Chen Weijie

Kelvin probe force microscopy and conductive atomic force microscopy are applied to explore local contact potential difference (CPD), current, and charge activities of the perovskite films with different contact layers. The average difference of the CPD values of the perovskite films obtained in dark and under illumination signifies the efficacy of the perovskite/contact layer heterojunctions.

Abstract

Hybrid halide perovskite based on CH₃NH₃PbI₃ and related materials has emerged as the most exciting development in the next generation photovoltaic technologies. There is still requirement for an effective method to establish a relationship between the charge transfer behaviors and photovoltaic properties. This study presents Kelvin probe force microscopy and conductive atomic force microscopy measurements of versatile perovskite films that participate in the formation of different heterojunctions, exploring local current, contact potential difference (CPD), and charge activities at the nanoscale. By comparing the values of CPD and current of these perovskite films in dark and under illumination, the charge transfer behaviors are locally illustrated, suggesting that the perovskite roles in these heterojunctions are strictly dependent on the contact layers. Furthermore, the average difference (ΔV) of the CPD values obtained in dark and under illumination for each heterojunction can be set to analyze the efficacy of the perovskite/contact layer interfaces. The ΔV polarity is related to the type of charge carrier (hole or electron), while the ΔV magnitude is related to the number of charge carrier. These results emphasize the importance of understanding of these heterojunction systems that could guide the design and optimization of the photovoltaic configuration.

Trivalent cations (Sb³⁺ and In³⁺) can be spontaneously distributed with a gradient in organometal halide perovskites from homogeneous precursors, because of their difference in ionic size and electrostatic interaction between the dopants and the host atoms. This phenomenon can facilitate charge separation and collection of photoelectrons, leading to excellent photovoltaic performance.

Abstract

A gradient heterosturcture is one of the basic methods to control the charge flow in perovskite solar cells (PSCs). However, a classical route for gradient heterosturctures is based on the diffusion technique, in which the guest ions gradually diffuse into the films from a concentrated source of dopants. The gradient heterosturcture is only accessible to the top side, and may be time consuming and costly. Here, the “intolerant” n‐type heteroatoms (Sb³⁺, In³⁺) with mismatched cation sizes and charge states can spontaneously enrich two sides of perovskite thin films. The dopants at specific sides can be extracted by a typical hole‐transport layer. Theoretical calculations and experimental observations both indicate that the optimized charge management can be attributed to the tailored band structure and interfacial electronic hybridization, which promote charge separation and collection. The strategy enables the fabrication of PSCs with a spontaneous graded heterojunction showing high efficiency. A champion device based on Sb³⁺ doped film shows a stabilized power‐conversion efficiency of 21.04% with a high fill factor of 0.84 and small hysteresis.

A new hybrid phenyl‐C61‐butyric acid methyl ester (PCBM)/stibonium‐carbon nanorods (Sb‐CNRs) passivation layer is developed for planar inverted perovskite solar cells by introducing 1D N‐type doped Sb‐CNRs into the PCBM nanofilm. The incorporation of N‐doped Sb‐CNRs into the PCBM film can extend the in‐built electric field to improve charge separation and construct a 1D transport channel to avoid the electron zigzag diffusion. Ultimately, the optimized device achieves an enhanced efficiency of 19.26%.

Abstract

To overcome the zigzag pathway transport of the electron diffusion process and eliminate the surface trap states of phenyl‐C61‐butyric acid methyl ester (PCBM) nanofilms in inverted perovskite solar cells, novel 1D N‐type doped carbon nanorods (CNRs) are developed by a stibonium (Sb) auxiliary ball milling method and introduced into the PCBM film to prepare the PCBM:Sb‐CNRs hybrid transport layer. In this way, the N‐type doped Sb‐CNRs can extend the built‐in electric field between CH₃NH₃PbI₃ and PCBM to facilitate the separation of electron/hole pairs. The discontinuous band with the built‐in potential in the PCBM/Sb‐CNRs heterojunction can boost interfacial charge redistribution and promote electrons diffusion from PCBM to electrode through 1D Sb‐CNRs network. As a result, the high device efficiency of 19.26% with enhanced air stability and little hysteresis are achieved. This work demonstrates a simple strategy to improve the efficiency and stability of perovskite photovoltaic devices using low‐cost carbon nanomaterials.

