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A halogen-free solvent 1,2,4-trimethylbenzene dissolves both unmodified fullerenes and conjugated polymers such as PTB7-Th. This unique feature enables the study on the effect of the substituent of...

Donor-acceptor (D-A) type fully conjugated block copolymer systems have been rarely reported due to the challenges in synthetic approaches to prepare well-defined low-polydispersity products. In this work, fully conjugated block copolymers are synthesized in a one-pot reaction through Stille coupling polycondensation, by utilizing the end-functional polymer copolymerization method. End-functional P3HT are copolymerized with AA (2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole) and BB (4,7-bis(5-(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole, TBT) type monomers, respectively. The orthogonal solubility between the very soluble P3HT donor and the insoluble PCDTBT acceptor block improves the purity of block copolymers as well as distinct nano-scale phase-separation compared with other reports on miscibility of donor and acceptor polymer block. Further purification via preparative GPC is carried out to remove the excess of unreacted P3HT and free PCDTBT as well as to achieve low polydispersity of block copolymers. The chemical structure of the P3HT-b-PCDTBT block copolymers are verified via 1H-NMR, and further confirmed by FTIR spectra. The block copolymer shows broad absorption and moderate optical band gap of 1.8 eV. Furthermore, the fully conjugated block copolymer films exhibit significant fine structures, much smoother film morphology compared to P3HT/PCDTBT polymer blends. By adding a small amount of block copolymer P3HT-b-PCDTBT as a compatibilizer into the bulk-heterojunction of P3HT:PC61BM blends, polymer solar cells with an 8% increase of short circuit current (J sc and 10% increase of power conversion efficiency (PCE) are achieved owing to the improvement of the active-layer film morphology. To the best of our knowledge, this is the first report on donor-acceptor type fully conjugated block copolymer as an effective ternary additive in polymer: fullerene bulk heterojunction solar cells.

We investigate a correlation between the type of solvent additives (SAs) with specific criteria such as aromatic additives (AAs) and non-aromatic additives (NAAs) and phase separation in the bulk heterojunction (BHJ) films comprising low-band gap polymer and fullerene derivatives. When AAs are used as SAs, the geometrical structures (π-π and lamellar stacking) of aggregated polymer chains do not significantly change. However, NAAs increase the lamellar stacking distance through a strong interaction with non-aromatic segments of polymers. Therefore, a well-phase separated BHJ morphology with the finer fibrils is developed, thereby leading to balanced charge mobilities and a reduced charge recombination in BHJ solar cells. Finally, the optimized solar cell exhibits a high power conversion efficiency of 7.9%.

This work was supported by the IAP 7/05 project FS2 (Functional Supramolecular Systems), granted by the Science Policy Office of the Belgian Federal Government (BELSPO). We are also grateful for financial support by the Research Program of the Research Foundation – Flanders (FWO) (projects G.0415.14N, G.0B67.15N and M.ERA-NET project RADESOL). MK acknowledges the Agency for Innovation by Science and Technology in Flanders (IWT) for his PhD grant. CWB is grateful to the Institute for Basic Science (IBS-R019-D1) and the BK21 Plus Program as funded by the Ministry of Education and the National Research Foundation of Korea for their generous financial support. The work in Austin was supported by the U.S. National Science Foundation (grant CHE-1402004 to JLS).

Colloidal zinc oxide (ZnO) nanoparticles are frequently used in the field of organic photovoltaics for the realization of solution-producible, electron-selective interfacial layers. Despite of the widespread use, there is a lack of detailed investigations regarding the impact of structural properties of the particles on the device performance. In this work, ZnO nanoparticles with varying surface-area-to-volume ratio were synthesized and implemented into polymer-fullerene bulk heterojunction solar cells with a gas-permeable top electrode. By comparing the electrical characteristics before and after encapsulation, it was found that the internal surface area of the ZnO layer plays a crucial role under conditions where oxygen can penetrate the solar cells. The adsorption of oxygen species at the nanoparticle surface causes band bending and electron depletion next to the surface. Both effects result in the formation of a barrier for electron injection and extraction at the ZnO/bulk heterojunction interface and were more pronounced in case of small ZnO nanocrystals (high surface-area-to-volume ratio). Different transport-related phenomena in the presence of oxygen are discussed in detail, i.e., Ohmic losses, expressed in terms of series resistance, as well as the occurrence of space-charge-limited currents, related to charge accumulation in the polymer-fullerene blend. Since absorption of UV light can cause desorption of adsorbed oxygen species, the electrical properties depend also on the illumination conditions. With the help of systematic investigations of the current versus voltage characteristics of solar cells under different air exposure and illumination conditions as well as studies of the photoconductivity of pure ZnO nanoparticle layers, we gain detailed insight into the role of the ZnO nanoparticle surface for the functionality of the organic solar cells., Comment: 12 pages, 9 figures; Supplemental Material: 10 pages, 7 figures

