Organic photovoltaics (OPVs) evolve in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) has in the last decade been increased by almost a factor of ten approaching 10%. A main concern has been the stability that was previously measured in minutes, but can now, in favorable circumstances, exceed many thousands of hours. This astonishing achievement is the subject of this article, which reviews the developments in stability/degradation of OPVs in the last five years. This progress has been gained by several developments, such as inverted device structures of the bulk heterojunction geometry device, which allows for more stable metal electrodes, the choice of more photostable active materials, the introduction of interfacial layers, and roll-to-roll fabrication, which promises fast and cheap production methods while creating its own challenges in terms of stability.

Stability of polymer solar cells.

In recent years, there has been an unprecedented rise in the performance of metal halide perovskite solar cells. They are now in a position to compete on performance with traditional crystalline solar cells, and as such the most pressing questions concern the long term operational stability of this class of solar cell. Here, recent developments in understanding and overcoming stability concerns of metal halide perovskite solar cells are highlighted. An overview of possible instability issues due to electrical, atmospheric, heat, and light stresses is provided and the different implications to the most commonly used device architectures are discussed.

Stability of Metal Halide Perovskite Solar Cells

Perovskite solar cells have attracted significant research efforts due to their remarkable performance, with certified power conversion efficiency now reaching 22%. Solution-processed perovskite thin films are polycrystalline, and grain boundaries are thought to be responsible for causing recombination and trapping of charge carriers. Here we report an effective and reproducible way of treating grain boundaries in CH3NH3PbI3 films deposited by means of a Lewis acid–base adduct approach. We show by high-resolution transmission electron microscopy lattice images that adding 6 mol% excess CH3NH3I to the precursor solution resulted in a CH3NH3I layer forming at the grain boundaries. This layer is responsible for suppressing non-radiative recombination and improving hole and electron extraction at the grain boundaries by forming highly ionic-conducting pathways. We report an average power conversion efficiency of 20.1% over 50 cells (best cell at 20.4%) together with significantly reduced current–voltage hysteresis achieved by this grain boundary healing process. The grain boundaries in thin-film perovskite solar cells are responsible for non-radiative carrier recombination, which is deleterious for the optoelectronic performance. Son et al. show how to passivate the grain boundaries by using excess CH3NH3I in the precursor solution, achieving efficiencies of 20.4%.

Self-formed grain boundary healing layer for highly efficient CH 3 NH 3 PbI 3 perovskite solar cells

Organic solar cells (OSCs) present some advantages, such as simple preparation, light weight, low cost and large-area flexible fabrication, and have attracted much attention in recent years. Although the power conversion efficiencies have exceeded 10%, the inferior device stability still remains a great challenge. In this review, we summarize the factors limiting the stability of OSCs, such as metastable morphology, diffusion of electrodes and buffer layers, oxygen and water, irradiation, heating and mechanical stress, and survey recent progress in strategies to increase the stability of OSCs, such as material design, device engineering of active layers, employing inverted geometry, optimizing buffer layers, using stable electrodes and encapsulation. Some research areas of device stability that may deserve further attention are also discussed to help readers understand the challenges and opportunities in achieving high efficiency and high stability of OSCs towards future industrial manufacture.

Stability of organic solar cells: challenges and strategies

Organic solar cells have the potential to be low-cost and efficient solar energy converters, with a promising energy balance. They are made from carbon-based semiconductors, which exhibit favourable light absorption and charge generation properties, and can be manufactured by low temperature processes such as printing from solvent-based inks, which are compatible with flexible plastic substrates or even paper. In this review, we will present an overview of the physical function of organic solar cells, their state-of-the-art performance and limitations, as well as novel concepts to achieve a better material stability and higher power conversion efficiencies. We will also briefly review processing and cost in view of the market potential.

Polymer-Fullerene Bulk Heterojunction Solar Cells

Oxygen doping of P3HT:PCBM blends: Influence on trap states, charge carrier mobility and solar cell performance

The trap distribution in the conjugated polymer poly(3-hexylthiophene) was investigated by fractional thermally stimulated current measurements. Two defect states with activation energies of about 50 and 105 meV and Gaussian energy distributions were revealed. The first is assigned to the tail of the intrinsic density of states, whereas the concentration of the second trap is directly related to oxygen exposure. The impact of the oxygen induced traps on the charge transport was examined by performing photo-induced charge carrier extraction by linearly increasing voltage measurements that exhibited a strong decrease in the mobility with air exposure time.

Trap distribution and the impact of oxygen-induced traps on the charge transport in poly(3-hexylthiophene)

The trap states in three fullerene derivatives, namely PC61BM ([6,6]-phenyl C61 butyric acid methyl ester), bisPC61BM (bis[6,6]-phenyl C61 butyric acid methyl ester) and PC71BM ([6,6]-phenyl C71 butyric acid methyl ester), are investigated by means of thermally stimulated current measurements (TSC). Thereby, the lower limit of the trap densities for all studied methanofullerenes is on the order of 1022 m−3, with the highest trap density in bisPC61BM and the lowest in PC61BM. Fractional TSC measurements on PC61BM reveal a broad trap distribution instead of discrete trap levels, with activation energies ranging from 15 meV to 270 meV and the maximum at about 75 meV. The activation energies of the most prominent traps in the other two fullerene derivatives are significantly higher, at 96 meV and 223 meV for PC71BM and 184 meV for bisPC61BM, respectively. The influence of these findings on the performance of organic solar cells is discussed.

Electronic Trap States in Methanofullerenes

The trap distribution in the conjugated polymer poly(3-hexylthiophene) was investigated by fractional thermally stimulated current measurements. Two defect states with activation energies of about 50 meV and 105 meV and Gaussian energy distributions were revealed. The first is assigned to the tail of the intrinsic density of states, whereas the concentration of second trap is directly related to oxygen exposure. The impact of the oxygen induced traps on the charge transport was examined by performing photo-induced charge carrier extraction by linearly increasing voltage measurements, that exhibited a strong decrease in the mobility with air exposure time.

Trap Distribution and the Impact of Oxygen-induced Traps on the Charge Transport in Poly(3-Hexylthiophene)

The trap states in three fullerene derivatives, namely PC61BM ([6,6]-phenyl C61 butyric acid methyl ester), bisPC61BM (bis[6,6]-phenyl C61 butyric acid methyl ester) and PC71BM ([6,6]-phenyl C71 butyric acid methyl ester), are investigated by thermally stimulated current measurements (TSC). Thereby, the lower limit of the trap densities for all studied methanofullerenes exhibits values in the order of 10^22 m^-3 with the highest trap density in bisPC61BM and the lowest in PC61BM. Fractional TSC measurements on PC61BM reveal a broad trap distribution instead of discrete trap levels with activation energies ranging from 15 meV to 270 meV and the maximum at about 75 meV. The activation energies of the most prominent traps in the other two fullerene derivatives are significantly higher, being at 96 meV and 223 meV for PC71BM and 184 meV for bisPC61BM, respectively. The influence of these findings on the performance of organic solar cells is discussed.

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Oxygen doping of P3HT:PCBM blends: Influence on trap states, charge carrier mobility and solar cell performance

Trap distribution and the impact of oxygen-induced traps on the charge transport in poly(3-hexylthiophene)

Electronic Trap States in Methanofullerenes

Trap Distribution and the Impact of Oxygen-induced Traps on the Charge Transport in Poly(3-Hexylthiophene)

Electronic Trap States in Methanofullerenes