in the performance of organic photovoltaic (OPV) devices can facilitate their use in internet of things applications. However, achieving excellent photostability and high efficiency using stable, efficient OPV devices in indoor settings is considerably difficult. To address this issue, a zinc oxide (ZnO) electron transport layer (ETL) was modified with a self-assembled monolayer of 4-aminobenzoic acid (ABA) in the present study, and the impact of this modification was correlated with the indoor performance of an OPV device with the PM6:L8-BO photoactive layer. The ABA-treated ZnO ETL exhibited a significant reduction in the work function (from 4.51 to 4.04 eV), surface roughness (from 0.201 to 0.177 nm), and hydrophilicity of an indium-tin-oxide electrode; this aided in selectively extracting charge carriers from the device and minimizing trap-assisted recombination losses. Additionally, the ABA treatment of the ZnO ETL considerably enhanced the electron mobility and recombination resistance. It reduced the trap density, thereby enabling the ZnO/ABA-based device to achieve improved performance. Consequently, the ZnO/ABA-based device exhibited a noteworthy 14.68% higher maximum power output than that of the device without any ZnO surface modification under 1000 lx halogen (HLG) illumination (P out, max = 354.48 and 309 µA cm−2, respectively). Moreover, under thermal illumination conditions (1000 lx HLG lighting), the ZnO/ABA-based device sustained ∼74% of its initial power conversion efficiency over 120 h, significantly higher than its ABA-free equivalent (∼55%).

Enhancing performance of organic photovoltaic and photodetector devices using non-atomically doped ZnO electrodes with superior optical properties

Enhancing performance of organic photovoltaic and photodetector devices using non-atomically doped ZnO electrodes with superior optical properties

THE OPTIMIZATION OF P3HT:PCBM THIN FILM THICKNESS FOR ORGANIC SOLAR CELLS

This study presents the complex relationship between film thickness and the properties of P3HT:PCBM thin films, looking at how different thicknesses affected their structural and optical properties. Poly(3, hexylthiophene) (P3HT) and poly(3, hexylthiophene) (PCMB) were blended in a 0.6:1 ratio and deposited using spin coating techniques. The research involve investigating the resulting changes in surface topography, optical properties, and electrical behaviour by systematically adjusting the film thickness through multiple coating repetitions ranging from one to three repetitions. Profilometric analysis revealed a direct relationship between film thickness and surface roughness, highlighting the importance of thickness in interfacial interactions within solar cells. Optical studies revealed fascinating trends in absorbance, transmittance, absorption coefficient, and extinction coefficient, highlighting the films' behaviour at various wavelengths. Surprisingly, regardless of thickness, the optical band gap remained constant, providing valuable insights for optimizing photon energy conversion. Measurements of the Hall effect revealed the effect of thickness on carrier mobility, concentration, conductivity, resistivity, and Hall voltage. Thinner films had higher mobility but lower carrier concentration, revealing a delicate balance critical for optimizing charge transport. The comprehensive analysis of the study sheds light on the complex interplay between film thickness and various properties, providing useful guidance for designing efficient P3HT:PCBM organic solar cells. This study's findings highlight the importance of precise control over film dimensions in improving the performance and efficiency of organic photovoltaic devices. This investigation paves the way for tailored approaches in the fabrication of next-generation organic solar cells, pushing the limits of renewable energy technology.

