Jiaojiao Li

Rapid thermal processing of ZnTe:Cu contacted CdTe solar cells

Cadmium telluride (CdTe) is a leading absorber for thin-film solar cells. However, state-of-the art open circuit voltages (Voc) of CdTe thin film solar cells fall ~350 mV below the value expected based on the band gap of CdTe. One of the reasons is due to barriers at the back contact. In this paper, we report on the development of a the back contact process for ZnTe:Cu contacted device that employs rapid thermal processing (RTP) to precisely control the activation and distribution of Cu. It is shown that 30 s annealing steps significantly improve fill factor and Voc without compromising the current density. Devices with >14% efficiency and >825 mV Voc are obtained under optimal conditions.

The effect of copper on the sub-bandgap density of states of CdTe solar cells

Two optical sub-bandgap transitions in CdTe thin-film solar cells have been identified using detailed transient photocapacitance and transient photocurrent spectroscopy measurements. A broad response centered at EV + 0.9 eV directly correlates with the quantity of Cu present in the absorber layer, while a second response at EV + 1.2 eV does not depend on Cu or Zn and may be an intrinsic defect. These results demonstrate the influence of Cu on the sub-bandgap density of states of CdTe, and they are critical to understanding, modeling, and improving its optoelectronic properties.

Atom probe tomography for nanoscale characterization of CdTe device absorber layers and interfaces

Cadmium telluride (CdTe) solar cells are a leading thin film technology with relatively high efficiencies. However, even the highest published efficiency CdTe cell is well below the theoretical achievable efficiency. Atomic scale characterization would provide important feedback on optimization of CdTe cells for further efficiency improvements. Atom probe tomography (APT), with both high spatial resolution and ppm composition sensitivity, is a technique well-suited for providing these details. It is demonstrated here that the compositions measured for CdTe and ZnTe by APT are sensitive to the analysis conditions, in particular the incident laser energy. Experiments demonstrating the relationships of the analysis parameters are presented. Using optimized values, APT analyses of the absorber layer and interfaces in CdTe devices were performed.

Structural and chemical characterization of the back contact region in high efficiency CdTe solar cells

Cadmium telluride (CdTe) is the leading commercialized thin-film photovoltaic technology. Copper is commonly used in back contacts to obtain high efficiency, but has also been implicated as a harmful factor for device stability. Thus it is critical to understand its composition and distribution within complete devices. In this work the composition and structure of the back contact region was examined in high efficiency devices (~16%) contacted using a ZnTe:Cu buffer layer followed by gold metallization. The microstructure was examined in the as-deposited state and after rapid thermal processing (RTP) using high resolution transmission electron microscopy and EDX chemical mapping. After RTP the ZnTe exhibits a bilayer structure with polycrystalline, twinned grains adjacent to Au and an amorphous region adjacent to CdTe characterized by extensive Cd-Zn interdiffusion. The copper that is co-deposited uniformly within ZnTe is found to segregate dramatically after RTP activation, either collecting near the ZnTe/Au interface or forming CuxTe clusters in CdTe at defects or grain boundaries near the interface with ZnTe. Chlorine, present throughout CdTe and concentrated at grain boundaries, does not penetrate significantly into the back contact region during RTP activation.

CdTe solar cells employing CdS1−yTey window layers

Oxygenated cadmium sulfide (CdS:O) is an alternative CdTe window layer that delivers improved blue response relative to CdS. Our recent study revealed that CdS: O completely transforms during device fabrication into a layer containing cadmium sulfate clusters interspersed among CdS1−yTey nanocrystals. This motivated us to study CdTe solar cells employing pre-formed CdS1−yTey alloy windows without sulfate present. The intrinsic properties of alloys deposited by co-evaporation are evaluated and then used in place of CdS in standard device fabrication. Interestingly we find that device efficiency is nominally unchanged, but there are significant tradeoffs between current collection, fill factor, and open circuit voltage with alloy composition. XRD analysis of interdiffusion with CdTe is performed and the results show that it is largely suppressed in alloy-based devices.

Activation of CdTe solar cells using molecular chlorine

There has been renewed interest in developing alternatives to CdCl2 activation, a critical but expensive step in CdTe manufacturing. A common element among the alternative agents that have been explored is chlorine, which facilitates recrystallization, passivates grain boundaries, and leads to enhanced efficiency. In this paper we present an evaluation of molecular chlorine (Ch) for CdTe activation. The broad process space was first screened by evaluating the change in the (111) texture coefficient. It is shown that recrystallization occurs quite quickly (~1 minute), and the required concentration of Cl2 is quite low (<;100 ppm). Devices fabricated at promising conditions displayed good rectification with device efficiencys >11%.

CdTe solar cells employing CdS 1−y Te y window layers

Oxygenated cadmium sulfide (CdS:O) is an alternative CdTe window layer that delivers improved blue response relative to CdS. Our recent study revealed that CdS: O completely transforms during device fabrication into a layer containing cadmium sulfate clusters interspersed among CdS1-yTey nanocrystals. This motivated us to study CdTe solar cells employing pre-formed CdS1-yTey alloy windows without sulfate present. The intrinsic properties of alloys deposited by co-evaporation are evaluated and then used in place of CdS in standard device fabrication. Interestingly we find that device efficiency is nominally unchanged, but there are significant tradeoffs between current collection, fill factor, and open circuit voltage with alloy composition. XRD analysis of interdiffusion with CdTe is performed and the results show that it is largely suppressed in alloy-based devices.

The impact of different metallization layers on CdTe solar cells contacted with ZnTe:Cu buffer layers

It is difficult to form ohmic contact with CdTe because of its high work function (5.7 eV), so a buffer layer is oftenused to decrease the barrier height. Previously, we demonstrated high efficiency solar cells using rapid thermal processing (RTP) to activate back contacts comprised of a ZnTe:Cu buffer and gold metallization layer. However, since Au is not practical for commercialization in this work we explore chromium and titanium as more practical alternatives. It was found that comparable performance could be obtained with each metal, but that the optimal Cu loading scaled as one would expect based on solubility. Comparisons of current-voltage and quantum efficiency behavior among devices produced with insufficient, optimal, and excess Cu dosing are used to provide insight into the role(s) of this critical impurity for device performance. Reliability tests were carried out under different stressing conditions and the results show the relation of the degradation with both the metal and the presence of illumination.