PCBM Molecules Research Articles

Optimizing Spin-Coat Speed for Fabrication of P3HT: PCBM Solar Cells

The effect of spin speed on the orientation of the conjugated polymer chains becomes a matter of concern while fabricating bulk heterojunction devices such as solar cells. The stability and performance strongly depend on the intertwining of the two molecules that form the bulk heterojunctions. In the case of P3HT: PCBM, it is the long chains of P3HT that encompasses the PCBM molecules. This work investigates the fabricating conditions of Poly (3-hexylthiophene) (P3HT) on the glass substrates in terms of spin coating speeds to obtain favourable geometry of P3HT. For this purpose, three different samples were prepared, keeping spin rate as 6850, 8150 and 9700 rpm, respectively. It was observed that at high spin rates, a large centrifugal force acts upon polymer chains, unfolding them into highly oriented structures. These P3HT chains are free from twists, kinks and are not intermingled. The results of the present work advocate fabricating P3HT: PCBM films at spin speeds less than 6850 rpm.

Assembly of galvinoxyl doped in polymer-fullerene photovoltaic blends.

Galvinoxyl (Gx) is a stable free radical used as a dopant in active layers of organic solar cells. Here, the nanoscale arrangement of Gx molecules in a composite of the PCDTBT polymer and modified C60 fullerene, PCBM, was studied using a two-pulse electron spin echo (ESE) technique. The results show that the Gx molecules assemble into clusters, which can be described by the model of 8 molecules on the surface of a sphere with a radius of 2.0 nm. Such a structure can arise due to the octahedral packing of 6 PCBM molecules surrounded by 8 Gx molecules. ESE decays also indicate that these clusters repel each other, forming a quasi-regular nanostructure in the matrix. The Gx concentration of 2 wt% at which clusters appear correlates with the literature data on the Gx-induced enhancement of photocurrent, which provides structural insight into the possible molecular mechanism of this enhancement.

Improving morphology of P3HT:PCBM bulk heterojunction solar cells with anisotropic shaped silica nanoparticles

Using coarse-grained molecular dynamics simulations we study blends of Poly(3-hexylthiophene-2,5-diyl) (P3HT), [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) and Silica nanoparticle (SiNP) to understand the effect of adding SiNP on morphology of P3HT:PCBM in Bulk heterojunction (BHJ) solar cells. We use an approximately 3 nm anisotropic shaped SiNP and predicted the morphology of BHJ upon its incorporation. The SiNP arrange themselves into anisotropic structures depending on the concentration of P3HT, PCBM and SiNP respectively creating a network like morphology. PCBM molecules utilize the surface energy of SiNP and gather at its surface forming a morphology which is beneficial for device efficiency. Our results suggest that an optimum weight fraction of all the three components leads to higher surface area of contact, optimum domain size and high percolation of domains throughout the system. The effective control of all the morphological parameters help in improving the charge generation, extraction and transport to electrodes, thereby improving the performance of BHJ solar cells.

Fullerene derivatives—Promising blue light absorbers suppressing visual hazards for efficient indoor light harvesters

With the rise of Internet of Things, indoor organic photovoltaic (IOPV) devices as the promising power supply unit of off-grid electronics have made great progress in recent years. However, as the only acceptable end-goal, their commercial large-scale production is still a long way off although the absolute efficiency values achieve significant breakthroughs and have exceeded 30%. One of the main obstacles to practical application is the impact of IOPV devices on human vision under indoor conditions. Because the IOPV devices usually require expansive areas deployed for larger output power under indoor cases, a semitransparent strategy is commonly adopted to reduce the effects on indoor light environments. Conventional semitransparent bulk heterojunctions (BHJs) with wide-bandgap donors and near-infrared absorbing acceptors generate a huge proportion of residual blue light, which may cause human visual fatigue and injury. Herein, we propose a fullerene derivative blending strategy to suppress the blue light hazards caused by the state-of-the-art IOPV devices and improve the overall device performance. A series of high-efficient non-fullerene BHJs were selected as model systems, and a low dosage of PC71BM molecules can effectively reduce the blue light hazard efficacy (KB,v). Furthermore, with the desirable electron transport property and strong absorbance of PC71BM in the wavelength range of 400–500 nm, the ternary PM6:Y6:PC71BM devices enjoy remarkably improved power conversion efficiencies. This work provides an effective approach to simultaneously suppress KB,v values and improve performance of IOPV devices.