Conjugated polymer-fullerene-based bulk-heterojunction (BHJ) organic solar cells (OSCs) have attracted tremendous attention over the past two decades because of their potential to develop low-cost and easy methods to produce energy from light. The complicated microstructure and morphology with randomly organized architecture of these polymer-fullerene-based active layers (ALs) is a key factor that limits photovoltaic performance. In this study, a binary-solvent annealing (BSA) approach was established to improve the poly(3-hexylthiophene):indene-C60 bisadduct-based AL for efficient BHJ-type OSCs by varying the second solvents with different boiling points (BP). Thus, we were able to change the evaporation behavior of cosolvents and consequently obtain the various microstructural properties of the AL. An in-depth study was conducted on the solvent-evaporation driven morphology of the active layer under various cosolvent conditions and its effect on the photovoltaic parameters of OSCs. Under the BSA processes, we found that the specimens with low-BP second solvents allows us to observe a more ideal AL for increasing photon absorption and efficient charge transport and collection at the respective electrodes, resulting in enhanced PCE of the corresponding OSCs. By contrast, the specimens with high-BP second solvents exhibit random microstructures, which are detrimental to charge transport and collection and lead to diminished PCE of the corresponding OSCs. By appropriately selecting the composition of a binary solvent, BSA can be employed as an easy method for the effective manipulation of the microstructures of ALs. BSA is a promising technique for the performance enhancement of not only OSCs but also other organic/polymeric-based electronic devices.

1,2,4-trimethylbenzene is a halogen-free solvent which dissolves unmodified and modified fullerenes and conjugated polymers. The unique feature enables the study of how the substituent of fullerene affects the thermal robustness in bulk heterojunction solar cells using a conjugated polymer PTB7-Th. While 175 °C is the best annealing temperature for a solar cell with unmodified C70, it deteriorates a device with a substituted C70 (C70-PCBM). Additionally, annealing at 175 °C does not change the surface of PTB7-Th:C70 film but makes the surface of PTB7-Th:C70-PCBM film bumpy. The results suggest that the substituent promotes the migration of fullerene in polymer:fullerene solid composite.

Thin film polymer solar cells have been fabricated using regioregular poly(3-hexylthiophene) (P3HT) within [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction active layer. The influence of the thermal annealing temperature and the time duration on the performance of P3HT/PCBM bulk heterojunction solar cells based on different thickness of active layers has been investigated. The results demonstrate that thermal annealing is a thermally activated kinetic process. The effect is almost the same for high temperature annealing for a short time and low temperature annealing for a long time at a temperature greater than the glass temperature of P3HT. The device with a thicker active layer requires a higher temperature or a longer time annealing treatment to achieve the optimised phase separation for efficient charge separation and transport and thereby leads to optimal performance. The device with a thinner active layer requires a lower temperature or a shorter time annealing treatment...

Blends of poly(3-hexylthiophene) (P3HT) and C61-butyric acid methyl ester (PCBM) are widely used as a model system for bulk heterojunction active layers developed for solution-processable, flexible solar cells. In this work, vertical concentration profiles within the P3HT:PCBM active layer are predicted based on a thermodynamic analysis of the constituent materials and typical solvents. Surface energies of the active layer components and a common transport interlayer blend, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), are first extracted using contact angle measurements coupled with the acid-base model. From this data, intra- and interspecies interaction free energies are calculated, which reveal that the thermodynamically favored arrangement consists of a uniformly blended "bulk" structure capped with a P3HT-rich air interface and a slightly PCBM-rich buried interface. Although the "bulk" composition is solely determined by P3HT:PCBM ratio, composition near the buried interface is dependent on both the blend ratio and interaction free energy difference between solvated P3HT and PCBM deposition onto PEDOT:PSS. In contrast, the P3HT-rich overlayer is independent of processing conditions, allowing kinetic formation of a PCBM-rich sublayer during film casting due to limitations in long-range species diffusion. These thermodynamic calculations are experimentally validated by angle-resolved X-ray photoelectron spectroscopy (XPS) and low energy XPS depth profiling, which show that the actual composition profiles of the cast and annealed films closely match the predicted behavior. These experimentally derived profiles provide clear evidence that typical bulk heterojunction active layers are predominantly characterized by thermodynamically stable composition profiles. Furthermore, the predictive capabilities of the comprehensive free energy approach are demonstrated, which will enable investigation of structurally integrated devices and novel active layer systems including low band gap polymers, ternary systems, and small molecule blends.

Planar structure organic solar cells (ITO/PEDOT:PSS/P3HT:PCBM/Al) were studied to determine the optimal concentration of polymer and fullerene in the photoactive layer blend. The bulk heterojunction of these devices consisted of the electron donating polymer poly-3(hexylthiophene-2,5-diyl) (P3HT), and the electron accepting fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Four unique photoactive layer solutions were prepared by dissolving various amounts of the organics in 2 mL chlorobenzene, while keeping the weight ratio of P3HT:PC61BM at 1:0.8. The fabricated solar cells were characterized electrically to determine power conversion efficiency, fill factor, and current density. Subsequent characterization techniques including atomic force microscopy, ultraviolet-visible spectrophotometry, and variable angle spectroscopic ellipsometry were performed on P3HT:PC61BM films to determine surface roughness, optical absorption, and extinction coefficient. Through this systematic study, it was determined that an appropriate selection of P3HT:PC61BM concentration allows for significant increases in the overall device performance of organic solar cells.

