Research

In Laboratory of Thin Films and Thin Film Photovoltaics our focus is on the fabrication of two types of emerging solar cells: Perovskite and Polymer Solar Cells.

Lead Halide Perovskite solar cell

 

Lead Halide Perovskite solar cell is a recently developed solution-processed emerging PV solar cell. A perovskite structure (Fig. 1a) denotes any material structure with the same type of crystal structure as calcium titanium oxide (CaTiO3), also known as ABX3 structure. It is named after Russian scientist Lev Aleksevich von Perovski. From the same family, methyl-ammonium lead halide (CH3NH3PbX3) is an organic−inorganic light absorbing semiconductor material with a perovskite polycrystalline structure (Fig. 1b); X is a halide atom (I, Cl, Br, or a combination of some of them). The component methyl-ammonium fills the coordinated space between the octahedrals that form in these three-dimensional structures.


Figure 1: (a) Structure of ABX3 perovskite crystal structure; (b) Methyl-ammonium lead halide perovskite as an energy harvesting molecule. 

Perovskite solar cell was first used as a variant of the dye-sensitized solar cell configuration, thus a nanostructured TiO2 perovskite-sensitized solar cell. Subsequently, the perovskite solar cells have been assembled in a variety of morphologies, either in nano hetero-junction or planar thin film, with different chemical compositions and preparation routes.
Figure 2 shows two configurations of perovskite solar cells. While in perovskite solar cells made on meso-porous TiO2 layer (Figure 2a), electron transfer seems to be done by the TiO2 porous layer, in Al2O3-based cell (Figure 2b), the Al2O3 layer acts as a scaffold for the perovskite layer and the electron-hole or exciton generation and charge transfer occurs within the perovskite molecule itself. A hole transporting material (HTM), such as spiro-OMeTAD (2,20,7,70-tetrakis-(N,N-di-p-methoxyphenylamine)9,90-spirobi-fluorene), is usually deposited on top of the perovskite layer. A gold electrode is thermally deposited on top of the HTM to complete the device.


Figure 2: Two architectures of perovskite solar cells with structure similar to the classical solid state dye sensitized solar cell. (a) TiO2 porous layer based and (b) Al2O3 layer based cell. Regardless of the meso-porous layer, a compact TiO2 layer is still required for both the collection of the generated electrons and hole blocking.

 

 

Polymer solar cell


Polymer solar cell is an attractive organic solar cell due to its simplicity, transparency and ease of fabrication from a solution on glass windows or flexible substrates. In a polymer solar cell, an active layer is sandwiched between two collecting electrodes. One of the electrodes must be transparent to allow transmission of solar radiation to the active layer. Currently indium tin oxide (ITO) is the widely used transparent anode, which is deposited on glass substrates. As the second electrode, silver, aluminum and other metals that have the right work function are used. The active layer is a blend of electron donor and electron acceptor materials. The common and currently used approach is to blend the donor and acceptor materials in a solution, transfer the solution to the substrate, and let the solvent evaporate to leave behind a thin solid film. Figure 3 shows the layer structure of a conventional polymer solar cell where poly(3-hexylthiophene-2,5-diyl) known as P3HT is the donor material and phenyl-C61-butyric acid methyl ester known as PCBM is the acceptor. As an alternative to conventional polymer solar cell devices shown in Figure 3, PEDOT:PSS may be deposited on top of P3HT:PCBT layer, which may result in a better bonding and a higher conversion efficiency and also a transparent device. This is called the inverted architecture. Once photons strike on the solar cell surface and reach the active layer, the polymer molecule in the donor becomes excited and a so-called excitons form. The excitons diffuse to the boundaries of the donor-acceptor blend where they dissociate to an electron and a hole due to a change in the energy level of the two materials at the boundaries. The electrons and holes then transfer to the opposite electrodes under the influence of different work functions. To minimize charge recombination, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) known as PEDOT:PSS is used between the anode and the active layer as a buffer layer to block the electron transfer in the wrong directions. Other materials, such as metal oxides, are also used as hole blocking layers between the cathode and the active layer.

Figure 3: The two prototypes for polymer solar cells. The upper one is the normal geometry and the lower one is the inverted geometry.

