Semi-monolithic Integration of All-Chalcopyrite Multijunction Solar Conversion Devices via Thin-Film Bonding and Exfoliation.

We report on a semi-monolithic integration method to circumvent processing incompatibility between materials of dissimilar classes and combine them into multijunction devices for photovoltaic and photoelectrochemical applications. Proof-of-concept all-chalcopyrite tandems were obtained by consecutive transfer of fully integrated unpatterned 1.85 eV CuGa3Se5 and 1.13 eV CuInGaSe2 PV stacks from their Mo/soda lime glass substrates onto a new single host substrate. This transfer approach consists of two key steps: (1) bonding of the solar stack (face down) onto a handle (e.g., SnO2:F, FTO) using a transparent conductive composite and (2) delamination of the solar stack at the chalcopyrite/Mo interface by employing a wedge-based exfoliation technique. Upon transfer onto FTO, a CuGa3Se5 champion device demonstrated near-coincident photocurrent density-voltage characteristic with a baseline measurement. Then, the exfoliated CuGa3Se5 single-junction stack transferred onto FTO served as the new host onto which a second fully processed CuInGaSe2 stack was bonded (face down) onto and liberated from its Mo/SLG substrate, leading to a complete transfer of both sub-cells onto one FTO substrate. A champion semi-monolithic tandem device exhibited a power conversion efficiency of 5.04% with an open-circuit voltage, a short-circuit current density, and a fill factor of 1.24 V, 7.19 mA/cm2, and 56.7%, respectively. This first-time demonstration of a fully operational semi-monolithic device provides a new avenue to combine thermally, mechanically, and/or chemically incompatible thin-film material classes into tandem photovoltaic and photoelectrochemical devices while maintaining state-of-the-art sub-cell processing.

Gradient Self-Doped CuBi2O4 with Highly Improved Charge Separation Efficiency.

A new strategy of using forward gradient self-doping to improve the charge separation efficiency in metal oxide photoelectrodes is proposed. Gradient self-doped CuBi2O4 photocathodes are prepared with forward and reverse gradients in copper vacancies using a two-step, diffusion-assisted spray pyrolysis process. Decreasing the Cu/Bi ratio of the CuBi2O4 photocathodes introduces Cu vacancies that increase the carrier (hole) concentration and lowers the Fermi level, as evidenced by a shift in the flat band toward more positive potentials. Thus, a gradient in Cu vacancies leads to an internal electric field within CuBi2O4, which can facilitate charge separation. Compared to homogeneous CuBi2O4 photocathodes, CuBi2O4 photocathodes with a forward gradient show highly improved charge separation efficiency and enhanced photoelectrochemical performance for reduction reactions, while CuBi2O4 photocathodes with a reverse gradient show significantly reduced charge separation efficiency and photoelectrochemical performance. The CuBi2O4 photocathodes with a forward gradient produce record AM 1.5 photocurrent densities for CuBi2O4 up to -2.5 mA/cm2 at 0.6 V vs RHE with H2O2 as an electron scavenger, and they show a charge separation efficiency of 34% for 550 nm light. The gradient self-doping accomplishes this without the introduction of external dopants, and therefore the tetragonal crystal structure and carrier mobility of CuBi2O4 are maintained. Lastly, forward gradient self-doped CuBi2O4 photocathodes are protected with a CdS/TiO2 heterojunction and coated with Pt as an electrocatalyst. These photocathodes demonstrate photocurrent densities on the order of -1.0 mA/cm2 at 0.0 V vs RHE and evolve hydrogen with a faradaic efficiency of ∼91%.

Photoelectrochemical water reduction over wide gap (Ag,Cu)(In,Ga)S2 thin film photocathodes.

