Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend

Developing a better understanding of the evolution of morphology in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies greater than 10%. In the most extensively characterized system, the poly(3‐hexyl thiophene) (P3HT):[6,6]‐phenyl‐C₆₁‐butyric‐acid‐methyl‐ester (PCBM) bulk heterojunction, the origins and evolution of the blend morphology during processes such as thermal annealing are not well understood. In this work, we use a model system, a bilayer of P3HT and PCBM, to develop a more complete understanding of the miscibility and diffusion of PCBM within P3HT during thermal annealing. We find that PCBM aggregates and/or molecular species are miscible and mobile in disordered P3HT, without disrupting the ordered lamellar stacking of P3HT chains. The fast diffusion of PCBM into the amorphous regions of P3HT suggests the favorability of mixing in this system, opposing the belief that phase‐pure domains form in BHJs due to immiscibility of these two components.     

Controlling Hierarchy in Solution‐processed Polymer Solar Cells Based on Crosslinked P3HT

Understanding and controlling the morphology of donor/acceptor blends is critical for the development of solution processable organic solar cells. By crosslinking a poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) film we have been able to spin‐coat [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) onto the film to form a structure that is close to a bilayer, thus creating an ideal platform for investigating interdiffusion in this model system. Neutron reflectometry (NR) demonstrates that without any thermal treatment a smaller amount of PCBM percolates throughout the crosslinked P3HT when compared to a non‐crosslinked P3HT film. Using time‐resolved NR we also show thermal annealing increases the rate of diffusion, resulting in a near‐uniform distribution of PCBM throughout the polymer film. XPS measurements confirm the presence of both P3HT and PCBM at the annealed film's surface indicating that the two components are intermixed. Photovoltaic devices fabricated using this bilayer approach and suitable annealing conditions yielded comparable power conversion efficiencies to bulk heterojunction devices made from the same materials. The crosslinking procedure has also enabled the formation of patterned P3HT films by photolithography. Pillars with feature sizes down to 2 μm were produced and after subsequent deposition of PCBM and thermal annealing devices with efficiencies of up to 1.4% were produced.     

Selective Dye Loading at the Heterojunction in Polymer/Fullerene Solar Cells

Selective dye loading at the polymer/fullerene interface was studied for ternary blend bulk heterojunction solar cells, consisting of regioregular poly(3‐hexylthiophene) (RR‐P3HT), a fullerene derivative (PCBM), and a silicon phthalocyanine derivative (SiPc) as a light‐harvesting dye. The photocurrent density and power conversion efficiency of the ternary blend solar cells were most improved by loading SiPc with a content of 4.8 wt%. The absorption and surface energy measurements suggested that SiPc is located in the disordered P3HT domains at the RR‐P3HT/PCBM interface rather than in the PCBM and crystal P3HT domains. From the peak wavelength of SiPc absorption, the local concentration of SiPc ([SiPc]_Local) was estimated for the RR‐P3HT:PCBM:SiPc ternary blends. Even for amorphous films of regiorandom P3HT (RRa‐P3HT) blended with PCBM and SiPc, [SiPc]_Local was higher than the original content, suggesting dye segregation into the RRa‐P3HT/PCBM interface. For RR‐P3HT:PCBM:SiPc blends, [SiPc]_Local increased with the increase in the P3HT crystallinity. Such interfacial segregation of dye molecules in ternary blend films can be rationally explained in terms of the surface energy of each component and the crystallization of P3HT being enhanced by annealing. Notably, the solvent annealing effectively segregated dye molecules into the interface without the formation of PCBM clusters.     

