Structure Of Organic Solar Cells Research Articles

D‐A Polymer with a Donor Backbone ‐ Acceptor‐side‐chain Structure for Organic Solar Cells

AbstractWe report the design, synthesis, and properties of a novel type of donor (D)‐acceptor (A) polymer, poly(3‐(([2,2′:5′,2′′‐terthiophen]‐3‐yl‐5,5“‐diyl)methylene)‐1‐(2‐octyldodecyl)indolin‐2‐one) (PTIBT), with a donor backbone and acceptor side chains (Type II D‐A polymer) as donor for organic solar cells (OSCs) as opposed to the conventional D‐A polymers having both donor and acceptor units on backbone (Type I D‐A polymers). PTIBT having a backbone consisting of thiophene donor units and side chains containing indolin‐2‐one acceptor units was synthesized very conveniently in three steps. This polymer has a high dielectric constant of 7.70, which is beneficial for the exciton diffusion and dissociation in the active blend layer in an OSC. In addition, PTIBT was found to have a low‐lying HOMO energy level of −5.41 eV and a wide band gap of 1.80 eV in comparison to its counterpart Type I D‐A polymer. In organic thin film transistors (OTFTs), PTIBT showed typical p‐type semiconductor performance with hole mobilities of up to 1.81×10−2 cm2 V−1 s−1. When PTIBT and ITIC were used as donor and acceptor to form a blend active layer, the best OSC device showed a JSC of 15.19 mAcm−2, a VOC of 0.66 V, and a fill factor of 0.57, resulting in a power conversion efficiency (PCE) of up to 5.72%.

FDTD simulation studies on improvement of light absorption in organic solar cells by dielectric nanoparticles

In this paper, we present a systematic design and analysis of organic solar cell (OSC) by embedding dielectric nanoparticles layer at anode. Using numerical simulations, we show that there is improvement in the light absorption in the active layer of the device using nanoparticles. The nanoparticles will act as scattering medium for the incident light and hence trap the light inside the device. For the light scattered at different angles, the optical path length in the active medium is increased and this leads to more absorption of light in the active layer and thereby increase in the efficiency. The scattering efficiency and hence the absorption of solar radiation for generation of current depends on the particle size, interparticle separation and the refractive index contrast between the particles and the embedding medium. Mie theory has been used to calculate the scattering efficiency of nanoparticles. The effect on light absorption and current density of OSC due to nanoparticles has been carried out using finite difference time domain analysis. It is shown that the proposed OSC structure increases the light absorption in the active layer of the device by 40% and short circuit current density by 34%.

Impact of concentration variation and thermal annealing on performance of multilayer OSC consisting of sandwiched P3HT layer between PEDOT:PSS and P3HT:PCBM

In this paper, comprehensive study on variation in composition ratio and thermal annealing of multilayer organic solar cells (OSC) has been demonstrated. Our previous work reported that in comparison to the conventional OSC structure, multilayer OSC consisting of the pure donor phase between the hole transport layer and active layer has shown a significant improvement in the power conversion efficiency (PCE). Since photovoltaic performance of the OSC strongly influenced by composition ratio, the impact of poly 3-hexylthiophene (P3HT) and 6,6-phenyl-C61-butyric acid methyl ester (PCBM) weight ratio has been analyzed. It has been found that, the short-circuit current density (JSC) of the device is highest for the P3HT:PCBM composition ratio of 1.0:0.8 and decreases slightly for 1.0:1.0 weight ratio. This study has also been accompanied with the thermal annealing on spin coated P3HT:PCBM which further enhances the PCE of the device. The maximum JSC and the PCE of 9.47 mA/cm2 and 3.26% has been achieved for the multilayer OSC having P3HT:PCBM composition ratio of 1.0:0.8 and thermally annealed at 140 °C. This is attributed largely by the tuned phase morphology of the donor-acceptor. It has been found that, the P3HT:PCBM weight ratio of 1.0:0.8 results into efficient photon absorption and charge separation in the bulk heterojunction OSC. The reduced concentration of PCBM in 1.0:0.8 weight ratio leads to the strong interchain interaction of P3HT molecules which enhances the charge separation and collection ability of the bulk heterojunction OSC.

