Matthew P. Lumb

Optimal Bandgap Combinations—Does Material Quality Matter?

The balance of photogeneration and recombination gives rise to an optimum bandgap for any solar cell. The radiative limit represents the lowest permissible level of recombination in a solar cell and, therefore, places an upper limit on the voltage that can be attained. Introducing additional nonradiative recombination results in a loss in voltage that can only be compensated for by moving to higher bandgaps. Consequently, the optimal bandgap for solar energy conversion will rise with increasing nonradiative recombination rate. This balance was recognized by Shockley and Queisser for single-junction solar cells and is here extended to multijunction solar cells. A rise in optimal bandgaps has been observed in simulated single-, double-, and triple-junction devices as nonradiative recombination increases. Optimal bandgaps between excellent and poor diode quality devices are shown to differ by 100s of meV under 1-sun illumination with both terrestrial and extraterrestrial spectra but exhibit no significant change at high concentration due to the dominance of the radiative component in the recombination dynamics.

Extending the 1-D Hovel Model for Coherent and Incoherent Back Reflections in Homojunction Solar Cells

In this paper we extend the analytical drift-diffusion model, or Hovel model, to model the electrical characteristics of solar cells incorporating a back mirror. We use a compact summation approach to derive modified optical generation functions in Homojunction solar cells, considering both coherent and incoherent reflections from the back reflector. These modified generation functions are then used to derive analytical formulae for the current-voltage characteristics of mirrored solar cells. We simulate the quantum efficiency of a simple GaAs np diode with a planar gold back reflector, and compare the results with the standard Hovel model using a generation function given by the Beer-Lambert law. Finally, we use the model to simulate the performance of a real GaAs solar cell device fabricated using an epitaxial-lift-off procedure, demonstrating excellent agreement between the simulated and measured characteristics.

Six not-so-easy pieces in intermediate band solar cell research

Abstract. The concept of intermediate band solar cell (IBSC) is, apparently, simple to grasp. However, since the idea was proposed, our understanding has improved and some concepts can now be explained more clearly than when the concept was initially introduced. Clarifying these concepts is important, even if they are well known for the advanced researcher, so that research efforts can be driven in the right direction from the start. The six pieces of this work are: Does a miniband need to be formed when the IBSC is implemented with quantum dots? What are the problems for each of the main practical approaches that exist today? What are the simplest experimental techniques to demonstrate whether an IBSC is working as such or not? What is the issue with the absorption coefficient overlap and the Mott’s transition? What would the best system be, if any?

Sintered CdTe Nanocrystal Thin Films: Determination of Optical Constants and Application in Novel Inverted Heterojunction Solar Cells

In this paper, we report a novel heterojunction solar cell based on sintered CdTe and CdSe nanocrystal thin films using solution-based deposition. For the absorber layer, CdTe thin films were made using a layer-by-layer deposition process consisting of spin coating colloidal CdTe nanocrystals followed by a sintering step. The optical constants for these sintered CdTe films were accurately determined through a combination of optical modeling and measurements. For the all-solution processed p-n heterojunction, we focus on CdSe nanocrystal thin films due to their excellent compatibility with solution processing. For the optimized inverted structure (glass/ITO/CdSe/CdTe/Cr/Au), a high open-circuit voltage (Voc) of 593 ± 32 mV with an efficiency of 1.9 ± 0.2% was obtained under simulated one sun illumination. These preliminary results demonstrate that this novel inverted heterojunction structure has the potential to produce a high-quality CdSe/CdTe junction for utilization in heterojunction solar cells.

Optical properties of Si-doped and Be-doped InAlAs lattice-matched to InP grown by molecular beam epitaxy

In this paper, we determine the optical constants and carrier mobilities of Si-doped and Be-doped InAlAs lattice matched to InP. The samples were grown using molecular beam epitaxy and characterized using Hall measurements, variable angle spectroscopic ellipsometry, and room temperature photoluminescence spectroscopy. A Moss-Burstein shift in the fundamental absorption edge was observed in both Si-doped and Be-doped materials. We fitted a multiple-oscillator, critical point model to the dielectric function of the materials extracted using the spectroscopic ellipsometry. The tabulated input parameters of this model allow for accurate calculations of the dielectric function of doped InAlAs to be made, which is useful information for simulating a variety of InP-based optoelectronic devices.

