Bulk Heterojunction Structure Research Articles

Bulk Heterojunction Nanomorphology of Fluorenyl Hexa-peri-hexabenzocoronene–Fullerene Blend Films

In this study, the nanomorphology of fluorenyl hexa-peri-hexabenzocoronene:[6,6]-phenyl C61-butyric acid methyl ester (FHBC:PC61BM) absorber layers of organic solar cells was investigated. Different electron microscopical techniques, atomic force microscopy, and grazing incidence wide-angle X-ray scattering were applied for a comprehensive nanomorphology analysis. The development of the nanomorphology upon sample annealing and the associated change of the device performance were investigated. It was shown that the annealing process enhances the phase separation and therefore the bulk heterojunction structure. Due to π-π stacking, the FHBC molecules assemble into columnar stacks, which are already present before annealing. While the nonannealed sample consists of a mixture of homogeneously distributed PC61BM molecules and FHBC stacks with a preferential in-plane stack orientation, crystalline FHBC precipitates occur in the annealed samples. These crystals, which consist of hexagonal arranged FHBC stacks, grow with increased annealing time. They are distributed homogeneously over the whole volume of the absorber layer as revealed by electron tomography. The FHBC stacks, whether in the two phase mixture or in the pure crystalline precipitates, exhibit an edge-on orientation, according to results from grazing incidence wide-angle X-ray scattering (GIWAXS), dark-field transmission electron microscopy (DF TEM) imaging and selective area electron diffraction (SAED). The best solar cell efficiencies were obtained after 20 or 40 s sample annealing. These annealing times induce an optimized degree of phase separation between donor and acceptor material.

플라스틱 기판에 제작된 유기박막태양전지의 출력특성 경시변화

<TEX>$PCDTBT:PC_{71}BM$</TEX>과 <TEX>$PTB7:PC_{71}BM$</TEX>을 유기고분자 활성층 재료로 이용한 Bulk Hetero-Junction (BHJ) 구조의 유기박막태양전지를 플라스틱 기판 위에 각각 제작하여, 시간변화에 따른 단락전류밀도(<TEX>$J_{SC}$</TEX>), 개방전압(<TEX>$V_{OC}$</TEX>), 곡선인자(FF) 및 전력변환효율(PCE) 등 출력특성의 변화에 대해 고찰하였다. 유기박막태양전지의 출력특성 파라메터는 시간 경과에 따라 모두 감소하는 경향을 나타내었으며, 특히 개방전압의 감소폭이 컸다. 이러한 개방전압 감소의 원인은 빛에 대한 장시간의 노출과 산소를 포함하는 수분과의 접촉에 의한 LUMO 준위와 HOMO 준위 차의 감소가 그 원인이라 생각되며, 그 메커니즘에 대해 고찰하였다. 또한 유기박막태양전지 소자의 직렬 및 병렬 저항 값은 감소하다가 다시 증가하는 경향을 보였다. 이는 LUMO 준위와 HOMO 준위 차가 감소함에 의한 것과 공액 고분자 활성층 내부에서의 열적과정 손실에 기인하여 전극과 고분자의 계면에서의 접촉저항의 증가 때문이라고 생각된다. 유기박막태양전지의 전력변환효율은 초기에 급격한 감소를 보이다가 시간이 지날수록 감소폭이 차츰 둔화되어 한계치에 도달한 후, 포화되는 경향을 보였다. 이것이 유기박막태양전지가 실제 구동에서 발생시킬 수 있는 최소 출력특성값인 것으로 판단된다. Two types of organic thin film solar cell devices with bulk hetero-junction (BHJ) structure were fabricated on plastic substrates using conjugated polymers of <TEX>$PCDTBT:PC_{71}BM$</TEX> and <TEX>$PTB7:PC_{71}BM$</TEX> blended as active channel layer. Time-variant characteristics of the organic thin film solar cell devices were investigated: short circuit current density (<TEX>$J_{SC}$</TEX>); open circuit voltage (<TEX>$V_{OC}$</TEX>); ; fill factor (FF); power conversion efficiency (PCE, ŋ). All the performance parameters were degraded by progress of the measurement time, while <TEX>$V_{OC}$</TEX> showed the most drastic decrease with time. Possible factors to cause the time-variant alteration of performance parameters were discussed to be clarified.

