Chen Weijie

The CZTSe surface is sulfurized at room temperature. After etching in concentrated ammonia solution followed by (NH₄)₂S vapor modification, the surface defects are passivated and the band alignment at the CZTSe/CdS interface is modified. The efficiency of the solar cell is increased to 9.04% with the (NH₄)₂S vapor treatment for 25 min.

The interfaces are very important for kesterite‐structured solar cells. In this study, a facile room temperature chemical sulfurization process is developed to modify the surface of the CZTSe films, which can prevent the decomposition of kesterite film at high temperature during the traditional sulfurization process and thus introducing the deep defects in the absorber layer. It is found that the (NH₄)₂S vapor sulfurizes the surfaces of the CZTSe thin films previously etched by ammonia. Consequently, the surface defects are passivated by the incorporated sulfur, and the interfacial band alignment is improved at the junction. The open circuit voltage (V _OC) of the solar cell device is improved from 364 mV to 406 mV, with the cell efficiency increased to 9.04% when the (NH₄)₂S vapor treatment time is optimized at 25 min. This study affords a new perspective for enhancing the performance of kesterite‐based thin film solar cells using a facile surface sulfurization approach.

Solution‐processed metal oxide nanocrystals present unique properties as efficient carrier transport layers in photovoltaic devices. In this review, solution‐processed metal oxide nanocrystal‐based carrier transport layers in organic solar cells and perovskite solar cells, and their low‐temperature solution‐processed synthesis approaches are summarized.

Abstract

There has been rapid progress in solution‐processed organic solar cells (OSCs) and perovskite solar cells (PVSCs) toward low‐cost and high‐throughput photovoltaic technology. Carrier (electron and hole) transport layers (CTLs) play a critical role in boosting their efficiency and long‐time stability. Solution‐processed metal oxide nanocrystals (SMONCs) as a promising CTL candidate, featuring robust process conditions, low‐cost, tunable optoelectronic properties, and intrinsic stability, offer unique advantages for realizing cost‐effective, high‐performance, large‐area, and mechanically flexible photovoltaic devices. In this review, the recent development of SMONC‐based CTLs in OSCs and PVSCs is summarized. This paper starts with the discussion of synthesis approaches of SMONCs. Then, a broad range of SMONC‐based CTLs, including hole transport layers and electron transport layers, are reviewed, in which an emphasis is placed on the improvement of the efficiency and device stability. Finally, for the better understanding of the challenges and opportunities on SMONC‐based CTLs, several strategies and perspectives are outlined.

An ultrathin ferroelectric oxide PbTiO₃ layer is incorporated between the electron transport material and the halide perovskite in the hole transport material (HTM) free carbon‐based perovskite solar cell (C‐PSCs). The achieved power conversion efficiency is as high as 16.37%, which is the highest record for HTM‐free C‐PSCs to date, mainly ascribable to the ferroelectric layer enhanced open circuit voltage.

Abstract

The hole transport material (HTM) free carbon based perovskite solar cells (C‐PSCs) are promising for its manufactural simplicity, but they currently suffer from low power conversion efficiencies (PCE) largely because of the voltage loss. Here, a new strategy to increase the PCE by incorporating an ultrathin ferroelectric oxide PbTiO₃ layer between the electron transport material and the halide perovskite is reported. The resulting C‐PSCs have achieved PCEs up to 16.37%, which is the highest record for HTM‐free C‐PSCs to date, mainly ascribable to the ferroelectric layer enhanced open circuit voltage. Detail measurements and analysis show an enhanced built‐in potential in the C‐PSCs as well as suppression of the non‐radiative recombination due to the ferroelectric PbTiO₃ layer incorporation, accounting for the boosted V _OC and photovoltaic performance.

A new method to control current–voltage hysteresis of planar structured FA_0.83Cs_0.17PbI₃ perovskite solar cells (PSCs) is presented while enhancing device efficiency through tailoring the crystal structure of the perovskite compound with a guanidinium cation (Gu⁺)‐dopant. New insights into the correlation of the dynamics of device hysteresis with the interfacial charge recombination and accumulation in the PSCs are revealed.

