Victor V. Plotnikov

Wiring-up carbon single wall nanotubes to polycrystalline inorganic semiconductor thin films: low-barrier, copper-free back contact to CdTe solar cells.

We have discovered that films of carbon single wall nanotubes (SWNTs) make excellent back contacts to CdTe devices without any modification to the CdTe surface. Efficiencies of SWNT-contacted devices are slightly higher than otherwise identical devices formed with standard Au/Cu back contacts. The SWNT layer is thermally stable and easily applied with a spray process, and SWNT-contacted devices show no signs of degradation during accelerated life testing.

10% efficiency solar cells with 0.5 µm of CdTe

Fabricating high-efficiency CdS/CdTe solar cells with ultra-thin (below 1µm) absorber layers is a challenging yet highly desirable step in improving CdTe technology. Typically solar cell performance decreases due to shunting, incomplete absorption (deep penetration loss), fully depleted CdTe layers or interference between the main and the back contact junction when the CdTe layer thickness approaches a certain limit. While some of these losses are fundamental, others can be minimized by careful optimization of the fabrication steps. We present the results of such optimization.

Semiconducting carbon single-walled nanotubes as a cu-free, barrier-free back contact for CdTe solar cell

Copper diffusion from the back contact degrades the performance of CdTe solar cells over time and increases the levelized cost of electricity production from CdTe photovoltaics. Recently, carbon single-wall nanotubes (SWNTs) were shown to be a Cu-free, stable alternative that preserves the device efficiency (Phillips et al., Nano Letter, 2013). Large diameter tube samples containing a mixture of semiconducting (s-SWNT) and metallic (m-SWNT) species were used in the previous work, and the mechanisms leading to a low back barrier for majority carrier flow were not clear. The good performance of the back contact was ascribed to the interaction between the s-SWNTs in the film and the polycrystalline facets of the CdTe surfaces. In that case, the s-SWNT species had small bandgaps (~0.6-0.8 eV). Here, in an attempt to develop a more detailed understanding of the SWNT/CdTe back contact, we employed SWNT samples that are predominantly semiconducting (95%) and of larger bandgap (~1.1-1.3 eV). The power conversion efficiency of these unoptimized devices was 11.5 % with a s-SWNT back contact, as compared to 11.2% with a standard Cu/Au back contact.

CdTe cell stability vs. CdS thickness

We have studied the stability in air of sputtered CdTe/CdS cells with a focus on the thickness of the CdS layer and the substrate used during cell fabrication. The stability of cell efficiencies under light soak is compared for CdTe cells on both HRT-coated Pilkington TEC 15 glass and standard TEC15 glass for a range of seven different CdS layer thicknesses from 0 to 230 nm. Little difference in stability can be found between cells with or without the HRT layer for all CdS thicknesses. The buffer layer helps to produce high initial performance and uniformity of cells by maintaining high VOC for very thin CdS layers. But there is no indication that the buffer layer improves the stability of unencapsulated, sputtered cells under one-sun light soak at open circuit. Devices with the thicker CdS layers show better stability under one-sun light soak in air.

Semitransparent PV windows with sputtered CdS/CdTe thin films

Photovoltaic devices have been increasingly relied upon for distributed power generation at various levels (portable power, residential, commercial and municipal). One particularly appealing option for PV devices is their incorporation as building-integrated or vehicle-integrated structures which would serve both electrical generation and aesthetic requirements. Two particular embodiments of this application are semitransparent PV windows and curtain walls for residential/commercial buildings as well as PV sunroofs for vehicles. The technology of semitransparent PV modules with good photo-conversion efficiency and electrical performance is non-trivial. This report discusses the use of ultrathin devices based upon CdS/CdTe films augmented with color shifting technologies to meet these requirements.

Superstrate design for increased CdS/CdTe solar cell efficiencies

The optical and electrical properties of the glass superstrate and window layer materials play critical roles in the performance of CdTe photovoltaic devices. The compositional modifications made to tailor the transmission spectra in these materials so as to maximize the Jsc of the device will be reviewed. Progress towards improving the electrical characteristics (e.g., carrier mobility) of the transparent conductive oxides as well as materials design to improve the interface behavior at the window layer will be presented. Recent results showing the impact of these changes on achieving thinner CdS window layers and improved device performance in devices made by both thermal and energetic deposition methods will be discussed.

Dependence CdS/CdTe solar cells efficiency and nonuniformity on CdS layer thickness

We have prepared cell with a variable thickness of CdS layer to get an understanding of the optimum thickness as well as it lowest limits for practical purposes. We used quantum efficiency current-voltage and light beam induce current testing to investigate cell performance as well as nonuniformity. The best results are achieved with 0.1 micron CdS layer. This number does depend slightly on the quality of CdCl2 treatment and the thickness of the CdTe layer. For the case of magnetron sputtered cells, nonuniformities have a relatively weak dependence on CdS layer thickness. We find also that some nonuniformities arise from postdeposition sample treatment and handling.

Studies of Cu location near the back contact of CdS/CdTe solar cells

We have used the x-ray fluorescence signal from copper selectively excited with a ∼10 keV photon beam from the Argonne Advanced Photon Source to probe low levels of copper in sputtered CdTe cells. Through a peel-off technique that separates the evaporated Cu/Au back contact from the CdTe, we are able to determine where the Cu resides after cell preparation is finished. With a 3 to 4 nm Cu layer under the 20 nm of gold, we find that about 10% stays with the CdTe after contact peel-off. Recontacting these cells with fresh gold yields cell performance very close to the originally prepared contacts indicating that only 3–4 Å of copper is needed for a good contact that exhibits no rollover in the first quadrant of the I–V curve.