Lila Raj Dahal

High-Speed Imaging/Mapping Spectroscopic Ellipsometry for In-Line Analysis of Roll-to-Roll Thin-Film Photovoltaics

An expanded-beam spectroscopic ellipsometer has been developed and applied toward in situ high-speed imaging/mapping analysis of large area spatial uniformity for multilayer coated substrates in roll-to-roll thin-film photovoltaics (PV). Slower speed instrumentation available in such analyses applies a 1-D detector array for spectroscopic mapping and involves width-wise translation of the ellipsometer optics over the moving coated substrate surface, measuring point-by-point in a time-consuming process. The expanded-beam instrument employs instead a 2-D detector array with no moving optics, exploiting one array index for spectroscopy and the second array index for line imaging across the width of a large area sample. Thus, the instrument enables imaging width-wise and mapping length-wise for uniformity evaluation at the high linear substrate speeds required for real-time, in situ, and online analysis in roll-to-roll thin-film PV. In this investigation, we employ the expanded beam technique to characterize the uniformity of the Ag, ZnO, and n-type hydrogenated amorphous silicon (a-Si:H) layers of an a-Si:H n-i-p structure deposited on a flexible polyimide substrate in the roll-to-roll configuration. Spectroscopic ellipsometry data across a line image were collected as the substrate was translated by a roll-to-roll mechanism. Coated areas as large as 12 cm × 45 cm were analyzed in this study for layer thickness and optical properties by applying the appropriate analytical models for the complex dielectric functions of the Ag, ZnO, and n-type a-Si:H layers.

Quantum efficiency simulations from on-line compatible mapping of thin-film solar cells

Through-the-glass and film side spectroscopic ellipsometry (SE) are being developed as in situ, on-line, and off-line mapping tools for large area thin film photovoltaics. Given that such instrumentation allows one to extract thicknesses, as well as parameterized optical functions versus wavelength, there exists the possibility to utilize this information further to predict the optical quantum efficiency (QE) and optical losses, the latter including the reflectance and inactive layer absorbances. By spatially resolving this information, one can gain a better understanding of the origin of performance differences between small area cells and large area modules. We have demonstrated these techniques for thin film hydrogenated amorphous silicon (a-Si:H) and Cu(In1−xGax)Se2 solar cell structures. For solar cells on glass superstrates, film-side SE can be supplemented with through-the-glass SE, which helps to increase the sensitivity of the analysis to the critical transparent conducting oxide and window layer properties. A comparison of predicted and experimental QE can reveal optical and electronic losses and light trapping gains.

Optical mapping of large area thin film solar cells

The mapping capability of multichannel spectro-scopic ellipsometry (SE) has been demonstrated with examples from hydrogenated amorphous silicon (a-Si:H) and CdTe thin film photovoltaics (PV) technologies on glass. Maps as large as 40 x 80 cm2 have been obtained. For a-Si:H, maps of the bulk i-layer thickness and band gap as well as surface roughness layer thickness have been determined. For CdTe, a map of the CdS window layer thickness has been determined with the prospect of grain structure mapping. In both cases, maps of the thickness and properties of the underlying transparent conducting oxide (TCO) layers have been determined. These first results demonstrate the ability of mapping SE to guide scale-up of thin film PV deposition processes.

Application of real time spectroscopic ellipsometry for analysis of roll-to-roll fabrication of Si:H solar cells on polymer substrates

