Bulk Heterojunction Composites Research Articles

Effect of External Magnetic Field on Bulk Heterojunction Polymer Solar Cells.

Polymer solar cells (PSCs) with a bulk heterojunction (BHJ) device structure have incredible advantages, such as low-cost fabrication and flexibility. However, the power conversion efficiency (PCE) of BHJ PSCs needs to be further improved to realize their practical applications. In this study, boosted PCEs from PSCs based on BHJ composites incorporated with Fe3 O4 magnetic nanoparticles (MNPs), aligned by an external magnetic field (EMF), are reported. It is found that the coercive electric field within the Fe3 O4 MNPs generated by the EMF has a strong and positive influence on the charge generation, which results in a more than 10% increase in free charge carriers. Moreover, the coercive electric field speeds up the charge carrier transport and suppresses charge carrier recombination within PSCs. In addition, a shortened extraction time makes charge carriers more likely to make it to the electrodes. As a result, more than 15% enhancement in PCE is observed from the PSCs based on the BHJ composite incorporated with the Fe3 O4 MNPs and the EMF as compared with that based on the BHJ composite thin film. This work indicates that the incorporation of MNPs and the EMF is a facile way to enhance the PCEs of PSCs.

Solution-processed bulk heterojunction broadband photodetectors based on perovskites incorporated with PbSe quantum dots

Broadband photodetectors (PDs) have great applications in both scientific and industrial sectors. In this study, we report room-temperature operated solution-processed bulk heterojunction (BHJ) broadband PDs based on perovskites incorporated with highly electrically conductive PbSe quantum dots (QDs). The p-type perovskites incorporated with the n-type PbSe QDs forming the BHJ composite thin film not only extend the spectral response up to the infrared region but also balance charge transport of the photoactive layer, resulting in boosted photocurrent and suppressed dark current, consequently, enhanced device performance of the broadband PDs. As a result, the solution-processed BHJ broadband PDs exhibit a responsibility of 10 mA/W, a detectivity of 10 11 Jones (1 Jones = 1 cm ⋅ Hz 1/2 /W), and a linear dynamic range of 53 dB in the spectral response ranging from 350 nm to 2500 nm, where the BHJ broadband PDs are operated at room temperature. These studies indicate that we provide a facile way to develop room-temperature operated solution-processed broadband PDs. • High electrically conductive n-type PbSe quantum dots and p-type CH 3 NH 3 PbI 3 perovskites. • p-type CH 3 NH 3 PbI 3 perovskites incorporated with n-type PbSe quantum dots forming bulk heterojunction composites. • Solution-processed room-temperature operated broadband photodetectors. • Responsibility, detectivity and a linear dynamic range of broadband photodetectors.

Quantitative Analysis of Charge Dissociation by Selectively Characterizing Exciton Splitting Efficiencies in Single Component Materials

AbstractExciton dissociation between donor and acceptor is the decisive process to determine the photovoltaic performance of organic solar cells (OSCs). The rather fast dynamics of photo‐induced charge generation in well‐optimized bulk heterojunction (BHJ) composites complicate the quantitative analysis of the charge generation efficiency for donor and acceptor units. Herein, we report time‐resolved photoluminescence (TRPL) investigations and demonstrate their potential as a quantitative and contactless characterization technique allowing to separately determine the exciton splitting efficiency of donor and acceptor moieties in selected single component materials. We demonstrate that the exciton splitting efficiency for donor or acceptor moieties can be separately adjusted in these materials by post‐treatment, while the corresponding BHJ composites undergo excessive phase separation upon external stress. By relating the separate exciton splitting efficiencies to their photovoltaic performance, the limiting factor in corresponding OSCs, either charge generation or recombination, is identified.

Effect of ZnO ETL and MoO3 HTL with PCDTBT:PC70BM-based BHJ organic solar cells

In this work, the authors fabricated and characterized efficient inverted organic solar cells by modifying them with a highly transparent sol–gel-derived zinc oxide (ZnO) film as an electron transport layer and optimized molybdenum trioxide (MoO3) as a hole transport layer with a very low-bandgap polymer, poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT), in bulk heterojunction (BHJ) composites with the fullerene derivative [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) BHJ in various solvents. Variations in the sol concentration, thickness, surface and optical characteristics of inverted organic solar cell glass/indium tin oxide/zinc oxide/PCDTBT:PC70BM/molybdenum trioxide/silver (Ag) structures were investigated and analyzed. The thickness of zinc oxide increased as the sol concentration increased from 10 to 100 nm. With the modification of the PCDTBT:PC70BM solvent, the device efficiency improved from 4.22 to 7.43%, which was accompanied by an improvement in the open-circuit voltage (V oc) from 0.87 to 1.03 V and enhancement of the short-circuit current density (J sc) from 9.1 to 13.5 mA/cm2.

