Optimization of solution-processed oligothiophene:fullerene based organic solar cells by using solvent additives.
ABSTRACT: The optimization of solution-processed organic bulk-heterojunction solar cells with the acceptor-substituted quinquethiophene DCV5T-Bu 4 as donor in conjunction with PC61BM as acceptor is described. Power conversion efficiencies up to 3.0% and external quantum efficiencies up to 40% were obtained through the use of 1-chloronaphthalene as solvent additive in the fabrication of the photovoltaic devices. Furthermore, atomic force microscopy investigations of the photoactive layer gave insight into the distribution of donor and acceptor within the blend. The unique combination of solubility and thermal stability of DCV5T-Bu 4 also allows for fabrication of organic solar cells by vacuum deposition. Thus, we were able to perform a rare comparison of the device characteristics of the solution-processed DCV5T-Bu 4 :PC61BM solar cell with its vacuum-processed DCV5T-Bu 4 :C60 counterpart. Interestingly in this case, the efficiencies of the small-molecule organic solar cells prepared by using solution techniques are approaching those fabricated by using vacuum technology. This result is significant as vacuum-processed devices typically display much better performances in photovoltaic cells.
Project description:Phthalocyanines and their main group and metal complexes are important classes of organic semiconductor materials but are usually highly insoluble and so frequently need to be processed by vacuum deposition in devices. We report two highly soluble silicon phthalocyanine (SiPc) diester compounds and demonstrate their potential as organic semiconductor materials. Near-infrared (?(EL) = 698-709 nm) solution-processed organic light-emitting diodes (OLEDs) were fabricated and exhibited external quantum efficiencies (EQEs) of up to 1.4%. Binary bulk heterojunction solar cells employing P3HT or PTB7 as the donor and the SiPc as the acceptor provided power conversion efficiencies (PCE) of up to 2.7% under simulated solar illumination. Our results show that soluble SiPcs are promising materials for organic electronics.
Project description:Squaraine dyes have shown promising properties for high performance organic solar cells owing to their advantages of intense absorption and high absorption coefficients in the visible and near-infrared (NIR) regions. In this work, to directly compare the photovoltaic performance of solution- and vacuum-processed small-molecule bulk heterojunction (SMBHJ) solar cells, we employed a squaraine small molecular dye, 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (DIBSQ), as an electron donor combined with fullerene acceptors to fabricate SMBHJ cells either from solution or vacuum deposition process. The solution-processed SMBHJ cell possesses a power conversion efficiency (PCE) of ~4.3%, while the vacuum-processed cell provides a PCE of ~6.3%. Comparison of the device performance shows that the vacuum-processed SMBHJ cells possess higher short-circuit current density, fill factor and thus higher PCE than the solution-processed devices, which should be assigned to more efficient charge transport and charge extraction in the vacuum-processed SMBHJ cells. However, solution-processed SMBHJ cells demonstrate more pronounced temperature-dependent device performance and higher device stability. This study indicates the great potential of DIBSQ in photovoltaic application via both of solution and vacuum processing techniques.
Project description:Solution-processable D-?-A-?-D structured two organic small molecules bearing thienyl diketopyrrolopyrrole (TDPP) and furanyl diketopyrrolopyrrole (FDPP) as central acceptor units and cyano on the ?-bridge and phenothiazine as the terminal donor units, coded as TDPP-PTCN and FDPP-PTCN, are designed and synthesized. The C-H arylation and Suzuki coupling protocols have been adopted for synthesizing the molecules. Solution-processed organic solar cells (OSCs) were constructed with these molecules as the donors and phenyl-C71-butyric acid methyl ester as the acceptor yielding power conversion efficiencies (PCE) of 4.0% for FDPP-PTCN and 5.2% for TDPP-PTCN, which is the highest PCE reported so far from the small molecular DPP-phenothiazine-based architecture for solution-based OSCs. The effect of heteroatom substitution on thermal stability and optoelectronic and photovoltaic performances is also systematically investigated herein. This work demonstrates that replacement of oxygen with sulfur in these kinds of small molecules remarkably improves the photovoltaic performance of OSCs.
Project description:A two-dimensional conjugated small molecule (SMPV1) was designed and synthesized for high performance solution-processed organic solar cells. This study explores the photovoltaic properties of this molecule as a donor, with a fullerene derivative as an acceptor, using solution processing in single junction and double junction tandem solar cells. The single junction solar cells based on SMPV1 exhibited a certified power conversion efficiency of 8.02% under AM 1.5 G irradiation (100 mW cm(-2)). A homo-tandem solar cell based on SMPV1 was constructed with a novel interlayer (or tunnel junction) consisting of bilayer conjugated polyelectrolyte, demonstrating an unprecedented PCE of 10.1%. These results strongly suggest solution-processed small molecular materials are excellent candidates for organic solar cells.