Abstract

Current–voltage hysteresis of perovskite solar cells (PSCs) has raised the concern of accurate performance measurement in practice. Although various theories have been proposed to elucidate this phenomenon, the origin of hysteresis is still an open question. Herein, the use of guanidinium cation (Gu⁺)‐dopant is demonstrated to tailor the crystal structure of mixed‐cation formamidinium‐cesium lead triiodide (FA_0.83Cs_0.17PbI₃) perovskite, resulting in an improved energy conversion efficiency and tunable current–voltage hysteresis characteristic in planar solar cells. Particularly, when the concentration of Gu‐dopant for the perovskite film increases, the normal hysteresis initially observed in the pristine PSC is first suppressed with 2%‐Gu‐dopant, then changed to inverted hysteresis with a higher Gu‐dopant. The hysteresis tunability behavior is attributed to the interplay of charge/ion accumulation and recombination at interfaces in the PSC. Furthermore, compared to the cell without Gu⁺‐dopant, the optimal content of 2% Gu⁺‐dopant also increases the device efficiency by 14%, reaching over 17% under one sun illumination.

F4‐TCNQ is applied to manipulate the morphological, electrical, and photovoltaic properties of nonfullerene solar cells. Adding a trace amount of F4‐TCNQ yields a higher current density and fill factor, in comparison to the reference device. The combined techniques evidence that the addition of F4‐TCNQ increases charge lifetime, charge mobility, and mean‐square composition variation.

Abstract

Fluorinated molecule 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ) and its derivatives have been used in polymer:fullerene solar cells primarily as a dopant to optimize the electrical properties and device performance. However, the underlying mechanism and generality of how F4‐TCNQ affects device operation and possibly the morphology is poorly understood, particularly for emerging nonfullerene organic solar cells. In this work, the influence of F4‐TCNQ on the blend film morphology and photovoltaic performance of nonfullerene solar cells processed by a single halogen‐free solvent is systematically investigated using a set of morphological and electrical characterizations. In solar cells with a high‐performance polymer:small molecule blend FTAZ:IT‐M, F4‐TCNQ has a negligibly small effect on the molecular packing and surface characteristics, while it clearly affects the electronic properties and mean‐square composition variation of the bulk. In comparison to the control devices with an average power conversion efficiency (PCE) of 11.8%, inclusion of a trace amount of F4‐TCNQ in the active layer has improved device fill factor and current density, which has resulted into a PCE of 12.4%. Further increase in F4‐TCNQ content degrades device performance. This investigation aims at delineating the precise role of F4‐TCNQ in nonfullerene bulk heterojunction films, and thereby establishing a facile approach to fabricate highly optimized nonfullerene solar cells.

Four new polymers containing the novel asymmetrical backbone, thienobenzodithiophene, are synthesized and applied in high‐performance nonfullerene solar cells. The asymmetrical backbone can dramatically effect the polymer geometric configuration and modulate the polymer aggregation and crystallinity. This work reveals that the versatile asymmetric backbone is an excellent moiety to construct light‐harvesting copolymers and to modulate the microstructure for highly efficient PSCs.

Abstract

In this work, a new asymmetrical backbone thienobenzodithiophene (TBD) containing four aromatic rings is designed, and then four polymers PTBD‐BZ, PTBD‐BDD, PTBD‐FBT, and PTBD‐Tz are synthesized. The planar and high degree of π‐conjugation configuration can guarantee effective charge carrier transport and the distinct flanked dihedral angles between the TBD core and conjugated side chain can subtly regulate the molecular aggregation and crystallinity. The four polymer/3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]‐dithiophene (ITIC) blending films exhibit predominantly face‐on orientation. The photovoltaic devices based on wide bandgap polymers PTBD‐BZ and PTBD‐BDD achieve power conversion efficiencies (PCEs) as high as 12.02% and 11.39% without any post‐treatment. For the medium bandgap polymers PTBD‐FBT and PTBD‐Tz, the devices also show good PCEs of 10.18% and 11.02% with high V _OC of 0.94 and 1.02 V, respectively, which indicates simultaneously achieving a V _OC > 1 V and a high J _SC is feasible to further improve the PSCs' performance by modifying this new backbone. This work reveals that the versatile asymmetric backbone is an excellent moiety to construct light‐harvesting copolymers and to modulate the microstructure for highly efficient PSCs.