Novel Small Four-armed Molecules with Triphenylamine-bridged Structure for Organic Solar Cells Featuring High Open-circuit Voltage

In view of few attention on star-shaped molecules containing triphenylamine(TPA) unit as π-linker, a series of small four-armed molecules, consisting of octyloxy-substituted 2,1,3-benzothiadiazole(DOBT) or 4-octyl-2-thienyl functionalized DOBT as the core, TPA as π-bridge and 4-methylphenyl or 4-methoxyphenyl groups as terminal units, was designed and synthesized. The effects of πbridges and substitute groups on molecular photoe-lectric performance and photovoltaic performance were fully explored. With the help of the additional thiophene-linkers incorporation, 3-octylthienyl substituted molecule with end-capping 4-methylphenyl(T-BTTPAM) and 3-octylthienyl substituted molecule with end-capping 4-methoxyphenyl(T-BTTPAOM) showed stronger and broader absorption, as well as higher charge mobilities compared to the molecules without thiophene-linkers(BTTPAM and BTTPAOM). Additionally, changing substitute groups from methyl to methoxy helped BTTPAOM and T-BTTPAOM achieve better absorption properties than BTTPAM and T-BTTPAM, respectively. When paired with PC61BM as the electron acceptor to fabricate solution-processed photovoltaic devices, the four materials gave high open-circuit voltage(Voc) values over 0.90 V. These results demonstrate that our materials are promising candidates as donor materials for organic solar cells(OSCs), and further device optimization is in progress in our laboratory.

Thermal stability analysis of buffered layer P3HT/P3HT:PCBM organic solar cells

Here to determine the thermal stability of buffered layer organic solar cell (BL-OSC), the effect of post anneal treatment has been studied. To investigate the effect of post annealing, the organic solar cells (OSCs) are annealed at the 120°C for different time duration. It has been observed that the BL-OSC structure exhibits the better thermal stability. Further, as the authors vary the post-annealing time duration from 0 min to 20 min, the power conversion efficiency (PCE) in the case BL-OSC drops by ~20%, whereas in conventional OSC, the PCE drops by ~35%. This annealing dependent study shows that, in conventional OSC structure an increase in phase segregation between donor and acceptor molecules reduces exciton dissociation and charge separation, this leads to sharp increase in series resistance and significant reduction in fill factor of the device. Whereas in the case of BL-OSS, there is a minimum reduction in the fill factor, which also determines the superior carrier collection and low recombination on the elevated annealing conditions. Further, the experimental results show that, in comparison with the conventional OSC structure, inserting the pure P3HT interlayer between PEDOT:PSS (hole transport layer) and P3HT:PCBM (photoactive layer) improves the PCE of the device by 34%.

Development of flexible, stable, and efficient inverted organic solar cells harvesting light in all directions

In this work, we designed low-cost, liquid-free, efficient, and highly flexible fiber-shaped inverted structure organic solar cells (FOSCs) over a flexible polyethylene terephthalate (PET) monofilament substrate. We also prepared a graphene–ZnO (G-ZnO) composite, wherein G sheets were compacted into a bunched-up structure through the binding force of Zn atoms with the C atoms of G. This composite was then utilized as a bifunctional layer i.e. electron transport and downconversion spectral in the FOSCs. The FOSCs based on the G-ZnO (D-1) demonstrated a power conversion efficiency (PCE) of 2.13% out of which 4.89% and 5% was retained after 8000 times bending and 120 h storage in ambient environmental conditions, respectively. The non-G-ZnO FOSCs (D-2) demonstrated a PCE of 1.78% and retained 5% and 6% of the initial value after 6000 bends and 48 h of storage in ambient environmental conditions, respectively. This better performance of D-1 compared to that of D-2 is due to the interfacial functionalization of G-sheets and ZnO nanoparticles inside the G-ZnO composite. Because of these interfacial chemical bonds, the G sheets were in close contact with each other and attached firmly through the ZnO molecules. As a result, these compacted G layers could serve as a strong barrier resisting the penetration of water molecules inside the device, thereby leading to an improved lifetime for the device. Additionally, the longitudinal and cross linkage of G-sheets could improve the mechanical properties of the G-ZnO composite, which in turn enhanced the flexibility of D-1. Finally, these interface functionalizations could work as linking bridges, providing an additional pathway for the transportation of free charge carriers. Therefore, D-1 demonstrated a higher Jsc by collecting a greater number of charges at the electrode compared to D-2, because the latter lacked similar functionalization.