Ultrasensitive and high gain solution-processed perovskite photodetectors by CH3NH3PbI2.55Br0.45:Zn2SnO4 bulk heterojunction composite

Hybrid perovskite photodetectors, the novel alternative devices transform incident light into electrical signal, have been rapidly developed in the past years. However, intrinsic unbalanced charge carrier transport within hybrid perovskite materials, restricting further boosting device performance of perovskite photodetectors, has rarely been addressed. In this study, we report room temperature–operated solution-processed bulk heterojunction perovskite photodetectors with ultrahigh sensitivity and great photo-gain. It is found that the introduction of Zn2SnO4 nanoparticles into CH3NH3PbI2.55Br0.45 perovskites results in enlarged photocurrent and suppressed dark current for bulk heterojunction perovskite photodetectors. As a result, a responsivity of over 1500 mAW−1 and projected detectivity of approximatively 1014 Jones (1 Jones = cm Hz1/2 W−1) from 380 to 760 nm are observed from bulk heterojunction perovskite photodetectors. In addition, bulk heterojunction perovskite photodetectors exhibit an excellent linear dynamic range of 124 dB and a great photo-gain of 535. All these results indicate that high-performance perovskite photodetectors could be realized through novel bulk heterojunction device structure.

Nickel oxide and polytetrafluoroethylene stacked structure as an interfacial layer for efficient polymer solar cells

An efficient polymer solar cell (PSC) has been demonstrated by incorporating an ultrathin interfacial insulating organic layer of polytetrafluoroethylene (PTFE) between a photoactive layer and hole-collecting buffer layer (NiOx). The photoactive layer is made with bulk heterojunction composites of poly[N-9’’-hepta-decanyl-2,7-carbazolealt-5,5-(4′,7′-di-2-thienyl- 2′,1′,3′-ben-zothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC71BM). The PTFE layer not only improves the energy level alignment by forming an interfacial dipole at the interface but also reduces the inherent incompatibility between the hydrophobic active layer and hydrophilic NiOx layer, thereby benefiting to the charge extraction and transport in the solar cells. As a result, with the NiOx/PTFE stacked structure, all the photovoltaic performance parameters are significantly improved, leading to a higher power conversion efficiency (PCE) of up to 7.11% compared to the control device without PTFE layer (PCE 5.50%). The PTFE layer provides a superior alternative to interfacial engineering of the metal oxide/organic semiconductor interface in polymer solar cells and other organic electronic devices.

Strontium Fluoride and Zinc Oxide Stacked Structure as an Interlayer in High-Performance Inverted Polymer Solar Cells.

Enhanced power conversion efficiency is reported in inverted polymer solar cells when an ultrathin layer of strontium fluoride (SrF2) is evaporated on the surface of the solution-processed zinc oxide (ZnO) electron transport layer. The photoactive layer is made up of bulk heterojunction composites of poly[4,8-bis(5(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b:4,5- b']dithiopheneco-3-fluorothieno[3,4- b]-thiophene-2-carboxylate] and [6,6]-phenyl-C71-butyric acid methyl ester. The ZnO film acts as an effective electron transport layer, whereas the ultrathin SrF2 layer improves the energy level alignment and enhances the built-in potential via the formation of an interfacial dipole layer at the interfaces between the ZnO film and the photoactive layer, resulting in an enhanced electron extraction efficiency and a decreased carrier recombination loss. Furthermore, the SrF2 layer reduces the inherent incompatibility between the hydrophilic ZnO film and the hydrophobic photoactive layer. As a result, all the photovoltaic performance parameters are remarkably improved, leading to a high efficiency of up to 10.46% (with a fill factor of 71.38%), corresponding to a ca. 21% improvement over the reference device performance (8.64%). The use of a ZnO/SrF2 stacked interlayer provides a simple, but effective, approach to obtain high-efficiency inverted PSCs.

Low-Temperature Solution-Processed Mg:SnO2 Nanoparticles as an Effective Cathode Interfacial Layer for Inverted Polymer Solar Cell

An efficient inverted polymer solar cell (PSC)-based on bulk heterojunction composites of poly(3-hexylthiophene) (P3HT) and phenyl C61-butryricacid methyl ester (PCBM) has been demonstrated by incorporating facile low-temperature solution-processed Mg-doped SnO2 (Mg:SnO2) nanoparticles as the cathode interfacial layer. Compared to the pure SnO2, the PSCs based on Mg:SnO2 interfacial layer exhibits excellent properties with a power conversion efficiency (PCE) of up to 4.08%, increased from 2.77%, corresponding to a significant 47.29% PCE enhancement. The improved photovoltaic performance is ascribed to the increased electron mobility, elevated electrical conductivity and optimized surface morphology, which makes it an excellent growth platform for a flat and high quality photoactive layer. Furthermore, we show the Mg:SnO2 interfacial layers to dramatically improve the electron extraction and effectively suppress the photogenerated carrier recombination. The low-temperature solution-processed SnO2 with Mg d...

