Organic-inorganic hybrid perovskite solar cells represent an exceptional candidate for next-generation photovoltaic technology. However, the presence of surface defects in perovskite crystals limits ...the performance as well as the stability of perovskite solar cells. We have employed a series of carbazole and benzothiadiazole (BT) based donor-acceptor copolymers, which have different lengths of alkoxy side-chains grafted on the BT unit, as the dopant-free hole transport materials (HTMs) for perovskite solar cells. We demonstrate that although these side-chains can reduce the π−π stacking structural order of these copolymers to affect the hole transport properties, the methoxy unit introduces a desired defect passivation effect. Compared to the Spiro-OMeTAD-based device, the copolymer with methoxy side-chains on the BT unit (namely PCDTBT1) as the HTM achieved superior power conversion efficiency and stability due to efficient hole transport and the suppression of trap-induced degradation, whilst the copolymer with octyloxy side-chains on the BT unit (namely PCDTBT8) as the HTM lead to poor performance and stability.
A series of carbazole and benzothiadiazole (BT) based copolymers have been employed as the dopant-free hole transport materials (HTMs) for perovskite solar cells. The side-chains of these HTM can reduce the π−π stacking structural order but can also introduce desired defect passivation effect. Display omitted
•Perovskite solar cells with PCEs over 19% were achieved using dopant-free HTMs.•The thiophene and methoxy units introduce a desired defect passivation effect.•PCDTBT1 passivates surface defects of perovskite and suppresses charge recombination.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
We report comparative indoor and outdoor stability testing of organic solar cells based on a blend between a donor-acceptor polyfluorene copolymer and a fullerene derivative. The outdoor testing was ...conducted for a period over 12,000 hours in Sheffield, England, with a Ts80 lifetime determined in excess of 10,000 hours (420 days). Indoor lifetime testing was performed on solar cells using a solar simulator under a constant irradiance of 1000 W/m
for more than 650 hours. We show that under the conditions explored here, device degradation under the two sets of conditions is approximately dependent on the absorbed optical energy dose.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
We present measurements of the outdoor stability of PCDTBT:PC71BM based bulk heterojunction organic solar cells for over the course of a year. We find that the devices undergo a burn-in process ...lasting 450 hours followed by a TS80 lifetime of up to 6200 hours. We conclude that in the most stable devices, the observed TS80 lifetime is limited by thermally-induced stress between the device layers, as well as materials degradation as a result of edge-ingress of water or moisture through the encapsulation.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Palladium (Pd) is commonly used as a catalyst in the polymerisation of conjugated polymers such as polyN-9′-heptadecanyl-2,7-carbozole-alt-5,5-(4′,7′-di-2-thenyl-2′,1′,3′-benzothiadiazole) (PCDTBT). ...Here we explore the effect of residual catalyst on the performance of organic photovoltaic devices (OPVs) based on a PCDTBT:fullerene thin-film blend. We find that as the relative concentration of Pd increases, the power conversion efficiency of the PV is reduced, dropping from 4.55% to 2.42% as the Pd concentration was increased to 2570 ppm (relative to that of the PCDTBT). This reduction in efficiency resulted primarily from a reduction in PV fill factor and shunt-resistance, indicating the presence of current-shunts within the device. Using optical microscopy, laser beam induced current mapping and scanning electron microscopy, we are able to demonstrate that such current shunts are associated with micron-sized aggregates of Pd-containing nanoparticles. We show that the presence of high concentrations of Pd within a PCDTBT OPV contribute to a larger drop in efficiency during the initial ‘burn-in’ period.
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•We investigate the effects of palladium on PCDTBT:PC70BM photovoltaic devices.•PCE drops by 47% at peak palladium content compared to the lowest concentration.•PCE loss is due to an increase in current shunts with increasing palladium.•Greater PCE loss during the ‘burn-in’ period of decay for higher palladium contents.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The evolution of film structure is reported during solution casting of PCDTBT:PC70BM 1:4 wt%, a polymer:fullerene blend system that finds application in an organic photovoltaic device. Using the ...complimentary techniques of grazing‐incidence wide‐angle X‐ray scattering and spectroscopic ellipsometry, a number of distinct processes that occur during film formation are identified. This includes the growth of fullerene molecules into nanoscale aggregates, the onset of which coincides with the solubility limit of the material in the casting solvent being reached. An apparent delay in Bragg scatter from the PCDTBT‐rich phase of the film suggests that, for the film composition studied here, the aggregation of PC70BM precedes weak self‐organisation of the conjugated polymer. This behaviour is compared with the drying dynamics of a number of different polymer:fullerene blends that each contain a high weight fraction of fullerene molecules, and a range of comparable solid concentrations are identified beyond which the precipitation of fullerene aggregates from solution occurs. These observations provide an insight into the development of structure in relatively amorphous polymer:fullerene blends for organic photovoltaic applications and potentially assists the future optimisation of this category of materials.
