Small Molecule Solution-Processed Bulk Heterojunction Solar Cells

Oligothiophene-based photovoltaic materials for organic solar cells: rise, plateau, and revival

The morphology of organic bulk heterojunction (BHJ) solar cells decisively influences the device performance and efficiency and therefore is an important factor that needs to be investigated to gain a better understanding and improvement of the devices. Especially the nanoscale morphology of the active layer plays an important role as it determines the charge separation at the interfaces and the percolation pathways to the electrodes. This nanoscale morphology depends not only on the involved materials but also on their molecular weight and their treatment like thermal annealing and solvent vapor annealing. Transmission electron microscopy (TEM) is an approved technique to study the morphology of organic solar cells. Due to the similarity of the organic materials involved in the BHJ active layer regarding the chemical composition and the formation of homogeneously thin films, the contrast in TEM images is often uniform and no significant structures can be seen. Thus conventional imaging techniques are often not sufficient to identify and distinguish the polymers and the fullerene derivatives. Here we will demonstrate that energy‐filtered TEM (EFTEM) is a powerful technique to visualize the material distribution in organic BHJ active layers. We present three concepts using different information gained in EFTEM investigations: i) the elemental information, ii) the plasmonic information, and iii) pre‐carbon imaging. We demonstrate that the results of these three concepts are in good agreement and that the morphology can be reliably and consistently determined using EFTEM. To corroborate the reliability of these three concepts we present different material systems. Figure 1 shows the results of the EFTEM investigation for a P3HT:PCBM BHJ film. Due to the different plasmon energies of the materials (P P3HT = 21.9 eV, P PCBM = 24.5 eV) the respective plasmonic energy regions represent P3HT and PCBM in the blend. The elemental maps of sulfur and carbon are used to represent P3HT and PCBM, respectively, due to the different elemental compositions (S P3HT = 4.0 at%, S PCBM = 0.0 at%, C P3HT = 40.0 at%, C PCBM = 81.8 at%). Additionally, the pre‐carbon image represents P3HT as the carbon signal is suppressed and the sulfur signal enhanced. These EFTEM investigations clearly elucidate the morphology of this blend exhibiting P3HT fibers with diameters of 10 nm. Comparing the results of the three concepts clearly shows the good agreement of the determined morphology. Furthermore, we demonstrate the capabilities of low‐energy scanning transmission electron microscopy (STEM). STEM at low electron energies exhibits enhanced material contrast and can therefore be used to visualize the material distribution of polymers and fullerene derivatives in a BHJ film. Figure 2 shows a STEM BF image of the same P3HT:PCBM BHJ film at a high tension of 15 kV. The P3HT fibers are clearly visible and consistent with the EFTEM investigation. The origin of material contrast will be discussed. Low‐energy STEM is a highly promising alternative for determination of the morphology of organic BHJ solar cells as it features a high throughput SEM based technique. Using EFTEM and low‐energy STEM the morphology of various organic BHJ solar cells can be elucidated, leading to a better understanding and improvement of the device performance.

Benzodithiophene-Based Small-Molecule Donors for Next-Generation All-Small-Molecule Organic Photovoltaics

The performance of organic bulk heterojunction (BHJ) solar cells depends strongly on the nanoscale morphology formed by the donor and acceptor materials. However, the majority of device models for organic BHJ solar cells are based on an effective-medium formulation that does not capture details of the underlying morphology. In order to link more detailed models with effective-medium models, we derive a spatially smoothed formulation for organic BHJ solar cells based on volume-averaging of a mathematical model that considers charge carrier transport, generation, and recombination in both the acceptor and donor phases. The formulation captures two essential morphological characteristics of the organic BHJ layer that are not found in existing effective-medium models: the effective interfacial area and the volume fraction ratio between donor and acceptor materials. In addition, effective charge carrier mobilities and diffusion coefficients are identified, which are determined for an “ideal” interpenetrated BHJ solar cell.

