The electron transport layer (ETL) is essential to improve device performance of organic solar cells (OSCs). Here, L-cystine, which is inexpensive and environmentally friendly, was successfully employed in OSCs of PTB7-Th: PC71BM as an ETL. The introduction of L-cystine can achieve an enormous enhancement in performance: Voc (open circuit voltage) from 0.74 to 0.79 V, Jsc (short-circuit current density) from 14.76 to 16.89 mA cm2, fill factor from 51.45% to 70.73%, and power conversion efficiencies from 5.62% to 9.44%, which is better than PFN (8.52%) as an ETL. The excellent PCE is attributed to the WF of Al was shifted from -4.23 to -3.87 eV by Lcysteine modified, thereby more effectively extracting and collecting carriers. This study demonstrates that the natural and non-toxic L-cystine is a strong competitor for ETL in high-efficiency OSCs.

Interfacial engineering plays a key role in the performance enhancement of organic solar cells (OSCs), which has a significant impact on the charge collection and transport in the devices. Herein, the conventional devices based on PTB7-Th:PC71BM were prepared by introducing a non-conjugated small molecule tetrasodium iminodi-succinate (IDS) as an electron transport layer (ETL). It is shown that IDS as ETL improves the charge transfer and collection and simultaneously inhibits the charge recombination in the devices. Consequently, the optimal power conversion efficiency (PCE) of the device with IDS as ETL reaches up to 9.45%, with an open circuit voltage (V-OC) of 0.80 V, a short circuit current density (J(SC)) of 16.88 mA cm(-2) and a fill factor (FF) of 70.01%. This work provides a new approach to optimize the PCEs of the OSCs.

Solvent additives play an important role in optimizing the morphology of the photoactive layer and improving the photovoltaic performance of polymer solar cells (PSCs).

Ternary copolymerization strategy is considered an effective method to achieve high-performance photovoltaic conjugated polymers. Herein, a donor-acceptor1-donor-acceptor2-type random copolymer, named PBDTNS-TZ-BDD (T1), containing one electron-rich unit alkylthionaphthyl-flanked benzo[1,2-b/4,5-b'] di-thiophene (BDTNS) as D and two electron-deficient moieties benzo[1,2-c/4,5-c']dithiophene-4,8-dione (BDD) and fluorinated benzotriazole as A, was synthesized to investigate the excitonic dynamic effect. Also, the D-A-type alternating copolymer PBDTNS-BDD (P1) was also prepared for a clear comparison. Although the UV-Vis spectra and energy levels of P1 and T1 are similar, the power conversion efficiencies (PCEs) of the related devices are 11.50% (T1/ITIC) and 8.89% (P1/ITIC), respectively. The reason for this is systematically investigated and analyzed by theoretical calculation, photoluminescence, and pump-probe transient absorption spectroscopy. The density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculation results show that the terpolymer T1 with a lower exciton binding energy and a longer lifetime of spontaneous luminescence can synergistically increase the number of excitons reaching the donor/acceptor interface. The results of the pump-probe transient absorption spectroscopy show that the yield of charge separation of T1/ITIC is higher than that of the P1/ITIC blend film, and improved PCE could be achieved via copolymerization strategies. Moreover, the fabrication of the T1-based device is also simple without any additive or postprocessing. Therefore, it provides a promising and innovative method to design high-performance terpolymer materials.

The fill factor (FF) as an important photovoltaic parameter is related to the property of transport layers and active layers for organic solar cells (OSCs). Herein, we propose a simple method to improve FF by using inorganic-organic composite hole transport layer (HTL) in OSCs. Through incorporating inorganic metal oxide, vanadium pentoxide (V2O5) nanoparticles into PEDOT:PSS, the morphology and conductivity of the composite HTL are improved, due to V2O5 nanoparticles can fill the pinholes in PEDOT:PSS and expose more PEDOT chains to the surface core-shell structure. The PTB7-Th:PC71BM-based OSCs with different HTLs were prepared. The power conversion efficiency and FF of the OSCs for the composite HTL can be greatly improved compared with the traditional HTL, which can reach up to 9.44% and 70.14%, respectively. The results of water contact angle and grazing incidence wide-angle X-ray scattering show that the hydrophobicity of the composite HTL is improved, which can not only improve the physical contact but also greatly affect the subsequent active layer formation, thus increasing the crystallinity of PTB7-Th:PC71BM. These results demonstrate that V2O5:PEDOT:PSS has good application prospects as a HTL to realize high-efficient OSCs.

A conjugated alcohol-soluble polymer based on perylene diimide (PDI) and fluorene (PPDI-FN) was designed and synthesized. PPDI-FN can be soluble in methanol solution under the existence of tiny acetic acid, providing a possibility to prepare polymer solar cells (PSCs) by low-cost non-interfacial mixed solution method. The conventional PSCs based on PTB7-Th:PC71BM were fabricated with PPDI-FN as electron transport layer (ETL). Compare with PSCs based on PTB7-Th:PC71BM without ETL, the introduction of PPDI-FN can achieve an enormous enhancement in performance: V oc (open circuit voltage) from 0.75 to 0.80 V, J sc (short-circuit current density) from 15.57 to 16.72 mA/cm2, fill factor) from 55.96% to 68.87%, and power conversion efficiencies from 6.53% to 9.21%, which is better than PFN (8.62%) as an ETL. The performance of PPDI-FN in the device was studied; the results showed a decreased work function and increased electron transportation and conductivity, which are benefits for the cathode to enhance charge extraction efficiency and decrease recombination losses. These results showed that PDIs derivative PPDI-FN has good application prospects as an ETL to realize high-efficient PSCs.

Self-doped polymer cathode interface materials for organic solar cells have been widely investigated to enhance the ohmic contact between the electrode and the photoactive layer.