A high‐performance (12.9%) non‐fullerene organic solar cell processed using a sequential bilayer deposition method from non‐halogenated solvents is reported. Using this method, the organic solar cell can be scaled up to a larger area (1 cm²) while maintaining a high performance of 11.4% by doctor‐blade coating. This method offers a truly compatible processing technique for printing large area organic solar cell modules.

Abstract

While the performance of laboratory‐scale organic solar cells (OSCs) continues to grow over 13%, the development of high‐efficiency large area OSCs still lags. One big challenge is that the formation of bulk heterojunction morphology is an extremely complicated process and the formed morphology is also a highly delicate balance involving many parameters such as domain size, purity, miscibility, etc. The morphology control becomes much more challenging when the device area is scaled up. In this work, a highly efficient (12.9%) nonfullerene organic solar cell processed using a sequential bilayer deposition method from nonhalogenated solvents, is reported. Using this bilayer processing method, the organic solar cells can be scaled up to a larger area (1 cm²) while maintaining a high performance of 11.4% using doctor‐blade‐coating technique. Moreover, as the acceptor is hidden behind the polymer donor, the possibility of degradation by sunlight is lessened. Thus, improved photostability is observed in the bilayer structure device when compared with the bulk heterojunction device. This method offers a truly compatible processing technique for printing large‐area OSC modules.

The bulk and surface defects of perovskite films are suppressed by using SnO₂/TiO₂ double layer oxide, addition of methylammonium chloride (MACl) as a crystallization aid to the precursor solution, and surface passivation of perovskite films with iodine solution, due to the formation of high‐quality large‐grain perovskite films and retardation of radiationless carrier recombination.

Abstract

The presence of bulk and surface defects in perovskite light harvesting materials limits the overall efficiency of perovskite solar cells (PSCs). The formation of such defects is suppressed by adding methylammonium chloride (MACl) as a crystallization aid to the precursor solution to realize high‐quality, large‐grain triple A‐cation perovskite films and that are combined with judicious engineering of the perovskite interface with the electron and hole selective contact materials. A planar SnO₂/TiO₂ double layer oxide is introduced to ascertain fast electron extraction and the surface of the perovskite facing the hole conductor is treated with iodine dissolved in isopropanol to passivate surface trap states resulting in a retardation of radiationless carrier recombination. A maximum solar to electric power conversion efficiency (PCE) of 21.65% and open circuit photovoltage (V _oc) of ≈1.24 V with only ≈370 mV loss in potential with respect to the band gap are achieved, by applying these modifications. Additionally, the defect healing enhances the operational stability of the devices that retain 96%, 90%, and 85% of their initial PCE values after 500 h under continuously light illumination at 20, 50, and 65 °C, respectively, demonstrating one of the most stable planar PSCs reported so far.

Publication date: 21 November 2018

Source: Joule, Volume 2, Issue 11

Author(s): Yanfa Yan

By combining optical modeling and experimental results, the authors provide a full optical analysis for semitransparent organic solar cells (STOSCs). Defined as the sum of external quantum efficiency and transmittance, the term “quantum utilization efficiency (QUE)” is proposed as a parameter to describe light energy use in the semitransparent devices, which provides a new angle for analyzing STOSCs.

