Bulk Heterojunction Organic Photovoltaics Research Articles

Engineering of the energetic structure of the anode of organic photovoltaic devices utilizing hot-wire deposited transition metal oxide layers

In this work we use hydrogen deposited molybdenum and tungsten oxides (chemically described as H:MOxx≤3 where M=Mo or W) to control the energetics at the anode of bulk heterojunction (BHJ) organic photovoltaics (OPVs) based on poly(3-hexylthiophene):[6,6]-phenyl butyric acid methyl ester (P3HT:PC71BM) blends. Significantly improved current densities and open circuit voltages were achieved as a result of improved hole transport from the P3HT highest occupied molecular orbital (HOMO) toward indium tin oxide (ITO) anode. This was attributed to the formation of shallow gap states in these oxides which are located just below the Fermi level and above the polymer HOMO and thus may act as a barrier-free path for the extraction of holes. Consequently, these states can be used for controlling the energetic structure of the anode of OPVs. By using ultraviolet photoelectron spectroscopy it was found that dependent on the deposition conditions these gap states and work function of the metal oxides may be tailored to contribute to the precise alignment of the HOMO of the organic semiconductor (OSC) with the Fermi level of the anode electrode resulting in further enhancement of the device performance.

Fast printing and in situ morphology observation of organic photovoltaics using slot-die coating.

The mini-slot-die coater offers a simple, convenient, materials-efficient route to print bulk-heterojunction (BHJ) organic photovoltaics (OPVs) that show efficiencies similar to spin-coating. Grazing-incidence X-ray diffraction (GIXD) and GI small-angle X-ray scattering (GISAXS) methods are used in real time to characterize the active-layer formation during printing. A polymer-aggregation-phase-separation-crystallization mechanism for the evolution of the morphology describes the observations.

"Supersaturated" self-assembled charge-selective interfacial layers for organic solar cells.

To achieve densely packed charge-selective organosilane-based interfacial layers (IFLs) on the tin-doped indium oxide (ITO) anodes of organic photovoltaic (OPV) cells, a series of Ar2N-(CH2)n-SiCl3 precursors with Ar = 3,4-difluorophenyl, n = 3, 6, 10, and 18, was synthesized, characterized, and chemisorbed on OPV anodes to serve as IFLs. To minimize lateral nonbonded -NAr2···Ar2N- repulsions which likely limit IFL packing densities in the resulting self-assembled monolayers (SAMs), precursor mixtures having both small and large n values are simultaneously deposited. These "heterogeneous" SAMs are characterized by a battery of techniques: contact angle measurements, X-ray reflectivity, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), cyclic voltammetry, and DFT computation. It is found that the headgroup densities of these "supersaturated" heterogeneous SAMs (SHSAMs) are enhanced by as much as 17% versus their homogeneous counterparts. Supersaturation significantly modifies the IFL properties including the work function (as much as 16%) and areal dipole moment (as much as 49%). Bulk-heterojunction OPV devices are fabricated with these SHSAMs: ITO/IFL/poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][2-[[(2-ethylhexyl)oxy]carbonyl]-3-fluorothieno[3,4-b]thiophenediyl]]:phenyl-C71-butyric acid methyl ester (PTB7:PC71BM)/LiF/Al. OPVs having SHSAM IFLs exhibit significantly enhanced performance (PCE by 54%; Voc by 35%) due to enhanced charge selectivity and collection, with the PCE rivaling or exceeding that of PEDOT:PSS IFL devices -7.62%. The mechanism underlying the enhanced performance involves modified hole collection and selectivity efficiency inferred from the UPS data. The ITO/SAM/SHSAM surface potential imposed by the dipolar SAMs causes band bending and favorably alters the Schottky barrier height. Thus, interfacial charge selectivity and collection are enhanced as evident in the greater OPV Voc.

An electron-deficient small molecule accessible from sustainable synthesis and building blocks for use as a fullerene alternative in organic photovoltaics.