Current cost drivers and potential avenues to reduce cost for organic solar modules by constructing a comprehensive bottom‐up cost model are examined. Moreover, the impact on the cost of alternative materials and constructions, process throughputs, module efficiency, and module lifetime, etc. is presented, and avenues for the further reduction of the minimum sustainable price and levelized cost of energy values are discussed.

Abstract

Organic photovoltaics (OPVs) have become a potential candidate for clean and renewable photovoltaic productions. This work examines the current cost drivers and potential avenues to reduce costs for organic solar modules by constructing a comprehensive bottom‐up cost model. The direct manufacturing cost (MC) and the minimum sustainable price (MSP) for an opaque single solar module (SSM) (MC = 187 ¥ m⁻², MSP = 297 ¥ m⁻²) and for a tandem solar module (MC = 224 ¥ m⁻², MSP = 438 ¥ m⁻²) are analyzed in detail. Within this calculation, the most expensive layers and processing steps are identified and highlighted. Importantly, the low levelized cost of energy (LCOE) value for an SSM with a 10% power conversion efficiency in a 20‐year range from 0.185 to 0.486 ¥ kWh⁻¹, with a national average of 0.324 ¥ kWh⁻¹ in China under an average solar irradiance of 1200 kWh m⁻² year⁻¹. Moreover, the impact on the cost of alternative materials and constructions, process throughputs, module efficiency, and module lifetime, etc., is presented and avenues to further reduce the MSP and LCOE values are indicated. The analysis shows that OPVs can emerge as a competitive alternative to established power generation technologies if the remaining issues (e.g., active layer material cost, module efficiency, and lifetime) can be resolved.

Charge recombination at grain boundaries is a key factor limiting the performance of low‐bandgap mixed tin–lead halide perovskite solar cells. It is found that bromine incorporation can passivate grain boundaries and lower the dark current density by two to three orders of magnitude. The champion cell shows an open‐circuit voltage deficit of 0.384 V and power conversion efficiency exceeding 19%.

Abstract

The unsatisfactory performance of low‐bandgap mixed tin (Sn)–lead (Pb) halide perovskite subcells has been one of the major obstacles hindering the progress of the power conversion efficiencies (PCEs) of all‐perovskite tandem solar cells. By analyzing dark‐current density and distribution, it is identified that charge recombination at grain boundaries is a key factor limiting the performance of low‐bandgap mixed Sn–Pb halide perovskite subcells. It is further found that bromine (Br) incorporation can effectively passivate grain boundaries and lower the dark current density by two–three orders of magnitude. By optimizing the Br concentration, low‐bandgap (1.272 eV) mixed Sn–Pb halide perovskite solar cells are fabricated with open‐circuit voltage deficits as low as 0.384 V and fill factors as high as 75%. The best‐performing device demonstrates a PCE of >19%. The results suggest an important direction for improving the performance of low‐bandgap mixed Sn–Pb halide perovskite solar cells.

Publication date: February 2019

Source: Nano Energy, Volume 56

Author(s): Yiqiang Zhang, Xiaotao Liu, Pengwei Li, Yanyan Duan, Xiaotian Hu, Fengyu Li, Yanlin Song

Abstract

Graphical abstract

The energy level mismatch between a FA0.6MA0.4 Sn0.6Pb0.4I3 absorber and PEDOT:PSS based hole transporting layer are reduced and an improved V_OC over 50 mV and hence power conversion efficiencies of up to 15.85% are achieved.

Small bandgap Sn‐Pb mixed perovskite is generally regarded as the most promising structure to further enhance the power conversion efficiency of perovskite solar cells. However, the open circuit voltages (V _OCs) are usually lower than expected. In this work, by doping the generally used hole transporting layer (HTL) poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) with a perfluorinated ionomer (PFI), we can tune the work function of it from −5.02 to −5.19 eV. This reduces the energy level mismatch between the FA_0.6MA_0.4 Sn_0.6Pb_0.4I₃ (FAMA) absorber and HTL, giving rise to enhanced built‐in voltage and better carrier extraction. The V _OC improves to over 50 mV, up to 0.783 V, resulting in an improved champion power conversion efficiency (PCE) of 15.85%. Moreover, the devices based on modified HTLs show improved stability at maximum power point. These results demonstrate that energy level tuning of the HTL is a promising strategy for the improvement of the PCEs of Sn‐Pb mixed inverted PSCs, and the addition of PFI is an effective method to tune the work function of PEDOT:PSS.

