Bimolecular Recombination in a Low Bandgap Polymer:PCBM Blend Solar Cell with a High Dielectric Constant

The strength of dielectric screening is one of the most intriguing yet least studied contributing factors to the operation and performance limit of organic solar cell devices. Increasing the dielectric constant of semiconducting polymers may close the performance gap between inorganic and organic solar cell devices. Here, a dielectric constant of 16.7 is reported for a DPP-based low bandgap polymer DT-PDPP2T-TT and 7 for its 1:3 blend with [60]­PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) using frequency and voltage dependent capacitance and charge extraction by linearly increasing voltage (CELIV) techniques. The charge mobility within the blend device (1.8 × 10^–3 cm² V^–1 s^–1) is found to be among the highest reported by CELIV. Bimolecular recombination and charge carrier lifetime in efficient photovoltaic devices are measured and compared to poly­(3-hexylthiophene) (P3HT):PCBM (1:1 w/w) and poly­[2,6-(4,4-bis­(2-ethylhexyl)-4<i>H</i>-cyclopenta­[2,1-b;3,4-b′]­dithiophene)-<i>alt</i>-4,7­(2,1,3-benzothiadiazole)] (PCPDTBT):PCBM (1:2 w/w) devices. When normalized to mobility, the bimolecular recombination coefficient in DT-PDPP2T-TT:PCBM is a factor of 2 lower than in P3HT:PCBM and an order of magnitude lower than in PCPDTBT:PCBM. The recombination mechanism is found to be close to diffusion-controlled Langevin recombination. The reduced recombination is explained by a smaller Coulomb capture radius, which, together with higher charge mobility, leads to efficient charge extraction in photovoltaic devices with large active layer thicknesses approaching 300 nm

Four Orders of Magnitude Acceleration of Electron Recombination at the Dye-TiO₂/Electrolyte Interface Severely Limiting Photocurrent with High-Oxidation-Potential Cu^2+/1+ Complexes

High-oxidation-potential Cu1+ complexes have been claimed to exhibit fast electron transfer toward oxidized dyes (dye)•+ on electrode surfaces (regeneration) at a low driving force. Herein, we show that the regeneration of the surface-bound (dye)•+ with heteroleptic Cu1+ complex electrolytes with a high oxidation potential, defined by the lowest oxidation potential redox peak, with a dye regeneration driving force of 170 meV is slow. On the other hand, the recombination reaction between the electrons in the semiconductor and the (dye)•+ is accelerated by 4 orders of magnitude in the presence of a Cu2+ complex. Thus, using the Cu2+/1+ complex mixture in an electrolyte solution, such previously unknown acceleration of recombination could easily be misinterpreted as dye regeneration. This significant finding helps to explain the previously unknown origin of the photocurrent drop of solar cells using high-redox-potential Cu2+/1+ complexes with high apparent regeneration yield. Although the origin of such unprecedented acceleration of kinetics is not yet fully identified, partial oxidation of the dye layer by the Cu2+ complexes is one of the key sources. The study is expected to influence the design of new redox mediators at a low driving force and specifically how their regeneration efficiency is evaluated. The huge acceleration of kinetics could provide a new avenue to control electron transfer rates at semiconductor/organic molecule interfaces relevant to a number of electrochemical applications involving electron transfer at semiconductor/electrolyte junctions, including photocatalysis, photosynthetic systems, and sensing

Tuning Non-Langevin Recombination in an Organic Photovoltaic Blend Using a Processing Additive

The effect of altering the acceptor and exchanging a key atom in the polymer structure on the extent of non-Langevin (suppressed) recombination has been examined using the polymer/fullerene photovoltaic blend PDTSiTTz:PC60BM. Time-of-flight data show that changing the acceptor from PC60BM to PC70BM maintains the non-Langevin recombination. In contrast, altering the donor polymer by exchanging the silicon bridging atom for a carbon considerably reduces the non-Langevin behavior. Importantly, the addition of a processing additive, diiodooctane (DIO), allows a partial recovery of this non-Langevin recombination. The addition of DIO also decreases the ionization potential of the polymer, which not only explains the drop in open circuit voltage but may also contribute to the partial recovery of non-Langevin behavior observed. It is proposed that localized, more crystalline areas of lower ionization potential (or higher electron affinity) within a mixed/amorphous phase may act as energy sinks for the holes (electrons), thus potentially inhibiting bimolecular recombination. Such a phenomenon could contribute to non-Langevin behavior in organic photovoltaic blends

