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Efficient surface passivation of perovskite solar cells (PSC) using treatment with ammonium salts is demonstrated as an efficient method to enhance the device performance, owing to the affinity between the amine group and [PbI6]4− octahedron. However, due to their high solubility in polar solvents (DMF/DMSO), ammonium salts are more difficult to use in passivation of the interface between the electron transport layer and perovskite thin film in n‐i‐p structured PSCs. In this report, this work successfully links the amine group with a fullerene through a series of increasing carbon chain length, from two to twelve methylene units (FC‐X, X = 2, 6, 12), and then introduce the synthesized molecules as interface passivation layers into SnO2‐based planar n‐i‐p PSCs. Results show that the interface passivation effect is highly dependent on the side‐chain length, and the longer chain length amine‐functionalized fullerene is more beneficial for the device performance. A power conversion efficiency as high as 21.2% is achieved by using FC‐12. The surface energy, perovskite crystallite size and electron transfer capacity correlate with the linker chain length. This work develops an amine‐induced anchored crystallization of perovskite to unravel the mechanism of this passivation effect. As expected, enhanced device stability is also observed in the FC‐12 passivated PSCs.

Passive thermoregulation is an important strategy to prevent overheating in thermally challenging environments. Can the diversity of optical properties found in Christmas beetles (Rutelinae) be an advantage to keep cool? We measured changes in temperature of the elytra of 26 species of Christmas beetles, exclusively due to direct radiation from a solar simulator in visible (VIS: 400-700 nm) and near infrared (NIR: 700-1700 nm) wavebands. Then, we evaluated if the optical properties of elytra could predict their steady state temperature and heating rates, while controlling for size. We found that higher absorptivity increases the heating rate and final steady state of the beetle elytra in a biologically significant range (3 to 5°C). There was substantial variation in the absorptivity of Christmas beetle elytra; and this variation was achieved by different combinations of reflectivity and transmissivity in both VIS and NIR. Size was an important factor predicting the change in temperature of the elytra after 5 min (steady state) but not maximum heating rate. Lastly, we show that the presence of the elytra covering the body of the beetle can reduce heating of the body itself. We propose that beetle elytra can act as a semi-insulating layer to enable passive thermoregulation through high reflectivity of elytra, resulting in low absorptivity of solar radiation. Alternatively, if beetle elytra absorb a high proportion of solar radiation, they may reduce heat transfer from the elytra to the body through behavioral or physiological mechanisms.

Bulk-heterojunction nanoparticle dispersions in water or alcohol can be employed as eco-friendly inks for the fabrication of organic solar cells by printing or coating. However, one major drawback is the need for stabilizing surfactants, which facilitate nanoparticle formation but later hamper device performance. When surfactant-free dispersions are formulated, a strong limitation is imposed by the dispersion concentration due to the tendency of nanoparticles to aggregate. In this work, pyridine end-capped poly(3-hexylthiophene) (P3HT-Py) is synthesized and included as an additive for the stabilization of P3HT:indene-C60 bis-adduct (ICBA) nanoparticle dispersions. In the presence of acetic acid (AcOH), a surface-active pyridinium acetate end-capped P3HT ion pair, P3HT-PyH+AcO-, is formed which effectively stabilizes the dispersion and hence allows the formation of dispersions with smaller nanoparticle sizes and higher concentrations of up to 30 mg/mL in methanol. The dispersions exhibit an enhanced shelf-lifetime of at least 60 days at room temperature. After the deposition of light-harvesting layers from the nanoparticle dispersions, the ion-pair formation is reversed at elevated temperatures leading to regeneration of P3HT-Py and AcOH. The AcOH evaporates from the active layer, while the performance of the corresponding solar cells is not affected by the residual P3HT-Py in the devices. Enhanced nanoparticle stability is achieved with only 0.017 wt % pyridine in the P3HT/ICBA formulation.

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination—poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ∼487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ∼2000 times less than predicted by Langevin theory. This in turn leads to the buildup of electric charge in donor and acceptor domains—away from the interface—resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films due to the different scaling of electroabsorption and photoinduced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of >8% because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies the bulk heterojunction can go beyond its established role in charge photogeneration and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination.

