First Solar (United States)

Colloidal semiconductornanocrystals (NCs) offer a cost-effectiveplatform for light-energy conversion in X-ray scintillators, photovoltaics,lasers, and display technologies. Yet, device-relevant NCs often requirecomplex heterostructured compositions, where lattice imperfectionscompromise the efficiency and stability of photoconversion processes.Here, we show that a simple synthetic detour through a eutectic stateof II–VI semiconductor NCs (e.g., CdSe, ZnSe) with halide salts(e.g., CdCl2, ZnCl2) overcomes this limitationby melting and reconstructing NC lattices into defect-free alloyedand core/shell architectures. Applied to ternary CdSeTe NCs, thisprocess produces downconverters with record brightness and minimalline widths, delivering a 3-fold increase in film-side external quantumefficiency of commercial CdTe photovoltaic modules (First Solar Inc.).Meanwhile, eutectic processing of CdSe-based core/shell emitters yieldsan 8-fold enhancement in their photoluminescence stability under backlightoperation, addressing the reliability bottleneck for display technologies.Together, these findings establish eutectic NC processing as a scalableroute to efficient, durable photoconversion materials for energy applications.

Four doping conditions were explored for CdTe-based solar cells: p-type As and P, n-type Al, and no intentional doping. In each case, the CdTe absorber was alloyed with Se, but only near the normal front-side, light entry for the cells, while the dopants were added from the back. Cells with p-dopants showed efficiencies up to 20% with front-side illumination, but the n-doped and undoped ones were close to zero. With back-side illumination, this was reversed with undoped up to 8% and p-doped ones only about 2%. These results are explained by Kelvin-probe measurements of electric-field profiles, which showed that the diode field was near the front for the higher-efficiency p-doping, but near the back for undoped and n-doped.

Cd(Se,Te) photovoltaics (PV) are the most widely deployed thin-film solar technology globally, yet continued efficiency improvements are stymied by challenges at the device hole contacts. The inclusion of solution-processed oxide layers such as AlGaOx in the contact stack has yielded improved device open-circuit voltages (VOC) and fill factors (FF). However, contradictory mechanisms by which these layers improve the device properties have been proposed by the research community. We demonstrate in this work that an underappreciated property of such spin-coated layers is the preferential deposition at grain boundaries, a process that isolates the grain boundaries during contact metallization. The effects of grain-boundary isolation are probed by varying the coverage of solution-processed AlGaOx "barrier" layers on the Cd(Se,Te) surface, quantified by scanning Auger microscopy. Examining coverage-dependent VOC and FF, it was observed that isolating the grain boundaries during metallization is sufficient to prevent damage to the absorber that occurs in devices lacking a barrier layer, while additional coverage contributes to the increased series resistance. Such an effect is agnostic to the material used as a barrier layer, as long as the material does not itself damage the absorber. Spin-coated SiOx was used in place of AlGaOx for an equally beneficial effect. This grain-boundary isolation phenomenon is also observed during Mo deposition and in absorbers that have been contacted with a nitrogen-doped ZnTe layer. The mechanisms by which metallization may degrade the absorber are discussed, as are contact design strategies leveraging barrier layers, which may lead to improved device efficiencies.

CdTe solar cells suffer from a non-ideal back contact which can strongly affect the equivalent circuit model and complicate capacitance measurements. Here, five different back contact materials are deposited on identical CdTe absorbers and their influence on capacitance measurements is characterized. A five-element equivalent circuit model is shown to represent a CdTe solar cell with a non-ideal back contact, and capacitance-frequency (C-f) measurements on each sample clearly show the influence of this contact. Capacitance-voltage (C-V) measurements on each sample show the influence of frequency on measured capacitance. In the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{AlGaO}_{\mathbf{x}}/\mathbf{Au}$</tex> sample, temperature dependent C-fs show the back barrier height to be 240 meV, and ongoing measurements will give a comparison of back barrier heights for all samples. This study provides insight into the efficacy of various back contacts and highlights potential errors in the interpretation of capacitance measurements due to the presence of the back contact.