Entomologiceskie i parazitologiceskie issledovanija v povolz'e : sbornik nauciych trudov

Electron lifetimes in dye-sensitized solar cells employing a porphyrin dye, an organic dye, a 1:1 mixture of the two dyes, and a dichromophoric dye design consisting of the two dyes using a nonconjugated linker were measured, suggesting that the dispersion force of the organic dyes has a significant detrimental effect on the electron lifetime and that the dichromophoric design can be utilized to control the effect of the dispersion force

Trap-Assisted Transport and Non-Uniform Charge Distribution in Sulfur-Rich PbS Colloidal Quantum Dot-based Solar Cells with Selective Contacts

This study reports evidence of dispersive transport in planar PbS colloidal quantum dot heterojunction-based devices as well as the effect of incorporating a MoO₃ hole selective layer on the charge extraction behavior. Steady state and transient characterization techniques are employed to determine the complex recombination processes involved in such devices. The addition of a selective contact drastically improves the device efficiency up to 3.15% (especially due to increased photocurrent and decreased series resistance) and extends the overall charge lifetime by suppressing the main first-order recombination pathway observed in device without MoO₃. The lifetime and mobility calculated for our sulfur-rich PbS-based devices are similar to previously reported values in lead-rich quantum dots-based solar cells. Nevertheless, strong Shockley–Read–Hall mechanisms appear to keep restricting charge transport, as the equilibrium voltage takes more than 1 ms to be established

El Médico : revista de medicina clínica y experimental

Porphyrins are some of the most studied chromophores employed in photo-electrochemical energy conversion devices. However, the molar extinction coefficient of most simple porphyrins is small within the 450–550 nm wavelength region, referred to here as the absorption gap, which limits the light harvesting efficiency of thin photoelectrodes. The purpose of this work is to fill the absorption gap by covalently attaching additional chromophores with complementary absorption in the 450–550 nm wavelength region. To this end, three carbazole-fused thiophene-substituted zinc porphyrin dyes were synthesized, and their photophysical properties were investigated using UV–vis absorption, photoluminescence, resonance Raman, and electrochemical methods, supported by density functional theory calculations. All three dyes showed much-improved light harvesting up to 550 nm when attached to TiO₂ photoelectrodes, resulting in doubling the short circuit current of dye-sensitized solar cells using the Co²⁺/Co³⁺ electrolyte. The highest power conversion efficiency of 4.7% was achieved using dithieno­[3,2-b:2′,3′-d]­thiophene attached to carbazole as the additional chromophore. All three carbazole-fused thiophene dichromophoric porphyrin dyes studied have attained increased electron lifetimes contributing to their higher open circuit voltage (<i>V</i>_OC) compared to that of a simple porphyrin. Absorbed photon to collected electron efficiency together with charge extraction studies suggests that the performance of the carbazole-fused thiophene dyes is limited by electron injection

Cation Exchange at Semiconducting Oxide Surfaces: Origin of Light-Induced Performance Increases in Porphyrin Dye-Sensitized Solar Cells

The origin of simultaneous improvements in the short-circuit current density (<i>J</i>_sc) and open-circuit voltage (<i>V</i>_oc) of porphyrin dye-sensitized TiO₂ solar cells following white light illumination was studied by systematic variation of several different device parameters. Reduction of the dye surface loading resulted in greater relative performance enhancements, suggesting open space at the TiO₂ surface expedites the process. Variation of the electrolyte composition and subsequent analysis of the conduction band potential shifts suggested that a light-induced replacement of surface-adsorbed lithium (Li⁺) ions with dimethylpropylimidazolium (DMPIm⁺) ions was responsible for an increased electron lifetime by decreasing the recombination with the redox mediator. Variation of the solvent viscosity was found to affect the illumination time required to generate increased performance, while similar performance enhancements were not replicated by application of negative bias under dark conditions, indicating the light exposure effect was initiated by formation of dye cation molecules following photoexcitation. The substituents and linker group on the porphyrin chromophore were both varied, with light exposure producing increased electron lifetime and <i>V</i>_oc for all dyes; however, increased <i>J</i>_sc values were only measured for dyes containing binding moieties with multiple carboxylic acids. It was proposed that the initial injection limitation and/or fast recombination process in these dyes arises from the presence of lithium at the surface, and the improved injection and/or retardation of fast recombination after light exposure is caused by the Li⁺ removal by cation exchange under illumination