Performance Analysis and Thermal Reliability Study of Multilayer Organic Solar Cells

The progress in organic solar cell (OSC) technology mainly focuses on improving the power conversion efficiency (PCE), longer lifetime and the development of scalable cost-effective manufacturing. In this paper, the impact of P3HT layer inserted between PEDOT:PSS hole transport layer (HTL) and P3HT:PCBM active layer has been comprehensively studied. The comparative analysis of the proposed OSC structure with the conventional OSC illustrates that, the multilayer structure significantly improves the short circuit current density ( ${J} _{SC}$ ), fill factor (FF) and PCE of the device. This improvement in the device performance is mainly because of the increased photo absorption, reduction in the recombination losses due to improved vertical phase gradient and the efficient collection of photogenerated charge carriers at the electrodes. In addition to this, in order to determine the impact of thermal stress on the device performance, the thermal reliability study on the proposed multilayer OSC and the conventional OSC has been carried out. It has been found that, with annealing at 140 °C temperature for 10 minutes duration, the maximum PCE of 3.27% and 2.44% has been achieved for proposed multilayer OSC and the conventional OSC respectively. It has also been observed that, in comparison to conventional OSC the proposed multilayer OSC is less susceptible to the thermal degradation. Thus, the proposed multilayer OSC not only improves the device performance but also exhibits better thermal stability under different thermal conditions.

Electrochemical growth of Y doped ZnO nanorods for use in inverted type organic solar cells as electron transport layer

In this study, Yttrium (Y) doped and undoped Zinc oxide (ZnO) nanorods (NRs) were synthesized by the electrochemical method on indium tin oxide (ITO) substrate. Y doped and undoped ZnO-NRs were used as an electron selective layer in inverted type organic solar cell structure. Effect of Y doping in ZnO was investigated via x-ray diffraction (XRD) and scanning electron microscopy (SEM). Poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) was used as an active polymer blend. Inverted type organic solar cells were fabricated with the configuration ‘ITO/Y:ZnO-NRs/P3HT:PCBM/Ag’. Active layer was coated on ZnO-NRs by using drop-casting method to completely fill the gaps between the ZnO-NRs. Short circuit current density of the solar cells was increased from 10.6 to 16.4 mA cm−2 by using 3% Y doped ZnO-NRs as electron selective layer, where as the solar cell efficiency was changed from 2.35% to 3.87%. It was experimentally found that charge injection and selection in the Zn-ONRs were improved by doping the ZnO-NRs with Y atoms.

Influence of material migration on the mechanical integrity of inverted organic solar cells

We have identified modifications of material distributions in inverted organic solar cell structures during accelerated aging processes that are potentially linked to the loss of the mechanical integrity of the devices. Non-encapsulated devices were treated with ISOS-D-3 ageing (the so called damp heat exposure at 85 °C/85% room humidity in the dark). After performing pull-off tests, the exposed surfaces were analyzed by X-ray photoemission spectroscopy and atomic force microscopy. For fresh devices, the results revealed that mechanical failure occurs close to the PEDOT:PSS/P3HT:PCBM interface, in agreement with the literature. However, after a short exposure to damp heat, XPS investigations indicate an enrichment of PCBM at the PEDOT:PSS/P3HT:PCBM interface and a change in the PEDOT:PSS surface composition. Both phenomena may explain the rapid drop in device performances and the increased tendency towards delamination.