Real‐Time Photoluminescence Studies of Structure Evolution in Organic Solar Cells

A method to study the structural evolution of organic bulk heterojunctions via real‐time, in‐situ, steady‐state photoluminescence (PL) is presented. In‐situ PL, in combination with real‐time transmission and reflection measurements, allows us to quantitatively describe the progression of intimate mixing during blade coating of two OPV systems: the common model system poly(3‐hexylthiophene‐2,5‐diyl)/phenyl‐C61‐butyric‐acid‐methyl ester (P3HT/PCBM), and the higher power conversion efficiency system 7,7′‐(4,4bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(6‐fluoro‐5‐(5′‐hexyl‐[2,2′‐bithiophen]‐5‐yl)benzo[c][1,2,5]thiadiazaole), p‐DTS(FBTTh₂)₂/[70]PCBM. Evaluating the time dependence of the PL intensity during drying using a 3D‐random‐walk diffusion model allows for the quantitative determination of the ratio of characteristic domain size to exciton diffusion length during solidification in the presence of the processing additives 1‐chloronaphtalene (CN), 1,8‐octanedithiol (ODT), and 1,8‐diiodooctane (DIO). In both cases, the obtained results are in good agreement with the typically observed fibril widths and grain sizes, for P3HT and p‐DTS(FBTTh₂)₂, respectively.     

Spontaneous Charge Transfer and Dipole Formation at the Interface Between P3HT and PCBM

In the pursuit of developing new materials for more efficient bulk‐heterojunction solar cells, the blend poly (3‐hexylthiophene):[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) serves as an important model system. The success of the P3HT:PCBM blend comes from efficient charge generation and transport with low recombination. There is not, however, a good microscopic picture of what causes these, hindering the development of new material systems. In this report UV photoelectron spectroscopy measurements on both regiorandom‐ (rra) and regioregular‐ (rr) P3HT are presented, and the results are interpreted using the Integer Charge Transfer model. The results suggest that spontaneous charge transfer from P3HT to PCBM occurs after heat treatment of P3HT:PCBM blends. The resulting formation of an interfacial dipole creates an extra barrier at the interface explaining the reduced (non‐)geminate recombination with increased charge generation in heat treated rr‐P3HT:PCBM blends. Extensive photoinduced absorption measurements using both above‐ and below‐bandgap excitation light are presented, in good agreement with the suggested dipole formation.     

Influence of Phonon Scattering on Exciton and Charge Diffusion in Polymer‐Fullerene Solar Cells

Thermally activated transport and phonon scattering in P3HT:PCBM (poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenylC61‐butyric acid methyl ester) bulk heterojunction (BHJ) organic solar cells is studied via temperature‐dependent external‐quantum‐efficiency (EQE) spectroscopy. The hopping barriers for combined exciton and charge transport are balanced for the individual blended materials in a sample, which possesses a blending ratio and a morphology that give rise to a maximal power‐conversion efficiency. Increasing the PCBM weight fraction leads to a reduction of exciton hopping barriers in PCBM, while for P3HT exciton hopping barriers remain constant. This reduction of PCBM exciton hopping barriers is attributed to a higher PCBM crystallinity in the PCBM‐rich solar cell as compared to the BHJ with the optimized blending ratio. The morphology‐dependent difference in exciton hopping activation energies between P3HT and PCBM is attributed to a higher impact of phonon scattering in P3HT than in PCBM, as concluded from the much stronger decrease of P3HT‐related temperature‐dependent external quantum efficiencies above room temperature in the PCBM‐rich BHJ solar cell. All EQE data of P3HT:PCBM‐based BHJ solar cells is modeled consistently over a broad temperature range by a simple analytical expression involving temperature activation and phonon scattering, without the need to distinguish two separate hopping regimes.     

Impact of Fullerene Molecular Weight on P3HT:PCBM Microstructure Studied Using Organic Thin‐Film Transistors

The clustering and diffusion of C₇₁‐butyric acid methyl ester (PC₇₁BM) in poly(3‐hexylthiophene) (P3HT) has been studied using single layer blend and bilayer organic field‐effect transistors (OFETs) and by atomic force microscopy (AFM). P3HT:PC₇₁BM blend based OFETs were found to undergo phase‐segregation upon annealing, which was detectable as a fall in electron mobility with increasing annealing temperature. By employing carefully designed bilayer P3HT:PC₇₁BM OFETs, the diffusion‐properties of PC₇₁BM in P3HT could additionally be inferred from electron mobility measurements. It was found that the prerequisite annealing temperatures for detectable PC₇₁BM clustering and diffusion in P3HT was approximately 20 °C higher than for PC₆₁BM. The diffusion coefficient of PC₆₁BM in P3HT was found to be several times higher that that of PC₇₁BM. The present work provides unique insights into the diffusion process of fullerenes in conjugated polymers and could prove highly valuable for future materials development and device optimization.     