PbS Quantum Dots Embedded in a ZnS Dielectric Matrix for Bulk Heterojunction Solar Cell Applications

We demonstrate a novel bulk heterojunction structure based on a mesoporous TiO2 substrate, PbS quantum dots (QDs), and a ZnS dielectric medium. The galena PbS QD arrays embedded in an amorphous ZnS matrix are adopted to fill up the mesoporous TiO2 electrode with an in situ approach, i.e., successive ionic layer adsorption and reaction. The inorganic ZnS resembles the capping material normally used in colloidal QDs to control the size of PbS.

Interfacial polymer ferroelectric dipole induced electric field effect on the photovoltaic performance of organic solar cells

Effect of a dipolar electrical field on the photovoltaic properties of polymer solar cell devices in the bulk heterojunction structure based on 1:1 blend of donor-regioregular poly(3-hexylthiophene), P3HT, and acceptor-phenyl-C61-butyric acid methyl ester, PCBM, polymer semiconductor material is studied. With a thin layer of poled ferroelectric P(VDF + TrFE) copolymer film having oriented dipoles inserted at the interface between poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and the P3HT:PCBM composite layer, significant improvement in the short-circuit photocurrent and open circuit voltage was observed consistent with the increase in the poling voltage. Remnant polarization charge due to dipoles produces localized electric field of 300–700 V · μm−1. The presence of the electric field helps increase the charge transfer exciton dissociation rates at the P3HT (donor)–PCBM (acceptor) interfaces, reduces the exciton recombination probability and increases the carrier extraction yield resulting in the enhancement in the photocurrent. Solar cell device in a structure having a poled P(VDF + TrFE) copolymer film sandwiched between the two P3HT:PCBM composite layers shows only a marginal improvement in the photovoltaic properties. This is mainly attributed to shielding of dipolar field from free charge carries and the inferior nanoscale morphology of the P3HT:PCBM composite layer.