Layer-by-layer (LBL) sequential solution processing of the active layer has been proven as an effective strategy to improve the performance of organic solar cells (OSCs), which could adjust vertical phase separation and improve device performance. Although perylene diimide (PDI) derivatives are typical acceptors with excellent photoelectric properties, there are few studies on PDI-based LBL OSCs. Herein, three PDI acceptors (TBDPDI-C5, TBDPDI-C11, and SdiPDI) were used to fabricate LBL and bulk heterojunction (BHJ) OSCs, respectively. A series of studies including device optimization, photoluminescence (PL) quenching, dependence of light intensity, carrier mobility, atomic force microscopy (AFM), transmission electron microscopy (TEM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and depth analysis X-ray photoelectron spectroscopy (DXPS) were carried out to make clear the difference of the PDI-based LBL and BHJ OSCs. The results show that LBL OSCs possess better charge transport, higher and more balanced carrier mobility, less exciton recombination loss, more favorable film morphology, and proper vertical component distribution. Therefore, all the three PDI acceptor-based LBL OSCs exhibit higher performance than their BHJ counterparts. Among them, TBDPDI-C5 performs best with a power conversion efficiency of 6.11% for LBL OSCs, higher than its BHJ OSC (5.14%). It is the first time for PDI small molecular acceptors to fabricate high-efficiency OSCs by using an LBL solution-processed method.

Star-shaped benzotriindole-based donor-acceptor molecules: Synthesis, properties and application in bulk heterojunction and single-material organic solar cells

Charge transfer and charge extraction mechanisms are two prevalent issues in the growing field of organic solar cells. Due to their complexity in nature, new methods need to be involved in addressing the fundamental properties associated with organic polymer solar cells. This dissertation has focused on developing a new method to estimate the charge collection lengths and surface recombination lengths of organic polymer solar cells. Photocurrent spectra have been analyzed systematically to observe the dependence on thickness of the active material. A red shift of the peak of the normalized photocurrent with respect to the device thickness has been further analyzed for two major material systems used in organic polymer solar cells, namely MDMO-PPV: PCBM and P3HT: PCBM. A theoretical model that measures the charge extraction of bulk hetero junction solar cell structures has been used taking into account of three main parameters including charge carrier collection length, absorption variation and surface recombination. This model has led to estimate two important parameters associated with charge transfer, recombination and extraction of organic solar cells which will provide opportunities for improvements in the performance of organic electronic devices. Key results are summarized as follows. A complete analysis of photocurrent spectra has been done to see its variation with active material thickness of well-known two material systems of bulk heterojunction organic solar cells. Results of these preliminary measurements suggest that peak of the photocurrent for both systems red shift with increasing thickness. Charge extraction model is introduced to explain the initial red shift of the photocurrent. This model fits well with the experimental results. Further analysis of the model suggests that the charge collection lengths can be estimated for organic polymer structures. Theoretical model gives higher collections lengths for MDMO-PPV solar cells while a lower collection length for P3HT solar cells. This model also has the capability to estimate the surface recombination length of organic bulk heterojunction solar cells. Different interfacial layers have been used to fit to the model calculation. These results suggest that the least surface recombination lengths were achieved with solar cells of PEDOT-PSS. This method can be used to optimize the interfacial layers to improve the efficiency in organic solar cells. AC photocurrent measurements have been carried out to observe the frequency dependence of organic solar cells. Main results show that increasing response time from the light source increases the performance of the solar cells. Further analysis of these

The production of electricity by harnessing the energy of the sun is done at almost no pollution and at very low cost. Due to the economical and ecological advantages, organic solar cells are evolved. This paper presents the affect on power conversion efficiency (PCE) by adding an active layer in a bulk heterojunction (BHJ) organic solar cell along-with one thin intermediate layer. Firstly, the efficiency is demonstrated by single layer BHJ with P3HT:PCBM material as an active layer. Furthermore, one additional active layer of MEH-PPV:PCBM material is added in BHJ with former material, an intermediate layer of organic material hexdecafluoro copper phthalocyanine (F 16 CuPc) is also introduced and the affect on efficiency is studied using Silvaco Atlas TCAD. Also, aluminium-doped-zinc-oxide (AZO) material which is a transparent-conducting-oxide (TCO) is used as anode layer. The Power-Conversion-Efficiency (PCE) of a solar cell depends on short-circuit-current-density (Jsc), open-circuit-voltage (Voc) and fill_factor (FF). The increment of the short circuit current is observed in multilayer in comparison to BHJ. Therefore, PCE is increased from 7.4% to 10.3%. Hence, the addition of active layer in BHJ increases the PCE of organic solar cell or photovoltaic cell (OPV).

