Chun-Yu Chang

Tuning perovskite morphology by polymer additive for high efficiency solar cell.

Solution processable planar heterojunction perovskite solar cell is a very promising new technology for low cost renewable energy. One of the most common cell structures is FTO/TiO2/CH3NH3PbI3-xClx/spiro-OMeTAD/Au. The main issues of this type of solar cell are the poor coverage and morphology control of the perovskite CH3NH3PbI3-xClx film on TiO2. For the first time, we demonstrate that the problems can be easily resolved by using a polymer additive in perovskite precursor solution during the film formation process. A 25% increase in power conversion efficiency at a value of 13.2% is achieved by adding 1 wt % of poly(ethylene glycol) in the perovskite layer using a 150 °C processed TiO2 nanoparticle layer. The morphology of this new perovskite was carefully studied by SEM, XRD, and AFM. The results reveal that the additive controls the size and aggregation of perovskite crystals and helps the formation of smooth film over TiO2 completely. Thus, the Voc and Jsc are greatly increased for a high efficiency solar cell. The amount of additive is optimized at 1 wt % due to its insulating characteristics. This research provides a facile way to fabricate a high efficiency perovskite solar cell by the low temperature solution process (<150 °C), which has the advancement of conserving energy over the traditional high temperature sintering TiO2 compact layer device.

Isoindigo-based copolymers for polymer solar cells with efficiency over 7%

A series of isoindigo-based low-band-gap copolymers (PnTI) containing an extended thiophene unit in the donor segment of the polymer were synthesized. The results show that the extended thiophene unit with centrosymmetric conformation simultaneously broadens the polymer absorption and enhances the crystallinity and, thus, hole mobility. Consequently, with additional improved solubility, the polymer P6TI exhibits the highest PCE of 7.25% (and a high Jsc of 16.24 mA cm−2) among isoindigo-based low-band-gap copolymers. This work demonstrates that by simply adjusting the donor segment and with relatively simple synthetic schemes, a material for high-performance and scalable PSCs will become available.

Molecular Structure Effect of Pyridine-Based Surface Ligand on the Performance of P3HT:TiO2 Hybrid Solar Cell

Colloid TiO(2) nanorods are used for solution-processable poly(3-hexyl thiophene): TiO(2) hybrid solar cell. The nanorods were covered by insulating ligand of oleic acid (OA) after sol-gel synthesis. Three more conducting pyridine type ligands: pyridine, 2,6-lutidine (Lut) and 4-tert-butylpyridine (tBP) were investigated respectively to replace OA. The power conversion efficiency (PCE) of the solar cell was increased because the electronic mobility of pyridine-type ligand-modified TiO(2) is higher than that of TiO(2)-OA. The enhancement of PCE is in the descending order of Lut > pyridine > tBP because of the effective replacement of OA by Lut. The PCE of solar cell can be further enhanced by ligand exchange of pyridine type ligand with conjugating molecule of 2-cyano-3-(5-(7-(thiophen-2-yl)-benzothiadiazol-4-yl) thiophen-2-yl) acrylic acid (W4) on TiO(2) nanorods because W4 has aligned bandgap with P3HT and TiO(2) to facilitate charge separation and transport. The electronic mobility of two-stage ligand exchanged TiO(2) is improved furthermore except Lut, because it adheres well and difficult to be replaced by W4. The amount of W4 on TiO(2)-tBP is 3 times more than that of TiO(2)-Lut (0.20 mol % vs. 0.06 mol %). Thus, the increased extent of PCE of solar cell is in the decreasing order of tBP > pyridine > Lut. The TiO(2)-tBP-W4 device has the best performance with 1.4 and 2.6 times more than TiO(2)-pyridine-W4 and TiO(2)-Lut-W4 devices, respectively. The pKa of the pyridine derivatives plays the major role to determine the ease of ligand exchange on TiO(2) which is the key factor mandating the PCE of P3HT:TiO(2) hybrid solar cell. The results of this study provide new insights of the significance of acid-base reaction on the TiO(2) surface for TiO(2)-based solar cells. The obtained knowledge can be extended to other hybrid solar cell systems.