Plasmonic Metal Nanoparticles with Core-Bishell Structure for High-Performance Organic and Perovskite Solar Cells.

To maximize light coupling into the active layer, plasmonic nanostructures have been incorporated into both active layers of organic solar cells (OSCs) and perovskite solar cells (PSCs) with the aim of increasing light absorption, but reports have shown controversial results in electrical characteristics. In this work, we introduce a core-bishell concept to build plasmonic nanoparticles (NPs) with metal-inorganic semiconductor-organic semiconductor nanostructure. Specifically, Ag NPs were decorated with a titania/benzoic-acid-fullerene bishell (Ag@TiO2@Pa), which enables the NPs to be compatible with fullerene acceptors or a perovskite absorber. Moreover, coating the Ag@TiO2 NP with a fullerene shell can activate efficient plasmon-exciton coupling and eliminate the charge accumulation, thus facilitating exciton dissociation and reducing the monomolecular recombination. The improved light absorption and enhanced carrier extraction of devices with Ag@TiO2@Pa nanoparticles are responsible for the improved short-circuit current and fill factor, respectively. On the basis of the synergistic effects (optical and electrical), a series of plasmonic OSCs exhibited enhancement of 12.3-20.7% with a maximum power conversion efficiency of 13.0%, while the performance of plasmonic PSCs also showed an enhancement by 10.2% from 18.4% to 20.2%. This core-bishell design concept of plasmonic nanostructures demonstrates a general approach to improving the photovoltaic performance with both optical and electrical contributions.

Improvement of inverted structure organic solar cells by Ar plasma treatment on P3HT:PC61BM active layer

In polymer solar cells, contact between layers and morphology of the layer’s surfaces has great and critical effect on performance of the solar cell. In this research, we have modified P3HT:PCBM active layer surface morphology by mild plasma treatment using cold atmospheric pressure Argon plasma jet to fabricate the inverted structure polymer solar cell and have perused its influence by various powers. At first, without plasma treatment measured efficiency of 2.15% was obtained. At the maximum influence of the plasma treatment efficiency of 3.26% was achieved that its improvement was obvious until a certain plasma power. The morphology of active layer surface was observed by atomic force microscope and devices absorption spectrum before and after plasma treatment was measured by spectrophotometer. Characterization of the plasma-induced surface changes will be performed using ATR-FTIR spectroscopy. Maximum improvement of solar cell characters was obtained in the short circuit current that improves from 7.60 to 9.22 mA cm−2.

Numerical studies of P3HT : PCBM organic solar cell with an additional polymer layer using drift diffusion model

Solar energy can replace the conventional energy sources in the near future. Organic solar cells have been showing tremendous improvement in their device performance for the past few years. Organic materials with their semiconducting properties and mechanical aspects being similar to that of conventional plastic materials present an affordable alternative for new generation solar cells to wipe out the energy crisis. Among the various structures for organic solar cells, bulk heterojunction (BHJ)solar cells have attracted attention for their improved mechanism for electron-hole pair separation, mechanical flexibility etc. In this paper, organic solar cell based on P3HT : PCBM BHJ is studied and the influence of active layer thickness on the device performance is investigated using numerical simulations with the drift diffusion concept. We further analyse the variation of device parameters by adding an additional polymer active layer of donor material P3HT between the BHJ and PEDOT : PSS layer. The numerical simulations are performed using the general purpose photovoltaic device model. The device parameters are assigned with the intention of analysing a solar cell with high mobility and less recombination rate leading to high efficiency. The solar cell shows maximum power conversion efficiency (PCE)of 9.312% when the active layer thickness is 210nm. On adding additional P3HT layer, the optimized cell shows maximum PCE of 11.13% under AM 1.5G spectrum when the polymer active layer thickness is 15nm and BHJ is 195nm

Introducing an identical benzodithiophene donor unit for polymer donors and small-molecule acceptors to unveil the relationship between the molecular structure and photovoltaic performance of non-fullerene organic solar cells

We designed an acceptor ITIC-SF by fluorinating the thiophene ring in the benzodithiophene segment of ITIC-S and investigated its effect on the morphology and performance.

