Morphology‐Limited Free Carrier Generation in Donor/Acceptor Polymer Blend Solar Cells Composed of Poly(3‐hexylthiophene) and Fluorene‐Based Copolymer

Abstract

fullerene derivative such as [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). [ 2,3 ] On the other hand, polymer/polymer BHJ solar cells, which consist of a polymer donor and a polymer acceptor, have been increasingly researched given the potential advantages they could have over conventional polymer/ fullerene BHJ devices. [ 4,5 ] The added fl exibility in molecular design introduced by polymer acceptors is particularly important, as it helps to improve short-circuit current density ( J SC ), open-circuit voltage ( V OC ), and the fi ll factor (FF) through fi ne tuning optical, electronic, and morphological properties. However, despite these attractive features, the development of polymer/polymer BHJ solar cells still lags far behind that of their polymer/fullerene counterparts, [ 6 ] with power conversion effi ciencies (PCEs) around 2% until recently. [ 4,5 ] Owing to considerable efforts in synthesizing polymer acceptors and optimizing the blend morphology, the PCEs of polymer/polymer BHJ solar cells have been improved up to more than 5% in the last few years. [ 7 ] Fluorene and benzothiadiazole-based copolymers are among the most promising of these acceptor alternatives [8][9][10][11] because of their large absorption coeffi cients in the visible region and higher energy levels for the lowest unoccupied molecular orbital (LUMO); these features in turn increase light absorption and V OC , respectively. Recently, we reported a polymer/ polymer BHJ solar cell containing poly(3-hexylthiophene) (P3HT) as the donor and poly[2,7-(9,9-didodecylfl uorene)-alt -5,5-(4′,7′-bis(2-thienyl)-2′,1′,3′-benzothiadiazole)] (PF12TBT) as the acceptor ( Figure 1 a). [9][10][11] This system gave an energy difference of more than 1 eV between the highest occupied molecular orbital (HOMO) of the donor and the LUMO of the acceptor, retaining the 0.8 eV offset in the HOMO-HOMO and the LUMO-LUMO levels needed for effi cient charge transfer ( Figure 1 ). As expected, much higher V OC of 1.26 V was yielded when compared to P3HT/PCBM devices, which typically yield values of only about 0.6 V. [9][10][11] In addition, thermal annealing improved both the J SC and the FF, resulting in a PCE of 2.7%. [ 10,11 ] However, the maximum external quantum effi ciency (EQE) of the P3HT/PF12TBT BHJ solar cells was just 30%, far below the 70-80% values reported for P3HT/PCBM devices. [ 2 ] This means that the high V OC of this The charge generation and recombination dynamics in polymer/polymer blend solar cells composed of poly(3-hexylthiophene) (P3HT, electron donor) and poly[2,7-(9,9-didodecylfl uorene)-alt -5,5-(4′,7′-bis(2-thienyl)-2′,1′,3′-benzothiadiazole)] (PF12TBT, electron acceptor) are studied by transient absorption measurements. In the unannealed blend fi lm, charge carriers are effi ciently generated from polymer excitons, but some of them recombine geminately. In the blend fi lm annealed at 160 °C, on the other hand, the geminate recombination loss is suppressed and hence free carrier generation effi ciency increases up to 74%. These ...

Ternary miscibility with specific interactions in novel blends comprising biodegradable polymers and a natural polyphenol

“…These polymers are widely applied in the daily life and investigated [1][2][3][4][5][6]. For example, they can be used in packing, electrical products, and composites [7][8][9][10][11][12]. In recent years, novel polymeric materials showing biodegradability, or biodegradable polymers, have attracted considerable interest [13][14][15].…”

Thermal effect on the morphology and performance of organic photovoltaics

“…In the simulation, a certain number of excitons were created at the start under the conditions that no charge injection, leak, nor collection and no applied voltage were imposed. The initial number of excitons was set to 100 and 1000: the latter corresponds to a density of 3 Â 10 17 cm À3 , which is consistent with the experimental value; 14 the former was for comparison, and note that we can simulate dilute conditions that experimental observation is difficult. There have been studies that discuss the dynamics of carriers-the effect of mobility on charge separation, 45 and the kinetic model of charge separation and recombination.…”

“…20 (vii) When a charge carrier reaches the electrode surface, it is able to be collected and then the current flows. The charge collection rate is also calculated by the Marcus formula (eqn (14)), but the negative of the charge injection barrier is considered for DE. Collection of holes from the cathode and electrons from the anode contribute to the photocurrent.…”

“…8,9 Transient absorption spectroscopy was also performed to track the efficiency and dynamics of excitons and charge carriers, and influences of morphologies were examined. 14 Meanwhile, various theoretical studies on OPVs have been presented to obtain a detailed understanding of the mechanism of photocurrent generation. 15 Quantum chemical calculations estimate properties such as energy levels and distributions of molecular orbitals, absorbance, [16][17][18][19] and rate of an elementary process.…”

The effect of processing additives for charge generation, recombination, and extraction in bulk heterojunction layers of all-polymer photovoltaics

“…To achieve such optimal morphology in all-polymer solar cells, several strategies, such as thermal annealing, solvent vapor annealing, and solvent selection, have been adopted, which, however, have demonstrated success only in a limited number of polymers. 9,10 Compared to these treatments, solvent additives are an effective approach, showing higher compatibility for polymers and successfully controlled morphology. By mixing a few volume percent of processing additives with the host solvent, dichlorobenzene (DCB) or chlorobenzene, the efficiencies of many polymers can be improved dramatically.…”