Charge Transport and Photocurrent Generation in Poly(3‐hexylthiophene): Methanofullerene Bulk‐Heterojunction Solar Cells

The effect of controlled thermal annealing on charge transport and photogeneration in bulk‐heterojunction solar cells made from blend films of regioregular poly(3‐hexylthiophene) (P3HT) and methanofullerene (PCBM) has been studied. With respect to the charge transport, it is demonstrated that the electron mobility dominates the transport of the cell, varying from 10^–8 m² V^–1 s^–1 in as‐cast devices to ≈3 × 10^–7 m² V^–1 s^–1 after thermal annealing. The hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases by more than three orders of magnitude, to reach a value of up to ≈ 2 × 10^–8 m² V^–1 s^–1 after the annealing process, as a result of an improved crystallinity of the film. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red‐shift, improving the spectral overlap with solar emission, which results in an increase of more than 60 % in the rate of charge‐carrier generation. Subsequently, the experimental electron and hole mobilities are used to study the photocurrent generation in P3HT:PCBM devices as a function of annealing temperature. The results indicate that the most important factor leading to a strong enhancement of the efficiency, compared with non‐annealed devices, is the increase of the hole mobility in the P3HT phase of the blend. Furthermore, numerical simulations indicate that under short‐circuit conditions the dissociation efficiency of bound electron–hole pairs at the donor/acceptor interface is close to 90 %, which explains the large quantum efficiencies measured in P3HT:PCBM blends.     

Abrupt Morphology Change upon Thermal Annealing in Poly(3‐Hexylthiophene)/Soluble Fullerene Blend Films for Polymer Solar Cells

The in situ morphology change upon thermal annealing in bulk heterojunction blend films of regioregular poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C₆₁ (PCBM) is measured by a grazing incidence X‐ray diffraction (GIXD) method using a synchrotron radiation source. The results show that the film morphology—including the size and population of P3HT crystallites—abruptly changes at 140 °C between 5 and 30 min and is then stable up to 120 min. This trend is almost in good agreement with the performance change of polymer solar cells fabricated under the same conditions. The certain morphology change after 5 min annealing at 140 °C is assigned to the on‐going thermal transition of P3HT molecules in the presence of PCBM transition. Field‐emission scanning electron microscopy measurements show that the crack‐like surface of blend films becomes smaller after a very short annealing time, but does not change further with increasing annealing time. These findings indicate that the stability of P3HT:PCBM solar cells cannot be secured by short‐time annealing owing to the unsettled morphology, even though the resulting efficiency is high.     

A Simple and Effective Modification of PCBM for Use as an Electron Acceptor in Efficient Bulk Heterojunction Solar Cells

[6,6]‐phenyl‐C‐61‐butyric acid methyl ester (PCBM) and poly(3‐hexylthiophene) (P3HT) are the most widely used acceptor and donor materials, respectively, in polymer solar cells (PSCs). However, the low LUMO (lowest unoccupied molecular orbital) energy level of PCBM limits the open circuit voltage (V_oc) of the PSCs based on P3HT. Herein a simple, low‐cost and effective approach of modifying PCBM and improving its absorption is reported which can be extended to all fullerene derivatives with an ester structure. In particular, PCBM is hydrolyzed to carboxylic acid and then converted to the corresponding carbonyl chloride. The latter is condensed with 4‐nitro‐4’‐hydroxy‐α‐cyanostilbene to afford the modified fullerene F . It is more soluble than PCBM in common organic solvents due to the increase of the organic moiety. Both solutions and thin films of F show stronger absorption than PCBM in the range of 250–900 nm. The electrochemical properties and electronic energy levels of F and PCBM are measured by cyclic voltammetry. The LUMO energy level of F is 0.25 eV higher than that of PCBM. The PSCs based on P3HT with F as an acceptor shows a higher V_oc of 0.86 V and a short circuit current (J_sc) of 8.5 mA cm^?2, resulting in a power conversion efficiency (PCE) of 4.23%, while the PSC based on P3HT:PCBM shows a PCE of about 2.93% under the same conditions. The results indicate that the modified PCBM, i.e., F , is an excellent acceptor for PSC based on bulk heterojunction active layers. A maximum overall PCE of 5.25% is achieved with the PSC based on the P3HT: F blend deposited from a mixture of solvents (chloroform/acetone) and subsequent thermal annealing at 120 °C.     

