Rajendra R. Khanal

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.

Iron Pyrite Nanocrystal Film As A Copper-Free Back Contact For Polycrystalline CdTe Thin Film Solar Cells

We report the application of thin film nanocrystalline (NC) FeS2 as the copper-free back contact for CdTe solar cells. The FeS2-NC layer is prepared from solution directly on the CdTe surface using drop-casting coupled with a hydrazine treatment at ambient temperature and pressure, and requires no thermal treatment. Copper-free solar cells based on the CdS/CdTe/FeS2-NC/Au architecture exhibit device efficiencies 490% that of a standard Cu/Au back contact devices. The FeS2-NC back contact solar cells show good thermal stability under initial tests. Devices prepared with untreated FeS2-NC back contacts display a strong “S-kink” behavior which we correlate with a high hole-transport barrier arising from inter-NC organic surfactant molecules. & 2015 Elsevier B.V. All rights reserved.

Investigation of degradation mechanisms of perovskite-based photovoltaic devices using laser beam induced current mapping

Solution processed thin film photovoltaic devices incorporating organohalide perovskites have progressed rapidly in recent years and achieved energy conversion efficiencies greater than 20%. However, an important issue limiting their commercialization is that device efficiencies often drop within the first few hundred hours of operation. To explore the origin of the device degradation and failure in perovskite solar cells, we investigated the spatial uniformity of current collection at different stages of aging using two-dimensional laser beam induced current (LBIC) mapping. We validated that the local decomposition of the perovskite material is likely due to interactions with moisture in the air by comparing photocurrent collection in perovskite devices that were maintained in different controlled environments. We show that the addition of a poly(methyl methacrylate)/single-wall carbon nanotube (PMMA/SWCNT) encapsulation layer prevents degradation of the device in moist air. This suggests a route toward perovskite solar cells with improved operational stability and moisture resistance.

Spray pyrolysis of semi-transparent backwall superstrate CuIn(S,Se) 2 solar cells

We explored the feasibility of depositing CuIn(S,Se)2 (CIS) absorbers using a recently developed hydrazine-based spray pyrolysis method in the backwall superstrate configuration. The devices were fabricated in a novel configuration consisting of a solution-processed CIS/CdS p-n junction sandwiched between two transparent conducting layers and photovoltaic responses was demonstrated with illumination through either side of the device. The structural, compositional, and electrical properties of the devices were investigated. The current voltage characteristics and quantum efficiency of the backwall superstrate and conventional substrate devices were studied. The performance of the backwall superstrate devices was less than that of the control devices due to the relatively thick absorber thickness and high sheet resistance of the transparent back contact. Further improvement in the crystal quality and stoichiometry of CIS deposition should lead to high efficiency devices with thinner and better quality absorbers.

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.

Simultaneous shunt protection and back contact formation for CdTe solar cells with single wall carbon nanotube layers

Thin film photovoltaic (PV) devices and modules prepared by commercial processes can be severely compromised by through-device low resistance electrical pathways. The defects can be due to thin or missing semiconductor material, metal diffusion along grain boundaries, or areas containing diodes with low turn-on potentials. We report the use of single wall carbon nanotube (SWCNT) layers to enable both protection against these defects and back contact formation for CdTe PV devices. Samples prepared with a SWCNT back contact exhibited good efficiency and did not require shunt protection, while devices prepared without shunt protection using a standard metal back contact performed poorly. We describe the mechanism by which the SWCNT layer functions. In addition to avoiding the need for shunt protection by other means, the SWCNT film also provides a route to higher short circuit currents.

Enhancing the efficiency of CdTe solar cells using a nanocrystalline iron pyrite film as an interface layer

We use thin film nanocrystalline (NC) iron pyrite as an interface layer at the back contact of CdS/CdTe solar cells. In both spattered and CSS deposited CdTe devices, improvements in Voc and FF were obtained after the inclusion of the NC FeS2 layer. Repeated tests show that Voc increases by >30 mV and FF increases by ~3.5% for a standard CdTe device having efficiency of ~13%. The devices tested at STC show a relative increase in the power conversion efficiency in the range of 5% to 9%.

Carbon nanotube reinforced cu metal matrix composites for current collection from space photovoltaics

There is a strong desire to reduce the weight and increase the power output of photovoltaics devices for space applications while simultaneously reducing cost. The inverted metamorphic multijunction (IMM) cell is an advanced III-V device architecture that provides routes to higher on-orbit power production. A higher specific power is provided by a combination of high efficiency and a thin, low-weight device. As crystalline devices become thinner, however, they are more prone to fracture/cleaving failure. In the most benign case, fracture may simply disconnect a portion of the cell from the power providing circuit. More seriously, fracture can also lead to an open string. These problems may be mitigated by the development of fracture/cleave-tolerant devices. In this paper we consider a novel fabrication route to form metal matrix composites (MMCs) comprised of carbon nanotubes embedded in metals such as Ag and Cu. The MMCs will be used to fabricate grid-lines and back-metal layers for IMM cells with a goal of maintaining cell active area even after fracture/cleavage failures.

Single wall carbon nanotube electrodes for hydrogenated amorphous silicon solar cells

Carbon single wall nanotube (SWNT) films were applied as electrodes to replace the p-layer in hydrogenated amorphous silicon (a-Si:H) solar cells. Devices were fabricated by transferring vacuum-filtered SWNT films of varying thickness onto a-Si:H layers grown by plasma enhance chemical vapor deposition on Pilkington TEC 15 glass substrates. Cells incorporating SWNTs were illuminated from each side (glass / SWNT). A cell illuminated through a 25 nm thick SWNT film yielded short circuit current density, open circuit voltage, and efficiency of 5.47 mA/cm2, 0.793 V, and 1.46%, respectively. Maximum quantum efficiency of 48% was measured at 475 nm for the same device.

Evolution of the optical response of sputtered CdS:O as a function of temperature

The linear absorption spectrum of an oxygenated CdS (CdS:O) thin film was measured as a function of annealing temperature. Subtle changes in the absorption spectrum were observed for short annealing times at temperatures up to 275 °C. Much more significant changes were observed as the temperature was increase from 275 °C to 325°C. Interestingly, at some point, increasing the temperature does not result in a change in the spectrum, indicating complete conversion into CdS. This evolution is also observed when the CdS:O film is capped with a thin film of CdTe. This suggests that an unreacted CdS:O layer is not likely to survive post deposition processing in CdTe solar cells.