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Most work on CdTe-based solar cells has focused on devices with a superstrate structure. This focus is due to the early success of the superstrate structure in producing high-efficiency cells, problems of suitable ohmic contacts for lightly doped CdTe, and the simplicity of the structure for manufacturing. The development of the CdCl 2 heat treatment boosted CdTe technology and perpetuated the use of the superstrate structure. However, despite the beneficial attributes of the superstrate structure, devices with a substrate structure are attractive both commercially and scientifically. The substrate structure eliminates the need for transparent superstrates and thus allows the use of flexible metal and possibly plastic substrates. From a scientific perspective, it allows better control in forming the junction and direct access to the junction for detailed analysis. Research on such devices has been limited. The efficiency of these devices has been limited to around 8% due to low open-circuit voltage (V oc ) and fill factor. In this paper, we present our recent device development efforts at NREL on substrate-structure CdTe devices. We have found that processing parameters required to fabricate high-efficiency substrate CdTe PV devices differ from those necessary for traditional superstrate CdTe devices. We have worked on a variety of contact materials including Cu-doped ZnTe and Cu x Te. We will present a comparative analysis of the performance of these contacts. In addition, we have studied the influence of fabrication parameters on junction properties. We will present an overview of our development work, which has led to CdTe devices with V oc values of more than 860 mV and NREL-confirmed efficiencies approaching 11%.

CdS/CdTe photovoltaic solar cells were made on two different transparent conducting oxide (TCO) structures in order to identify differences in fabrication, performance, and reliability. In one set of cells, chemical vapor deposition (CVD) was used to deposit a bi-layer TCO on Corning 7059 borosilicate glass consisting of a F-doped, conductive tin-oxide (cSnO2) layer capped by an insulating (undoped), buffer (iSnO2) layer. In the other set, a more advanced bi-layer structure consisting of sputtered cadmium stannate (Cd2SnO4; CTO) as the conducting layer and zinc stannate (Zn2SnO4; ZTO) as the buffer layer was used. CTO/ZTO substrates yielded higher performance devices however performance uniformity was worse due to possible strain effects associated with TCO layer fabrication. Cells using the SnO2-based structure were only slightly lower in performance, but exhibited considerably greater performance uniformity. When subjected to accelerated lifetime testing (ALT) at 85 - 100 °C under 1-sun illumination and open-circuit bias, more degradation was observed in CdTe cells deposited on the CTO/ZTO substrates. Considerable C-V hysteresis, defined as the depletion width difference between reverse and forward direction scans, was observed in all Cu-doped CdTe cells. These same effects can also be observed in thin-film modules. Hysteresis was observed to increase with increasing stress and degradation. The mechanism for hysteresis is discussed in terms of both an ionic-drift model and one involving majority carrier emission in the space-charge region (SCR). The increased generation of hysteresis observed in CdTe cells deposited on CTO/ZTO substrates suggests potential decomposition of these latter oxides when subjected to stress testing.

We study the performance of CdS/CdTe thin film PV devices processed with a ZnTe:Cu/Ti contact to investigate how carrier lifetime (τ) in the CdTe layer is affected by Cu diffusion from the contact. Time-resolved photoluminescence (TRPL) measurements show that τ decreases slightly as the contacting temperature increases in the temperature regime that produces “insufficient” Cu concentration in CdTe (∼room temperature to ∼250° C). However, τ increases significantly once the contact temperature is in the range that yields “optimum” Cu concentration and high-performance devices (∼280 to ∼320°C). At higher substrate temperatures (≫∼300° C), τ drops precipitously, consistent with a region previously identified as producing “excessive” Cu concentration - and poor device performance. The observed τ increase within the “optimum temperature” range not only suggests why high-performance devices may form at these contact temperatures using many different contact processes (including paste contacts), but may provide a significant clue as to why Cu-contact formation processes impart a broad latitude in other process parameters.

We have investigated the effect of deposition conditions of SnO2 films, deposited by chemical vapor deposition using tin tetrachloride and tetramethyltin precursors, on the film properties. The type of precursor and the deposition temperature affect the morphology of the films. The structure of the films is determined by the deposition temperature: films deposited at low temperatures show a mixed SnO and SnO2 phase. The processing temperature and type of substrate determine the impurity content in the films. Electrical properties (e.g., the carrier mobility) and optical properties of the films are affected by the structure and the impurity content in these layers.

Polycrystalline thin films of CdTe grown on glass/SnO2/CdS substrates are studied using X‐ray diffraction, atomic force microscopy, and time resolved photoluminescence decay techniques. CdS films were deposited by chemical solution. CdTe films were grown by close‐spaced sublimation at substrate temperatures between 475–625 °C. CdTe thin films deposited at temperatures higher than 525 °C show preferential orientation in the 〈111〉 direction. The Grain size of the films increases with substrate temperature and the films are faceted for all the temperatures. The PL decay constant increases with substrate temperature up to 575 °C for as‐deposited films on soda‐lime substrates. Films on borosilicate substrates show an increase up to the highest temperature used (625 °C). There is systematic increase in the PL decay constant after CdCl2 heat treatment, and the range of values is 1–1.5 nsec for soda‐lime samples and 1–2 nsec for borosilicate samples.

We report on the deposition of thin cadmium sulfide (CdS) layers from aqueous solutions and their optical properties. CdS layers have been deposited on soda lime glass, tin oxide coated glass and copper indium diselenide (CuInSe2) thin films. A systematic increase in the absorption is found to occur with increasing concentration of the buffer salt used in the bath. CdS/CdTe thin film solar cells have been fabricated by close spaced sublimation of CdTe, yielding 11.3% devices.

The authors have studied the electrical properties of CdTe∕CdS solar cells using conductive atomic force microscopy (C-AFM) applied to cross sections of the device. This novel technique uses the sharp tip of an atomic force microscope to contact the sample and apply an electrical potential, allowing the study of device properties with spatial resolution second to none. The CdTe∕CdS∕SnO2/substrate structures were treated with CdCl2 and etched with bromine/methanol or nitric/phosphoric acid solution. Finally, a Cu-containing back contact was applied to the surface of the device. The C-AFM analysis showed the existence of high-conductivity regions in CdTe close to the film surface, while the regions close to the junction with CdS remain resistive. The width of the conductive area in general varied laterally and, occasionally, reached the junction in some spots, causing microshunts in the devices. By analyzing the fracture of the cross sections, they observed that the conductive areas are concentrated at grai...