Efficient perovskite/organic integrated solar cells with extended photoresponse to 930 nm and enhanced near-infrared external quantum efficiency of over 50

Revealing the impact of thermal annealing on the perovskite/organic bulk heterojunction interface in photovoltaic devices

Perovskite/organic bulk heterojunction (BHJ) integrated solar cells have tremendous development potential to exceed the Shockley-Queisser limit efficiency of single-junction photovoltaics, due to the merits of spectra response extension. However, the presence of energy level barriers and severe non-radiative recombination at the interface between perovskite and BHJ greatly hindered the transport and collection of charge carriers, usually leading to large Voc and photocurrent loss, as well as the stability degradation of integrated devices. Therefore, investigating the interface properties of perovskite/BHJ is crucial for understanding the charge transport process and enhancing device performance. In this study, we effectively regulated the interface properties and charge transport in perovskite/BHJ integrated devices using a thermal annealing process. Using Kelvin probe microscopy, photoluminescence, and transient absorption spectroscopy, we revealed that moderate annealing treatment would contribute to forming close interface contact and provide more channels or pathways for charge transfer, which is advantageous for the interface charge collection and device performance. In addition, the lone pair electrons of acyl, thiophene and pyrrole function groups in polymer PDPP3T and PCBM can act as the Lewis base and provide electrons to the under-coordinated lead atoms or clusters in the perovskite, effectively passivating traps on the surface and grain boundaries of the perovskite through Lewis acid-base coordination. Finally, we improved the photovoltaic conversion efficiency of the device to 21.57% with enhanced stability using an optimized thermal annealing process. This study provides a comprehensive understanding of the integrated perovskite/BHJ interface properties, which could be extended to other optoelectronic devices based on a similar integrated structure.

Integrated Ideal-Bandgap Perovskite/Bulk-Heterojunction Solar Cells with Efficiencies > 24

Here, the authors report a highly efficient integrated ideal-bandgap perovskite/bulk-heterojunction solar cell (IPBSC) with an inverted architecture, featuring a near infrared (NIR) polymer DTBTI-based bulk-heterojunction (BHJ) layer atop guanidinium bromide (GABr)-modified FA_0.7 MA_0.3 Pb_0.7 Sn_0.3 I₃ perovskite film as the photoactive layer. The IPBSC shows cascade-like energy level alignment between the charge-extractionlayer/perovskite/BHJ and efficient passivation effect of BHJ on perovskite. Thanks to the well-matched energy level alignment and high-quality ideal bandgap-based perovskite film, an efficient charge transfer occurs between the charge-extraction-layer/perovskite/BHJ. Moreover, the NIR polymer DTBTI on the perovskite film leads to an improved NIR light response for the IPBSC. In addition, the O, S and N atoms in the DTBTI polymer yield a strong interaction with perovskite, which is conducive to reducing the defects of the perovskite and suppressing charge recombination. As a result, the solar cell achieves a power conversion efficiency (PCE) of 24.27% (certificated value at 23.4% with 0.283-volt voltage loss), currently the recorded efficiency for both IPBSCs and Pb-Sn alloyed PSCs, and which is over the highest efficiency of perovskite-organic tandem solar cell. Moreover, the thermal, humidity and long-term operational stabilities of the IPBSCs are also significantly improved compared with the control PSCs.

Double Cascading Charge Transfer at Integrated Perovskite/Organic Bulk Heterojunctions for Extended Near-Infrared Photoresponse and Enhanced Photocurrent

Nowadays, nearly 48.7% near-infrared (NIR) irradiation (>800 nm) of the full solar spectrum has actually not been fully utilized since the state-of-the-art perovskite film usually can only absorb the most UV-vis sunlight radiation. Herein, high efficiency integrated Cs_0.15 FA_0.85 PbI₃ perovskite/organic bulk (PC₆₁ BM:D18:Y6) heterojunction solar cells with enhanced low energy photon harvest until 931 nm and a high maintained open circuit voltage of 1.04 V is successfully obtained. In particular, the favorable double cascading charge transfer paths pave an interesting possibility to spatially separate electrons upon visible light excitation and holes upon NIR photon absorption simultaneously at interfaces, significantly suppressing non-radiative bimolecular recombination and reaching the photocurrent density as high as 27.48 mA cm^-2 and power conversion efficiency of 20.31%. Besides, the strong hydrophobicity of the ternary organic film has effectively prevented ambient humidity penetration and improves the stability of the perovskite in the continuous aging test (humidity > 60%) compared with the control device. This work has opened a significantly new window to improve the NIR light harvest for next generation highly efficient solar cells with full spectrum response.

