The remarkable development in photovoltaic (PV) technologies over the past 5 years calls for a renewed assessment of their performance and potential for future progress. Here, we analyse the progress in cells and modules based on single-crystalline GaAs, Si, GaInP and InP, multicrystalline Si as well as thin films of polycrystalline CdTe and CuInxGa1−xSe2. In addition, we analyse the PV developments of the more recently emerged lead halide perovskites together with notable improvements in sustainable chalcogenides, organic PVs and quantum dots technologies. In addition to power conversion efficiencies, we consider many of the factors that affect power output for each cell type and note improvements in control over the optoelectronic quality of PV-relevant materials and interfaces and the discovery of new material properties. By comparing PV cell parameters across technologies, we appraise how far each technology may progress in the near future. Although accurate or revolutionary developments cannot be predicted, cross-fertilization between technologies often occurs, making achievements in one cell type an indicator of evolutionary developments in others. This knowledge transfer is timely, as the development of metal halide perovskites is helping to unite previously disparate, technology-focused strands of PV research. Nearly all types of solar photovoltaic cells and technologies have developed dramatically, especially in the past 5 years. Here, we critically compare the different types of photovoltaic technologies, analyse the performance of the different cells and appraise possibilities for future technological progress.

/pdf/photovoltaic-solar-cell-technologies-analysing-the-state-of-4yccykvci3.pdf

Photovoltaic solar cell technologies: analysing the state of the art

CdTe solar cells have the potential to undercut the costs of electricity generated by other technologies, if the open-circuit voltage can be increased beyond 1 V without significant decreases in current. However, in the past decades, the open-circuit voltage has stagnated at around 800–900 mV. This is lower than in GaAs solar cells, even though GaAs has a smaller bandgap; this is because it is more difficult to achieve simultaneously high hole density and lifetime in II–VI materials than in III–V materials. Here, by doping the CdTe with a Group V element, we report lifetimes in single-crystal CdTe that are nearly radiatively limited and comparable to those in GaAs over a hole density range relevant for solar applications. Furthermore, the deposition on CdTe of nanocrystalline CdS layers that form non-ideal heterointerfaces with 10% lattice mismatch impart no damage to the CdTe surface and show excellent junction transport properties. These results enable the fabrication of CdTe solar cells with open-circuit voltage greater than 1 V. Solar cells based on CdTe are a promising low-cost alternative to mainstream Si devices, but they usually produce voltages below 900 mV. Burst et al. now show that open-circuit voltages greater than 1 V can be achieved by doping the CdTe with a group V element.

https://www.nature.com/articles/nenergy201615.pdf?proof=t

CdTe solar cells with open-circuit voltage breaking the 1 V barrier

CdTe-based solar technology has achieved one of the lowest levelized costs of electricity among all energy sources as well as state-of-the-art field stability. Yet, there is still ample headroom to improve. For decades, mainstream technology has combined fast CdTe deposition with a CdCl2 anneal and Cu doping. The resulting defect chemistry is strongly compensated and limits the useful hole density to ~1014 cm−3, creating a ceiling for fill factor, photovoltage and efficiency. In addition, Cu easily changes energy states and diffuses spatially, creating a risk of instabilities that must be managed with care. Here, we demonstrate a significant shift by doping polycrystalline CdSexTe1 − x and CdTe films with As while removing Cu entirely from the solar cell. The absorber majority-carrier density is increased by orders of magnitude to 1016–1017 cm−3 without compromising the lifetime, and is coupled with a high photocurrent greater than 30 mA cm−2. We demonstrate pathways for fast dopant incorporation in polycrystalline thin films, improved stability and 20.8% solar cell efficiency. CdTe solar cells have relied for decades on copper, which creates limited hole density, stability issues and a ceiling for voltage and efficiency. Now, Metzger et al. demonstrate As-doped Cu-free polycrystalline CdTe cells with enhanced hole density and dopant stability, achieving 20.8% efficiency.

Exceeding 20% efficiency with in situ group V doping in polycrystalline CdTe solar cells

United Nations adopted Sustainable Development Goals to ensure prosperity, reduction of poverty and mitigation of climate change. Energy is a part of these goals. Today, energy demand heavily relies on the fossil fuel sources: petroleum, natural gas, coal etc., which causes greenhouse gas emission and environmental deterioration. In order to avoid environmental impacts, current energy systems need to transform into clean and renewable sources of energy systems. Deployment of solar energy, an omnipresent renewable energy source, is gaining popularity due to the easiness of installation, availability and competitive cost. For effective utilization and higher penetration of solar energy, knowledge about technology and performance of solar energy system is required. In this paper, different solar photovoltaic (SPV) technology and mathematical modeling to characterize the SPV systems are comprehensively presented. The performance analysis on the basis of standard parameters like performance ratio, yield energy, reference energy, capacity utilization factor etc. and on the basis of exergy as well as energy efficiency are also presented. Variation in environmental conditions significantly affects the outdoor performance and operation of SPV systems. Therefore, study of degradation and failure modes is important for accurate prediction of performance of SPV systems and has been discussed.

