chalcopyrite ternary thin films of copper - indium - diselenide ( cuinse 2 ) and copper - indium - gallium - diselenide ( cuin 1 - x ga x se 2 ), both of which are generically referred to as cu ( in , ga ) se 2 , or cigs , have become the subject of considerable interest and study for semiconductor devices in recent years . sulfur can also be , and sometimes is , substituted for selenium , so the compound is sometimes also referred to even more generically as cu ( in , ga )( se , s ) 2 so as to encompass all of these possible combinations . these devices are also referred to as i - iii - vi 2 devices according to their constituent elemental groups . these devices are of particular interest for photovoltaic device or solar cell absorber applications . for photovoltaic applications , the p - type cigs layer is combined with an n - type cds layer to form a p - n heterojunction cigs / cds device . the direct energy gap of cigs results in a large optical absorption coefficient , which in turn permits the use of thin layers on the order of 1 - 2 μm . an additional advantage of cigs devices is their long - term stability . viewing fig1 , all layers are deposited on a substrate 105 which may comprise one of a plurality of functional materials , for example , glass , metal , ceramic , or plastic . it is contemplated that the substrate thickness may range from approximately 10 . 0 μm - 10 mm , and may be rigid or flexible . preferably , the substrate functions as the back contact for interconnection . deposited directly on the substrate 105 is a barrier layer 110 . the barrier layer 110 comprises a thin conductor or very thin insulating material and serves to block the out diffusion of undesirable elements or compounds from the substrate to the rest of the cell . this barrier layer 110 may comprise chromium , titanium , silicon oxide , titanium nitride and related materials that have the requisite conductivity and durability . it is preferable to have a thinner barrier layer 110 . the next deposited layer is the back contact layer 120 comprising non - reactive metals such as molybdenum . the back contact layer is the electrical contact for the solar cell . the layer may further serve to prevent the diffusion of chemical compounds from the other layers to the solar cell structure . the layer also serves as a thermal expansion buffer between the substrate 105 and the solar cell structure . the next layer is deposited upon the back contact layer 120 and is a p - type semiconductor layer 130 to improve adhesion between the absorber and the back contact . the p - type semiconductor layer 130 may be a i - iii a , b - vi isotype semiconductor , but the preferred composition is cu : ga : se , cu : al : se or cu : in : se alloyed with either of the previous compounds . in this embodiment , the formation of a p - type absorber layer 155 involves the inter - diffusion of a number of discrete layers . ultimately , as seen in fig1 , the p - type semiconductor layers 130 and 150 combine into a single composite layer 155 which serves as the prime absorber of solar energy . in this embodiment , alkali materials 140 are added for the purpose of seeding the growth of subsequent layers as well as increasing the carrier concentration and grain size of the absorber layer 155 , thereby increasing the conversion efficiency of the pv cell . referring still to fig1 , the next layer comprises another semiconductor layer 150 , also known as the cigs absorber layer . the layer 150 may comprise a compound or compounds , that includes a type i element ( such as cu , or ag ), and / or a type iii element ( such as in , ga , or al ) and / or a type vi element ( such as se , and / or s ). preferably , the p - type layer 150 comprises a i -( iii a , iii b )- vi 2 layer ( iii a = in , iii b = ga , al ) where the 0 . 0 & lt ; iiib /( iiia + iiib )& lt ; 0 . 4 . preferably , the p - type absorber layer comprises cuin 1 - x : ga x : se 2 where x ranges between 0 . 2 to 0 . 3 wherein the thickness ranges from about 1 μm to about 3 μm . the semiconductor layer 150 is formed by delivery of the i , iii , and vi precursor materials or the reacted i - iii - vi compound on top of the alkali materials 140 . the semiconductor layer or layers may be formed as a mixture or a series of thin layers . in an alternate embodiment , the semiconductor layers may consist of a graded absorber layer comprising multiple layers of various combinations of sputter target precursors . for example , cu 2 se : ga 2 se 3 : in 2 se 3 or any like combinations . in an alternate embodiment none of the layers include se . group i , iii , vi precursor materials are subsequently reacted at temperatures of about 400 ° c . to about 600 ° c . to form a i - iii - vi 2 compound material . the presence of the p - type semiconductor 130 enables optimal i - iii - vi 2 compound formation kinetics by providing a like chemical and physical surface on which the p - type absorber layer 155 can be formed . at temperatures of about 400 ° c . to about 600 ° c ., the p - type semiconductor layer 130 and p - type semiconductor layer 150 will inter - diffuse by the exchange of the type iii elements . additionally , the na contained in the alkali materials 140 will diffuse out and into the semiconductor layer 150 , thus improving the growth of the p - type absorber layer 155 of the completed device . once deposited , the layers are thermally treated at a temperature of about 400 ° c .