We introduce new hole-selective contacts for next-generation perovskite photovoltaics and point to design paths for molecular engineering of perfect interfaces.

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.

Perovskite-based tandem solar cells have proven to be suitable candidates to increase the power conversion efficiency (PCE) of conventional single-junction photovoltaic devices, such as those based on silicon and Cu(In,Ga)Se 2 (CIGSe) absorbers, beyond the Shockley-Queisser single-junction PCE limit. Here, we present a highly efficient monolithic perovskite/CIGSe tandem solar cell with a solution processed perovskite top cell fabricated directly on an asgrown, rough CIGSe bottom cell. To prevent potential shunting due to the rough CIGSe surface, a thin NiO x layer is conformally deposited via atomic layer deposition (ALD) on the ITO front contact of the CIGSe bottom cell. The performance is further improved by an additional layer of the p-type polymer PTAA at the NiO/perovskite interface. This novel hole transport bilayer enables a 21.6% stabilized PCE of the monolithic perovskite/CIGSe tandem device at 0.778 cm 2 active area. We use TEM/EDX measurements to investigate the deposition uniformity and conformality of the NiO x and PTAA layers. By comparing the performance of single-junction subcells with absolute photoluminescence measurements, we determine the contribution of the individual subcells to the tandem V OC , revealing that further fine-tuning of the recombination layers between the two subcells might improve the tandem V OC further. Finally, based on the obtained results we give guidelines on how to further improve monolithic perovskite/CIGSe tandems towards predicted PCE estimates above 30%.

We propose and test monolithic perovskite/CIGS tandem solar cells for readily stowable, ultra-lightweight space photovoltaics. We design operando and ex situ measurements to show that perovskite/CIGS tandem solar cells retain over 85% of their initial power-conversion efficiency after high-energy proton irradiation. While the perovskite sub-cell is unaffected after this bombardment, we identify increased non-radiative recombination in the CIGS bottom cell and nickel-oxide-based recombination layer. By contrast, monolithic perovskite/silicon-heterojunction cells degrade to 1% of their initial efficiency due to radiation-induced defects in silicon.

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).

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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.

We compare the dopant concentration of polycrystalline Cu(In,Ga)Se 2 thin film absorbers derived from Hall and capacitance-voltage measurements. Although both measurements techniques appear to be reliable, dopant concentrations determined by capacitance-voltage analysis are significantly lower and vary with probing depth into the absorber. The doping profiles and differences between both measurement techniques are consistent with Cd in-diffusion from the CdS buffer layer during solar cell fabrication. Different doping profiles obtained after a variation of the CdS deposition process support this scenario.