Improving Performance of Organic Photovoltaic Devices under Low Illuminations

Improving Performance of Organic Photovoltaic Devices under Low Illuminations

Theoretical study on organic photovoltaic heterojunction FTAZ/IDCIC

Understanding organic photovoltaic (OPV) work principles and the materials’ optoelectronic properties is fundamental for developing novel heterojunction materials with the aim of improving power conversion efficiency (PCE) of organic solar cells. Here, in order to understand the PCE performance (&amp;gt;13%) of OPV device composed of the non-fullerene acceptor fusing naphtho[1,2-b:5,6-b′]dithiophene with two thieno[3,2-b]thiophene (IDCIC) and the polymer donor fluorobenzotriazole (FTAZ), with the aid of extensive quantum chemistry calculations, we investigated the geometries, molecular orbitals, excitations, electrostatic potentials, transferred charges and charge transfer distances of FTAZ, IDCIC and their complexes with face-on configurations, which was constructed as heterojunction interface model. The results indicate that, the prominent OPV performance of FTAZ:IDCIC heterojunction is caused by co-planarity between the donor and acceptor fragments in IDCIC, the the charge transfer (CT) and hybrid excitations of FTAZ and IDCIC, the complementary optical absorptions in visible region, and the large electrostatic potential difference between FTAZ and IDCIC. The electronic structures and excitations of FTAZ/IDCIC complexes suggest that exciton dissociation can fulfill through the decay of local excitation exciton in acceptor by means of hole transfer, which is quite different from the OPVs based on fullerenes acceptor. The rates of exciton dissociation, charge recombination and CT processes, which were evaluated by Marcus theory, support the efficient exciton dissociation that is also responsible for good photovoltaic performance.

Compromising Charge Generation and Recombination of Organic Photovoltaics with Mixed Diluent Strategy for Certified 19.4% Efficiency.

The ternary blend is demonstrated as an effective strategy to promote the device performance of organic photovoltaics (OPVs) due to the dilution effect. While the compromise between the charge generation and recombination remains a challenge. Here, a mixed diluent strategy for further improving the device efficiency of OPV is proposed. Specifically, the high-performance OPV system with a polymer donor, i.e., PM6, and a nonfullerene acceptor (NFA), i.e., BTP-eC9, is diluted by the mixed diluents, which involve a high bandgap NFA of BTP-S17 and a low bandgap NFA of BTP-S16 (similar with that of the BTP-eC9). The BTP-S17 of better miscibility with BTP-eC9 can dramatically enhance the open-circuit voltage (VOC ), while the BTP-S16 maximizes the charge generation or the short-circuit current density (JSC ). The interplay of BTP-17 and BTP-S16 enables better compromise between charge generation and recombination, thus leading to a high device performance of 19.76% (certified 19.41%), which is the best among single-junction OPVs. Further analysis on carrier dynamics validates the efficacy of mixed diluents for balancing charge generation and recombination, which can be further attributed to the more diverse energetic landscapes and improved morphology. Therefore, this work provides an effective strategy for high-performance OPV for further commercialization.

Dithieno[3,2-a:2′,3′-c]phenazine based hole-transporting materials for efficient perovskite solar cells: Effects of donors numbers

Incorporation of electron-deficient polycyclic aromatics into molecular skeleton has been proved to be an effective strategy for improving the device performance of organic photovoltaics. In this context, three low-cost hole-transporting materials (HTMs) endowed with dithieno[3,2-a:2′,3′-c]phenazine core were successfully synthesized and employed for perovskite solar cells (PSCs). A comparative evaluation in relation to the numbers of peripheral donors was systematically investigated by measurement of their photophysical, electrochemical and photovoltaic performance. It is revealed that the low-symmetrical WH04 featuring three triphenylamine (TPA) donors exhibited a deeper HOMO level, a higher hole-transporting capacity and smoother film morphology than the molecules with two or four terminal donors. As a result, the WH04-based PSCs realized the highest power conversion efficiency of 20.52%, accompanied with excellent long-term device stability, which is competitive with spiro-OMeTAD based devices. We believe that molecular engineering of donors numbers is envisioned as an effective strategy for constructing highly efficient D-A-D type HTMs for PSCs.

Effects of stirring temperature of P3HT:PCBM solution on device performance of organic photovoltaics

Effects of stirring temperature of P3HT:PCBM solution on device performance of organic photovoltaics

Systematic Investigation of Core and Endcap Selection on the Development of Functional π-Conjugated Materials

