PCBM System Research Articles

Photoinduced charge transfer assisted through external electric field and ternary hydrogen bonding strategies

Understanding the mechanisms governing interfacial charge transfer in photoactive layer is crucial for optimizing photogenerated charge separation efficiency. In this study, the interfacial charge transfer process is regulated by applying external electric field (Fext) and ternary hydrogen bonding strategies. We observe significant changes in the excited state properties and charge transfer parameters at the interface under Fext conditions. This facilitates an in-depth exploration of the effects of Fext on the microscopic details of the interface at the molecular level. Implementing the ternary hydrogen bonding strategy significantly enhances the electron-attracting ability of fullerene. Comparative analysis of the PTB7-Th:PC71BM and PTB7-Th:C7:PC71BM systems under Fext = 0, reveals that the ternary hydrogen bonding strategy increases the charge transfer pathway and significantly improves the intermolecular charge separation rate (KCS). Additionally, we evaluated the relationship between hydrogen bond strength and Fext, demonstrating that Fext significantly enhances the hydrogen bond strength between C7:PC71BM. These studies provide deeper insights into the role of Fext and ternary hydrogen bonding strategies in charge transfer process, providing a valuable theoretical framework for designing more efficient organic solar cells (OSCs).

Exploring photoexcited spin states for fullerene-derivatives based organic bulk heterojunction solar cells using magneto-photocurrent

Fullerene-derivatives based bulk heterojunctions hold an exceptionally important role on the roadmap of highly efficient organic solar cells (OSCs). In recent years, the utilization of the non-fused ring acceptors based OSCs has further improved photovoltaic power conversion efficiencies. Among these, one of the fundamental issues is to explore and to understand the spin-related polaron dissociation at charge transfer states because they act as the central unit for the photovoltaic action. It is also eagerly important to quantify some internal fields, such as hyperfine fields and the spin–orbit coupling. The aim of the work is to develop a method for unraveling the photoexcited spin states, particularly for the fullerene-derivative based OSC. Furthermore, it helps to elucidate a long-standing issue regarding the relatively high production of photocurrent for the P3HT:PC71BM system, which is indeed contrary to its counterpart the P3HT:PC61BM system. Their corresponding Jablonski diagrams have been determined in order to understand interior spin dynamics. The method of the study offers an alternative route for an understanding of device performance from the spin-related aspect.

Different Energetics at Donor:Acceptor Interfaces in Bilayer and Bulk‐Heterojunction Polymer:Non‐Fullerene Organic Solar Cells

To understand the limitations placed on the open‐circuit voltage of bulk heterojunction (BHJ) organic solar cells, the energy levels of neat donor and acceptor samples are often characterized and applied to study BHJ blends. However, energy levels derived from neat samples may not necessarily reflect those at the donor:acceptor interface in blends. The properties of organic semiconductors are sensitive to microstructural changes, with non‐fullerene acceptors (NFAs) in particular known to exhibit different thin‐film polymorphs. To investigate the influence of differences in molecular packing in neat and blend films, temperature‐dependent current–voltage characteristics are measured for bilayer (BL) and BHJ devices. Herein, the fullerene acceptor PC71BM is compared—whose energy levels are expected to be less sensitive to molecular packing—with the NFA ITIC, paired with the same donor polymer PTB7‐Th. It is found that the interfacial energy levels differ for BL and BHJ devices for the PTB7‐Th:ITIC system but remain the same for the PTB7‐Th:PC71BM system. Furthermore, X‐ray scattering measurements identify that ITIC exhibits a different packing mode in neat films and in BHJ blends. Such microstructure‐dependent differences between neat and blend samples need to be considered when studying energy losses in NFA BHJ solar cells.