We demonstrate P doping of CdSeTe and compare resulting absorbers and solar cell devices to As doping, for graded and uniform Se profiles. We demonstrate improved Voc*FF product for P doped devices, particularly for high Se absorbers where voltage dependent collection limits FF. Improvement in Voc from P doping is associated with reduced Urbach tails compared to As doping, which directly translates to higher entitlement for ideal Voc. Doping levels resulting from P tend to be lower than As for similar incorporation levels, indicating reduced dopant activation ratio for P. Both photoluminescence and cathodoluminescence measurements for P doped absorbers indicate shallower and suppressed sub-bandgap luminescence compared to As doping, consistent with a shallower acceptor level for P.

High-Performance Perovskite Solar Cells In article number 2310404, Viktor V. Brus and co-workers find that multi-component high-performance perovskite solar cells reveal excellent radiation resistance even under extreme high-intense short-pulse proton irradiation, that significantly exceeds harsh space conditions, including intense coronal ejection events usually associated with solar flares.

This report provides a snapshot of emerging photovoltaic (PV) technologies. It consists of concise contributions from experts in a wide range of fields including silicon, thin film, III-V, perovskite, organic, and dye-sensitized PVs. Strategies for exceeding the detailed balance limit and for light managing are presented, followed by a section detailing key applications and commercialization pathways. A section on sustainability then discusses the need for minimization of the environmental footprint in PV manufacturing and recycling. The report concludes with a perspective based on broad survey questions presented to the contributing authors regarding the needs and future evolution of PV.

There are few doping concentration measurement techniques that are contactless and usable for all semiconductors. In this article, we demonstrate the use of injection-dependent quasi-steady-state photoluminescence to simultaneously measure the external radiative efficiency, minority-carrier lifetime, and activated dopant concentration of solar cell absorber layers. We first demonstrate this measurement on Si, for which established lifetime and doping measurement techniques exist, and determine a doping density of 4.2 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup>, which is within a factor of 2 from the 2.1 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> value calculated from quasi-steady state photoconductance. Then, we demonstrate the use of the technique to measure doping concentrations of CdSeTe over two orders of magnitude and of perovskite down to nearly 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup>—materials that are much more difficult to accurately assess with other techniques.

Tandem solar cells have received a lot attention from academia and industrial researchers as the potential next-generation PV technology, with higher efficiency above the limit of single-junction solar cells. Thin-film/thin-film (TF/TF) tandems are attractive due to similar toolset and processes producing the top and bottom cells, which improve scalability and promote cost reduction compared to TF/wafer tandem technologies. TF/TF/tandems additionally offer more absorber bandgap flexibility that promotes photovoltaic conversion efficiency optimization. Many materials not suitable for single junction solar cells can be explored as tandem top or bottom cells. To assess the practical efficiency potential of tandem solar cells limited by non-ideal material and device quality, we present a Shockley–Queisser-like efficiency calculation for tandem devices consisting of non-ideal top and bottom cells and with a range of absorber band gaps. The non-ideality is introduced through an experimentally measurable external radiative quantum efficiency (ERE). We find that a range of top and bottom cell band gaps enabling the highest tandem efficiency shifts from the ideal Shockley–Queisser case and depends on the top and bottom cell ERE. Furthermore, tandem cell efficiency greater than 37% can be achieved with very modest top/bottom cell EREs, for example of only 0.008%/0.5% which is typical for CdTe/CIS cells. Our results indicate that high efficiency tandem solar cells have good probability to be manufactured at high volume within a foreseeable future, despite non-ideal material and device quality due to early stages of development or constraint by manufacturing requirements. Finally, we review a number of mature and emerging thin film absorber material candidates for tandem applications. We discuss properties of these materials and the corresponding device performance as well as the associated technological challenges. We concludes on the promise of each of these materials for tandem applications that is expected to provide guidance to the photovoltaic research community.