James M. Burst

Minority Carrier Lifetime Analysis in the Bulk of Thin-Film Absorbers Using Subbandgap (Two-Photon) Excitation

We describe a new time-resolved photoluminescence (TRPL) analysis method for the determination of minority carrier lifetime τB. This analysis is based on subbandgap excitation (two-photon excitation, or 2PE) and allows selective lifetime determination at the surface or in the bulk of semiconductor absorbers. We show that for single-crystal CdTe, τB could be determined even if surface recombination velocity is >105 cm s-1. Two-photon excitation TRPL measurements indicate that radiative lifetime in undoped CdTe is >>66 ns. We also compare one-photon excitation (1PE) and 2PE TRPL data for polycrystalline CdS/CdTe thin films.

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.

Intrinsic surface passivation of CdTe

Recombination is critically limiting in CdTe devices such as solar cells and detectors, with much of it occurring at or near the surface. In this work, we explore different routes to passivate p-type CdTe surfaces without any intentional extrinsic passivation layers. To provide deeper insight into the passivation routes, we uniquely correlate a set of characterization methods: surface analysis and time-resolved spectroscopy. We study two model systems: nominally undoped single crystals and large-grain polycrystalline films. We examine several strategies to reduce surface recombination velocity. First, we study the effects of removing surface contaminants while maintaining a near-stoichiometric surface. Then we examine stoichiometric thermally reconstructed surfaces. We also investigate the effects of shifting the surface stoichiometry by both “subtractive” (wet chemical etches) and “additive” (ampoule anneals and epitaxial growth) means. We consistently find for a variety of methods that a highly ordered ...

Properties of reactively sputtered oxygenated cadmium sulfide (CdS:O) and their impact on CdTe solar cell performance

Oxygenated cadmium sulfide (CdS:O) is commonly used as the n-type window layer in high-performance CdTe heterojunction solar cells. This layer is deposited by reactive sputtering, but the optimal amount of oxygen in the sputtering ambient is highly dependent on the specific system and process employed. In this work, the intrinsic properties of CdS:O were measured as a function of the oxygen content (0%–10%) in the sputtering ambient and correlated to device performance with the goal of better defining optimal CdS:O properties for CdTe solar cells. Optimal performance was found using CdS:O films that contained ∼40 at. % oxygen as measured by Rutherford backscattering spectrometry. X-ray photoelectron spectroscopy confirmed these results and showed that oxygen is incorporated primarily as oxygenated sulfur compounds (SOx). Device efficiency improved from 10.5% using CdS to >14% with CdS:O due largely to increases in short-circuit current density as well as a modest improvement in open-circuit voltage. The t...

Single-crystal CdTe solar cells with Voc greater than 900 mV

We fabricated single-crystal CdTe photovoltaic devices in a heterojunction structure with an In-doped CdS window layer and ZnO/Al-doped ZnO front contact. By replacing the polycrystalline absorber layer of a CdTe solar cell with a single crystal, we were able to achieve open-circuit voltage (Voc) as high as 929 mV. Simulations and measurements indicate that increased minority-carrier lifetime and carrier concentration can explain this high Voc. Cu and Na both introduce transient effects in single-crystal CdTe similar to those observed in polycrystalline CdTe, suggesting that Group I dopants pose stability problems that are linked fundamentally to their defect chemistry in CdTe, regardless of the presence of grain boundaries.

Enhanced p-type dopability of P and As in CdTe using non-equilibrium thermal processing

One of the main limiting factors in CdTe solar cells is its low p-type dopability and, consequently, low open-circuit voltage (VOC). We have systematically studied P and As doping in CdTe with first-principles calculations in order to understand how to increase the hole density. We find that both P and As p-type doping are self-compensated by the formation of AX centers. More importantly, we find that although high-temperature growth is beneficial to obtain high hole density, rapid cooling is necessary to sustain the hole density and to lower the Fermi level close to the valence band maximum (VBM) at room temperature. Thermodynamic simulations suggest that by cooling CdTe from a high growth temperature to room temperature under Te-poor conditions and choosing an optimal dopant concentration of about 1018/cm3, P and As doping can reach a hole density above 1017/cm3 at room temperature and lower the Fermi level to within ∼0.1 eV above the VBM. These results suggest a promising pathway to improve the VOC and...

Cd-rich and Te-rich low-temperature photoluminescence in cadmium telluride

Low-temperature photoluminescence emission spectra were measured in cadmium telluride (CdTe) samples in which composition was varied to promote either Cd or Te-rich stoichiometry. The ability to monitor stoichiometry is important, since it has been shown to impact carrier recombination. Te-rich samples show transitions corresponding to acceptor-bound excitons (∼1.58 eV) and free-electron to acceptor transitions (∼1.547 eV). In addition to acceptor-bound excitons, Cd-rich samples show transitions assigned to donor-bound excitons (1.591 eV) and Te vacancies at 1.552 eV. Photoluminescence is a noninvasive way to monitor stoichiometric shifts induced by post-deposition anneals in polycrystalline CdTe thin films deposited by close-spaced sublimation.

Long carrier lifetimes in large-grain polycrystalline CdTe without CdCl2

For decades, polycrystalline CdTe thin films for solar applications have been restricted to grain sizes of microns or less whereas other semiconductors such as silicon and perovskites have produced devices with grains ranging from less than a micron to more than 1 mm. Because the lifetimes in as-deposited polycrystalline CdTe films are typically limited to less than a few hundred picoseconds, a CdCl2 treatment is generally used to improve the lifetime; but this treatment may limit the achievable hole density by compensation. Here, we establish methods to produce CdTe films with grain sizes ranging from hundreds of nanometers to several hundred microns by close-spaced sublimation at industrial manufacturing growth rates. Two-photon excitation photoluminescence spectroscopy shows a positive correlation of lifetime with grain size. Large-grain, as-deposited CdTe exhibits lifetimes exceeding 10 ns without Cl, S, O, or Cu. This uncompensated material allows dopants such as P to achieve a hole density of 1016 c...

Carrier density and lifetime for different dopants in single-crystal and polycrystalline CdTe

CdTe defect chemistry is adjusted by annealing samples with excess Cd or Te vapor with and without extrinsic dopants. We observe that Group I (Cu and Na) elements can increase hole density above 1016 cm−3, but compromise lifetime and stability. By post-deposition incorporation of a Group V dopant (P) in a Cd-rich ambient, lifetimes of 30 ns with 1016 cm−3 hole density are achieved in single-crystal and polycrystalline CdTe without CdCl2 or Cu. Furthermore, phosphorus doping appears to be thermally stable. This combination of long lifetime, high carrier concentration, and improved stability can help overcome historic barriers for CdTe solar cell development.

Surface Passivation of CdTe Single Crystals

Low open-circuit voltages (850-870 mV), due to excessive bulk and surface recombination, currently limit CdTe photovoltaic efficiencies. Here, we study surface recombination in single crystals with single-photon excitation time-resolved photoluminescence (1PE-TRPL) to measure minority carrier lifetimes. Typically, minority carrier lifetimes of untreated undoped CdTe material as measured by 1PE-TRPL are ~100 ps or less, even though their bulk lifetimes as measured by two-photon excitation TRPL can reach 100 ns. Such short 1PE-TRPL lifetimes indicate very high surface recombination velocities exceeding 100 000 cm/s. Here, we examine treatments that can reduce surface recombination and discuss different ways of evaluating their efficacy.