This paper describes a solvothermal approach to synthesize CuInS2 quantum dots (QDs) and demonstrates their application as a potential electron accepting material for polymer-based hybrid solar cells, for the first time. The CuInS2 QDs with a size of 2-4 nm are synthesized by the solvothermal method with 4-bromothiophenol (HSPh) as both reduction and capping agents, and characterized by XRD, XPS, TEM, FT-IR, cyclic voltammetry (CV), and absorption and photoluminescence spectra. Results reveal that the CuInS2 QDs result from the solvothermal decomposition of a soluble organic sodium salt as an intermediate precursor formed by simple reactions among CuCl2, InCl3, HSPh and Na2S at room temperature; they have an ionization potential (IP) of -5.8 eV and an electron affinity (EA) of -4.0 eV and can quench effectively the luminescence of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). Due to the favorable IP and EA positions with respect to MEH-PPV, the CuInS2 QDs act as an effective electron acceptor for the hybrid solar cells based on MEH-PPV/CuInS2-QDs blends with a wide spectral response extending from 300 to 900 nm, by allowing the efficient charge separation for neutral excited states produced either on the polymer or on the QDs. The MEH-PPV/CuInS2-QDs solar cells exhibit a promising open circuit voltage (V-oc) of 0.62 V under the monochromic illumination of 15.85 mW cm(-2) at 470 nm. The charge transfer processes in the solar cells are also described

For characterization of polymer-based solar cells with vertically aligned ZnO nanorod arrays (ZnO-NAs) by intensity modulated photocurrent spectroscopy (IMPS), a dynamic IMPS model is developed, where the structure-related charge generation and transport dynamics are considered. The model describes the IMPS responses affected by the phase shift φ n (ω) due to the exciton diffusion property (ω 0 ) and the structurerelated device ideality factor N, the electron diffusion coefficient D e , the exciton dissociation rate S, and the device structure (e.g., nanorod length d and interspacing l). The main expectations of the model are confirmed by the experimental data of the polymer/ZnO-NA cells with d ) 180-650 nm, offering mechanistic information on the structure-related charge generation, charge transport, and device performance. The presence of the φ n (ω) makes IMPS response not spiral into the origin and the phase angle in its Bode plot not tend to 90°; the d-dependent direct diffusion (DD) and diffusion-reflection (DF) transport processes are normally involved in the travel of injected electrons to the collection electrode; the incident photon-to-current conversion efficiency (IPCE) and the transit time (τ D ) for DD transport under the influence of DF process reach their peak values at d ≈ 500 nm, and the φ n (ω) effect on electron transport is affected by ω 0 , D e , and S. Satisfactory fittings of measured IMPS responses to the model further reveal that the d dependence of the IPCE or the photocurrent actually originates from the S value governed by d-dependent exciton generation and dissociation; when changing d, a larger number of electrons for DD transport causes a smaller N or a more remarkable φ n (ω) effect; a longer τ D is accompanied by a larger RC effect of the ZnO electrode. Those results clearly suggest that a highly efficient polymer/ZnO-NA device requires d ≈ 500 nm and l ) 5-10 nm, along with a high interfacial exciton dissociation efficiency.

Polymer-based solar cells consisting of ZnO nanorods and poly(1-methoxy-4-(2-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) are investigated by current−voltage characterization and intensity modulated photovoltage spectroscopy (IMVS). The high quality ZnO nanorod arrays were prepared by electrodeposition, in which the length (L n) and the concentration of deep level defects of ZnO nanorods were controlled by deposition time (T d). Results show that increasing T d leads to ZnO nanorods with linearly increased L n but differently increased defect concentration for the MEH-PPV/ZnO solar cells, providing a peak device power conversion efficiency of 0.34% at AM 1.5 illumination (100 mW/cm2) for T d = 10 min. The electron lifetimes in MEH-PPV/ZnO nanorod devices at open circuit were studied by means of IMVS for the first time, and the influences of L n and defect concentration on the charge recombination kinetics and device performance were revealed. It is found that, in the MEH-PPV/ZnO devices with high quality ZnO nanorods with rather low defect concentration, both photocurrent and recombination rate are mainly dependent on the L n value as a result of the exponential attenuation of incident light intensity in the device, but the open circuit voltage V oc is more sensitive to the defect concentration. The present study provides new insights into designing the nanostructures for the hybrid photovoltaic devices based on vertically aligned one-dimensional nanoarrays.