Red Phosphorescent Carbon Quantum Dot Organic Framework-Based Electroluminescent Light-Emitting Diodes Exceeding 5% External Quantum Efficiency

Carbon quantum dots (CQDs) have developed into prospective nanomaterials for next-generation lighting and displays due to their intrinsic advantages of high stability, low cost, and environmental friendliness. However, confined by the spin-forbidden nature of triplet state transitions, the highest theoretical value of external quantum efficiency (EQE) of fluorescent CQDs is merely 5%, which fundamentally limits their further application in electroluminescent light-emitting diodes (LEDs). Soluble phosphorescent CQDs offer a means of breaking the shackle to achieve efficient monochromatic electroluminescence, especially red emission, which is a pivotal constituent in full-color displays. Here, the synthesis of red (625 nm) phosphorescent carbon quantum dot organic frameworks (CDOFs) with a quantum yield of up to 42.3% and realization of high-efficiency red phosphorescent electroluminescent LEDs are reported. The LEDs based on the CDOFs exhibited a red emission with a maximum luminance of 1818 cd m^-2 and an EQE of 5.6%. This work explores the possibility of a new perspective for developing high-performance CQD-based electroluminescent LEDs.

Photon management to reduce energy loss in perovskite solar cells

Despite the rapid development of perovskite solar cells (PSCs) over the past few years, the conversion of solar energy into electricity is not efficient enough or cost-competitive yet. The principal energy loss in the conversion of solar energy to electricity fundamentally originates from the non-absorption of low-energy photons ascribed to Shockley-Queisser limits and thermalization losses of high-energy photons. Enhancing the light-harvesting efficiency of the perovskite photoactive layer by developing efficient photo management strategies with functional materials and arrays remains a long-standing challenge. Here, we briefly review the historical research trials and future research trends to overcome the fundamental loss mechanisms in PSCs, including upconversion, downconversion, scattering, tandem/graded structures, texturing, anti-reflection, and luminescent solar concentrators. We will deeply emphasize the availability and analyze the importance of a fine device structure, fluorescence efficiency, material proportion, and integration position for performance improvement. The unique energy level structure arising from the 4fn inner shell configuration of the trivalent rare-earth ions gives multifarious options for efficient light-harvesting by upconversion and downconversion. Tandem or graded PSCs by combining a series of subcells with varying bandgaps seek to rectify the spectral mismatch. Plasmonic nanostructures function as a secondary light source to augment the light-trapping within the perovskite layer and carrier transporting layer, enabling enhanced carrier generation. Texturing the interior using controllable micro/nanoarrays can realize light-matter interactions. Anti-reflective coatings on the top glass cover of the PSCs bring about better transmission and glare reduction. Photon concentration through perovskite-based luminescent solar concentrators offers a path to increase efficiency at reduced cost and plays a role in building-integrated photovoltaics. Distinct from other published reviews, we here systematically and hierarchically present all of the photon management strategies in PSCs by presenting the theoretical possibilities and summarizing the experimental results, expecting to inspire future research in the field of photovoltaics, phototransistors, photoelectrochemical sensors, photocatalysis, and especially light-emitting diodes. We further assess the overall possibilities of the strategies based on ultimate efficiency prospects, material requirements, and developmental outlook.