Real time spectroscopic ellipsometry (RTSE) has been developed to monitor the cassette roll-to-roll deposition of thin film Si:H n-i-p solar cells on flexible polymer substrates coated with a back-reflector (BR). The methodology is first demonstrated here in growth studies from nucleation to the final thickness for a magnetron sputtered ZnO layer on top of opaque Ag in the BR structure. The methodology is then extended to plasma-enhanced chemical vapor deposition (PECVD) of the i and p-layers in succession on the BR/n-layer stack. RTSE data collection is initiated before the plasma is ignited so that the nucleation of the layers can be observed. The film thickness increases with time until a steady state is reached, after which the bulk layer thickness at the monitoring point is constant with time. This occurs when the elapsed deposition time equals the time required for the moving substrate to travel from the leading edge of the deposition zone to the monitoring point. Although a constant substrate speed is selected such that the final film thickness is achieved in the time required to move through the entire deposition zone, this speed does not permit study of film growth that occurs after the substrate passes the monitoring point. To address this deficiency, the substrate speed is reduced only over an initial length of the roll such that the final film thickness of interest is reached at the monitoring point. In this way, RTSE can be used to analyze the entire layer on an initial length of the roll before its full length is coated. The goal of this work is to develop and verify optimum deposition procedures based on optical monitoring of thin film Si:H solar cell structures in roll-to-roll multi-chamber deposition.

Correlations Between Mapping Spectroscopic Ellipsometry Results and Solar Cell Performance for Evaluations of Nonuniformity in Thin-Film Silicon Photovoltaics

An understanding of the relationship between materials property and thin-film solar cell performance variations over large areas is of interest for evaluating the impact of macroscopic nonuniformities in scale-up from laboratory cells to production modules. In this study, we have spatially correlated the properties of the hydrogenated silicon (Si:H) i- and p-layers—as mapped over a 13 cm×13 cm substrate area—with device performance parameters from an array of a-Si:H based n-i-p dot cells. To evaluate materials and device nonuniformities, a 16 × 16 array of dot cells has been fabricated over the substrate area, and this same area has been mapped by spectroscopic ellipsometry (SE).Analysis of the SE data over the full area provides maps of i-layer thickness and band gap, p-layer thickness and band gap, and p-layer surface roughness thickness for the n-i-p solar cell structure. The mapped values adjacent to the devices have been correlated with photovoltaic (PV) device performance parameters. When sufficient nonuniformity exists, these correlations enable optimization based on specific values of the fundamental properties. Alternatively, if the optimum set of properties has been identified, the impact of deviations due to macroscopic uniformities can be evaluated.

Plasmonic characteristics of Ag/ZnO back-reflectors for thin film Si photovoltaics

Real time spectroscopic ellipsometry (RTSE) and ex-situ normal incidence reflectance and scattering spectroscopies have been applied to analyze the plasmonic characteristics of Ag/ZnO back-reflector (BR) structures used in triple junction thin film Si:H photovoltaics. The structure explored here is relevant to the substrate/BR/(n-i-p)**3 configuration and consists of opaque Ag followed by up to ∼ 3500 Å of ZnO, both films prepared by sputtering onto Si wafers. The use of Si wafer substrates and RTSE enables controllability of the Ag final roughness layer thickness from ∼ 4 to 105 Å, i.e., from microscopic (specular) to macroscopic (low texture), in order to investigate its role in Ag/ZnO interface formation and its effect on the interface plasmonic characteristics. The analysis in ex situ measurement modes has also been extended to the optimized BR structure (with full texture). For each BR, the dielectric functions ɛ = ɛ1 + iɛ2 of all layers have been determined, including the Ag/ZnO interface layer, and the latter has been fit using a model that includes localized plasmon resonances arising from free electron oscillations within Ag interface protrusions. These resonances shift to lower energy with increasing ZnO/Ag interface thickness due to dipolar interactions, and can account for both re-radiation and dissipation in fully textured BR structures. The operative mechanism of these optimized back-reflectors for triple junction a-Si:H-based solar cells is interference-enhanced localized plasmon coupling -- and subsequent re-radiation -- in the range of 1.4–1.5 eV. In fact, this mechanism appears to be up to ∼ 80% efficient in optimized BRs. Coupling efficiency to surface plasmons is limited to ≪ 15% for 1.4–1.5 eV due to dominant protrusions in fully textured BRs.

