Exploratory Study Of High-performance Graphene-based Solar Cells

Graphene has promising applications in the fields of energy storage and conversion owing to its excellent optical and electronic properties. In this thesis, the applications of graphene in solar cells as active layer (i.e., schottky junction) and transparent conductive electrodes (TCEs) are studied.The application of graphene as active layer is investigated by using graphene/silicion (G/Si) schottky-junction solar cells as a model. The interfaces of G/Si solar cells are systematically explored to achieve high efficiency; in addition, solution-processed G/Si devices are fabricated to lower the cost. The applications of graphene as TCEs are also explored, that is, the preparation of graphene/ZnO (NRs) hybrid strucutre is studied and its use as TCEs in CZTSSe solar cells is demonstrated. The main results are summarized as follows:(1) Graphene oxide (GO) is first utilized as the interfacial material due to its tunable electronic properties. The PCE is enhanced by> 100% with the insertion of GO interlayers. Its functions are three-fold. First, it can suppress the interfacial recombination. Second, the schottky barrier height (SBH) is enhanced, thereby increasing Voc. Third, the charge-transfer resistance is reduced, improving FF. The PCE can be further improved to 12.3% using HNO3 doping and TiO2 antireflection coating.(2) GO only "works" after high-temperature annealing (400℃), which drives us to explore more effective interfacial materials. It is found that MoS2 can improve PCE effectively yet only low-temperature annealing (80℃) is required. The role of MoS2 is investigated in detail by varying MoS2 film annealing temperature. It is found that type Ⅱ heterojunction is formed between Si and MoS2(80), facilitating carrier transport and thereby improving PCE. On the contrary, valence band mismatch is formed between Si and MoS2(200) due to the increase in WFMoS2, which is responsible for the decrease in PCE. A PCE of 6.56% is obtained with the aid of silicon surface passivation.(3) Cost-effective G/Si solar cells are fabricated by spray coating. The graphene solution is sythesized using electrochemical exfoliation. The spray process is optimized by evaluating the effects of substrate temperature (Tsub) and graphene film thickness (8) on PCE. The optimized conditions are:Tsub=180％,δ=26 nm. A simple hydroquinone/methanol method is introduced to passivate silicon surface. UPS reveals the electron affinity of Si is lowered by 350 meV after surface passivation, which is responsible for the enhancement in PCE (～300%). A PCE of 4.41% is obtained for spray-coated G/Si solar cells, comparable to that of CVD-G/Si devices.(4) The deposition of ZnO seed layer on graphene by atomic layer deposition (ALD) is systematically studied. It is found that uniform and compact ZnO seed layer can be deposited on graphene using the interactions between graphene and metallic substrates together with a simple pre-H2O treatment. After obtaining G/ZnO (NRs) hybrid structure, graphene-based CZTSSe (i.e., G/ZnO/CZTSSe) solar cells are fabricated and a PCE of 1.01% is obtained, approaching the highest PCE of ITO-based devices with similar architectures, highlighting the great potential of graphene as TCEs in photovoltaic devices.