Gradient heating dissolution of active materials enhances the performance of organic solar cells

Chapter 11 - Organic Solar Cells Enhanced by Carbon Nanotubes

The performance of organic solar cells (OSCs) is determined by the delicate, meticulously optimized bulk-heterojunction microstructure, which consists of finely mixed and relatively separated donor/acceptor regions. In this work we examine the reliability and stability of bulk-heterojunction (BHJ) microstructures of a highly-efficiency OSC based on PCE11 as the donor and PCBM as the acceptor. The so called burn-in degradation is identified as a spinodal de-mixing due to the low miscibility of donor and acceptor, which is turned out to be a major challenge for the development of stable and efficient OSCs. Even though the microstructure can be kinetically tuned for achieving high-performance, the inherently low miscibility of donor and acceptor leads to spontaneous phase separation in the solid state, even at room temperature and in the dark. The construction of the spinodal phase diagrams highlight molecular incompatibilities between the donor and acceptor as a dominant mechanism for burn-in degradation, which is to date the major short-time loss reducing the performance and stability of organic solar cells [1].

We have studied the effect of spherical SiO2 nanoparticles with sizes of 20, 50, and 80 nm embedded in a PEDOT : PSS buffer layer on the performance of organic solar cells (OSCs) based on star-shaped oligomers. The current–voltage characteristics and absorption spectra of samples have been measured and analyzed; the morphology of the buffer layer surface with embedded nanoparticles has been studied. It has been shown that an increase in the OSC performance occurs in the case of embedded nanoparticles with a diameter of 20 and 50 nm and weakly depends on the concentration of the nanoparticles in the layer.

We report the enhanced performance of organic solar cells (OSCs) based on regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend by using dihydroxybenzene as additive in the active layer. The effect of the content of the additives on electrical characteristics of the device is studied. The device with 0.2 wt% dihydroxybenzene additive achieves the best power conversion efficiency (PCE) of 4.58% with Jsc of 12.5 mA/cm2, Voc of 0.65 V, and FF of 66.6% under simulated solar illumination of AM 1.5G (100 mW/cm2), compared with the control device with PCE of 3.39% (35% improvement compared with the control device). The XRD measurement reveals that the addition of additives induces the crystallization of P3HT and establishes good inter-network to increase the contact area of donor and acceptor, and then helps charge to be effectively transferred to the electrode to reduce the chance of recombination. All evidences indicate that the dihydroxybenzene is likely to be a promising new type additive that can enhance the performance of organic bulk heterojunction solar cells.

The performance of solution-processed organic solar cells is often limited by various recombination pathways. In this scenario ideality factor (nid) act as a signature for identification of different loss mechanisms in these solar cells. A detailed analysis of open circuit voltage (Voc) dependence on illumination intensity as well as electroluminescence (EL) dependence on the injected currents can provide the value of ideality factor. We have carried out this study for identification and quantification of dominant recombination loss mechanism in crystalline Si and organic solar cells. The obtained ideality factor by these two methods support each other which validates the result. On the basis of these studies, it has been concluded that the major energetic loss in bulk heterojunction (BHJ) based organic solar cell arises due to its low radiative efficiency (QLED) and non-radiative recombination loss mechanism at donor/acceptor interface and semiconductor/electrode interfaces. So the performance of the organic solar cells can be improved by optimizing the semiconductor-electrode energy alignment, and surface morphology near contacts

Organic solar cells have attracted considerable interest due to their great potential for the production of flexible and large-area solar cells at relatively low costs and easy-processing fabrication properties [1–2]. The present work reports the effect of organic salt, tetrabutylammonium hexafluorophosphate (TBAPF 6 ) doping on the performance of single layer bulk heterojunction organic solar cell with ITO/MEHPPV:PCBM/Al structure where indium tin oxide (ITO) was used as anode, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor, (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor and aluminium (Al) as cathode. The active layer was prepared by spin coating technique. The organic solar cells were characterized by current-voltage measurements under illumination with a halogen projector lamp at 100 mW/cm2 using Keithley 237 source measurement unit. As shown in Fig.1, the device doped with TBAPF 6 demonstrated a significant increment in the short circuit current density, J sc and open circuit voltage, V oc as compared to the undoped device. Under illumination of a halogen projector lamp at 100 mW/cm2, the undoped device showed a J sc of 0.54 µA/cm2, V oc of 0.24 V, and a fill factor (FF) of 16%. With the doping of TBAPF 6 , the J sc increased almost ten times to 6.41 µA/cm2. Besides, the V oc also improved significantly from 0.24 V to 0.50 V. The significant improvement was attributed to the increase of built-in electric field caused by accumulation of ionic species at the electrode/active layer interfaces. Therefore, TBAPF 6 doping has been shown to be a simple and cost-effective approach to increase the performance of organic solar cell.

D-A-D type small molecule donors based on BODIPY skeleton for bulk heterojunction organic solar cells

Annealing dependent performance of organic bulk-heterojunction solar cells: A theoretical perspective

To improve the performance of organic solar cells, various methods have been used to increase the light absorbance and electron transfer efficiency or decrease the internal resistance of the device. In this article, red dyes of phosphorescent materials are used to improve the performance of bulk heterojunction organic solar cells based on MDMO-PPV and PCBM. Solar cell devices doped with different red dyes showed higher performances in terms of current, voltage and conversion efficiency than those without red dyes. The efficiency was maximized in the devices with a 10% concentration of red dye 2, which was attributed to the longer exciton lifetimes that were induced by the triplet spin state of the red dyes allowing them to reach the p-n junction and thereby generate more photocurrent.

