Organic photovoltaics: The current challenges

Organic photovoltaics are remarkably close to reaching a landmark power conversion efficiency of 20%. Given the current urgent concerns regarding climate change, research into renewable energy solutions is crucially important. In this perspective article, we highlight several key aspects of organic photovoltaics, ranging from fundamental understanding to implementation, that need to be addressed to ensure the success of this promising technology. We cover the intriguing ability of some acceptors to undergo efficient charge photogeneration in the absence of an energetic driving force and the effects of the resulting state hybridization. We explore one of the primary loss mechanisms of organic photovoltaics - non-radiative voltage losses - and the influence of the energy gap law. Triplet states are becoming increasingly relevant owing to their presence in even the most efficient non-fullerene blends, and we assess their role as both a loss mechanism and a potential strategy to enhance efficiency. Finally, two ways in which the implementation of organic photovoltaics can be simplified are addressed. The standard bulk heterojunction architecture could be superseded by either single material photovoltaics or sequentially deposited heterojunctions, and the attributes of both are considered. While several important challenges still lie ahead for organic photovoltaics, their future is, indeed, bright

Lewis Base Passivation Mediates Charge Transfer at Perovskite Heterojunctions

Understanding interfacial charge transfer processes such as trap-mediated recombination and injection into charge transport layers (CTLs) is crucial for the improvement of perovskite solar cells. Herein, we reveal that the chemical binding of charge transport layers to CH3NH3PbI3 defect sites is an integral part of the interfacial charge injection mechanism in both n-i-p and p-i-n architectures. Specifically, we use a mixture of optical and X-ray photoelectron spectroscopy to show that binding interactions occur via Lewis base interactions between electron-donating moieties on hole transport layers and the CH3NH3PbI3 surface. We then correlate the extent of binding with an improvement in the yield and longer lifetime of injected holes with transient absorption spectroscopy. Our results show that passivation-mediated charge transfer has been occurring undetected in some of the most common perovskite configurations and elucidate a key design rule for the chemical structure of next-generation CTLs

Effect of Interfacial Energetics on Charge Transfer from Lead Halide Perovskite to Organic Hole Conductors

The control and optimization of interfacial charge transfer processes is crucial to the design of efficient perovskite solar cells. Herein, we measure the yield and kinetics of hole transfer across the methylammonium lead triiodide perovskite|polymeric hole transport material heterojunction, as a function of the interfacial energy offset, ΔE, between the highest occupied molecular orbital of the hole transport material and the valence band edge of the perovskite. A combination of steady-state and time-resolved photoluminescence, along with transient absorption spectroscopy, revealed that only a small driving energy (ΔE ∼ 0.07 eV) is required to induce highly efficient hole transfer. The findings of this paper suggest that further improvements in the open-circuit voltage, and so the power conversion efficiency, of perovskite solar cells could be achieved by incorporating hole transport materials that provide an interfacial energy offset in the range 0 < ΔE < 0.18 eV

The Effect of Interfacial Energetics on Charge Transfer from Lead Halide Perovskite to Organic Hole Conductors

The control and optimization of interfacial charge transfer processes is crucial to the design of efficient perovskite solar cells. Herein, we measure the yield and kinetics of hole transfer across the methylammonium lead triiodide perovskite|polymeric hole transport material heterojunction, as a function of the interfacial energy offset, ∆E between the highest occupied molecular orbital of the hole transport material and the valence band of the perovskite. A combination of steady-state and time-resolved photoluminescence, along with transient absorption spectroscopy revealed that only a small driving energy (∆E~0.07eV) is required to induce highly efficient hole transfer. The findings of this paper suggest that further improvements in the open-circuit voltage, and so the power conversion efficiency, of perovskite solar cells could be achieved by incorporating hole transport materials that provide an interfacial energy offset in the range 0 < ∆E < 0.18eV

2D phase purity determines charge-transfer yield at 3D/2D lead halide perovskite heterojunctions

Targeted functionalization of 3D perovskite with a 2D passivation layer via R-NH3I treatment has emerged as an effective strategy for enhancing both the efficiency and chemical stability of ABX3 perovskite solar cells, but the underlying mechanisms behind these improvements remain unclear. Here, we assign a passivation mechanism where R-NH3I reacts with excess PbI2 in the MAPbI3 film and unsaturated PbI6 octahedra to form (R-NH3)2(MA)n-1PbnI3n+1. Crucially, we show that precise control of the 2D (R-NH3)2(MA)n-1PbnI3n+1 layer underpins performance improvements: n = 1 yields over a 2-fold improvement in hole injection to the HTL; n > 1 deteriorates hole injection. Ultrafast transient absorption spectroscopy suggests this n-dependence is rooted in the fact that fast (<6 ns) hole injection does not occur between the 3D and 2D layers. These results help explain contemporary empirical findings in the field and set out an important design rule for the further optimization of multidimensional perovskite optoelectronics

Surface passivation of perovskite films via Iodide salt coatings for enhanced stability of organic lead halide perovskite solar cells

Organic–inorganic halide perovskite materials have emerged as attractive alternatives to conventional solar cells, but device stability remains a concern. Recent research has demonstrated that the formation of superoxide species under exposure of the perovskite to light and oxygen leads to the degradation of CH3NH3PbI3 perovskites. In particular, it has been revealed that iodide vacancies in the perovskite are key sites in facilitating superoxide formation from oxygen. This paper shows that passivation of CH3NH3PbI3 films with an iodide salt, namely phenylethylammonium iodide (PhEtNH3I) can significantly enhance film and device stability under light and oxygen stress, without compromising power conversion efficiency. These observations are consistent with the iodide salt treatment reducing iodide vacancies and therefore lowers the yield of superoxide formation and improves stability. The present study elucidates a pathway to the future design and optimization of perovskite solar cells with greater stability