The optimized thicknesses of the active individual layers in organic thin film solar cells are obtained using optical admittance analysis method (OAAM). We have used OAAM to simulate the optical properties of two bulk-heterojunction (BHJ) organic solar cells (OSCs) of structures: (1) ITO/PEDOT:PSS/P3HT:PCBM/Lif/Al and (2) ITO/PTB7:PCBM/Lif/Ag. The optimal thicknesses of 75 nm and 115 nm of P3HT:PCBM and PTB7:PCBM blend layers, respectively, are obtained by maximising the absorbance in these layers through this simulation, which agree very well with the experimental results. The simulated short-circuit current density J SC is plotted as a function of the active layer thickness for a few selected thicknesses of the Al cathode in these two OSCs and it is found that J SC becomes maximum when the thickness of Al cathode is 40 nm. Using these optimised thicknesses of the active layers in these two cells the short-circuit current density is found to increase in ITO/PEDOT:PSS/P3HT:PCBM/Lif/Al BHJ OSC by 4.8% and in ITO/PTB7:PCBM/Lif/Ag by 13.3%.

Numerical modeling of organic bulk heterojunction (BHJ) solar cells has been undertaken using the AMPS computer code and material parameters representative of the P3HT/PCBM cell. These simulations show that the V OC and thus the efficiency of these BHJs is currently controlled by interface recombination. These results show V OC tracks with the HOMO (D) − LUMO (A) difference with a relationship of the form V OC = V I + m[HOMO (D) − LUMO (A)], as is seen experimentally. The AMPS simulations show that the built-in potential and the contact barrier heights surprisingly do not affect V OC in these cells. These do, however, affect the cell fill factor FF and efficiency. If interface recombination is suppressed, then V OC values larger than HOMO (D) − LUMO(A) difference are possible since band bending must occur at the heterojunction to sustain the bulk and contact recombination that balances generation at open circuit. With interface recombination suppressed and contacts chosen for optimum FF, this modeling shows BHJs are capable of power conversion efficiencies of about 10% using material parameters corresponding to a P3HT/PCBM cell.

One of the key parameters in determining the power conversion efficiency (PCE) of bulk heterojunction (BHJ) organic solar cells (OSCs) is the open circuit voltage . The processes of exciting the donor and acceptor materials individually in a BHJ OSC are investigated and are found to produce two different expressions for . Using the contributions of electron and hole quasi-Fermi levels and charge carrier concentrations, the two different expressions are derived as functions of the energetics of the donor and acceptor materials and the photo-generated charge carrier concentrations, and calculated for a set of donor-acceptor blends. The simultaneous excitation of both the donor and acceptor materials is also considered and the corresponding , which is different from the above two, is derived. The calculated from the photoexcitation of the donor is found to be somewhat comparable with that obtained from the photoexcitation of the acceptor in most combinations of the donor and acceptor materials considered here. It is also found that the calculated from the simultaneous excitations of donor and acceptor in BHJ OSCs is also comparable with the other two . All three thus derived produce similar results and agree reasonably well with the measured values. All three depend linearly on the concentration of the photoexcited charge carriers and hence incident light intensity, which agrees with experimental results. The outcomes of this study are expected to help in finding materials that may produce higher and hence enhanced PCE in BHJ OSCs.

Sequential deposition method processed ternary organic solar cells with efficiency of 17.92%

