NOVELTY - Forming (M1) a semiconductor film on at least a portion of a substrate (44) involves: depositing a source solution containing a solvent and first and second type of metal ionic species dissolved in that on the substrate to form preparatory film comprising the first and second type of metal ionic species; forming a precursor film (47) by exposing the preparatory film to series of at least one reaction condition, where the condition comprises heating the preparatory film; and exposing the precursor film to a material comprising a Group 15 or Group 16 element to form a semiconductor film.    USE - For forming a semiconductor film on at least a portion of a substrate used in solar cell (claimed) such as copper indium gallium selenide (CIGS) solar cell.    ADVANTAGE - As compared to known methods, the present method provides ease of manufacture; has ability to form a semiconductor film with substantially uniform composition over a large surface area; and has low cost and improved efficiencies.    DETAILED DESCRIPTION - Forming (M1) a semiconductor film on at least a portion of a substrate (44) involves: (a) depositing a source solution containing a solvent and at least a first type of metal ionic species and a second type of metal ionic species dissolved in that on at least a portion of the substrate to form a preparatory film comprising the first type of metal ionic species and the second type of metal ionic species, where the substrate onto which the source solution is deposited is at a temperature at or above the boiling point of the solvent; (b) forming a precursor film (47) by exposing the preparatory film to series of at least one reaction condition, where the reaction conditions comprises heating the preparatory film; and (c) exposing the precursor film to a material comprising a Group 15 or Group 16 element to form a semiconductor film, where the semiconductor film comprises a reaction product formed via a reaction between 1) the Group 15 or Group 16 element and 2) the first type of metal species and/or the second type of metal species. INDEPENDENT CLAIMS are included for the following:    (1) exposing a substrate to a gas, involving positioning the substrate in a chamber, where the chamber comprises a gas inlet, a gas outlet, and a gas supply system associated with the gas inlet, where the chamber and/or the substrate is heated; exposing at least a portion of the substrate to the gas, which comprises a reactive material, via the gas supply system, where at least a portion of the gas interacts with the substrate and where a portion of the reactive material reacts with the substrate to form a chemically modified substrate and at least one gaseous byproduct; and recirculating the a portion of the gas comprising unreacted reactive material and at least a portion of the gaseous byproduct from the gas outlet back to the gas inlet using a gas treatment and recirculation system, where the gas treatment and recirculation system is configured to remove at least a portion of the gaseous byproduct from the gas being recirculated;    (2) an apparatus for exposing a substrate to a gas comprising a chamber comprising a substrate holder, a gas inlet, and a gas outlet, where the substrate holder is configured to be operatively coupled to a substrate; a gas supply system in fluid communication with the gas inlet and configured to supply gas to the chamber and distribute gas onto at least one surface of a substrate associated with the substrate holder; a heater configured to heat the chamber and/or substrate coupled to the substrate holder; and a gas treatment and recirculation system configured to recirculate gas from the gas outlet back to the gas inlet where the gas handling system comprises a moisture removal system configured to remove moisture from the gas being recirculated; and    (3) a solar cell comprising a semiconductor film formed using the method (M1).    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of a system for heat treatment of a preparatory film under an atmosphere of N2/air or vacuum to form a preparatory film comprising a mixture of oxides with substantially uniform composition.    Gas inlet valve (42)    Desired gas (43)    Substrate (44)    Precursor film (47)    Decomposition products of metal nitrates (48)