Wayne A. Buchanan

Novel photochemical vapor deposition reactor for amorphous silicon solar cell deposition

A novel photochemical vapor deposition (photo‐CVD) reactor having a flexible ultraviolet‐transparent Teflon curtain and a secondary gas flow to eliminate deposition on the window has been used to deposit amorphous silicon films and p‐i‐n solar cells. The background levels of atmospheric contaminants (H2O, CO2, N2) depend strongly on the vacuum procedures but not on the presence of a Teflon curtain in the reactor. Intrinsic films with a midgap density of states of 3×1015 eV−1 cm−3 and all‐photo‐CVD pin solar cells with efficiencies of 8.5% have been deposited.

Design of a vapor transport deposition process for thin film materials

A vapor transport process for continuous deposition of elemental and compound thin film materials is presented. The process saturates a carrier gas with a vapor from a subliming source. The saturated mixture is directed over a substrate at lower temperature, resulting in a supersaturation condition and subsequent film growth. The process geometry, comprising the dimensions of the saturation and deposition zones, carrier gas pressure and flow rate, and saturation zone temperature are determined by calculating worst-case characteristic times and simply insuring that the residence time of the carrier gas sufficiently exceeds these times. A model was used to design a system, which is currently being used to deposit 1–10μm thick CdTe films on a 10×10cm2 translating substrate. The process produces film thickness uniformity to within ±5% in the translation direction and across the deposition zone, with a material utilization of 50%. Linear translation speed of 12.5cm∕min has been demonstrated in depositing a 4.5...

High throughput processing of CdTe/CdS solar cells with thin absorber layers

This paper presents deposition and post-deposition processing methods amenable for rapid processing of large area CdTe-based photovoltaic modules. A process optimization rationale is given for superstrate CdS/CdTe solar cells. CdTe films with sub-micron thickness were deposited by vapor transport (VT) on commercially available soda-lime glass substrates and treated in CdCl2 vapor with processing time of 2 min per plate. CdTe film thickness was reduced by lowering source temperature, and treatment temperatures were selected based on 3D modeling of CdS diffusion into polycrystalline CdTe films. The CdTe microstructure and optical properties for 0.3 to 3 μm film thickness are quantified for films deposited in N2/O2 ambient at 550°C and growth rates ∼10 μm/min and are compared to baseline samples with 5 μm CdTe. Solar cells with 1.5 μm and 0.8 μm VT CdTe processed with 2 min vapor CdCl2 treatment and mild etching have conversion efficiencies of 11% and 8%, respectively.

Design considerations for low band gap a-Sige:H alloy solar cells

Abstract The best low band gap (E g = 1.3 eV) a-Sige:alloy grown to date still have relatively poor electronic transport compared to that of unalloyed amorphous silicon. The poor electron transport is found to be at least partially due to poor electron mobility. Solar cell design is used to mitigate the performance loss. Thin a-Sige:H solar cells with Q.E. over 40% at 800nm and over 10% at 900nm have been prepared.

High throughput processing of CdTe/CdS solar cells

Methods for achieving high throughput and materials yield in processing >13% efficient CdTe thin film solar cells are demonstrated, with emphasis on semiconductor deposition and post-deposition processing. Chemical surface deposition was used to deposit uniform 100 cm/sup 2/ area CdS films 50 nm thick at 0.2 nm/sec, with >80% utilization of solvated Cd species. Vapor transport at 20 Torr was used to deposit dense CdTe films, 1-7 /spl mu/m thick, onto CdS-coated substrates with >50% utilization in an unbaffled system. Uniform films have been deposited with translation at up to 12 cm/min and static equivalent growth rate >80 pm/min. Vapor CdCl/sub 2/ post-deposition treatments yielded solar cells with AM1.5 conversion efficiency >13% for 6 /spl mu/m thick CdTe and >8% for 1 /spl mu/m thick CdTe.

