π-Conjugated small molecules enable efficient perovskite growth and charge-extraction for high-performance photovoltaic devices

A π-conjugated small molecule N,N′-bis(naphthalen-1-yl)-N,N’+-bis(phenyl)benzidine (NPB) is introduced into a poly (bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA) hole transport layer in inverted perovskite solar cells (PSCs). The NPB doping induces better perovskite crystal growth owing to a strong π-π interaction with PTAA and cation-π interactions with CH3NH3+ (MA+). In addition, NPB doping not only improves the wettability of PTAA and regulates the perovskite crystallization to achieve a larger grain size, but also moves the valence band energy of the hole transport layer closer to the perovskite layer. Consequently, the fabricated PSCs delivered a power conversion efficiency (PCE) of 20.15%, with a short-circuit current density (JSC) of 22.60 mA/cm2 and open-circuit voltage (VOC) of 1.14 V. This outcome indicates that PTAA:NPB composite materials present great potential for fabricating high-performance PSCs

Initial Stages of Photodegradation of MAPbI3 Perovskite: Accelerated Aging with Concentrated Sunlight

The initial stages of photo‐degradation of CH3NH3PbI3 (MAPbI3) thin films prior to any significant change in light absorption are studied in experiments with independent control of sample temperature and intensity of concentrated sunlight from 50 to 500 suns. Photo‐stability of the MAPbI3 film is revealed to be extremely sensitive to the sample temperature. Under the combined action of light and heat (either by concentrated sunlight or by external heating), a strong reduction of the film photoluminescence (PL) without changes in the perovskite light absorption can be observed during the initial stages of degradation. In contrast, illumination of perovskite films (with intensity up to 500 suns) without heating (using chopped concentrated sunlight) induces considerable PL enhancement while the optical absorption spectrum remains unchanged. With accurate temperature control, aging under concentrated sunlight results in similar instability trends as that under 1 sun

Structure and Performance Evolution of Perovskite Solar Cells under Extreme Temperatures

Metal halide perovskite solar cells may work for application in extreme temperatures, such as those experienced under extraterrestrial conditions. However, device performances in extreme temperatures are poorly investigated. This work systematically explores the performance of perovskite solar cells between −160 and 150 °C. In situ grazing-incidence wide-angle X-ray scattering discloses perovskite phase transition and crystal disordering as dominant factors for the temperature-dependent device efficiency deterioration. It is shown that perovskite lattice strain and relaxation originating from extreme temperature variations are recoverable, and so are the perovskite structure and photovoltaic performances. This work provides insights into the functioning under extreme temperatures, clarifying bottlenecks to overcome and the potential for extraterrestrial applications

Raman mapping analysis for removal of surface secondary phases of CZTS films using chemical etching

Raman spectroscopy has been widely used as a non-destructive surface characterization method for the Cu 2ZnSnS4 (CZTS) thin films. Secondary phases, which often co-exist with CZTS, are detrimental to the device performance. In this work, removal of the secondary phases using sodium sulfide (Na 2S) aqueous solution etching in various time durations was investigated. Raman scattering mapping provides a direct visualization of phase distribution in CZTS-based materials on a relatively large scale (1 mm × 10 mm). Both as-grown and etched CZTS absorber layers were examined by Raman spectroscopy with a 532 nm excitation laser light in the range of 50–500 cm-1. A clear reduction of the secondary phases (mainly SnS) at the surface after etching was confirmed by Raman spectroscopy and scanning electron microscopy. Room temperature photoluminescence (PL) reveals a pronounced correlation between the amount of secondary phases and photoluminescence peak position. The PL spectra of the regions with more Sn-rich secondary phases show clearly a shift to high wavelength of the peak position, in comparison with regions with less Sn-rich secondary phases. These observed PL changes could be due to Sn-rich defects which may cause recombination processes

Highly efficient p-i-n perovskite solar cells that endure temperature variations

Daily temperature variations induce phase transitions and lattice strains in halide perovskites, challenging their stability in solar cells. We stabilised the perovskite black phase and improved the solar cell performance using the ordered dipolar structure of β-poly(1,1-difluoroethylene) to control the perovskite film crystallisation and the energy alignment. We demonstrated p-i-n perovskite solar cells with a record power conversion efficiency of 24.6% over 18 square millimetres and 23.1% over 1 square centimetre, which retained 96% and 88% of the efficiency after 1000-hours 1-sun maximum power point tracking at 25 and 75 °C, respectively. Devices under rapid thermal cycling between −60 °C and +80 °C showed no sign of fatigue, demonstrating the impact of the ordered dipolar structure on the operational stability of perovskite solar cells

Recent progress, fabrication challenges and stability issues of lead-free tin-based perovskite thin films in the field of photovoltaics

