Michael H.-C. Jin

Aerosol-Assisted Chemical Vapor Deposited Thin Films for Space Photovoltaics

Abstract Copper indium disulfide thin films were deposited via aerosol-assisted chemical vapor deposition using single source precursors. Processing and post-processing parameters were varied in order to modify morphology, stoichiometry, crystallography, electrical properties, and optical properties in order to optimize device-quality material. Growth at atmospheric pressure in a horizontal hot-wall reactor at 395 °C yielded best device films. Placing the susceptor closer to the evaporation zone and flowing a more precursor-rich carrier gas through the reactor yielded shinier, smoother, denser-looking films. Growth of (112)-oriented films yielded more Cu-rich films with fewer secondary phases than growth of (204)/(220)-oriented films. Post-deposition sulfur-vapor annealing enhanced stoichiometry and crystallinity of the films. Photoluminescence studies revealed four major emission bands (1.45, 1.43, 1.37, and 1.32 eV) and a broad band associated with deep defects. The highest device efficiency for an aerosol-assisted chemical vapor deposited cell was 1.03 percent.

CuInSe 2 solar cells prepared by using seleno-amide as selenium source

CuInSe2 (CISe) absorber films were prepared by selenizing Cu-In metallic films with seleno-amide in a tube furnace for solar cell fabrication. Seleno-amide used in this study is stable at room temperature and decomposes into H2Se at below 150 °C. Chalcogen amides allow safe transportation and the handling of chalcogen sources that can produce chalcogen hydrides at reasonably low temperature. Post-selenization annealing at 500°C for various time periods were carried out to reverse the indium inhomogeniety created during selenization and to promote further phase transformation. Cu/In ratio of CISe thin films was reduced from ~1.6 down to ~0.9 through the post-selenization annealing at 500°C for up to 3 hours. The first batch of cells fabricated with soda-lime glass/Mo/CISe/CdS/ZnO/ZnO:Ga structure exhibited power conversion efficiency up to 1.6% without any optimization.

Fabrication of P3HT/PCBM bulk heterojunction solar cells with DNA complex layer

This work demonstrates the use of deoxyribose nucleic acid - hexadecyl trimethyl ammonium (DNA-CTMA) thin-film as a hole transport layer in P3HT/PCBM (poly(3-hexylthiophene)/(1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C61) bulk heterojunction polymer solar cell. DNA-CTMA complex thin-film as a potential replacement of commonly used poly(3,4-ethylene-dioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) provides various advantages. While the band gap of PEDOT:PSS is about 1.7 eV and its absorption band overlaps with that of P3HT reducing photocurrent, the band-gap of the DNA complex layer was measured to be about 4.1 eV and the film showed transmittance greater than 90 % over the broad spectrum of UV and visible light. The lowest unoccupied molecular orbital level of about 1.1 eV provides much more efficient electron blocking than PEDOT:PSS potentially reducing recombination at anode side. In addition, butanol solution of the DNA-CTMA complex easily wets any oxide surface effectively forming uniform thin films. P3HT/PCBM bulk heterojunction solar cells fabricated with a DNA-CTMA layer showed rectifying behavior under dark and a photovoltage over 200 mV under AM1.0 light exposure.

Semi-transparent photovoltaic devices for smart window applications

Photovoltaic devices integrated into smart windows can provide power necessary to tune the transparency of the windows for energy savings and environmental control. Both wide-bandgap polymer and oxide thin-films can provide visibility necessary through windows as well as voltage to drive the window to vary its transparency. Mini-modules including polymer bulk heterojunction solar cells have been fabricated and the open-circuit voltage up to 7.7 V was obtained from a 6cm × 6cm-size module which contains two banks of the cells connected in parallel, and each bank has 14 cells connected in series. The rectifying heterojunctions of Cu2O/i-ZnO/AZO were also successfully demonstrated. The presence of i-ZnO is critical to reduce the significant tunneling current caused by considerable band offset and the inherent lattice mismatch at the junction interface.

Thermally induced morphological changes in PAT/PCBM bulk heterojunctions studied by time-resolved small- and wide-angle X-ray scattering

In polymer bulk heterojunctions (BHJs), the thermally induced phase separation between electron-donating and -accepting molecules is considered as a key process in determining the overall nano-morphology of the BHJ, which can limit the performance of the fully fabricated BHJ device. Although it is often understood that the spinodal decomposition or the constitutional solidification (crystallization) of polymer is the main mechanism for phase separation, there are still questions remained to be answered because the crystallization rate can be comparable to the kinetics of the spinodal decomposition when the BHJ is made of semicrystalline polymer and it is difficult to separate those mechanisms to establish fundamental principles. This study uses the in-situ small- and wide-angle X-ray scattering to collect the information on the crystallization of polymer in BHJs under thermal annealing and its phase separation at the same time. The data showed the faster diffusion of PCBM compared to the crystallization of the polymer in the BHJs and, in fact, the crystallization was strongly limited by the diffusivity of PCBM in the polymer.

Thermally activated sulfurization of In-Cu bilayers for CuInS 2 solar cells

Sulfur vapor or H2S is often used for the fabrication of CuInS2 thin film solar cells by sulfurizing Cu/In bimetallic layers at elevated temperature over 400 °C. In order to promote low temperature sulfurization and/or faster sulfurization, sulfur vapor was thermally activated before it reaches Cu/In metallic precursor layers for the formation of CuInS2 thin films. The thermal activation of sulfur at over 600 °C dissociates high molecular weight sulfur molecules primarily comprised of S8 into smaller size molecules. Thermodynamic calculation predicted more spontaneous sulfurization process with S2 molecules. However, experimental results indicated that more complex phase transformation process was occurring with the thermally activated sulfur molecules. While In-rich Cu/In bimetallic precursor layers only experienced minor compositional change in Cu/In ratio after sulfurization, Cu-rich films have lost significant amount of In resulting increased Cu/In ratio after sulfurization process. The compositional change of the Cu-rich bilayers was sensitive to the temperature at which sulfur molecules were activated (dissociated). In general, more indium loss was observed with a higher activation temperature. The Cu/In ratio was reduced down slightly below 1.0 by KCN etching of the sulfurized CuInS2 films indicating the existence of CuS phase and also indium binary phase, both observed in XRD patterns.

Fabrication of low band-gap polymer solar cells using chemical vapor deposition polymerization

For the first time, insoluble poly(isothianaphthene-3,6-diyl) (PITN(3,6)) thin-film has been successfully deposited from 3,4-diethynylithiophene via chemical vapor deposition polymerization. PITN(3,6) with an optical band-gap of about 1.8 eV, is a conjugated polymer with its backbone constructed through phenyl rings. The low band-gap was expected from an idea that the quinoid state of the polymer could be stabilized by the thiophene ring fused into the phenyl ring. Electrochemical analysis further provided the highest occupied molecular orbital and the lowest unoccupied molecular orbital levels with values of 5.0 eV and 3.2 eV respectively. PITN(3,6) was also synthesized through more conventional liquid-solution based synthesis (Bergmann cyclization). The structural analysis showed there were undesirable side reactions during the process leaving terminal alkyne groups and five membered thiophene rings within PITN(3,6) thin-film while PITN(3,6) deposited by CVDP showed very clean structure. Finally, a bi-layer heterojunction between carbonized poly(p-phenylenevinylene) and PITN(3,6) was fabricated. Without optimization, an open circuit voltage of about 300 mV was measured. Ultimately, CVDP can realize multi-layer organic optoelectronic devices on any platform because of its low substrate temperature and highly conformal coating capability.