In our research various versions of spray coating technique is used to fabricate solution processed solar cells. Figure 4 shows the process of spray coating of a solution-processed solar cell by ultrasonic atomization, which is a method to convert the solar cell material solution into tiny droplet by high frequency vibration of a nozzle. During spray coating, these tiny droplets are carried to the substrate by a gas, such as air or nitrogen (to avoid degradation of the solar cell materials). The droplets create a thin liquid film on the substrate; as a result of the high temperature of the substrate, the solvent evaporates and a thin solid film of solar cell material forms. In Figure 4, the substrate is also ultrasonically vibrated, to improve the uniformity of the film.

                   
     
Figure 4: Substrate vibration-assist ultrasonic atomization for spray coating of a solution-processed solar cell: (1) Ultrasonic nozzle tip, (2) 2D traveling arm, (3) solution inlet, (4) holder plate with controlled temperature, (5) substrate, (6) ultrasonic vibration transducer box, (7) function generator for ultrasonic piezoelectric transducer. The double arrows beside the plate show the direction of imposed vibration, which is lateral vibration.

 

OUR Research DIrections

Here we endeavor to improve the performance and stability of perovskite and polymer solar cells by improving the functionality of each solar cell layer, individually, and the complete device. We endeavor to perform most processes in air, but the spray coating process may be performed under nitrogen environment, and device packaging under a glove box, to avoid degradation. Solar cells are also fabricated by spin coating, as a reference method. Below lists our ongoing projects:

  1. a) Solvothermal Carbon Doping of PEDOT:PSS
Improving conductivity of solar cell layers, by solvothermal treatment of multi-walled carbon nanotubes (MWCNT)/PEDOT:PSS and Graphene doped PEDOT:PSS is currently being performed in our lab. Our preliminary results show that the conductivity of PEDOTT:PSS thin films doped with CNTs and Graphene is higher than that of un-doped PEDOTT:PSS thin films. The modified PEDOT:PSS film are being used for the fabrication of polymer and perovskite solar cells to improve the device performance.
 
  1. b) Conductivity and Stability Improvement of PEDOT:PSS
PEDOT:PSS itself has low conductivity as an electrode layer and its acid nature which comes from the additional sulfonate group of PSS making the solar cells to degrade fast. Therefore, using a novel idea developed in our lab, we are using long-chain tertiary amine (PEGO) to modify the PEDOT:PSS.


  1. c) Stability Improvement of Perovskite solar cells:
While perovskite solar cells have reached an efficiency of 17.9% within few years, improving the solar cell stability remains an unraveled problem. Replacing liquid electrolyte in perovskite solar cells with solid state films and using several coating layers has led to an increase in device stability. The thickness of each layer is an important factor to achieve highly stable and efficient solar cells. The most important layer affecting the stability of perovskite solar cell is the Hole Transporting Material (HTM) layer. HTM is used to improve the work function of perovskite solar cells. We are currently attempting to improve the stability of perovskite solar cells using various strategies, such as removing or replacing HTM layer with more stable materials and layers.

  1. d) Fabrication of Perovskite and Polymer Solar Cells on Vibrating Substrates :
Typically the perovskite precursor solution should make a favorable uniform wetting layer resulting in well distributed and large contact surface between perovskite and scaffold layers. We are currently using a novel approach developed and tested in our lab, i.e., using a vibrating substrate instead of a stationary substrate for controlled crystallization and improved performance of the perovskite layer (Fig. 5).

  1. e) Thin film field efect transistors :
Given that most thin film devices share similar fabrication routes and functionality We are expanding our activities, and now focus on the fabrication of thin film field effect transistors. Two innovative directions are followed. First, emerging perovskite semiconductors are used as the conducting channel of the transistor. And secondly, our developed imposed ultrasonic substrate vibration method is employed to improve the mobility and charge density of the perovskite layer.
 
  1. f) Fluid mechanics and heat transfer of thin liquid films :
The group also works on fluid mechanics and heat transfer aspects of thin liquid films We have developed a theory that explains how imposed ultrasonic vibration affects the stability and mixing of thin liquid films of solutions. It is observed that the imposed vibration even though tends to make the liquid film unstable, it causes mixing of precursor solutions, thus improves the functionality of ensuing thin solid films.


  1. g) Other areas of fluid-thermal sciences :
The group also works on general area Of modeling of liquid atomization and sprays in various applications, such as jet in cross flow and spray coating.


Figure 5: Controlling crystallization of perovskite layer using imposed ultrasonic vibration on the substrate.

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