Semitransparent organic solar cells (STOSCs) show great potential for application as power generating windows for buildings. The power conversion efficiency (PCE) and the average visible transmittance (AVT) are both important parameters with which to evaluate the overall performance of STOSCs. However, it is very challenging to simultaneously improve these two performance parameters because they are intrinsically contradictory to each other. In this work, the optical and photovoltaic properties of STOSCs are investigated based on two model samples including PTB7‐Th:PC₆₁BM and PTB7‐Th:PC₇₁BM by systematically tuning their device structures. By combining optical modeling and experimental results, a full optical analysis is provided for the STOSCs with details on photon harvesting, optical losses, transmission properties, energy distribution spectrum, electric field intensity distribution, and photon absorption rate distribution within the devices. Defined as the sum of the external quantum efficiency and the transmittance, the term “quantum utilization efficiency” is used as a subjective parameter to describe the light energy use in the semitransparent devices, which provides an alternative angle for analyzing STOSCs.

Non‐peripheral octamethyl‐substituted copper (II) phthalocyanine nanowires are incorporated in poly(3‐hexylthiophene) to form nanocomposite, which exhibited higher hole mobilities and well‐matched energy levels. A power conversion efficiency of 16.61% is achieved for a perovskite solar cell based on composite hole‐transport material which retains 90% of their initial efficiencies after 800 h of storage at 25 °C with a relative humidity of 75% without any encapsulations.

New efficient hole‐transport material (HTM) composites based on low‐cost easy‐preparation non‐peripheral octamethyl‐substituted copper (II) phthalocyanine (N‐CuMe₂Pc) nanowire and poly(3‐hexylthiophene) (P3HT) are developed for CH₃NH₃PbI₃ (MAPbI₃)‐based perovskite solar cells (PSCs). Compared with pristine P3HT, the prepared nanocomposite HTMs provided thin films with better qualities and reduced trap densities, and exhibited higher hole mobilities and well‐matched energy levels with the perovskite layer. Depending on the ratio of the two components, the power conversion efficiency (PCE) reached up to 16.61%, which is higher than the efficiency of the standard device based on doped spiro‐OMeTAD (16.13%). Moreover, the long‐term stability of the PSCs is also improving greatly. The best performing devices based on P1C1 HTM retained 90% of their initial efficiencies after 800 h of storage with a relative humidity of 75%. These results indicate N‐CuMe₂Pc nanowire/P3HT nanocomposites can be an effective HTM to realize superior performance in PSCs.

Based on the high humidity stable [(NH₄)_2.4(FA)₈Pb₉I_28.4]_0.85(MAPbBr₃)_0.15 mixed‐dimensional perovskite, the authors investigated its aging properties under different humidity levels. Through analyzing the performance changes during aging, the authors speculated, and verified the possible mechanism of its high moisture resistance, which is a result from the formation of NH₄PbX₃*(H₂O)₂ and two‐dimensional protective layers, and the conversion of δ‐phase FAPbI₃ into α‐phase under continuous illumination.

Recently, perovskite materials are widely applied in the photovoltaic field, whereas its practical application is hindered by the humidity instability. To solve this problem, the authors prepared a [(NH₄)_2.4(FA)₈Pb₉I_28.4]_0.85(MAPbBr₃)_0.15 mixed‐dimensional (MD) perovskite with superior humidity stability. Herein, we investigated the aging properties of three‐dimensional (3D) and MD perovskite under different humidity levels. Through analyzing the performance changes before and after aging tests, the possible mechanism of high moisture resistance for MD perovskite is speculated and verified. After undergoing cation exchange, the surface NH₄ ⁺ combines with H₂O to form NH₄PbX₃*(H₂O)₂ (X = I or Br), and then the two‐dimensional (2D) perovskite protective layers are formed on the surface of perovskite, which prevent H₂O from destroying the 3D perovskite structure. Meanwhile, under continuous illumination, the δ‐phase FAPbI₃ produced from inside FA⁺ may change into α‐phase FAPbI₃. Therefore, the MD perovskite maintains great 3D perovskite structure and displays outstanding humidity stability under high humidity. The devices retain their starting photoelectric conversion efficiency (PCE) for 4000 h under 40% relative humidity (RH) and 80% of PCE over 2000 h under 70% RH. This finding provides a promising prospect for solving the humidity instability of perovskite materials and will promote the development of PSCs.

