13·9%‐efficient CdTe polycrystalline thin‐film solar cells with an infrared transmission of ∼50%

To fabricate a high‐efficiency polycrystalline thin‐film tandem cell, the most critical work is to make a high‐efficiency top cell ( > 15%) with high bandgap (E_g = 1·5–1·8 eV) and high transmission (T > 70%) in the near‐infrared (NIR) wavelength region. The CdTe cell is one of the candidates for the top cell, because CdTe state‐of‐the‐art single‐junction devices with efficiencies of more than 16% are available, although its bandgap (1·48 eV) is slightly lower for a top cell in a current‐matched dual‐junction device. In this paper, we focus on the development of a: (1) thin, low‐bandgap Cu_xTe transparent back‐contact; and (2) modified CdTe device structure, including three novel materials: cadmium stannate transparent conducting oxide (TCO), ZnSnO_x buffer layer, and nanocrystalline CdS:O window layer developed at NREL, as well as the high‐quality CdTe film, to improve transmission in the NIR region while maintaining high device efficiency. We have achieved an NREL‐confirmed 13·9%‐efficient CdTe transparent solar cell with an infrared transmission of ∼50% and a CdTe/CIS polycrystalline mechanically stacked thin‐film tandem cell with an NREL‐confirmed efficiency of 15·3%. Copyright © 2005 John Wiley & Sons, Ltd.     

CuxS背接触层制备及在碲化镉薄膜太阳电池中应用研究

本文采用化学水浴法沉积CuxS薄膜,通过改变Cu元素比例研究其对碲化镉电池效率的影响。研究表明化学水浴法沉积的CuxS是非晶的,采用适当退火条件可以使其晶化,随着退火温度的提高,薄膜变得致密且结晶明显。CuxS薄膜厚度对电池性能有很大的影响,结果表明,随着CuxS薄膜厚度增加,电池性能先增加后减少。薄膜厚度为75nm时,CdS/CdTe电池性能最佳,达到了最高转化效率(η)为12.19%,填充因子(FF)为68.82%,开路电压(Voc)为820mV。     

Copper migration in cdte heterojunction solar cells

CdTe solar cells were fabricated by depositing a Au/Cu contact with Cu thickness in the range of 50 to 150Å on polycrystalline CdTe/CdS/SnO₂/glass structures. The increase in Cu thickness improves ohmic contact and reduces series resistance (R_s), but the excess Cu tends to diffuse into CdTe and lower shunt resistance (R_sh) and cell performance. Light I-V and secondary ion mass spectros-copy (SIMS) measurements were performed to understand the correlations between the Cu contact thickness, the extent of Cu incorporation in the CdTe cells, and its impact on the cell performance. The CdTe/CdS/SnO₂/glass, CdTe/ CdS/GaAs, and CdTe/GaAs structures were prepared in an attempt to achieve CdTe films with different degrees of crystallinity and grain size. A large grain polycrystalline CdTe thin film solar cell was obtained for the first time by selective etching the GaAs substrate coupled with the film transfer onto a glass substrate. SIMS measurement showed that poor crystallinity and smaller grain size of the CdTe film promotes Cu diffusion and decreases the cell performance. Therefore, grain boundaries are the main conduits for Cu migration and larger CdTe grain size or alternate method of contact formation can mitigate the adverse effect of Cu and improve the cell performance.     

薄膜太阳电池研究综述

薄膜太阳电池是最具发展潜力的新型能源之一,对缓解能源危机、保护人类生存环境提供了一种新的切实可行的方法。综述了目前国际上研究较多的几种薄膜太阳电池的最新进展,包括硅基薄膜(非晶硅、多晶硅)、多元化合物类(碲化镉、铜铟硒、铜铟镓硒、铜锌锡硫等)、有机薄膜太阳电池以及染料敏化太阳电池等。分析并总结了其在成本、转换效率等方面的优劣。为更有效地降低成本及提高电池效率,新技术、新结构的不断创新应该是未来薄膜太阳电池的发展趋势。     

