We demonstrate a monolithic perovskite/CIGS tandem solar cell with a certified power conversion efficiency (PCE) of 24.2%. The tandem solar cell still exhibits photocurrent mismatch between the subcells; thus optical simulations are used to determine the optimal device stack. Results reveal a high optical potential with the optimized device reaching a short-circuit current density of 19.9 mA cm −2 and 32% PCE based on semiempirical material properties. To evaluate its energy yield, we first determine the CIGS temperature coefficient, which is at −0.38% K −1 notably higher than the one from the perovskite subcell (−0.22% K −1 ), favoring perovskite in the field operation at elevated cell temperatures. Both single-junction cells, however, are significantly outperformed by the combined tandem device. The enhancement in energy output is more than 50% in the case of CIGS single-junction device. The results demonstrate the high potential of perovskite/CIGS tandem solar cells, for which we describe optical guidelines toward 30% PCE.

Non-fullerene acceptors based organic solar cells represent the frontier of the field, owing to both the materials and morphology manipulation innovations. Non-radiative recombination loss suppression and performance boosting are in the center of organic solar cell research. Here, we developed a non-monotonic intermediate state manipulation strategy for state-of-the-art organic solar cells by employing 1,3,5-trichlorobenzene as crystallization regulator, which optimizes the film crystallization process, regulates the self-organization of bulk-heterojunction in a non-monotonic manner, i.e., first enhancing and then relaxing the molecular aggregation. As a result, the excessive aggregation of non-fullerene acceptors is avoided and we have achieved efficient organic solar cells with reduced non-radiative recombination loss. In PM6:BTP-eC9 organic solar cell, our strategy successfully offers a record binary organic solar cell efficiency of 19.31% (18.93% certified) with very low non-radiative recombination loss of 0.190 eV. And lower non-radiative recombination loss of 0.168 eV is further achieved in PM1:BTP-eC9 organic solar cell (19.10% efficiency), giving great promise to future organic solar cell research.

The impact of a rubidium fluoride post deposition treatment (RbF-PDT) on the material and device properties of Cu(In,Ga)Se 2 (CIGS) thin films and corresponding solar cells is investigated. The structure and device properties of CIGS with different PDT duration are compared. With longer PDT duration, which equals a higher amount of RbF deposited on the CIGS absorber layer, a clear trade-off is observed between increasing open-circuitvoltage (V OC ) and decreasing fill factor (FF). An optimum of the PDT duration is found increasing the efficiency by about 0.8% (absolute) compared to the Rb-free reference device. The mechanisms behind the increased V OC are explored by various characterization methods and identified as a combination of increased carrier concentration and reduced recombination rates in the device. Possible origins for these mechanisms are discussed. Furthermore numerical simulations are used to analyze the detrimental effect of the PDT on the FF. It is found that thermally activated alkali migration into the transparent front contact could create acceptor states there, which could explain the observed FF-loss.

In this contribution, the effectiveness of an RbF post deposition treatment (PDT) is evaluated in dependence on the Cu content of the absorber layer of Cu(In,Ga)Se 2 solar cells. It is shown that the PDT only acts beneficially on the open-circuit voltage and fill factor (FF) on samples with rather high Cu content, while it deteriorates all parameters of the solar cells in samples with low Cu content. In order to clarify the behavior of the open-circuit voltage, the well-known exchange mechanism of Rb and Na during the PDT is analyzed as a function of the Cu content of the absorber layers and discussed in regard to theoretical publications. Furthermore, a model explaining the observed effect on the FF based on the formation of an RbInSe 2 (RIS) layer during the RbF-PDT is proposed. The model supposes that the RIS layer acts as a barrier for the photocurrent and therefore lowers the FF. It is evaluated theoretically in dependence of the properties of the RIS layer using one-dimensional solar cell capacitance simulator (SCAPS) simulations. Finally, the proposed model is also tested and confirmed experimentally by directly depositing RIS onto untreated Cu(In,Ga)Se 2 layers. Index Terms-Cu(In,Ga)Se 2 (CIGS) solar cells, heavy alkali metals, RbF-PDT, RbInSe 2 (RIS).

