C.-S. Jiang

Square‐Centimeter Solution‐Processed Planar CH3NH3PbI3 Perovskite Solar Cells with Efficiency Exceeding 15%

The preparation of uniform, high-crystallinity planar perovskite films with high-aspect-ratio grains over a square-inch area is demonstrated. The best power conversion efficiency (PCE) of 16.3% (stabilized output of ≈15.6%) is obtained for a planar perovskite solar cell (PSC) with 1.2 cm2 active area, and the PCE jumps to 18.3% (stabilized output of ≈17.5%) for a PSC with a 0.12 cm2 active area.

Local Built-in Potential on Grain Boundary of Cu(In,Ga)Se2 Thin Films

We report on a direct measurement of two-dimensional potential distribution on the surface of photovoltaic Cu(In,Ga)Se2 thin films using a nanoscale electrical characterization of scanning Kelvin probe microscopy. The potential measurement reveals a higher surface potential or a smaller work function on grain boundaries of the film than on the grain surfaces. This demonstrates the existence of a local built-in potential on grain boundaries, and the grain boundary is positively charged. The local built-in potential on the grain boundary is expected to increase the minority-carrier collection area from one to three dimensional. In addition, a work function decrease induced by Na on the film surface was observed.

Does the local built-in potential on grain boundaries of Cu(In,Ga)Se2 thin films benefit photovoltaic performance of the device?

In a previous paper [C.-S. Jiang et al., Appl. Phys. Lett. 84, 3477 (2004)], we reported the existence of a local built-in potential on grain boundaries (GBs) of photovoltaic Cu(In,Ga)Se2 (CIGS) thin films. However, whether the built-in potential benefits photovoltaic properties of the device has not been proven. Using a scanning Kelvin probe microscope, we found that, with increasing Ga content in the CIGS film, the built-in potential on the GB drops sharply in a Ga range of 28%–38%. Comparing the changes in the built-in potential, the device efficiency, and the CIGS band gap, we conclude that the built-in potential on the GB plays a significant role in the device conversion efficiency of NREL’s three-stage CIGS device.

How grain boundaries in Cu(In,Ga)Se2 thin films are charged: Revisit

Potential measurements on Cu(In,Ga)Se2 thin films using scanning Kelvin probe force microscopy have been reported extensively to address grain-boundary (GB) recombination by examining GB charging. However, the results are highly inconsistent. We revisit this issue by measuring high- and low-quality wide-bandgap films and using a complementary method of scanning capacitance microscopy. Our results show consistent positively charged GBs in our high-quality films with minimal surface defects, except for the Σ3[112] GBs, which are charge neutral. We discuss possible artifacts due to surface defects when examining the GB charging and the role of GBs in the device performance.

Investigation of potential and electric field profiles in cross sections of CdTe/CdS solar cells using scanning Kelvin probe microscopy

We investigated cross sections of working CdTe/CdS solar cells using scanning Kelvin probe microscopy (SKPM). The cross sections were prepared by polishing to avoid steps between the glass substrate and film that generally make the analysis difficult. However, this process resulted in strong pinning of the Fermi level. During the measurements, the cells were biased under different conditions, revealing the distribution of the electrical potential inside the device. We were able to identify different regions inside the device: in the region away from the CdTe/CdS junction, there was only a small variation in the potential; closer to the junction, the potential increased, due to the increase in the depletion regions with the reverse bias; at the junction, there was a sudden increase in the potential, which was attributed to interdiffusion between CdTe and CdS. By taking the first derivative of the potential, we were able to calculate the electric field inside the device. The maximum of the electric field, w...

Two-dimensional junction identification in multicrystalline silicon solar cells by scanning Kelvin probe force microscopy

We report on a two-dimensional investigation of the p-n junction in multicrystalline silicon solar cells using scanning Kelvin probe force microscopy (SKPFM). The junction location and depth were identified by SKPFM potential measurement and subsequent data analysis, where a procedure taking bias-voltage-induced changes in the potential and electric field was developed to avoid the effects of surface Fermi level pinning. Device simulation supported the junction identification procedure and showed a possible deviation of ∼40 nm in the junction identification. The two-dimensional electric-field images show that the shape of the junction follows the surface topography of the device, or, in other words, the junction depth is identical over the device.

Effects of dilution ratio and seed layer on the crystallinity of microcrystalline silicon thin films deposited by hot-wire chemical vapor deposition

Abstract We deposited microcrystalline silicon (μc-Si) by hot-wire chemical vapor deposition (HWCVD) at different thickness and dilution ratio, with and without seed layer. As the dilution ratio increased, we observed an increase in the amount of microcrystalline phase in the film, a change in the structure of the grains and a loss of the (220) preferential orientation. The films deposited over a seed layer had a larger fraction of crystalline phase than films deposited with the same parameters but without a seed layer. For high dilution ratios (R=100), most of the film grows epitaxially at the interface with the Si substrate, but a microcrystalline film slowly replaces the single-crystal phase. For low dilution ratios (R=14), the film starts growing mostly amorphously, but the amount of crystalline phase increases with thickness.

Local current flow in amorphous and nanocrystalline mixed-phase silicon solar cells

Local current flow in amorphous and nanocrystalline mixed-phase n-i-p silicon solar cells is measured using conductive atomic force microscopy (C-AFM) and correlated to the material structure from Raman measurement. The C-AFM images show that the current is very low over the entire surface of the fully amorphous region. High current spikes are observed in the mixed-phase region, where the current spike corresponds to aggregations of nanocrystallites. The size of the nanocrystalline aggregations is on the order of a half micrometer in diameter. The density of the current spike increases from the mixed phase to the heavily nanocrystalline regions. A thick amorphous silicon buffer layer inserted between the p and i layers significantly reduced the magnitude of the current spike. The C-AFM measurements suggest that the mixed-phase cells can be considered as a two-phase parallel-connected diode structure, consistent with our previously proposed model.

Distribution of built-in electrical potential in GaInP2/GaAs tandem-junction solar cells

Distributions of built-in potential in GaInP2/GaAs tandem-junction solar cells were investigated by scanning Kelvin probe microscopy. Two states of potential distribution resulting from flattening of band bending and charge accumulation on either the top or bottom p–n junction were observed under short circuit, depending on the illumination spectra. With an external bias voltage, the voltage change always happened on the junction with the charge accumulation, and the potential distribution between the two states became less sensitive to illumination spectra.

Electrical modification in Cu(In,Ga)Se2 thin films by chemical bath deposition process of CdS films

We have measured the two-dimensional electrical potential distribution on the surface of photovoltaic Cu(In,Ga)Se2 (CIGS) thin films using the nanoscale electrical characterization of scanning Kelvin probe microscopy. The potential peak on the grain boundaries becomes sharper after the sample is rinsed in high-purity water, and the height of the potential peak becomes smaller after chemical treatments in a solution similar to that used in the chemical bath deposition of CdS films. This demonstrates an effect of surface Na removal by the water rinsing and downward band bending on the CIGS film surface induced by the chemical treatment. This electrical modification is expected to benefit the properties of the electrical junction and, hence, CIGS/CdS device performance.