Plasmonic effects associated with localized surface plasmon (LSP) resonances such as strong light trapping, large scattering cross-section, and giant electric field enhancement have received much attention for the more efficient harvesting of solar energy. Notably, even as the thickness of the active layer is significantly reduced, the optical absorption capability of a solar cell could be maintained with the incorporation of plasmonic effects. This is especially important for the development of bulk heterojunction (BHJ) organic solar cells (OSCs), where the short exciton diffusion length, low carrier mobility, and strong charge recombination in organic materials strongly favors the use of optically thin active layers (<100 nm). However, the disappointing performance improvements obtained with plasmonic effects in the majority of BHJ OSCs realized to date suggests that plasmonic effects are yet to be fully taken advantage of; for example, in thick active layer OSCs (>100 nm), the optical absorption is already high, even in the absence of plasmonic effects, while in thin active layer OSCs (<100 nm), insufficient attention has been given to the analysis of plasmonic effects, such as the impact of plasmonic nanoparticle (NP) geometrical factors on the directional scattering efficiency. In this paper, we propose and demonstrate that the geometrical tuning of spheroidal plasmonic nanoparticles (NPs) could enable the full exploitation of plasmonic effects, providing dramatic improvements to the light absorption and energy harvesting capability of ultrathin film BHJ OSCs. Our theoretical analysis demonstrates a dramatic enhancement in optical absorption of ∼60% with spheroidal NPs embedded in a BHJ OSC device with ultrathin, <100 nm active layer, as compared to an NP absent reference device. These improvements are explained according to enhanced scattering of light into the active layer plane, spectral broadening of absorption resonances, in addition to an increased plasmonic modal volume, exhibited near LSP resonances of spheroidal NPs with optimal eccentricity. The result of our coupled optical-electrical device simulations also proves that the outstanding optical absorption enhancement obtained from the proposed device indeed translates into significant electrical performance gains; such as a ∼30% increase in the short-circuit current and ∼20% improvement in the power conversion efficiency (PCE).

We have simulated the effect of changing the end groups in BTP core with five organic units of 1,3-Indandione (IN), 2-thioxothiazolidin-4-one (Rhodanine), propanedinitrile (Malononitrile), (2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile) (CPTCN) and 2-(3-oxo-2,3-dihydroinden-1-ylidene (IC), and two halogenated units of (4F) IC and (4Cl) IC on the optical and photovoltaic properties of the BTP DA’D core molecular unit. Thus modified, seven molecular structures are considered and their optical properties, including HOMO and LUMO energies and absorption spectra are simulated in this paper. On the basis of HOMO and LUMO energies, it is found that two of the seven molecules, BTP-IN and BTP-Rhodanine, can act as donors and the other four, BTP-(4F) IC, BTP-(4Cl) IC, BTP-CPTCN and BTP-IC, as acceptors in designing bulk heterojunction (BHJ) organic solar cells (OSCs). Using these combinations of donors and acceptors in the active layer, eight BHJ OSCs, such as BTP-IN: BTP-(4F) IC, BTP-IN: BTP-(4Cl) IC, BTP-IN: BTP-CPTCN, BTP-IN: BTP-IC, BTP-Rhodanine: BTP-(4F) IC, BTP-Rhodanine: BTP-(4Cl) IC, BTP-Rhodanine: BTP-CPTCN and BTP-Rhodanine: BTP-IC, are designed, and their photovoltaic performance is simulated. The photovoltaic parameters Jsc, Voc and FF for all eight BHJ OSCs and their power conversion efficiency (PCE) are simulated. It is found that the BHJ OSC of the BTP-IN: BTP-CPTCN donor–acceptor blend gives the highest PCE (14.73%) and that of BTP-Rhodanine: BTP-(4F) IC gives the lowest PCE (12.07%). These results offer promising prospects for the fabrication of high-efficiency BHJ OSCs with the blend of both donor and acceptor based on the same core structure.

Bulk heterojunction (BHJ) organic solar cells (OSCs) have been experiencing larger attention because of its advantages in terms of the low-cost, light-weight, and flexibility. In BHJ organic solar cells, half of the total efficiency lost in the midst of all energy passage ways due to the photo generated charge carrier recombination within OSCs. So in order to mark this issue, we introduce spinel ferrites magnetic nanoparticles (MNPs) such as super paramagnetic and ferromagnetic MNPs. Additionally; these superparamagnetic and ferromagnetic MNPs are doped at active layer of OSCs and further improve the photovoltaic performance of the cell. However with the increase in efficiency, the enhancement in the short circuit current and reduction in the open circuit voltage of OSC has been observed in previous papers. In this paper, various essential parameters of the OSCs are explained and the role of spinel ferrites in the improvement of photovoltaic performance has been reviewed.