Polycrystalline Thin Film Solar Cells Research Articles

Development of semitransparent CdTe polycrystalline thin-film solar cells modified with a CuCl layer for BIPV

In the current context of renewable energy development, CdTe polycrystalline thin-film solar cells are expected to have broad prospects in fields such as Building Integrated Photovoltaics. It is crucial to fabricate sub-micron-thick, semitransparent CdTe solar cells for photovoltaic glass curtain walls that require a certain degree of transparency. As a result, we have employeded a high-quality deposition technique for sub-micron-thick CdTe polycrystalline films. We have analyzed the influence of post-treatment processes on the film properties and investigated the impact of solution-based CuCl back buffer layer preparation on device performance. By improving the quality of sub-micron-thick CdTe polycrystalline films, and optimizing the concentration and process of Cu doping, we have successfully fabricated semitransparent CdTe solar cells with an average visible light transmittance of 14.21%, ηFront: 6.2%, ηRear: 4.4%, and bifacial factor:0.7.

Tuning Band Alignment at Grain Boundaries for Efficiency Enhancement in Cu2ZnSnS4 Solar Cells.

Conducting atomic force microscopy has been performed for a fundamental understanding of the mechanism responsible for the lower power conversion efficiency (PCE) of Cu2ZnSnS4 (CZTS) solar cells than that of CuIn1-xGaxSe2 (CIGS) solar cells. The difference in efficiency is partly attributed to the distinctly different band alignment between the grain boundaries (GBs) and grain interior (GI) for the two materials. While CIGS shows type-II band alignment, CZTS was discovered to demonstrate type-I band alignment with the conduction band shifting downward while the valence band shifting upward at the GBs. The type-I band alignment in CZTS leads to both electron and hole trapping, enhancing their recombination, and lowers the PEC. Band engineering was realized by moderate oxidative annealing of CZTS. The preferential GB oxidation changes the band alignment into inverse type-I (i.e., the conduction band upward bending and valence band downward bending at GBs). The blocking of carrier recombination at GBs leads to 30% enhancement in PCE. Our work reveals the critical role that band alignment between the grain boundary and interior plays in polycrystalline thin film solar cells and suggests band alignment engineering as a practical approach to enhance PCE. Furthermore, conducting AFM has been shown to be a powerful tool for qualitative and semiquantitative characterization of band alignment in polycrystalline films.

Thermal Stability of Semiconductor Nanocrystal Solids: Understanding Nanocrystal Sintering and Grain Growth

Nanomaterials are naturally metastable with respect to bulk solids. This raises the very important fundamental problem of their morphological stability, especially when nanoscale crystallites are touching or nearly touching each other, such as in thin-film devices. In some cases, nanostructuring must be preserved under operational conditions (e.g., in quantum dot LEDs, lasers, photodetectors, and nanogranular thermoelectric devices). In other cases, we use nanocrystalline particles as precursors to a material with large crystalline grains and aim to sinter them as efficiently as possible (e.g., in polycrystalline thin-film solar cells). We carried out a systematic study of sintering and grain growth in materials composed of various sub-10 nm semiconductor grains. The boundaries between individual semiconductor grains have been chemically engineered using inorganic surface ligands. We found that the early stages of sintering and grain growth of nanocrystalline semiconductors are controlled by the ion mobility at the nanocrystal surfaces, while the late stages of grain growth are controlled by the mobility of the grain boundaries. This appears to be a general phenomenon for semiconductor nanocrystals, and it leads to several interesting and counterintuitive trends. For example, III–V InAs nanocrystals are generally much more resilient against sintering and grain growth compared to II–VI CdSe nanocrystals even though bulk CdSe has significantly higher melting point temperature than InAs (1268 °C vs 942 °C). Grain growth can be dramatically accelerated when coupled to solid−solid phase transitions. These findings expand our toolbox for rational design of nanocrystal materials for different applications.