- 600 ° c . after the thermal treatment of the p - type absorber layer 150 , the photovoltaic production process is continued by the deposition of an n - type junction layer 160 . this layer 160 will ultimately interact with the semiconductor layer 150 to form the necessary p - n junction 165 . preferably , the junction buffer layer is formed in the present invention by providing in , se , ga , for a period of time and at a lower temperature so that the material rather than react to form a cigs material , instead deposits an n type material . typically this takes place when temperature goes below about 450 ° c . and continues to about 300 ° c . one or more of the constituents of the n - type junction layer may diffuse in whole or in part into the p - type absorber layer aiding in the formation of the p - n junction . thickness ranges from about 50 nm to about 500 nm for this layer . the bandgap of the junction layer may or may not be greater than that of the p - type absorber layer . the junction layer may be formed at an ambient temperature that is less than the maximum temperature previously achieved , such as during the upstream p - type absorber layer formation step , specifically in the range of 300 ° c . to 450 ° c . in one embodiment , the lower temperature junction process may be delivered in the same chamber in which the p - type absorber layer is thermally formed . according to this embodiment , the completed p - type absorber layer is exposed to in , ga , se vapor for an additional period of time . concurrently , the temperature is lowered from a first temperature to a preferred range of about 300 ° c . to 450 ° c . more preferably , the lower temperature range is about 350 ° c . to 400 ° c . according to this embodiment , a new buffer layer ( inga ) yse is created . in this embodiment , a chamber may thus be configured to anneal the p - type absorber precursor materials in a higher temperature region , and subsequently form a junction layer in a lower - temperature downstream region in the same chamber . the next layer is an intrinsic transparent oxide layer 170 . the transparent intrinsic oxide layer 170 is deposited next to serve as a hetero - junction with the absorber . preferably , the transparent oxide layer 170 includes a ii - vi or a iiix viy compound that serves as the hetero - junction partner to the i - iii - vi 2 absorber . for example , an oxide is typically an oxide of in , sn or zn . preferably , the intrinsic layer 170 comprises a thickness of about 10 nm to about 50 nm . finally , a conducting transparent oxide layer 180 is deposited to function as the top of the electrode of the cell . the oxide is doped to make it both conductive and . transparent that serves to carry the current to the grid structure . for example , the transparent conducting oxide layer 180 may comprise zno or ito deposited by cvd or sputter . the top conducting layer is preferably transparent , conductive , and contains a compound that includes a type ii element ( such as cd or zn ), and / or a type iii element ( such as in , or al ), and / or a type iv element ( such as sn ), and / or a type vi element ( such as oxygen ). a grid structure is deposited on top of the conducting transparent oxide layer and is comprised of a metallic layer in a pattern designed to optimize collection and minimize obscuration . preferably , the grid includes a thin metal layer of type a to assure good ohmic contact between grid structure and transparent conducting oxide and a second metal type b to carry the current to the external circuit . a typical grid metal comprises type a : nickel ( 10 nm to about 50 nm ) and type b : aluminum or silver ( 3 to 5 um ). while the invention has been described with reference to particular embodiments , it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention . in addition , many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope of the invention . therefore , it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention , but that the invention will include all embodiments falling within the scope and spirit of the appended claims .