Perovskite solar cells with all‐organic transport layers exhibit efficiencies rivaling their counterparts that employ inorganic transport layers, while avoiding high‐temperature processing. Herein, ...it is investigated how the choice of the fullerene derivative employed in the electron‐transporting layer of inverted perovskite cells affects the open‐circuit voltage (VOC). It is shown that nonradiative recombination mediated by the electron‐transporting layer is the limiting factor for the VOC in the cells. By inserting an ultrathin layer of an insulating polymer between the active CH3NH3PbI3 perovskite and the fullerene, an external radiative efficiency of up to 0.3%, a VOC as high as 1.16 V, and a power conversion efficiency of 19.4% are realized. The results show that the reduction of nonradiative recombination due to charge‐blocking at the perovskite/organic interface is more important than proper level alignment in the search for ideal selective contacts toward high VOC and efficiency.
Perovskite solar cells have emerged as one of the most promising solar‐cell technologies for the next generation of photovoltaics. Despite simple and cheap processing, the solar cells exhibit comparably little loss in open‐circuit voltage. An approach to further reduce this loss by optimizing the charge selectivity of the fullerene‐based electron contact in efficient devices is shown.
Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit ...voltage (
V
OC
) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in
pin
- and
nip
-type cells including high efficiency devices (21.4%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the
V
OC
by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the
V
OC
of the device. Importantly, the
V
OC
equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the
V
OC
is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the
p
-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the
V
OC
. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces.
We quantify recombination losses in the bulk and interfaces for different perovskite compositions and popular charge transport layers.
A high doping technique has been widely used for record-efficiency crystalline silicon (Si) solar cells to minimize the series resistance losses and to form a back surface field. However, it requires ...high temperatures (up to 1000 degree C) and involves toxic gases, which may not be compatible for hybrid organic-silicon solar cells. Here, we report that a high power conversion efficiency (PCE) of 13.7% with a device area of 0.8 cm super(2) has been achieved for organic-nanostructured Si hybrid solar cells by inserting a cesium carbonate (Cs sub(2)CO sub(3)) layer between Si and the rear electrode aluminium (Al), which is realized by a solution process under low-temperature annealing (<150 degree C). Transient and constant current-voltage, capacitance-voltage, and scanning Kelvin probe microscope measurements are used to characterize the effect of the Cs sub(2)CO sub(3) layer on the device performance. The insertion of Cs sub(2)CO sub(3) not only decreased the contact resistance, but also generated a built-in electric field on the rear electrode. The recombination rates are suppressed at the back surface due to the deflection of minority carriers. These findings show a promising strategy to achieve high performance organic-silicon solar cells with a simple, low temperature and cost effective process.
One of the most promising routes for the future fabrication of solution-processable high-performance solar cells is to employ metal halide perovskites as photoactive material combined with organic ...semiconductors as charge extraction layers. An essential requirement to obtain high device performance is a proper energy level alignment across the device interfaces. Here, we investigate the interface between a triple cation perovskite and a prototypical electron acceptor molecule. Photoemission spectroscopy reveals a ground state charge transfer induced band bending on either side of the junction, which significantly alters the charge extraction barriers as compared to assumed vacuum level alignment and flat-band conditions. In addition, we demonstrate that upon white light illumination, the energy levels of the organic layer exhibit rigid shifts by up to 0.26 eV with respect to those of the perovskite, revealing a non-constant energy offset between the frontier energy levels of the two materials. Such level shifts of the organic transport layer are fully reversible upon switching on/off the light, indicating an electrostatic origin of this phenomenon caused by unbalanced distribution of photogenerated charge carriers. We therefore stress the importance of determining the energy level alignment at perovskite-based interfaces not only in the electronic ground state (dark) but also under device operating conditions (
operando
) to enable for a reliable correlation with the device performance.
Perovskite solar cells are among the most exciting photovoltaic systems as they combine low recombination losses, ease of fabrication, and high spectral tunability. The Achilles heel of this ...technology is the device stability due to the ionic nature of the perovskite crystal, rendering it highly hygroscopic, and the extensive diffusion of ions especially at increased temperatures. Herein, we demonstrate the application of a simple solution-processed perfluorinated self-assembled monolayer (p-SAM) that not only enhances the solar cell efficiency, but also improves the stability of the perovskite absorber and, in turn, the solar cell under increased temperature or humid conditions. The p–i–n-type perovskite devices employing these SAMs exhibited power conversion efficiencies surpassing 21%. Notably, the best performing devices are stable under standardized maximum power point operation at 85 °C in inert atmosphere (ISOS-L-2) for more than 250 h and exhibit superior humidity resilience, maintaining ∼95% device performance even if stored in humid air in ambient conditions over months (∼3000 h, ISOS-D-1). Our work, therefore, demonstrates a strategy towards efficient and stable perovskite solar cells with easily deposited functional interlayers.
Perovskite semiconductors are an attractive option to overcome the limitations of established silicon based photovoltaic (PV) technologies due to their exceptional opto‐electronic properties and ...their successful integration into multijunction cells. However, the performance of single‐ and multijunction cells is largely limited by significant nonradiative recombination at the perovskite/organic electron transport layer junctions. In this work, the cause of interfacial recombination at the perovskite/C60 interface is revealed via a combination of photoluminescence, photoelectron spectroscopy, and first‐principle numerical simulations. It is found that the most significant contribution to the total C60‐induced recombination loss occurs within the first monolayer of C60, rather than in the bulk of C60 or at the perovskite surface. The experiments show that the C60 molecules act as deep trap states when in direct contact with the perovskite. It is further demonstrated that by reducing the surface coverage of C60, the radiative efficiency of the bare perovskite layer can be retained. The findings of this work pave the way toward overcoming one of the most critical remaining performance losses in perovskite solar cells.
