Abstract
Thin film materials for photovoltaics such as cadmium telluride (CdTe), copper-indium diselenide-based chalcopyrites (CIGS), and lead iodide-based perovskites offer the potential of lower ...solar module capital costs and improved performance to microcrystalline silicon. However, for decades understanding and controlling hole and electron concentration in these polycrystalline films has been extremely challenging and limiting. Ionic bonding between constituent atoms often leads to tenacious intrinsic compensating defect chemistries that are difficult to control. Device modeling indicates that increasing CdTe hole density while retaining carrier lifetimes of several nanoseconds can increase solar cell efficiency to 25%. This paper describes
in-situ
Sb, As, and P doping and post-growth annealing that increases hole density from historic 10
14
limits to 10
16
–10
17
cm
−3
levels without compromising lifetime in thin polycrystalline CdTe films, which opens paths to advance solar performance and achieve costs below conventional electricity sources.
CdTe photovoltaics has achieved one of the lowest levelized costs of electricity among all energy sources. However, for decades, carrier lifetimes have been inferior to those of other prevalent solar ...cell materials. This quality has inhibited common methods to improve solar cell efficiency such as back‐surface fields, electron reflectors, or bifacial solar cells. In this work, a significant increase in carrier lifetime to values exceeding 200 ns in fully functional CdTe solar cells is demonstrated. The increased lifetime is achieved by large CdSeTe grains at the absorber/emitter interface, intragrain passivation in the absorber layer, and chemical passivation by forming nanoscale oxidized tellurium species at the transparent conducting oxide interface. The carrier lifetime is correlated to the open‐circuit voltage and enables paths for back‐surface manipulation and novel cell architectures to further improve CdTe photovoltaic performance.
For decades, carrier lifetimes in CdTe photovoltaics have been inferior to those of other prevalent solar cell materials. Here, lifetime values exceeding 200 ns with correspondingly increased open‐circuit voltage are achieved through enhanced grain growth, intragrain passivation, and emitter interface passivation by oxidized Te species, enabling paths for back‐surface manipulation and novel cell architectures to further improve CdTe photovoltaic performance.
Dislocation cores have long dominated the electronic and optical behaviors of semiconductor devices and detailed atomic characterization is required to further explore their effects. Miniaturization ...of semiconductor devices to nanometre scale also puts emphasis on a material's mechanical properties to withstand failure due to processing or operational stresses. Sessile junctions of dislocations provide barriers to propagation of mobile dislocations and may lead to work‐hardening. The sessile Lomer–Cottrell and Hirth lock dislocations, two stable lowest elastic energy stair‐rods, are studied in this paper. More specifically, using atomic resolution high‐angle annular dark‐field imaging and atomic‐column‐resolved X‐ray spectrum imaging in an aberration‐corrected scanning transmission electron microscope, dislocation core structures are examined in zinc‐blende CdTe. A procedure is outlined for atomic scale analysis of dislocation junctions which allows determination of their identity with specially tailored Burgers circuits and also formation mechanisms of the polar core structures based on Thompson's tetrahedron adapted to reactions of polar dislocations as they appear in CdTe and other zinc‐blende solids. Strain fields associated with the dislocations calculated via geometric phase analysis are found to be diffuse and free of `hot spots' that reflect compact structures and low elastic energy of the pure‐edge stair‐rods.
Polycrystalline photovoltaic materials offer low costs and good scalability; however, grain boundaries in these materials are extended defects, which can drastically increase carrier recombination. ...Interestingly, three leading polycrystalline thin-film technologies—cadmium telluride (CdTe), CuIn1-xGaxSe2 (CIGS), and perovskite solar cells (PSCs)—passivate absorber surfaces in the same way: via formation of low-dimensional, typically two-dimensional (2D), van der Waals materials. This has primarily occurred serendipitously through process optimization, but in some cases, 2D capping layers are intentionally incorporated to improve device performance. Here, evidence compiled from the literature, supplemented with new data where necessary, is presented to illustrate the existence of 3D/2D interfaces in CdTe, CIGS, and PSCs, and their correlation with improved passivation and device performance. This suggests that 3D/2D passivation might be a heretofore unappreciated key to successful polycrystalline thin-film photovoltaics. Finally, the desired attributes of successful low-dimensional layers are presented with rational design strategies for next-generation polycrystalline solar cells.
