Reappraising the Need for Bulk Heterojunctions In Polymer− Fullerene Photovoltaics: The Role of Carrier Transport In All-Solution-Processed P3HT/PCBM Bilayer … (original) (raw)
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The European Physical Journal Applied Physics, 2018
This paper reports a study on the effect of different parameters such as charge carriers mobility, electrodes work function, energy gap, series as well as shunt resistances on the performances of an organic photovoltaic cell based on polymer/fullerene P3HT: PCBM. Thus, numerical simulations have been investigated on ITO/PEDOT:PSS/P3HT: PCBM/LiF/Al structure with Analysis of Microelectronic and Photonic Structures the simulation one dimension (AMPS-1D) and General-Purpose Photovoltaic Device Model (GPVDM) softwares. The results show that the optimum efficiency is obtained for electron and hole motility values of 10−4 cm2 V−1 s−1 and 2 × 10−4 cm2 V−1 s−1, respectively. Moreover, we reported that the ohmic contact for both anode and cathode electrodes of the investigated device remains very important to get the maximum efficiency. Furthermore, when the gap energy increases, the efficiency is considerably improved, and reach's a value of about 5.421%. In addition, in this present wo...
We examine how thermal annealing affects the fullerene network in conjugated polymer bulk heterojunction (BHJ) solar cells. We begin by creating electron-only devices with a BHJ geometry by blending the fullerene derivative [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) with polystyrene (PS). These electron-only PS:PCBM diodes function even with a poly(ethylenedioxithiophene):poly(styrenesulfonate) (PEDOT:PSS) layer, indicating that PEDOT:PSS films do not serve as electron blocking layers. Atomic force microscopy shows that the degree of phase segregation in the PS:PCBM blend films is similar to that in the active layer of blends of PCBM with poly(3-hexylthiophene-2,5-diyl) (P3HT), so that the PS:PCBM blends provide a good model for the fullerene part of the BHJ network in P3HT:PCBM solar cells. We find that thermal annealing dramatically decreases the electron current that flows in the PS:PCBM diodes, suggesting that annealing leads to increased phase segregation that lowers the electron mobility on fullerenes in the BHJ geometry. We also find that annealing increases the photoluminesence of P3HT:PCBM blend films, indicating that thermal treatment produces increased phase segregation that leads to decreased exciton harvesting. The fact that annealing decreases both exciton harvesting and electron mobility implies that there is significant room to further improve polymer/fullerene photovoltaics by controlling the amount of phase segregation.
The correlation between the physical properties of spin-casting solvents, film morphology, nanoscale charge transport, and device performance was studied in poly(3hexylthiophene):phenyl-C 61 -butyric acid methyl ester (P3HT:PCBM) blends, spin cast with two halogenated aromatic solvents: chlorobenzene (CB) and ortho-dichlorobenzene (1,2-DCB). 1,2-DCB-based blends exhibited fine phase separation of ∼10 to 15 nm length scale with ordered self-assembly of P3HT whereas blends spin cast from CB showed coarse phase separation with large isolated clusters of ∼25 to 100 nm of donor-and acceptor-rich regions. Higher solubility of both P3HT and PCBM in 1,2-DCB and a slower drying rate of 1,2-DCB (because of higher boiling point) facilitated self-organization and ordering of P3HT and promoted finer phase separation. Higher local hole mobility in 1,2-DCB-based blend was attributed to efficient hole transport through the ordered network of P3HT chains. Moreover, higher local illuminated current (dark + photocurrent) in 1,2-DCB-based blend suggested efficient diffusion and dissociation of excitons due to finer phase separation. As a consequence, 1,2-DCB-based devices exhibited higher short circuit current density (J sc ), external quantum efficiency and power conversion efficiency in contrast to the CB-based device. It was also observed that the device performance was not limited by light absorption and exciton generation; rather morphology dependent processes subsequent to exciton generation, primarily charge transport to the electrodes, limited device performance. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). Dutta et al.: Connecting physical properties of spin-casting solvents with morphology... are generated primarily in the donor material. The excitons diffuse to the donor-acceptor (DA) interfaces where they dissociate, leading to formation of free electrons and holes. Under electric field, dissociated carriers are transported to the respective electrodes where they are collected. Holes flow through the donor material toward the anode and electron flows through the acceptor phase toward the cathode. The nanoscale morphology of the DA blend plays a crucial role in determining the photovoltaic performance of the device. 3-9 A blend of poly(3-hexylthiophene) (P3HT) as donor and phenyl-C 61 -butyric acid methyl ester (PCBM) as acceptor has been studied extensively as a prototypical BHJ cell, with reported efficiencies up to 5%. In an ideal scenario, phase separation in the blend should lead to the formation of an interpenetrating bicontinuous network of DA materials. The mean domain size of the acceptor PCBM and donor (P3HT) in the blends should lie within the exciton diffusion length (4 to 20 nm) 12-14 for efficient diffusion of excitons to DA interface. A high degree of DA phase separation with large interfacial area helps exciton dissociation while bicontinuous percolating pathways of interconnecting DA network offer efficient transport of free carriers to the respective electrodes. 15 However, an extremely fine phase separated DA blend is not ideal because it impedes the flow of both excitons and dissociated charge carriers and increases the probability of recombination. Thus, an optimum balance between interpenetrating and interconnecting DA components in the blend is necessary to maximize exciton dissociation and free carrier transport 16 and hence to maximize the cell efficiency.