Ether chain functionalized fullerene derivatives as cathode interface materials for efficient organic solar cells

The electron transport layer (ETL) plays a crucial role on the electron injection and extraction, resulting in balanced charge transporting and reducing the interfacial energy barrier. The interface compatibility and electrical contact via employing appropriate buffer layer at the surface of hydrophobic organic active layer and hydrophilic inorganic electrode are also essential for charge collections. Herein, an ether chain functionalized fullerene derivatives [6,6]-phenyl-C61-butyricacid-(3,5-bis(2-(2-ethoxyethoxy)-ethoxy)-phenyl)-methyl ester (C60-2EPM) was developed to modify zinc oxide (ZnO) in inverted structure organic solar cells (OSCs). The composited ZnO/C60-2EPM interface layer can help to overcome the low interface compatibility between ZnO and organic active layer. By introducing the C60-2EPM layer, the composited fullerene derivatives tune energy alignment and accelerated the electronic transfer, leading to increased photocurrent and power conversion efficiency (PCE) in the inverted OSCs. The PCE based on PTB7-Th: PC71BM was enhance from 8.11% on bare ZnO to 8.38% and 8.65% with increasing concentrations of 2.0 and 4.0 mg/mL, respectively. The fullerene derivatives C60-2EPM was also used as a third compound in P3HT:PC61BM blend to form ternary system, the devices with addition of C60-2EPM exhibited better values than the control device.

Physical properties of 2D and 3D ZnO materials fabricated by multi-methods and their photoelectric effect on organic solar cells

Abstract ZnO material is a crucial layer for organic solar cell due to its excellent photoelectric properties. However, the influence of ZnO fabricated by various methods as well as the effect of 2-dimensional (2D) and 3-dimensional (3D) configuration is still under debate. In this study, a comprehensive study of 2D ZnO made by sol-gel, atomic layer deposition (ALD) method, 3D ZnO nanorods grown from sol-gel seed layer and ALD seed layer was carried out. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the morphology of the films. X-ray photoelectron spectroscopy (XPS) were used to probe bonding nature as well as defects present in different forms of ZnO. Band gap and crystal quality were characterized by UV-vis spectra. The inverted structure of organic solar cells was fabricated using these forms of ZnO, and the I-V curves as well as the power conversion efficiency (PCE) were measured to evaluate the photoelectric property of the synthesized ZnO nanostructures. It is found that 2D ZnO made by the sol-gel method yields the best PCE of 2.53%.

Spirobifluorene-cored wide bandgap non-fullerene small molecular acceptor with 3D structure for organic solar cells

In this paper, a wide bandgap small molecule (SF(BR)4) based on spirobifluorene (SF) core, which is flanked with 2,1,3-benzothiadiazole and end-capped with 3-ethylrhodanine groups is synthesized and investigated as a non-fullerene acceptor in solution-processed organic solar cells (OSCs). The SF(BR)4 shows a wide optical bandgap of 2.10 eV and great advantages with respect to the low bandgap donor material poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)2,2′; 5′,2″; 5″,2‴-quaterthiophen-5,5‴-diyl)] (PffBT4T-2OD), such as well energy-level matching and complementary absorption. Furthermore, the three dimensional (3D) molecular geometry of SF(BR)4 helps to suppress the strong intermolecular aggregation in PffBT4T-2OD:SF(BR)4 blend, leading to efficient non-fullerene OSCs. The resultant devices show a power conversion efficiency (PCE) of 6.15%. To our best knowledge, the PCE is among the highest reported to date for 3D non-fullerene acceptors except perylene diimides derivatives.