Using in situ photoluminescence measurements during the spin-coating and annealing steps, we probed the formation of 2D layers on 3D triple cation perovskite films comparing phenylethylammonium and 2-thiophenemethylammonium iodide bulky cations. We elucidate the formation mechanisms of the surface layers for both cases and reveal two regimes during 2D layer formation: a kinetic-driven and a thermodynamic-driven process. These driving forces result in different compositions of the 2D/3D interface for each treatment; namely, different ratios of pure 2D (n = 1) and quasi-2D (n > 1) structures. We show that a higher ratio of quasi-2D phases is more beneficial for device performance, as pure-2D layers may hamper current extraction. Due to a more evenly distributed formation energy profile among 2D and quasi-2D phases, highly concentrated 2-thiophenemethylammonium iodide appears to be more suited for effective surface passivation than its phenylethylammonium analog.

Black-phase formamidinium lead iodide perovskite (FAPbI3), whilst the most promising species for efficient perovskite photovoltaics, is energetically unfavored at room temperature, and is thus always accompanied by undesirable yellow phases during crystallization 1,2,3,4 . The challenge to formulate the fast crystallization process of perovskite has limited the community in deriving unified guidelines for governing the formation of black-phase FAPbI3 5,6 . Here, through in-situ monitoring of the perovskite crystallization process, we report an oriented nucleation mechanism that acted as the key to avoid undesirable phases. This concept was applicable to improving the photovoltaic device performance under different film-processing scenarios. The small-area device demonstrated a power conversion efficiency of 25.4% (certified 25.0%), and the module (27.83 cm 2 ) achieved a champion aperture efficiency of 21.4% (certified). MainFormamidinium lead iodide perovskite (FAPbI3) features desirable bandgap and thermal resistance, and has thus emerged as the most promising candidate among the perovskite family for highly efficient photovoltaic devices 1,2,3,7,8 . However, the photoactive black-phase FAPbI3 is not energetically favorable at room temperature 4,9,10 .Polytype formation and other intermediate non-photoactive phases can readily occur, which undermines the photovoltaic performance. A few approaches have been developed to promote the formation of black-phase FAPbI3 at room temperature, such as adduct formation with PbI2 and solvent engineering using ionic liquids 11,12 .

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Latest record efficiencies of Cu(In,Ga)Se2 (CIGSe) solar cells were achieved by means of a rubidium fluoride (RbF) post-deposition treatment (PDT). To understand the effect of the RbF-PDT on the surface chemistry of CIGSe and its interaction with sodium that is generally present in the CIGSe absorber, we performed an X-ray photoelectron spectroscopy (XPS) study on CIGSe thin films as-deposited by a three-stage co-evaporation process and after the consecutive RbF-PDT. The sample transfer from the deposition to the XPS analysis chamber was performed via an ultra-high vacuum transfer system. This allows to minimize air exposure, avoiding oxide formation on the CIGSe surface, especially for alkali-treated absorbers. Beside an expected reduction of Cu-and Ga-content at the surface of RbF-treated CIGSe films, we find that Rb penetrates the CIGSe and, contrary to fluorine, it is not completely removed by subsequent ammonia etching. The remaining Rb contribution at 110.0 eV binding energy, which appears after the RbF-PDT is similar to the one detected on a coevaporated RbInSe2 reference sample and together with a new Se 3d contribution may hence belong to an Rb-In-Se secondary phase on the CIGSe surface. In addition, Na is driven towards the surface of the CIGSe absorber as a direct result of the RbF-PDT. This proves the ion-exchange mechanism in the absence of moisture and air/oxygen between heavy Rb atoms incorporated via PDT and lighter Na atoms supplied by the glass substrate. A remaining XPS signal of Na 1s is observed after etching the vacuum transferred RbF-CIGSe sample, indicating that Rb and/or F are not as much a driving force for Na as oxygen usually is.