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

Abstract

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.

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Introduction

Solution-processed inorganic thin-film solar cells based on nanocrystals (NCs) have attracted increased attention, largely due to the high extinction coefficient, solution processability, photostability and excellent electronic properties of NCs.^1–5 Since the pioneering work of Sargent and co-workers,⁶ a variety of NCs have been investigated for fabricating inorganic NCs solar cells. Generally, these NCs can be divided into two categories. One is nanocrystalline bulk semiconductors, such as CdTe, CuInGaSe₂ and CuZnSnS₄.^7–15 The other one is a quantum-confined system, such as PbS and PbSe.^16–25 Recently, power conversion efficiency (PCE) of 8–9% has been obtained by optimization of device architecture and material property.^26–28

In previous reports, the material preparation and device fabrication usually involve organic solvents, such as chloroform, toluene and pyridine. Compared to NCs dissolved in organic solvent, aqueous NCs are environment-friendly and cost-effective. In recent years, they are widely investigated in bioimaging and biosensing^29–32 while application of aqueous NCs in thin-film solar cells is scarce. Recently, the aqueous CdTe NCs were demonstrated as active layer to fabricate all inorganic solar cells and PCE of 3.9% was achieved.³³ Nanoscale CdTe–CdS heterojunction was established upon thermal treatment due to the reaction between metal cations and mercapto ligands.

In the case of CdTe NCs dissolved in organic solvent, the elimination of ligand upon annealing is nearly complete due to the weak interaction of Cd²⁺ and ligand.³⁴ For aqueous processed CdTe NCs films, elimination of the mercapto ligand competes with formation of CdS during thermal annealing.^33,35 As the elimination reaction and formation reaction are both affected by the temperature,^33,36 adjusting annealing temperature may change the CdTe/CdS ratio which may influence the charge dynamics and doping density. In this work, these factors and corresponding device performance are investigated in detail. PCE of 5.73% is achieved through optimising the active layer thickness and annealing temperature.

Experimental

Preparation of aqueous CdTe NCs

In a typical synthesis, 0.28 mL aqueous NaHTe (2/3 M) solution was injected into 12.5 mM N₂-saturated CdCl₂ solution in the presence of mercaptoethylamine (MA) in the pH range of 5.70–5.74. The molar ratio of Cd/MA/Te was set as 1 : 2.4 : 0.2. The resultant precursor solution was refluxed for 60 min to maintain the growth of CdTe NCs. After that, the NCs solution was concentrated and centrifugated at 6000 rpm for 5 min with the addition of isopropanol to remove ligands and superfluous salts. Subsequently, the NCs were dried in a vacuum oven and then dissolved in deionized water.

Device fabrication

The ITO was cleaned using chloroform, acetone, isopropanol and ethanol before drying in a N₂ flow. Next, the ITO substrate was treated by O₂ plasma for 5 min and immediately coated with the TiO₂ precursor at a speed of 2000 rpm. Afterwards, the samples were annealed in air at 500 °C to convert the TiO₂ precursor into anatase-phase TiO₂. The active layer with thickness of 80 nm was fabricated by spin-coating the aqueous solution of CdTe NCs at a speed of 700 rpm for 60 s in ambient condition, and subsequently annealed at desired temperature in the glove box for 2 min. This process was repeated to obtain desired thickness. Finally, a 5 nm MoO₃ film and 60 nm gold electrode was evaporated on top of the active layer through a mask at a pressure below 10⁻⁵ Torr, leading to an active area of 5 mm².