Cover Picture: Tuning the Dimensions of C60‐Based Needlelike Crystals in Blended Thin Films (Adv. Funct. Mater. 6/2006)

A new ordered structure of the C₆₀ derivative PCBM is obtained in thin films based on the blend PCBM:P3HT, as detailed by Swinnen, Manca, and co‐workers on p. 760. Needlelike crystalline PCBM structures, whose dimensions and spatial distribution ca be tuned by adjusting the blend ratio and annealing conditions, are formed. In typical solar‐cell applications of these blended films, these results indicate that during long‐term operation under normal conditions (50–70 °C) morphology changes and a decrease in cell performance could occur. A new ordered structure of the C₆₀ derivative PCBM ([6‐6]‐phenyl C₆₁‐butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3‐hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 μm in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as‐obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix.     

Time‐Dependent Morphology Evolution by Annealing Processes on Polymer:Fullerene Blend Solar Cells

Changes in the nanoscale morphologies of the blend films of poly (3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), for high‐performance bulk‐heterojunction (BHJ) solar cells, are compared and investigated for two annealing treatments with different morphology evolution time scales, having special consideration for the diffusion and aggregation of PCBM molecules. An annealing condition with relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization results in poor BHJ morphology because of prevention of the formation of the more elongated P3HT crystals. However, an annealing condition, accelerating PCBM diffusion after the formation of a well‐ordered morphology, results in a relatively stable morphology with less destruction of crystalline P3HT. Based on these results, an effective strategy for determining an optimized annealing treatment is suggested that considers the effect of relative kinetics on the crystallization of the components for a blend film with a new BHJ materials pair, upon which BHJ solar cells are based.     

Effects of Thermal Annealing Upon the Morphology of Polymer–Fullerene Blends

Grazing incidence X‐ray scattering (GIXS) is used to characterize the morphology of poly(3‐hexylthiophene) (P3HT)–phenyl‐C61‐butyric acid methyl ester (PCBM) thin film bulk heterojunction (BHJ) blends as a function of thermal annealing temperature, from room temperature to 220 °C. A custom‐built heating chamber for in situ GIXS studies allows for the morphological characterization of thin films at elevated temperatures. Films annealed with a thermal gradient allow for the rapid investigation of the morphology over a range of temperatures that corroborate the results of the in situ experiments. Using these techniques the following are observed: the melting points of each component; an increase in the P3HT coherence length with annealing below the P3HT melting temperature; the formation of well‐oriented P3HT crystallites with the (100) plane parallel to the substrate, when cooled from the melt; and the cold crystallization of PCBM associated with the PCBM glass transition temperature. The incorporation of these materials into BHJ blends affects the nature of these transitions as a function of blend ratio. These results provide a deeper understanding of the physics of how thermal annealing affects the morphology of polymer–fullerene BHJ blends and provides tools to manipulate the blend morphology in order to develop high‐performance organic solar cell devices.     

The Origin of the High Voltage in DPM12/P3HT Organic Solar Cells

Organic solar cells made using a blend of DPM12 and P3HT are studied. The results show that higher V_oc can be obtained when using DPM12 in comparison to the usual mono‐substituted PCBM electron acceptor. Moreover, better device performances are also registered when the cells are irradiated with sun‐simulated light of 10–50 mW cm^?2 intensity. Electrochemical and time‐resolved spectroscopic measurements are compared for both devices and a 100‐mV shift in the density of states (DOS) is observed for DPM12/P3HT devices with respect to PCBM/P3HT solar cells and slow polaron‐recombination dynamics are found for the DPM12/P3HT devices. These observations can be directly correlated with the observed increase in V_oc, which is in contrast with previous results that correlated the higher V_oc with different ideality factors obtained using dark‐diode measurements. The origin for the shift in the DOS can be correlated to the crystallinity of the blend that is influenced by the properties of the included fullerene.     

A new model for PCBM phase segregation in P3HT:PCBM blends

The phase segregation in P3HT:PCBM blend films has been investigated from an experimental and theoretical viewpoint. Optical microscopy, atomic force microscopy, scanning electron microscopy and X-ray diffraction show that thermal annealing of P3HT:PCBM blend films leads to the formation of PCBM aggregates. These aggregates are composed of dense randomly packed ∼50 nm PCBM crystallites with an overall aggregate density of ∼0.85 g cm⁻³. By applying the critical radius of nucleation for PCBM and the Stokes-Einstein equation for mobility of PCBM in a P3HT matrix, a model is developed which explains the formation of both crystallites and aggregates.     