We investigate the optoelectronic properties of four new π-conjugated materials, which feature either a 3,4-ethylenedioxythiophene (EDOT) or an EDOT-flanked benzothiadiazole aryl core with two N-annulated perylene diimide (NPDI) or theobromine endcaps. Endcap selection was an important factor for the preferred molecular geometry, where the use of NPDI endcaps induced a dragonfly-type conformation and the use of theobromine endcaps promoted a corkscrew-like conformation. Electrochemically, the choice of aryl π-conjugated core was found to have a strong influence on the molecule’s highest occupied molecular orbital (HOMO) energy level, while the choice of endcap had a more profound effect on the lowest unoccupied molecular orbital (LUMO) energy level. Although the optical absorption profiles were generally dominated by endcap contributions, using the larger EDOT-flanked benzothiadiazole aryl core was found to decrease the optical energy band gap. Proof-of-concept organic photovoltaic (OPV) devices, using a PM6:Y6 bulk heterojunction (BHJ) photoactive layer, were fabricated using the four new π-conjugated materials as the cathode interlayer (CIL). The molecules featuring the EDOT aryl core (A and B) enabled OPV device power conversion efficiencies (PCEs) by around 12%, comparable to control devices using PDINN as the CIL, while those featuring the EDOT-flanked benzothiadiazole aryl core (C and D) obtained PCEs of 10.5%. The observed differences in OPV device performance were attributed to superior solubility and subsequent CIL film formation in the case of A and B, compared to C and D, which, in turn, led to improved contact between the Ag top electrode and the BHJ photoactive layer.

Thermal Activation of PEDOT:PSS/PM6:Y7 Based Films Leads to Unprecedent High Short‐Circuit Current Density in Nonfullerene Organic Photovoltaics

AbstractFinding an effective approach to suppress trap formation is a potential route for enhancing the performance of nonfullerene organic photovoltaic (NF‐OPVs) devices. Here, an extraordinary short‐circuit current density (JSC) value of 32.65 mA cm‐2 is achieved, higher than the state‐of‐the art NF‐OPVs reported, reaching a high power conversion efficiency (PCE) of 17.92%. This remarkable enhancement is exhibited through the fine‐tuning of PEDOT:PSS/PM6:Y7 films and interface morphologies via applying the prethermal treatment approach (Pre‐TT) to the devices, which exhibit JSC and PCE enhancement of 21% and 8%, respectively, compared to the pristine devices. Accordingly, the dependence of the JSC upon the Pre‐TT approach through a range of morphological, optical, electrical, and advanced transient measurements is investigated. The Pre‐TT‐based films are found to possess optimal smooth blend morphology with better dispersity owing to reduced domain size. Moreover, the measurements show that the optimized treated devices present higher exciton dissociation probabilities and generation rate of the free charge carriers, showing an ideal balanced electron/hole mobility that reveals the JSC and PCE enhancement. Hence, Pre‐TT approach provides a facile passivation strategy that reduces the trap state density of the blend film, improves interface charge transfer, allows balanced electron/hole mobility, and thus promotes device performance.

Impact of the bilayer electron transport layer in the donor acceptor bulk heterojunctions for improved inverted organic photovoltaic performance

Functionalization at interface of the electron transport layer and the organic polymeric donor and non-fullerene acceptor bulk heterojunction layer in photovoltaic devices that have an inverted structure, is a unique strategy for improving performance. Here we demonstrate improved interfacial nanomorphology of the well-known PTB7-Th donor and IEICO-4F acceptor bulk heterojunction (BHJ) on an atomic layer deposited (ALD) ultrathin titanium oxide (TiO2) passivated sol–gel grown zinc oxide (ZnO) electron transport bilayer and highlight how this is correlated to the overall organic photovoltaic device performance. Synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS) has been used to demonstrate an enhanced face-on conformation within the BHJ with highly crystalline ZnO/TiO2/BHJ layers and this is greatest for the very thinnest of TiO2 layers on ZnO (ZnT1), from just two deposition cycles, relative to both four (ZnT2) and eight (ZnT3) cycles. Photovoltaic devices with BHJs grown over ZnT1, showed an overall average increase of 30.3 ± 0.5 % in the power conversion efficiency (PCE 10.3 ± 0.5 %), a 14.3 ± 1 % increase in the fill-factor, FF (60.1 ± 1 %), 2.9 ± 0.05 % improvement in Voc (0.70 ± 0.05 mV) and 13.6 ± 0.5 % higher JSc (24.2 ± 0.5 mA/cm2), compared to the control device,with a best efficiency of 11.1 ± 0.5 % PCE, 61.9 ± 1 % FF, 0.70 ± 0.5 mV Voc and 25.0 ± 0.5 mA/cm2Jsc. We conclude that the molecular crystal conformation of the organic donor acceptor BHJ layer can be partly controlled through the ETL along with an enhanced organic inorganic interface which strongly influences the OPV performance.

