Organic solar cell (OSC) is deemed as promising for future photovoltaics due to its potentially large-area and low-cost processing and the inherent mechanical and chemical flexibility. Transparent electrode is one of the essential aspects in designing efficient OSCs. More importantly, it is desirable for the electrodes to possess mechanical flexibility compatible with many organic materials. Widely utilized as transparent electrodes, indium tin oxide (ITO) is relatively expensive and chemically unstable. Furthermore, the brittleness of ITO can induce device failure under bending. 

Graphene has generated intensive research interests for next generation transparent electrodes for OSCs. Recently, it has been reported to be more mechanically robust than ITO. However, the interface engineering for graphene can be challenging as many commonly used approaches cannot be applied to graphene directly. While there are challenges in tuning the properties of graphene electrodes including surface wettability, work function alignment and carrier extraction, we have proposed efficient graphene electrodes in flexible OSC device addressing these issues. Based on our different interface modification approaches, we have demonstrated the capability of graphene functioning as both effective anode and cathode for OSCs. For graphene anode, thin oxidized Au nanoclusters are introduced to form favorable energy alignment for hole extraction, which avoids issues arising from using poly(3,4-ethylenedioythiophene):poly(styrenesulfonate). For graphene cathode, we employ a two-step Al-TiO2 composite to modify single-layer graphene. We find that the evaporated Al nanoclusters benefit the graphene cathode by simultaneously fulfilling two roles, namely improving the surface wettability and reducing the work function to facilitate electron extraction. As a result, Al-TiO2 modified graphene cathode gives rise to an enhanced power conversion efficiency of 2.58% in OSCs, which is two-fold of the previously best reported value. To further extend the versatility of graphene electrodes, we have also demonstrated efficient top graphene electrodes for semitransparent OSCs. By using a hybrid graphene/metal grid structure as the top electrode, low sheet resistance of 22 ohm/sq and high optical transmittance of 81.4% is attained, which is comparable to conventional ITO/glass electrode. The graphene top electrode is subsequently set in contact with the active layer stack, using a lamination process for forming favorable electrical contact. Consequently, an optimal power conversion efficiency of 3.1% is achieved in semitransparent OSCs.

Organic solar cell (OSC) is deemed as promising for future photovoltaics due to its potentially large-area and low-cost processing and the inherent mechanical and chemical flexibility. Transparent electrode is one of the essential aspects in designing efficient OSCs. More importantly, it is desirable for the electrodes to possess mechanical flexibility compatible with many organic materials. Widely utilized as transparent electrodes, indium tin oxide (ITO) is relatively expensive and chemically unstable. Furthermore, the brittleness of ITO can induce device failure under bending. 

Graphene has generated intensive research interests for next generation transparent electrodes for OSCs. Recently, it has been reported to be more mechanically robust than ITO. However, the interface engineering for graphene can be challenging as many commonly used approaches cannot be applied to graphene directly. While there are challenges in tuning the properties of graphene electrodes including surface wettability, work function alignment and carrier extraction, we have proposed efficient graphene electrodes in flexible OSC device addressing these issues. Based on our different interface modification approaches, we have demonstrated the capability of graphene functioning as both effective anode and cathode for OSCs. For graphene anode, thin oxidized Au nanoclusters are introduced to form favorable energy alignment for hole extraction, which avoids issues arising from using poly(3,4-ethylenedioythiophene):poly(styrenesulfonate). For graphene cathode, we employ a two-step Al-TiO2 composite to modify single-layer graphene. We find that the evaporated Al nanoclusters benefit the graphene cathode by simultaneously fulfilling two roles, namely improving the surface wettability and reducing the work function to facilitate electron extraction. As a result, Al-TiO2 modified graphene cathode gives rise to an enhanced power conversion efficiency of 2.58% in OSCs, which is two-fold of the previously best reported value. To further extend the versatility of graphene electrodes, we have also demonstrated efficient top graphene electrodes for semitransparent OSCs. By using a hybrid graphene/metal grid structure as the top electrode, low sheet resistance of 22 ohm/sq and high optical transmittance of 81.4% is attained, which is comparable to conventional ITO/glass electrode. The graphene top electrode is subsequently set in contact with the active layer stack, using a lamination process for forming favorable electrical contact. Consequently, an optimal power conversion efficiency of 3.1% is achieved in semitransparent OSCs.