Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current (Jsc ) and the fill factor (FF). Identifying the mechanism of ...recombination is, therefore, fundamentally important for increasing the power conversion efficiency. Light intensity and temperature-dependent current-voltage measurements on polymer BHJ cells made from a variety of different semiconducting polymers and fullerenes show that the recombination kinetics are voltage dependent and evolve from first-order recombination at short circuit to bimolecular recombination at open circuit as a result of increasing the voltage-dependent charge carrier density in the cell. The “missing 0.3 V” inferred from comparison of the band gaps of the bulk heterojunction materials and the measured open-circuit voltage at room-temperature results from the temperature dependence of the quasi-Fermi levels in the polymer and fullerene domains—a conclusion based on the fundamental statistics of fermions.
The use of molybdenum oxide as the anode interfacial layer in conventional bulk heterojunction polymer solar cells leads to an improved power conversion efficiency and also dramatically increases the ...device stability. This indicates that the engineering of improved anode interface materials is an important method by which to fabricate efficient and stable polymer solar cells.
Differential resistance analysis has been shown to be effective at studying the charge carrier losses in bulk heterojunction solar cells. The variations of differential resistance with light ...intensity and Voc provide strong evidence for the identification of the recombination kinetics, revealing bimolecular as the dominating recombination mechanism in PCDTBT:PC60BM system and showing an evolution to trap‐assisted recombination in presence of PC84BM trap states.
Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device, including reducing the open circuit voltage, ...the short circuit current and the fill factor. Steady state studies show a dramatic increase in the trap‐assisted recombination rate when 6,6‐phenyl C84 butyric acid methyl ester (PC84BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymer polyN‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and the fullerene derivative 6,6‐phenyl C61 butyric acid methyl ester (PC60BM). The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley–Read–Hall recombination; the latter is dramatically enhanced by the addition of the PC84BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the mechanism of the trap‐induced recombination loss.
PC84BM traps provide a definitive measure of trap‐assisted recombination in polymer:fullerene solar cells. The trap‐density‐dependent recombination studied here can be described by the result of bimolecular and Shockley–Read–Hall recombination. Dilute impurities act as an important loss mechanism that may be eliminated via further purification processes. Schematic illustration of the addition of PC84BM in small mass fraction to the polymer bulk‐heterojunction solar cell, showing the chemical structure of PC84BM and PC60BM.
Recombination in bulk heterojunction solar cells is explored by observing the result of prolonged white light illumination, thermal annealing to high temperature, and chemical doping. Measurements of ...the photocurrent spectral response, the steady state photocurrent‐voltage characteristics, transient photoconductivity and the dark forward bias current on polymer:fullerene solar cells provide information about the density of states and the electronic properties. Illumination generates deep localized states in the interface gap, which act as recombination centers and also increase the diode ideality factor. Annealing induces both nanostructural and electronic changes. The coarsening of the domain structure reduces the probability that excitons reach the interfaces and also reduces the charge transfer absorption. At the same time annealing broadens the exponential band tails and increases the recombination rate. Doping introduces shallow states near the fullerene conduction band, which also act as recombination centers. The results show that recombination is through localized states of different character, depending on the circumstances.
Recombination in bulk heterojunction solar cells is explored by observing the results of extended white light illumination, thermal annealing to high temperature, and chemical doping. Measurements of spectral response and other cell characteristics show that recombination is through localized states of different type, depending on the circumstances.
This article presents a critical discussion of the various physical processes occurring in organic bulk heterojunction (BHJ) solar cells based on recent experimental results. The investigations span ...from photoexcitation to charge separation, recombination, and sweep‐out to the electrodes. Exciton formation and relaxation in polyN‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and poly‐3(hexylthiophene) (P3HT) are discussed based on a fluorescence up‐conversion study. The commonly accepted paradigm describing the conversion of incident photons into charge carriers in the BHJ material is re‐examined in light of these femtosecond time‐resolved measurements. Transient photoconductivity, time‐delayed collection field, and time‐delayed dual pulse experiments carried out on BHJ solar cells demonstrate the competition between carrier sweep‐out by the internal field and the loss of photogenerated carriers by recombination. Finally, an emerging hypothesis is discussed: that bimolecular recombination accounts for the majority of recombination from short circuit to open circuit in optimized solar cells, and that bimolecular recombination is bias‐ and charge‐density‐dependent. The study of recombination loss processes in organic solar cells leads to insights into what must be accomplished to achieve the “ideal” solar cell.