Microscopy Visualization of Carrier Transport in CdSeTe/CdTe Solar Cells

Solar cells are essentially minority carrier devices, and it is therefore of central importance to understand the pertinent carrier transport processes. Here, we advanced a transport imaging technique to directly visualize the charge motion and collection in the direction of relevant carrier transport and to understand the cell operation and degradation in state-of-the-art cadmium telluride solar cells. We revealed complex carrier transport profiles in the inhomogeneous polycrystalline thin-film solar cell, with the influence of electric junction, interface, recombination, and material composition. The pristine cell showed a unique dual peak in the carrier transport light intensity decay profile, and the dual peak feature disappeared on a degraded cell after light and heat stressing in the lab. The experiments, together with device modeling, suggested that selenium diffusion plays an important role in carrier transport. The work opens a new forum by which to understand the carrier transport and bridge the gap between atomic/nanometer-scale chemical/structural and submicrometer optoelectronic knowledge.

Imaging CdCl2 defect passivation and formation in polycrystalline CdTe films by cathodoluminescence

Polycrystalline thin-film solar cells are attractive for low-cost photovoltaics, but their efficiencies are hindered by material quality issues. State-of-the-art CdTe solar cells use ${\mathrm{CdCl}}_{2}$ annealing treatments whose effects are still being discovered at a fundamental level. Here, a series of CdTe samples with different annealing temperatures is investigated with high-resolution hyperspectral cathodoluminescence mapping measured at both room temperature and low temperature on the same microscopic areas. A statistical analysis over a large number of grains is combined with a local analysis at grain boundaries. The results elucidate the dynamic interplay between grain boundary and intragrain defect passivation and formation, in the midst of grain growth. The ${\mathrm{CdCl}}_{2}$ annealing initially contributes to an increase of the grain size and the passivation of both grain boundaries and grain interiors, increasing the overall luminescence and diffusion length. For higher annealing temperatures, a further increase of grain size is counterbalanced by the rise of bulk defects. The results illustrate the tradeoffs that lead to an optimal annealing temperature, as well as new methods for understanding defect passivation and creation in thin film solar cells.

3D/2D passivation as a secret to success for polycrystalline thin-film solar cells

<h2>Summary</h2> 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), CuIn_1-xGa_xSe₂ (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.

Halide Perovskite Single Crystals: Optoelectronic Applications and Strategical Approaches

Halide perovskite is one of the most promising semiconducting materials in a variety of fields such as solar cells, photodetectors, and light-emitting diodes. Lead halide perovskite single crystals featuring long diffusion length, high carrier mobility, large light absorption coefficient and low defect density, have been attracting increasing attention. Fundamental study of the intrinsic nature keeps revealing the superior optoelectrical properties of perovskite single crystals over their polycrystalline thin film counterparts, but to date, the device performance lags behind. The best power conversion efficiency (PCE) of single crystal-based solar cells is 21.9%, falling behind that of polycrystalline thin film solar cells (25.2%). The oversized thickness, defective surfaces, and difficulties in depositing functional layers, hinder the application of halide perovskite single crystals in optoelectronic devices. Efforts have been made to synthesize large-area single crystalline thin films directly on conductive substrates and apply defect engineering approaches to improve the surface properties. This review starts from a comprehensive introduction of the optoelectrical properties of perovskite single crystals. Then, the synthesis methods for high-quality bulk crystals and single-crystalline thin films are introduced and compared, followed by a systematic review of their optoelectronic applications including solar cells, photodetectors, and X-ray detectors. The challenges and strategical approaches for high-performance applications are summarized at the end with a brief outlook on future work.

The History of Photovoltaics with Emphasis on CdTe Solar Cells and Modules

Among thin-film photovoltaic technology, cadmium telluride (CdTe) has achieved a truly impressive development that can commercially compete with silicon, which is still the king of the market. Solar cells made on a laboratory scale have reached efficiencies close to 22%, while modules made with fully automated in-line machines show efficiencies above 18%. This success represents the result of over 40 years of research, which led to effective and consolidated production processes. Based on a large literature survey on photovoltaics and on the results of research developed in our laboratories, we present the fabrication processes of both CdTe polycrystalline thin-film solar cells and photovoltaic modules. The most common substrates, the constituent layers, their interaction, the interfaces and the different “tricks” necessary to obtain highly efficient devices will be analyzed. A realistic industrial production process will be analytically described. Moreover, environmental aspects, end-of-life recycling and the life cycle assessment of CdTe-based modules will be deepened and discussed.