Nano Letters, 2011
PTB7 semiconducting copolymer comprising thieno [3,4-b]thiophene and benzodithiophene alternating repeat units set a historic record of solar energy conversion efficiency (7.4%) in polymer/fullerene bulk heterojunction solar cells. To further improve solar cell performance, a thorough understanding of structureÀproperty relationships associated with PTB7/fullerene and related organic photovoltaic (OPV) devices is crucial. Traditionally, OPV active layers are viewed as an interpenetrating network of pure polymers and fullerenes with discrete interfaces. Here we show that the active layer of PTB7/fullerene OPV devices in fact involves hierarchical nanomorphologies ranging from several nanometers of crystallites to tens of nanometers of nanocrystallite aggregates in PTB7-rich and fullerene-rich domains, themselves hundreds of nanometers in size. These hierarchical nanomorphologies are coupled to significantly enhanced exciton dissociation, which consequently contribute to photocurrent, indicating that the nanostructural characteristics at multiple length scales is one of the key factors determining the performance of PTB7 copolymer, and likely most polymer/fullerene systems, in OPV devices.
Applied Physics Letters, 2009
We demonstrate photocurrent enhancement of up to 20% in polymer:fullerene bulk heterojunction photovoltaic cells via the incorporation of a phosphorescent dopant, without degradation in the open-circuit voltage or fill factor of the device. The enhancement is shown to originate from multiple sources. First, the phosphor is able to populate the long-lived triplet state of the polymer, leading to longer diffusion length and a larger polymer contribution. Also, there is direct absorption on the dopant leading to enhanced spectral coverage. Finally, the dopant acts as a donor site and therefore increases the fullerene signal.
The European Physical Journal Applied Physics, 2011
The nanostructure of the active layer in polymer/fullerene bulk heterojunction solar cells is known to have a strong impact on the device performances. Controlling the polymer/fullerene blend morphology is therefore particularly important. In this work, a rod-coil block copolymer, based on a regio-regular poly(3-hexylthiophene) electron-donor rod block and a C 60-grafted coil block, is used as compatibilizer and its influences on the thin film morphology as well as the photovoltaic performances are investigated. It is shown that a small fraction of compatibilizer can enhance the device performances in an otherwise nonoptimized process. At higher fractions or long annealing times however, the fullerene-grafted copolymer is found to behave as a nucleation center and triggers the formation of fullerene crystals.
Applied Mechanics and Materials, 2013
Blend of P3HT/Fullerene thin films solar cell with two different percentage ratio of PCBM loading is investigated. Optical absorption spectroscopy is employed to elucidate the nature of PCBM cluster formation upon thermal annealing. Sandwich structures comprising of ITO/ Cs2CO3/ P3HT: PCBM/ LiF/ Al (electron only device), and ITO/ PEDOT:PSS/ P3HT:PCBM/ Au (hole only device) are fabricated using spin coating for the investigations concerning electron and hole mobilities. The impact of charge carrier mobilities on bimolecular recombination and ultimately the power conversion efficiency for two different PCBM loading is also investigated. A direct correlation between Langev in recombination rate and short circuit current density as a function of thermal annealing is realized. The maximum power conversion efficiency is measured at 150°C for P3HT: PCBM (1:1) solar cell.