A Nonconjugated Bridge in Dimer-Sensitized Solar Cells Retards Charge Recombination without Decreasing Charge Injection Efficiency

Dye sensitized solar cells (DSSCs) employing a dimer porphyrin, which was synthesised with two porphyrin units connected without conjugation, have shown that both porphyrin components can contribute to photocurrent generation, that is, more than 50 % internal quantum efficiency. In addition, the open-circuit voltage (<i>V</i>_oc) of the DSSCs was higher than that of DSSCs using monomer porphyrins. In this paper, we first optimized cell structure and fabrication conditions. We obtained more than 80% incident photon to current conversion efficiency from the dimer porphyrin sensitized DSSCs and higher <i>V</i>_oc and energy conversion efficiency than monomer porphyrin sensitized solar cells. To examine the origin of the higher <i>V</i>_oc, we measured electron lifetime in the DSSCs with various conditions, and found that the dimer system increased the electron lifetime by improving the steric blocking effect of the dye layer, whilst the lack of a conjugated linker prevents an increase in the attractive force between conjugated sensitizers and the acceptor species in the electrolyte. The results support a hypothesis; dispersion force is one of the factors influencing the electron lifetime in DSSCs

Driving Force Dependence of Electron Transfer Kinetics and Yield in Low-Band-Gap Polymer Donor–Acceptor Organic Photovoltaic Blends

The rate of photoinduced electron transfer (PET) (κ_PET), quantum yield of PET (QY_PET), and charge extraction yield (EQE) are determined for a series of donor–acceptor (DA) organic photovoltaic systems, comprising low-band-gap polymer donors and the phenyl-C₆₁-butyric acid methyl ester (PCBM) acceptor. The energetic alignment of these polymer donors relative to PCBM provides driving forces for PET (Δ<i>G</i>_PET) in the range of 0.18–0.57 eV. Femtosecond transient absorption (TA) spectroscopy was used to assess the PET kinetics and QY_PET, while time-resolved charge extraction (TRCE) measurements were employed to assess EQE. Near unity QY_PET was observed in DA blend films with a Δ<i>G</i>_PET of 0.57 and 0.30 eV, whereas no resolvable PET was observed with a Δ<i>G</i>_PET of 0.18 eV. For the DA blends that exhibit PET, both κ_PET and QY_PET appear independent of Δ<i>G</i>_PET, with an average κ_PET of 420 fs for the 70% PCBM blends. An increase in nanosecond charge separation yield (TA) and EQE (TRCE) between DA systems was observed, which appears not to be due to the PET process but rather the subsequent recombination processes. DA systems should be designed to minimize Δ<i>G</i>_PET, minimizing associated losses in device open-circuit potential; however, picosecond bimolecular recombination severely limits achievable charge extraction yields in these DA systems

Dye Regeneration Kinetics in Dye-Sensitized Solar Cells

The ideal driving force for dye regeneration is an important parameter for the design of efficient dye-sensitized solar cells. Here, nanosecond laser transient absorption spectroscopy was used to measure the rates of regeneration of six organic carbazole-based dyes by nine ferrocene derivatives whose redox potentials vary by 0.85 V, resulting in 54 different driving-force conditions. It was found that the reaction follows the behavior expected for the Marcus normal region for driving forces below 29 kJ mol^–1 (Δ<i>E</i> = 0.30 V). Driving forces of 29–101 kJ mol^–1 (Δ<i>E</i> = 0.30–1.05 V) resulted in similar reaction rates, indicating that dye regeneration is diffusion controlled. Quantitative dye regeneration (theoretical regeneration yield 99.9%) can be achieved with a driving force of 20–25 kJ mol^–1 (Δ<i>E</i> ≈ 0.20–0.25 V)