Electrical contacts for II-VI semiconducting devices

High resistivity II-VI semiconductors in general and CdTe and its associated materials like CdZnTe and CdMnTe in particular are suffering from ohmic contacting problem due to their high electron affinity and consequently large work function. Ni, Au, Pt and Pd have large work function and have possibility to match with the above materials. However, except Ni other materials have problems in large-scale commercial applications. In order to overcome the ohmic contacting problem to these semiconductors the following studies has been conducted in the text of the present paper. These are: (i) Work function engineering to modulate the work function through combination materials like, Cu, Au, Mo, W and Co. (ii) Introduction of a charge diluting intermediate semiconducting layer media in between the metal and CdTe to neutralize the bound polarized charges. These two aspects were applied in case of thin film CdTe-CdS solar cells to evaluate their contacting performances and their influences in solar cell parameters. Both cell performances and the contact characteristics of these contacting technologies were studied at depth and indicated their applicability in semiconducting devices.     

太阳电池中CdS薄膜的制备及其性能的研究

分别采用化学池沉积(CBD)和真空蒸发法,在三种衬底(玻片、ITO玻片、SnO2玻片)上沉积CdS薄膜,并利用扫描电镜(SEM)、透射光谱、X射线衍射(XRD)等方法对沉积膜进行了测试分析,同时阐述了两种不同方法下CdS膜的生长沉积机制。     

Copper (I) Oxide (Cu2O) based back contact for p‐i‐n CdTe solar cells

In this paper a promising solution for the notorious problem of manufacturing a stable low ohmic back contact of a CdTe thin film superstrate solar cell is presented without using elemental copper. Instead we have used a Cu₂O layer inserted between the CdTe absorber and metal contact (Au). In contrast to the barrier free band alignment gained by using the transitivity rules, XPS measurements show a barrier in the valence band of the Cu₂O layers directly after deposition, which results in a low performing JV curve. The contact can be improved by a short thermal treatment resulting in efficiencies superior to copper based contacts for standard CdS/CdTe hetero junction solar cells prepared on commercial glass/FTO substrates. By replacing the CdS window layer with a CdS:O buffer layer efficiencies of >15% could be achieved. Copyright © 2016 John Wiley & Sons, Ltd.     

Thin-film solar cells

The R&D status of cells and modules based on hydrogenated amorphous silicon (a-Si:H) and those based on CdTe and CuInSe₂ is reviewed. The stability of a-Si:H solar cells is still a major concern. Improvements have been achieved on an empirical basis by application of multijunction structures, optimization of interfaces, etc. Stabilized efficiencies of close to 10% have been reported. In parallel, the introduction of the ‘defect-pool model’ led to remarkable progress in understanding; it follows that a-SiGe:H instead of a-Si:H should be used for the i-layer (absorber). Improved cell engineering concepts, however, such as enhancement of the built-in electric field via reduction of the i-layer thickness and/or folded structures, are believed to be more promising. Polycrystalline thin-film cells based on CdTe and CuInSe₂ are not affected by inherent degradation mechanisms. the specific properties of these materials demand heterojunctions, and particular problems arise due to the polycrystallinity of the films and to the lattice mismatch and mismatch of the electronic band structures of the materials involved. These are discussed in conjunction with measures currently applied for optimizing solar cell performance. Both cell types exhibit eficiencies in the range 16-17%. Estimations of production costs and energy payback times of thin-film photovoltaic modules are reviewed (even below 1 US$ W_p⁻¹ and as low as 4 months, respectively) and environmental concerns, especially for Cd-containing cells, are summarized.     