Evolution of Structure,Optoelectronic Properties,and Device Performance of Polythiophene:Fullerene Solar Cells During Thermal Annealing

We use spectroscopic ellipsometry to study the evolution of structure and optoelectronic properties of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) photovoltaic thin film blends upon thermal annealing. Four distinct processes are identified: the evaporation of residual solvent above the glass transition temperature of the blend, the relaxation of non‐equilibrium molecular conformation formed through spin‐casting, the crystallization of both P3HT and PCBM components, and the phase separation of the P3HT and PCBM domains. Devices annealed at 150 °C for between 10 and 60 min exhibit an average power conversion efficiency of around 4.0%. We find that the rate at which the P3HT/PCBM is returned to room temperature is more important in determining device efficiency than the duration of the isothermal annealing process. We conclude that the rapid quenching of a film from the annealing temperature to room temperature hampers the crystallization of the P3HT and can trap non‐equilibrium morphological states. Such states apparently impact on device short circuit current, fill factor and, thus, operational efficiency.     

The Impact of Polymer Regioregularity on Charge Transport and Efficiency of P3HT:PCBM Photovoltaic Devices

The charge transport in pristine poly(3‐hexylthiophene) (P3HT) films and in photovoltaic blends of P3HT with [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) is investigated to study the influence of charge‐carrier transport on photovoltaic efficiency. The field‐ and temperature dependence of the charge‐carrier mobility in P3HT of three different regioregularities, namely, regiorandom, regioregular with medium regioregularity, and regioregular with very high regioregularity are investigated by the time‐of‐flight technique. While medium and very high regioregularity polymers show the typical absorption features of ordered lamellar structures of P3HT in the solid state even without previous annealing, films of regiorandom P3HT are very disordered as indicated by their broad and featureless absorption. This structural difference in the solid state coincides with partially non‐dispersive transport and hole mobilities µ_h of around 10^?4 and 10^?5 cm² V^?1 s^?1 for the high and medium regioregularity P3HT, respectively, and a slow and dispersive charge transport for the regiorandom P3HT. Upon blending the regioregular polymers with PCBM, the hole mobilities are typically reduced by one order of magnitude, but they do not significantly change upon additional post‐spincasting annealing. Only in the case of P3HT with high regioregularity are the electron mobilities similar to the hole mobilities and the charge transport is, thus, balanced. Nonetheless, devices prepared from both materials exhibit similar power conversion efficiencies of 2.5%, indicating that very high regioregularity may not substantially improve order and charge‐carrier transport in P3HT:PCBM and does not lead to significant improvements in the power‐conversion efficiency of photovoltaic devices.     

Stabilization of poly(3-hexylthiophene)/PCBM morphology by hydroxyl group end-functionalized P3HT and its application to polymer solar cells

A new concept to stabilize the morphology of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) blend through H-bond formation by using a hydroxyl group end-functionalized P3HT (HOC-P3HT-COH) as a compatibilizer is presented. Domain size of the PCBM crystals in the annealed P3HT/PCBM film is diminished with addition of HOC-P3HT-COH. Surface roughness of the P3HT/PCBM film also becomes smoother with addition of HOC-P3HT-COH. Thermal stability of solar cell device is improved significantly through the H-bond formation between HOC-P3HT-COH and PCBM. A high performance and thermal stable polymer solar cell with 4.06% power conversion efficiency under AM1.5G irradiation is fabricated with 5% HOC-P3HT-COH in P3HT/PCBM layer.     

The Effect of Poly(3‐hexylthiophene) Molecular Weight on Charge Transport and the Performance of Polymer:Fullerene Solar Cells

The time‐of‐flight method has been used to study the effect of P3HT molecular weight (M_n = 13–121 kDa) on charge mobility in pristine and PCBM blend films using highly regioregular P3HT. Hole mobility was observed to remain constant at 10^?4 cm²V^?1s^?1 as molecular weight was increased from 13–18 kDa, but then decreased by one order of magnitude as molecular weight was further increased from 34–121 kDa. The decrease in charge mobility observed in blend films is accompanied by a change in surface morphology, and leads to a decrease in the performance of photovoltaic devices made from these blend films.     

