Xiaohang Du

From planar-heterojunction to n–i structure: an efficient strategy to improve short-circuit current and power conversion efficiency of aqueous-solution-processed hybrid solar cells

In this paper, inverted hybrid solar cells (HSCs) with planar heterojunction (PHJ) and n–i structure are fabricated from an aqueous-solution-processed poly(p-phenylenevinylene) (PPV) precursor and 2-mercaptoethylamine (MA)-stabilized CdTe nanocrystals (NCs). Firstly, one densely stacked, smooth CdTe film with a wide absorption range and sufficient water resistance is obtained and used for fabricating the PHJ device. A PCE of 3.75% is obtained, which is comparable to the bulk-heterojunction (BHJ) devices. Secondly, the n–i structure is constructed by replacing the PPV layer with a mixed solution consisting of the PPV precursor and CdTe NCs. The n–i structure shows improved photocurrent with lowered fill factor (FF). The low FF is attributed to the severe phase separation of the PPV:CdTe layer, which is caused by the hydrophobicity of the annealed CdTe layer. Subsequently, the hydrophilicity of the annealed CdTe layer is improved by spin-coating an aqueous solution of CdCl2, which greatly increases the FF. Eventually, a record power conversion efficiency of 4.76% is achieved and the maximum Jsc can reach 16.08 mA cm−2.

Aqueous-Processed Inorganic Thin-Film Solar Cells Based on CdSexTe1–x Nanocrystals: The Impact of Composition on Photovoltaic Performance

Aqueous processed nanocrystal (NC) solar cells are attractive due to their environmental friendliness and cost effectiveness. Controlling the bandgap of absorbing layers is critical for achieving high efficiency for single and multijunction solar cells. Herein, we tune the bandgap of CdTe through the incorporation of Se via aqueous process. The photovoltaic performance of aqueous CdSexTe1-x NCs is systematically investigated, and the impacts of charge generation, transport, and injection on device performance for different compositions are deeply discussed. We discover that the performance degrades with the increasing Se content from CdTe to CdSe. This is mainly ascribed to the lower conduction band (CB) of CdSexTe1-x with higher Se content, which reduces the driving force for electron injection into TiO2. Finally, the performance is improved by mixing CdSexTe1-x NCs with conjugated polymer poly(p-phenylenevinylene) (PPV), and power conversion efficiency (PCE) of 3.35% is achieved based on ternary NCs. This work may provide some information to further optimize the aqueous-processed NC and hybrid solar cells.

Efficient aqueous-processed hybrid solar cells from a polymer with a wide bandgap

In this work, MPPV with a wide bandgap is synthesized and combined with CdTe NCs for aqueous-processed polymer–nanocrystal hybrid solar cells (HSCs). A PCE of 5.18% is achieved, which is the highest for solar cells via an aqueous process. The function of MPPV is deeply investigated which shows that MPPV can fill the voids between CdTe NCs to decrease the leakage current. Furthermore, hole transfer from CdTe NCs to MPPV occurs, thus leading to increased carrier lifetime. Finally, MPPV can promote the interfacial carrier injection. We demonstrate that efficient HSCs can be achieved from polymers with a wide bandgap. In this case, key factors limiting traditional HSCs may be eliminated. As a consequence, this work may provide an alternative way to develop HSCs.

High efficiency aqueous-processed MEH-PPV/CdTe hybrid solar cells with a PCE of 4.20%

Due to their low cost, environmental friendliness and efficiency, aqueous-processed polymer/nanocrystal hybrid solar cells (HSCs) have attracted much attention in recent years. To promote the development of aqueous-processed HSCs, the design and synthesis of new water soluble conjugated polymers (WSCPs) are required. In this work, aqueous-processed MEH-PPV/CdTe HSCs are firstly fabricated. A PCE of 4.20% is achieved, which is comparable to the oil-processed MEH-PPV/NC HSCs. The water-soluble MEH-PPV precursor could increase the miscibility between polymers and aqueous CdTe NCs, and decrease their phase size. Furthermore, by adjusting MEH-PPV/CdTe ratio and annealing temperature, the nanoscale morphology of MEH-PPV/CdTe HSCs will change, which impacts charge transfer and transport. These results provide an important approach for efficient aqueous-processed MEH-PPV/NC HSCs.

Improvement in Open-Circuit Voltage of Thin Film Solar Cells from Aqueous Nanocrystals by Interface Engineering.

In this work, improved solar cells from aqueous CdTe NCs is achieved by replacing evaporated MoOx with spiro-OMeTAD as a hole transfer layer. The increased Voc and Jsc can be attributed to interfacial dipole effect and reduced back recombination loss, respectively. A high PCE of 6.56% for solar cells from aqueous NCs is obtained by optimizing the microstructure further.