Influence of the (E)-N-(4-diphenylamino)benzylidene)benzo[d]thiazol-2-imine on the dielectric properties of P3HT, PCBM and P3HT:PCBM system in a wide temperature and frequency range

We investigated the relaxation processes occurring in the sub-hertz frequency range, difficult to access in experiments, using unconventional method of analysing the dielectric relaxation spectra recorded for highly conductive donor–acceptor materials. In particular, we investigated how the dielectric properties of the conventionally used donor–acceptor P3HT:PCBM system will change when PCBM is replaced by a new acceptor, the imine SC2, or when the imine SC2 is added to this system. The dielectric studies in the broad frequency and temperature range were performed and the relaxation processes connected with the charge movements and with the dipolar rotation (β relaxation) in systems based on polymer P3HT were extracted from the spectra. The relaxation times as well as the activation energies were obtained. Also the temperature dependence of the DC specific electric conductivity and high frequency permittivity for all materials studied were determined as well as the static permittivity for the P3HT:SC2 system.

Efficient Cathode Buffer Material Based on Dibenzothiophene-S,S-dioxide for Both Conventional and Inverted Organic Solar Cells.

A novel conjugated molecule (PBSON) based on a main chain composed of bis(dibenzothiophene-S,S-dioxide) fused cyclopentadiene and side chains containing amino groups is presented as an efficient cathode buffer material (CBM) for organic solar cells (OSCs). PBSON showed a deep highest occupied molecular orbital (HOMO) energy level of -6.01 eV, which was beneficial for building hole-blocking layers at the cathodes of OSCs. The energy bandgap of PBSON reached 3.17 eV, implying high transmittance to visible and near-infrared light, which meant PBSON should be suitable for the applications to most inverted OSCs. The scanning Kelvin probe microscopy measurement and theoretical calculation on the PBSON/cathode interfacial interaction proved the excellent work function-regulating abilities of PBSON for various cathodes, suggesting that PBSON could promote the formation of Ohmic contacts at the cathodes and thus improve the transport and collection of electron carriers for OSCs. The characterization of electron-only devices demonstrated the good electron-transporting performance of PBSON at the cathodes. In the conventional OSCs, it was hinted that PBSON might restrain the infiltrations of evaporated cathode atoms into the active films, consequently reducing the reverse leakage currents. As a result, PBSON was able to boost the power conversion efficiencies (PCEs) by 58.2 and 56.4% for both conventional and inverted OSCs of the typical PTB7:PC71BM system, respectively, as compared to the unadorned devices. In terms of the classical PTB7-Th:PC71BM system, substantial increases in PCEs could also be found with PBSON interlayers, which were 54.7 and 59.8% for the conventional device and inverted device, respectively. Therefore, PBSON is a kind of promising CBM for realizing both conventional and inverted OSCs of high performance.

Tuning Morphology of Active Layer by using a Wide Bandgap Oligomer‐Like Donor Enables Organic Solar Cells with Over 18% Efficiency

AbstractA wide bandgap oligomer‐like donor CNS‐6‐8 is synthesized and incorporated into the host PM6:Y6:PC71BM system to tune the morphology of the active layer for better device performance. Due to the good miscibility of CNS‐6‐8 with both host donor (PM6) and acceptors (Y6 and PC71BM), an optimized morphology is achieved with the appropriate phase separation size and enhanced crystallinity, which ultimately leads to more efficient exciton dissociation, charge transport, and lower nonradiative energy loss. As a result, the quaternary device achieves an improved efficiency of 18.07%, with a simultaneously increased open circuit voltage of 0.868 V, fill factor of 78.8%, and the comparable short‐circuit current density of 26.43 mA cm−2. This work indicates that the favorable 3D interpenetrating network morphology of Y6 containing blend films can be optimized by introducing small amount of a specific molecule with high crystallinity, thus providing an effective strategy to achieve better photovoltaic performance for state‐of‐the‐art Y6 analogs‐based organic solar cells.

Enhanced Efficiency and Excellent Thermostability in Organic Photovoltaics via Ternary Strategy with Twisted Conjugated Compound.

