Discriminating between bilayer and bulk heterojunction polymer:fullerene solar cells using the external quantum efficiency

The morphology of the active layer in polymer:fullerene solar cells is a key parameter for the performance. We compare bilayer poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT/PCBM) solar cell devices produced from orthogonal solvents before and after thermal annealing with P3HT:PCBM bulk heterojunction solar cells produced from a single solvent. By comparing the spectral shape and magnitude of the experimental and theoretically modeled EQEs we show that P3HT/PCBM bilayers made via orthogonal solution processing do not lead to bilayers with a sharp interface but that partial intermixing has occurred. Thermal annealing of these diffusive P3HT/PCBM bilayers leads to increased mixing but does not result in the same mixed bulk heterojunction morphology that is obtained when P3HT and PCBM are cast simultaneously from single solution. For thicker layers, the annealed bilayers significantly outperform the bulk heterojunction devices with the same nominal composition and same total thickness.     

Modified processing conditions for optimized organic solar cells with inkjet printed P3HT:PC61BM active layers

Inkjet printing can be used to deposit the functional layers of organic solar cells and it offers advantages over spin coating such as the possibility to print films with user-defined patterns. In this study, inkjet printing was utilized to deposit polymer:fullerene solar cell active layers and different drying and annealing conditions were examined in order to optimize device performance. Low fill factors of approximately 30% were found for devices with printed active layers that were dried at 100 °C and a considerable shift in the fill factor of up to 60% was seen after post-annealing at 150 °C. Changes in the fill factor corresponded to an increase in device efficiency from ～1.3% to ～2.4% after post-annealing. An alternative active layer drying procedure was used based on solvent annealing which resulted in high fill factors of 60% and efficiencies of ～2.4% without post-annealing. Blend films were examined with atomic force microscopy, ultra-violet visible spectroscopy and X-ray photoelectron spectroscopy. It was determined that solvent annealed, inkjet printed active layers are considerably rougher and show enhanced organization with respect to films that were dried at 100 °C. Two preparation routes are provided for devices with printed active layers with acceptable efficiencies based on quick drying and post-annealing or slow drying (solvent annealing).     

Photovoltaic properties of bulk heterojunction devices based on CuI-PVA as electron donor and PCBM and modified PCBM as electron acceptor

In this paper, we have investigated the bulk heterojunction organic solar cells based on CuI ?? polyvinyl alcohol (CuI-PVA) nanocomposite as electron donor and [6,6] ?? phenyl C₆₀ ?? butyric acid methyl ester (PCBM) or modified PCBM i.e. F as electron acceptor. The power conversion efficiencies (PCEs) of 0.46 % and 0.68 % were achieved for the photovoltaic devices based on as cast CuI-PVA:PCBM and CuI-PVA:F blend films, respectively. The higher PCEs of the organic solar cells based on F as electron acceptor resulted from the increase in both short circuit current (J _sc ) and open circuit voltage (V _oc ), due to the increased absorption of F in visible region and its higher LUMO level. After thermal annealing, the PCEs of the organic solar cells were further increased to 0.54 % and 0.80 % for CuI-PVA:PCBM and CuI-PVA:F blends, respectively. The increase in the PCEs was mainly due to the increase in J _sc , which has been attributed to the improvement in hole mobility and broadening of the absorption band in the longer wavelength region. The improved hole mobility resulted in more balanced charge transport in the devices based on the thermally annealed blends.     

Solvent effect on the thin film formation of polymeric solar cells

This research work investigates the influence of the solvent used upon the formation of polymeric photoactive layer. This photoactive layer consists of a blend of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), to produce P3HT:PCBM (at 1:1 ratio) blend thin films, in which the material solutions are deposited onto clean substrates via spin-coating technique. Three types of solvents were used namely: 100% dichlorobenzene (DCB), 100% chloroform (CH) and 50%:50% mixture of DCB and chloroform. The results reveal that the mixture of solvent produces a better solubility of the polymeric materials compared to that of the pure single solvent.     