Conventional PbI₂ is replaced by PbI₂/I₂ mixed precursor during the first step of sequential deposition, causing the formation of a PbI₂ porous nanostructure. By changing the content of I₂ in the precursor, the morphology of the PbI₂ film as well as the resulting perovskite film can be successfully modulated. With an optimal content of I₂, a high‐quality perovskite film with a pure phase and smooth surface is achieved, enabling the high performance of perovskite solar cells.

The quality of the perovskite film has a vital influence on the performance of perovskite solar cells and it is quite desirable to simultaneously manipulate the crystallization and morphology of the perovskite film. In this study, conventional PbI₂ is replaced with a PbI₂/I₂ mixed precursor during the first step of sequential deposition, causing the formation of a PbI₂ porous nanostructure. By changing the content of I₂ in the precursor, the morphology of the PbI₂ film as well as the resulting perovskite film can be successfully modulated. With an optimal content of I₂, a high‐quality perovskite film with a pure phase and smooth surface can be achieved. As a result, the conversion efficiency of perovskite solar cells using a PbI₂/I₂ mixed precursor can be as high as 18.63%, compared to 16.89% for the reference device through traditional sequential deposition with a pure PbI₂ precursor.

The utilization of poly(vinylpyrrolidone) (PVP) as a cathode interlayer is demonstrated in inverted and conventional devices via both the self‐organization method and the step‐by‐step preparation method. The driving forces for PVP migration are the high surface energy of the PVP and the strong intermolecular interaction between the PVP and the bottom cathode. In addition, the PVP‐modified devices have excellent stability in air and show insensitivity to PVP molecular weight.

Abstract

Herein, poly(vinylpyrrolidone) (PVP) is used as the cathode interlayer (CIL) through the self‐organization method in inverted organic solar cells (OSCs). By coating a solution of PVP and active layer materials onto a glass/indium tin oxide (ITO) substrate, the PVP can segregate to the near ITO side due to its high surface energy and strong intermolecular interaction with the ITO electrode. The power conversion efficiency (PCE) of the obtained OSC device reaches 13.3%, much higher than that of the control device with a PCE of only 10.1%. The improvement results from the increased exciton dissociation efficiency and the depressed trap‐assisted recombination, which can be attributed to the reduced work function of the cathode by the self‐organized PVP. Additionally, the molecular weight of the PVP has almost no influence on the device performance, and the PVP‐modified device presents superior stability. This method can also be applied in other highly efficient fullerene‐free OSCs, and with a fine selection of the active layer, a high PCE of 14.0% is obtained. Overall, this work demonstrates the great potential of the PVP‐based CIL in inverted OSCs fabricated via the self‐organization method.

Publication date: February 2019

Source: Nano Energy, Volume 56

Author(s): Haoran Chen, Yingdong Xia, Bo Wu, Feng Liu, Tingting Niu, Lingfeng Chao, Guichuan Xing, TzeChien Sum, Yonghua Chen, Wei Huang

Abstract

Graphical abstract

We investigated the effect of chloride on the optical and electronic properties of the LDRP perovskites employing PEA spacer. We found that the incorporation of chloride significantly improved the perovskite morphology with increased grain size, enhanced crystallinity, and uniform and smooth surface. Moreover, the charge transport characteristics were also remarkably improved and trap densities were significantly reduced. Finally, a high PCE of 12.78% was achieved in I-I-Cl system with negligible hysteresis and long-term stability compared to others, 10.94% in I-Cl-I, 9.32% in I-Cl-Cl, and 6.52% in I-I-I.

