Current advancements on charge selective contact interfacial layers and electrodes in flexible hybrid perovskite photovoltaics

Perovskite-based photovoltaic materials have been attracting attention for their strikingly improved performance at converting sunlight into electricity.The beneficial and unique optoelectronic characteristics of perovskite structures enable researchers to achieve an incredibly remarkable power conversion efficiency.Flexible hybrid perovskite photovoltaics promise emerging applications in a myriad of optoelectronic and wearable/portable device applications owing to their inherent intriguing physicochemical and photophysical properties which enabled researchers to take forward advanced research in this growing field.Flexible perovskite photovoltaics have attracted significant attention owing to their fascinating material properties with combined merits of high efficiency,light-weight,flexibility,semitransparency,compatibility towards roll-to-roll printing,and large-area mass-scale production.Flexible perovskite-based solar cells comprise of 4 key components that include a flexible substrate,semi-transparent bottom contact electrode,perovskite(light absorber layer)and charge transport(electron/hole)layers and top(usually metal)electrode.Among these components,interfacial layers and contact electrodes play a pivotal role in influencing the overall photovoltaic performance.In this comprehensive review article,we focus on the current developments and latest progress achieved in perovskite photovoltaics concerning the charge selective transport layers/electrodes toward the fabrication of highly stable,efficient flexible devices.As a concluding remark,we briefly summarize the highlights of the review article and make recommendations for future outlook and investigation with perspectives on the perovskite-based optoelectronic functional devices that can be potentially utilized in smart wearable and portable devices.

Modulating perovskite crystallization and band alignment using coplanar molecules for high-performance indoor photovoltaics

The proper bandgap and exceptional photostability enable CsPbI_(3) as a potential candidate for indoor photovoltaics(IPVs),but indoor power conversion efficiency(PCE) is impeded by serious nonradiative recombination stemming from challenges in incomplete DMAPbI_(3) conversion and lattice structure distortion.Here,the coplanar symmetric structu re of hexyl sulfide(HS) is employed to functionalize the CsPbI_(3) layer for fabricating highly efficient IPVs.The hydrogen bond between HS and DMAI promotes the conversion of DMAPbI_(3) to CsPbI_(3),while the copianar symmetric structure enhances crystalline order.Simultaneously,surface sulfidation during HS-induced growth results in the in situ formation of PbS,spontaneously creating a CsPbI_(3) N-P homojunction to enhance band alignment and carrier mobility.As a result,the CsPbI_(3)&HS devices achieve an impressive indoor PCE of 39.90%(P_(in):334.6 μW cm^(-2),P_(out):133.5 μW cm^(-2)) under LED@2968 K,1062 lux,and maintain over 90% initial PCE for 800 h at ^(3)0% air ambient humidity.

Efficient Monolithic Perovskite/Silicon Tandem Photovoltaics

Tunable bandgaps make halide perovskites promising candidates for developing tandem solar cells(TSCs),a strategy to break the radiative limit of 33.7%for single-junction solar cells.Combining perovskites with market-dominant crystalline silicon(c-Si)is particularly attractive;simple estimates based on the bandgap matching indicate that the efficiency limit in such tandem device is as high as 46%.However,state-of-the-art perovskite/c-Si TSCs only achieve an efficiency of~32.5%,implying significant challenges and also rich opportunities.In this review,we start with the operating mechanism and efficiency limit of TSCs,followed by systematical discussions on wide-bandgap perovskite front cells,interface selective contacts,and electrical interconnection layer,as well as photon management for highly efficient perovskite/c-Si TSCs.We highlight the challenges in this field and provide our understanding of future research directions toward highly efficient and stable large-scale wide-bandgap perovskite front cells for the commercialization of perovskite/c-Si TSCs.