Strain within grains and at grain boundaries (GBs) in polycrystalline thin-film absorber layers limits the overall performance because of higher defect concentrations and band fluctuations. Yet, the ...nanoscale strain distribution in operational devices is not easily accessible using standard methods. X-ray nanodiffraction offers the unique possibility to evaluate the strain or lattice spacing at nanoscale resolution. Moreover, the combination of nanodiffraction with additional techniques in the framework of multimodal scanning X-ray microscopy enables the direct correlation of the strain with material and device parameters such as the elemental distribution or local performance. This method is applied for the investigation of the strain distribution in CdTe grains in fully operational photovoltaic solar cells. It is found that the lattice spacing in the (111) direction remains fairly constant in the grain cores but systematically decreases at the GBs. The lower strain at GBs is accompanied by an increase of the total tilt. These observations are both compatible with the inhomogeneous incorporation of smaller atoms into the lattice, and local stress induced by neighboring grains.
Using two-photon tomography, carrier lifetimes are mapped in polycrystalline CdTe photovoltaic devices. These 3D maps probe subsurface carrier dynamics that are inaccessible with traditional optical ...techniques. They reveal that CdCl
treatment of CdTe solar cells suppresses nonradiative recombination and enhances carrier lifetimes throughout the film with substantial improvements particularly near subsurface grain boundaries and the critical buried p-n junction.
Model CdTe systems –both single-crystalline (sx) and poly-crystalline (px) –are investigated experimentally as a means to understand the role of competing material properties and processing steps in ...improving the performance of standard thin-film solar cells. Previous device optimization work is reviewed explaining the close space sublimation growth technique and ongoing analysis using scanning transmission electron microscopy. This is followed by motivation for molecular beam epitaxy (MBE) growth studies of CdTe and the results of fundamental material investigations. The results show that (a) the minority carrier lifetimes in hetero-epitaxial layers is limited by surface recombination, (b) source selection and anneals can be tuned to achieve p-type carrier density of 6x1015 cm-3, and (c) counter-intuitively, the increase in p-density is associated with increased mobility in lower crystal quality samples, suggesting the role of anneal. Finally, controlled and re-growth of px-CdTe by MBE studies are discussed with results indicating that shorter lifetimes are directly correlated with the increased surface/interface density.
Advancing CdTe solar cell efficiency requires improving the open-circuit voltage (V OC ) above 900 mV. This requires long carrier lifetime, high hole density, and high-quality interfaces, where the ...interface recombination velocity is less than about 10 4 cm/s. Using CdTe single crystals as a model system, we report on CdTe/CdS electrical and structural interface properties in devices that produce open-circuit voltage exceeding 950 mV.
We have investigated
in situ
p
-type doping of ZnTe and CdZnTe on Si(211) by molecular beam epitaxy using a radiofrequency (RF)-nitrogen plasma source for application to multijunction II–VI-based ...solar cells. CdZnTe would be used as a wide-gap top cell in a monolithic multijunction device, and ZnTe or CdZnTe could be used for the
p
-side of tunnel junctions. Highly
p
-type material is required for producing the high-quality tunnel junctions crucial for maintaining current flow, and
p
-doping of order 10
17
cm
−3
is required for the generation of a large built-in potential in the absorber region of solar cells. Our uniformly doped films exhibited good Hall characteristics, especially considering the large lattice mismatch between Si and either ZnTe or CdZnTe. Crystal quality was examined by x-ray diffraction. Nitrogen incorporation was examined as a function of the source-gas dilution with argon. A sample with layers of CdZnTe doped using 1% to 100% nitrogen was grown on nominally undoped CdZnTe and analyzed using secondary-ion mass spectrometry. The nitrogen incorporation differed by only a factor of 10, despite the factor of 100 difference in the nitrogen concentration in the plasma, indicating a saturation effect.
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