An electron-deficient small molecule accessible from sustainable isoindigo and phthalimide building blocks was synthesized via optimized synthetic procedures that incorporate microwave-assisted synthesis and a heterogeneous catalyst for Suzuki coupling, and direct heteroarylation carbon-carbon bond forming reactions. The material was designed as a non-fullerene acceptor with the help of DFT calculations and characterized by optical, electronic, and thermal analysis. Further investigation of the material revealed a differing solid-state morphology with the use of three well-known processing conditions: thermal annealing, solvent vapor annealing and small volume fractions of 1,8-diiodooctane (DIO) additive. These unique morphologies persist in the active layer blends and have demonstrated a distinct influence on device performance. Organic photovoltaic-bulk heterojunction (OPV-BHJ) devices show an inherently high open circuit voltage (Voc ) with the best power conversion efficiency (PCE) cells reaching 1.0 V with 0.4 v/v % DIO as a processing additive.

Improvement of the photovoltaic properties for P3HT:PCBM system annealed at the temperature in liquid crystalline regions induced by liquid crystal molecules

The poly(3-hexylthiophene):[6, 6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction organic photovoltaics (BHJ OPVs) showed a maximum value of 3.5 % by incorporating of 6 wt% nematic liquid crystals (NLCs) of 4-octyloxy-4′-cyanobiphenyls (8OCB) after annealing at the temperature of 70 °C in liquid crystalline transitions. The 8OCB was immiscible with P3HT, and the melting temperature of P3HT was not influenced at 8OCB doping amount below 10 wt%. Based on the UV–vis analysis, the conjugation length and chain order of P3HT should be the highest at 8OCB content of 6 wt% after annealing at 70 °C. The reflection peak of P3HT shifted to lower positions as the annealing temperature increasing from 70 to 130 °C, indicating that PCBM and 8OCB molecules could be incorporated into the lamellar spacing of P3HT crystallites at high annealing temperatures. The improvement of the solar cell device after annealing at 70 °C is probably due to the optimized phase separation morphology and relatively higher hole mobility in P3HT:PCBM:8OCB system. The 8OCB molecules in the liquid crystalline state are supposed to be a good template for the crystallization of P3HT, as well as the phase separation between P3HT and PCBM.

Bridging donor–acceptor energy offset using organic dopants as energy ladders to improve open-circuit voltages in bulk-heterojunction solar cells

A systematic way to improve the open circuit voltage of bulk-heterojunction (BHJ) organic photovoltaics is demonstrated by incorporating energy ladder material to form a ternary blend with cascade energy structures. We propose that doping organic molecules with appropriate energetic alignment to bridge the donor–acceptor energy offset can facilitate exciton dissociation and hence improve open-circuit voltage (Voc) and power conversion efficiency. The cascade energy alignment structure is measured via ultraviolet photoemission spectroscopy. The Voc and power conversion efficiency of devices using the proposed schemes can be enhanced by 10% and 20%, respectively. The results provide a new path toward higher power conversion efficiency in BHJ systems by engineering the energy structures of the materials in the active layers.

Competitive Absorption and Inefficient Exciton Harvesting: Lessons Learned from Bulk Heterojunction Organic Photovoltaics Utilizing the Polymer Acceptor P(NDI2OD‐T2)