PEDOT:PSS 정공 수송층에 금 나노입자를 첨가한 유기태양전지의 제작 및 특성 연구

본 논문은 캐리어의 이동도 및 전도도를 개선하고, 흡수된 빛의 이동 경로를 증가시켜 광흡수도를 높이기 위하여 정공 수송층 재료에 금 나노입자를 첨가하여 유기태양전지를 제작하였다. 광활성층으로는 P3HT와 PCBM의 bulk-heterojunction 구조를 사용하였다. 유기태양전지에서 금 나노입자를 첨가한 정공 수송층의 효과를 관찰하기 위하여 금 나노입자의 첨가량(0, 0.5, 1.0 wt% Au)과 열처리온도(상온, <TEX>$110^{\circ}C$</TEX>, <TEX>$130^{\circ}C$</TEX>, <TEX>$150^{\circ}C$</TEX>)에 따른 광학적 전기적 특성을 조사하였다. 최대전력변환효율을 갖는 유기태양전지는 0.5 wt% 금 나노입자 첨가한 소자와 <TEX>$130^{\circ}C$</TEX>에서 열처리한 소자에서 관찰되었다. 이때 유기태양전지의 전기적 특성은 금 나노입자를 0.5 wt% 첨가한 경우, 단락전류밀도, 곡선인자 및 전력변환효율은 각각 10.2 <TEX>$mA/cm^2$</TEX>, 55.8% 및 3.1%로 나타났으며, <TEX>$130^{\circ}C$</TEX>에서 열처리한 경우, 12.0 <TEX>$mA/cm^2$</TEX>의 단락전류밀도와 64.2%의 곡선인자를 가지며, 4.0%의 전력변환효율이 관찰되었다. In this paper, organic solar cells(OSCs) based on bulk-heterojunction structures were fabricated by spin coating method using polymer P3HT and fullerene PCBM as a photoactive layer. The fabricated OSCs had a simple glass/ITO/PEDOT:PSS/P3HT:PCBM/Al structures. The photoactive layer of mixed P3HT:PCBM was formed with 1:1 weight ratio. The hole transport layer(HTL) was used conducting polymer PEDOT:PSS concentration with gold nanoparticles. The annealing temperature and concentration of nanoparticles in HTL were verified to improve the OSC characterization. The percentage of gold nanoparticles in HTL were 0.5 wt% and 1.0 wt%, and the surface morphology, electrical properties and absorption intensities were investigated. The devices were 0.5 wt%, and the highest 3.1% of the powder conversion efficiency(PCE), 10.2 <TEX>$mA/cm^2$</TEX> of the maximum short circuit current density(<TEX>$J_{SC}$</TEX>), 0.535V of the open circuit voltage(<TEX>$V_{OC}$</TEX>) and 55.8% of the fill factor(F.F) could be obtained when the nanoparticle concertration was 0.5 wt%. The annealing temperature of HTL was <TEX>$110^{\circ}C$</TEX>, <TEX>$130^{\circ}C$</TEX>, <TEX>$150^{\circ}C$</TEX> in vacuum oven and measured the absorption intensities, surface morphology, crystallinity and electrical properties were investigated. The best property was obtained in HTL annealed at <TEX>$130^{\circ}C$</TEX> for gold nanoparticles of 0.5 wt%, showing that <TEX>$J_{SC}$</TEX>, <TEX>$V_{OC}$</TEX>, F.F and PCE were about 12.0 <TEX>$mA/cm^2$</TEX>, 0.525V, 64.2% and 4.0%, respectively.

Synthesis of a Phenothiazine-Quinoxaline Polymer for Photovoltaic Applications

Polymer solar cells (PSCs) have attracted considerable attention because of their unique advantages of low cost, light weight, and potential use in flexible devices. Based on the concept of a bulk heterojunction (BHJ) structure, PSCs made by blending poly(3-hexylthiophene) (P3HT) as a p-type material and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an n-type material have been most intensively investigated and have shown power conversion efficiencies (PCEs) up to 5-6%. PCE of photovoltaic devices is proportional to the shortcircuit current density (Jsc), the open-circuit voltage (Voc), and the fill factor (FF). The Jsc of a solar cell is strongly affected by the absorption of the active layer, and thus an ideal p-type polymer should have a broad and strong absorption spectrum, which requires the polymer to have a low band-gap. The Voc is tightly related to the energy difference between the highest occupied molecular orbital (HOMO) of the p-type polymer and the lowest unoccupied molecular orbital (LUMO) of PCBM. Thus, the p-type polymer should have a low band-gap with appropriate energy levels of HOMO and LUMO. A facile method to synthesize low band-gap polymers is to combine electron-rich (electron-donor, D) and electrondeficient (electron-acceptor, A) monomers, forming alternating D-A type polymers. Quinoxaline was reported to be a good acceptor that can be combined with appropriate donors. For example, a thiophene-bis(3-octyloxyphenyl)quinoxaline polymer (TQ1) exhibited PCEs up to 6.0%. Alternating copolymers consisting of bis(3-alkoxyphenyl)quinoxaline and either dialkoxybenzene (LBPP) or fluorine (APFO) with a thiophene spacer showed PCEs up to 2.9% Another alternating copolymers of quinoxaline and either thieno[3,2-b]thiophene or carbazole with a thiophene spacer showed PCEs of 2.27% and 1.8%, respectively. A copolymer of 2,3-diphenylquinoxaline-based combined with a ladder type oligo-p-phenylene with a thiophene spacer showed a PCE of 3.04%. Some other types of quinoxaline-based copolymers with much lower PCEs were also reported. Poly(10-hexyl-10H-hexylphenothiazine-3,7-diyl) is a very strong electron donor and has a high ionization potential and has thus been used as a hole injection material in polymer light-emitting diodes. D-A type polymers consisting of phenothiazine and quinoxaline are also expected to be p-type materials with low band-gaps, but such polymers have not yet been reported in the literature. Thus, we attempted to synthesize a copolymer (PPTQX) consisting of alternating phenothiazine (PT) and dithienylquinoxaline (QX) segments for photovoltaic applications. As shown in Scheme 1, diboronic ester (compound 1) was reacted with dibromide (compound 2) in the presence of a palladium catalyst via Suzuki coupling to give the corresponding polymer, PPTQX. The polymer was purified using a Soxhlet extraction. The H NMR of the isolated polymer is shown in Figure 1, where the two different sets of aromatic protons (a, b) and four aliphatic protons (d) in the QX segment are easily identified along with two aliphatic protons (c) in the PT segment, indicating that PPTQX has been successfully synthesized. The numberand weight-average molecular weights of PPTQX were 7,000 and 12,000, respectively. The relatively low molecular weight of the polymer may be a result of the low reactivity of the QX monomer since its reacting sites are sterically hindered due to the two large alkoxyphenyl substituents. Molecular weights of LBPP and APFO, which were synthesized from QX monomers similar to ours, were also relatively low (Mn 12,000-15,000). 11

