Charge-transfer (CT) state electroluminescence is investigated in several polymer:fullerene bulk heterojunction solar cells. The ideality factor of the electroluminescence reveals that the CT emission in polymer:fullerene solar cells originates from free-carrier bimolecular recombination at the donor-acceptor interface, rather than a charge-trap-mediated process. The fingerprint of the presence of nonradiative trap-assisted recombination, a voltage-dependent CT electroluminescence quantum efficiency, is only observed for the P3HT:PCBM system, which is explained by a reduction of the competing bimolecular recombination rate. These results are in agreement with measurements of the illumination-intensity dependence of the open-circuit voltage.
The charge transport and recombination in white-emitting polymer light- emitting diodes (PLEDs) are studied. The PLED investigated has a single emissive layer consisting of a copolymer in which a green and red dye are incorporated in a blue backbone. From single-carrier devices the effect of the green- and red-emitting dyes on the hole and electron transport is determined. The red dye acts as a deep electron trap thereby strongly reducing the electron transport. By incorporating trap-assisted recombination for the red emission and bimolecular Langevin recombination for the blue emission, the current and light output of the white PLED can be consistently described. The color shift of single-layer white-emitting PLEDs can be explained by the different voltage dependencies of trap-assisted and bimolecular recombination.
The charge transport in blue light-emitting polyspirobifluorene is investigated by both steady-state current-voltage measurements and transient electroluminescence. Both measurement techniques yield consistent results and show that the hole transport is space-charge limited. The electron current is found to be governed by a high intrinsic mobility in combination with electron traps. Numerical simulations on light-emitting diodes reveal a shift in the recombination zone from the cathode to the anode with increasing bias.
The effect of on-chain ketone defects on the charge transport of the polyfluorene derivative poly(9,9-dioctylfluorene) (PFO) is investigated. Using MoO3 as ohmic hole contact, the hole transport in a pristine PFO diode is observed to be limited by space-charge, whereas fluorenone contaminated PFO (PFO-F) is shown to be trap limited by the occurrence of an exponential trap distribution with a trap depth of 0.18 eV. The electron transport in PFO is also observed to be trap limited, but in order to describe the electron transport of PFO-F, an additional trap level with a depth of 0.46 eV must be introduced. The obtained energy levels of the fluorenone trapping sites are in close agreement with cyclic voltammetry (CV) measurements reported in literature. As a result, the fluorenone defects are shown to simultaneously act as hole- and electron trap. Moreover, through ideality factor measurements, the green emission associated with these defects is observed to originate from trap-assisted recombination.
Lowering of the optical band gap of conjugated polymers in bulk heterojunction solar cells not only leads to an increased absorption but also to an increase of the optimal active layer thickness due to interference effects at longer wavelengths. The increased carrier densities due to the enhanced absorption and thicker active layers make low band gap solar cells more sensitive to formation of space charges and recombination. By systematically red shifting the optical parameters of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] and 6,6-phenyl C61-butyric acid methyl ester, we simulate the effect of a reduced band gap on the solar cell efficiencies. We show that especially the fill factor of low band gap cells is very sensitive to the balance of the charge transport. For a low band gap cell with an active layer thickness of 250 nm, the fill factor of 50% for balanced transport is reduced to less than 40% by an imbalance of only one order of magnitude. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011
Here, the performance of bulk-heterojunction solar cells based on a series of bisadduct analogues of commonly used derivatives of C60 and C70, such PCBMs and their thienyl versions, is investigated. Due to their higher lowest unoccupied molecular orbital an increase in open-circuit voltage and thus performance is expected. It is shown that the occurrence of a multitude of different isomers results in a decrease in the electron transport for some of the materials. Surprisingly, the solar-cell characteristics are very similar for all materials. This apparent discrepancy is explained by a significant amount of shallow trapping occurring in the fullerene phase that does not hamper the solar cell performance due the filling of these shallow traps during illumination. Furthermore, the trisadduct analogue of [60]PCBM has been investigated, which, despite an even further increase in open-circuit voltage, results in a significantly reduced device performance due to a strong deterioration of the electron mobility in the fullerene phase.
The charge transport and photogeneration in solar cells based on the low bandgap-conjugated polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b; 3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and fullerenes is studied. The efficiency of the solar cells is limited by a relatively low fill factor, which contradicts the observed good and balanced charge transport in these blends. Intensity dependent measurements display a recombination limited photocurrent, characterized by a square root dependence on effective applied voltage, a linear dependence on light intensity and a constant saturation voltage. Numerical simulations show that the origin of the recombination limited photocurrent stems from the short lifetime of the bound electron-hole pairs at the donor/acceptor interface.
Here, controlled p-type doping of poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) deposited from solution using tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) as a dopant is presented. By using a co-solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current–voltage characteristics of MEH-PPV-based hole-only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the density dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of 1 F4-TCNQ dopant per 600 repeat units of MEH-PPV leads to a free carrier density of 4 × 1022 m−3. Neglecting the density-dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4-TCNQ doped MEH-PPV and a silver electrode.
Plastic solar cells bear the potential for large-scale power generation based on materials that provide the possibility of flexible, lightweight, inexpensive, efficient solar cells. Since the discovery of the photoinduced electron transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, this material combination has been extensively studied in organic solar cells, leading to several breakthroughs in efficiency, with a power conversion efficiency approaching 5 %. This article reviews the processes and limitations that govern device operation of polymer:fullerene BHJ solar cells, with respect to the charge-carrier transport and photogeneration mechanism. The transport of electrons/holes in the blend is a crucial parameter and must be controlled (e.g., by controlling the nanoscale morphology) and enhanced in order to allow fabrication of thicker films to maximize the absorption, without significant recombination losses. Concomitantly, a balanced transport of electrons and holes in the blend is needed to suppress the build-up of the space–charge that will significantly reduce the power conversion efficiency. Dissociation of electron–hole pairs at the donor/acceptor interface is an important process that limits the charge generation efficiency under normal operation condition. Based on these findings, there is a compromise between charge generation (light absorption) and open-circuit voltage (VOC) when attempting to reduce the bandgap of the polymer (or fullerene). Therefore, an increase in VOC of polymer:fullerene cells, for example by raising the lowest unoccupied molecular orbital level of the fullerene, will benefit cell performance as both fill factor and short-circuit current increase simultaneously.