Using charge modulated electroabsorption spectroscopy (CMEAS), for the first time, the energy level alignment of a polymer:fullerene bulk heterojunction photovoltaic cell is directly measured. The charge-transfer excitons generated by the sub-bandgap optical pumping are coupled with the modulating electric field and introduce subtle changes in optical absorption in the sub-bandgap region. This minimum required energy for sub-bandgap charge genreation is defined as the effective bandgap.

This work covers the use of solution-processed metal oxides as interface layers for organic solar cells. To study the interface properties, intrinsic and Al-doped ZnO x were chosen as reference systems. From the class of n-type metal oxides, ZnO x was chosen because it can be doped when it is solution processed. Furthermore, the influence of thin modification layers applied on top of the metal oxides is investigated.

Inverted polymer solar cells based on P3HT/PCBM bulk heterojunction were prepared on flexible polyethylene naphthalate (PEN) substrate. The effect of annealing of the PEN/ITO/ZnO multilayer and ZnO/P3HT:PCBM on the structural, morphological, photophysical and photovoltaic properties was investigated and scrutinized directly on the OPV devices using atom probe tomography (APT), scanning electron microscopy (SEM) and microfocus X-ray techniques. We carried out a 3D reconstruction of the interfaces of the multilayer containing PEN/ITO, ZnO/ITO and P3HT:PCBM/ZnO to address the interface micro-structure and its influence on the morphology of the photoactive film. The analyses show that the morphology of the interfaces is affected by the structure of each layer of the BHJ devices causing orientation of P3HT crystals with PCBM aggregates and ZnO, which in turn leads to a significant change of the charge transport across each layer and therefore photovoltaic performances.

In this work, we present resonant tail-state regime absorption enhanced organic photovoltaics. We combine periodically structured ${\mathrm{TiO}}_{2}$ bottom electrodes with P3HT-PCBM bulk-heterojunction solar cells in an inverted device configuration. The wavelength-scale patterns are transferred to the electron-selective bottom electrodes via direct laser interference patterning, a fast method compatible with roll-to-roll processing. Spectroscopic and optoelectronic device measurements suggest polarization-dependent absorption enhancement along with photocurrent generation unambiguously originating from the population of tail states. We discuss the effects underlying these absorption patterns with the help of electromagnetic simulations using the discontinuous Galerkin time domain method. For this, we focus on the total absorption spectra along with spatially resolved power loss densities. Our simulations stress the tunability of the absorption resonances towards arbitrary wavelength regions.

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.

An amorphous polymer, poly(BTD-TPA), which consists of benzothiadiazole and triarylamine units, can be successfully utilized to fabricate bulk heterojunction (BHJ) organic photovoltaics (OPVs), and the OPV performance can be demonstrated to be independent of the casting solvent or thermal annealing temperature. The OPV based on poly(BTD-TPA):PC 70 BM (1:4) that was fabricated using chloroform (boiling point of 61 °C) and annealed at 60 °C for 10 min exhibited a power conversion efficiency (PCE) of 2.81% under simulated solar irradiation through an air mass of 1.5 at 100 mW cm −2 . On the other hand, the OPV fabricated using o -dichlorobenzene (boiling point of 181 °C) and annealed at 110 °C for 10 min exhibited a PCE of 2.65%. Almost the same PCEs and incident photon to current conversion efficiencies (IPCEs) were obtained in both OPVs. The use of an amorphous film of poly(BTD-TPA) in the fabrication of OPVs offers great advantages over the use of a polycrystalline film of regioregular poly(3-hexylthiophene) (P3HT) in terms of high reproducibility of the OPV performance.

A series of n-type fulleropyrrolidine derivatives as the acceptors, including Th-C60, PFTh-C60, and OPFTh-C60, have been synthesized via the key step of the typical Prato reaction to investigate the steric hindrance effect of various phenylfluorenyl moieties on the electronic structures, aggregate morphologies, and device performances of solar cells. Conjugation-interrupted linkage obviously does not change the energy bandgaps and lowest unoccupied molecular orbital (LUMO) energy levels in PFTh-C60 and OPFTh-C60 models with respect to that of precursor Th-C60 according to UV–vis spectra and cyclic voltammetry. In contrast, dramatically different phase separation behaviors in the bulk heterojunction (BHJ) film blending with poly(3-hexylthiophene) (P3HT) were observed by atomic force microscopy. A prototype OPFTh-C60-based BHJ polymer solar cell (PSC) with the configuration of ITO/PEDOT:PSS/P3HT:OPFTh-C60 (1:1) (200 nm)/Ca/Al has the performance with the short-circuit current (Isc) of 8.68 mA/cm2, open-circ...

In recent years, steady improvements in organic photovoltaic (OPV) performance have been reported with power conversion efficiency (PCE) reaching 9%. Despite this rapid improvement, the mechanisms limiting the existing performance are not completely known; therefore a better understanding of the loss processes is necessary. In this respect a crucial role may be played by the charge transfer (CT) state, which is the intermediate state between the excitons and the fully dissociated charges. In this review article we outline the dynamics of this intermediate state in light of their influence on the photovoltaic performance, with central emphasis on highly efficient polymer-fullerene bulk heterojunction solar cells.