Boosting the Conversion Efficiency Over 20% in MAPbI3 Perovskite Planar Solar Cells by Employing a Solution-Processed Aluminum-Doped Nickel Oxide Hole Collector

Recently, nickel oxide (NiO_x) thin films have been used as an efficient and robust hole transport layer (HTL) in inverted planar perovskite solar cells (IP-PSCs) to replace costly and unstable organic transport materials. However, the power conversion efficiency (PCE) of most IP-PSCs using NiO_x HTLs is rather limited below 20% due to insufficient electronic conductivity of the NiO_x. In this work, solution-processed Al-doped NiO_x (ANO) films are suggested as HTLs for low-cost and stable IP-PSCs. The electrical conductivity of the NiO_x film is significantly enhanced by Al doping, which effectively reduces the nonradiative recombination losses at the HTL-perovskite interfaces and boosts hole extraction/transportation. The device with undoped NiO_x shows the best PCE of 16.56%, whereas ANO HTL (5% doping) contributes to achieving a PCE of 20.84%, which outperforms other CH₃NH₃PbI₃ IP-PSCs with NiO_x-based HTLs reported to date. Moreover, a reliability test (1728 h storage) shows that the performance stability is enhanced by approximately 11% by employing ANO HTLs. This investigation into ANO HTLs provides a new guideline for the further development of highly efficient and reliable IP-PSCs using low-cost and robust metal oxide HTLs.

Integrated Perovskite/Bulk-Heterojunction Organic Solar Cells

The recently emerged integrated perovskite/bulk-heterojunction (BHJ) organic solar cells (IPOSCs) without any recombination layers have generated wide attention. This type of device structure can take the advantages of tandem cells using both perovskite solar and near-infrared (NIR) BHJ organic solar materials for wide-range sunlight absorption and the simple fabrication of single junction cells, as the low bandgap BHJ layer can provide additional light harvesting in the NIR region and the high open-circuit voltage can be maintained at the same time. This progress report highlights the recent developments in such IPOSCs and the possible challenges ahead. In addition, the recent development of perovskite solar cells and NIR organic solar cells is also covered to fully underline the importance and potential of IPOSCs.

Enhanced Near-Infrared Photoresponse of Inverted Perovskite Solar Cells Through Rational Design of Bulk-Heterojunction Electron-Transporting Layers

How to extend the photoresponse of perovskite solar cells (PVSCs) to the region of near-infrared (NIR)/infrared light has become an appealing research subject in this field since it can better harness the solar irradiation. Herein, the typical fullerene electron-transporting layer (ETL) of an inverted PVSC is systematically engineered to enhance device's NIR photoresponse. A low bandgap nonfullerene acceptor (NFA) is incorporated into the fullerene ETL aiming to intercept the NIR light passing through the device. However, despite forming type II charge transfer with fullerene, the blended NFA cannot enhance the device's NIR photoresponse, as limited by the poor dissociation of photoexciton induced by NIR light. Fortunately, it can be addressed by adding a p-type polymer. The ternary bulk-heterojunction (BHJ) ETL is demonstrated to effectively enhance the device's NIR photoresponse due to the better cascade-energy-level alignment and increased hole mobility. By further optimizing the morphology of such a BHJ ETL, the derived PVSC is finally demonstrated to possess a 40% external quantum efficiency at 800 nm with photoresponse extended to the NIR region (to 950 nm), contributing ≈9% of the overall photocurrent. This study unveils an effective and simple approach for enhancing the NIR photoresponse of inverted PVSCs.

Expanding the Light Harvesting of CsPbI2Br to Near Infrared by Integrating with Organic Bulk Heterojunction for Efficient and Stable Solar Cells

