Hopping conduction observed in thermal admittance spectroscopy

Interdiffusion and Doping Gradients at the Buffer/Absorber Interface in Thin-Film Solar Cells

An accurate determination of the net dopant concentration in photovoltaic absorbers is critical for understanding and optimizing solar cell performance. The complex device structure of multilayered thin-film solar cells poses challenges to determine the dopant concentration. Capacitance-voltage ( C- V) measurements of Cu(In,Ga)Se₂ thin-film solar cells typically yield depth-dependent apparent doping profiles and are not consistent with Hall measurements of bare absorbers. We show that deep defects cannot fully explain these discrepancies. We instead find that the space charge region capacitance follows the model of a linearly graded junction in devices containing a CdS or Zn(O,S) buffer layer, indicating that elemental intermixing at the absorber/buffer interface alters the dopant concentration within the absorber. For absorbers covered with MgF₂, C- V measurements indeed agree well with Hall measurements. Photoluminescence measurements of Cu(In,Ga)Se₂ absorbers before and after deposition of a CdS layer provide further evidence for a significant reduction of the near-surface net dopant concentration in the presence of CdS. We thus demonstrate that interdiffusion at the absorber/buffer interface is a critical factor to consider in the correct interpretation of doping profiles obtained from C- V analysis in any multilayered solar cell and that the true bulk dopant concentration in thin-film devices might be considerably different.

Defect levels in Cu(In,Ga)Se2 studied using capacitance and photocurrent techniques

This work contributes to the discussion on defect levels in Cu(In,Ga)Se2 photovoltaic material. CuInSe2- and Cu(In,Ga)Se2- based Schottky junctions, solar cells and thin films were investigated using complementary capacitance and current spectroscopic techniques. Depending on the applied technique and type of investigated structure, six different signals were observed. Out of the signals identified, three were ascribed to responses from bulk defects-two electron and one hole trap. The remainder were discussed in light of available in-literature models including carrier mobility freeze-out and non-ohmic back junction.

Measuring long-range carrier diffusion across multiple grains in polycrystalline semiconductors by photoluminescence imaging

Thin-film polycrystalline semiconductors are currently at the forefront of inexpensive large-area solar cell and integrated circuit technologies because of their reduced processing and substrate selection constraints. Understanding the extent to which structural and electronic defects influence carrier transport in these materials is critical to controlling the optoelectronic properties, yet many measurement techniques are only capable of indirectly probing their effects. Here we apply a novel photoluminescence imaging technique to directly observe the low temperature diffusion of photocarriers through and across defect states in polycrystalline CdTe thin films. Our measurements show that an inhomogeneous distribution of localized defect states mediates long-range hole transport across multiple grain boundaries to locations exceeding 10 μm from the point of photogeneration. These results provide new insight into the key role deep trap states have in low temperature carrier transport in polycrystalline CdTe by revealing their propensity to act as networks for hopping conduction.

Understanding the role of defects on semiconductor carrier transport should help improve their performance in devices. Using photoluminescence techniques, Alberi et al. image the carrier diffusion in polycrystalline CdTe and find that long-range transport is mediated by the distribution of defect states.

Vacancies in CuInSe2: new insights from hybrid-functional calculations

We calculate the energetics of vacancies in CuInSe(2) using a hybrid functional (HSE06, HSE standing for Heyd, Scuseria and Ernzerhof), which gives a better description of the band gap compared to (semi)local exchange-correlation functionals. We show that, contrary to present beliefs, copper and indium vacancies induce no defect levels within the band gap and therefore cannot account for any experimentally observed levels. The selenium vacancy is responsible for only one level, namely, a deep acceptor level ε(0/2-). We find strong preference for V(Cu) and V(Se) over V(In) under practically all chemical conditions.