Voltage-dependent white-light bias external quantum efficiency (EQE) measurements are employed to compare the relative voltage-dependence of photocurrent generation of polymer and fullerene phases in polymer bulk heterojunction solar cells. By measuring EQE spectra as a function of applied bias, voltage dependent changes in the shape of EQE spectra can be detected and compared with the absorption profiles of polymer and fullerene. Several high-efficiency systems are studied including poly(3-hexylthiophene):PC60BM ([6,6]-phenyl-C61-butyric acid methyl ester) blends and blends of low band gap polymers with PC70BM. For a number of systems (but not all), voltage-dependent but light-intensity-independent changes in the shape of the EQE spectrum are observed that originate from differences in the voltage dependence of photocurrent for polymer and fullerene absorption. For one system, a stronger voltage-dependence of photocurrent generation following fullerene absorption is found, whereas for another system a st...

In organic bulk heterojunction solar cells, a deviation of the ideality factor of the dark current from unity is commonly put forward as evidence for the presence of trap-assisted recombination. We demonstrate that the non-ideality of the dark characteristics is determined by deeply trapped carriers in the transport-dominating constituent of the donor:acceptor blend, rather than a trap-assisted recombination mechanism. The light-intensity dependence of the open-circuit voltage confirms the absence of trap-assisted recombination and demonstrates that the dominant recombination mechanism in the investigated polymer:fullerene solar cells is bimolecular. (C) 2011 American Institute of Physics. [doi:10.1063/1.3651752]

We report quantitative measurements of ordering, molecular orientation, and nanoscale morphology in the active layer of bulk heterojunction (BHJ) organic photovoltaic cells based on a thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7), which has been shown to yield very high power conversion efficiency when blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC(71)BM). A surprisingly low degree of order was found in the polymer-far lower in the bulk heterojunction than in pure PTB7. X-ray diffraction data yielded a nearly full orientation distribution for the polymer π-stacking direction within well-ordered regions, revealing a moderate preference for π-stacking in the vertical direction ("face-on"). By combining molecular orientation information from polarizing absorption spectroscopies with the orientation distribution of ordered material from diffraction, we propose a model describing the PTB7 molecular orientation distribution (ordered and disordered), with the fraction of ordered polymer as a model parameter. This model shows that only a small fraction (≈20%) of the polymer in the PTB7/PC(71)BM blend is ordered. Energy-filtered transmission electron microscopy shows that the morphology of PTB7/PC(71)BM is composed of nanoscale fullerene-rich aggregates separated by polymer-rich regions. The addition of diiodooctane (DIO) to the casting solvent, as a processing additive, results in smaller domains and a more finely interpenetrating BHJ morphology, relative to blend films cast without DIO.

The efficiency of solar cells made from a conjugated polymer blended with a fullerene derivative has risen from around 1 % to over 9 % in the last ten years, making organic photovoltaic technology a viable contender for commercialization. The efficiency increases have resulted from the development of new materials with lower optical gaps, new polymer:fullerene combinations with higher charge separated state energies, and new approaches to control the blend microstructure, all driven by a qualitative understanding of the principles governing organic solar cell operation. In parallel, a device physics framework has been developed that enables the rational design of device structures and materials for improved organic photovoltaic devices. We review developments in both materials science and device physics for organic photovoltaics.

While regulation of the nanoscale microstructure of the active layers in organic bulk heterojunction (BHJ) solar cells, particularly for conjugated polymer–fullerene blend systems, has been shown to be highly important when maximizing power conversion efficiency, little is known about the role of disordered polymer chains in the photovoltaic (PV) behaviors and electrochemical potential drops of polymer–fullerene interfaces. In this study, the microstructural-dependent PV properties of a series of poly(3-hexylthiophene) (P3HT):fullerene (i.e., [6,6]-phenyl-C61-butyric acid methyl ester, or PCBM) blending films with different compositions have been investigated using several experiments (i.e., absorption spectroscopy, Raman spectroscopy, X-ray diffraction, and atomic force microscopy) and theoretical methods (i.e., spectroscopic simulation and quantum mechanical calculations). A strong correlation exists between amorphous P3HT chain properties, characterized by degree of conjugation (Leff), and PV parameter...

Polymer solar cells based on bulk heterojunction (BHJ) structures, featuring conjugated polymers as donors and fullerene derivatives as acceptors, [ 1 ] are being developed for their potential application in the low-cost fabrication of large-area devices. In recent reports, BHJ solar cells incorporating crystalline or low-bandgap conjugated polymers [ 2 ] and fullerene derivatives have exhibited maximum power conversion effi ciencies (PCEs) of up to 8%. [ 3 ] The morphology [ 4 ] of the active layer in a BHJ solar cell incorporating a polymer/fullerene thin fi lm plays a critical role affecting the device performance; phase-separated domains in the active layer provide not only interfaces for charge separation of photogenerated excitons but also percolation pathways for charge carrier transport to the respective electrodes, critically affecting the device’s PCE. The nanoscale morphology of a polymer/fullerene thin fi lm is greatly affected by (i) the fi lm processing conditions, [ 5 ] (ii) the molar ratio (composition) of the polymer and the fullerene, [ 6 ] and (iii) the nature of the solvent additive (if any). [ 7 ] In particular, BHJ polymer solar cells can exhibit improved device performance after undergoing thermal or solvent annealing or the incorporation of solvent additives, all of which alter the fi lm morphology to a more favorable state relative to that of the as-cast fi lm or the fi lm in the absence of the additive, presumably resulting from (i) self-organization of the polymer units into ordered structures and (ii) appropriate aggregation of fullerene domains to provide percolation networks for charge carrier transport. [ 6 , 8 ] Among these approaches, the addition of solvent additive during the processing of the active layer is the simplest and most effective means of optimizing a BHJ device’s morphology; it infl uences the size of the fullerene domains and enhances the crystallinity of the self-organized polymers by improving the solubility of