Mapping local photocurrents in polymer/fullerene solar cells with photoconductive atomic force microscopy

The performance of organic solar cells is highly dependent on film morphology. However, directly correlating local film structures with device performance remains challenging. We demonstrate that photoconductive atomic force microscopy (pcAFM) can be used to map local photocurrents with 20 nm resolution in donor/acceptor blend solar cells of the conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyl-oxy)-1,4-phenylene vinylene] (MDMO-PPV) with the fullerene (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) spin-coated from various solvents. We present photocurrent maps under short-circuit conditions (zero applied bias) as well as under various applied voltages. We find significant variation in the short-circuit current between regions that appear identical in AFM topography. These variations occur from one domain to another as well as on larger length scales incorporating multiple domains. These results suggest that the performance of polymer-fullerene blends can still be improved through better control of morphology.     

Investigation of aged organic solar cell stacks by cross-sectional transmission electron microscopy coupled with elemental analysis

Polymer solar cells are of great interest as candidates for future low-cost and lightweight energy sources. One of the major reliability problems of these devices is the thermal instability of the blend morphology typically composed of poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT and PCBM, respectively). Phase segregation of the blend has been extensively investigated by transmission electron microscopy (TEM) on free-standing films. In this study, we investigate in cross-section the morphology reorganization of P3HT:PCBM layers confined between poly(3,4-ethylenedioxythiophene)poly-(styrenesulfonate) (PEDOT:PSS) and a metal electrode similar to functional solar cell devices. The strengths of different TEM imaging and compositional analysis modes for the investigation of organic solar cells is illustrated by studying the evolution of the material stack with ageing conditions. Combining TEM imaging of the layer stack with energy-dispersive X-ray and energy loss electron spectroscopy, we not only gain insight into the phase segregation process but also explore the interdiffusion in the layer stack. More than 100 °C annealing leads to the formation of elongated protrusions ranging 100–500 nm. Thinning of the neighboring areas indicates lateral diffusion in the stack. Interestingly, the metal cathode remains still conformal over these large aggregates. The particles protrude through the metal layer only after prolonged (>100 h) annealing at higher temperatures when they reach several micrometer in height and are identified as crystalline PCBM-like material. Hence, almost full phase separation occurs by PCBM agglomeration and diffusion over large distances. Elemental analysis confirms that diffusion of the electrode materials (In, Sn and Yb) into the P3HT:PCBM stack remains below the detection limit.     

Evolution of vertical phase separation in P3HT:PCBM thin films induced by thermal annealing

The achievement of the desirable morphology at the nanometer scale of bulk heterojunctions consisting of a conjugated polymer with fullerene derivatives is a prerequisite in order to optimize the power conversion efficiency of organic solar cells. The various experimental conditions such as the choice of solvent, drying rates and annealing have been found to significantly affect the blend morphology and the final performance of the photovoltaic device. In this work, we focus on the effects of post deposition thermal annealing at 140 °C on the blend morphology, the optical and structural properties of bulk heterojunctions that consist of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM). The post thermal annealing modifies the distribution of the P3HT and the PCBM inside the blend films, as it has been found by Spectroscopic Ellipsometry studies in the visible to far-ultraviolet spectral range. Phase separation was identified by AFM and GIXRD as a result of a slow drying process which took place after the spin coating process. The increase of the annealing time resulted to a significant increase of the P3HT crystallinity at the top regions of the blend films.     

Improvements of fill factor in solar cells based on blends of polyfluorene copolymers as electron donors

The photovoltaic characteristics of solar cells based on alternating polyfluorene copolymers, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3), and poly(2,7-(9,9-didodecyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-4), blended with an electron acceptor fullerene molecule [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), have been investigated and compared. The two copolymers have the same aromatic backbone structure but differ by the length of their alkyl side chain. The overall photovoltaic performance of the solar cells is comparable irrespective of the copolymer used in the active layer. However, the fill factor (FF) values of the devices are strongly affected by the copolymer type. Higher FF values were realized in solar cells with APFO-4 (with longer alkyl side chain)/PCBM bulk heterojunction active layer. On the other hand, devices with blends of APFO-3/APFO-4/PCBM were found to render fill factor values that are intermediate between the values obtained in solar cells with APFO-3/PCBM and APFO-4/PCBM active film. Upon using APFO-3/APFO-4 blends as electron donors, the cell efficiency can be enhanced by about 16% as compared to cells with either APFO-3 or APFO-4. The transport of holes in each polymer obeys the model of hopping transport in disordered media. However, the degree of energetic barrier against hopping was found to be larger in APFO-3. The tuning of the photovoltaic parameters will be discussed based on studies of hole transport in the pure polymer films, and morphology of blend layers. The effect of bipolar transport in PCBM will also be discussed.     

Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends

Control of blend morphology at the microscopic scale is critical for optimizing the power conversion efficiency of plastic solar cells based on blends of conjugated polymer with fullerene derivatives. In the case of bulk heterojunctions of regioregular poly(3-hexylthiophene) (P3HT) and a soluble fullerene derivative ([6,6]-phenyl C61-butyric acid methyl ester, PCBM), both blend morphology and photovoltaic device performance are influenced by various treatments, including choice of solvent, rate of drying, thermal annealing and vapour annealing. Although the protocols differ significantly, the maximum power conversion efficiency values reported for the various techniques are comparable (4-5%). In this paper, we demonstrate that these techniques all lead to a common arrangement of the components, which consists of a vertically and laterally phase-separated blend of crystalline P3HT and PCBM. We propose a morphology evolution that consists of an initial crystallization of P3HT chains, followed by diffusion of PCBM molecules to nucleation sites, at which aggregates of PCBM then grow.     

Printing highly efficient organic solar cells

The technological attraction in organic solar cells is their compatibility to printing processes. However, up to today, nearly no literature on "printed" organic solar cells have been published and the major body of the research work was done by spin coating or blading techniques. Transferring the spin-coating or doctor blading process currently used for the fabrication of bulk heterojunction solar cell to a printing process holds morphological challenges that have not been observed or reported up to today. We highlight these challenges and we show that inkjet printing of organic bulk heterojunction solar cells requires completely novel approaches and skill sets compared to the current state of the art. By adjusting the chemical properties of the poly(3-hexylthiophene) polymer donor and by using our recently developed inkjet solvent mixture, we have gained control over the nanomorphology of poly(3-hexylthiophene):fullerene blends during the printing process and report a new record power conversion efficiency of 3.5% for inkjet printed poly(3-hexylthiophene):fullerene based solar cells.     

Comparative study: the effect of annealing conditions on the properties of P3HT:PCBM blends

This paper presents a detailed study on the role of various annealing treatments on organic poly(3-hexylthiophene) and [6]-phenyl-C₆₁-butyric acid methyl ester blends under different experimental conditions. A combination of analytical tools is used to study the alteration of the phase separation, structure and photovoltaic properties of the P3HT:PCBM blend during the annealing process. Results showed that the thermal annealing yields PCBM “needle-like” crystals and that prolonged heat treatment leads to extensive phase separation, as demonstrated by the growth in the size and quantity of PCBM crystals. The substrate annealing method demonstrated an optimal morphology by eradicating and suppressing the formation of fullerene clusters across the film, resulting in longer P3HT fibrils with smaller diameter. Improved optical constants, PL quenching and a decrease in the P3HT optical bad-gap were demonstrated for the substrate annealed films due to the limited diffusion of the PCBM molecules. An effective strategy for determining an optimized morphology through substrate annealing treatment is therefore revealed for improved device efficiency.     

Increasing the efficiency of polymer solar cells by silicon nanowires

Silicon nanowires have been introduced into P3HT:[60]PCBM solar cells, resulting in hybrid organic/inorganic solar cells. A cell efficiency of 4.2% has been achieved, which is a relative improvement of 10% compared to a reference cell produced without nanowires. This increase in cell performance is possibly due to an enhancement of the electron transport properties imposed by the silicon nanowires. In this paper, we present a novel approach for introducing the nanowires by mixing them into the polymer blend and subsequently coating the polymer/nanowire blend onto a substrate. This new onset may represent a viable pathway to producing nanowire-enhanced polymer solar cells in a reel to reel process.     

Morphology Control Enables Efficient Ternary Organic Solar Cells

Ternary organic solar cells are promising alternatives to the binary counterpart due to their potential in achieving high performance. Although a growing number of ternary organic solar cells are recently reported, less effort is devoted to morphology control. Here, ternary organic solar cells are fabricated using a wide‐bandgap polymer PBT1‐C as the donor, a crystalline fused‐ring electron acceptor ITIC‐2Cl, and an amorphous fullerene derivative indene‐C₆₀ bisadduct (ICBA) as the acceptor. It is found that ICBA can disturb π–π interactions of the crystalline ITIC‐2Cl molecules in ternary blends and then help to form more uniform morphology. As a result, incorporation of 20% ICBA in the PBT1‐C:ITIC‐2Cl blend enables efficient charge dissociation, negligible bimolecular recombination, and balanced charge carrier mobilities. An impressive power conversion efficiency (PCE) of 13.4%, with a high fill factor (FF) of 76.8%, is eventually achieved, which represents one of the highest PCEs reported so far for organic solar cells. The results manifest that the adoption of amorphous fullerene acceptor is an effective approach to optimizing the ternary blend morphology and thereby increases the solar cell performance.     

