PCBM-based Bulk Heterojunction Research Articles

Theoretical Performance Analysis of Inverted P3HT: PCBM Based Bulk Hetero-Junction Organic Solar Cells through Simulation

In this study, the role of active layer thickness, hole transport layer thickness, and electron mobility on the performance of P3HT: PCBM-based inverted organic solar cells has been investigated. The simulation has been done for device structure ITO/ZnO/P3HT: PCBM/MoO3/Ag using the general-purpose photovoltaic device model (GPVDM) program tool. The short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) of the cell were determined by varying the thickness of the active layer from 140 nm to 260 nm, the hole transparent layer from 10 nm to 40 nm, and electron mobility from 0.5 × 10−3 cm2V−1s−1 to 6.5 × 10−3 cm2V−1s−1. The PCE improvement was observed at 220 nm and 20 nm active layer and hole transporting layer thickness, respectively, for 4.5 × 10−3 cm2V−1s−1 electron mobility. The results confirmed that the thickness of the active layer, hole transport layer, and charge carrier mobility plays an important role in the performance improvement of organic solar cells.

Review on performance analysis of P3HT:PCBM-based bulk heterojunction organic solar cells

Due to their low production costs, small weight, printability, solution processing, and the possibility of using flexible substrates, organic solar cells (OSCs) exhibit strong potential to be used in future solar cell technology. Major specifications for the commercialization of OSCs are the long life span of OSC devices, enhanced environmental stability and a sufficiently high power conversion efficiency (PCE). The development of low energy gap organic polymers, transport materials and multi-layer system architecture has been effective in achieving higher PCE. However, overcoming environmental stability and sustaining a longer life span are a major challenge. The instability of OSCs is the dynamic mechanism, mostly induced due to the combined impact of oxygen, incident light, and ambient, as well as processing, temperature. Studies have reported that thermal annealing of the active layer tends to a shift in the phase morphology, which typically occurs either because of the vertical phase separation or because of the process of donor–acceptor phase segregation. In this paper, various factors that affect the performance of the OSC have been comprehensively studied. Factors such as thermal stress, vertical phase segregation, material composition and tradeoff between thickness and light absorption have been discussed. This paper mainly reviews the measures carried out to improve the efficiency of conventional polymer-fullerene-based bulk-heterojunction OSCs and reports the different techniques to address these issues.

Thickness Optimization and Photovoltaic Properties of Bulk Heterojunction Solar Cells Based on PFB–PCBM Layer

We report on the fabrication and study of bulk heterojunction (BHJ) solar cells based on a novel combination of a donor–acceptor poly(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N0-diphenyl)-N,N′di(p-butyl-oxy-pheyl)-1,4-diamino-benzene) (PFB) and [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) blend composed of 1:1 by volume. indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate (PEDOT:PSS)/PFB–PCBM/Ag BHJ solar cells are fabricated by a facile cost-effective spin-coating technique. The thickness of the active film (PFB–PCBM) plays an important role in the efficiency of light absorption, exciton creation, and dissociation into free charges that results in higher power conversion efficiency (PCE). In order to optimize the PCE as a function of active layer thickness, a number of solar cells are fabricated with different thicknesses of PFB–PCBM films at 120, 140, 160, 180, and 200 nm, and their photovoltaic characteristics are investigated. It is observed that the device with a 180 nm thick film demonstrates a maximum PCE of 2.9% with a fill factor (FF) of 53% under standard testing conditions (STC) (25 °C, 1.5 AM global, and 100 mW/cm2). The current–voltage (I-V) properties of the ITO/PEDOT:PSS/PFB–PCBM/Ag BHJ devices are also measured in dark conditions to measure and understand different parameters of the heterojunction. Atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) absorption spectroscopy for the PFB–PCBM film of optimal thickness (180 nm) are carried out to understand the effect of surface morphology on the PCE and bandgap of the blend, respectively. The AFM micrographs show a slightly non-uniform and rough surface with an average surface roughness (Ra) of 29.2 nm. The UV-vis measurements of the PFB–PCBM blend exhibit a reduced optical bandgap of ≈2.34 eV as compared to that of pristine PFB (2.88 eV), which results in an improved absorption of light and excitons generation. The obtained results for the ITO/PEDOT:PSS/PFB–PCBM (180 nm)/Ag BHJ device are compared with the ones previously reported for the P3HT–PCBM blend with the same film thickness. It is observed that the PFB–PCBM-based BHJ device has shown two times higher open circuit voltage (Voc) and, hence, enhanced the efficiency.

