Electron accumulation across the perovskite layer enhances tandem solar cells with textured silicon.

Morphology control and device optimization for efficient organic solar cells

The charge carriers of single-junction solar cells can be fluently extracted and then collected by electrodes, leading to weak charge carrier accumulation and low energy loss (Eloss ). However, in tandem solar cells (TSCs), it is a considerable challenge to obtain a balance between the densities of the holes and electrons extracted from the two respective subcells to facilitate an efficient recombination in the interconnecting layer (ICL). Herein, a charge-carrier-dynamic management strategy for inorganic perovskite/organic TSCs is proposed, centered on the simultaneous regulation of the defect states of CsPbI1.9 Br1.1 perovskite in the front subcell and hole transport ability from the perovskite to ICL. The target hole density on the perovskite surface and the hole loss before reaching the ICL are significantly improved. As a result, the hole/electron density offset in the ICL can be effectively narrowed, leading to a balanced charge carrier recombination, which reduces the Eloss in TSCs. The resulting inorganic perovskite/organic 0.062-cm2 TSC exhibits a remarkable power conversion efficiency (PCE) of 23.17% with an ultrahigh open-circuit voltage (Voc ) of 2.15V, and the PCE of the 1.004-cm2 device (21.69%) exhibited a weak size-dependence. This charge-carrier-dynamic management strategy can also effectively enhance the operational and ultraviolet-light stabilities of the TSCs.

ABX3 Perovskites for Tandem Solar Cells

Third generation multi-layer tandem solar cells for achieving high conversion efficiencies

Inorganic–organic hybrid perovskites offer wide optical absorption, long charge carrier diffusion length, and high optical-to-electrical conversion, enabling more than 25% efficiency of single-junction perovskite solar cells. All-perovskite four-terminal (4T) tandem solar cells have gained great attention because of solution-processability and potentially high efficiency without a need for current-matching between subcells. To make the best use of a tandem architecture, the subcell bandgaps and thicknesses must be optimized. This study presents a drift-diffusion simulation model to find optimum device parameters for a 4T tandem cell exceeding 33% of efficiency. Optimized subcell bandgaps and thicknesses, contact workfunctions, charge transport layer doping and perovskite surface modification are investigated for all-perovskite 4T tandem solar cells. Also, using real material and device parameters, the impact of bulk and interface traps is investigated. It is observed that, despite high recombination losses, the 4T device can achieve very high efficiencies for a broad range of bandgap combinations. We obtained the best efficiency for top and bottom cell bandgaps close to 1.55 eV and 0.9 eV, respectively. The optimum thickness of the top and bottom cells are found to be about 250 nm and 450 nm, respectively. Furthermore, we investigated that doping in the hole transport layers in both the subcells can significantly improve tandem cell efficiency. The present study will provide the experimentalists an optimum device with optimized bandgaps, thicknesses, contact workfunctions, perovskite surface modification and doping in subcells, enabling high-efficiency all-perovskite 4T tandem solar cells.

Passivating defects at the wide‐bandgap perovskite/C60 interface without impeding interfacial charge transport can effectively enhance the efficiency of perovskite/silicon tandem solar cells (TSCs). Herein, we study the impact of benzene‐derivative ligands with elaborately modulated binding strength and acidity on wide‐bandgap perovskites for high‐performance perovskite/silicon TSCs. Specifically, the acidity/alkalinity and binding strength are preliminarily tuned using different functional groups of ‐PO₃H₂, ‐COOH, and ‐NH₂, and further finely adjusted by altering the chain lengths between the benzene ring and the functional groups. The results show that strong binding is indispensable for effectively suppressing voltage loss. However, the commonly used benzylphosphonic acid (BPPA) for firm surface binding exhibits too strong acidity that can etch the perovskite surface, resulting in halide‐vacancy defects and pronounced hysteresis. Increasing the side chain length of BPPA to (2‐phenylethyl)phosphonic acid not only enables a suitable acid dissociation constant (pKa) to avoid acid‐induced etching but also achieves robust anchoring to the perovskite surface with a parallel adsorption orientation, which reduces the charge transport barrier at the interface. These properties enable strong‐adsorption surface termination (SAST) of the perovskite surface while preventing acid‐induced etching. As a result, the SAST strategy achieves a remarkable efficiency of 32.13% (certified 31.72%) for hysteresis‐free perovskite/silicon TSCs.