Freezing of Rotation of P3HT Crystallites by PCBM Molecules during Thermal Relaxation of the Blend Thin Films

AbstractPoly(3‐hexylthiophene), or P3HT, is a semicrystalline polymer with charge conduction anisotropy with respect to its different axes, which calls for gaining better control of texture for targeted device applications. A blend system of P3HT and [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) is studied for understanding the role of PCBM in texture control of P3HT crystallites. 2D grazing incidence diffraction (2D‐GIXD) combined with high resolution grazing incidence X‐ray diffraction (1D‐GIXD) is performed on thin films of pristine P3HT and blend (P3HT/PCBM) thin films prepared by spin coating. Thermal annealing at 110 °C is used for relaxing the films. The lamellar P3HT crystallites preferably orient along the surface plane with lamellar stacking in a vertical direction and have a distribution of orientation with a limiting angle of 44°. In the pristine P3HT system, the rotations of crystallites are confirmed by the retreat of the limiting angle. The observed intensity cross‐over for precursor and annealed samples is again the signature of the rotations of crystallites. For the blend system, the intensity cross‐over is absent and there is no sign of retreat of the limiting angle, which explicitly confirms the blocking of rotation lamellar crystallites. This study provides, for the first time, the clear evidence with unambiguous analysis of blocking of rotation of crystallites due to presence of PCBM, which is explained with an analogy of the “ball‐and‐stick” mechanism. Atomic force microscopy (AFM) studies of surface morphology correlates with the above findings.

Self-stability of un-encapsulated polycrystalline MAPbI3 solar cells via the formation of chemical bonds between C60 molecules and MA cations

The day-dependent photovoltaic performance of P3CT-Na based CH 3 NH 3 PbI 3 (MAPbI 3 ) solar cells can be improved by adding small molecules (urea, PCBM and C 60 ) into the MAPbI 3 crystal thin films to passivate the defects at the grain boundaries. The experimental results show that the passivation mechanism of the different additives does not only depend on the type of additives but also be related to the chemical interaction between additives and MAPbI 3 crystals. Atomic-force microscopic images, day-dependent photoluminescence (PL) spectra, and temperature-dependent PL shows that the chemical bonds between C 60 molecules and MA cations can be formed and thereby stabilizing the MAPbI 3 crystal thin films. In other words, the stable and high-efficiency P3CT-Na based MAPbI 3 solar cells can be realized when the surface defects of MAPbI 3 crystal grains are passivated with the MA-C 60 -MA cations. Our results provide a way to increase the device stability of the solution-processed polycrystalline MAPbI 3 thin film based solar cells. • MA-C 60 -MA cations are formed, which resulted in stable perovksite solar cells. • Migrations of C 60 and PCBM molecules were observed by using PL spectra. • The highest PCE increases from 16.28% to 19.27% on the 2nd day. • The PCE of un-encapsulated solar cells increases from 15.49% to 15.86% within 21 days.

Improving inter-phase charge transfer via defect passivation for efficient Quasi-2D (BA)2(FA)8Pb9I28 perovskite solar cells

There has been extensive investigation into phase orientation and phase distribution for improving charge transfer in 2D perovskites-based PSCs, but very few reports have focused on the enhancement of inter-phase charge transfer by passivation. In this work, a synergetic strategy is developed by introducing hydrochloric acid and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as additives to modulate phase distribution and passivate defects in (BA)2(FA)n−1PbnI3n+1 (n = 9) quasi-2D perovskite films. In this process, perovskite films with suppressed phase segregation and PCBM molecules coated along the grain boundaries are obtained. PCBM is found to reduce the trap density by compensating defects, which accelerate inter-phase charge transfer and restrain nonradiative recombination, resulting in improved performance of quasi-2D PSCs. As a consequence, a high PCE of 15.01% with negligible hysteresis for quasi-2D PSCs is achieved.

How the Size and Density of Charge-Transfer Excitons Depend on Heterojunction’s Architecture