In this paper, performance in hybrid solar cells based on ZnO nanorod array (ZnO-NA) is significantly improved by formation of a heterostructured ZnO/CdScore/shell nanorod array (ZnO-CdS-NA), the CdS shell effects on device performance including charge transport and recombination dynamics are discussed, and a model concerning ineffective polymer phase is proposed for understanding the charge generation upon CdS shell formation. The ZnO-CdS-NAs with varied CdS shell thickness (L) were prepared by depositing CdS quantum dots on the ZnO nanorods in the ZnO-NA. Solar cells were prepared by filling the interspaces between the nanorods in ZnO-NA or ZnO-CdS-NAs with poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV). Compared to MEH-PPV/ZnO-NA devices, both open-circuit voltage (V oc ) and short-circuit current (J sc ) in MEH-PPV/ZnO-CdS-NA solar cells were dramatically improved depending on L,resulting in a peak efficiency of ca. 1.23% under AM 1.5 illumination (100 mW/cm 2 ) with a 7-fold increment for L = 6 nm. In particular, the experimental L-dependence of J sc agreed with the expectation from the proposed model and the V oc was improved from ca. 0.4 V for ZnO-NA up to around 0.8 V. Results demonstrate that in the MEH-PPV/ZnO-CdS-NA devices, the J sc correlates mainly with the charge generation subjected to the exciton generation altered by CdS shell formation, in which the polymer absorption is dominantly contributive; however, the V oc is determined by the energy difference between the highest occupied molecular orbital level of MEH-PPV and the conduction band edge of ZnO but significantly correlates with the quasi-Fermi levels of the electrons in ZnO nanorods.

This paper reports the chemical modification effects at charge separation interface on the performance of the hybrid solar cells consisting of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) as an electron donor (D) and vertically aligned ZnO nanorod arrays as an electron acceptor (A). Results show that, with increasing the modification time T s for grafting dye Z907 onto the ZnO surface from 0 to 8 h, the charge transfer efficiency at the MEH-PPV/ZnO interface keeps increasing, the short circuit current (J sc ) increases and reaches a peak value at T s = 8 h, but the open circuit voltage (V oc ) increases within T s = 1-3 h and reduces with further increasing T s up to g6 h. By controlling the T s , a peak power conversion efficiency of η = 0.61% at AM 1.5 illumination (100 mW/cm 2 ) is obtained for T s = 6 h. It is revealed that the Z907 modification mainly contributes to the enhanced J sc by increasing the charge separation efficiency as a result of the improved electronic coupling property at the D/A interface rather than the light harvesting; on the other hand, the Z907 modification reduces (T s = 1-3 h) or increases (T s g 6 h) the surface defect concentration of the ZnO nanorods, resulting in the increased or reduced V oc (or electron lifetime τ e ). It is demonstrated that trapping electrons by the surface defects may facilitate the charge separation at the D/A interface in the MEH-PPV/ZnO devices, and both V oc and τ e correlate to the occupation of injected electrons in conduction band and surface defects. Further analysis provides the relation between V oc and τ e in those devices.

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To develop solution-processed and novel device structures is of great importance for achieving advanced and low-cost solar cells. In this paper, we report the solution-processed solar cells based on inorganic bulk heterojunctions (BHJs) featuring a bulk crystalline Sb2S3 absorbing layer interdigitated with a TiO2 nanoarray as an electron transporter. A solution-processed amorphous-to-crystalline transformation strategy is used for the preparation of Sb2S3/TiO2-BHJs. Steady-state and dynamic results demonstrate that the crystalline structure in the Sb2S3 absorbing layer is crucial for efficient devices, and a better Sb2S3 crystallization favors a higher device performance by increasing the charge collection efficiency for a higher short-circuit current, due to reduced interfacial and bulk charge recombinations, and enhancing the open-circuit voltage and fill factor with the reduced defect states in the Sb2S3 layer as well. Moreover, an evident contribution to photocurrent generation from the photogenerated holes in the Sb2S3 layer is revealed by experimental and simulated dynamic data. These results imply a kind of potential non-excitonic BHJ for energy conversion.

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