Passivating defects at the wide-bandgap perovskite/C60 interface without impeding interfacial charge transport can effectively enhance the efficiency of perovskite/silicon tandem solar cells (TSCs). Herein, we study the impact of benzene-derivative ligands with elaborately modulated binding strength and acidity on wide-bandgap perovskites for high-performance perovskite/silicon TSCs. Specifically, the acidity/alkalinity and binding strength are preliminarily tuned using different functional groups of -PO₃H₂, -COOH, and -NH₂, and further finely adjusted by altering the chain lengths between the benzene ring and the functional groups. The results show that strong binding is indispensable for effectively suppressing voltage loss. However, the commonly used benzylphosphonic acid (BPPA) for firm surface binding exhibits too strong acidity that can etch the perovskite surface, resulting in halide-vacancy defects and pronounced hysteresis. Increasing the side chain length of BPPA to (2-phenylethyl)phosphonic acid not only enables a suitable acid dissociation constant (pKa) to avoid acid-induced etching but also achieves robust anchoring to the perovskite surface with a parallel adsorption orientation, which reduces the charge transport barrier at the interface. These properties enable strong-adsorption surface termination (SAST) of the perovskite surface while preventing acid-induced etching. As a result, the SAST strategy achieves a remarkable efficiency of 32.13% (certified 31.72%) for hysteresis-free perovskite/silicon TSCs.

Ordered Si nanowire (SiNW) arrays can be fabricated by metal-assisted chemical etching. The metal mesh films (MMFs) are extremely important for achieving a high quality of the SiNWs. We have developed a two-step chemical deposition method to obtain compact porous Ag MMFs. By the separation of the nucleation and growth stages of the metal in the two-step deposition processes, the overgrowth of the metals to form randomly aggregated irregular metal particles can be overcome. Hexagonally arranged polystyrene (PS) latex microspheres have been employed as a template for the deposition of porous Ag MMFs. The spacing of the pores in the Ag MMFs is determined by the diameter of PS microspheres, and the pore size can also be tuned by changing Ar plasma etching time. One of the main advantages of the two-step deposition method lies in that Ag MMFs can be produced with PS microspheres that are not limited to a single layer, which dramatically simplifies the tedious processes of producing a monolayered PS template. The two-step chemical deposition method shows great potential in metal-assisted chemical etching.

All‐perovskite tandem solar cells have garnered considerable attention because of their potential to outperform single‐junction cells. However, charge recombination losses within narrow‐bandgap (NBG) perovskite subcells hamper the advancement of this technology. Herein, we introduce a lithium salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), for modifying NBG perovskites. Interestingly, LiTFSI bifunctionally passivates the surface and bulk of NBG by dissociating into Li+ and TFSI− ions. We found that TFSI− passivates halide vacancies on the perovskite surface, reducing nonradiative recombination, while Li+ acts as an interstitial n‐type dopant, mitigating the defects of NBG perovskites and potentially suppressing halide migration. Furthermore, the underlying mechanism of LiTFSI passivation was investigated through the density functional theory calculations. Accordingly, LiTFSI facilitates charge extraction and extends the charge carrier lifetime, resulting in an NBG device with power conversion efficiency (PCE) of 22.04% (certified PCE of 21.42%) and an exceptional fill factor of 81.92%. This enables the fabrication of all‐perovskite tandem solar cells with PCEs of 27.47% and 26.27% for aperture areas of 0.0935 and 1.02 cm2, respectively.image