Characterization

UV-visible absorption spectra were obtained using a Shimadzu 3600 UV-visible-NIR spectrophotometer. Transmission electron microscopy (TEM) was conducted using a Hitachi H-800 electron microscope at an acceleration voltage of 200 kV with a CCD camera. Current density versus voltage (J–V) characteristics were measured by a computer-controlled Keithley 2400 source meter measurement system under 100 mW cm⁻² illumination with an AM1.5G filter. The incident photon to current efficiency (IPCE) was recorded under illumination of monochromatic light from the xenon lamp using a monochromator (Jobin Yvon, TRIAX 320) and detected by a computer-controlled Stanford SR830 lock-in amplifier with a Stanford SR540 chopper. The transient absorption (TA) setup consisted of 400 nm pump pulses doubled from 800 nm laser pulses (∼100 fs duration, 250 Hz repetition rate) generated from a mode-locked Ti: sapphire laser/amplifier system (Solstice, Spectra-Physics) and broadband white-light probe pulses generated from 2 mm-thick wafer. The relative polarization of the pump and the probe beams was set to the magic angle. The TA data were collected by a fiber coupled spectrometer connected to a computer. XRD data was collected at 298 K on a Bruker SMART-CCD diffractometer. Energy disperse spectroscopy (EDS) data were collected on 240 nm thick CdTe films using Bruker energy disperse spectroscopy. The EIS measurements were conducted from a CHI 660E electrochemical workstation in dark conditions at zero bias voltage with a frequency ranging from 0.1 Hz to 100 kHz. For TPV measurement, the sample chamber consisted of a platinum wire gauze electrode (with the transparency of ca. 50%) as top electrode, a glass substrate covered with ITO as bottom electrode, and a 10 μm thick mica spacer as electron isolator. The samples were excited with a laser radiation pulse (wavelength of 355 nm and pulse width of 5 ns) from a third harmonic Nd:YAG laser (Polaris, New Wave Research, Inc.). Intensity of the pulse was regulated with a neutral grayfilter and determined by an EM500 single-channel joulemeter (Molectron, Inc.). The TPV signals were registered by a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix) with a preamplifier. The energy band value was measured in an integrated ultrahigh vacuum system equipped with multitechnique surface analysis system (VG ESCALAB MK II spectrometer) UPS. UPS was measured with the He(I) (21.2 eV) line using a negative bias voltage applied to the samples in order to shift the spectra from the spectrometer threshold.

Results and discussion

CdTe NCs were synthesized through the aqueous approach with a CdTe core and mercaptoethylamine (MA) as ligand. As shown in Fig. 1a, the absorption peak of the as-synthesized CdTe NCs is 530 nm and the average size is about 3 nm. Based on our previous work, CdTe–CdS heterostructure NCs were formed only when the annealing temperature was increased to 300 °C.³³ Therefore, higher temperature is investigated in this work. For convenience, CdTe–CdS heterostructure NCs are termed as CdTe NCs in the following discussions. Fig. 1b shows the absorption spectra of CdTe NCs films annealed at different temperature which all exhibit the absorption property of bulk CdTe. Transmission electron microscopy (TEM) measurement is used to monitor the size of the annealed CdTe NCs films. When the annealing temperature is 300, 350 and 400 °C, the average size is 34, 51 and 79 nm which distinctly exceed the exciton Bohr radius of CdTe. As shown in the X-ray diffraction (XRD) patterns (Fig. 1c), the peaks at 24.2°, 39.8°, and 47.1° corresponding to the (111), (220), and (311) faces of the CdTe NCs are found. This indicates that the CdTe NCs grow with a zinc blende structure.

Fig. 1 (a) UV-vis absorption spectrum and TEM image of the as-synthesized CdTe NCs. Scale bar: 100 nm. (b) UV-vis absorption spectra and (c) XRD patterns of CdTe NCs films on quartz with different annealing temperature. TEM images and size distribution histograms of CdTe NCs films annealed at (d) 300 °C, (e) 350 °C and (f) 400 °C. Scale bar: 100 nm.

Energy dispersive spectroscopy characterization is used to determine the composition of the annealed CdTe NCs films (Table 1). As the temperature increases, the content of CdS decreases which indicates that more mercapto ligands are removed at higher temperature. On one hand, the rate of annealing-induced growth is much faster than the formation of CdS.^7,33 Therefore, the increase of size will decrease the specific surface area of NCs which will reduce the superfacial binding sites of MA. On the other hand, elevating temperature will increase the evaporation rate of MA.