The performance of organic solar cells has improved significantly in recent years due to the use of non-fullerene acceptors (NFA). While processing additives are typically added to the active layer blends to enhance device performance in NFA organic solar cells, their impact on device degradation remains unclear. In this work we have compared the performance, in pristine and degraded state, between air-processed slot-die coated NFA ITO-free organic solar cells with and without the processing additive DIO, using a structure of PET/Ag/ZnO/PBDB-T:ITIC/FHC PEDOT:PSS. We observed an improvement in the power conversion efficiency of the devices when adding DIO, from 4.03% up to 4.97%. The evolution of the performance for both devices under ISOS-L1 life testing protocol reveals that the drop in efficiency is mainly due to a decay of JSC for both cells. In the short time scale the efficiency of non-DIO cells decays faster than the DIO cells, whereas in the long time scale the efficiency of non-DIO cells tends to stabilize sooner. Carrier mobilities estimated from impedance measurements decrease with time at similar rate for both degraded samples. Besides, DIO devices present a steep increase of the series resistance with time causing a decrease of the FF and thus of the efficiency. Moreover, in both degraded devices, the open-circuit voltage saturates with increasing illumination intensity. Numerical simulations reveal that a reduced anode work function of 5 eV is needed to fit experimental data.

We present our investigation of organic solar cells by modeling and simulation after numerically solving Poisson and continuity equations that describe the electric property of semiconductors. Specifically, simulations reveal that Langevin type recombination, which describes the loss mechanism in pristine materials with low mobility, is not proper to predict the performance of BHJ organic solar cells and will lead counterintuitive simulation results. Then, the recombination mechanism has been studied in bulk heterojunction (BHJ) organic solar cells by simulating intensity-dependent current-voltage (J-V) measurements. The simulation results indicate that primary loss mechanism is monomolecular recombination. Moreover, the unbalanced carrier transport in organic solar cells is explored and the simulation suggests that increasing hole mobility is an effective method to improve the performance of organic solar cells.

The use of processing additives is one of the most widely used approaches to optimizing the bulk heterojunction (BHJ) morphology for boosting the photovoltaic performance of organic solar cells (OSCs). Recently, insulating polymer-based additives have drawn great attention owing to the merits of good morphology-directing abilities, simple post treatments, rich functionality and enhanced device stabilities. In this contribution, a known fluoropolymer poly-p-trifluoromethylstyrene (PTFS) is selected as a solid additive. Employing PTB7-Th:PDI2 (ethylene-annulated perylene diimide dimer) as the prototype active layer, the devices processed with PTFS shows a best power conversion efficiency (PCE) of 6.64 %, which is the highest efficiency for nude PDI2-based OSCs. For comparison, the photovoltaic performance OSCs processed with polystyrene (PS) or without additive were investigated, PCEs of 5.29 % and 5.76 % were obtained, respectively. It is worth noting that the utilization of PTFS can also improve the thermal stability of the devices pronouncedly. Moreover, PTFS based additive also displays enhanced performance in other devices with different active layers, illustrating a good universality in OSCs. The enhancement of the PCE and thermal stability may result from the relatively lower surface energy of PTFS, which can boost the light absorbance capability as well as promote a favorable phase separation of the active layer.

The effect of nanoimprinted structures on the performance of organic bulk heterojunction solar cells was investigated. The nanostructures were formed over the active layer employing the soft lithographic technique. The measured incident photon-to-current efficiency revealed that the nanostructured morphology over the active layer can efficiently enhance both light harvesting and charge carrier collection due to improvement of the absorption of incident light and the buried nanostructured cathode, respectively. The devices prepared with the imprinted nanostructures exhibited significantly higher power conversion efficiencies as compared to those of the control cells.

Improving the photovoltaic performance directly by innovative device architectures contributes much progress in the field of organic solar cells. Photovoltaic device using different kinds of heterojunction with the given set of organic semiconductors paves the way to a better understanding of the working mechanism of organic heterojunction. Here, we report on the fabrication of a new device structure without employing extra material. A thin film of the donor material (chloroaluminum phthalocyanine (ClAlPc)) is inserted between ClAlPc:C60 bulk heterojunction (BHJ) and C60 layer by glancing angle deposition. A ClAlPc/C60 planar heterojunction co-exists with ClAlPc:C60 BHJ simultaneously in this device. Higher efficiency is obtained with this novel device structure. The effects of this additional ClAlPc layer on open-circuit voltage and fill factor in photovoltaic cells are studied. This work provides a new route to improve the device performance of organic solar cells.

Al nanoparticles (NPs) are incorporated in the active layers to enhance the performance of organic solar cells (OSCs). The improved short circuit current Jsc and power conversion efficiency (PCE) for OSCs with Al NPs are observed. A final PCE of 3.66wt% is achieved, which is an improvement of more than 30wt% compared to a standard cell with a PCE of 2.84wt%. When the mass of Al NPs is 10wt% of OSCs, the device performance is best. The optical performance of OSCs reveals that the absorption from sunlight increases. The external quantum efficiency spectra suggests that the Al NPs in the active layers influence the efficiency of converting photons into electrons, which leads to the improvements in the photocurrent. The enhanced photovoltaic performance induced by incorporating Al NPs in the active layer is discussed in the terms of increasing charge separation at the donor-acceptor interface and the effectively decreasing transmission distance of charge in polymer.