ABSTRACT
In organic photovoltaic (OPV) cells, besides the organic active layer, the electron-transporting layer (ETL) has a primordial role in transporting electrons and blocking holes. In planar heterojunction-OPVs (PHJ-OPVs), the ETL is called the exciton blocking layer (EBL). The optimum thickness of the EBL is 9 nm. However, in the case of inverted OPVs, such thickness is too high to permit efficient electron collection, due to the fact that there is no possibility of metal diffusion in the EBL during the top metal electrode deposition. In the present work, we show that the introduction of a thin potassium layer between the indium tin oxide (ITO) cathode and the EBL increases dramatically the conductivity of the EBL. We demonstrate that K not only behaves as a simple ultrathin layer allowing for the discrimination of the charge carriers at the cathode/organic material interface but also by diffusing into the EBL, it increases its conductivity by 3 orders of magnitude, which allows us to improve the shape of the J-V characteristics and the PHJ-inverted OPV efficiency by more than 33%. Moreover, we also show that PHJ-inverted OPVs with K in their EBLs are more stable than those with Alq3 alone.
ABSTRACT
This work investigates the impact of the elemental sulfur evaporation during or after KF-post deposition treatment (KF-PDT) on the resulting Cu(In,Ga)Se2/chemical bath deposited(CBD)-CdS interface. Chemical composition of the various interfaces were determined through Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger spectroscopy (XAES). Cu(In,Ga)Se2 absorber which experienced KF-PDT in selenium atmosphere (KSe sample) exhibits the formation of the well-reported In-Se based topping layer. Additional exposure to elemental sulfur, resulting in KSe+S sample, induces the partial sulfurization of this overlayer and/or of the absorber. After short immersion into the CdS bath, the resulting In-rich surfaces of KSe and KSe+S are likely to turn into few atomic layers of Cd-In-(Se/S)-O whose [S]/[Se]+[S] ratio and O content depend on their respective post deposition treatment. In contrast, KF-PDT performed in S atmosphere does not show an In-rich surface, making the early stage of CdS growth similar to that observed on untreated CIGSe.
ABSTRACT
An efficient approach for improving the photoelectrical conversion efficiency (PCE) of the bulk heterojunction (BHJ) solar cells, based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acidmethyl ester (PC61BM), by incorporating PbSe nanorods decorated with graphene (G) into their active layer has been reported for the first time. Pristine PbSe and PbSe:G composites (with different amount of graphene) are synthesized via hydrothermal process and the formation mechanism is explained. The systematic investigation indicates that the crystallite size of PbSe:G increases with increasing graphene content. The PCE of the classical BHJ solar cells based on P3HT:PC61BM is improved from 2.32 up to 2.57% by the incorporation of pristine PbSe. It is also enhanced by the incorporation of PbSe:G up to certain composition of graphene in which a maximum PCE value of 5.16% is achieved. The external quantum efficiency of the BHJ solar cells is also investigated. The photovoltaic parameters are discussed based on the morphology variation detected by scanning electron microscope and atomic force microscope of the active layers together with their UV-VIS absorption measurements.
ABSTRACT
The removal of secondary phases from the surface of the kesterite crystals is one of the major challenges to improve the performances of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this contribution, the KCN/KOH chemical etching approach, originally developed for the removal of CuxSe phases in Cu(In,Ga)(S,Se)2 thin films, is applied to CZTSe absorbers exhibiting various chemical compositions. Two distinct electrical behaviors were observed on CZTSe/CdS solar cells after treatment: (i) the improvement of the fill factor (FF) after 30 s of etching for the CZTSe absorbers showing initially a distortion of the electrical characteristic; (ii) the progressive degradation of the FF after long treatment time for all Cu-poor CZTSe solar cell samples. The first effect can be attributed to the action of KCN on the absorber, that is found to clean the absorber free surface from most of the secondary phases surrounding the kesterite grains (e.g., Se0, CuxSe, SnSex, SnO2, Cu2SnSe3 phases, excepting the ZnSe-based phases). The second observation was identified as a consequence of the preferential etching of Se, Sn, and Zn from the CZTSe surface by the KOH solution, combined with the modification of the alkali content of the absorber. The formation of a Cu-rich shell at the absorber/buffer layer interface, leading to the increase of the recombination rate at the interface, and the increase in the doping of the absorber layer after etching are found to be at the origin of the deterioration of the FF of the solar cells.