Table 1 The composition of CdTe NCs films with different annealing temperature obtained from EDS measurement. Measuring error is shown in the bracket

Temperature (°C)	Cd (%)	Te (%)	S (%)
300	46.02 (0.74)	43.23 (0.75)	10.75 (0.09)
350	46.88 (0.83)	43.14 (0.82)	9.98 (0.09)
400	46.93 (0.83)	44.24 (0.84)	8.84 (0.09)

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As charge separation and recombination in heterostructured semiconductor NCs are influenced by the ratio of the two materials combined at the heterointerface,³⁷ the reduction of CdS/CdTe ratio may influence the charge transfer between CdTe and CdS. Between bulk CdTe and CdS, electrons are delocalized across CdTe and CdS while holes are defined in CdTe.³⁸ Therefore, the hole transfer from CdS to CdTe should be independent on the CdS/CdTe ratio and is overlooked. Fig. 2a–c show the TA spectra of annealed CdTe NCs films recorded at different time intervals after laser excitation at 800 nm. The TA samples were spin coated on the quartz substrates with thickness of 80 nm and annealed at desired temperature. The bleaching band at 825 nm can be assigned to the 1S(e)–1S(h) transition of CdTe while the bleaching band at 510 nm is the lowest exciton transition of CdS. As the energy of laser at 800 nm is not intense enough to excite CdS, the emergence of the bleaching band of CdS indicates the electron transfer from CdTe to CdS. Besides, no other phases such as CdTe_xS_1−x are formed during annealing as no bleaching band except 825 nm and 510 nm are observed in the TA spectra. Fig. S1† shows the TA kinetics of the annealed CdTe NCs films at the wavelength of the CdTe and CdS states. The TA kinetics curves keep constant at their maximum in the range of 1–5 ps followed by a fast decay. Therefore, TA spectra recorded at 2 ps are selected to compare the electron transfer from CdTe to CdS (Fig. 2d). As the annealing temperature increases, the intensity ratio between CdS and CdTe states decreases, which indicates the electron transfer from CdTe to CdS is partly suppressed. The decreased spatial charge separation will increase the opportunity of recombination, thus leading to the reduction of carrier lifetime. Transient photovoltage (TPV) measurement is conducted to investigate the carrier recombination of the CdTe NCs (Fig. 3a). The samples were spin coated on the indium tin oxides (ITO) substrate with thickness of 240 nm and excited at wavelength of 365 nm. Obviously, an accelerated decay is observed when higher temperature is used, indicative of faster carrier recombination and shorter carrier lifetime.

Fig. 2 Transient absorption spectra of the CdTe NCs films with annealing temperature of (a) 300 °C, (b) 350 °C and (c) 400 °C recorded at different time after laser excitation at 800 nm (unit: ps). (d) Transient absorption spectra of the CdTe NCs films with different annealing temperature recorded at 2 ps.

Fig. 3 (a) Transient photovoltage responses and (b) SCLC curves of the CdTe NCs films with different annealing temperature.

A commonly accepted problem in NCs solar cells is non-radiative loss due to surface trap states which shows great impact on carrier dynamics.²³ In our system, elevating annealing temperature decreases the specific surface area of the NCs which should lead to reduction of the surface trap states density. In this case, less electrons are trapped, that is to say, more electrons participate in the process of delocalization across CdTe and CdS phases. This is contrary to the above-mentioned phenomenon. Therefore, the surface trap states can not account for the blocked electron transfer. However, the function of CdS phase is similar to that of shallow defect in NCs. For NCs with shallow defect, the electrons can be trapped into the shallow defect and detrapping of the trapped electrons can also occur. Decreasing the number of shallow defects will decrease the number of trapped electrons and the carrier lifetime.³⁹ In consideration of the CdS/CdTe ratio and the bandgap, the CdTe phase should contribute mainly to the absorption and can be regarded as the main body of the NCs. The electron transfer between CdTe and CdS is similar to the trapping and detrapping process of electrons. And the effect caused by decreased content of CdS is the same as that caused by decreased shallow defect density. Nevertheless, the content of CdS in the NCs is much higher than that of shallow defects. As a consequence, the CdS phase can be continuous to form electron transport pathway. This is different from the shallow defects which are discrete.