Influence of thermal annealing-induced molecular aggregation on film properties and photovoltaic performance of bulk heterojunction solar cells based on a squaraine dye

Squaraine (SQ) dyes have been considered as efficient photoactive materials for organic solar cells. In this work, we purposely controlled the molecular aggregation of an SQ dye, 2,4-bis[4-(N,N-dibutylamino)-2-dihydroxyphenyl] SQ (DBSQ-(OH)₂) in the DBSQ(OH)₂:[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) blend film by using the thermal annealing method, to study the influence of the molecular aggregation on film properties as well as the photovoltaic performance of DBSQ(OH)₂:PCBM-based bulk heterojunction (BHJ) solar cells. Our results demonstrate that thermal annealing may change the aggregation behavior of DBSQ(OH)₂ in the DBSQ(OH)₂:PCBM film, and thus significantly influence the surface morphology, optical and electrical properties of the blend film, as well as the photovoltaic performance of DBSQ(OH)₂:PCBM BHJ cells.     

The role of solvent and morphology on miscibility of methanofullerene and poly(3-hexylthiophene)

A bicontinuous, percolating bulk heterojunction morphology is integral to organic polymer solar cells. Understanding the factors affecting the miscibility of photovoltaic polymers with a fullerene electron acceptor molecule is a key to controlling the morphology. Starting from discreet pure phases - a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) bilayer film - the evolution of the P3HT-PCBM interface was studied with particular attention to the role of residual solvent in P3HT on PCBM interdiffusion. This investigation shows that in the bilayer geometry PCBM can rapidly diffuse into amorphous P3HT, but phase separation is maintained if the P3HT layer is cast from a very volatile solvent or if it is annealed prior to casting the PCBM overlayer to complete the bilayer geometry.     

Improvement of organic solar cell efficiency by solution-processed doping of pentacene in PEDOT:PSS layer

In the current research, organic solar cells (OSCs) with various concentrations of pentacene in Poly(ethylenedioxythiopene):Poly(styrenesulfonate) (PEDOT:PSS) interface layer were investigated for better hole extraction. The ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al-fabricated solar cell fabricated via brush coating provides superior photovoltaic, electrical and optical characteristics when compared with the ITO/PEDOT:PSS/P3HT:PCBM/Al solar cell. The ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al solar cells deliver a V_OC ~350?mV and 2.57% efficiency. It is observed that the optimized concentration of pentacene doping in PEDOT:PSS layer, along with an active layer of P3HT and PC₆₀BM, doubles the efficiency of the device, when compared with pristine PEDOT:PSS layer. The degradation studies of the fabricated bulk heterojunction OSCs reveal that the degrading abilities of ITO/Pentacene?+?PEDOT:PSS/P3HT:PCBM/Al solar cells are 60% more better than those of ITO/PEDOT:PSS/P3HT:PCBM/Al devices. Thus, this work will ultimately contribute toward fully solution processed painted device, which will provide low-cost manufacturing and improved stability of pentacene-based organic photovoltaics.     

Isomer‐Pure Bis‐PCBM‐Assisted Crystal Engineering of Perovskite Solar Cells Showing Excellent Efficiency and Stability

A fullerene derivative (α‐bis‐PCBM) is purified from an as‐produced bis‐phenyl‐C₆₁‐butyric acid methyl ester (bis‐[60]PCBM) isomer mixture by preparative peak‐recycling, high‐performance liquid chromatography, and is employed as a templating agent for solution processing of metal halide perovskite films via an antisolvent method. The resulting α‐bis‐PCBM‐containing perovskite solar cells achieve better stability, efficiency, and reproducibility when compared with analogous cells containing PCBM. α‐bis‐PCBM fills the vacancies and grain boundaries of the perovskite film, enhancing the crystallization of perovskites and addressing the issue of slow electron extraction. In addition, α‐bis‐PCBM resists the ingression of moisture and passivates voids or pinholes generated in the hole‐transporting layer. As a result, a power conversion efficiency (PCE) of 20.8% is obtained, compared with 19.9% by PCBM, and is accompanied by excellent stability under heat and simulated sunlight. The PCE of unsealed devices dropped by less than 10% in ambient air (40% RH) after 44 d at 65 °C, and by 4% after 600 h under continuous full‐sun illumination and maximum power point tracking, respectively.     