Efficient lateral-structure perovskite single crystal solar cells with high operational stability

The power conversion efficiency of perovskite polycrystalline thin film solar cells has rapidly increased in recent years, while the stability still lags behind due to its low thermal stability as well as the fast ion migration along the massive grain boundaries. Here, stable and efficient lateral-structure perovskite solar cells (PSCs) are achieved based on perovskite single crystals. By optimizing anode contact with a simple surface treatment, the open circuit voltage and fill factor dramatically increase and promote the efficiency of the devices exceeding 11% (0.05 to 1 Sun) compared to that of 5.9% (0.25 Sun) of the best lateral-structure single crystal PSCs previously reported. Devices show excellent operational stability and no degradation observed after 200 h continuous operation at maximum power point under 1 Sun illumination. Devices with scalable architectures are investigated by utilizing interdigital electrodes, which show huge potential to realize low cost and highly efficient perovskite photovoltaic devices.

Toward CdCl2 activation on CdSe thin films for absorber layer applications

The polycrystalline thin film solar cells often possess problem of grain boundaries which lower the efficiency of device, but several mechanisms/treatments are available to passivate these grain boundaries and enhance the performance of device concerned. In cadmium-based devices, the passivation could be undertaken by chloride activation where grain boundaries are passivated along with grain growth. In this report, the physical properties of the CdCl2-activated CdSe thin films are optimized. The CdSe thin films of thickness 550 nm are developed on microscopic glass and indium-doped tin oxide glass substrates employing electron beam evaporation technique followed by post-wet chloride treatment and subsequently annealing in air atmospheric condition at temperature of 170 °C, 320 °C and 470 °C. The structural analysis confirmed the polycrystalline nature with cubic phase of the CdSe films which changed into hexagonal at high temperature. The absorbance increases with the activation and optical energy band gap is found to decrease from 2.03 to 1.71 eV. The electrical analysis shows ohmic nature of the pristine and treated films. The surface morphology demonstrates grain growth with treatment, and the energy-dispersive spectroscopy (EDS) patterns indicate the existence of cadmium, selenium and chlorine elements which ensure deposition and chloride activation to the films. The outcomes designate that CdCl2-passivated CdSe films have appropriate candidature as an absorber layer to the solar cell device.

Cd-Free Cu(In,Ga)(Se,S)2 Thin-Film Solar Cell With Record Efficiency of 23.35%

In this article, the excellent properties of state-of-the-art Cd-free Cu(In,Ga)(Se,S)2 (CIGSSe) solar cells with Zn(O,S,OH) x /Zn0.8Mg0.2O double buffer layers, deposited by a combination of chemical bath deposition and atomic layer deposition techniques, are presented. By the replacement of conventional CdS buffer layers with this double buffer layer, the open-circuit voltage ( V oc) deficit of the devices could be significantly reduced, and V oc increased by approximately 15 mV. In addition, the fill factor and short-circuit current were also improved, increasing the device efficiency by approximately 0.5 absolute percent compared with devices with CdS buffers. The Cd-free double buffer layer improved the device efficiency regardless of the bandgap of the CIGSSe absorber. The minority carrier lifetime ( τ ) measured via time-resolved photoluminescence became longer, indicating that carrier recombination is mitigated using the double buffer layer. Based on the device parameters extracted by fitting the Suns– V oc characteristics to the double-diode model, the longer τ could be attributed to the decreased recombination rate in the space-charge region, rather than in the bulk and at the interface. The best performing cell was evaluated by a reliable third party, the National Institute of Advanced Industrial Science and Technology; this cell achieved a new world record efficiency of 23.35% for 1-cm2-sized thin-film polycrystalline solar cells. The device parameters of this cell are also discussed in this article.