Modeling and analysis of high-performance, multicolored anti-reflection coatings for solar cells

In this work solar cell anti-reflection coatings tuned to give a specific hue under solar illumination are investigated. We demonstrate that it is possible to form patterned coatings with large color contrast and high transmittance. We use colorimetric and thin film optics models to explore the relationship between the color and performance of bilayer anti-reflection coatings on Si, and predict the photocurrent generation from an example Si solar cell. The colorimetric predictions were verified by measuring a series of coatings deposited on Si substrates. Finally, a patterned Si sample was produced using a simple, low-cost photolithography procedure to selectively etch only the top layer of a bilayer coating to demonstrate a high-performance anti-reflection coating with strong color contrast.

Solution processing of CdTe nanocrystals for thin-film solar cells

We describe solution-processed sintered nanocrystal solar cells. As the absorber layer, thin-films of CdTe nanocrystals were deposited using a layer-by-layer spin coating process. For Schottky barrier solar cells (ITO/CdTe/Ca/Al), an efficiency of 3.0±0.3% with V_oc= 0.53±0.03 V, J_sc= 13.2±0.2 mA/cm², and FF=43.1±4.2% was measured under AM1.5G conditions. In order to overcome the limitations associated with the Schottky barrier structure, heterojunction solar cells incorporating solution-processed CdSe nanocrystals as an n-type layer were fabricated and characterized. Under darkness, heterojunction devices (ITO/CdTe/CdSe/Al) showed an improved current rectification ratio of 4.53×10² at ±1V in comparison to Schottky barrier solar cells, indicating that a well-defined junction is formed between CdSe and CdTe. Under illumination (AM1.5G), the devices exhibited an average efficiency of ~2% with V_oc=0.50±0.02 V.

Transparent conducting oxide-based, passivated contacts for high efficiency crystalline Si solar cells

In this work, we investigate a transparent conducting oxide (TCO)-based, passivated contact for the potential use as a passivated tunnel contact to p-type Si. As a surface passivation layer, the Al₂O₃ films with varying the thickness are deposited using plasma-enhanced atomic layer deposition (PEALD) at 200 °C, followed by post-deposition annealing. For a ~15 nm thick Al₂O₃ layer, a high level of surface passivation is achieved, characterized by the effective surface recombination velocity (S_eff,max) of <;30 cm/s. The samples with ultrathin Al₂O₃ layer <;3 nm, however, shows degradation in passivation quality, reaching the S_eff,max<;500 cm/s. When Al-doped zinc oxide (ZnO:Al) as TCO contact is directly deposited onto a ~10.6 nm thick Al₂O₃ coated p-Si via RF magnetron sputtering, the final passivation quality (p-Si/Al₂O₃/ZnO:Al) is characterized by the saturation current density at contact (J_0,contact) of 92.1 fA/cm² with the implied open-circuit voltage (iVoc) of 653 mV, showing the passivation quality is not severely degraded after sputtering without thermal treatment. Further process optimization of PEALD is in progress to produce an improved quality of surface passivation with the S_eff,max<;10 cm/s for ultrathin passivation layers less than 2 nm, enabling a passivated tunneling contact.

Rapid thermal annealing of InAlAsSb lattice-matched to InP for top cell applications

The effect of rapid thermal annealing on the optical properties of In_xAl_1-xAs _ySb _1-y was analyzed and compared to that for In_0.52 Al_0.48As. Initial ellipsometry and photoluminescence experiments performed before the annealing indicate the presence of a low energy Urbach tail in the absorption spectrum. Rapid thermal annealing produces a blue-shift in the PL emission when annealed at 650°C for 60s and a decrease in the full-width-half-maximum, which originates from a reduction of the emission from the longer wavelength states. For the In_0.52 Al_0.48As, the emission energy and the full-width-half-maximum remain constant during the annealing study. The elimination of sub-bandgap states in In_0.52 Al_0.48As is critical for achieving a realistic path towards high efficiency multijunction cells lattice-matched to InP.

Analysis of gaas photovoltaic device losses at high MOCVD growth rates

Gallium arsenide material has been deposited via metal organic chemical vapor deposition (MOCVD) at growth rates varying between 14 μm/hr and 56 μm/hr. Photovoltaic device results indicate a 6-7% relative decrease in efficiency between 14 and 56 μm/hr GaAs solar cells, due to a reduction in short-circuit current and open-circuit voltage. By simulating the experimental characterization data, it is established that performance losses are associated with rear surface recombination velocity and Shockley-Read-Hall lifetime. The relative impact of these loss mechanisms will be quantified and conclude with discussions on their mitigation.