Investigation of Non-ideality Factors for a P3HT: PCBM Based Bulk Heterojunction Organic Solar Cell in Presence of Silver Nanoparticles

Solar cells are a potential option to meet the growing energy requirements of humans. Organic solar cells (OSCs) represent a class of solar cells that is a part of the third generation solar cell technology. The quest for obtaining enhanced OSC efficiencies has led to the incorporation of metallic nanoparticles (NPs) in the OSCs. Metallic NPs increase the incident light absorption instances, thus increasing the obtainable cell efficiencies. Different parameters and factors need to be considered for obtaining the optimum NP specifications. Investigations of the mechanism of light absorption after the introduction of NPs in the OSC are critical. Hence theoretical simulations for such OSCs are important. An overview of the different solar cell characterization techniques is presented in this paper. Simulations are carried out for these characterization techniques to study the behavior of the P3HT:PCBM based OSC in which silver NPs are incorporated in the active layer. The simulations are carried out for the cell structure in the presence of different non-ideality factors. The non-idealities include mobility limitations, presence of traps, recombination losses, low generation, presence of non-ideal values of series and shunt resistances, the effect of doping, etc. The simulated characterization techniques can be utilized for the performance study and parameter extraction of these NP incorporated OSCs.

Effect of PQT-12 interface layer on the performance of PCDTBT: PCBM bulk heterojunction solar cells

The effect of PQT-12 interface layer on the performance of PCDTBT:PCBM based bulk heterojunction (BHJ) organic solar cells (OSCs) is reported in this paper. The PQT-12 interface layer is obtained by cost and material effective technique, floating film transfer method (FTM). It is shown that the use of FTM-based PQT-12 HTL significant improves the power conversion efficiency (PCE) and external quantum efficiency (EQE) of the OSCs over only PEDOT:PSS HTL based OSCs. The OSC has the maximum values of JSC (=5.62 mA cm−2), VOC (=562 mV) and FF (=0.33) in the PQT-12 incorporated structure due to efficient carrier transfer at the interface. The PCE of nearly twice of the only PEDOT:PSS HTL OSC is obtained in case of the PQT-12/PEDOT:PSS structure.

Influence of thermal annealing-induced molecular aggregation on film properties and photovoltaic performance of bulk heterojunction solar cells based on a squaraine dye

Squaraine (SQ) dyes have been considered as efficient photoactive materials for organic solar cells. In this work, we purposely controlled the molecular aggregation of an SQ dye, 2,4-bis[4-(N,N-dibutylamino)-2-dihydroxyphenyl] SQ (DBSQ-(OH)2) in the DBSQ(OH)2:[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend film by using the thermal annealing method, to study the influence of the molecular aggregation on film properties as well as the photovoltaic performance of DBSQ(OH)2:PCBM-based bulk heterojunction (BHJ) solar cells. Our results demonstrate that thermal annealing may change the aggregation behavior of DBSQ(OH)2 in the DBSQ(OH)2:PCBM film, and thus significantly influence the surface morphology, optical and electrical properties of the blend film, as well as the photovoltaic performance of DBSQ(OH)2:PCBM BHJ cells.

Measurement of contact surface photo-voltage from forward bias C-V characteristics of P3HT:PCBM based BHJ solar cells

The surface photovoltage (SPV) and carrier accumulation in open circuit conditions are sensitive indicators of the contact quality and band bending. However, these are not accessible directly in experiments. We analyze the capacitance-voltage (C-V) characteristics in P3HT:PCBM based bulk heterojunction to investigate SPV for both conventional (transparent anode) and inverted (transparent cathode) architectures. The C-V characteristics under dark conditions are analyzed using drift-diffusion model to extract built-in voltage, while under white light illumination it yields SPV. The SPV and the corresponding carrier density at the anode are quantified as a function of light intensity for conventional structure and compared with those of inverted structure to distinguish between the mechanisms in the two cases. The inverted structure with higher open circuit voltage is marked with less SPV compared to the conventional one. The technique of obtaining SPV as described here and the underlying mechanism can be used to suggest modifications in device architecture.