Solar cells based on a poly(p-phenylene vinylene) (PPV) derivative and zinc oxide nanoparticles can reach a power conversion efficiency of 1.6 %. The transport of electrons and holes in these promising devices is characterized and it is found that the electron mobility is equal to 2.8 × 10–9 m2 V–1 s–1, whereas the hole mobility amounts to 5.5 × 10–10 m2 V–1 s–1. By modeling the current–voltage characteristics under illumination it is found that the performance of PPV/zinc oxide solar cells is limited by the charge-carrier mobilities. Subsequently, how to further improve the efficiency is discussed.
The photogeneration mechanism in blends of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) and poly[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV) is investigated. The photocurrent in the MDMO-PPV:PCNEPV blends is strongly dependent on the applied voltage as a result of a low dissociation efficiency of the bound electron–hole pairs. The dissociation efficiency is limited by low carrier mobilities, low dielectric constant, and the strong intermixing of the polymers, leading to a low fill factor and a reduced photocurrent at operating conditions. Additionally, electrons trapped in the PCNEPV phase recombine with the mobile holes in the MDMO-PPV phase at the interface between the two polymers, thereby affecting the open-circuit voltage and increasing the recombination losses. At an intensity of one sun, Langevin recombination of mobile carriers dominates over trap-assisted recombination.
The effect of controlled thermal annealing on charge transport and photogeneration in bulk-heterojunction solar cells made from blend films of regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene (PCBM) has been studied. With respect to the charge transport, it is demonstrated that the electron mobility dominates the transport of the cell, varying from 10–8 m2 V–1 s–1 in as-cast devices to ≈3 × 10–7 m2 V–1 s–1 after thermal annealing. The hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases by more than three orders of magnitude, to reach a value of up to ≈ 2 × 10–8 m2 V–1 s–1 after the annealing process, as a result of an improved crystallinity of the film. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red-shift, improving the spectral overlap with solar emission, which results in an increase of more than 60 % in the rate of charge-carrier generation. Subsequently, the experimental electron and hole mobilities are used to study the photocurrent generation in P3HT:PCBM devices as a function of annealing temperature. The results indicate that the most important factor leading to a strong enhancement of the efficiency, compared with non-annealed devices, is the increase of the hole mobility in the P3HT phase of the blend. Furthermore, numerical simulations indicate that under short-circuit conditions the dissociation efficiency of bound electron–hole pairs at the donor/acceptor interface is close to 90 %, which explains the large quantum efficiencies measured in P3HT:PCBM blends.
Selective ambipolar transport in solution-processed polymer ferroelectric field-effect transistors (FeFETs) is reported. Depending on the polarization state of the ferroelectric, either remanent hole or electron accumulation is achieved in the transistor, as illustrated by a butterfly-shaped current–voltage (I–V) transfer curve (see Figure). For memory purposes, the polarity of the channel can be easily read using the change in drain current in response to a small gate voltage.
The active part of present polymer light-emitting diodes (PLEDs) consists of only a single layer. Multilayer devices have the advantage that the electron and hole transport can be balanced and that the recombination can be removed from the metallic cathode, leading to higher efficiencies. A major problem for polymer-based multilayer devices is the solubility of the materials used; a multilayer can not be fabricated when a spin-cast layer dissolves in the solvent of the subsequent layer. We demonstrate the development of high-mobility poly(p-phenylenevinylene) (PPV)-based hole-transport layers with tunable solubility by chemical modification. Enhanced charge-transport properties are achieved by using symmetrically substituted PPVs; copolymers of long and short side chains enable us to tune the solubility without loss of the enhanced charge transport. Dual-layer PLEDs, in which the holes are efficiently transported via this copolymer towards the luminescent layer, exhibit an enhanced efficiency at high voltages (> 10 V) and a strongly improved robustness against electrical breakdown.
![2,1,3-Benzothiadiazole, 4,4'-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl]bis[5-fluoro-7-(5'-hexyl[2,2'-bithiophen]-5-yl)-](/data/chemimg/164900/1402460-84-7.png)
![2,1,3-Benzothiadiazole, 4,4'-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl]bis[5-fluoro-7-(5'-hexyl[2,2'-bithiophen]-5-yl)-](/data/chemimg/164900/1402460-84-7_b.png)
![Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)]](/data/chemimg/107700/210347-52-7.png)
![Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)]](/data/chemimg/107700/210347-52-7_b.png)
![Poly[[2-[(3,7-dimethyloctyl)oxy]-5-methoxy-1,4-phenylene]-1,2-ethenediyl]](http://img.cochemist.com/ccimg/177800/177716-59-5.png)
![Poly[[2-[(3,7-dimethyloctyl)oxy]-5-methoxy-1,4-phenylene]-1,2-ethenediyl]](http://img.cochemist.com/ccimg/177800/177716-59-5_b.png)
![Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]](/data/chemimg/102300/920515-34-0.png)
![Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]](/data/chemimg/102300/920515-34-0_b.png)
![Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]](http://img.cochemist.com/ccimg/138200/138184-36-8.png)
![Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]](http://img.cochemist.com/ccimg/138200/138184-36-8_b.png)