Efficient two-terminal all-perovskite tandem solar cells enabled by high-quality low-bandgap absorber layers

Efficient two-terminal all-perovskite tandem solar cells enabled by high-quality low-bandgap absorber layers, Published online: 26 November 2018; doi:10.1038/s41560-018-0278-x

The β‐position of bithiophene imide is modified by incorporating F atoms to generate a novel building block s‐FBTI2, a well desired “stronger acceptor” for developing electron‐transporting semiconductors. The polymer s‐FBTI2‐FT shows a remarkable electron mobility approaching 3.0 cm² V⁻¹ s⁻¹ in organic thin‐film transistors and a highly promising power conversion efficiency of 6.50% in all‐polymer solar cells.

Bithiophene imide (BTI) is a promising building block for constructing n‐type organic semiconductors. The β‐positions of thiophene in BTI offer an exceptional opportunity for further structural expansion and optimization. Herein, a novel fluorinated BTI, s‐FBTI2, is designed and successfully synthesized, and its incorporation into a polymer backbone led to the resulting semiconductor s‐FBTI2‐FT with improved polymer backbone planarity enabled by the intramolecular non‐covalent S···F interactions and optimized electronic structure attributed to the high electronegativity of F atoms. When applied in organic thin‐film transistors (OTFTs), s‐FBTI2‐FT shows a unipolar n‐type transport with a remarkable electron mobility approaching 3.0 cm² V⁻¹ s⁻¹, which is >3‐fold higher than that of the polymer analogue without F. Moreover, all‐polymer solar cells (all‐PSCs) with s‐FBTI2‐FT as the electron acceptor polymer achieve a power conversion efficiency of 6.50% with a remarkably high open‐circuit voltage of 1.04 V, which is substantially greater than that of solar cells based on the nonfluorinated analogue acceptor showing negligible photovoltaic performance. The results demonstrate that s‐FBTI‐FT is one of best‐performing n‐type polymer semiconductors reported till today in terms of both OTFT and all‐PSC performances, and fluorination offers an effective approach for optimizing optoelectronic properties of BTI‐based polymers for device performance improvement.

Three nonfullerene acceptors based on a fused benzo[1,2‐b:4,5‐b′]diselenophene as the electron‐rich central core with zero to four bromine atoms on the electron‐deficient group are synthesized for polymer solar cells (PSCs). The PM6:BDSeIC2Br based device achieves a PCE of 12.5% with a relatively low Eloss of 0.52 eV, which is the highest PCE in brominated INCN end‐capped NF‐SMAs based binary PSCs.

In this work, three near‐infrared (NIR) absorption nonfullerene small‐molecule acceptors (NF‐SMAs) (BDSeIC, BDSeIC2Br, and BDSeIC4Br) based on a fused benzo[1,2‐b:4,5‐b′]diselenophene unit as the electron‐rich central core and 2‐(2,3‐dihydro‐3‐oxo‐1H‐inden‐1‐ylidene)propanedinitrile (INCN) without or with one or two bromine substituents as the electron‐deficient group have been synthesized for polymer solar cells. Compared to BDSeIC without bromine substitution, these multibrominated materials BDSeIC2Br and BDSeIC4Br exhibit lower energy levels, stronger absorption in the range of 500–900 nm, better crystalline quality, and enhanced electron mobility. The optimal BDSeIC2Br‐based devices with PM6 as the donor, achieved a high power conversion efficiency (PCE) of up to 12.5% with a relatively low energy loss (E _loss) of 0.52 eV. The PCE of 12.5% for the BDSeIC2Br‐based devices are much higher than those devices based on PM6:BDSeIC (7.1%) or PM6:BDSeIC4Br (9.6%) blend films, and it is the highest reported PCE in binary PSCs with the brominated INCN end‐capped NF‐SMAs. Such outstanding PCE of BDSeIC2Br‐based device is attributed to more balanced electron/hole mobility, higher charge dissociation and charge collection efficiency, and more proper phase separation features. These results indicate that introducing a benzo[1,2‐b:4,5‐b′]diselenophene core unit and bromine substiution on the end groups is an effective way to achieve high‐performance NIR absorption NF‐SMAs.