Poly-(3)hexylthiophene nanowire networks for versatile fabrication of bulk heterojunctions with increased active volume

Spontaneous demixing of poly-(3)hexylthiophene (P3HT) and polystyrene (PS) generates a dense array of crystalline P3HT nanowires embedded in a PS matrix. The PS phase is subsequently removed, to form a network of P3HT nanowires with a greatly enhanced surface area-to-volume ratio relative to a planar P3HT film. Photovoltaic devices fabricated from these networks backfilled with either organic ([6,6]-phenyl-C61-butyric acid methyl ester (PCBM)) or inorganic (cadmium selenide (CdSe) nanocrystals) material have dramatically increased short-circuit current and power conversion efficiencies relative to planar-bilayer-prepared devices, with increases of ∼8× and ∼3× for PCBM and CdSe nanocrystals, respectively, suggesting that these devices have increased P3HT active volume. This two-step backfill technique can produce nanostructured all-organic and hybrid organic/inorganic bulk heterojunction structures that cannot be formed with traditional one-step casting methods.

Enhanced Performance of PDPP3T/${\rm PC}_{60}{\rm BM}$ Solar Cells Using High Boiling Solvent and UV - Ozone Treatment

Using higher temperature without any additives, poly(diketopyrrolopyrrole-terthiophene) was successfully dissolved with PC60BM in a commonly used high-boiling point solvent namely chlorobenzene (CB). Use of CB allowed us to heat the blend solution at a higher temperature, which leads to improved nanoscale phase separation between donor and acceptor, which is important for an efficient exciton dissociation. Devices were fabricated in both regular and inverted bulk heterojunction structure for comparison by spin coating an active layer. Inverted structure solar cells exhibited higher power conversion efficiency than regular structure with significant improvement in current density, possibly attributed to increased light absorption in the active layer due to less parasitic absorption in the PEDOT:PSS and the Ca layer, which were used in normal structures. Furthermore, ultraviolet (UV) ozone treatment of ZnO film improved the solar cell performance. A UV ozone treatment of 5 min was found to be the optimal time resulting in the highest device efficiency (4.45%) with short circuit current density of 9.3 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , open circuit voltage of 0.69, and fill factor 69.5% without the need for any solvent additives.