The effects of partial replacement of Cu with Ag in a Cu(In,Ga)S2 (CIGS) thin film on its structural, optical, electrostructural, and photoelectrochemical (PEC) properties were investigated, in order to improve its performance for PEC water reduction under sunlight illumination. Results from X-ray diffraction (XRD) analyses revealed the successful partial replacement of Cu with Ag to form solid-solutions with different Ag/(Ag + Cu) ratios (A(x)CIGS, x = Ag/(Ag + Cu) = 0.1, 0.2, 0.3 and 0.4), as confirmed by a gradual change in the (112) reflections to smaller 2θ angles with increasing Ag/(Ag + Cu) ratio. Analyses of the photoabsorption properties of the materials using photoacoustic spectroscopy indicated changes in the band gap energies associated with increasing the Ag/(Ag + Cu) ratio. In addition, valence band maximum potentials of A(x)CIGS were deepened gradually with increasing Ag/(Ag + Cu) ratio. After modifying these A(x)CIGS films with a CdS ultrathin (ca. 70 nm) layer and a Pt catalyst, the PEC water reduction properties were evaluated in an electrolyte solution with the pH adjusted to 6.5, under simulated sunlight (AM 1.5G) radiation. Compared to the CdS- and Pt-modified Ag-free A(x)CIGS (A(0)CIGS) films, appreciable enhancements in the PEC properties were observed for electrodes based on A(x)CIGS (x > 0) films, and the best PEC performance was obtained for the electrode based on the A(0.2)CIGS film. However, the electrode derived from the A(x)CIGS film with Ag/(Ag + Cu) ratios higher than 0.3 showed diminished PEC properties due to the partial conversion of its semiconducting properties from p-type to n-type.

Investigation of the Electric Structures of Heterointerfaces in Pt- and In2S3-Modified CuInS2 Photocathodes Used for Sunlight-Induced Hydrogen Evolution.

Copper indium disulfide (CuInS2) modified with an In2S3 layer and a Pt catalyst showed a more efficient photoelectrochemical (PEC) property for hydrogen evolution from a nearly neutral (pH 6) 0.2 M NaH2PO4 solution under simulated sunlight illumination (AM 1.5G) than that of a CuInS2 electrode modified with a CdS layer and a Pt catalyst. Analysis of the PEC properties of In2S3-modified CuInS2 (In2S3/CuInS2) and CdS-modified CuInS2 (CdS/CuInS2) in solutions containing an electron scavenger (Eu(3+)) showed identical enhancement of the PEC properties of In2S3/CuInS2 when compared to those of CdS/CuInS2, indicating the formation of a favorable heterointerface in In2S3/CuInS2 for efficient charge separation. Spectroscopic evaluation of conduction band offsets revealed that In2S3/CuInS2 had a notch-type conduction band offset, whereas a cliff-type offset was formed in CdS/CuInS2: these results also revealed a better interface electric structure of In2S3/CuInS2 than that of CdS/CuInS2.

Cu(In,Ga)(S,Se)2 thin film solar cell with 10.7% conversion efficiency obtained by selenization of the Na-doped spray-pyrolyzed sulfide precursor film.

Selenium-rich Cu(In,Ga)(S,Se)2 (CIGSSe) thin films on an Mo-coated soda-lime glass substrate were fabricated by spray pyrolysis of an aqueous precursor solution containing Cu(NO3)2, In(NO3)3, Ga(NO3)3, and thiourea followed by selenization at 560 °C for 10 min. We studied the effects of intentional sodium addition on the structural and morphological properties of the fabricated CIGSSe films by dissolving NaNO3 in the aqueous precursor solution. The addition of sodium was found to affect the morphology of the final CIGSSe film: the film had denser morphology than that of the CIGSSe film obtained without addition of NaNO3. Photoelectrochemical measurements also revealed that the acceptor density of the nondoped CIGSSe film was relatively high (N(a) = 7.2 × 10(17) cm(-3)) and the addition of sodium led to a more favorable value for solar cell application (N(a) = 1.8 × 10(17) cm(-3)). As a result, a solar cell based on the sodium-modified CIGSSe film exhibited maximum conversion efficiency of 8.8%, which was significantly higher than that of the cell based on nondoped CIGSSe (4.4%). In addition, by applying MgF2 antireflection coating to the device, the maximum efficiency was further improved to 10.7%.