Comparison of the Morphology Development of Polymer–Fullerene and Polymer–Polymer Solar Cells during Solution‐Shearing Blade Coating

In this work, the detailed morphology studies of polymer poly(3‐hexylthiophene‐2,5‐diyl) (P3HT):fullerene(PCBM) and polymer(P3HT):polymer naphthalene diimide thiophene (PNDIT) solar cell are presented to understand the challenge for getting high performance all‐polymer solar cells. The in situ X‐ray scattering and optical interferometry and ex situ hard and soft X‐ray scattering and imaging techniques are used to characterize the bulk heterojunction (BHJ) ink during drying and in dried state. The crystallization of P3HT polymers in P3HT:PCBM bulk heterojunction shows very different behavior compared to that of P3HT:PNDIT BHJ due to different mobilities of P3HT in the donor:acceptor glass. Supplemented by the ex situ grazing incidence X‐ray diffraction and soft X‐ray scattering, PNDIT has a lower tendency to form a mixed phase with P3HT than PCBM, which may be the key to inhibit the donor polymer crystallization process, thus creating preferred small phase separation between the donor and acceptor polymer.     

Intra‐Molecular Donor–Acceptor Interaction Effects on Charge Dissociation,Charge Transport,and Charge Collection in Bulk‐Heterojunction Organic Solar Cells

This article reports experimental studies on internal charge dissociation, transport, and collection by using magnetic field effects of photocurrent (MFE_PC) and light‐assisted dielectric response (LADR) in highly‐efficient organic solar cells based on photovoltaic polymer PTB2 and PTB4 with intra‐molecular “donor–acceptor” interaction. The MFE_PC at low‐field (< 150 mT) indicates that intra‐molecular “donor‐acceptor” interaction generates charge dissociation in un‐doped PTB2 and PTB4 films, which is similar to that in lightly doped P3HT (Poly(3‐hexylthiophene)) with 5 wt% PCBM (1‐(3‐methyloxycarbonyl)‐propyl‐1‐phenyl (6,6) C₆₁). After PTB2 and PTB4 are mixed with PCBM to form bulk‐heterojunctions, the MFE_PC at high‐field (> 150 mT) reveals that the charge‐transfer complexes formed at PTB2:PCBM and PTB4:PCBM interfaces have much lower binding energies due to stronger electron‐withdrawing abilities, as compared to the P3HT:PCBM device, towards the generation of photocurrent. Furthermore, the light‐assisted dielectric response: LADR indicates that the PTB2:PCBM and PTB4:PCBM solar cells exhibit larger capacitances relative to P3HT:PCBM device under photoexcitation. This reflects that the PTB2:PCBM and PTB4:PCBM bulk heterojunctions have more effective charge transport and collection than the P3HT:PCBM counterpart. As a result, our experimental results indicate that intra‐molecular “donor‐acceptor” interaction plays an important role to enhance charge dissociation, transport, and collection in bulk‐heterojunction organic solar cells.     