Application of Fe-doped SnO2 nanoparticles in organic solar cells with enhanced stability

Solution-processed P3HT:PCBM-based bulk heterojunction organic solar cells were constructed with Fe-doped tin oxide (Fe/SnO2) magnetic nanoparticles (NP) incorporated to the active layer. Performance and stability of devices were evaluated. Devices reinforced with Fe/SnO2 nanoparticles showed improvement in short-circuit current density Jsc up to 10%, when compared with devices without the addition of NPs. Improved Jsc was attributed to the extension of photogenerated exciton lifetime as a result of the magnetic field provided by the NPs. Life time and stability tests revealed extended T80 for samples reinforced with NPs for both indoor and outdoor tests, which can be attributed to slightly improvements of fill factor during aging tests. Moreover, NP-reinforced devices have shown the tendency to overcome the burn-in regime faster. These results are indications that the addition of diluted magnetic semiconductor oxide nanoparticles in the active layer of organic solar cells has the potential to contributed to the extension of lifetime and improvement of stability of these devices.

Combined influence of plasmonic metal nanoparticles and dual cathode buffer layers for highly efficient rrP3HT:PCBM-based bulk heterojunction solar cells

The combined influence of plasmonic metal nanoparticles and dual cathode buffer layers resulted in high-performance rrP3HT:PCBM based BHJ solar cells (PCE ∼5.65%).

Effect of Dual Cathode Buffer Layer on the Charge Carrier Dynamics of rrP3HT:PCBM Based Bulk Heterojunction Solar Cell.

In bulk heterojunction (BHJ) solar cells, the buffer layer plays a vital role in enhancing the power conversion efficiency (PCE) by improving the charge carrier dynamics. A comprehensive understanding of the contacts is especially essential in order to optimize the performance of organic solar cells (OSCs). Although there are several fundamental reports on this subject, a proper correlation of the physical processes with experimental evidence at the photoactive layer and contact materials is essential. In this work, we incorporated three different additional buffer layers, namely, tris(8-hydroxyquinolinato) aluminum (Alq3), bathophenanthroline (BPhen) or bathocuproine (BCP) with LiF/Al as conventional cathode contact in both rrP3HT:PC61BM and rrP3HT:PC71BM blend BHJ solar cells and their corresponding photovoltaic performances were systematically correlated with their energy level diagram. The device with dual cathode buffer layer having ITO/PEDOT:PSS/blend polymer/BCP/LiF/Al configuration showed the best device performance with PCE, η = 4.96%, Jsc = 13.53 mA/cm(2), Voc = 0.60 V and FF= 61% for rrP3HT:PC71BM and PCE, η = 4.5% with Jsc = 13.3 mA/cm(2), Voc = 0.59 V and FF = 59% for rrP3HT:PC61BM. This drastic improvement in PCE in both the device configurations are due to the combined effects of better hole-blocking capacity of BCP and low work function provided by LiF/Al with the blend polymer. These results successfully explain the role of dual cathode buffer layers and their contribution to the PCE improvement and overall device performance with rrP3HT:PCBM based BHJ solar cell.

Perovskite-polymer hybrid solar cells with near-infrared external quantum efficiency over 40%

In the past several years, conjugated polymers and organometal halide perovskites have become regarded as promising light-absorbing materials for next-generation photovoltaic devices and have attracted a great deal of interest. As the main part of this contribution, we describe the enhancement of near-infrared (NIR) photoresponse of well-known CH3NH3PbI3−x Cl x -based solar cells by the integration of bulk heterojunction (BHJ) small band gap polymer:fullerene absorbers. Particularly, the integration of a commercially available polymer PDPP3T and PCBM-based BHJ boosts the peak external quantum efficiency (EQE) by up to 46% in the NIR region (800−1000 nm), which is outside of the photoresponsive region (300−800 nm) of conventional perovskite solar cells. This substantial improvement in the EQE over the NIR region offers an additional current density of ∼5 mA cm−2 for the control perovskite solar cell, and a high power conversion efficiency (PCE) of over 12% was obtained in the perovskite/BHJ-based solar cells. In addition, the insertion of the BHJ absorber consisting of a small band gap polymer PDTP-DFBT and PCBM also results in nearly 40% EQE for the perovskite/BHJ solar cell. The results also reveal that controlling over the polymer/PCBM weight ratio for a BHJ absorber is the key to achieving the optimal efficiency for this type of perovskite-polymer hybrid solar cell.