Highly efficient aqueous-processed polymer/nanocrystal hybrid solar cells with an aqueous-processed TiO2 electron extraction layer

In this work, a novel environmentally friendly approach to deposit TiO2 films via an aqueous process is developed. The atomic force microscopy (AFM) measurement indicates that water-dispersed TiO2 nanocrystals (NCs) show excellent film-forming properties on an ITO-coated glass substrate. In addition, the femtosecond transient absorption (TA) and conductivity measurements suggest that the aqueous-processed TiO2 film shows excellent electron extraction ability and good conductivity. The aqueous-processed polymer/NC hybrid solar cells based on the aqueous-processed TiO2 electron extraction layer present superior photovoltaic performance and a PCE of 5.53% is achieved, which is the highest for hybrid solar cells fabricated via an aqueous process. Finally, the solar cells with the aqueous-processed TiO2 electron extraction layer exhibit better humidity resistance compared to the TiO2 film deposited via a sol–gel method.

Aqueous-Processed Insulating Polymer/Nanocrystal Hybrid Solar Cells

A novel kind of hybrid solar cell (HSC) was developed by introducing water-soluble insulating polymer poly(vinyl alcohol) (PVA) into nanocrystals (NCs), which revealed that the most frequently used conjugated polymer could be replaced by an insulating one. It was realized by strategically taking advantage of the characteristic of decomposition for the polymer at annealing temperature, and it was interesting to discover that partial decomposition of PVA left behind plenty of pits on the surfaces of CdTe NC films, enlarging surface contact area between CdTe NCs and subsequently evaporated MoO3. Moreover, the residual annealed PVA filled in the voids among spherical CdTe NCs, which led to the decrease of leakage current. An improved shunt resistance (increased by ∼80%) was achieved, indicating the charge-carrier recombination was effectively overcome. As a result, the new HSCs were endowed with increased Voc, fill factor, and power conversion efficiency compared with the pure NC device. This approach can be applied to other insulating polymers (e.g., PVP) with advantages in synthesis, type, economy, stability, and so on, providing a novel universal cost-effective way to achieve higher photovoltaic performance.

Efficient inorganic solar cells from aqueous nanocrystals: the impact of composition on carrier dynamics

Solution-processed thin-film solar cells based on aqueous nanocrystals (NCs) are attractive due to their environmental friendliness and cost effectiveness. Furthermore, nanoscale heterostructures can be formed upon annealing which is beneficial for prolonged carrier lifetime and effective carrier transport. Herein, we demonstrate that the carrier dynamics can be controlled by adjusting the composition of heterostructure NCs. Efficient thin-film solar cells are fabricated based on aqueous CdTe NCs and a power conversion efficiency (PCE) of 5.73% which is a record for thin-film solar cells fabricated from aqueous materials. This work should be instructive for application of aqueous NCs in thin-film solar cells.

Constructing Post-Permeation Method to Fabricate Polymer/Nanocrystals Hybrid Solar Cells with PCE Exceeding 6.

A post-permeation method is constructed for fabricating bulk-heterojunction hybrid solar cells. Porous CdTe film is prepared by annealing the mixture solution of aqueous CdTe nanocrystals and cetyltrimethyl ammonium bromide, after which the post-permeation of polymer is employed. By this method, kinds of polymers can be applied regardless of the intermiscibility with the nanoparticles. The inorganic nanocrystals and the polymer can be treated under respective optimized annealing temperatures, which can facilitate the growth of nanocrystals without damaging the polymers. A high power conversion efficiency of 6.36% in the polymer/nanocrystals hybrid solar cells is obtained via systematical optimization.

High-Efficiency Aqueous-Processed Polymer/CdTe Nanocrystals Planar Heterojunction Solar Cells with Optimized Band Alignment and Reduced Interfacial Charge Recombination

Aqueous-processed nanocrystal solar cells have attracted increasing attention due to the advantage of its environmentally friendly nature, which provides a promising approach for large-scale production. The urgent affair is boosting the power conversion efficiency (PCE) for further commercial applications. The low PCE is mainly attributed to the imperfect device structure, which leads to abundant nonradiative recombination at the interfaces. In this work, an environmentally friendly and efficient method is developed to improve the performance of aqueous-processed CdTe nanocrystal solar cells. Polymer/CdTe planar heterojunction solar cells (PHSCs) with optimized band alignment are constructed, which results in reduced interfacial charge recombination, enhanced carrier collection efficiency and built-in field. Finally, a champion PCE of 5.9%, which is a record for aqueous-processed solar cells based on CdTe nanocrystals, is achieved after optimizing the photovoltaic device.