We have characterized the size, intensity, density, and distribution of charge-transfer (CT) excitons as a function of the acceptor–donor architecture of prototypical organic interfaces. This characterization was done by computational analysis of 17 models of varying numbers, positions, and orientations of the donor and acceptor molecules. The models’ building blocks were phenyl-C61-butyric acid methyl ester (PCBM) fullerene acceptors and dual-band donor polymers composed of thiophene, benzothiadiazole, and benzotriazole subunits. The electronic structure of the donor–acceptor complexes was computed with the time-dependent long-range-corrected density-functional tight-binding method and analyzed with the fragment-based one-electron transition density matrix. In all models, the complexes with edge-on orientation have denser spectra of low-energy CT states lying below the absorption bands compared to the complexes with face-on orientation. This CT-state distribution in edge-on complexes provides a gate to efficiently populate cold CT excitons. Moreover, the cold CT excitons have a higher degree of charge separation in the edge-on than in the face-on complexes. The CT amount and the CT exciton size generally increase with the energy of the CT states, although the electron remains localized on a single molecule in cold CT states. Delocalization over two PCBM molecules was observed for high-energy CT states. The exciton size also depends on the orientation. Larger excitons are produced by the delocalization of the electrons perpendicularly to the interface. When the delocalization is parallel, the smaller electron–hole distances yield moderately sized CT excitons.

Impact of the Electron Acceptor Nature on the Durability and Nanomorphological Stability of Bulk Heterojunction Active Layers for Organic Solar Cells.

A systematic study is conducted to compare the performances and stability of active layers employing a high performance electron donor (PBDB-T) combined with state-of-the-art fullerene (PC71 BM), nonfullerene (ITIC), and polymer (N2200) electron acceptors. The impact of the chemical nature of the acceptor on the durability of organic solar cells (OSCs) is elucidated by monitoring their photovoltaic performances under light exposure or dark conditions in the presence of oxygen. PC71 BM molecules exhibit a higher resistance toward oxidation compared to nonfullerene acceptors. Unencapsulated PBDB-T:PC71 BM OSCs display relatively stable performances at room temperature when stored in air for 3 months. However, when exposed to temperatures above 80°C, their active materials demix causing notable reductions in the short-circuit densities. Such detrimental demixing can also be seen for PBDB-T:ITIC active layers above 120°C. Although N2200 chains irreversibly degrade when exposed to air, thermally induced demixing does not occur in PBDB-T:N2200 active layers annealed up to 200°C. In summary, fullerene OSCs may be the best currently available choice for unencapsulated room temperature applications but if oxidation of the polymer acceptors can be avoided, all polymer active layers should enable the fabrication of highly durable OSCs with lifetimes matching the requirements for OSC commercialization.

The diverse passivation effects of fullerene derivative on hysteresis behavior for normal and inverted perovskite solar cells

[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is usually used to passivate the trap states and ion defects for suppressing hysteresis in both normal and inverted perovskite solar cells. Here, we find that the normal cell with the perovskite film fabricated on the PCBM film shows an evident hysteresis, while the inverted cell with the perovskite film covered by the PCBM film shows nearly no hysteresis, and the PCBM-Perovskite interface impacts on the hysteresis vastly. The poor PCBM surface wettability results in an inefficient carrier extraction at the interface and a red-shift of photoluminescence emission for the perovskite film coated on the PCBM film, which results from the plentiful trap states and ion defects inside the perovskite film due to its poor crystallization. Thus, the trapping/de-trapping processes and slow ion migration eventually lead to the hysteresis. For the perovskite film covered by the PCBM film, due to the good wettability of the perovskite film, PCBM molecules easily permeate into the perovskite film to passivate the trap states and ion defects, which improves the carrier extraction and thus eliminates the hysteresis of the inverted cell.

Role of PCBM in the Suppression of Hysteresis in Perovskite Solar Cells

AbstractThe power conversion efficiency of inorganic–organic hybrid lead halide perovskite solar cells (PSCs) is approaching that of those made from single crystalline silicon; however, they still experience problems such as hysteresis and photo/electrical‐field‐induced degradation. Evidences consistently show that ionic migration is critical for these detrimental behaviors, but direct in‐situ studies are still lacking to elucidate the respective kinetics. Three different PSCs incorporating phenyl‐C61‐butyric acid methyl ester (PCBM) and a polymerized form (PPCBM) is fabricated to clarify the function of fullerenes towards ionic migration in perovskites: 1) single perovskite layer, 2) perovskite/PCBM bilayer, 3) perovskite/PPCBM bilayer, where the fullerene molecules are covalently linked to a polymer backbone impeding fullerene inter‐diffusion. By employing wide‐field photoluminescence imaging microscopy, the migration of iodine ions/vacancies under an external electrical field is studied. The polymerized PPCBM layer barely suppresses ionic migration, whereas PCBM readily does. Temperature‐dependent chronoamperometric measurements demonstrate the reduction of activation energy with the aid of PCBM and X‐ray photoemission spectroscopy (XPS) measurements show that PCBM molecules are viable to diffuse into the perovskite layer and passivate iodine related defects. This passivation significantly reduces iodine ions/vacancies, leading to a reduction of built‐in field modulation and interfacial barriers.

