Yun Seog Lee

Atomic Layer Deposited Gallium Oxide Buffer Layer Enables 1.2 V Open-Circuit Voltage in Cuprous Oxide Solar Cells

The power conversion efficiency of solar cells based on copper (I) oxide (Cu2 O) is enhanced by atomic layer deposition of a thin gallium oxide (Ga2 O3 ) layer. By improving band-alignment and passivating interface defects, the device exhibits an open-circuit voltage of 1.20 V and an efficiency of 3.97%, showing potential of over 7% efficiency.

3.88% Efficient Tin Sulfide Solar Cells using Congruent Thermal Evaporation

Tin sulfide (SnS), as a promising absorber material in thin-film photovoltaic devices, is described. Here, it is confirmed that SnS evaporates congruently, which provides facile composition control akin to cadmium telluride. A SnS heterojunction solar cell is demons trated, which has a power conversion efficiency of 3.88% (certified), and an empirical loss analysis is presented to guide further performance improvements.

Hall mobility of cuprous oxide thin films deposited by reactive direct-current magnetron sputtering

Cuprous oxide (Cu2O) is a promising earth-abundant semiconductor for photovoltaic applications. We report Hall mobilities of polycrystalline Cu2O thin films deposited by reactive dc magnetron sputtering. High substrate growth temperature enhances film grain structure and Hall mobility. Temperature-dependent Hall mobilities measured on these films are comparable to monocrystalline Cu2O at temperatures above 250 K, reaching 62u2002cm2/Vu2009s at room temperature. At lower temperatures, the Hall mobility appears limited by carrier scattering from ionized centers. These observations indicate that sputtered Cu2O films at high substrate growth temperature may be suitable for thin-film photovoltaic applications.

High Photocurrent in Silicon Photoanodes Catalyzed by Iron Oxide Thin Films for Water Oxidation

Silicon splits: The application of silicon to water oxidation is limited due to unfavorable interface properties. However, these can be circumvented by using a high-performance silicon photoanode with a catalytically active iron oxide thin film (see picture). This approach results in photocurrents as high as 17u2005mAu2009cm(-2) under 1u2005sun and zero overpotential conditions.

Band offsets of n-type electron-selective contacts on cuprous oxide (Cu2O) for photovoltaics

The development of cuprous oxide (Cu2O) photovoltaics (PVs) is limited by low device open-circuit voltages. A strong contributing factor to this underperformance is the conduction-band offset between Cu2O and its n-type heterojunction partner or electron-selective contact. In the present work, a broad range of possible n-type materials is surveyed, including ZnO, ZnS, Zn(O,S), (Mg,Zn)O, TiO2, CdS, and Ga2O3. Band offsets are determined through X-ray photoelectron spectroscopy and optical bandgap measurements. A majority of these materials is identified as having a negative conduction-band offset with respect to Cu2O; the detrimental impact of this on open-circuit voltage (VOC) is evaluated through 1-D device simulation. These results suggest that doping density of the n-type material is important as well, and that a poorly optimized heterojunction can easily mask changes in bulk minority carrier lifetime. Promising heterojunction candidates identified here include Zn(O,S) with [S]/[Zn] ratios >70%, and Ga...

Nitrogen-doped cuprous oxide as a p-type hole-transporting layer in thin-film solar cells

We demonstrate the potential of a nitrogen-doped cuprous oxide (Cu2O:N) film as a p-type hole-transporting layer for photovoltaic devices. To reduce back-contact resistance and create an electron-reflecting back surface field, high carrier density and appropriate work function are desired for the layer. Its electrical and optical properties can be appropriately tuned via nitrogen-doping to create a semi-transparent tunnel junction to a back-contact. We fabricate Cu2O-based heterojunction thin-film solar cells and insert a 20 nm-thick Cu2O:N hole-transporting layer between a silver back-contact and a Cu2O light-absorbing layer. The insertion of a 20 nm-thick Cu2O:N layer results in sizeable enhancements of fill-factor and power conversion efficiency of the solar cells. Cu2O:N thin-films may also be useful in other photovoltaic material systems, improving their back-contact properties as well as widening the range of possible back-contact materials.