The thermal stability of high-efficiency organic photovoltaics (OPVs) is critical to the manufacturing of this technology. Therefore, targeted strategies and approaches shall be developed to improve efficiency and stability, simultaneously. Herein, a seleno twisted benzodiperylenediimides (TBD-PDI-Se) acceptor-doping strategy is taken advantage of to demonstrate the ternary bulk heterojunction OPVs with excellent stability, achieving outstanding power conversion efficiency of 14.43% and 17.25% based on PM6:IT-4F and PM6:Y6 ternary blend devices, respectively, which are superior to the corresponding binary devices. As evidenced by the active layer morphology, exciton dynamic study and the characterizations of the enabled device, the ternary blend device keeps nearly 90% original efficiency (t= 1000 h) under continuous constant heating at 140°C. Furthermore, the application of acceptor as the third component in PBDB-T:ITIC, J71:ITIC, and PBDB-T:PC71 BM systems is also verified, proving the good universality of acceptor-doping ternary strategy.

Naphtho[2,3-c]thiophene-4,9-dione based polymers for efficient fullerene solar cells

Designing and synthesizing new organic photovoltaic materials is an inexhaustible driving force for the development of polymer solar cells (PSCs). For an excellent molecular structure with a certain conjugated skeleton, studying its analogues is of considerable significance for fully exploiting its photovoltaic potential. In this work, naphtho[2,3-c]thiophene-4,9-dione (NTD), an analogue of benzo [1,2-c:4,5-c’]dithiophene-4,8-dione (BDD), was designed and synthesized. Based on this building block, alkoxy side chains and alkylthio side chains were grafted on NTD to build NTD-O and NTD-S unit respectively, and then two new D-A conjugated donor polymers, PBNO and PBNS, were synthesized with NTD-O and NTD-S units as acceptor units and asymmetric benzodithiophene unit (asy-BDTBP) as donor units. Later, the photoelectric properties of these two materials in fullerene PSCs were studied. Compared with BDD based benzodithiophene copolymer (asy-BDTBP-BDD), PBNO and PBNS show similar LUMO energy levels, −3.59 eV and −3.63 eV, respectively, PBNO and PBNS show higher HOMO energy levels, which are −5.25 eV and −5.37 eV, respectively. PBNO blended with fullerene acceptor PC71BM exhibited similar power conversion efficiency (PCE) compared to the copolymer asy-BDTBP-BDD (4.64%). The PCE is 4.71% with short-circuit current density (JSC) of 9.85 mA cm−2, open-circuit voltage (VOC) of 0.784 V and fill factor (FF) of 60.98%. Encouragingly, PBNS presented an obviously enhanced PCE in PC71BM system PSCs, and PCE boosted to 7.58% (JSC, 12.40 mA cm−2; VOC, 0.865 V; FF, 70.83%). This work shows that analogues of excellent molecules have huge photovoltaic potential, and that side-chain modification has a significant impact on improving the photovoltaic performance of PSCs.

High-Performance Inverted Structure Broadband Photodetector Based on ZnO Nanorods/PCDTBT:PCBM:PbS QDs

This article reports the performance improvement of a broadband photodetector using penetrating ZnO nanorods arrays (NRAs)-based electron transport layer (ETL) into a nanocomposite active layer of poly-[N-9''-heptadecanyl-2.7-carbazole-alt-5.5(4',7'-di-2-thienyl-2',1',3 -benzothiadiazole)] (PCDTBT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and PbS quantum dots (QDs) grown on an fluorine-doped tin oxide (FTO)-coated glass substrate. A thin layer of MoOx on the active layer was used as the hole transport layer (HTL) of the proposed photodetector. The device showed a high responsivity of ~213.77, ~28.57, and ~7.22 A/W at three selected wavelengths in ultraviolet (380 nm), visible (550 nm), and near-infrared (860 nm) regions, respectively. High external quantum efficiency (EQE) of more than 1000% was measured for each selected wavelength at low -1.5 V external bias. The superior EQE in the proposed device was attributed to both the surface trap-induced carrier injection into the ZnO NRA and ultrafast exciton dissociation existing in the PCDTBT:PCBM system.

