A search for new back contacts for CdTe solar cells.

There is widespread interest in reaching the practical efficiency of cadmium telluride (CdTe) thin-film solar cells, which suffer from open-circuit voltage loss due to high surface recombination velocity and Schottky barrier at the back contact. Here, we focus on back contacts in the superstrate configuration with the goal of finding new materials that can provide improved passivation, electron reflection, and hole transport properties compared to the commonly used material, ZnTe. We performed a computational search among 229 binary and ternary tetrahedrally bonded structures using first-principles methods and transport models to evaluate critical material design criteria, including phase stability, electronic structure, hole transport, band alignments, and p-type dopability. Through this search, we have identified several candidate materials and their alloys (AlAs, AgAlTe2, ZnGeP2, ZnSiAs2, and CuAlTe2) that exhibit promising properties for back contacts. We hope that these new material recommendations and associated guidelines will inspire new directions in hole transport layer design for CdTe solar cells.

Scalable ultrafast epitaxy of large-grain and single-crystal II-VI semiconductors.

A general problem for semiconductor applications is that very slow deposition on expensive single-crystal substrates yields high crystalline quality with excellent electro-optical properties, but at prohibitive costs and throughput for many applications. In contrast, rapid deposition on inexpensive substrates or nanocrystalline films yields low costs, but comparatively inferior crystallinity, carrier transport, and recombination. Here, we present methods to deposit single-crystal material at rates 2-3 orders of magnitude faster than state-of-the-art epitaxy with low-cost methods without compromising crystalline or electro-optical quality. For example, single-crystal CdTe and CdZnTe films that would take several days to grow by molecular-beam epitaxy are deposited in 8 minutes by close-spaced sublimation, yet retain the same crystalline quality measured by X-ray diffraction rocking curves. The fast deposition is coupled with effective n- and p-type in-situ doping by In, P, and As. The epitaxy can be extended to nanocrystalline substrates. For example, we recrystallize thin CdTe films on glass to deposit large grains with low defect density. The results provide new research paths for photovoltaics, detectors, infrared imaging, flexible electronics, and other applications.

SnO2-Catalyzed Oxidation in High-Efficiency CdTe Solar Cells.

Interfaces at the front of superstrate CdTe-based solar cells are critical to carrier transport, recombination, and device performance, yet determination of the chemical structure of these nanoscale regions has remained elusive. This is partly due to changes that occur at the front interfaces during high temperature growth and substantive changes occurring during postdeposition processing. In addition, these buried interfaces are extremely difficult to access in a way that preserves chemical information. In this work, we use a recently developed thermomechanical cleaving technique paired with X-ray photoelectron spectroscopy to probe oxidation states at the SnO2 interface of CdTe solar cells. We show that the tin oxide front electrode promotes the formation of nanometer-scale oxides of tellurium and sulfur. Most oxidation occurs during CdCl2/O2 activation. Surprisingly, we show that relatively low-temperature anneals (180-260 °C) used to diffuse and activate copper acceptors in a doping/back contact process also cause significant changes in oxidation at the front of the cell, providing a heretofore missing aspect of how back contact processes can modify device transport, recombination, and performance. Device performance is shown to correlate with the extent of tellurium and sulfur oxidation within this nanometer-scale region. Mechanisms responsible for these beneficial effects are proposed.

Overcoming Carrier Concentration Limits in Polycrystalline CdTe Thin Films with In Situ Doping.

Thin film materials for photovoltaics such as cadmium telluride (CdTe), copper-indium diselenide-based chalcopyrites (CIGS), and lead iodide-based perovskites offer the potential of lower solar module capital costs and improved performance to microcrystalline silicon. However, for decades understanding and controlling hole and electron concentration in these polycrystalline films has been extremely challenging and limiting. Ionic bonding between constituent atoms often leads to tenacious intrinsic compensating defect chemistries that are difficult to control. Device modeling indicates that increasing CdTe hole density while retaining carrier lifetimes of several nanoseconds can increase solar cell efficiency to 25%. This paper describes in-situ Sb, As, and P doping and post-growth annealing that increases hole density from historic 1014 limits to 1016-1017 cm-3 levels without compromising lifetime in thin polycrystalline CdTe films, which opens paths to advance solar performance and achieve costs below conventional electricity sources.

3D Lifetime Tomography Reveals How CdCl2 Improves Recombination Throughout CdTe Solar Cells.

Using two-photon tomography, carrier lifetimes are mapped in polycrystalline CdTe photovoltaic devices. These 3D maps probe subsurface carrier dynamics that are inaccessible with traditional optical techniques. They reveal that CdCl2 treatment of CdTe solar cells suppresses nonradiative recombination and enhances carrier lifetimes throughout the film with substantial improvements particularly near subsurface grain boundaries and the critical buried p-n junction.

Protonation effects on the branching ratio in photoexcited single-walled carbon nanotube dispersions.

The ensemble PL quantum yield for raw single-walled carbon nanotubes (SWNTs) dispersed in sodium cholate (SC) is approximately 5 times greater than that for the same raw SWNTs dispersed in sodium dodecyl sulfate (SDS) and approximately 10 times greater than the quantum yield of purified SWNTs dispersed in SC. Absorbance and Raman spectra indicate that purified SC-dispersed SWNTs and raw SDS-dispersed SWNTs are hole-doped by protonation. Experiments comparing PL emission efficiency using E2 and E1 excitation show that protonation significantly affects the E2 --> E1 relaxation process, which has typically been assumed to occur with unit efficiency. The E2 --> E 1 relaxation is 5 times more efficient in producing E 1 PL when SWNTs are unprotonated and protected by the SC surfactant. The results provide clear evidence that extrinsic factors, such as residual acids and the specific nature of SWNT-surfactant and SWNT-solvent interactions, can significantly affect measured SWNT luminescence quantum yields.

Multiple exciton generation in colloidal silicon nanocrystals.

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.

Extrinsic and intrinsic effects on the excited-state kinetics of single-walled carbon nanotubes.

We characterized the photoluminescence (PL) decay of 15 different, solubilized single-walled carbon nanotubes with tube diameters that ranged from 0.7 to 1.1 nm using time-correlated single photon counting. Each nanotube species was excited resonantly at the second excited state, E2, and PL was detected at the lowest energy exciton emission, E1. In a 10 ns window, the PL decays were described well by a biexponential fitting function with two characteristic time constants, suggesting that at least two kinetically distinct relaxation processes were observed. The dominant decay component increased from 60 to 200 ps with increasing tube diameter, while the lesser component, which contributed up to 8% of the total decay, increased from 200 ps to 4.8 ns. The observation of the second, longer decay time component is examined in terms of two possible models: an extrinsic behavior that implicates sample inhomogeneity and an intrinsic process associated with interconversion between kinetically distinct bright and dark exciton states. A common conclusion from both models is that nonradiative decay controls the PL decay by a process that is diameter dependent.