Direct Evaluation of Intrinsic Optoelectronic Performance of Organic Photovoltaic Cells with Minimizing Impurity and Degradation Effects

A correlation between the photovoltaic performance and dynamics of transient photoconductivity is investigated by flash‐photolysis time‐resolved microwave conductivity (FP‐TRMC). This electrode‐less technique offers chances to mitigate barriers for direct, speedy, and robust evaluation of bulk heterojunction (BHJ) film. We examined the blend ratio, process (solvent and thermal annealing), and impurity (a metal complex of Pd) and degradation effects in BHJ films consisting of poly(3‐hexylthiophene) (P3HT) and methanofullerene (PCBM). The minimum charge carrier mobility of 0.22 cm²V^?1s^?1 was found in P3HT:PCBM = 1:1 film along with 3.26% power conversion efficiency. The revealed good correlation is not only applicable to process optimization, but also expected as a facile screening method to survey the potential of optoelectronic materials.     

Polymer Blend Solar Cells Based on a High‐Mobility Naphthalenediimide‐Based Polymer Acceptor: Device Physics,Photophysics and Morphology

A high electron mobility polymer, poly{[N,N’‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5’‐(2,2’‐bithiophene) (P(NDI2OD‐T2)) is investigated for use as an electron acceptor in all‐polymer blends. Despite the high bulk electron mobility, near‐infrared absorption band and compatible energy levels, bulk heterojunction devices fabricated with poly(3‐hexylthiophene) (P3HT) as the electron donor exhibit power conversion efficiencies of only 0.2%. In order to understand this disappointing photovoltaic performance, systematic investigations of the photophysics, device physics and morphology of this system are performed. Ultra‐fast transient absorption spectroscopy reveals a two‐stage decay process with an initial rapid loss of photoinduced polarons, followed by a second slower decay. This second slower decay is similar to what is observed for efficient P3HT:PCBM ([6,6]‐phenyl C₆₁‐butyric acid methyl ester) blends, however the initial fast decay that is absent in P3HT:PCBM blends suggests rapid, geminate recombination of charge pairs shortly after charge transfer. X‐ray microscopy reveals coarse phase separation of P3HT:P(NDI2OD‐T2) blends with domains of size 0.2 to 1 micrometer. P3HT photoluminescence, however, is still found to be efficiently quenched indicating intermixing within these mesoscale domains. This hierarchy of phase separation is consistent with the transient absorption, whereby localized confinement of charges on isolated chains in the matrix of the other polymer hinders the separation of interfacial electron‐hole pairs. These results indicate that local, interfacial processes are the key factor determining the overall efficiency of this system and highlight the need for improved morphological control in order for the potential benefit of high‐mobility electron accepting polymers to be realized.     

Nanofiber‐Based Bulk‐Heterojunction Organic Solar Cells Using Coaxial Electrospinning

Nanofibers consisting of the bulk heterojunction organic photovoltaic (BHJ–OPV) electron donor–electron acceptor pair poly(3‐hexylthiophene):phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) are produced through a coaxial electrospinning process. While P3HT:PCBM blends are not directly electrospinnable, P3HT:PCBM‐containing fibers are produced in a coaxial fashion by utilizing polycaprolactone (PCL) as an electrospinnable sheath material. Pure P3HT:PCBM fibers are easily obtained after electrospinning by selectively removing the PCL sheath with cyclopentanone (average diameter 120 ± 30 nm). These fibers are then incorporated into the active layer of a BHJ–OPV device, which results in improved short‐circuit current densities, fill factors, and power‐conversion efficiencies (PCE) as compared to thin‐film devices of identical chemical composition. The best‐performing fiber‐based devices exhibit a PCE of 4.0%, while the best thin‐film devices have a PCE of 3.2%. This increase in device performance is attributed to the increased in‐plane alignment of P3HT polymer chains on the nanoscale, caused by the electrospun fibers, which leads to increased optical absorption and subsequent exciton generation. This methodology for improving device performance of BHJ–OPVs could also be implemented for other electron donor–electron acceptor systems, as nanofiber formation is largely independent of the PV material.     