Profiling exciton generation and recombination in conventional and inverted bulk heterojunction organic solar cells

Applying the optical transfer matrix method and the drift-diffusion equations, the efficient light absorption, exciton generation, and recombination rate in bulk heterojunction (BHJ) organic solar cells (OSCs) with conventional and inverted configurations are studied. Analysing the influence of the electric field component of the electromagnetic radiation propagating through the layered structure of BHJ OSCs and using contour plots of normalized modulus squared of the electric field, the constructive interference points (CIPs) which represent the positions of maximum absorption of photons and hence generation of excitons within the active layer are investigated for both the conventional and inverted OSCs. Also, the influence of the thicknesses of other layers in both the inverted and conventional structures is investigated. It is found that except the thickness of MoO3 in the inverted structure the thicknesses of other layers do not have any significant influence on CIPs. The maximum CIP occurs at an active layer thickness of 190 nm, regardless of the thickness of the second layer, which is MoO3, Ag, or ITO in the inverted structure and PEDOT:PSS, Al, or ITO in the conventional structure. The results of 3D plots of the normalized modulus squared of the electric field reveal that the absorption of photons at the end of the active layer in the inverted structure is higher than that in the conventional structure for all the effective wavelengths and different active layer thicknesses. It is expected that this study provides a deeper understanding of exciton generation within the two structures.

Simultaneously improving efficiency and transparency of semitransparent organic solar cells by constructing semitransparent microcavity

Semitransparent microcavity is constructed by sandwiching an active layer between the WO3/Ag/WO3 multilayer electrode and the thin Ag electrode capped by one-dimensional photonic crystals (1DPCs) to simultaneously improve light absorption and transmission of the semitransparent organic solar cells (OSCs). Optical simulations demonstrate that the upper limit to power conversion efficiency (PCEmax) of the semitransparent OSCs is improved up to 8.37%, an improvement of 14.0% from that of the conventional device without the microcavity; simultaneously, the transparency of the device reaches a relatively high value of 21.7% with an improvement of 8.0%. In addition, it is revealed that PCEmax, transparency and see-through color of the devices bear strong relevance to the photonic bandgap of 1DPCs. Transparency for the semitransparent OSCs can be tuned from 2.4% to 33% and the see-through color of the semitransparent OSCs can be tuned in a broad range by tailoring the center wavelength of the photonic bandgap. Results from the paper demonstrate the semitransparent microcavity is an efficient light trapping structure for semitransparent OSCs and can find application in the design and fabrication of semitransparent OSCs with high efficiency and high transparency.

Homo-tandem structures to achieve the ideal external quantum efficiency in small molecular organic solar cells.

In this study, we report a homo-tandem structure of small molecular organic solar cells (SMOSCs) using identical single-junction devices as sub-cells. The trade-off between the absorption and internal quantum efficiency (IQE) of single-junction devices tends to limit the external quantum efficiency (EQE). However, multiple-stacked thin cells with maximized IQE in homo-tandem structures amplify the absorption to achieve the ideal EQE. As a result, a high power conversion efficiency of 7.81% is achieved in tetraphenyldibenzoperiflanthene (DBP):C70-based homo-tandem SMOSCs, which is 21.8% higher than that in a single-junction device.

Significance of Polymeric Nanowire-Network Structures for Stable and Efficient Organic Solar Cells

Evolution of favourable nanomorphology which can withstand external stimuli is a critical issue for efficient and stable organic solar cells. Here, we demonstrate a novel strategy for the stabilization of nanomorphology of organic solar cells by inducing polymeric nanowire network structures. Thermal annealing of poly(3- hexylthiophene-2,5-diyl) nanowires, highly crystalline, 1-dimensional structures held together through interchain π-π stacking, led to the formation of nanowire network structures confirmed through small angle neutron scattering measurements. The physically interconnected network structures form robust electron donor domains and impose confinement which suppresses the aggregation of the electron acceptor, [6,6]-phenyl-C61-butylric acid methyl ester. Organic solar cells having the nanowire network structures showed increased power conversion efficiencies and dramatically enhanced thermal stability compared to bulk heterojunction (BHJ) and non-network nanowire-based devices. Furthermore, the performance of the nanowire network-based devices was inversely related to the size of the networks, attesting to the significance of nanoconfined geometry formed within nanowire network structures.