The challenges of experimentally probing the physical and electronic structures of the highly intermixed organic semiconductor blends that comprise active layers in high-performance organic photovoltaic (OPV) cells ultimately limit the fundamental understanding of the device performance. We use Fouriertransform IR (FTIR)-absorption spectroscopy to quantitatively determine the interfacial electric fi eld in blended poly(3-hexylthiophene) (P3HT):phenylC61-butyric acid methyl ester (PCBM) thin fi lms. The interfacial electric fi eld is ≈ 0.2 V nm − 1 in the as-spun fi lm and blends annealing at temperatures as high as 150 ° C, which is the optimal annealing temperature in terms of OPV performance. The fi eld decreases to a negligible value upon further annealing to 170 ° C, at which temperature PCBM changes from amorphous to crystalline and the open-circuit voltage of the solar cell decreases from 0.62 to 0.4 V. In addition, our measurements also allow determination of the absolute degree of crystallinity within the acceptor material. The roles of interfacial fi eld and local crystallinity in OPV device performance are discussed.

Lowering of the optical band gap of conjugated polymers in bulk heterojunction solar cells not only leads to an increased absorption but also to an increase of the optimal active layer thickness due to interference effects at longer wavelengths. The increased carrier densities due to the enhanced absorption and thicker active layers make low band gap solar cells more sensitive to formation of space charges and recombination. By systematically red shifting the optical parameters of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylenevinylene] and 6,6-phenyl C(61)-butyric acid methyl ester, we simulate the effect of a reduced band gap on the solar cell efficiencies. We show that especially the fill factor of low band gap cells is very sensitive to the balance of the charge transport. For a low band gap cell with an active layer thickness of 250 nm, the fill factor of 50% for balanced transport is reduced to less than 40% by an imbalance of only one order of magnitude. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 708-711, 2011

We present a combined numerical charge transport and morphology model to describe the current density–voltage ( j – V ) characteristics of three different, benchmark polymer:fullerene bulk heterojunction organic solar cells in which the device performance critically depends on the processing conditions or composition of the active layer. We fithat an accurate description of the j – V characteristics over a broad bias range can be obtained when the actual complex, three-dimensional (3D) phase separation is represented by a simplifi ed 2D or even 1D description. The morphological device model allows predicting the potential for increasing device performance by further optimizing the morphology. The optimal simplifi ed morphology consists of two, relatively thin alternating vertically oriented slabs, that allow for fast lateral separation of photocreated holes and electrons. This morphology can effectively be described as 1D.

Solution-processed bulk heterojunction organic photovoltaic (OPV) devices have gained serious attention during the last few years and are established as one of the leading next generation photovoltaic technologies for low cost power production. This article reviews the OPV development highlights of the last two decades, and summarizes the key milestones that have brought the technology to today's efficiency performance of over 7%. An outlook is presented on what will be required to drive this young photovoltaic technology towards the next major milestone, a 10% power conversion efficiency, considered by many to represent the efficiency at which OPV can be adopted in wide-spread applications. With first products already entering the market, sufficient lifetime for the intended application becomes more and more critical, and the status of OPV stability as well as the current understanding of degradation mechanisms will be reviewed in the second part of this article.

High photovoltaic device performance is demonstrated in ambient-air-processed bulk heterojunction solar cells having an active blend layer of organic poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C{sub 61}-butyric acid methyl ester (PCBM), with power conversion efficiencies as high as 4.1%, which is comparable to state-of-the-art bulk heterojunction devices fabricated in air-free environments. High-resolution transmission electron microscopy is combined with detailed analysis of electronic carrier transport in order to quantitatively understand the effects of oxygen exposure and different thermal treatments on electronic conduction through the highly nanostructured active blend network. Improvement in photovoltaic device performance by suitable post-fabrication thermal processing results from the reduced oxygen charge trap density in the active blend layer and is consistent with a corresponding slight increase in thickness of an {approx}4 nm aluminum oxide hole-blocking layer present at the electron-collecting contact interface.

The organic magnetoconductance (MC) effects in poly(3-hexylthiophene): [6,6]-phenyl-C61-butyricacid methylester based bulk heterojunction solar cells were studied in dark and under illumination. The correlations between the MC and current character were revealed in this study. Results show that the dark current always exhibits a negative MC whereas a sign change in MC under illumination occurs at the bias around the open circuit voltage Voc. We suggest that the positive MC in photocurrent is due to the field dependent conversion of singlet electron–hole pairs to triplet states and the negative MC is associated with space charge limited current with traps. Other possible mechanisms about the magnetoconductance effects are also discussed.

A polymer solar-cell based on a bulk hetereojunction design with an internal quantum efficiency of over 90% across the visible spectrum (425 nm to 575 nm) is reported. The device exhibits a power-conversion efficiency of 6% under standard air-mass 1.5 global illumination tests.