Nonradiative recombination induced by C60 is limiting the performance of pin type perovskite solar cells and remains poorly understood. In this manuscript, the possible recombination pathways are systematically examined and it is discovered that across‐interface recombination dominates. A point contact strategy is suggested to circumvent the loss, paving the way to improved pin type perovskite solar cells.
Abstract
Inverted perovskite solar cells still suffer from significant non-radiative recombination losses at the perovskite surface and across the perovskite/C
60
interface, limiting the future ...development of perovskite-based single- and multi-junction photovoltaics. Therefore, more effective inter- or transport layers are urgently required. To tackle these recombination losses, we introduce ortho-carborane as an interlayer material that has a spherical molecular structure and a three-dimensional aromaticity. Based on a variety of experimental techniques, we show that ortho-carborane decorated with phenylamino groups effectively passivates the perovskite surface and essentially eliminates the non-radiative recombination loss across the perovskite/C
60
interface with high thermal stability. We further demonstrate the potential of carborane as an electron transport material, facilitating electron extraction while blocking holes from the interface. The resulting inverted perovskite solar cells deliver a power conversion efficiency of over 23% with a low non-radiative voltage loss of 110 mV, and retain >97% of the initial efficiency after 400 h of maximum power point tracking. Overall, the designed carborane based interlayer simultaneously enables passivation, electron-transport and hole-blocking and paves the way toward more efficient and stable perovskite solar cells.
With power conversion efficiencies of perovskite-on-silicon and all-perovskite tandem solar cells increasing at rapid pace, wide bandgap (>1.7 eV) metal-halide perovskites (MHPs) are becoming a major ...focus of academic and industrial photovoltaic research. Compared to their lower bandgap (≤1.6 eV) counterparts, these types of perovskites suffer from higher levels of non-radiative losses in both the bulk material and in device configurations, constraining their efficiencies far below their thermodynamic potential. In this work, we investigate the energy losses in methylammonium (MA) free high-Br-content wide bandgap perovskites by using a combination of THz spectroscopy, steady-state and time-resolved photoluminescence, coupled with drift-diffusion simulations. The investigation of this system allows us to study charge-carrier recombination in these materials and devices in the absence of halide segregation due to the photostabilty of formamidinium-cesium based lead halide perovskites. We find that these perovskites are characterised by large non-radiative recombination losses in the bulk material and that the interfaces with transport layers in solar cell devices strongly limit their open-circuit voltage. In particular, we discover that the interface with the hole transport layer performs particularly poorly, in contrast to 1.6 eV bandgap MHPs which are generally limited by the interface with the electron-transport layer. To overcome these losses, we incorporate and investigate the recombination mechanisms present with perovskites treated with the ionic additive 1-butyl-1-methylpipiderinium tetrafluoroborate. We find that this additive not only improves the radiative efficiency of the bulk perovskite, but also reduces the non-radiative recombination at both the hole and electron transport layer interfaces of full photovoltaic devices. In addition to unravelling the beneficial effect of this specific treatment, we further optimise our solar cells by introducing an additional LiF interface treatment at the electron transport layer interface. Together these treatments enable MA-free 1.79 eV bandgap perovskite solar cells with open-circuit voltages of 1.22 V and power conversion efficiencies approaching 17%, which is among the highest reported for this material system.
We identify the limiting factors of wide bandgap metal halide perovskite solar cells. To overcome these losses, we developed an efficient optimisation strategy and outline the necessary steps for the continued development of these perovskites.
Engineering the interface between the perovskite absorber and the charge-transporting layers has become an important method for improving the charge extraction and open-circuit voltage (V OC) of ...hybrid perovskite solar cells. Conjugated polymers are particularly suited to form the hole-transporting layer, but their hydrophobicity renders it difficult to solution-process the perovskite absorber on top. Herein, oxygen plasma treatment is introduced as a simple means to change the surface energy and work function of hydrophobic polymer interlayers for use as p-contacts in perovskite solar cells. We find that upon oxygen plasma treatment, the hydrophobic surfaces of different prototypical p-type polymers became sufficiently hydrophilic to enable subsequent perovskite junction processing. In addition, the oxygen plasma treatment also increased the ionization potential of the polymer such that it became closer to the valance band energy of the perovskite. It was also found that the oxygen plasma treatment could increase the electrical conductivity of the p-type polymers, facilitating more efficient charge extraction. On the basis of this concept, inverted MAPbI3 perovskite devices with different oxygen plasma-treated polymers such as P3HT, P3OT, polyTPD, or PTAA were fabricated with power conversion efficiencies of up to 19%.
Photovoltaic cells based on halide perovskites, possessing remarkably high power conversion efficiencies have been reported. To push the development of such devices further, a comprehensive and ...reliable understanding of their electronic properties is essential but presently not available. To provide a solid foundation for understanding the electronic properties of polycrystalline thin films, we employ single-crystal band structure data from angle-resolved photoemission measurements. For two prototypical perovskites (CH3NH3PbBr3 and CH3NH3PbI3), we reveal the band dispersion in two high-symmetry directions and identify the global valence band maxima. With these benchmark data, we construct “standard” photoemission spectra from polycrystalline thin film samples and resolve challenges discussed in the literature for determining the valence band onset with high reliability. Within the framework laid out here, the consistency of relating the energy level alignment in perovskite-based photovoltaic and optoelectronic devices with their functional parameters is substantially enhanced.
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