Exclusion of PCBM molecules from C60 domain on Au (111)

To investigate the influence of asymmetric intermolecular interactions of phenyl-C61-butyric acid methyl ester (PCBM), the behaviors of C60 and PCBM molecules deposited on the same Au (111) surface were observed by scanning tunneling microscopy under ultra-high vacuum conditions. After annealing, the system formed polygonal islands, the peripheries of which were occupied by PCBM molecules having their functional groups directed outside of the island, whereas the inside was composed of hexagonally packed C60 molecules. Such exclusion of PCBM from the C60 domain is attributed to asymmetric intermolecular interactions of PCBM: namely, the cage of PCBM has an affinity for C60 and the cages of other PCBM owing to van der Waals interactions. However, the functional groups of PCBM prevented C60 from attaching. The PCBM molecules at the periphery pinned the island’s edges along the stripes of a herringbone reconstruction region owing to the weak affinity of the tail for the fcc region to determine the shape of the islands.

Hybrid PCDTBT:PCBM:Graphene-Nanoplatelet Photoabsorbers

We study the photoactive properties of poly [N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT): [6,6]-Phenyl-C71-butyric acid methyl ester (PCBM) thin films under ultraviolet irradiation as well as their photo-oxidation, thermal and electrical properties. We compare their stability and performance with composites obtained by integrating graphene nanoplatelets (GNPs). Compared to the pristine polymer, an increase in light absorption and photoluminescence quenching were observed in PCDTBT:PCBM:GNPs. This indicates the occurrence of electron transfer from the conjugated polymer to the empty state of the GNP, through the interfacial bonding based on CH-π and ππ interactions, thereby reducing the photodegradation of the active layer. This is manifested by a significant decline in photo-oxidation, followed by improved thermal stability and suppressed phase separation and aggregation of the PCBM molecules. Atomic force microscopy imaging of the PCDTBT:PCBM:GNPs nanocomposite revealed that graphene flakes enhance the order of the polymeric structures. Finally, we discuss the effect of the GNPs content on the electrical conductivity and electron mobility of the photoactive layer. Our findings offer significant insights into the PV properties and photo physics of hybrid organic bulk-heterojunction solar cells, paving the way towards enhancing their durability and long term performance.

Bottlebrush compatibilizers with polythiophene backbones and dielectric/conductive side brushes in naphthothiadiazole solar systems

Poly(3-thiophene ethanol) (P3ThEt)-g-polystyrene (PSt) and P3ThEt-g-polyaniline (PANI) bottlebrushes were used as morphology compatibilizers in poly[benzodithiophene-bis(decyltetradecyl-thien) naphthothiadiazole] (PBDT-DTNT):phenyl-C61-butyric acid methyl ester (PC61BM) and poly[bis(triiso-propylsilylethynyl) benzodithiophene-bis(decyltetradecyl-thien) naphthobisthiadiazole] (PBDT-TIPS-DTNT-DT):PC61BM photovoltaics. Although 10 wt% of P3ThEt-g-PSt and P3ThEt-g-PANI bottlebrushes improved the cell features (4.39–4.42% versus 3.24%), a 20 wt% was found to be the optimal compatibilizer content and performances of 5.34 and 5.26% were detected, respectively. Polythiophenic backbones covered with either coily PSt brushes or rod PANI brushes enabled the simultaneous growth of donor and acceptor assemblies in these systems. Therefore, the hole (4.6 × 10−3 and 4.9 × 10−3 cm2/V·s) and electron (5.1 × 10−2 and 5.5 × 10−2 cm2/V·s) mobilities and also other photovoltaic properties (12.68 and 12.71 mA/cm2; 61 and 60%; 0.69 and 0.69 V) were improved. In contrast, PBDT-TIPS-DTNT-DT based devices did not benefit from bottlebrush compatibilizers as much as PBDT-DTNT based ones and performance peaked at 3.81% for PBDT-TIPS-DTNT-DT:PC61BM:10 wt% P3ThEt-g-PSt system. Herein, addition of rod-coil and rod-rod bottlebrushes further affected PC61BM clustering than polymer crystallization with complicated structures and TIPS side groups. Only 10 wt% of bottlebrushes, especially rod-coil P3ThEt-g-PSt ones, was enough in regulating the PCBM molecules and enhancing cell parameters. Thereafter, further raising the compatibilizer population saturated the active layer and, consequently, device functions especially hole mobility plummeted. The smallest charge transfer resistances (Rtr) also belonged to PBDT-DTNT:PC61BM:20 wt% P3ThEt-g-PSt (=273 Ω·cm2) and PBDT-DTNT:PC61BM:20 wt% P3ThEt-g-PANI (=276 Ω·cm2). In optimum PBDT-DTNT:PC61BM and PBDT-TIPS-DTNT-DT:PC61BM systems, the recombination time values ranged in 18.3–18.4 ns and 12.1–14.8 ns, respectively.