Effects of phosphorous doping to poly (methylphenyl silane) and fabrication of thin film solar cells

Effects of phosphorus bromine (PBr3) doping to polysilane were investigated for spin-coating thin films. Phosphorus doped poly (methylphenyl silane) (PMPS) provided n-type semiconductor behavior, which was confirmed by Hall effect measurements. Desorption of phenyl and methyl groups in doped PMPS thin films was observed after annealing at 300 °C from Raman scattering measurements and theoretical calculations. The band gap energy of PMPS was measured to be 3.2 eV. Decrease of photoluminescence intensity of PMPS was observed by phosphorus doping. Microstructures of the doped PMPS thin films were investigated by using X-ray diffraction, which indicated doped PMPS had an amorphous structure after annealing. A solar cell with PMPS(PBr3):poly[3-hexylthiophene] bulk-heterojunction structure was fabricated, and provided a photovoltaic behavior. Formation mechanism and carrier transport mechanism of the doped PMPS thin films were proposed.

Opto-electrical and density functional theory analysis of poly(2,7-carbazole-alt-thieno[3,4-c]pyrrole-4,6-dione) and photovoltaic behaviors of bulk heterojunction structure

To increase open-circuit voltage (Voc), we introduced thieno[3,4-c]pyrrole-4,6-dione (TPD) moieties which has unsubstituted and substituted thiophene spacers. Poly(2,7-carbazole-alt-TPD) derivatives, namely PCDTTPD and PCDHTTPD, were copolymerized with 2,7-carbazole through Suzuki coupling reaction. An increase in the molecular weight and a decrease in the highest occupied molecular orbital (HOMO) energy level are confirmed. Steric hindrance caused by rotational dynamics was measured by density functional theory calculation. When the resulting polymers, PCDHTTPD, were used to fabricate a bulk heterojunction device with PC71BM, Voc increased to 0.84V, whereas the short-circuit current density (Jsc) decreased to 0.93mA/cm2 because of poor charge dissociation.

Synthesis and photovoltaic performance of a fluorene-bithiophene copolymer

We present the synthesis of a copolymer structure, poly(9,9′-n-di-hexyl-2,7-fluorene-alt-2,5-bithiophene), referred to herein as LaPPS43, and its physico-chemical characterization. Thin films of this polymer mixed with phenyl-C61-butyric acid methyl ester (PCBM) were used as the active layer in photovoltaic devices using the ITO/PEDOT:PSS/LaPPS43:PCBM/Ca/Al bulk heterojunction structure. The devices of different active layer thicknesses were electrically studied using J-V curves and the Photo-Celiv technique. The obtained results show that LaPPS43 combined with PCBM is a promising system for photovoltaic devices. Device performance is discussed in terms of the mean drift distance x for charge carriers. Photophysical data showed that the excitonic species are all localized in the aggregated forms. The mechanism of exciton formation and dissociation is also discussed.

Microstructures and Photovoltaic Properties of Polysilane-Based Solar Cells

Polysilane-based organic solar cells with bulk heterojunction and heterojunction structures were fabricated by spin-coating using a mixture solution. Microstructures of the solar cells were investigated by X-ray diffraction analysis and transmission electron microscopy, which indicated a nanocomposite and amorphous structure. The effects of phosphorus and boron doping into polysilanes were investigated by electrical measurements, Raman scattering analysis and Hall effect measurements. Here, we discuss the photovoltaic properties and microstructures of the solar cells. Energy levels in the present solar cells are also discussed.

Structural control of bulk heterojunction films based on oligothiophene with sterically-bulky groups

Organic photovoltaic solar cells based on oligothiophene and fullerene have been investigated intensively. Until now, the morphologies of bulk heterojunction (BHJ) structures based on oligothiophene and fullerene have been difficult to control because the oligothiophene molecules have lacked substituent groups and therefore have interacted strongly with one another to the point of aggregating into microfibril structures.We succeeded in fabricating homogeneous oligothiophene:C60 BHJ films by adding sterically-bulky groups to oligothiophene in order to control the extent of aggregation. We found that the aggregation onset and maximum fill factor occurred at higher oligothiophene concentrations in BHJ films with C60 for the substituted molecules – the ratio that provides as much oligothiophene as possible for good photovoltaic performance but not so much as to incur aggregation-fill factor was improved. Here in, we report the morphologies and photovoltaic performances of single-component oligothiophene films and BHJ films based on oligothiophenes.