Organic solar cells utilizing the small molecule donor 7,7′‐(4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(6‐fluoro‐4‐(5′‐hexyl‐[2,2′‐bithiophen]‐5‐yl)benzo[c][1,2,5] thiadiazole) (p‐DTS(FBTTh2)2and the polymer acceptor poly{[N,N′‐bis(2‐octyldodecyl)‐1,4,5,8‐naphthalenedicarboximide‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)}(P(NDI2OD‐T2)) are investigated and a power conversion efficiency of 2.1% is achieved. By systematic study of bulk heterojunction (BHJ) organic photovoltaic (OPV) quantum efficiency, film morphology, charge transport and extraction and exciton diffusion, the loss processes in this blend is revealed compared to the blend of [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) and the same donor. An exciton diffussion study using Förster resonant energy transfer (FRET) shows the upper limit of the P(NDI2OD‐T2) exciton diffusion length to be only 1.1 nm. The extremely low exciton diffusion length of P(NDI2OD‐T2), in combination with the overlap in donor and acceptor absorption, is then found to significantly limit device performance. These results suggest that BHJ OPV devices utilizing P(NDI2OD‐T2) as an acceptor material will likely be limited by its low exciton diffusion length compared to devices utilizing functionalized fullerene acceptors, especially when P(NDI2OD‐T2) significantly competes with the donor molecule for photon absorption.

Junction capacitance and donor-acceptor interface of organic photovoltaics

To obtain quantitative information of the bicontinuous interface between the donors and acceptors (D/A), the junction capacitance of various organic photovoltaic (OPV) structures was measured by direct current biased impedance spectroscopy, without breaking the vacuum after device fabrication. A correlation between the extracted interfacial area and power conversion efficiency (PCE) was established for bilayer and bulk heterojunction OPVs. The latter was analyzed by two opposite models, one of which follows the same trend as the bilayer, such that the PCE increases with the enlarged interface. However, the increase is much less than expected, which may be caused by the loose D/A domains, “wasted” due to the poor electrical connection. The finding provides not only a better understanding of the device structure but also an opportunity for the further research of the structure-performance relationship of OPVs.

Qualitative analysis of bulk-heterojunction solar cells without device fabrication: an elegant and contactless method.

The enormous synthetic efforts on novel solar cell materials require a reliable and fast technique for the rapid screening of novel donor/acceptor combinations in order to quickly and reliably estimate their optimized parameters. Here, we report the applicability of such a versatile and fast evaluation technique for bulk heterojunction (BHJ) organic photovoltaics (OPV) by utilizing a steady-state photoluminescence (PL) method confirmed by electroluminescence (EL) measurements. A strong relation has been observed between the residual singlet emission and the charge transfer state emission in the blend. Using this relation, a figure of merit (FOM) is defined from photoluminescence and also electroluminescence measurements for qualitative analysis and shown to precisely anticipate the optimized blend parameters of bulk heterojunction films. Photoluminescence allows contactless evaluation of the photoactive layer and can be used to predict the optimized conditions for the best polymer-fullerene combination. Most interestingly, the contactless, PL-based FOM method has the potential to be integrated as a fast and reliable inline tool for quality control and material optimization.

Scratch resistance and durability enhancement of bulk heterojunction organic photovoltaics using ultra-thin alumina layers

Atomic layer deposition (ALD) was studied for ultra-thin (≤100nm) barrier coating deposition onto organic photovoltaics (OPVs) to enhance device lifetime. Herein, we report the first known investigation of the mechanical characteristics of AlOx encapsulated OPVs using nanoindentation and scratch testing with deposition temperature as a tunable parameter. The higher organic content in the AlOx film, grown at lower temperatures, enhanced the interfacial bonding at the AlOx–OPV interface and provided modulus and hardness performance exceeding that of a common polymer encapsulant. Furthermore, AlOx׳s organic content impeded fracture propagation, confirming the durability enhancement associated with this approach.

Free Carrier Generation in Organic Photovoltaic Bulk Heterojunctions of Conjugated Polymers with Molecular Acceptors: Planar versus Spherical Acceptors

