QWERTY smartphones once held an impressive share of the business device market. Companies such as Nokia, BlackBerry, HTC, Toshiba and others have created devices for business people who have extensive correspondence with partners. But under the onslaught of touch screens, such smartphones began to rapidly lose ground. On-screen buttons are convenient for those who write a little: they allow more efficient use of the front panel area, appearing only when needed. But they failed to become a full-fledged replacement for mechanical keys.
Touch input makes it difficult to “touch” typing, has more errors, slows down the typing speed, and forces you to constantly look at the virtual keyboard. If you need to type a lot of text information - an external keyboard comes to the rescue, connected via USB or Bluetooth. The second option is more preferable as it is compatible with any Android device. Wired accessories require OTG support and may not work everywhere.
Our review includes interesting Bluetooth keyboards for smartphones that you can buy now. The selection includes both quite classic accessories and rather unusual solutions. Unfortunately, this market is not developed as well as some users would like. Few manufacturers make devices that are comfortable and stylish at the same time. Therefore, we will try to highlight the most attractive keyboards.
Seenda IBK-03 is a foldable Bluetooth keyboard for smartphones that combines compactness and convenience. When folded, its dimensions are comparable to those of a phablet. In working position, you get a device comparable to a netbook keyboard. The included stand allows you to attach your smartphone to your desk. Thanks to the built-in 200 mAh battery, the keyboard will work up to 3 weeks in standby mode or up to a day with active typing. Seenda IBK-03 price is about $ 25.
Another small "keyboard" is Cvrk-A224. With it, you can not only type long messages from your smartphone, but also recharge it. 
The device is also a 5000mAh portable battery. The dimensions of the Cvrk-A224 are 13x6.5 cm and the price is about $ 30. The gadget is connected via Bluetooth, but it also has two USB ports (full-size and Micro). The first is for recharging the smartphone, the second is for recharging the keyboard itself.
The Samsung Keyboard Cover is available for popular South Korean devices such as Galaxy S6 Edge +, S7 Edge, Note 5 and more. Its cost is from 40 to 80 dollars (depending on the seller, country and smartphone model). A distinctive feature of the gadget is the lack of digital connection interfaces. It is a passive device that works without electricity, therefore, its use does not accelerate the consumption of the battery. 
The kit consists of a bumper for the back cover and a button block overlaid on top of the screen. When connected, the smartphone interface adapts, the lower part of the display turns off (fortunately, SuperAMOLED can do that), and the sensor remains active. Button presses are transmitted to the screen through the capacitive pads located on the back of the accessory.
Norwegian startup one2TOUCH offers a slim and compact keyboard case for smartphones that does not require charging. The accessory is a regular flip case, but slightly thicker (about 4 mm). A slim QWERTY keyboard is integrated into the cover of the case. To connect to a smartphone, NFC technology is used, thanks to which it became possible to reject batteries and wires. The gadget receives energy from the induction field generated by the NFC-module of the mobile device. Unfortunately, the price of the device is still unknown, and its mass production has not yet been established. 
Logitech, a well-known manufacturer of computer peripherals, also could not resist the market for accessories for smartphones and tablets. The universal keyboard K400 Plus, designed for such devices, is not that compact (as much as 33 cm), but a convenient device for typing large texts. Power is supplied from a pair of AA batteries. In addition to the keys, the accessory features a touch-sensitive touchpad and can also function as a mouse on a smartphone. The price of the device is $ 30 without the case and about $ 50 with it. 
Another interesting Bluetooth keyboard for a smartphone is produced under the iWerkz brand. The foldable accessory, starting at $ 30, comes in a variety of colors and comes with a stand for your mobile device. 
The keyboard is powered by a built-in battery, the capacity of which is not specified. The manufacturer claims up to a month of standby time or 44 hours of typing.
The TK-FBP029BK is another compact external smartphone keyboard that will fit any device up to 150mm in height (up to 5.5 "diagonal). It has a flip cover that can be used as a stand. 
It works via Bluetooth, AAA batteries ("mini finger") are used as a battery. The claimed battery life is up to 2 months (depending on the quality of the batteries).