Synthesis of fluorene‐based conjugated polyelectrolytes was achieved via Suzuki polycondensation in water and completely open to air. The polyelectrolytes were conveniently purified by dialysis and analysis of the materials showed properties expected for fluorene‐based conjugated polyelectrolytes. The materials were then employed in solar cell devices as an interlayer in conjunction with ZnO. The double interlayer led to enhanced power conversion efficiency of 10.75 % and 15.1 % for polymer and perovskite solar cells, respectively.

Following an initial work on the isolation of a single geometric isomer from an indene-C70 bisadduct (IC70BA) mixture, we report the full fractionation and identification of the bisadduct species in the material. Eleven fractions of IC70BA isomers were separated by high-performance liquid chromatography. A number of fractions contained relatively pure isomer species and their configuration were deduced using a variety of analytical techniques including (1)H and (13)C NMR and UV-vis spectroscopy. The electrochemical properties and the organic solar cell device performance were investigated for fractions where a reasonable quantity of sample could be isolated.

A syndiotactic poly(methyl methacrylate) bottlebrush polymer has been shown to complex with C60 fullerene and assemble into nanoparticles that can be dispersed in polar organic solvents. This composite material was used as an electrode interlayer in organic solar cell (OSC) devices leading to enhanced device performance.

Bimolecular recombination in bulk heterojunction organic solar cells is the process by which nongeminate photogenerated free carriers encounter each other, and combine to form a charge transfer (CT) state which subsequently relaxes to the ground state. It is governed by the diffusion of the slower and faster carriers toward the electron donor–acceptor interface. In an increasing number of systems, the recombination rate constant is measured to be lower than that predicted by Langevin's model for relative Brownian motion and the capture of opposite charges. This study investigates the dynamics of charge generation, transport, and recombination in a nematic liquid crystalline donor:fullerene acceptor system that gives solar cells with initial power conversion efficiencies of >9.5%. Unusually, and advantageously from a manufacturing perspective, these efficiencies are maintained in junctions thicker than 300 nm. Despite finding imbalanced and moderate carrier mobilities in this blend, strongly suppressed bimolecular recombination is observed, which is ≈150 times less than predicted by Langevin theory, or indeed, more recent and advanced models that take into account the domain size and the spatial separation of electrons and holes. The suppressed bimolecular recombination arises from the fact that ground‐state decay of the CT state is significantly slower than dissociation.

Well‐defined semiconducting bottlebrush polymers are prepared using controlled polymerization methods. The electron donor semiconductor poly(3‐hexylthiophene), P3HT, is synthesized using Grignard metathesis polymerization. With appropriate norbornene end caps, the P3HT macromonomers are used to form the bottlebrush material via ring opening metathesis polymerization. The bottlebrush polymers are characterized and compared with linear P3HT materials, as well as high‐molecular‐weight commercial P3HT. It is found that longer polythiophene chains impart better semiconducting properties resulting in higher solar‐cell device performance. image

A single isomer of fullerene bisadduct, PC61PF, was obtained from commercially available fullerene derivative, PC61BM, in one pot over two steps. The tether-directed remote functionalization approach provided a very simple and fast method to produce a single isomer of fullerene bisadduct with good yield and easy purification. Bulk heterojunction organic solar cells containing the bisadduct was fabricated and tested.

A high molecular weight donor-acceptor conjugated polymer is synthesized using the Suzuki polycondensation method. Using this polymer, a single-junction bulk-heterojunction solar cell is fabricated giving a power conversion efficiency of 9.4% using a fullerene-modified ZnO interlayer at the cathode contact.

