Photophysics in emerging photovoltaics

Emerging photovoltaic technologies continue to march forward with power conversion efficiencies of lead halide perovskite solar cells (PSCs) nearing 26%, perovskite-Si tandems now exceeding the single junction detailed balance limit of 33%, and organic solar cells (OSCs) showing efficiencies nearing 20%. Yet, some of the photophysics underpinning PSCs and OSCs remain unresolved. A better understanding will enable more efficient and photostable solar cells and optoelectronic devices in the future. This special issue collects the latest cutting-edge research in the photophysics of PSCs and OSCs, contributed by experts on spectroscopy, theory, device physics, and materials science.

Organic chromophores, with their ubiquity in both biological and synthetic systems, serve as the backbone for excited-state chemical physics. There is a fertile exchange of ideas based on novel characterization techniques with strong relevance to emerging photovoltaic concepts. The papers in this collection show the utility of the close integration of theory and experiment in elucidating many of these unknown phenomena. Stuart and Huang¹ employ transient absorption spectroscopy of common acenes in solution to demonstrate that intersystem crossing has previously been misattributed as singlet fission in many scenarios, showing the importance of properly accounting for intersystem crossing in studies of singlet fission. Bittner and Silva² combine theoretical analysis with ultrafast double-quantum coherence spectroscopy to provide a detailed investigation into the stability of biexcitons in hybrid HJ aggregates of conjugated polymers. Kong and Xia³ investigate the excited-state charge transfer in novel donor–pi–acceptor chromophores using ultrafast spectroscopy and theoretical calculations, elucidating the roles that conformational relaxation in the excited state plays in carrier dynamics. This close integration of theory and experiment is critical for concerted progress in the field of molecular photophysics.

A dominant theme of the papers in this collection concerns the development and application of complex theoretical formalisms to accelerate and uncover the light–matter interactions in molecules used in high-performance novel photovoltaics. As the theoretical description of excited states in molecular aggregates is well known to be computationally demanding, Liu and Andrienko⁴ introduce a model Hamiltonian for thiophene hexamers and several non-fullerene acceptors that qualitatively predicts experimentally measured spectra at a dramatically reduced cost. Similarly, internal conversion and intersystem-crossing transitions are notoriously difficult to calculate, but Shi and Troisi⁵ introduce an approach to accelerate these calculations via a systematic reduction in the number of vibrational modes, reproducing experimental trends and opening the door to the virtual screening of nonradiative rates in molecular materials. Cainelli and Tanimura⁶ investigate the influence of mixing of Frenkel and charge-transfer states on the spectral properties of perylene bisimide derivatives, showcasing the importance of CT states in exciton localization. Sahoo and Patterson⁷ employ configuration interaction singles calculations on polythiophene chains to disentangle the competing effects of single-chain and aggregate interactions on the experimental electroabsorption profiles. Huang and Risko⁸ report on quantum-chemical calculations of the excited states of trialkyltetrelethynyl acene aggregates as a function of molecular packing, correlating packing motifs with the potential for promoting singlet fission. Mondelo-Martell and Burghardt⁹ report on quantum dynamical simulations of inter-chain exciton transport in P3HT using the multi-layer multi-configuration time-dependent Hartree method, calculating exciton diffusion coefficients that increase non-linearly with temperature. The richness and diversity of these theoretical developments demonstrate that the theoretical analysis of molecular photochemistry is alive and well in the community.

The photophysics of perovskites continues to be a productive avenue for both fundamental insights and the practical understanding of this class of photovoltaic materials. Chen and Lu¹⁰ utilize transient absorption spectroscopy to elucidate the photophysics of unconventional 1D perovskites with functional organic chromophores with strong electron accepting functionality. Handa and Kanemitsu¹¹ showcase the ability of time-resolved pulse-excitation techniques to quantify the steady-state carrier density and lifetimes of metal halide perovskite solar cells, identifying a scaling law between monomolecular recombination lifetime and open-circuit voltage. Zhao and Yuan¹² investigate the cation exchange mechanism in Cs_1−XFA_XPbI₃ perovskite quantum dots, observing a fast cation exchange enabled by variations in concentration and host solvent polarity that lead to enhanced power conversion efficiencies. Zhang and Zhu¹³ investigate the photophysics of type II heterostructures formed with lead halide perovskites and two-dimensional transition metal dichalcogenides, observing a long-lived charge separation. Ventosinos and Schmidt¹⁵ examine halide perovskite thin-films with the moving grating technique, proposing a new model for the density of states in the forbidden gap. With the abundance of emerging experimental approaches for dissecting the photophysics of perovskites, we anticipate additional insights into the high performance of these materials manifesting in the near future.

Equally fascinating work in this collection showcases the photophysics of inorganic, thin-film, and heterostructured systems of interest for photovoltaic applications, where many of the photophysical phenomena are entirely uncharted and experimental efforts are leading the way. Kipkorir and Kamat¹⁴ employ transient absorption measurements that elucidate the kinetic aspects of electron transfer processes in AgInS₂ and CdS capped AgInS₂. Yang and Yang¹⁶ use transient absorption spectroscopy to investigate the recombination dynamics of photocarriers in a multilayer indium selenide nanofilm, showing that exciton–exciton annihilation is inefficient in multilayers due to weaker Coulomb interactions. Schwinn and Chen¹⁷ utilize femtosecond transient absorption spectroscopy to reveal photoinduced charge transfer and interlayer exciton formation in a mixed dimensional type-II heterojunction between monolayer MoS₂ and VOPc, observing interlayer excitons that lead to long-lived charge separated states.

Theoretical efforts to disentangle the complex photophysics of perovskites, heterostructures, and low dimensional systems are also of critical importance to several emerging photovoltaic technologies. Park and Limmer¹⁸ employ quasiparticle path integral molecular dynamics to characterize how phonons screen electron–hole interactions in solids, generically reducing exciton binding energies and increasing their radiative lifetimes. Wong and Tisdale¹⁹ develop a detailed, open-source numerical framework for modeling the spatiotemporal dynamics of free charge carriers in bulk semiconductors associated with transient photoluminescence microscopy data of CdS and MAPbBr₃ crystals. Mosquera and Borys²⁰ develop a model for coupling surface plasmon polaritons with two-dimensional excitons in atomically thin semiconductors, showing the emergence of strongly interacting states known as plexcitons and suggesting guidance for their experimental detection and characterization.

The articles contained in this collection demonstrate the diversity of physical insights and method developments occurring within the photophysics community and spanning both theory and experiment of organic molecules, inorganic solids, heterostructures/heterojunctions, and thin-films. With the rapid pace of improvement in the efficiencies of novel photovoltaic materials and devices, the challenge for the community will be to keep up with the novel photophysical mechanisms underscoring high performance, such that rational modifications toward further enhancements can be achieved. We hope that the collection of excellent work herein is enjoyed by the community interested in the photophysics of emerging photovoltaic materials.

The guest editors thank the authors who contributed, the journal editors, and the staff who assisted this special topic.