Optimization of Si:H multijunction n-i-p solar cells through the development of deposition phase diagrams

Phase diagrams have been developed to guide very high frequency (vhf) plasma-enhanced chemical vapor deposition (PECVD) of intrinsic hydrogenated amorphous silicon (a-Si:H), amorphous silicon-germanium alloys (a-Si1-xGex:H), and nanocrystalline silicon (nc-Si:H) for use as the top, middle, and bottom cell i-layer components, respectively, of triple junction n-i-p solar cells. These phase diagrams have been used in conjunction with cross-sectional transmission electron microscopy (XTEM) to identify nucleation and growth behavior in order to gain a better understanding of phase evolution. The substrates for phase diagram development by real time spectroscopic ellipsometry (RTSE) are crystalline Si wafers that have been over-deposited with either n-type a-Si:H for the top and middle cell amorphous i-layers, or n-type nc-Si:H for the bottom cell nanocrystalline i-layer. Identical n/i cell structures were co-deposited on textured (stainless steel)/Ag/ZnO, which serve as substrate/back-reflectors, in order to relate the RTSE-developed phase diagrams to the performance parameters of single-junction solar cells. This study has reaffirmed that the highest efficiencies for a-Si:H and a-Si1-xGex:H solar cells are obtained when the i-layers are prepared under previously-described maximal H2 dilution conditions. The phase diagram for the bottom cell using a nc-Si:H n-layer reveals for the first time a bifurcation at a critical R value between mixed-toamorphous phase transitions [(a+nc) → a] at low R and mixed-to-single phase nanocrystalline transitions at high R [(a+nc) → nc]. The highest efficiency single-step nc-Si:H solar cell is obtained at minimal R while remaining on the nanocrystalline side of the identified bifurcation where suitable grain boundary passivation is assured.

Through-the-glass spectroscopic ellipsometry of superstrate solar cells and large area panels

We have advanced the technique of through-the-glass spectroscopic ellipsometry (SE) toward the nondestructive, non-invasive analysis of large area coated glass plates and completed solar modules in the superstrate configuration. The focus of this work involves reducing the effects of artifacts due to changes in the polarization state of light as it traverses the glass to the film side. By including the effects of (i) strain in the glass, (ii) differences in soda lime glass optical properties at the tin side versus the film side, and (iii) possible collection of both tin side and film side reflections, the accuracy in the determination of film properties in through-the-glass measurements can be improved. For example, measurements of the index of refraction spectra of the uncoated film side glass using a through-the-glass method agree with direct measurements from the uncoated film side to within ±0.004 over the full spectral range of through-the-glass measurements (∼300 to 1600 nm).

Optimization of a-Si:H p-i-n solar cells through development of n-layer growth evolution diagram and large area mapping

A growth evolution diagram has been developed to guide plasma-enhanced chemical vapor deposition (PECVD) of n-type hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) for use as the n-layer component of p-i-n a-Si:H superstrate solar cells. The substrates for such growth evolution diagram development by real time spectroscopic ellipsometry (RTSE) are crystalline silicon wafers that have been over-deposited with intrinsic a-Si:H layers. This diagram was applied to guide n-layer depositions on 15 cm × 15 cm glass/TCO/p/i superstrates in order to relate the diagram to the performance parameters of single-junction a-Si:H solar cells. Over the 15 cm × 15 cm TCO coated glass superstrate area, a 16 × 16 array of p-i-n dot cells has been fabricated, and this area has been mapped at high resolution by spectroscopic ellipsometry (SE). Analysis of such SE data over the full area provides maps of the p-layer effective thickness, i-layer thickness and band gap, and n-layer thickness and nanocrystalline Si:H vol. fraction. In addition, J-V measurements were performed on the 16 × 16 array of dot cells. The goal of this study is to identify and understand the relationships between basic materials property and thin film solar cell performance variations over large areas, and to evaluate impacts of non-uniformities on module performance.

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.