Recombination in polymer-fullerene bulk heterojunction solar cells
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.
CHARGE RECOMBINATION IN DYE-SENSITIZED NANOCRYSTALLINE TIO2 SOLAR CELLS
Charge recombination between dye-sensitized nanocrystalline TiO2 electrodes and the I3-/I- couple in nonaqueous solution is described. The sensitizer was [RuL2(NCS)2] (L = 2,2‘-bipyridyl-4,4‘-dicarboxylic acid). An apparent inequality between the dark current and the recombination current is ascribed to a voltage shift caused by a potential drop at the SnO2/TiO2 interface, ohmic losses in the SnO2 and TiO2, and an overpotential for the redox reaction at the Pt counter electrode. Treating the dye-coated TiO2 electrodes with pyridine derivatives (4-tert-butylpyridine, 2-vinylpyridine, or poly(2-vinylpyridine)) improves significantly both the open-circuit photovoltage Voc (from 0.57 to 0.73 V) and the cell conversion efficiency (from 5.8 to 7.5%) at a radiant power of 100 mW/cm2 (AM 1.5) with respect to the untreated electrode. An analytical expression relating Voc to the interfacial recombination kinetics is derived, and its limitations are discussed. Analysis of Voc vs radiant power data with this expressi...
Recombination in quantum dot sensitized solar cells.
Quantum dot sensitized solar cells (QDSCs) have attracted significant attention as promising third-generation photovoltaic devices. In the form of quantum dots (QDs), the semiconductor sensitizers have very useful and often tunable properties; moreover, their theoretical thermodynamic efficiency might be as high as 44%, better than the original 31% calculated ceiling. Unfortunately, the practical performance of these devices still lags behind that of dye-sensitized solar cells. In this Account, we summarize the strategies for depositing CdSe quantum dots on nanostructured mesoporous TiO(2) electrodes and discuss the methods that facilitate improvement in the performance and stability of QDSCs. One particularly significant factor for solar cells that use polysulfide electrolyte as the redox couple, which provides the best performance among QDSCs, is the passivation of the photoanode surface with a ZnS coating, which leads to a dramatic increase of photocurrents and efficiencies. However, these solar cells usually show a poor current-potential characteristic, so a general investigation of the recombination mechanisms is required for improvements. A physical model based on recombination through a monoenergetic TiO(2) surface state that takes into account the effect of the surface coverage has been developed to better understand the recombination mechanisms of QDSCs. The three main methods of QD adsorption on TiO(2) are (i) in situ growth of QDs by chemical bath deposition (CBD), (ii) deposition of presynthesized colloidal QDs by direct adsorption (DA), and (iii) deposition of presynthesized colloidal QDs by linker-assisted adsorption (LA). A systematic investigation by impedance spectroscopy of QDSCs prepared by these methods showed a decrease in the charge-transfer resistance and increased electron lifetimes for CBD samples; the same result was found after ZnS coating because of the covering of the TiO(2) surface. The increase of the lifetime with the ZnS treatment has also been checked independently by open-circuit potential (V(oc)) decay measurements. Despite the lower recombination rates by electron transfer to electrolyte as well as the higher light absorption of CBD samples, only a moderate increase of photocurrent compared with colloidal QD samples is obtained, indicating the presence of an additional, internal recombination pathway in the closely packed QD layer.
Intensity dependence of current-voltage characteristics and recombination in high-efficiency solution-processed small-molecule solar cells.
Solution-processed small-molecule p-DTS(FBTTh2)2:PC71BM bulk heterojunction (BHJ) solar cells with power conversion efficiency of 8.01% are demonstrated. The fill factor (FF) is sensitive to the thickness of a calcium layer between the BHJ layer and the Al cathode; for 20 nm Ca thickness, the FF is 73%, the highest value reported for an organic solar cell. The maximum external quantum efficiency exceeds 80%. After correcting for the total absorption in the cell through normal incidence reflectance measurements, the internal quantum efficiency approaches 100% in the spectral range of 600-650 nm and well over 80% across the entire spectral range from 400 to 700 nm. Analysis of the current-voltage (J-V) characteristics at various light intensities provides information on the different recombination mechanisms in the BHJ solar cells with different thicknesses of the Ca layer. Our analysis reveals that the J-V curves are dominated by first-order recombination from the short-circuit condition to the maximum power point and evolve to bimolecular recombination in the range of voltage from the maximum power point to the open-circuit condition in the optimized device with a Ca thickness of 20 nm. In addition, the normalized photocurrent density curves reveal that the charge collection probability remains high; about 90% of charges are collected even at the maximum power point. The dominance of bimolecular recombination only when approaching open circuit, the lack of Shockley-Read-Hall recombination at open circuit, and the high charge collection probability (97.6% at the short circuit and constant over wide range of applied voltage) lead to the high fill factor.
Bimolecular recombination in polymer/fullerene bulk heterojunction solar cells
Bimolecular recombination in organic semiconductors is known to follow the Langevin expression, i.e., the rate of recombination depends on the sum of the mobilities of both carriers. We show that this does not hold for polymer/fullerene bulk heterojunction solar cells. The voltage dependence of the photocurrent reveals that the recombination rate in these blends is determined by the slowest charge carrier only, as a consequence of the confinement of both types of carriers to two different phases.