Light intensity dependence of the photocurrent in organic photovoltaic devices

The competition between recombination and extraction of carriers defines the charge collection efficiency and, therefore, the overall performance of organic photovoltaic devices, including solar cells and photodetectors. In this work, we describe different components of the steady-state light intensity-dependent photocurrent (IPC) and charge collection efficiency under operational conditions. Further, we demonstrate how different loss mechanisms can be identified based on their unique signatures in the IPC. In particular, we show how IPC can be used to distinguish first-order, trap-assisted recombination from other first-order photocurrent loss mechanisms, which dominate at the low-intensity characteristic of indoor light-harvesting applications. The theoretical framework is presented and verified by a one-dimensional drift-diffusion device model. Finally, the extended IPC methodology is validated on organic thin-film photovoltaic devices. We conclude that the relatively straightforward measurement of IPC over a large dynamic range can be a powerful tool for understanding solar and indoor device fundamentals. • IPC is a tool for understanding photovoltaic device fundamentals • Photocurrent loss mechanisms are identified based on their signatures in IPC • The theoretical framework is verified with a drift-diffusion device model • Experimental demonstration of IPC method on state-of-the-art organic solar cells Zeiske et al. present a combined theoretical and experimental study of intensity-dependent photocurrent (IPC), a tool for understanding solar and indoor device fundamentals, to identify different photovoltaic device performance-limiting photocurrent loss mechanisms based on their unique signatures in IPC.

Understanding interfacial energy structures in organic solar cells using photoelectron spectroscopy: A review

Organic solar cells (OSCs) have received considerable attention as a promising clean energy-generating technology because of their low cost and great potential for large-scale commercial manufacturing. With significant advances in new charge-transport material design, interfacial engineering, and their operating conditions, power conversion efficiencies of OSCs have continued to increase. However, a fundamental understanding of charge carrier transport and especially how ionic moieties affect carrier transport is still lacking in OSCs. In this regard, photoelectron spectroscopy has provided valuable information about interfacial electronic structures. The interfacial electronic structure of OSC interlayers greatly impacts charge extraction and recombination, controls energy level alignment, guides active layer morphology, improves material’s compatibility, and plays a critical role in the resulting power conversion efficiency of OSCs. Interfacial engineering incorporating inorganic, organic, and hybrid materials can effectively enhance the performance of organic photovoltaic devices by reducing energy barriers for charge transport and injection while improving compatibility between metal oxides and donor–acceptor based active layers or transparent conducting electrodes. This article provides a review of recent developments in interfacial engineering underlying organic photovoltaic devices of donor–acceptor interfaces.

Submolecular Resolution Imaging of P3HT:PCBM Nanostructured Films by Atomic Force Microscopy: Implications for Organic Solar Cells.

The efficiency of organic bulk-heterojunction (BHJ) solar cells depends greatly on both the bulk and surface structure of the nanostructured bicontinuous interpenetrating network of materials, known as the active layer. The morphology of the top layer of a coated film is often resolved at the scale of a few nanometers, but fine details of the domains and the order within them are more difficult to identify. Here, we report a high-resolution atomic force microscopy (AFM) investigation of various stoichiometries of the well-studied poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) active layer mixture. Images of the surface were obtained using AC-mode AFM exciting higher-order resonance frequencies of a standard silicon probe, a promising technique for acquiring real-space images of organic-based thin films with nanoscale and even submolecular resolution. We provide firm evidence of the nanoscale organization of the P3HT polymer and of the P3HT:PCBM stoichiometric mixtures at the surface-air interface of the BHJ architecture. Our study shows the characteristic periodicity of the regioregular P3HT identified in the nanoscale domain areas with submolecular resolution. Such areas are then distorted in place when adding different quantities of PCBM forming stoichiometric mixtures. When the samples were exposed to ambient light, the morphologies were very different, and submolecular resolution was not achieved. This approach is shown to provide a precise view of the active layer's nanostructure and will be useful for studies of other materials as a function of various parameters, with particular attention to the role of the acceptor in tuning morphology for understanding optimum performance in organic photovoltaic devices.