Qijie Guo

Fabrication of 7.2% Efficient CZTSSe Solar Cells Using CZTS Nanocrystals

Earth abundant copper-zinc-tin-chalcogenide (CZTSSe) is an important class of material for the development of low cost and sustainable thin film solar cells. The fabrication of CZTSSe solar cells by selenization of CZTS nanocrystals is presented. By tuning the composition of the CZTS nanocrystals and developing a robust film coating method, a total area efficiency as high as 7.2% under AM 1.5 illumination and light soaking has been achieved.

Synthesis of Cu2ZnSnS4 Nanocrystal Ink and Its Use for Solar Cells

Cu(2)ZnSnS(4) (CZTS) and Cu(2)ZnSnSe(4) (CZTSe) based solar cells are promising candidates for low cost solar cells due to the natural abundance and low toxicity of the constituent elements. Here, we present the first reported synthesis of colloidal CZTS nanocrystals using a simple solution-phase method. Solar cells fabricated using selenized CZTS nanocrystal inks had a power conversion efficiency of 0.74% under AM1.5G illumination.

Development of CuInSe2 Nanocrystal and Nanoring Inks for Low-Cost Solar Cells

The creation of a suitable inorganic colloidal nanocrystal ink for use in a scalable coating process is a key step in the development of low-cost solar cells. Here, we present a facile solution synthesis of chalcopyrite CuInSe 2 nanocrystals and demonstrate that inks based on these nanocrystals can be used to create simple solar cells, with our first cells exhibiting an efficiency of 3.2% under AM1.5 illumination. We also report the first solution synthesis of uniform hexagonal shaped single crystals CuInSe 2 nanorings by altering the synthesis parameter.

Sulfide Nanocrystal Inks for Dense Cu(In1−xGax)(S1−ySey)2 Absorber Films and Their Photovoltaic Performance

Recent developments in the colloidal synthesis of high quality nanocrystals have opened up new routes for the fabrication of low-cost efficient photovoltaic devices. Previously, we demonstrated the utility of CuInSe(2) nanocrystals in the fabrication of CuInSe(2) thin film solar cells. In those devices, sintering the nanocrystal film yields a relatively dense CuInSe(2) film with some void space inclusions. Here, we present a general approach toward eliminating void space in sintered nanocrystal films by utilizing reactions that yield a controlled volume expansion of the film. This is demonstrated by first synthesizing a nanocrystal ink composed of Cu(In(1-x)Ga(x))S(2) (CIGS). After nanocrystal film formation, the nanocrystals are exposed to selenium vapor during which most of the sulfur is replaced by selenium. Full replacement produces a approximately 14.6% volume expansion and reproducibly leads to good dense device-quality CIGSSe absorber films with reduced inclusion of void space. Solar cells made using the CIGSSe absorber films fabricated by this method showed a power conversion efficiency of 4.76% (5.55% based on the active nonshadowed area) under standard AM1.5 illumination.

A generalized and robust method for efficient thin film photovoltaic devices from multinary sulfide nanocrystal inks

A generalized and robust method using multinary sulfide nanocrystals for the fabrication Cu(In, Ga)(S, Se)2 CIGSSe and Cu2Zn, Sn(S, Se)4 CZTSSe thin films and photovoltaic (PV) devices has been developed. By direct synthesis of the multinary sulfide nanocrystals with controlled stoichiometry, superior composition uniformity can be achieved inherently. Using standard device structure, PV devices yield total area power conversion efficiencies (PCE) as high as 12.5% and 7.2% for CIGSSe and CZTSSe respectively, under AM1.5 illumination without anti-reflective coating. By incorporating Ge into the CZTS nanocrystal and adjusting the Sn/Ge ratio, band gap optimization of the CZTSSe absorber film can be accomplished. Despite little optimization, CZTGeSSe has yielded promising results by improving the total area PCE to 8.4%.

Selenization of copper indium gallium disulfide nanocrystal films for thin film solar cells

Recently, numerous non-vacuum methods have been demonstrated for the fabrication of Cu(InxGa1−x)Se2 (CIGSe) thin films and solar cells via selenization of various precursor materials. However, composition control and uniformity at all scales remains a challenge. Here we present a promising alternative approach using Cu(In1−xGax)S2 (CIGS) nanocrystal inks for low cost fabrication of CIGSSe absorber films. By using nanocrystal inks of CIGS as the precursor for selenization, the composition can be controlled at all scales. Nanocrystals with varying Ga concentration can also be used to form graded band gap CIGSSe absorber film. Initial devices fabricated using a graded CIGSSe absorber layer showed a photon to electricity conversion efficiency of 6.23% corresponding to an active area efficiency of 7.10% under AM1.5 illumination.

Solar cells via selenization of CuInS 2 nanocrystals: Effect of synthesis precursor

Solar cells are fabricated by selenizing CuInS2 nanocrystals synthesized from various precursors to determine the effect of precursor on device performance. Copper and indium iodides, chlorides, acetates, acetylacetonates and nitrates are used to synthesize sphalerite CuInS2 nanocrystals, which are drop cast onto Mo coated glass. Selenization results in chalcopyrite CuIn(S,Se)2 films, which are fabricated into devices exhibiting ∼5% efficiency regardless of precursor. Variations in nanocrystals are investigated with TEM, XRD and EDS while film formation is studied with SEM and XRD. Devices fabricated with a SLG/Mo/CISSe/CdS/i-ZnO/ITO/Ag structure are studied with JV under simulated AM1.5G, EQE and CV. It is determined that impurities resulting from the choice of nanocrystal synthesis precursor did not significantly limiting device performance during this study.