Direct Correlation of the Organic Solar Cell Device Performance to the In‐Depth Distribution of Highly Ordered Polymer Domains in Polymer/Fullerene Films

This study correlates the device performance of organic solar cells and the electronic charge transport within polymer/fullerene films, directly to the optical order of the polymer. The optical order was measured by spectroscopic ellipsometry and evaluated by our previously derived model. We were able to determine the in‐depth distribution of higher and lower ordered poly(3‐hexylthiophene) (P3HT) domains within an [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) matrix. The over the film thickness integrated volume fraction of highly ordered P3HT domains could be directly correlated to the corresponding solar cell device performance. We are able to describe various thermally annealing conditions between room‐temperature and 200 °C.     

The Importance of Fullerene Percolation in the Mixed Regions of Polymer–Fullerene Bulk Heterojunction Solar Cells

Most optimized donor‐acceptor (D‐A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than ～80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrificing internal quantum efficiency, the device power conversion efficiency (PCE) could be significantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐octylthieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) with 7.3% PCE and find that bimolecular recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimolecular recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphology of these BHJs before and after thermal annealing and determine that thermal annealing drastically reduces the concentration of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concentration of PCBM may reduce the number of percolating electron transport pathways within these mixed regions and create morphological electron traps that enhance charge‐carrier recombination and limit device quantum efficiency. These findings suggest that (i) the concentration of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum efficiency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimolecular recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells.     

Multi‐Length‐Scale Morphologies in PCPDTBT/PCBM Bulk‐Heterojunction Solar Cells

Additives are known to improve the performance of organic photovoltaic devices based on mixtures of a low bandgap polymer, poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (PCPDTBT) and [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM). The evolution of the morphology during the evaporation of the mixed solvent, which comprises additive and chlorobenzene (CB), is investigated by in‐situ grazing incidence X‐ray scattering, providing insight into the key role the additive plays in developing a multi‐length‐scale morphology. Provided the additive has a higher vapor pressure and a selective solubility for PCBM, as the host solvent (CB) evaporates, the mixture of the primary solvent and additive becomes less favorable for the PCPDTBT, while completely solubilizing the PCBM. During this process, the PCPDTBT first crystallizes into fibrils and then the PCBM, along with the remaining PCPDTBT, is deposited, forming a phase‐separated morphology comprising domains of pure, crystalline PCPDTBT fibrils and another domain that is a PCBM‐rich mixture with amorphous PCPDTBT. X‐ray/neutron scattering and diffraction methods, in combination with UV–vis absorption spectroscopy and transmission electron microscopy, are used to determine the crystallinity and phase separation of the resultant PCPDTBT/PCBM thin films processed with or without additives. Additional thermal annealing is carried out and found to change the packing of the PCPDTBT. The two factors, degree of crystallinity and degree of phase separation, control the multi‐length‐scale morphology of the thin films and significantly influence device performance.     

Morphological Characterization of a Low‐Bandgap Crystalline Polymer:PCBM Bulk Heterojunction Solar Cells

Understanding the morphology of polymer‐based bulk heterojunction (BHJ) solar cells is necessary to improve device efficiencies. Blends of a low‐bandgap silole‐containing conjugated polymer, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b;2′,3′‐d]silole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,5′‐diyl] (PSBTBT) with [6,6]phenyl‐C61‐butyric acid methyl ester (PCBM) were investigated under different processing conditions. The surface morphologies and vertical segregation of the “As‐Spun”, “Pre‐Annealed”, and “Post‐Annealed” films were studied by scanning force microscopy, contact angle measurements, X‐ray photoelectron spectroscopy, near‐edge X‐ray absorption fine structure spectroscopy, dynamic secondary ion mass spectrometry, and neutron reflectivity. The results showed that PSBTBT was enriched at the cathode interface in the “As‐Spun” films and thermal annealing increased the segregation of PSBTBT to the free surface, while thermal annealing after deposition of the cathode increased the PCBM concentration at the cathode interface. Grazing‐incidence X‐ray diffraction and small‐angle neutron scattering showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating, and thermal annealing increased the ordering of PSBTBT and enhanced the segregation of the PCBM, forming domains ～10 nm in size, leading to an improvement in photovoltaic performance.     

