Charge Transport and Photocurrent Generation in Poly(3-hexylthiophene): Methanofullerene Bulk-Heterojunction Solar Cells (original) (raw)
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Applied Physics Letters, 2009
We demonstrate photocurrent enhancement of up to 20% in polymer:fullerene bulk heterojunction photovoltaic cells via the incorporation of a phosphorescent dopant, without degradation in the open-circuit voltage or fill factor of the device. The enhancement is shown to originate from multiple sources. First, the phosphor is able to populate the long-lived triplet state of the polymer, leading to longer diffusion length and a larger polymer contribution. Also, there is direct absorption on the dopant leading to enhanced spectral coverage. Finally, the dopant acts as a donor site and therefore increases the fullerene signal.
Advanced Functional Materials, 2008
Photocurrent generation by charge-transfer (CT) absorption is detected in a range of conjugated polymer:[6,6]-phenyl C 61 butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (EQE) spectrum by means of Fourier-transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak groundstate charge-transfer complexes in the studied polymer:fullerene blends. The effective band gap (E g ) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all Submitted to 2 2 devices, under various preparation conditions, the open-circuit voltage (V oc ) scales linearly with E g . The redshift of the CT band upon thermal annealing of regioregular poly(3hexylthiophene):PCBM and thermal aging of poly(phenylenevinylene)(PPV):PCBM photovoltaic devices correlates with the observed drop in open-circuit voltage of hightemperature treated versus untreated devices. Increasing the weight fraction of PCBM also results in a redshift of E g , proportional with the observed changes in V oc for different PPV:PCBM ratios. As E g corresponds with the effective bandgap of the material blends, a measurement of the EQE spectrum by FTPS allows us to measure this energy directly on photovoltaic devices, and makes it a valuable technique in the study of organic bulk heterojunction solar cells. Submitted to 3 3
Advanced Energy Materials, 2014
conjugated polymer or small molecule as the donor material and a fullerene derivative as the electron acceptor. Efforts to raise the power conversion effi ciency by increasing the open-circuit voltage (V OC) have primarily focused on fi nding donor materials with lower-lying highest occupied molecular orbital (HOMO) levels [ 4 ] or fullerene derivatives with higher-lying lowest unoccupied molecular orbital (LUMO) levels. [ 5 ] Several research groups [ 6 ] have shown that a maximum of approximately 1.0 V for the V OC exists for effi cient OPV devices using fullerene derivatives as the electron acceptor. This limit is due to the inability to effi ciently split excitons on the fullerene molecule when the energy of the charge transfer (CT) state is less than 0.15 eV below the fullerene singlet excited state energy of 1.7 eV. In polymerfullerene systems where the fullerene has the smaller singlet energy, excitons formed in the polymer can reach the fullerenes via energy transfer, and ineffi cient hole transfer from the fullerenes to the polymer results in a large current loss in devices with V OC values exceeding 1.0 V. In order to relax this ceiling on the V OC and fi nd effi cient devices with voltages that can yield high effi ciencies in both single and multi-junction devices, new electron acceptors are needed with higher energy singlet excited states. [ 3b , 6d , 7 ] In addition to this restriction on the V OC , fullerene derivatives are relatively expensive [ 8 ] and C 60 derivatives do not absorb light well. One study [ 8a ] has shown that the PC 60 BM commonly used in bulk heterojunction (BHJ) organic solar cells could account for 12% of the overall OPV module cost and that C 70-based derivatives would be even more expensive. Current research into new electron acceptors [ 9 ] has covered an array of polymers [ 10 ] and small molecules. [ 11 ] While most devices prepared with these acceptors have effi ciencies near or below 2%, a few, including those based on evaporated devices incorporating halogenated boron subphthalocyanine molecules, [ 11p ] dimeric perylene diimide small molecules, [ 11r ] and solution-processed all-polymer devices based on naphthalene diimide [ 10a ] have achieved effi ciencies greater than 4%. V OC values approaching and even exceeding 1.0 V have been achieved in a few of these devices [ 11b , 11h ] but the typical values for the short-circuit current (J SC) and fi ll factor are There is a need to fi nd electron acceptors for organic photovoltaics that are not based on fullerene derivatives since fullerenes have a small band gap that limits the open-circuit voltage (V OC), do not absorb strongly and are expensive. Here, a phenylimide-based acceptor molecule, 4,7-bis(4-(N-hexyl-phthalimide)vinyl)benzo[c]1,2,5-thiadiazole (HPI-BT), that can be used to make solar cells with V OC values up to 1.11 V and power conversion effi ciencies up to 3.7% with two thiophene polymers is demonstrated. An internal quantum effi ciency of 56%, compared to 75-90% for polymer-fullerene devices, results from less effi cient separation of geminate charge pairs. While favorable energetic offsets in the polymer-fullerene devices due to the formation of a disordered mixed phase are thought to improve charge separation, the low miscibility (<5 wt%) of HPI-BT in polymers is hypothesized to prevent the mixed phase and energetic offsets from forming, thus reducing the driving force for charges to separate into the pure donor and acceptor phases where they can be collected.
Journal of Polymer Science Part B: Polymer Physics, 2011
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 0 ,7 0-dimethyloctyloxy)-p-phenylenevinylene] and 6,6-phenyl C 61-butyric acid methyl ester, we simulate the
Advanced Functional Materials, 2011
We present a combined numerical charge transport and morphology model to describe the current density-voltage ( j -V ) characteristics of three different, benchmark polymer:fullerene bulk heterojunction organic solar cells in which the device performance critically depends on the processing conditions or composition of the active layer. We fi nd that an accurate description of the j -V characteristics over a broad bias range can be obtained when the actual complex, three-dimensional (3D) phase separation is represented by a simplifi ed 2D or even 1D description. The morphological device model allows predicting the potential for increasing device performance by further optimizing the morphology. The optimal simplifi ed morphology consists of two, relatively thin alternating vertically oriented slabs, that allow for fast lateral separation of photocreated holes and electrons. This morphology can effectively be described as 1D.
Advanced Materials, 2014
be bound by Coulombic interaction. Direct evidence for the formation of strongly correlated interfacial charge pairs came from electron spin resonance studies on polymer-fullerene blends. As most donor-acceptor blends exhibit additional sub-bandgap absorption and emission features, it was proposed that these interfacial excitations are charge transfer states (CT states) coupled to the ground state via radiative transitions. The reasons for highly effi cient charge generation despite the existence of Coulombically bound geminate pairs is subject of ongoing debate. Correlations have been found between the free carrier generation yield and the energy difference