Sensitivity analysis of current generation in organic solar cells—comparing bilayer, sawtooth, and bulk heterojunction morphologies

Organic solar cells (OSC) show great potential as a low-cost energy source. In addition, their mechanical flexibility allows the added advantage of use on a wide variety of surfaces. In recent years, progress in experimental strategies and modeling approaches have enabled enhancing the power conversion efficiencies of OSC. In particular, simulation based analysis has played a significant role in improving our understanding of the charge transport phenomena in the active layer of these devices. The excitonic drift-diffusion (EDD) model has been used widely to simulate the generation and transport properties of bulk heterojunction (BHJ) solar cells. The EDD model – which is derived from the Boltzmann transport is dependent on a number of input parameters such as (1) material properties (mobility and permittivity), (2) operating conditions (illumination and device thickness), and (3) active layer morphology.A comprehensive sensitivity analysis of the short-circuit current, Jsc, to the input parameters is performed. This helps in rank ordering the input parameters and operating conditions – by strength and relevance – on their impact on Jsc. We particularly focus our investigations on understanding how the active layer morphology affects the sensitivity of Jsc. To accomplish this we analyze three classes of morphologies: bilayer, BHJ, and sawtooth. The results show significant differences in sensitivities between BHJ, sawtooth, and bilayer morphologies. Short-circuit current in BHJ structure shows higher sensitivity to material properties than either sawtooth and bilayer structure, suggesting that the necessity for finer control of material properties to counteract the increased disorder in the active layer morphology. The electrode current is found to be most sensitive to illumination intensity for all three morphologies. We report some interesting trends that may help choose the most sensitive parameters to vary for designing OSC's with better performance.

Effect of Side Chain of Oligothiophene Derivatives on Bulk Heterojunction Structure in Organic Photovoltaic Devices

The effect of the molecular structure on photovoltaic performance was investigated by comparing two types of active layers. One is an active layer formed by sexithiophene (6T) and C60 molecules which don’t have side chains. Another one is an active layer formed by a,ω-bis(2-hexyldecyl)sexithiophene (BHD6T) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) molecules which have side chains. The 6T:C60 active layer exhibited an excessive crystallization of 6T, which led to the low photovoltaic performance. In the BHD6T:PCBM active layer, the crystallization of BHD6T was suppressed. The crystallization of BHD6T was also easily controlled by thermal annealing, which led to improved photovoltaic performance.

Performance Comparison of Conventional and Inverted Organic Bulk Heterojunction Solar Cells From Optical and Electrical Aspects

The conventional and inverted organic solar cells (OSC and IOSC) based on the bulk heterojunction structure are investigated from both optical and electrical aspects. When the optical aspect is considered only, with the increase of the active layer thickness, the number of photons absorbed in the active layer and the external quantum efficiency tend to increase with the obvious interference behavior for both OSC and IOSC. However, compared to OSC, IOSC shows a better performance except for the thicknesses around which the interference maxima of OSC are obtained. When the electrical aspect is also considered, an effective area in the active layer will be induced by the charge drift length (L), and only the photons absorbed in this effective area have contribution to the photocurrent. By considering optical and electrical aspects together, OSC and IOSC show different behaviors. Compared to IOSC, OSC performs better for relatively thick active layers. Simultaneously, the optical modulation effect is also investigated by introducing an optical spacer layer. It is found that the optical spacer layer can notably enhance the performance of OSC with thin and thick active layers, while it could only degrade the performance of IOSC with relatively thick active layers.