A Conclusive View on Charge Generation,Recombination, and Extraction in As‐Prepared and Annealed P3HT:PCBM Blends: Combined Experimental and Simulation Work

Time‐delayed collection field (TDCF) and bias‐amplified charge extraction (BACE) are applied to as‐prepared and annealed poly(3‐hexylthiophene):[6,6]‐phenyl C₇₁ butyric acid methyl ester (P3HT:PCBM) blends coated from chloroform. Despite large differences in fill factor, short‐circuit current, and power conversion efficiency, both blends exhibit a negligible dependence of photogeneration on the electric field and strictly bimolecular recombination (BMR) with a weak dependence of the BMR coefficient on charge density. Drift‐diffusion simulations are performed using the measured coefficients and mobilities, taking into account bimolecular recombination and the possible effects of surface recombination. The excellent agreement between the simulation and the experimental data for an intensity range covering two orders of magnitude indicates that a field‐independent generation rate and a density‐independent recombination coefficient describe the current–voltage characteristics of the annealed P3HT:PCBM devices, while the performance of the as‐prepared blend is shown to be limited by space charge effects due to a low hole mobility. Finally, even though the bimolecular recombination coefficient is small, surface recombination is found to be a negligible loss mechanism in these solar cells.     

Tail State‐Assisted Charge Injection and Recombination at the Electron‐Collecting Interface of P3HT:PCBM Bulk‐Heterojunction Polymer Solar Cells

The systematic insertion of thin films of P3HT and PCBM at the electron‐ and hole‐collecting interfaces, respectively, in bulk‐heterojunction polymer solar cells results in different extents of reduction in device characteristics, with the insertion of P3HT at the electron‐collecting interface being less disruptive to the output currents compared to the insertion of PCBM at the hole‐collecting interface. This asymmetry is attributed to differences in the tail state‐assisted charge injection and recombination at the active layer‐electrode interfaces. P3HT exhibits a higher density of tail states compared to PCBM; holes in these tail states can thus easily recombine with electrons at the electron‐collection interface during device operation. This process is subsequently compensated by the injection of holes from the cathode into these tail states, which collectively enables net current flow through the polymer solar cell. The study presented herein thus provides a plausible explanation for why preferential segregation of P3HT to the cathode interface is inconsequential to device characteristics in P3HT:PCBM bulk‐heterojunction solar cells.     

The Roles of Poly(Ethylene Oxide) Electrode Buffers in Efficient Polymer Photovoltaics

The role of poly(ethylene oxide) polymer is investigated as an effective buffer with Al electrodes to markedly improve the electrode interface and enhance the open‐circuit voltage (V_OC) and the power conversion efficiency (PCE, η) of poly(3‐hexylthiophene) (P3HT):[6,6]‐phenyl C61‐butyric acid methyl ester (PCBM)‐based bulk‐heterojunction (BHJ) solar cells. A unique process is developed by thermally co‐evaporating the poly(ethylene glycol) dimethyl ether (PEGDE, Mn ca. 2000) polymer with Al metal simultaneously at different ratios in vacuum (10^?6 Torr) to prepare the electrode buffers. The instant formation of a carbide‐like junction at the ethylene oxide/Al interface during the thermal evaporation is of essential importance to the extraction of electrons through the Al electrode. The performance of P3HT:PCBM‐based solar cells can be optimized by modulating the co‐evaporation ratios of the PEGDE polymer with Al metal due to the changes in the work functions of the electrodes. The V_OC and η for devices fabricated with Al electrode are 0.44 V and 1.64%, respectively, and significantly improve to 0.58 V and 4.00% when applying the PEGDE:Al(2:1)/Al electrode. This research leads to a novel electrode design – free of salts, additives, complicated syntheses, and having tunable work function – for fabricating high‐performance photovoltaic cells.     

Plasmonic Back Reflectors: A Small Molecule Non‐fullerene Electron Acceptor for Organic Solar Cells

Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2‐[{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazol‐4‐yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.