Lead-halide perovskite materials have fascinated incredible attention among the research communities due to their favorable electrical and optical properties for optoelectronics and photovoltaic application such as significant light absorption coefficient, higher values of diffusion length, carrier mobility, carrier lifetime, etc. However, lead toxicity as well as its low material stability makes it still far from commercialization. Finding of lead ion replacement is therefore needed to form environmental friendly perovskite materials that are called lead-free perovskites. Among different substitutions, tin is the most potential candidate that is also non-toxic. In this review, the recent development of tin-based lead-free perovskite thin films in photovoltaics research area during the period 2014 till now is summarized. Moreover, the technical challenges and the stability issues faced by them are also discussed. Further, it is suggested that more efforts are needed for the advancement of tin-based lead-free perovskite thin films based solar cells to bring up this field as a cost competent technology for long term sustainability

Twisted Carotenoids Do Not Support Efficient Intramolecular Singlet Fission in the Orange Carotenoid Protein

Singlet exciton fission is the spin-allowed generation of two triplet electronic excited states from a singlet state. Intramolecular singlet fission has been suggested to occur on individual carotenoid molecules within protein complexes provided that the conjugated backbone is twisted out of plane. However, this hypothesis has been forwarded only in protein complexes containing multiple carotenoids and bacteriochlorophylls in close contact. To test the hypothesis on twisted carotenoids in a “minimal” one-carotenoid system, we study the orange carotenoid protein (OCP). OCP exists in two forms: in its orange form (OCPo), the single bound carotenoid is twisted, whereas in its red form (OCPr), the carotenoid is planar. To enable room-temperature spectroscopy on canthaxanthin-binding OCPo and OCPr without laser-induced photoconversion, we trap them in a trehalose glass. Using transient absorption spectroscopy, we show that there is no evidence of long-lived triplet generation through intramolecular singlet fission despite the canthaxanthin twist in OCPo

In situ investigation of perovskite solar cells’ efficiency and stability in a mimic stratospheric environment for high-altitude pseudo-satellites

Perovskite solar cells with high power-per-weight have great potential to be used for aerospace applications such as satellites or high-altitude pseudo-satellites. The latter are unmanned aircraft exclusively powered by solar energy, typically flying in the stratosphere where the conditions of pressure, temperature and illumination are critically different from that on the earth's surface. In this work, we evaluate the performance and stability of high efficiency perovskite solar cells under a mimic stratospheric environment. In situ measurements under controlled conditions of pressure, temperature and illumination were developed. We show that the cells can operate efficiently in a large range of temperature from −50 °C to +20 °C, with a maximum power conversion efficiency at −20 °C, which is ideal for use in the stratosphere. Besides, performances are maintained after a number of temperature cycles down to −85 °C, representative of temperature variations due to diurnal cycles. An efficient encapsulation is developed, which could be critical to avoid the accelerated degradation of the cells under vacuum. Finally, a promising stability for 25 days of day–night cycles was demonstrated, which suggests that perovskite solar cells could be used to power high altitude pseudo-satellites

An insight into the air stability of the benchmark polymer:Fullerene photovoltaic films and devices: A comparative study

In this study, a comparative analysis of the instabilities and degradation routes of organic solar cell (OSCs) employing the three benchmarked donor polymers namely poly(3-hexylthiophene) (P3HT), poly[N-900- hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT) and Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2 ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7) along with [6,6]-phenylC71 butyric acid methyl-ester (PC71BM) acceptor have been conducted using the extracted photovoltaic parameters in conjunction with the X-ray photoelectron spectroscopy (XPS), optical and morphological analysis. During the 14 days air stability test, the power conversion efficiency (PCE) decreased by 78.85%, 65.83% and 83.36% for P3HT:PC71BM, PCDTBT:PC71BM and PTB7:PC71BM based devices, respectively. However, the degradation study of the bulk heterojunction (BHJ) films was prolonged to 28 days in order to further elucidate the degradation factors affecting the device performance. XPS, optical and morphological studies enabled detailed information on the device degradation mechanisms and confirmed the oxidation of photoactive layer after ageing, morphological deterioration and fall in absorbance, particularly, the PTB7:PC71BM blend that showed the rapid degradation among all three. The results obtained in the current study advance the understanding of the stability/degradation mechanisms pertaining to the three most commonly used BHJ materials and hence, will help to improve the OSCs for longer lifetime

Down‐conversion materials for organic solar cells: Progress, challenges, and perspectives

Organic solar cells (OSCs) in terms of power conversion efficiency (PCE) and operational lifetime have made remarkable progress during the last ten years by improving the active layer materials and introducing new interlayers. The newly developed wide bandgap organic donor and low bandgap acceptor molecules covered the absorption from the visible to the near-infrared region. Whereas the incident high energy region (UV) is not in favour of OSCs. Its absorption causes thermalization losses and photoinduced degradation, which hinders the PCE and lifetime of OSCs. Recently, lanthanide and non-lanthanide-based down-conversion (DC) materials have been introduced, which can effectively convert the high energy photons (UV) to low energy photons (visible) and resolve the spectral mismatch losses that limit the absorption of OSCs in high energy incident spectrum. Furthermore, the DC materials also protect the OSCs from UV-induced degradation. The DC materials were also proposed to cross the Shockley-queisser efficiency limit of the solar cell. In this review, the need for DC materials and their processing method for OSCs have been thoroughly discussed. However, the main emphasis has been given on developing lanthanides and non-lanthanides-based DC materials for OSCs, their applications, and their impact on photovoltaic device performance, stability, and future perspectives