Reliability improvement of bulk-heterojunction organic solar cell by using reduced graphene oxide as hole-transport layer

This work covered the application of thermally reduced graphene oxide (rGO) as a hole-transport layer (HTL) to enhance stability of normal P3HT:PCBM-based bulk-heterojunction solar device. This device showed approximately the same performance of device using PEDOT:PSS as HTL (3.14%). After 3 days, exposure of an unsealed device in 1 sun illumination, rGO device resulted in 67% degradation of performance, whereas samples fabricated with PEDOT:PSS or without HTL degraded to more than 88% in efficiency. By simulation into an equivalent circuit, impedance analysis demonstrated that this degradation was strongly depended on the buffer materials. The use of the thermally rGO as buffer material could reduce the change of resistance in both HTL and bulk layer as well as avoid the accumulation of space charges in the active film.

Carrier Lifetime Extension via the Incorporation of Robust Hole/Electron Blocking Layers in Bulk Heterojunction Polymer Solar Cells

We report the achievement of a power conversion efficiency (PCE) improvement in P3HT:PCBM-based bulk-heterojunction type polymer solar cells using photocrosslinked P3HT (c-P3HT) as the electron blocking/hole extraction layer and titanium oxide nanoparticles (TiO2) as the hole blocking/electron extraction layer. Devices prepared with a 20 nm thick c-P3HT layer showed an improved PCE of 3.4% compared to devices prepared without the c-P3HT layer (PCE = 3.0%). This improvement was attributed to an extension in the carrier lifetime and an enhancement in the carrier mobility. The incorporation of the c-P3HT layer lengthened (by more than a factor of 2) the carrier lifetime and increased (by a factor of 5) the hole mobility. These results suggest that the c-P3HT layer not only prevented non-geminate recombination but it also improved carrier transport. The PCE was further improved to 4.0% through the insertion of a TiO2 layer that acted as an effective hole-blocking layer at the interface between the photoactive layer and the cathode. This work demonstrates that the incorporation of solution-processable hole and electron blocking/extraction layers offers an effective means for preventing nongeminate recombination at the interfaces between a photoactive layer and an electrode in bulk-heterojunction-type polymer solar cells.

Evolution of Lateral Structures during the Functional Stack Build-up of P3HT:PCBM-Based Bulk Heterojunction Solar Cells

Bulk heterojunction (BHJ) solar cells from 1,2-dichlorobenzene solution processed regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) are prepared and investigated at different steps of the multilayer stack build-up of the device. The inner structure is probed from the molecular to the mesoscale with grazing incidence small/wide-angle X-ray scattering (GISAXS/GIWAXS) and X-ray reflectivity (XRR). The surface morphology is detected with atomic force microscopy (AFM). Therefore, an in-depth knowledge of the three-dimensional morphology of the bulk heterojunction solar cell, starting from the cleaned ITO substrate up to the final post-treated solar cell, is generated. The active layer structure is influenced by the annealing as well as by the top contact deposition. Structures coarsen during the evaporation of the metal contacts. The P3HT crystal structure strongly depends on the device processing as well. These morphological changes together with the diffusion of aluminum atoms to the active layer are of importance for the device efficiency.

Improved Performance of P3HT:PCBM-Based Solar Cells Using Nematic Liquid Crystals as a Processing Additive under Low Processing Temperature conditions

Nematic liquid crystals (NLCs) are introduced to P3HT:PCBM-based bulk heterojunction polymer solar cells (PSCs) as processing additives for low processing temperatures. To investigate why the addition of NLCs improves the efficiency of PSCs, the photon absorbance properties are observed using UV–vis spectroscopy and photoluminescence. Then, the degree of crystallinity of P3HT is examined through X-ray diffraction analysis. In addition, changes in the nanostructure and surface morphology of the P3HT:PCBM blend film are observed through atomic force microscopy and transmission electron microscopy.