Enhancement of the light-harvesting efficiency in poly(3-hexylthiophene)/fullerene derivative (P3HT/PCBM) bulk heterojunction solar cells has been demonstrated by the introduction of near-infrared phthalocyanine molecules as the third component at the P3HT/PCBM interface. The introduction of silicon phthalocyanine derivative (SiPc) increased the short-circuit current density and hence improved the overall power conversion efficiency by 20%, compared to the P3HT/PCBM control device. For P3HT/PCBM/SiPc devices, two distinct external quantum efficiency (EQE) peaks were observed at wavelengths for the absorption bands of SiPc as well as P3HT before and after thermal annealing, suggesting that SiPc molecules are located at the P3HT/PCBM interface because of crystallization of the P3HT and PCBM domains. Furthermore, the EQE for the device increased even at wavelengths for the absorption band of P3HT by the introduction of SiPc molecules. This indicates that P3HT excitons can be dissociated into charge carriers more efficiently in the presence of SiPc molecules at the P3HT/PCBM interface by energy transfer from P3HT to SiPc molecules. These findings suggest that there are two origins for the increase in the photocurrent by the introduction of SiPc; SiPc molecules serve not only as a light-harvesting photosensitizer but also as an energy funnel for P3HT excitons at the P3HT/PCBM interface.

Photocurrent generation by charge-transfer (CT) absorption is detected in a range of conjugated polymer–[6,6]-phenyl C61 butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (EQE) spectrum by means of Fourier-transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak ground-state charge-transfer complexes in the studied polymer–fullerene blends. The effective band gap (Eg) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all devices, under various preparation conditions, the open-circuit voltage (Voc) scales linearly with Eg. The redshift of the CT band upon thermal annealing of regioregular poly(3-hexylthiophene):PCBM and thermal aging of poly(phenylenevinylene)(PPV):PCBM photovoltaic devices correlates with the observed drop in open-circuit voltage of high-temperature treated versus untreated devices. Increasing the weight fraction of PCBM also results in a redshift of Eg, proportional with the observed changes in Voc for different PPV:PCBM ratios. As Eg corresponds with the effective bandgap of the material blends, a measurement of the EQE spectrum by FTPS allows us to measure this energy directly on photovoltaic devices, and makes it a valuable technique in the study of organic bulk heterojunction solar cells.

Polymer/fullerene bulk heterojunction solar cells with poly[3-hexylthiophene] (P3HT), poly [2-methoxy-5-(20-ethylhexoxy)-1,4-phenylenevinylene] (MEH-PPV), and 6,6-phenyl C61-butyric acid methyl ester (PCBM) were produced and characterized. A device based on P3HT and PCBM provided better efficiency, fill factor, and short-circuit current compared to those of a device based on MEH-PPV and PCBM. The solar cell with P3HT and PCBM structure showed a higher photoresponse in the range of 400–650 nm. Energy levels of the molecules were calculated and discussed.

Optical interference effects are important for the total absorption as well as the profile of the exciton generation rate in polymer:fullerene bulk heterojunction solar cells. For solar cells with an active layer of poly2-methoxy-5-3,7-dimethyloctyloxy-1,4-phenylenevinylene as electron donor and 6,6-phenyl C61 butyric acid methyl ester as electron acceptor, the total exciton generation rate can be directly extracted from the saturated photocurrent. It is demonstrated that for solar cells with an active layer thickness smaller than 250 nm, a constant exciton generation profile, based on this extracted total rate, gives identical electrical characteristics as compared to exciton generation profiles from an optical model. For thicker cells interference effects have to be taken into account, since a uniform generation profile leads to an overestimation of recombination losses and space-charge formation. © 2008 American Institute of Physics. DOI: 10.1063/1.2905243

The effect of post-thermal treatment under reverse bias on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(20-ethylhexyloxy)- p -phenylene vinylene] (MEH-PPV) and fullerene (C60) composites is investigated. The experimental results show that treated devices deliver increased short-circuit current density ( J sc ) and conversion efficiency ( η p ) values. Especially, for the device annealed under the bias of −6 V, the short-circuit current density ( J sc ) and the conversion efficiency ( η p ) are enhanced by a factor of 6 and 5.4, respectively. It is concluded that the enhancement of charge mobility induced by polymer chains modified orientation along the direction of electric field leads to a significant increase in the photovoltaic parameters.

A series of alkyl 4-benzoylbutyrate p-tosylhydrazones were synthesized and reacted with C60 in the presence of sodium methoxide. Interestingly, n-butyl 4-benzoylbutyrate p-tosylhydrazone causes a high yield of the undesired compound-[6,6]-phenyl C61-butyric acid methyl ester (PCBM). Photovoltaic cells with these derivatives as electron acceptors were fabricated. The surface morphology of poly(3-hexylthiophene)(P3HT)/C60 derivatives was characterized by atomic force microscopy. The nano-scale phase separation was observed in P3HT/PCBM film through a slow-growth process. This phenomenon is indistinguishable in P3HT/PCBiB ([6,6]-phenyl C61-butyric acid iso-butyl ester) film because of the higher solubility of PCBiB in P3HT. The power conversion efficiency of the device that was made of P3HT/PCBiB blend is 2.8%, which is lower than that of P3HT/PCBM-based device (4.0%).

