Dual electrochemical and photochemical aniline treatment for CdTe solar cells

Large-area electronic devices such as thin-film solar cells are well-known to develop nonuniformities and even pinholes in the semiconducting layers. These nonuniformities lead to the formation of weak diodes that can seriously degrade the device performance and pinholes that can shunt the back electrode to the front electrode of the cell. Since it is essentially impossible to avoid such non-uniformities, one solution is to block conduction through these pinholes with a resistive material such as polyaniline, using an electro-polymerization technique. After the polymer film deposition, this new layer acts as an insulating layer, reducing current flow from the back contact (e.g. Cu/Au) to the transparent conductive oxide and/or the n-type CdS layer, thus limiting the spatial extent of the current sink. Less dramatic than pinholes that lead to shunts, nonuniformities in the semiconducting layers are also detrimental to the CdTe device, leading to weak (low V_OC) diodes that will conduct forward current when surrounding material is producing reverse, photogenerated current. Under strong illumination, the effect can be similar to a shunt. These weak diodes rob current from the neighboring cells thereby decreasing the device-generated power. Roussillon et al. showed that blocking such weak diodes can be done with a photochemical polymerization treatment. Using the device stack, glass/SnO₂:F/CdS/CdTe/Cu/Au (CdS and CdTe are sputtered), we will illustrate in this paper the difference between the two types of shunt passivation treatments and demonstrate the advantage of using both treatments sequentially for shunt passivation of polycrystalline, thin-film solar cells.