As shown in the XRD patterns, the increase of annealing temperature strengthens the intensity of the diffraction peaks. This implies the crystallinity of CdTe NCs increases which is beneficial for reducing crystal boundary. Besides, the increase of size will decrease the hopping process required for electron transport between NCs, thus improving the carrier mobility. To confirm this point, space charge limited current (SCLC) method is used to measure the hole mobility of the annealed NCs films (Fig. 3b). The SCLC method has been demonstrated as an effective way to measure the hole mobility of PbS NCs.⁴⁰ The hole-only device structure is ITO/CdTe/MoO₃/Au. The hole mobility is calculated according to eqn (2):⁴¹

J = 9ε₀ε_rμ(V − V_bi − V_r)²/8L³ (1)

where ε₀ is the permittivity of free space, ε_r is the dielectric constant of CdTe, μ is the hole mobility, V is the applied voltage, V_r is the voltage drop due to contact resistance and series resistance across the electrodes, V_bi is the built-in voltage, and L is the film thickness. The hole mobility of the CdTe NCs films annealed at 300, 350 and 400 °C by this method is 1.32 × 10⁻⁵ cm² V⁻¹ S⁻¹, 3.8 × 10⁻⁴ cm² V⁻¹ S⁻¹ and 1.5 × 10⁻³ cm² V⁻¹ S⁻¹.

The essence of solar cells is charge generation, recombination and transport.⁴² The exciton Bohr radius of CdTe is 6.8 nm and the binding energy is just 10 meV.⁷ Therefore, charge generation is facile at room temperature for all the samples discussed here. As mentioned above, elevating temperature will lower the carrier lifetime which will promote recombination loss. Nevertheless, the improved carrier mobility will decrease the recombination loss. Therefore, how annealing temperature influences the device performance is investigated in the following part.

Thin-film solar cells with device structure of ITO/TiO₂/CdTe/MoO₃/Au are fabricated. The impact of active layer thickness and annealing temperature on the device performance is investigated (Table 2). The active layer with thickness of 80 nm was fabricated by spin-coating the aqueous solution of CdTe NCs in ambient condition, and subsequently annealed at desired temperature in the glove box for 2 min (prolonged annealing time shows no impact on the performance). The active layer thickness is controlled by the number of spin-coating process. Fig. 4a shows the J–V characteristics of the solar cells with annealing temperature of 300 °C. When the active layer thickness is 80 nm, the as-fabricated devices show PCE of 1.85% with V_oc of 0.487 V, J_sc of 8.2 mA cm⁻² and FF of 0.460. As the thickness increases to 160 nm, J_sc increases dramatically while V_oc and FF increase slightly, thus leading to an improved PCE of 3.45%. The increase of J_sc can be attributed to increase of photogenerated carriers as more photons are absorbed. As shown in Fig. 1, large amounts of voids exist in the CdTe NCs film, which will cause the occurrence of leakage current. The J–V curves of the devices measured in the dark are shown in Fig. S2.† The dark current decreases as the thickness increases which indicates the voids are blocked effectively through the multilayer fabrication process. However, further increase of the thickness to 240 nm leads to slight increase of J_sc and decrease of FF. This should be attributed to the low carrier mobility which hinders the charge transport.