Morphological Stabilization by In Situ Polymerization of Fullerene Derivatives Leading to Efficient,Thermally Stable Organic Photovoltaics

The successful design and synthesis of two styryl‐functionalized fullerene derivatives, [6,6]‐phenyl‐C₆₁‐butyric acid styryl dendron ester (PCBSD) and [6,6]‐phenyl‐C₆₁‐butyric acid styryl ester (PCBS) is presented. The polymerizable PCBS or PCBSD materials are incorporated into a poly(3‐hexylthiophene) (P3HT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) blend to form an active layer of ternary blend. The blending systems are first thermally annealed at 110 C for 10 min to induce optimal morphology, followed by heating at 150 C for 10 min to trigger the in situ polymerization of styrene groups. Through chemical crosslinking of PCBSD, the initial morphology of the blend (P3HT:PCBM:PCBSD = 6:5:1 in weight) can be effectively fixed and stably preserved. The device based on this blend shows extremely stable device characteristics, delivering an average power conversion efficiency (PCE) of 3.7% during long‐term thermal treatment. By molecular engineering to reduce the insulating portion, PCBS with higher C₆₀ content (71 wt%) possesses better electron‐transport properties than PCBSD (58 wt%). Encouragingly, at a low doping concentration of PCBS in the blend (P3HT:PCBM:PCBS = 6:5:1 in weight), linear‐polymerized PCBS can stabilize the morphology against thermal heating. This device exhibits more balanced charge mobility to achieve an average PCE of 3.8% over 25 h heating at 150 °C.     

Tuning the Dimensions of C60‐Based Needlelike Crystals in Blended Thin Films

A new ordered structure of the C₆₀ derivative PCBM ([6‐6]‐phenyl C₆₁‐butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3‐hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 μm in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as‐obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix.     

High‐Resolution Spectroscopic Mapping of the Chemical Contrast from Nanometer Domains in P3HT:PCBM Organic Blend Films for Solar‐Cell Applications

A high‐resolution near‐field spectroscopic mapping technique is successfully applied to investigate the influence of thermal annealing on the morphology of a poly(3‐hexylthiophene) and [6,6]‐penyl‐C₆₁ butyric acid methyl ester (P3HT:PCBM) blend film. Based on the simultaneously recorded morphological and spectroscopic information, the interplay among the blend film morphology, the local P3HT:PCBM molecular distribution, and the P3HT photoluminescence (PL) quenching efficiency are systematically discussed. The PL and Raman signals of the electron donor (P3HT) and acceptor (PCBM) are probed at an optical resolution of approximately 10 nm, which allows the chemical nature of the different domains to be identified directly. In addition, the local PL quenching efficiency, which is related to the electron transfer from P3HT to PCBM, is quantitatively revealed. From these experimental results, it is proposed that high‐resolution near‐field spectroscopic imaging is capable of mapping the local chemical composition and photophysics of the P3HT:PCBM blends on a scale of a few nanometers.     

Quantitative Analysis of Bulk Heterojunction Films Using Linear Absorption Spectroscopy and Solar Cell Performance

A fundamental understanding of the relationship between the bulk morphology and device performance is required for the further development of bulk heterojunction organic solar cells. Here, non‐optimized (chloroform cast) and nearly optimized (solvent‐annealed o‐dichlorobenzene cast) P3HT:PCBM blend films treated over a range of annealing temperatures are studied via optical and photovoltaic device measurements. Parameters related to the P3HT aggregate morphology in the blend are obtained through a recently established analytical model developed by F. C. Spano for the absorption of weakly interacting H‐aggregates. Thermally induced changes are related to the glass transition range of the blend. In the chloroform prepared devices, the improvement in device efficiency upon annealing within the glass transition range can be attributed to the growth of P3HT aggregates, an overall increase in the percentage of chain crystallinity, and a concurrent increase in the hole mobilities. Films treated above the glass transition range show an increase in efficiency and fill factor not only associated with the change in chain crystallinity, but also with a decrease in the energetic disorder. On the other hand, the properties of the P3HT phase in the solvent‐annealed o‐dichlorobenzene cast blends are almost indistinguishable from those of the corresponding pristine P3HT layer and are only weakly affected by thermal annealing. Apparently, slow drying of the blend allows the P3HT chains to crystallize into large domains with low degrees of intra‐ and interchain disorder. This morphology appears to be most favorable for the efficient generation and extraction of charges.     