Theoretical efficiency limit and realistic losses of indoor organic and perovskite photovoltaics [Invited]

Indoor organic and perovskite photovoltaics(PVs)have been attracting great interest in recent years.The theoretical limit of indoor PVs has been calculated based on the detailed balance method developed by Shockley–Queisser.However,realistic losses of the organic and perovskite PVs under indoor illumination are to be understood for further efficiency improvement.In this work,the efficiency limit of indoor PVs is calculated to 55.33%under indoor illumination(2700 K,1000 lux)when the bandgap(E_(g))of the semiconductor is 1.77 eV.The efficiency limit was obtained on the basis of assuming 100%photovoltaic external quantum efficiency(EQ_(EPV))when E≥E_(g),there was no nonradiative recombination,and there were no resistance losses.In reality,the maximum EQEPV reported in the literature is 0.80–0.90.The proportion of radiative recombination in realistic devices is only 10^(−5)–10^(−2),which causes the open-circuit voltage loss(ΔV_(loss))of 0.12–0.3 V.The fill factor(FF)of the indoor PVs is sensitive to the shunt resistance(R_(sh)).The realistic losses of EQE_(PV),nonradiative recombination,and resistance cause the large efficiency gap between the realistic values(excellent perovskite indoor PV,32.4%;superior organic indoor PV,30.2%)and the theoretical limit of 55.33%.In reality,it is feasible to reach the efficiency of 47.4%at 1.77 eV for organic and perovskite photovoltaics under indoor light(1000 lux,2700 K)with V_(OC)=1.299 V,J_(SC)=125.33μA/cm^(2),and FF=0.903 when EQE_(PV)=0.9,EQE_(EL)=10^(−1),R_(s)=0.5Ωcm^(2),and R_(sh)=10^(4) kΩcm^(2).

Deposition technologies of perovskite layer enabling large-area photovoltaic modules

The commercialization of perovskite solar cells(PSCs)is expected.However,the selection of fabrication technology remains unclear,especially with different technologies corresponding to different area ranges.This study presents a summary of recent technologies related to device area and photovoltaic parameters for certain area ranges.Blade-coating,slot-die coating,and bar-coating technologies are suitable for PSCs whose area is greater than or equal to 100 cm^(2).Meanwhile,meniscus-coating,spray-coating,and roll-to-roll technologies are appropriate for flexible large-area PSCs.The definition of large area has been updated to one above 10 cm^(2).In conclusion,we provide a perspective for future large-area perovskite photovoltaics.

Long-Chain Gemini Surfactant-Assisted Blade Coating Enables Large-Area Carbon-Based Perovskite Solar Modules with Record Performance

Carbon-based perovskite solar cells show great potential owing to their low-cost production and superior stability in ambient air.However,scaling up to high-efficiency carbon-based solar modules hinges on reliable deposition of uniform defect-free perovskite films over large areas,which is an unsettled but urgent issue.In this work,a long-chain gemini surfactant is introduced into perovskite precursor ink to enforce self-assembly into a network structure,considerably enhancing the coverage and smoothness of the perovskite films.The long gemini surfactant plays a distinctively synergistic role in perovskite film construction,crystallization kinetics modulation and defect passivation,leading to a certified record power conversion efficiency of 15.46%with Voc of 1.13 V and Jsc of 22.92 mA cm^(-2)for this type of modules.Importantly,all of the functional layers of the module are printed through a simple and high-speed(300 cm min^(-1))blade coating strategy in ambient atmosphere.These results mark a significant step toward the commercialization of all-printable carbon-based perovskite solar modules.

Effect of temperature on the efficiency of organometallic perovskite solar cells

In recent years perovskite solar cells have attracted an increasing scientific and technological interest in the scientific community. It is important to know that the temperature is one of the factors which have a strong effect on the efficiency of perovskite solar cell. This study communicates a temperature analysis on the pho- tovoltaic parameters of CH3NH3Pbl3-based perovskite solar cell in a broad interval from 80 to 360 K. Strong temperature-dependent photovoltaic effects have been observed in the type of solar cell, which could be mainly attributed to CH3NH3PbI3, showing a ferroelectric-paraelectric phase transition at low temperature （T 〈 160 K）. An increase in temperature over the room temperature decreased the perovskite solar cell performance and reduced its efficiency from 16Z to 9%. The investigation with electronic impedance spectroscopy reveals that at low temperature （T 〈 120 K） the charge transport layer limits the device performance, while at high temperature （T 〉 200 K）, the interfacial charge recombination becomes the dominant factor.