A comparative study of the photophysical performance of the prototypical fullerene derivative PC61BM with a planar small-molecule acceptor in an organic photovoltaic device is presented. The small-molecule planar acceptor is 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile, termed K12. We discuss photoinduced free charge-carrier generation and transport in blends of PC61BM or K12 with poly(3-n-hexylthiophene) (P3HT), surveying literature results for P3HT:PC61BM and presenting new results on P3HT:K12. For both systems we also review previous work on film structure and correlate the structural and photophysical results. In both cases, a disordered mixed phase is formed between P3HT and the acceptor, although the photophysical properties of this mixed phase differ markedly for PC61BM and K12. In the case of PC61BM the mixed phase acts as a free carrier generation region that can efficiently shuttle carriers to the pure polymer and fullerene domains. As a result, the vast majority of excitons quenched in P3HT:PC61BM blends yield free carriers detected by the contactless time-resolved microwave conductivity (TRMC) method. In contrast, approximately 85% of the excitons quenched in P3HT:K12 do not result in free carriers over the nanosecond timescale of the TRMC experiment. We attribute this to poor electron-transport properties in the mixed P3HT:K12 phase. We propose that the observed differences can be traced to the respective shapes of PC61BM and K12: the three-dimensional nature of the fullerene cage facilitates coupling between PC61BM molecules irrespective of their relative orientation, whereas for K12 strong electronic coupling is only expected for molecules oriented with their π systems parallel to each other. Comparison between the eutectic compositions of the P3HT:PC61BM and P3HT:K12 shows that the former contains enough fullerene to form a percolation pathway for electrons, whereas the latter contains a sub-percolating volume fraction of the planar acceptor. Furthermore, the planar K12 co-assembles with P3HT into a disordered, glassy phase that partly accounts for the poor electron-transport properties, and may also enhance recombination due to the strong intermolecular interactions between the donor and the acceptor. The implication for the performance of organic photovoltaic devices with the two acceptors is also discussed.

Reduced Bimolecular Charge Recombination Loss in Thermally Annealed Bilayer Heterojunction Photovoltaic Devices with Large External Quantum Efficiency and Fill Factor

Thermally annealed bilayer heterojunction was shown to be an alternative approach to form the bulk-heterojunction (BHJ) structure and yield superior device performance compared to blend-solution-processed BHJ organic photovoltaic (OPV) devices. The best poly(3-hexylthiophene) (P3HT)/(6,6)- phenyl-C61-butyric acid methyl ester (PCBM) based bilayer structure OPV showed an external quantum efficiency approaching 82%, a high fill factor of 74%, and the power conversion efficiency of 5.1%, which are higher than those of the blend-solution-processed BHJ OPV devices. In contrast to the BHJ formed from blend solution, richer acceptor, e.g. PCBM, close to the cathode, or different morphology, was generally found in thermally annealed bilayer films. However, the physical mechanism for improved performance in thermally annealed bilayer films was rarely investigated in previous studies. Here, the amount of long-lived photogenerated charges with lifetime longer than 1 μs, which was characterized by the time delayed collection field method, was slightly enhanced by about 5% in the thermally annealed bilayer devices. Nevertheless, the bimolecular recombination coefficient was strongly decreased by 350% with transient photovoltage/ photocurrent measurements, conclusively demonstrating that the significantly reduced bimolecular recombination loss is a major contribution to the increased efficiency in the thermally annealed bilayer OPVs.

Acceptor blending ratio dependence of bulk heterojunction organic photovoltaic devices

Bulk heterojunction (BHJ) organic photovoltaic (OPV) devices are generally composed of a blend film of a π-conjugated polymeric electron donor and a C60 derivative electron acceptor intercalated between an anode and a cathode, typical of a two-dimensional device structure. Often, a regioregular poly(3-hexylthiophene) (rr-P3HT) and indene-C60 bisadduct (ICBA) blend film is used as the BHJ active layer instead of the conventional P3HT and phenyl-C61-butyric acid methyl ester (PCBM) blend film due to the ICBA device’s higher power conversion efficiency (PCE), which is correlated with its higher open-circuit voltage (VOC). To determine the quantitative influences of those electron acceptors in the BHJ OPV devices, we prepared various devices of acceptors, either PCBM or ICBA, with a wide range of blending ratios (from 0 to 100%) for estimating their typical optoelectronic properties, e.g., UV-visible absorption spectra, photoluminescence intensity, surface morphology, and photovoltaic device parameters. Obtained data were compared to each other and were analyzed as a function of their acceptor blending ratio.

