Electrical characterization of Cu composition effects in CdS/CdTe thin-film solar cells with a ZnTe:Cu back contact

Li, Jian V.; Duenow, Joel N.; Kuciauskas, Darius; Kanevce, Ana; Dhere, R. G.; Young, Matthew R.; Levi, Dean H.

doi:10.1109/pvsc-vol2.2012.6656756

Cited by 5 publications

(5 citation statements)

References 20 publications

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“…Hall measurements are often difficult to perform on p -type poly-CdTe films because of their low free-carrier concentrations and mobility. To probe the trap state behaviour in our material, admittance spectroscopy (AS) was carried out on completed poly-CdTe devices with comparable Cu concentrations 29 . Differential capacitance spectra as a function of temperature yield a hole trap activation energy of 152 meV, which is consistent with the acceptor-binding energy determined from the PL measurements.…”

Section: Resultsmentioning

confidence: 99%

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

et al. 2013

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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.

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Section: Resultsmentioning

confidence: 99%

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

et al. 2013

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“…The observed concentration of the Cu i ++ defect corresponded well to the Cu diffusion profile monitored by secondary ion mass spectroscopy (SIMS) with a Cu piled up at the CdTe/CdS interface. 6,24 It is, therefore, possible that this deep defect level is originated from the Cu-containing back contact due to diffusion of excess Cu that was evaporated. Since sets of cell 1 samples had higher concentration of copper with 15 nm evaporated copper in addition to ZnTe:Cu (2 atomic% Cu) paste, possible formation copper-related defect (E V + 0.57 eV) can be inferred, where E v is the energy at valence band edge.…”

Section: Resultsmentioning

confidence: 99%

Evaluation of Cu Back Contact Related Deep Defects in CdTe Solar Cells

Kharangarh

Misra

Georgiou

et al. 2012

ECS J. Solid State Sci. Technol.

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CdTe solar cells with back contacts formed by either (i) 15nm Cu evaporation followed by application of carbon conductive paste embedded with micron sized Cu particles in ZnTe powder and (ii) only with the above mentioned conducting paste, were evaluated. A Cu-related deep level defect with an activation energy of E a ∼ = 0.57 eV was observed for Cu evaporated back contact cells and an intrinsic defect with an activation energy E a ∼ = 0.89 eV was found for cells prepared only by ZnTe:Cu embedded carbon paste. Frequency dispersion in C-V measurements confirms the presence of Cu-related deep level traps for cells with Cu evaporated back contact whereas no such defects were observed in carbon paste contact. The behavior was believed to be due to diffusion of excess Cu from the contact. It was further observed that majority carrier deep level traps (Cu-related or intrinsic) contribute differently to the degradation of electronic properties of the CdTe solar cells.

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“…As most commonly practiced, ZnTe:Cu layers are deposited by sputtering at elevated temperature (240-360 ºC) in processes whose duration are on the order of hours [14,15]. The amount of copper is controlled by varying the composition of the sputter target or the layer thickness.…”

Section: Introductionmentioning

confidence: 99%

“…It is well known that copper is a fast diffuser, with reported coefficients of ~10 -9 cm 2 /s at the temperatures employed [16,17]. In addition to limiting throughput, another drawback of this procedure is that deposition and diffusion occur simultaneously, making process control difficult and resulting in copper migration throughout the device [15]. In bulk CdTe a very small amount of copper may be beneficial [18,19], however excessive amounts lead to deep level defects and recombination centers [20][21][22].…”

Section: Introductionmentioning

confidence: 99%

Controlled activation of ZnTe:Cu contacted CdTe solar cells using rapid thermal processing

Diercks

Ohno

et al. 2015

Solar Energy Materials and Solar Cells

110

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Back contacts can significantly limit CdTe solar cell performance, reducing both open circuit voltage (V oc) and fill factor (FF). Copper is an essential component of effective back contacts, but its presence in the CdTe absorber creates detrimental recombination centers. Rapid thermal processing (RTP) is demonstrated as a highly effective approach for reducing back contact barriers in CdTe solar cells contacted with ZnTe:Cu buffer layers, substantially improving both FF (>73%) and V oc (>850 mV). Current density and quantum efficiency remain essentially unchanged, but a five-fold increase in minority carrier lifetime is observed which is attributed to passivation of recombination sites in the back contact region. Quantitative analysis of secondary ion mass spectrometry shows that the majority of Cu segregates to the Au metallization layer and that the ZnTe buffer appears to inhibit the Cu diffusion into CdTe. 3D imaging of the back contact region using atom probe tomography shows that optimized devices are characterized by preferential segregation of copper to both the Au|ZnTe and CdTe|ZnTe interfaces, perhaps in the form of Cu x Te. With its low thermal budget the RTP process has been successfully applied to multiple device architectures. including devices with certified efficiencies in excess of 16%.

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Electrical characterization of Cu composition effects in CdS/CdTe thin-film solar cells with a ZnTe:Cu back contact

Cited by 5 publications

References 20 publications

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

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

Evaluation of Cu Back Contact Related Deep Defects in CdTe Solar Cells

Controlled activation of ZnTe:Cu contacted CdTe solar cells using rapid thermal processing

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