Atomic layer deposition of molybdenum oxide:for silicon heterojunction solar cells (2015)

Having a current record efficiency of 24.7% and a potential efficiency of 28%, silicon heterojunction (SHJ) solar cells are a very promising type of solar cell and are therefore receiving considerable interest from the photovoltaics (PV) community. However, one of the limiting factors of the SHJ cell is the parasitic absorption occurring in the front layers of the cell. Especially the hole-selective p-type a-Si:H and passivating intrinsic a-Si:H layers are responsible for substantial losses in generated current, limiting the efficiency of the cell. Recently, MoOx (x 3) has been suggested as a viable candidate material to replace the p-type a-Si:H hole-selective layer, on the basis of its wide band gap (3 eV) and high work function (6.6 eV). Although thermal evaporation was used for the deposition of the MoOx in the first studies (Battaglia et al, Applied Physics Letters, 2015), atomic layer deposition (ALD) is considered a more suited technique due to its excellent thickness control and the possibility to deposit uniformly over large areas. In this work, a novel ALD process is reported for the deposition of MoOx, which is compatible with SHJ technology and shows promising results for the application in SHJ solar cells. The newly developed process uses (NtBu)2(NMe2)2Mo as precursor and O2 plasma as reactant and yields good material properties for deposition temperatures between 50 °C and 350 °C. Between 50 °C and 250 °C amorphous films were deposited at a growth per cycle (GPC) around 0.80 A° , while at higher deposition temperatures the films were polycrystalline and the GPC increased up to 1.88 A° . Rutherford backscattering measurements showed C and N contamination levels below the detection limit of 3 at.% and 2 at.%, respectively, indicating the high quality of the material. In addition to an extensive study of the ALD process and the resulting material properties, the potential of the novel process for application in SHJ solar cells was explored by depositing ALD MoOx layers in test structures and using a combination of optical simulations and lifetime measurements. The test structures showed very promising initial results, achieving an implied open-circuit voltage (iVoc) of 710 mV for a film stack of a-Si:H(i)/MoOx/In2O3:H, with just 3 nm a-Si:H(i). In addition, optical simulations showed an estimated short-circuit current density Jsc of 41.7 mA/cm2 for a solar cell with 5 nm MoOx as hole selective layer, in contrast to an estimated 40.7 mA/cm2 for the conventional SHJ solar cell with 10 nm a-Si:H(p). Although two very preliminary tests of the ALD MoOx films in solar cells yielded poor results, additional experiments are planned in order to verify the expected gain in Jsc and improved solar cell performance is expected. Having a current record efficiency of 24.7% and a potential efficiency of 28%, silicon heterojunction (SHJ) solar cells are a very promising type of solar cell and are therefore receiving considerable interest from the photovoltaics (PV) community. However, one of the limiting factors of the SHJ cell is the parasitic absorption occurring in the front layers of the cell. Especially the hole-selective p-type a-Si:H and passivating intrinsic a-Si:H layers are responsible for substantial losses in generated current, limiting the efficiency of the cell. Recently, MoOx (x 3) has been suggested as a viable candidate material to replace the p-type a-Si:H hole-selective layer, on the basis of its wide band gap (3 eV) and high work function (6.6 eV). Although thermal evaporation was used for the deposition of the MoOx in the first studies (Battaglia et al, Applied Physics Letters, 2015), atomic layer deposition (ALD) is considered a more suited technique due to its excellent thickness control and the possibility to deposit uniformly over large areas. In this work, a novel ALD process is reported for the deposition of MoOx, which is compatible with SHJ technology and shows promising results for the application in SHJ solar cells. The newly developed process uses (NtBu)2(NMe2)2Mo as precursor and O2 plasma as reactant and yields good material properties for deposition temperatures between 50 °C and 350 °C. Between 50 °C and 250 °C amorphous films were deposited at a growth per cycle (GPC) around 0.80 A° , while at higher deposition temperatures the films were polycrystalline and the GPC increased up to 1.88 A° . Rutherford backscattering measurements showed C and N contamination levels below the detection limit of 3 at.% and 2 at.%, respectively, indicating the high quality of the material. In addition to an extensive study of the ALD process and the resulting material properties, the potential of the novel process for application in SHJ solar cells was explored by depositing ALD MoOx layers in test structures and using a combination of optical simulations and lifetime measurements. The test structures showed very promising initial results, achieving an implied open-circuit voltage (iVoc) of 710 mV for a film stack of a-Si:H(i)/MoOx/In2O3:H, with just 3 nm a-Si:H(i). In addition, optical simulations showed an estimated short-circuit current density Jsc of 41.7 mA/cm2 for a solar cell with 5 nm MoOx as hole selective layer, in contrast to an estimated 40.7 mA/cm2 for the conventional SHJ solar cell with 10 nm a-Si:H(p). Although two very preliminary tests of the ALD MoOx films in solar cells yielded poor results, additional experiments are planned in order to verify the expected gain in Jsc and improved solar cell performance is expected.