Layered double hydroxides (LDHs) goes into polymer solar cells: LDH nanosheets are applied for the first time in bulk heterojunction inverted polymer solar cells (BHJ‐iPSCs) as a novel low‐temperature solution‐processed cathode interfacial layer (CIL), leading to an obvious efficiency enhancement relative to that of the device based on the commonly used ZnO CIL due to the improved interfacial contact between the ITO and active layer. Thus, the present limit on the metal oxides as the commonly used CIL is broke.

Metal oxides such as zinc oxide (ZnO) have been commonly used as the cathode interfacial layer (CIL) of bulk heterojunction inverted polymer solar cells (BHJ‐iPSCs), for which a high‐temperature annealing treatment is usually required to improve their CIL performance. Layered double hydroxides (LDHs) are a class of inorganic two‐dimensional (2D) nanomaterials composed of positively charged brucite‐like layers intercalated with charge‐balancing anions and water molecules, showing potential applications in catalysis, adsorption, electrochemical energy storage and conversion, etc., but have never been applied in PSCs. Herein, for the first time LDH nanosheets are applied in BHJ‐iPSC devices as a novel CIL substituting the commonly used ZnO, affording an obvious efficiency enhancement relative to ZnO‐based device. Ultrathin Mg_xAl‐NO₃‐LDH nanosheets are prepared by ultrasonication‐assisted liquid exfoliation of bulk Mg_xAl‐NO₃‐LDHs prepared via a co‐precipitation method, and deposited onto an ITO substrate as a CIL by a low‐temperature solution‐processed technique. Based on Mg_xAl‐NO₃‐LDH CIL, BHJ‐iPSC devices with the poly(4,8‐bis‐alkyloxybenzo(l,2‐b:4,5‐b′)‐dithiophene‐2,6‐diylalt‐(alkylthieno(3,4‐b) thiophene‐2‐carboxylate)‐2,6‐diyl):[6,6]‐phenyl C71‐butyric acid methyl ester (PBDTTT‐C:PC₇₁BM) photoactive layer exhibits an improvement of power conversion efficiency relative to those based on ZnO CIL. This is primarily originated from the increase of fill factor due to the improved interfacial contact between the ITO and active layer, facilitating the interfacial electron transport.

A novel device structure of double‐side‐passivated perovskite solar cells is devised through intentionally distributing PbI₂ to both front/rear‐side surfaces and grain boundaries of perovskite film and a stabilized efficiency of 22% is achieved. Double‐side‐passivation effectively boosts the limits of open circuit voltage toward a record potential loss of 0.38 V for 1.53 eV‐bandgap perovskites.

An ideal crystal quality in the grain interior of perovskite polycrystalline films is well recognized; therefore, understanding interfacial impact and the ways to limit interfacial recombination is critical to fabricating highly efficient solar cells. In perovskite solar cells, PbI₂ has been used to passivate defects at grain boundaries, yet a systematic PbI₂ passivation engineering to boost the high‐performance perovskite solar cells has not been fully explored. Here, a novel device structure comprised of double‐side‐passivated perovskite solar cells (DSPC) is devised through intentionally distributing PbI₂ to both the front/rear‐side surfaces and grain boundaries of the formamidinium‐lead‐iodide‐based (FAPbI₃‐based) perovskite film. The minority carrier lifetime in double‐side‐passivated perovskite is extended to 1.1 μs with single‐exponential decay using time‐resolved photoluminescence. This result indicates a generic passivation effect of PbI₂ on perovskite interfaces, resembling SiO₂ passivation in silicon solar cells. Correspondingly, the best photovoltaic device with TiO₂‐based planar structure presents a stabilized efficiency of 22%. Moreover, DSPC effectively boosts the limits of open circuit voltages toward a record potential loss of 0.38 V for 1.53 eV‐bandgap perovskites. The architecture of double‐side‐passivated perovskite opens up new opportunities to exceed the efficiency of state‐of‐the‐art perovskite solar cells.

