Device Performance of Emerging Photovoltaic Materials (Version 3)

Following the 2nd release of the “Emerging PV reports,” the best achievements in the performance of emerging photovoltaic devices in diverse emerging photovoltaic research subjects are summarized, as reported in peer-reviewed articles in academic journals since August 2021. Updated graphs, tables, and analyses are provided with several performance parameters, e.g., power conversion efficiency, open-circuit voltage, short-circuit current density, fill factor, light utilization efficiency, and stability test energy yield. These parameters are presented as a function of the photovoltaic bandgap energy and the average visible transmittance for each technology and application, and are put into perspective using, e.g., the detailed balance efficiency limit. The 3rd installment of the “Emerging PV reports” extends the scope toward triple junction solar cells.     

Defect Engineering in Multinary Earth‐Abundant Chalcogenide Photovoltaic Materials

Application of zinc‐blende‐related chalcogenide absorbers such as CdTe and Cu(In,Ga)Se₂ (CIGSe) has enabled remarkable advancement in laboratory‐ and commercial‐scale thin‐film photovoltaic performance; however concerns remain regarding the toxicity (CdTe) and scarcity (CIGSe/CdTe) of the constituent elements. Recently, kesterite‐based Cu₂ZnSn(S,Se)₄ (CZTSSe) materials have emerged as attractive non‐toxic and earth‐abundant absorber candidates. Despite the similarities between CZTSSe and CIGSe/CdTe, the record power conversion efficiency of CZTSSe solar cells (12.6%) remains significantly lower than that of CIGSe (22.6%) and CdTe (22.1%) devices, with the performance gap primarily being attributed to cationic disordering and associated band tailing. To capture the promise of kesterite‐like materials as prospective “drop‐in” earth‐abundant replacements for closely‐related CIGSe, current research has focused on several key directions to control disorder, including: (i) examination of the interaction between processing conditions and atomic site disorder, (ii) isoelectronic cation substitution to introduce ionic size mismatch, and (iii) structural diversification beyond the zinc‐blende‐type coordination environment. In this review, recent efforts targeting accurate identification and engineering of anti‐site disorder in kesterite‐based CZTSSe are considered. Lessons learned from CZTSSe are applied to other complex chalcogenide semiconductors, in an effort to develop promising pathways to avoid anti‐site disordering and associated band tailing in future high‐performance earth‐abundant photovoltaic technologies.     

Imidazolium‐Substituted Polythiophenes as Efficient Electron Transport Materials Improving Photovoltaic Performance

In the field of polymer solar cells, improving photovoltaic performance has been the main driver over the past decade. To achieve high power conversion efficiencies, a plethora of new photoactive donor polymers and fullerene derivatives have been developed and blended together in bulk heterojunction active layers. Simultaneously, further optimization of the device architecture is also of major importance. In this respect, we report on the use of specific types of electron transport layers to boost the inherent I–V properties of polymer solar cell devices, resulting in a considerable gain in overall photovoltaic output. Imidazolium‐substituted polythiophenes are introduced as appealing electron transport materials, outperforming the currently available analogous conjugated polyelectrolytes, mainly by an increase in short‐circuit current. The molecular weight of the ionic polythiophenes has been identified as a crucial parameter influencing performance.     

Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. Thus, understanding of spatial variations in composition as well as electrical properties of OPV materials is of paramount importance for moving PV technology forward.^1,2 In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution. Currently, materials properties measurements performed using commercially available AFM-based techniques (PeakForce, conductive AFM) generally provide only qualitative information. The values for resistance as well as Young''s modulus measured using our method on the prototypical ITO/PEDOT:PSS/P3HT:PC₆₁BM system correspond well with literature data. The P3HT:PC₆₁BM blend separates onto PC₆₁BM-rich and P3HT-rich domains. Mechanical properties of PC₆₁BM-rich and P3HT-rich domains are different, which allows for domain attribution on the surface of the film. Importantly, combining mechanical and electrical data allows for correlation of the domain structure on the surface of the film with electrical properties variation measured through the thickness of the film.     

