The aim of this work is to investigate the degradation of perovskite solar cells (PSCs) by means of impedance spectroscopy, a highly sensitive characterization technique used to establish the electrical response of a device in a nondestructive manner. In this paper, PSCs with two different electron transport layers (ETLs) are studied: PSCs with undoped SnO₂ as an ETL are compared to PSCs with an ETL composed of graphene-doped SnO₂ (G-SnO₂). Experimental data were collected immediately after fabrication and after one week, monitoring both impedance spectroscopy and dark current-voltage (I-V) curves. It was observed that, in the case of the undoped PSCs, the degradation of the solar cells affected both the AC behavior of the devices, modifying the associated Nyquist plots, and the DC behavior, observable from the dark I-V measurements. Conversely, the solar cells with G-SnO₂ showed no variation. Considering the Nyquist plots, a quantitative analysis was performed by comparing the parameters of a proper equivalent circuit model. The results were coherent with those achieved in the DC analysis, thus proving that the analysis of impedance spectra, supported with dark I-V curves, allows one to gain a deeper knowledge of the degradation phenomena of perovskite solar cells. This study opens the door for further improvement of these devices through a better understanding of their electrical behavior.

In this work, front contacts for graphene-based solar cells are made by means of colloidal graphite instead of gold. The performance is characterized by exploiting impedance spectroscopy and is compared to the standard gold contact technology. Impedance data are analysed through equivalent circuit representation in terms of lumped parameters, suitable to describe the complex impedance in the frequency range considered in the experiments. Using this approach, capacitance–voltage of the considered graphene–silicon solar cell is found and the barrier height forming at the graphene–silicon interface is extracted.

In this article, the effect of graphene/silicon Schottky junction solar cells (G/Si SJSCs) encapsulation is investigated in terms of capacitance–voltage ( C – V ) curves in the forward bias region. Capacitance behavior can give accurate information on interface properties, especially when it is considered at positive voltage. The detection of a double peak on such curve is related to nonuniform interface properties at the graphene/silicon interface. To passivate the interface, G/Si SJSCs have been exposed to HNO $_{\text{3}}$ vapors and the effectiveness of HNO $_{\text{3}}$ doping has been exploited as well as its volatility, assessing the need for encapsulation. C – V curves collected over time from doped unencapsulated G/Si SJSCs clearly exhibited a modification of capacitance behavior, revealing a fading of doping beneficial effects over a week. On the contrary, C – V curves acquired from encapsulated samples evidence that capacitance behavior did not modify over time.

Substoichiometric molybdenum oxide (MoO_x) has good potential as a hole-collecting layer in solar cells. In this paper, we report on the application of ultrathin evaporated MoO_x as a hole collector at the back side of two distinct photovoltaic technologies: polymeric and silicon heterojunction (SHJ). In the case of polymer solar cells, we test MoO_x as a hole transport layer in devices with inverted architecture. The higher transparency of the MoO_x film, compared to the commonly used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), allows an enhanced back reflected light into the photoactive layer, thus boosting the photogeneration, as found from the illuminated J-V and external quantum efficiency (EQE) curves. The higher fill factor (FF) of the MoO_x-based device also suggests an improved charge collection efficiency compared to the cells with PEDOT:PSS. As for SHJ solar cells, we show that MoO_x offers the means for dopant-free hole collection with both p-type and n-type Si wafers. In the present comparison over planar test structures with Ag back reflecting electrodes, we observe an efficiency gain of approximately 1% absolute against a baseline with a conventional p-type amorphous silicon hole collector. The gain is linked to the increased V_OC, which is likely due to the reduced recombination at the Si wafer.

Perovskite/silicon tandem solar cells have strong potential for high efficiency and low cost photovoltaics. In monolithic (two-terminal) configurations, one key element is the interconnection region of the two subcells, which should be designed for optimal light management and prevention of parasitic p/n junctions. We investigated monolithic perovskite/silicon-heterojunction (SHJ) tandem solar cells with a p/n nanocrystalline silicon/silicon-oxide recombination junction for improved infrared light management. This design can additionally provide for resilience to shunts and simplified cell processing. We probed modified SHJ solar cells, made from double-side polished n-type Si wafers, which included the proposed front-side p/n tunnel junction with the p-type film simultaneously functioning as selective charge transport layer for the SHJ bottom cell, trying different thicknesses for the n-type layer. Full tandem devices were then tested, by applying a planar n-i-p mixed-cation mixed-halide perovskite top cell, fabricated via low temperature solution methods to be compatible with the processed Si wafer. We demonstrate the feasibility of this tandem cell configuration over a 1 cm² area with negligible J-V hysteresis and a V_OC ~1.8 V, matching the sum of the V_OC-s contributed by the two components.

Characterizing the electron transport layer (ETL)/perovskite interface in perovskite solar cells (PSCs) is of paramount importance for their overall performance. In this article the effect of different concentrations of graphene nanoplatelets in addition to SnO ₂ is investigated by considering degradation over time. PSCs behavior is monitored by collecting dark current–voltage curves as fabricated and after two months. A deeper insight is gained through impedance spectroscopy analysis. From Nyquist plots equivalent circuit models and the corresponding time constants are extracted. Moreover, resistive part of the impedance associated with high frequency has been related to static shunt resistance, assessing one of the considered ETL doping concentration as the more suitable choice to reduce degradation.