Microstructural dependence of electron and hole transport in low-temperature-grown polycrystalline-silicon thin-film solar cells

Carrier transport properties of undoped polycrystalline silicon (poly-Si) thin films prepared by ${\mathrm{SiH}}_4–{\mathrm{H}}_2$ plasma at low temperature have been investigated. The ac-conductivity measurement technique has been applied to poly-Si $i$ layers with an $\mathrm{n-i-n}$ junction structure in order to characterize the electron conductivity along the growth direction. Furthermore, the hole conductivity has been measured with $\mathrm{p-i-p}$ junction structures. The temperature dependence of ac conductivity reveals that poly-Si films with relatively low crystalline volume fraction ${\mathrm{(X}}_c\text{∼50\%)}$ exhibit intrinsic character, while the poly-Si films with high $X_c$ $\text{(>50\%)}$ exhibit n-type character with activation energies less than 0.15 eV. Based on these results, the relationship among microstructure, carrier transport, and photovoltaic performance of poly-Si solar cells is discussed.