Synthesis and photovoltaic properties of novel C60 bisadducts based on benzo[2,1,3]-thiadiazole

A novel C60 solar cell acceptor (BTOQC, benzo[2,1,3]-thiadiazole-o-quinodimethane-C60 bisadducts) based on benzo[2,1,3]thiadiazole has been synthesized as model to study how the thiadiazole group will affect the device performance in bulk heterojunction organic photovoltaics (BHJ-OPV) with poly(3-hexylthiophene) (P3HT) as donor. The optoelectronic, electrochemistry, and photovoltaic properties of the novel bisadduct BTOQC have been fully investigated. The best device performance of this fullerene derivative in our research was obtained as 2.50% with a high Voc of 0.74 V.

Research Progress of Non-Fullerene Small-Molecule Acceptor Materials for Organic Solar Cells

Bulk heterojunction organic photovoltaics have been the subject of intensive academic interest over the past two decades. Numerous recent efforts have been directed towards this area with the vision of developing next-generation low-cost solar cells. In the field of bulk heterojunction organic photovoltaics, fullerene and its derivatives are an important class of n-type electron acceptor materials. However, their disadvantages such as narrow wavelength absorption, high affinity, poor solubility, have severely limited their wide application as electron acceptors for organic solar cells and have largely hampered further improvement of the device performance. Recently, a number of research efforts have been focused on the develop- ment of novel non-fullerene n-type small-molecule acceptors. Various design rules and interesting new materials have been explored. The non-fullerene n-type small-molecule acceptors usually appear to possess lots of attractive advantages, such as adjustable energy levels, facile synthesis, good solubility, low processing cost. More important, when compared to the fullerene and its derivatives, this kind of small-molecule acceptors has wider spectral absorption that allows to absorb more sunlight to generate electricity. Recent breakthroughs rely mostly on the development of novel high-performance acceptor materials and optimization of the device structures. The up-to-date power conversion efficiencies exceeding 4% with using non-fullerene small-molecule acceptor materials in bulk heterojunction organic solar cells have been achieved. In this review, recent advances of non-fullerene small-molecule n-type acceptor materials for organic solar cells are reviewed, including rylene diimide-based acceptors, pentacene-based acceptors, benzothiadiazole-based acceptors, 1,4-diketopyrrolo-(3,4-c)- pyrrole (DPP)-based acceptors, fluorene-based acceptors, fluoranthene-fused imide-based acceptors and so on. Meanwhile, the future trends on material design and development have also been discussed. This review on illustrating the influence of the molecular structures and corresponding photovoltaic properties would thus be helpful to further unravel the role of elec- tron acceptors and shed light on exploring efficient n-type electron acceptor materials for high performance organic photo- voltaic devices. Keywords n-type small-molecular acceptors; wide spectral absorption; solution processing; organic photovoltaics

Small molecule BODIPY dyes as non-fullerene acceptors in bulk heterojunction organic photovoltaics

A series of acceptor-donor-acceptor molecules containing terminal BODIPY moieties conjugated through the meso position were synthesized. Deep LUMO energy levels and good visible absorption led to their use as acceptors in bulk heterojunction solar cells. Inverted devices were fabricated, reaching efficiencies as high as 1.51%.

