Shuangyong Sun

The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells based on methylammonium lead iodide (CH3NH3PbI3) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Our cell has a power conversion efficiency (PCE) of 5.2% under simulated AM 1.5G irradiation (100 mW cm−2) and an internal quantum efficiency of close to 100%, which means that nearly all the absorbed photons are converted to electrons and are efficiently collected at the electrodes. This implies that the exciton diffusion, charge transfer and charge collection are highly efficient. The high exciton diffusion efficiency is enabled by the long diffusion length of CH3NH3PbI3 relative to its thickness. Furthermore, the low exciton binding energy of CH3NH3PbI3 implies that exciton splitting at the CH3NH3PbI3/PC61BM interface is very efficient. With further increase in CH3NH3PbI3 thickness, a higher PCE of 7.4% could be obtained. This is the highest efficiency attained for low temperature solution-processable bilayer solar cells to date.

Organic Photovoltaic Devices Using Highly Flexible Reduced Graphene Oxide Films as Transparent Electrodes

The chemically reduced graphene oxide (rGO) was transferred onto polyethylene terephthalate (PET) substrates and then used as transparent and conductive electrodes for flexible organic photovoltaic (OPV) devices. The performance of the OPV devices mainly depends on the charge transport efficiency through rGO electrodes when the optical transmittance of rGO is above 65%. However, if the transmittance of rGO is less than 65%, the performance of the OPV device is dominated by the light transmission efficiency, that is, the transparency of rGO films. After the tensile strain (∼2.9%) was applied on the fabricated OPV device, it can sustain a thousand cycles of bending. Our work demonstrates the highly flexible property of rGO films, which provide the potential applications in flexible optoelectronics.

Perovskite-based solar cells: impact of morphology and device architecture on device performance

Organic–inorganic metal halide perovskites have recently shown great potential for application in solar cells with excitingly high performances with an up-to-date NREL-certified record efficiency of 20.1%. This family of materials has demonstrated considerable prospects in achieving efficiencies comparable to or even better than those of thin film solar cells. The remarkable performances thus far seem not to be limited to any specific device architecture. Both mesoscopic and planar cells showed good device performance and this eventually leads to the inevitable comparison between both architectures. Regardless of device architecture, device performance is highly dependent on the film morphology. The factors influencing the film morphology such as the deposition method, material composition, additives and film treatment will be discussed extensively in this review. The key to obtaining good-quality film morphology and hence performance is to essentially lower the energy barrier for nucleation and to promote uniform growth of the perovskite crystals. A comparison of the material selection for various layers as well as their corresponding impact on the perovskite film and device behavior in both device architectures will be presented.

A new insight into controlling poly(3-hexylthiophene) nanofiber growth through a mixed-solvent approach for organic photovoltaics applications

One dimensional (1-D) nanostructures of conjugated polymers, such as nanofibers, offer the possibility of directed charge transport and improved absorption due to better chains ordering. Poly(3-hexylthiophene) (P3HT) nanofibers can be synthesized by utilizing its interaction with marginal solvents. This work explores the effect of different poor solvents in driving P3HT chain self-assembly into nanofibers and also the effect of a small amount of good solvent in such a poor solvent system in controlling the nanofiber morphology. The organic photovoltaic (OPV) devices based on the blend of P3HT nanofibers and PCBM showed an improved short circuit current when anisole was used compared to p-xylene. Surprisingly, the presence of a small amount of good solvent such as chlorobenzene (CB) in anisole resulted in a higher degree of crystallinity and thinner nanofibers compared to purely anisole system. These are evident from the absorption, scattering and morphology data. The presence of CB delayed crystallization, which is evident from the synchrotron small angle X-ray scattering (SAXS) measurements. This modification of fiber morphology with CB addition into P3HT/anisole results in an improved power conversion efficiency (PCE) of 2.3%; an improvement of more than 50% compared to the pure anisole system. Our investigation provides a new insight into self-assembly of polymers in a mixed solvent system, paving the way to new approaches of controlled self-assembly of organic nanofibers.