Precise Structural Development and its Correlation to Function in Conjugated Polymer: Fullerene Thin Films by Controlled Solvent Annealing

The impact of controlled solvent vapor exposure on the morphology, structural evolution, and function of solvent‐processed poly(3‐hexylthiophene):[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) bilayers is presented. Grazing incident wide angle X‐ray scattering (GIWAXS) shows that the crystallization of P3HT increases with solvent exposure, while neutron reflectivity shows that P3HT simultaneously diffuses into PCBM, indicating that an initial bilayer structure evolves into a bulk heterojunction structure. Small angle neutron scattering (SANS) shows the agglomeration of PCBM and the formation of a PCBM pure phase when solvent annealing for 90 min. The structural evolution can be described as occurring in two stages: the first stage combines the enhanced crystallization of P3HT and diffusion of PCBM into P3HT, while the second stage entails the agglomeration of PCBM and formation of a PCBM pure phase. The phase separation of PCBM from P3HT is not driven by P3HT crystallinity, but is due to the concentration of PCBM exceeding the miscibility limit of PCBM in P3HT. Correlation of the morphology to photovoltaic activity shows that device performance significantly improves with solvent annealing for 90 min, indicating that both sufficient P3HT crystallization and formation of a PCBM pure phase are crucial in the optimization of the morphology of the active layer.     

Annealing effects on the electrical properties of spin coated poly (3-hexylthiophene) (P3HT) thin films

Poly (3-hexylthiophene-2, 5-diyl) (P3HT) thin films were prepared using spin coating technique and the effect of annealing on the bias switching for memory applications were studied. Due to annealing, the threshold voltage for switching was reduced considerably. In bias switching, threshold voltage was least for the sample annealed at 100 °C. Addition of phenyl-C₆₁-butyric acid-methyl-ester (PCBM) into P3HT also reduced the threshold voltage. It was also found that the devices with gold (Au) top electrode switched at a lower threshold voltage compared to their aluminium (Al) counterparts.     

High resolution electrical characterisation of organic photovoltaic blends

In this work, electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM) are applied to perform high-resolution electrical characterisation of organic photovoltaic films. These films are composed of the C₆₀-derivative PCBM blended with hole conductive conjugated polymers PPV derivatives or P3HT. It is demonstrated that both EFM and C-AFM are able to electrically evidence phase separation in the blends, suggesting in addition higher density of carriers along interfaces. Correlation between the EFM contrast and the photovoltaic properties of the blends was observed. Local spectroscopy (I-V curves) completes the C-AFM investigations, analysing charge transport mechanisms in the P3HT:PCBM blend. Significant modifications of the local electrical properties of P3HT are shown to occur upon blending. Space charge limited current is evidenced in the blend and a hole mobility of 1.7 × 10⁻² cm² V⁻¹ s⁻¹ is determined for P3HT.     

Effects of PCBM concentration on the electrical properties of the Au/P3HT:PCBM/n-Si (MPS) Schottky barrier diodes

In this study, the gold/poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester/n-type silicon (Au/P3HT:PCBM/n-Si) metal–polymer–semiconductor (MPS) Schottky barrier diodes (SBDs) were investigated in terms of the effects of PCBM concentration on the electrical parameters. The forward and reverse bias current–voltage (I–V) characteristics of the Au/P3HT:PCBM/n-Si MPS SBDs fabricated by using the different P3HT:PCBM mass ratios were studied in the dark, at room temperature. The main electrical parameters, such as ideality factor (n), barrier height (Φ_B0), series resistance (R_s), shunt resistance (R_sh), and density of interface states (N_ss) were determined from I–V characteristics for the different P3HT:PCBM mass ratios (2:1, 6:1 and 10:1) used diodes. The values of n, R_s, Φ_B0, and N_ss were reduced, while the carrier mobility and current were increased, by increasing the PCBM concentration in the P3HT:PCBM organic blend layer. The ideal values of electrical parameters were obtained for 2:1 P3HT:PCBM mass ratio used diode. This shows that the electrical properties of MPS diodes strongly depend on the PCBM concentration of the P3HT:PCBM organic layer. Moreover, increasing the PCBM concentration in P3HT:PCBM organic blend layer improves the quality of the Au/P3HT:PCBM/n-Si (MPS) SBDs which enables the fabrication of high-quality electronic and optoelectronic devices.