The formation of large-grain CulnSe2 films by selenization by high-rate Se transport under moderate vacuum conditions

A novel method for forming large-grain CuInSe₂ thin films by the reaction of In and Cu with Se inside a sealed ampoule is presented. A Se source and a selenization precursor consisting of a co-evaporated In+Cu film deposited onto an alumina substrate were placed at opposite ends of a closed quartz crucible sealed at 100 Torr. By establishing a small thermal gradient between the ends of the ampoule, Se was transported to the precursor where selenization of the In-Cu layer occurred. The maximum substrate temperature used was 475°C. Resulting film microstructures were extremely well formed, consisting of 3 to 5 μm grains of densely packed CuInSe₂ crystallites. It is suggested that such enhanced grain growth may be the result of the preferential formation of the Serich liquid, L3, under conditions of high Se flux. Such a mechanism may be exploited for fabricating large-grain CuInSe₂ films at temperatures compatible with substrates-of-choice in solar cell manufacturing.     

Evaluating the economic viability of CdTe/CIS and CIGS/CIS tandem photovoltaic modules

We analyze the potential cost competitiveness of two frameless, glass–glass thin‐film tandem photovoltaic module structures, cadmium telluride (CdTe)/CuInSe₂ (CIS) and CuIn_0.3Ga_0.7Se₂ (CIGS)/CIS, based on the demonstrated cost of manufacturing the respective component cell technologies in high volume. To consider multiple economic scenarios, we base the CdTe/CIS module efficiency on the current industrial production of CdTe modules, while for CIGS/CIS, we use an aspirational estimate for CIGS efficiency. We focus on four‐terminal mechanically stacked structures, thus avoiding the need to achieve current matching between the two cells. The top cell in such a tandem must have a transparent back contact, which has not been successfully implemented to date. However, for the purpose of understanding the economic viability of both tandems, we assume that this can be implemented at a cost similar to that of sputtered indium tin oxide. The cost of both tandem module structures was found to be nearly identical on an equal‐area basis and approximately $30/m² higher than the single‐junction alternatives. Both tandem modules are about 4% (absolute) more efficient than a module by using the top‐cell material alone. We find that these tandem modules might reduce total system cost by as much as 11% in applications having a high area‐related balance‐of‐system cost, such as area‐constrained residential systems; however, the relative advantage of tandems decreases in the cases where balance‐of‐system costs are lower, such as in commercial and utility scale systems. Copyright © 2017 John Wiley & Sons, Ltd.     

Comparison of indoor and outdoor performance measurements of recent commercially available solar modules

The strong growth of the PV market is accompanied by an increasing number of “new” PV technologies and concepts now mature for commercialization. A correct calibration of these devices is in some cases very difficult, because indoor and outdoor performance measurements often lead to different results. In this paper we compare the indoor and outdoor performance measurements of a set of recent commercially available PV modules (conventional and high‐efficiency c‐Si, single‐, double‐, and triple‐junction thin film (TF) technologies) and we observe that the maximum power P_max of some devices measured indoors using our large area pulsed solar simulator is usually lower than the power measured outdoors under natural sunlight. The major effects which lead to these discrepancies are identified, as follows: (a) spectral mismatch errors, very significant for CdTe, and all a‐Si TF technologies; (b) measurement‐related sweep‐time effects, which seem to strongly influence the performance of high efficiency c‐Si devices and to a lesser extend of all a‐Si TF technologies; and (c) short‐time light‐soaking effects, which influence the performance of CIS and to a lesser extent CdTe. Copyright © 2010 John Wiley & Sons, Ltd.     

Absorptivity of semiconductors used in the production of solar cell panels

The dependence of the absorptivity of semiconductors on the thickness of the absorbing layer is studied for crystalline silicon (c-Si), amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium diselenide (CuInSe₂, CIS), and copper gallium diselenide (CuGaSe₂, CGS). The calculations are performed with consideration for the spectral distribution of AM1.5 standard solar radiation and the absorption coefficients of the materials. It is shown that, in the region of wavelengths λ = λ _g = hc/E _g , almost total absorption of the photons in AM1.5 solar radiation is attained in c-Si at the thickness d = 7−8 mm, in a-Si at d = 30–60 μm, in CdTe at d = 20−30 μm, and in CIS and CGS at d = 3−4 μm. The results differ from previously reported data for these materials (especially for c-Si). In previous publications, the thickness needed for the semiconductor to absorb solar radiation completely was identified with the effective light penetration depth at a certain wavelength in the region of fundamental absorption for the semiconductor.     

