Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to-power conversion efficiencies exceeding 20%. This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally provided by soda lime glass substrates used for processing of champion devices. The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in significantly inferior device performance. Here we present a new sequential post-deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters. Ion exchange processes, well known in other research areas, are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction.

Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells

Publisher Summary This chapter reviews the thermal conductivity of nonmetallic crystals at temperatures comparable to or higher than the Debye temperature. It deals with the intrinsic behavior of such pure crystals at high temperatures. In such crystals, the dominant carriers of thermal energy are phonons and the dominant scattering mechanism to be considered is the intrinsic phonon–phonon scattering. This is a small section of the much larger problem of the thermal conductivity of nonmetallic solids and clearly it neglects possible heat transport by photons, charge carriers, polarons, and magnons. It also neglects other possible phonon scattering mechanisms such as isotopes, impurities, vacancies, charge carriers, dislocations, grain boundaries, and crystal boundaries. It presents the absolute value of the thermal conductivity, K, as determined by phonon–phonon scattering, the temperature dependence of K, the volume dependence of K, the change in K upon melting, and the minimum value of K. The chapter discusses a composite curve for the thermal conductivity versus temperature of pure KCl measured at a constant pressure of, say, one atmosphere.

The Thermal Conductivity of Nonmetallic Crystals

Chemical deposition method for metal chalcogenide thin films

It is shown that substitutional doping of an amorphous semiconductor is possible and can provide control of the electronic properties over a wide range. a-Si and Ge specimens have been prepared by the decomposition of silane (or germane) in a radio-frequency (r.f.) glow discharge. Doping is achieved by adding carefully measured amounts of phosphine or diborane, between 5 × 10−6 and 10−2 parts per volume, to obtain n- or p-type specimens. The room temperature conductivity of doped a-Si specimens can be controlled reproducibly over about 10 orders of magnitude, which corresponds to a movement of the Fermi level of 1·2 eV. Ion probe analysis on phosphorus doped specimens indicates that about half the phosphine molecules in the gaseous mixture introduce a phosphorus atom into the Si random network; it is estimated that 30–40% of these will act as substitutional donors. The results also show that the number of incorporated phosphorus atoms saturates at about 3 × 1019 cm−3, roughly equal to the number ...

Electronic properties of substitutionally doped amorphous Si and Ge

Self-heating is a severe problem for high-power gallium nitride (GaN) electronic and optoelectronic devices. Various thermal management solutions, for example, flip-chip bonding or composite substrates, have been attempted. However, temperature rise due to dissipated heat still limits applications of the nitride-based technology. Here we show that thermal management of GaN transistors can be substantially improved via introduction of alternative heat-escaping channels implemented with few-layer graphene-an excellent heat conductor. The graphene-graphite quilts were formed on top of AlGaN/GaN transistors on SiC substrates. Using micro-Raman spectroscopy for in situ monitoring we demonstrated that temperature of the hotspots can be lowered by ∼20 °C in transistors operating at ∼13 W mm(-1), which corresponds to an order-of-magnitude increase in the device lifetime. The simulations indicate that graphene quilts perform even better in GaN devices on sapphire substrates. The proposed local heat spreading with materials that preserve their thermal properties at nanometre scale represents a transformative change in thermal management.

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Graphene quilts for thermal management of high-power GaN transistors

Thermal conductivity of copper films

The thermal conductivity of 2000-9000-\AA{}-thick amorphous and crystalline films of Ge and GeTe has been measured in the temperature range 100-550\ifmmode^\circ\else\textdegree\fi{}K No thickness dependence has been observed in these films down to 2000 \AA{} The results show that crystalline Ge and amorphous Ge and GeTe films have a negligible electronic contribution in the measured thermal conductivity However, as expected, crystalline GeTe films exhibit significant electronic contribution which is \ensuremath{\sim}25% of the measured value of the thermal conductivity The thermal conductivity of both amorphous Ge and GeTe increases slowly with increasing temperature in contrast to the rapid decrease in the crystalline films In the latter case, the lattice component of the thermal conductivity decreases approximately inversely with temperature The observed temperature dependence of the thermal conductivity can be understood on the basis of a combination of (i) umklapp-scattering mechanism, (ii) scattering due to defects, and (iii) scattering by grain boundaries The last term, which is primarily responsible for the observed thermal resistance in the case of amorphous films, leads to a temperature-independent phonon mean free path The observed increase of thermal conductivity with temperature for amorphous films is, therefore, attributed to the increase of specific heat with temperature The values of the phonon mean free path in amorphous films as deduced from the thermal-conductivity data are \ensuremath{\sim}3 \AA{} in amorphous GeTe and \ensuremath{\sim}5 \AA{} in amorphous Ge films These values suggest a relatively longer short-range order (coherently scattering regions) in amorphous Ge, as compared with amorphous GeTe

Thermal conductivity of amorphous and crystalline Ge and GeTe films

Experimental determination of the thermal conductivity of thin films

CdxZn1–xS alloy films are prepared by evaporation from a mixture of CdS and ZnS powder in suitable proportions and their structural properties are investigated as a function of substrate temperature. Both the structure and the lattice parameter of the structures exhibit a strong dependence on temperature of deposition. The variation in lattice parameters of the alloy films as a function of substrate temperature is qualitatively explained on the basis of non-stoichiometry arising due to the presence of excess metal atoms. The presence of excess metal atoms is a consequence of the fact that CdS and ZnS dissociate during evaporation and the constituents have different sticking coefficients on the substrate during condensation depending on the deposition conditions particularly substrate temperature.

Structure of vacuum, evaporated CdxZn1–xS thin films

ZnxCd1xS alloy films are prepared in the entire composition range from pure CdS to pure ZnS using a chemical solution spray technique. Lattice constants and optical band gap of the films are measured. Photoconductivity is studied as a function of film composition. Dark electrical resistivity of the alloy films is determined and the effect of vacuum annealing established. The temperature dependence of resistivity is used to explain qualitatively the conduction mechanism of the alloy films. Heterojunction solar cells using these alloy films are fabricated and an enhanced open circuit voltage is observed.



Mit einer Spruhtechnik chemischer Losungen werden ZnxCd1xS-Legierungsschichten im gesamten Zusammensetzungsbereich von reinem CdS bis reinem ZnS hergestellt. Gitterkonstanten und optische Bandlucke der Schichten werden gemessen. Die Photoleitung wird als Funktion der Schichtzusammensetzung gemessen. Die elektrische Dunkelleitfahigkeit der Legierungsschichten wird bestimmt und ein Einflus der Vakuumtemperung festgestellt. Die Temperaturabhangigkeit der Leitfahigkeit wird benutzt, um den Leitungsmechanismus der Legierungsschichten qualitativ zu erklaren. Heteroubergangs-Solarzellen werden mit diesen Legierungsschichten hergestellt, wobei eine Vergroserung der Leerlaufspannung beobachtet wird.

Prem Nath

Papers

Thermal conductivity of copper films

Thermal conductivity of amorphous and crystalline Ge and GeTe films

Experimental determination of the thermal conductivity of thin films

Structure of vacuum, evaporated CdxZn1–xS thin films

Properties of ZnxCd1−xS films prepared by solution spray technique