Porphyrin oriented self-assembled nanostructures for efficient exciton dissociation in high-performing organic photovoltaics

Herein we report on enhanced organic solar cell performance through the incorporation of cathode interfacial layers consisting of self-organized porphyrin nanostructures with a face-on configuration. In particular, a water/methanol-soluble porphyrin molecule, the free base meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, is employed as a novel cathode interlayer in bulk heterojunction organic photovoltaics. It is demonstrated that the self-organization of this porphyrin compound into aggregates in which molecules adopt a face-to-face orientation parallel to the organic semiconducting substrate induces a large local interfacial electric field that results in a significant enhancement of exciton dissociation. Consequently, enhanced photocurrent and open circuit voltage were obtained resulting in overall device efficiency improvement in organic photovoltaics based on bulk heterojunction mixtures of different polymeric donors and fullerene acceptors, regardless of the specific combination of donor–acceptor employed. To highlight the impact of molecular orientation a second porphyrin compound, the Zn-metallated meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, was also studied and it was found that it forms aggregates with an edge-to-edge molecular configuration inducing a smaller increase in the device performance.

Investigation of Thermal Instability of Additive-Based High-Efficiency Organic Photovoltaics

Most photovoltaics operate at high temperature under sunlight. In this work, the thermal instability of diiodooctane-based high-efficiency bulk heterojunction (BHJ) organic photovoltaics (OPVs) is studied. The BHJ layers were heated to various temperatures to investigate the changes in their physical properties using atomic force microscopy phase images. The mobilities of the carriers were characterized at various temperatures using the space-charge-limited current method, and the carrier lifetime was calculated by applying impedance spectroscopy to the simulated equivalent circuit of the OPV devices.

Computational modelling of donor–acceptor conjugated polymers through engineered backbone manipulations based on a thiophene–quinoxaline alternating copolymer

The rapid progress of bulk heterojunction organic photovoltaics has been boosted by (i) design and synthesis of novel conjugated donor materials, (ii) control and optimization of device fabrication, and (iii) the development of new device architectures such as tandem and ternary solar cells. Computationally driven material design has attracted increasing interest to accelerate the search for optimal conjugated photovoltaic materials, and the exploration of chemical methodologies is highly desirable in pushing the efficiency further towards the theoretical limit. Based on the motif of donor–acceptor polymers, around 50 comparable polymers were constructed and investigated, derived from an easily accessible thiophene–quinoxaline alternating polymer donor showing power conversion efficiency up to 7%. We performed a systematic density functional theory (DFT) study on the heteroatom effects of combining fluorine, nitrogen and chalcogen substitutions onto the donor/acceptor units as well as the effect of extending π-conjugation in the donor moiety, in order to gain insight into how structural modifications to the conjugated backbone can affect the molecular structure and electronic properties of a conjugated polymer. It is found that the trends in the energy levels and band gaps of these polymers correlate well with their structural modifications. Finally, by examining the systematically evaluated data in the energy diagram, we proposed three important ways of energy level modulation, showing potential chemical methodologies that can be applicable to further modify and optimize existing polymer backbones. Especially such energy level modulation can be applied to meet the particular requirements of different device architectures (including tandem and ternary solar cells) on the donor components, such as a prominent photocurrent or photovoltage combined with a high efficiency, to further maximize the overall performance of organic photovoltaics. This will provide valuable guidance and chemical methodologies for a judicious material design of conjugated polymers for solar cell applications with desirable photovoltaic characteristics.

Impact of dithienyl or thienothiophene units on the optoelectronic and photovoltaic properties of benzo[1,2,5]thiadiazole based donor–acceptor copolymers for organic solar cell devices

We report a comparative study on four donor–acceptor benzothiadiazole-based copolymers containing dithienyl or thienothiophene moieties for application in organic photovoltaic (OPV) devices. Bulk-heterojunction OPV devices are fabricated having power conversion efficiencies ranging between 4 and 6%. Morphological, spectroscopic and charge-transport measurements are used to investigate the influence of either the dithienyl or thienothiophene moieties on the structure and photophysical properties of the copolymer and copolymer:PC71BM blend films and rationalise the solar cell characteristics. Although all copolymer:PC71BM blends exhibit comparable hole polaron yields, solar cell devices with the highest power conversion efficiencies are correlated with increased charge-carrier mobility of the copolymer and enhanced aggregation of PC71BM in the blend.