ABSTRACT The performance of polymer solar cells is strongly dependent on the morphology of the ph... more ABSTRACT The performance of polymer solar cells is strongly dependent on the morphology of the photoactive layer, which can be optimized by tuning the polymer side chain pattern. Whereas most studies focus on length and bulkiness, the side chain density receives much less attention. In this work, the effect of the number of side chains on PCPDTQx(2F) low bandgap copolymers on material properties and solar cell characteristics is investigated. The active layer morphology is strongly affected, affording more favorable finely intermixed blends when decreasing the side chain density. As a result, the efficiency increases to a maximum of 5.63% for the device based on the copolymer with intermediate side chain density. Moreover, removal of the side chains also has a positive effect on device stability under prolonged thermal stress. A single structural parameter—alkyl side chain density—is hence used for simultaneous enhancement of both solar cell efficiency and lifetime.
ABSTRACT Although degradation mechanisms in organic photovoltaic devices continue to receive incr... more ABSTRACT Although degradation mechanisms in organic photovoltaic devices continue to receive increased attention, it is only recently that the initial light-induced failure, or so-called burn-in effect, has been considered. Both prototypical polythiophene:fullerene and polycarbazole:fullerene systems exhibit an exponential performance loss of ≈40% upon 150 h of continuous solar illumination. While the decrease in both the short-circuit current (JSC) and open-circuit voltage (VOC) is the origin of performance loss in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC60BM), in poly(N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)):[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC70BM) the decline of the fill factor dominates. By systematic variation of the interface layers, active layer thickness, and acceptor in polythiophene:fullerene cells, the loss in JSC is ascribed to a degradation in the bulk of the P3HT:PC60BM, while the drop in VOC is reversible and arises from charge trapping at the contact interfaces. By replacing the C60 fullerene derivative with a C70 derivative, or by modifying the electron transport layer, the JSC or VOC, respectively, are stabilized. These insights prove that the burn-in process stems from multiple concurrent failure mechanisms. Comparing the ageing and recovery processes in P3HT and PCDTBT blends results in the conclusion that their interface failures differ in nature and that burn-in is a material dependent, rather than an intrinsic, failure mechanism.
Here we report the fabrication of nanofibre-based organic phototransistors (OPTs) using preformed... more Here we report the fabrication of nanofibre-based organic phototransistors (OPTs) using preformed poly(3-hexylthiophene) (P3HT) nanofibres. OPT performance is analysed based on two important parameters: photoresponsivity R and photosensitivity P. Before testing the devices as OPTs, the normal organic field-effect transistor (OFET) operation is characterized, revealing a surface-coverage-dependent performance. With R reaching 250 A W(-1) in the on-state (V(GS) = -40 V) and P reaching 6.8 × 10(3) in the off-state (V(GS) = 10 V) under white light illumination (I(inc) = 0.91 mW cm(-2)), the best nanofibre-based OPTs outperform the OPTs fabricated from a solution of P3HT in chlorobenzene, in which no preformed fibres are present. The better performance is attributed to an increase in active layer crystallinity, a better layer connectivity and an improved edge-on orientation of the thiophene rings along the polymer backbone, resulting in a longer exciton diffusion length and enhanced charge carrier mobility, linked to a decreased interchain coupling energy. In addition, the increased order in the active layer crystallinity induces a better spectral overlap between the white light emission spectrum and the active layer absorption spectrum, and the absorption of incident light is maximised by the favourable parallel orientation of the polymer chains with respect to the OPT substrate. Combining both leads to an increase in the overall light absorption. In comparison with previously reported solution-processed organic OPTs, it is shown here that no special dielectric surface treatment or post-deposition treatment of the active device layer is needed to obtain high OPT performance. Finally, it is also shown that, inherent to an intrinsic gate-tuneable gain mechanism, changing the gate potential results in a variation of R over at least five orders of magnitude. As such, it is shown that R can be adjusted according to the incident light intensity.