Table 2 The detailed device parameters of the CdTe NCs solar cells with different annealing temperature and thickness. Twelve devices are tested to obtain average values

Temperature (°C)	Thickness (nm)	Jsc (mA cm^−2)	Voc (mV)	FF (%)	PCE (%)
300	80	8.2 ± 0.1	487 ± 7	46.0 ± 1.0	1.85 ± 0.08
300	160	13.9 ± 0.2	513 ± 10	51.3 ± 1.0	3.45 ± 0.33
300	240	14.8 ± 0.4	523 ± 7	46.6 ± 1.1	3.71 ± 0.17
350	80	8.1 ± 0.3	456 ± 6	51.2 ± 1.3	1.88 ± 0.13
350	160	13.6 ± 0.1	510 ± 10	52.6 ± 1.2	3.65 ± 0.12
350	240	15.3 ± 0.3	528 ± 12	52.3 ± 1.7	4.12 ± 0.33
400	80	8.0 ± 0.3	410 ± 10	49.9 ± 1.4	1.67 ± 0.09
400	160	13.6 ± 0.1	530 ± 10	58.0 ± 1.9	4.05 ± 0.11
400	240	16.0 ± 0.2	543 ± 13	57.9 ± 1.0	4.96 ± 0.11
400	320	17.2 ± 0.4	558 ± 3	58.6 ± 1.3	5.63 ± 0.10
400	400	16.9 ± 0.4	531 ± 4	53.4 ± 1.5	4.79 ± 0.27

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Fig. 4 (a) J–V characteristics of the CdTe NCs solar cells annealed at 300 °C with different active layer thickness. J–V characteristics of the CdTe NCs solar cells with different annealing temperature and active layer thickness of (b) 80 nm, (c) 160 nm and (d) 240 nm.

In this case, devices with active layer treated at higher temperature are fabricated. The relationship between annealing temperature and device performance is correlated intimately with the film thickness. When the thickness is 80 nm, the increase of annealing temperature leads to negligible change of PCE due to the trade-off between decreased V_oc and increased FF (Fig. 4b). As shown in Fig. 1d–f, the size of voids between NCs increases with the increase of temperature. This will increase the leakage current which in turn lowers the V_oc. By contrast, when the leakage current is inhibited by multilayer fabrication process, the increase of annealing temperature leads to improved device performance (Fig. 4c and d). In particular, the increase of FF and PCE is more obvious when the thickness increases. As the temperature is elevated from 300 °C to 400 °C, the FF shows increase of 8.5% for film thickness of 80 nm. When the film thickness is 160 nm and 240 nm, the ratio of increase is 13.1% and 24.2%, respectively.

The relationship between annealing temperature and device performance shows that the positive effect of improved carrier mobility outpaces the side effect of reduced carrier lifetime. To further confirm this point, electrochemical impedance spectra (EIS) measurement is conducted to investigate the charge dynamics. The EIS measurements were conducted with a frequency ranging from 0.1 Hz to 100 kHz. Fig. 5 shows the EIS and their magnification in the high frequency region of the solar cells with different annealing temperature measured in the dark under zero bias. In the measurements, mainly one semicircle is observed on the Nyquist plot. The diameter of the semicircle exhibits a large dependence on the annealing temperature. Upon increasing the temperature, the shunt resistance (R_sh) increases while the sheet resistance (R_s) decreases. The decrease of the R_s is consistent with the increased carrier mobility. The increase of R_sh implies the decrease of recombination loss which coincides with the improvement of FF and PCE.

Fig. 5 (a) Electrochemical impedance spectra of the CdTe NCs solar cells with different annealing temperature and (b) their magnification in the high frequency region.

Further elevating the temperature to 425 °C, negligible change of device performance is observed (Fig. S3†). After further optimizing the film thickness to 320 nm at temperature of 400 °C, we obtain our champion device which displays PCE of 5.73% with V_oc of 0.564 V, J_sc of 17.8 mA cm⁻² and FF of 0.571 (Fig. 6a). The devices show good reproducibility with average PCE of 5.63% which is obtained from twelve devices. To the best of our knowledge, this PCE is a record for thin-film solar cells fabricated from aqueous NCs. Fig. 6b shows the corresponding external quantum efficiency (EQE) spectrum which presents a wide photoresponse. The EQE spectrum shows peak values of 70% and broad band conversion efficiency of exceeding 55% at wavelengths less than 700 nm. The J_sc based on integration of EQE is 17.9 mA cm⁻² which is in accordance with the measured value.