Solution-Processed Nanocrystalline TiO2 Buffer Layer Used for Improving the Performance of Organic Photovoltaics

In this study, we use solution-processable crystalline TiO(2) nanoparticles as a buffer layer between the active layer and aluminum cathode to fabricate the P3HT:PCBM-based bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. The employment of the presynthesized TiO(2) nanoparticles simplifies the fabrication of OPV devices because of the elimination of an additional hydrolysis step of precursors in air. The fabricated OPV devices with the thermally stable TiO(2) buffer layer are subjected to the further postdeposition thermal annealing, resulting in a power conversion efficiency (PCE) as high as 3.94%. The improved device performance could be attributed to the electron transporting and hole blocking capabilities due to the introduced TiO(2) buffer layer.

A high voltage solar cell using a donor–acceptor conjugated polymer based on pyrrolo[3,4-f]-2,1,3-benzothiadiazole-5,7-dione

We report an improved synthesis of the electron-accepting unit pyrrolo[3,4-f]-2,1,3-benzothiadiazole-5,7-dione (BTI) and the synthesis and characterisation of two donor–acceptor copolymers containing it: poly[6-dodecyl-4,8-bis(2-thienyl)pyrrolo[3,4-f]-2,1,3-benzothiadiazole-5,7-dione-alt-9,9-dioctyl-2,7-bis(thien-2′-yl)-9H-fluorene] (P(BTI-F)), and poly[6-dodecyl-4,8-bis(thien-2′-yl)pyrrolo[3,4-f]-2,1,3-benzothiadiazole-5,7-dione-alt-4,8-bis(octyloxy)benzo[1,2-b:4,5-b′]dithiophene] (P(BTI-B)). By the incorporation of an imide group onto a benzothiadiazole moiety, the HOMO level is lowered. To explore the effectiveness of conjugation along the polymer backbone, two electron donors: fluorene and benzobithiophene, were introduced. These represent a weak and a strong electron-donor, respectively. Both polymers have low lying HOMOs and hence are thermally stable and also result in high open-circuit voltages (Voc) in photovoltaic applications. Compared to P(BTI-B), P(BTI-F) has a larger bandgap due to the incorporation of a weaker donor unit and thus a deeper lying HOMO level. Devices based on P(BTI-F) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) can obtain a remarkably high Voc of 1.11 V, but a PCE of only 1.61%. By contrast, polymer P(BTI-B) has stronger absorption in the longer wavelength range and can achieve a well-dispersed nanomorphology with PC61BM for efficient exciton dissociation, achieving PCE of 3.42%, with a Jsc of 9.71 mA cm−2, Voc of 0.75, and FF of 0.47.

New 3D supramolecular Zn(II)-coordinated self-assembled organic networks†

New 3D supramolecular networks S1 and S2 were prepared by Zn(II) coordination of the tetraphenylmethane-based p-type and n-type molecules bearing four terpyridine ligands. XRD and BET results indicate they are relatively amorphous and non-porous with a high degree of interpenetration within the networks. These could be disassembled by adding more Zn(II) ions and re-assembled to form extended 3D networks S3–6 by inserting linear n-type or p-type linking units. BET data suggests that these expanded networks are more porous than the original networks S1–2, but the low porosity and surface area suggest a high degree of interpenetration remains within the expanded networks. The optical properties of these materials were compared to the linear polymers P1–3 made by Zn(II)-mediated assembly of the same linear linking units. The emission spectra of both the 3-D and 1-D cases with the same linking unit matched each other, confirming the incorporation of the linker units into the expanded assemblies. This shows that metal–ligand mediated self-assembly can be used to make two component systems in which the optical properties can be tuned by selection of the units. The assembly was also performed in the presence of CdSe nanocrystals to form nanocomposites.