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

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

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

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

Dependence of Charge Separation Efficiency on Film Microstructure in Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C₆₁ Butyric Acid Methyl Ester Blend Films

Herein we address the factors controlling photocurrent generation in P3HT:PCBM blend films as a function of blend composition and annealing treatment. Absorption, photoluminescence, and transient absorption spectroscopy are used to distinguish the role of exciton dissociation, charge pair separation, and charge collection. Variations in blend film microstructure with composition and annealing treatment are studied using X-ray diffraction. While the trend in photocurrent generation with composition and annealing [Muller, et al., <i>Adv. Mater.</i> <b>2008</b>, <i>20</i>, 3510] does not follow the trend in exciton dissociation, it closely follows the trend in charge pair generation. Moreover, charge pair generation efficiency is positively correlated to the degree of polymer crystallization and the appearance of large domains of both polymer and fullerene phases. We argue that larger domains assist charge pair separation by increasing the probability of escape from the P3HT:PCBM interface, thus reducing geminate charge recombination