Fig. 6 (a) J–V characteristic and (b) EQE curve of the champion device with thickness of 320 nm and annealing temperature of 400 °C.

We now turn to discuss the operation mechanism of the as-fabricated CdTe NCs solar cells. Efficient solution-processed inorganic NCs solar cells are generally fabricated based on p–n junction where long depletion width is vital for effective charge transport.^43,44 According to the ultraviolet photoemission spectroscopy measurement (Fig. S4†), the work function of the as-prepared TiO₂ film is close to its conduction band, which indicates that TiO₂ is highly n-type. Meanwhile, CdTe should be p-type due to the excess amount of bivalent anions (S and Te). Therefore, formation of a p–n junction is suggested for our system. As mentioned above, the content of S decreases, which may affect the doping density and therefore the depletion width.

Capacitance–voltage (C–V) analysis is a useful approach for providing an estimation of the net doping density and depletion width. Fig. 7a shows the Mott–Schottky plot of the devices with different annealing temperature. The negative value of the slope matches well with the p-type of CdTe NCs. The net doping density is calculated by the following equation:⁴⁵

N = 2/(qεA²) × dV/d(1/C²) (2)

where N is the net doping density, q is the elementary charge, ε is the permittivity of CdTe, A is the device area, C is the capacitance, and V is the applied voltage. The calculated net doping density of CdTe NCs films treated at 300, 350 and 400 °C by this method are 5.3 × 10¹⁶ cm⁻³, 4.4 × 10¹⁶ cm⁻³ and 2.0 × 10¹⁶ cm⁻³. As the as-prepared TiO₂ film is highly n-type, the depletion region is mainly located on the CdTe NCs films. In this case, the depletion width is inversely proportional to the net doping density of CdTe NCs films. Fig. 7b shows the capacitance–voltage characteristics of the devices with different annealing temperature. It is worth noting that the capacitance decreases with increase of the annealing temperature. The depletion width at zero bias can be calculated by the following equation:⁴⁶

W = εA/C₀ (3)

where W is the depletion width, ε is the permittivity of CdTe, A is the device area, and C₀ is the capacitance at zero bias. The calculated depletion width of the devices treated at 300, 350 and 400 °C by this method is 112 nm, 132 nm and 186 nm. The reduction of net doping density broadens the depletion width which is beneficial for effective charge transport.

Fig. 7 (a) Mott–Schottky curves and (b) capacitance–voltage curves of the CdTe NCs solar cells with different annealing temperature. The frequency is 1000 Hz. (c) The proposed operation mechanism of the as-fabricated CdTe NCs solar cells.

The proposed operation mechanism of the as-fabricated CdTe NCs solar cells is illustrated in Fig. 7c. Bulk heterojunction (BHJ) and p–n depletion junction coexist in this system. Upon light excitation, spatial charge separation occurs between CdTe and CdS which is beneficial for prolonging carrier lifetime. In the depletion region, effective carrier transport can be achieved by the establishment of electrical field. In the neutral region, the recombination loss will be reduced with the assist of prolonged carrier lifetime.

Conclusions

In conclusion, adjusting the composition of the CdTe–CdS heterostructure NCs can be achieved by tuning annealing temperature. Besides, the variation of composition is demonstrated to correlate intimately with charge transfer, recombination, doping density and depletion region. Finally, the operation mechanism of the as-fabricated CdTe NCs solar cells is illustrated. Considering the general application of mercapto ligands during the synthesis of aqueous NCs, the discovery reported in this work may be universal. Therefore, we think this work should be instructive for application of aqueous NCs in thin-film solar cells.

Acknowledgements

This work was financially supported by the National Science Foundation of China (NSFC) under Grant No. 51433003, 21221063, 91123031, 51373065, and the National Basic Research Program of China (973 Program) under Grant No. 2012CB933800, 2014CB643503.

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Footnote

† Electronic supplementary information (ESI) available: UPS spectrum of TiO₂, TA and EDS characterization of the CdTe NCs films. See DOI: 10.1039/c5ra15805b

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