Hybrid organic-inorganic perovskites are one of the most promising material classes for photovoltaic energy conversion. In solar cells, the perovskite absorber is sandwiched between n- and p-type contact layers which selectively transport electrons and holes to the cell’s cathode and anode, respectively. This thesis aims to advance contact layers in perovskite solar cells and unravel the impact of interface and contact properties on the device performance. Further, the contact materials are applied in monolithic perovskite-silicon heterojunction (SHJ) tandem solar cells, which can overcome the single junction efficiency limits and attract increasing attention. Therefore, all contact layers must be highly transparent to foster light harvesting in the tandem solar cell design. Besides, the SHJ device restricts processing temperatures for the selective contacts to below 200°C. A comparative study of various electron selective contact materials, all processed below 180°C, in n-i-p type perovskite solar cells highlights that selective contacts and their interfaces to the absorber govern the overall device performance. Combining fullerenes and metal-oxides in a TiO2/PC60BM (phenyl-C60-butyric acid methyl ester) double-layer contact allows to merge good charge extraction with minimized interface recombination. The layer sequence thereby achieved high stabilized solar cell performances up to 18.0% and negligible current-voltage hysteresis, an otherwise pronounced phenomenon in this device design. Double-layer structures are therefore emphasized as a general concept to establish efficient and highly selective contacts. Based on this success, the concept to combine desired properties of different materials is transferred to the p-type contact. Here, a mixture of the small molecule Spiro-OMeTAD [2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluoren] and the doped polymer PEDOT [poly(3,4-ethylenedioxythiophene)] is presented as a novel hole selective contact. PEDOT thereby remarkably suppresses charge recombination at the perovskite surface, allowing an increase of quasi-Fermi level splitting in the absorber. Further, the addition of Spiro-OMeTAD into the PEDOT layer is shown to enhance charge extraction at the interface and allow high efficiencies up to 16.8%. Finally, the knowledge on contact properties is applied to monolithic perovskite-SHJ tandem solar cells. The main goal is to optimize the top contact stack of doped Spiro-OMeTAD/molybdenum oxide(MoOx)/ITO towards higher transparency by two different routes. First, fine-tuning of the ITO deposition to mitigate chemical reduction of MoOx and increase the transmittance of MoOx/ITO stacks by 25%. Second, replacing Spiro-OMeTAD with the alternative hole transport materials PEDOT/Spiro-OMeTAD mixtures, CuSCN or PTAA [poly(triaryl amine)]. Experimental results determine layer thickness constrains and validate optical simulations, which subsequently allow to realistically estimate the respective tandem device performances. As a result, PTAA represents the most promising replacement for Spiro-OMeTAD, with a projected increase of the optimum tandem device efficiency for the herein used architecture by 2.9% relative to 26.5% absolute. The results also reveal general guidelines for further performance gains of the technology.

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Indacenodithiophene-based wide bandgap copolymers for high performance single-junction and tandem polymer solar cells

Improved stability and efficiency of two-terminal monolithic perovskite-silicon tandem solar cells will require reductions in recombination losses. By combining a triple-halide perovskite (1.68 electron volt bandgap) with a piperazinium iodide interfacial modification, we improved the band alignment, reduced nonradiative recombination losses, and enhanced charge extraction at the electron-selective contact. Solar cells showed open-circuit voltages of up to 1.28 volts in p-i-n single junctions and 2.00 volts in perovskite-silicon tandem solar cells. The tandem cells achieve certified power conversion efficiencies of up to 32.5%.

Upscaling and stabilizing efficient wide‐bandgap perovskite solar cells (PSCs) are critical for the commercialization of tandem photovoltaics. Herein, solvent engineering is applied for scalable deposition of wide‐bandgap (≈1.72 eV) perovskite by the introduction of N‐methyl‐2‐pyrrolidone (NMP) additives, which enables compact and phase‐stable FA0.83Cs0.17Pb(I0.7Br0.3)3 perovskite even without the use of antisolvent. By further passivation with a 2‐thiophenemethylammonium bromide (2‐ThMABr)‐based quasi‐2D perovskite (n = 2) on the surface of 3D perovskite, a champion power conversion efficiency (PCE) of 19.46% with a higher open‐circuit voltage of 1.219 V for a small‐sized wide‐bandgap PSC is achieved. It also exhibits excellent long‐term stability, maintaining 93% of its initial PCE after 2000 h storage in the air without encapsulation. In addition, this wide‐bandgap perovskite is also easy to be upscaled via a blade‐coating strategy, which demonstrates high PCEs of 16.07% and 13.03% with active areas of 46.5 and 123.0 cm2, respectively. Furthermore, the application of this wide‐bandgap perovskite for four‐terminal (4‐T) perovskite/silicon (Si) tandem solar cells also demonstrates high PCEs of 23.85% and 19.51% with active areas of 0.16 and 1.0 cm2. The work demonstrates a great potential toward large‐area efficient and stable wide‐bandgap PSCs and perovskite/Si tandem cells.

A two-step deposition method has been developed that enables the conformal coating of textured surfaces with perovskite films. This allows the realization of perovskite/silicon tandem solar cells with increased short-circuit current density.