All-inorganic perovskite (CsPbX₃, X = Br or I) solar cells demonstrate superior stability, while the power conversion efficiency (PCE) lags behind the organic-inorganic hybrid counterparts mainly due to the limitation of narrow absorption bands. To broaden their absorption spectrum and improve their PCE, all-inorganic perovskite/organic integrated solar cells utilizing CsPbI₂Br as an ultraviolet-visible light absorber and PBDTTT-E-T:IEICO as a near-infrared light absorber are demonstrated in this work. The integrated solar cells exhibit a broadened photoresponse to over 900 nm, attributed to the integration of PBDTTT-E-T:IEICO. The additional absorption enhances the short-circuit current density from 14.78 to 15.98 mA/cm², resulting in greatly improved PCE of 14.03% for integrated solar cells, much higher than that of the control perovskite solar cells (12.53%) and organic solar cells (7.51%). An in-depth understanding of the charge-transfer dynamic process in the CsPbI₂Br/PBDTTT-E-T:IEICO film is comprehensively analyzed by photoinduced transient absorption spectroscopy. Furthermore, the air stability and thermal stability of the integrated solar cells are greatly enhanced. For unencapsulated integrated solar cells, the PCE still preserves 95% of its initial value after aging for 300 h in an ambient environment and retains about 90% of its original value even after aging at 85 °C for 180 h in nitrogen.

Optical Management with Nanoparticles for a Light Conversion Efficiency Enhancement in Inorganic γ-CsPbI3 Solar Cells

Recently, γ-CsPbI₃ perovskite solar cells (PSCs) have shown potential applications in optoelectronic devices, due to their high thermal stability. However, the incomplete utilization of the solar spectra especially in the near-infrared (ca. 46%) range significantly limits the power conversion efficiency (PCE). Herein, core-shell-structured NaLuF₄:Yb,Er@NaLuF₄ upconversion nanoparticles (UCNPs) have been successfully synthesized and integrated into the hole transport layer for improving PCE in γ-CsPbI₃ PSCs. Compared with the reference one, the short-circuit current density ( J_SC) and PCE of the optimized device reached up to 19.17 mA/cm² (18.81 mA/cm²) and 15.86% (15.51%), respectively. Actually, due to the ultralow photoluminescence quantum yield (PLQY, < 1%) obtained in UCNPs now, we proved the generally recognized upconversion effect of UCNPs in solar cells (adjusting the light absorption edge from the visible toward NIR range for extending the spectral absorption) was negligible. A further study found the UCNPs in the PSCs primarily served as scattering centers, which is beneficial to extend the sunlight optical path by combining with scattering and reflecting sunlight, leading to producing more photoelectric current. This study suggests a new insight into understanding the underlying mechanism of UCNPs in the PSCs and provides a promising strategy via light scattering effect to enhance the device performance.

Enhancing charge transport in an organic photoactive layer via vertical component engineering for efficient perovskite/organic integrated solar cells

Suitable vertical component distribution within an organic bulk-heterojunction (BHJ) is vital for effective exciton dissociation and smooth charge transport in perovskite/organic integrated solar cells (ISCs). Herein, a bi-continuous interpenetrating network of organic donor/acceptor materials is constructed simply by optimizing their weight ratio, and is further applied in perovskite/organic ISCs. Time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) and scanning Kelvin probe microscopy (SKPM) strongly confirm that this method can effectively restrict vertical stratification and build a desired bi-continuous framework within the organic photoactive layer, which can effectively suppress two potential recombination losses from the viewpoint of kinetics, leading to the PCE increasing from 12.63% to 15.47% for ISCs based on the structure of MAPbI3/PBDB-T : IEICO. Meanwhile, our ISCs combining a UV-vis harvesting layer of MAPbI3 and a near-infrared absorbing layer of PBDB-T : IEICO exhibit a photo-response extending to the whole visible and infrared spectrum (up to 900 nm). This work verifies that tuning the donor/acceptor weight ratio is a feasible strategy for optimizing the morphology of BHJ absorbers and suppressing charge recombination for efficient perovskite/BHJ ISCs.

All-solution-processed perovskite light-emitting diodes with all metal oxide transport layers

All-solution-processed perovskite light-emitting diodes (PeLEDs) with all metal oxide transport layers were successfully realized based on an ITO/NiOx/CsPbBr3/ZnMgO/Al conventional device structure. A unique perovskite-polymer composite method enables the deposition of solution-processed ZnMgO nanoparticles on the perovskite film. As a result, we achieved highly efficient PeLEDs with a maximum luminance of 17 017 cd m-2, and the efficiency showed little roll-off with increasing current density.