An investigation of annealing methods for benzodithiophene terthiophene rhodanine based all small molecule organic solar cells

Abstract Mainstream organic solar cells (OSCs) suffer a great variation of photovoltaic performance among different batches of polymers, which brings an opportunity for all-small-molecule OSCs to take leading position of industrialization. In recent years, benzodithiophene terthiophene rhodamine (BTR), as small molecule donor, has played an important role in this field. Here we investigated two typical BTR based all-small-molecule OSCs processed with different annealing methods, to explore the morphology optimization brought by them. As a result, BTR:PC71BM system was optimized by solvent vapor annealing (SVA) reaching an excellent fill factor (FF) of 79.1% via tuning molecular packing intensity, while BTR:Y6 with temperature annealing (TA) yielded a power conversion efficiency (PCE) of 12.125% whose molecular packing orientation had been changed. Additionally, by crossing using SVA and TA methods, we found that these two method can't be utilized together to further improve the PCE for either system. Therefore, our work offers better PCEs for these two reported combinations and further studies the compatibility between specific BTR based active layers and designated annealing methods, providing deeper understanding of device engineering on all-small-molecule OSCs.

Elimination of Charge Transfer Energy Loss by Introducing a Small-Molecule Secondary Donor into Fullerene-Based Polymer Solar Cells

The fullerene acceptor, which has historically been used in bulk-heterojunction (BHJ) organic solar cells, is being replaced by non-fullerene acceptors due to its problem of high energy loss (Eloss). To reduce the energy loss in BHJ organic solar cells as a means to improve open-circuit voltage (Voc), several approaches have been applied to diminish recombination loss, notably enhancements to film morphology and decreasing of trap states by improving the crystallinity of active components. However, the adjustment of trap density by means of morphology control only has an inevitable limit. In this work, we have investigated the use of a ternary configuration as an alternative way to mitigate the energy loss in fullerene-based BHJ solar cells. A ternary system based on the wide-band-gap polymer PBDTTPD-HT, the small molecule DRCN5T, and PC71BM showed cascading charge transfer from PBDTTPD-HT through DRCN5T to PC71BM, an indirect electron transfer that avoided the deep charge transfer state between PBDTTPD-HT and PC71BM. DRCN5T:PC71BM mixture has a high ECT level close to the singlet state energy of DRCN5T, leading to low energy loss between DRCN5T and PC71BM and thereby greatly reducing the probability of first-order recombination. Consequently, the incorporation of a small amount of DRCN5T as a secondary donor into the PBDTTPD-HT:PC71BM system enhanced overall PCE, an effect mostly attributed to enhancement of Voc by means of eliminating charge transfer energy losses.

Black phosphorous quantum dots as an effective interlayer modifier in polymer solar cells

Black phosphorus quantum dots (BPQDs), as 2D van der Waals crystals, possess remarkable electronic and optoelectronic properties. Their excellent ultra-high mobility and thickness-dependent tunable direct bandgap can meet well the needs of polymer solar cells (PSCs) for adjustable energy levels in different active layer systems. The formation of better energy alignment in the devices is significantly beneficial to the charge transfer, exciton dissociation and reduce charge recombination, which can lead to improved power conversion efficiency (PCE). Herein, it is introduced to modify hole transport layer (HTL) as interfacial modifier (IM) in fullerene and non-fullerene PSCs. The cascade band structure is formed successfully between the anode and polymer donor. In PTB7-Th:PC71BM system, PCE is increased from 8.12% to 9.11%. In PM6:IT-4F system, PCE is enhanced from 11.65% to 12.81%. BPQDs are utilized as HTL IM in conventional device in our study, which can open up a promising route for photovoltaic research.