Thin‐film formation in solution‐cast PCDTBT:PC70BM 1:4 wt% blend films is characterised using a combination of spectroscopic ellipsometry and grazing‐incidence wide angle X‐ray scattering. A multi‐step process of structure evolution is observed which includes the coalescence of PC70BM molecules and an apparent delay in weak ordering within the PCDTBT‐rich phases of the film.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
A new donor–acceptor polymer based on 9,9‐dioctylfluorene is synthesized and tested in organic photovoltaic devices. Results show that the polymer exhibits good solubility in a range of organic ...solvents and has a high hole mobility. When blended with a PC70BM acceptor and fabricated into a bulk heterojunction, photovoltaic devices having a maximum power conversion efficiency (PCE) of 6.2% and a peak external quantum efficiency of 74% are created. Such efficiencies are realized without any necessity for solvent additives or thermal annealing protocols.
A novel low bandgap, fluorene‐based polymer with a soluble benzothiadiazole moiety is synthesized and presented. By including additional thiophenes, the optical bandgap is reduced. Initial studies show good solubility and hole mobility. Bulk heterojunction devices realize efficiencies up to 6.2% without the need for a solvent additive or thermal annealing treatment.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We have used Soxhlet solvent purification to fractionate a broad molecular weight distribution of the polycarbazole polymer PCDTBT into three lower polydispersity molecular weight fractions. Organic ...photovoltaic devices were made using a blend of the fullerene acceptor PC₇₁BM with the molecular weight fractions. An average power conversion efficiency of 5.89% (peak efficiency of 6.15%) was measured for PCDTBT blend devices with a number average molecular weight of Mn = 25.5 kDa. There was significant variation between the molecular weight fractions with low (Mn = 15.0 kDa) and high (Mn = 34.9 kDa) fractions producing devices with average efficiencies of 5.02% and 3.70% respectively. Neutron reflectivity measurements on these polymer:PC₇₁BM blend layers showed that larger molecular weights leads to an increase in the polymer enrichment layer thickness at the anode interface, this improves efficiency up to a limiting point where the polymer solubility causes a reduction of the PCDTBT concentration in the active layer.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
We report the fabrication of high performance organic solar cells by spray‐coating the photoactive layer in air. The photovoltaic blends consist of a blend of carbazole and benzothiadiazole based ...donor–acceptor copolymers and the fullerene derivative PC70BM. Here, we formulate a number of photovoltaic inks using a range of solvent systems that we show can all be deposited by spray casting. We use a range of techniques to characterize the structure of such films, and show that spray‐cast films have comparable surface roughness to spin‐cast films and that vertical stratification that occurs during film drying reduces the concentration of PCBM towards the underlying PEDOT:PSS interface. We also show that the active layer thickness and the drying kinetics can be tuned through control of the substrate temperature. High power conversion efficiencies of 4.3%, 4.5% and 4.6% were obtained for solar cells made from a blend of PC70BM with the carbazole‐based co‐polymers PCDTBT, P2 and P1. By applying a low temperature anneal after the deposition of the cathode, the efficiency of spray‐cast solar‐cells based on a P2:PC70BM blend is increased to 5.0%. Spray coating holds significant promise as a technique capable of fabricating large‐area, high performance organic solar cells in air.
High performance organic solar cells from the blend of donor–acceptor copolymers and PC70BM were made by spray‐coating the photoactive layer in air. A high power conversion efficiency of 5% was achieved after device optimization. Neutron scattering measurements show that vertical stratification occurs during the film drying process, and the concentration of PCBM reduces towards the underlying PEDOT:PSS interface.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Neutron reflectivity and X‐ray scattering have been used to characterise the structure of photovoltaic optimised PCDTBT:PCBM (1:4) blend films. A negative gradient of the PCBM forms spontaneously ...upon film casting – a fortuitous structure that is well matched for efficient charge extraction. Mild thermal annealing at 70 °C does not significantly modify the film structure, and any improvement in device efficiency is likely to be through the removal of trapped casting solvent.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
A series of donor/acceptor carbazole copolymers comprising alternating 6,7-diphenyl-4,9-bis-(thiophen-2-yl)-1,2,5thiadiazolo3,4-gquinoxaline and 3,6-dimethyl-9-alkyl-9H-carbazole repeat units (P1), ...or 3,6-dimethyl-9-triarylamino-9H-carbazole repeat units (P2), or 9-triarylamino-9H-carbazole repeat units (P3) has been prepared following Suzuki polymerization procedures. P3 absorbs light up to 1200 nm and has an energy gap of 1.1 eV, while P1 and P2 have energy gaps of 1.3 and 1.25 eV, respectively. Photovoltaic cells with ITO/PEDOT:PSS/P3:PCBM (1:1 w/w)/Ca showed an open-circuit voltage of 0.4 V under white light illumination, power conversion efficiency of 0.61%, and short-circuit current of 5.2 mA*cm-2.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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