Project description:We report a spray deposition technique as a screening tool for solution processed solar cells. A dual-feed spray nozzle is introduced to deposit donor and acceptor materials separately and to form blended films on substrates in situ. Using a differential pump system with a motorised spray nozzle, the effect of film thickness, solution flow rates and the blend ratio of donor and acceptor materials on device performance can be found in a single experiment. Using this method, polymer solar cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C61 butyric acid methyl ester (PC61BM) are fabricated with numerous combinations of thicknesses and blend ratios. Results obtained from this technique show that the optimum ratio of materials is consistent with previously reported values confirming this technique is a very useful and effective screening method. This high throughput screening method is also used in a single-feed configuration. In the single-feed mode, methylammonium iodide solution is deposited on lead iodide films to create a photoactive layer of perovskite solar cells. Devices featuring a perovskite layer fabricated by this spray process demonstrated a power conversion efficiencies of up to 7.9%.
Project description:Organic solar cells (OSCs) offer the possibility of harnessing the sun's ubiquitous energy in a low-cost, environmentally friendly and renewable manner. OSCs based on small molecule semiconductors (SMOSCs)--have made a substantial improvement in recent years and are now achieving power conversion efficiencies (PCEs) that match those achieved for polymer:fullerene OSCs. To date, all efficient SMOSCs have relied on the same fullerene acceptor, PCBM, in order to achieve high performance. The use of PCBM however, is unfavourable due to its low lying LUMO level, which limits the open-circuit voltage (VOC). Alternative fullerene derivatives with higher lying LUMOs are thus required to improve the VOC. The challenge, however, is to prevent the typical concomitant decrease in the short circuit current density (JSC) when using a higher LUMO fullerene. In this communication, we address the issue by applying methano indene fullerene, MIF, a bis-functionalised C60 fullerene that has a LUMO level 140?mV higher than PCBM, in solution processed SMOSCs with a well known small molecule donor, DPP(TBFu)2. MIF-based devices show an improved VOC of 140?mV over PC61BM and only a small decrease in the JSC, with the PCE increasing to 5.1% (vs. 4.5% for PC61BM).
Project description:Four solution-processable D-A-D structured organic molecules with diketopyrrolopyrrole (DPP) as acceptor unit and triphenylamine (TPA) or (4-hexyl)thieno [3,2-b]thiophene (HTT) as donor unit, DPP8-TPA, DPP8-TPA-OR, DPP6-HTT and DPP8-HTT, were designed and synthesized for the application as donor materials in solution-processed organic solar cells (OSCs). The molecules show broad absorption and relatively lower highest occupied molecular orbital energy levels. Photovoltaic properties of the molecules were investigated by fabricating the bulk-heterojunction OSCs with the molecules as donor and PC71BM as acceptor. Power conversion efficiency of the OSC based on DPP8-HTT reached 1.5% under the illumination of AM1.5, 100 mW cm(-2).
Project description:The synthesis and characterization of four novel donor-acceptor-donor ?-extended oligomers, incorporating naphtha(1-b)thiophene-4-carboxylate or benzo(b)thieno(3,2-g) benzothiophene-4-carboxylate 2-octyldodecyl esters as end-capping moieties, and two different conjugated core fragments, is reported. The end-capping moieties are obtained via a cascade sequence of sustainable organic reactions, and then coupled to benzo(c)(1,2,5)thiadiazole and its difluoro derivative as the electron-poor ?-conjugated cores. The optoelectronic properties of the oligomers are reported. The novel compounds revealed good film forming properties, and when tested in bulk-heterojunction organic photovoltaic cell devices in combination with PC61BM, revealed good fill factors, but low efficiencies, due to their poor absorption profiles.
Project description:Donor-acceptor organic solar cells often show low open-circuit voltages (V OC) relative to their optical energy gap (E g) that limit power conversion efficiencies to ~12%. This energy loss is partly attributed to the offset between E g and that of intermolecular charge transfer (CT) states at the donor-acceptor interface. Here we study charge generation occurring in PIPCP:PC61BM, a system with a very low driving energy for initial charge separation (E g-E CT?~?50?meV) and a high internal quantum efficiency (? IQE?~?80%). We track the strength of the electric field generated between the separating electron-hole pair by following the transient electroabsorption optical response, and find that while localised CT states are formed rapidly (<100?fs) after photoexcitation, free charges are not generated until 5?ps after photogeneration. In PIPCP:PC61BM, electronic disorder is low (Urbach energy <27?meV) and we consider that free charge separation is able to outcompete trap-assisted non-radiative recombination of the CT state.
Project description:Post-treatment processes improve the performance of organic photovoltaic devices by changing the microscopic morphology and configuration of the vertical phase separation in the active layer. Thermal annealing and solvent vapor (or chemical) treatment processes have been extensively used to improve the performance of bulk-heterojunction (BHJ) organic photovoltaic (OPV) devices. In this work we introduce a new post-treatment process which we apply only electrical voltage to the BHJ-OPV devices. We used the commercially available P3HT [Poly(3-hexylthiophene)] and PC61BM (Phenyl-C61-Butyric acid Methyl ester) photovoltaic materials as donor and acceptor, respectively. We monitored the voltage and current applied to the device to check for when the post-treatment process had been completed. This electrical treatment process is simpler and faster than other post-treatment methods, and the performance of the electrically treated solar cell is comparable to that of a reference (thermally annealed) device. Our results indicate that the proposed treatment process can be used efficiently to fabricate high-performance BHJ-OPV devices.