ABSTRACT The performance of polymer solar cells is strongly dependent on the morphology of the ph... more ABSTRACT The performance of polymer solar cells is strongly dependent on the morphology of the photoactive layer, which can be optimized by tuning the polymer side chain pattern. Whereas most studies focus on length and bulkiness, the side chain density receives much less attention. In this work, the effect of the number of side chains on PCPDTQx(2F) low bandgap copolymers on material properties and solar cell characteristics is investigated. The active layer morphology is strongly affected, affording more favorable finely intermixed blends when decreasing the side chain density. As a result, the efficiency increases to a maximum of 5.63% for the device based on the copolymer with intermediate side chain density. Moreover, removal of the side chains also has a positive effect on device stability under prolonged thermal stress. A single structural parameter—alkyl side chain density—is hence used for simultaneous enhancement of both solar cell efficiency and lifetime.
ABSTRACT Although degradation mechanisms in organic photovoltaic devices continue to receive incr... more ABSTRACT Although degradation mechanisms in organic photovoltaic devices continue to receive increased attention, it is only recently that the initial light-induced failure, or so-called burn-in effect, has been considered. Both prototypical polythiophene:fullerene and polycarbazole:fullerene systems exhibit an exponential performance loss of ≈40% upon 150 h of continuous solar illumination. While the decrease in both the short-circuit current (JSC) and open-circuit voltage (VOC) is the origin of performance loss in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC60BM), in poly(N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)):[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC70BM) the decline of the fill factor dominates. By systematic variation of the interface layers, active layer thickness, and acceptor in polythiophene:fullerene cells, the loss in JSC is ascribed to a degradation in the bulk of the P3HT:PC60BM, while the drop in VOC is reversible and arises from charge trapping at the contact interfaces. By replacing the C60 fullerene derivative with a C70 derivative, or by modifying the electron transport layer, the JSC or VOC, respectively, are stabilized. These insights prove that the burn-in process stems from multiple concurrent failure mechanisms. Comparing the ageing and recovery processes in P3HT and PCDTBT blends results in the conclusion that their interface failures differ in nature and that burn-in is a material dependent, rather than an intrinsic, failure mechanism.
Here we report the fabrication of nanofibre-based organic phototransistors (OPTs) using preformed... more Here we report the fabrication of nanofibre-based organic phototransistors (OPTs) using preformed poly(3-hexylthiophene) (P3HT) nanofibres. OPT performance is analysed based on two important parameters: photoresponsivity R and photosensitivity P. Before testing the devices as OPTs, the normal organic field-effect transistor (OFET) operation is characterized, revealing a surface-coverage-dependent performance. With R reaching 250 A W(-1) in the on-state (V(GS) = -40 V) and P reaching 6.8 × 10(3) in the off-state (V(GS) = 10 V) under white light illumination (I(inc) = 0.91 mW cm(-2)), the best nanofibre-based OPTs outperform the OPTs fabricated from a solution of P3HT in chlorobenzene, in which no preformed fibres are present. The better performance is attributed to an increase in active layer crystallinity, a better layer connectivity and an improved edge-on orientation of the thiophene rings along the polymer backbone, resulting in a longer exciton diffusion length and enhanced charge carrier mobility, linked to a decreased interchain coupling energy. In addition, the increased order in the active layer crystallinity induces a better spectral overlap between the white light emission spectrum and the active layer absorption spectrum, and the absorption of incident light is maximised by the favourable parallel orientation of the polymer chains with respect to the OPT substrate. Combining both leads to an increase in the overall light absorption. In comparison with previously reported solution-processed organic OPTs, it is shown here that no special dielectric surface treatment or post-deposition treatment of the active device layer is needed to obtain high OPT performance. Finally, it is also shown that, inherent to an intrinsic gate-tuneable gain mechanism, changing the gate potential results in a variation of R over at least five orders of magnitude. As such, it is shown that R can be adjusted according to the incident light intensity.
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