The insertion layer of cadmium sulfide (CdS) between polymer–fullerene blend and Al electrode is used to enhance the short-circuit current ( I sc ) and the power conversion efficiency (PCE). The solar cells based on the blend of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and C 60 with the function layer of CdS (∼10 nm) shows the open-circuit voltage ( V oc ) of ∼0.7 V, short-circuit current ( I sc ) of ∼4.6 mA/cm 2 , filling factor (FF) of ∼0.28, and the power conversion efficiency (PCE) of ∼5.3% under monochromatic light (532 nm) photoexcitation of about 16.7 mW/cm 2 . Compared to cells without the CdS layer, the power conversion efficiency increases about an order of magnitude. The thickness of CdS layer was varied from 10 to 40 nm using e-beam deposition, and we obtained optimum current density–voltage characteristics for 10 nm thick CdS layer.

Plastic solar cells bear the potential for large-scale power generation based on materials that provide the possibility of flexible, lightweight, inexpensive, efficient solar cells. Since the discovery of the photoinduced electron transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, this material combination has been extensively studied in organic solar cells, leading to several breakthroughs in efficiency, with a power conversion efficiency approaching 5 %. This article reviews the processes and limitations that govern device operation of polymer.-fullerene BHJ solar cells, with respect to the charge-carrier transport and photogeneration mechanism. The transport of electrons/holes in the blend is a crucial parameter and must be controlled (e.g., by controlling the nanoscale morphology) and enhanced in order to allow fabrication of thicker films to maximize the absorption, without significant recombination losses. Concomitantly, a balanced transport of electrons and holes in the blend is needed to suppress the build-up of the space-charge that will significantly reduce the power conversion efficiency. Dissociation of electron-hole pairs at the donor/acceptor interface is an important process that limits the charge generation efficiency under normal operation condition. Based on these findings, there is a compromise between charge generation (light absorption) and open-circuit voltage (V-oc) when attempting to reduce the bandgap of the polymer (or fullerene). Therefore, an increase in V-oc of polymer.-fullerene cells, for example by raising the lowest unoccupied molecular orbital level of the fullerene, will benefit cell performance as both fill factor and short-circuit current increase simultaneously.

High charge-separation efficiency combined with the reduced fabrication costs associated with solution processing and the potential for implementation on flexible substrates make 'plastic' solar cells a compelling option for tomorrow's photovoltaics. Attempts to control the donor/acceptor morphology in bulk heterojunction materials as required for achieving high power-conversion efficiency have, however, met with limited success. By incorporating a few volume per cent of alkanedithiols in the solution used to spin-cast films comprising a low-bandgap polymer and a fullerene derivative, the power-conversion efficiency of photovoltaic cells (air-mass 1.5 global conditions) is increased from 2.8% to 5.5% through altering the bulk heterojunction morphology. This discovery can potentially enable morphological control in bulk heterojunction materials where thermal annealing is either undesirable or ineffective.

The effect of a thermal annealing treatment on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and fullerene (C60) composites is investigated. Upon thermal annealing at 120 ∘C, short-circuit current ( I sc ) and power conversion efficiency ( η ) are more than tripled, while a sharp rise by eight times in I sc and η is found for the device annealed at 200 ∘C. It is concluded that the improved phase separation between MEH-PPV and C60 leads to the enhancement of I sc and η at 120 ∘C, while thermodynamic molecule arrangement at the higher temperature of ∼200 ∘C induces a significant increase in all photovoltaic parameters of composite devices except the open-circuit voltage ( V oc ) .

The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.

The insertion layer of TiO2 between polymer-fullerene blend and LiF/Al electrode is used to enhance the short-circuit current Isc and fill factor (FF). The solar cell based on the blend of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and C60 with the modifying layer of TiO2 (about 20 nm) shows the open-circuit Voc of about 0.62 V, short circuit current Isc of about 2.35 mA/cm2, filling factor FF of about 0.284, and the power conversion efficiency (PCE) of about 2.4% under monochromatic light (500 nm) photoexcitation of about 17 mW/cm2. Compared to cells without the TiO2 layer, the power conversion efficiency increases by about 17.5%. Similar effect is also obtained in cells with the undoped MEH-PPV structure of ITO/PEDOT:PASS/MEH-PPV/(TiO2)LiF/Al. The improved solar cell performance can be attributed to enhanced carrier extraction efficiency at the active layer/electrode interfaces when TiO2 is inserted.

Achievement of extremely high stability for inverted-type polymer:fullerene solar cells is reported, which have bulk heterojunction (BHJ) layers consisting of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by employing UV-cut filter (UCF) that is mounted on the front of glass substrates. The UCF can block most of UV photons below 403 nm at the expense of approximate to 20% reduction in the total intensity of solar light. Results show that the PTB7-Th:PC71BM solar cell with UCF exhibits extremely slow decay in power conversion efficiency (PCE) but a rapidly decayed PCE is measured for the device without UCF. The poor device stability without UCF is ascribed to the oxidative degradation of constituent materials in the BHJ layers, which give rise to the formation of PC71BM aggregates, as measured with high resolution and scanning transmission electron microscopy and X-ray photoelectron spectroscopy. The device stability cannot be improved by simply inserting poly(ethylene imine) (PEI) interfacial layer without UCF, whereas the lifetime of the PEI-inserted PTB7-Th:PC71BM solar cells is significantly enhanced when UCF is attached.