Enhancing organic–inorganic hybrid solar cell efficiency using rod–coil diblock polymer additive

Organic–inorganic hybrid bulk heterojunction (BHJ) solar cells have attracted much attention due to their low cost fabrication, flexibility, and long life. However, the compatibility between organic and inorganic materials is still an issue that needs to be solved to achieve high power conversion efficiency (PCE). The larger size and dense characteristics of inorganic nanocrystals make it hard to control the morphology and phase separation of organic–inorganic hybrid thin films by conventional processes like thermal annealing and solvent annealing. In this study, we have carried out a systematic investigation using an additive: rod–coil diblock copolymer poly(3-hexyl thiophene)-b-poly(2-vinyl pyridine) (P3HT-b-P2VP) to P3HT:TiO2 to make a ternary system. That improves the compatibility between the P3HT homopolymer and the TiO2 nanorod hybrid materials and results in enhanced performance of the hybrid solar cell. The hydrophobic characteristics of the P3HT segment of the copolymer are compatible with the P3HT homopolymer, and the P2VP segment, containing a pyridine moiety is more compatible with hydrophilic TiO2. The results of atomic force microscopy and X-ray diffraction spectroscopy studies of hybrid films reveal that the crystallization behavior of the homopolymer P3HT in the film can be tuned by incorporating different weight ratios of P3HT-b-P2VP. The efficiency of charge separation is also improved as observed by greater photoluminescence quenching. Furthermore, the power conversion efficiency of the solar cell fabricated from this new hybrid system was increased threefold as compared with the one without the additive (1.20% vs. 0.42%), which indicates that the amphiphilic P3HT-b-P2VP can effectively modulate the interfacial interactions between the conducting polymer and nanocrystals in both solution and film to have the appropriate morphology for high efficient solar cells.

Enhancing performance of P3HT:TiO2 solar cells using doped and surface modified TiO2 nanorods

Here we demonstrated an approach to increase performance of P3HT:TiO2 solar cell either by electron deficient boron or electron rich bismuth doping into TiO2 nanorods. The B doping increases the absorption, crystallinity and electron mobility of TiO2 nanorods. The Bi-doped TiO2 has higher J(sc) as compared with B-doped TiO2, mainly due to the improvement of electron density and increased absorption of TiO2 nanorods. The devices were fabricated from TiO2 nanorods being surface modified by organic dye W-4. The dye facilitates the bandgap alignment and compatibility between TiO2 and P3HT. The power conversion efficiency of solar cell has been increased by 1.33 times and 1.30 times for Bi-doped TiO2 and B-doped TiO2, respectively, as compared with that of as-synthesized TiO2. The results suggest the optical and electronic properties of TiO2 can be tuned by various dopants to enhance the device performance.

Low-temperature solution processable n–i–p perovskite solar cell

Recently, the perovskite solar cells draw lots of attentions because of low cost, solution-processable and promising high efficiency. The power conversion efficiency (PCE) is rapidly reached to a certified value of 20.1%. However, the conventional electron transport layer (ETL) of the device needs to be sintered over 500 °C which is high energy-consumption process and cannot use on flexible substrate. Therefore, many research groups endeavor to develop low-temperature solution processable electron transport material in perovskite solar cells. In this review, we present systematic and critical discussions on the evolution of perovskite solar cells from meso-superstructure solar cells (MSSCs) to low-temperature solution processed planar heterojunction (PHJ) perovskite solar cells. This review will provide a guideline for the fabrication of high PCE perovskite solar cell using solution process at temperature of 150 °C or under, which has potential to achieve large scale fabrication by roll-to-roll technique for commercial products in near future.

BiFeO3/YSZ bilayer electrolyte for low temperature solid oxide fuel cell

We have demonstrated BiFeO3 (BFO) as a potential bilayer electrolyte for 650 °C low temperature solid oxide fuel cell application. The stoichiometric perovskite BFO is synthesized by wet chemistry, calcined at 500 °C and sintered at 850 °C. The crystalline structure is confirmed by X-ray diffraction spectroscopy, the atomic ratios (Bi : Fe) of 1.02 and 1.00 are determined by X-ray energy dispersive spectroscopy and inductively coupled plasma-mass spectroscopy, respectively. The X-ray photoelectron spectroscopy analysis indicates the presence of oxygen vacancies which can partially reduce Fe3+ and result in relatively high dielectric constant (6252 at 100 kHz) and ionic conductivity (>10−2 S cm−1 at 650 °C). The BFO is coated with an yttria-stabilized zirconia (YSZ) protective layer to avoid hydrogen reduction of BFO. This bilayer electrolyte exhibits a 1.6 times increase in maximum power density as compared with pure YSZ when a Ni–YSZ anode and lanthanum strontium cobalt ferrite (LSCF) cathode are used in the fuel cell at 650 °C.