Wyatt K. Metzger

Superior radiation resistance of In1−xGaxN alloys: Full-solar-spectrum photovoltaic material system

High-efficiency multijunction or tandem solar cells based on group III–V semiconductor alloys are applied in a rapidly expanding range of space and terrestrial programs. Resistance to high-energy radiation damage is an essential feature of such cells as they power most satellites, including those used for communications, defense, and scientific research. Recently we have shown that the energy gap of In1−xGaxN alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a full-solar-spectrum material system for multijunction solar cells. We find that the optical and electronic properties of these alloys exhibit a much higher resistance to high-energy (2 MeV) proton irradiation than the standard currently used photovoltaic materials such as GaAs and GaInP, and therefore offer great potential for radiation-hard high-efficiency solar cells for space applications. The observed insensitivity of the semiconductor characteristics to the radiation damage is explained by the location of the band edge...

Grain-boundary recombination in Cu(In,Ga)Se2 solar cells

Two-dimensional simulations are performed to investigate the impact of grain boundaries (GBs) on Cu(In,Ga)Se2 (CIGS) solar-cell performance. Charged defect levels and compositional variations at GBs are considered. Neutral grain boundaries in the CIGS layer are predicted to be most detrimental if they are parallel to the main junction and located within the depletion region. For columnar GBs with a grain size near 1μm, the effective grain-boundary recombination velocity must be less than 104cm∕s to allow for record-efficiency devices. The majority-hole repulsion (additional donors at the GB) and the resulting band bending have a small effect on current collection but substantially lower the open-circuit voltage, and the combined effect is generally a lowering of the solar-cell efficiency. Minority-electron repulsion (additional acceptors at the GB) will partially mitigate GB recombination. A downshift of the valence-band energy, as predicted by the observed Cu depletion at CIGS GBs, can effectively block ...

Time-resolved photoluminescence studies of CdTe solar cells

We show that time-resolved photoluminescence measurements of completed polycrystalline CdTe solar cells provide a measure of recombination near the CdTe/CdS metallurgical interface that is strongly correlated to the open-circuit voltage in spite of complex carrier dynamics in the junction region. Oxygen in the growth ambient during close-spaced sublimation generally reduces this recombination rate; grain size does not have a strong effect.

Long lifetimes in high-efficiency Cu(In,Ga)Se2 solar cells

Time-resolved photoluminescence measurements on polycrystalline Cu(In,Ga)Se2 (CIGS) thin films corresponding to high-efficiency solar cells indicate recombination lifetimes as long as 250ns, far exceeding previous measurements for this material. The lifetime decreases by two orders of magnitude when exposed to air. Charge separation effects can be observed on CIGS∕CdS∕ZnO devices in low-intensity conditions. The ZnO layer forms a robust junction critical for charge separation, whereas the CdS layer alone forms a much weaker junction. Recombination at the CIGS/CdS interface is negligible. The results significantly adjust the previous picture of recombination in CIGS solar cells.

The role of amorphous silicon and tunneling in heterojunction with intrinsic thin layer (HIT) solar cells

This work analyzes heterojunction with intrinsic thin layer (HIT) solar cells using numerical simulations. The differences between the device physics of cells with p- and n-type crystalline silicon (c-Si) wafers are substantial. HIT solar cells with n-type wafers essentially form a n/p/n structure, where tunneling across the junction heterointerfaces is a critical transport mechanism required to attain performance exceeding 20%. For HIT cells with p-type wafers, only tunneling at the back-contact barrier may be important. For p-wafer cells, the hydrogenated amorphous silicon (a-Si:H) between the indium tin oxide (ITO) and crystalline silicon may act as a passivating buffer layer but, otherwise, does not significantly contribute to device performance. For n-wafer cells, the carrier concentration and band alignment of this a-Si:H layer are critical to device performance.

The impact of charged grain boundaries on thin-film solar cells and characterization

We use two-dimensional computer simulations to examine how charged columnar grain boundaries (GBs) affect transport, recombination, characterization, and performance in polycrystalline Cu(In,Ga)Se2 solar cells. Although the simulations show that charged GBs can increase photocurrent by forming minority-carrier collection channels, this generally occurs at the expense of overall efficiency. Carrier dynamics induced by the GBs significantly alter time-resolved photoluminescence, near-field scanning optical microscopy, electron-beam-induced current microscopy, and quantum efficiency spectra. Consequently, these experiments can place bounds on the role and strength of GB charge in polycrystalline materials. Simulations of these experiments indicate that GB charge sufficient to significantly increase photocurrent collection is generally inconsistent with the actual observations for Cu(In,Ga)Se2 solar cells.

A comparison of MBE- and MOCVD-grown GaInNAs

We have been able to discount three point defects as the factor limiting GaInNAs material quality by comparing samples grown by two different growth techniques. Samples with vastly different concentrations of hydrogen and carbon have very similar properties in terms of deep levels, mobilities, and minority-carrier lifetimes. In addition, growth of hydrogen-free samples and corresponding measurements of vacancies provide strong evidence that gallium vacancies have an effect, but are not a limiting defect.

CdCl2 treatment, S diffusion, and recombination in polycrystalline CdTe

Time-resolved photoluminescence measurements on glass∕SnO2∕CdTe and glass∕SnO2∕CdTe∕CdS structures indicate that the CdCl2 process, without any S present, significantly reduces recombination. However, S diffusion is required for lifetimes comparable to those observed in high-efficiency solar cells. Low-temperature photoluminescence, cathodoluminescence, and scanning electron images indicate how defect chemistry, grain-boundary passivation, and morphology are affected by S diffusion and the CdCl2 treatment.

Required material properties for high-efficiency CIGS modules

Relatively high proven efficiencies of CIGS devices are often cited regarding its choice as a semiconductor for photovoltaic manufacturing. Module efficiency is an important parameter, as a number of factors in the cost per watt are driven downward by increasing efficiency. Some of these factors include materials costs, throughput for a given capital investment, and installation costs. Thus, realizing high-efficiency (e.g. 15%) large-area CIGS modules is key in both reducing cost per watt and differentiating the technology from other thin films. This paper discusses the material properties required of each layer of the CIGS device such that large-area CIGS modules can achieve efficiencies 15%, which is substantially higher than the current industrial state-of-the-art. The sensitivity of module performance to the important material parameters is quantified based on both experimental data and modeling. Necessary performance differences between small-area devices and large-area modules imposed by geometry are also quantified. Potential technical breakthroughs that may relax the requirements for each layer are discussed.

14%-efficient flexible CdTe solar cells on ultra-thin glass substrates

Flexible glass enables high-temperature, roll-to-roll processing of superstrate devices with higher photocurrents than flexible polymer foils because of its higher optical transmission. Using flexible glass in our high-temperature CdTe process, we achieved a certified record conversion efficiency of 14.05% for a flexible CdTe solar cell. Little has been reported on the flexibility of CdTe devices, so we investigated the effects of three different static bending conditions on device performance. We observed a consistent trend of increased short-circuit current and fill factor, whereas the open-circuit voltage consistently dropped. The quantum efficiency under the same static bend condition showed no change in the response. After storage in a flexed state for 24 h, there was very little change in device efficiency relative to its unflexed state. This indicates that flexible glass is a suitable replacement for rigid glass substrates, and that CdTe solar cells can tolerate bending without a decrease in device performance.