Design of thin film solar cells based on a unified simple analytical model

Polycrystalline thin film solar cells made with absorber materials such as CdTe, CIGS, CZTS and metalorganic halides (perovskites) are currently important alternatives for the silicon solar cell technology, which still dominates the photovoltaic market. Then, it is important to have tools which can be used to design this kind of solar cells. For this purpose, we have developed a unified simple analytical model that can be applied to thin film solar cells. The model is based on the basic physics of hetero-junction devices, but it takes into consideration that the space charge region can extend along the major part of the cell length, particularly for very thin cells, causing important effects that typically are not observed in conventional junction devices. Photo-generated carriers are collected by electric field-drift instead of diffusion, and simultaneously strong recombination at this region may dominate the electrical I–V characteristic of the cell. Since the space-charge region width varies with the applied voltage, the illumination current density and the saturation dark current density are no longer independent of the voltage as is assumed for conventional solar cells. When the model is applied to CIS and CdTe solar cells as examples, it is found that it is possible to design very thin film solar cells (absorber less than 1 μm thick) with high efficiencies, whenever the recombination velocity at the back surface becomes small (102 cm/s), instead of the highrecombination velocities present at ohmic contacts (107 cm/s). This fact implies the cost reduction of thin film solar cells by reducing absorber material thickness, and therefore it poses a challenge to develop deposition methods for very thin CdTe and CIGS absorber materials without pinholes, so that improved efficiencies are obtained when the surface recombination velocity is made small at the back by having a p+ or an electron blocking region before the ohmic contact. This result also explains the high efficiencies achieved by very thin perovskite solar cells.

Heterogeneity at multiple length scales in halide perovskite semiconductors

Materials with highly crystalline lattice structures and low defect concentrations have classically been considered essential for high-performance optoelectronic devices. However, the emergence of high-efficiency devices based on halide perovskites is provoking researchers to rethink this traditional picture, as the heterogeneity in several properties within these materials occurs on a series of length scales. Perovskites are typically fabricated crudely through simple processing techniques, which leads to large local fluctuations in defect density, lattice structure, chemistry and bandgap that appear on short length scales ( 10 μm). Despite these variable and complex non-uniformities, perovskites maintain exceptional device efficiencies and are, as of 2018, the best-performing polycrystalline thin-film solar cell material. In this Review, we highlight the multiple layers of heterogeneity ascertained using high-spatial-resolution methods that provide access to the relevant length scales. We discuss the impact that the optoelectronic variations have on halide perovskite devices, including the prospect that it is this very disorder that leads to their remarkable power-conversion efficiencies. Despite their excellent macroscopic operational parameters, halide perovskites exhibit heterogeneity in materials properties at all lateral and vertical length scales. In this Review, we discuss the nature of heterogeneity in halide perovskites and assess the impact of these non-uniformities on their optoelectronic properties and how the heterogeneity may even be beneficial for device properties.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

Record Efficiency for Thin-Film Polycrystalline Solar Cells Up to 22.9% Achieved by Cs-Treated Cu(In,Ga)(Se,S)2

An efficiency of 22.9% for 1-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -sized Cu(In,Ga)(Se,S) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> solar cells has been independently verified, establishing a record device efficiency for thin-film polycrystalline solar cells. The main improvement in the solar cell device is due to a reduction in the deficit of the open-circuit voltage (Voc), which is notably suppressed by modifying the absorber formation. This is presumably due to reduced defect density, as suggested by the enhanced photoluminescence performance. Such improvement in the absorber quality allowed for an opportunity to benefit from the effects of a wider absorber bandgap. The reverse saturation current density and Voc were significantly improved. Meanwhile, heavier alkali treatment on the absorber surface using cesium was adopted to further boost the device performance. As a result, the significant enhancements in Voc and fill factor led to the achievement of this record-breaking efficiency. These findings have been systematically reproduced and will be leveraged to improve the module performance of Solar Frontier's production in the near future.

Interface Engineering for 25% CdTe Solar Cells

Buffer layers, such as CdS and MgxZn1–xO (MZO), are critical for CdTe and other thin film polycrystalline solar cells. A ternary compound, such as MZO, allows for interface engineering by adjusting...

