CdTe:Bi films deposited by closed space vapor transport under variable pressure and doping levels: evidences of the possible formation of an intermediate band O. Vigil-Gala ´n 1 • Maykel Courel 1 • F. Cruz-Gandarilla 1 • D. Seuret-Jime ´nez 2 Received: 5 November 2015 / Accepted: 14 February 2016 / Published online: 18 February 2016 Ó Springer Science+Business Media New York 2016 Abstract In this work, the results about the properties of CdTe:Bi thin films grown by the Closed Space Vapor Transport (CSVT) method are presented. Two procedures were developed for the CdTe:Bi films deposition: (a) using powders with different Bi concentrations at a constant pressure in the CSVT chamber and (b) varying the total final Ar pressure in the CSVT chamber. The CdTe powders used in our experimental conditions were obtained by using CdTe:Bi crystals grown by the vertical Bridgman method, varying the nominal Bi-dopant concentration in the 1.0 9 10 17 to 4 9 10 19 at/cm 3 range. Finally, the possible influence of both parameters on the existence of an inter- mediate band in CdTe:Bi thin films and CdS/CdTe solar cell characteristics is analyzed. 1 Introduction CdTe semiconductor compound has attracted a great deal of attention for solar cell applications, due to the adequate properties of the CdTe/CdS heterojunctions [1–3]. Despite Cu has been widely chosen as an acceptor dopant for CdTe, it has been demonstrated that in the long-run, the efficiency is doomed to decrease since Cu can be segregated to grain boundaries [1]. As a result, other elements such as Bi have been considered as a dopant for CdTe thin film. It is well known that CdTe solar cell efficiency improvements can be achieved if defects in CdTe semiconductor are reduced. On the other hand, it has been shown that by means of intro- ducing controlled impurities in semiconductors, an inter- mediate band (IB) can be formed which could promote solar cell performance. Several doped semiconductors such as O-doped (Zn,Mn)Te [4], N-doped Ga(As,P) [5], O-doped ZnTe [5], and V-doped SnS 2 [6] have been pro- posed to form the IB materials. What is more, the potential of the intermediate band solar cell concept to improve the efficiency of thin-film chalcopyrite solar cells, especially those based on CuGaS 2 with the incorporation of Ti or Fe has been proposed [7]. The IB concept consists of a narrow isolated band inside the band-gap; which allows the absorption of two extra photons in addition to that absorbed by the host semiconductor band-gap [8]. In this way, an increase in short circuit current density value should be expected. However, in order to obtain an IB, the role of doping atom concentration should be taken into account. In fact, there is a required minimum doping atom concentra- tion for which an IB is formed. For lower doping con- centrations, only centers instead of an IB are formed contributing to radiative or non-radiative losses. On the contrary, for relatively higher doping concentrations, the transport properties of carriers in semiconductors are degraded as well as its structural properties. Therefore, the trade-off between the IB formation and the degrading of the electrical and structural properties of semiconductors is an open research for improving the solar cell efficiency. Recently, the physical properties of CdTe:Bi crystals have been studied [9]. In this work, the amphoteric prop- erties of Bi in CdTe were demonstrated with the following properties: for low Bi concentrations, the Bi acts as a donor impurity, compensating the Cd vacancies and leading to high resistivity crystals. Besides, Bi Cd donor specie domi- nates the conductivity properties. For high dopant & D. Seuret-Jime ´nez diego@seuret.com; dseuret@uaem.mx 1 Escuela Superior de Fı ´sica y Matema ´ticas – I.P.N., Edificio No. 9 U.P.A.L.M., 07738 Mexico, D.F., Mexico 2 Universidad Auto ´noma del Estado de Morelos, CIICAp, Ave. Universidad 1001, CP 62209 Cuernavaca, Morelos, Mexico 123 J Mater Sci: Mater Electron (2016) 27:6088–6095 DOI 10.1007/s10854-016-4534-1