The impact of sodium on the sub-bandgap states in CZTSe and CZTS

We compare the optically active sub-bandgap states in polycrystalline Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS) thin films as a function of sodium content. In all samples studied, we find that CZTSe has a lower concentration of radiative defect-derived states compared to CZTS and that the states are also shallower in CZTSe compared to CZTS. Further, we find that sodium impacts the relative ratios of two sub-bandgap peaks in the 4u2009K photoluminescence (PL) spectra of CZTSe (one at ∼0.85u2009eV and another at ∼0.92u2009eV). We propose that both of these sub-bandgap peaks stem from intrinsic point defects in CZTSe rather than from electronic states introduced by sodium; this is supported by a measurement on a sodium-free single-crystal of CZTSe. We also show that films with stronger emission through the shallower sub-bandgap states at 4u2009K display room-temperature PL closer to the bandgap energy. For all sodium quantities studied, one broad PL peak is observed in the 4u2009K PL spectrum of CZTS which also shifts towards the...

Phase transition-induced band edge engineering of BiVO4 to split pure water under visible light

Significance Hydrogen has been recognized as one of the most promising energy carriers for the future, because it can generate enormous energy by clean combustion chemistry without any greenhouse gas emissions. Water splitting under visible light irradiation is an ideal route to cost-effective, large-scale, and sustainable hydrogen production, but it is challenging, because it requires a rare photocatalyst that carries a combination of suitable band gap energy, appropriate band positions, and photochemical stability. To create this rare photocatalyst, we engineered the band edges of BiVO4 by simultaneously substituting In3+ for Bi3+ and Mo6+ for V5+ in the host lattice of monoclinic BiVO4, which induced partial phase transformation from pure monoclinic BiVO4 to a mixture of monoclinic BiVO4 and tetragonal BiVO4. Through phase transition-induced band edge engineering by dual doping with In and Mo, a new greenish BiVO4 (Bi1-XInXV1-XMoXO4) is developed that has a larger band gap energy than the usual yellow scheelite monoclinic BiVO4 as well as a higher (more negative) conduction band than H+/H2 potential [0 VRHE (reversible hydrogen electrode) at pH 7]. Hence, it can extract H2 from pure water by visible light-driven overall water splitting without using any sacrificial reagents. The density functional theory calculation indicates that In3+/Mo6+ dual doping triggers partial phase transformation from pure monoclinic BiVO4 to a mixture of monoclinic BiVO4 and tetragonal BiVO4, which sequentially leads to unit cell volume growth, compressive lattice strain increase, conduction band edge uplift, and band gap widening.

Low contact resistivity of metals on nitrogen-doped cuprous oxide (Cu2O) thin-films

Forming low-resistivity contacts on cuprous oxide (Cu2O) is an essential step toward demonstrating its suitability as a candidate solar cell material. We measure the contact resistivity of three noble metals (Au, Ag, and Pd) on sputtered Cu2O thin-films with a range of nitrogen doping levels. Using the circular transmission line model, specific contact resistivity as low as 1.1u2009×u200910−4 Ωu2009·u2009cm2 is measured for Pd contacts on heavily doped Cu2O films. Temperature-dependent current-voltage measurements and X-ray photoemission spectroscopy are used to determine the barrier heights formed at metal/Cu2O interfaces. Thermionic emission is observed to dominate for undoped films, whilst field emission dominates for heavily doped films, highlighting the importance of carrier concentration on contact resistivity. Finally, we demonstrate that low contact resistivity can be achieved on heavily doped Cu2O films using Earth-abundant metals, such as Cu and Ni.

Textured conducting glass by nanosphere lithography for increased light absorption in thin-film solar cells

Nanoscale surface texturing in thin-film solar cells has been shown to enhance device efficiency by increasing light absorption through reduced reflectance and increased light scattering across a broad range of wavelengths and angles. However, light trapping in the industrial thin-film cells is still sub-optimal and creating optimized nanoscale texture over a large area remains challenging. In this article, we present a well-controlled low-cost process to fabricate a periodic nanocone texture optimized for maximum light absorption in thin-film microcrystalline silicon solar cells. The texture is fabricated using nanosphere lithography with the period controlled by the nanosphere diameter and the texture shape and aspect ratio controlled by the reactive ion etching conditions. Finite-difference time-domain optical simulations are used to optimize the texture in the state-of-the-art microcrystalline cells, and optical absorption measurements show that the same cells fabricated on the optimized nanocone-textured substrates exhibit a relative short-circuit current increase of close to 30% compared to a reference state-of-the-art cell with a randomly textured zinc oxide layer. This nanocone texturing technique is compatible with standard thin-film cell fabrication processes and can also be used for other thin-film cells (CIGS, CdTe, CZTS, etc) to maximize light absorption and minimize layer thickness enabling more efficient carrier collection and lower overall cost.