High performance organic solar cells based on ZnO: POT2T as an effective cathode interfacial layer

Interface engineering in organic solar cells (OSCs) plays an important role in improving electron extraction and suppressing carrier recombination. In this article, the improvement of device was achieved by doping POT2T into ZnO as a new electron transport layer (ETL). For PTB7-Th: PC71BM system, the device fabricated by using ZnO as ETL achieved a PCE of 9.03% with a JSC of 17.33 mA/cm2, a VOC of 0.783V and a FF of 64.17%. After doping 5wt% POT2T in ZnO, the JSC increased to 18.01 mA/cm2 and the FF increased slightly to 69.87%, as a result, the PCE increased to 9.84% with about 10% enhancement. The enhancement is attributed to the improved electron transport and conductivity, optimized ZnO surface morphology after adding POT2T, resulting in higher JSC and FF. Therefore, doping POT2T into ZnO as an ETL is a simple and effective method for obtaining high efficient OSCs.

Modeling process–structure–property relationship in organic photovoltaics using a robust diffuse interface approach

Organic photovoltaics (OPVs) can potentially provide a cost-efficient means of harnessing solar energy. However, optimum OPV performance depends on understanding the process–structure–property (PSP) correlation in organic semiconductors. In the working of bulk-heterojunction OPVs, the morphology plays a crucial role in device performance. In order to understand PSP linkage, a theoretical framework has been developed. We first established process–structure correlations by generating a range of morphologies with various blend ratios of donor and acceptor organic semiconductors for various annealing periods. Second, we calculated the effective electronic properties corresponding to the simulated structures using a diffuse interface approach that is numerically more robust and straightforward than the classical sharp interface method. This novel framework, wherein both the process–structure and the structure–property relationship have been established using the diffuse interface approach, completes the theoretical PSP linkage, allowing the optimization of process parameters for device applications. The theoretical PSP linkage is then benchmarked qualitatively with experimental results on a model P3HT:PCBM system. We have been able to identify the morphological characteristics that maximize device performance. This work is carried out in the broad overview of the integrated computational materials engineering framework wherein the processing parameters are optimized by determining the process–structure–property relationships.

Phase Behavior in the Active Layer of Small Molecule Organic Photovoltaics: State Diagram of p-DTS(FBTTh2)2:PC71BM

A comprehensive study was undertaken to obtain a more fundamental understanding of the phase behavior of the p-DTS(FBTTh2)2:PC71BM system, used in small molecule organic solar cells, with a strong ...

Functionalized Amphiphilic Diblock Fullerene Derivatives as a Cathode Buffer Layer for Efficient Inverted Organic Solar Cells.

The amphipathic interface layer sandwiched between cathode and active layers had always played a role to balance interface compatibility and interfacial energy barriers in inverted organic solar cell (OSC) devices. Two functionalized amphiphilic diblock fullerene derivatives named C60-2DPE and C60-4HTPB were synthesized and applied as an interface layer in modifying zinc oxide (ZnO). Based on their amphipathic characteristics, the solvent treatment was introduced to cause an obvious self-assembly of the two materials on ZnO. The introduced cathode buffer layer could improve the interface compatibility between ZnO and the organic active layer effectively with its amphipathic blocks. Based on the PTB7-Th:PC71BM system, the OSC devices with a functionalized fullerene derivative layer could reach a power conversion efficiency of 9.21 and 8.86% for C60-2DPE and C60-4HTPB , respectively.

Impact of 1,8-diiodooctane on the morphology of organic photovoltaic (OPV) devices – A Small Angle Neutron Scattering (SANS) study

The impact of the additive 1,8-diiodooctane on the morphology of bulk-heterojunction solar cells based on the systems P3HT:PC71BM, PTB7:PC71BM and PTB7-Th:PC71BM is studied using a combination of Small Angle Neutron Scattering (SANS) and Atomic Force Microscopy (AFM). The results clearly show that while in the P3HT:PC71BM system, the additive DIO promotes a slight coarsening of the phase domains (type I additive), in the systems PTB7:PC71BM and PTB7-Th:PC71BM, DIO promotes a large decrease in the size of the phase domains (type II additive). SANS is demonstrated as being particularly useful at detecting the minor morphological changes observed in the P3HT:PC71BM system, which can be hardly seen in AFM. This work illustrates how SANS complements AFM and both techniques when used together provide a deeper insight into the nanoscale structure in thin organic photovoltaic (OPV) device films.