The relationship between hydrogen content, weak bond density and Staebler-Wronski defects in amorphous silicon

Abstract P-i-n solar cells were degraded by current injection at high temperature until steady state was achieved and quenched to room temperature for SW defect determination. The H content in the i-layer of the devices was systematically varied through reactor pressure. The effect of the H content on the steady state SW density is found to be associated with the weak bond density rather than with an alteration in the recombination rate.

Improving performance of superstrate p-i-n a-Si solar cells by optimization of n/TCO/metal back contacts

A comprehensive study of the n-layer and back contact for superstrate (glass/textured SnO/sub 2/-p-i-n/TCO/metal) a-Si solar cells is presented. In particular, the difference between a-Si and /spl mu/c-Si n-layers are compared. These results show that the efficiency can be improved from 7% to 10% (absolute) by optimizing the back contact layers to incorporate a good optical back reflector. A rectifying contact is formed between the TCO and a-Si n-layer which reduces FF. A /spl mu/c-Si n-layer eliminates the blocking n/TCO contact. Results suggest that the n/TCO interface has a controlling influence. ZnO gives /spl sim/1 mA/cm/sup 2/ higher J/sub sc/ compared to ITO. The best contacts are /spl mu/c-Si/ZnO/metal.

Performance and analysis of amorphous silicon p-i-n solar cells made by chemical-vapor deposition from disilane

The photovoltaic performance of amorphous silicon p‐i‐n solar cells made by chemical‐vapor deposition (CVD) from disilane is reported and analyzed. Intrinsic layers were deposited at rates from 0.2 to 50 A/s at temperatures from 380 to 460 °C with and without boron doping. Device performance was insensitive to substantial differences in disilane purity. A cell efficiency of 4% was achieved. The primary limitation to higher efficiency was low fill factor ( 18 Ω cm2). Analysis of the series resistance indicated a contact‐related resistance of 4–12 Ω cm2 and a photoconductive resistance composed of intrinsic layer thickness‐independent (10 Ω cm2) and thickness‐dependent terms. Analysis of the voltage dependence of the current collection indicated a fill factor of 60% would be expected in the absence of series resistance. The maximum short‐circuit current of 12.5 mA/cm2 (normalized to 100 mW/cm2) resulted with a boron‐doped i layer deposited at 440 °C at 3.3 A/s. Modeling ...

Cu-In-Ga metal precursors sputter deposited from a single ternary target for Cu(lnGa)(SeS) 2 film formation

Precursor films used for the growth of Cu(InGa)Se2 were sputter deposited from a single Cu-In-Ga compound target to determine if better control of composition and improved reproducibility compared to precursors deposited from separate sputter targets could be obtained. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize changes in phase and morphology of the ternary sputter target, precursor films, and reacted films over a series of deposition runs and compared with films deposited from two targets. The surface morphology and composition of the target and precursor films were characterized over a range of sputtering conditions and compared to the bulk target composition over time. The precursor films contain Cu9(In1-xGax)4 and In phases with In-rich nodules. The target has a Cu-rich surface composition compared to the bulk that indicates preferential sputtering of In. Cu(InGa)(SeS)2 films were formed by a three-step H2Se/Ar/H2S reaction process. The composition of reacted films using ternary target precursors is comparable to the bulk target composition and good reproducibility is demonstrated. Finally, solar cells fabricated using the reacted films gave a maximum efficiency of 13.4%.

Vapor transport deposition of cadmium telluride films

Polycrystalline CdTe thin films are grown by vapor transport deposition onto moving substrates using a linear cross-web source for operation at low vacuum and high growth temperature. The source design, based on delivery of a carrier gas saturated with Cd and Te/sub 2/ vapors, overcomes the limitations of other vapor deposition methods. Models for vapor transport deposition and radiative transfer within the source facilitate a quantitative understanding of source behavior leading to control of film deposition. The deposition has been characterized over a wide range of operating parameters. Dense, single phase device quality CdTe films have been fabricated. Deposition of other compound semiconductor materials has also been demonstrated.