Overcoming Microstructural Limitations in Water Processed Organic Solar Cells by Engineering Customized Nanoparticulate Inks

AbstractThe application of conjugated polymer and fullerene water‐based nanoparticles (NP) as ecofriendly inks for organic photovoltaics (OPVs) is reported. A low bandgap polymer diketopyrrolopyrrole–quinquethiophene (PDPP5T‐2) and the methanofullerene PC71BM are processed into three types of nanoparticles: pristine fullerene NPs, pristine polymer NPs, and mixed polymer:fullerene NPs, allowing the formation of bulk heterojunction (BHJ) composites with different domain sizes. Mild thermal annealing is required to melt the nanospheres and enable the formation of interconnected pathways within mixed phases. This BHJ is accompanied by a shrinkage of film, whereas the more compact layers show enhanced mobility. Consistently reduced recombination and better performance are found for mixed NP, containing both, the polymer and the fullerene within a single NP. The optimized solar cell processed by ultrasmall NPs delivers a power conversion efficiency of about 3.4%. This is among the highest values reported for aqueous processed OPVs but still lacks performance compared to those being processed from halogenated solvents. Incomplete crystallization is identified as the main root for reduced efficiency. It is nevertheless believed that postprocessing does not cut attraction from printing aqueous organic NP inks as a trendsetting strategy for the reliable and ecofriendly production of organic solar cells.

Negligible effect of processing additive in polymer bulk heterojunction photovoltaic cells with unmodified fullerene

Although processing additives such as 1,8-diiodooctane are known to effectively modify the microscopic structure of polymer bulk heterojunction composite using PCBM-type fullerene derivatives and yield efficient photovoltaic cells, it seems not yet to be clarified which part of the PCBM-type fullerene derivatives, i.e. bucky ball or side group, is essential to the phenomena. Here, the effect of processing additive 1,2-dimethylnaphthalene (DMN), which is reported to be effective for polymer bulk heterojunction composite film with [6,6]-phenyl-C71-butyric acid methyl ester (C70-PCBM) coated from 1,2,4-trimethylbenzene, on the composite film with unmodified C70 is examined. It has been observed that DMN shows negligible effects on the photovoltaic performance including the low-light performance as well as the surface morphology. Therefore, not the bucky ball but the side group in the fullerene derivatives seems to be critical to the positive effect of processing additives such as DMN.

Density functional theory design D-D-A type small molecule with 1.03 eV narrow band gap: effect of electron donor unit for organic photovoltaic solar cell

ABSTRACTSix new low-band-gap copolymers of donor–donor–acceptor (D-D-A) architecture have been designed using density functional theory and time-dependent density functional theory methods in order to use them in organic photovoltaic cell (OPVC). Phenanthro[3,4-d:9,10-d′]bis([1,2,3]thiadiazole)-10,12-dicarbonitrile moiety has been used as an acceptor for all compounds. We insert benzo[1,2-b:4,5-b′]dithiophene and N,N-diphenylbenzo[1,2-b:4,5-b′]dithiophen-2-amine units as donor to complete designing of copolymers. In order to tuning the optical and electronic properties, we have modified the donor unit by substituted with amine, methoxyamine, N-methylenethiophen-2-amine, methoxy, alkoxy moieties. The band gap (Eg), HOMO and LUMO values and plots, open circuit voltage (VOC) as well as optical properties have been analysed for designed copolymers. The optimised copolymers exhibit low-band-gap lying in the range of 1.03–2.24 eV. DPTD-6 copolymer presents the optimal properties to be used as an active layer due to its low Eg (1.03 eV) and a moderate VOC (0.56 eV). Thus, OPVC based on this copolymer in bulk-heterojunction composites with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor has been modelled. Eg and VOC values of composite material DPTD-6:PCBM are found as 1.32 and 0.65 eV, respectively. A model band diagram has been established for OPVC, simulating the energy transfer between active layers.