Solution-processed organic photovoltaic cells (OPVs) hold great promise to enable roll-to-roll printing of environmentally friendly, mechanically flexible and cost-effective photovoltaic devices. Nevertheless, many high-performing systems show best power conversion efficiencies (PCEs) with a thin active layer (thickness is ~100 nm) that is difficult to translate to roll-to-roll processing with high reproducibility. Here we report a new molecular donor, benzodithiophene terthiophene rhodanine (BTR), which exhibits good processability, nematic liquid crystalline behaviour and excellent optoelectronic properties. A maximum PCE of 9.3% is achieved under AM 1.5G solar irradiation, with fill factor reaching 77%, rarely achieved in solution-processed OPVs. Particularly promising is the fact that BTR-based devices with active layer thicknesses up to 400 nm can still afford high fill factor of ~70% and high PCE of ~8%. Together, the results suggest, with better device architectures for longer device lifetime, BTR is an ideal candidate for mass production of OPVs.

The effect of molecular weight of a conjugated polymer on its photophysical properties and solar cell device performance was investigated.

Fully printed perovskite solar cells are demonstrated with slot-die coating, a scalable printing method. A sequential slot-die coating process is developed to produce efficient perovskite solar cells and to be used in a large-scale roll-to-roll printing process. All layers excluding the electrodes are printed and devices demonstrate up to 11.96% power conversion efficiency. It is also demonstrated that the new process can be used in roll-to-roll production.

Small molecules with dithieno[3,2-b;2',3'-d]thiophene as central building block and octyl cyanoacetate and octyl cyanoacetamide as different terminal building blocks have been designed and synthesized. The amide containing small molecule can form intermolecular hydrogen bonding between N-H...O = C of the amide group. The photovoltaic properties and active layer morphologies of the two molecules in bulk heterojunction solar cells are compared to study the influence of hydrogen bonding on the active layer morphology. New methanofullerene compound containing amide group has also been synthesized and compared with conventional fullerene electron acceptors.

Three new conjugated polymers based on 5-decylthiazol-2-yl substituted benzodithiophene have been synthesized by Stille cross-coupling polymerization. 1,3-Dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione (M1), 2,5-diethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (M2) and 4,6-dibromo-thieno[3,4-b]thiophene-2-dodecyl carboxylate (M3) were used as acceptor building blocks for the synthesis of conjugated donor-acceptor polymers. The thermal, optical, electrochemical, and photovoltaic properties of the synthesized polymers were studied.

A new benzo[1,2-b:4,5-b']dithiophene (BDT) building block with 4,8-disubstitution using 2-(2-ethylhexyl)-3-hexylthieno[3,2-b]thiophene as the substituent has been designed and synthesized. The new building block has been copolymerized with benzothiadiazole (BT) and 5,6-difluorobenzothiadiazole (fBT) by Suzuki and Stille coupling polymerization to synthesize donor-acceptor conjugated polymers. The optical and electrochemical properties of the synthesized copolymers were studied. Bulk heterojunction solar cells were fabricated using the donor-acceptor copolymers in conjunction with PC71BM and exhibited up to 4.20% power conversion efficiency.

Self-complementary hydrogen-bonding domains were incorporated as the electron deficient unit in "push-pull", p-type small molecules for organic photovoltaic active layers. Such compounds were found to enhance the fill factor, compared with similar non-self-organized compounds reported in the literature, leading to higher device efficiencies. Evidence is presented that the ability of these molecules to form one-dimensional hydrogen-bonded chains and subsequently exhibit hierarchical self-assembly into nanostructured domains can be correlated with improved device efficiency.

There is an increasing demand for organic semiconducting materials with the emergence of organic electronic devices. In particular, large-area devices such as organic thin-film photovoltaics will require significant quantities of materials for device optimization, lifetime testing and commercialization. Sourcing large quantities of materials required for the optimization of large area devices is costly and often impossible to achieve. Continuous-flow synthesis enables straight-forward scale-up of materials compared to conventional batch reactions. In this study, poly(3-hexylthiophene), P3HT, was synthesized in a bench-top continuous-flow reactor. Precise control of the molecular weight was demonstrated for the first time in flow for conjugated polymers by accurate addition of catalyst to the monomer solution. The P3HT samples synthesized in flow showed comparable performance to commercial P3HT samples in bulk heterojunction solar cell devices.