A step towards high-performance photovoltaics via three-component P3HT/PANI-graft-rGO nanocomposites

Polyaniline-grafted reduced graphene oxide (PANI-graft-rGO) was synthesized by in situ oxidative polymerization of aniline using amine-terminated rGO (rGO-NH2) as an initiator. Nanosheets were applied as templates for crystallization of regioregular poly(3-hexylthiophene) (P3HT) chains. The resulting nano-hybrids entitled supra(rGO-NH2/P3HT) and supra(PANI-graft-rGO/P3HT) were utilized in the active layers of P3HT:phenyl-C71-butyric acid methyl ester (PC71BM) solar cells to improve the photovoltaic characteristics up to (6.79 mA/cm2, 43%, and 0.65 V) and (13.21 mA/cm2, 64%, and 0.67 V), leading to the efficiencies of 1.90% and 5.66%, respectively. In similar conditions and only via replacing the rGO-NH2 with PANI-graft-rGO, the photovoltaic features were elevated. It could be assigned to the impact of PANI grafts in facilitating th[e surface crystallization of P3HT host chains and thereby in well-arranging the active layer constituents. The PANI mediators may act as both acceptors (accepting the electrons from rGO) and donors (donating the electrons to P3HT crystals) in configuration of supra(PANI-graft-rGO/P3HT) nano-hybrids, thus made the supra(PANI-graft-rGO/P3HT) nanostructures the appropriate candidates in manipulation of cell factors. This phenomenon was comprehended from the escalation in contents of hole (µh) and electron (µe) mobilities and also short circuit current density (Jsc) values. The highest µe (8.9 × 10–3 cm2/V.s) and µh (4.5 × 10–3 cm2/V.s) were acquired for electron-only (Al/supra(PANI-graft-rGO/P3HT):P3HT:PC71BM/Al) and hole-only (indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/supra(PANI-graft-rGO/P3HT):P3HT:PC71BM/Pd) devices. In similar conditions, the charge mobilities were lower in rGO-NH2 based systems either with PC71BM molecules or without them.

Morphological and photophysical study in hybrid ternary organic nanoparticles blends

The electron donor-acceptor based hybrid ternary nanostructures remains a frontier area of research in designing novel light harvesting devices. Here we report the morphological and photophysical study in Intra-triad and Inter-triad nanoparticles by using electron donating donor-acceptor-donor (D-A-D) type Diketopyrrolopyrrole (DPP) molecule which imparts the advantages of both polymers and small molecules and non fullerene electron acceptor, Naphthalene diimide (NDI) and fullerene based acceptor, Phenyl-C61-butyric acid methyl ester (PCBM). Both these nanostructures were fabricated by modified mini-emulsion technique. The Intra-triad nanostructure contains DPP, NDI and PCBM molecules inside the same matrix and in case of Inter-triad nanostructure, individual nanoparticles are attached together by electrostatic force. The morphological optimizations in hybrid nanostructures are performed with the help of AFM studies. We have further studied the structure mediated charge transfer phenomena and lifetime decay profiles by time resolved photoluminescence and steady state spectroscopy measurements. Due to the favorable energy levels, DPP molecules have been employed here as donor molecule for cascade energy transfer in ternary heterotriad geometric system. We have further studied the structure mediated charge transfer phenomena and lifetime decay profiles by time resolved photoluminescence and steady state spectroscopy measurements. Due to the favorable energy levels, DPP molecules have been employed here as donor molecule for cascade energy transfer in ternary heterotriad geometric system. The steady state and time resolved spectroscopy reveal interesting photophysics about the ternary heterostructures. The Intra-triad system has shown improved charge transfer properties compared to Inter-triad system and could be a futuristic material for opto-electronic applications.

