Highly conductive ZnO films with high near infrared transparency

We present an approach for deposition of highly conductive nominally undoped ZnO films that are suitable for the n-type window of low band gap solar cells. We demonstrate that low-voltage radio frequency (RF) biasing of growing ZnO films during their deposition by non-reactive sputtering makes them as conductive as when doped by aluminium (110(-3) cm). The films prepared with additional RF biasing possess lower free-carrier concentration and higher free-carrier mobility than Al-doped ZnO (AZO) films of the same resistivity, which results in a substantially higher transparency in the near infrared region (NIR). Furthermore, these films exhibit good ambient stability and lower high-temperature stability than the AZO films of the same thickness. We also present the characteristics of Cu(InGa)Se-2, CuInSe2 and Cu2ZnSnSe4-based solar cells prepared with the transparent window bilayer formed of the isolating and conductive ZnO films and compare them to their counterparts with a standard ZnO/AZO bilayer. We show that the solar cells with nominally undoped ZnO as their transparent conductive oxide layer exhibit an improved quantum efficiency for > 900 nm, which leads to a higher short circuit current density J(SC). This aspect is specifically beneficial in preparation of the Cu2ZnSnSe4 solar cells with band gap down to 0.85 eV; our champion device reached a J(SC) of nearly 39 mAcm(-2), an open circuit voltage of 378mV, and a power conversion efficiency of 8.4 %. Copyright (c) 2015John Wiley & Sons, Ltd.