Possibility to Use Low Temperature Pulsed RF Sputtered Indium Tin Oxide for the Fabrication of Organic Solar Cell

In this work we have used pulsed RF sputtering method to deposit indium tin oxide (ITO) for the fabrication of P3HT:PCBM based bulk heterojunction polymer solar cell. We have deposited ITO at low substrate temperature (100°C) and for different pulse modes. Oxygen was used as an admixture to the sputtering gas argon, and the percentage was varied from 0 to 6%. During deposition, plasma was studied by optical emission spectroscopy (OES) method. For our present range of deposition conditions lowest resistivity of ITO is around 2 × 10−4 Ω-cm, and it is deposited in High-Low mode with 1% of oxygen added to argon. The effect of oxygen admixture on electrical and optical properties of ITO thin films has been studied for different pulse modes. ITO films have been optimised by measuring their resistivity, transparency, and X-ray diffraction. Finally we have applied the ITO film for the fabrication of P3HT:PCBM based solar cell.

Interlayer adhesion in roll-to-roll processed flexible inverted polymer solar cells

The interlayer adhesion of roll-to-roll processed flexible inverted P3HT:PCBM bulk heterojunction (BHJ) polymer solar cells is reported. Poor adhesion between adjacent layers may result in loss of device performance from delamination driven by the thermomechanical stresses in the device. We demonstrate how a thin-film adhesion technique can be applied to flexible organic solar cells to obtain quantitative adhesion values. For the P3HT:PCBM-based BHJ polymer solar cells, the interface of the BHJ with the conductive polymer layer PEDOT:PSS was found to be the weakest. The adhesion fracture energy varied from 1.6 J/m 2 to 0.1 J/m 2 depending on the composition of the P3HT:PCBM layer. Post-deposition annealing time and temperature were shown to increase the adhesion at this interface. Additionally the PEDOT:PSS cells are compared with V 2O 5 cells whereby adhesive failure marked by high fracture energies was observed.

Device lifetime improvement of polymer-based bulk heterojunction solar cells by incorporating copper oxide layer at Al cathode

Organic solar cells are commonly susceptible to degradation in air. We present that insertion of a thin layer of thermally evaporated copper oxide (CuOx) between the organic active layer and the Al cathode can greatly extend the lifetime of P3HT:PCBM based bulk heterojunction solar cells. The performance can be further improved by applying an interfacial bilayer of CuOx/LiF. Our results suggest that the CuOx functions not only as a charge transport layer but also as a protection layer, which prevents formation of thick organic-Al interdiffusion area. This leads to a more air-resistive cathode/organic interface.

Solution-Processed Nanocrystalline TiO2 Buffer Layer Used for Improving the Performance of Organic Photovoltaics

In this study, we use solution-processable crystalline TiO(2) nanoparticles as a buffer layer between the active layer and aluminum cathode to fabricate the P3HT:PCBM-based bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. The employment of the presynthesized TiO(2) nanoparticles simplifies the fabrication of OPV devices because of the elimination of an additional hydrolysis step of precursors in air. The fabricated OPV devices with the thermally stable TiO(2) buffer layer are subjected to the further postdeposition thermal annealing, resulting in a power conversion efficiency (PCE) as high as 3.94%. The improved device performance could be attributed to the electron transporting and hole blocking capabilities due to the introduced TiO(2) buffer layer.

Consequences of Anode Interfacial Layer Deletion. HCl-Treated ITO in P3HT:PCBM-Based Bulk-Heterojunction Organic Photovoltaic Devices

In studies to simplify the fabrication of bulk-heterojunction organic photovoltaic (OPV) devices, it was found that when glass/tin-doped indium oxide (ITO) substrates are treated with dilute aqueous HCl solutions, followed by UV ozone (UVO), and then used to fabricate devices of the structure glass/ITO/P3HT:PCBM/LiF/Al, device performance is greatly enhanced. Light-to-power conversion efficiency (Eff) increases from 2.4% for control devices in which the ITO surface is treated only with UVO to 3.8% with the HCl + UVO treatment--effectively matching the performance of an identical device having a PEDOT:PSS anode interfacial layer. The enhancement originates from increases in V(OC) from 463 to 554 mV and FF from 49% to 66%. The modified-ITO device also exhibits a 4x enhancement in thermal stability versus an identical device containing a PEDOT:PSS anode interfacial layer. To understand the origins of these effects, the ITO surface is analyzed as a function of treatment by ultraviolet photoelectron spectroscopy work function measurements, X-ray photoelectron spectroscopic composition analysis, and atomic force microscopic topography and conductivity imaging. Additionally, a diode-based device model is employed to further understand the effects of ITO surface treatment on device performance.