Researchers from Helmholtz-Zentrum Berlin (HZB), Kaunas University of Technology (KTU) in Lithuania, and additional partners have developed a non-fullerene electron transport material (ETM) built around a carborane core, reporting improved performance over the standard C60 layer in both single-junction perovskite cells and perovskite/silicon tandems. The material has been patented and is already commercially available.
C60 remains the default ETM in high-efficiency p-i-n perovskite single-junction and multi-junction devices, and nearly all published record-efficiency p-i-n perovskite/silicon tandems have used it. Its appeal lies in fast electron extraction from the absorber, high electron mobility, and compatibility with thermal evaporation, which allows thin, conformal layers to be deposited over large substrate areas. But it carries several drawbacks: substantial non-radiative recombination at the perovskite/C60 interface that caps open-circuit voltage (VOC), high material cost, mechanically weak interfaces prone to delamination, sensitivity to ambient oxygen, and high parasitic absorption - the last of which is especially costly in tandem architectures, where light enters through the ETM.
Interest in non-fullerene alternatives has been growing, but most reported candidates have been demonstrated in single-junction devices with lower-bandgap perovskites, and many rely on spin-coating, which limits large-area processing and compatibility with textured surfaces. Carborane-based molecules have appeared in perovskite photovoltaics before - as interfacial passivation layers, post-treatments, thermal-regulation interlayers, and components of bilayer hole-transporting contacts - but in each case as an auxiliary layer rather than a stand-alone charge transport material.
The new molecule pairs a meta-carborane core with two 9-fluorenylidene malononitrile groups (mCB-FMN) and is made from commercially available reagents via a three-step synthesis. It is deposited by thermal evaporation, at lower temperatures than C60 - reducing the energy required and the thermal load on the equipment - and forms uniform films even at the 4 nm thickness used in the devices, as confirmed by KPFM and SEM.
Characterization pointed to favorable energetic alignment with the perovskite alongside lower optical absorption than C60. Transient surface photovoltage, transient photoluminescence, and steady-state PL measurements indicated efficient electron extraction with strongly suppressed interfacial non-radiative recombination. DFT calculations suggested that the nitrile nitrogen interacts with undercoordinated Pb2+ sites and PbI antisite defects at the perovskite surface, implying a chemical passivation effect. The team also reported reduced oxygen-induced degradation at the interface, improved nucleation of the ALD-grown SnOx buffer (seen in in-situ ellipsometry, XPS, and SEM), and higher tensile strength across the perovskite/ETM/SnOx stack in mechanical adhesion tests.
In opaque p-i-n single-junction devices, replacing C60 with mCB-FMN improved power conversion efficiency by 1.5% absolute, driven by a 110 meV gain in VOC. In a proof-of-concept monolithic perovskite/silicon tandem, the mCB-FMN device reached 31.3% PCE - 2.4% absolute above the C60 reference - with gains in both VOC and short-circuit current density, the latter helped by the material's wide optical bandgap and correspondingly lower parasitic absorption.
The researchers specified a remaining gap: while mCB-FMN films showed improved stability under illumination and heat, reduced air sensitivity, and excellent scan stability, operational stability under continuous maximum power point tracking still trails the C60 reference. Their analysis attributes this to degradation at the perovskite/mCB-FMN interface - likely perovskite-induced chemical decomposition of the ETM - rather than to instability of the molecule itself. Spacer layers or targeted modifications to the molecule are suggested as routes to address it. A molecular fragment study and variation of the carborane core, both included in the work, are offered as guidance for designing future ETMs in this class.
"We have developed a very high-performance substitute material for fullerenes in perovskite solar cells, and we have demonstrated its benefits through different measurements," said HZB's Lea Zimmermann, first author of the study.
The material was selected for the "Best Scientific Content Award" at the 2025 TandemPV International Workshop, and a European patent application (EP 25175871.0) has been filed covering mCB-FMN, its derivatives, and their use in solar cells. According to Steve Albrecht, who led the study, Dyenamo has brought the material to market to enable wider use.
Albrecht's group previously worked on self-assembling monolayers (SAMs) for the hole-transporting side of the cell, in collaboration with international partners, and is now pursuing the same for the electron transport layer. The team says it is developing further materials in this class, which it believes could have broad implications for tandem solar cells.