Performance assessment and degradation analysis of solar photovoltaic technologies: A review

Already, several technologies of polycrystalline thin‐film photovoltaic materials have achieved certified record small‐cell power conversion efficiencies exceeding 22%. They are CdTe, Cu(In,Ga)(S,Se)2 (CIGS), and metal halide perovskite (PSC), each named after the light-absorbing semiconductor material. Thin-film solar cells and modules require very little active material due to their very high absorption coefficient. Efficient production methods with low materials waste, moderate temperatures, attractive cost structures, and favorable energy payback times will play a strong role in market development as thin-film technologies reach full maturity, including mass production and the standardization of production machineries. In fact, the first two technologies have already been developed up to the industrial scale with a market share of several GW. In this review article, we outline similarities and differences between these high‐efficiency thin‐film technologies from both the materials and the industrial point of view. We address the materials characteristics and device concepts for each technology, including a description of recent developments that have led to very high efficiency achievements. We provide an overview of the CIGS industry players and their current status. The newcomer PSC has demonstrated its potential in the laboratory, and initial efforts in industrial production are underway. A large number of laboratories are experimenting through a wide range of options in order to optimize not only the efficiency but also stability, environmental aspects, and manufacturability of PSC. Its high efficiency and its high bandgap make PSC particularly attractive for tandem applications. An overview of all these topics is included here along with a list of materials configurations.

Thin‐film solar cells exceeding 22% solar cell efficiency: An overview on CdTe-, Cu(In,Ga)Se2-, and perovskite-based materials

After a long period of stagnancy for record cell efficiency and several years of growth of the industry, CdTe solar cell efficiency has been rapidly increasing recently. First Solar (FSLR) fabricated an 18.7% NREL certified cell at the end of 2012 and reports an increase to a certified 19.0% in this paper. Although the improvements were dominated by increases in short-circuit current and fill factor, there is now evidence that the open-circuit voltage of polycrystalline CdTe is not fundamentally limited to ~850 mV, and first devices exceeding 900 mV have been demonstrated. In-depth device characterization indicates that the responsible third-level metric is the increase of carrier lifetime, which can be characterized by transient photoluminescence. The progress at hand suggests that the near-term achievable target for CdTe solar cells should be raised from 19% to 22%. A detailed numerical model is used to translate cell results into predicted module efficiency. These simulations are in agreement with FSLR's recently certified 16.1% total-area module efficiency record. Forward-looking simulations show that with already demonstrated technology components, a module efficiency exceeding 17% is attainable. Disruptive changes and implementation of new device architectures can provide further room for improvement for cell efficiency beyond 22%.

CdTe Solar Cells at the Threshold to 20% Efficiency

While the electrical and optical properties of most crystalline materials are determined by the point defects, the association of these defects into complexes may further alter material properties, introducing new important phenomena. The properties of isolated point defects in CdTe have attracted significant research efforts, yet understanding of the complex defects in this material remains insufficient. This paper investigates the thermodynamic aspects of defect association in chlorinated copper-doped CdTe absorbers from first principles, using a supercell approach with the range-separated hybrid exchange–correlation functional. Based on the complex association energies calculated for 76 defect reactions, we propose the most favorable pair complexes formed in Cl- and Cu-doped CdTe absorbers. Most of the complexes studied in this work appear to be harmful for p-doping and may be responsible for the performance instabilities observed in CdTe devices. We also discuss a plausible passivation mechanism that mitigates TeCd recombination centers during Cl treatment and consider the formation of larger defect clusters and segregation of the point defects on extended defects.

Defect interactions and the role of complexes in the CdTe solar cell absorber

Comparative study of As and Cu doping stability in CdSeTe absorbers

To overcome the limitations of thermodynamic defect models, we introduce a multilevel kinetic approach to simulate the activation of $p$ doping in CdTe by group V elements using arsenic as a ``model'' dopant. On the lowest level, we calculate thermodynamic and kinetic parameters of point defects, complexes, and reactions from first principles. On the intermediate level, we use these parameters to calculate the kinetic rates of defect reactions. Finally, we simulate the time evolution of defects and free carriers. Our results show the importance of kinetic factors in defect chemistry models. We reveal the primary arsenic activation pathway to be a fast reaction in which the tellurium atoms get kicked out and replaced on the regular anion sites by interstitial arsenic species. We discover the important role of ($\mathrm{A}{\mathrm{s}}_{\mathrm{i}}\mathrm{A}{\mathrm{s}}_{\mathrm{Te}}$) and ($\mathrm{A}{\mathrm{s}}_{\mathrm{i}}\mathrm{A}{\mathrm{s}}_{\mathrm{i}}$) complexes that arise during activation anneal to form kinetically stabilized transient states that not only compensate the doping but also can produce deep recombination levels. We expect that our modeling approach and the gained insight into the atomic processes behind the doping formation will advance the defect chemistry modeling of electronic properties of materials.

https://link.aps.org/accepted/10.1103/PhysRevMaterials.2.103803

Beyond thermodynamic defect models: A kinetic simulation of arsenic activation in CdTe

A discontinuous or reduced thickness window layer can improve the efficiency of CdTe-based or other kinds of solar cells.

Photovoltaic window layer

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