Bandgap Engineering Enhances the Performance of Mixed‐Cation Perovskite Materials for Indoor Photovoltaic Applications

Indoor photovoltaics (IPVs) are attracting renewed interest because they can provide sustainable energy through the recycling of photon energy from household lighting facilities. Herein, the Shockley–Queisser model is used to calculate the upper limits of the power conversion efficiencies (PCEs) of perovskite solar cells (PeSCs) for two types of artificial light sources: fluorescent tubes (FTs) and white light–emitting diodes (WLEDs). An unusual zone is found in which the dependence of the PCEs on the bandgap (E_g) under illumination from the indoor lighting sources follows trends different from that under solar irradiation. In other words, IPVs exhibiting high performance under solar irradiation may not perform well under indoor lighting conditions. Furthermore, the ideal bandgap energy for harvesting photonic power from these indoor lighting sources is ≈1.9 eV—a value higher than that of common perovskite materials (e.g., for CH₃NH₃PbI₃). Accordingly, Br^? ions are added into the perovskite films to increase their values of E_g. A resulting PeSC featuring a wider bandgap exhibits PCEs of 25.94% and 25.12% under illumination from an FT and a WLED, respectively. Additionally, large‐area (4 cm²) devices are prepared for which the PCE reaches ≈18% under indoor lighting conditions.     

Photovoltaic Materials and Devices Based on the Alloyed Kesterite Absorber (AgxCu1–x)2ZnSnSe4

The photovoltaic absorber Cu₂ZnSn(S_xSe_1–x)₄ (CZTSSe) has attracted interest in recent years due to the earth‐abundance of its constituents and the realization of high performance (12.6% efficiency). The open‐circuit voltage in CZTSSe devices is believed to be limited by absorber band tailing caused by the exceptionally high density of Cu/Zn antisites. By replacing Cu in CZTSSe with Ag, whose covalent radius is ≈15% larger than that of Cu and Zn, the density of I–II antisite defects is predicted to drop. The fundamental properties of the mixed Ag‐Cu kesterite compound are reported as a function of the Ag/(Ag + Cu) ratio. The extent of band tailing is shown to decrease with increasing Ag. This is verified by comparing the optical band gap extrapolated from transmission data with the position of the room‐temperature photoluminescence peak; these values converge for the pure‐Ag compound. Additionally, the pinning of the Fermi level in CZTSSe, attributed to heavy defect compensation and band tailing, is not observed in the pure‐Ag compound, offering further evidence of improved electronic structure. Finally, a device efficiency of 10.2% is reported for a device containing 10% Ag (no antireflection coating); this compares to ≈9% (avg) efficiency for the baseline pure‐Cu CZTSe.     

Cyclopenta[c]thiophene‐4,6‐dione‐Based Copolymers as Organic Photovoltaic Donor Materials

A class of “push‐pull” conjugated copolymers based on cyclopenta[c]thiophene‐4,6‐dione (CTD) and benzodithiophene (BDT) is synthesized for application as an electron donor in organic photovoltaics (OPV). Given the diverse electronic and structural tunability of the CTD unit, specific CTD‐containing copolymers are chosen with the aid of theoretical calculations from a broad array of potential candidate materials. Evaluation of the chosen materials as OPV absorbers includes characterization of the optical, electronic, and structural properties of the polymer films using UV‐vis absorbance, photoluminescence, cyclic voltammetry, and X‐ray diffraction techniques. In addition, the contactless time‐resolved microwave conductivity (TRMC) technique is used to measure the photoconductance of polymer/fullerene blends. Excellent correlation between measured photoconductance and OPV device efficiency is demonstrated with these materials and TRMC is discussed as a tool for screening potential active layer materials for OPV devices.     

中国茜草科资料之二

在编研《中国植物志》茜草科的过程中,对龙船花属（Ｉｘｏｒａ）植物作进一步检查和整理,共发现６个新分类群Ｉｘｏｒａｉｎｓｉｇｎｉｓ,Ｉ．ａｍｐｌｅｘｉｃａｕｌｉｓ,Ｉ．ａｕｒｉｃｕｌａｒｉｓ,Ｉ．ｐａｒａｏｐａｒａ,Ｉ．ｆｏｎｃｈｅｗｉ,Ｉ．ｇｒａｃｉｌｉｓ;新种的模式标本保存在中国科学院华南植物研究所标本馆（ＩＢＳＣ）。     

Photovoltaic properties of three new cyanine dyes for dye-sensitized solar cells.