Elaboration of PCBM Coated P3HT Nanoparticles: Understanding the Shell Formation

Solvent displacement, or nanoprecipitation, is a well-known process to develop colloidal dispersions in water. Using two successive and selective nanoprecipitation steps, we developed a method to generate [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) shell on poly(3-hexylthiophene) (P3HT) core nanoparticles (P3HT@PCBM). We report herein on the understanding of the shell formation during this process. Using several techniques (dynamic light scattering, zeta-potential, photoluminescence), we evidenced that after the first solvent displacement with dimethyl sulfoxide (DMSO), the PCBM molecules still dissolved in the medium are already in close interaction with the P3HT nanoparticles (NP). Such proximity of the P3HT core with PCBM molecules in the DMSO dispersion explains why PCBM aggregates around the nanoparticles during the second solvent displacement with water. A fast electron transfer from P3HT to PCBM was identified by transient absorption spectroscopy, confirming the core-shell morphology even for low PCBM concentration. This study opens the route for the development of well-defined nano-objects dispersed in water for fabrication of organic photovoltaic devices with eco-friendly processes.

Nanoscale investigation of bulk heterojunction organic solar cell by scanning capacitance force microscopy

We developed a frequency-modulation atomic force microscopy (FM-AFM) system combined with Kelvin probe force microscopy (KFM) and scanning capacitance force microscopy (SCFM). Using the developed system, we investigated the surface structure and electrical characteristics of a bulk heterojunction (BHJ) sample consisting of a mixture of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT). The BHJ structure consisted of PCBM and P3HT molecules. These materials are often used as the active and charge transport layers in an organic solar cell, respectively. The results suggest that we succeeded in visualizing the phase separation in the BHJ sample using SCFM measurements. We illustrated the energy band diagrams from the KFM results and showed that band bending occurs from the organic material side at the interface with the indium–tin oxide substrate. Furthermore, the KFM and SCFM results suggested that charge separation occurs in the BHJ structure.

Roles of Interfacial Tension in Regulating Internal Organization of Low Bandgap Polymer Bulk Heterojunction Solar Cells by Polymer Additives

AbstractThe role of a tertiary polymer‐based additive is investigated in increasing the efficiency of inverted low bandgap polymer:fullerene bulk heterojunction (BHJ) solar cells. Charge separation in polymer BHJ solar cells relies on the phase separation between electron accepting fullerene derivatives and photoactive polymers. Proper distribution of individual phases of suitable crystallinities within the active layer is a key factor for efficient charge transport/extraction and high photovoltaic performance. Here, it is demonstrated that the minor addition of a tertiary amorphous polymer, polystyrene (PS), with optimized molecular weights can increase the overall photovoltaic efficiency of poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]:phenyl‐C61butyric acid methyl ester (PCDTBT:PCBM) inverted BHJ solar cells, through the interfacial‐tension‐driven increase in crystallinities of photoactive phases and redistribution of PCBM molecules away from the top hole‐collecting anode interface. Complementary studies correlating polymer interfacial tension, blend internal structure, charge transport, and photovoltaic characteristics show that tertiary, high molecular‐weight polymers can serve as effective additives for improving the performance of low bandgap polymer solar cells.

Electronic and vibrational spectroscopy studies of PffBT4T π-conjugated donor–acceptor copolymer

We used a variety of optical spectroscopies to investigate the charge excitations and correlated infrared (IR)-active and Raman-active vibrations in poly[(difluoro-benzothiadiazol-diyl)-alt-(di(2-octyldodecyl)-quaterthiophen-diyl)], PffBT4T, a π-conjugated donor–acceptor (DA) copolymer, which, when blended with fullerene PCBM molecules, serves as an active layer in high-performance photovoltaic solar cells. The applied optical spectroscopies in films of pristine PffBT4T and PffBT4T/PCBM blend include absorption, photoluminescence, electroabsorption, photoinduced absorption (PA), and resonant Raman scattering. We found that the PffBT4T copolymer chain contains 11 strongly coupled Raman-active vibrational modes, which are renormalized upon photogeneration of charge polarons onto the chain. As the lower energy polaron absorption band overlaps with the renormalized vibrational modes, they appear in the PA spectrum as antiresonance (AR) lines superposed onto the induced polaron absorption band. We show that the Raman scattering, doping induced, and photoinduced AR spectra in PffBT4T are well explained by the amplitude mode model (AMM), where a single vibrational propagator describes the renormalized Raman modes and their related photoinduced AR intensities in detail. Surprisingly, we found that two of the IR-active modes in the pristine copolymer must be included in the AMM propagator for explaining the complete photoinduced AR spectrum. This feature is unique to DA-copolymers and indicates that some intrachain C2v symmetry breaking occurs because of the different electron affinities of the donor and acceptor moieties.