Publication date: February 2019

Source: Nano Energy, Volume 56

Author(s): Fuhua Hou, Can Han, Olindo Isabella, Lingling Yan, Biao Shi, Junfan Chen, Shichong An, Zhongxin Zhou, Wei Huang, Huizhi Ren, Qian Huang, Guofu Hou, Xinliang Chen, Yuelong Li, Yi Ding, Guangcai Wang, Changchun Wei, Dekun Zhang, Miro Zeman, Ying Zhao

Abstract

Graphical abstract

Publication date: 20 February 2019

Source: Joule, Volume 3, Issue 2

Author(s): Kai Yao, Shifeng Leng, Zhiliang Liu, Linfeng Fei, Yongjian Chen, Sibo Li, Naigeng Zhou, Jie Zhang, Yun-Xiang Xu, Lang Zhou, Haitao Huang, Alex K.-Y. Jen

Context & Scale

Summary

Graphical Abstract

A planar electrocatalyst with CoO _x embedded in nitrogen‐doped graphitic carbon (N‐C‐CoO _x ) was created through the direct pyrolysis of a metal–organic complex. The N‐C‐CoO _x catalyst shows high oxygen reduction activity, indicated by excellent half‐wave (0.84 V vs. RHE) and onset (1.01 V vs. RHE) potentials.

Abstract

Efficient and durable nonprecious metal electrocatalysts for the oxygen reduction (ORR) are highly desirable for several electrochemical devices, including anion exchange membrane fuel cells (AEMFCs). Here, a 2D planar electrocatalyst with CoO _x embedded in nitrogen‐doped graphitic carbon (N‐C‐CoO _x ) was created through the direct pyrolysis of a metal–organic complex with a NaCl template. The N‐C‐CoO _x catalyst showed high ORR activity, indicated by excellent half‐wave (0.84 V vs. RHE) and onset (1.01 V vs. RHE) potentials. This high intrinsic activity was also observed in operating AEMFCs where the kinetic current was 100 mA cm⁻² at 0.85 V. When paired with a radiation‐grafted ETFE powder ionomer, the N‐C‐CoO _x AEMFC cathode was able to achieve extremely high peak power density (1.05 W cm⁻²) and mass transport limited current (3 A cm⁻²) for a precious metal free electrode. The N‐C‐CoO _x cathode also showed good stability over 100 hours of operation with a voltage decay of only 15 % at 600 mA cm⁻² under H₂/air (CO₂‐free) reacting gas feeds. The N‐C‐CoO _x cathode catalyst was also paired with a very low loading PtRu/C anode catalyst, to create AEMFCs with a total PGM loading of only 0.10 mg_Pt‐Ru cm⁻² capable of achieving 7.4 W mg⁻¹ _PGM as well as supporting a current of 0.7 A cm⁻² at 0.6 V with H₂/air (CO₂ free)—creating a cell that was able to meet the 2019 U.S. Department of Energy initial performance target of 0.6 V at 0.6 A cm⁻² under H₂/air with a PGM loading <0.125 mg cm⁻² with AEMFCs for the first time.

Hybrid cation (guanidinium/formamidinium) tin‐based perovskites that give a new performance record for lead‐free perovskite solar cells (power conversion efficiency = 9.6%) are demonstrated. The fabricated devices show an incredible light‐soaking stability for continuous 1 sun illumination for 1 h, and the device passes all harsh verification steps to attain a certified efficiency of 8.3% for a fresh cell.

Abstract

The stability of a tin‐based perovskite solar cell is a major challenge. Here, hybrid tin‐based perovskite solar cells in a new series that incorporate a nonpolar organic cation, guanidinium (GA⁺), in varied proportions into the formamidinium (FA⁺) tin triiodide perovskite (FASnI₃) crystal structure in the presence of 1% ethylenediammonium diiodide (EDAI₂) as an additive, are reported. The device performance is optimized at a precursor ratio (GAI:FAI) of 20:80 to attain a power conversion efficiency (PCE) of 8.5% when prepared freshly; the efficiencies continuously increase to attain a record PCE of 9.6% after storage in a glove‐box environment for 2000 h. The hybrid perovskite works stably under continuous 1 sun illumination for 1 h and storage in air for 6 days without encapsulation. Such a tin‐based perovskite passes all harsh standard tests, and the efficiency of a fresh device, 8.3%, is certified. The great performance and stability of the device reported herein attains a new milestone for lead‐free perovskite solar cells on a path toward commercial development.