PffBT4T-2OD Based Solar Cells with Aryl-Substituted N-Methyl-Fulleropyrrolidine Acceptors.

Novel C60 and C70 N-methyl-fulleropyrrolidine derivatives, containing both electron withdrawing and electron donating substituent groups, were synthesized by the well-known Prato reaction. The corresponding highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels were determined by cyclic voltammetry, from the onset oxidation and reduction potentials, respectively. Some of the novel fullerenes have higher LUMO levels than the standards PC61BM and PC71BM. When tested in PffBT4T-2OD based polymer solar cells, with the standard architecture ITO/PEDOT:PSS/Active-Layer/Ca/Al, these fullerenes do not bring about any efficiency improvements compared to the standard PC71BM system, however they show how the electronic nature of the different substituents strongly affects the efficiency of the corresponding organic photovoltaic (OPV) devices. The functionalization of C70 yields a mixture of regioisomers and density functional theory (DFT) calculations show that these have systematically different electronic properties. This electronic inhomogeneity is likely responsible for the lower performance observed in devices containing C70 derivatives. These results help to understand how new fullerene acceptors can affect the performance of OPV devices.

Cyclopentadithiophene cored A-π-D-π-A non-fullerene electron acceptor in ternary polymer solar cells to extend the light absorption up to 900 nm

Abstract Conjugated small molecular non-fullerene electron acceptors (NFA) are considered as one of the critical materials for achieving high performance and low cost of polymer solar cells, and received much attention in the last few years. However, most of the NFAs are based on large fused π-aromatic core, which requires complicate synthesis efforts. In addition, the relatively weak light absorption limited to 800 nm of most the NAFs limits the energy harvesting capability of the solar cells. In this paper, we report an A-π-D-π-A type molecule cored with a cyclopentadithiophene unit, which can be easily synthesized in two steps from commercially available starting materials. This compound shows a broad absorption up to 900 nm in thin solid film, which is ascribed to the relatively high highest occupied molecular orbital (HOMO) energy level as confirmed by cyclic voltammery and theoratical calculation. Application of the compound in polymer solar cells was also investigated both in binary and in ternary systems. The optimized power conversion efficiency (PCE) in binary solar cell with PTB7-Th as donor is 5.76% with an open circuit voltage (VOC) of 0.838 V, a short circuit current (JSC) of 14.81 mA/cm2 and a fill factor (FF) of 46.4%. In the ternary solar cells which includes a second acceptor, PC71BM, the highest PCE achieved is 9.39% with a VOC of 0.803 V, a JSC of 19.01 mA/cm2, a FF of 61.6%, which is over 20% enhancement compared to the PTB7-Th:PC71BM system (PCE of 7.58%). This work develops a simple small molecule non-fullerene acceptor which can largely enhance the photo response in near infrared region to improve the performance of fullerene based organic solar cell.

Efficient Interface Engineering Enhances Photovoltaic Performance of a Bulk-Heterojunction PCDTBT:PC71BM System

High-performance conventional polymer solar cells based on a 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 (PC71BM) system are achieved via cathodic interfacial engineering by a bispyridinium salt small molecule (FPyBr). The bispyridinium salt can form a directed dipole at the cathode interface and thus decrease the cathode work function, leading to an improved built-in potential and open-circuit voltage. The good energy level alignment and hole-blocking ability of FPyBr at the cathode may suppress the charge recombination and promote the electron extraction process to improve the device performance. A smooth and uniform FPyBr film can be formed on the active layer, resulting in a good interfacial contact. As a result, a power conversion efficiency of 7.68% can be realized with FPyBr, which is significantly higher than for bare Al, solvent-treated and poly[(9,9-bis(3'-(N,N-dimethylamino)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]-modified devices. To the best of our knowledge, this result is one of the highest photovoltaic performance based on PCDTBT:PC71BM system. Therefore, FPyBr is a promising cathode modifier for high-performance polymer solar cells.