Exciton Dynamics of P3HT:PCBM Blend Films with Different Polymer Regioregularities Using Transient Absorption Spectroscopy

The poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blending is one of the most intensively studied active material systems for bulk-heterojunction (BHJ) organic photovoltaic devices. For the active layer with BHJ structure, the charge tranfer (CT) and exciton dissociation processes between P3HT and PCBM are the important initial steps to form free charge carriers in the active layer. To compare the CT related species from the transient absorption dynamics in P3HT:PCBM blend systems with different orderings of the polymer, we investigated the charge carrier dynamics in P3HT:PCBM blend films with regiorandom and regioregular polymer structures using femtosecond transient absorption spectroscopy.

Influence of gradient doping on photoelectric conversion efficiency of organic photovoltaic devices

On the basis of the traditional bulk heterojunction structure, we introduce a method of gradient doping in the mixing layer to improve photoelectric conversion efficiency (ηp) of organic photovoltaic device (OPV). One of the devices with the structure of ITO/CuPc (10 nm)/CuPc:C60 (2:1) (4 nm)/CuPc:C60 (1.5:1) (4 nm)/CuPc:C60 (1:1) (4 nm)/CuPc:C60 (1:1.5) (4 nm)/CuPc:C60 (1:2) (4 nm)/C60 (30 nm)/Bphen (8 nm)/Al (100 nm) show the improvement on performance:short-circuit current JSC = 9.18 mA/cm2, photoelectric conversion efficiency ηp = 1.35% under AM1.5 solar illumination. Compared with ηp of the traditional bulk heterojunction OPV ITO/CuPc(10 nm)/CuPc:C60 (1:1) (20 nm)/C60(30 nm)/Bphen(8 nm)/Al(100 nm), the ηp of the present structure is improved by 25%, which is attributed to the improvement on percolating paths of donor and acceptor molecules across the entire mixing layer, thus increasing the charge collection efficiency. Consequently, the overall device series resistance is reduced and the photoelectric conversion efficiency is enhanced.

A fullerene dyad with a tri(octyloxy)benzene moiety induced efficient nanoscale active layer for the poly(3-hexylthiophene)-based bulk heterojunction solar cell applications

A bulk tri(octyloxy)benzene moiety grafted onto fullerene (PCBB-C8) was successfully synthesized, which could effectively induce poly(3-hexylthiophene) (P3HT) to form highly ordered bulk heterojunction structure without any external treatment. This ordered active layer exhibits good photovoltaic performance.

Visualizing physical, electronic, and optical properties of organic photovoltaic cells

The most interesting active layer system for organic photovoltaic cells is the bulk heterojunction (BHJ). The general structure of a BHJ is a network of domains, which contain blended donor and acceptor molecules, often in pure and mixed phases. Directly visualizing this nanoscale phase separation of BHJs and corresponding morphological parameters has attracted much interest and has led to novel insights into their physical properties. Imaging methods that have been proven to be of great importance are scanning probe, scanning X-ray, and electron microscopy. Early works successfully employed conventional contrast mechanisms at high resolution for correlation of structure and function. Many imaging techniques also offer analytical capabilities using specific interactions of the probes with different material domains. Resulting material contrasts can be converted into morphological maps by correlation with the intrinsically different physical, electronic, and optical sample properties. Spatially resolved visualization of these properties is crucial to provide answers to questions about fundamental processes that determine the solar cell performance. A major paradigm shift was initiated by the discovery that instead of a completely separated network of donor and acceptor domains, a multiphase system is beneficial for enhanced photo-physical properties. Thereby phases differ in ordering, orientation and composition. In this review, we provide an overview of techniques and recent approaches for imaging BHJ structures at the nanoscale. Advantages and disadvantages of the underlying contrast mechanisms are summarized. In addition, we introduce best practices for the application of transmission electron microscopy (TEM) by comparing enhanced conventional and analytical modes.