Thermal stability of low-bandgap copolymers PTB7 and PTB7-Th and their bulk heterojunction composites

The impact of PC61BM and PC71BM on the thermal stability of two low-bandgap conjugated polymers widely used in organic photovoltaic applications, namely PTB7 and PTB7-Th (also known as PBDTTT-EFT), was studied under nitrogen and air atmospheres. The thermal stability effect observed is mainly additive, i.e., the stability of the composites is similar to the weighted average of the stabilities of the individual components. The thiophene-substituted polymer (PTB7-Th) displays higher thermal stability, both in nitrogen and air, than its ether-substituted analogue (PTB7). The corresponding bulk heterojunction composites display the same order of relative thermal stabilities, i.e., PTB7-Th:PC71BM (1:1.5) > PTB7:PC71BM (1:1.5), which interestingly is also the order of lifetime stabilities reported in the literature for the corresponding OPV devices. These results suggest that simple and rapid thermogravimetry tests might be used as a valid rapid screening test to infer on the relative lifetime stability of OPV devices.

Inverted polymer solar cells with Zn2SnO4 nanoparticles as the electron extraction layer

In this study, we report narrow-size distribution Zn2SnO4 (ZSO) nanoparticles, which are produced by low-temperature solution-processed used as the electron extraction layer (EEL) in the inverted polymer solar cells (i-PSCs). Moreover, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) is used to modify the surface properties of ZSO thin film. By using the ZSO NPs/PFN as the EEL, the i-PSCs fabricated by poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′] dithio-phene-2,6-diyl-altethylhexyl-3-fluorothithieno [3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7) blended with (6,6)-phenyl-C71-butyric acid methylester (PC71BM) bulk heterojunction (BHJ) composite, exhibits a power conversion efficiency (PCE) of 8.44%, which is nearly 10% enhancement as compared with that of 7.75% observed from the i-PSCs by PTB7:PC71BM BHJ composite using the ZnO/PFN EEL. The enhanced PCE is originated from improved interfacial contact between the EEL with BHJ active layer and good energy level alignment between BHJ active layer and the EEL. Our results indicate that we provide a simple way to boost efficiency of i-PSCs.

Characteristics of PTB7:C70 bulk heterojunction photocell prepared with halogen-free solvent at low light illumination

Increasing social demands for clean and cheap renewable energy sources have been promoting the researches on polymer photocells based on bulk heterojunction composites consisting of conjugated polymers and fullerenes. The replacement of traditional chloroaromatic solvents such as chlorobenzene with a halogen-free solvent 1,2,4-trimethylbenzene (TMB), which enables the preparation of bulk heterojunction composites with unmodified fullerenes, potentially reduces the economic and environmental costs of organic bulk heterojunction photocells. In this study, photovoltaic characteristics of PTB7:C70 bulk heterojunction photocells prepared with TMB at low light intensity are studied. It has been found that the power conversion efficiency (PCE) of a photocell with ITO/PEDOT:PSS/PTB7:C70/Al structure whose PCE is approximately 3 % under AM1.5G 1 sun (100 mW/cm−2) illumination increases with decreased illumination light intensity, being 5.9 % at 10−3 sun illumination. An analysis of current–voltage characteristics suggests that the increased PCE is mainly due to increased shunt resistance and decreased saturation current of the diode in the model. The results indicate that the solution-processed organic photocell based on unmodified fullerene prepared with halogen-free solvent is a promising candidate for environmental-friendly device for indoor light harvesting.

Organic-Inorganic Hybrid Ternary Bulk Heterojunction of Nanostructured Perovskite-Low Bandgap Polymer-PCBM for Improved Efficiency of Organic Solar Cells.

A new organic-inorganic ternary bulk heterojunction (TBHJ) hybrid configuration comprised of nanostructured (CH3)3NHPbI3 (MAPbI3) perovskite-low bandgap PCPDTBT-PCBM was investigated. Well-organized TBHJ films were readily prepared by sequential spin-casting of sparsely covered MAPbI3 nano dots and PCPDTBT-PCBM bulk heterojunction (BHJ) composites on ITO/PEDOT:PSS substrates. The TBHJ hybrid device configuration comprising diiooctane (DIO) treated MAPbI3 perovskite nano dots and a PCPDTBT-PCBM BHJ composite processed with DIO additive exhibited excellent performances. The DIO additive played a key role in developing perovskite structures of MAPbI3 nano dots and induced the (110) directional crystallinity growth of longitudinal constructive morphologies such as nano rods. The improved photocurrent and fill factor compared to those of conventional BHJ devices led to an increase in efficiency of ∼28%. This improved photovoltaic performance originated from the higher quantum efficiencies contributed by the charge transfer from nanostructured MAPbI3 perovskite to PCBM. These TBHJs composed of nanostructured MAPbI3 perovskite, PCPDTBT, and PCBM also facilitated the exciton dissociation in the multi-BHJ system between MAPbI3 perovskite, PCPDTBT, and PCBM.