Chemical incorporation of copper into indium selenide thin‐films for processing of CuInSe2 solar cells

A chemical method of incorporating copper into indium selenide thin‐films has been investigated, with the goal of creating a precursor structure for conversion into CuInSe₂ (CIS) layers suitable for solar cell processing. The precursor and converted layers have been investigated with scanning electron microscopy (SEM), X‐ray diffraction (XRD), Raman spectroscopy and X‐ray photoelectron spectroscopy (XPS). From these measurements, the incorporation of copper into the indium selenide layers is concluded to proceed by an ion‐exchange reaction. This reaction results in the formation of a precursor layer with a graded compositional depth‐profile containing the crystalline phases In₂Se₃ and Cu_2−xSe. Selenisation of the precursor layer homogenises the composition and forms chalcopyrite CIS. These CIS layers exhibit a dense microstructure with rough surface morphology, which is ascribed to a non‐optimal selenisation process. Solar cells with the structure ZnO: Al/i‐ZnO/CdS/CIS/Mo/glass have been processed from the selenised layers and have exhibited efficiencies of up to 4% under simulated AM1·5 illumination. Copyright © 2008 John Wiley & Sons, Ltd.     

A system dynamics model of tellurium availability for CdTe PV

The routine availability of key component materials has been highlighted as a potential constraint to both extensive deployment and reduction in production costs of thin‐film photovoltaic (PV) technologies. This paper examines the effect of material availability on the maximum potential growth of thin‐film PV by 2050 using the case of tellurium (Te) in cadmium telluride (CdTe) PV, currently the dominating thin‐film technology with the lowest manufacturing cost. The use of system dynamics (SD) modelling allows for a dynamic treatment of key Te supply features and prospects for reductions in PV demand via material efficiency improvements, as well as greater transparency and a better understanding of future recycling potential. The model's projections for maximum Te‐constrained CdTe PV growth by 2050 are shown to be higher than a number of previous studies using static assumptions—suggesting that a dynamic treatment of the resource constraints for CdTe inherently improves the outlook for future deployment of this technology. In addition, the sensitivity analysis highlights certain complex correlations between the maximum potential CdTe growth by 2050 and the rated lifetime of PV modules as well as the reported size of global Te resources. The highest observed sensitivities are to the recovery rate of Te from copper anode slimes, the active layer thickness, the module efficiency and the utilisation rate of Te during manufacturing, all of which are highlighted as topics for further research. Copyright © 2013 John Wiley & Sons, Ltd.     

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Diode characteristics in state‐of‐the‐art ZnO/CdS/Cu(In1−xGax)Se2 solar cells

We report a new state of the art in thin‐film polycrystalline Cu(In,Ga)Se₂‐based solar cells with the attainment of energy conversion efficiencies of 19·5%. An analysis of the performance of Cu(In,Ga)Se₂ solar cells in terms of some absorber properties and other derived diode parameters is presented. The analysis reveals that the highest‐performance cells can be associated with absorber bandgap values of ∼1·14 eV, resulting in devices with the lowest values of diode saturation current density (∼3×10⁻⁸ mA/cm²) and diode quality factors in the range 1·30 < A < 1·35. The data presented also support arguments of a reduced space charge region recombination as the reason for the improvement in the performance of such devices. In addition, a discussion is presented regarding the dependence of performance on energy bandgap, with an emphasis on wide‐bandgap Cu(In,Ga)Se₂ materials and views toward improving efficiency to > 1;20% in thin‐film polycrystalline Cu(In,Ga)Se₂ solar cells. Published in 2005 John Wiley & Sons, Ltd.     

