D.M. Meysing

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.

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...

The effect of copper on the sub-bandgap density of states of CdTe solar cells

Two optical sub-bandgap transitions in CdTe thin-film solar cells have been identified using detailed transient photocapacitance and transient photocurrent spectroscopy measurements. A broad response centered at EV + 0.9 eV directly correlates with the quantity of Cu present in the absorber layer, while a second response at EV + 1.2 eV does not depend on Cu or Zn and may be an intrinsic defect. These results demonstrate the influence of Cu on the sub-bandgap density of states of CdTe, and they are critical to understanding, modeling, and improving its optoelectronic properties.

Properties of oxygenated cadmium sulfide (CdS:O) and their impact on CdTe device performance

In this work, we report on the development of a reactive sputtering process for CdS:O for high efficiency CdTe solar cells. X-ray diffraction, UV-Vis-NIR spectrophotometry, and Rutherford backscattering spectrometry were used to characterize the crystal structure, composition, and optical properties, respectively. All films were slightly Cd-rich, while the bulk oxygen content increased up to 45 at. % in direct proportion to the O2 partial pressure. Optical absorption in cells was reduced by increasing the oxygen fraction in the sputtering ambient. Optimal performance was obtained from cells with CdS sputtered in a 6% O2/Ar ambient, yielding efficiency >14% and VOC >840 mV.

The use of Corning® Willow™ glass for flexible CdTe solar cells

New flexible glass products should enable the fabrication of high efficiency, flexible CdTe devices because of their high optical transmission and compatibility with the high temperature processing conditions often used for making high performance CdTe solar cells. Here, we will report on our preliminary results using Corning® Willow™ Glass in a high temperature CdTe device fabrication process. For all device studies, we used MOCVD deposited SnO2:F/SnO2 bilayers as the transparent conducting oxide/buffer. We investigated CdS window layers deposited by both room temperature sputtering and chemical bath deposition (CBD). Using 550°C CdTe layers deposited by close-spaced sublimation (CSS) on both types of CdS layers, we made CdTe devices with efficiencies above 12% with Willow Glass. These efficiencies are comparable to identically processed devices on rigid glass, confirming that Willow Glass is compatible with all high temperature CdTe processing steps.

Chemical and mechanical techniques enabling direct characterization of the CdS/CdTe heterojunction region in completed devices

The composition and structure of the CdS/CdTe heterojunction is critical to device performance. However, it is difficult to access this region in devices employing the conventional superstrate configuration. In this work, we report on the development of two techniques for exposing the CdS window layer in completed CdTe solar cells. First, we report on a chemical etch that selectively removes CdTe and exposes the CdS back surface. In addition, we demonstrate a thermo-mechanical lift-off technique that allows clean separation at the TCO/CdS interface. These techniques enable simple, quick sample preparation for characterization of the heterojunction region of completed devices.

Structural and chemical characterization of the back contact region in high efficiency CdTe solar cells

Cadmium telluride (CdTe) is the leading commercialized thin-film photovoltaic technology. Copper is commonly used in back contacts to obtain high efficiency, but has also been implicated as a harmful factor for device stability. Thus it is critical to understand its composition and distribution within complete devices. In this work the composition and structure of the back contact region was examined in high efficiency devices (~16%) contacted using a ZnTe:Cu buffer layer followed by gold metallization. The microstructure was examined in the as-deposited state and after rapid thermal processing (RTP) using high resolution transmission electron microscopy and EDX chemical mapping. After RTP the ZnTe exhibits a bilayer structure with polycrystalline, twinned grains adjacent to Au and an amorphous region adjacent to CdTe characterized by extensive Cd-Zn interdiffusion. The copper that is co-deposited uniformly within ZnTe is found to segregate dramatically after RTP activation, either collecting near the ZnTe/Au interface or forming CuxTe clusters in CdTe at defects or grain boundaries near the interface with ZnTe. Chlorine, present throughout CdTe and concentrated at grain boundaries, does not penetrate significantly into the back contact region during RTP activation.

High-efficiency flexible CdTe superstrate devices

Flexible, superstrate CdTe devices combine the advantages of a commercially demonstrated, low-cost manufacturing process with a lightweight, flexible form factor. Here, we present data on cell efficiencies greater than 16%, and the critical processing changes that have enabled recent efficiency increases. The devices in this study were made on Corning® Willow® Glass, which is a highly transparent, flexible, hermetic, and dimensionally stable substrate that can withstand high processing temperatures. To date, we have produced devices with several different combinations of front and back contacts on this glass and have found that it is compatible with most of our standard processing steps. One of our best devices to date has a certified efficiency of 16.2%, with a short-circuit current density (Jsc) of 25.6 mA/cm2, an open-circuit voltage of 820 mV, and a fill factor (FF) of 77.3%. The increased Jsc in this cell is due to an improved sputtered CdS:O deposition process, and the high FF is due to a co-evaporated ZnTe:Cu back contact.

Performance of transparent conductors on flexible glass and plastic substrates for thin film photovoltaics

High-performance transparent conductive indium-tin-oxide (ITO) films on flexible glass have been flextested to 25-50k bend cycles without breakage, and with ~0.1% change in sheet resistance. In contrast, commercial ITO/PET samples undergo ~50-100% increase in sheet resistance in the same test, indicating that such coatings/substrates may not be acceptable for use in some products or fabrication procedures. The flexible glass substrate enables high-temperature processing, which facilitates the high performance of the coatings. Measurements of the volume resistivity and water vapor transmission rate (WVTR) indicate that Corning® Willow® Glass is suitable as a PV substrate material without need for barrier coatings or glass lamination.

The effect of back contact and rapid thermal processing conditions on flexible CdTe device performance

Flexible CdTe solar cells on ultra-thin glass substrates can enable new applications that require high specific power, unique form-factors, and low manufacturing costs. To be successful, these cells must be cost competitive, have high efficiency, and have high reliability. Here we present back contact processing conditions that enabled us to achieve over 16% efficiency on flexible Corning® Willow® Glass substrates. We used co-evaporated ZnTe:Cu and Au as our back contact and used rapid thermal processing (RTP) to activate the back contact. Both the ZnTe to Cu ratio and the RTP activation temperature provide independent control over the device performance. We have investigated the influence of various RTP conditions to Cu activation and distribution. Current density-voltage, capacitance-voltage measurements along with device simulations were used to examine the device performance in terms of ZnTe to Cu ratio and rapid thermal activation temperature.