Formation of phenyl-C<Subscript>61</Subscript>-butyric acid methyl ester nanoscale aggregates after supercritical carbon dioxide annealing

In this study, we successfully developed a novel method to create [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) nanoscale aggregates using supercritical carbon dioxide (scCO₂) annealing and fabricated bulk heterojunction (BHJ) solar cells with the nanoscale PCBM to improve device performance. PCBM forms nanoscale aggregates with a size of approximately 70 nm after scCO₂ annealing at 11 MPa and 50 °C for 60 min. However, PCBM remains amorphous after thermal annealing (TA) at 150 °C for 5 min. The morphology, structure, and crystallinity of poly(3-hexylthiophene) (P3HT) in the scCO₂-treated P3HT film are nearly the same as those in the TA-treated P3HT film. In the P3HT/PCBM blend, the formation of PCBM nanoscale aggregates by scCO₂ treatment decreases the disturbance for P3HT crystallization and improves diffusion and regular packing of P3HT molecular chains. This increases the crystallinity of P3HT so that it becomes higher than that in the TA-treated blend film. The nanoscale aggregates of PCBM and the higher crystallinity of P3HT give the scCO₂-treated P3HT/PCBM BHJ solar cells a maximum power conversion efficiency (PCE) of 2.74%, which is much higher than that of the as-cast device (PCE is 1.70%) and a little higher than the highest PCE (2.64%) of thermally annealed devices. These results indicate that scCO₂ is an effective, mild, and environmental method to modulate the nanoscale aggregates of PCBM and to improve the PCE of BHJ solar cells. However, the size of the PCBM aggregates is a little larger than the most suitable size of the exciton diffusion length, leading to limited improvement of the PCE.     

Hot Hydrocarbon-Solvent Slot-Die Coating Enables High-Efficiency Organic Solar Cells with Temperature-Dependent Aggregation Behavior

Organic solar cells (OSCs) have made rapid progress in terms of their development as a sustainable energy source. However, record-breaking devices have not shown compatibility with large-scale production via solution processing in particular due to the use of halogenated environment-threatening solvents. Here, slot-die fabrication with processing involving hydrocarbon-based solvents is used to realize highly efficient and environmentally friendly OSCs. Highly compatible slot-die coating with roll-to-roll processing using halogenated (chlorobenzene (CB)) and hydrocarbon solvents (1,2,4-trimethylbenzene (TMB) and ortho-xylene (o-XY)) is used to fabricate photoactive films. Controlled solution and substrate temperatures enable similar aggregation states in the solution and similar kinetics processes during film formation. The optimized blend film nanostructures for different solvents in the highly efficient PM6:Y6 blend is adopted to show a similar morphology, which results in device efficiencies of 15.2%, 15.4%, and 15.6% for CB, TMB, and o-XY solvents. This approach is successfully extended to other donor–acceptor combinations to demonstrate the excellent universality of this method. The results combine a method to optimize the aggregation state and film formation kinetics with the fabrication of OSCs with environmentally friendly solvents by slot-die coating, which is a critical finding for the future development of OSCs in terms of their scalable production and high-performance.     

Vertical concentration gradients in bulk heterojunction solar cells induced by differential material solubility

Highly soluble fullerene derivatives (HSFD) and a low soluble polymer (LSP) were investigated as modifiers of the active layer morphology in conventional P3HT/PCBM bulk heterojunction solar cells. The observed changes in photovoltaic and electrical characteristics of the devices after addition of one or two modifiers suggest that they induced favourable vertical phase separation in the blends simply due to different solubilities of the components. In particular, HSFD is supposed to accumulate at the top of the film serving as a hole-blocking interlayer at the cathode/active layer interface. On the contrary, LSP seems to form electron-blocking buffer layer at the bottom of the device at the active layer/anode interface. Thus, the differential material solubility was suggested as a tool for adjustment of vertical morphology of organic bulk heterojunction solar cells.