Three carboxylated cyanine dyes, 2-[(1-butyl-3,3-dimethyl-5-carboxylindoline-2-ylidene)propenyl]-[1-butyl-3,3-dimethyl-7-(1-ethyl-1H-1,2,3-triazole-4-yl]-1H-benz[e]indolium iodide (), 2-[(1-butyl-3,3-dimethyl-5-carboxyl-indoline-2-ylidene)propenyl]-{1-butyl-3,3-dimethyl-7-[(4-piperidine-N-ethyl-1,8-naphthalimide)-1H-1,2,3-triazole-4-yl]}-1H-benz[e]indolium iodide (Cy2) and 2-[(1-butyl-3,3-dimethyl-5-carboxyl-indoline-2-ylidene)propenyl)-[1-butyl-3,3-dimethyl-7-{(4-piperidine-N-butyl-1,8-naphthalimide)-1H-1,2,3-triazole-4-yl}]-1H-benz[e]indolium iodide (Cy3), have been synthesized and characterized with regard to their structures and electrochemical properties. Upon adsorption onto a TiO(2) electrode, the absorption spectra of the three cyanine dyes are all broadened to both red and blue sides compared with their respective spectra in an acetonitrile and ethanol mixture. Cy2 and Cy3, containing a naphthalimide group, have stronger absorption intensities and broader absorption spectra than , which consequently leads to better light-to-electricity conversion properties. Among the three cyanine dyes, generated the highest photoelectric conversion yield of 4.80% (J(sc) = 14.5 mA cm(-2), V(oc) = 500 mV, FF = 0.49) under illumination with 75 mW cm(-2) white light from a Xe lamp.     

Improved Black Silicon for Photovoltaic Applications

The morphology and the electronic properties of monocrystalline Si (c‐Si) with a nano‐textured “black” surface, obtained by a metal‐catalyzed wet etching process, and the improvement by an additional chemical treatment are examined with regard to solar cell applications. Photoluminescence and optical reflectivity measurements show the presence of a nano‐porous Si (np‐Si) phase in the as‐prepared nano‐texture. It is found that an additional wet chemical treatment with the standard clean 1 of the common RCA cleaning process removes the np‐Si fraction and significantly alters the surface of the nano‐structure. Cross‐sectional scanning electron microscopy images reveal a pronounced reduction of the surface area, to values of only 3–6 times that of a planar surface. Electron spin resonance measurements were performed to investigate the type and quantity of defects induced by the nano‐texturing process. The optimized nano‐texture exhibits a Si dangling bond density comparable to planar c‐Si wafers. Electrically detected magnetic resonance spectra reveal an additional paramagnetic defect present in the nano‐textured Si, linked to a hydrogen‐ or oxygen‐related double donor. In addition, initial results on the passivation of surface defects via atomic layer deposition of Al₂O₃ are presented. Photoconductance decay measurements of passivated samples show a tenfold increase of the effective lifetime for nano‐textures which have received the additional etching treatment. The improved electronic quality of the nano‐textured surface makes it an interesting candidate for application as an anti‐reflection surface in solar cells.     

Economic Feasibility of Recycling Photovoltaic Modules

The market for photovoltaic (PV) electricity generation has boomed over the last decade, and its expansion is expected to continue with the development of new technologies. Taking into consideration the usage of valuable resources and the generation of emissions in the life cycle of photovoltaic technologies dictates proactive planning for a sound PV recycling infrastructure to ensure its sustainability. PV is expected to be a “green” technology, and properly planning for recycling will offer the opportunity to make it a “double‐green” technology—that is, enhancing life cycle environmental quality. In addition, economic feasibility and a sufficient level of value‐added opportunity must be ensured, to stimulate a recycling industry. In this article, we survey mathematical models of the infrastructure of recycling processes of other products and identify the challenges for setting up an efficient one for PV. Then we present an operational model for an actual recycling process of a thin‐film PV technology. We found that for the case examined with our model, some of the scenarios indicate profitable recycling, whereas in other scenarios it is unprofitable. Scenario SC4, which represents the most favorable scenario by considering the lower bounds of all costs and the upper bound of all revenues, produces a monthly profit of $107,000, whereas the least favorable scenario incurs a monthly loss of $151,000. Our intent is to extend the model as a foundation for developing a framework for building a generalized model for current‐PV and future‐PV technologies.     

Batch‐to‐Batch Variation of Polymeric Photovoltaic Materials: its Origin and Impacts on Charge Carrier Transport and Device Performances

A detailed investigation of the impact of molecular weight distribution of a photoactive polymer, poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT), on photovoltaic device performance and carrier transport properties is reported. It is found that different batches of as‐received polymers have substantial differences in their molecular weight distribution. As revealed by gel permeation chromatography (GPC), two peaks can generally be observed. One of the peaks corresponds to a high molecular weight component and the other peak corresponds to a low molecular weight component. Photovoltaic devices fabricated with a higher proportion of low molecular weight component have power conversion efficiencies (PCEs) reduced from 5.7% to 2.5%. The corresponding charge carrier mobility at the short‐circuit region is also significantly reduced from 2.7 × 10^?5 to 1.6 × 10^?8 cm² V^?1 s^?1. The carrier transport properties of the polymers at various temperatures are further analyzed by the Gaussian disorder model (GDM). All polymers have similar energetic disorders. However, they appear to have significant differences in carrier hopping distances. This result provides insight into the origin of the molecular weight effect on carrier transport in polymeric semiconducting materials.