Separating grain-boundary and bulk recombination with time-resolved photoluminescence microscopy

Two-photon excitation (2PE) microscopy allows contactless and non-destructive cross-sectional analysis of grain-boundary (GB) and grain-interior (GI) properties in polycrystalline solar cells, with measurements of doping uniformity, space-charge field distribution, and carrier dynamics in different regions of the device. Using 2PE time-resolved microscopy, we analyzed charge-carrier lifetimes near the GBs and in the GI of polycrystalline thin-film CdTe solar cells doped with As. When the grain radius is larger than the minority-carrier diffusion length, GI lifetimes are interpreted as the bulk lifetimes τB, and GB recombination velocity SGB is extracted by comparing recombination rates in the GI and near GBs. In As-doped CdTe solar cells, we find τB = 1.0–2.4 ns and SGB = (1–4) × 105 cm/s. The results imply the potential to improve solar cell voltage via GB passivation and reduced recombination center concentration in the GI.

Design of thin film solar cells based on a unified simple analytical model

Abstract Polycrystalline thin film solar cells made with absorber materials such as CdTe, CIGS, CZTS and metalorganic halides (perovskites) are currently important alternatives for the silicon solar cell technology, which still dominates the photovoltaic market. Then, it is important to have tools which can be used to design this kind of solar cells. For this purpose, we have developed a unified simple analytical model that can be applied to thin film solar cells. The model is based on the basic physics of hetero-junction devices, but it takes into consideration that the space charge region can extend along the major part of the cell length, particularly for very thin cells, causing important effects that typically are not observed in conventional junction devices. Photo-generated carriers are collected by electric field-drift instead of diffusion, and simultaneously strong recombination at this region may dominate the electrical I–V characteristic of the cell. Since the space-charge region width varies with the applied voltage, the illumination current density and the saturation dark current density are no longer independent of the voltage as is assumed for conventional solar cells. When the model is applied to CIS and CdTe solar cells as examples, it is found that it is possible to design very thin film solar cells (absorber less than 1 μm thick) with high efficiencies, whenever the recombination velocity at the back surface becomes small (102 cm/s), instead of the high recombination velocities present at ohmic contacts (107 cm/s). This fact implies the cost reduction of thin film solar cells by reducing absorber material thickness, and therefore it poses a challenge to develop deposition methods for very thin CdTe and CIGS absorber materials without pinholes, so that improved efficiencies are obtained when the surface recombination velocity is made small at the back by having a p+ or an electron blocking region before the ohmic contact. This result also explains the high efficiencies achieved by very thin perovskite solar cells.

Thin single crystal perovskite solar cells to harvest below-bandgap light absorption

The efficiency of perovskite solar cells has surged in the past few years, while the bandgaps of current perovskite materials for record efficiencies are much larger than the optimal value, which makes the efficiency far lower than the Shockley–Queisser efficiency limit. Here we show that utilizing the below-bandgap absorption of perovskite single crystals can narrow down their effective optical bandgap without changing the composition. Thin methylammonium lead triiodide single crystals with tuned thickness of tens of micrometers are directly grown on hole-transport-layer covered substrates by a hydrophobic interface confined lateral crystal growth method. The spectral response of the methylammonium lead triiodide single crystal solar cells is extended to 820 nm, 20 nm broader than the corresponding polycrystalline thin-film solar cells. The open-circuit voltage and fill factor are not sacrificed, resulting in an efficiency of 17.8% for single crystal perovskite solar cells.

Charged Grain Boundaries and Carrier Recombination in Polycrystalline Thin-Film Solar Cells

We present analytical relations for the dark recombination current of a $pn^+$ junction with positively charged columnar grain boundaries in the high defect density regime. We consider two defect state configurations relevant for positively charged grain boundaries: a single donor state and a continuum of both acceptors and donors. Compared to a continuum of acceptor+donor states, or to the previously studied single acceptor+donor state, the grain boundary recombination of a single donor state is suppressed by orders of magnitude. We show numerically that superposition holds near the open-circuit voltage $V_{\rm oc}$, so that our dark $J(V)$ relations determine $V_{\rm oc}$ for a given short circuit current $J_{\rm sc}$. We finally explicitly show how $V_{\rm oc}$ depends on the grain boundary defect state configurations.