Alan E. Delahoy

New features of the temperature dependence of photoconductivity in plasma-deposited hydrogenated amorphous silicon alloys

The temperature and flux dependences of photoconductivity have been investigated for plasma‐deposited hydrogenated amorphous silicon alloys produced under a variety of processing conditions. In undoped films, new features such as thermal quenching and supralinearity are observed. Such behavior is critically dependent on the position of the Fermi level, and is not observed in alloys doped by the addition to the plasma of PH3, B2H6, O2+N2 mixtures, or air. Interpretation of the data is based on a model of competing recombination centers.

Textured tin oxide films produced by atmospheric pressure chemical vapor deposition from tetramethyltin and their usefulness in producing light trapping in thin film amorphous silicon solar cells

Abstract The phenomenon of light trapping was studied by producing and characterizing amorphous silicon solar cells on textured tin oxide films produced from tetramethyltin, bromotrifluoromethane and oxygen. The amount of texturing was systematically increased by increasing the film thickness. The investigation revealed that only about 5% to 10% diffuse scattering is necessary to obtain the entire beneficial effect of light trapping; higher values give rise to poorer cell performance. Conditions leading to the deposition of tin oxide films with various types of surface texturing and the subsequent solar cell results are reported.

An 8% efficient layered Schottky‐barrier solar cell

An 8.1% efficient 1‐cm2 Schottky‐barrier solar cell has been fabricated in our laboratory using a layered Schottky barrier on 2‐Ω cm p‐type silicon. Reproducible results have been obtained on the layered structure which involves 44‐A Cr adjacent to the silicon to obtain good photovoltaic voltage and a 58‐A Cu overlayer to decrease cell resistance. The layered structure provides good control of barrier height, resistance, and optical transmission. Application of this approach should produce a 15% efficient Schottky solar cell and be readily applied to thin‐film silicon solar cells when high‐quality thin silicon films have been developed.

A laboratory technique for the evaluation of electrochemical transparent conductive oxide delamination from glass substrates

Abstract A rapid laboratory technique has been developed to evaluate the susceptibility of the transparent conductive oxide (TCO) layer to delaminate from its glass substrate in a thin film PV module. The test sample is stressed through the use of heat, humidity, and a DC bias to drive sodium ions to the TCO–glass interface. Delamination reactions occur at the interface causing the TCO to lose adhesion. The technique takes less than 15 min to complete and is useful in predicting the propensity of a thin film module to fail via electrochemical delamination of the TCO. Potential mechanisms for the adhesion failure are discussed.

Self-consistency and self-sufficiency of the photocarrier grating technique

The recently suggested photocarrier grating technique appears to be the most reliable method available for the determination of the ambipolar diffusion length in hydrogenated amorphous silicon. We show that the technique can be made simpler than originally suggested, and that it is self‐sufficient in the sense that all the required parameters can be determined by the same experimental setup. It is demonstrated that the various results obtained by the technique are self‐consistent and that extremely accurate values of the ambipolar diffusion length can be determined.

Impurity effects in a‐Si:H solar cells due to air, oxygen, nitrogen, phosphine, or monochlorosilane in the plasma

Solar cells fabricated using plasma-deposited a-Si:H alloys can be seriously degraded by the incorporation of certain impurities during deposition of the a-Si:H materials. Nominally intrinsic layers are adversely affected by the addition to the plasma of air, N/sub 2/ + O/sub 2/ mixtures (although by neither N/sub 2/ nor O/sub 2/ separately), PH/sub 3/ or SiH/sub 3/Cl (monochlorosilane). For example, the conversion efficiency of Pd/p-i-n solar cells is lowered from 3.1 to 1.5% by the presence in the plasma of 3000 ppM air during deposition of the i-layer. In this case, modification of the a-Si:H gap state density owing to synergistic effects of oxygen and nitrogen in the plasma leads to a collapse of the space charge region and the reduction of the ..mu.. tau product for holes. The deleterious effects on device performance of phosphine and monochlorosilane are also discussed.

High‐efficiency Cr‐MIS solar cells on single and polycrystalline silicon

Cr‐MIS solar cells having a 2‐cm2 area have been fabricated to produce 12.2% efficiency on single crystal and 8.8% efficiency on polycrystalline Si. Surface‐state data were used to predict open‐circuit voltages of 0.60 and 0.50 V, respectively, for the single‐crystal and polycrystalline Si. Spectral response measurements and Cr metal thickness confirm the differences in short‐circuit current density using these two types of Si.

Deposition schemes for low cost transparent conductors for photovoltaics

Production requirements for transparent conductors for thin film photovoltaics are reviewed, with particular reference to magnetron sputtered zinc oxide. On-going optimization processes for ZnO:Al prepared on 0.43 m{sup 2} substrates by bipolar, reactive sputtering from ZnO:Al{sub 2}O{sub 3} targets and by reactive sputtering from a Zn:Al target are described. Optical emission from the plasma is used to study the state of the sputtering target. Attention is drawn to the spatial dependence of deposition conditions under the cathode.

Barrier height modification in silicon Schottky (MIS) solar cells

A study of experimental data on Cr-oxide-p-Si solar cells has led to a metal evaporation procedure which gives 0.50 V < Voc< 0.56 V. This voltage is independent of the method used in oxide formation when oxide thickness ranges from 10 to 30 Å. It is concluded that slow deposition of the Cr on an oxide interface leads to a lowered metal work function and thus an increased Voc. A high n-value and fixed charge in the oxide are not necessary to obtain a high Voc.

A one-unit photovoltaic electrolysis system based on a triple stack of amorphous silicon (pin) cells

Abstract An arrangement is suggested in one-unit photovoltaic electrolysis whereby the land area needed can be halved.