Truxene-based Hole-transporting Materials for Perovskite Solar Cells

Three star-shaped truxene-based small molecules（namely TXH,TXM,TXO） were synthesized,characterized and used as hole-transporting materials（HTMs） for perovskite solar cells（Pv SCs）. The device based on TXO delivered a respectable power conversion efficiency（PCE） of 7.89% and a high open-circuit voltage（Voc） of 0.97 V,which far exceeded the values of the devices based on other two small molecules. The highest PCE for the device based on TXO is mainly contributed from its lowest series resistance（Rs） value and largest short-circuit current（Jsc） value under the same circumstances. All these results indicate that TXO is a promising HTM candidate for Pv SCs.

Improving power conversion efficiency of perovskite solar cells by cooperative LSPR of gold-silver dual nanoparticles

Enhancing optical and electrical performances is effective in improving power conversion efficiency of photovoltaic devices. Here, gold and silver dual nanoparticles were imported and embedded in the hole transport layer of perovskite solar cells. Due to the cooperative localized surface plasmon resonance of these two kinds of metal nanostructures, light harvest of perovskite material layer and the electrical performance of device were improved, which finally upgraded short circuit current density by 10.0%, and helped to increase power conversion efficiency from 10.4% to 11.6% under AM 1.5G illumination with intensity of 100 m W/cm;. In addition, we explored the influence of silver and gold nanoparticles on charge carrier generation, dissociation, recombination, and transportation inside perovskite solar cells.

Towards operation-stabilizing perovskite solar cells:Fundamental materials,device designs,and commercial applications

Over the last decade,perovskite solar cells(PSCs)have drawn extensive atten-tion owing to their high power conversion efficiency(single junction:26.1%,perovskite/silicon tandem:33.9%)and low fabrication cost.However,the short lifespan of PSCs with initial efficiency still blocks their practical applications.This operational instability may originate from the intrinsic and extrinsic deg-radation of materials or devices.Although the lifetime of PSCs has been prolonged through component,crystal,defect,interface,encapsulation engineering,and so on,the systematic analysis of failure regularity for PSCs from the perspective of materials and devices against multiple operating stressors is indispensable.In this review,we start with elaboration of the predominant degradation pathways and mechanism for PSCs under working stressors.Then the strategies for improving long-term durability with respect to fundamental materials,interface designs,and device encapsulation have been summarized.Meanwhile,the key results have been discussed to understand the limitation of assessing PSCs stability,and the potential applications in indoor photovoltaics and wearable electronics are demonstrated.Finally,promising proposals,encompassing material processing,film formation,interface strengthening,structure designing,and device encapsulation,are provided to improve the operational stability of PSCs and promote their commercialization.

High-performance large-area perovskite photovoltaic modules

Perovskite solar cells(Pero-SCs)exhibited a bright future for the next generation of photovoltaic technology because of their high power conversion efficiency(PCE),low cost,and simple solution process.The certified laboratory-scale PCE has reached 25.7%referred to small scale(<0.1 cm^(2))of Pero-SCs.However,with the increase of the area to module scale,the PCE drops dramatically mainly due to the inadequate regulation of growing large-area perovskite films.Therefore,there is a dire need to produce high-quality perovskite films for large-area photovoltaic modules.Herein,we summarize the recent advances in perovskite photovoltaic modules(PPMs)with particular attention paid to the coating methods,as well as the growth regulation of the high-quality and large-area perovskite films.Furthermore,this study encompasses future development directions and prospects for PPMs.

Determination of bandgaps of photoactive materials in perovskite solar cells at high temperatures by in-situ temperature-dependent resistance measurement

Normally, it is difficult to directly measure the bandgaps of perovskite based on methylammonium(MA) or formamidinium(FA) at high temperatures due to material decomposition. We prevent the decomposition by keeping the synthesized perovskite films(MAPbI_3 and MAPbI_3) in organic iodide vapors, then measure the in-situ resistance of the films at varied temperatures, and further evaluate the bandgaps of these two materials. The evaluated bandgaps are consistent with the results from ultraviolet-visible(UV-vis) absorption spectrum. The bandgap of MAPbI_3 decreases with temperature above 95 ℃, whereas that of FAPbI_3 first increases with temperature from 95 ℃ to 107 ℃ and then decreases with temperature above 107 ℃.