Performance Comparison Between Polycrystalline Thin-film and Single-crystal Solar Cells

Recent progress has increased the efficiency of poly crystalline thin-film solar cells to roughly two-thirds that of single-crystal cells of comparable bandgap. The largest remaining difference is the excessive forward recombination current, which is larger by a factor of 20 for the best CuInSe₂-based cells, and by more than a factor of 100 for CdTe cells. Collection loss, related to diffusion length, is nearly as important for the CuInSe₂-based cells. It is an open question how much additional reduction in these two tosses should he anticipated. However, other losses in thin-film cells due to window-layer absorption, reflection and grid coverage will almost certainly be reduced, so that efficiencies equal to 80% of crystalline counterparts should be a viable 5-year target.     

High-Efficiency Polycrystalline CdS/CdTe Solar Cells on Buffered Commercial TCO-Coated Glass

Multiple polycrystalline CdS/CdTe solar cells with efficiencies greater than 15% were produced on buffered, commercially available Pilkington TEC Glass at EPIR Technologies, Inc. (EPIR, Bolingbrook, IL) and verified by the National Renewable Energy Laboratory (NREL). n-CdS and p-CdTe were grown by chemical bath deposition (CBD) and close space sublimation, respectively. Samples with sputter-deposited CdS were also investigated. Initial results indicate that this is a viable dry-process alternative to CBD for production-scale processing. Published results for polycrystalline CdS/CdTe solar cells with high efficiencies are typically based on cells using research-grade transparent conducting oxides (TCOs) requiring high-temperature processing inconducive to low-cost manufacturing. EPIR’s results for cells on commercial glass were obtained by implementing a high-resistivity SnO₂ buffer layer and by optimizing the CdS window layer thickness. The high-resistivity buffer layer prevents the formation of CdTe-TCO junctions, thereby maintaining a high open-circuit voltage and fill factor, whereas using a thin CdS layer reduces absorption losses and improves the short-circuit current density. EPIR’s best device demonstrated an NREL-verified efficiency of 15.3%. The mean efficiency of hundreds of cells produced with a buffer layer between December 2010 and June 2011 is 14.4%. Quantum efficiency results are presented to demonstrate EPIR’s progress toward NREL’s best-published results.     

Perspectives of crystalline Si thin film solar cells: a new era of thin monocrystalline Si films?

A large number of competing approaches are currently being investigated around the world to develop crystalline silicon thin film solar cells on foreign substrates. These approaches can be broadly classified according to the crystalline state of the Si films employed: (i) thin film solar cells based on nano‐ or microcrystalline Si‐films; (ii) cells fabricated from large‐grained polycrystalline Si and (iii) recent approaches utilizing the transfer of monocrystalline Si films. The paper discusses prospects and limitations of these approaches and describes device results based on the transfer of quasi‐monocrystalline Si films. Using Si absorber films epitaxially grown on quasi‐monocrystalline Si, we achieve a conversion efficiency of 13˙6% for a 4 cm² sized thin film solar cell on glass. In contrast to the limited performance of polycrystalline Si thin film solar cells imposed by the presence of grain boundaries, transfer approaches are expected to result in thin film solar cell efficiencies in the range of 15 – 18% depending on process maturity and complexity. The transfer of monocrystalline Si films therefore opens a new avenue to an efficient and competitive Si‐based thin film technology. Copyright © 2000 John Wiley & Sons, Ltd     

Potential und Chancen der Photovoltaik aus industrieller Sicht

Photovoltaic (PV) energy production is an established and reliable technology. From the cost perspective, PV installations today are mainly operated economically in non-grid connected applications. In order to improve the cost situation, the present silicon wafer technology will be further developed for ultrathin tricrystalline cells. In the next step the follow-up generation of chalkopyrite-based CIS thin film solar cells aims at low-cost mass production.     

Studies of the micro- and nanostructure of polycrystalline CdTe and CuInSe2 using atomic force and scanning tunneling microscopy

Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) have been used to study the micro- and nanostructure of CdTe and CuInSe₂ thin films used for photovoltaic cells. Topographic images are comparable with those reported previously using conventional scanning electron microscopy (SEM)—to the limit of spatial resolution of the SEM technique. For higher magnifications, nanoscale structures and features have been observed for the first time with AFM and STM, and these observations have implications for the suitability and preparation of these semiconductors for high-efficiency solar cell realization.