Interfacial Recombination Processes in Dye-Sensitized Solar Cells and Methods To Passivate the Interfaces
Conventional dye-sensitized solar cells function efficiently only with a single redox couple, I-/I2, because of the unusually slow kinetics for I2 reduction on SnO2 and TiO2 surfaces. When faster redox couples such as ferrocene/ferrocenium are employed, the rapid interfacial recombination of photoinjected electrons with the oxidized half of the redox couple eliminates the photovoltaic effect. To make use of other, perhaps more appropriate, redox couples in these cells, the interfacial recombination processes must be understood and controlled. Charge recombination at the SnO2/solution interface is clearly distinguishable from recombination at the nanoporous TiO2/solution interface. Dark current measurements probe mainly the former reaction, although the latter may be the dominant recombination mechanism under illumination. We introduce two methods for passivating the interfaces that decrease the recombination rates by orders of magnitude. One method involves electropolymerization of an insulating film of p...
Transport-Limited Recombination of Photocarriers in Dye-Sensitized Nanocrystalline TiO2 Solar Cells
The effect of lithium intercalation on the transport dynamics and recombination kinetics in dye-sensitized nanoparticle TiO2 solar cells at lithium levels below 5 atom % was investigated by photocurrent and photovoltage transient and spectroelectrochemical techniques. Titanium dioxide films were doped electrochemically in the dark and under illumination. It was discovered that when Li+ is present in the electrolyte, lithium intercalates irreversibly into dye-sensitized TiO2 films at open circuit (ca. −0.7 V) under normal solar light intensities. Photocurrent transients of doped nonsensitized TiO2 films indicate that lithium doping decreases the diffusion coefficient of electrons through the nanoparticle network. Photocurrent and photovoltage transients of sensitized TiO2 films provide the first evidence that electron transport limits recombination with the redox electrolyte in working cells. As the Li density in the films increases, the diffusion and recombination times of photoelectrons increase proporti...
Bimolecular recombination losses in polythiophene: Fullerene solar cells
Transient absorption spectroscopy is employed to monitor charge carrier decay dynamics in an annealed poly(3-hexylthiophene): methanofullerene solar cell. Comparisons of device and film data and data under applied bias demonstrate that these dynamics are dominated by bimolecular recombination. These data allow us to quantify the rate constant for bimolecular recombination, found to be strongly carrier density dependent, and thus determine the bimolecular recombination flux. By comparison with the device short-circuit photocurrent we conclude that the open-circuit voltage is primarily limited by bimolecular recombination.
Charge transport versus recombination in dye-sensitized solar cells employing nanocrystalline TiO2 and SnO2 films.
We report a comparison of charge transport and recombination dynamics in dye-sensitized solar cells (DSSCs) employing nanocrystalline TiO(2) and SnO(2) films and address the impact of these dynamics upon photovoltaic device efficiency. Transient photovoltage studies of electron transport in the metal oxide film are correlated with transient absorption studies of electron recombination with both oxidized sensitizer dyes and the redox couple. For all three processes, the dynamics are observed to be 2-3 orders of magnitude faster for the SnO(2) electrode. The origins of these faster dynamics are addressed by studies correlating the electron recombination dynamics to dye cations with chronoamperometric studies of film electron density. These studies indicate that the faster recombination dynamics for the SnO(2) electrodes result both from a 100-fold higher electron diffusion constant at matched electron densities, consistent with a lower trap density for this metal oxide relative to TiO(2), and from a 300 mV positive shift of the SnO(2) conduction band/trap states density of states relative to TiO(2). The faster recombination to the redox couple results in an increased dark current for DSSCs employing SnO(2) films, limiting the device open-circuit voltage. The faster recombination dynamics to the dye cation result in a significant reduction in the efficiency of regeneration of the dye ground state by the redox couple, as confirmed by transient absorption studies of this reaction, and in a loss of device short-circuit current and fill factor. The importance of this loss pathway was confirmed by nonideal diode equation analyses of device current-voltage data. The addition of MgO blocking layers is shown to be effective at reducing recombination losses to the redox electrolyte but is found to be unable to retard recombination dynamics to the dye cation sufficiently to allow efficient dye regeneration without resulting in concomitant losses of electron injection efficiency. We conclude that such a large acceleration of electron dynamics within the metal oxide films of DSSCs may in general be detrimental to device efficiency due to the limited rate of dye regeneration by the redox couple and discuss the implications of this conclusion for strategies to optimize device performance.
Boosting power conversion efficiencies of quantum-dot-sensitized solar cells beyond 8% by recombination control.
At present, quantum-dot-sensitized solar cells (QDSCs) still exhibit moderate power conversion efficiency (with record efficiency of 6-7%), limited primarily by charge recombination. Therefore, suppressing recombination processes is a mandatory requirement to boost the performance of QDSCs. Herein, we demonstrate the ability of a novel sequential inorganic ZnS/SiO2 double layer treatment onto the QD-sensitized photoanode for strongly inhibiting interfacial recombination processes in QDSCs while providing improved cell stability. Theoretical modeling and impedance spectroscopy reveal that the combined ZnS/SiO2 treatment reduces interfacial recombination and increases charge collection efficiency when compared with conventional ZnS treatment alone. In line with those results, subpicosecond THz spectroscopy demonstrates that while QD to TiO2 electron-transfer rates and yields are insensitive to inorganic photoanode overcoating, back recombination at the oxide surface is strongly suppressed by subsequent inorganic treatments. By exploiting this approach, CdSe(x)Te(1-x) QDSCs exhibit a certified record efficiency of 8.21% (8.55% for a champion cell), an improvement of 20% over the previous record high efficiency of 6.8%, together with an additional beneficial effect of improved cell stability.
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