Thermally stable polymer:fullerene solar cells are enabled by locking the bulk‐heterojunction morphology through to incorporation of a thermally triggered bisazide cross‐linker. The concept is applicable to a wide range of absorber polymers. Even after 200 h of thermal stress at 120 °C, the solar cells retain up to 90% of their initial performance.

After enhancing the power conversion efficiencies of organic solar cells beyond 10%, their long term stability has become the most urgent challenge in order to eventually integrate organic solar cells into end‐user products. Solar devices may have to endure harsh conditions already during their fabrication, typically requiring lamination temperatures up to 120 °C, critical for the initial performance of organic solar cells. In this work, polymer:fullerene bulk‐heterojunctions are fabricated with significantly enhanced thermal stability at 120 °C and beyond, by locking the bulk‐heterojunction morphology through incorporating the novel cross‐linkable bisazide 1,2‐bis((4‐(azidomethyl)phenethyl)thio)ethane (TBA‐X). Bulk‐heterojunctions comprising various light‐harvesting polymers and the industrially relevant fullerene acceptor PC₆₁BM are investigated. Upon thermal annealing, the reference blends without the cross‐linking TBA‐X exhibit only moderate thermal stability and a relative loss of more than 70% of their initial performance, mainly originating from aggregation of the fullerene. In contrast, polymer:fullerene blends comprising TBA‐X retain up to 90% of their initial performance despite the harsh thermal annealing at 120 °C for up to 200 h.

Melamine hydroiodide (MLAI) has been successfully introduced for preparing hetero structured MAPbI₃ perovskite for enhancing the photovoltaic performance and stability of perovskite solar cells in a robust humid ambient atmosphere (35 °C, 60–70% relative humidity). The power conversation efficiency of perovskite solar cells based on MLAI functionalized perovskite is 25% higher than that of pristine MAPbI₃ perovskite with nearly no hysteresis and high stability.

Despite the remarkable performance of organometallic halide perovskite solar cells (PSCs), their ultimate stability is still a major issue that inhibits the commercialization of this eminent technology. Herein, melamine hydroiodide (MLAI) is added to function methyl ammonium (CH3NH3⁺, MA⁺) lead iodide perovskite for fabricating structured perovskite with enhanced photovoltaic performance and stability in the harsh ambient atmosphere (35 °C, 60–70% relative humidity). Nearly no new phase formed even incorporated 25 mol.% MLAI induces the strain in the perovskite crystal structure. The MLAI‐structured perovskite film shows a denser and smoother surface than the pristine MAPbI₃ perovskite. Planar PSCs based on 2 mol.% MLAI‐functionalized perovskite show 17.2% power conversion efficiency with nearly no hysteresis which is much higher than pristine MAPbI₃ PSCs. Most importantly, the solar cell devices based on 2 mol.% MLAI‐functionalized perovskite still retain over 90% of the initial performance after being kept in ambient atmosphere for more than 560 h without encapsulation.

Next‐generation solar cells consisting of organic materials are studied. To develop novel dyes for dye‐sensitized solar cells, the essential dye structures are explored to attain high efficiency. Additionally, the interfaces in the perovskite solar cells are characterized via electrochemical methods, and newly developed laser deposition methods for perovskite layers are discussed.

Abstract

Next‐generation organic solar cells such as dye‐sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) are studied at the National Institute of Advanced Industrial Science and Technology (AIST), and their materials, electronic properties, and fabrication processes are investigated. To enhance the performance of DSSCs, the basic structure of an electron donor, π‐electron linker, and electron acceptor, i.e., D–π–A, is suggested. In addition, special organic dyes containing coumarin, carbazole, and triphenylamine electron donor groups are synthesized to find an effective dye structure that avoids charge recombination at electrode surfaces. Meanwhile, PSCs are manufactured using both a coating method and a laser deposition technique. The results of interfacial studies demonstrate that the level of the conduction band edge (CBE) of a compact TiO₂ layer is shifted after TiCl₄ treatment, which strongly affects the solar cell performance. Furthermore, a special laser deposition system is developed for the fabrication of the perovskite layers of PSCs, which facilitates the control over the deposition rate of methyl ammonium iodide used as their precursor.