Charge carrier mobility (μ), recombination kinetics, and lifetime (τ) have been investigated with the photo-induced charge carrier extraction by linearly increasing voltage technique (photo-CELIV) in blends of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-C61(PCBM). Different MDMO-PPV/PCBM ratios have been studied showing that increasing the PCBM content induces an increase of the photo-CELIV mobility up to two orders of magnitude. Simultaneously, the lifetime of the charge carriers decreases in such a way that the product μ × τ appears almost constant independently of the blend composition. Recombination kinetics close to the Langevin one is observed for all PCBM concentrations studied.

There has been an intensive search for cost-effective photovoltaics since the development of the first solar cells in the 1950s. [1–3] Among all alternative technologies to silicon-based pn-junction solar cells, organic solar cells could lead the most significant cost reduction. [4] The field of organic photovoltaics (OPVs) comprises organic/inorganic nanostructures like dyesensitized solar cells, multilayers of small organic molecules, and phase-separated mixtures of organic materials (the bulkheterojunction solar cell). A review of several OPV technologies has been presented recently. [5] Light absorption in organic solar cells leads to the generation of excited, bound electron– hole pairs (often called excitons). To achieve substantial energy-conversion efficiencies, these excited electron–hole pairs need to be dissociated into free charge carriers with a high yield. Excitons can be dissociated at interfaces of materials with different electron affinities or by electric fields, or the dissociation can be trap or impurity assisted. Blending conjugated polymers with high-electron-affinity molecules like C60 (as in the bulk-heterojunction solar cell) has proven to be an efficient way for rapid exciton dissociation. Conjugated polymer–C60 interpenetrating networks exhibit ultrafast charge transfer (∼40 fs). [6,7] As there is no competing decay process of the optically excited electron–hole pair located on the polymer in this time regime, an optimized mixture with C60 converts absorbed photons to electrons with an efficiency close to 100%. [8] The associated bicontinuous interpenetrating network enables efficient collection of the separated charges at the electrodes. The bulk-heterojunction solar cell has attracted a lot of attention because of its potential to be a true low-cost photovoltaic technology. A simple coating or printing process would enable roll-to-roll manufacturing of flexible, low-weight PV modules, which should permit cost-efficient production and the development of products for new markets, e.g., in the field of portable electronics. One major obstacle for the commercialization of bulk-heterojunction solar cells is the relatively small device efficiencies that have been demonstrated up to now. [5] The best energy-conversion efficiencies published for small-area devices approach 5%. [9–11] A detailed analysis of state-of-the-art bulk-heterojunction solar cells [8] reveals that the efficiency is limited by the low opencircuit voltage (Voc) delivered by these devices under illumination. Typically, organic semiconductors with a bandgap of about 2 eV are applied as photoactive materials, but the observed open-circuit voltages are only in the range of 0.5–1 V. There has long been a controversy about the origin of the Voc in conjugated polymer–fullerene solar cells. Following the classical thin-film solar-cell concept, the metal–insulator–metal (MIM) model was applied to bulk-heterojunction devices. In the MIM picture, Voc is simply equal to the work-function difference of the two metal electrodes. The model had to be modified after the observation of the strong influence of the reduction potential of the fullerene on the open-circuit volt

Within the different organic photovoltaic devices the conjugated polymer/fullerene bulk heterojunction approach is one of the foci of today's research interest. These devices are highly dependent on the solid state nanoscale morphology of the two components (donor/acceptor) in the photoactive layer. The need for finely phase separated polymer–fullerene blends is expressed by the limited exciton diffusion length present in organic semiconductors. Typical distances that these photo-excitations can travel within a pristine material are around 10–20 nm. In an efficient bulk heterojunction the scale of phase separation is therefore closely related to the respective exciton diffusion lengths of the two materials involved. Once the excitons reach the donor/acceptor interface, the photoinduced charge transfer results in the charge separation. After the charges have been separated they require percolated pathways to the respective charge extracting electrodes in order to supply an external direct current. Thus also an effective charge transport relies on the development of a suitable nanomorphology i.e. bicontinuous interpenetrating phase structures within these blend films. The present feature article combines and summarizes the experimental findings on this nanomorphology–efficiency relationship.

By applying the specific fabrication conditions summarized in the Experimental section and post-production annealing at 150 °C, polymer solar cells with power-conversion efficiency approaching 5 % are demonstrated. These devices exhibit remarkable thermal stability. We attribute the improved performance to changes in the bulk heterojunction material induced by thermal annealing. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the improved contact to the electron-collecting electrode facilitate charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency by lowering the series resistance of the polymer solar cells.

The nanoscale phase separation in polymer/fullerene bulk heterojunction plastic solar cells requires the use of high resolution techniques for imaging. In this study we used a high-resolution transmission electron microscope (HR-TEM) together with selected area electron diffraction (SAED) to visualize the polymer and fullerene distributions and their amorphous or crystalline organization in the film. While pristine polymer films exhibited no crystalline order, the fullerene organized in nanocrystals. Upon annealing of the blend film, the accompanying phase separation reaches the micron level and fullerene phases aggregate to larger single crystals, as could be seen by SAED.