A comprehensive picture of Cu doping in CdTe solar cells

Abstract

The importance of Cu for CdTe solar cell absorber doping has been increasingly recognized in recent years. Currently different models are being discussed how to understand the case of Cu Cd substitutional doping in polycrystalline CdTe solar cells. In this work, an understanding is developed, which is based on a low concentration deep acceptor doped CdTe layer (N a $ 5 Â 10 14 cm À3 ,E a $ 300 meV above the valence band). Despite their non-shallow nature, Cu Cd acceptors are fully or at least heavily (>30%) ionized. The low hole concentration in CdTe ($1 Â 10 14 cm À3) originates directly from low Cu solubility in CdTe bulk material and is not caused by partial ionization or compensation as proposed by earlier models. The three to four orders of magnitude difference between bulk acceptor concentration and average Cu concentration in polycrystalline CdTe is attributed to grain boundary segregation of Cu. Our model is derived from substrate and superstrate CdTe solar cell measurements, controlled CdTe doping and quenching, Hall Effect measurements of CdTe films, numerical and analytical calculations, and a broad literature survey. Based on these results, routes to improve the conversion efficiency of CdTe solar cells are discussed. V

Structural and electronic properties of CdTe1-xSex films and their application in solar cells

“…Before the resistivity and Hall effect measurements, the layers were annealed at 210 °C for 10 min with subsequent rapid cooling on a copper plate (quenching). Previous measurements [5] have shown that quenching decreases the resistivity in CdTe by an order of magnitude probably due to a supersaturation of Cu’ Cd in the CdTe bulk. This supersaturation decays exponentially within hours at room temperature [5].…”

“…Previous measurements [5] have shown that quenching decreases the resistivity in CdTe by an order of magnitude probably due to a supersaturation of Cu’ Cd in the CdTe bulk. This supersaturation decays exponentially within hours at room temperature [5]. Each sample was quenched separately, the TLM measurements were performed within 2 h after quenching, and the Hall effect measurements were performed within 3 h after quenching.…”

“…The achievable doping concentration in II–VI compound semiconductors is believed to be limited by dopant solubility and formation of compensating defects [5]. Copper is a common dopant used in CdTe solar cells to improve p-type conductivity.…”

“…As a consequence, excessive copper amounts are detrimental to the resistivity and the hole density [8]. Additionally, the majority of Cu in solar cells is segregated at grain boundaries [5]. This can influence the minority carrier life time which is limited by recombination at bulk defects and at grain boundaries in the CdTe layer [9].…”

Brief review of cadmium telluride-based photovoltaic technologies

“…Copper source layers include Cu-doped graphite paste and low-resistivity compounds, such as Cu 2 Te and Cu-doped ZnTe, that form low-resistance primary contacts with contact potential barriers <0.3 eV. Amount of Cu introduced is critical and has an optimum value based on the device structure 28 and the nature of the CdTe layer. After the doping step, high-efficiency devices are reported 27 to have an ionized acceptor density in the range of 0.5 to 1 × 10 15 ∕cm 3 .…”