The light beam induced current (LBIC) method was adopted to nondestructively map the photoresponse of real planar organic–inorganic hybrid perovskite solar cells (PSCs). It is found that the photoresponse of the devices is not uniform even though the morphology of the perovskite films from scanning electron microscope (SEM) or atomic force microscope (AFM) images shows uniform character. This nonuniformity of the photoresponse of the devices is further exacerbated after degradation, which can be well traced by the LBIC method. The indistinguishable morphology change during the device degradation indicates that the degradation of the device is not mainly determined by the morphology of the perovskite layer, but by the interface between the perovskite and the electrode. By using the LBIC method, the worse performing area of the device is identified and then removed accordingly. The current density of the device can be enhanced from 19.44 mA cm−2 to 21.72 mA cm−2 after this clearance of the worse performing area.

•The effect of additive residue on the photodegradation was investigated.•Additive residue accelerates the oxidation of PTB7 under illumination.•The oxidation of PTB7 results in less carrier generation.•Additive residue has little effect on the absorption after illumination.Solvent additives are indispensable to achieve highly efficient organic solar cells. The additive residue is unavoidable especially when the devices are prepared at room temperature and atmospheric pressure. In this paper, we introduce 1,10-diiododecane (DID) as the additive, which has high boiling point, and investigate the effects of additive residue on the photodegradation of organic materials and photoelectric properties of solar cells after light illumination. The iodine from the residue of DID in the active layer could be confirmed by X-ray photoelectron spectroscope (XPS) measurements. Structural changes in the films upon illumination are probed using Fourier Transform Infrared Spectrometer (FTIR). The residual DID is found to dramatically decrease the photostability of the active layer and device performance under light illumination compared with those without additive residue, which are exemplified in current density–voltage (J-V) and electrochemical impedance measurements. Furthermore, the absorption of the film with additive residue is unchanged after light illumination, indicating that the conjugation of the polymer is not affected by the residue.Additive residue accelerates PTB7 degradation after 2 h illumination, resulting in worse device performance. However, this accelerated degradation would gradually disappear with exhausting of additive residue.Download high-res image (298KB)Download full-size image

•The effect of varied alkyl chain length of additives on the PC71BMs was systematically investigated for the first time.•The length of alkyl chain of additives influences on the surface morphology of PTB7:PC71BM bulk heterojunction film.•The electrical properties of PTB7:PC71BM solar cells have been found to closely correlate with chain length of additives.•A new four-step route is finally proposed to further optimize the performance of additive based polymer solar cell.The impact of alkyl chain length of different additives, such as 1,4-diiodobutane (DIB), 1,6-diiodohexane (DIH), 1,8-diiodooctane (DIO) and 1,10-diiododecane (DID), on the PC71BM distribution in PTB7:PC71BM-based polymer solar cells, is systematically investigated, for the first time. Among these additives, DIO is found to have the optimum alkyl chain length that maximizes the performance of PTB7:PC71BM based polymer solar cells, attaining a power conversion efficiency as high as 8.84%, which is almost four times higher than that without any additives. For DID additives (longer alkyl chain length than DIO), a drop in efficiency to 7.91% was observed. Experimental investigations show that the microstructure of the bulk and the surface layer as well as the surface morphology of the PTB7:PC71BM polymer film can be controlled simultaneously by varying the alkyl chain length of additives. Results also show that the substantial improvement in performance is attributed to the improved 1) phase segregation, 2) PC71BM distribution uniformity in the bulk of the PTB7:PC71BM film, 3) surface smoothness and 4) high PTB7 content at the interface between the active layer and the top electrode.Schematic illustration of the morphologies and components of PTB7:PC71BM films without and with DIB, DIH, DIO and DID additives for different efficiency respectively.

One-step spin coating is a simple method which has been widely used in fabricating perovskite solar cells. However, this method was vastly demonstrated in glove box wherein the influence of moisture is negligible. Thus the use of one-step spin-coating in ambient air has not been comprehensively investigated. In this work, we employ one-step spin-coating method to coat perovskite films in ambient air (with humidity above 50%), and then the perovskite films are annealed in vacuum or air. Experimental results show that by using vacuum annealing, a power conversion efficiency of 12.98% is attained, and this is 45% higher than that attained by air annealing method. This improvement is mainly attributed to the fast solvent evaporation process in vacuum during annealing, which induces high supersaturation that leads to higher coverage of perovskite film.

Nanosecond timescale transient photocurrent (ns-TPC) measurements on organic solar cells (OSCs) are commonly used in combination with numerical simulation to study charge transport and recombination phenomenon in these devices. But the ns-TPC measurement itself is influenced by the RC-effects of the test circuit. Thus the RC-constant of the test circuit is needed to mathematically eliminate the RC-effects to reconstruct an accurate TPC signal. Nowadays, an estimated RC-constant is used by researchers to reconstruct the TPC signal. So, a reliable method is needed to experimentally determine the RC-constant accurately to reconstruct the accurate TPC signal. Here, a simple method, by analyzing the transient response of the test circuit after a square voltage pulse excitation, is used to experimentally determine the RC-constant in ns-TPC measurements on typical planar hetero-junction small-molecule organic solar cells and typical bulk hetero-junction polymer solar cells. In the meantime, in order to verify the correctness of the experimentally determined RC-constant, three verification methods, which are valid under specific conditions, are selectively adopted to verify whether the experimentally determined RC-constant is reliable. Finally, all the results given by the verification methods show that this simple method could be used as a reliable method to experimentally determine the correct RC-constant in ns-TPC measurements on OSCs.

In this work, we investigate the effect of the thickness of the polyethylenimine ethoxylated (PEIE) interface layer on the performance of two types of polymer solar cells based on inverted poly(3-hexylthiophene) (P3HT):phenyl C61-butryric acid methyl ester (PCBM) and thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Maximum power conversion efficiencies of 4.18% and 7.40% were achieved at a 5.02 nm thick PEIE interface layer, for the above-mentioned solar cell types, respectively. The optimized PEIE layer provides a strong enough dipole for the best charge collection while maintaining charge tunneling ability. Optical transmittance and atomic force microscopy measurements indicate that all PEIE films have the same high transmittance and smooth surface morphology, ruling out the influence of the PEIE layer on these two parameters. The measured external quantum efficiencies for the devices with thick PEIE layers are quite similar to those of the optimized devices, indicating the poor charge collection ability of thick PEIE layers. The relatively low performance of devices with a PEIE layer of thickness less than 5 nm is the result of a weak dipole and partial coverage of the PEIE layer on ITO.

In this work, we investigate the effect of the thickness of the polyethylenimine ethoxylated (PEIE) interface layer on the performance of two types of polymer solar cells based on inverted poly(3-hexylthiophene) (P3HT):phenyl C61-butryric acid methyl ester (PCBM) and thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Maximum power conversion efficiencies of 4.18% and 7.40% were achieved at a 5.02 nm thick PEIE interface layer, for the above-mentioned solar cell types, respectively. The optimized PEIE layer provides a strong enough dipole for the best charge collection while maintaining charge tunneling ability. Optical transmittance and atomic force microscopy measurements indicate that all PEIE films have the same high transmittance and smooth surface morphology, ruling out the influence of the PEIE layer on these two parameters. The measured external quantum efficiencies for the devices with thick PEIE layers are quite similar to those of the optimized devices, indicating the poor charge collection ability of thick PEIE layers. The relatively low performance of devices with a PEIE layer of thickness less than 5 nm is the result of a weak dipole and partial coverage of the PEIE layer on ITO.