Effects of magnetic nanoparticles and external magnetostatic field on the bulk heterojunction polymer solar cells.

The price of energy to separate tightly bound electron-hole pair (or charge-transfer state) and extract freely movable charges from low-mobility materials represents fundamental losses for many low-cost photovoltaic devices. In bulk heterojunction (BHJ) polymer solar cells (PSCs), approximately 50% of the total efficiency lost among all energy loss pathways is due to the photogenerated charge carrier recombination within PSCs and low charge carrier mobility of disordered organic materials. To address these issues, we introduce magnetic nanoparticles (MNPs) and orientate these MNPS within BHJ composite by an external magnetostatic field. Over 50% enhanced efficiency was observed from BHJ PSCs incorporated with MNPs and an external magnetostatic field alignment when compared to the control BHJ PSCs. The optimization of BHJ thin film morphology, suppression of charge carrier recombination, and enhancement in charge carrier collection result in a greatly increased short-circuit current density and fill factor, as a result, enhanced power conversion efficiency.

In situ studies of the molecular packing dynamics of bulk-heterojunction solar cells induced by the processing additive 1-chloronaphthalene

In situGIWAXS was used to monitor the molecular packing dynamics and phase separation of bulk-heterojunction composites with a processing additive 1-chloronaphthalene.

New acceptor–donor–acceptor (A–D–A) type copolymers for efficient organic photovoltaic devices

Three new conjugated systems alternating acceptor–donor–acceptor (A–D–A) type copolymers have been investigated by means of Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) at the 6-31g (d) level of theory. 4,4′-Dimethoxy-chalcone, also called the 1,3-bis(4-methoxyphenyl)prop-2-en-1-one (BMP), has been used as a common acceptor moiety. It forced intra-molecular S⋯O interactions through alternating oligo-thiophene derivatives: 4-AlkylThiophenes (4-ATP), 4-AlkylBithiophenes (4-ABTP) and 4-Thienylene Vinylene (4-TEV) as donor moieties. The band gap, HOMO and LUMO electron distributions as well as optical properties were analyzed for each molecule. The fully optimized resulting copolymers showed low band gaps (2.2–2.8eV) and deep HOMO energy levels ranging from −4.66 to −4.86eV. A broad absorption [300–900nm] covering the solar spectrum and absorption maxima ranges from 486 to 604nm. In addition, organic photovoltaic cells (OPCs) based on alternating copolymers in bulk heterojunction (BHJ) composites with the 1-(3-methoxycarbonyl) propyl-1-phenyl-[6,6]–C61 (PCBM), as an acceptor, have been optimized. Thus, the band gap decreased to 1.62eV, the power conversion efficiencies (PCEs) were about 3–5% and the open circuit voltage Voc of the resulting molecules decreased from 1.50 to 1.27eV.

Optimization of photovoltaic device based on poly(3-hexylthiophene):C60 bulk heterojunction composites prepared with halogen-free solvent

The combination of halogen-free solvent and neat fullerene is expected to reduce the economical and environmental costs of polymer photovoltaic devices. Recently, it has been found that a naturally produced solvent, 1,2,4-trimethylbenzene, enables the preparation of bulk heterojunction composites with neat C60. However, the power conversion efficiency (PCE) of such devices reported so far was less than 1%. In this research, the annealing effects on and film thickness dependence of the performance of photovoltaic devices based on a composite consisting of equivalent weights of poly(3-hexylthiophene) and C60 have been studied to improve the device performance. The optimized device showed a PCE of 1.21%, suggesting that neat C60 is a promising candidate electron acceptor material for polymer bulk heterojunction composites for environmentally friendly and cost-effective photovoltaic devices.

Effect of energetic disorder on the open-circuit voltage in organic bulk heterojunction composites

Under open-circuit condition, the current is not extracted and the photogenerated carriers in principle disappear only by recombination. We study the open-circuit voltage and transient photovoltage under the effect of bulk recombination in a medium with energetic disorder by using the multiple trapping (MT) model. The key parameter in the MT model is the dispersion parameter given by the ratio of thermal energy to the characteristic energy of trap states. We show that depends linearly on the logarithm of the light intensity and the slope depends on the of the MT model. Under the continuous irradiation of light, the photovoltage response to the weak perturbation by a pulsed light obeys pseudo-first-order decay. The rate as a function of is independent of the dispersion parameter. However, it obeys the power law as a function of light intensity, and the exponent is given by , which reduces to 1/2 in the absence of energetic disorder.