Recent experiments that investigate the recombination loss processes in organic bulk heterojunction solar cells, ranging from photoexcitation to charge transfer, recombination, and collection, are described. The study of recombination loss processes in optimized and intentionally unoptimized systems leads to insights into what must be accomplished to achieve the “ideal” solar cell.
The role of work function and thermodynamic selectivity of hole collecting contacts on the origin of open circuit voltage (VOC) in bulk heterojunction organic photovoltaics is examined for ...poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and 6,6‐phenyl‐C71 butyric acid methyl ester (PC71BM) solar cells. In the absence of a charge selective, electron blocking contact, systematic variation of the work function of the contact directly dictates the VOC, as defined by the energetic separation between the relative Fermi levels for holes and electrons, with little change in the observed dark saturation current, J0. Improving the charge selectivity of the contact through an increased barrier to electron injection from the fullerene in the blend into the hole contact results in a decreased reverse saturation current (decreased J0 and increased shunt resistance, RSH) and improved VOC. Based on these observations, we provide a set of contact design criteria for tuning the VOC in bulk heterojunction organic photovoltaics.
Contact work function and thermodynamic selectivity both play key roles in determining the open circuit voltage in PCDTBT:PC71BM OPVs. Systematically increasing the work function of the contact increases the VOC; adding a selective contact with the same work function further increases the VOC due to a change in the space charge region at the active layer/contact interface.
A spin‐cast method is presented for the formation of phosphonic acid functionalized small molecule layers on solution‐processed ZnO substrates for use as electron collecting interlayers in organic ...photovoltaics. Phosphonic acid interlayers modify the ZnO work function and the charge carrier injection barrier at its interface, resulting in systematic control of V OC in inverted bulk heterojunction solar cells. Surface modification is shown to moderate the need for UV light‐soaking of the ZnO contact layers. Lifetime studies (30 days) indicate stable and improved OPV performance over the unmodified ZnO contact, which show significant increases in charge extraction barriers and series resistance. Results suggest that enhanced stability using small molecule modifiers is due to partial passivation of the oxide surface to molecular oxygen adsorption. Surface passivation while maintaining work function control of a selective interlayer can be employed to improve net efficiency and lifetime of organic photovoltaic devices. The modified cathode work function modulates V
OC via static energetic barriers and modulates contact conductivity by creating reversible and irreversible S‐shape current‐voltage characteristics as a result of kinetic barriers to charge transport.
Deposition of benzyl phosphonic acids and alkanethiol self‐assembled monolayers improve initial device performance, and have beneficial effect at mitigating the light‐soaking effect present after aging inverted architecture organic bulk heterojunction devices incorporating ZnO contact layers in air. The effect of a kinetic/transport barrier and a static energetic barrier resulting in formation of S‐shaped J–V curves is isolated.
The electronic structure of a bi‐layer hole extraction contact consisting of nickel oxide (NiOx) and molybdenum trioxide (MoO3) is determined via ultraviolet and X‐ray photoemission spectroscopy. The ...bi‐layer presents ideal energetics for the extraction of holes and suppression of carrier recombination at the interface. The application of the NiOx/MoO3 bi‐layer as the anode of organic bulk heterojunction solar cells based on PCDTBT/PC71BM leads to improved device performance, which is explained by an intricate charge transfer process across the interface.
Nickel oxide/molybdenum oxide bi‐layers enhance hole collection in organic photovoltaic devices. Device improvement in PCDTBT:PC71BM based solar cells is attributed to the electronic structure of the oxide interlayer. The electron blocking capability of NiOx is combined with the high work function of MoO3, which enable efficient coupling of the electrode to the hole transport level in the polymer.
Key content
Twin pregnancies are associated with a three‐fold greater perinatal mortality than singleton pregnancies. Prematurity is a main contributor, with 50% of twin pregnancies delivering before ...37 weeks and 10% delivering before 32 weeks of gestation.
The aetiology of preterm delivery in twin pregnancies is likely multifactorial and different from that of singletons.
Cervical cerclage reduces preterm birth rates in singletons but has mixed results in twins with some studies showing harm.
The use of progesterone to prevent preterm birth in singletons has conflicting results and has not been proven to prevent preterm birth in twins. Studies continue to determine whether the cervical pessary is effective in preventing preterm birth in multiple pregnancies.
There is a paucity of data available on the prevention of preterm birth in triplets/higher order multiples but similar principles to twin pregnancy apply.
Learning objectives
To review the burden of preterm birth in multiple pregnancy.
To understand the methods available for preventing preterm birth in multiple pregnancies and the evidence surrounding the use of each one.
To be aware of the use of the Arabin pessary.
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